A thermal power station is a power station in which heat energy is converted to electricity. Typically, heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam turbine connected to an electrical generator. The low-pressure exhaust from the turbine passes through a steam condenser and is recycled back to where it was heated. This is known as a Rankine cycle. Natural gas can also be burnt directly in a gas similarly connected to a generator.
Water power stations (which generate hydroelectricity) are excluded from this category, since they convert the potential energy of water into electricity via a water turbine. Photovoltaic and wind plants are likewise excluded.
The design of thermal power stations depends on the intended energy source: fossil fuel, nuclear and geothermal power, solar energy, biofuels, and waste incineration are all used. Certain thermal power stations are also designed to produce heat for industrial purposes, for district heating, or for desalination of water, in addition to generating electrical power. Almost all coal-fired power stations, petroleum, nuclear, geothermal, solar thermal electricity, and waste incineration plants, as well as all natural gas power stations, are thermal. Natural gas is frequently burned in gas turbines as well as boilers. The waste heat from a gas turbine, in the form of hot exhaust gas, can be used to raise steam by passing this gas through a heat recovery steam generator (HRSG). The steam is then used to drive a steam turbine in a combined cycle plant that improves overall efficiency. Power stations burning coal, fuel oil, or natural gas are often called fossil fuel power stations. Some biomass-fueled thermal power stations have appeared as well. Non-nuclear thermal power stations, particularly fossil-fueled plants that do not use co-generation, are sometimes referred to as conventional power stations.
Commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation. Virtually all electric power stations use three-phase electrical generators to produce alternating current (AC) electric power at a frequency of 50 Hz or 60 Hz. Large companies or institutions may have their own power stations to supply heating or electricity to their facilities, especially if steam is created anyway for other purposes. For the majority of the twentieth century, most ships were powered by steam-powered power stations [citation needed]. Shipboard power stations usually directly couple the turbine to the ship's propellers through gearboxes. Power stations in such ships also provide steam to smaller turbines, driving electric generators to supply electricity. With few exceptions, nuclear marine propulsion is used only in naval vessels. There have been many turbo-electric ships in which a steam-driven turbine drives an electric generator that powers an electric motor for propulsion.
Co-generation plants, often called combined heat and power (CHP) facilities, produce both electric power and heat for process heat or space heating, such as steam and hot water.
Almost all coal-fired power stations, petroleum, nuclear, geothermal, solar thermal electricity, and waste incineration plants, as well as all natural gas power stations, are thermal.
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