1. Reading: Cengel & Boles, Chapter 9
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

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