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Energy Engg. Energy Engg. Document Transcript

  • 1.STEAM POWER PLANT Different types of fuels used for steam generation.:- Generally there are three types of fuels can be burnt in any type of steam power plant. They are 1) Solid fuels 2) Liquid fuels 3)Gaseous fuels. Gaseous fuels: The gaseous fuels widely used in steam power plants are natural gas, Blast furnace gas. Gaseous fuels may be either natural gases or manufactured gases. Since the cost of manufactured gases is high only natural gases are used for power generation. Natural gas is colorless , odorless and is non poisonous its calorific value lies between 25000KJto 50000 KJ/ m3 . The various manufactured gases are coal gas, Coke oven gas , Blast furnace gas and producer gas. These manufactured gases play an less important role in the steamgeneration. Advantages: 1) Excess air required is less 2) Uniform mixing of fuels and air is possible 3) Handling is much more easier compared to the coal. 4) The load changes can be met easily. 5) There is no problem of ash disposal. 6) Operational labourer required is less. Disadvantages: 1) Storage of gaseous fuels is not easy compared to liquid fuels due to the risk of explosions. 2) The plant must be located near the natural gas field other wise transportation cost increases. Liquid fuels: The liquid fuels used in the thermal plant to generate the steam instead of coal as it offers the following advantages over the coal. 1) The calorific value of liquid fuels is about 40% higher than that of coal or solid fuels. 2) The storage space required for liquid fuels is less. 3) Instantaneous ignition and extinction of fire is possible. 4) Stand by losses are minimum. 5) Efficiency of the boiler is high. 6) Ash handling system can be eliminated. 7) Oil can be easily metered. The rate of fuel supply to furnace is easy to control. Disadvantages 1) The overall combustion efficiency of the liquid fuel fired power plant is less compared to the coal fired power plants 2) The availability of the liquid fuel resources are very limited as compared to the coal resources Ex. Heavy oils, Bunker C oil , Viscous residue oil, Petroleum and its by products . Solid Fuels: Example for solid fuels is Coal. The term coal refers to the rocks in the earth’s crust , produced by the decaying of the plant materials accumulated overt the milli ns of years ago. Different types of coals that are o used for steam generation are 1) Lignite 2) Sub Bituminous 3) Bituminous coal 4) Semi Anthracite coal 5) Anthracite coal. 1)Lignite: It is lowest grade of coal . Its having 30% of moisture calorific value is about 14650 to 19300 KJ/ Kg. Due to the high moisture content and low calorific value lignite is not easy to transport over long distances. It is usually burnt by the utilities at the mine sites. 2) Sub Bituminous: Its calorific value is slightly less than that of the Bituminous coal. Its calorific value is in between 193000 to 26750 KJ / Kg. The moisture content is about 15 to 30%. It is brownish black in colour. These coals usually burned in the pulverized form. 3) Bituminous coal: Bituminous coal is widely used in all purposes. It is used in steam generation and in the production of the coal gas and producer gas. Its moisture content may vary from 6 to 12 %. Its calorific value ranges from 25600 KJ/Kg to 32600 KJ /Kg. Bituminous coal burn easily especially in pulverized form. 4) Semi Anthracite 1
  • It is an intermediate coal between Bituminous and Anthracite coal. It ignites more easily than anthracite to give a short flame changing from Yellow to Blue . It is having the following properties’ Moisture content = 1 to 2% Volatile matter = 10 to 15 % Calorific value= 36000 to 36960 KJ / Kg 5) Anthracite: It is the most mature and hard form of solid fossil fuel. It is having an fixed Carbon content ranging from 92 to 98%. Anthracite is good domestic fuel for heating and is some times used for steam generation. Selection of coal for steam generation. Selecting an suitable coal for steam generation is an very difficult task. The firing qualities of coal are every important when we are considering an combustion equipments. Slower burning coal generates high fuel bed temperature s and therefore requires forced draught fan. The fast burning coals require large combustion chamber. Such coals are suitable for meeting an sudden demand for steam. The most important factors which are to be considered in the selection of coal are sizing and caking, Swelling properties and ash fusion temperature . Some times the selection of coal depends on the ash content also. The following properties of the coal are to be considered for the selection of the coal for steam generation in steam power plants.1) Burning rate of coal 2) Sizing, Caking , Swelling properties of coals. 3) Finess of coal. Selection of site The following factors should be considered while selecting the site for a steam power station 1) Availability of fuel. 2) Nature of load 3) Cost of land. 4) Availability of water. 5) Transport facilities. 6) Ash disposal facilities. 7) Availability of labour. 8) Size of the plant. 9) Load center. 10) Public problems. 11) Future extensions. Combustion equipments for steam generators. The combustion equipment is one of the important component of the steam generator. The combustion equipment used must have the ability to meet the following requirements. 1) Through mixing of fuel and air. 2) Maintaining of optimum air fuel ratio leading to complete combustion over the full load range. 3) ready and accurate response to the load demand 4) continuous and reliable ignition of the fuel. Coal handling : The coal handling plant needs extra attention , while designing a thermal power station, as almost 50% to 60% of the total operating costs consists of fuel purchasing and handling . Fuel system is designed in accordance with the type and nature off fuel. Plants may use coal oil or gas as the fuel. The different stages in coal handling are shown below. 2
  • Coal delivery The method of transporting coal to a power station depends on the location of the plant, but may be one or more of the following : rail, road , river or sea. Plants situated near river or sea may make use of the navigation facilities. Stations which cannot make use of these facilities may be supplied coal either by trucks or by rail. Transportation of trucks is usually used in case the mines are not available. In case rail transport is to be adopted , the necessary siding for receiving the coal should be brought as near the station is possible. Unloading: Just what kind of equipment will do the best job for unloading depends first of all on how the coal is received. If the coal is delivered in dump trucks and if the plant site is favourable we may not need additional unloading equipment. When coal transported by using by sea or rivers unloading bridge or tower and portable conveyors are used. In case the coal received by rail in hoppers cars, again the coal may be unloaded quickly by using any of the facilities such as car shakers , Car throwing equipments , Car dumpers(Rotary), coal accelerators . Preparation: If the coal is brought to the site un sized and sizing is desirable for storage or firing purposes. The coal preparation plant may be located either near the coal receiving point or at the point of actual use. The coal preparation plant may include the following equipments a) Crushers b) Sizers c) Dryers and d) Magnetic separators. Coal preparation plant is as shown below. 3
  • The raw coal is crushed in to required size using crushers. The crushed coal is passed over the sizer which removes unsized coal and feeds back to the crusher. The crushed coal is further passed to the drier to remove the moisture from the supplied coal . Before supplying the coal to the storage hopper , the iron scrap and particles are removed with the help of magnetic separators. Transfer: Transfer means the handling of the coal between the unloading point and the final storage point from where it is discharged to the firing equipment. The equipments used for the transfer of coal may be any one of the following or a suitable combination there of:a) Belt conveyors b) Screw conveyors c) Bucket Elevators d) Grab bucket Elevators e) Skip hoists and f) Flight conveyors. Belt conveyor Cross section of belt drive The belt conveyors are suitable for transporting large quantities of coal over large distances. It consists of endless belt made up of rubber, canvas or balata running over a pair of end drums or pulleys and supported by series of rollers provided at regular intervals. The return idlers which support the empty belt are plain rollers and are spaced wide apart. Belt conveyors can be used successfully up to 20 degree inclination to the horizontal . The load carrying capacity of the belt may vary from 50 to 100 tons /h and it can easily be transported through 400 meters. Advantages: 1) It is most economical method of coal transfer. 2) The rate of coal can be regulated by varying the speed of the belt. 4
  • 3) The repair and maintenance charges are minimum. 4) The coal can be protected. 5) The power consumption is minimum. Disadvantages; 1) It is not suitable for short distances and greater heights. Screw conveyor It consists of an helicoids screw fitted to a shaft as shown in the figure. The driving mechanism is connected to one end of the shaft and the other end of the shaft is supported in an enclosed ball bearing. The screw while rotating in a trough transfers coal from one end to the other end as shown in figure. The diameter of screw is 15 cm to 50 cm and its speed varies from 70 to 120 rpm and the maximum capacity is 125 tones per hour. Advantages: 1) It requires minimum space and is cheap in cost 2) It is most simple and compact 3) It can be made dust tight. Disadvantages: 1) The power consumption is high. 2) The maximum length limited to 30 meters. 3) The wear and tear is very high therefore life of the equipment is less. Bucket elevators: These are used extensively for vertical lifts, through their for horizontal runs is not ruled out. These elevators consists of relatively small size buckets closely spaced on an endless chain. The coal is carried by the buckets from the bottom and discharged at the top. Centrifugal type and continuous type bucket elevators are most commonly used. The maximum height of the elevator is limited to 30.5 m and maximum inclination to the horizontal is limited to 60 degree. The speed of the chain required in first case is 75 m/min and continuous type is 35 m/min for 60 tonnes capacity per hour. Advantages: 1) Less power is required. 5
  • 2) Coal can be discharged at elevated places. 3) Less floor area is required. Disadvantages: Its capacity is limited to 60 tons per hour and hence not suitable for large capacity stations. Grab bucket Grab bucket conveyor is form of hoist which lifts and transfers the load on a single rail or track form one point to another. This can be used with crane or tower as shown in figure A 2-3 cu-m bucket operating over a distance of 60 m transfer nearly 100 tons of coal per hour. Its initial cost is high but operation cost is less. Flight conveyor This type of conveyor is generally used for transfer of coal when filling of number of storage bins situated under the conveyor is required. It consists of one or two strands of chain to which steel scrappers are attached The scraper scraps the coal through a trough and the coal is discharged in the bottom of the trough as shown in figure. Advantages: 1) It requires small head room 2) The speed can be regulated . 3) It can be used for as well as coal transfer. 4) It requires less attention. Disadvantages: 1) There is excessive wear and tear and hence the life of the conveyor is less. 2) The repair and maintenance charges are high. 3) The restricting the operating speed to 300m/min is required to reduce the abrasive action. 4) Power consumption is high per unit of coal or ash handled. 6
  • Skip hoist: It is used in high lifts and handling is not continuous. It consists of vertical or inclined hoist way, a bucket or a car guided by the frame, and a cable for hoisting the bucket. Advantages: 1) It requires very low maintenance. 2) Power requirement is low. 3) It can handle larger size clinkers. 4) It can be used for handling ash as well as coal 5) It needs minimum floor area. Disadvantages: 1) The initial cost is high 2) This is not suitable for continuous supply of coal. 3) There is excessive wear of skips and ropes which need frequent replacements. Out door storage: Whether the storage is large or small , it needs protection against losses by weathering and by spontaneous combustion. With proper methods adopted even largeroutdoor storage can remain safe. In order to avoid the oxidation of coal the compact layers are formed. To avoid spontaneous combustion air is allowed move evenly through the layers. Indoor storage or Live storage: This is usually a covered storage provided in plants, sufficient to meet day’s requirement of the boiler. Storage is usually done in bunkers made of steel or reinforced concrete having enough capacity to store the requisite of coal. From the coal bunkers coal is transferred to the boiler grates. Weighing: A frequent part of in plant handling is keeping tabs on quantity and quality of coal fired. For weighing weigh bridge is used. Coalis weighed in transit also by using belt scale. Fuel Firing methods Selection of firing method adopted for a particular power plant depends on the follow factors. ing 1) Characteristics of fuel available. 2) Capacity of the plant. 3) Load factor of the power plant. 4) Nature of load fluctuations . 5) Reliability and efficiency of the various combustion equipments. Depending upon the combustion equipments used boilers can be classified as 1) Solid fuel fired. 2) Liquid fuel fired. 3) Gaseous fuel fired Solid fuel firing The classification of combustion system used for coal burning given below. 7
  • Hand firing system is the simplest method for solid fuel firing but it can not be used in modern power plant. The most commonly used methods forfiring the coal are 1) Stoker fire 2) Pulverized fire. Stoker fire: Stoker is fuel burning mechanism used for burning fuel on grate. This type of burning mechanism is suitable where the coal is burned. Stokers are classified as 1) Over feed stokers 2) Under feed stoker. In over feed stoker the direction of air and coal are opposite to one another. The coal is supplied on to the grates above the point of air admission. In under feed stoker coal is fed from underneath the grate between the two tuyers. The direction of fuel and air is same. Over feed stoker. 8
  • Typical overfeed stoker is as shown in the figure. Coal is fed on to the grate above the point of air admission. The pressurized air coming from the FD fan enters under the bottom of the grate. The air passing through the grate opening is heated by absorbing the heat from the ash and grate itself , where as the grate and ash get cooled . As hot air passes through the incandescent coke layer O2 reacts with Carbon to form Carbon dioxide. This is an exothermic reaction and releases heat required for continuation of combustion process. It continues till all the oxygen is consumed. If the incandescent layer thick, CO2 may be partly reduced to CO(CO2 + C – 2CO) The gasses leaving the incandescent layer are N2 , CO, CO2, H2 . A slight water reaction may take place with the moisture in air (H2O + C – H2 + CO) . This is an endothermic reaction and may bring down the temperatureof the bed and gas. Stream of gases then passes through the distillation zone where volatile matter is added from raw coal and then moisture is picked up in the drying zone and finally emerges above the fuel bed. The gases leaving the upper surface of the fuel bed contain combustible volatile matter, N2, CO2, CO, H2 and H2O , If the combustion of Carbon , Hydrogen and volatile matter is to be completed following have to provide . a) Sufficient fresh air or secondary air is supplied . b) Ignition point should be in the range 10000C – 13000 C. c) Creating turbulence by supplying secondary air at right angles to the up flowing gas stream from fuel bed. It does not help supplying if the secondary air supplied along with primary air, since more primary air produces only more carbon monoxide. The presence of the Carbon monoxide in the exhaust gases indicates the in complete combustion leads to decrease in the efficiency of combustion equipments. Types of Over feed stoker 1) Traveling grate stoker 2) 2) Spreader stoker 1) Traveling grate stoker 9
  • The traveling grate stoker is as shown in the figure. This type of stoker has the grate which is moving from one end of the furnace to the other end. This grate may be chain grate type or bar grate type chain grate stoker is made up of series of Cast Iron chain links connected by pins to form an endless chain. The bar grate stoker is made up of a series of Cast Iron sections mounted on a carrier bars. The carrier bars are mounted and ride on two endless drive chains. he traveling grate stoker consist of an endless chain which forma support for the fuel bed . The chain travels over the two sprocket wheels which are at the front and rear end of the furnace. The front end sprocket wheel is connected to variable speed drive mechanism. The grate can be raised or lowered as needed. Simultaneous adjustment of grate speed, fuel bed thickness, and air flow control, the burning rate so that nothing but ash remains on the grate by the time it reaches furnace rear. The ash falls on to the ash pit, as the grate turns to make the return trip. A coal gate at the rear of the coal hopper regulates coal. As the raw coal or green coal on the grate enters the furnace, surface coal gets ignited from heat of furnace flame and radiant heat rays reflected by ignition arch. The fuel bed becomes thinner towards the rear of furnace as combustible matter burns off. The secondary air supplied helps in mixing the gases and supplies oxygen to complete combustion. The coal should have minimum ash content which will form an layer on the grate . It helps in protecting grate from over heating. Advantages: 1) Simple and initial cost is low. 2) Its maintenance costs are low 3) It is self cleaning stoker. 4) Heat releases rates can be easily controlled. 5) It gives high heat release rates per unit volume of furnace . Disadvantages This ca not be use for high capacity boilers 200t / hr or more. The temperature of preheated air is limited to 1800c. The clicker troubles are common. The ignition arches are required. The loss of fuel in ash can not be avoided. Spreader Stoker 10
  • The coal from coal hopper is fed by a rotating feeder, a drum fitted with short blades on its surfaces, to the spreader or distributor below. Which projects the coal particles on to the grate holding an ignited fuel bed. The finer particles burn in suspension and the coarse Particles are consumed on the grate. The speed of the feeder directly proportional to the steam out put of the boiler. The secondary air helps in creating turbulence and completing combustion. In high capacity boilers may have traveling grate stoker in addition to spreader stoker. The grate consists of Cast Iron links underneath the grate connect all the bars to a lever. Moving lever makes the ash fall through to the ash pit below. Spreader stokers capable of burning any type of coal. Advantages: 1) Almost any type of the coal can be burnt. 2) Clinkering problem is less. 3) It is having quick response to varying load. 4) The quantity of excess air required is less. 5) The operation cost is low. Disadvantages: 1) The problem of fly ash is high. It requires an dust collector to prevent the environment pollution. 2) Coal particles trapping mechanism is necessary to prevent their escape with excess air. 3) Its operating efficiency decreases with varying sizes of coal. Vibrating stoker Its operation of similar to that of the chain grate stoker except that fuel feed and fuel bed movement are accomplished by vibration. The vibration and the inclination of the grate cause the fuel to move through the furnace towards the ash pit. The vibrating conditions of the fuel bed permits the use of wider range of fuels. Vibrating grate stokers are suitable for medium volatile Bituminous coal and Lignite but at reduced burn rates. Under feed stoker 11
  • In this type of stokers the fuel and air move in the same direction. In this case coal is fed from underneath the grate by screw conveyor or by a ram. Primary air after passing through the holes in the grate meets the raw coal . As the air diffuse through the bed of raw coal picks up moisture and then pass through the distillation zone where volatile matter is added. When gas stream next passes through the incandescent coke region, volatile matter burns readily with the secondary air fed at the top. The gases in this type stoker are at higher temperature than over feed stoker. The under feed method is best suited for burning semi Bituminous and Bituminous coals high in volatile matter. Types of Under feed stokers. 1) Single retort Stoker 2) Multi retort stoker. 1) Single retort stoker The arrangement of single retort stoker is shown in figure in the form of two views. The fuel is placed in large hopper on the front of the furnace and then further fed by reciprocating ram or screw conveyor in to the bottom of the horizontal trough. Air is supplied through the tuyers provided along upper edge of the 12
  • grate. The ash and clicker are collected on the ash plate provided with dumping arrangement. The coal feeding capacity of a single retort stoker varies from 100 to 2000 Kg / Hr. 2) Multi retort stoker Tuyers Incandescent zone Distilat ion zone Green coal Stoker ram Extension grat e D per am W box ind inlet D per am Pushers D arge sh isch A D gauge aft connection Multi retort stoker is as shown in figure . It consists of series of alternate retorts and tuyers boxes for supply of air . Each retort is fitted with reciprocating ram for feeding and pusher plates for uniform distribution of coal. Coal falling from hopper is pushed forward during inward stroke of the stoker ram. Then distributing ram pushes the entire coal down length of the stoker. The ash formed is collected at the end as shown in the figure. The number of retorts may be vary from 2 to 20 with burning capacity varying from 300 kg to 2000 Kg /hr/retort. Advantages 1) High thermal efficiency compared to chain grate stoker. 2) Combustion rate is high 3) Combustion is continuous 4) Grate is self cleaning 5) Smoke less operation 6) Stokers are suitable for non clinker high volatile and low ash content coal. Disadvantages 1) It requires large building space. 2) Clicker problems are high. 3) Low grade coals with high ash content can not be burn economically. 4) Initial cost of the unit is high. Pulverized fuel firing system. In pulverized fuel firing system the coal is grinded in to a fine powder form with the help of grinding mill and then projected in to the combustion chamber with the help of hot air current. This hot air is known as the primary air. The amount of the air required for complete combustion is supplied separately in the 13
  • combustion chamber. It helps in creating turbulence, so that uniform and intimate mixing of coal particles and air can take place inside combustion chamber. The efficiency of the pulverized fuel firing system mostly depends upon the size of the particles of the coal in the coal powder . The finess of the coal particles should be such that 70% of it would pass through 200 mesh sieve and 98% through a 50 mesh sieve. Coal handling for pulverized fuel plant is shown in figure Advantages: 1) Any grade of coal can be used. 2) Stand by losses are reduced and banking losses are eliminated. 3) Efficiency of combustion is high compared to other methods of solid fuel firing methods. 14
  • 4) Boiler unit can be started up from cold rapidly and efficiently. 5) Practically free from slagging and clinker troubles. 6) Furnace has no moving parts subjected high temperatures. 7) The furnace volume required is less. 8) This system works successfully with or in combination with gas and oil. 9) Greater capacity to meet the peak loads. 10) Practically no ash handling problems. 11) The structural arrangements and flooring are simple. 12) The external heating surfaces are free from corrosion. Disadvantages: 1) Coal preparation plant is necessary. 2) High capital cost. 3) Handling of fly ash makes the system uneconomical. 4) Special equipment is needed to start this system. 5) Larger building space is needed especially with central system.. 6) Skilled operators are required. 7) Refractory material surfaces are affected by high furnace temperatures. 8) Atmospheric pollution created by the fly ash is can not be completely eliminated. 9) The possibility of explosion is more as coal burns like gas. 10) The maintenance of furnace brick work is costly. There are two methods of pulverized fuel firing. They are unit system and central or Bin system. In unit system each burner of the plant has its own pulveriser and handling units. In central or Bin system fuel is pulverized in the central plant and then distributed to each burner with the help of high pressure air current. Unit system In unit system each burner of the plant has its own pulverizer and handling units. The pulveriser an together with feeder , separator and fans may be arranged to form an complete unit or mill. The number of units required depends on the capacity of the boiler. Raw coal from coal hopper fed to the pulverizing mill through feeder . Hot air from or flue gases passed through the feeder to dry the coal before feeding to the pulveriser. The pulverized coal is carried from the mill with the help of induced draught fan as shown in figure. This further carries the coal through the pipes to the burner. Secondary air supplied to the burner before fuel entry in to the combustion chamber is as shown in figure helps in creating the turbulence as well as supplying additional air required for completing the combustion of the coal particles in the furnace. Advantages: 15
  • 1) It is simple in layout and cheaper than central system. 2) It allows direct control of combustio rate from the pulveriser. n 3) Maintenance charges are less. 4) The coal transportation is simple. Disadvantages. 1) The performance of pulverizing mill is poor. 2) Degree of flexibility is less than central system. 3) The fault in the preparation unit may put entire steam generator out of use. 4) There is excessive wear and tear of the blades of fan as it handles air and coal particles. 5) Strict maintenance of the mill is required because the entire plant operation depends on it Central or Bin system The central system or Bin system fuel is pulverized in the central plant and then distributed to each burner with the help of high pressure air current. Crushed and sized coal is fed to the drier from coal bunker by gravity as shown in figure. The dried coal fed to the pulverizing mill with the help of air, as shown in figure ,separated in the cyclone separator. The separated pulverized coal is transferred to the central bunker using conveyor as shown in figure. Oversized coal particles are fed back to the pulverizing mill for further processing. The storage bin may contain 12 to 24 hours of supply of pulverized coal. The energy consumption is 15 to 25 KW-Hr / Ton of coal pulverized. Advantages: 1) The reliability of plant is high. 2) The central system is flexible . Supply of the coal can be maintained to the burners without any interruption. 3) Burner operation is independent of coal preparation. 4) The pulverising mill may work at the part load because of storage capacity available in the storage bin. 5) Power consumption per ton of coal handled is less . 6) As the fans handle only air there is no problem of excessive wear and tear. 7) The labourers required is less . 16
  • Disadvantages. 1) Initial cost is high and occupies a larger space. 2) The overall power consumption per ton of coal handled is higher than unit system due to high power consumption by auxiliaries. 3) The operation and maintenance charges are higher than unit system of same capacity. 4) There is possibility of fire hazard due to the stored pulverized coal. Equipments or components of the pulverized coal fired plant. The main equipments used in the pulverized coal fires plant are 1) Primary crushers . 2) Magnetic separators. 3) Coal driers . 4) Pulverizing mill 5) Burners. Primary crushers Crushing of coal is required when we are handling un sized coal. Plant using pulverized coal generally specifies the top size, larger than what can not be handled by the pulveriser, making crushing necessary to prepare coal for pulverization following types of crushers are used. 1) Ring crushers 2) Hammer mill crushers. 3) Bread ford breaker. 4) Rotary breaker 5) Single roll crushers. Ring crushers In this type of crushers coal is fed at top of crushers and is crushed by the action of ring that pivot off centre on a rotor. Adjustable plate helps in varying the size of discharge coal. It can be used as off or on plant site. Hammer coal crusher 17
  • In this type of coal crusher also the coal is fed from the top and is crushed by the action of swinging hammers that are pivoted on a rotor. Swinging hammers are attached to the central drum. As the drum rotates coal particles coming in between the swinging hammers and adjustable plates crushed . The crushed sized coal falls out of the crusher through the opening provided at the bottom. The adjustable plates used to vary the size of discharge coal. Brad ford crushers. It is used in large capacity plant. It comprises of large cylinder consisting of perforated steel screen plates to which lifting shelves are attached inside . The cylinder rotates slowly at about 20 rpm and receives feed at the one end. The coal is lifted by the shelves , the breaking action is accomplished by the repeated lifting and dropping of the coal until its size permits it to discharge through the perforation made . The size of the perforation determine the size of crushed coal. The main advantage is rejection of the foreign matter and to produce relatively uniform size coal particles. Rotary breakers. 18
  • The crushing of coal takes place between the rotating cylinder and rollers. The crushing action is combination of both lifting and dropping of the coal and also by the crushing action of coal between rollers and rotating cylinder. Single roll crushers The crushing of coal takes place between adjustable plate and rotating single roller having teeth on the circumference. The size of the coal particle can be varied by varying the gap between adjustable plate and rotating roller. Pulveriser: Pulverisers are devices that are used to produce coal in the powder form. They are also called as pulverizing mills. The pulverizing process consists of three stages namely i) Feeding ii) Drying iii) Grinding . Feeding system controls automatically air required for drying and transporting pulverized fuel to the burner depending on the boiler demand. For pulverization of coal has to be dry and dusty. Dryer are an integral part of the pulverizing equipment. For drying coal part of primary air passing through the air preheater at 3500c is utilized. The third stage of pulverization process is the grinding and equipment used for this action is known as the grinding mill. Four different types of pulverizing mills are used . a) Ball and race mill b) Bowl mill. c) Ball mill. d) Hammer mill. a) Ball and race mill 19
  • This is also known as the contact mill. The coal is crushed between two moving surfaces ball and race. The upper race is stationary and the lower race is driven by worm and gear, holds the steel balls between them. The coal is allowed to fall on the inside of the race from feeder or hopper. Moving balls and race catches coal between them to crush in to a powder. Springs are used to hold down the upper race and adjust the force needed for crushing. Hot air supplied picks up the coal dust as it flows between the ball and races and then enters in to the classifier, moving and fixed vanes make the entering air to form a cyclonic flow which helps to through the oversized particles on to the wall of classifier. The oversized particles slide down for further grinding in the mill. The coal particles of required size carried to burners with air from the top of the classifier. b) Bowl mill 20
  • The bowl mill grinds the coal between a whirling bowl & rollers mounted on pivoted axis. The pulveriser consists of stationary rollers and power driven balls in which pulverization takes place as the coal passes between the bowl and rollers. The hot primary air supplied in to the bowl picks up coal parcels and passes through the classifier. Where oversized coal particles falls back to bowl for further grinding. The required size coal particles along the primary air supplied to the burner. c) Ball Mill with double classifier 21
  • The line diagram of the ball mill is as shown in figure . It consist of a large cylinder partly filled with varying sized steel balls. The coal from coal hopper fed in to the cylinder with the help of crew conveyor. At the same time required quantity of hot air from air preheater is also enters. As the cylinder rotates pulverization takes place between the balls and the coal. The stream of hot air picks up the pulverized coal and pass through the classifier. The oversized coal particles thrown out of the air stream in the classifier and fine coal particles are passed to the burner through exhaust fan. Ball mill capable of pulverizing 10 tons of coal / hr containing 4% moisture requires 28 tons of steel balls and consumes 20- 25 KW – Hr energy per ton of coal pulverized. d) Hammer mill The hammer mills have swinging hammers connected to an inner ring and placed within the rotating drum. The coal to be pulverized is fed in to the path of hammers. Grinding is done by the combination of impact on large particles and attrition on small particles . The hot air is supplied to dry the coal as well as carrying coal particles to burners. It is compact low in cost and simple in operation. How ever its maintenance is costly and its capacity is limited. The power consumption is high when fine powder is required. 22
  • Pulverised fuel burners. Burners are devices use to burn coal particle by uniform mixing of coal and air and creation of turbulence within the furnace. The air which carries pulverized coal in to the furnace through the burner is primary air. The secondary air required for completing combustion is supplied separately around the burner or else where in the furnace. The main requirements of pulverized fuel burners are. 1) It should mix thoroughly primary air with coal particles and secondary air. 2) It should create turbulence and maintain stable combustion. 3) It should control the flame shape and it travel in the furnace. 4) The velocity of primary air and coal particles should be same as that of flame velocity to avoid flash back. 5) The burner should have ability to with stand overheating due internal fires and excessive abrasive wear. Types of pulverized burners are 1) Long flame or U- Flame burners. Or streamlined burners. 2) Turbulent burners. 3) Tangential burners. 4) Cyclone burners 1) Long flame burners. The tertiary air supplied around the burner to provide better mixing of primary air and fuel. The burner discharges air and fuel mixture vertically down wards with no turbulence to provide long flame. Heated secondary air supplied at right angles to the flame creates turbulence that required rapid combustion. This type of burners are suitable for burning low volatile slower burning coal particles. 2) Turbulent Burners. 23
  • These burners are also called as short flame burners. Turbulent burners can project flame horizontally or at small inclination to the furnace. The fuel – primary air mixture and secondary hot air are arranged to pass through the burner in such a way tat there is good mixing and the mixture is projected in highly turbulent form in to the furnace. The mixture burns intensely and combustion is completed in a short distance. The burning rate of turbulent burners is high compared to other types of burners. Turbulent burners are preferred for high volatile coal and they are used in modern power plants. 3) Tangential burners. It consists of four different burners located at 4 corners of the furnace. The discharge of fuel and air mixture directed tangentially to an imaginary circle in the centre of the furnace. The swirling action creates necessary turbulence required for completing the combustion in short period. The tips of the burners can be angled through a small vertical arc. So as to raise or lower the position of turbulent combustion region in the furnace. It helps in maintaining constant super heat temperature of steam as load varies. This arrangement can provide 1000c difference in furnace gas exit temperature. Advantages 1) Parts of burners are well protected. 2) High combustion efficiency and turbulence existing throught the furnace. 3) Liquid, gaseous and pulverized fuel can be readily fired either separately or in combination. 24
  • Cyclone Burner It consists of horizontal cylinder of water cooled construction , 2 to 3 meters in diameter and 2.5 m in length . The horizontal axis of the burner is slightly deflected downward towards the boiler. These burners are externally attached to the furnace. The cyclone burner receives pulverized coal carried by the primary air tangentially to the cylinder at outer end creates strong and highly turbulent Vortex. Secondary air enters in to the cylinder tangentially to complete the combustion. These burners can be rotated by 30 degree up and down it helps in controlling the super heater temperature.The fuel supplied burns quickly with high heat liberate rates with temperature around 20000c . The ash forms the molten film over the inner wall surface and molten ash flows to an ash disposal system. The cyclone burners give best results with low grade fuel. Advantages 1) Crushed coal can be used instead of costly pulverized coal. 2) It can burn low grades of coal. 3) Percentage of excess of air required is less 4) Combustion efficiency is high 5) Combustion rates ca be easily controlled by varying fuel and air supply 6) High furnace temperature can be obtained. Ash handling system 25
  • General layout of ash handling system Large quantity of ash is produced by the power plants. Which are burning coals having high ash content. The ash should be discharged and dumped at sufficient distance from the power plant because of the following reasons. 1) The ash content is dusty 2) It is very hot when it comes out of the furnace 3) It produces poisonous gases and corrosive acids whenmixed with water. The amount of ash produced is as large as 20% of total coal burnt during the day. In order to handle this large quantity of ash use of mechanical handling equipment becomes necessary. Any ash handling system consists of the following operations. 1) Removal of ash from the furnace. 2) Carrying of ashes from ash hopper to storage with the help of conveyor. 3) Quenching of hot ash before carrying is desirable and necessary as it offers the following advantages. a) Reduces the temperature . b) Reduces dustiness of ash. c) Reduces the corrosive action. d) Disintegrate large clinkers in to smaller one . e) It act as sealing against the air entering in to boiler. Ash handling equipment. The main requirements of good ash handling plants are listed below. 1) It should be capable of handling large volume of ash. 2) It should be capable of handling large clickers with minimumattention. 3) The plant should have high rates of handling. 4) The operation should be noise less as much as possible. 5) It should deal effectively both hot and wet ash. 6) The initial cost, operating and maintenance charges should be minimum as per as possible. The generally used ash handling systems are classified in to four groups. 1) Mechanical handling system 2) Hydraulic handling system. 26
  • 3) Pneumatic handling system. 4) Steam jet system. Mechanical ash handling system. This system of handling ash is used in low capacity power plants. The hot ash coming out of furnace allowed to fall on to the belt conveyor moving through the water trough . Cooled ash carried continuously by belt conveyor to the ash bunker . The ash is removed from the ash bunker to the dumping site with the help of trucks. Hydraulic handling system. In this system ash is carried with the flow of water . The hydraulic system, is subdivide in to low velocity system and high velocity system. Low velocity system. In this system water trough is provided just below the boiler and water is made to flow through the trough. The ash falling directly in to the drain and it is carried by water to the sump. In the sump ash is separated from water , separated water is used again while the ash collected in the sump is removed to the dumping yard. The capacity of this system is 50 tons/ hr. 27
