Science Fair Project Encyclopedia
Geothermal power is generally harnessed in one of three ways. Large scale electrical generation is possible in areas near geysers or hot springs by utilizing naturally occurring steam, superheated ground water or using geothermal heat to heat a heat-transfer fluid. Experiments are testing whether a fourth method, deep wells into "hot dry rocks" can be economically used to heat water pumped down from the surface. A hot dry rock project in the United Kingdom was abandoned after it was pronounced economically unviable in 1989. HDR programs are currently developed in Australia, France, Switzerland and Germany.
Geothermal-generated electricity was first produced at Larderello, Italy, in 1904. Since then, the use of geothermal energy for electricity has grown worldwide to about 8,000 megawatts of which the United States produces 2700 megawatts. The largest dry steam field in the world is The Geysers, about 90 miles north of San Francisco began in 1960 which produces 2000 MWe. Calpine Corporation is currently the United States largest producer of renewable geothermal energy.
Geothermal power is generated in over 20 countries around the world including Iceland (producing 17% of its electricity from geothermal sources), the United States, Italy, France, New Zealand, Mexico, the Philippines, Indonesia and Japan. Canada's government (which officially notes some 30,000 earth-heat installations for providing space heating to Canadian residential and commercial buildings) reports a test geothermal-electrical site in the Meager Mountain - Pebble Creek area of B.C, where a 100 MWe facility might be developed at that site.
In some locations, the natural supply of water producing steam from the hot underground magma deposits has been exhausted and processed waste water is injected to replenish the supply. In at least one location, this has resulted in small but frequent earthquakes (see external link below)
Although geothermal sites are capable of providing heat for many decades, eventually they are depleted as the ground cools.  The government of Iceland states It should be stressed that the geothermal resource is not strictly renewable in the same sense as the hydro resource. It estimates that Iceland's geothermal energy could provide 15 TWh per year over 100 years, compared to the current production of 1.2 TWh per year. 
- US Department of Energy pages on geothermal energy
- A University of Alaska article on geothermal energy
- Earthquakes due to geothermal energy production - A real-time one week duration earthquake map that always shows a number of small earthquakes due to water injection and geothermal production - observe the upper left corner (Cloverdale) - this is the site of a geothermal plant called "The Geysers"
- Hot Dry Rock Geothermal Energy Technology
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