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Power Plants

Power plants


The term power plant refers to any installation at which electrical energy is generated. Power plants can operate using any one of a number of fuels: oil, coal , nuclear material, or geothermal steam, for example. The general principle on which most power plants operate, however, is the same. In such a plant, a fuel such as coal or oil is burned. Heat from the burning fuel is used to boil water, converting it to steam. The hot steam is then used to operate a steam turbine.

A steam turbine is a very large machine whose core is a horizontal shaft of metal. Attached to the horizontal shaft are many fan-shaped blades. As hot steam is directed at the turbine, it strikes the blades and causes the horizontal shaft to rotate on its axis. The rotating shaft is, in turn, attached to the shaft of an electric generator. The pressure of hot steam on turbine blades, is therefore, ultimately converted into the generation of a electric current. One of the first steam turbines designed to be used for the generation of electricity was patented by Charles Parson in 1884. Twenty years later, entrepreneurs had already put such turbines to use for the generation of electricity for street lines, subways, train lines and individual appliances.

Most early power plants were fueled with coal. Coal was the best-know, most abundant fossil fuel at the time. It had several disadvantages, however. It was dirty to mine, transport, and work with. It also did not burn very cleanly. Areas around a coal-fired power plant were characterized by clouds of smoke belching from smokestacks and films of ash deposited on homes, cars, grass, and any other exposed surface.

By the 1920s interest in oil-fired power plants began to grow. The number of these plants remained relatively low, however, as long as coal was inexpensive. But, by the 1960s, there was a resurgence of interest in oil-fired plants, largely in response to increased awareness of the many harmful effects of coal combustion on the environment . One of the most troublesome effects of coal combustion is acid rain . Coal contains trace amounts of elements which produce acid-forming oxides when burned. The two most important of these elements are sulfur and nitrogen . During the combustion of coal, sulfur is oxidized to sulfur dioxide and nitrogen to nitric oxide. Both sulfur dioxide and nitrogen oxide undergo further changes in the atmosphere , forming sulfur trioxide and nitrogen dioxide. Finally, these two oxides combine with water in the atmosphere to produce sulfuric and nitric acids.

This series of chemical reactions in the atmosphere may take place over hundreds or thousands of miles. Oxides of sulfur and nitrogen that leave a power plant smokestack are then carried eastward by prevailing winds. It may be many days or weeks before these oxides are carried back to earthnow as acidsin rain, snow, or some other form of precipitation. During the 1960s and 1970s, many authorities became concerned about the possible effects of acid precipitation on the environment. They argued that the problem could be solved only at the sourcethe electric power-generating plant.

One approach to the reduction of pollutants from a power plant is the installation of equipment that will remove undesirable materials from waste gases. An electrostatic precipitator is one such device which removes particulates. It attaches electrical charges to the particulates in waste gases and then applies an opposing electric field to a plate that attracts the particulates and removes them from the waste gas stream. Scrubbers , filters , and cyclone collectors also remove harmful pollutants from waste gases.

Another approach to acid rain and other power-plantgenerated pollution problems is to switch fuel from coal to oil or natural gas . Although petroleum and natural gas also contain sulfur and nitrogen, the concentration of these elements is much less than it is in many forms of coal. As a result, a number of electrical utilities began to retrofit their generating plants in the 1960s and early 1970s to allow them to burn oil or natural gas rather than coal. This change-over is reflected in the increased use of petroleum by electrical utilities in the United States between 1965 and 1975. Consumption increased more than six-fold in that ten year period, from about 100 million barrels per year to more than 6,700 million barrels per year. Interestingly enough, the use of coal by utilities did not drop off during the same period, but continued to rise from about 200 million short tons to over 300 million short tons annually.

This pattern of fuel switching turned out to be short-lived, however. Although oil and natural gas are relatively clean fuels, they also appealed to utilities because of their relatively low cost. The oil embargo instituted by the Organization of Petroleum Exporting Countries (OPEC) in 1973 altered that part of the equation. Worldwide oil prices jumped from $1.36 per million Btu in 1973 to $2.23 per million Btu in 1975, an increase of 64 percent per million Btu in just two years. A decade later, the price of oil had more than doubled. And, the situation was even worse for natural gas. The cost of a million Btu of this resource skyrocketed from 36.7 cents in 1970 to 69.3 cents in 1975 to $2.23 in the mid-1980s.

