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# Solar Thermal Power Plant Final Year Project Report

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### Solar Thermal Power Plant Final Year Project Report

1. 1. Solar Thermal Power PlantConcept, Design, Simulation and Fabrication Sulaiman Dawood Barry Syed Mohammed Umair Saad Ahmed Khan Arsalan Qasim 09 1
2. 2. 2
4. 4. DSG Advantages: ........................................................................................................ 27 DSG Disadvantages: .................................................................................................... 27 HTF Advantages: ........................................................................................................ 27 HTF Disadvantages: .................................................................................................... 27 Combined cycle Advantages: ...................................................................................... 28 Combined cycle Disadvantages: .................................................................................. 28Conclusion ................................................................................................................................ 29INTRODUCTION TO DESIGN CALCULATIONS ................................................................. 32 Objective ........................................................................................................................................... 32 First Law of Thermodynamics ............................................................................................................ 32 Second Law of Thermodynamics ........................................................................................................ 32 What is a Thermodynamic Cycle? ...................................................................................................... 32 Source ............................................................................................................................................... 33 Sink ................................................................................................................................................... 33 Efficiency of a cycle............................................................................................................................ 33 Heat Engine ....................................................................................................................................... 33TYPES OF EXTERNAL COMBUSTION CYCLES................................................................. 34 Carnot Cycle ...................................................................................................................................... 34 Ideal Cycle ......................................................................................................................................... 34 Rankine Cycle .................................................................................................................................... 35DISCUSSION OF CYCLE SELECTION ................................................................................. 36 Close Cycle Vs Open Cycle.................................................................................................................. 36EXPANDERS TYPES, COMPARISON AND SELECTION .................................................... 38 Drawbacks of Turbo-machines ........................................................................................................... 38 Advantages of Displacement Machines .............................................................................................. 38 Factors in selection of a Positive Displacement Machine .................................................................... 39 Disadvantages of Positive Displacement Machines ............................................................................ 39 Torque meter .................................................................................................................................... 39PROJECT DESIGN SCHEMATIC ........................................................................................... 41 Parabolic Trough ............................................................................................................................... 41 Absorber Pipe .................................................................................................................................... 42 Over Head Tank ................................................................................................................................. 42 4
5. 5. Steam Engine..................................................................................................................................... 42 Operation .......................................................................................................................................... 42CAD MODEL ........................................................................................................................... 43 Complete Assembly ........................................................................................................................... 43 Base Frame........................................................................................................................................ 44 Base Frame Dimensions ..................................................................................................................... 45 Tube Holder....................................................................................................................................... 47 Parabola ............................................................................................................................................ 48 Absorber Tube ................................................................................................................................... 50MATERIAL SELECTION ........................................................................................................ 52 Absorber Tube and Gauge fittings ...................................................................................................... 52 Parabola ............................................................................................................................................ 52 Base Frame........................................................................................................................................ 52 Glass Mirrors ..................................................................................................................................... 52 Teflon String ...................................................................................................................................... 52 Brass ................................................................................................................................................. 52 Black Nickel Coating........................................................................................................................... 52MANUFACTURING PLAN ..................................................................................................... 53 Market Survey ................................................................................................................................... 53 Tooling Techniques ............................................................................................................................ 53 Assembling Of Base Frame................................................................................................................. 53 Assembling Of Parabola ..................................................................................................................... 53 Assembling Of Absorber Tube ............................................................................................................ 54SOLAR CALCULATIONS ...................................................................................................... 55COMPARISON OF DIFFERENT WORKING FLUIDS ........................................................... 58THERMODYNAMIC CALCULATIONS & MODELING ....................................................... 59 Problem Definition: ........................................................................................................................... 59 Assumptions: ..................................................................................................................................... 60 Mass Flow Rate ................................................................................................................................. 61 Inner Surface Temperatures .............................................................................................................. 64 Super-heater Analysis ........................................................................................................................ 64 5
