What is a combined cycle plant?

Combined cycle power plants transform the thermal energy of natural gas into electricity through the joint work of a gas turbine and a steam turbine. The process involves the start-up of two consecutive cycles: the Brayton cycle, which corresponds to the conventional gas turbine, and the Rankine cycle, which operates with the steam turbine.

Combined cycle power plants are characterized by being more flexible than conventional ones. This means that it can operate at full load or part loads, down to a minimum of approximately 45% of maximum power. In addition, they are more efficient -greater efficiency for a wider power range-, their emissions are lower, the consumption of cooling water is lower and they save energy in the form of fuel. Finally, they are built faster than other plants and require less surface area per installed MW, which reduces the visual impact.

Parts of a combined cycle plant

The combined cycle power plant is mainly made up of three elements: gas turbine, steam turbine and recovery boiler. Next, we break down the details of each one of them.

The gas turbine is the fundamental part of the combined cycle. It is an internal combustion turbine formed mainly by:

• Compressor. Its function is to inject pressurized air for the combustion of gas and cooling of hot areas.
• Combustion chamber. Place where natural gas is mixed with pressurized air, producing combustion.
• Turbine. Space where the expansion of gases from the combustion chamber occurs. It consists of three or four stages of expansion. The temperature of the inlet gases can reach 1,400ºC, while the outlet gases reach 600ºC.

Steam turbine. It transforms the energy of the steam into kinetic energy of the rotor. It usually consists of three bodies and is based on conventional technology. It is very common for the steam turbine and the gas turbine to be coupled to the same shaft, so that they drive the same electric generator.

Another fundamental element of combined cycle plants is the recovery boiler. It is a conventional boiler where the heat from the gases that come from the gas turbine is used in a water-steam cycle.

Source: ENDESA Foundation