Electric Power

Huge electric generators in power plants produce almost all the world's electricity. The majority of these plants burn coal, oil, or natural gas to run the generators. Most other plants drive the generators by means of nuclear energy or the force of falling water. Wires carry the electricity from power plants to the cities or other areas where it is needed. The electricity is then distributed to individual consumers.

Electric power is measured in units called watts. For example, it takes 100 watts of electric power to operate a 100-watt light bulb. Ten 100-watt bulbs require 1,000 watts, or 1 kilowatt. The amount of energy used is expressed in kilowatt-hours. A kilowatt-hour equals the amount of work done by 1 kilowatt in one hour. If you burn ten 100-watt bulbs for one hour or one 100-watt bulb for 10 hours, you use 1 kilowatt-hour of electric energy.

The world's electric power plants can produce more than 2 1/4 billion kilowatts of electricity at any given time. The United States leads all other countries in generating capacity. American power plants can generate as much as 672 million kilowatts. Canadian plants can produce about 96 million kilowatts.

An electric generator has a stationary part called a stator and a rotating part called a rotor. In the huge electric generators used in power plants, the stator consists of hundreds of windings of wire. The rotor is a large electromagnet that receives electricity from a small separate generator called an exciter. An external source of mechanical energy, such as a turbine, turns the rotor. The magnetic field created by the rotor turns as the rotor turns. As this field rotates, it produces a voltage in the wire windings of the stator that causes a flow of electric current. See Electric Generator.

The major types of electric power plants are (1) fossil-fueled steam electric power plants, (2) hydroelectric power plants, and (3) nuclear power plants. Various other kinds of power plants produce smaller amounts of electricity.

Fossil-fueled steam electric power plants generate about 63 percent of the world's electric power and about 70 percent of the electric power produced in the United States. Such plants burn coal, oil, or natural gas. These substances are called fossil fuels because, like fossils, they developed from the remains of prehistoric plants and animals. The fuel is burned in a combustion chamber to produce heat. The heat, in turn, is used to convert water in a boiler to steam. The steam then flows through a set of tubes in a device called a superheater. Hot combustion gases surround the steam-filled tubes in the superheater, increasing the temperature and pressure of the steam in the tubes.

Steam electric power plants create steam by heating water in a nuclear reactor or in a combustion chamber, where coal, oil, or gas is burned. The steam turns a turbine that runs a generator. The generator has a rotating electromagnet called a rotor and a stationary part called a stator. A separate generator called an exciter powers the rotor, creating a magnetic field that produces an electric charge in the stator. The charge is transmitted as electricity. A transformer boosts the voltage. Exhaust steam passes cool water pipes in a condenser and turns back to water for reheating. The water that has absorbed the steam's heat in the condenser is piped to a cooling tower. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Oxford Illustrators Limited.

The superheated, high-pressure steam is used to drive a huge steam turbine. A steam turbine has a series of wheels, each with many fanlike blades, mounted on a shaft. As the steam rushes through the turbine, it pushes against the blades, causing both the wheels and the turbine shaft to spin. The spinning shaft turns the rotor of the electric generator, thereby producing electricity. See Turbine.

After the steam has passed through the turbine, it enters a condenser. In the condenser, the steam passes around pipes carrying cool water. The water in the pipes absorbs heat from the steam. As the steam cools, it condenses into water. This water is then pumped back to the boiler to be turned into steam again.

At many power plants, the water in the condenser pipes, which has absorbed heat from the steam, is pumped to a spray pond or a cooling tower to be cooled. At a spray pond, the water is sent through nozzles that form a spray of droplets. The spray increases the surface area of the water that is exposed to the air, quickly cooling the water. A cooling tower has a series of decks. The water spills down from one deck to another, cooling as it comes into contact with the air. The cooled water is recycled through the condenser or discharged into a lake, river, or other body of water.

Fossil-fueled steam electric power plants are efficient and reliable. But they can cause pollution. Some power plants do not use cooling towers or spray ponds. They release heated water into lakes, ponds, rivers, or streams. Such thermal pollution may harm plant and animal life in these bodies of water. In many areas, laws limit the discharge of heated water by power plants.

The smoke from burning fossil fuels contains chemicals and tiny particles that cause air pollution if they are released into the atmosphere. Most power plants that burn these fuels use pollution control equipment to limit the release of pollutants. However, the use of such equipment has not fully eliminated the air pollution created by plants that burn fossil fuels.

Hydroelectric power plants generate about 20 percent of the world's electric power and about 9 percent of the electricity produced in the United States. Such plants convert the energy of falling water into electric energy. A hydroelectric plant uses water that is stored in a reservoir behind a dam. The water flows through a tunnel or pipe to the plant's water turbine, or hydraulic turbine. As the water rushes through the turbine, it spins the turbine shaft, which drives the electric generator. See Turbine.