  • High velocity or high pressure system. The ash hoppers below the boilers are fitted with water nozzles at the top and on the sides. The top nozzle quench ash and side nozzle provide driving force to carry the ash through a trough. The cooled ash with high velocity water is carried to the sump. The water is re circulated again after separating it out from the ash. Capacity of the system is 120 tons /hr and distance is 1000 meters. Advantages 1) The system is clan , dustless, totally enclose and pollution free. 2) The ash can be discharged at a considerable distance. 3) Its handling capacity is large hence it can be used in large capacity powerplants. 4) Working parts do not come in contact with ash. 5) It can also be used to handle molten ash. Pneumatic handling system. 28
  • In primary and secondary separation working on cyclone principle and then it is collected in the ash hopper as shown in the figure. The clean air is discharged from the top of the secondary air separator in to the atmosphere through the exhauster. Exhauster may be mechanical type with filter or washer to ensure that the exhauster handles clean air or it may use steam jet or water jet for its operation. Mechanical exhausters are used in large power stations. While steam exhausters are used in small and medium power stations. The pneumatic system can handle abrasive as well as fine materials such as fly ash as soot. The capacity of system varies from 15 -25 tons/hr. Advantages: 1) The system is flexible. 2) There is no spillage and re handling. 3) No chances of ash freezing and sticking of the materials , ash can be discharged freely by gravity. 4) Dustless operation as the system is totally closed. 5) Cost / ton of ash handled is comparatively less. Disadvantages. 1) Wear and tear of pipes is high and hence the maintenance costs are high. 2) The operation is noisy compared to other systems . Steam jet system In this type of ash handling system, a jet of high pressure steam is passed in the direction of ash travel through a conveying pipe in which ash from the boiler ash hopper is fed. The ash is deposited in the ash hopper . The velocity is given to the steam by forcing it through the pipe under pressure greater than that of atmosphere. Advantages: 1) It does not requires any auxiliary drivers. 2) Capital coat and maintenance costs are low. 3) It requires less space. 4) Equipment can be installed in any position Disadvantages 1) Noisy operation. 2) Wear and tear of pipes is high. 3) Capacity of this system is limited to 15 tons/hr 29
  • Dust collection Any gas borne matter larger than 1 micron (0.001mm) in diameter we called it as dust. If the particles are mainly ash particles then it is called fly ash. If the particles are in turn mixed with some quantity of carbon, then the matter is known as the cinders. The size of cinders is usually greater than 100 micron. Incomplete combustion volatile components of fuel produces smoke, consists of particles smaller than 10 micron. The removal of dust and cinders from flue gas can be achieved by using dust collectors. These are classified as 1) Mechanical dust collectors. 2) Electrical dust collectors. Mechanical dust collectors. The basic principle used in the mechanical dust collection is as shown in the figure. a) Sudden velocity decreasing method: Enlarging cross sectional area off the dust carrying pipe helps in slow down of the gas so that dust particles will have the chance to settle out are allowed to fall down. b) Abrupt change of flow direction.: When gas makes a sharp change in flow direction the heavier particles tend to keep goinig in original direction and so settle out. c) Impingement upon small baffles: The larger dust particles may be knocked out of the gas stream by impingement on baffles. These are used to drop large cinders from the gases. Mechanical dust collectors can be further classified as wet type and dry type. The wet type dust collectors are also called as scrubbers . Scrubbers operate with water sprays to wash dust from the air. Large quantity of wash water is required for central power stations and this system is rarely used. This also produces waste water that may require chemical neutralization before it may be discharged in to the natural water bodies. Scrubbers may be 1) Packed type 2)Spray type 3) Impingement type Dry collectors Dry collectors are the most commonly used . One example for dry dust collectors is cyclone separator. In this type of mechanical dust collector, a high velocity gas stream carrying the dust particles enters at high velocity and tangential to the conical shell. This produces a whirling motion of the gas within the chamber and throws heavier dust particles to the sides and fall out of the gas stream and are collected at the bottom 30
  • of the collector. The gas from the conical shell is passed through the secondary chamber as shown in figure for final separation. Advantages 1) Maintenance cost is low. 2) Efficiency is higher for bigger size particles. 3) Its efficiency increases with increasing the load. Disadvantages 1) It requires more power than other collectors. 2) It is not flexible. 3) Pressure loss is comparatively high. 4) The collection efficiency decreases as the finess of dust particles increases. 5) It requires large head room. Electrostatic precipitator. Electrostatic precipitator are extensively used in removal of fly ash from electric utility boiler emissions. The dust laden gas is passed between oppositely charged conductors and it becomes ionized. As the dust laden gas passed through these charged electrodes, both negative and positive ions are formed. The ionized gas is further passed through the collecting unit which consists of set of vertical plates. Alternates plates are charged and earthed. As the alternate plates are grounded, high intensity electrostatic field exerts a force on positively charged dust particles and drives them towards the grounded plate. The deposited dust 31
  • particles are removed from the plates by giving the shaking motion to the plates . Dust removed collected in the dust hoppers. Advantages 1) It is more effective in removing small particle. 2) Its efficiency s high. 3) The drought losses are least. 4) It provides ease of operation. Disadvantages: 1) Use of electrical equipment for converting AC in to DC is necessary. 2) The space required is larger than wet system. 3) Collectors must be protected from sparking . 4) The running costs are high. High pressure boilers. For generating steam up to 30 bar pressure with flow rates up to about 30 tones / hr can be achieved with the use of shell boilers using fire tube principles. These boilers are known as the low pressure boilers. High pressure boilers are operating with pressures ranging from 30 bar to 300 bars, steam flow rate vary between 30 to 650 tones /hr and maximum temperature is around 6000c having furnace height varying from 32 to 62 m. In high pressure boilers water tube principle is used. The boilers operating at pressure 221 bar . Then such boilers are known as the sub critical boilers. The boilers operating above 221 bar steam pressure are called super critical boilers. 32
  • The unique feature of high pressure boilers are i) Method of water circulation. Use of natural circulation is limited to sub critical boilers with pressures less than 221 bar. In high pressure boilers forced circulation of water is used instead of natural circulation. With the increase in pressure in the boiler, the pressure difference causing the natural flow of water decreases and this becomes zero at critical pressure of steam 221 bar, because the density of water and steam is same at critical pressure. Therefore the use of forced circulation becomes necessary. Forced circulation of water is achieved with the help of pumps these pumps are known as forced circulation pumps. ii) Arrangement of drums and tubing. In order to avoid large resistance to flow of water these boilers have a parallel set of tubes arrangement. They have small steam separating drum or may be entirely free of drum. iii) Improved method of heating. The following methods are used to improve the heating. 1) heat added to produce steam can be avoid by eliminating latent heat of evaporation at pressure above critical (221 Bar). 2) Super heated steam is used to heat water by mixing. 3) Heat transfer coefficients can be improved by increasing gas and water velocities above sonic velocity. Advantages: 1) Scale formation is avoided due to the use of high velocity of water. 2) Light weight tubes can be used. 3) Reduction in number of tubes used. 4) Boilers are capable of meeting rapid load changes. 5) Completely eliminates the hig head which is needed for natural circulation. h 6) Since all parts are heated uniformly which eliminates danger of overheating and setting up thermal stresses. 7) Construction time required is less. Lamont boiler 33
  • The schematic arrangement of Lamont boiler is as shown in figure .The feed water from hot well is pumped in to the steam separating drum with the help of the feed pump. The circulating pump draws water from the drum and delivers it under pressure to the headers. These headers distribute the water to the steam generating tubes or evaporator, part of the water evaporated is separated in the steam separator drum. The steam from the top of the drum is allowed to enter super heater s located in the path of hot gases. As the steam is drawn from the super heater , an equivalent quantity of feed water is supplied through the economizer in to the drum. The large quantity of water circulated prevents the tubes from being overheated. These boilers can be built to generate 45 to 50 tones/hr of super heated stem at pressure of 130 bar and at a temperature of 5000c . The major disadvantage of this boiler is the formation of scale due the presence of dissolved gases in the water it decreases heat transfer rate and efficiency of the boiler. Loeffler boiler. 34
  • The major difficulty of salt deposition and sediments experienced in Lamont boiler was solved in Loeffler boiler by preventing the flow of water to the boiler tubes. In Loeffler boiler, feed water passes through an economizer before its entry in to the evaporating drum. Superheated steam mixing with drum water and evaporates it in to saturated steam. The saturated steam flows through radiant and convective super heaters. About 2/3rd of the steam returns to the demand 1/3rd leaves as the steam generator out put. The steam coming out of from HP turbine is passed through reheater before supplying to LP turbine. The steam generating capacity of this boiler is 100 tones/hr at 140 bar pressure. It is best suited for land and sea transport power generation. Benson: One of the difficulty experienced in Lamont boiler is the formation and attachment of bubbles on the inner surfaces of the heating tubes. In Benson boiler the difficulty of bubble formation experienced in Lamont boiler is avoided by raising the boiler pressure to critical pressure (221.6Bar) . The arrangement of the boiler components is as shown if the figure. The Benson boiler is the drum less once through boiler. This boiler takes the feed water in at one end and discharges it as superheated steam at the other end. Feed water flows through the radiant tube section to evaporate partly. Where major part of the water is converted in to steam. The remaining water is evaporated in the convection evaporator tubes. The saturated high pressure steam is further passes through super heater before leaving the unit. Major problem that experienced with this boiler is the salt deposition. To avoid this difficulty the boiler is normally flashed out after every 4000 working hours to remove the salt. Capacity of this boiler is 150 tones / hr of steam generation with pressure 300 bar at 6000c. Advantages 1) There may be no pressure limitation and it may be as high as super critical. 2) Absence of drum and hence cost is less. 3) Evaporation is quick. 4) Light in weight. 5) Space re4quired is less. 6) Expansion problem is less compared to drum type boiler. Disadvantage 1) The deposition of salt in evaporator tube is common. 2) Over heating of tubes incase of insufficient water supply. 35
  • 3) It requires close coordination between steam generation and feed water supply. 4) There is a greater chance of corrosion of evaporator tubes. Velox boiler. The Velox boiler is a high pressure , forced circulation pressurized or forced combustion boiler with the limitation of firing with oil or gas. Air is compressed to about 2.5 bar in an compressor run by gas turbine before being supplied to an furnace. Compressed air helps in generating high velocity gas and also at the same time release of greater amount of heat . The heat transferred from gases to water while passing through the annulus to generate the steam. The mixture of water and steam thus formed then passes in to separator. The separated steam is further passed in to the super heater and then supplied to the prime move. The water removed from mixture is again passed in to the water tubes with the help of a pump. The gases coming out of the combustion chamber are used for superheating steam in super heater. The gases coming out of the gas turbine are used to heat water in economizer. The capacity of this boiler is limited to 100 tones /hr Advantages: 1) High combustion rates are possible. 2) Low excess air is required. 3) It is very compact. 4) It can be quickly started. Schmidt Boiler 36
  • It consist of two separate circuits. In primary circuit steam is produced from distilled water. The generated steam is passed through an heating coil located in evaporator drum. The steam produced in the evaporator drum from impure water is passed through the super heater and then supplied to the prime mover. The high pressure condensate formed in the submerged heating coil is circulated through the low pressure feed water pre heater to raise the fed water temperature to its saturation temperature. Advantages: 1) Overheating of tubes is completely eliminated. 2) It is capable of taking wide fluctuation of loads. 3) Removal of salts deposited is easy. Diesel Engine power generation. 37
  • Introduction: Diesel electric engine plants which are in the range of 2 to 50 MW capacity used as central stations for small capacity and they are universally adopted to supplement hydroelectric or thermal stations where stand by generating plants are essential for starting from cold conditions. The demand for diesel engine plants increased in some countries due to the difficulties in establishing the new hydroelectric steam , steam power plants and enlargement of old power plants. The diesel engines are most efficient than any other heat engines of comparable sizes . It is cheap in first cost. It can be started quickly and brought in to the service. Its manufacturing periods areshort. Applications: The following are the very important applications of the diesel power plants. 1) Peak load plants: Since the diesel plants can be started quickly and it has no stand by losses it can be used as peak load plant in combination with thermal and hydel plant. 2) Stand by unit: This can be used as stand by unit to supply part load when required. There are many situation like when main unit fails or can not cope with demand, due to less rain fall in a particular year, hydroelectric power plant can not meet the demand. Thus diesel units are installed as stand by unit to supply power in parallel to generate the short fall of power. 3) Central stations: Due to the ease in installation, starting stopping, diesel electric plants can be used as central stations where the capacity required is very small. 4) Starting stations: The diesel units are used to run the auxiliaries for starting the large thermal power plants. 5) Mobile units: These plants are mounted on the trailer are used to supply the power to construction works that are carried out in remote areas where there is no power. 6) Nursing stations: In remote areas where there is no supply of power from main grid diesel electric plant can be installed. When the power becomes available by main grid, this plant can be shifted to other place such as diesel electric plants is generally called nursery uni or stations. t 7) Emergency plant: Diesel electric plant can be used as the emergency plant to meet the power requirement when ever there is an power interruption, in the emergency situations like tunnel lighting, hospitals, Telecommunication and watersupply. Advantages: 1. Design and installation are very simple 2. Can respond to varying loads without any difficulty. 3. The stand by losses are less. 4. Occupy less space . 5. Can be started and put on load quickly . 6. Require less quantity of water for cooling purposes. 7. Overall capital cost is lesser than that for steam plants . 8. Require less operating supervising staff as compared to that for steam plants. 9. These plants can be located very near to the load centre. 10. Lubrication system is more economical as compared with that of a steam power plant. Disadvantages: 1.High operating cost 2) High maintenance and lubrication cost. 3) Diesel units capacity is limited . These plants cannot be constructed in large size. 4) In a diesel power plant noise is a serious problem. 5) Diesel plants cannot supply over loads continuously. 6) The life of the diesel plant is quite small. General layout of diesel engine power plant: 38
  • The essential components of diesel engine power plants are shown in the figure. It consists of the following components. 1) Engine: This is the main component of the plant which develops required power by burning fuel. Which is directly coupled to the generator 2) Air filters and super chargers: Air filters remove dust from the air which is taken by the engine. Super charger is used to increase the pressure of air supplied to the engine to increase the engine power. 3) Exhaust system: This includes the silencers, connecting ducts. The high temperature of exhaust gas is utilized for heating oils or air supplied to the engine. 4) Fuel system: It includes the storage tank, fuel pump, fuel transfer pump, strainer and heaters. The fuel is supplied to the engine according to the engine load on the plant. 5) Cooling system: It includes water circulating pumps, cooling towers, or spray ponds, water filters on plants. This system helps in maintaining the temperature of engine within the allowable limits. 6) Lubrication system: It includes the oil pumps, oil tank filters, coolers and connecting pipes. It helps in reducing the wear and tear of the moving parts. 7) Starting system: This includes compressed air tank. The function of this system is to start the engine from cold condition by supplying compressed air. 8) Governing system: The function of the governing system is to maintain the speed of the engine constant irrespective of load. This can be done by varying the fuel supply to the engine according to the load. Types of diesel engines: Different types of diesel engines are used 1) Four stroke 2) Two stroke 3) Horizontal engine 4) Vertical engine5) Single cylinder 6) Multi cylinder 7) Indirect ignition 8) Direct ignition 9) naturally aspirated 10) Super charged engine 11) Air cooled engine 12) Water cooled engine etc. Four stroke multi cylinder engines are generally used. Methods of starting diesel engines: 39
  • It is difficult to start even smallest diesel engine by hand cranking as the compression pressures required are extremely high. Therefore some mechanical system must be used to start the engine. There are three methods are used in starting the diesel engines they are: 1) By an auxiliary engine: An auxiliary engine mounted close to the main engine drives the diesel engine plant through clutch and gear . Once the engine stared auxiliary starting device automatically disengages. 2) Storage battery and electric motor: By using an electric motor, in which a storage battery of 12 to 36 volts is used to supply power to an electric motor that drives the engine. 3) Compressed air system: In this system compressed air is used to start the diesel engine. The compressed air about 17 bar. The compressed air from this air tank is admitted to the few engine cylinder making them to work like reciprocating air motor to run the engine shaft. Fuel is admitted in to the remaining cylinders and ignited to start the engine. Compressed air system is widely used to start the diesel engine plants. Different systems of diesel engine power plants 1) Fuel storage and supply system: The fuel storage system and supply system depends on the type of fuel, size of plant and type of engine used and so on. The fuel oil may be delivered at the plant site by many means such as trucks, railway wagons or Barges and oil tanks with the help of unloading pump the fuel oil is delivered to the main tank from which oil is pumped to small service storage tank known as engine day tank through the strainers. In order to reduce the pumping power input oil is heated either by hot water or steam which reduces the viscosity and so the power input. From storage tanks oil flows under gravity to the engine pumps. This type of system is as shown in figure and it is generally preferred for medium size or big size plants. The location of storage tank above ground or below ground depends on the local conditions. The heating requirements depends on the climate conditions. Fuel Injection system: It is an heart of the engine and its failure means stopping of engine. It performs the following functions. 40
  • 1) It filters the fuel ensuring oil free from dirt 2) It measures the correct quantity of the fuel to be injected in each cylinder.] 3) It times the injection process in relation to the crank shaft revolution. 4) It regulates the fuel supply. 5) It atomizes the fuel oil under high pressure for better mixing with hot air leading to efficient combustion. 6) It distributes the atomized fuel properly in the combustion chamber. There are two methods used in atomizing the fuel. 1) Air injection system 2) Mechanical injection system Air injection system is obsolete and mechanical injection is invariably used. In mechanical injection or solid injection system fuel oil is forced to flow through the spray nozzles at pressure above 100 bar. There are three types of solid injection system. 1) Common rail injection system 2) Individual pump injection system. 3) Distributor system. 1) Common rail injection system This system consists of a high pressure pump which distributes fuel to a common rail or header to which all the fuel injectors are connected. In common rail system, the fuel injectors are operated mechanically. The metering and timing of fuel injection is accomplished by the spray valve. Then amount of fuel to be injected in to the cylinder in controlled by the lift of the needle valve in the injector. The quantity of the fuel injected depends on the duration of valve opening size and number of holes in the nozzle tip, fuel pressure and air pressure in the cylinder. 2) Individual injection system: 41
  • As the name employs, the system has an independent high pressure pump for each cylinder with meters, pumps and controls the timing of fuel injection. Each cylinder is provided with one injector and the pump. The fuel is brought to the individual pump from storage tank or day tank through filters, low pressure pump. The high pressure pump is equipped with control mechanism of injecting fuel at the proper time , a rocker arm actuates the plunger and thus injects the fuel in to the cylinder . This is the most popular fuel injection system used. 3) Distributor fuel injection system. The above figure shows the arrangement of distributor system. In this system, a metering and high pressure pump is used to pump the metered quantity of fuel in to the rotating distributor which distributes the fuel to 42
  • the individual cylinders at the correct timing. The number of injection strokes per cycle for the pump is equal to the number of cylinders. The fuel is fed to high pressure pump from storage tank through course filter, low pressure pump and the fine filter. Air supply system: A large diesel power plant requires considerable amount of air as 4 to 8 m3 / Kw-hr . The air contains considerable amount of dust and , therefore it is necessary to remove this dust from air before entering in to the cylinder which would otherwise cause excessive wear in the engine. An air supply system of diesel power plant begins with intake located out side the building provided with filters. The filters may be oil impingement or oil bath or drag type depending upon concentration of dust in the air. In cold climate conditions the air intake system needs heating and necessary heating of air is provided by using the heat from the exhaust gases. Exhaust system: The purpose of exhaust system is to discharge the engine exhaust to the atmosphere with minimum noise. The following fig shows the exhaust system. The exhaust manifold connects the engine exhaust outlets to the exhaust pipe which is provided with a muffler or silencer to dampen the fluctuating pressure of exhaust line which in turn reduces the noise . To isolate the exhaust system from engine vibration flexible exhaust pipe is used. A provision is made to extract heat from exhaust system if the heating is required for fuel oil heating or building heating or process heating. Cooling system: 43
  • The temperature of the gases inside the cylinder vary from 3500c to 27500c . If there is no external cooling, the cylinder walls and piston tend to attain the temperature of the gases which may be of the order of 10000c to 15000c . The cooling of the engine is necessary forthe following reasons. 1) To prevent the disintegration of the lubricating oil. 2) To avoid the seizure of the piston. 3) Overheating of cylinder may lead to pre ignition of fuel air mixture which affects the performance of the engine. 4) The strength of the materials used for various engine parts decreases with temperature. It may lead to the cracking of the different parts of the engine due to uneven heating. 5) At high head temperature , the volumetric efficiency and hence the power out put of the engines are reduced. If the engines are not properly cooled temperatures inside the engine causes disintegration of the lubricating oil on the liners , wrapping of valves and pistons takes place. Proper cooling of engine is necessary to extend the life of the plant. Therefore exit temperature of the cooling water must be controlled. If it is too low, lubricating oil will not spread properly and wearing of piston and cylinder takes place . If it is too high, the lubricating oil burn. Therefore maximum exit temperature of water is limited to 700c. Based on cooling medium used cooling systems are classified in to 1) Air cooling system. 2) Liquid or indirect cooling system. 1) Air cooling system: It can be used in very small engines and portable engines by providing fins on the cylinder. The fins are arranged in such a way that they are at right angles to the cylinder axis and air flow should be such that the fin surfaces are exposed to maximum air flow. Advantages: 1) The cooling system is simple as no cooling pipes, radiators are involved. 2) For a given power the weight of air cooled engines is lesser than the liquid cooled engines. 3) The design of air cooled engines is simple as no water jackets are required surrounding the engine. 4) The engine is free from freezing problems as in the case of water cooled engines. Disadvantages: 1) The noise level is high. 2) Non uniform cooling. 3) The power produced by air cooled engines 2) Liquid or indirect cooling system. In this method of cooling engines the water jackets are provided in the cylinder wall and cylinder head through which cooling liquid can be circulated. Heat is transferred from the cylinder barrels and cylinder 44
  • head to the liquid by conduction and convection. The cooling fluid it self is cooled by transferring heat to the air in a radiator or in cooling towers. Diesel engines are always water cooled. Various methods used forcirculating water around cylinder are Big diesel engines are always liquid cooled. Liquid cooling system is further classified as 1) Open cooling system. 2) Natural circulating system. 3) Forced circulation system. 4) Evaporation cooling system. Open cooling system This system is suitable only for those plants where plenty of water is available. The hot water coming out of the engine is not cooled for reuse but it is discharged. Diesel engine Hot water discharged PUMP in to river system Natural circulation system (Thermo syphon cooling) This system is closed one and its design is such that water may circulate naturally due to density difference in water at different temperatures. The following fig shows the natural circulation system. It consists of water jacket radiator and a fan. When water is heated its density decreases and tends to rise while the colder molecules tend to sink. Circulation of water then is obtained as water heated in the water jacket tends to rise and water cooled in the radiator with the help of air passing over radiator either by ram effect or by fan or jointly tends to sink. Thermostatic controlled circulation system: 45
  • It is as shown in the figure. It is also closed one. The system consists of pump, water jacket in the cylinder, radiator, fan and a thermostat. The coolant is circulated through the water jacket with the help of pump which is usually a centrifugal type, driven by engine. The thermostat provided in the engine upper nose regulates the temperatures of cooling water. Stand by diesel power plant up to 200 KVA use this type of cooling system. In case of bigger plant, the hot water is cooled in a cooling tower and recalculated again and again. There is a need of small quantity of cooling make up water. Evaporative cooling system: In this method water is allowed to evaporate by absorbing the latent heat of evaporation from cylinder walls. Evaporated water is once again converted in to liquid water by allowing it flow through the radiators. Where it gives up the latent heat absorbed in the cylin walls to the air flowing across the radiator tubes. der General layout of the water cooling system 46
  • A common water cooling system used in diesel plant is as shown in figure. The water which is not pure will cause deposits at temperature about 500c. Therefore it is necessary to purify the water before entering in to the system and to prevent the growth of algae. Which may reduce the heat transfer due to fouling. The cooling water is treated with Calgon to control the scaling in the different parts of the system and it is also chlorinated once per shift to prevent algae growth which would cause the rapid tube fouling . To prevent corrosion of tubes Sodium Chromate is added. Based on the water circuit system cooling system is divided in to single circuit system and double circuit system. In the single circuit system may be subjected to corrosion in the cylinder jackets because of the dissolved gases in the cooling water. The double circulating system completely eliminates internal jacket corrosion but corrosion may exist in raw water circuit Lubrication system: The purpose of lubrication system is to provide sufficient quantity of cool filtered oil to give positive and adequate lubrication to all moving parts of the engine. The following are the function of lubrication. 1) To reduce the friction which reduces the power required to overcome the same. 47
  • 2) To reduce the wear and tear between the moving parts. 3) To cool the surfaces by carrying away heat generated by friction. 4) To carry away the particles of carbon and metal scrap. 5) To provide sealing between the moving piston and cylinders. 6) To reduce engine noise and to increase the engine life. 7) To avoid corrosion and deposits. General outline of lubrication system The forced feed circulation is generally used to lubricate all the parts. The general equipments which are used in lubrication system are pumps, oil cleaners, oil coolers, storage, sump tanks and safety devices. The frictional losses of engine will appear as the heating of the lubricating oil during its circulation through the engine. It is necessary to remove this heat for proper functioning of the lubrication. The lubricating oil is cooled in an oil cooler before supplying to engine. The cooling is done by using the water from the sump of the cooling tower. The oil from the engine is filtered by passing through the metal screen strainers and ultimate cleaning is accomplished by passing oil through the centrifugal cleaners . The oil is heated to increase the fluidity of the oil before passing through the cleaning system by using hot water or steam circulating in the heaters. The lubrication system is classified as a) Mist lubrication system. b) Wet lubrication system 1) Splash lubricating system 2) Pressure feed system 3) Splash pressure feed system c) Dry sump lubrication. a) Mist lubrication system: This system is used for two stroke engines. In this type of lubrication system small quantity of lubricating oil is mixed in the fuel tank. These engines are lubricated by adding 2 to 3 percent lubricating oil in the tank The oil and fuel mixture is induced through carburetor . The gasoline is vaporized and the oil in the form of mist , goes via crankcase in to the cylinder. The oil which impinges on the crankcase walls lubricate the main and connecting rod bearing and rest of the oil which passes on the cylinder during the charging and scavenging periods, lubricates the piston and , piston rings and the cylinder b) Wet lubrication system: 1) Splash lubricating system: 48
  • This system is used on small four stroke stationary engines. A splasher or dipper is provided under each connecting rod which dips in to the oil in the trough at every revolution of crank shaft and oil is splashed all over the interior of the crank shaft. Surplus oil eventually flows back to the oil sump. Oil level in the trough is maintained by means of a oil sump which takes oi from sump through a filter. l 2) Pressure feed lubrication system The oil is drawn from sump through strainer which prevents foreign particles and is pumped with help of gear pump submerged in the oil. An oil hole drilled in the crankshaft from the centre of each crankpin to the centre of an adjacent main journal , through which oil can pass from main bearing to the crank pin bearing. The cylinder walls, cam, piston and piston rings are lubricated by oil spray from around piston pins and main connecting rod bearings. A pressure regulator fitted at the delivery point of the pump helps in regulating pressure of lubricating oil in the circuit. Excess oil is returned back to the sump. A pressure regulating valve is also provided on the delivery side of this pump to prevent 49
  • excessive pressure. This system finds favor from most of the engine manufacturers as it allows high bearing pressure and rubbing speeds. 3) Splash pressure feed lubrication In this case lubricating oil is supplied under pressure to main and cam shaft bearings . Splash is used to lubricate crankpin bearings. c) Dry sump lubrication system In the dry sump lubrication system the supply of oil is carried in an external tank with the help of scavenging pump through strainer and filter. The pump is placed out of the sump . The capacity of the scavenging pump is always greater than oil feed pump. The supply tank is usually placed behind the radiator. The dry sump is generally used in large stationary marine engines. The oil pressure may vary from 3 to 8 Kgf / cm2 . Dry sump lubrication system is generally adopted for high capacity engines. DRAUGHT Draught is mechanism of creation of small pressure difference that is required maintain the constant flow of air for combustion of fuel and to discharge the gases through the chimney to atmosphere. Draught can be produced by using chimney, fans , steam or air jet or combination of these . The purpose of draught is to 1) Helps in allowing desired volume of air flow in to the furnace. 2) Helps in overcoming the resis tances offered to the flow of air through the furnace. 3) Discharge gases at sufficient height to avoid pollution to atmosphere. Classification of draught system. 50
  • I) Natural draught. II) Artificial draught a) Steam jet draught i) Induced draught ii) Balanced draught. b) Mechanical draught . i) Induced draught ii) Forced draught. iii) Balanced draught. Natural draught: If the draught is produced only with the help of chimney it is known as natural draught. Artificial draught: If the draught produced by other than chimney it is known as artificial draught. Artificial draught can be produced either by using fan or using steam jet. Depending on the positions of fans or steam jet they are further classified as Induced , forced and balanced draught. Mechanical draught: This type of draught is produced by using fans in the gas flow path depending on the locations and number of fans we have , a) Forced draught fan system: The forced draught system fans are installed at the base of the boiler. This draught system is known as the positive draught system. The fans or blowers installed at the base of the boiler forces the air through the fuel bed, Economiser, air preheater and to the chimney. The furnace has to be gas tight to prevent the leakage of gases in the boiler house . Since the FD fans handle cold air, so they consume less power and less maintenance problems. b) Induced draught 51
  • In this type of draught system a fan or blower is located at the base of the chimney. The ID fan sucks the burned gases from furnace and the pressure inside the furnace is reduced below atmosphere and induces the atmospheric air to flow through the furnace. The draught produced is independent of the temperature of the gases . This draught is similar to the natural draught system in action but the total draught produced is the sum of draught produced by the fans and chimney. Advantages of Forced draught over induced draught : 1) Forced draught fans does not requirewater cooled bearing. 2) The tendency of air leak in to the furnace reduced. 3) The life of the FD fan blades is hight. 4) The power required for FD fans is less compared to Induced draught fans. c) Balanced draught. It is the combination of the both forced draught and induced draught system. In this system FD fan over comes the resistance of fuel bed and air pre heater. The induced draught fan removes the gases from the furnace maintaining the pressure in the furnace just below atmosphere. Advantages of mechanical draught. 1) Easy control over the combustion process. 2) The efficiency of thermal power plant improves. 3) It reduces the chimney height. 4) Low grade fuel can be used with high intensity of mechanical draught. 52
  • 5) The fuel burning capacity of grate is enhanced. Steam jet draught system. This type of draught is produced by introducing the steam in the flow path of the gases depending on the location of steam jet we have induced jet draught and forcedjet draught systems. Induced jet draught: This is used in locomotive boilers. In this method of draught the steam from the boiler is led to smoke box through the nozzle to produce draught but when it is running air enters through the damper and forces it way through the grate besides the induced draught is also produced by utilizing the exhaust steam from the cylinder through the nozzles placed in the smoke box. Forced jet draught In this case steam nozzle is placed in a diffuser pipe. The steam from the boiler after being throttled to 2 bar enters the nozzle and emerging out with a great velocity dragging air with it. The mixture of air and steam with high kinetic energy passes in to the diffuser pipe where the kinetic energy is converted in to the pressure energy which forces the air out of chimney. Natural draught: The natural draught is produced by chimneyor stack. It is caused by the density difference between the atmospheric air and the hot gas in the stack. This type of draught is useful for small capacity boiler and it does not play much important role in the high capacity thermalpower plant. consider the chimney above grate level is H. The pressure acting on the grate from chimney side P1 = Pa + Wg H = Pa + g gH (1) Where Pa – Atmospheric pressure at chimney top. g gH – Pressure due to the column of hot gas of height H meters.(Chimney side) And g = Average mass density of hot gas Kg/m3 Similarly the pressure acting on the grate on open side P2= Pa + a gH (2) a gH – Pressure acting on the grate on open side by the column of cold air out side the chimney of height H meters. a – Average mass density of cold air out side the chimney. The net pressure acting on the grate P = P2 – P1 = gH(a -  g ) (3) 53
