Combined Cycle

1 min read Last updated Sat Jun 27 2026 08:46:52 GMT+0000 (Coordinated Universal Time)

Exhaust heat from a gas turbine (Brayton cycle) is recovered to drive a steam turbine (Rankine cycle). Higher total efficiency than either cycle alone, often exceeding 55%.

Energy Relation

Heat rejected from the Brayton cycle becomes the heat input for the Rankine cycle.

Q_\text{Rankine,in} = Q_\text{Brayton,out}

No additional fuel is burned in the heat recovery stage.

Efficiency

Let $\eta_B$ and $\eta_R$ be the Brayton and Rankine thermal efficiencies.

\eta_\text{combined} = 1 - (1 - \eta_B)(1 - \eta_R)

Combined efficiency exceeds either individual cycle because waste heat from the topping cycle does useful work in the bottoming cycle.

Heat Recovery Steam Generator

The Heat Recovery Steam Generator (HRSG) transfers exhaust heat from the gas turbine to produce steam for the Rankine cycle.

Examples:

Natural gas combined cycle (NGCC) power plants
Co-generation plants producing heat and electricity simultaneously

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