1. Thermal_Power_Plant_2 Prepared by: NMG

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, Carnot
The smaller the TL the larger the th, Carnot
Nimymech\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 CYCLES
The simple ideal Rankine cycle.

7. Energy Analysis of the Ideal Rankine Cycle
Steady-flow energy equation
The 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 pressure
Must have at least 10°C DT between condenser and cooling water or air temperature for effective heat transfer
Watch quality at exit to prevent turbine problems (shouldn’t go less than about 88%)
Superheat the steam more
Tmax ~ 620° due to metallurgical considerations
Increase boiler pressure (with same Tmax)
Pmax ~ 30 Mpa
4. Reheat of steam
Regeneration

9. HOW CAN WE INCREASE THE EFFICIENCY OF THE RANKINE CYCLE?
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.

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 Cycle
Allows us to increase boiler pressure without problems of low quality at turbine exit

15. Rankine Cycle with Reheat
Component Process First Law Result
Boiler 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 by
Nimymech\RankineCycleReheating\Source\shell.swf
Nimymech\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. Regeneration
Preheats steam entering boiler using a feedwater heater, improving efficiency
Also deaerates the fluid and reduces large volume flow rates at turbine exit.

19. nimymech\openFWH.swf
nimymech\RankineCycleRegeneration\shell.swf

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.

21. Simple Rankine Cycle
nimymech\steamPowerPlant.swf
Reheat
Nimymech\RankineCycleReheating\Source\shell.swf
Nimymech\Animation\ReheatCycle.swf
Regeneration
nimymech\openFWH.swf
nimymech\RankineCycleRegeneration\shell.swf