  • This difference in pressure is responsible for causing flow of air through the combustion chamber and discharge of gases through the chimney it is known as static draught. This draught can be increased either by increasing height of chimney or by reducing the density of gases. If the acting pressure is in terms of mm of water(head). hw x Ww = H (Wa – Wg) hw w g = gH(a -  g ) (hw x 1000x g)/1000 = gH(a -  g )  hw = H(a -  g ) mm of water (4) Draught in terms of hot column of gas . Let Hg is the hot column of gas in meters. Hg x Wg = H (Wa – Wg) Hg x g g = gH(a -  g) Hg = H(a/g - 1) Meters of hot column of gas (5) Calculation of chimney height and cross section: If m kg be the mass of air supplied per kg of fuel . Then m+1 will be the mass of flue gases. Assuming volume of air and gas is same at same temperature, at 00c or 2730 K and at atmospheric pressure one kg of air occupies volume equal to v = RT/P = 287 x 273 / 1.013 x 105 = 0.7734 m3 (1) The volume of the gases at higher temperaturecan be calculated as follows. Let Tg – Mean absolute temperature of flue gases 0K Ta – Mean absolute temperature of out side air 0K. Volume of one kg of air at temperature Ta = (0.7734 x Ta) / 273 (2) Volume of m kg of air at temperature Ta = (0.7734 x Ta x m) / 273 (3) Volume of m+1 kg of flue gases at temperature Tg = (0.7734 x Tg x m) / 273 (4) Hence density of air at temperature Ta = Mass / Volume(air) a = (m x 273) / (0.773 x Ta x m ) = 353 / Ta (5) Similarly density of gases at temperature Tg = Mass of hot flue gas / Volume of hot flue gas = (m+1 x 273) / (0.7734 x m x Tg) g = [353 x / Tg] x [( m+1) / m] kg / m3 (6) The draught is the difference in pressure between hot gas columnin chimney of height H and cold air column of same height H and thus P = P2-P1 = gH(a -  g) (7) Substituting the value of a and  g in to the equation from equations 5 and 6 P = 353 gH [ 1/Ta – (m+1)/m x (1/Tg)] N/m2 (8) If hmm be the draught measured in water column then h = H(a -  g) mm of water. = 353 H [ 1/Ta – (m+1)/m x (1/Tg)] (9) If Hg is the height of a column of hot gas expressed in meters which would produce the pressure P in N/ m2 Then, Hg = {P(N/ m2)} / {Density ((Kg/ m3) x g } From equation 8 Hg = 353 gH [ 1/Ta – (m+1)/m x (1/Tg)] / { [353 x / Tg] x [( m+1) / m] } x g = H x (m/ m+1) x Tg [ 1/Ta – (m+1)/m x 1/Tg] = H [Tg /Ta x ( m/m+1)–1] in meters of hot gas (10) Cross sectional area of chimney: Velocity of flue gases in ideal chimneyis C = (2gHg)1/2 (1) 54
  • In practical chimney we can not avoid draught losses. Let Hg be the losses in the chimney equivalent to hot gas column in meter then the velocity of gas in the chimney is C = (2gHg- Hg)1/2 = 4.4294 x (Hg) 1/2 ( 1- Hg / Hg) 1/2 = K (Hg) 1/2 (2) Where K= 4.4294 x ( 1- Hg / Hg) 1/2 From experiments it is found that K = 1.1 for steel , K= 0.825 for Brick. From continuity equation the mass of hot gases flowing through any cross section of the chimney Mg = A x C x g  A = (Mg / g ) x (1/C) = (Mg / g ) x [1/ {K(Hg) 1/2 } ] (3) From equation 3 cross sectional area of chimney can be calculated and from this area the diameter of base of chimney can be calculated using formula A= D2/4  The diameter of the chimney = (A x 4/)1/2 Condition for maximum discharge Theoretical velocity of flue gases produced by static draught is C = (2gHg)1/2 (1) Where Hg is the height of a column of flue gases corresponding to draught pressure. Hg = H [Tg /Ta x ( m/m+1)–1] in meters of hot gas column (2) Substituting the value of Hg in to the equation (1) C = {2g H [Tg /Ta x ( m/m+1)–1] }1/2 (3) We know that Pv = RT or g = P / R T g = K / T g (4) Tg – Temperature of hot flue gas g density of gases K – Constant. This shows that the density of gasses is inversely proportional to its temperature. Mass of gases flowing through chimney is given by Mg = Area x Velocity x Density. = A {2gH[(m/m+1) x Tg/Ta -1}1/2 x K/Tg = Ax K/Tg { 2gH[(m/m+1) x Tg/Ta -1}1/2 = Ax K’/Tg { 2gH[(m/m+1) x Tg/Ta -1}1/2 (5) Where K’ = A x K We can write the above equation as Mg = K’x (2g/Tg2)1/2 { 2gH [(m/m+1) Tg/Ta -1] }1/2 Using Kg = K’x(2g)1/2 Mg = Kg { (m/m+1) 1/TgTa – 1/Tg2] 1/2 (6) In the above equation 6 , Tg and Mg are only two variables all other parameters are constants. To find the condition for maximum discharge differentiate Mg with respect to Tg and equate to zero. Thus for maximum discharge. dMg/dTg = 0 = d/dTg { Kg [ (m/m+1) 1/TgTa – 1/Tg2] ½ } = Kg x ½ x -(m/m+1) x 1/ Ta x 1/Tg2 + 2/Tg3 / { [(m/m+1) x 1/TgTa-1/Tg2] 1/2 }=0 (m/m+1) x 1/ Ta x 1/Tg2 = 2/Tg3 Tg / Ta = (m+1)/m x 2 Thus we can see that the absolute temperature of the chimney gas bears a certain ratio to the absolute temperature of the out side air. Using this value of Tg/Ta in to the equation of height of chimney. H max = H [ (m/m+1) x Tg / Ta -1] Replacing Tg/ Ta = 2 ( m+1/m) = H [ m/m+1 x 2 ( m+1) /m-1] Hmax = H (7) Thus for maximum discharge of flue gases draught produced is equal to the height of the chimney therefore. Maximum discharge (Mg)max = A x g (2gHg)1/2 = A . (P / RTg) x (2gHg)1/2 = A P / RTg x Ta /Ta (2gHg)1/2 Substitute the value of Tg / Ta in to the above equation 55
  • = (APm/2RTa) x (2gHg)1/2 / (m+1) (8) The draught in mm of water column is h = 353H[1/Ta –( m+1) / m x 1/Tg] For maximum discharge the condition is T g = 2Ta (m+1)/m Substitute this value in to the equation 9 h = 353H [1/Ta – ( m+1) / m x (m/m+1)x 1/ (2xTa)] = 176.5H/Ta mm of water . Efficiency of the chimney. The temperature of the flue gases leaving the chimney in case natural draught is higher than that of the flue gases leaving in case of artificial draught. This leads to the certain minimum temperature needed to produce a given draught for a given height of chimney. This shows that the draught is created at the cost of thermal efficiency of the boiler plant. Therefore efficiency of the chimney is defined as the ration of the energy equivalent of draught produced by artificial draught fan system expressed in meter head to the energy equivalent expressed in per kg of gases of additional heat carried away by the flue gases in the natural draught system. Let Tg be the temperature of the flue gases in the chimney for natural draught at 00C. Tg’ Temperature of flue gases in chimney for artificial draught 00C. Hg is the column of the flue gases equivalen of draught produced by artificial draught meter head. t o Cp is specific heat J/Kg K of gas. chimney = Hg/JCp(Tg-Tg’) = H [ (m/m+1) x Tg/Ta -1 ] / JCp (Tg – Tg’) Power required to drive the fan ( Artificial draught / Mechanical draught) V – Volume of the flow gases through the fan m3/min . H= Draught produced by the fan mm of water. P = Draught in N/m2 = Efficiency of fan. We have P = w gh = 1000/1000 x (gh) The work done on the gas W = PV / 60 = ghV/60 Watts Power required to drive the fan = ghV / (60 x 1000 x ) Problems: 1) A 200 m high 4 m dia stack emits 1000kg/s of 1000c gases in to 50c air. The prevailing wind velocity is 50 Km/h. The atmosphere is in a condition of neutral stability. Calculate the height of the gas plume. Soln : Data- H = 200 m , D = 4 m , Mg = 1000 kg/s , Tg = 1000c Ta = 50c Vw = 50 x 1000/3600 = 13.89 m/s . Using correlation of Carson and Moses. H = 2.62 (Qe)1/2 / Vw – 0.029 VsD/Vw Where D = Stack dia, m , Vw = Wind velocity , m/s , Qe = heat emission from plume , watts = MgCp(Tg-Ta) = 1000x1.005 (100-5) = 95475 KW Vs = Stack gas exit velocity , m/s, = Mg/gA Where g = Pg/RTg = 101.325 / (0.287 x 373) = 0.9465 Kg/m3 . A =  x D2 / 4 =  x 42 / 4 = 12.56m2 Vs = 1000/ (0.9465 x 12.56) = 84.12 m/s 56
  • H = 2.62 (Qe)1/2 / Vw – 0.029 VsD/Vw = 2.62 x (95475x1000) ½ / 13.89 – 0.029 x 84.12 x 4 / 13.89 = 1842.9 m 2) A textile factory has a battery of 6 Lancashire boilers, each supplying 6t/h of steam at 16 bar, 2500c from feed water at 300c. The boilers burn fuel oil of calorific value 43.96 MJ/kg with an overall efficiency of 75%, for efficient combustion 16 kg of air per kg of fuel is required for which draught of 20mm of water gauge is required at the base of the chimney. The flue gases leave the boilers at 3200c . The average temperature of the gases in the stack may be taken to be 3000c . The atmosphere is at 300c. Assuming the velocity of gases at stack exit to be negligible , determine the height of the stack and the diameter at its base. Soln: Data n = 6, Ms = 6000 / 3600 = 1.67 kg/s, Ps = 16 bar , Ts = 2500c ,Tfw = 300c , CV = 43.96MJ/Kg o = 75% , Ma = 16 Kg/s h = 20mm of water , Tgs = 3200c . Tg = 3000c , Ta = 300c . h = 353 gH [ 1/Ta – (m+1)/m x (1/Tg)] 20 x 10-3 x g9.81 x 103 = 353 x 9.81xH [ 1/303 – (16+1)/16 x (1/573)] H = 39.18 m boiler = Ms x n (hs-hfd) / Mf x CV 0.75 = 6000 x 6 ( 2919.2 – 125.8) / Mf x 43960 Mf = 36000 x 2793.4 /( 43960 x 0.75x3600 ) = 0.8472 Kg/s Ma = 16 x 0.8472 = 13.5552 Kg/s Mfg = 17 x .8472 = 14.4 kg/s Mfg = fg x A x Cg = (Pfg / R Tg ) x (D2 / 4) x (2gH)1/2 14.4 = (101.3 / 0.287 x 573) x ( x D2 / 4) x (2x 9.81 x39.18)1/2 D = 1.0735 m. 3) Calculate the height of a chimney to produce a static draught of 15 mm of water column. The temperature of the gas in the chimney is 2700c. The barometric reading is 760 mm of Hg. The value of R of air and gases may be taken as 0.287 and 0.255 KJ/Kg K Soln: Data: h = 15 mm of water , Tg = 2700c, P = 760mm of Hg = 1.013 x 102 KPa , Ra = 0.287 KJ/Kg K , Rg = 0.255 KJ/Kg K. Density of air at 270c , is given by a = P/RaTa = 1.013 x 102 / 0.287 x 300 = 1.1765 Kg/m3 Density of flue gases at 2700c is given as g = P/RgTg = 1.013 x 102 / 0.255 x 543 = 0.7135 Kg/m3 The pressure difference is given by h = H(a -g ) mm of water 15 = H ( 1.1765 – 0.7135) Height of the chimney H = 32.4 m 4) A chimney of 30 m of height discharges gases at 3560c, when the outside air temperature is 2300C , 16 kg of air per kg of fuel required to burn the coal on the grate calculate the following . (a) Draught in mm of water column. (b) Equivalent draught in m of hot column (c) Volume of hot gases passing through the chimney per second if 1360 kg of coal is burnt per hour over the grate. (d) The base diameter of the chimney if the velocity C of the gases at the base of the chimney is given by Hg = 1.63 C2/2 Soln: Data: - H = 30 m , Tg = 3560c , Ta = 230c , m = 16 Kg/kg of fuel. Mtf = 1360 kg /hr (a) The draught in mm of water column h = 353 H [ 1/Ta – (m+1)/m x (1/Tg)] 57
  • = 353 x 30 [ 1/ 296 – (16+1)/16 x (1/629)] = 17.88mm of water column. (b) Draught in meter of hot gas column is given by Hg = H [Tg /Ta x ( m/m+1)–1] = 30 [629 /296 x ( 16/16+1)–1] = 30 m meter of gas column. ( c) Volume of gases passing through the chimney. Mass of the flue gas per kg of coal burnt = (m+1) = 16+1 = 17 kg /Kg of fuel. Total mass of the flue gases Mg = 17 x 1360 = 23120 Kg /h Density of flue gases is given by g = [353 x / Tg]x [( m+1) / m] kg / m3 = [353 x / 629]x [( 16+1) / 16] kg / m3 = 0.5962 kg / m3 Volume of the gases passing through the chimney = Mg / g = 23120/0.5962 = 38778.9 m3/hr = 10.77 m3/s (d) Diameter of the base of the chimney Hg = 1.63 C2/2 30 = 1.63 C2/2  C = (30 x 2 /1.63)1/2 = 6.067 m/s Area = D2/4 = Volume / velocity = 10.77 / 6.067 = 1.775 m2  D = ( 4 x 1.775 / )1/2 = 1.503 m (5) Design the principal dimension such as a height and diameter of the chimney of a boiler plant which has average coal consumption of 1500 kg of coal per hour gives 16 kg of dry flue gases per kg of fuel. The draught losses may be taken as follows. Fuel bed = 4.5 mm of water column Boiler flues = 7.8 mm of water column Straight breaching = 3 mm of water column Bend in breaching = 0.4 mm of water column Velocity of gas flow = 0.5 mm of water column The average temperature of outside air is 270c and the mean temperature of the flue gases in the chimney is 2500c. The actual draught may be taken as 0.8 of theoretical draught and the velocity of flow coefficient as 0.35 . Soln : Data : Mtf= 1500 Kg /hr , m+1 = 16 , hfbd = 4.5 mm of H20 , hbf = 7.8 mm of H20 , hsb= 3 mm of H20, hbb = 0.4 mm of H20 , hvg = 0.5 mm of H20 , Ta = 270c , Tg = 2500c , ha = 0.8 x ht , Ccvf = 0.35 The actual draught needed is hfbd + hbf + hsb+ hbb + hvg = 4.5 + 7.8 + 3 + 0.4 + 0.5 = 16.2 mm of water The theoretical draught to be produced = Actual draught / 0.8 = 16.2 / 0.8 = 20.25 mm of water. Mass of the flue gases = 16 = m+1 kg / kg of fuel m = 16-1 = 15 kg of air /kg of fuel The theoretical draught in mm of water h = 353 H [ 1/Ta – (m+1)/m x (1/Tg)] 20.25 = 353 x H [ 1/300 – (16)/15 x (1/523)]  H = Height of the chimney = 44.34 m Density of flue gas g = [353 x / Tg]x [( m+1) / m] kg / m3 = [353 x / 523]x [( 15+1) / 15] kg / m3 = 0.7199 kg / m3 Head for velocity of 0.5 mm of water = 0.5 / 0.7199 = 0.694 m of gas column. 58
  • Velocity of the flue gas Cfg = (2gHv) ½ = (2x9.81x 0.694) ½ = 3.69 m/s Actual velocity = Ca = (1-Ccvf) x Cfg = ( 1-0.35) x 3.69 = 2.3985 m/s Mass of the dry flue gases Mtf = 1500 x 16 /3600 = 6.67 kg/s. Volume of flue gases = Mtf / g = 6.67/0.7199 = 9.265 m3/s Area of chimney = Area = D2/4 = Volume / Velocity = 9.265 / 2.3985 = 3.86 m2 Diameter of chimney = D = ( 3.86 x 4 / ) ½ = 2.216 m 6) Calculate the power of a motor required to drive an induced draught fan for the following data Draught to be produced = 40 mm of water Temperature of outside air = 300K Mean temperature of flue gases = 470 K Efficiency of the fan = 80% Air supplied = 16 kg /kg of fuel Coal consumption = 1500 Kg/h What would be the power consumption if the induced draught fan replaced by a forced draught fan of the same efficiency . Soln: Data: h = 40 mm of water , Ta = 300 K , Tg = 470 K f = 80% , m = 16 kg /kg of fuel , M = 1500 kg /h The power required for the induced fan is Power = P = ghV / (60 x 1000 x ) = ghM.mTgVo /(60x1000x x273) The volume of one kg of air at NTP is given by Using Vo = RTo / P = 0.287 x 273 /(1.013 x 100) = 0.7734 m3/kg Quantity of the fuel fired per minute M = 1500/60 = 25 kg/min  Power = 9.81x 40 x 25 x 16 x 470 x 0.7734 / ( 60 x 1000 x 273 x 0.8) = 4.353KW (b) Power required for forced draught fan = ghV / (60 x 1000 x ) = ghM.mTaVo /(60x1000x x273) = 9.81x40x25x16x300x0.7734 / (60x1000x 0.8x273) = 2.779 KW Power of I.D fan/ Power of F.D fan = 4.353 / 2.779 = 1.566 7) A forced draught fan employed in a boiler installation deliver at 12 m/s against a draught of 30 mm of water column across the fuel bed or the grate. 10,000 kg of coal per hour is burnt which requires 14 kg of air per kg of fuel burnt . Boiler house pressure and temperature are 1bar and 200C . The efficiency of the fan is 80%. Calculate the power required to drive the fan. Soln: Data : C = 12 m/s , h= 30 mm of water , M = 10000 / 60 =166.67 Kg/min , m = 14 Kg/Kg of fuel burnt , P= 1 bar Ta= 200C Draught to be impart the velocity head in mm of water column. = 1000 C2 a / (2gw) = 1000 x 12 x 12 x (1.013x100/0.287x273) / (2x 9.81x1000) = 9.489 mm Draught across the fuel bed = 30 mm of water column. Hence the total draught to be developed by the forced draught fan H= 30 + 9.489 = 39.489 mm The power required to drive the fan P = ghV / (60 x 1000 x ) V = MtgRT/P = 14 x 10000 x 0.287 x 293 /1.013x 10 0 = 1962.12 m3 /min Power = 9.81 x 39.489 x 1962.12 /( 60 x1000x 0.8) = 15.835 KW 8) A chimney having a height of 42 m produces a natural draught equivalent of 33 m of hot gas column . The test analysis shows that mass of flue gases per kg of coal is 21 kg/ kg of fuel. The mean temperature of boiler house is 270c . Find the following (a) The temperature of flue gases leaving the chimney. (b) Extra heat being carried away by the gases if the artificial draught , the gas temperature could be reduced to as low as 1200c. The temperature limit is being imposed due to condensation of gases below this temperature 59
  • Cp may be taken as 1.045 for hot gases. (c) The chimney efficiency. (d) Temperature of chimney gases and the corresponding draught for the condition of maximum discharge through the chimney. Soln: Data- H = 42 m , Hg = 33 m of hot gas column, m = 21-1 = 20 kg /kg of fuel , Ta = 27 0C, Tg’ = 1200C Cp = 1.045 KJ/kg K (a) Temperature of flue gases Hg = H [ (m/m+1) x Tg/Ta-1] 33= 42[20/21x ( Tg / 300) -1] , Tg = 562.5 K  Temperature of the flue gases leaving the chimney = 562.5 K (b) Extra heat being carries away by 1 kg of flue gas if artificial draught is used is = Cp (Tg-Tg’) = 1.045(562.5-393) = 177.13 KJ/Kg of flue gas ( c) Efficiency of chimney: chimney = Hg / Cp (Tg -Tg’) = 33 / 1.045 ( 562.5 – 393) = 18.63 % (d) Temperature of flue gases for maximum discharge is Tg = 2 (m+1/m) Ta = 2 x ( 21/20 x 300) = 630 K For maximum discharge the draught is H max dis = 176.5 x H/Ta = 176.5 x 42 /300 = 24.71 mm of water . Also Hg for maximum discharge = H = 42 m of hot gas. 9) A set of steam generators operate under the following conditions: Steam condition at boiler outlet 16 bar , 250oC , Feed water temperature : 30o , Steam generation rate : 30 tones per hour, Overall efficiency of the boiler : 80%, Air fuel ratio by mass : 16.35, Required draught : 20 mm of water at the base of chimney, Calorific value of the fuel used : 44 MJ / Kg, Exhaust gas temperature at the exit of the boiler 3470C , Average temperature of the gas in the chimney : 3270C, Pressure and temperature of atmosphere : 1 bar , 270 C,. Neglecting the velocity of gases at stack exit , determine the height of the stack and the diameter at its base.(July / Aug 2006) 10 Soln : Data – Ps -= 16bar, Ts = 250oC , Tfw = 30o , Ms = 30 x 1000 / 3600 = 8.33 Kg/s , boiler = 80 % , ma / mf =16.35 , h = 20 mm, CV = 44 x 106 J / kg , Tge = 347 + 273 = 6200K , Tg = 327 + 273 =600 0K , Pa = 1 bar , Ta = 27 + 273 = 3000K , h = 353 H [ 1/Ta – (m+1)/m x (1/Tg)] 20 =353 H [ 1/ 300 – (16.35+1)/16.35 x (1/ 600)]  H = 36.21 m Properties of steam at 16 bar 2500C , hs = 2801 KJ / Kg , hfd = 125 KJ /Kg boiler = Ms (hs-hfd) / Mf x CV 0.8 = 8.33 ( 2801 – 125) / Mf x 44 x 103  Mf = 0.633 Kg/s . Flue gases formed per sec Mfg = Mf x 17.35 = 0.633 x 17.35 = 10.98 kg/s Mfg = fg x A x Cg = (Pfg / R Tg ) x (D2 / 4) x (2gH)1/2 10.98 = (100 / 0.287 x 600) x ( x D2 / 4) x (2x 9.81 x39.18)1/2 D = 0.672 m Accessories for steam generators: 60
  • Different devices that are used for the safety and to improve the steam generators are classified in to two groups a) Boiler mountings b) Boiler accessories a) Boiler mountings: Boiler mountings are those devices which are mounted over the body of the boiler itself for the safety of the boiler and for complete control of the process of steam generation. Some of the mountings are 1) Two safety valves 2) Water level indicators 3) Pressure gauges 4) Fusible plug 5) Steam stop valve 6) Feed check valve 7) Blow off cock 8) Inspector test gauge 9) Man and mud hole. b) Boiler accessories : These are those devices which are installed either inside or out side the boiler to increase the efficiency of the plant and or for proper working of the plant. The following accessories are used in the boiler. 1) Air preheater 2) Economizers 3)Super heaters 4) De super heaters 5) Steam separators 6) Pressure reducing valve Super heaters : The function of the super heater is to remove the last traces of moisture from the saturated steam coming out of the boiler and to increase its temperature sufficiently above saturation temperature. Super heating of steam helps in improving the overall efficiency and it also avoids too much condensation in the last stages of the turbine which avoids the blade erosion. Super heaters helps in recovering as large as 40% of heat in steam generators. Modern super heaters are made up of special high strength steel alloys 0 0 ( Chromium Molybdenum) it can be operated in the temperature range of 540 C - 650 C . If the operating temperature is between 4600 C - 5100 C Carbon steel is sufficient. Super heaters are classified in to three categories as a) Convection zone heaters. b) Radiation zone heaters. c) External heaters. Convection zone super heaters are usually of the horizontal type, heat transfer takes place from hot gases to the super heater due to the convection . Due to chances of condensation during short time, shut down super heaters in the convection zone are invariably made as drainable type. Radiant super heaters receive heat by direct radiation. Heat available to the radiant super heater does not increase at the same rate as steam mass flow within the tubes thus steam temperature decreases. External super heaters are once through design. If it is not possible to locate the super heater within the boiler , it may be located externally. Based on mechanical construction there are three kinds of super heaters namely pendant , inverted and horizontal as shown below. 61
  • Pendant type super heaters are those that are hung from above they have the advantage of firm structural support but disadvantage of flow blockage by condensed steam after cold shut down. To avoid this problem inverted tube arrangement is used. To combine the advantages of both horizontal type super heaters are used. Super heaters may be made in the form of coils or platens .The coil may be single, double with one tube and double coil with two tubes as shown below. Super heater temperature control For successful operation of prime mover it is necessary to supply steam as far as possible at constant temperature and pressure . Steam pressure can be easily regulated by means of safety valve invariably mounted on the boiler. Control of steam temperature is really a problem with boiler designers. Accurate steam temperature control is necessary for avoiding over stressing of super heater tubes and to maintain overall efficiency as high as possible. Common methods used for controlling super heat temperature of the steam are listed below. 1) By passing the furnace gas around the super heaters. 62
  • In this method part of the flue gases are by passed with the help of damper as shown in the figure . Even though the this method can be used successfully to control steam temperatures but finding an suitable material to with stand erosion and high temperatures in the gas passage have limited the use of damper method of control. 2) Tilting burner in furnace. The temperature of the steam coming out of super heaters is controlled by tilting the burners up or down through a range of 300C . By tilting the burner down ward in a furnace much of heat is given to the water walls by the gas and the gas entering the super heater system is relatively cool. If the burner tilted up ward, then the heat given to the boiler water wall is less and hot gas enters the super heat region to increase the steam temperatures. 3) Desuper heaters using water spray The temperature of the steam can be controlled by injecting the water either before the super heater or between the sections of super heaters . 4) Pre condensing control: 63
  • The temperature of the steam can be controlled by condensing the steam coming out of boiler with a small condenser with the help of feed water as shown in the figure . Automatic control regulates the amount of feed water passing through the condenser. 5) Gas re circulation: The gas coming out of the economizer is partially re circulated in to the furnace with the help of a fan as shown in figure. The re circulated gas forms an blanket inside the furnace wall. This reduces the heat absorption by water walls and increases the heat absorption by super heater. 6) Twin furnace arrangement. Twin furnace arrangement is as shown in the figure below. It is an extension of separately fired super heater. Varying the firing rates between the furnaces controls the super heat temperature. 7) Coil immersion in the boiler drum 64
  • The arrangement of this system is shown in the figure. A portion of thee steam from low temperature section of super heater by passed to a coil immersed in lower drum of the boiler under the control of by pass valve, later it is actuated by the final temperature of the steam. Thus making system automatic. The desuper heated steam in the boiler drum is returned and mixed with non desuper heated steam in the junction header and final super heating takes place in the second stage super heater . This method is used in Bacock and Wilcox boiler. The advantages of this method are 1) There is no erosive action on the equipment by gases 2) Over heating of the metal at high temperatures. Economizers: Economizer is a feed water heater deriving heat from the flue gases discharged from the boiler. The function of economizer is to heat the feed water coming up to nearly saturation temperature corresponding to the boiler pressure by utilizing energy of the hot flue gases leaving the super heater or re heater which is generally at temperatures varying from 3700C to 5700C. Economizers are usually placed between last super heater and the air preheaters. They have been built in vertical coils of continuous tubes connected between inlet and outlet headers with each sections formed in to an several horizontal paths connected by 1800 Vertical at a pitch of 45 to 50 mm spacing depending on the type of fuel and ash characteristics. Types of Economizers: Basically there are two types of economizers 1) Plain tube economizer: Plain tubes are generally used in Lancashire boiler working with natural draught. The tubes are made up of cast Iron to prevent corrosive action by the flue gases. The waste flue gases flow out side the economizer tubes and heat is transferred to the feed water flowing inside the tubes. The external surfaces of the tubes are continuously cleaned with the help of soot scrappers to maintain an constant heat transfer. 2) Gilled tube economizer : 65
  • A reduction in economizer size together with increased heat transmission can be obtained by casting rectangular gills on the bare tube walls. Up to 50 bar pressure cast iron gilled tube economizers can be used. Greater than 50 bar pressure applications steel tubes are used but gills are made up of cast iron are shrunk to them. Advantages of Economizers: 1) It helps in reduction of soot and fly ash. 2) Fuel savings will be higher than theoretically calculated. 3) It helps in reduction of thermal stresses in the pressurized parts of the boiler and promotes the better internal mixing. 4) Hardness problem of water can be minimized Air preheaters Air preheater utilize some of the heat energy left in the gases before exhausting them to atmosphere. The heat carried with the flue gases coming out of economizer is further utilized for preheating the air before supplying in to the combustion chamber. It helps in improving the efficiency of the boiler. Air preheaters are necessary equipments used for supply of hot air for drying the coal in pulverized mill and satisfactory combustion of fuel in furnace. Advantages of preheating air are 1) Improved combustion 2) Successful use of low grade fuel. 3) Increased thermal efficiency 4) Saving in fuel consumption 5) Increased steam generation capacity. Air preheaters are divided in to two groups 1) Regenerative ( Ljungstorm preheater ) 2) Recuperative preheater. 1) Regenerative preheater : 66
  • In this type of preheater heat is transferred from the hot flue gases, first to an intermediate heat storage medium, then to air. The most common is the rotary air preheater known as Ljungstrom preheater. Rotor driven by a electric motor. The rotor is having several radial members they form sectors. The sectors are filled with heating surface composed of steel sheets. They constitute a heat storage medium of the preheater. A stationary seal covers the equivalent of two opposite sectors. Half of the remaining sectors are exposed at any instant to the hot gases which are moving in one direction . The sectors in the other half are exposed to the air which is moving in the opposite direction. As the rotating sectors enters the gas zone they are progressively heated by the gas. They store heat as sensible heat. When they enter air zone they progressively give up this heat to the air . The seal system reduces the leakage. 2) Recuperative air preheater: In this type of air preheaters heat is directly transferred to the air from hot gases across the heat exchange surface. There are two types of recuperative air preheaters a) Tubular air preheaters b) Plate type air preheater a) Tubular air preheaters: 67
  • Tubular air preheater is as shown in the figure. It works similar to the counter flow heat exchanger . The flue gases flow through the tubes and air is passed over the outer surface of the tubes. The horizontal baffles are provided as shown in the figure to increase time of contact which will help for higher heat transfer. In some designs tube row staggering is used to improve the air distribution. The steel tubes of 3 to 10 m in height and 6 to 8 cm in diameters are commonly used. The gases reverse their direction near the bottom of the air heater, and a soot hoper is fitted to the bottom of air preheater casing to collect soot. Plate type air preheater: G S IN A A OT IR U G SO T A U A IN IR It consists of rectangular flat plates spaced from 1.5 to 2.5 cm apart leaving alternate air and gas passages. This type of air heaters is not used in modern installation as it more expensive compared to the tubular air heater. Cooling towers and cooling ponds 68
  • Necessity of cooling the condenser water 1) Cooling water system is one of the most important system of power plant and its availability predominantly decides the plant site. 2) as the cooling water takes the latent heat of steam in the condenser, the temperature of water increases. The hot water coming out of the condenser can not be used again and again in a closed system without pre- cooling. This is because the hot water coming out of condenser if again used it will not absorb the heat as two reaches near to saturation temperature of steam at condenser pressure an the condenser vacuum can not be maintained. Therefore it is absolutely necessary to pre cool the water coming out of condenser before using again. Cooling towers and ponds are used for cooling water coming out of the condenser to make it suitable for reusing in the plant. Cooling towers: The purpose of cooling towers is to cool the warmedwater discharged from condenser and feed the cooled water back to the condenser. By this way cooling water requirement get reduced to make up water supply only. The cooling towers may be wet type or dry type. Wet cooling towers employ a hot water distributing system that showers or sprays the water evenly over a lattice work of closely set horizontal bars called fills or packaging. Since the water splashes down from one fill level to the next by gravity . There is a through mixing of water falling with air moving through the fill. The intimate mix between water and air results in enhancement of heat and mass transfer(Evaporation) which cools the water. The cold water gets collected at the bottom of tower in concrete basin from where it is pumped back to the condenser in a closed system or returned to the water body in open system. The resulting hot and moist air leaves the tower at the top. Wet cooling towers are classified as either i) Mechanical draught cooling towers ii) Natural draught cooling towers i) Mechanical draught cooling towers. 69
  • In this case air is moved by one or more mechanically driven fans. The fan could be of forced draught (FD) type or Induced draught (ID) type. The FD fan is mounted on the lower sides to force air in to the tower while in ID type ID fan is located on the top of the tower. FD type is thermodynamically superior but it is having s=disadvantages like leakage, recirculation of hot and moist air and frost accumulation at fan in lets during winter operation. Therefore induced draught type wet cooling towers are commonly used. In this type air enters through the large openings provided at the bottom of the tower at slow velocity and passes through the fill. The fan located at the top of the tower exhausts the hot humid air in to the atmosphere. The fans are propeller type and driven by electric motor. The blades of the fan are usually made of cast aluminum, stainless steel or fiber glass to safe guard against the corrosion. Advantages: 1) It is independent of climate conditions. 2) Low initial capital costs and low physical profile. 3) More efficient and more safe. Disadvantages: 1) High power consumption by fans 2) Noise produced is enormous. 3) Operating and maintenance costs are more. Natural draught cooling towers 70
  • In this type of cooling towers there are no fans. These towers depend for air flow upon the natural driving pressure caused by the difference in densities between the cool out side air and the hot humid air inside. The driving pressure differential is expressed as Pd = (o-i) gH , Where H is height of tower above the fill, o and I are densities of air out side and inside . Since (o-i) is relatively small , so H must be large enough to cause desired Pd and as a result natural draught cooling towers are very tall towers .Towers are usually of hyperbolic profiles and due to this natural draught towers are called by name hyperbolic towers. The advantage of this type of towers is that their greater resistance to outside wind loading compared to other shapes. The natural draught towers may be counter flow type or cross flow type. In counter flow the fill is inside where as in cross flow the sits out side the tower. Advantages: (i) Low operating and maintenance cost . (ii) It gives more or less trouble free operation (iii) Considerable less ground area required. (iv) Towers may be as high as 125 m and 100 m in diameter at the base with the capability of withstanding winds of very high speed. Disadvantages: (i) High initial cost (ii) Its performance depends on the atmospheric conditions. Dry cooling towers: Dry cooling towers are those in which water pass through the finned tubes over which the cooling air is passed. A dry cooling tower can be either mechanical draught or natural draught. They are very suitable where there is scarcity of water. The plant could be located on fuel source site to avoid transportation cost . They are less expensive and maintenance costs are low. The main disadvantage is less efficient than wet type, work at high back pressure which decreasesthe plant out put and efficiency. Dry cooling towers are of two types direct and indirect Direct dry cooling towers: In this type of cooling tower turbine exhaust steam is admitted in to a steam header through large ducts and is condensed as it flows downward through a large number of finned tubes or coils arranged in parallel which are cooled by the atmospheric air flowing in a natural draught cooling towers or forced draught fan. 71
  • The condensate flows by gravity and gets collected in condenser receiver from where it is pumped in to the plant feed water system. Indirect dry cooling tower There are three design concepts available for indirect dry cooling towers i) With conventional surface condenser. It uses a conventional surface condenser in which circulating water goes through the finned tubes cooled by atmospheric air in the tower. The finned tubes may be either cooled by air through natural draught or induced draught system. This design is similar to the design of two heat exchangers in series and thus two temperatures drops , one between steam and water and another one between water and air. If this type of towers is used in the plant efficiency will be low compared to once through system. ii) Using an open or direct contact condenser: It eliminates the intermediate water loop of first design concept and uses an direct or indirect contact condenser. Since the operation is in the closed mode circulating water can be mixed with steam from plant and hence use of open type condenser is justified. The exhaust steam from turbine enters the open type 72
  • condenser. Where cold water is sprayed in to the steam for intimate mixing. The condensate falls to the bottom of the condenser from which most of it is pumped by recirculation pump under positive pressure to finned tubing in the tower. This part of the condensate is cooled and is returned to the condenser sprayers. The condensate equal to the steam flow , is pumped to the plant water system by the condensate pump. In this case also cooling towers may be natural or forced draught. A water recover turbine may be used to recover some of the work of pump. This concept of indirect dry cooling tower is efficient more economical and more feasible for large plants. 3) Using Vaporizing coolant. This design uses vaporizing coolant instead of water. Nearly saturated liquid ammonia enters surface condenser and is vaporized to saturated vapor . Vapor flows to finned coils and is condensed to saturated liquid and finally pumped to the condenser. Since the heat transfer coefficient is very high compared to the convective heat transfer in single phase fluid. Thus the use of ammonia reduces the size and power requirement of the equipment. Advantages of dry cooling towers: 1) There is no thermal pollution and loss of water due to evaporation. 2) Power plant can be located closer to the load centre. 3) There is minimum air pollution. 4) There is no fog , no blowdown treatment , no windage loss of water, no evaporative loss of water and no thermal discharge to water source. Disadvantages: 1) Their performance is dependent on the atmospheric conditions and so turbine exhaust temperature s are much higher resulting in a substantial loss of turbine efficiency , most critical in warm climates. 2)Due to low heat transfer coefficient , dry cooling towers require enormous volumes of air, large surface areas and are less effective at high natural air temperatures. Cooling lakes or cooling ponds. Cooling lakes are also known as the cooling ponds .These are the oldest and simplest type of artificially made heat rejection system. Hot circulating water from the condenser is simply dumped in to an artificially made lake and left to cool. Cooled water from the lake is returned to the water circulating system. Cooling is accomplished naturally by evaporation by thermal radiation to sky and convection by wind. It requires large land area. The main disadvantages of cooling lake is that very low cooling effectiveness and costs of structure. Some of the advantages are 1) Simplicity 2) Low maintenance 3) It can be operated for extended periods without make up water 4) Only mechanical equipment needed is pump Types of cooling ponds. The common types of cooling ponds used in practice are listed below. 1) Single deck and double deck systems. i) Single deck system 73
  • In this type of cooling ponds the spray nozzles are arranges at the same elevation as shown in figure. Its effectiveness is less than the double deck system. ii) Double deck system. In this type spray nozzles are arranged at different elevation as shown in the figure. Its cooling effect is more than single deck system as water comes in contact with air at low temperature. 2) Natural and directed flow system. i) Natural flow system In natural flow system water coming out from the condenser is directly allowed to flow in to the pond. This system rarely used. 74
  • ii) Directed flow system In this type of cooling pond the hot water coming out of condenser enters the middle channel. Then it reaches the far end of channel and divides in to two currents. Baffles walls are used to traverse water several times in the pond before uniting at the intake point. The cooling achieved is more effective. GAS TURBINE POWER PLANT The power generation by gas turbines is now quite attractive due to its low capital cost and its high reliability in operation. Another outstanding feature is its capability of quick starting and using wide variety of fuels from natural gas to residual oils or powdered coal. Due to the better materials being made available and with adequate blade cooling made inlet gas temperature to gas turbine blades can now exceed 12000C, as a result of which overall efficiency of a gas turbine plant can be about 35% almost same as that of a conventional steam power plant. Advantages 1) Simple in construction has having only 20% of moving parts in compared to reciprocating engine. 2) Power plant is vibration free. 3) The weight of the plant per KW out put is low 4) Once the turbine is brought up to the rated speed by starting motor and fuel is ignited the gas turbine will accelerate from cold start to full load without warm up time. 5) Any hydro carbon fuel from high Octane gasoline to heavy diesel oils and pulverized coal can be used effectively. 6) Floor space requirement is less. 7) A gas turbine can be started up as well as shut down quickly like a diesel engine. 75
