In combined cycle power plant Gas turbine and steam cycle work together. Steam Turbine runs on Rankin Cycle and Gas Turbine runs on Brayton Cycle. The diagram of the cycle is shown here.
· Fuel is burned on the starting stage of gas turbine.
· Fuel is compressed and combustion takes place in the combustion chamber.
· These combustion product which is a very high temperature gas mixture goes through the blades of the gas turbine. The gas turbine blades move and produces work by running the generator. The portion of the energy from the combustion products is converted to electrical energy.
· The waste heat or the exhaust gas is used to run the steam turbine. The waste heat is recovered by the heat recovery steam generator (HRSG) or boiler. These HRSG produces steam and this steam is directed towards the steam turbine blades. As a result the steam turbine moves and produces some more electricity.
In these combined cycle power plants topping cycle is the gas turbine or Brayton cycle and the bottoming cycle is the Steam Turbine Cycle or Rankin Cycle. The cycle 1-2-3-4-1 shows the topping cycle (GT cycle) which represents that heat and work transfers take place at higher temperature. The Steam Turbine Cycle a-b-c-d-e-f-a shows that it operates at lower temperature. It is known as the bottoming cycle. In constant pressure process 4-1 (here gas turbine rejects heat) the waste heat of the exhaust is used in a heat recovery steam generator.
In the processes a to b , b to c and c to d some of this waste heat from the GT exhaustgas is absorbed by the feed water as well as wet and super-heated steam.