How Electricity is Generated
A generator is a device used by electric companies that converts
mechanical energy into electrical energy. The process is based on
the relationship between magnetism and electricity. In 1831, Faraday
discovered that when a magnet is moved inside a coil of wire,
electrical current flows in the wire.
A typical generator used by electric companies uses an
electromagnet—a magnet produced by electricity—not a traditional
magnet. The generator has a series of insulated coils of wire that
form a stationary cylinder. This cylinder surrounds a rotary
electromagnetic shaft. When the electromagnetic shaft rotates, it
induces a small electric current in each section of the wire coil.
Each section of the wire becomes a small, separate electric
conductor. The small currents of individual sections are added
together to form one large current. This current is the electric
power that is transmitted from electric companies to the consumer.
Electric companies use either a turbine, engine, water wheel, or
other similar machine to drive an electric generator or a device
that converts mechanical or chemical energy to generate electricity.
Steam turbines, internal-combustion engines, gas combustion
turbines, water turbines, and wind turbines are the most common
methods to generate electricity. Most electric companies are about
35 percent efficient. That means that for every 100 units of energy
that go into a plant, only 35 units are converted to usable
Most of the electricity produced by electric companies in the United
States is produced in steam turbines. A turbine converts the kinetic
energy of a moving fluid (liquid or gas) to mechanical energy. Steam
turbines have a series of blades mounted on a shaft against which
steam is forced, thus rotating the shaft connected to the generator.
In a fossil-fueled steam turbine, the fuel is burned in a furnace to
heat water in a boiler to produce steam. Coal, petroleum (oil), and
natural gas are burned in large furnaces to heat water to make steam
that in turn pushes on the blades of a turbine.
Did you know that most electricity generated by electric companies
in the United State comes from burning coal? In 2007, nearly half
(48.5%) of the country's 4.1 trillion kilowatthours of electricity
used coal as its source of energy.
Natural gas, in addition to being burned to heat water for steam,
can also be burned to produce hot combustion gases that pass
directly through a turbine, spinning the blades of the turbine to
generate electricity. Gas turbines are commonly used when an
electricity companies usage is in high demand. In 2007, 21.6% of the
nation's electricity was fueled by natural gas.
Petroleum can also be used by electric companies to make steam to
turn a turbine. Residual fuel oil, a product refined from crude oil,
is often the petroleum product used in electric plants that use
petroleum to make steam. Petroleum was used to generate about two
percent (2%) of all electricity generated in U.S. electricity plants
Nuclear power is a method in which steam is produced by heating
water through a process called nuclear fission. In a nuclear power
plant, a reactor contains a core of nuclear fuel, primarily enriched
uranium. When atoms of uranium fuel are hit by neutrons they fission
(split), releasing heat and more neutrons. Under controlled
conditions, these other neutrons can strike more uranium atoms,
splitting more atoms, and so on. Thereby, continuous fission can
take place, forming a chain reaction releasing heat. The heat is
used to turn water into steam, that, in turn, spins a turbine that
generates electricity. Nuclear power was used to generate 19.4% of
all the country's electricity in 2007.
Hydropower, the source for 5.8% of U.S. electricity generation in
2007, is a process in which flowing water is used to spin a turbine
connected to a generator. There are two basic types of hydroelectric
systems that produce electricity. In the first system, flowing water
accumulates in reservoirs created by the use of dams. The water
falls through a pipe called a penstock and applies pressure against
the turbine blades to drive the generator to produce electricity. In
the second system, called run-of-river, the force of the river
current (rather than falling water) applies pressure to the turbine
blades to produce electricity.
Geothermal power comes from heat energy buried beneath the surface
of the earth. In some areas of the country, enough heat rises close
to the surface of the earth to heat underground water into steam,
which can be tapped for use at steam-turbine plants. This energy
source generated less than 1% of the electricity in the country in
Solar power is derived by electric companies from the energy of the
sun. However, the sun's energy is not available full-time and it is
widely scattered. The processes used to produce electricity using
the sun's energy have historically been more expensive than using
conventional fossil fuels. Photovoltaic conversion generates
electric power directly from the light of the sun in a photovoltaic
(solar) cell. Solar-thermal electric generators use the radiant
energy from the sun to produce steam to drive turbines. In 2007,
less than 1% of the nation's electricity was based on solar power.
Wind power is derived by electric companies from the conversion of
the energy contained in wind into electricity. Wind power, less than
1% of the nation's electricity in 2007, is a rapidly growing source
of electricity. A wind turbine is similar to a typical wind mill.
Biomass includes wood, municipal solid waste (garbage), and
agricultural waste, such as corn cobs and wheat straw. These are
some other energy sources for producing electricity. These sources
replace fossil fuels in the boiler. The combustion of wood and waste
creates steam that is typically used in conventional steam-electric
plants. Biomass accounts for about 1% of the electricity generated
in the United States.