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power, electric,energy dissipated in an electrical or electronic circuit or device per unit of time. The electrical energy supplied by a current to an appliance enables it to do work or provide some other form of energy such as light or heat. Electric power is usually measured in WattsWatts,
residential section of south central Los Angeles. Named after C. H. Watts, a Pasadena realtor, the section became part of Los Angeles in 1926. Artist Simon Rodia's celebrated Watts Towers are there.
..... Click the link for more information. , kilowatts (1,000 watts), and megawatts (1,000,000 watts). The amount of electrical energy used by an appliance is found by multiplying its consumed power by the length of time of operation. The units of electrical energy are usually watt-seconds (joules), watt-hours, or kilowatt-hours. For commercial purposes the kilowatt-hour is the unit of choice.
Sources of Electrical Energy
Electrical energy occurs naturally, but seldom in forms that can be used. For example, although the energy dissipated as lightning exceeds the world's demand for electricity by a large factor, lightning has not been put to practical use because of its unpredictability and other problems. Generally, practical electric-power-generating systems convert the mechanical energy of moving parts into electrical energy (see generatorgenerator,
in electricity, machine used to change mechanical energy into electrical energy. It operates on the principle of electromagnetic induction, discovered (1831) by Michael Faraday.
..... Click the link for more information. ). While systems that operate without a mechanical step do exist, they are at present either excessively inefficient or expensive because of a dependence on elaborate technology. While some electric plants derive mechanical energy from moving water (hydroelectric power), the vast majority derive it from heat engines in which the working substance is steam. Roughly 89% of power in the United States is generated this way. The steam is generated with heat from combustion of fossil fuelsfuel,
material that can be burned or otherwise consumed to produce heat. The common fuels used in industry, transportation, and the home are burned in air. The carbon and hydrogen in fuel rapidly combine with oxygen in the air in an exothermal reaction—one that liberates
..... Click the link for more information. or from nuclear fission (see nuclear energynuclear energy,
the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity. In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E is energy, m
..... Click the link for more information. ; nuclear reactornuclear reactor,
device for producing controlled release of nuclear energy. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is
..... Click the link for more information. ).
Steam as an Energy Source
The conversion of mechanical energy to electrical energy can be accomplished with an efficiency of about 80%. In a hydroelectric plant, the losses occur in the turbines, bearings, penstocks, and generators. The basic limitations of thermodynamicsthermodynamics,
branch of science concerned with the nature of heat and its conversion to mechanical, electric, and chemical energy. Historically, it grew out of efforts to construct more efficient heat engines—devices for extracting useful work from expanding hot gases.
..... Click the link for more information. fix the maximum efficiency obtainable in converting heat to electrical energy. The necessity of limiting the temperature to safe levels also helps to keep the efficiency down to about 41% for a fossil-fuel plant. Most nuclear plants use low-pressure, low-temperature steam operation, and have an even lower efficiency of about 30%. Nuclear plants have been able to achieve efficiency up to 40% with liquid-metal cooling. It is thought that by using magnetohydrodynamicmagnetohydrodynamics
, study of the motions of electrically conducting fluids and their interactions with magnetic fields. The principles of magnetohydrodynamics are of particular importance in plasma physics. See nuclear energy.
..... Click the link for more information. "topping" generators in conjunction with normal steam turbines, the efficiency of conventional plants can be raised to close to 50%. These devices remove the restrictions imposed by the blade structure of turbines by using the steam or gasses produced by combustion as the working fluid.
The heat generated by an electric-power plant that is not ultimately converted into electrical energy is called waste heat. The environmental impact of this waste is potentially catastrophic, especially when, as is often the case, the heat is absorbed by streams or other bodies of water. Cooling towers help to dispose waste heat into the atmosphere. Associated with nuclear plants, in addition to the problem of waste heat, are difficulties attending the disposal and confinement of reaction products that remain dangerously radioactive for many thousands of years and the adjustment of such plants to variable demands for power. Public concern about such issues—fueled in part by the accidents at the Three Mile Island nuclear plant in Harrisburg Pennsylvania in 1979, and the nuclear plant explosion in the Soviet Union at Chernobyl in 1986—forced the U.S. government to introduce extensive safety regulations for nuclear plants. Partly because of those regulations, nuclear plants are proving to be uneconomical. Several are being shut down and replaced by conventionally fueled plants.
Alternative Energy Sources
Fuel cellsfuel cell,
electric cell in which the chemical energy from the oxidation of a gas fuel is converted directly to electrical energy in a continuous process (see oxidation and reduction).
..... Click the link for more information. develop electricity by direct conversion of hydrogen, hydrocarbons, alcohol, or other fuels, with an efficiency of 50% to 60%. Although they have been used to produce electric power in space vehicles and some terrestrial locations, several problems have kept them from being widely used. Most important, the catalyst, which is an important component of a fuel cell, especially one that is operating at around room temperature, is very expensive. Controlled nuclear fusion could provide a virtually unlimited source of heat energy to produce steam in generating plants; however, many problems surround its development, and no appreciable contribution is expected from this source in the near future.
Solar energy has been recognized as a feasible alternative. It has been suggested that efficient collection of the solar energy incident on 14% of the western desert areas of the United States would provide enough electricity to satisfy current demands. Two main solar processes could be used. Photovoltaic cells (see solar cellsolar cell,
semiconductor devised to convert light to electric current. It is a specially constructed diode, usually made of forms of crystalline silicon or of thin films (as of copper indium gallium selenide or amorphous silicon).
