Vapor Power Cycles

1
Vapor Power Cycles

• Reading Cengel Boles, Chapter 9

2
Vapor Power Cycles

• Produce over 90 of the worlds 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 gt Tambient
• Pcond lt 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. 620C
• 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