Power station

A power station or power plant is a facility for the generation of electric power. 'Power plant' is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term "energy center" because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, into electrical energy. However, "power plant" remains the most commonly used term in American English, while "power station" is the most common term in Commonwealth English.

At the centre of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.

Thermal power stations

In thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power plants produce steam, and these are sometimes called steam power plants. Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, thermal power plants also produce low-temperature heat. If no use is found for the heat, it is lost to the environment. If reject heat is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant.

Classification

Thermal power plants are classified by the type of fuel and the type of prime mover installed. Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator. Fossil fueled power plants may also use a steam turbine generator. Natural gas fired plants may use a combustion turbine, known as a 'peaker', which provides rapid startup for peak loads, although older ones typically use steam turbine generators and must be started and stopped slowly. Combined cycle plants have a gas turbine fired by natural gas, with a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and most new baseload power plants are combined cycle plants fired by natural gas. A relatively small amount of electric power is produced by internal combustion piston engines, also called reciprocating engines. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas plants. Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals and other critical facilities also use them to provide backup power in case of a power outage. Microturbines and reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

Cooling towers

Cooling towers are huge hyperbolic chimney-like structures that release the waste heat to the atmosphere. The very large cooling towers at nuclear power plants are often mistaken for the reactors, which are actually contained in thick, reinforced concrete containment domes that are typically much smaller. Cooling towers may be employed at any type of thermal power station, but large towers are found at either nuclear or fossil fueled stations. Geothermal, biomass and waste to energy plants tend to use smaller cooling towers. Whenever possible, electric companies prefer to use cooling water from the ocean or a nearby lake or river. The water is taken in, heated by the heat exchanger, and discharged back into the water body. This type of cooling is less costly and more efficient, and it also saves a substantial amount of fresh water from being used to cool the plant provided wet cooling towers are being used. However, it can cause the temperature of the water to rise significantly. Although a small percentage of plants use reclaimed water, some people worry about the organisms killed when they are sucked into the water intake and the effects of the increased water temperature on the aquatic environment.

Other sources of energy

Other power stations use the energy from wave or tidal motion, wind, sunlight or the energy of falling water, hydroelectricity.

Hydroelectricity

Hydroelectric dams impound a reservoir of water and release it through one or more water turbines to generate electricity.

Pumped storage

A pumped storage hydroelectric power plant is a net consumer of energy but increases the value of electricity. Water is pumped to a high reservoir during the night when the demand, and price, for electricity is low. During hours of peak demand, when the price of electricity is high, the stored water is released to produce electric power. Some pumped storage plants are actually not net consumers of electricity because they release some of the water from the lower reservoir downstream, either continuously or in bursts.

Solar

A solar photovoltaic power plant converts sunlight directly into electrical energy, which may need conversion to alternating current for transmission to users. This type of plant does not use rotating machines for energy conversion. Solar thermal electric plants are another type of solar power plant. They direct sunlight using either parabolic troughs or heliostats. Parabolic troughs direct sunlight onto a pipe containing a heat transfer fluid, such as oil, which is then used to boil water, which turns the generator. The central tower type of power plant uses hundreds or thousands of mirrors, depending on size, to direct sunlight onto a receiver on top of a tower. Again, the heat is used to produce steam to turn turbines. There is yet another type of solar thermal electric plant. The sunlight strikes the bottom of the pond, warming the lowest layer which is prevented from rising by a salt gradient. A rankine cycle engine exploits the temperature difference in the layers to produce electricity. Not many solar thermal electric plants have been built. Most of them can be found in the Mojave Desert, although Sandia National Laboratory, Israel and Spain have also built a few plants.

Wind

Wind turbines can be used to generate electricity in areas with strong, steady winds. Many different designs have been used in the past, but all modern turbines being produced today use the Dutch three-bladed, upwind design. The newer turbines produce power much more cheaply and are more reliable and efficient. They are also much larger, so it takes fewer turbines to produce the same amount of power and the blades move far slower, which is less visually distracting. However, the old turbines can still be seen at some wind farms, particularly at Altamont Pass and Tehachapi Pass.

Electrical generation

Most power stations are designed to generate AC power with three phases of 50 Hz or 60 Hz. The United States, Canada, the Caribbean, parts of Latin America and many Pacific Islands use a frequency of 60 Hz. Europe and continental Asian countries use 50 Hz. In some power stations in Germany, Austria and Switzerland ( traction powerstation) there is a second generator for single phase AC current with 16.7 Hz for the traction power network. In a few places (such as Brazil), both 50 Hz and 60 Hz generators are installed. Sometimes high voltage direct current lines are used when connecting power grids operating at different frequency or for other reasons related to power flow.