In combined cycle power plants, gas turbines are combined with steam turbines. Once the gas has flown through the gas turbines, the remaining heat in the exhaust gas is recovered using a heat exchanger in order to operate a steam process. The core of a combined cycle power plant is the gas turbine with a compressor that draws in ambient air and compresses it to a pressure between 20 and 30 bar. In the combustor, the natural gas is combusted with this compressed air and this creates temperatures of up to 1,500 °C. In the turbine, the hot exhaust gas then expands to the ambient pressure and is then partly used to drive the compressor but is mainly used to drive the generator.
In a combined cycle power plant, the exhaust gas must still have a temperature greater than 600 °C after passing through the gas turbine in order to ensure an economic steam process. It is then fed through a heat recovery boiler (heat exchanger) before being released to the environment through a chimney. The heat recovery boiler generates steam that is compressed to several hundred bar, which then expands in the steam turbine before condensed in a condenser. The collected water is then pumped under pressure and re-fed to the heat exchanger. This creates a closed water/steam cycle.