  • 8) A gas turbine plant used in conjunction with a bottoming steam plant in the combined cycle mode to yield an overall fuel to electrical efficiencyof 55%. 9) The requirement of cooling water is less. 10) The ash disposal problem does not exist in gas turbine plant. 11) Installation cost is much less compared to thermal plant. 12) The capital cost per KW is considerably less than that of thermal plant. Disadvantages: 1) Highly sensitive to component efficiency like compressor and turbine. 2) Efficiency depends on the ambient conditions. 3) High rate of air is required to limit the gas turbine inlet temperature as result of exhaust losses are high. 4) Compressor work required is very large and hence the power produced by the plant decreases. 5) Air filters have to be very high quality so that no dust enters to erode and corrode turbine blades. Gas turbine power plant. Components of the gas turbine power plants are 1) Gas turbine. 2) Compressor mounted on the same shaft. 3) Combustion chamber. 4) Intercoolers and regenerators. 5) Accessories such as starting ,lubricating , oil , dust collection system. Etc. 1) Gas turbine: A gas turbine consists of blades mounted of the shaft and enclosed in a casing. The gas turbines may be of either axial flow type or tangential flow type. Multistage turbines are most common in any gas turbine power stations because it helps in reducing the stresses in the blades and increases the overall life of the turbine. It is essential to provide cooling arrangement for the blades to enhance the life of blades as the blades are continuously subjected to high temperature gases. The blades can be cooled by different methods , the common method being the air cooling. The air is passed through the holes provided through the blade. 2) Compressors. The compressor compress the atmospheric air and increases the pressure then supplies in to the combustion chamber. The compressors which are commonly used are a) Centrifugal type b) Axial flow type. Multi stage compressors are generally used where ever there is an requirement of high pressure. The pressure ratio of 2 to 3 is possible with single stage compressor and it can be increased up to 20 with multi stage compressor. The compressor may have single or double inlet. The single inlet compressors are designed to handle the air in the range of 15 to 300 m3/min and double inlets are preferred above 300 m3/min. Even though the multistage compressor increases the delivery pressure of the air but it decreases the overall efficiency. Axial flow compressors are commonly used in a gas turbine installations. 3) Combustion chamber: 76
  • The primary function of the combustion chamber is to make reaction between the fuel and air being supplied by the compressor. The process of combustion in combustion chamber completes in four steps 1) Formation of reactive mixture. 2) Ignition. 3) Flame propagation. 4) Cooling of combustion products with air. The nozzle sprays the fuel under pressure in an atomized conical spray. Atomized fuel mixes thoroughly with high pressure air from compressor. The fine mixture of the air and fuel ignited with the help of an igniter . Outer shell of the combustion chamber guides the flame and combustion products in the required path. 4)Inter coolers and regenerators. Inter coolers are used in gas turbine plants when ever there is an requirement of high pressure air . High pressure air is obtained by using multistage compressor. Intercoolers are placed in between the two stages of compression. The cooling of compressed air is generally done with the use cooling water. A cross flow type intercooler is generally preferred for effective heat transfer. In regenerator heat transfer takes place between the exhaust gases and cool air. It helps in improving the thermal efficiency of the plant by recovering the part of the heat carried away by the exhaust gases. In regenerator both air and hot gases flow in the opp osite directions to each other. 5) Auxiliary systems of gas turbine plant. Starting and ignition systems. Starting and ignition systems are two separate systems that are required to ensure a gas turbine plant that are required to ensure a gas turbine starting and working satisfactorily. During engine starting the two systems must operate simultaneously. The following are the various gas turbine plant starters. a) Electrical AC cranking motors of sufficient capacity are used.DC starters motor takes power from bank of batteries of sufficient capacity. Engaging and disengaging clutches are used. b) Pneumatic starters or air starter. Air starting is used most generally as it is light simple and economical to operate. The air starter motor contains turbine that transmits power to the starter out put shaft which is connected to the engine.When this starter turbine is operated by supply of an air . As the engine is directly coupled to this air turbine shaft, engine starts. After engine starting stator motor automatically disengages from engine. c) Combustion starter. The unit is started with electric starter. The starter turbine is directly geared to the gas turbine shaft through the reduction gear. d) Hydraulic starting system : In this method of starting hydraulic starter motor is used to start the main engine. Open cycle gas turbine power plant. 77
  • A simple open cycle gas turbine power plant is as shown in the figure. It consists of a compressor, combustion chamber and a turbine. Compressor takes the air from atmosphere and raises its pressure. This high pressured air heated in the combustion chamber by burning the fuel and its temperature increases. The heated gases coming out of the combustion chamber are allowed in to the turbine where it expands resulting in developing power by rotating the rotor of the turbine. Part of the power developed by the turbine is used in driving the compressor and other accessories and remaining is used for power generation. Since the ambient air enters the compressor and gases coming out of the turbine exhausted to the atmosphere the working medium continuously replaced, cycle is known as the open cycle gas turbine power plant. This type of the power plant is most generally used . Advantages: 1) Warm up time required is less 2) Almost any hydrocarbon fuels can be burnt . 3) Low weight and size. The weight in kg/kw power developed is less. 4) It occupies very little space. 5) The plant can be used as peak load plant. 6) The cooling medium requirement is not necessary . Disadvantages 1) The part load efficiency of the plant decreases as the part of the power developed by the turbine is used to run the compressor. 2) The plant efficiency is completely dependent on the atmospheric conditions. 3) The heat rejection to the atmosphere is very high and hence this reduces overall thermal efficiency of the plant. 4) Atmospheric dust that entering the compressor and deposition of carbon and ash on the turbine blades reduces the plant efficiency. Methods of improving the thermal efficiency of the open cycle gas turbine power plant. The following methods are used to increase the out put and thermal efficiency of the ploant. 1.) Intercooling 2. Reheating 3. Regeneration 78
  • Intercooling : A compressor in gas turbine cycle utilizes the major percentage of power developed by the gas turbine. The work required to run the compressorcan be reduced by compressing the air in two stages and incorporating an intercooler between the two as shown in the fig. Intercooler Fuel ( Heat) 2' 3 4' 5 LP C H C P T Work 1 6' Reheating: Out put of a gas turbine can be improved by expanding the gases in two stages with a reheater between the two as shown in the fig. The High pressure turbine drives the compressor and the LP turbine provides the useful power out put . Fuel ( Heat ) Fuel ( Heat) 3 4' C b1 om C b2 om 2' 5 P L C H T P P L T Work 1 6' A in ir Exhaust Regeneration: The exhaust gases from a gas turbine carry a large quantity of heat with them since their temperature is far above the ambient temperature. They can be used to heat the air coming from the compressor thereby reducing the mass of fuel supplied in the combustion chamber. 6 H exchanger eat 5' 3 2' Work C T 1 Closed Cycle gas turbine power plant. 79
  • In this type of the plant working medium is continuously used without exhausting to the atmosphere. The arrangements of the components of the closed cycle gas turbine plant is as shown in figure . The working fluid coming out of the compressor is heated in the heater by external source at constant pressure. The high pressure and temperature air coming out of the external heater is allowed to pass through the gas turbine. The gas coming out from the turbine is cooled to its original temperature in the cooler using external cooling source before passing to the compressor. Advantages: 1) Back pressure problem of open cycle is eliminated. 2) The machine can be smaller and cheaper. 3) It avoids erosion of turbine blades. 4) Its operation is independent of atmospheric conditions. 5) High density working fluid increases the heat transfer properties. 6) Cheaply available fuels can be used. 7) Maintenance cost is low and reliability is high. 8) Higher thermal efficiencies can be obtained. 9) Power required for starting the plant is low. Disadvantages: 1) Substantial quantity of cooling water is required to operate pre cooler. 2) Load response is poor. 3) Very large heat exchanger is required and hence the cost of the power produced increases. 80
  • Nuclear power plants Introduction: There is a common trend throughout the world to use nuclear energy as a source of power. This is because of the rapid depletion of conventional energy sources. Transportation network and large storage facility are not required which is one of the major hurdle in coal based thermal power plants. How ever recently there is stiff opposition for the installation of nuclear power plants due to a fear of radiation hazards. Atomic structure: All matter consists of unit particles called atoms. An atom consists of a relatively heavy , positively charged, electrons orbiting around the nucleus. The nucleus consists of protons and neutrons, which together are called nucleons. Protons are positively charged , while the neutrons are electrically neutral. The number of protons in the nucleus is called the atomic number, Z. The total number of nucleons in the nucleus is called the mass number, A. The atomic mass unit , is a unit of mass approximately equal to 81
  • 1.66x10-27 kg. Mass of Neutron is 1.008665amu, Protons is 1.007277amu and Electron is 0.005486amu. ( The mass of Protons=1837xmas of Electron, Neutrons = 1839 x Electron. ). An element is distinguished by its atomic number . Some elements exist in more than one form, with the same atomic number but with different mass numbers. These are known as the isotopes of an element. For example Uranium exists in three isotopic forms 92U233, 92U235, 92U238. (Atoms which are having different number of neutrons than the number of protons are known as isotopes.) Binding energy: It is the energy required to keep the protons together in the nucleus of an atom or It is the energy required to overcome the binding forces of nucleus is called as binding energy. The binding energy is very large compared with chemical bond energy. When two nuclear particles are combined to form nucleus . It is observed that there is a different mass of the resultant nucleus and the sum of the masses of the two parent nuclear particles will be different. This decrement of mass is called mass defect Einstein’s theory of relativity shows that mass is convertible into energy and this energy is given by the formula . E = mc2 , E – Energy(J), m- mass defect(Kg), c – Velocity of light. (3 x 108 m/s) Energy can also be measured in electron volt ( 1. e.v = 1.6021 x 10-19 J). The energy equivalent of 1g of mass is E = 1x10-3 x (3 x 108 m/s) 2 = 9 x1013 J Similarly the energy equivalent of 1 amu of mass is E = 1.66x10-27 x (3 x 108 m/s) 2 = 9 x1013 J = 14.94 x 10-11 J = 9.31 x 10 8 eV = 931 MeV Therefore , if 1 amu of mass could be completely converted to energy , 931 MeV would be yielded. The amount of mass defect is directly proportional to the amount of energy released. Binding energy per nucleons increases with increase in number of nucleons. For example binding energy per nucleon for H2 is 1.109MeV and for He4 it is 28.24 = 7.05 MeV. A curve representing the variation of nuclear binding energy per nucleon with the mass number is shown in figure. The curve indicates that peak value of about 8.8 MeV at nearly 60 mass number. As the mass number increases still further , the binding energy curve falls gradually to 7.6 MeV for U238. An atom with even number of protons of mass number is more stable because of pairing of protons and neutrons. Example 92U238 atom having 92 protons and 146 neutrons is quite stable and requires very high energy neutrons for fission, Where as 92U235 atom having 92 protons and 143 neutrons can be fissioned even by low energy neutrons. Radioactive decay and half life: All isotopes of heavier elements less stable emits radiation till a more stable nucleus is reached. Thus a spontaneous disintegration process , called radioactive decay occurs. For various elements decay time is different , which follows certain law . This law is known as radioactive decay law. The law states that the small amount of disintegration of the isotope in a small period is directly proportional to the total number of radioactive nuclei and proportionality constant. N= Number of radioactive nuclei present at any time t, No = Initial number of nuclei,  = Proportionality constant. Then according to the decay law N = -Nt (1) dN/dt = -N (2) Negative sign indicates that during disintegration number of nuclei decreasing. Integrating the equation 2 No dN/N = - o t dt N (3) loge N – loge No = -t loge N/No = -t 82
  • N/No = e-t N=No e-t (4) dN/dt = -N = - No e-t (5) Equation 5 shows that the decay scheme follows the exponential law. If A = Activity at time t, A1 = Initial activity, k = detection coefficient then A = k(-dN/dt) = kN = kNo e-t = A e-t (6) Half life: Half time represents the rate of decay of the radioactive isotopes. The half life is the time required for half of the parent nuclei to decay or to disintegrate. Using N =No/2 and t = t1/2 in equation 6 we get. No/2 = No e-t1/2  e-t = ½ 1/2  t = log e 2 = 0.693 1/2 t = 0.693 /  1/2 (7) Nuclear fission In this type of process heavy nucleus is divided in two equal number of fragments. Fission can be caused by bombarding with high energy  particles, Protons, X-rays as well as neutrons. How ever neutrons are most suitable for fission, they require less kinetic energy to collide with nuclei. Two or three neutrons are released for each neutron absorbed in fission, and can thus keep reaction going . Isotopes like U233, U235 and Pu239 can be fissioned by neutrons of all energies , where as isotopes U238, Th232 and Pu240 are fissionable by high energy only. When neutron enters nucleus of U235 the nucleus splits in to two fragments and also releases 2 to 3 neutrons per fission. The difference in the binding energy between the products of fission and the original nucleus is evolved during the fission reaction. This is known as nuclear fission. The breaking of U235 can takes place in different ways , forming a variety of different products. Each way of splitting U 235 nucleus ejects different numbers of neutrons 1,2,3. As an average of 2.5 neutrons released per neutron absorbed. Out of 2.5 neutrons , nearly 0.2 to 0.3 neutron is lost due to escape at the surface and out of remaining 2.2 neutrons are allowed to continue chain reaction. The reaction rate will increase exponentially and enormous amount of heat energy will be released. Such reaction is known as uncontrolled chain reaction . When only one neutron after every fission is allowed to continue to cause fission reaction , it is known as controlled chain reaction. This is the type of nuclear fission reaction used for power production and energy evolved remains at constant level. For sustaining of the chain reaction at least there must be an one neutron available for absorption. This condition can be conveniently expressed in the form of multiplication product or reproduction factor of the system which may be defined as K = No of neutrons in any particular fission/ No.of neutrons in the preceding fission. If K > 1 , chain reaction will continue and if K<1 , chain reaction can not be maintained. When K<1 system is known as sub critical and when K>1 the system is known as super critical and when K=1 , the system is known as critical and this is the desirable requirement for power reactors. 83
  • Prom gam rays pt a Fission fragments U235 P pt neutron rom Incident N eutron Fission fragment P pt neuttron rom Chain reaction figure Nuclear fusion Nuclear fusion is the process of combining or fusing two lighter nuclei in to a stable and heavier nuclide. In this process also large quantity of energy released because mass of the product nucleus is less than the masses of the two nuclei which are fused. Several reactions between nuclei of low mass can be initiated by accelerating one or the other nucleus in a suitable manner. These are often fusion process accompanied by release of energy . How ever the nuclear fusion reaction can not be regarded as much significance for the utilisation of nuclear energy. To have a practical value fusion reaction must be self sustaining ,i.e., more energy must be released than is consumed in initiating the reaction. For initiating the nuclear fusion reaction very high stellar temperature of 30 million 0K is needed. 84
  • Above figure shows the schematic diagram of futuristic deuterium-tritium fusion reactor . The plasma is contained inside an evacuated tube of 4m. The surrounding vacuum wall through which 14 MeV neutrons from the plasma pass, is maintained at about 7500C . Out side this wall are two concentric regions, viz, the lithium breeding moderator and magnetic shield. Tritium is manufactured in the lithium blanket. Large cryogenic superconducting magnets of 7 to 8 m diameter maintain the magnetic shield. The binary vapour power cycle consists of a potassium topping cycle and a conventional steam cycle. It includes tritium recovery system. Advantages of fusion power plants: 1) The supply of deuterium is almost inexhaustible. 2) Radioactive wastes are not produced. 3) It is very safe to operate. 4) High energy conversion efficiency can be achived. 5) Low heat rejection to the environment takes placeper KW of electricity generated. Comparison between nuclear fission and fusion Fission Fusion Heavy nucleus splits in to two nuclei of equal mass Lighter nuclei fuse together to form heavy nucleus and energy released. with the release of energy. About one thousandth of the mass is converted in to It is possible to have four thousandth of mass energy. converted in to energy. Nuclear reaction residual problem is great Residual problem is much less. Amount of radioactive material in a fission reactor is Radioactive material produced is much less than that high. of the fission reaction. Health hazards are high in the event of accidents. Health hazards is much less. It is possible to construct self sustained chain It is extremely difficult to construct controlled reaction reactors. fusion reactors. 85
  • Manageable temperatures are obtained Un manageable temperatures Raw fissionable material is not available in plenty Reserves of deuterium, the fusion element is available in large quantity. Fuels used in the reactor: The fuels which are commonly used are natural uranium containing 0.7% U235 or enriched uranium containing 1.5 to 2.5 % U235. In addition to natural nuclear fuels some of artificial or man made fuels such as Pu239, Pu241,U 233 are also used. Considering the necessary requirement of fission process and its availability economically the fuels used in reactors are uranium, plutonium and thorium. U235 is easily available nature with concentrations up to 0.7% and its content increases up to 90% in enriched uranium. The nuclear fuels is available in three states solid, liquid and gas. In reactors fuel is mostly used in solid state or in the form of solution dissolved in water. The liquid metal reactors are in practical use. The fuel used in the reactors is in the form of rods or plates . The fuel rods are surrounded by the moderator. The fuel rods are clad with stainless steel or zirconium to prevent oxidation. The minimum amount of fuel required to maintain chain reaction is known as critical mass. The fuel core must contain at least the critical mass and more often, slightly larger than the critical mass in order to maintain the chain reaction. Elements of the nuclear reactor: The essential components of nuclear reactor are as follows: 1) Fuel rods 2) Control rods 3) Moderator 4) Reflector. 5) Coolants. 6) Shielding 7) Control mechanisms 8) Measuring systems. 1)Fuel rods 86
  • Fuels which are commonly used are natural uranium and enriched uranium cast in the form of rods and plates. The fuel rods are clad with stainless steel to prevent the oxidation. The fuel rods are surrounded by the moderator. The minimum amount of the fuel must be maintained in the reactor in order to sustain the chain reaction this is known as the critical mass. The fuel rods must contain at least the critical mass and slightly larger than the critical mass in order to maintain the chain reaction. 2) Control rods The purpose of the control rod is to maintain the value of multiplication factor as one this is the minimum condition required to maintain the nuclear fission. This maintains the steady state heat generation in the reactor. The control rod helps to vary the out put according to the load and shut – down the reactor under emergency conditions. When the shutting down of the reactor is required the control rods, absorb more number of neutrons than emitted and the fission reaction dies out. The material which are commonly used for control rods are cadmium, Boron etc. The control rods are automatically operated. 3) Moderator The function of the moderator is to reduce the energy of the neutrons evolved during fission from 2Mev to 0.25 Mev in order to maintain the chain reaction. By the slowing down of high energy neutrons, possibility of escape of neutrons is reduced and possibility of absorption of neutrons to cause further fission is increased. This also reduces the quantity of the fuel required to maintain the chain reaction. The common moderators used are ordinary water , heavy water , graphite and beryllium. 4) Reflector The neutrons which may escape from the surface of the core without taking part in fission can be reflected back in to the core to take part in the chain reaction . This is done by a reflector. The required properties of a good reflector are low neutron absorption , high capacity to reflect and resistance to oxidation and radiation. The moderators which are commonly used also work as reflectors. A blanket of reflector can reduce the critical mass required to maintain the chain reaction. 5) Coolants The purpose of the coolants is to transfer the heat generated in the reactor core and use it for steam generation. The coolant circulated in the reactor core keeps the temperature of the fuel below safe level by continuous removal of energy from the core. The coolant used must have very high specific heat to carry more heat per kg of coolant used. It should not absorb neutrons, It must be non corrosive , non oxidizing and non toxic. Ordinary water , heavy water, sodium, potassium and carbon dioxide are the common coolants used in power generating reactors. 6) Shielding The reactor is source of intense radio activity and these radiations are very harm full to the human life. Therefore it is necessary to prevent the escape of these radiations to the atmosphere. The inner core is made of 50 to 60cm thick steel plate and it is further thickened by few meters using concrete. The thermal shield is cooled by circulation of water. 7) Control mechanisms The control system is also necessary to prevent the chain reaction from becoming violent and consequently damaging the reactor. It is an essential part of a reactor and serves the following purposes i) Starting the reactor , ii) Maintaining the reactor at that level ,iii) Shutting down of the reactor during emergency conditions. The control system works on the principle of absorbing the excess neutron with the s help of control rods either made up of boron steel or cadmium strips. 8) Measuring systems Main instruments required in nuclear reactor are thermocouples for measuring temperatures instrument for determining the thermal neutron flux. 87
  • Types of Nuclear reactors: 1) Pressurised water reactor. (PWR) In pressurized water reactor , heat generated in the nuclear core is removed by water circulating at high pressure through the primary circuit. The heat is transferred from primary to secondary circuit in a heat exchanger , or boiler, there by generating the steam in the secondary circuit. As such the steam in the turbine is not radioactive and need not be shielded. The pressure in the primary circuit maintained high using pressuriser so that boiling of water will not takes place. In order to vary the pressure in the primary circuit electric heating coils are used in the pressuriser. PWR produces only saturated steam. By providing separate furnace steam formed from the reactor could be super heated. Advantages: 1) The coolant used is cheap and easily available. 2) The reactor is compact, small in size and power density is high. 3) Fission products remain in the reactor and are not circulated. 4) There is a complete freedom to inspect and maintain the turbine, feed water heaters, and condensers during the operation. 5) Small number of control rods are required. 6) The fuel costs are less as the reactor extracts more energy per unit weight of fuel Disadvantages: 1) High primary circuit pressure requires strong pressure vessel and so high capital costs. 2) Severe corrosion problems. 3) Reprocessing of fuel is very difficult. 4) The reactor must be shut down for recharging. 5) Fuel fabrication is very difficult. 6) Thermal efficiency of secondary loop is very poor. 88
  • 7) Designing of the vessel against the thermal stresses is very difficult. Boiling water reactor (BWR) Apart from heat source the BWR generation cycle is similar to that found in the thermal power plants. In this type of reactor , enriched uranium is used as fuel and water is used as coolant, moderator and reflector like PWR except the steam is generated in the reactor itself instead of separate steam boiler. The plant can be safely operated using natural convection within the core or forced circulation as shown . Advantages: 1) The cost of the pressure vessel is less compared to vessel required for PWR. 2) This reactor does not requires separate steam generator therefore the cost is further reduced. 3) The metal temperature remains low for given out put conditions. 4) The reactor is capable of meeting the small fluctuating load requirements. 5) Thermal efficiency is high compared to PWR. 6) BWR is more stable than the PWR. Disadvantages: 1) Steam leaving the reactor is slightly radioactive therefore shielding of turbine and piping is required. 2) Power density of the reactor is only 50% of PWR. 3) Part of the steam is wasted at low loads. 4) Enrichment of the fuel for the reactor is extremely costly process. 5) More biological protection is required. 6) Possibility of burn out of fuel is more in this reactor than PWR 3) CANDU ( Canadian-Deuterium-Uranium ) Reactor. 89
  • CANDU is pressurized heavy water reactor first developed in Canada. The coolant heavy water is passed through the fuel pressure tubes and heat exchanger. The heavy water is circulated in the primary circuit in the same way as with a PWR and steam is raised in the secondary circuit transferring the heat in the heat exchanger to the ordinary water. The reactor is controlled by the moderator level hence control rods are not required. In CANDU reactor refueling is carried out even as the reactor is in operation. The high temperature coolant leaving the reactor passes out of the outlet header to a steam generator of conventional inverted U tube and is then pumped back in to the reactor through the inlet header .The steam is generated at temperature about 2650C. The reactor vessel and the steam generator system are enclosed by a concrete containment structure. A water spray in the containment would result from large break in the coolant circuit. Advantages: 1) The fuel need not be enriched one. 2) The cost of vessel is less. 3) No control rods are required. 4) Low moderator level increases the effectiveness in slowing down of neutrons. 5) Construction time required is less compared to BWR and PWR. 6) The cost of moderator used is less. Disadvantages: 1) The power density is considerably low compared to BWR and PWR. 2) It requires high standard of design , manufacture and maintenance. 3) The leakage is a major problem. 4) The cost of heavy water is extremely high. 5) The size of the reactor is very large. Sodium Graphite reactor.(Liquid metal cooled reactor) 90
  • Sodium graphite reactor is one of the typical liquid metal reactor. In this reactor sodium works as a coolant and graphite works as moderator. It consists of three circuits, primary circuit, secondary circuit and Steam circuit. In primary circuit liquid sodium which circulates through the reactor core and gets heated. This heated liquid sodium gets cooled in the intermediate heat exchanger and returns to the reactor core . The secondary circuit has an alloy of sodium and potassium in liquid form. This coolant absorbs heat from the sodium circulating in the primary circuit in the intermediate heat exchanger. The heated coolant then passes through the boiler and supplies heat required for generation of steam. The steam generated in this boiler is super heated. The sodium potassium liquid in the secondary circuit from the boiler is supplied back in to the intermediate heat exchanger with the help of pump. Advantages: 1) The thermal efficiency is high . 2) The cost of graphite moderator is low. 3) Excellent heat removal capability. 4) The size of the reactor is small. 5) High temperatures are available at low pressure. 6) Super heating of steam is possible. 7) High conversion ratio. 8) The coolant sodium need not be pressurized. Disadvantages: 1) Sodium reacts violently with water in the air. 2) Heat exchanger must be leak proof. 3) The problem of thermal stresses can not be maintained. 4) Intermediate system is necessary to prevent the reaction of sodium with water. 5) The leak of sodium is very dangerous as compared with other coolants. 6) It is necessary to shield the primary and secondary circuits with concrete blocks as sodium is highly radioactive. 91
  • Fast breeder reactor. The fast breeder reactor derives its name from its ability to breed , that is to create more fissionable material than it consumes. When U235 is fissioned it produces additional heat and neutrons. If some U238 is kept in the reactor , part of additional neutrons available , after reaction with U235 convert U238 in to fissionable plutonium. The general arrangements of the sodium fast breeder reactor is as shown in the figure. In fast breeder reactor, enriched uranium or plutonium is kept in reactor core without moderator. The vessel is surrounded by thick blanket of depleted fertile uranium. The ejected excess neutrons are absorbed by the fertile blanket and converts it in to fissile material. The heat produced in the reactor core is carried by liquid metal sodium. Advantages: 1) High breeding gain is possible. 2) High power density. 3) It has high boiling point. 4) It has low vapour pressure at most temperatures. 5) Absorption of neutrons is low. 6) High burn – up of fuel is achievable. 7) Small core is sufficient. 8) The moderator is not required. Disadvantages: 1) Requires highly enriched fuel . 2) Neutron flux is high at the centre of the core. 3) The specific power of the reactor is low. 4) Handling of hot radioactive sodium is major problem. 5) Safety must be provided against the melt down. 92
  • Homogeneous graphite reactor and gas cooled reactor.(HGGCR) In gas cooled reactors most commonly inert gases such as helium and carbon dioxide are used as coolants and graphite as moderator . The graphite tubes fitted with fuel rods or fuel tubes fitted in tubes or rods made up of graphite and fuel mixed together are used. The gas is passed through the tubes and carry the heat . The fuel used is either enriched uranium or natural uranium. Two types of reactors are used . a) Indirect circuit gas cooled reactor. The arrangement of this type of reactor is as shown in the fig. The gas is passed through the reactor to carry the heat generated by fission and the hot gas is further used for generating the super heated steam. The Hinkley power station in England is working on this principle. Advantages: 1) Fuel processing is simple. 2) There is no need for limiting the fuel element temperature. 3) Graphite remains stable even at high temperatures under high intensity radiation. 4) There is chances of explosion in the reactor due to the use of carbon dioxide as the coolant. 5) There is no corrosion problem. 6) It gives better neutron economy. Disadvantages: 1) Power density is too low. Therefore reactor vessel is very large. 2) The leakage of gas is the main problem. 3) The loading of the fuel is more elaborate and costly. 4) The coolant circulation absorbs as large as 10 to 20% of plant capacity where as only 5% is required in water cooled reactor. 5) The critical mass is high. 6) The control is more complicated. b) Direct circuit gas cooled reactor. 93
  • Direct gas cooled reactor is as shown in the fig. The high pressure , high temperature gas coming out of the reactor is directly fed in to the gas turbine for power generation. This is similar to the closed Brayton Cycle except that heat required to heat the fluid is generated in the reactor instead of in the combustion chamber. Advantages: 1) Thermal efficiency is high. 2) The capital cost is low. 3) The reactor can be made more compact as high density gas can be used. 4) The use of gas turbine offers greater flexibility for selection of site Disadvantages: 1) The system design is more complicated. 2) The components must be designed to bear higher stresses as high pressure gases are used. This increases the capital cost of the plant. Advantages of Nuclear power plants 1) Nuclear power plants need less space compared to other types of power plants. 2) Better performance at higher load factors. 3)There is saving in cost of the fuel transportation. 4) The operation is more reliable. 5) Nuclear power plants operation is independent of the weather conditions. 6) Advantage is more with large size power plants. 7) The expenditure on metal structures piping , storage mechanisms is much lower for a nuclear power plant than a coal burning power plant. 8) The nuclear power plants, besides producing large amount of power , produce valuable fissile material which is produced when the fuel is renewed. Disadvantages: 1) The capital cost is high. 2) The danger of nuclear radiations always persists in the nuclear plants. 3) The maintenance cost is high. 4) The disposal of fission products is major problem.. 5) Working conditions in the power plants always detrimental to heath ofworkers. Selection of site for Nuclear power plants: 1) Proximity to load. 2) Population distribution. 94
  • 3) Land use. 4) Geology. 5) Hydrology 6) Seismology. 7) Safety Radiation hazards. Human beings are continuously exposed to radiation from cosmic rays and various radioactive materials in the earth and air. Small amounts of radiation can be tolerated but exposure to radiations above certain level is dangerous to health and life. Living tissues are affected in three different ways when exposed to radiations they are i) Ionization: The formation of ion – pair in tissue requires 32.5 MeV of energy. About 3100 ion – pairs are formed when single 1MeV beta particle is stopped by tissue. This absorption results in complete damage of tissues in the body man, or beast or bird. ii) Displacement: If the energy of the impinging particle is sufficiently high, an atom in the tissue is displaced from its normal lattice position with possible adverse effects. iii) Absorption: Absorption of neutron by a tissue nucleus results in forming a radioactive nucleus and change the chemical nature of the nucleus. This severe alteration of the tissue causes malfunctioning of the cell and cell damage may have severe biological disorders including genetic modifications. Ultimate effect of all these hazards on human being is to damage the living cells of body by ionization. The result of such damage may be immediate , effects like burns, even death, or delayed effects like lukaemia , a anemia or cancer or may be genetic giving hereditary effects. Shielding: The common nuclear radiation emitting from nuclear reactors are in theform of -rays,neutrons,X-rays,- Rays and -Rays. The  and  radiations are absorbed in a smaller thickness of the shielding.  radiations require higher thickness shielding because of their higher high level of energy and frequency they can penetrate more . Neutrons have high power of penetration and do not follow any defined path through the shield materials. The shield should be designed to absorb or reduce  and neutron radiations. The nuclear radiation if it is not prevented , it will have very bad effects on the human life and biological plants. The desirable properties of the good shielding materials are. 1) It must have ability to absorb more  radiation with minimum thickness. 2) It must be fire resistant. 3) The strength of the material should remain constant under the influence of radiations. 4) It must have high density and it must contain light materials. 5) Density of the material must remain constant. The use of best neutron absorber shield is beneficial. The combination of light and heavy elements in the shield is best , the use of laminated construction or the use of iron concrete. The latter consists of iron mixed in barytes concrete, or alternatively limonite is used partially to replace barytes in the mix. Example for shielding materials include Water, Iron, cement and concrete, Tantalum, Lead, Bismuth and Boron. Nuclear waste disposal Used fuel in a nuclear power plant is highly radioactive and can contaminate air or water and if absorbed by a living organisms, it can cause biological damage. Disposal of radioactive waste is therefore a problem which requires consideration right from the planning stage. The nuclear wastes from the reactor are classified as i) High level waste( above 1000 Curie) ii) Medium level waste (100 to 1000 Curie) iii) Low level waste ( below 100 Curie). The spent fuel is withdrawn from the reactor and placed in a water pond where heat is removed. The pond water is treated to remove radiations. The spent fuel is then transferred to the processing plant where cladding that contains the fuel is removed and the fuel is dissolved in the nitric acid. The U235 (20 to 90% ) and Pu239 are then removed leaving the solution the solution known as highly active liquid waste. The 95
  • separated U235 and Pu239 are further purified and either stored for future use or fabricated in to fresh fuel for reactor. The waste from the cooling fond is the transferred to intermediate storage and kept there for a period of about 30 to 100 years where most of radioactive nature is reduced to a considerably low level. Then waste is permanently shifted to the final storage . Various methods used for the disposal of radioactive waste are given below. a) Storage in tanks on site. Solid and liquid wastes are stored in concrete or stainless steel tanks at site . During storage period the radioactivity decays and then the waste is disposed of either in the sea or buried under the ground. b) Dilution: Disposal of liquids after dilution to safe limits, in the rivers or sea is also done. Gases are also left off in air after dilution. Before disposal in the dilutent the radioactivity of the gas or liquid being discharged is reduced to acceptable levels. c) Sealed containers: Radioactive liquid and solid wastes are put in sealed containers which prevent the radioactive contamination . These sealed containers are then disposed of at sea where they are quickly and completely covered with mud in the bottom. d) Underground burial. Another alternative is the burial of wastes direct in the ground . How ever burial ground must be isolated from the public and water must not be able to seep through as it may cause radioactive contamination of drinking water supplies. Nuclear power plants in India: 1. Tarapur power plant: Located in Maharastra , has a capacity of 380 MW with the steam pressure and temperature of 35 bar and 2400C. 2.Rana Pratap Sagar power plant: Located near Kota in Rajasthan, has a capacity of 400 MW with steam pressure and temperature of 40 bar and 2500C. 3. Kalpakkam power plant: Located near Chennai in Tamilnadu, has a capacity of 470 MW. 4. Narora power plant : located near narora in UP , with , a capacity of 470 MW , steam pressure temperature of 40 bar and 2500C. 5. Kakrapar atomic power plant : Located near the Surat in Gujarat with a capacity of 470 MW, steam pressure temperature of 40 bar and 2500C. 6. Kaiga atomic power plant: Kaiga situated near Karwar in Karnataka. With a capacity of 440 MW , steam pressure temperature of 40 bar and 2500C. Problems: 1) Each Fission of U235 yields 190 MeV of useful energy. Assuming that 85% of neutrons absorbed by U235 cause fission, the rest being absorbed by non fission capture to produce an isotope U-236 , estimate the fuel consumption of U-235 per day to produce 3000 MW of thermal power. Soln: Data:- 190 MeV/ fis sion , P= 3000 MW Each fission yields 190 x 106ev x 1.60 x10-19 J/ eV = 3.04 x 10 -11 J Fission rate / W = 1/ 3.04 x 10 -11 = 3.3 x 1010 /s 96
