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