Combined cycle power unit is combined with gas turbine generator (Brayton cycle) and steam turbine generator (Rankine cycle). The gas turbine drives the generator generating electricity, then the residue energy contained in exhausted flue gas is recovered by heat recovery steam generator (HRSG). The steam generated by HRSG rotates the blades in steam turbine and drives the generator to generate electricity.

If the unit is designed to equip with a bypass stack, the flue gas exhausts from gas turbine and passes via bypass stack to atmosphere. In this operation mode, HRSG and steam turbine are stopped running, just the gas turbine operates independently, which is known as “simple cycle power generation”. As combined with steam cycle, which is known as “combined cycle operation”. Except the advantages of high efficiency, fast start-stop and fast load response, the combined cycle power unit is more friendly to environment due to less pollutant emission by usage of natural gas or light diesel as fuel. The cost of power generation for combined cycle power unit is higher than that of coal-fired steam power unit and nuclear power unit. Therefore, in the whole power generation system, it plays as the main role of middle load power supplier.