  • In one days operation(86400 s) of reactor per MW of thermal power, the number of U 235 nuclei burned is = (106 w) ( 3.3 x1010 fission / W-s) ( 86400 s/day) / 0.85 fission / absorption = 3.35 x 1021 absorptions / day Mass of Uranium consumed to produce 1 MW power is = (3.35 x 10 21 / day) (235 g / g mol) / 6.023 x 1023 ( nuclei/g mol) = 1.3 g /day Therefore the fuel consumption of U235 to produce 3000 MW is 1.3 x3000 /1000 = 3.9 Kg / day 2) A nuclear reactor consumes 12 kg of U235 per day. Calculate its power out put if the average energy released per U235 fission is 200 MeV. Soln: Data:- m = 12 kg / per day of U235, Average energy released per U235 fission = 200 MeV Number of atoms in 235 kg of U235 = 6.02x1026 (AN) Hence number of atoms contained in 12 Kg ofU235 = (6.02 x 1026 / 235) x 12 = 3.07 x 1025 Total fission energy produced by these atoms = 200 x 3.07 x 1025 x 1.60 x10-13 =982.41012 J Time taken to consume 12 kg of U235 = one day =- 24 x 3600 = 86400 s  Power produced = 982.41012 J / 86400 = 11.37 x 109 W 3) 500 MW of electrical power is required for a city . If this is to be supplied by a nuclear reactor of efficiency 20 percent , using U235 as the nuclear fuel, calculate the amount of fuel required for one days operation. Soln: Data- P = 500 MW / day ,  = 20%, Energy consumed by the city in one day = 500 x 106 x 24 x 3600 =4320 x 1010 J Power to be produced by the reactor = Energy per day / Efficiency = 4320 x 1010 / 0.2 = 2160 x 1011 J Energy released per atom = 200 x 1.6 x 10-13 = 32 x 10-12 J Total number of atoms to be fissioned = Energy to be produced/ Energy released = 2160 x 1011 J / 32 x 10-12 J = 67.5 x 1023 6.02 x 10 number of atoms contained in the 235 kg of U235 , hence 67.5 x 1023 atoms are contained in a 26 mass = 235 x 67.5 x 1023 / 6.02 x 1026 = 2.635 kg 4) Calculate the binding energy and mass defect per nucleon of oxygen . Given mp= 1.007277 amu, mn = 1.008665 amu, me = 0.00055 amu atomic mass of oxygen  16 = 15.99491 amu. Soln: A molecule of oxygen has 8 proton, 8 neutrons and 8 electrons therefore mass defect m = 8x 1.007277 + 8 x 1.008665 + 8 x 0.00055 – 15.99491 = 0.13703 amu Binding energy = m x energy equivaelent of 1 amu = 0.13703 x 931 = 127.6 MeV Binding energy per nucleon = Binding energy / Number of nuclei = 127.6 / 16 = 7.97 MeV 97
  • Hydro electric power plants Introduction: The development of the hydroelectric power plant plays an very important role in the development of country. The power generated by the water is cheapest as it is perpetual source of energy. Hydro electric generation plants help for irrigation and flood control in addition to power generation. Nearly 30 % of the total power of the world is generated using hydro plants. In hydroelectric power plants the energy of water is utilized to drive the turbine which , in turn, runs the generator to produce electricity . Rain falling on the earth’s surface has potential energy relative to the oceans towards which it flows. This energy is converted in to shaft work where it falls through an appreciable vertical distance. The theoretical power available from falling water can be calculated using the formula P= gQH (1) Where P = Hydraulic power in watts, 98
  • g = 9.81 m/s2  = Density of water, (1000 kg / m3 ) Q = Flow discharge , m3/s H = Height in meter. The electrical energy produced in KWh canbe written in the form of W = 1000 x 9.81 x Q x H x  x t = 9.81 x Q x H x  x t KWh (2) Where  is the efficiency of turbine generator assembly and t is the time in hours. From equation 1 and 2 we can observe that the generating capacity of the hydroelectric power plant is dependent on the quantity of the water and potential head available at that particular site. If the head available is high then quantity of water required for the power generation is less where as when head availability is low then quantity of water to be required for per watt of power generation is high . The head availability depends on the topography of the dam . The quantity of the water available depends on the rainfall and runoff of the catchment area of the dam. Run off and its measurements: The part of the water in the rain fall which is flowing through the catachment area on the surface of the earth is known as the run off. As the rain falls upon the drainage basin, a portion of it is evaporated directly by the sun , another large portion is absorbed by the growing plants and crops and some water percolates into the ground. The remaining portion of the rain fall flows over the surface of the of earth is known as the run off. In general run off is calculated by R=P–L, Where R (Run off) = Rs + Rc , Rs- Run off over the surface Rc is the run off reaching the catachment area through pervious earth. P = Precipitation by rain fall L = All losses Measurement of run off Run can be measured daily, monthly , yearly. It can be measured by the following methods. 1) From rain fall records. 2) Empirical formulae. 3) Run off curves and tables. 4) Discharge observation method. 1) From rain fall records. In this method rain fall activity recorded over long period of time and the average of rain fall over catachment area is determined. Then considering all the factors affecting the run off ,a coefficient is calculated for that catachment . Now simple equation can be used to find out the run off over the catachment Run off = Rain fall x coefficient 2) Empirical formulae: In this method direct relationship between the rain fall and run off is established with fairly accurate results. Some of the formulas used for calculating the run off are a) Khosla’s formula: R = P – 4.811T , R- Run off, P – Annual rain fall in mm , T – temp in oC b) Linglis Formula: For ghat region R = 0.88 P – 304.8 For plain region R = (P-177.80 P/ 2540 c) Lacey’s formula: R = P/{( 1+ 3084F/PS)} R- run off in mm , P – Annual rain fall in mm, F – Monsoon duration factor 99
  • S- Catachment area factor 3) Run off curves and tables. The formulae given above can not be used universally due the variation in characteristics for different catachment areas and rain fall. How ever for the same region characteristics remain unchanged . Based on this run off coefficients are derived once for all. Then graph is plotted in which one axis represents rain fall and other run off . The curves obtained are called run off curves. Alternatively a table can be prepared to give the run off for a certain value of rain fall for a particular region. 4) Discharge observation method. By actual measurement of discharge at outlet of a drainage basin run off over a catachment can be computed . The water flow volume through a selected channel of fixed cross section is measured by measuring the velocity of water at enough points for different water levels. The mean velocity at each section is measured with float method or current meter directly. Then run off through the cross section of the river is given by Q = A1V1+A2V2+…….. +AnVn Where A1,A2,..An Are the areas of the sections, V1,V2..Vn are the mean velocities. Hydro graph Hydro graph is plot of discharge through a river versus time for specified period. The time period for discharge hydrograph may be day, week, or month. Each hydro graph has a reference to a particular site. Besides the variation in flow indicated by a hydrograph , it also indicates the power available from the stream at different times of the day, week, month or year. Extreme conditions of flow can also be studied from hydro graph. Behavior of flash stream on a hydrograph is indicated by the steep rise and fall of the curve. A hydro graph also helps in the studies of the effect of storage on flow. We can obtain the following information from the hydrographs. 1) Rate of flow at any instant during the duration period 2) Total volume of flow up to that instant . 3) The mean annual run off or mean run off . 4) Maximum and minimum run –off or mean run off for each month. 5) The maximum rate of run – off during the floods duration and frequency of the flood. 100
  • Crest Rising limb Falling limb ischarge in m /sec 3 D Time Flow duration curve A flow duration curve is another useful form to represent the run off data for the given time. This curve is plotted between flow available during a period versus the fraction of time. The flow may be expressed in the form cubic meters per second per week or any other convenient unit of time knowing the available head of water , total energy of flow can be computed. By changing the ordinate to power instead of discharge , the power duration curve is obtained and the area under the curve would then represent the average yield of power from hydro power project. Thus by flow duration curve it is possible to know the total power available at the site . Q m/ s , 3 Tim % e 101
  • 240 225 210 195 180 165 150 Flow rate m / s 135 3 Flow duration curve 120 105 90 Flow duration line Q 75 n FQ D n E 60 45 30 C 15 B Qm O AQm 0 10 20 30 40 50 60 70 80 90 100 Percentage of time Form the fig given above Qm is the minimum flow rate that would be available all the time (100% of time) .and the area OABC represents the minimum power available always , often termed as primary power. The additional out put available at higher water flows is called secondary power. If the flow rate of Qn is required for all the times as indicated by the area under flow demand line DEF, then it would be possible to meet this uniform demand of flow rate for all the times only if storage equal to area BEF is provided. An alternative to this is to install a thermal power unit of BF capacity to work as supplement to the hydro electric power unit. The curve also shows that natural flow sufficient to meet the flow demand Qn is available for 53.5 percent of time or 195 days in the year of lowest flow of the record. In the absence of any storage area BCDE represents the secondary power that would be available from the river. Mass curve: The mass curve is a plot of cumulative volume of water that can be stored from stream flow versus time in days, weeks, or months. The unit used for indicating the storage are the cubic meters or the day second meters. Mass curve is an integral curve of the hydrograph which expresses the area under the hydrograph from one time to another time. Mathematically the flow mass curve is expressed as t2 V =  Qt , dt t1 Where V is the volume of run off and Qt is the discharge in m3/s as a function of time. A typical mass curve is a shown in figure below. The slope of the curve at any point indicates the rate of flow at that particular of time. If the curve is horizontal flow is zero and if there is a high rate of flow the curve rises steeply. Relatively dry periods are indicated as concave depressions on the mass curve. 102
  • C ulative discharge um Time Calculation of storage capacity and spill way capacity from mass curve. The slope of the straight line AB joining the end points of the mass curve represents the average discharge over the total period. The straight line CD parallel to AB and tangent to the mass curve at its lowest point g is called a use line. The storage volume required to supply water continuously is given by the greatest ordinate between the use line and mass curve. If it is required to determine the required storage for some other required uniform flow rate, straight lines such as fj, and hi are drawn tangents to the high points of the mass curve , with a slope equal to the desired flow. The required storage for continuous supply is given by the maximum ordinate between such lines and mass curve. B j D f i ulated flow h Slope of demand line M curve ass Accum G Storage capacity A C Tim (Years) e Storage and pondage: 103
  • The flow rate of stream varies considerably with the time. In rainy season stream is in floods it carries huge quantity of water as compared to other times of the year when quantity of water carried by it is considerably less. However the demands for the power do not correspond to such variations of the natural flow of stream . A such arrangement in the form of storage and pondage of water is required for the regulation of the flow of water so as it make it available in requisite quantity to meet the power demand at a given time. Storage: Storage may be defined as storing of considerable amount of excess run off during seasons of surplus flow for use in dry seasons. This accomplished by constructing the dam across the stream at suitable site and building a storage reservoir on the upstream side of the dam. Storage increases the capacity of the river over an extended period of 6 months as much as 2 years. The following figure shows the location of the storage with respect to the power house. River S orage reservoir t Dam Hydro power house River Pondage: Pndage may be defined as a regulating body of water in the form of a relatively small pond or reservoir provided at the plant. The Pondage is used to regulate the variable water flow to meet power demand. It takes care of short term fluctuations which may occur due to a) Sudden increase or decrease of load on the turbine. B) Sudden changes in the flow of water, say by breaches in the conveyance channel c) Change of water demand by turbines and the natural flow of water from time to time. Pondage increases the capacity of a river over a short time , such as a week. The following figure shows the location of the pondage with respect to the power house. 104
  • River Power channel Intake Weir Hydro power house Tail race Fore bay to provide Pondage Short Penstocks Pondage fig: Classification of hydroelectric power plants Hydro electric power stations may be classified as follows. 1) According to availability of head. a) High head power plants b) Medium head power plants c) Low head power plants 2) According to nature load a) Base load plants b) Peak load plants. 3.According to the quantity of water available. a) Pump storage plants b) Storage type plants c) Mini and micro hydel plants d) Plant with pondage e) Plant without pondage. 1) According to availability of head. a) High head plants In high head plants operating head is 100 m and above. Water is usually stored in lakes on high mountains during the rainy season . The rate of water discharge from the water is maintained at such rate that water must available throughout the year. 105
  • The above figure shows the high head power plant layout. In order to maintain the safety of the dam surplus water is discharged through the spillway. Flow is controlled by the head gates at the tunnel intake .Tunnel is constructed through the mountain with surge chamber at near exit. . Butterfly valves are used to regulate the water in the penstocks , and gate valves at the turbines. This type of the plant can also constructed under ground. Pelton wheel is the commonprime mover used in the high head power plants. b) Medium head power plants. These plants operate under the heads varying from 30 m to 100 m. Forebay is constructed at the beginning of the penstock serves as water reservoir. This type of the plant commonly uses Fracis turbine as the prime mover. In such plants water is carried in open canals from main reservoir to the fore bay and then to the power house through the penstocks. The forebay itself works as the surge tank in this plant. c) Low head power plant. In low head power plants working head is less than 30 m. A dam is constructed across a river and a sideway stream diverges from the river at the dam. Over this stream power house is constructed. Later this channel joins the river further down stream. This type of plant uses vertical shaft Francisturbine or Kaplan turbine. 2) According to nature of load 106
  • a) Base load plant: These plants supply constant power to the grid without any interruption. They work throughout the day. Base load plants are often remote controlled with which least staff required for such plants. Run –of-river plants without pondage may sometimes work as base load plant but the capacity is less. b) Peak load plats: They supply power only during the certain hours of the day when the load is more than the average. Thermal power plants work with hydel plants in tandem to meet the base load and peak load during various seasons. Some of such plants supply the power during the average load but also supply peak load as and when it is there. The run-off river plants may be made for peak load by providing pondage. Pump storage plants Water after working in the turbine stored in the tail race pond. During low load periods this water is pumped back in to the head reservoir using an extra power available. This water can be again used for generating power during peak load periods. Pumping of water may be done seasonally or daily depending upon the conditions of the site and the nature of the load on the plant. Such plants are usually interconnected with steam or diesel engine plants so that off peak capacity of interconnecting stations is use in pumping water and the same is used during the peak load periods. Advantages: 1) There will be an increase in the plant capacity with low cost. 2) Operating efficiency of the plant is high. 3) There is an improvement in the load factor. 4) The hydroelectric plant becomes partly independent of stream flow conditions. In this type of plants reversible turbine pump units are used. These units can be used as turbine while generating power and as pump while pumping water to storage. With the use of reversible turbine pump sets, additional capital investment on pump and its motor can be saved . Essential elements of the hydroelectric power plants 107
  • The following are the essential elements of the hydro electric power plants 1) Catchment area 2) Dam 3) Reservoir 4) Spill ways 5) Penstock 6) Surge tanks 7) Draft tubes 8) power house 9) Switch yard for power transmission. Catachment area: The whole area behind the dam draining into a stream or river across which the dam has been constructed is called the catchmentarea. Dam:A dam performs the following two basic functions. 1) It develops reservoir of desired capacity to store water 2) It builds up a head for power generation. Various types of the dams are used depending on the requirement and geographical area. 1) Gravity dams: These dams are constructed in stone masonry or in concrete. 2) Earth dams: For small projects of up to 70 m height , dams constructed of earth fill or embankment are used. 3) Rock fill dams: It is made up of all sizes and has a trapezoidal shape with a wide base, having water tight section to reduce seepage. Spill ways: When water level in the reservoir rises, the stability of the dam is endangered. To relive the reservoir of this excess water, a structure is provided in the body of a dam or close to it. This safe guarding 108
  • structure is called spillway. Variety of spill ways are used example Overall spillway , Chute or trough spillway, Side channel spillway, saddle spillway, Shaft spillway and Siphon spillway. Penstocks: It is an closed conduit used for supplying water to the turbine from forebay under pressure. Penstocks are used where slope is too great for canal . Surge tanks or other measures are necessary to prevent damage in closed conduits due to abnormal pressures. The regulating forebay has a small storage capacity to care for minor flow fluctuations . It has an automatic spillway to discharge overflow when turbine shut down suddenly. In different ways we can arrange to supply water to the turbines. i) One penstock for one turbine. In such a case water is supplied independently to each turbine from a separate penstock ii) Single penstock for the entire plant.: In this case penstock should have as many branches as the number of hydraulic turbines. iii) Multiple penstocks but each penstock should supply water to at least two hydraulic turbines. Penstock materials and their suitability. i) Reinforced concrete: These penstocks are suitable up to 18 m head . Beyond this pressure concrete can not with stand the pressure. ii) Wood stave penstocks: In this type of penstocks treated wood is placed side by side to form cylinder and held together by the steel hoops. These penstocks are used for heads up to about 75 m. iii) Steel penstocks: Penstocks made up of steel can be used for any head , with the thickness varying with the pressure and diameter. The strength of the penstocks can be expressed as horse power it can carry. High pressure penstocks are fabricated in 6 to 8 meters lengths in order to minimize transportation difficulties. Welded joints are used instead of riveted joints because of the higher frictional losses in latter case. Penstocks are generally supported by concrete piers cadles., although they may be laid on or in ground. . Water hammer Water hammer is defined as the change in pressure rapidly above or below normal pressure caused by sudden changes in the rate of water flow through the pipe according to the demand of prime mover. When the gates supplying the water to the turbines are suddenly closed owing to the action of governor , when the load on the generator is suddenly reduced, there is sudden rise in pressure in the upstream of the pipe supplying the water to the turbine. This sudden change of pressure and its fluctuations in the pipe line during reduction of load on the turbine is known as water hammer. The turbine gates suddenly opens because turbine needs more water due to increased demand on the generator and therefore, during increased 109
  • load conditions , water has to rush through the pipe and there is tendency to cause a vacuum in the pipe supplying the water. The pipe supplying the water must have the capacity with stand variations in the water pressures. The water hammer can occurs at all points in the penstock between the forebay or surge tank and the turbines. Surge tank Surge tank is open reservoir or tank in which the water level rises or falls to reduce the pressure swings so that they are not transmitted in full to a closed circuit. Important functions of the surge tank are 1) It reduces the distances between the free water surface and turbine thereby reducing the water hammer effect of the penstock and also protect the up streamtunnel from high pressure rises. 2) It serves as the supply tank to the turbine when the water in the pipe is accelerating during increased load conditions as a storage tank when the water is decelerating during the reduced load conditions. 3) It acts as relief valve when ever there is variations in water pressure in the penstocks. Surge tank should be located as near to the power house as is feasible to reduce the length of the penstock thereby reducing water hammer effect. It is generally located at the junction of tunnel and penstock in order reduce its height. Types of surge tanks. 1) Simple surge tank. The simple surge tank is cylindrical in shape and attached to the penstock as shown in the figure. It is always desirable to place the surge tank over the ground surface on the penstock pipe. If suitable site is not available the height of the tank should be increased with the help of a support. This type of the surge tank is uneconomical due to its large size and its action is also sluggish as compared with other types of tanks. It is most expensive and seldom used in preference to other types. 2) Inclined surge tank. When a surge tank is inclined at an angle to the horizontal its effective water surface area increases and therefore , lesser height surge tanks are required of the same diameter if tit is inclined or lesser diameter tank is required for the same height. It is more costlier than the ordinary type as construction is difficult ant it is seldom used unless the topographical conditions are in favour. 3) Expansion chamber surge tank. 110
  • This type of surge tank has an expansion tank at top and expansion gallery at the bottom, these expansions limit the extreme surges. The upper expansion chamber must be above the maximum reservoir level and bottom gallery must be below the lowest steady running level in the surge tank. In addition the intermediate shaft should have minimum diameter. 4) Restricted orifice surge tank. It is also called throttled surge tank. The orifice provided helps in creating appreciable friction loss when the water is flowing to or from the tank. When the load on the turbine is reduced , the surplus water passes through the throttle and a retarding head equal to the loss due to throttle is built up in the conduit. The size of the throttle can be designed for any designed retarding head. The effect of throttle is very limited except at large change of lad because the additional frictional loss is proportional to the square of the velocity in the port. The change in the velocity will not be considerable unless the change of load is not large. It is very rapid in action, but the pressure rises are also equally rapid , therefore, it is less effective than simple surge tank in relieving the water hammer. The main disadvantage of this type of the surge tank is that , considerable portion of water hammer pressure is transmitted directly in to the low pressure conduit.. 5) Differential surge tank. A differential surge tank has riser with a small hole at its lower end through which water enters in it. The function of the surge tank depends upon the area of hole. With change of load , the water level in the riser rises or falls very rapidly thus producing a rapid deceleration or acceleration of the conduit flow. Though rapid in action, the differential surge tank gives reasonably low pressure rises and surges low amplitude. This type of surge tank is having an advantage of preventing increasing surges under all conditions. 111
  • Draft tubes: Draft tube allows the turbine to be set above the tailrace to facilitate inspection and maintenance and diffuser action regains the major portion of the kinetic energy or velocity head at runner outlet, which would otherwise go waste as an exit loss. The draft tube can be straight conical tube , or an elbow type is more common. Power house: A power house should have a stable structure and its layout should be such that adequate space is provided around the equipment for convenient dismantling and repair. The power provides the space for following equipments. i) Hydraulic turbines ii) Electric generators iii) Governors iv) Gate valves v) Relief valves vi) Water circulation pumps vii) Air duct viii) Switch board and instruments ix) Storage batteries x) Cranes. Advantages: 1) Operating cost of the plant including auxiliaries is extremely low. 2) As maintenance cost of the plant is less costly. 3) Less labour is required to operate the plant.. 4) No nuisance of smoke , exhaust gases, soot etc exists in this case. 5) The cost of the land required is less. 6) The plant efficiency does not change with age. 7) Plant life is much longer than that of the thermal power plant. 8) Less number of skilled workers are required. 9) In addition to the power generation these plan are also used for flood control and irrigation purposes. ts 10) No fuel charges. Disadvantages: 1) Initial cost of the plant including the cost of dam is high. 2) Power production may be curtailed or even discontinued in time of drought. Thus power plant is not reliable. 112
  • 3) The suitable sites are always away from the load center and hence transmission losses are more. 4) Vast area of fertile ,agriculture and forest land may be submerged. 5) The plant construction time is long. Brief Description of some of the important hydel power stations in India. Important hydro plants in India are Sl State / Name of the power plant Installed capacity (MW) No Andhra Pradesh 1 Machkand (Stage I and II) 114 2 Upper silern 120 3 Lower Silern 600 4 Srisailam 770 5 Narjuna sagar pumped storage 100 Assam 1 Umian 54 Gujrat 1 Ukai 300 Himachal pradesh 1 Baira suil 200 Jammu Kashmir 1 Slal 200 Karnataka 1 Tungabhadra 72 2 Sharavathi 890 3 Kalinadi 396 Kerala 1 Parambikulam Aliyar 185 2 Sabarigiri 300 3 Iddiki 390 Maharastra 1 Koyana 860 Orissa 1 Hirakud 270 2 Balimela 480 Punjab 1 Bhakra nangal 1084 2 Beas sutlej link 780 Rajasthan 1 Chambal 287 Uttar pradesh 1 Rihand 300 2 Yamuna 424 Tamil nadu 1 Kundah 425 2 Kodiar 100 113
  • Problems: 1) At a particular site ( in millions of m3) of a river in 12 months from January to December are 30,25,20,0,10,50,80,100,110,65,45and 30 respectively. i) Draw hydro graph the flow duration curve on the graph sheet and find the average monthly flow. ii) Estimate the power developed in MW if the available head is 90 m and the overall efficiency of generation is 87.4% assume each month 30 days. Soln: H = 90 m, o = 87.4% Month Discharge in millions of Month Discharge in millions of cubic meter / month cubic meter / month January 30 July 80 February 25 August 100 March 20 September 110 April 0 October 65 May 10 November 45 June 50 December 30 Hydro graph 114
  • 120 10 1 100 90 eter 80 illions of cubic m 70 Average flow 47.083 60 50 ischarge in m 40 30 D 20 10 0 J F M A M J J A S O N D Average discharge for the flow Qav = (30+25+20+0+10+50+80+100+110+65+45+30)/ 12 = 47.0834 millions of cubic meter /month = 47.083 x 106 / ( 30 x 24 x 3600) =18.165 m3 /s ii) Flow duration curve Discharge in millions of cubic Total number of months during Percentage time meter / month which flow is available 0 12 100 10 11 91.7 20 10 83.3 25 9 75 30 8 66.7 45 6 50 50 4 33.3 80 3 25 100 2 16.7 110 1 8.3 115
  • 10 1 100 onth 90 eter per m 80 70 illions of cubic m 60 50 ischarge in m 40 30 20 D 10 0 10 20 30 40 50 60 70 80 90 100 Tim ( % ) e Flow duration curve Power developed in MW P = o g Qav H /106 = 0.874 x 1000 x 9.81x 18.165 x 90 / 106 = 14.02 MW 2) The mean monthly discharge for 12 months at a particular site of river is tabulated below Month Discharge in millions of Month Discharge in millions of cubic meter / month cubic meter / month May 500 October 2000 June 200 November 1500 March 1500 December 1500 July 2500 January 1000 August 3000 February 800 September 2400 March 600 i) Draw hydro graph and flow duration curve for the above and find average monthly flow. ii) Determine the power available at mean flow of water if available head is 80 m at the site and overall efficiency of generation is 80%. Take 30 days in a month. Soln: H = 80 m, o = 80% 116
  • 3000 2800 2600 2400 onth 2200 eter / m 2000 illions of cubic m 1800 1600 1400 Average flow ischarge in m 1200 1458.33 millions of cubic meter / month 1000 D 800 600 400 200 0 A M J J A S O N D J F M Average monthly flow Qav = (500+200+1500+2500+3000+2400+2000+1500+1500+1000+800+600) /12 = 1458.33 x 106 m3 / month = 1458.33 x 106 m3 / (30x24x3600) = 562.63 m3 /s Flow duration curve Discharge in millions of cubic Total number of months during Percentage time meter / month which flow is available 200 12 100 500 11 91.7 600 10 83.3 800 9 75 1000 8 66.7 1500 7 58.3 2000 4 33.3 117
  • 2400 3 25 2500 2 16.7 3000 1 8.3 Flow duration curve 3000 2800 2600 2400 onth 2200 eter / m 2000 illions of cubic m 1800 1600 1400 ischarge in m 1200 1000 D 800 600 400 200 0 10 20 30 40 50 60 70 80 90 100 Tim ( % ) e ii) Power developed in MW P = o g Qav H /106 = 0.80 x 1000 x 9.81x 562.63 x 80 / 106 = 353.24 MW 3) The data for twelve months flow at a particular site is given below. Month Discharge in millions of Month Discharge in millions of cubic meter / month cubic meter / month 1 100 7 190 2 50 8 40 3 20 9 30 4 80 10 200 5 10 11 170 6 10 12 80 Find a) The required reservoir capacity for the uniform flow of 50 millions cu-m per month throughout the year b) Spill way capacity. c) Average flow capacity if whole water is used and required capacity of the reservoir for this condition . 118
  • Month Discharge in millions of cubic Cumulative volume in millions of meter / month cu-m 1 100 100 2 50 150 3 20 170 4 80 250 5 10 260 6 10 270 7 190 460 8 40 500 9 30 530 10 200 730 11 170 900 12 80 980 230 millions cu- m 1000 b 900 800 onth d eter / m 700 34 millions cu- m/m 600 illions of cubic m 500 82 millions cu - m/m 400 S = 86 m pill illions cu- m 300 ischarge in m 200 z 100 x y D 0 a 1 2 c 3 4 5 6 7 8 9 10 11 12 From the graph storage capacity = 82 x 106 m3 Spill way capacity required = 86 x 106 m3 Join the points a and b then the slope of the line ab represents the uniform discharge throughout the year. = 980 / 12 x 106 = 81.7 x 106 m3 / month 119
  • Draw the line cd parallel to ab which touche the mass curve to its lowest point . The maximum departure s of the line cd from the mass curve represents the requiredstorage capacity for the uniform supply of 81.7 x 106 m3 / month . In this case , storage capacity required = 230 millions of cu-m. Non conventional energy sources: Solar energy: Introduction: Of all the renewable energy sources , solar energy received the greatest attention the decade of the 1970s. Many regarded it as the solution for reducing the use of fossil and nuclear fuels and for a cleaner environment. Solar energy , in sheer size, does have the potential to supply all energy needs: electrical, thermal, process, and chemical , and even transportation fuels. It is however , very diffuse, cyclic, process, and chemical , and often undependable . It is therefore needs system and components and that can gather and concentrate it efficiently for conversion to any of these uses and that can do the conversion as efficiently as possible . Solar radiation outside the earth’s atmosphere: The energy incident on the earth out side its atmosphere is called extraterrestrial radiation. Energy radiated by the sun as electromagnetic waves of which 99 percent have wave lengths in the range of 0.2 to 4.0 micrometers. Solar energy reaching the top of the earths atmosphere consists of about 8 percent ultraviolet radiation , 46 percent visible light (0.39 to 0.78 micrometer), and 46 percent infrared radiation ( long wave length more than 0.78 micrometer). The sun is very large sphere of very hot gases , heat being generated by various kinds of fusion reactions. Its diameter is 1.39 x 106 km . While that of the earth is 1.27 x 104 km. The earth rotates around the sun in the elliptical orbit with major and minor axes differing by 1.7 percent. The mean distance between the two is 1.50 x 108 km . It subtends at an angle of only 32 minutes at the earth’s surface. This is because it is also at a very large distance. Thus the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its centre to its edge. For all practical purposes , therefore , the sun’s rays may be considered parallel when they reach the earth . The sun has an effective black body temperature , as seen from the earth of 5762 K. The rate at which solar energy arrives at the top of the atmosphere is called the solar constant (S = 1.353 KW / m2 ) or This is the amount of energy received in unit time on a unit area perpendicular to the sun’s direction at the mean distance of the earth from the sun. Energy received at the earth’s atmosphere varies Because of the variation in the distance and activity . This can be approximated by the equation I/ Isc = 1+ 0.033 x cos x [360 (n-2) ] / 365  1+ 0.033 x cos x 360n / 365 Where n is the day of the year . I is the change in the solar constant 120
  • Spectral distribution of solar radiation intensity It will be noted from fig which shows spectral distribution of solar radiation intensity at the outer limit of atmosphere, that the maximum value of 2074 W / m2 -m occurs at wave length of 0.48 m and that 99 percent of the sun’s radiation is obtained up to a wavelength of 4 m. Solar radiation at the earth’s atmosphere Solar energy falling on the earth’s surface is called terrestrial radiation . Terrestrial radiation varies significantly both daily because of the earth’s rotation and seasonally because of the change in sun’s declination angle. The terrestrial radiation is said to be attenuated by two mechanisms , scattering and absorption . Scattering (Diffusing) is a mechanism by which part of a radiation beam is scattered laterally and is , therefore attenuated by the air molecules, water vapour, and the dust in the atmosphere. The scattered or diffuse radiation is mostly of shorter wave length. 121
  • The absorption of the solar radiation in the atmosphere takes place mainly by ozone O3, water vapour and carbon dioxide CO2, X-ray and other very short wave length radiation of the sun are absorbed in ionosphere by N2, O2 and other components. The terrestrial solar radiation incident on the earth’s surface is composed of two parts 1) Beam radiation ( Without scattering and absorption) 2) Diffuse radiation . The incident radiation on the earth’s surface is presented in terms of dimensionless parameter known as air mass ma . It is defined as the ratio of optical thickness of the atmosphere ( through which beam radiation passes to the surface) to its optical thickness if the sun were at the zenith( Directly above). Thus ma = 0 means extraterrestrial , ma=1 indicates sea level on the earth when sun is at zenith ma = 2 when sun is at zenith angle equal to 600. The air mass and zenith angle are related by ma = ( cosz) -1. The available terrestrial solar energy at a given time and place is influenced not only by time of the day or year, location and scattering but also by cloudiness . All effects may be combined in one parameter called clearness index Ci. It is defined as the ratio of the average radiation on a horizontal surface for a given period to the average extraterrestrial radiation for the same period. The values of Ci varies widely from nearly 30 to 70% in some localities on the earth, with its value going down to zero in some location because of bad weather even in the day time. The total amount of extraterrestrial power Pe received is given by the solar constant S times the projected area of the earth. Pe = SR2 Where R = Radius of the earth = 6.378 x 106 m, S = 1.353 KW / m2 , Pe = 1.353 x  x (6.378 x 106 ) 2 = 1.73 x 1014 KW Energy per year = 1.73 x 1014 x 8766 h/year = 1.516 x 1018 KWh / Year = 1.516 x 1018 x 3.6 = 5.47 x 1018 MJ / year 122
  • Under favourable atmosphere conditions the maximum intensity observed at noon on an oriented surface at sea level is 1 KW / m2 at an altitude of 1000 m. The value rises to about 0.5 KW / m2, and in higher mountains values slightly above 1.1 KW / m2 are obtained compared with 1.353 KW / m2 in outer space. The upper curve represents the outer limit of atmosphere. The other lower curve applies to earth’s surface during clear days for a sea level location. Dotted curve shown for a black body at 59000 K . The lower two curves are for diffuse components for some haze and clear sky conditions respectively. Pyrometers: Pyrometers are the instruments which are used for measuring the solar radiation. measurements of solar radiation are important because of the increasing number of solar cooling applications and the need for accurate solar radiation data to predict performance .Two basic types of instruments are used for solar radiation measurement. 1) A Pyrheliometer: Which is used for measuring radiation to determine the beam intensity as a function of incident angle. 2) A Pyranometer : This instrument measures the total hemispherical solar radiation. The pyranometers are the most common. Pyrheliometer: In this type of instrument sensor disc located at the base of the tube whose axis is aligned with the direction of the sun’s rays. Thus diffuse radiation is completely blocked from the sensor surface. Three different types of Pyrhelliometers are widely used 1) The Angstrom Pyrheliometer 2) Silver disc pyrheliometer 3) Eppley pyrheliometer. Angstrom pyrheliometer: In this type of pyrheliometer , a thin blackened shaded manganin strip is heated electrically until it is at the same temperature as similar strip which is exposed to solar radiation. Under steady state condition the energy used for heating is equal to the absorbed solar energy . The thermocouple on the back of the each strip , connected to galvanometer . The energy H of direct solar radition is calculated by means of thye formula. Hdn = Ki2. Hdn is direct radiation incident on an area normal to sun’s rays , I heating current in amperes, K is a dimension and instrument constant = R / W R – is the resistance / unit length of the absorbing strip  is the absorbing coefficient of the absorbin strip. g Pyranometer: 123
  • This instrument measures total or global radiation over a hemispherical field of view. In most pyranometrers , the sun’s radiation is allowed to fall on a black surface to which the hot junctions of a thermocouple are attached . The cold junctions of a thermocouple are attached . The cold junctions of the thermocouple are located in such way that they do not receive the radiation. As a result an emf proportional to the solar radiation is generated . This emf (0 to 10 mV) recorded over a period of time there are following types of pyranometers. 1) Eppley Pyranometer 2) Yellot Solarimeter ( Photovoltalic cell) 3) Mol – Gorczyhelki solarimeter 4) Bimetallic action graphs of the Rabitzsch type Eppley pyranometer : It works on the principle of thermocouple . There is a temperature difference between the black and white surface. The detection of the temperature difference is achieved by thermocouple . Some models are made up wedges . The disks or wedges are enclosed in a hemispherical glass cover. Flat plate collectors (Liquid or air) Flat plate collectors are suitable for the applications which are having operating temperatures 900C and below. Generally these are non concentrating type. They are made up of rectangular panels of about 1.7 to 2.9 sq m in area and are relatively simple to construct and erect. Flat plate collectors can absorb both direct and diffuse radiation, hence these are effective even on cloudy days when there is no direct radiation. 124
