THERMAL POWER CYCLESCompiled & presented by :UTKARSH PRAKASHReliance Power-GET „2011
THERMAL POWER PLANT The basic energy cycleA thermal power station is a power plant in involved in the plant is as follows :which the prime mover is steam driven. Wateris heated, turns into steam and spins a steamturbine which drives an electrical generator. Afterit passes through the turbine, the steam is Chemical Energycondensed in a condenser and recycled to whereit was heated. The greatest variation in the designof thermal power stations is due to the different Mechanical Energyfuel sources. Some thermal power plants alsodeliver heat energy for industrial purposes, fordistrict heating, or for desalination of water aswell as delivering electrical power. Electrical Energy
POWER CYCLES CARNOT CYCLE RANKINE CYCLE DIESEL CYCLE OTTO CYCLE BRAYTON CYCLE STIRLING CYCLE COMBINED CYCLES
LAWS OFTHERMODYNAMICS The zeroth law of thermodynamics recognizes that if two systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other, thus supporting the notions of temperature and heat. The first law of thermodynamics distinguishes between two kinds of physical process, namely energy transfer as work, and energy transfer as heat. The internal energy obeys the principle of conservation of energy but work and heat are not defined as separately conserved quantities. ∆Q= ∆U + p.dv Equivalently, the first law of thermodynamics states that perpetual motion machines of the first kind are impossible. The second law of thermodynamics distinguishes between reversible and irreversible physical processes. It says that the full conversion of heat to the equivalent amount of work is not possible. Equivalently, perpetual motion machines of the second kind are impossible. The third law of thermodynamics concerns the entropy of a perfect crystal at absolute zero temperature, and implies that it is impossible to cool a system to exactly absolute zero.
CARNOT CYCLEThe Carnot cycle can be thought of as the most efficient heat engine cycle allowed byphysical laws. The most efficient heat engine cycle is the Carnot cycle, consisting oftwo isothermal processes and two adiabatic processes. When the second law ofthermodynamics states that not all the supplied heat in a heat engine can be used todo work, the Carnot efficiency sets the limiting value on the fraction of the heat whichcan be so used. In order to approach the Carnot efficiency, the 1-2 isothermal heat processes involved in the addition in a boiler heat engine cycle must be 2-3 isentropic expansion reversible and involve no in a turbine change in entropy. This 3-4 isothermal heat means that the Carnot rejection in a condenser cycle is an idealization 4-1 isentropic compression in a compressor T-s diagram of Carnot vapor cycles. 7
CARNOT CYCLE EFFICIENCYIf W= net work output of the system in Carnot cycle, and as the system is carried outthrough a cycle then there is no change in the internal energy of thesystem, therefore QH – Qc = W QH= TH (S2- S1)The efficiency η is defined to be: (Work output)/(Heat input) η= W/QH = (QH-Qc)/QH also,Where,W is the work done by the system (energy exiting the system as work),QH is the heat put into the system (heat energy entering the system),TC is the absolute temperature of the cold reservoir,and TH is the absolute temperature of the hot reservoir.
CARNOT CYCLE FEASIBILTY Carnots theorem: No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs. The Carnot cycle is the most efficient cycle operating between two specified temperature limits but it is not a suitable model for power cycles. Because: Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the maximum temperature that can be used in the cycle (374 C for water) Process 2-3 The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear. Process 4-1 It is not practical to design a compressor that handles two phases.
RANKINE TERMINOLOGY The Rankine cycle most closely describes the process by which steam- operated heat engines most commonly found in power generation plants to generate power.
RANKINE CURVE Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy. Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. Process 4-1: The wet vapor thenProcess 3-4: The dry saturated vapor enters a condenser where it isexpands through a turbine, generating power. condensed at a constantThis decreases the temperature and pressure temperature to become a saturatedof the vapor liquid.
Thermal Efficiency of RankineCycle: Heat Input = Q23 = H3 – H2 Heat Rejected = Q41 = H4 – H1 Work Output = W34 = H3 – H4 Work done by Pump = W12 = H2 – H1 Work output – Pump work W34 – W12 η= = Heat Input Q23 “the rankine cycle has a lower efficiency compared to corresponding Carnot cycle 2‟-3-4-1‟ with the same maximum and minimum temperatures.”
