Vapor Power Cycles

1. Vapor Power Cycles Reading: Cengel & Boles, Chapter 9

2. Vapor Power Cycles • Produce over 90% of the world’s electricity • Four primary components • boiler: heat addition • turbine: power output • condenser: heat rejection • pump: increasing fluid pressure • Heat sources • combustion of hydrocarbon fuel • e.g., coal, natural gas, oil, biomass • nuclear fission or fusion • solar energy • geothermal energy • ocean thermal energy

3. Carnot Vapor Power Cycle • Consists of four reversible processes inside the vapor dome (see Figure 9-1 in text) and yields maximum • Carnot vapor power cycle is not a practical model since • isothermal heat addition can only occur at temperatures less than Tcr • pumps or compressors cannot handle two-phase mixtures efficiently • turbines suffer severe blade erosion from liquid droplets in two-phase mixtures

4. The Rankine Cycle • The Rankine cycle serves as a more practical ideal model for vapor power plants: • pumping process is moved to the compressed liquid phase • boiler superheats the vapor to prevent excessive moisture in the turbine expansion process • Steam (H2O) is, by far, the most common working fluid; however, low boiling point fluids such as ammonia and R-134a can be used with low temperature heat sources.

5. Analysis of Rankine Power Cycles • Typical assumptions: • steady-state conditions • negligible KE and PE effects • negligible P across boiler & condenser • turbine, pump, and piping are adiabatic • if cycle is considered ideal, then turbine and pump are isentropic • Energy balance for each device has the following general form:

6. Analysis of Rankine Power Cycles, cont. • Pump (q = 0) • Boiler (w = 0) • Turbine (q = 0)

7. Analysis of Rankine Power Cycles, cont. • Condenser (w = 0) • Thermal Efficiency • Back Work Ratio (rbw)

8. Increasing Rankine Cycle Efficiency • It can be shown that • To increase cycle efficiency, want: • high average boiler temperature, which implies high pressure • low condenser temperature, which implies low pressure • This holds true for actual vapor power cycles as well

9. Increasing Rankine Cycle Efficiency, cont. • Methods used in all vapor power plants to increase efficiency: 1) Use low condenser pressure • decreases Tout • limitation: Tout > Tambient • Pcond < Patm requires leak-proof system • increases moisture content in turbine 2) Use high boiler pressure • increases Tin • limitation: approx. 30 MPa • increases moisture content in turbine

10. Increasing Rankine Cycle Efficiency, cont. 3) Superheat vapor in boiler to high temperature • increases Tin • limitation: approx. 620°C • decreases moisture content in turbine 4) Use multistage turbine with reheat • allows use of high boiler pressures without excessive moisture in turbine • limitation: adds cost, but 2-3 stages are usually cost-effective

11. Increasing Rankine Cycle Efficiency, cont. 5) Preheat liquid entering boiler using feedwater heaters (FWHs) • bleed 10-20% of steam from turbine and use to preheat boiler feedwater • limitation: adds cost, but as many as 6-8 units are often cost-effective • open feedwater heaters: steam directly heats feedwater in a mixing chamber; can also be used to deaerate the water • closed feedwater heaters: steam indirectly heats feedwater in a heat exchanger; condensed steam is routed to condenser or a lower pressure FWH