A combined cycle couples a high-temperature gas-turbine cycle to a lower-temperature vapor-power cycle so that energy rejected by the first becomes useful heat input to the second.

In a common arrangement, a Brayton-cycle gas turbine produces power first. Its hot exhaust then passes through a heat-recovery steam generator that supplies a Rankine-cycle steam turbine.

If fuel supplies heat rate $\dot Q_{in}$ and the gas and steam systems produce net powers $\dot W_G$ and $\dot W_S$, the overall thermal efficiency is

$$\eta=\frac{\dot W_G+\dot W_S}{\dot Q_{in}}.$$

The key is not that energy can be reused without limit. The exhaust leaves the gas turbine at a lower temperature, but it can still transfer useful heat to a bottoming cycle before final rejection to the environment.

Combined cycles illustrate system-level thermodynamic design: matching the temperature ranges of coupled subsystems can increase total work output even though each component obeys the same first- and second-law constraints as before.