The combined-cycle power plant consists of two means of generation: combustion turbine and steam turbine. The combustion turbine is similar to a jet engine whose high-temperature and high-pressure exhaust spins a turbine whose shaft is connected to a generator. The hot exhaust is then coupled through a heat recovery steam generator (HRSG) that is used to heat water, thus producing steam to drive a secondary steam turbine generator. The combustion turbine typically uses natural gas as the fuel to drive the turbine blades.
 The advantage of a combined-cycle (CC) system is that in addition to the electrical energy produced by the fuel combustion engine, the exhaust from the engine also produces electrical energy. Another potential benefit of CC plants is that the end user can have steam made available to assist in other functions such as building heat and hot water and production processes that
 require steam (such as paper mills). Therefore, from one source of fuel (i.e., natural gas), many energy services are provided (electrical energy, steam, hot water, and building heat). Some CCs can reach efficiencies near 90%.
 Figure shows a combined-cycle power plant.