Combined-cycle units use a gas turbine to generate electricity in a first stage. The hot exhaust gases from the gas turbine are then used to provide all or a portion of the heat for the boiler that produces steam to drive a steam generator turbine for further electricity generation.
This type of arrangement is more efficient than either the combustion turbines or steam generating units separately. The thermal efficiency of a combined cycle is around 50%. These units may have multiple gas turbines driving one steam turbine.