6. 6. Boiler Analysis ................................................................................................................................... 74 Heat Loss Analysis ............................................................................................................................. 83 Natural Convection Analysis ....................................................................................... 85 Forced Convection Analysis ........................................................................................ 89 Glass Tube Analysis............................................................................................................................ 95 Heat Input and Area Required ......................................................................................................... 105 Cost Analysis.................................................................................................................................... 111 Plant Start Up Analysis.............................................................................................. 111ANALYSIS AT DIFFERENT PRESSURES........................................................................... 114 Variation of Superheater Surface Temperature and Steam Exit Temperature with Pressure ............ 114 Variation of Plant Carnot Efficiency, Efficiecny with Bare Tube and Glass Tube with Pressure .......... 115 Heat Loss with Pressure ................................................................................................................... 116 Variation of total area with pressure ratio. ...................................................................................... 117 Variation of parabola width with pressure ....................................................................................... 118 Mass flow rate versus pressure ratio ............................................................................................... 119Manufacturing Operations ....................................................................................................... 120 Engine design Calculations ............................................................................................................... 129 Pump............................................................................................................................................... 130 Property of the Pump ................................................................................................. 131 PARABOLIC REFLECTOR.................................................................................................................... 132Instrumentation ....................................................................................................................... 133 Water Level Detector....................................................................................................................... 133 Thermocouple ................................................................................................................................. 133 Flash Valve ...................................................................................................................................... 133 Pressure Guage ............................................................................................................................... 133 Flow meter ...................................................................................................................................... 133FEA Analysis .......................................................................................................................... 135 Support Stress and Strain Analysis ................................................................................................... 135 Super-heater Flow analysis .............................................................................................................. 136REFRENCES .......................................................................................................................... 140 6
7. 7. INTRODUCTIONThe need of energyWith the advancement of science and the usage of many electronic gadgets, life becomes verydifficult without electricity. Hence, ample supply of electricity that can match the powerrequirements of industry is the key for national progress and prosperity.Fossil fuels are non-renewable resources because they take millions of years to form, andreserves are being depleted much faster than new ones are being formed. The production and useof fossil fuels also raise environmental concerns. Therefore, a global movement toward thegeneration of renewable energy is under way to help meet increased energy needs.Wood, wind, water, and sun power have been used for cooking, heating, milling and other tasksfor millennia. During the Industrial Revolution of the eighteenth and early nineteenth centuries,these forms of renewable energy were replaced by fossil fuels such as coal and petroleum.Attention has refocused on renewable energy sources since the 1960s and 1970s, not onlybecause of concern over fossil fuel depletion, but also because of apprehension over acid rainand global warming from the accumulation of carbon dioxide in the atmosphere.Fossil fuels are becoming ever more expensive especially after the oil embargo of the 1970’s.Very recently the price of oil shot up to about \$120 per barrel which is definitely unbearable forthe economy. Also, the supply of oil is uncertain. Even if oil supply is continuous, the cost ofimporting oil is tremendous (which will deplete the National Exchequer) and Pakistan thereforehas to borrow from institutions like IMF and World Bank which deepens the debt problem. Inyear 2006, Pakistan imported crude worth 6.7 Billion Dollars. In such a situation, solar power isthe need of the hour since these problems will then be eliminated. Also, the land of Pakistan isparticularly well endowed for solar energy projects since it has vast tracts of desert regions thatreceive large amounts of unbroken sunshine throughout the year.Renewable energy resources are cleaner and far more abundant than fossil resources, but theytend to be dispersed and more expensive to collect. Many of them, such as wind and solarenergy, are intermittent in nature, making energy storage or distributed production systemsnecessary. Therefore, the direct cost of renewable energy is generally higher than the direct costof fossil fuels. At the same time, fossil fuels have significant indirect or external costs, such aspollution, acid rain, and global warming. 7
8. 8. About Solar Power:According to Wikipedia, the earth is blasted with 89 peta-watts (1015 W) of sunlight which isplentiful, almost 6,000 times more than the 15 terawatts of average electrical power consumed byhumans.Solar power is the generation of electricity from sunlight. This can be direct as with photo-voltaics (PV), or indirect as with concentrating solar power (CSP), where the suns energy isfocused to boil water which is then used to provide power. The power gained from sun can beused to eliminate or atleast cut down the need for purchased electricity (usually electricity gainedfrom burning fossil fuels) or, if the energy harnessed from sun exceeds a homes requirements,the extra electricity can be sold back to the homes supplier of energy, typically for credit.The advantages of solar energy are as follows  Solar power is pollution-free during use. Production end-wastes and emissions are manageable using existing pollution controls.  Solar electric generation is economically superior where grid connection or fuel transport is difficult, costly or impossible.The largest solar power plants, like the 354 MW (SolarEnergy Generating Systems) SEGS, are concentratingsolar thermal plants which consists of nine solar powerplants in Californias Mojave Desert, where insolation isamong the best available in the United States but recentlymulti-megawatt photovoltaic plants have been built.Completed in 2008, the 46 MW Moura photovoltaic powerstation in Portugal and the 40 MW Waldpolenz SolarPark in Germany are characteristic of the trend toward Figure 1: Solar Energy Generating Systemslarger photovoltaic power stations. Much larger ones are solar power plants III-VII at Kramer Junction, Californiaproposed, such as the 100 MW Fort Peck Solar Farm, the550 MW Topaz Solar Farm, and the 600 MW RanchoCielo Solar Farm.Solar power is a predictably intermittent energy source,meaning that whilst solar power is not available at alltimes, we can predict with a very good degree ofaccuracy when it will and will not be available.Some technologies, such as solar thermal concentratorshave an element of thermal storage, such as moltensalts. These store spare solar energy in the form of heatwhich is made available overnight or during periods 8 Figure 2: Waldpolenz Solar Park, Germany
9. 9. that solar power is not available to produce electricity.Why do we need solar power?Recent reports on the current status of the reserves of fossil fuels point to the need to switch toalternative energies such as Solar Power.Even without considering environmental impacts, it is clear that at some stage we will not beable to meet our ever increasing energy needs from a finite supply of these non-renewableresources. 9
10. 10. Objective/Goals projectThe aim of our project is to design, simulate and fabricate a lab scale solar thermal power plantthat utilizes solar energy for the generation of electrical energy of atleast 40 Watts. Moreover,the Plant is aimed to achieve the following goals  Least running cost,  High reliability to demonstrate students on regular basis the use of solar power during the day.  Validate the solar thermal plant construction cost is less than Rs. 50,000 for the first plant of 100 WattsWhat do we stand to gain?Considering the exponential growth in the prices of fossil fuel and hence the utilities using fuel,alternative methods need to be found immediately.With our project we can:  Highlight the potential of solar energy use in Pakistan  Create a platform for the future students to work on 10