  • Flat plate collectors are of two different types 1) Liquid heating collectors 2) Air or gas heating collectors All flat plate collectors have five main components. 1) A transparent cover which may be one or more sheets of glass or radiation transmitting plastic film or sheet. 2) Tubes, fins passages or channels are integral with the collector absorber plate. which carry the water, air or other fluid. 3) The absorber plate , normally metallic or with a black surface . 4) Insulation Which should be provided at the back and sides to minimize the heat losses. Standard insulating materials such as fiber glass or styro – foam are used for this purpose. 5) Casing or container which enclose the other components and protects them from the weather. Liquid flat plate collector: It consists of a flat surface with high absorptivity for solar radiation, called adsorbing surface. Absorber metal plate is made up of copper , steel or aluminum.. Thickness of the metal sheet 1 to 2 mm , while tubes , which are also made from a metal , range in diameter from 1 to 1.5 cm. They are soldered , brazed or clamped to the bottom of the absorber plate with pitch ranging from 5 to 15 cm. The most widely used material for the absorber plate is corrugated galvanized sheet . The use of conventional standard panel radiators shown in the figure. The methods of bonding and clamping tubes to flat or corrugatedsheet are shown in fig 125
  • Heat is transferred from the absorber plate to a point of use by circulation of fluid across the solar heated surface. Thermal insulation of 5 to 10 cm thickness is usually placed behind the absorber plate to prevent the heat losses from the rear surface. The front covers are generally glass that is transparent to in coming solar radiation and opaque to the infra red re-radiation from the absorber. Glass is generally used for the transparent covers but certain plastic films may also be used. The thickness of 3 and 4 mm are commonly used. Usual practice is to have two covers. Air space between the cover and the absorber plate largely prevents loss of heat from the plate by convection. Cover glass permits the passage of solar radiations with smaller wave lengths but opaque to larger wavelength . As a result heat is trapped in the air space between the cover and the absorber plate in a manner similar to to green house. The loss of solar radiation due to absorption in cover plate can be minimized by using clear glass with low iron content.. The generated in the absorber is removed by continuous flow of a heat transparent medium, either water or air. When water is used it is most commonly passed through metal tubes with either circular or rectangular cross section. The tubes are connected to a common header at each end of the collector. In order to maximize the exposure to solar radiation , collectors are almost invariably sloped. Cooler water enters at the bottom header , flows upward through the tubes where it is warmed by the absorber , and leaves by way of the top header. 126
  • Some of the problems associated with liquid(Water) collectors are i) Freezing of water during cold nights ii) Corrosion of metal tubes by the water . Corrosion can be minimized by using copper or Aluminum tubes . iii) Leaks in a water circulation system require immediate attention. Air collector or Solar air heaters: Fig shows the solar air heater where an air stream is heated by the back side of the collector plate . Contact surface can be increased by attaching fins. Back side of the collector is insulated with mineral wool . The most favourable orientation of the collector , for heating only is facing due south at an inclination angle to the horizontal equal to the latitude plus 150. Air used as heat transport medium in solar collectors have some advantages over water. 1) It eliminates both freezing and corrosion problems ii) Small air leaks are less concern than water leaks iii) It can be directly used for space heating . The air may passed through a space between the absorber plate and insulator with baffles arranged to provide a long flow path as shown in the fig. Possible applications of solar heaters are drying or curing of agriculture products , space heating for comfort , regeneration of dehumidifying agents, seasoning of timber, curing of industrial products such as plastics. The flow of air may be straight through, serpentine, above or below or on both sides of the absorber plate or through porous absorber material. Air heaters are classified in the following two categories. 1) Non porous absorber 2) Porous absorber 1) Non Porous absorber solar air heater: In this type of solar air heater air stream does not flow through the absorber plate . Air may flow above and or behind the absorber plate. Transmission of solar radiation is similar to that of liquid type flat plate collector but due to heat transfer coefficients efficiencies are lower than liquid solar heaters . Some of the non porous type air heaters are shown below. 127
  • 2) Porous absorber solar air heater : Porous absorber solar air heaters are used for overcoming the difficulties faced in non porous solar air heaters . Important problems faced in porous type are 1) Excessive radioactive losses 2) Pressure drop along the duct formed 3) Larger amount of energy required to push the air through. Some of the porous absorber type air heaters are shown below. 128
  • Applications of solar air heaters: 1) Heating buildings 2) Heating in green houses 3) Air conditioning buildings . 4) Using as the source of heat for a heat engine such as Brayton or Stirling cycle 5) Drying agricultural produce Advantages of flat plate collectors: 1) Both beam and diffuse radiation can be used. 2) They do not require orientation towards the sun. 3) They require little maintenance. 4) They are mechanically simpler than concentrating reflectors . Solar energy storage: Storing of solar energy is necessary due to the following reasons. 1) It helps in absorbing solar energy when it is highest and the later used when the need is greatest. 2) It makes possible to meet electrical load demand during the times when insolation is below normal or non existent. 129
  • 3) It improves the reliability 4) It permits better match between solar energy input and demand output. Solar energy storage may be broadly classified as 1) Thermal storage : i) Sensible heat a) Water storage b) Pebble bed storage ii) Latent heat 2) Electrical storage i) Capacitor storage ii) Inductor storage iii) Battery storage 3) Chemical storage i) Chemical ii) Thermo chemical 4) Mechanical energy storage i) Pumped hydro electric storage ii) Compressed air iii) Fly wheel 5) Electromagnetic storage Thermal energy storage: In this method energy is stored by heating melting or vaporization of the material, and the energy becomes available as heat , when the process is reversed. In two different ways thermal energy can be stored i) As Sensible heat: Energy storage result in the rising of material temperature without changing phase. ii) As Latent heat : Energy storage results only in the phase change without rising the temperature. Sensible Heat storage: Fig Sensible heat storage involves no change in phase over the temperature change in the storage system. The basic equation governing the energy storage system operating over a finite temperature difference is Qs = mCp (T1-T2) Where Qs = Total thermal energy capacity for a system m = Mass of the storage medium in kg Cp = Specific heat of storage system T1, T2 = Temperature limits of storage medium The ability to store thermal energy in a given container of volume V is Qs / V =  Cp T Where  is the density of the storage medium. Materials which are generally used for this type energy storage i) Water ii) Rock, gravel or crushed stone iii) Iron shot iv) Iron, red iron oxide or iron ore (magnetic) v) Concrete vi) Refractory materials like magnesium oxide , Aluminum oxide and silicon oxide. 130
  • The most easiest way to store thermal energy is by storing the water directly in well insulated tank. Water storage is having several advantages over the other methods 1) It is an inexpensive , easily available. 2) It has high thermal storage capacity. 3) Pumping cost is small 4) No other heat transferring fluid is required. Rocks can also be used for storing heat . Rocks does have the following advantages over water. 1) Rock is more easily contained than water. 2) The system cost is low. 3) Much higher temperatures can be stored . 4) Heat transfer coefficients between solid and air is high. 5) The of storage material is low. Latent heat storage: A typical latent heat storage arrangement is shown in fig in which , the storage material is placed in long thin containers e.g cylinders, and the gas is passed through narrow spaces between the tubes. The advantage is that this system is more compact than the sensible heat system. In this method heat is stored in a material when it melts and extracted from the material when it freezes . The material which is used for latent heat storage must satisfy the following criteria 1) Phase change must accompanied by high laten heat. t 2) Phase change must be reversible. 3) The cost of materials and its containers must be reasonable . 4) It must have the capacity to store large quantity of heat. 5) Preparation of material must simple. 6) Must be harmless. 7) The material must have high thermal conductivity. 8) Containment of the material , transfer of heat into it and out of it must be easy. If these criteria can be met then reduction in volume and weight in storage system is possible. The materials which are having the ability to store latent heat are Glauber’s salt ( Na2SO4. 10H20), Water, Fe(No3)2 6 H20, and salt eutectics are mostly used. 131
  • Solar pond: Solar ponds is very shallow , 5cm to 2m deep with a absorbing black surface at the bottom. Bottom of the pond is well insulated against the loss of heat to the ground. Transparent fibre glass cover provided over the pond permits the solar radiation but reduces losses by radiation and convection. If pure water is used in the pond temperature rise is limited only to a few degrees due to loss of energy in natural convection currents. The temperature in the pond can be increased by using salt water with increasing salinity towards the bottom from top. In this method pure water is placed at the top of the pond which acts as the insulator against the loss of thermal energy . Salts like magnesium chloride , sodium chloride or sodium nitrate are dissolved in the water, the concentration varying from 20 to 30% at the bottom to almost zero at the top. In salt solar pond has three zones with following salinity with depth. I) Surface convection zone (0.3 – 0.5 m) , salinity <5% ii) Non convective zone 1 to 1.5 m , salinity increases with depth iii) Storage zone or lower convective zone 1.5 to 2 m, salinity  20% . Typical value of salt concentration at the top surface is 20 Kg/ m3, increasing to 300 and 260 Kg/m3 , for magnesium chloride and sodium chloride respectively at the bottom. The salt water next to the absorbing surface when heated its density decreases but this density still remains higher than that of the water above . This avoids the mixing of the bottom hot salt water with the top less salinity water .It helps in maintenance of the stability of the solar pond . Hence the top surface of the solar pond remains cooler compared to bottom and acts as the insulator against the loss of energy stored. In this method it is possible to achieve the temperatures as large as 93oC. Thus solar pond can be defined as the artificially constructed pond in which significant temperature rises are caused to occur in the lower regions by preventing convection. It is necessary to add periodically concentrated solutions at the bottom, and wash the surface with fresh water to maintain the concentration gradient in the presence of diffusion effects. Extraction of solar energy from solar ponds. 132
  • The solar energy from the solar pond is used to drive a Rankine cycle heat engine. Hot water from the bottom level of the pond is pumped to the evaporator where the organic fluid is vapourised . The vapour than flows under pressure to the turbine where it expands and work thus obtained runs an electric generator producing electricity. The exhaust vapour is then condensed in a condenser and the liquid is pumped back to the evaporator and the cycle is repeated. Applications of solar pond: 1) Heating and cooling of buildings 2) Production of power. 3) Industrial process heat. 4) Desalination . 5) Heating animal housing and drying crops on farms. 6) Heat fro biomass conversion. Photovoltaic conversion: Photovolatalic energy conversion is a direct conversion technology that produces electricity directly from sunlight without the use of a working fluid such as steam or gas and a mechanical cycles such as Rankine or Brayton. The basic unit of a photovoltaic system is the solar cell. Solar cells are usually made up of silicon . A combination of solar cells designed to increase the electric power out put is called a solar module or solar array. Solar cells or photovoltaic cells generate electricity when they absorb light by means of the photovoltaic effect ,that is conversion of light in to electricity. As photons are received , free electrical charges are generated that can be collected on contacts applied to the surfaces of the semiconductors. The theoretical efficiencies are in the order of 25%. But actual operating efficiencies are less than half of this value. Photovoltaic cells could be used in either small or large power plants . The cost of energy storage and power conditioning equipments result in power generation by large power stations uneconomical . Solar cells can be used to operate irrigation pumps, navigational signals, highway call systems, rail road crossing warnings etc. A photovoltaic system consists of i) Solar cell array ii) Load leveler iii) Storage system iv) Tracking system. All solar cells are interconnected in certain series/ parallel combinations to form modules. These modules are sealed to protect against the corrosion, moisture, pollution and weathering. Solar PV 133
  • system can produce an output only if sunlight is present. If it is required to be used during non sunshine hours, a suitable systems of storage batteries will be required Principle of operation of solar cell( Photovoltaic effect) Solar cell consists of combination of p-type and n-type semiconductors. N- type semiconductor has excess number of electrons , where as p-type semiconductor has deficiency of electrons or holes. Such a piece of semiconductor with one side of the p-type and the other of the n-type is called a p-n junction. In this junction after the photons are absorbed the free electrons of the n-side will tend to flow to the p-side , and the holes of p-side will tend to flow to the n region to compensate for their respective deficiencies. This diffusion will create an electric field flowing from the n region to the p- region . If electrical contacts are made with the two semiconductors materials and the contacts are connected through an external electrical conductor , free electrons will flow from n-type material through conductor to the p-type material.. The flow of electrons through the external conductor constitutes an electric current which will continue as long as more free electrons and holes are being formed by solar radiation WIND ENERGY Introduction : Wind energy is rightfully an indirect form of solar energy since wind is induced chiefly by the uneven heating of the earth’s crust by the sun. Winds can classified as planetary and local. Planetary winds are caused by greater heating of the earth’s surface near the equator than near the northern or southern poles. Local winds are caused by two mechanisms first is the differential heating of the land and water. During the day land mass becomes hotter than water , air near to the surface of the land heats up and rises ,the cooler heavier air above the water moves in to replace it. This is the mechanism of shore breeze. T the night direction of the breeze reversed . The second mechanism of local wind is caused by the hills and mountains. The air above the slopes heats up during the day and cools down at night, more rapidly than the air above the low lands. This causes heated air during the day to rise along the slopes and relatively cool heavy air to flow down at night. 134
  • It has been estimated that about 2 percent of all solar radiation falling on the face of the earth is converted to kinetic energy in the atmosphere and that 30 percent of this kinetic energy occurs in the lowest 1000m of elevation.. It is thus said that total kinetic energy of the wind in this lowest kilometer, if harnessed , can satisfy several times the energy demand of a country. It is also claimed that wind power is pollution free and that its source of energy is free. Solar energy is cyclic and predictable , and even dependable in some parts of the globe, wind energy . however, is erratic, unsteady, and often not reliable , except in very few areas. Properties of wind: 1) Wind power is pollution free. 2) Fuel provisions and transport are not required in wind power systems. 3) Wind energy is a renewable source of energy. 4) Wind energy when produced on small scale is cheaper, but competitive with conventional power generating systems when produced on large scale. 5) Wind energy is highly erratic in nature. 6) Wind energy is unsteady. 7) Due to its irregularity it needs storage devices. 8) Wind power generating sy stems produce ample noise. 9) Wind speeds increases with height. 10) Average wind speeds are greater in hilly and coastal areas than they are well inland. 11) Velocity of wind over the water remains almost constant. Availability of wind in energy in India Data quoted by some scientist that for India wind speed value lies between 5 Km/hr to 15-20 Km/hr. These low seasonal winds imply high cost of exploitation of wind energy. India has potential of over 20,000 MW for power generation and rank one of the promising courtiers for tapping this source Wind power projects aggregate capacity of 8MW including 7 wind farms projects of capacity 6.85 MW have been established in different parts of the country of which 3MW capacity has been completed in 1989 by DNES. Wind farms are operating successfully and have already fed 150 lakh units of electricity to the respective grids. Over 25 MW additional power capacity from wind is under implementation. Under demonstration program 271 wind pumps have been installed up to 1989. Sixty small wind battery chargers of capacity 300 watt to 4KW are under installation. Likewise to stand alone wind electric generators of 10 to 25 KW are under installation. Wind velocity and power from the wind Wind posses energy by virtue of its motion. A device which is capable of absorbing this energy and converting in to useful work is known as the wind converter or wind turbine. The power out put from the wind energy converter is dependent on the i) The wind speed ii) The cross section wind swept by the rotor iii) The overall conversion efficiency of the rotor., transmission system and generator or pump. Wind mill works on the principle of converting kinetic energy of the wind in to mechanical energy. The total power of a wind stream is equal to the rate of incoming kinetic energy of that stream KE or Ptot = m KEi = m Vi2 / 2gc (1) Where Ptot = Total power , W M = Mass flow rate Vi = Incoming wind velocity , m/s gc = Conversion factor = 1.0 kg / (N.s2) The mass flow rate is given by the continuity equation m = AVi 135
  • Where  = Density of incoming wind , A = Cross sectional area of stream , m2 Thus Ptot = AVi3 / 2gc (2) Thus the total power of a wind stream is directly proportional to its density, area, and the cube of its velocity . Generally the swept area is circular of diameter D in horizontal axis aero turbines, then A =D /42 Using this in equation 2 Available total wind power Ptot =  D2 Vi3 / 8gc (3) Thus doubling the diameter of the rotor will result in a four fold increase in the available wind power. The combined effect of wind speed and rotor diameter variations ar shown in fig. 6 60m 4 Tot al power in MW R or diam er ot et 40 m 2 20 m 0 5 10 15 W speed in m/s ind Wind machines intended for generating substantial amounts of power should have large rotors and be located in areas of high wind speed. In wind turbines only fraction of the power available in the wind can be converted in to useful power. As the wind passes through the rotor , rotor absorbs fraction of the kinetic energy available with wind and its speed decreases to a minimum in the rotor wake. Subsequently wind regains its speed and energy at a sufficient distance from the rotor . While the speed is decreasing , air pressure in the wind stream changes in different manner It first increases as the wind approaches the rotor and then drops sharply by an amount p as it passes through and energy is transferred to the rotor. Finally pressure increases to ambient pressure . 136
  • Turbine wheel Pa Pressure Pi Pe Pb i a b e Vi Va elocity Vt Vt Vb V Ve i Distance e Maximum power: Consider a horizontal axis , propeller type wind mill or turbine has thickness a-b , incoming wind pressure and velocity , far upstream of the turbine , are Pi and Vi and that exit wind pressure and velocity , far down stream of the turbine , are Pe and Ve, respectively. Ve is less than Vi because kinetic energy is extracted by the turbine. Assumptions : 1) Incoming air between I and a as a thermodynamic system. 2) Density remains constant . 3) Change in potential energy is zero. 4) No heat or work added or removed between i and a The general energy equation reduces to kinetic and flow energy terms only. Thus energy equation between i and a Piv + Vi2 / 2gc = Pav + Va2 / 2gc (1) Or Pi + Vi / 2gc = Pav + Va / 2gc 2 2 (2) Where v and  are the specific volume and its reciprocal, the density, respectively both considered constant. Similarly for the exit region b-e Pb + Vb2 / 2gc = Pev + Vb2 / 2gc (3) As the pressure of wind is high at the entry and low at exit , the equations 2 and 3 can be written as 137
  • Pa – Pb = [Pi + (Vi2 - Va2 ) / 2gc] - [Pe + (Ve2 – Vb2 ) / 2gc] (4) It is reasonable to assume that, far from the turbine at e, the wind pressure returns to ambient or Pe= Pi Velocity within the turbine , Vt , does not chandge because the blade width a-b is thin compared with the total distance considered, so that equation 4 reduces to Pa – Pb = (Vi2 – Ve2 ) / 2gc (5) The axial force Fx , in the direction of the wind stream , on a turbine wheel with projected area , perpendicular to the stream A is given by Fx = (Pa – Pb)A = A (Vi2 – Ve2 ) / 2gc (6) This force equal to the momentum of the wind ( From Newton’s second law) Fx = (mV) / gc Where m is the mass flow rate is given by m = AVt (7) Thus Fx = AVt ( Vi-Ve) / gc (8) Equating equations 6 and 8 Vt = ( Vi +Ve) / 2gc (9) Assuming no changes in potential , internal energies and system between i and e as adiabatic , the general energy equation reduces to steady flow work W = Kei –Ke = (Vi2 – Ve2 ) / 2gc (10) The power P is the rate of work P = m (Vi2 – Ve2 ) / 2gc = AVt (Vi2 – Ve2 ) / 2gc (11) Using equation 10 P = A( Vi +Ve) (Vi2 – Ve2 ) / 4gc (12) The optimum value of Ve can be calculated by differentiating equation 12 with respect to Ve for a given value of Vi and equating to zero. i.e., dp/dVe = 0 which gives 3Ve2 + 2ViVe – Vi2 = 0 This is solved for a positive Ve to give Ve.opt ( The quadratic equation gives two solution i.e., Ve = Vi and Ve = 1/3 Vi , only second solution is physically acceptable ). Ve.opt = Vi /3 (13) Combining equation 12 gives Pmax = A( Vi +Vi /3 ) [Vi2 – (1/3 Vi )2 ] / 4gc = [A (4Vi/ 3) x ( 8 Vi2 / 9 )] / 4gc = (A Vi3 8)/ 27gc Fraction of the wind power that can be extracted by the rotor is called the power coefficient thus max = Power to the wind rotor / power available in the wind = 16/27gc [ AVi3 / 2gc ] = 16/27 Ptot = 0.5926 (14) Forces on the blades and thrust on turbines: Two different types of forces are acting on the blades they are circumferential forces in the direction of wheel rotation that provide the troque and the axial forces in the direction of the wind stream that provide an axial thrust that must be counteracted by proper mechanical design. Circumferential force or torque T is obtained from T = P/  = P / (DN) Where T – Torque , N  = Angular velocity of the turbine wheel m/s D = Diameter of the turbine wheel = (4/)1/2 A , m N = Wheel speed in rpm The real efficiency  = P / Ptot = AVi3 / 2gc Or P = AVi3 / 2gc (1) For a turbine operating at power P, the torque T is given by T = AVi3 / (2gc DN ) 138
  • = (  D2 /4)Vi3 / (2gc DN ) =  DVi3 / (8gc N ) (2) At maximum efficiency (max = 16/27) , the torque has maximum value Tmax which is equal to Tmax = 2  DVi3 / (27gc N ) (3) The axial force or axial thrust given by Fx =A (Vi2 – Ve2 ) / 2gc =  D2 (Vi2 – Ve2 ) / (8 gc) (4) The axial force on a turbine wheel operating at maximum efficiency where Ve = Vi/3 Fxmax = 4AVi2/ (9gc) =  D2 Vi2/ (9gc) (5) From the above equation it is clear that axial forces are proportional to the square of the diameter of the turbine wheel, this limits turbine wheel diameter of large size. Problems associated with the wind power: 1) Location of the site The selected site must be gig enough with reasonable average high wind velocity. 2) Variation in the wind velocity : Wind velocity varies with the time and varies in direction and also varies from the bottom to top of a large rotor . This causes fatigue in blades. 3) Need of storage system: At zero wind velocity conditions, the power generated will be zero and this means some storage system will have to be incorporated along with the wind mill. 4) Strong supporting structures: Since the wind mill generator will have to be located at height, the supporting structure will have to be designed with stand high wind velocity and impacts . This will add to the initial costs of the wind mill. 5) Occupation of large areas of the land: Large areas of land will become unavailable due to wind mill gardens. The whole area will have to be protected to avoid accidents. 6) Nature of ground: Ground surface should be stable . Erosion problem should not be there, as it could possible later wash out the foundations of WECS, destroying the whole system. 7) Wind structure at the proposed site: For better performance of the wind turbine velocity (Vt) curve must flat,i.e., a smooth steady wind that blows all the time is necessary. But a typical site is always less than ideal Wind specially near the ground is turbulent and gusty, and changes rapidly in direction and in velocity. 8) Availability of the wind curve :It determines the maximum energy in the wind and hence the principal initially controlling factor in predicting the electrical output and hence revenue return from the WECS machine. If there are long periods of calm the WECS reliability will be lower than if the calm periods are short.. In making such reliability estimates it is desirable to have measured Vt curve over about 5 year period for the highest confidence level in the reliability estimates. 9) Availability of high average wind speed: Wind velocity is the critical parameter. The power in the wind Ptot, through cross sectional area for a uniform wind velocity V, is P = K Vi 3 . It is evident ,because of the cubic dependence on wind velocity that small increases in Vi remarkably affects the power in the wind. 10) Other problems: Other problems like icing, salt spray or blowing dust should not present at the site , as they may affect aero turbine blades . Type of the wind machine and their characteristics: All the win machines are classified as I) Based on the axis of rotor rotation 1) Horizontal axis wind machine : Axis of rotation is horizontal , aero turbine plane is vertical facing the wind 2) Vertical axis wind machines : The axis of rotation is vertical Ex Darrieus wind mill II) According to size : 139
  • 1) Small scale (up to 2KW ) : These might be used on farms remote applications, and other places requiring relatively low power. 2) Medium size machines: ( 2-100 KW) These turbines are used for supplying electricity to several residence and local use. 3) Large scale(100 KW and above) They are used to generate power for distribution in central power grids III) According to out put power 1) DC output : Dc generator, alternator rectifier 2) AC out put : i) Variable frequency ii) Constant frequency IV) According to rotational speed: 1) Constant sped with variable pitch blades 2) Nearly constant speed with fixed pitch blades 3) Variable speed with fixed pitch blades. V) According to utilization 1) Battery storage. 2) Direct connection 3) Other forms of storage 4) Interconnection with conventional electric utility grids. Horizontal axis machines: 1) Horizontal axis machine with two aerodynamic blades: This machine schematically shown in the figure. In this type rotor drives a generator through a step up gear box. The components are mounted on a bed plate which is attached on a pintle at the top of the tower. When machine is in operation blades are subjected to aerodynamic , gravitational and inertial loads. If the blades are made up of metal , flexing reduces fatigue life which may cause serious damage to both blades and tower. If the vibrational frequency of the rotor coincides with the natural frequency of the tower , the system may shake itself in to pieces. On economical point of view more than two blades are not recommended. Rotors which are having more than two would have slightly higher power coefficients. 2) Horizontal axis propeller type with single blade: 140
  • In this arrangement a long blade is mounted on a rigid hub. Induction generator and gear box are also shown. Extremely long blades create various problems like gravity and sudden shifts in wind directions. To reduce rotor cost , use of low cost counter weight is used which balance long blade centrifugally. Advantages : 1) Simple blade controls, lower blade weight and cost, lower gear box cost. 2) Counter weight costs less than a second blade. 3) Counter weight can be inclined to reduce blade coning. 4) Pitch gearing do not carry centrifugal force. Disadvantages: 1) Vibration level is too high. 2) Unconventional appearance. 3) Large blade root bending moment. 4) Staring torque is reduced. 5) One per rev coriolis torque produced due to flapping. 3) Horizontal axis multi bladed type 141
  • Multi bladed horizontal axis wind turbine is shown in the fig . Blades are made from sheet metal or aluminum. The rotors have high strength to weight ratios and have been known to service hours of freewheeling operation in 60 Km/hr winds. They have good power coefficients, high starting torque and added advantage of simplicity and low cost. 4) Horizontal axis Dutch type: Dutch type wind machine is shown in the figure. This is one of the oldest designs . The blade surfaces are made from an array of wooden slats which rotates at high speeds. 5) Sail type : It is of recent origin. The blade surface is made from cloth, nylon or plastics arranged as mast and pole or sail wings. There is also variation in the number of sail used. Vertical axis Wind machines 142
  • One of the main advantage of the vertical axis type wind machine is that they do not have to be turned in to the wind stream as the wind direction changes. Because their operation is independent of wind direction, vertical axis machines are called panemones. Most of the vertical axis machines are drag devices. Such devices have relatively high starting torque compared to lift devices , but have low, tip to wind speeds and lower power outputs per given rotor size, weight and cost. Vertical axis machines are difficult to control in strong winds The transmission and generator are on the ground rather than at the top of a tall tower. Advantages: 1) They will react to the wind in any direction and therefore they do not need yawing equipment to turn the rotor in to the wind. 2) They require less structural support.. 3) Rotors are not subjected to cyclic gravity loads. 4) Installation and maintenance is simple. Two different types of vertical axis rotors are common ,Savonius and Darrieus Savonius Rotor: It consists of two half cylinders facing opposite directions such a way as to have almost an S – shaped cross section. These two semi-circular drums are mounted on a vertical axis perpendicular to the wind direction with a gap at the axis between the two drums. Irrespective of the wind direction the rotor rotates such as to make the convex sides of the buckets head in to the wind. Form the rotor shaft we can take the power for use like water pumping , battery charging etc. The force of the wind is greater on the cupped face than the rounded face. The wind curving around the back side of the cupped face exerts a reduced pressure much as the wind does over the top of an air foil and this helps to drive the rotor. Characteristics of Savonius Rotor 1) Self starting 2) Low speed 3) Low efficiency 143
  • Advantages: 1) It eliminates the expensive power transmission system from the rotor to axis. 2) It has its low cut in speed. 3) Cost of the vertical axis wind turbine is lower than that of standard wind turbines. 4) It has simple structure , hence easy to manufacture . 5) Overall weight of the turbine may be substantially less than that of conventional system. 6) Yaw and pitch controls are not needed to bring it into the wind or operate in high winds. Disadvantages: 1) This type of machine is too solid it leads to excessive weight. 2) It is useful for a very tall installation . Darrieus Type machine : It has two or three , thin curved blades with airfoil cross section and constant chord length. Both ends of the blades are attached to a vertical shaft. Thus the force on the blade due to rotation is pure tension This provide stiffness to help withstand the forces it experiences. The blades can thus be made lighter than in the propeller type. When rotating these airfoil blades provide a torque about the central shaft in response to a wind stream. This shaft torque being transmitted to a generator at the base of the central shaft for power generation. Darrieus type rotors are lift devices, characterized by curved blades with airfoil cross section. They have low solidity, but high tip to wind speeds and , therefore relatively high power outputs per given rotor weight and cost. Characteristics: 1) No self starting 2) High speed 3) High efficiency 4) Potentially low capital cost. Advantages: 1) The rotor blades can accept wind from any direction. 2) It eliminates tower structure and can be operated close to the ground level. 3) It eliminates the yaw control requirement for its rotor to capture wind energy. 4) Pitch control is not required this reduces the cost. 5) The tip speed ratio and power coefficients are considerably better than those of the S – rotor . Disadvantage: 144
  • 1) It requires external mechanical aids for start up . 2) Wind energy conversion sy stem is some what lower than that of conventional horizontal rotor. 3) The less energy out put. 4) Vibratory stresses level encountered are high. 5) Special high torque breaking system is needed Design principles of Horizontal axis wind turbines Some of the main design considerations of the horizontal axis wind turbines are outlined below. Rotor: A wind turbine rotor may have any number of blades which may be made from wood, metal or composites of several materials. From a performance point of view , taller the tower better because wind speeds increase with height. Horizontal axis rotors can be either lift or drag devices. Lift devices are generally preferred because they develop more power than the drag devices. Lift devices use slender blades with an aerofoil section that generate aerodynamic lift when placed in an air current. Small rotors can rote at higher speeds with blade tip speed 8-10 times that of wind speeds. Drag devices are less efficient wind energy converters and always turn more slowly than the wind. Drag devices are capable of generating higher torques. They are less suitable for power generation. For lift type devices solidity ratio is usually kept lower (0.1 to 0.01). Solidity ratio is the ratio of projected area of the rotor to swept area of the rotor. Lift type rotors often use tapered and/ or twisted blades to reduce the bending strains on the roots of the blades. Maximum efficiency can be achieved by maintaining high lift and drag ratio. The ratio of the speed of the rotor blade tips to the speed of the wind is called the tip – speed ratio. . Maximum efficiency can be achieved when ever the tip-speed ratio is at optimum level. The tip speed ratio calculated numerically as TSR = Vtip / Vi Where Vtip = Speed of the rotor tip, Vi = Free wind speed. Rotor with high TSR needs less material and can have relatively slender blades. Rotor with low TSR needs more number of blades. As the TSR value increases the number of blades required decreases. Numerically solidity can be expressed as S= NC / D (1) Where N is the number of blades C is the average breadth of a blade D is the diameter of the circle described by a blade. If the characteristics of both load and rotor torque speed are known, the system performance can be defined. Torque coefficient T = T / Tmax (2) Where T = Shaft torque , Tmax is the torque at the maximum efficiency For a propeller turbine of radius R Tmax = F max R (3) And Fmax = Ai Vi / 2 2 (4) So Tmax = Ai Vi R/ 2 2 (5) For a working machine the torque T = T Tmax We know tip speed ration  = Vt / Vi = R / Vi (6) Using equation 4 and 5 Tmax = Ai Vi2  Vi / 2 = Ptot  /  (7) Shaft power derived from turbine P so P = T (8) = T Tmax Now from equation P = Ptot  thus using 2 , 3 and 8 becomes Ptot  = T Tmax = T Ptot  145
  •  = T  Note that in practice power coefficient  and torque coefficients T will both be function of  and are not constants . By the Betz criterion the maximum value of  is 0.593, so in the ideal case max = 0.593/  (9) Machine with higher speeds have slightly higher maximum  but much lower T , particularly for starting .The choice of the rotor is made mainly on the pump’s load characteristics. Number of blades: Wind turbine have been built with up to six propeller type blades but two and three bladed propellers are most common . One bladed rotor with counterweight has advantages , including lower weight and cost and simpler controls, over multi blade type. Two bladed systems are receiving major attention. Blade design: Wind turbine blades have an airfoil type cross section and a variable pitch. They are slightly twisted from the outer tip to the root . Better performance can be obtained with blades that are narrower at the tip than at the root. As shown in the fig the force that propels the blades of conventional wind mill comes from the chord of the airfoil, being tilted away from the direction of motion. In large two bladed wind turbines , the blades are inclined at a small angle called the coming angles to the vertical This design decreases the bending load and helps in avoiding damage to supporting tower under severe wind conditions. The design of the blade must be capable withstanding several forces like , Vibrational, gravitational , forces arising from variation in wind speeds , turbulence , pressure etc. Consequently aerodynamic performance is sacrificed to sum extent in the design of a rotor with adequate strength. The limiting dimensions of the blade depend on the design and constructional materials , but the maximum practical diameter of a two rotor may perhaps be in the range of 90 to 110 m . For small rotors blades are made of laminated wood, covered with skin of aluminum . It possible to construct the blades up to 34 m in diameter using plastic reinforced with glass fibre. The very largest rotor blades have been made of steel to provide adequate strength. Yaw control: The area of the wind streams swept by the wind turbine is maximum,, when blades face in to the wind. This achieved by control arrangement , is which when the wind direction changes , motor rotates the turbine slowly about the vertical axis so as to facethe blades in to the wind. 146
  • Aerodynamic consideration in the design of wind mill ENERGY FROM THE OCEANS TIDAL POWER Introduction: Wind generates large ocean waves with energies that can be used to generate power. Ocean wave energy is said to be solar energy twice removed. Ocean waves vary widely with time and place on amplitude and frequency, and hence in their energies, much like the wind that causes them. Tides are primarily cause by lunar , and only secondarily by solar , gravitational forces acting together with those of the earth on the ocean waters to create tidal flows. These manifest themselves in the rise and fall of waters with ranges that vary daily and seasonally and come at different times from day to day. They also vary widely from place to place , being as low as few centimeters but may exceed 8 to 10 m in some parts of the world. The potential energy of the tides can be trapped to generate power , but at extremely high capital costs. Mechanics of tides: 147
