Electric power has become an indispensable form of energy throughout much of the world; significant interruptions of electric service create serious problems in industrial, commercial, and residential activities. Even systems that use forms of energy other than \Telectricity\t are likely to contain controls or equipment that run on electric power. For example, modern home heating systems may burn natural gas, oil, or coal, but most systems have combustion and temperature controls that require electricity in order to operate. Similarly, most industrial and manufacturing processes require electric power, and the computers and business machines of many offices and commercial establishments are paralyzed if electric service is interrupted. During the first part of the 20th century, only about 10% of the total energy generated in the United States was converted to electricity. By 1985 more than 37% was converted; estimates are that electric power might account for 40% of the total by 1990. Developing countries are usually not as dependent on electricity as are the more industrialized nations, but the growth rate of electricity use in some of those countries is comparable to the rate of growth in the early years of electricity availability in the United States. GROWTH OF THE ELECTRIC POWER INDUSTRY The first commercial electric-power installations in the United States were constructed in the latter part of the 19th century. The Rochester, N.Y., Electric Light Co. was established in 1880. In 1882, Thomas A. Edison's Pearl Street steam-electric station began operation in New York City and within a year was reported to have had 500 customers for the lighting services it supplied. A short time later a central station powered by a small waterwheel began operation in Appleton, Wis. In 1886 the feasibility of sending electric power greater distances from the point of generation by using \Talternating current\t (AC) was demonstrated at Great Barrington, Mass. The plant there utilized transformers to raise the voltage from the generators for a high-voltage transmission line. The electric power industry of the United States grew from small beginnings such as these to become, in less than 100 years, the most heavily capitalized industry in the country. It now comprises about 3,100 different corporate entities, including systems of private investors, federal and other government bodies, and cooperative-user groups. Less than one-third of the corporate groups have their own generating facilities; the others are directly involved only in the transmission and distribution of electric power. For several decades electric power use in the United States grew at an average annual rate of about 7%, a rate that results in a doubling every 10 years. The rate of growth remained constant, with only minor year-to-year variations, until the early 1970s, when fuel shortages and rising concern over possible environmental damage, together with reduced expansion of the U. S. economy, slowed the growth rate. In the period from 1974 to 1985 the annual increase in electricity use varied between 1.7% and 6.2%. Although total energy use in the United States has either declined or remained unchanged since 1973, electricity use has continued to grow in the past several years. Table 1, showing annual production of electric energy per capita in the United States from 1920 through 1980, indicates the steady growth that resulted from technological developments, an expanding economy, an increasing population, and the increased availability of electrically powered devices. STANDARD ELECTRIC GENERATING PLANTS Virtually all commercial electric energy is now produced by generators driven by steam from the burning of fossil fuels or from nuclear sources or by hydropower. In 1984 the United States produced a total of 2,416 billion kWh; approximately 73% of this was produced by the burning of fossil fuels, about 14% from nuclear sources, and about 13% from hydroelectric power. Geothermal and solar energy produced only 0.4%. Many other developed nations also depend mainly on fossil fuels, but some countries now depend more heavily on \Tnuclear energy\t. France for example, generates about 70% of its electricity from nuclear power plants; power costs in that nation are the lowest in Europe. A basic steam-power plant includes a furnace or reactor for raising the temperature of the water in a \Tboiler\t, or steam generator, until it changes into steam, and a turbine, which drives the generator to produce electric power. Throughout the history of the electric power industry, improvements in design, metallurgy, fabrication techniques, and control systems have permitted continual increases in the size, operating temperatures, pressures, and efficiencies of electric generating units. These improvements and increasing demands for electric power have led generating facilities to develop from the early steam-engine-driven generator, which could produce a few kilowatts (kW), today's giants, with outputs as high as 1,300,000 kW. Hydroelectric, or waterpower, generators have grown