Production of electrical and thermal energy will be performed in so called combined cycle power plant, which refers to the process composed of gas turbine and steam turbine part. Main parts, of course, are gas turbines and steam turbine. The main purpose of this kind of plant is to use the heat produced on the output of gas turbines. As exhaust gases from gas turbines have extremely high temperatures, around 600°C, it can be used to heat up water and to produce water steam for steam turbine. In that way, efficiency of the whole plant is increasing and can reach up to 60% efficiency.
In combined plant the compressor compress the air and transmit it to combustion section together with combustion fuel. Exhaust gases of very high temperature are lead from combustion section to the steam turbine where expanding, providing useful work on shaft connected to the rotor of gas turbine. Shaft initiates electricity generator and produces electricity which is transferred to the grid. After expansion, exhaust gases from gas turbines go to HRSG (Heat recovery steam generators).
One of a very good characteristics of gas turbine is that there is a very high ratio of air/fuel as it must be added several times more of the air for cooling of gas turbine blades. Therefore on the outlet of gas turbine remain a lot of unused air and for that reason this excess of the air is using for combustion of additional fuel in HRSG. In HRSG supplied water is heating to vaporization and overheating to anticipated parameters. This hot steam is passing from HRSG to steam turbine where expands and deliver mechanical work to generator of electricity. Afterwards, the steam, now with low parameters, goes into the condenser where condensates. After condensation, the water returns into the HRSG by water supply pumps for the re-heating.