electric power, 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 Watts, 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 generator). 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 fuels or from nuclear fission (see nuclear energy; nuclear reactor).
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 thermodynamics 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 magnetohydrodynamic "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 cells 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 cell) 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.
Windmills, 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 of.
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 transformers 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 inductors and capacitors 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).
Power is defined as the energy that is consumed or converted in a certain amount of time. In a simple electrical circuit, the power is found by multiplying the voltage and current. An electric current is the movement of charged particles measured in amperes and the voltage of the force driving them. Current that flows in one direction only, such as the current in a battery-powered flashlight, is called direct current. Current that flows back and forth, reversing direction again and again, such as household current, is called alternating current. Household electricity bills are computed on the basis of how many thousand-watt hours (kWh) of energy are consumed over a certain period of time. Today's home consumes, on average, between twelve hundred and two thousand kWh per month.
Most of the world's electric power is generated in steam plants. In a steam turbine generator, fossil fuel, such as coal, oil, natural or synthetic gas are the most common fuels used. Coal-based generation produces about 45 percent of all electricity generated in the United States, and natural or synthetic gas about 35 percent. The remaining, approximately 20 percent of generated electricity derives mostly from nuclear power plants, but includes wind, solar, biomass, diesel, geothermal, hydro, and other sources.
In a power plant, electricity is generated when a loop of conducting wire rotates in a magnetic field. Burning coal or gas produces hot steam that is forced through a turbine, causing it to spin. The spinning motion drives the generating coils within a magnetic field to produce electricity. Modern electricity-generating plants usually have a series of turbines to more effectively utilize the steam heat. The hot water returning to the boiler is used to preheat the fuel, allowing more efficient firing. See the illustration for a diagram of how electricity is generated by burning coal.
An electric power system consists of six main components: the electric power generating plant; a set of transformers at the plant to raise the generated electricity to the high voltages used on the transmission lines; the transmission lines; the substations at which the power is stepped down to the voltage that can be distributed to consumers; the distribution lines; and the transformers that lower the distributed voltage to the level needed by residential, industrial, and commercial users.
New gas turbine generators (analogous to big jet engines) are now being built that burn natural or synthetic gas as it is injected directly into the turbine system. This reduces heat loss and increases the efficiency of the fossil fuel.
Among the most modern systems are coal gasification or biomass gasification, which produce synthetic gases (syngas) by refining coal or biomass in a high-heat, pressurized system (gassifiers). Syngas is a more efficient fuel and contains less pollutant than either biomass or coal. Nitrogen and sulfur products are captured in the conversion process and become industrial and agricultural chemicals. At present, these systems remain expensive to build and much of the technology is still being improved. However, gasification systems are becoming more competitive with coal- or gas-fired steam plants as the costs of pollution abatement continue to rise.
As energy is converted to electricity, it flows to a transmission station where transformers change a large current and low voltage into a small current and high voltage. The electricity flows over high voltage transmission lines to a series of transmission stations where the voltage is stepped down by transformers to levels appropriate for distribution to customers.
Coal has the lowest heat values (British thermal units (BTUs) or BTU per ton) of any of the common fuel sources in the world today. When it is burned to generate steam, the major pollutants are sulfur, nitrogen, very fine ash, and mercury. The amounts of sulfur and nitrogen emitted when coal is burned depend on the kind of coal and where that coal is mined. In the United States, high-sulfur coal is mined in the Appalachian region, New York to Kentucky and the region south of the Great Lakes, Illinois, Iowa, and Kansas. These are the bituminous coal types, with high BTU per ton. Low-sulfur coal is mined in the Midwest and the intermountain regions (Wyoming, Colorado, Utah, and the Dakotas). This coal is mostly bituminous and subbituminous. Subbituminous coal has a lower BTU per ton rating. The nitrogen content of coal varies significantly and does not have the unique geographic distribution of sulfur. Finally, in the Dakotas, there is lignite, which is literally carbon-based earth. It has a very low BTU per ton rating, and is one of the most abundant coal types in the northern Great Plains.
Pollution from electric power generation depends on the type and source of fuel. The emissions, when not captured, produce oxides of nitrogen, commonly referred to as NOx, and sulfate aerosols from sulfur and oxygen, commonly referred to as SOx. Both pollutants are chemically unstable when emitted into the atmosphere and combine with oxygen and moisture to form the SOx and NOx particulates that are recognized as the pollutants. NOx is highly reactive with other pollutants found in urban and industrial areas and, with sunlight, forms smog. SOx is often attributed as the primary source of acid deposits across the landscape, particularly in the northeastern United States, which is downwind from power plants in the Midwest.
