Power generation is still heavily dependent on the steam turbine. Even gas turbine power stations usually are provided with a waste heat boiler and steam turbine to maximize the efficiency with which fuel is converted into electricity. Steam turbines operate in the Rankine thermodynamic cycle: Work is put into the system to pressurize water to boiler pressure. Then, heat is put in to evaporate the water to produce high pressure steam. The high pressure steam then is expanded through the turbine to obtain a useful work output. Finally, heat must be rejected to condense the low pressure steam back to water. This takes place in a condenser, which usually is water cooled. The cooling water may be seawater (if the power station is on the coast) or fresh water from a river, borehole or town supply. Turbines designed with fresh water cooling can have a single-pass, once-through system design with direct discharge. Alternatives include single-pass designs with evaporative cooling prior to discharge or a design where the cooling water is recycled via evaporative cooling towers.
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