What Is Geothermal Energy?
Geothermal energy comes
from the heat within the earth. The word "geothermal" comes from the
Greek words geo, meaning earth," and therme, meaning
"heat." People around the world use geothermal energy to produce
electricity, to heat buildings and greenhouses, and for other purposes.
The earth's core lies
almost 4,000 miles beneath the earth's surface. The double-layered core is made
up of very hot molten iron surrounding a solid iron center. Estimates of
the temperature of the core range from 5,000 to 11,000 degrees Fahrenheit (F).
Heat is continuously produced within the earth by the slow decay of radioactive
particles that is natural in all rocks.
Surrounding the earth's
core is the mantle, thought to be partly rock and partly magma. The
mantle is about 1,800 miles thick. The outermost layer of the earth, the
insulating crust, is not one continuous sheet of rock, like the shell of an
egg, but is broken into pieces called plates. These slabs of continents
and ocean floor drift apart and push against each other at the rate of about
one inch per year in a process called continental drift.
Magma (molten rock) may come quite close
to the surface where the crust has been thinned, faulted, or fractured by plate
tectonics. When this near-surface heat is transferred to water, a usable form
of geothermal- energy is created.
Geothermal energy is
called a renewable energy source because the water is replenished by rainfall,
and the heat is continuously produced by the earth.
History of Geothermal Energy
Many ancient peoples,
including the Romans, Chinese, and Native Americans, used hot mineral springs
for bathing, cooking, and heating. Water from hot springs is now used
world-wide in spas, for heating buildings, and for agricultural and industrial
uses. Many people believe hot mineral springs have natural healing powers.
Using geothermal energy
to produce electricity is a relatively new industry. It was initiated by a
group of Italians who built an electric generator at Lardarello in 1904. Their
generator was powered by the natural steam erupting from the earth.
The first attempt to
develop geothermal power in the United States came in 1922 at The Geysers steam
field in northern California. The project failed because the pipes and turbines
of the day could not stand up to the abrasion and corrosion of the particles
and impurities that were in the steam. Later, a small but successful
hydrothermal plant opened at the Geysers in 1960. Today 28 plants are operating
Electricity is now
produced from geothermal energy in 21 countries, including the United States.
Where Is Geothermal Energy Found?
What does geothermal
energy look like? Some visible features of geothermal energy are volcanoes, hot
springs, geysers, and fumaroles. But you cannot see most geothermal energy.
Usually geothermal energy is deep underground. There may be no clues above
ground to what exists below ground.
Geologists use many
methods to find geothermal resources. They may study aerial photographs and
geological maps. They may analyze the chemistry of local water sources and the
concentration of metals in the soil. They may measure variations in gravity and
magnetic fields. Yet the only way they can be sure there is a geothermal
resource is by drilling wells to measure underground temperatures.
The earth is a hotbed of
geothermal energy. The most active geothermal resources are usually found along
major plate boundaries where earthquakes and volcanoes are concentrated. Most
of the geothermal activity in the world occurs in an area known as the
"Ring of Fire." The Ring of Fire rims the Pacific Ocean and is
bounded by Japan, the Philippines, the Aleutian Islands, North America, Central
America, and South America.
Today's Geothermal Energy
There are four main kinds
of geothermal resources: hydrothermal, geo-pressured, hot dry rock, and magma.
Today hydrothermal resources are the only kind in wide use. The other three
resources are still in the infant stages of development.
have the common
ingredients of water (hydro) and heat (thermal). These geothermal
reservoirs of steam or hot water occur naturally where magma comes close enough
to the surface to heat ground water trapped in fractured or porous rocks, or
where water circulates at great depth along faults. Hydrothermal resources are
used for different energy purposes depending on their temperature and how deep
"Direct Use" or Heating
When the temperature of a
hydrothermal resource is around 50F and up, it can be used directly in spas or
to heat buildings, grow crops, warm fish ponds, or for other uses. Hydrothermal
resources suitable for heating occur throughout the United States and in almost
every country in the world. Most of the people in Iceland and over 500,000
people in France use geothermal heat for their public buildings, schools, and
homes. In the United States, geothermal heat pumps are used in 45 states to
heat and cool homes and buildings. Idaho, Oregon, Nevada, and some other states
use geothermal energy to heat entire districts.
Heat from geothermal
resources is also used to dry ceramics, lumber, vegetables, and other products.
When the temperature of a
hydrothermal resource is around 220F and up, it can be used to generate electricity.
Most electricity-producing geothermal resources have temperatures from 300 to
700F, but geothermal reservoirs can reach nearly 1,000F.
