A thermal power station is a power plant in which heat energy is converted to electric power. In most of the world the prime mover issteam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources, fossil fuel dominates here, although nuclear heat energy and solar heat energy are also used. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy.[1] Certain thermal power plants also are designed to produce heat energy for industrial purposes of district heating, or desalination of water, in addition to generating electrical power. Globally, fossil-fuel power stationsproduce a large part of man-made CO2 emissions to the atmosphere, and efforts to reduce these are varied and widespread.
Almost all coal, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power plants are thermal. Natural gas is frequently combusted 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 plants burning coal, fuel oil, or natural gas are often called fossil-fuel power plants. Some biomass-fueled thermal power plants have appeared also. Non-nuclear thermal power plants, particularly fossil-fueled plants, which do not use co-generation are sometimes referred to as conventional power plants.
Commercial
electric utility power stations are usually constructed on a large scale and designed for continuous operation. Virtually all Electric power plants 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 plants to supply
heating or electricity to their facilities, especially if steam is created anyway for other purposes. Steam-driven power plants have been used to drive most ships in most of the 20th century until recently. Steam power plants are now only used in large nuclear
naval ships. Shipboard power plants usually directly couple the turbine to the ship's propellers through gearboxes. Power plants in such ships also provide steam to smaller turbines driving electric generators to supply electricity.
Nuclear marine propulsion is, with few exceptions, used only in naval vessels. There have been many
turbo-electric ships in which a steam-driven turbine drives an electric generator which powers an
electric motor for
propulsion.
Combined heat and power plants (CH&P plants), often called
co-generation plants, produce both electric power and heat for process heat or space heating. Steam and hot water
The initially developed reciprocating steam engine has been used to produce mechanical power since the 18th Century, with notable improvements being made by James Watt. When the first commercially developed central electrical power stations were established in 1882 at Pearl Street Station in New York and Holborn Viaduct power station in London, reciprocating steam engines were used. The development of the steam turbine in 1884 provided larger and more efficient machine designs for central generating stations. By 1892 the turbine was considered a better alternative to reciprocating engines;[2] turbines offered higher speeds, more compact machinery, and stable speed regulation allowing for parallel synchronous operation of generators on a common bus. After about 1905, turbines entirely replaced reciprocating engines in large central power stations.
The largest reciprocating engine-generator sets ever built were completed in 1901 for the Manhattan Elevated Railway. Each of seventeen units weighed about 500 tons and was rated 6000 kilowatts; a contemporary turbine set of similar rating would have weighed about 20% as much
The energy efficiency of a conventional thermal power station, considered salable energy produced as a percent of the
heating value of the fuel consumed, is typically 33% to 48%.
[citation needed][4] As with all heat engines, their efficiency is limited, and governed by the laws of
thermodynamics. By comparison, most
hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity.
[5]
The energy of a thermal not utilized in power production must leave the plant in the form of heat to the environment. This
waste heat can go through a
condenser and be disposed of with
cooling water or in
cooling towers. If the waste heat is instead utilized for
district heating, it is called
co-generation. An important class of thermal power station are associated with
desalination facilities; these are typically found in desert countries with large supplies of
natural gasand in these plants, freshwater production and electricity are equally important co-products.
The
Carnot efficiency dictates that higher efficiencies can be attained by increasing the temperature of the steam. Sub-critical fossil fuel power plants can achieve 36–40% efficiency.
Super critical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 4400 psi (30.3 MPa) and multiple stage reheat reaching about 48% efficiency. Above the
critical point for
water of 705 °F (374 °C) and 3212 psi (22.06 MPa), there is no
phase transition from water to steam, but only a gradual decrease in
density.
Currently most of the nuclear power plants must operate below the temperatures and pressures that coal-fired plants do, in order to provide more conservative safety margins within the systems that remove heat from the nuclear fuel rods. This, in turn, limits their thermodynamic efficiency to 30–32%. Some advanced reactor designs being studied, such as the
very high temperature reactor,
advanced gas-cooled reactor and
supercritical water reactor, would operate at temperatures and pressures similar to current coal plants, producing comparable thermodynamic efficiency.
The direct cost of electric energy produced by a thermal power station is the result of cost of fuel, capital cost for the plant, operator labour, maintenance, and such factors as ash handling and disposal. Indirect, social or environmental costs such as the economic value of environmental impacts, or environmental and health effects of the complete fuel cycle and plant decommissioning, are not usually assigned to generation costs for thermal stations in utility practice, but may form part of an environmental impact assessment.
Typical diagram of a coal-fired thermal power station