from the 12-kW machines of 1882 to the 600,000-kW units at the Grand Coulee station in Washington state (see \Thydroelectric power\t). Peak-Load Problem All electric-utility systems experience cyclic load patterns involving higher demands for electric power at some hours of the day and some seasons of the year than at others. Such considerations affect the design of a utility's generating capacity plant because some types of generating equipment are better suited to supplying base, or continuous, loads and may not operate satisfactorily or economically over a varying load cycle; others are better designed for the variable loading, intermittent use, and frequent start-up and shutdown required by such patterns of operation. Hydroelectric plants are often well adapted to intermittent operation and may be useful for supplying peaking power. They can be constructed only in special locations, however, and they must often rely on fuel plants to supply peaking needs. Steam plants especially designed for peaking service have been installed in a few systems, and internal combustion units have sometimes been used for such service. Combustion Turbines More recently, combustion turbine generators have become popular as peaking units, not only because of their quick-start and intermittent-operation capabilities, but often because of the short time they require for installation. In recent years many U.S. utilities have found themselves deficient in generating capacity when the installation of new facilities has been delayed by problems in procurement, licensing, or construction. Lengthy delays have occurred in many planned nuclear, fossil-fueled, and hydroelectric facilities. Procurement and construction of a large steam-electric station takes from 5 to 10 years even after advance procedural requirements have been met. Thus a combustion turbine unit of 30,000 kW or more, which can be installed and operated within a year or two after procurement, is an attractive alternative. Such units can also provide emergency service during power outages and are often valuable as sources of start-up power for conventional generating plants following \Lblackout\ls. Capital costs of combustion turbines are lower than those of conventional steam units, but fuel efficiency is usually not as high and maintenance is more expensive. NEW FORMS OF ELECTRIC GENERATION A continued search has gone on for many years to find new forms of electric power generation that would be suitable for electric-utility use, provide higher efficiencies, and reduce damage to the environment. The key issue is whether alternative systems can produce power at an acceptable cost. Thus far, however, no new methods have been developed sufficiently to be practical for utility use other than those that depend on rotating machinery to convert input energy into mechanical energy that is then converted into electrical energy. Recently, however, the power industry has begun to examine other possibilities. Magnetohydrodynamics In \Tmagnetohydrodynamics\t (MHD), an ionized gas or liquid metal is passed through a magnetic field to generate electricity. Experimental units have been constructed both in the United States and elsewhere, but none is yet capable of being substituted for conventional generators. Electrogasdynamics Electrogasdynamics (EGD) utilizes a gas stream to carry charged particles through an electric field. The electric field opposes the motion of the particles and slows them down, thereby increasing their charge by converting their kinetic (motion) energy into direct current. As has been done with MHD, some encouraging results have been obtained with EGD but many problems remain to be solved before EGD can replace conventional generating-plant equipment. Thermoelectric and Thermionic Generators Generators working on the principle of \Tthermoelectricity\t or of \Tthermionic\t emission are static devices that can convert heat directly into electricity without converting it into mechanical energy first. Both have been used in some small applications with low power requirements, but neither seems promising as a source of large amounts of utility power in the near future. Fuel Cells Fuel cells are thought by some to have a bright future as sources of residential and industrial electric power. The fuel cell is an electrochemical device that converts the chemical energy of the fuel directly into a direct-current electrical output, somewhat like a continuous-process battery. Hydrogen-fueled cells have been used in space applications, and a group of natural-gas utilities has sponsored an extensive research program to develop a natural-gas fuel cell, but so far none is competitive with conventional utility-electric service. Solar Power Interest in \Tsolar energy\t has grown in recent years as the public has become increasingly aware that fossil fuels such as oil and coal cannot be replaced as they are consumed and as concern increases over air pollution and environmental effects of the normal combustion processes. Producing electric power from solar energy is still costly. The