11. 11. Current Methods of solar power productionAround the world the following methods are being use to harness solar power;  PHOTOVOLTAIC CELLS  SOLAR THERMAL POWER PLANTS o Fresnel mirror and lens collectors o Parabolic trough/dish collectors o Flat plate collectors o Solar power towers o Solar updraft towers o Solar pondsWe will now discuss in detail these methods.Photovoltaic cells:Solar photovoltaics (PVs) are arrays of cells containing amaterial that converts solar radiation into directcurrent electricity. Materials presently used forphotovoltaics include amorphous silicon, polycrystallinesilicon, microcrystalline silicon, cadmium telluride,and copper indium selenide/ sulfide.At the end of 2008, the cumulative global PVinstallations reached 15,200 megawatts. Roughly 90% ofthis generating capacity consists of grid-tied electricalsystems. Such installations may be ground-mounted (and Figure 3: An array of photovoltaic cellssometimes integrated with farming and grazing) or builtinto the roof or walls of a building, known as Building Integrated Photovoltaics or BIPV forshort. Solar PV power stations today have capacities ranging from 10-60 MW although proposedsolar PV power stations will have a capacity of 150 MW or more. 11
12. 12. Advantages  PV installations can operate for many years with little maintenance or intervention after their initial set-up, so after the initial capital cost of building any solar power plant, operating costs are extremely low compared to existing power technologies.  PV is economically superior where grid connection or fuel transport is difficult, costly or impossible. Long-standing examples include satellites, island communities, remote locations and ocean vessels.  When grid-connected, solar electric generation replaces some or all of the highest-cost electricity used during times of peak demand (in most climatic regions). This can reduce grid loading, and can eliminate the need for local battery power to provide for use in times of darkness. These features are enabled by net metering.  Grid-connected solar electricity can be used locally thus reducing transmission/distribution losses (transmission losses in the US were approximately 7.2% in 1995).  Compared to fossil and nuclear energy sources, very little research money has been invested in the development of solar cells, so there is considerable room for improvement. Nevertheless, experimental high efficiency solar cells already have efficiencies of over 40%and efficiencies are rapidly rising while mass-production costs are rapidly falling.Disadvantages  Photovoltaics are costly to install. While the modules are often warranted for upwards of 20 years, an investment in a home-mounted system is mostly lost if you move.  Solar electricity is seen to be expensive. Once a PV system is installed it will produce electricity for no further cost until the inverter needs replacing but the timetable for payback is too long for most.  Solar electricity is not available at night and is less available in cloudy weather conditions from conventional silicon based-technologies. Therefore, a storage or complementary power system is required. However, the use of germanium (more expensive than silicon) in amorphous silicon-germanium thin-film solar cells provides residual power generating capacity at night due to background infrared radiation.  Solar cells produce DC which must be converted to AC (using a grid tie inverter) when used in current existing distribution grids. This incurs an energy loss of 4-12%. 12
13. 13.  Silicon solar cell manufacturing is not available in Pakistan and quite expensive to import and install so it not a feasible option to generate the required output.Solar power towersThe solar power tower (also known as Central Towerpower plants or Heliostat power plants or powertowers) is a type of solar furnace using a tower toreceive the focused sunlight. It uses an array of flat,movable mirrors (called heliostats) to focus the sunsrays upon a collector tower (the target).Early designs used these focused rays to heat water, andused the resulting steam to power a turbine. However,designs using liquid sodium in place of water have been Figure 4: Solar-two Mojave Desert, Californiademonstrated; this is a metal with high heat capacity,which can be used to store the energy before using it to boil water to drive turbines. Thesedesigns allow power to be generated when the sun is not shining.The 10 MWe Solar One and Solar Two heliostat demonstration projects in the Mojave Deserthave now been decommissioned. The 15 MW Solar Tres Power Tower in Spain builds on theseprojects. In Spain the 11 MW PS10 solar power tower and 20 MW PS20 solar power tower havebeen recently completed. In South Africa, a solar power plant is planned with 4000 to 5000heliostat mirrors, each having an area of 140 m². A site near Upington has been selected.Disadvantage  Large areas of land are required  Technology requires storage for stable power output  Cost of such energy is about three times higher than conventional of power generation as with all technologies  The tall tower is also difficult to construct.  Each mirror needs its own heliostat which is very expensive.Advantage  High temperatures can be achieved which lead to higher efficiencies.  Flat mirrors can be used which are very cheap compared to curved mirrors. 13
14. 14. Fresnel lens collectorsA Fresnel lens is a type of lens developed by French physicist Augustin-JeanFresnel for lighthouses; a similar design had previously been proposedby Buffon and Condorcet as a way to make large burning lenses.The design enables the construction of lenses of large aperture and short focal length without theweight and volume of material that would be required in conventional lens design. Compared toearlier lenses, the Fresnel lens is much thinner, thus passing more light andallowing lighthouses to be visible over much longer distances. The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by breaking the lens into a set of concentric annular sections known as Fresnel zones. In the first (and largest) variations of the lens, each zone was actually a different prism. Though a Fresnel lens might look like a single piece of glass, closer examination reveals that it is many small pieces. It was not until modern computer-Figure 5: Working Operation of Fresnel Lens controlled milling equipment (CNC) could turn out large complex pieces that these lenses weremanufactured from single pieces of glass.For each of these zones, the overall thickness of the lens is decreased, effectively chopping thecontinuous surface of a standard lens into a set of surfaces of the same curvature, withdiscontinuities between them. This allows a substantial reduction in thickness (and thus weightand volume of material) of the lens, at the expense of reducing the imaging quality of the lens.A Concentrating Linear Fresnel Reflector is a typeof solar power collector. Instead of using parabolicreflectors, Linear Fresnel Reflectors focus solar energywith a series of essentially flat mirrors on a stationarylinear water-filled receiver for the purpose of collectingheat to generate steam and power a steam turbine. Figure 6: A physical model of fresnel mirror collector 14
15. 15. Since March 2009, the Fresnel solar power plant PE 1, designed and constructed by the Germancompany Novatec Biosol, is in commercial operation. The solar thermal power plant is based onlinear Fresnel collector technology and has an electrical capacity of 1.4 MW. Beside aconventional power block, PE 1 comprises a solar boiler with mirror surface of around 18,000m².The steam is generated by concentrating direct solar irradiation onto a linear receiver which is7.40m above the ground. An absorber tube is positioned in the focal line of the mirror field inwhich water is evaporated directly into saturated steam at 270°C and at a pressure of 55 bars bythe concentrated solar energy.Advantage  More rugged than parabolic mirrors  Light weight  Flat, hence occupy little volume, and hence easy to set up.Disadvantage  Unavailable locally  Maybe expensive for large sizes 15
