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Vapor and Combined Power Cycles

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Ideal Regenerative Rankine Cycle. Regeneration: effective use of energy ... Ideal Regenerative Rankine Cycle. 2nd Law Analysis. Ideal Rankine cycle is ... – PowerPoint PPT presentation

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Title: Vapor and Combined Power Cycles


1
Vapor and Combined Power Cycles
  • The steam cycle and more

2
Carnot Cycle
  • The standard all others are measured against
  • Not realistic model for vapor cycles

3
Rankine Cycle, Ideal
  • 1-2 isentropic compression (pump)
  • 2-3 constant pressure heat addition (boiler)
  • 3-4 isentropic expansion (turbine)
  • 4-1 constant pressure heat rejection (condenser)

4
Rankine Cycle, Ideal
5
Rankine Cycle Energy Analysis
  • Energy balance, each process
  • For pump

6
Rankine Cycle Energy Analysis
  • For boiler
  • For turbine
  • For condenser

7
Rankine Cycle Energy Analysis
  • Thermal efficiency
  • Heat rate amount of heat (Btu) to generate 1 kWh
    of electricity

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Real vs. Ideal Cycle
15
Real vs. Ideal Cycle
  • Major difference is irreversibilities in pump and
    turbine

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Increase Efficiency?
  • Lower condenser pressure
  • Increase superheat temperature

21
Increase Efficiency?
  • Increase boiler pressure

22
Reheat
  • Materials limit temperature of steam, but can we
    take advantage of higher steam pressures and not
    have quality of steam issues?

23
Reheat
  • Equations become
  • Purposes of reheat keep turbine inlet temps
    within limits, increase quality of steam in last
    stages of turbine

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Ideal Regenerative Rankine Cycle
  • Regeneration effective use of energy
  • Open (direct contact) feedwater heaters (mixing
    chambers)
  • Closed feedwater heaters (heat exchangers)

31
Ideal Regenerative Rankine Cycle
32
Ideal Regenerative Rankine Cycle
33
Ideal Regenerative Rankine Cycle
34
2nd Law Analysis
  • Ideal Rankine cycle is internally reversible
  • Analysis indicates where irreversibilities are
  • Again for steady-flow system

35
2nd Law Analysis
  • For a cycle

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Cogeneration
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Combined Gas-Vapor Power Cycle
  • Use of two cycles to maximize efficiency
  • Gas power cycle topping a vapor power cycle
  • Combined cycles have higher efficiency than
    either independently
  • Works because
  • Gas turbine needs high combustion temp to be
    efficient, vapor cycle can effectively use
    rejected energy

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