  • Tides are produced mainly by the gravitational attraction of the sun and the moon on the water of ocean. Major part of the tides about 70% are produced due to the moon and 30% to the sun. As the earth rotates , the position of a given area to the moon changes, and so also do the tides. There are thus a periodic succession of high and low tides. A high tide will be experienced at a point which is directly under the moon. At the same time diametrically opposite point on the earth’s surface also experiences a high tide due to dynamic balancing . Thus a full moon as well as no moon produce a high tide. In a period of 24 hrs 50 minutes, there are therefore , two high tides and two low tides; These are called semi – diurnal tides. The rise and fall of the water level follows sinusoidal curve, shown with point A indicating the high tide point and point B indicating the low tide point . The difference between high and low water level is called the range of the tide. At full moon , when sun, moon and earth are approximately in a line, the tidal range is exceptionally large, the high tides are higher and low tides are lower than average. These high tides are called spring tides, on the other hand , near the first and third quarters of the moon, when sun and moon are at right angles with respect to the earth, neap tides occur. The tidal range is then exceptionally small: the high tides are lower and low tides are higher than the average. Hence range is not constant. 148
  • Fig The tidal ranges vary from one earth location to another. Ocean waves mechanics: Ocean waves are caused directly by the indirect solar energy like the wind. Wave energy at its most active, however, can be much more concentrated than the solar energy. Devices that convert energy from waves can therefore produce much higher power densities that solar devices. Up to now no major development programme has been carried out through any country .Small devices are available however , and are in limited use as power supplies for buoys and navigational aids. Some of the important stes of wave energy are Molakai and Alenihaha channels in the Hawalian islands, where 2 to 3 m high waves are typical during the normal trade wind periods, Pacific coast of north America, the Arabian sea of India and Pakistan , the north Atlantic coast of Scotland . Advantages: 1) The degree of power concentration is 10 to 100 times larger than wind energy. 2) It is free and renewable energy source. 3) Wave energy devices do not use up large;and masses unlike solar or wind. 4) These devices are relatively pollution free. Disadvantages: 1) The construction cost is more , life time and reliability is less . 2) Wave energy converting devices must be capable of with standing severe peak stresses in storms. 3) Wave energy devices construction is relatively complicated. 4) Capital investment need is very large compared to other types of plant. Energy and power from the waves: Fig 149
  • A two dimensional sinusoidal progressive wave as shown in fig . is represented by the sinusoidal simple harmonic wave shown at time t = 0 and at time t. The wave may be expressed by the following relation involving some parameters y = a sin [ 2x /  - 2t /  ] Where y = height above its mean level in m, a= Amplitude in m ,  = Wave length in m, t = time in seconds ,  = Periods in seconds 2 = [ x / - t /  ] = phase angle The relationship between wave length and periods is approximately  = 1.56 2 (1) The above expression can be written as y = a sin(mx – nt) (2) Where m = 2 /  and n = 2 /  = phase rate. 2a = height (From Crest to trough) Energy and power from waves: Total energy of wave is the sum of its potential and kinetic energies. Potential energy: The potential energy arises from the elevation of the water above the (y = 0) considering the differential volume ydx, it will have a mean height y/2. Thus the potential energy is dPE = mgy/2gc = (ydxL) gy/(2gc) = gy2L dx / (2gc) (3) Where m = mass of the liquid in y dx, Kg g = Gravitational acceleration , m/s2 gc = Conversion factor (1.0 kg.m / ( N.s2)  = Water density k/m3 L = Arbitrary width of the two dimensional wave , perpendicular to the direction or wave propagation x, m Combining equations 2 and 3 we get  PE = (ga2L) / (2gc)  sin2(mx – nt)dx 0  2 PE = (ga L) / (2gc) [1/2 (mx ) – (1/4) sin2mx ] 0 = (ga2L) / (4gc) (4) The potential energy ensity per unit area A = L PE / A = (ga2) / (4gc) (5) Kinetic energy: Kinetic energy of the wave is that of the liquid between two vertical planes perpendicular to the direction of wave propagation x and placed one wave length apart. From hydrodynamic theory it can be expressed as KE = (ga2L) / (4gc) (6) 2 Kinetic energy density = KE/A = (ga ) / (4gc) (7) Total energy and power density can be written as E/A = (ga2) / (2gc) 150
  • P/A = (ga2) / (2gc) x f (8) Where P is the energy per unit time , f is the frequency . Wave energy conversion devices: Several types of wave energy conversion devices are used some of the important devices are 1) Wave energy conversion by floats: The wave motion is basically horizontal but the motion of water is primarily vertical. This latter motion is made use of by floats to obtain mechanical power. A square float moves up and down with the water, guided by four vertical manifolds that are part of the platform. The platform is stabilized within the water by four large under water floatation tanks so that it is supported by buoyancy forces and no significant .vertical or horizontal displacement of the platform due to wave action occurs. Attached to the float there is a piston that moves up and down inside a cylinder that is attached to the platform and is therefore relatively stationary .The piston cylinder arrangement is used as a reciprocating air compressor. The downward movement of the piston draws air in to the cylinder via an inlet check valve. The upward motion compresses the air and sends it through an outlet check valve to the four under water floatation tanks via four manifolds. The floatation tanks serve dual purpose of buoyancy and air storage . The compressed air in the buoyancy storage tanks is in turn used to drive an air turbine that drives an electrical generator . The electric current is transmitted to the shore via underwater cable. 2) High level reservoir wave machine 151
  • In these machines, instead of compressing air the water itself is pressurized and stored in a high pressure accumulator or pumped to a high level reservoir, from which it flows through a water turbine - electrical generator . This is done by transforming large volumes oflow pressure water wave crest into small volumes of high pressure water by the use of a composite piston. This piston composed of a large diameter main piston and a small diameter piston at its center. On the trough of the wave , the composite piston is pushed downward by the gas pressure above the main piston, Which thus acts also as a spring. When there is peak of the wave piston pressurizes the water and is elevated to a natural reservoir above the wave generator, which would have to be near a shoreline, or to an artificial water reservoir. The water in the reservoir is made to flow though a turbine back to sea level . Calculation shows that a 20 m diameter generator of this type can produce 1 MW. 3) Dolphin type wave power machine 152
  • The major components of this system are a dolphin, a float , a connecting rod , and two electrical generators. The float has two motions . the first is rolling motions about its own fulcrum with connecting rod. The other is relatively vertical or heaving motion about the connecting rod fulcrum. It causes relative revolving movements between the connecting rod and the stationary dolphin. In both cases, the movements are amplified and converted by gears in to continuous rotary motions that drive the two electrical generators. The system is envisaged to be used for electric power generation, pumping for desalination plant, or for uranium extraction from sea. Because it completely eliminates waves, it can provide suitable sites for fish farming , port facilities etc. 4) Dam – Atoll device Fig It is a massive and robust device that appears to overcome some of the disadvantages of many other devices, namely, complexity and fragility in heavy seas. It is said to be strong enough to survive any ocean storm. The principle of operation is based on the observed action of waves as they approach atolls (small volcanic islands) in an ocean. The waves wraps them selves around the atolls from all the sides , ending in a spiral in the center, driving a turbine before discharging laterally outward. A module , 80m in diameter and 20 meter in high, is said to be capable of generating 1 to 1.5 MW in 7 to 10 – s period waves. Harnessing tidal energy: The power generation from tides involves flow between an artificially developed basin and the basic scheme can be elaborated by having two or more basins. Accordingly we can have two different types of arrangements . 1) Single basin arrangement 2) Double basin arrangement. 1) Single basin or pool system The simple – pool tidal system has one pool or basin behind a dam that is filled from the ocean at high tide and emptied to it at low to tides. Both filling and emptying processes take place during short periods of time: the filling when ocean is at high tide while the water in the pool is at low tide level, the emptying when the ocean is at low tide and the pool at high tide level. The flow of water in both directions is used to drive a number of reversible water turbines, each driving an electrical generator. Electric power would thus be generated during two short periods during each tidal period, of 12 h, 25 min or once every 6h, 12.5 min. The generation of power in a single basin system can be carried out either as a) Single ebb – cycle system or b) Single tide cycle system or c) Double cycle system. 153
  • a) Single ebb – cycle system. When high tide comes , the sluice gates are opened to permit the sea water to enter the basin or reservoir, while the turbines sets are shut. The reservoir thus starts filling while its level rises, till the maximum tide level is reached. At the beginning of the ebb tide the sluice gates are closed. Then the generation of power takes place when the sea is ebbing (Flowing back of tide) and the water from the basin flows through the turbine in to the lower level sea. The generation of power can be continued till there is sufficient head difference between the level of water in the reservoir and the sea. The turbines are closed when the level of water becomes same on both the sides; sluice gates are opened to repeat the cycle. b) Single tide cycle system: In a single tide cycle system , the generation of power is carried out when sea at flood tide. The water of the sea is admitted in to the basin through the turbines. As the flood tide period is over and the sea level starts falling again, the generation is stopped. The basin is drained in to the sea through the sluice ways. This system needs large size plant, operating for short period and hence less efficient as compared to ebb tide operation. c) Double cycle system In this system power generation is carried out during both high tide as well as ebb tides. The flow of water in both the directions is used to drive a number of reversible water turbines, each driving an electrical generator. Electric power would thus be generated during two short period during each tidal period of 12 h, 25 min or once every 6h, 12.5 min Fig 154
  • Though the double cycle system has only short duration interruptions in the turbine operation, yet a continuous generation of power is still not possible. Further the periods of power generation coincide occasionally with periods of peak demand. 2) Double basin arrangement: Two basin system is one that is much less dependent on tidal fluctuation but at the expense of more complex and hence more costly dam construction. A inland basin is enclosed by dam A and divides into a high pool and a low pool by dam B. By proper gating in the dam A, the high pool gets periodically filled at high tide from the ocean and the low pool gets periodically emptied at low tide. Water flows from the high to the low pool through the turbines that are situated in the dam B. The power generation thus continue simultaneously with the filling up the high pool The capacities of these two pools are large enough in relation to the water flow between them that the fluctuations in the head are minimized, which results in continuous and much more uniform power generation. At the end of the flood tide when high pool is full and the water level in it is maximum, its sluice gates are closed. When ebb tide level gets lower than the water level in lo pool , its sluice gates are opened whereby the water level in low pool, which was rising and reducing the operating head, starts falling with the ebb. This continues until the head and water level in high pool is sufficient to run the turbines. With the next flood tide cycle repeats. With this twin pool system , a longer and more continuous period of generation per day is possible. Estimation of power in the single basin system: For tidal rang R , and an intermediate head h at a given time during the emptying process, the differential work done by the water is equal to the potential energy at the tine or 155
  • A A rea High t ide level dh Range R Basin h Ocean at low tide Reversible turbine and gates dW = dm.h g/gc (1) But dm = -.A. dh (2) So that dW = - .A. h dh g/gc (3) Where W = work done by the water g = Acceleration due to gravity m= Mass of water flowing through the turbine Kg h = head in m  = Water density , A = Basin surface area , considered The total theoretical work during a full emptying period is obtained by integrating equation 3 0 0 W =  dW = - .A. g/gc  h dh = .A. g R2 /2gc (4) R R Thus the work is proportional to the square of the tidal range. The average theoretical power delivered by the water is W divided by the total time it takes each period to repeat itself or 6 h, 12.5 min or 22,350 s thus Pav = .A. g R2 /(44700gc ) . Assuming average sea water density = 1025 kg/ m3 , the average power per unit basin area is given by Pav /A = 9.80 x 1025 R2 / 44700 = 0.225 R2 The actual power generated by the real tidal system is less than the average theoretical power. The actual power generated may be about 25 to 30 percent of the theoretical power. Estimation of power in double cycle system Let V be the volume of the basin V = A ho (1) Where A is the average cross sectional area of the basin in M2 , and ho is the difference between maximum and minimum water levels.  Average discharge Q = A ho / t (2) t is the total duration of generation in one filling / emptying operation. Now power generated at any instant P = Qh x o x 0.736 / 75 KW (3) H is the available head at the instant , then the total energy 156
  • t t =  P dt =  Qh x o x 0.736 / 75 Kw per tidal cycle (4) 0 0 Then yearly power generation t =  Qh x o x 0.736 x 705 / 75 KW h / year (5) 0 Advantages of tidal power: 1) Tidal power is inexhaustible in nature. 2) Tidal power generation is free from pollution. 3) The requirement of valuable land is less. 4) Peak power demand can be met if it effectively works in combination with hydroelectric or thermal system. 5) It can provide better recreational facilities to visitors and holiday makers , in addition to the possibility of fish farming in the tidal basins. Limitations: 1) Generating power is always dependent on the ti al range. d 2) The generating efficiency of the turbines affected by the variations in the operating head. 3) Power generation is intermittent in nature. 4) The selecting of suitable turbine operating under varying head condiion is difficult. t 5) Load sharing of power with the grid is very difficult due variation in power cycle. 6) Maintenance cost of the machinery is high due to the corrosive nature ofsea water. 7) Construction in sea is found difficult 8) Cost of power generation is not favourable compared to other sources of energy. 9) It may affect fishing and navigation. Ocean Thermal energy conversion (OTEC) Introduction: The concept of ocean temperature energy conversion (OTEC) is based on the utilization of the temperature difference in a heat engine to generate power. In tropics , the ocean surface temperature often exceeds 25oc , while 1 km below the temperature is usually no higher than 10oC.Water density decreases with increase in temperature . Thus there will be no thermal convection currents between warmer , lighter water at the top and deep cooler heavier water so warm water stays at the top and the cool water stays at the bottom. The maximum temperature difference on the earth is in the tropics and is about 15 oC The surface temperatures vary both with latitude and season, both being maximum in tropical, subtropical, and equatorial waters i.e., between the two tropics, making these waters the most suitable for OTEC systems. In OTEC systems the average temperature difference may be 200C compared to 5000C for modern fossil power plants. Taking the temperature difference of 200C and a surface temperature of 27oC, the Carnot cycle efficiency would be c = (T1 – T2) / T1 = 20 / (27 + 273) = 6.67% The extremely low efficiency of an OTEC system implies extremely large power plant heat exchangers and components. There are two basic designs of OTEC system: the open cycle , also known as the claude cycle, and the closed cycle, also known as the Anderson cycle . Open cycle or Claude cycle. 157
  • The Claude plant used an open cycle in which water itself plays the multiple role of heat source, working fluid, coolant, and heat sink. Schematic flow and corresponding T-S diagrams are shown in fig. In the cycle warm surface water admitted into an evaporator in which pressure is maintained at a value slightly below the saturation pressure corresponding to that water temperature. Water entering the evaporator; therefore, finds itself superheated at the new pressure. The warm water at 270c has saturation pressure of 0.0356 bar, point 1. The evaporator pressure is 0.0317 bar. This temporarily superheated water undergoes volume boiling causing that water to partially flash to steam to an equilibrium two phase condition at new pressure and temperature 0.0317 bar and 25oC, point 2. Process 1-2 is throttling hence constant enthalpy process. The low pressure in the evaporator can be maintained by using vacuum pump. The steam is separated from the water as the saturated vapor at the point 3. The remaining water is saturated at 4 and is discharged as brine back to ocean. The quality of the steam at 3 is low pressure high specific volume. It expands in a specially designed turbine, condenser pressure and temperature at 5 are 0.017 bar and 150C . The condenser used is direct contact type , in which the exhaust at 5 is mixed with cold water from the deep cold water pipe at 6, which results in a near saturated water at 7. That water is now discharged to ocean. 158
  • Disadvantages: 1) Volume flow rates of water required are high. 2) The special types of turbines are required. 3) The size of the turbines required is very large. 4) Use of degasifiers required to remove dissolved gases in the sea water. 5) The cost of the open cycle system is more compared to closed cycle system. 6) The cost of the turbine is about half of the overall cost of power plant. Closed or Anderson , OTEC cycle. The closed cycle utilizes the ocean’s warm surface and cold deep waters as heat source and sink, respectively, but requires a separate working fluid that receives and rejects heat to the source and sink via heat exchangers. The working fluid may be ammonia, propane or Freon. When high pressure liquid ammonia enters the evaporator absorbs heat from the water which is circulating and converted in to high pressure vapour. This vapour expanded in to low pressure vapour in the turbine. Low pressure ammonia vapour is condensed in to low pressure liquid ammonia in condenser. In order to remove the heat from vapour in the condenser cold water from depth of sea is used. Low pressure liquid ammonia is converted in to high pressure liquid ammonia using pump and supplied back in to the evaporator for repeating the cycle. The operating pressure is much higher compared open cycle thus smaller and hence less costly . But it requires very large heat exchangers. Instead of usual heavier and more expensive shell and tube heat exchangers, In Anderson cycle thin plate heat exchangers are used. Problems associated with OTEC 1) OTEC plants sites are always located away from the load centers . 2) The availability of suitable temperature differences between surface water and deep cold water is restricted to equatorial regions. 3) The power transmission cost from the OTEC plant to load center is very high. 4) The power generation system gives less efficiency. 5) Large heat exchangers are required and hence the cost of the power generated increases. 6) The bio fouling is a major problem encountered in most power plants. 159
  • 7) In the manufacture of heat exchangers costly , non corrosive materials must be used this further increases the overall cost of the plant. 8) The initial investment required is high. 9) Construction of the plant in the rough sea is very difficult. Geothermal energy. “Geo” means earth and “therm” means heat energy i.e. geothermal energy is heat energy from the earth. Geothermal energy is recoverable in some form such as steam or hot water. The earth crust now averages about 20 to 40 km in thickness. Below that crust, the molten mass called magma , is still in the process of cooling. Earth tremors caused the magma to come close to the earth’s surface in certain places and crust fissures to open up . The hot magma near the surface thus causes active volcanoes ,hot springs and geysers where water exists. It also causes the steam to vent through the fissures ( fumaroles) . A typical geothermal field is shown in the figure. The hot magma near the surface (A) solidifies into igneous rock (B). The heat of the magma is conducted upward to this igneous rock. The ground water that finds its way down to this rock through fissures in it will be heated by the heat of the rock or by mixing with hot gases and steam emanating from the magma . The heated water will then rise convectively upward and into a porous and permeable reservoir C above the igneous rock. This reservoir is capped by a layer of impermeable solid rock D that traps the hot water in the reservoir . The solid rock however has fissures E that acts as vent of the giant underground boiler . The vents show up at the surface as geysers, fumaroles F . or hot springs G. A well H traps steam from the fissures for use in a geothermal power plant. It can be seen that geothermal steam is of two kinds: that originating from the magma itself , called magma tic steam , and that from the ground water heated by the magma called meteoritic steam. The latter is the largest source of geothermal steam. There are three basic kinds geothermal sources a) Hydrothermal b) Geopressured and c) Petrothermal a) Hydrothermal sources: 160
  • Hydrothermal sources are those in which water is heated by contact with the hot rock. Hydrothermal systems are in turn subdivided into 1) Vapor dominated and 2) Liquid dominated. 1) Vapor dominated: In these systems the water is vaporized into steam that reaches the surface in a relatively dry condition at about 2050C and rarely above 8 bar. This system is the most suitable for use in turboelectric power plants, with least cost. It does, however, suffer problems similar to those encountered by all geothermal systems, namely , the presence of corrosive gases and erosive material and environmental problems . Vapor dominated systems, however , are a rarity . These systems account for about 5 percent of all geothermal sources. Vapor dominated power plant: Vapor dominated geothermal systems are the most developed of all geothermal systems. They have the lowest cost and the least number of problems. The vapor dominated power plant is as shown in the fig. Dry steam from the well (1) at 2000C is used . It is nearly saturated and may have a shut off pressure up to 35 bar. Pressure drops through the well causes it to slightly superheat at the well head 2. The pressure there rarely exceeds 7 bar . It then goes through a centrifugal separator to remove particulate matter and then enters the turbine after additional pressure drop 3. Processes 1-2 and 2-3 are essentially throttling process with constant enthalpy. The steam expands through the turbine and enters the condenser at 4. The condenser used is of direct contact type. Turbine exhaust steam at 4 mixes with cooling water (7) that comes from a cooling tower. The mixture of 7 and 4 is saturated water (5) that is pumped to the cooling tower (6) . The greater part of the cooled water at 7 is recircualted to the condenser. The balance, which would normally be returned to the cycle in a conventional plant, is rejected in to the ground either before or after the cooling tower. No make up water is necessary. 161
  • 2) Liquid dominated systems: In these systems the hot water circulating and trapped underground is at a temperature range 174 to 3150C. When tapped by wells drilled in the right places and to the right depths, the water flows either naturally to the surface or is pumped up to it . The drop in pressure usually to 8 bar or less , causes it to partially flash to a two phase mixture of low quality , liquid dominated. It contains relatively large concentrations of dissolved solids ranging between 3000 to 25000 ppm and sometimes higher. The power production is adversely affected by these solids due to formation of scaling, reducing flow and heat transfer . The liquid dominated systems , however are much more plentiful than vapor dominated systems . Liquid dominated power plants: The two different methods are used for generating power i) The flashed system ii) Binary cycle system. i) The flashed system: The schematic diagram of this system is as shown in the figure. The water from the underground reservoir at 1 reaches the well head at 2 at a lower pressure. Process 1-2 is essentially a constant enthalpy throttling process that results in two phase mixture of low quality at 2. This is further throttled in flash separator resulting in a still low but slightly higher quality at 3. This mixture is now separated in to dry saturated steam at 4 and saturated brine at 5. The latter is rejected in to the ground. The dry steam usually at pressure of less than 8 bar , is expanded in a turbine to 6 and mixed with cooling water in direct contact condenser with mixture at 7 is going to a cooling tower. The greater part of the cooled water at 7 is recircualted to the 162
  • condenser. Remaining portion of the mixture is rejected in the ground. In order to improve the efficiency in splashing two stage flashing is used instead of single stage flashing (double flash) ii) Binary cycle system The figure shows the schematic diagram of binary cycle system. Hot water or brine from the underground reservoir circulates through a heat exchanger and is pumped back to the ground. In the heat exchanger it 163
  • transfers its heat to the organic fluid thus converting it to superheated vapor that is used in a standard closed Rankine cycle . The vapor drives the turbine and is condensed in a surface condenser ; the condensate is pumped back to the heat exchanger . The condenser is cooled by the water from the natural source, if available, or a cooling tower circulation system. The blow down from the tower may be rejected to the ground with cooled brine. Makeup of the cooling tower water must be provided. In binary cycle there is no problems of corrosion or scaling . Such problems are confined to well casing and the heat exchanger . The heat exchanger is shell and tube unit so that no contact between brine and working fluid takes place. b) Geo pressured systems Geopressured systems are sources of water, or brine, that has been heated in a manner similar to hydrothermal water, except that geopressured water is trapped in much deeper underground acquifers, at depth between 2400 m to 9100 m . This water is relatively at low temperature(1600C) and under very high pressure of 1000 bar. It has relatively high salinity. In addition , it is saturated with natural gas , mostly methane CH4 . Such water is thought to have thermal and mechanical potential to generate electricity . Temperature , however is not high enough and the depth so great that there is little economic justification of drilling for this water for its thermal potential alone. How ever it is possible to generate electricity by recovering dissolved methane. Petrothermal systems: Magma lying close the earth’s surface heats overlying rock . When no ground water exists, there is simply hot, dry rock(HDR) . The known temperatures of HDR vary between 150 to 2900C . This energy is called petrothermal energy , represents by far the largest source of geothermal energy of any type. Much of the HDR occurs at relatively moderate depths, but it is largely impermeable. In order to extract thermal energy out of it , water will have to be pumped into it and back out to the surface. It is necessary for the heat transport mechanism that a way be found to render the impermeable rock into a permeable structure with a large heat transfer surface. Rendering the rock permeable is to be done by fracturing it. Fracturing methods that have been considered involve drilling wells into the rock and then fracturing by (1) high pressure water (2) Nuclear explosives. High pressure water method: Fracturing by high pressure water is done by injecting water into HDR at very high pressure. This water widens existing fractures and creates new ones through rock displacement. This method is successfully used by the oil industry to facilitate the path of under ground oil. Nuclear explosives: Fracturing by nuclear explosives is scheme that has been considered as part of a program for using such explosives for peaceful uses, such as natural gas estimation and oil stimulation, creating cavities for large storage, canal and harbor construction and many other applications. The principal hazards associated with this are the ground shocks , the danger of radioactivity releases to the environment, and the radioactive material that would surface with heated water and steam. Geothermal plants in the world. Some of the important geothermal plants in the world are 1) 540MW plant at Larderello, Italy. 2) USA generating 1514 MW of power using geothermal sources (Claifornia, 50MW) 3) New Zealand , 353 MW ( Wairakei power station ,175 MW) 4) Japan, 266 MW.( 5) Mexico 180 MW 6) El Salvador 95 MW 7) Iceland 63 MW 164
  • 8) USSR 211 MW ( Muntnovsky power station, 200 MW) 9) Phillipines , 665 MW ( Tiwi power station, 55MW) 10) Turkey , 0.5 MW 11) Hungary, 363 MW 12) France , 5 MW Problems associated with geothermal conversion: Environmental problems: Some effluents contain boron, fluorine and arsenic. All these are very harmful to plants and animal life in concentrations as low as two parts per million. Suitable waste treatment plants to prevent degradation of water quality will have to be installed to treat these new and increased sources of pollution. Before entry of steam in to the turbines removal of condensable gases such as CO2, Methane, H2, N2, NH3 and H2S is necessary it requires additional equipments. Re injection: Re injection is necessary to avoid discharging large quantities of heat into rivers , with consequent hazards to fisheries and farming activities, endanger down stream drinking water supplies. Huge quantity of underground water removal cause land subsidence. Noise: Noise is another problem.. The noise cause a serious health hazard Workers on new well sites have to wear ear plugs or muff lest their hearing damaged. Water borne poisons: The in wet fields some times contain toxic mercury , arsenic, ammonia etc, which would if discharged could contaminate water down stream. Air borne poisons: From various points harmful substances may escape into the air at thermal sites. These may contain radioactive materials also. Systematic monitoring is advisable in this case. Heat pollution: Geothermal power plants produce large quantity of waste heat . The proper way of discharging this heat is necessary y to avoid damage to local climate, water bodies fisheries etc. Silica: Reinjection of the silica loaded water could affect the permeability of the substrate thus it requires construction of settlement ponds . Subsidence: The withdrawal of huge quantities of underground fluids cause substantial ground subsidence, which could cause fitting and stressing of pipelines and surface structures. The remedy for this problem to some extent is the reinjection. However large extractions and reinjections also pose the possibility of seismic disturbances Seismity: Some fears have been expressed that prolonged geothermal exploitation could trigger off earthquakes especially at the zones of high shear stress . Escaping steam: Huge volumes of flash steam escaping into the air could cause dense fog to occur, which may drift across to nearly roads and cause traffic hazards. Erosion: The water with sand cause scaling and erosion problems in the pipe lines. Application of geothermal energy: There are three main applications of the steam and hot water from the wet geothermal reservoirs. 1) Generation of electric power 2) Industrial process heat and 3) Space heating for various kinds of buildings. The major benefit of geothermal energy is its varied application and versatility. 165
  • Advantages: 1) Geothermal energy is renewable source of energy. 2) Geothermal energy is least polluting compared to other conventional energy sources. 3) Geothermal plants have higher annual load factors. 4) It is cheaper compared to the energies obtained from other sources. 5) The greatest advantage of geothermal power is that it can be used in multiple uses. Disadvantages: 1) Overall efficiency for power production is low. 2) The withdrawal of large amounts of steam or water from a hydrothermal reservoir may result in surface subsidence. 3) The gases present in the steam must be removed by chemical action before discharging into atmosphere. 4) Drilling operation is noisy. 5) Large areas are needed for exploitation of geo thermal energy as much of it is diffused. Energy from bio mass Photosynthesis and oxygen production: Photosynthesis is the process in which radiant solar energy of sun is absorbed by the green pigment chlorophyll in the plant and is stored within the plant in the form of energy rich compounds like sugars and starches. So we can harvest and burn such plants to burn to produce steam in a similar manner as in thermal power stations ultimate to produce electric power. Such an energy plantation would be a renewable resource and an economical means of harnessing solar energy. How ever photosynthesis concept is less attractive as the average efficiency of solar energy conversion in plants is about 1% compared to 10% for photovoltaic cells. In photosynthesis reaction , water and CO2 molecules broken down and a carbohydrate is formed with the release of pure oxygen with the absorption of sunlight by the chlorophyll in plants. The process can be expressed as CO2 + H2O + Light + Chlorophyll (H2CO)6 + O2 + Chlorophyll Or 6CO2 + 12H2O C6 H12 O6+ 6H2O + 6O2 The chlorophyll activated by the absorption of sun light and passes its energy on to the water molecules. The hydrogen atom is then released and reacts with the carbon dioxide molecule to produce H2CO and oxygen. H2CO is the basic molecule in the formation of carbohydrate. The necessary conditions for photo synthesis are 1) Light : Only a part of the solar radiation (40 -45%) of 400 700 Ao wave length is used in photosynthesis. This range of light is called photo synthetically active radiation (PAR) . 2) CO2 concentration: Carbon dioxide is the primary raw material for photo synthesis. It is observed that if CO2 concentration is increased , increase in the yield of several crops , upto a certain limit. 3) Temperature: Photosynthesis is restricted to the temperature range which can be tolerated by the proteins i.e. 0oC to 60oC . The process of photosynthesis has two main steps: 1) Spliting of H2O molecule into H2 and O2 under the influence of chlorophyll and sunlight. This phase reaction is called light reaction. O2 escapes and H2 is transformed in to unknown compounds. 2) In the second phase , hydrogen is transformed from this unknown compound to CO2 to form starch or sugar. Formation of starch or sugar are dark reaction not requiring sunlight. Energy plantation: Energy plantation is the method of tapping maximum solar energy by growing plants. Photosynthesis occurring in naturally , stores more than ten times much energy annually , in plant farm than is consumed 166
  • by all mankind. But very little of this energy is tapped. Fuel wood accounts for about 60% of all energy consumed in the country . Social forestry programme comprises the schemes a) Mixed plantation on waste lands, and b) Reforestration of degraded forests. Jojaba evergreen shrub around 1.7 m height grows wild in the semi-arid region of USA . Its seeds contain about 50 to 80% of oil and its plantation in USA . The tree species namely Acacia, Tortila, Albizzia , Lebbak, Prasois, Juliflora and likewise have been identified adaptable to the hot – arid regions in our country. The plant namely Erythrina and Leocaena which are known to be fast growing plants are proposed for the subtropical regions . Ethyl alcohol , the most promising compound, for mixing with gasoline, can be easily prepared from starch and carbohydrates available from plants on other sources of bio- mass. Sycamore is a promising tree that yield up to 16 ton / acre per year . All of it is used except the foliage, which contains nutrients and is returned to the soil. A harvesting sycamore produces a number of sprouts that are themselves ready for harvesting in 2 to 3 years. Up to 1990-91 over 14 lakh family size bio gas plants have been set up in the country by DNES only. Its annual production of 1100 million cubic meters of gas equivalent to 38.18 lakh fuel wood is saved . The benefits to society from the biogas plants already in excess of Rs 300 corers per year. Under programme on improved chulhas (NPIC) 42 lakh tones of wood saved . The value of this is equivalent to 168 corers per year. The DNES taken up projects worth 5 MW aggregate capacity split into mechanical and electrical application systems through gasifiers/ stirling engines working on biomass at various locations in the country. Under the biomass programme energy plantation projects have been taken up with a view to fulfill the needs of fuel , fodder , and power generation together with good potential for rural employment. Very encouraging results have been obtained in the production of fast growing species of biomass in the arid areas. Bio gas production from organic wastes by anaerobic fermentation Bio gas is the mixture containing 55-65 % of methane, 30 -40 % of carbon dioxide and the rest being the impurities. Bio gas can produced from anaerobic decomposition of plant and human waste. Its calorific value is between 20935 KJ/Kg to 23028 KJ/ kg or 38131 KJ / m 3 . Bio gas is produced by digestion, pyrolysis or hydrogasification. Digestion is a biological process that occurs in the absence of oxygen and in the presence of anaerobic organisms at ambient pressures and temperatures of 35 – 70oC. The container in which this digestion takes place is known as the digester. Bio gas is generated through fermentation or bio digestion of various wastes by a variety of anaerobic and facultative organisms. Anerobic fermentation produces CO2, CH4, H2 and traces of other gases along with a decomposed mass. In bio gas plant the main is to generate methane and hence anaerobic digestion is used. Here comlex organic molecule is broken down to sugar , alcohols, pesticides and amino acids by acid producing bacteria. These products are then used to produce methane. By another category of bacteria. The anaerobic digestion or fermentation consists of three phases. 1) Enzymatic hydrolysis: Where fats, starches and proteins contained in cellulosic biomass are broken down into simple compounds. 2) Acid formation : Where the micro organisms of facultative and anaerobic group collectively called as acid farmers, hydrolyse and ferment used to break simple compounds in to acids and volatile solids. The initial acid phase of digestion may last about two weeks and during this period a large amount of carbon dioxide is given off. 3) Methane formation: Where organic acids formed above are then converted into methane and carbon dioxide by the anaerobic bacteria called methane fermentors. For the efficient fermentation these acid farmers and methane fermentors must remain in a state of dynamic equilibrium. The methane forming bacteria are sensitive to pH ,and conditions should be mildly acidic (pH 6.6 to 7.0) The general equation for anaerobic digestion is CxHyOz + [ x – y/4 – z /2] H2O [ x/2 – y/8 + z/4 ] CO2 + x/2 +y/8 – z/4] CH4 Foe cellulose this becomes (C6H10O5)n + n H2O 3n Co2 + 3n CH4 Some organic material (lignin) and all inorganic inclusions do not digest. The reaction is exothermic. Gas yield is about 0.2 to 0.4 m3 per kg dry digestible input at STP . 167