16. 16. Parabolic troughA parabolic trough is a type of solar thermalenergy collector. It is constructed as a longparabolic (usually coated silver or polished aluminum) witha Dewar tube running its length at the focalpoint. Sunlight is reflected by the mirror and concentratedon the Dewar tube. The trough is usually aligned on anorth-south axis, and rotated to track the sun as it movesacross the sky each day. Figure 7: Figure 7: An Array of Parabolic Trough Collector at the National Solar Energy CenterAlternatively the trough can be aligned on an east-westaxis; this reduces the overall efficiency of the collector, dueto cosine loss, but only requires the trough to be aligned with the change in seasons, avoiding theneed for tracking motors. This tracking method works correctly at the spring andfall equinoxes with errors in the focusing of the light at other times during the year (themagnitude of this error varies throughout the day, taking aminimum value at solar noon). There is also an errorintroduced due to the daily motion of the sun across thesky, this error also reaches a minimum at solar noon. Dueto these sources of error, seasonally adjusted parabolictroughs are generally designed with a lower solarconcentration ratio. In order to increase the level ofalignment, some measuring devices have also beeninvented.Heat transfer fluid (usually oil) runs through the tubeto absorb the concentrated sunlight. This increases thetemperature of the fluid to some 400°C. The heat transferfluid is then used to heat steam in a standard turbinegenerator. The process is economical and, for heating thepipe, thermal efficiency ranges from 60-80%. The overallefficiency from collector to grid, i.e. (Electrical OutputPower)/ (Total Impinging Solar Power) is about 15%,similar to PV (Photovoltaic Cells) but lessthan Stirling dish concentrators. 16 Figure 8: A diagram of a parabolic trough solar farm (bottom), and an end view of how a parabolic collector focuses sunlight onto its focal point.
17. 17. Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used duringnight hours, but the amount of fossil fuel used is limited to a maximum 27% of electricityproduction, allowing the plant to qualify as a renewable energy source. Because they are hybridsand include cooling stations, condensers, accumulators and other things besides the actual solarcollectors, the power generated per square meter of area ranges enormously.Types of mirrorsUsually, mirrors are used which are parabolic and are of a single piece. In addition, V-typeparabolic troughs exist which are made from 2 mirrors and placed at an angle towards eachother.Mirror coatingsIn 2009, scientists at the National Renewable Energy Laboratory (NREL) and SkyFuel teamed todevelop large curved sheets of metal that have the potential to be 30% less expensive thantodays best collectors of concentrated solar power by replacing glass-based models witha silverpolymer sheet that has the same performance as the heavy glass mirrors, but at a muchlower cost and much lower weight. It also is much easier to deploy and install. The glossy filmuses several layers of polymers, with an inner layer of pure silver.Energy storageAs this renewable source of energy is inconsistent by nature, methods for energy storage havebeen studied, for instance the single-tank (thermocline) storage technology for large-scale solarthermal power plants. The thermocline tank approach uses a mixture of silica sand and quartziterock to displace a significant portion of the volume in the tank. Then it is filled with the heattransfer fluid, typically a molten nitrate salt.Existing plantsThe largest operational solar power system at present is one of the SEGS plants and is locatedat Kramer Junction in California, USA, with five fields of 33 MW generation capacities each. 17
18. 18. The 64 MW Nevada Solar One also uses this technology. In the new Spanish plant, Andasol 1solar power station, the Eurotrough-collector is used. This plant went online in November2008 and has a nominal output of 49.9 MW.Large solar thermal power stations include the 354 MW Solar Energy Generating Systems powerplant in the USA, Nevada Solar One (USA, 64 MW), Andasol 1 (Spain, 50 MW), Andasol 2(Spain, 50 MW), PS20 solar power tower (Spain, 20 MW), and the PS10 solar power tower(Spain, 11 MW).The solar thermal power industry is growing rapidly with 1.2 GW under construction as of April2009 and another 13.9 GW announced globally through 2014. Spain is the epicenter of solarthermal power development with 22 projects for 1,037 MW under construction, all of which areprojected to come online by the end of 2010. In the United States, 5,600 MW of solar thermalpower projects have been announced. In developing countries, three World Bank projects forintegrated solar thermal/combined-cycle gas-turbine power plants in Egypt, Mexico, andMorocco have been approved. 18
19. 19. SOLAR PONDSA solar pond is simply apool of saltwater whichcollects and stores solarthermal energy. Thesaltwater naturally forms avertical salinitygradient also known as a"halocline", in which low-salinity water floats on topof high-salinity water. Thelayers of salt solutions Figure 9: A solar pond schematicincrease in concentration(and therefore density)with depth. Below a certain depth, the solution has a uniformly high salt concentration.There are 3 distinct layers of water in the pond: The top layer, which has a low salt content. An intermediate insulating layer with a salt gradient, which establishes a density gradient that prevents heat exchange by natural convection. The bottom layer, which has a high salt content.If the water is relatively translucent, and the ponds bottom has high optical absorption, thennearly all of the incident solar radiation (sunlight) will go into heating the bottom layer.When solar energy is absorbed in the water, its temperature increases, causing thermalexpansion and reduced density. If the water were fresh, the low-density warm water would floatto the surface, causing convection current. The temperature gradient alone causes a densitygradient that decreases with depth. However the salinity gradient forms a densitygradient that increases with depth, and this counteracts the temperature gradient, thus preventingheat in the lower layers from moving upwards by convection and leaving the pond. This meansthat the temperature at the bottom of the pond will rise to over 90 °C while the temperature at thetop of the pond is usually around 30 °C. A natural example of these effects in a saline water bodyis Solar Lake, Sinai, Israel. 19
20. 20. The heat trapped in the salty bottom layer can be used for many different purposes, such as theheating of buildings or industrial hot water or to drive an organic Rankine cycle turbineor Stirling engine for generating electricity.Advantages and disadvantages The approach is particularly attractive for rural areas in developing countries. Very large area collectors can be set up for just the cost of the clay or plastic pond liner. The evaporated surface water needs to be constantly replenished. The accumulating salt crystals have to be removed and can be both a valuable by-product and a maintenance expense. No need of a separate collector for this thermal storage system. Not suitable on a small scale. 20
21. 21. SOLAR UPDRAFT TOWERSchematic presentation ofa solar updraft towerThe solar updrafttower is a proposed typeof energy power. Itcombines three old andproven technologies: thechimney effect,the greenhouse effect,and the wind turbine. Airis heated by sunshine andcontained in a very Figure 10: Solar Updraft Tower Schematiclarge greenhouse-likestructure around the base of a tall chimney, and the resulting convection causes rising airflow torise through the updraft tower. The air current from the greenhouse up the chimneydrives turbines, which produce electricity. A successful research prototype operated in Spain inthe 1980s, and many modeling studies have been published as to optimization, scale, andeconomic feasibility.The generating ability of a solar updraft power plant depends primarily on two factors: the sizeof the collector area and chimney height. With a larger collector area, a greater volume of air iswarmed to flow up the chimney; collector areas as large as 7 km in diameter have beenconsidered. With a larger chimney height, the pressure difference increases the stack effect;chimneys as tall as 1000 m have been considered.Heat can be stored inside the collector area greenhouse to be used to warm the air later on.Water, with its relatively high specific heat capacity, can be filled in tubes placed under thecollector increasing the energy storage as needed.Turbines can be installed in a ring around the base of the tower, with a horizontal axis, asplanned for the Australian project and seen in the diagram above; or—as in the prototype inSpain—a single vertical axis turbine can be installed inside the chimney. 21
22. 22. Carbon dioxide is emitted only negligibly while operating, but is emitted more significantlyduring manufacture of its construction materials, particularly cement. Net energy payback isestimated to be 2–3 years.A solar updraft tower power station would consume a significant area of land if it were designedto generate as much electricity as is produced by modern power stations using conventionaltechnology. Construction would be most likely in hot areas with large amounts of very low-valueland, such as deserts, or otherwise degraded land.A small-scale solar updraft tower may be an attractive option for remote regions in developingcountries. The relatively low-tech approach could allow local resources and labor to be used forits construction and maintenance. 22
23. 23. Comparative study Solar Solar Parabolic Solar Fresnel PV cells updraft power trough pond collector tower tower collector Cost Very high Very high High Low High mediumMaintenance Very little Medium Little Little Little little Area Very Very little Very large Very large Medium medium required large Reliability High medium medium High High high Not Not Material Easily available available available available availableavailability available locally locally Large Large Large Small Small to scale Small scale scale scale scale large scale electricity electricityApplications electricity electricity electricity electricity generation generation generatio generation generation generation or water only n only only only heating 23