..... Click the link for more information. ) convert sunlight directly into electrical energy. Another method would use special coatings that absorb sunlight readily and emit infrared radiation slowly, making it possible to heat fluids to 1,000°F; (540°C;) by solar radiation. The heat in turn can be converted to electricity. Some of this heat would be stored to allow operation at night and during periods of heavy cloud cover. The projected efficiency of such a plant would be about 30%, but this fairly low efficiency must be balanced against the facts that energy from the sun costs nothing and that the waste heat from such a plant places virtually no additional burden on the environment. The principal problem with this and other exotic systems for generating electricity is that the time needed for their implementation may be considerable.
apparatus that harnesses wind power for a variety of uses, e.g., pumping water, grinding corn, driving small sawmills, and driving electrical generators. Windmills were probably not known in Europe before the 12th cent.
..... Click the link for more information. , once widely used for pumping water, have become viable for electric-power generation because of advances in their design and the development of increasingly efficient generators. Windmill "farms," at which rows of windmills are joined together as the source of electrical energy, serve as a significant, though minor, source of electrical energy in coastal and plains areas. However, the vagaries of the wind make this a difficult solution to implement on a large scale.
See also energy, sources ofenergy, sources of,
origins of the power used for transportation, for heat and light in dwelling and working areas, and for the manufacture of goods of all kinds, among other applications.
..... Click the link for more information. .
Transmission of Electrical Energy
Electrical energy is of little use unless it can be made available at the place where it is to be used. To minimize energy losses from heating of conductors and to economize on the material needed for conductors, electricity is usually transmitted at the highest voltages possible. As modern transformerstransformer,
electrical device used to transfer an alternating current or voltage from one electric circuit to another by means of electromagnetic induction. The simplest type of transformer consists of two coils of wire, electrically insulated from one another and arranged so
..... Click the link for more information. are virtually loss free, the necessary steps upward or downward in voltage are easily accomplished. Transmission lines for alternating current using voltages as high as 765,000 volts are not uncommon. For voltages higher than this it is advantageous to transmit direct current rather than alternating current. Recent advances in rectifiers, which turn alternating current into direct current, and inverters, which convert direct into alternating, have made possible transmission lines that operate at 800,000 volts and above. Such lines are still very expensive, however.
Electric utilities are tied together by transmission lines into large systems called power grids. They are thus able to exchange power so that a utility with a low demand can assist another with a high demand to help prevent a blackout, which involves the partial or total shutdown of a utility. Under such a system a utility experiencing too great a load, as when peak demand coincides with equipment failure, must remove itself from the grid or endanger other utilities. During periods in which demand exceeds supply a utility can reduce the power drawn from it by lowering its voltage. These voltage reductions, which are normally of 3%, 5%, or 8%, result in power reductions, or brownouts, of about 6%, 10%, or 15%, causing inefficient operation of some electrical devices. The power distribution system, because of its generation of low-frequency electromagnetic fields, has been suggested as a possible source of health problems.
Reactive power is a concept used by engineers to describe the loss of power in a system arising from the production of electric and magnetic fields. Although reactive loads such as inductorsinductor,
electric device consisting of one or more turns of wire and typically having two terminals. An inductor is usually connected into a circuit in order to raise the inductance to a desired value.
..... Click the link for more information. and capacitorscapacitor
device for the storage of electric charge. Simple capacitors consist of two plates made of an electrically conducting material (e.g., a metal) and separated by a nonconducting material or dielectric (e.g.
..... Click the link for more information. dissipate no power, they drop voltage and draw current, which creates the impression that they actually do. This "imaginary power" or "phantom power" is called reactive power. It is measured in a unit called Volt-Amps-Reactive (VAR). The actual amount of power being used, or dissipated, is called true power, and is measured in the unit of watts. The combination of reactive power and true power is called apparent power, and it is the product of a circuit's voltage and current. Apparent power is measured in the unit of Volt-Amps (VA). Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. Reactive power is significant because it must be provided and maintained to insure continuous, steady voltage on transmission networks. Reactive power thus is produced for maintenance of the system and not for end-use consumption. Power losses incurred in transmission from heat and electromagnetic emissions are included in the total reactive power requirement as are the needs of power hungry devices, such as electric motors, electromagnetic generators, and alternators. This power is supplied for many purposes by condensers, capacitors, and similar devices, which can react to changes in current flow by releasing energy to normalize the flow. If elements of the power grid cannot get the reactive power they need from nearby sources, they will pull it across transmission lines and destabilize the grid. In this way, poor management of reactive power can cause major blackouts.
See K. W. Li and A. P. Priddy, Power Plant System Design (1985); L. F. Drbal et al., Power Plant Engineering (1996).
a physical quantity, a measure of the time rate of transmitting or transforming electric energy.
In DC electric circuits electric power is given as P = EI, where E is the voltage in volts and I is the current in amperes. In AC circuits the product of instantaneous values of voltage e and current i is equal to the instantaneous power: p = ei. The value of p is a variable quantity that represents the power at a given instant. The average value of the instantaneous electric power during the time period T is called the active power
In single-phase circuits with sinusoidal current P = El cos φ, where E and I are the effective values of voltage and current and φ is the phase shift angle between voltage and current. Active electric power characterizes the time rate of the irreversible conversion of electric energy into other kinds of energy (heat, light, and so on). Electric power that characterizes the time rate of energy transmission from the current source to the receiver and back is called reactive power Q = El sin φ. The apparent power is equal to the product of effective values for circuits with periodically changing currents. The relationship between apparent power and active and reactive power is expressed by the equation S2 = P2 + Q2. For circuits with nonsinusoidal currents, electric power is equal to the sum of the corresponding average powers of the individual harmonics
For three-phase circuits electric power is defined as the sum of the powers of individual phases. With symmetrical loading
Where El is the line current, Il, is the line voltage, and φφ is the phase dishift angle between voltage and current.
P. V. ERMURATSKII