Reasons for Considering Rankine Cycle as an Ideal Cycle For SteamPower Plants:1) It is very difficult to build a pump that will handle a mixture of liquid and vaporat state 1’ (refer T-s diagram) and deliver saturated liquid at state 2’. It ismuch easier to completely condense the vapor and handle only liquid in thepump.2) In the rankine cycle, the vapor may be superheated at constant pressure from3 to 3” without difficulty. In a Carnot cycle using superheated steam, thesuperheating will have to be done at constant temperature along path 3-5.During this process, the pressure has to be dropped. This means that heat istransferred to the vapor as it undergoes expansion doing work. This is difficultto achieve in practice.
Second law analysis of Rankine cycle The Rankine cycle is not a totally reversible cycle, it is only internally reversible, since heat transfer through a finite temperature difference (between the furnace and the boiler or between the condenser and the external medium) can results in irreversibilities. The second law of thermodynamics can be used in order to reveal the regions where the largest irreversibilities within Rankine cycle occur. It will be possible, therefore, to act on these regions to reduce the irreversibilities. To do this we must compute the exergy destruction for each component of the cycle.
MEAN TEMPERATUREMETHOD In rankine cycle heat is added at a constant pressure but at infinite temperatures If TM1 is the mean temperature of the heat addition as shown in the 6 Tm1 figure so that the area under the 7 curve 2 to 3” is equal to the area under 6 and 7 then the heat added T2 is Q23” = Tm1 (S3”- S2) Tm1 = (H3”- H2)/(S3” – S2) Heat rejected, Q4”1 = H4” – H1 = T2 (S4” – S1)“The higher the mean temperatureof heat addition, higher will be the η = 1 – Q23”/Q4”1Rankine cycle efficiency.” η = [1 – Tm1/T2]
DEVIATION OF ACTUAL VAPOUR POWER CYCLES FROM IDEALIZED CYCLEThe actual vapor power cycle differs from the ideal Rankine cycle as aresult of irreversibilities in various components.Fluid friction and heat loss to the surroundings are the two commonsources of irreversibilities. Isentropic efficiencies(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.(b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.
HOW TO IMPROVE EFFICIENCY The basic idea behind all the modifications to increase the thermal efficiency of a power cycle is the same: Increase the average temperature at which heat is transferred to the working fluid in the boiler, or decrease the average temperature at which heat is rejected from the working fluid in the condenser. Lowering the Condenser Pressure (Lowers Tlow,avg) To take advantage of the increased efficiencies at low pressures, the condensers of steam power plants usually operate well below the atmospheric pressure. There is a lower limit to this pressure depending on the temperature of the cooling medium Side effect: Lowering the condenser pressure increases the moisture content of the steam at the final stages of the turbine.The effect of lowering the condenser pressure on the ideal Rankine cycle.18
Superheating the Steam to High Temperatures (Increases Thigh,avg) Both the net work and heat input increase as a result of superheating the steam to a higher temperature. The overall effect is an increase in thermal efficiency since the average temperature at which heat is added increases. Superheating to higher temperatures decreases the moisture content of the steam at the turbine exit, which is desirable. Constraint: The temperature is limited byThe effect of superheating the metallurgical considerations. Presently thesteam to higher temperatures highest steam temperature allowed at theon the ideal Rankine cycle. turbine inlet is about 620 C. 19
Increasing the Boiler Pressure (Increases Thigh,avg) Today many modern steam power plants operate at supercritical pressures (P > 22.06 MPa) and For a fixed turbine inlet have thermal efficiencies of about temperature, the cycle shifts to the 40% for fossil-fuel plants and 34% left and the moisture content of for nuclear plants. steam at the turbine exit increases. This side effect can be corrected by reheating the steam.The effect of increasing the boiler A supercritical Rankine cycle.pressure on the ideal Rankine cycle. 20
THE IDEAL REHEAT CYCLEHow can we take advantage of the increased efficiencies at higher boiler pressureswithout facing the problem of excessive moisture at the final stages of the turbine?1. Superheat the steam to very high temperatures. It is limited metallurgically.2. Expand the steam in the turbine in two stages, and reheat it in between (reheat) The ideal reheat Rankine cycle. 21
The single reheat in a modern power plantimproves the cycle efficiency by 4 to 5% byincreasing the average temperature at whichheat is transferred to the steam.The average temperature during the reheatprocess can be increased by increasing thenumber of expansion and reheat stages. Asthe number of stages is increased, theexpansion and reheat processes approach anisothermal process at the maximumtemperature. The use of more than two reheatstages is not practical. The theoreticalimprovement in efficiency from the secondreheat is about half of that which results froma single reheat. The average temperature at which heat is transferred duringThe reheat temperatures are very close or reheating increases as theequal to the turbine inlet temperature. number of reheat stages isThe optimum reheat pressure is about one- increased.