A hydroelectric power plant uses the force of falling water from a reservoir to turn a turbine that drives a generator. An exciter powers the rotor. As the rotor and its magnetic field turn, an electric charge is created in the stator. A transformer increases the voltage of the current coming from the stator. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Oxford Illustrators Limited.

Hydroelectric power plants called pumped-storage hydroelectric plants can store energy by operating in reverse. When the demand for electricity is low, such plants can use their generators as motors to turn the turbines. The turbines then function as pumps, raising water to the reservoir. The water can be used at a later time to produce electricity.

Hydroelectric power plants cost less to operate than fossil-fueled plants and do not pollute the air. The number of hydroelectric power plants is limited, however, by the availability of water power and suitable locations for dams and reservoirs.

Nuclear power plants generate about 17 percent of the world's electric power and about 21 percent of the electricity generated in the United States. Nuclear plants produce electricity in much the same way that fossil-fueled plants do. But instead of a fuel-burning combustion chamber, a nuclear power plant has a device called a nuclear reactor. A nuclear reactor produces enormous amounts of heat by fissioning (splitting) the nuclei of atoms of a heavy element. Most nuclear plants use the element uranium as the fuel in their reactors.

Heat from the nuclear fission is used to convert water into steam. The steam drives the steam turbine that runs the electric generator. After the steam has left the turbine, it is condensed and recycled through the plant. Many nuclear power plants use cooling towers to cool the water from the condenser pipes.

A nuclear power plant requires much less fuel than a fossil-fueled plant to produce an equal amount of electricity. Nuclear plants also cause much less air pollution. However, they contain dangerous radioactive materials. As a result, the plants must install special safety systems to help prevent and quickly deal with accidents that could cause the release of radiation. Nuclear power plants cost more to build than fossil-fueled plants, partly because of the expense of the safety systems. Nuclear plants also create radioactive wastes that remain hazardous for thousands of years and therefore must be disposed of with extreme caution.

Other sources of electric power produce relatively small amounts of electricity. Geothermal power plants use steam from the depths of the earth to run turbines that drive electric generators. Some power plants harness wind energy by using windmills to drive electric generators. A number of power plants use the energy of the ocean tides to turn turbines that run generators. Some burn wood or agricultural wastes to drive generators. A few power plants convert the sun's energy into electricity by means of devices called solar cells. Producing electric power with solar cells is expensive. However, scientists and engineers are studying ways to improve solar cells in order to produce large quantities of electric power more economically. See Solar Energy.

A number of electric power plants have gas turbines or diesel engines to drive auxiliary generators. Such generators supply the extra power needed in times of high demand. Diesel engines are also used to drive generators in isolated areas not served by power companies. Many hospitals, factories, and apartment buildings have diesel engines to drive generators in case the distribution of electricity from power plants is disrupted.

An electric power delivery system has power lines to carry current and transformers to change its voltage. Step-up transformers boost voltages so that current can be transmitted long distances. Substations and transformers reduce voltages to levels needed by consumers. Some industrial users and transportation systems that require high voltages have their own transformers. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Oxford Illustrators Limited.

Transmission. Most electricity travels from power plants along overhead wires called transmission lines. Laying underground or underwater cables generally costs more than stringing overhead wires. Cables are therefore used much less often than overhead wires.

As electric current moves along transmission lines, the lines resist the current flow. The resistance causes the current to lose energy by heating the lines. Power plants limit energy losses by transmitting electricity at high voltages. As voltage is increased, the amount of current needed to transmit a particular amount of electric power decreases. Because less current flows through the line, there is less energy lost due to resistance.

Electric current may be either direct current (DC) or alternating current (AC). Direct current flows in only one direction. Alternating current reverses direction many times each second. It is easier to boost the voltage of alternating current than that of direct current. Alternating current is therefore easier to transmit than direct current. For this reason, electric power plants generate alternating current.

The typical power plant generator can produce about 1 million kilowatts of electric power at up to 22,000 volts. Devices called step-up transformers then boost this voltage as high as 765,000 volts for transmission.

Distribution. Some large industries require high-voltage current and receive it directly from transmission lines. But high-voltages are unsafe in homes, offices, and most factories. The voltage must therefore be decreased before electricity is distributed to them.

High-voltage electricity is carried by the transmission lines to subtransmission substations near the area where the power will be used. These substations have devices called step-down transformers that reduce the voltage to 12,500 to 138,000 volts. The voltage is then further reduced at distribution substations to 2,000 to 34,500 volts. Distribution lines may carry this medium-voltage current directly to commercial, industrial, or institutional users. Distribution lines also carry electric power to distribution transformers on poles, on the ground, or in underground vaults. Distribution transformers reduce the voltage to the levels needed by most users. Wires from the transformers run to homes, stores, offices, and other users. Nearly all such consumers in the United States and Canada receive electric power at about 110 or 220 volts.