24. 24. Constraints:Our major constraints are TIME, MONEY, MANPOWER and SPACE. On this basis we cannotuse PV cells(High Cost), Solar Ponds(Large Scale), Solar Updraft and Power Towers(LargeScale and High Cost), Fresnel collectors(Complexity of Tracking for each Mirror) or Flat platecollectors(Unavailability). Therefore we will go with parabolic troughs.Power generation methods using parabolic troughs:The following methods are being used around the world using parabolic troughs:  Steam heated with a heat transfer fluid.  Steam heated directly by solar radiation.  Combined cycle power generation using both solar and fossil fuel.SEGS with HTFA solar electric generating system (SEGS), shown in Fig. 10, refers to a class of solar energysystems that use parabolic troughs in order to produce electricity from sunlight (Pilkington,1996). The parabolic troughs are long parallel rows of curved glass mirrors focusing the sun’senergy on an absorber pipe located along its focal line. These collectors track the sun by rotatingaround a north–south axis. The heat transfer fluid (HTF), oil, is circulated through the pipes.Under normal operation the heated HTF leaves the collectors with a specified collector outlettemperature and is pumped to a central power plant area. There, the HTF is passed throughseveral heat exchangers where its energy is transferred to the power plant’s working fluid, whichis water or steam. The heated steam is used in turn to drive a turbine generator to produceelectricity. Figure 11: A Schematic model of SEGS using HTF 24
25. 25. SEGS with DSG:This is the same as before except that there is not HTF and the water is heated to steam directlyin the collectors. Figure 12: DSG operation in Recirculation mode 25
26. 26. Combined Power Cycle Figure 13: A schematic model of Combined Power CycleAs seen from the above diagram the combined cycle heats the water partly by solar energy andpartly by fossil fuel. In this way the plant can run even on night or cloudy forecast when there isno sun and on normal days the running cost of the fuel will be reduced due to lesser fuel input. 26
27. 27. Comparision of all 3 Parabolic Trough Power PlantsDSG Advantages:  Complexity reduced  Efficiency increased due to no intermediate heat transfer  Costly synthetic oil eliminated  Oil can be flammable at high temperatures. No oil used in DSG  Oil breaks down at temperatures near to its maximum working temperature.  Using high conductivity metal like copper minimizes the problem of thermal stress greatly  Environmental friendly as only water is used as working fluid. No danger of contamination from oil.  Copper tubes easily available along with mirror strips for parabola.DSG Disadvantages:  Cannot be used in absence of sunlight.  No thermal storage possible hence can be used only for part load power.  Copper can be expensive  Control of DSG plants is difficult  Materials like steel will have greater thermal stresses on them while working with two phase flowHTF Advantages:  Thermal storage can be done so that energy will be available even if sunlight is not available.  Higher temperatures can be achieved which will lead to greater heat transfer in the heat exchangers.  Molten salt is very good at transferring heat, it is a liquid at atmospheric pressure and has high heat storage capacity.HTF Disadvantages:  Oil is flammable at high temperatures. 27
28. 28.  It will break down at high temperatures and hence its viscosity will increase causing damage to pumps and pipes.  Increase in pump work will also take place if quality of oil degrades.  Oil can contaminate the environment.Combined cycle Advantages:  It can be used all year round without dependency on weather.  Fuel cost of the plant will be reduced as major part of the heating is done by the solar collectors.  Thermal energy storage may not be required, depending on power requirement,Combined cycle Disadvantages:  It will be very complex.  Separate gas fired boilers needed.  Will cause pollution free.  Fuel cost will keep rising and hence running costs will be expensive. 28
29. 29. ConclusionConsidering the above weight matrix and the comaprision of advantages and disadvantages ofeach plant in parabolic trough category, the DSG is considered to be the most feasible option andhence will be carried forward for fabrication. On the following page is the Gantt Chart thatshows our plan that will be considered to achieve our objective. 29
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31. 31. 31
32. 32. INTRODUCTION TO DESIGN CALCULATIONSObjectiveTo design and fabricate a Lab - Scale Solar Thermal Power Plant for demonstration of principleof Direct Steam Generation (DSG) by production of 40 W of net power.First Law of ThermodynamicsThe first law of thermodynamics is also called the Principle of Conservation of Energy. It statesthat energy can neither be created nor destroyed but it can change from one form to another.Therefore, according to the first law, the net energy input in a cycle in the form of heat must beconverted to the net work output so that the principle of energy is conserved.Second Law of ThermodynamicsAll the work can be converted to heat but the vice versa is not possible unless a part of heat isrejected to the sink.What is a Thermodynamic Cycle?A thermodynamic cycle obeys the two fundamental lawsof thermodynamics. It is defined as a process in whichthere is transfer of heat and work; while its physical state(like temperature and pressure) parameter of a workingfluid changes and finally coming back its initial state aftercompleting the whole cycle.In a thermodynamic cycle the work required or producedis the indicated by the area of closed loop Pressure-Volume Diagram (also called the Indicator Diagram) asshown in the figure 1.Any theoretical thermodynamic cycle is an ideal one with Figure 14: Example of a PV diagramsome assumptions because none of the practical heat [1]engine (explained below) strictly follows the cycle. Yet still, understanding physical concepts ofdifferent cycles are essential in order to gain the highest possible efficiencies for a particularcycle. There are various types of thermodynamic cycles available like Brayton, Rankine, Otto,Carnot, Sterling, Vapor-Compression Cycle, etc. In a thermodynamic cycle, the state propertiesare a function of thermodynamic states only where as heat and work are path dependentfunctions.The net work produced by a thermodynamic cycle is given by: (1)For power producing cycle, the PV-diagram shown in Figure 1 has clockwise loop and the workcalculated in equation (1) has a positive value. However, for power consuming cycle, the loophas anticlockwise direction and equation (1) has a negative value. The former cyclearrangement is for heat engine where as the latter arrangement is for heat pump. 32
33. 33. Since the project is aimed to producing a useful power output therefore heat engines arediscussed next. However, some of the common terms commonly used in heat engines areintroduced first.SourceA source is at a higher temperature surrounding which gives input energy to a heat engineSinkA sink is at a lower temperature in which a heat engine rejects heat.Efficiency of a cycleAn efficiency of a cycle is defined as the percentage of net work-out to the heat input. (2)Heat EngineA heat engine is a device that converts heat into work,by utilizing the temperature difference between thesource and sink. While the heat enters the engine fromthe source, the working medium (usually liquid or gas)converts the part of energy received to work in somepart of the cycle where as the rest of the energy isrejected to the sink.Generally, the greater the temperatures differencebetween the source and the sink, the higher the thermalefficiency of a cycle. Since the sink medium is normallyearth’s environment and its temperature is always about300 K, therefore, in order to achieve higher efficienciesthe source temperature have to be raised. Figure 15: Heat Engine Diagram [2]These engines operate on a particular thermodynamic cycle as mentioned earlier. Moreover,the working cycles may be open to atmospheric or sealed from the outside (Open or ClosedCycles). The cycle on which a heat engine operates is called Power Cycle. The Power Cyclesare further classified into two categories: 1. Internal Combustion Cycles 2. External Combustion CyclesSince, the solar power can only be used for external combustion cycles, therefore internalcombustion cycles will not be discussed further. 33
34. 34. TYPES OF EXTERNAL COMBUSTION CYCLESThere are various cycles available in which the external combustion cycles can be used toproduce useful power output. These are as follows 1. Carnot Cycles 2. Ideal Cycle 3. Rankine CycleCarnot CycleA Carnot cycle is comprised of entirely reversible processes which includes isothermal heataddition, isentropic expansion, isothermal heat rejection and isentropic compression to completethe cycle. The temperature is only the determining the factor for thermal efficiency and equation(2) reduces to (3)Where TL is the lowest cycle temperature, and TH is the highest cycle temperature.The work produced by the cycle is determined by the rectangular area bounded by the T-sdiagram or it can be calculated by (4)Where sH and sL are the entropies at highest and lowest temperature respectively.The efficiency of a Carnot Cycle is highest known; however, controlling the state points are verydifficult to manage practically, therefore it cannot be employed for calculations. Even ifsomehow managed inside the saturation curve, the heavy liquid/vapor mixture compression isnot appropriate as it will damage the compressor and engine severely.Ideal CycleAn Ideal Cycle is made up of the following four processes,constant volume heat addition, isobaric expansion,constant volume heat rejection and finally isobariccompression to complete the cycle. The power of an idealcycle is determined by the rectangular area bounded by thepressure volume curve or it can also be determined by thefollowing equation (5)The problem faced in this cycle that in order to add and Figure 16: An illustration of anreject heat, the whole cycle has to be completely stopped ideal cycle heat engine [3]and therefore it would be very inappropriate as it would cause too much transients and the cyclewould not be stable. Thus it is not used. 34