  • Advantages of anaerobic digestion: 1) The bio gas generated is having appreciable value of calorific value and can therefore , be used as an energy source to produce steam or hot water. 2) It produces smaller quantity of excess sludge. 3) The running cost are very less compared to aerobic conversion. 4) Since the system is enclosed the odours are contained. 5) A well adopted anaerobic sludge can be presented unfed for a considerable period of time without appreciable deterioration. 6) It reduces the number of pathogens produces , so reducing subsequent disposal problems. 7) The sludge produced has higher nitrogen content giving it increasing value as a fertilizer. 8) The nutrient requirement is low. Bio gas plants: Bio gas plats are mainly classified as 1) Continuous and batch type. 2) The dome and drum type. 3) Different variations in the drum type. Continuous and batch type Continuous plant: In continuous plant there is a single digester in which raw material are charged regularly and the process goes on without interruption except for their repair and cleaning etc. The continuous process may be completed in a single stage or separated in two stage. i) Single stage: process: The entire process of conversion complex organic compounds into biogas is completed in a single chamber. This chamber is regularly fed with raw material while the spent residue keeps moving out. The biogas is stored at the top portion of the chamber. The collected gas is regularly tapped for using in different applications. ii) Double stage process: Acedogenic stage methanogenic stages are separated into two chambers. Thus the first stage of acid production is carried out in separate chamber and only the diluted acids are fed into the second chamber where bio- methanisation takes place and the biogas can be collected from the second chamber. 168
  • Batch plant The feeding is between intervals , the plant is emptied once the process of digestion is complete. In this type several digesters are charged along with lime, urea etc. and allowed to produce gas for 40-50 days. These are charged and emptied one by one in a synchronous manner which maintains a regular supply of the gas through the a common gas holder. Obviously such a plant would be expensive to install and unless operated on large scale it would not be economical. The main features of the batch plant are gas production is intermittent, several digesters are needed, it is best suitable for fibrous material, needs addition of fermented slurry, plant is expensive and has more problems compared to continuous type. 2) Dome and drum type In dome type of bio gas plants , digester and the gas holder both are combined . The fixed dome is best suited for batch process especially when daily feeding is adopted in small quantities. The fixed dome digester is usually built below ground level and is suitable for cooler regions. In drum type , digester and gas holder both are separated. Digester is of masonry construction and gas holder is of M.S plates. The fixed dome plant is called Chinese plant. There are different shapes in both the designs, cylindrical , rectangular spherical etc. The digester may be vertical or horizontal. They can be constructed above the ground or below the ground. 3) Different variations in the drum type: There are two main variations in the floating drum design. One with water seal and the other without water seal. Water sealing makes the plant completely anaerobic and corrosion of the gas holder drum is also reduced. The horizontal plats are suited for high ground water level or rocky areas. Floating drum biogas plant: (KVIC) It mainly consists of two parts 1) Digester or pit 2) The gas holder or the gas collector. Digester: Digester is also called fermentation plant, it is a sort of well of masonry work, dug and built below the ground level. The depth of this well varies from 3.5 m to 6 meters, and diameter from 1.35 m to 6m , depending on the gas generating capacity and the quantity of raw material fed for each day. The digester well is divided in to two semi cylindrical compartments by means of partition wall . The level of the partition wall is lower than the level of the digester rim .Two slanting cement pipes reach the bottom of 169
  • the well on either side of the partition wall. One pipe serves as the inlet and the other as outlet. An inlet chamber near the digester at the surface level serves for mixing dung and water which is done mechanical or manually. The mixture of dung and water in the proportions of 4:5 by volume, called slurry, flows down the inlet pipe to the bottom of the primary compartment of the digester. The digester is designed to hold the 60 days raw material. The outlet chamber is again at surface level, just a few cms below the level of the inlet chamber. If both compartments of the digester are full and more slurry is added from the inlet, then equivalent amount of fermented slurry flows out of the outlet and discharged in to the composite pit. Gas holder : It is a drum constructed of mild steel sheets, cylindrical in shape with a conical top and radial supports at the bottom. It sinks into the slurry due to its own weight and rests upon the ring constructed for this purpose . As the gas is generated the holder rises and floats freely on the surface of the slurry. As the pipe is provided at the top of the holder for flow of the gas for usage. To prevent the holder from tilting a central guide pipe is fitted to the frame and is fixed at the bottom in the masonry work . The holder is capable of holding pressure equivalent upto 9 cms of water column. The holder also acts as the seal for the gas . The construction of this plant is very simple and the gas comes out with constant pressure. The only maintenance required is the painting of the gas collector at regular intervals Advantages: 1) It has less scum trouble. 2) No separate pressure equalizing devices are required. 3) In it the danger of mixing oxygen with the gas to form an explosive mixture is minimized. 4) Higher gas production per cum of the digester volume is achieved.. 5) No problem of gas leakage. 6) Constant gas pressure. Disadvantage: 1) It has higher cost , as cost is dependent steel and cement. 2) It has poor insulation aginst heat and hence it troubles in colder regions and periods. 3) Gas holder requires painting once or twice in a year . 170
  • 4) The overall maintenance cost of the plant is more compared to fixed dome type. 2) Fixed dome or Chinese digester: . The Chinese digester or Janata model or fixed dome digester is a drum less type similar in construction to the KVIC model except that the steel drum is replaced by fixed dome roof of masonry construction. The dome roof in Chinese model requires specialized design and skilled masonry construction . A poorly constructed roof generally leads to leakage from top and junction of the roof with the digester wall, thereby causing drop in gas yield. Therefore, at least three layers of extra careful plasters are must to prevent any gas leakage. The cement plaster work is very laborious and also adds to the cost of construction. More ever quality and correct proportions of the raw materials during the construction have to be maintained properly to achieve biogas impermeability. In areas where soil swell and shrink considerably there are chances of developing cracks in the brick masonry construction. Advantages: 1) It has low cost compared to floating drum type. 2) It has no corrosion problem. 3) The plant is well insulated against the heat and hence constant temperature can be maintained. 4) Almost all bio mass can be fed. 5) No maintenance. Disadvantages: 1) This plant construction requires skilled masons. 2) Gas production per cum of the digester volume is less. 3) Scum formation is problem as no stirring arrangement. 4) It has variable gas pressure. Bio gas plant for water hyacinth Behavior of water hyacinth under biodegradation is different from that of cattle dung. Cattle dung has a specific gravity almost equal to water and remains wherever it has been fed into the digester while water hyacinth floats over water surface when fresh and as digestion proceeds partially and fully decomposed 171
  • material settles down at the bottom. So, traditional bio gas plants based on cattle dung as feed material could not be used for water hyacinth . It sis observed that deliberate attempts are required to bring an intimate contact of microbes with fresh and floating material for decomposition. The final decomposed material obtained is in powder form. The main modifications done to traditional biogas plant using cow dung are. 1) The inlet is provided near the top of the digester with proper sealing.. 2) The slurry outlet is provided from the bottom of the digester. The bottom should be hopper to facilitate the discharge of digested slurry. 3) There is stirring arrangement to bring the intimate of microbes with substrate. In this plant , the 550 gms chopped dried water hyacinth is fed daily with 20 litres of water. 400 litres of bio gas generated. Chopped wet water hyacinth initially mixed with digested slurry from a continuously operated gobar gas plant. Bio gas plant based on dried water hyacinth would be very useful substituting the conventional fuel for cooking. Kachra gas plant: 172
  • A family size biogas plant based on continuous fermentation process was designed and tested at Gujarat Agricultural University Ananad. This plant was named as “ Kachra gas plant”. The plant is as shown in the fig. The brief description of the plant is as follows. Feeding: The feeding material must be chopped to few cm sized pieces when it consists of fresh water plant materials. The feeding material can charged in any position. For example , paddy straw or wheat straw or water hyacinth was pushed with a stick at the rate of 10 kg chopped material every day through the corner of the digester. Thus the problem of slurry making in the beginning is not involved. Stirring: Stirring is the most important operation , since the material floats in a thick layer (30-40cm) . The stirring should be so designed that it should be able to submerge the floating material. In the above plant horizontal stirrer is provided which is mounted on a 4 cm dia water pipe shaft. Operation of the plant : The plant is initially filled with water in which few buckets of cow dung or dirty drain water or well rotten compost are added. The fresh fibrous plant material is processed through the chaff cutter and about 10 kg chopped material is spread on the ground . This is frequently sprayed with water to keep it moist. Thus the material is charged in to the digester after ten days of decomposition . About one kg of urea may also be added to the digester . The evolved gas is regularly let out into air for about a week. During this period the gas never be tested for burning due to possible danger of its explosion and accident. Problems related to Bio-gas plants: 1) Handling of effluent is major problems if the person is not having sufficient open space or compost pits to get the slurry dry. 2) The methanogenic bacteria are sensitive to the temperature variations . During winter as the temperature falls , there is decrease in the activity of the methanogenic bacteria and subsequently fall in the gas production rate. Many methods have been suggested to overcome this temperature problem. 1) Using solar heated hot water to make slurry 2) Green house effect 3) Manual or auto stirring 4) Addition of nutrients 5) Covering the bio gas plant by straw bags . 3) Improper way of preparing slurry may results failure of bio gas plant due to accumulation of fatty acids and drop in pH . 4) Some persons add urea fertilizer in large quantities due to which toxity of ammonia nitrogen may cause a decrease in gas production. 5) pH and fatty acids play an important role in anaerobic digestion and should remain under optimum range otherwise this may cause upsetting of digester and even its failure. 6) Leakage of gas from gas holder especially in case of Janata type biogas plants is a major and very common problem. The immediate detection and repair of gas leakage is always required . 173
  • Application of bio gas The bio gas can be utilized effectively for household cooking, lighting, operating small engines, utilizing power or pumping water, chaffing fodder and grinding flour by using already known technology. In the rural areas popularizing the bio gas is the only way to save house wife from the irritating smoke of the dung cakes and wood. It helps solving the several health hazard problems like respiratory diseases and trachoma of eyes. Bio gas can be burned using bio gas burners with mixing ratio of bio gas and air (1:10). Bio gas lamp needs a mantle, which is made of a Ramic fibre . With one cum of bio gas we can save electricity equivalent to burn 60 watt lamp for 6 hours. One horse power engine can work for two hours roughly with a one cum of bio gas. This quantity of gas can cook three meals for family of about five. It is possible to build power house at the places of bio gas generation so that electricity can be produced and the same supplied to the grid. Bio gas can be used to operate both CI and SI engines . 425 litres of bio gas is required to operate1 HP engine for one hour. In sewage treatment plants the gas is utilized as fuel for the boilers that supply hot water for heating the digesters, for running gas engines which may be coupled to pumps, blowers or generators. The other main product of the biogas plant is the organic manure. This comes out at outlet as slurry which is quite rich in nitrogen . When the slurry can not be used with irrigation water it can be used for rapid fermentation of compost. Application of biogas engines: Bio gas can be used to operate both CI and SI engines. CI engines can run on dual fuel(biogas + diesel) and injection of the diesel is necessary for igniting the mixture of air and bio gas inside the cylinder. But the starting of the engine is carried out using only diesel. SI engine can be operated on biogas after initially starting on petrol. The existing diesel engines can be directly converted to use biogas , with slight modification, saving thereby 80% of diesel oil. It is possible to reduce the diesel oil consumption by further research. The petrol engines can be used to burn the bio gas by simple modification of carburetor. The SI engine needs following modification. It includes the provisions for the entry of bio gas , throttling of intake air and advancing the ignition timing. Bio gas can be admitted to a stationary SI engine through the intake manifold and air flow control valve can be provided on the air cleaner pipe connecting the air cleaner and caburettor for throttling the intake air, as shown in the fig. In this case the intake air is required to be manual throttled in the initial stage. 174
  • The CI engine which is running on the dual fuel needs necessary engine modifications includes provision for the entry of biogas with intake air, advancing the injection timing and provision of a system to reduce diesel supply. The entry of biogas and mixing of gas with intake air can be achieved by providing a mixing of biogas with air before entering into the cylinder. The admittance of bio gas in to the engine cylinder increases the engine speed and therefore , a suitable system to reduce the diesel supply by actuating the control rack needs to be incorporated. It is concluded that i) Bio gas is a suitable for conventional engine fuels with little modifications in both SI and CI engines . petrol replacement of the order of 100% and diesel replacement of about 80% is possible using bio gas ii) SI engines develop 85% of rated power where as CI engine develop full power on biogas. Thus application of biogas in CI engine is a better alternative. iii) By reducing the CO2 content in bio gas the engine performance can be improved . iv) The injection timing of SI engine using bio gas fuel can be advanced by 4-5 degrees for better engine performance. v) The injection timing CI engines operating on dual fuel shall be kept between 31-33 degrees before TDC for better performance. vi) It is economical to use biogas in engine keeping in view the present trend of increase in prices of conventional fuel and their shortage. 175
  • vii) In sewage treatment plants the biogas engines are used for running the compressors, pumps, blowers or generators. ADDITIONAL ENERGY SOURCES Fuel cells: A cell or combination of cells capable of generating an electric current by converting the chemical energy of a fuel directly into electrical energy. It consists od positive and negative electrodes with an electrolyte between them. Fuel in suitable form is supplied to the negative electrode and oxygen, often from air, to the positive electrode. When the cell operates , the fuel is oxidized and the chemical reaction provides the energy that is converted in to electrici y . The fuel cells differ from conventional electric cells in the respect t that the active material are not contained within the cell but are supplied from outside. The most commonly used fuel cell is Hydrogen oxygen fuel cell. The main types of the fuel cells are 1) Hydrogen(H2) fuel cell. 2) Hydrazine (N2H4) fuel cell 3) Hydrocarbon fuel cell . 4) Alcohol ( Methanol) fuel cell. Main uses of fuel cells are in powder production, automobile vehicles and in special military use. 1) Hydrogen oxygen fuel cell 176
  • The main components of a fuel cell are i) A fuel electrode (Anode). ii) An oxidant or air electrode (Cathode), and iii) an electrolyte. Hydrogen is supplied on to the negative electrode whereas oxygen is supplied to the positive electrode. Solid electrical conductors acts as current collector and provide terminal at each electrode. Porous nickel and carbon electrodes are generally used in fuel cells. In between positive and negative electrodes an aqueous solution of an alkali or acid is used. The porous electrode has large number of sites, where the gas, electrolyte and electrode are in contact. The reactions are very slow in order to accelerate the reactions finely divided platinum or platinum like metal deposited on or incorporated with porous electrode. The operating temperature of the fuel cell is less than 2000C.Electric current is drawn from the cell in the usual manner by connecting a load between the electrode terminals. At the negative electrode , hydrogen gas is converted in to hydrogen ions and an equivalent number of electrons thus H2 2H+ + 2 e- The catalyst on this electrode enables the hydrogen molecules to be absorbed , which reacts with the hydroxyl ions (OH-) in the electrolyte to form water. When the cell is operating and producing current , the electrons flow through the external load to the positive electrode. Here they react with the oxygen (O2) and water (H2O) from the electrolyte to form negatively charged hydroxyl (OH-) ions; thus 1/2O2 + H2O + 2e- 2 OH- The hydrogen and hydroxyl ions then combine in the electrolyte to produce water. H+ + OH H2O The electrolyte most commonly used is 40% KOH solution because of its high electrical conductivity and it is less corrosive than acids. This shows that hydroxyl ions produced at one electrode are involved in the reaction at the other. Also electrons are absorbed from oxygen electrode and released to the hydrogen electrode . When the cell is operating the overall process is the chemical combination of hydrogen and oxygen to form water that is H2 + 1/2O2 H2O The oxygen and hydrogen are converted to water, which is the waste product of the cell. If the electrodes are on open circuit, the hydrogen electrode accumulates a surface layer of negative charges. These attract potassium ions, K+ , of the electrolyte, providing an electrical double layer. Similarly the loss of electrons from oxygen electrode results in a layer of positive charges, which in turn attracts hydroxyl ions, OH- , from the electrolyte. 177
  • If the circuit is closed, the electron can now leave the electrodes pass through the connecting circuit to the oxygen electrodes, and take part in the reaction of equation above. In this way useful electrical current is directly obtained from the hydrogen to the oxygen electrode. Hydrogen fuel cells are of two types 1) Low temperature cell: The electrolyte operating temperature is 900C, and it is pressurized up to 4 atmospheres. 2) High pressure cell: The operating pressure is about 45 atmospheres and temperature is upto 3000C It is possible to create useful potential of 100 to 1000 volts and power levels of 1 KW to 100 MW nearly by connecting a number of cells. Fossil fuel cells The fossil fuel cells are near future of modified hydrogen – oxygen cell, in which a gaseous or liquid hydrocarbon is the source of hydrogen . In this cell , coal serve as the primary energy source. The cells based of fuels have three main components. 1) The fuel processor which converts the fossil fuel into a hydrogen – rich gas. 2) The power section consisting of the actual fuel cell and 3) The inverter for changing the DC generated by fuel cell into AC . 178
  • AIR FO S FU L S IL E E FU L YD O E H R GN DC AC PO E S LE TIO WR E C N IN E TE VR R P OE S R R CSO S A TE M M IN C M O E TS O FU L C LL S TE A O P N N F E E YS M The most highly developed fossil fuel cells are phosphoric acid cells, molten carbonate cells, solid electrolyte cell. The phosphoric acid cell utilizes concentrated aqueous solution of phosphoric acid as the electrolyte. The primary fuel is the light hydrocarbon, such as natural gas or Naptha. The operating temperature is 150 to 2000C and discharge voltage is 0.7 to 0.8 Volts. i) Molten carbonate cell – High temperature fuel cells. These are high temperature fuel cells with a molten carbonate ( Na, K, Li carbonates) as electrolytes. A special feature of these cells is that, during operation, they can oxidize carbon monoxide in to carbon dioxide as well as hydrogen to water. Hence gaseous mixture of hydrogen and CO, which are relatively inexpensive to manufacture, can be used in the cell. The common electrolyte in the high temperature fuel cells under development in a molten mixture of alkali metals carbonate at temperature of 600 – 700 o C. The mixture of H2 and CO is supplied is supplied to the negative electrode and O2 to the positive electrode. The discharge emf of cell is about 0.8 Volts. The electrolyte held in a sponge like ceramic matrix. Metallic electrodes are placed in direct contact with solid electrolyte . Hydrocarbon fuel such as methane or kerosene is used. The fuel is reacted inside the cell to produce H 2 and CO . At the fuel electrode , H2 and CO react with CO3 ions in electrolyte, releasing electrons to electrode and forming H2) and CO2 as shown in the fig. The reactions as follows At the fuel electrodes H2 + CO3 -- = H20 + CO2 + 2e CO + CO3-- = 2CO2 + 2e At the oxygen electrode O2 + 2CO2 + 4e = 2CO3-- The overall cell reactions in the cell 179
  • H2 + CO + O2 = H2O + CO2 The discharged gases consists mainly steam and carbon dioxide products and nitrogen from the air. The hot gases could be used to industrial heat , operate gas turbines , to produce steam in waste heat boiler to drive the steam turbine. ii)Solid oxide electrolyte cells: Ceramic solid, ceramic oxides are able to conduct electricity at high temperatures and can serve as electrolytes for fuel cells. These cells could utilize the dame fossil fuels as the molten carbonate cells. The processing operation is same as the carbonate cells. The possible electrolyte is zirconium dioxide containing small amount of another oxide to stabilize the crystal structure. The electrode material might be porous nickel and the operating temperaturein the range of 600 – 1000oC . iii) Aluminum oxygen (Air cell) This is unusual in the respect that the metal aluminum is effectively the fuel which is consumed during operation and replaced as required. Aluminum forms the negative electrode of the cell and oxygen (from the air ) is the positive electrode; the electrolyte is an aqueous solution of sodium hydroxide. The chemical reaction is Al (-) + ¾ O2 (air) (+) + ¾ 4H2O = Al ( OH)3 Aluminum, oxygen( from air) and water ( from electrolyte) combine to form aluminum hydroxide ( Al (OH3)). The aluminum (Negative) electrode are made up of the metal containing a small amount of gallium, and the air (Positive) electrode are carbon coated with an chemical catalyst , possibly silver. Before entering battery, the air is scrubbed to remove CO2. The operating temperature of the battery is about 50 to 60oC . Ion exchange membrane cell ( Low temp cell) The basic design of the cell consists of a solid electrolyte non- exchange membrane, electro catalysts and gas feed tubes as shown in the fig. In this cell, the electrolyte is solid electrolyte in the form of ion exchange membrane . The membrane is non permeable to the reactant gases, hydrogen, and oxygen, which thus prevents them from coming into contact. The two electrodes , which consists of the electro catalyst and a plastic material in the form of wire metallic screens. They are bonded on either side of the electrolyte layer . The hydrogen compartment of the cell is enclosed . The hydrogen gas enters the compartment through a small inlet and circulates through out the collectors and distributes itself evenly over the electrode. On opposite side, oxygen or air enters the 180
  • compartment , an oxygen side, the current collectors hold wicks which absorb water. The ion exchange membrane electrolyte is acidic in nature . The current carrier in solution is hydrogen ions. The hydrogen ions produced by reaction at anode 2H2 = 4H+ + 4e- These electrons are transferred to cathode through the electrolyte and reach the cathode via the external circuit. The oxygen ions at cathode O2 + 4H+ + 4e- = 2H2O Thus Overall cell reactions 2H2 + O2 = 2H20 This cell operates about 40-60oC. The thermodynamic reversible potential for the reaction is 1.23 volts at 25oC. Regenerative fuel cells . A regenerative fuel cell is one in which the fuel cell product is recovered in to its reactants by one of several possible methods – thermal , chemical , photochemical , electrical or radio chemical. Since there are two stage in the regenerative fuel cell. 1) Conversion of fuel cell reactants into products while producing electrical energy and 2) Reconversion of fuel cell products into reactants , it is clear that the overall efficiency of a regenerative fuel cell is the product of the efficiencies of these two stages . One best example for the regenerative cells is the photochemical regenerative fuel cell. In this method the products of the fuel cell reaction are transformed into its reactants by light. The sequence of the reactions which are taken place in this fuel cell can be represented as follows. Electrochemical : A + B = AB + Electricity Photochemical : AB + light = A + B Overall : Light + electricity The nitric =oxide chlorine fuel cell , in which the overall reaction is 2NO + Cl2 = 2NOCl The product nitosyl chloride is decomposed photochemically to chlorine and nitrous oxide. The system is schematically represented in fig. The cell has reversible potential of 0.21 Volt the reactants may 181
  • be regretted from NOCl , in the liquid phase by light. In the gas phase regeneration is easier although there sis some problem of separating the NO and Cl2. Advantages of fuel cells: 1) Conversion efficiencies are very high. 2) Require little attention and less maintenance. 3) Can be installed near the use point, thus reducing electrical transmission requirements and accompanying losses. 4) Fuel cells does not make any noise. 5) A little time is needed to go into operation. 6) Space requirement considerably less in comparison to conven tional power plants. Disadvantages: 1) High initial COST. 2) Low service life. Applications of fuel cells: 1) Domestic use. 2) Automotive vehicles. 3) Central power stations . 4) Special applications. Magneto hydro dynamic power generation. Introduction: MHD power generation is a is a new system of electric power generation which is said to be of high efficient and low pollution. Magneto – Hydro dynamic is concernedwith the flow of conducting fluid in the presence of magnetic and electric field. The fluid may be gas at elevated temperature or liquid metal like sodium and potassium. A MHD generator is device used for converting energy of a fuel directly into electrical energy without a conventional electric generator. In advanced counties MHD generators are widely used but in developing countries like India it is still under construction. MHD construction work is in progress at Trichi in Tamilnadu ,BHEL, associated cement corporation . Working principle: 182
  • The principle of working of MHD is based on the Faraday’s laws of electro magnetic induction which states that “ a changing magnetic field induces an electric field in any conductor located in it”. MHD generator arrangement is as illustrated in fig provides d.c power directly. As in the case of conventional generator conductor crosses the lines of the magnetic field and a voltage is induced. Similarly , in a magneto hydrodynamic field a voltage is induced. The ionized gas acts like an electrical conductor. The gas used may have a temperature between 2000 to 3000 K. In MHD generator gaseous conductor (ionized gas ) is used. If this gas is passed at high velocity through a power full magnetic field, a current is generated and can be extracted by placing electrodes in a suitable position in the stream. The direct conversion of kinetic energy into electrical energy by the flow of an electrically conducting fluid, through a stationary magnetic field. If the flow direction is at right angles to the magnetic field direction, an electromotive force (or electric voltage) is induced in the direction at right angles to both flow and field directions as depicted in the fig . This is the basic principle of MHD conversion. MHD generator: A schematic of MHD generator is as shown in the fig. The conducting flow fluid is forced between the plates with a kinetic energy and pressure differential sufficient to overcome magnetic induction force Find. An ionized gas is employed as the conducting fluid. Ionisation is produced either by thermal means i.e. by an elevated temperature or by seeding with substance like cesium or potassium vapours which ionize at relatively low temperatures. The presence of the negatively charged electrons in the seeding material makes the carrier gas an electrical conductor. The other way is to incorporate a liquid metal in to a flowing carrier gas . Since the metal is a good electrical conductor , the gas metal mixture can be used as the working fluid in an MHD generator. In the overall power cycle , the MHD generator takes the place of a turbine in a conventional vapour or gas turbine cycle. Still , a compressor must be used to elevate the pressure, heat is added at high pressure and the flow is accelerated before entering the converter. The MHD power cycle with T- S diagram is as shown below. Classification of MHD systems: The MHD systems are broadly classified as 1) Open cycle systems. 2) Closed cycle systems. i) Seeded inert gas system. ii) Liquid metal systems. 1) Open cycle systems 183
  • The following fig shows an open cycle MHD system. Here the fuel (Such as oil , coal , natural gas) is burnt in the combustion chamber, air required for combustion is supplied from air preheater. The hot gases produced by the combustion chamber are then seeded with a small amount of an ionized alkali metal ( Cesium or potassium) to increase the electrical conductivity of the gas. The ionisati0on of potassium takes place due to the gases produced at temperature of about 2300 – 2700 degree centigrade by combustion. The hot pressurized working fluid so p[produced leaves the combustion chamber and passes through a convergent divergent nozzle. The gases coming out of the nozzle at high velocity then enter the MHD generator. The expansion of the hot gases takes place in the generator surrounded by powerful magnets. The MHD generator produces direct current. By using an inverter this direct current can be converted into alternating current. 2) Closed cycle systems: (Liquid metal ) 184
  • A liquid metal closed cycle system is shown in fig . A liquid metal ( Potassium) is used as working fluid in this system The liquid potassium after being heated in the breeder reactor is passed through the nozzle where its velocity increased. The vapour formed due to nozzle action are separated in the separator and condensed and then pumped back to the reactor as shown in fig. Then the liquid metal with high velocity is passed through MHD generator to produce D.C power. The liquid potassium coming out of MHD generator is passed through the heat exchanger to use its remaining heat to run a turbine and then pumped back to the reactor. 2) Closed cycle systems: (Seeded inert gas ) In closed cycle system carrier gas (argon / helium) operates in a form of Braytoin cycle. The coal is gasified and the gas is burnt in the combustion chamber to provide heat. In the primary heat exchanger this heat is transferred to the carrier gas argon / helium (working fluid)of the MHD cycle. The combustion 185
  • products after passing through the air preheater and air purifier are discharged to atmosphere. As the combustion system is separated from the working fluid there is no problem of seed recovery. The hot argon gas is seeded with cesium and resulting working fluid is passed through the MHD generator at high speeds. The d.c. power out put of the generator is converted in to A.C by the inverter and is then fed into the grid. The hot fluid from MHD enters secondary heat exchanger, which serves as the waste heat boiler to generate steam. This steam is partly utilized to drive a turbine generator and for driving a turbine which runs the argon compressor . The out put is also fed to the main grid. The working fluid is returned back to primary heat exchanger after passing through compressor and intercooler. A closed cycle system operates at lower temperature compared to open cycle system. Advantages of MHD systems: 1) More reliable since there are no moving parts. 2) In MHD system the efficiency can be about 50% as compared to less than 40% fro most efficient steam turbine plants. 3) Power produced is free from pollution. 4) As soon as it is started it can reach the full power level. 5) The size of the plant is considerably smaller than convent onal fossil fuel plants. i 6) Less overall operational cost. 7) The capital cost of MHD plants is comparable to those of conventional system plants. 8) Better utilization of fuel. 9) Suitable for peak power generation and emergencyservice. 10) Large amount can be generated. Disadvantages: 1) The MHD systems suffer from the reverse flow of electrons through the conducting fluids around the ends of the magnetic field. 2) There will be a high friction losses and heat transfer losses. 3) The resistivity of the gas near the electrodes is very high. 4) MHD system needs very large magnets and this is a major expense. 5) Coal, when used as fuel , poses the problem of molten ash which may short circuit the electrodes. Thermoelectric power conversion The basis for this method of power generation is seeback effect that a loop of two dissimilar metals developed an e.m.f. when the two junctions are kept at different temperatures. This effect has long been used in thermocouples to measure temperatures. This phenomenon offers one method of producing electrical energy directly from the heat of combustion, but its thermal efficiency is very low, of the order 1 to 3 percent. Efficiency of thermoelectric generator depends upon the temperatures of hot and cold junctions in any heat engine. Where fuels are very cheap, the device based on thermoelectric power can generate power for stand by or even base load plants. Basic principle of thermoelectric power conversion: Thermo electric generator is a device which converts heat energy into electrical energy through semiconductor or conductor. The direct conversion of heat energy into electrical energy is based on the Seeback Thermo electric effect. Consider two dissimilar materials joined together in the form of a loop so that there are two junctions. Such a system is shown in the figure. If a temperature difference is maintained between these two junctions, an electric current will flow round the loop. 186
  • The magnitude of the current will depend on both the materials used and the temperature difference of the junction (T = T2 –T1). If the circuit is broken an open circuit voltage V appers across the terminals of the break. The thermo emf , V produced by the device is given by V = S1-2 T (1) Where S1-2 is the Seeback coefficient. For larger temperature difference the above equation can be written in more accurate form as T2 V = S1-2 dT (2) T1 Seeback coefficient is the temperature coefficient of thermo emf or the rate of change of thermo – emf with temperature. S1-2 = Lt V / T = dV / dT (3) T 0 Depending on the choice of the materials , the drop in the potential may be either positive or negative in the direction of the drop of the temperature. The sign as well as magnitude of the Seeback coefficient is significant. Seeback effect arise because the concentration of the charge carriers in a conductor depends upon the temperature. The presence of a temperature gradient in material causes a carrier concentration gradient and an electric field is established, which causes the net flow of charge carriers under open circuittions to be zero.The Seeback effect has been used for great many years in the thermocouple, which is used in the measurement of temperature. Thermoelectric power generator: The simple arrangement for utilizing the Seeback effect is shown in fig. 187
  • The thermocouple material A and B are joined at the hot end, but the other ends are kept cold; an electric voltage is generated between the cold ends. A direct current will flow in a circuit or load connected between these ends. For a given thermocouple, the voltage and electric power output are increased by increasing the temperature difference the hot and cold ends. In a practical thermoelectric converter, several couples are connected in series to increase both voltage and power. If voltage is not sufficient to operate any device or equipment, it can be increased with the help of inverter transformer combination. Thermoelectric converter is a form of heat engine. Heat is taken up at an upper temperature and part is converted into electrical energy; the remainder is discharged at a lower temperature .The thermal efficiency of the thermoelectric converter mainly depends on the temperature of hot junction. Thermoelectric power generators have been built with power outputs ranging from a few watts to kilowatts. The source of heat is immaterial and hence it can be used in areas outside the regular electric power distribution system. An important application is the use of radioactive decay heat to generate power in space and other remote locations. Thermoionic conversion Another method of conversion of heat energy directly into electric energy is thermoionic conversion. It utilizes the thermoionic emission effect i.e., the emission of electrons from heated metal surfaces. The energy required to extract electron from the metal is known as the work function of the metal. In principle 188
  • , thermoionic consists of two metals with different work functions sealed into an evacuated vessel. Electrode with large work function is maintained at a higher temperature than one with the smaller work function . The vessel or container is filled with ionized cesium vapour. Heating one electrode, electrons are emitted, that travel to the opposite, colder electrode. The hotter electrode emits electrons and so acquires a positive charge, where as the colder electrode collects the electrons and becomes negatively charged. A voltage thus develops, between the two electrodes and a direct current will flow in an external circuit connecting them. The size of the converter is limited and this is suitable only for small scale power production. A thermoionic converter is a form of heat engine, in principle, heat is taken in at the upper temperature, part is converted into electrical energy, and the remainder is discharged at the lower temperature. The thermoionic converter will continue to generate electric power as long as heat is supplied to the emitter and a temperature difference is maintained between it and the collector. The efficiency of 10 percent can be achieved by maintaining hot electrode at 1000oC and cesium in the vessel. Higher efficiencies , possibly upto 40 percent can be obtained by operating at still higher temperatures. Heat sources such as fossil or nuclear or solar can be used in a thermoionic generator. 189