A means for converting stored energy into work. Stationary power plants such as electric generating stations are located near sources of stored energy, such as coal fields or river dams, or are located near the places where the work is to be performed, as in cities or industrial sites. Power plants range in capacity from a fraction of a horsepower (hp) to over 106 kW in a single unit. Large power plants are assembled, erected, and constructed on location from equipment and systems made by different manufacturers. Smaller units are produced in manufacturing facilities.
Most power plants convert part of the stored raw energy of fossil fuels into kinetic energy of a spinning shaft. Some power plants harness nuclear energy. Elevated water supply or run-of-the-river energy is used in hydroelectric power plants. For transportation, the plant may produce a propulsive jet, as in some aircraft, instead of the rotary motion of a shaft. Other sources of energy, such as fuel cells, winds, tides, waves, geothermal, ocean thermal, nuclear fusion, photovoltaics, and solar thermal, have been of negligible commercial significance in the generation of power despite their magnitudes.
There is no practical way of storing the mechanical or electrical output of a power plant in the magnitudes encountered in power plant applications, although several small-scale concepts have been researched. As of now, however, the output must be generated at the instant of its use. This results in wide variations in the loads imposed upon a plant. The capacity, measured in kilowatts or horsepower, must be available when the load is imposed. Much of the capacity may be idle during extended periods when there is no demand for output. Hence much of the potential output, measured as kilowatt-hours or horsepower-hours, cannot be generated because there is no demand for output. Kilowatts cannot be traded for kilowatt-hours, and vice versa. See also Energy storage.
The efficiency of energy conversion is vital in most power plant installations. With thermal power plants the basic limitations of thermodynamics fix the efficiency of converting heat into work. The cyclic standards of Carnot, Rankine, Otto, Diesel, and Brayton are the usual criteria on which heat-power operations are variously judged. Performance of an assembled power plant, from fuel to net salable or usable output, may be expressed as thermal efficiency (%); fuel consumption (lb, pt, or gal per hp-h or per kWh); or heat rate (Btu supplied in fuel per hp-h or per kWh). American practice uses high or gross calorific value of the fuel for measuring heat rate or thermal efficiency and differs in this respect from European practice, which prefers the low or net calorific value.
In scrutinizing data on thermal performance, it should be recalled that the mechanical equivalent of heat (100% thermal efficiency) is 2545 Btu/hp-h and 3413 Btu/kWh (3.6 megajoules/kWh). Modern steam plants in large sizes (75,000–1,300,000 kW units) and internal combustion plants in modest sizes (1000–20,000 kW) have little difficulty in delivering a kilowatt-hour for less than 10,000 Btu (10.55 MJ) in fuel (34% thermal efficiency). For condensing steam plants, the lowest fuel consumptions per unit output (8200–9000 Btu/kWh or 8.7–9.5 MJ/kWh) are obtained in plants with the best vacuums, regenerative-reheat cycles using eight stages of extraction feed heating, two stages of reheat, primary pressures of 4500 lb/in.2 gage or 31 megapascals gage (supercritical), and temperatures of 1150°F (620°C). An industrial plant cogenerating electric power with process steam is capable of having a thermal efficiency of 5000 Btu/kWh (5.3 MJ/kWh).
Combustion turbines used in combined cycle configurations have taken a dominant role in new power generation capacity. The reason is the higher efficiency and lower emissions of the power plant in this arrangement. The rapid pace in advances in combustion turbine technology (such as higher firing temperatures that improve the Brayton cycle efficiency) has driven combined cycle efficiency to nearly 60% when using natural gas as fuel, while attaining low emission rates. Low fuel consumption (5700–6000 Btu/kWh or 6.0–6.3 MJ/kWh) is obtained by using higher firing temperatures, steam cooling on the combustor and gas turbine blades, a reheat steam cycle with a three-pressure heat recovery steam generator, and higher pressure and temperature of the steam cycle. These conditions are balanced with the need to keep the exhaust flue gas temperature as low as practical to achieve low emissions.
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