35. 35. Rankine CycleThe well known Rankine cycle is the used in most traditional power plants. The working fluid ispumped to a boiler where it is evaporated, passed through a turbine and is finally re-condensed.This cycle is sometimes referred to as a practical Carnot cycle as, when an efficient turbine isused, the TS diagram begins to resemble the Carnot cycle. The main difference is that heataddition and rejection are isobaric in the Rankine cycle and isothermal in the theoretical Carnotcycle. A pump is used to pressurize liquid instead of gas. This requires a very small fraction ofthe energy compared to compressing a gas in a compressor (as in the Carnot cycle).In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbinewould generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4would be represented by vertical lines on the Ts diagram and more closely resemble that of theCarnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheatregion after the expansion in the turbine, which reduces the energy removed by the condensers. Figure 17: Ts diagram of a typical Rankine cycle operating between pressures of 0.06 bar and 50 bar [4] 35
36. 36. DISCUSSION OF CYCLE SELECTIONThe Rankine Cycle is the most appropriate one for the power plant because of the better controlof the states conditions, and hence the performance of the plant.Close Cycle Vs Open CycleWe performed simulation test of an ideal Rankine Cycle with water as a working fluid operatingon closed cycle with the liquid being compressed with at quality of 0.1 from 1 atm to higherpressure with open cycle where the liquid is pumped from 25 oC and 1 atm to the same pressure Efficiency Vs Turbine Pressure 0.07 0.06 0.05 Efficiency Closed 0.04 Cycle 0.03 Open 0.02 Cycle 0.01 0 102 110 120 130 140 150 160 170 180 190 200 210 220 230 Turbine Pressure Figure 18: Comparison of Open and Closed Rankine CycleIt is evident from the graph that the difference in efficiencies between the cycles is not great atlower pressure ratios. However, the gap widens when the plant is operated on higher pressures.For our project we have selected an open Rankine cycle. This is because of the followingreasons:  Our project is a small prototype for the validation of the concept of DSG.  For the sake of simplicity we do not want to go into the complexity of designing a condenser 36
37. 37.  According to the following graph there is only a small difference in the efficiencies; hence an open cycle is selected due to its simplicity. 37
38. 38. EXPANDERS TYPES, COMPARISON AND SELECTIONThere are two main types of expanders: 1. Turbo-machines, and 2. Displacement type machinesDifference between them must be clear to select an appropriate one.Drawbacks of Turbo-machinesUsing turbo-machines have several drawbacks when used in low power applications. Theperformances of most rotary machines are related to their peripheral speed (or tip speed) U[m/s], rather than directly to the shaft speed. They have an optimal tip speed, usuallyindependent from the machine size. For Positive Displacement this value ranges typically from 1to 10 m/s, while for turbo-machines, this value is close to 300 m/s.The tip speed is given by: , (6)Where R is the radius of the rotary machine and N is the number of revolutions.When used in smaller units, the turbo-machines have a lower radius R, and their optimalrotational speed is therefore increased. This very high shaft speed causes high mechanicalstresses (e.g. due to centrifugal loading), bearing friction losses, reduction of the bearing life,necessity for higher reduction gear, etc.Advantages of Displacement Machines  In contrast, the tip speed of a displacement type machine is inherently lower, and the drawbacks presented above disappear.  The pressure ratio of a single stage turbo-machine has a low value (typically 1.5), while the displacement machine can have as high pressure ratios as desired. This latter solution is hence preferred for the single stage expansion usually used in the low power Rankine cycle. 38
39. 39.  Volumetric machines are much more resistant to an eventual liquid phase in the fluid than turbines: their rugged design and their low rotational speed make them less sensitive to contamination by liquid droplets.In one paper, the scroll machine has been selected among all the displacement type machinesfor its reduced number of moving parts, reliability, wide output power range, and goodavailability. Compared to the piston compressor, the scroll also shows the advantage of nothaving admission valvesA few papers also present the Wankel engine and the screw expander as appropriatetechnologies for organic Rankine cyclesFactors in selection of a Positive Displacement Machine 1. Swept Volume 2. Internal built-in ratioThe Internal built-ratio has to be adapted to the range of pressure ratios imposed to theexpander.Disadvantages of Positive Displacement MachinesThe biggest disadvantage is the leakage. It reduces the output power of a machine working asexpander, as the fluid flows directly from the high pressure region to the low pressure regionwithout producing any useful work.Torque meterIn order to measure the expander mechanical power, a torque meter on the expander shaft isrequired. The accuracy on the measurement of the torque is to be known. A tachometermeasures the rotational speed of the torque meter shaft.The mechanical power is calculated by: (7) 39
40. 40. Where ηcour is the efficiency of the transmission and is the revolutions per minute of thetorque meter. 40
41. 41. PROJECT DESIGN SCHEMATICFollowing is the brief description of the small scale direct steam generation power plant: Overhead tank Steam Engine PRV Boiler pipe Super heater Parabolic trough Figure 19: Schematic Model of PlantThe plant will have the following major components: 1. Parabolic trough with mounting 2. Over head tank/pump 3. Absorber pipe 4. Steam engineParabolic TroughThis trough will have an area of 5m2. Its length will be 1.6m. The trough will be pivoted about itsfocal point which will be at a distance of 1.5m. The mounting frame is a 2m by 2m angle ironstructure, with 2m high posts for the pivot of the trough. It will also have a sub-frame for theabsorber pipe, to place it at the focal point of the parabola.The parabola will be made with 2.54cm by 160cm mirror strips placed closely together. It will bebacked by a galvanized iron sheet bent into a close approximation of a parabola. An exactparabola with continuous mirror sheet is both difficult to make and very expensive.The parabola focus will be horizontal to the ground, and the entire assembly will be fitted withwheel casters for mobility. 41
42. 42. Absorber PipeThis is a 90% copper tube with a nominal diameter of 1.75 in and a thickness of 1.6mm. Itslength is 160cm of which 12 cm is the super heater. It is electroplated with black nickel which isa solar selective coating with 0.90-0.95 absorptivity and 0.15-0.16 emissivity.The super heater and boiler are isolated from each other except for a spring loaded valve whichallows flow of saturated steam. The tube has a flash valve in the beginning in order to removeair from the boiler. There is also an inlet valve to control the flow of water coming into the pipe.There is a glass tube with an anti reflection coating around the copper tube to minimizeconvection losses.Over Head TankIn order to achieve our working pressure of 140 kPa, we will use a tank at a height of 4m abovethe absorber pipe which will provide us with a pressure slightly more than our requirement. Wecan also use a pump but it is difficult to find one matching our requirements, hence currently theover head tank will be used which is a cheaper and simpler option.Steam EngineThis is a single cylinder engine which we will use to produce power output of approximately 40watts.OperationWe are using the principle of the Basic Rankine Cycle; except that we are exhausting to theatmosphere. So there is no condenser involved. Feed water is supplied by over head tank intothe tube.  First the tube will be filled with water,  The inlet water valve is closed,  Heating is done until steam is observed from the flash valve,  Close the flash valve,  Open inlet valve,  Allow steam pressure to build up to approximately 140kPa (operating pressure),  Open solenoid valve at 140kPa to allow steam to enter super-heater at 109°C -110°C  Open super-heater exit valve when pressure is 140kPa and temperature is 130°C 42
43. 43. CAD MODELComplete Assembly Figure 20: Complete Working Model of the Power Plant 43