price must decrease before the use of solar power for electricity generation becomes widespread. The usefulness of solar energy for the production of large amounts of power depends upon achieving a high efficiency at a sufficiently low cost. Work is progressing on ideas ranging from small, independent solar collection devices that absorb energy and produce heat to relatively large solar-collector fields both on the Earth's surface and in space (see \Tsolar\t \Tcollector\t). Research on \Lsolar cell\ls (photovoltaic cells), which convert sunlight into electricity, has been considerable. New manufacturing processes, based on computer chip technology, are being developed; if manufacturing costs can be reduced, this approach may become practical in sunnier areas in the United States and in other countries. In experimental operation, photovoltaic cell systems have shown to be reliable at maintenance costs acceptable to the electric power industry. Geothermal Power Pockets of heat inside the Earth have been tapped as \Tgeothermal energy\t sources for electric-power generation in California, Italy, Iceland, New Zealand, the United Kingdom, and a few other locations, and more may be developed if increased costs of other processes make the geothermal alternatives more economically competitive. Geothermal power plants operate on the same principles as other thermal power plants. They utilize the heat from steam or hot water found below the Earth's surface to power turbines that drive electric generators. Drawbacks to the process include the presence of salts in the hot water or steam and the extreme drilling depths required at some locations. Wind Power Wind energy has received considerable attention in recent years as a method of electricity production (see WINDMILLS AND WIND \Tpower\t). Thousands of wind power machines, most of which have a capacity of 50-200 kW, have been installed in the United States, particularly in California. Wind energy supplies about 1% of California's electric power. Larger wind turbines, such as some of those which operate in the United Kingdom, have a capacity of several thousand kilowatts; their reliability has generally not yet proven satisfactory, however. Although the generation of electricity from wind power conserves fossil fuels and does not contribute to air pollution, wind power is intermittent in places where power is needed most. The difficulties in constructing long-lived turbines that can withstand strong winds and the high cost of wind power are the two main reasons why wind power will probably play only a small role in power production in the future. Tidal Power \Ttidal energy\t plants have been constructed in a few places outside the United States, but none has yet produced spectacular results. No such installations have been built in the United States. In several cities in the United States and elsewhere in the world, energy-recovery plants with massive incinerators convert municipal solid waste into electricity for sale to electric utilities. FUEL USE FOR POWER GENERATION IN THE UNITED STATES The central-station generating plants built throughout the United States were generally designed to use the most accessible and economical fuels. Hydroelectric plants were built at locations where dams could be built to impound the water needed to supply the hydraulic energy for the turbogenerators. Power plants near coalfields were likely to have coal-fired furnaces, whereas others were more likely to utilize oil or natural gas as the primary fuel. In time the price of fuel became an important factor in the generating process as fuel transportation systems developed. Many coal-burning plants were converted to use either oil or gas as competition between the fuels and fuel suppliers increased. Air pollution from coal-fired plants became a major issue in some parts of the country in the 1960s and '70s, leading more utilities to switch to gas or oil. Later, however, shortages of oil and gas required some plants to be converted back to coal, either because high costs or uncertain supplies of the desired fuel meant that it was not available or because of governmental regulatory action. To reduce pollutants to within new statutory limits, some utilities shifted to new--and frequently distant--sources of coal, and some installed sophisticated and expensive devices to cleanse pollutants from plant emissions. The locations for power plants run by nuclear energy are usually determined not by the source of their fuel but by land availability, access to suitable sources of cooling water, and other physical considerations. At one time the \Tbreeder reactor\t, a nuclear reactor that makes more fuel than it uses, was looked upon as a major potential source of energy for electric power production in the United States, but recent concern about the spread of plutonium sufficient supplies of uranium and has resulted in a decreased interest in breeder-reactor power plants. Sharp differences of opinion exist concerning the safety of nuclear plants, and safety has become a growing public concern, particularly