A gas and steam combined cycle power plant or gas and steam turbine power plant (gas and steam power plant for short) is a power plant in which the principles of a gas turbine power plant and a steam power plant are combined. A gas turbine serves as a heat source for a downstream waste heat boiler, which in turn acts as a steam generator for the steam turbine.
This combined mode of operation achieves a higher level of efficiency in the thermodynamic cycle than gas turbines in open operation or in conventionally fired steam power plants. With electrical efficiencies of up to 60%, combined cycle power plants are among the most efficient conventional power plants (see gas-fired power plant).
Principle combined cycle power plant
Combined cycle power plants can be used very flexibly to generate their own electricity for commercial and industrial companies, regional suppliers (municipal utilities) and in power plant management: thanks to short start-up times and the possibility of rapid load changes, they are ideal base and medium load power plants for production companies and municipal utilities. For energy supply companies, these power plants are primarily operated in the medium load range and even in the peak power range if required.
In the English-speaking world, this is referred to as a combined cycle power plant (CCPP) or combined cycle gas turbine (CCGT).
In the GUD power plant, electricity is generated with one to four gas turbines and a steam turbine, whereby either each turbine drives a generator (multi-shaft plant) or a gas turbine with the steam turbine (can be uncoupled) on a common shaft drives the generator (single-shaft plant). The hot exhaust gases from the gas turbines are used to generate steam in a heat recovery steam boiler. The steam is then expanded using a conventional steam turbine process. The gas turbine accounts for approx. 2/3 of the electrical power and the steam process for 1/3.
For smaller plant capacities up to approx. 5 MWel, an evaporator and a steam turbine can also be used to increase the electrical efficiency instead of the heat recovery steam boiler and ORC (Organic Rankine Cycle) turbine can also be installed downstream of the gas turbine.Gaseous or liquid fuels such as natural gas, biogas or fuel oil are used to operate the gas turbine.
Source : Wikipedia http://de.wikipedia.org/wiki/GuD