44. 44. Base Frame Figure 21: Mounting Structure (Base) Isometric View 44
45. 45. Base Frame Dimensions Figure 22: Base Frame Top View (with dimensions) 45
46. 46. Figure 23: Base Frame Front View (with dimensions)Figure 24: Close up of Front View upper section to elaborate the dimensions 46
47. 47. Tube Holder Figure 25: Dimensions of Tube Holding Stand 47
48. 48. Parabola Figure 26: Parabola Collector Isometric View Figure 27: Sketch View of Parabola indicating basic Dimensions 48
49. 49. Figure 28: Parabola Skeleton indicating Length wise dimensionsFigure 29: Bearing Dimensions for the Parabola to rotate about the focus 49
50. 50. Figure 30: Holder for BearingsAbsorber Tube Figure 31: Absorber tube with different components 50
51. 51. Figure 32: Close up View for Superheater Section Figure 33: Boiler Inlet Zoomed in ViewThe length of the tube is 160cm.Super heater section is 12 cm.Diameter of the pipe is 4.445cm 51
52. 52. MATERIAL SELECTIONThe following materials have been selected for our projectAbsorber Tube and Gauge fittings Copper is our choice of material. We have selected this due to the high thermal diffusivity of this metal which is very important for our application in order to reduce thermal stresses generated due to the large difference of heat transfer coefficients of water and steamParabola GI sheet and wood strips will be used to form the surface of the parabola. Aluminum pipes will be bent according to parabola shape to support the surface. These materials are cheap and easy to work with.Base Frame Galvanized Iron L section will be used for this part as it is easily available and cheap, and can be worked on very easily.Glass Mirrors These are to be used in the form of 1inch wide strips to approximate the curvature of a parabola. They are a cheaper alternative to using a continuous curved glass sheet and more robust then Mylar.Teflon String This will be used when fitting gauges and valves to ensure leak proof fitting.Brass This will be used for fittings of gauges and valves.Black Nickel Coating This is a solar selective absorber, with high absorptivity of 0.9-0.95 and an emissivity of 0.15-0.2 at 100°C. These special optical properties are very important for our solar energy usage. 52
53. 53. MANUFACTURING PLANThe manufacturing will be done in the following steps  Market survey for materials/equipment and subsequent purchase  Tooling techniques for the different materials  Assembling of base frame  Assembling of parabola  Assembling of absorber tube  Complete assemblyMarket SurveyThis is needed to search for and obtain the materials required for our project.We have obtained copper pipe, galvanized iron and aluminum pipes and wood and havecompleted market survey for the different gauges that we will be using.Tooling TechniquesThe following processes will be used  Abrasive machining for cleaning galvanized iron surfaces and for metal cutting  Shielded Metal Arc Welding, for joining galvanized iron pieces for base frame and parabola structure.  Metal Drilling for placement of screws and bolts  Pipe Bending for aluminum pipes  Oxy-Acetylene Gas welding for copper pipe fittingsAssembling Of Base FrameThe base will be made first. The galvanized iron L section will be cut according to thedimensions specified earlier in the report. Then we will weld them according to our requirement.Then the supporting columns of the parabola will be erected by similar procedure, care has tobe taken to ensure they are perfectly perpendicular to the base.Then the frame for the absorber tube is to be welded along with the base for the steam engine.Finally bearings for supporting the parabola are to be bolted onto the base frame at thespecified position.Assembling Of ParabolaThe aluminum pipes will be bent according to the dimensions.Supporting GI L sections will be cut and bolted onto the aluminum. 53
54. 54. The GI Sheet will be bent on the pipes and riveted. Wood strips will be placed on underside ofthe sheet to prevent uneven surface.Parabola mounting pieces will be then welded to the GI L section.Mirror strips will be stuck to the GI sheet with double sided adhesive tape and their angleadjusted by hand if required.Assembling Of Absorber TubeThe copper pipe will first be electroplated with black nickel coating.Bushes will be made for the ends of the pipe and threads cut into the bushes. The bushes willbe Gas welded to the pipe.The end caps will be made which will have holes for inlet and outlet. The caps will have thesame threading as the bushes, so that they can be screwed together.Another bush will be on the top surface of the tube for fitting the flash valveInlet and exit valves and gauges will be fitted. 54
55. 55. SOLAR CALCULATIONS Solar constant = [5]Assuming earth to be a flat disc, with radius R, all the flux would be falling on it.Total solar flux incident on the earth =Since earth is a sphere with radius R, the average flux falling on it would beAlbedo: fraction of solar energy reflected by the earth’s surface.Average albedo of the earth = 0.31Latitude of Karachi = 24° 51’ = 24.85° 55
56. 56. Karachi 342 W/m2 Θ=24.89° Figure 34: For reference of Solar Flux Incident on KarachiProjected area for 342 W/m2 of equator = 1m2Therefore, for Karachi, projected area =Hence, flux falling at Karachi =Since on average 3% of light is reflected back, thereforeThis flux is falling during the whole day. Therefore average flux during the day onlyFrom solar power map, Karachi receives annually 1900 to 2000 kWh/m2 56
57. 57. (Verified) Figure 35: SOLAR INSOLATION MAP [6] 57
58. 58. COMPARISON OF DIFFERENT WORKING FLUIDSThis is the most vital choice for the power because it has the major contribution indetermining the efficiency of the overall cycle. Below is the bar-chart of differentworking fluids operating at the same pressure in a closed Rankine Cycle with 15degree superheat and it shows how the efficiency varies with working fluid. Efficiency for Same Working Pressure (140 kPa) for different working fluids in an Ideal Rankine Cycle 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0 Steam R11 R113 R123 R134a R22 n-pentane Working Fluids Figure 36: Variation of Efficiencies with Working FluidsAlthough the efficiency for steam is less than the rest, it is used due to less complexity of the systemsuch as sealing, safety and cost issues.As the graph suggests, if we use steam, efficiency is 2.1% as compared to 3.7% for R22. 58
59. 59. THERMODYNAMIC CALCULATIONS & MODELINGPlant Design At 140 kPa (Absolute) Pressure 2 5 3 Boiler Superheater Expander Pump / Overhead Tank Saturated Vapor Leaving boiler 4 1 Figure 37: Schematic of Plant for Mathematical ModelingProblem Definition:Assuming steady-state conditions, we are required to design a Solar Thermal Power Plant usingdirect steam generation that gives a net Power Output of 40 Watts. The water enters the pumpwith an inlet temperature of 25°C and pressure of 101.325 kPa. Same conditions can beassumed if the pump is substituted with an overhead tank. The water is pumped to 140 kPawhere it is subjected to boil in boiler section and superheated by 15°C in the super-heater. Thesuperheated steam is then fed to the engine where it is expanded to produce work and thenexhausted to the atmosphere. Calculate: (a) The mass flow rate at a pressure of 140 kPa, and the dimensions of super-heater and boiler. Assume that the rate of heat absorbed by the fluid is same throughout the length of the pipe. (b) Determine the inside surface temperature for super-heater and boiler section, if problem exists suggest some practical solution. (c) Determine the variation of Reynolds Number, heat transfer co-efficient of steam and Lengths of thermal boundary layer for different levels of liquid and the boiler. Suggest best possible liquid level, support your answers with reason (d) Total Heat Lost along with radiation with bare tube having  natural convection losses,  with wind speed of 2 m/s, and 59
60. 60.  Glass tube having diameter 2.5 inches.  Show the results with different varying wind speeds at 140 kPa without glass tube and with glass tube. (e) Calculate the installation trough area required and the cycle efficiency for each condition mentioned above. (f) Calculate the time required for the copper pipe to heat up to the required temperature and its linear expansion assuming the cross section does not vary and compare it with steel pipe.Assumptions:  Isentropic efficiency of the engine 70 %  Pump isentropic efficiency (The tank may also have some isentropic efficiency due to which we may require higher installation) 80%  Average Heat flux incident on Karachi is 0.446 KW/m2.  Boiler and super-heater are made from copper tube having 1.6m length with 1.75 inch average diameter and a wall thickness of 1.6 mm,  It is coated with black chrome having absorptivity of 0.90 and emissivity of 0.15  The ambient temperature is 25°C  Gravitational acceleration to 9.81 m/s2. 60
61. 61. Mass Flow Rate Figure 38: T-s Diagram, signifying the states and operating pressure State 1 State 2 State 3 State 4 61
62. 62. Assuming efficiencies of the pump and the engine (h is enthalpy) ( is the pump work and is the mass flow rate)Now to determine the length of super-heater, it is necessary to determine the rate of heatabsorbed by the working fluid. 62
