2 We consider power cycles where the working fluid undergoes a phase change. The best example of this cycle is the steam power cycle where water (steam) is the working fluid.Carnot Vapor Cycle
3 The heat engine may be composed of the following components. The working fluid, steam (water), undergoes a thermodynamic cycle from The cycle is shown on the following T-s diagram.
4 The thermal efficiency of this cycle is given as Note the effect of TH and TL on th, Carnot.The larger the TH the larger the th, CarnotThe smaller the TL the larger the th, CarnotNimymech\CarnotCycle\shell.swf
5 To increase the thermal efficiency in any power cycle, we try to increase the maximum temperature at which heat is added.Reasons why the Carnot cycle is not used:Pumping process 1-2 requires the pumping of a mixture of saturated liquid and saturated vapor at state 1 and the delivery of a saturated liquid at state 2.To superheat the steam to take advantage of a higher temperature, elaborate controls are required to keep TH constant while the steam expands and does work.To resolve the difficulties associated with the Carnot cycle, the Rankine cycle was devised.
6 RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLESThe simple ideal Rankine cycle.
7 Energy Analysis of the Ideal Rankine Cycle Steady-flow energy equationThe efficiency of power plants in the U.S. is often expressed in terms of heat rate, which is the amount of heat supplied, in Btu’s, to generate 1 kWh of electricity.The thermal efficiency can be interpreted as the ratio of the area enclosed by the cycle on a T-s diagram to the area under the heat-addition process.nimymech\steamPowerPlant.swf
8 To increase system efficiency Lower condenser pressureMust have at least 10°C DT between condenser and cooling water or air temperature for effective heat transferWatch quality at exit to prevent turbine problems (shouldn’t go less than about 88%)Superheat the steam moreTmax ~ 620° due to metallurgical considerationsIncrease boiler pressure (with same Tmax)Pmax ~ 30 Mpa4. Reheat of steamRegeneration
9 HOW CAN WE INCREASE THE EFFICIENCY OF THE RANKINE CYCLE? The basic idea behind all the modifications to increase the thermal efficiencyof 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 mediumSide 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.
10 Effect of Lowering Condenser Pressure on the Ideal Rankine cycle
11 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.The temperature is limited by metallurgical considerations. Presently the highest steam temperature allowed at the turbine inlet is about 620°C.The effect of superheating the steam to higher temperatures on the ideal Rankine cycle.
12 Increasing the Boiler Pressure (Increases Thigh,avg) For a fixed turbine inlet temperature, the cycle shifts to the left and the moisture content of steam at the turbine exit increases. This side effect can be corrected by reheating the steam.A supercritical Rankine cycle.Today many modern steam power plants operate at supercritical pressures (P > MPa) and have thermal efficiencies of about 40% for fossil-fuel plants and 34% for nuclear plants.The effect of increasing the boiler pressure on the ideal Rankine cycle.
13 Effect of Increasing Boiler Pressure on the Ideal Rankine cycle
14 Reheat CycleAllows us to increase boiler pressure without problems of low quality at turbine exit
15 Rankine Cycle with Reheat Component Process First Law ResultBoiler Const. P qin = (h3 - h2) + (h5 - h4)Turbine Isentropic wout = (h3 - h4) + (h5 - h6)Condenser Const. P qout = (h6 - h1)Pump Isentropic win = (h2 - h1) = v1(P2 - P1)The thermal efficiency is given byNimymech\RankineCycleReheating\Source\shell.swfNimymech\Animation\ReheatCycle.swf
16 THE IDEAL REHEAT RANKINE CYCLE How can we take advantage of the increased efficiencies at higher boiler pressures without 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.
17 RegenerationPreheats steam entering boiler using a feedwater heater, improving efficiencyAlso deaerates the fluid and reduces large volume flow rates at turbine exit.
20 Open Feedwater Heaters An open (or direct-contact) feedwater heater is basically a mixing chamber, where the steam extracted from the turbine mixes with the feedwater exiting the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure.The ideal regenerative Rankine cycle with an open feedwater heater.