Aka. steam power cycle. Energy of a vapor is converted to mechanical work output. The working fluid experiences phase transition between liquid and gas. Used in steam power plants. Highly pressurized and high temperature steam is the most common working fluid.
Rankine cycle is a vapor power cycle.
Analysis
T-S diagram is used. The phase transition allows isothermal heat addition and rejection at constant pressure, distinguishing vapor cycles from gas cycles.
On the T-S diagram:
- Heat addition occurs along the saturation dome (isothermal in the two-phase region)
- The enclosed area represents net work output
- Superheat extends the cycle above the dome to raise turbine inlet temperature
Advantages over Gas Cycles
- Phase transition enables large enthalpy changes at constant temperature
- Condensation allows the pump to handle liquid, requiring far less work than compressing a gas
- Steam has high latent heat, increasing energy input per unit mass
Efficiency
Limited by the Carnot efficiency at the operating temperature range. Real cycles are less efficient due to irreversibilities in the turbine and pump.