63. 63. Let Ltotal be the total length and Lsuperheater be the super-heater length.Now the mass entering the super-heater will have the enthalpy of saturated vapor where as forthe mass leaving the super-heater will have the enthalpy of state 3.Therefore, AndAssuming that the rate of energy absorbed per unit length by the fluid is same throughout theheating section, we can apply the energy balance on super-heater as shown:And for the boiler, we know that the mass entering in will be at state 2 and the leaving mass willhave saturated vapor enthalpy. Applying similar procedure as above 63
64. 64. Inner Surface TemperaturesGiven Average Diameter of pipe (dnominal); andPipe thickness is known to be:Let do and di be outside and inside diameters, respectively.Let the cross-section of the pipe in which the steam flows be Axsn:Super-heater AnalysisFirst, it must be observed whether the flow is fully developed or not in order to observe if theprofile of temperature is fully developed.As already mentioned above, all the fluid entering the super-heater will be in saturated vaporphase, however for the above problem we have to determine the properties at averagetemperature of steam entering and leaving the super-heater. 64
65. 65. Let Tmean be the average temperature for steam in super-heater. (Mean temperature) (Mean specific volume) (Volume flow rate) (Velocity) (Density) (Viscosity) (Prandtl number) (Reynolds number)For pipe flow, Reynolds number has the following criteria 65
66. 66. LaminarTransitionalTurbulentSince the Reynolds number obtained is just greater that Laminar Criteria, therefore, for most ofthe time the flow is considered to be laminar in the super-heater section.Since all the required parameters for hydrodynamic boundary layer and thermal are determined,so now we determine the entry length.Let the entry lengths for hydro-dynamic boundary and thermal boundary for super-heater be Lh,laminar sup and Lt, laminar sup, respectively.Since the entry length for each boundary layer is much greater than the length of superheater,therefore the neither profile is fully developed. In order to determine the heat transfer co-efficient, the Nusselt number available for pipes subjected to constant flux is used. (Nusselt number)Finally, for the inside surface temperature, we determine the inside surface area of pipe andapply energy balance. 66
67. 67. Let the inner surface area of the pipe for super-heater be As, superheater insideLet Ts, superheater inside be the inside surface temperature of a pipe.Applying Energy Balance on superheaterSubstituting the values, we obtainHowever, for constant temperature developing flow, we have 67
68. 68. Substituting the values, we obtainCAUTION!! The value of Temperature obtained above is still too high and it may be possibledue to very low heat transfer co-efficient of steam. This high temperature has materialconstraints as well as it would cause high loss of energy resulting from convection and radiation(with major contribution of radiation loss). Below is shown the graph of Pipe SurfaceTemperature Vs. heat transfer co-efficient of steam. 68
69. 69. Variation of Superheater Surface Temperature with respect to Heat Transfer Co-efficent of Steam 1600 1400 1200 Surface Temperature (°C) 1000 800 600 400 200 0 10 40 80 120 160 200 240 280 320 360 400 440 480 Heat Transfer Co-efficient of Steam (W/m2-K) Figure 39: Variation of Surface Temperature with Heat Transfer CoefficientIt is highly recommended to reduce the surface temperature to not more than 5 degreecentigrade of the outlet temperature which can be achieved by increasing the heat transfercoefficient which can be achieved through high turbulence inside the pipe which would be acompromise on Pressure loss.However, creating turbulence in 1.89 cm section of super-heater is not practical. Therefore, nowwe try to change the length of the super-heater and disregard the assumption of constant rate ofheat absorption per unit length.We fix the length of the super-heater to 0.05 which is approximately 2.5 times greater andobserve the effect of surface temperature with respect to it. 69
70. 70. The parameters mentioned below do not change because they are dependent on inlet andoutlet steam conditions and pipe diameter which are kept fixed: 70
71. 71. Since the entry length for each boundary layer is much greater than the length of super-heater,therefore the neither profile is fully developed. Since the pipe is made of copper high thermaldiffusivity, it is expected to have constant surface temperature. Therefore, to determine the heattransfer co-efficient, the Nusselt number available for pipes subjected to constant surfacetemperature and developing flow is used.Let the inner surface area of the pipe for super-heater be As, superheater insideHowever, for constant temperature developing flow, we haveLet Ts, superheater inside be the inside surface temperature of a pipe. Applying Energy Balance onsuper-heater 71
72. 72. Substituting the values, we obtainIt is clear that increasing length has a dramatic effect on the Surface temperature, now we plot aLength of Super-heater Vs Surface temperature Superheater Surface Temperature against its Length 1800 1600 Surface Temperature (°C) 1400 1200 1000 800 600 400 200 0 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 Superheater Length (m) Figure 40: Variation of Surface Temperature Againt Superheater LengthHowever, from the graph above, we cannot conclude the optimum length because although thetemperature is seen to decrease, the surface area of the pipe also increases, which wouldincrease the heat loss for the pipe. However, the melting point of copper is 1083°C. Therefore it 72
73. 73. is reasonable to choose a length at which the temperature is at least half of the melting point sotherefore, the rest of the calculation is performed on a length of 0.1 m with a surfacetemperature of 527.3°C.The pressure lost during the first case is determined in any case assuming the flow for most partto be laminar. (Friction factor) (Pressure difference)The extra pumping work required in the above case is not significant, so it can be ignored forthe rest of the calculations.Let the outside surface temperature of the copper be Ts, superheater outside which is determined byapplying the resistance method: 73
74. 74. Discussion: There is no difference between the inside and the outside surface temperature dueto very low thermal resistance as calculated above.Boiler AnalysisThe water level in boiler is always designed to be maintained to be half filled. The analysis forsurface temperature is carried in two parts, first for saturated vapor and second for the boilingliquidFollowing part is done for vapor onlySince it was mentioned earlier that the liquid level of boiler is maintained half, therefore the areaavailable for steam to flow is also halved as shown in the diagram. Let A vapor boiler be the crosssectional area available for steam to flow 74
75. 75. However, we know that steam is forming in the boiler so the steam velocity at the inlet of boilerwill be zero and the above velocity is at exit. Therefore the average velocity of steam in boiler isThe characteristic Length is found bySince, in the above case the length chosen for super-heater was 0.1 which reduces the lengthof the boiler to be equal to 1.5 m. In this case also, no profile is fully developed. 75
76. 76. One thing to note in boiler is that the steam is subjected to constant temperature heating whenin contact with water. Therefore, we find heat transfer co-efficient for steam in the boiler atconstant surface temperature.Since the profile is developing we use the following formula for to determine heat transfer co-efficient for constant temperatureLet r be the ratio of volume of water in the boiler to the Volume of boiler tubeThe following table and its respective graphs show the variation of heat transfer co-efficient ofsteam, Reynolds Number and Entry Lengths with changing water level in the boiler. 76
77. 77. Re boiler h const, temp L laminar, thermal L laminar, hydro 0 1198 3.188 2.569 2.5660.1 1232 3.404 2.446 2.4420.2 1273 3.661 2.322 2.3190.3 1323 3.974 2.195 2.1920.4 1385 4.369 2.061 2.0590.5 1464 4.833 1.918 1.9160.6 1568 5.602 1.761 1.7590.7 1716 6.685 1.582 1.580.8 1952 8.426 1.365 1.3640.9 2443 12.54 1.069 1.068 Heat Transfer Coefficient Vs Water Level 14 12 Heat Transfer Co-efficients W/m2-K 10 8 6 4 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Figure 41: Heat Transfer Coefficient Variation with Different Water Levels in Boiler 77
78. 78. Reynolds Number Vs Water Level 2600 2400 2200 2000 1800 1600 1400 1200 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure 42: Reynold Number Variation with Different Water Levels in Boiler Entry Length of Thermal Bondary Layer Vs Water LevelEntry Length of Thermal Bondary Layer (m) 3 2.5 2 1.5 1 0.5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure 43: Entry Length of Flow with water Level. Note: Flow Becomes fully developed if r>0.7 78
79. 79. From the graphs above it is observed that as the level of water in the boiler is raised, the heattransfer co-efficient increase as well as the Reynolds Number.However, the entry length of thermal boundary layer decreases and at r = 0.7 it is approximatelyof the same length as of the tube and on further increasing the water level the boundary lengthbecomes shorter than the tube and the flow becomes fully developed.Thus, it is recommended to use r ranging from 0.7 to 0.9.The average flux on boiler is determined from Figure 44: Typical Pool Boiling curve for water at 1 atm [7] 79