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

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

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

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.

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:

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

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

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

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

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

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