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Geothermal power (from the Greek roots geo, meaning earth, and thermos, meaning heat) is energy generated from heat stored in the earth, or the collection of absorbed heat derived from underground.

Prince Piero Ginori Conti tested the first geothermal generator on 4 July 1904, at the Larderello dry steam field in Italy. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California, United States. The Philippines and Iceland are the only countries to generate a significant percentage of their electricity from geothermal sources; in both countries 15-20% of power comes from geothermal plants. As of 2008, geothermal power supplies less than 1% of the world's energy. The most common type of geothermal power plants (binary plants) are closed cycle operations and release essentially no Greenhouse gas emissions; geothermal power is available 24 hours a day with average availabilities above 90% (compared to about 75% for coal plants).