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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

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vapor power cycles

Vapor Power Cycles

Reading: Cengel & Boles, Chapter 9

vapor power cycles1
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
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
  • 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
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
Analysis of Rankine Power Cycles, cont.
  • Pump (q = 0)
  • Boiler (w = 0)
  • Turbine (q = 0)
analysis of rankine power cycles cont1
Analysis of Rankine Power Cycles, cont.
  • Condenser (w = 0)
  • Thermal Efficiency
  • Back Work Ratio (rbw)
increasing rankine cycle efficiency
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
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
increasing rankine cycle efficiency cont1
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 cont2
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