Vapor and Combined Power Cycles

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Vapor and Combined Power Cycles

• The steam cycle and more

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Carnot Cycle

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

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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)

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Rankine Cycle, Ideal
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Rankine Cycle Energy Analysis

• Energy balance, each process
• For pump

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Rankine Cycle Energy Analysis

• For boiler
• For turbine
• For condenser

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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
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Real vs. Ideal Cycle

• Major difference is irreversibilities in pump and
  turbine

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Increase Efficiency?

• Lower condenser pressure
• Increase superheat temperature

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Increase Efficiency?

• Increase boiler pressure

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Reheat

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

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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)

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Ideal Regenerative Rankine Cycle
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Ideal Regenerative Rankine Cycle
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Ideal Regenerative Rankine Cycle
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2nd Law Analysis

• Ideal Rankine cycle is internally reversible
• Analysis indicates where irreversibilities are
• Again for steady-flow system

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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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