Internal combustion units include stationary gas turbines, also known as combustion turbines, and reciprocating internal combustion engines. These units are generally less than 100 megawatts in size and they are considered to be less efficient than steam turbines.
However, since the gas turbine generators do not have boilers or a steam supply and condensers, the capital costs are much lower for a gas turbine unit than for a steam electric unit.
Gas turbine units have quick startup times, compared with steam-turbine units and, because of their relatively small size, they can be installed in a variety of sites, which make them suitable for generation at peak times when demand exceeds the installed capacity of major power stations or for emergency and reserve power requirements.
An important amount of heat is carried away with the exhaust gases that are emitted to the atmosphere from the turbine. Gas turbines are mainly used in the so-called combined cycle plants.