Two main types of
hydrothermal resources are used to generate electricity:
Dry steam reservoirs are rare but highly
efficient at producing electricity. The Geysers in California is the largest
and best known dry steam reservoir. Here, steam is obtained by drilling wells
from 7,000 to 10,000 feet deep. In a dry steam reservoir, the natural steam is
piped directly from a geothermal well to power a turbine generator. The spent
steam (condensed water) can be used in the plant's cooling system and injected
back into the reservoir to maintain water and pressure levels.
Hot water geothermal reservoirs are the most
common type. In a liquid-dominated reservoir, the hot water has not vaporized
into steam because the reservoir is saturated with water and is under pressure.
To generate electricity, the hot water is piped from geothermal wells to one or
more separators where the pressure is lowered and the water flashes into
steam. The steam then propels a turbine generator to produce electricity. The
steam is cooled and condensed and either used in the plant's cooling system or
injected back into the geothermal reservoir.
A binary cycle power
plant is used when the water in a hot water reservoir is not hot enough to
flash into steam. Instead, the lower-temperature hot water is used to heat a
fluid that expands when warmed. The turbine is powered from the expanded,
pressurized fluid. Afterwards, the fluid is cooled and recycled to be heated
over and over again.
Geothermal Energy Production and Economics
Geothermal energy is put
to work in many places around the world. The best known geothermal energy
sources in the United States are located in western states and Hawaii. Some
moderately hot geothermal resources also exist in the Dakotas, along the
Atlantic coast, and in Arkansas and Texas. Someday we may be able to use these
Most geothermal energy is
produced in four states--California, Nevada, Utah, and Hawaii. Today the total
installed capacity of geothermal power plants in the United State is 3,200
megawatts (MW) That's the energy equivalent of three nuclear power plants.
American geothermal power plants range in size from a few hundred kilowatts to
more than 130 megawatts.
In 1994, geothermal
energy produced 18 billion kilowatt hours (kWh) of electricity, or 0.3 percent
of the electricity used in this country. Still, this was enough to serve the
electrical energy needs of over three million households. California gets six
percent of its electricity from geothermal energy, more than any other state.
Geothermal supporters say
geothermal energy production will grow in the 1990s despite the fact that
geothermal energy production peaked in 1987 and has since declined. Geothermal
supporters say at least 400 MW more capacity is planned for the next five years
and estimate that geothermal energy could provide 10 percent of the electrical
capacity of the western United States by the turn of the century.
Geothermal power plants
can produce electricity as cheaply as some conventional power plants. It costs
4.5 to seven cents per kWh to produce electricity from hydrothermal systems. In
comparison, new coal-fired plants produce electricity at about four cents per
costs for geothermal power plants are high because geothermal wells and power
plants must be constructed at the same time.
But the cost of producing
electricity over time is lower because the price and availability of the fuel
is stable and predictable. The fuel does not have to be imported or transported
to the power plant. The power plant literally sits on top of its fuel source.
Geothermal power plants
are also excellent sources of base load power. Base load power is power
that electric utility companies must deliver all day long. Base load geothermal
plants sell electricity all the time, not only during peak use times when the
demand for electricity is high.
Until recently, utilities
were required to buy the least-cost electricity, without regard to
environmental impacts. Federal and state energy and environmental agencies are
studying ways to give preference to nonpolluting energy sources such as
Geothermal Energy and the Environment
Geothermal energy is a
renewable energy source that does little damage to the environment.
Geothermal steam and hot
water do contain naturally occurring traces of hydrogen sulfide (a gas that
smells like rotten eggs) and other gases and chemicals that can be harmful in
high concentrations. Geothermal power plants use "scrubber" systems
to clean the air of hydrogen sulfide and the other gases. Sometimes the gases
are converted into marketable products, such as liquid fertilizer. Newer
geothermal power plants can even inject these gases back into the geothermal
Geothermal power plants
do not burn fuels to generate electricity as do fossil fuel plants. Geothermal
power plants release less than one to four percent of the amount of carbon
dioxide (C02) emitted by coal plants.
Emissions of sulfur
compounds from motor vehicles and fossil fuel plants are also major
contributors to acid rain. Geothermal power plants, on the other hand, emit
only about one to three percent of the sulfur compounds that coal and oil-fired
power plants do. Well-designed binary cycle power plants have no emissions at
Geothermal power plants
are compatible with many environments. They have been built in deserts, in the
middle of crops, and in mountain forests.
Geothermal development is
often allowed on federal lands because it does not significantly harm the
environment. Before permission is granted, however, studies must be made to
determine what effect a plant may have on the environment. Geothermal features
in national parks, such as the geysers and fumaroles in Yellowstone and Lassen
National Parks, are protected by law, so geothermal energy is not tapped in