Mercury is emitted as elemental mercury vapor. It settles only a short distance from the stacks of power plants. However, it very quickly changes to a methyl mercury form, and when it settles into water, streams, lakes, or cooling ponds, it is absorbed by plants and transferred up the food chain to fish and waterfowl eaten by humans. Although the total annual tonnage is small, science is showing that extremely small amounts of mercury can cause significant harm to humans, particularly the unborn and very young children.
Most ash from burning coal is collected at the bottom of the fire box. However, very fine ash can float out of the smokestack. The particle size that concerns present-day regulators falls in the 10 micron and 2.5 micron range. A micron is one-thousandth of a millimeter. Airborne particulate this small may be contributing to increases in childhood asthma. Electric power generation is not the only source of such particulates in urban and suburban areas. Vehicle emissions from gasoline and diesel engines are also significant contributors.
The ash residual from burning coal is often suitable for the production of road surfaces, some forms of concrete, and lightweight blocks used to reduce erosion along rivers and streams. Once considered a pollutant or waste and dumped into open pit coal mines, coal ash is now becoming a valuable commodity.
Sulfur and nitrogen are captured by passing the hot gases from the combustion chamber through filters and water baths or by selective catalytic converters, thus removing them from the heat passed up the smokestack. The fine ash from the burning process is also filtered by a huge vacuum system with bags able to filter particles as fine as face powder. The concern about emissions of mercury is leading to the design of new systems capable of capturing the mercury vapor before it is released from the smokestack.
Environmental air quality standards are continually changing as new information about potential harm is published. It is a continual struggle between electric power generators and regulators to write and meet pollution standards that protect the environment and human health. Changes to a modern coal-based generator or even a natural gas generator cost thousands of dollars per megawatt of generating capacity. This means that every update, which must be designed onsite, as there are no standardized units, results in millions of dollars in additional costs. A steam generation system is designed to last at least fifty years, with initial investments close to a billion dollars, but because continually shifting requirements for pollution reduction systems cannot be incorporated in its design at the time of construction, the costs of later upgrades are almost inevitably incurred.
Electricity consumption has continued to rise approximately 2 to 5 percent per year as more and more electrical appliances are required to meet daily needs. Paying attention to the efficiency of each appliance, from computers to air conditioners, helps reduce the rate of increase. The higher the efficiency, the less total growth in individual consumer electricity use. More efficient lights, such as compact fluorescent bulbs, can effectively reduce the per capita use of electricity. Most electrical equipment manufacturers now provide comparisons of various appliances, machines, or other power equipment so informed consumers can make efficient choices. The U.S. Environmental Protection Agency (EPA) has a program called Energy Star that rates the efficiency of various appliances, computers, and other equipment. Those manufacturers that are compliant with high-efficiency standards receive an Energy Star stamp of approval.
Deregulation offers opportunities to independent power producers developing green electric power companies, for example, wind, biomass, solar, geothermal, and hydroelectricity generation, that wish to assure consumers their power source will not contribute to the increasing consumption of fossil fuels or emission of greenhouse gases. Such opportunities will, however, continue to come at a slightly increased price over the next decade before technologies to produce green power become more efficient, more generated power of this kind is widely available, and the costs of fossil fuels become more prohibitively expensive.
From 1950 to 1999, the most recent year for which data are available, annual world electric power production and consumption rose from slightly less than 1,000 billion to 14,028 billion kWh. The most commonly used form of power generation also changed. In 1950 about 66 percent of electricity came from thermal (steam-generating) sources and approximately 33 percent from hydroelectric sources. In 1998 thermal sources produced 63 percent of the power, but hydropower had declined to 19 percent, and nuclear power accounted for 17 percent of the total. The growth in nuclear power slowed in some countries, notably the United States, in response to concerns about safety. Nuclear plants generated 20 percent of U.S. electricity in 1999; in France, the world leader, the figure was 76 percent. See the pie chart for 2002 information on the net generation of electricity by fuel source.
see also Abatement; Acid Rain; Air Pollution; Cleanup; Coal; Energy; Energy, Nuclear; Fossil Fuels; Petroleum; Renewable Energy.
Gary R. Evans