Providing reliable service. Equipment failures or damage caused by storms or accidents can interrupt local service of electric power. Such interruptions are known as power blackouts. Engineers called load dispatchers keep track of the flow of current through the transmission network. When a blackout occurs, the load dispatcher may restore service to the affected area by rerouting current along usable lines.

The demand for electricity often varies greatly from hour to hour. For example, sudden, dark storm clouds will increase demand because many lights will be turned on. The load dispatcher forecasts changes in demand and adjusts the generation and transmission of power accordingly. When demand exceeds the generating capacity of a power plant, the load dispatcher may reduce the voltage to prevent a blackout. Such a situation, called a brownout, may damage electrical equipment or cause it to operate less efficiently.

The transmission networks of most electric companies are interconnected, forming a power pool. Power pools enable companies to receive power from one another in an emergency. Computers control the flow of electricity through transmission networks.

In the United States, there are about 3,300 electric utilities. Stockholders own about 200 of them. These stockholder-owned electric companies account for about 75 percent of the nation's generating capacity. City governments own about 1,900 electric utilities. The remaining utilities are owned by cooperatives, public power districts, or state or federal government organizations. Most cooperatives were established to supply electricity to rural areas not served by other electric utilities. Public power districts consist of several counties that jointly produce and distribute electricity.

One of the largest federal power projects is the Tennessee Valley Authority (TVA). Its power plants generate over 125 billion kilowatt-hours of power a year.

Local and state utility commissions set the rates that electric utilities charge their customers. A number of federal agencies govern the design and licensing of power plants, regulate the interstate sale of electric power, and set and enforce pollution control standards. But in 1992, the U.S. government began to take steps to deregulate the electric power industry and to allow electric utilities to compete for customers.

In Canada, the governments of the various provinces own most major electric utilities. However, stockholders own large electric utilities in Alberta and Prince Edward Island. Provincial public utility boards regulate Canada's electric utilities. The federal Atomic Energy Control Board oversees the nation's nuclear energy industry. The National Energy Board controls the export of electric power. About 9 percent of the electricity generated in Canada is sold in the United States.

In 1879, the California Electric Light Company in San Francisco began operating the world's first central power plant that sold electricity to private customers. Also in 1879, the American inventor Thomas A. Edison perfected a lamp that glowed and gave off light when a filament in the lamp was heated by an electric current. Edison's incandescent lamp burned much longer than an arc lamp, and it quickly created a growing demand for electricity. In 1882, Edison opened the Pearl Street Station, a steam electric power plant, in New York City. Shortly after it opened, the plant was providing direct current to light more than 1,000 incandescent lamps. In 1895, huge hydroelectric generators at Niagara Falls began providing area industries with electric power.

Growth of the electric power industry. By the end of the 1800's, there were over 3,600 electric utilities in the United States. However, they did not all provide electricity at the same voltages. Studies conducted by electrical engineers in 1891 resulted in the standardization of voltages. Utilities could then form power pools.

By the early 1930's, electric utilities served about two-thirds of the U.S. population. But only about 10 percent of U.S. farms had electricity. President Franklin D. Roosevelt established the Rural Electrification Administration (REA) in 1935 to expand electric service in rural areas. Today, nearly all American farms have electricity. Half are served by REA-financed electric power systems.

The first full-scale nuclear power plant began operation in 1956 at Calder Hall in northwestern England. In 1966, the world's first tidal power plant opened on the Rance River near St.-Malo, France.

Electric power today. Power companies must plan carefully for expansion to meet the ever-increasing demand for electric power. However, construction of new power plants is costly and takes several years. Many planned nuclear power plants in the United States have been canceled because of soaring construction costs as well as safety concerns.

The supply of fossil fuels will eventually run out unless economical substitutes are developed for them. Many scientists believe that energy from the earth, sun, wind, and oceans can be used more extensively to produce electric power economically in the future. Some utilities have already begun to use solar, geothermal, tidal, or wind power in addition to regular energy sources to generate electricity.

Electric utilities rely increasingly on computerized control systems. The use of such systems and the widespread interconnection of power lines among electric companies enable the utilities to provide extremely reliable service.

Contributor: M. E. El-Hawary, Ph.D., Prof. of Electrical Engineering, Technical Univ. of Nova Scotia.

Related articles include:

Electric Current; Electric Generator; Electricity; Transformer; Turbine.

How long must you burn one 100-watt light bulb to use 1 kilowatt-hour of electric energy?

What does a load dispatcher do?

Why do power plants transmit electricity at high voltages?

What is a power pool?

What are the three major types of electric power plants?

Why do power plants generate alternating current?

When and where did the first full-scale nuclear power plant begin operation?

What source of energy do geothermal power plants use?

Which type of electric power plant produces most of the world's electric power?

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