since the reactor accident (1979) at Three Mile Island in Pennsylvania. As a result, the future of nuclear power in the United States appears uncertain. Increasing concern about the contribution of the burning of fossil fuels to the \Tgreenhouse effect\t has led to a reevaluation of nuclear power, however. Nuclear power plants do not emit "greenhouse gases" such as carbon dioxide. In 1987, production of electric energy by utilities in the United States totaled 2,570 billion kilowatt-hours (kW h). Of this, 56.9% was produced by coal-burning plants, 4.6% by oil, 10.6% by gas, 9.7% by hydroelectric plants, and 17.7% by nuclear plants. The remainder came from geothermal, wood, waste, and solar plants. This distribution of sources represents a significant decrease in oil use by utilities and an increase in coal and uranium use. The highly industrialized nature of the United States, together with its population, economic development, and overall size, have made it the world's largest user of electric energy. In 1986, about 35% of the total electricity sold in the United States was used for residential purposes, 35% for industrial activities, and 26% for commercial purposes. The remainder was used for farm purposes and miscellaneous uses, and some was lost in the generation, transmission, and distribution system. ELECTRIC POWER TRANSMISSION Electric power transmission systems consist of step-up transformer stations to connect the lower-voltage power-generating equipment to the higher-voltage transmission facilities; high-voltage transmission lines and cables for transferring power from one point to another and pooling generation resources; switching stations, which serve as junction points for different transmission circuits; and step-down transformer stations that connect the transmission circuits to lower-voltage distribution systems or other user facilities. In addition to the transformers, these transmission substations contain circuit breakers and associated connection devices to switch equipment into and out of service, lightning arresters to protect the equipment, and other appurtenances for particular applications of electricity. Highly developed control systems, including sensitive devices for rapid detection of abnormalities and quick disconnection of faulty equipment, are an essential part of every installation in order to provide protection and safety for both the electrical equipment and the public. Overhead Transmission Lines Many of the first high-voltage transmission lines in the United States were built principally to transmit electrical energy from hydroelectric plants to distant industrial locations and population centers. High-voltage transmission lines were originally designed to permit the construction of large generating units and central stations on attractive, remote sites close to fuel sources and supplies of cooling water. Today, however, they connect different power networks in order to achieve greater economy by exchanges of low-cost power, to achieve savings in reserve generating capacity, to improve the reliability of the system, and to take advantage of diversity in the peak loads of different systems and thereby reduce operating costs. At one time power lines in the 33-kV or 44-kV class were classified as high-voltage lines. As loads increased and transmission distances became greater, transmission voltages were increased. Electrical losses increase proportionately to the square of the current--the higher the voltage of the line, the lower the current needed to carry an equivalent amount of power. Moreover, one high-voltage line can usually carry as much power as several lower-voltage ones, so the use of higher voltages reduces the number of lines required and conserves the space required for rights-of-way. Voltage levels increased to 69, 115, 138, and 161 kV in various sections of the United States. Before World War II the highest-voltage lines in the United States were 230 kV, with the exception of one 287-kV line from Boulder Dam to Los Angeles. In the early 1950s several 345-kV lines were constructed. By 1964 the first 500-kV lines in the United States were being completed, and in 1969 the first 765-kV line was put into service. All of these involved AC systems. In 1970 a 1,380-km (856-mi), 800-kV direct-current (DC) line was placed in commercial service to connect northwestern U.S. hydroelectric sources with the Los Angeles area. Such systems offer an economical means of transferring large quantities of power over long distances. They also avoid stability problems sometimes encountered by AC systems; DC systems are sometimes used to connect AC systems even over short transmission distances. Underground Transmission Cables Many transmission circuits utilize underground \Lcable\ls, although these installations have been limited largely to locations where rights-of-way for overhead lines could not be obtained or where overhead lines were not feasible because they would have interfered with other activities. In general the costs of underground circuits are several times those of