Vapor and Combined
Power Cycles
The steam cycle and more…
Carnot Cycle
� The standard all others are measured against
� Not realistic model for vapor cycles
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)
Rankine Cycle, Ideal
Rankine Cycle Energy Analysis
� Energy balance, each process
� For pump
Rankine Cycle Energy Analysis
� For boiler
� For turbine
� For condenser
Rankine Cycle Energy Analysis
� Thermal efficiency
� Heat rate: amount of heat (Btu) to
generate 1 kWh of electricity
Real vs. Ideal Cycle
Real vs. Ideal Cycle
� Major difference is irreversibilities in pump and turbine
Increase Efficiency?
� Lower condenser pressure
� Increase superheattemperature
Increase Efficiency?
� Increase boiler pressure
Reheat
� Materials limit temperature of steam, but can we take advantage of higher steam pressures and not have quality of steam issues?
Reheat
� Equations become:
� Purposes of reheat: keep turbine inlet
temps within limits, increase quality of steam in last stages of turbine
Ideal Regenerative Rankine Cycle
� Regeneration: effective use of
energy
� Open (direct contact)
feedwater heaters
(mixing chambers)
� Closed feedwater heaters (heat
exchangers)
Ideal Regenerative Rankine Cycle
Ideal Regenerative Rankine Cycle
Ideal Regenerative Rankine Cycle
2nd Law Analysis
� Ideal Rankine cycle is internally reversible
� Analysis indicates where irreversibilities are
� Again for steady-flow system:
2nd Law Analysis
� For a cycle:
Cogeneration
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