Suddenly, the conversion of power plants from coal to oil and natural gas no longer seemed like such a wonderful idea. And the utilities industry began once more to focus on the fossil fuel in most plentiful supplycoal. Between 1975 and 1990, the fraction of power plants powered by oil and natural gas dropped from 12% to 4% for the former and from 24% to less than 10% for the latter. In the same period, the fraction of plants powered by coal rose from 46%57%.

Power plants using fossil fuels , of whatever kind, are not the world's final solution to the generation of electricity. In addition to the environmental problems they cause, there is a more fundamental limitation. The fossil fuels are a nonrenewable resource. The time will comesooner or laterwhen coal, oil, and natural gas supplies will be depleted. Thus, research on alternatives to fossil-fuel-fired plants has gone on for many decades.

Thirty years ago, many people believed that nuclear power was the best solution to the world's need to produce large amounts of electricity. Indeed, the number of nuclear power plants in the United States grew from 6 in 1965 to 95 in 1985. By 1992, 421 nuclear plants were in commercial operation worldwide, supplying 17% of the world's electricity. But enthusiasm for nuclear power plants began to wane in the 1980s. One reason for this trend was the serious accidents at the Three Mile Island Nuclear Reactor (Middletown, Pennsylvania) in 1979 and at the Chernobyl Nuclear Power Station (Chernobyl, Ukraine) in 1986. Another reason was the growing concern about disposal of the ever-increasing volume of dangerous radioactive wastes produced by a nuclear power plant. In any case, no new nuclear power plant has been ordered in the United States since the Three Mile Island accident and 65 plant orders have been canceled since that event. Forty-nine nuclear plants are now under construction in other parts of the world, but this number is only a quarter as many as were under construction a decade ago. Indeed, between 1990 and 1991, the total installed nuclear generating capacity worldwide declined for the first time since commercial nuclear power generation began. Given current economic and political conditions, a major revival of the nuclear power industry seems unlikely.

A nuclear power plant operates on much the same principle as does a fossil-fuel power plant. In a nuclear power plant, water is heated not by the combustion of a fuel like coal or oil, but by nuclear fission reactions that take place within the reactor core. Some scientists anticipate that another type of nuclear power plantthe fusion reactormay provide a long-term answer to the world's electrical power needs. In a fusion reactor, atoms of light elements are fused together to make heavier elements, releasing large amounts of energy in the process. Nuclear fusion is the process by which stars make their energy and is also the energy source in the hydrogen bomb. Research efforts to develop an economic and safe method of controlling fusion power have been underway for nearly 40 years. Although progress has been made, a full-scale commercial fusion power plant currently appears to be many decade in the future.

Other types of power plants also have been under investigation for many years. For example, it is possible to operate a steam turbine with hot gases that come directly from geothermal vents. In locations where geysers or other types of vents exist, geothermal power plants are an economical, safe and dependable alternative to fossil-fueled and nuclear power plants. In the early 1990s, for example Pacific Gas and Electric in northern California obtained about eight percent of its electrical power from geothermal plants.

Finally, power plants that do not use heat at all are possible. One of the oldest forms of power plant is, in fact, the hydroelectric power plant. In a hydroelectric power plant, the movement of water (as from a river) is directed again turbine blades, causing them to rotate. In 1970, 16 percent of all electric power in the United States came from hydropower although that portion has now fallen to less than eight percent.

Proposals for wind power generated-electricity, tidal power plants, and other alterative energy sources as substitutes for fossil fuels and nuclear power have been around for decades. During the 1960s and 1970s, governments aggressively encouraged research on these new technologies, offering both direct grants and tax breaks. Since 1980, however, the United States government has lost interest in alternative methods of power generation, preferring instead to encourage the growth and development of more traditional fuels, such as coal, oil and petroleum.

See also Geothermal energy; Ocean thermal energy conversion; Solar energy; Tidal power; Wind energy

[David E. Newton ]


RESOURCES

BOOKS

Brown, L. R., et al. Vital Signs, 1992. New York: Norton, 1992.

McGraw-Hill Encyclopedia of Science & Technology. 7th ed. New York: McGraw-Hill, 1992.

PERIODICALS

Balzhiser, R. E., and K. E. Yeager. "Coal-fired Power Plants for the Future." Scientific American 257 (September 1987): 100107.

Swain, H. "Power Plants Threaten Shenandoah's Air." National Parks 65 (March-April 1991): 14.

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