Geothermal energy comes from within the earth. It may be the result of the decay of radioactive substances, chemical reactions, friction from the movement of the continents or heat present when the earth formed.
Most of this heat is at depths beyond the reach of current technology. One of the most famous examples of geothermal energy is the geyser Old Faithful in Yellowstone National Park in the United States.
The four basic forms of geothermal energy are dry steam, hot water (or wet steam), hot dry rock and geopressurized systems. Dry steam occurs only in a few places, but it is the only one of the forms that is in commercial use.
Dry steam
The Geysers plant north of San Francisco, California, uses dry steam to run turbine generators, producing more than 500 megawatts of electric power. Operators pipe dry steam from natural underground reservoirs through a conventional steam turbine generator to produce electricity. The system converts the steam to water in a condenser and returns the water to the earth.
Hot water
Hot rock far beneath the earth's surface heats underground water to temperatures up to 2,200 degrees Fahrenheit. Pressure keeps the water in liquid form. The hot water flows to the surface through wells. Deprived of its pressure, it becomes steam to drive a steam turbine directly or to heat another fluid to run a turbine. Hot water geothermal energy provides central heating for all the buildings in Reykjavik, Iceland.
Hot dry rock
Extracting energy from subterranean hot dry rock means introducing a heat exchange fluid (water) to carry the heat from the rock to the power plant. Scientists inject water deep into fractured hot rock. Then they use the heated water as geothermal water for conversion to useful energy.
Geopressurized systems
Reservoirs of hot water mixed with methane gas, trapped underground, offer the energy potentials of both pressure and burnable methane, as well as the heat energy available from any geothermal resource.