comparable overhead circuits. Insulation problems are very different with underground cables from those with overhead lines, in which air serves as a major insulating medium. A number of different types of cable designs and insulations have been used in the United States. Solid synthetic insulating materials have given satisfactory results in the lower voltage ranges, but for high-voltage applications the principal insulator is gas, or an, oil-paper combination. Some extruded synthetic insulations have recently been developed that use materials such as polyethylene. One common kind of gas- and oil-insulated cable, known as self-contained cable, uses a conductor formed around a hollow core that is later filled with oil under low pressure. The conductor is insulated with an oil-impregnated paper, and the entire assembly is covered with a metal sheath. Three such cables are required, one for each phase of a three-phase power circuits normally used for alternating current transmission throughout the world. Another cable system, known as pipe-type, utilizes conductors insulated with oil-impregnated paper and covered with metallic and synthetic sheathing tapes. Three of these cables are pulled into a single pipe that is then filled with either gas or oil under high pressure. In the United States, the pipe-type system has been used most. One significant problem with underground AC circuits is the continuous flow of charging current between the energized conductors and the metallic cable sheaths. Unless expensive compensation devices are used, this charging current can utilize the entire current-carrying capacity of the cable within a few miles of circuit and introduce other operating problems as well. Although these problems do not occur with DC cable systems, DC transmission involves the additional cost of converters. Aesthetic concerns and difficulties in obtaining rights-of-way have increased the pressures to place power circuits underground, and the future will probably see a significant expansion in the use of underground systems. The most extensive extra-high-voltage (EHV) underground cable system at present is the 345 kV network that supplies the New York City area. The Federal Power Commission reported 760,378 circuit km (472,477 mi) of transmission line at voltages of 69 kV and above in service in the United States at the end of June 1976, and 4,580 circuit km (2,846 mi) of underground transmission cables. FUTURE NEEDS AND CONSIDERATIONS The electric utility industry has a large number of research and development programs under way to produce new technology, methods, and equipment for the years ahead. Much of this work in the United States is now carried out under the auspices of the Electric Power Research Institute (EPRI), an organization supported by most of the major utilities. The technology involved in reducing the cost of the production and delivery of electricity while minimizing environmental pollution is under study at EPRI. Major effort is being directed toward developing less expensive methods of underground transmission. Progress has already been made in the development of new oil-filled and solid-insulation cable designs, and some experts believe that the technology now exists to place almost any power line underground but that the costs are generally prohibitive. Some research projects involve cryogenic (extremely low temperature) systems, which present a lower resistance to current flow and permit transfer of large amounts of power with low transmission losses. Considerable work has also been done on the development of technology and equipment for ultra-high-voltage transmission systems, which are expected to utilize AC voltages in the 1,000-1,500 kV range. As awareness grows of the need to conserve energy resources, increasing interest also is being shown in the development of small-scale hydroelectric power plants. In the United States, legislation now favors the development of such plants. The Public Utilities Regulatory Policies Act (1978) states that utilities must buy electric power fed into their lines from small, privately owned generators. Such small-scale facilities can make more efficient user of power resources. In the mid-1980s, with the possibility of government deregulation and the development of new technologies for the production of electricity, the electric power industry in the United States was forced to seek more economical ways of generating electricity. Bibliography: Atteberry, P. H., Power Mechanics (1986); Collier, Hugh, Developing Eletrical Power (1984); Fardo, S. W., and Patrick, D. R., Electrical Power Systems Technology (1985); Grannis, Gary E., Modern Power Mechanics (1979); Hiss, Philip, Power Generation (1977); Stephenson, G. E., Power Technology, 4th ed. (1986); Walker, John R., Exploring Power Technology (1976); Weedy, B. M., Electric Power Systems, 2d ed. (1972); Weeks, W. L., Transmissions and Distribution of Electrical Energy (1981); Wood, A. J., and Wollenberg, B. F., Power Generation, Operation, and Control (1983). See also: \Tenergy sources\t; \Tfuel\t.