Definition of Geothermal Power
Geothermal relates to the heat of the Earth's interior. This abundant source of natural heat can be used to generate power or energy. Once heat has been mined, electricity can be produced with the same basic turbine/generator technology used since 18th century steam engines.
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History
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Geothermal comes from the Greek words for Earth, geo, and heat, thermos. Geothermal power has been used for heating and bathing since ancient Roman times, but today it is better known for generating electricity.
Significance
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Geothermal power is a source of renewable energy, which is free of potentially dangerous emissions, and an attractive alternative to dirtier fuel sources. In addition, geothermal heating, theoretically, can be used in most areas of the world, because underground air temperature is stable, usually around 55 degrees Fahrenheit.
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Resource Types
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There are four categories of geothermal resource types: hydrothermal, magma, geo-pressured and hot dry rock. Hydrothermal and magma resources are found in areas of high tectonic activity. Hydrothermal forms when moisture percolates to a heat source and convectively rises, appearing as hot springs, geysers and similar surface activity. Magma is often difficult to reach because of problems with extreme heat and drilling equipment. Geo-pressured sources are usually found in areas of sandstone-shale formations. These have relatively low thermal conductivity, which can result in a steep underground pressure gradient. This gradient, with the presence of water, can form a sort of deep aquifer. Tapping this source can create subsidence, and the water often is extremely saline. Hot dry rock is available anywhere if a well is drilled deep enough. All is needed is rock that can heat liquid to about 302 degrees Fahrenheit for power generation, and 112 to 212 degrees Fahrenheit for heating. As a result, hot dry rock can be used for a closed-loop system such as a binary power plant.
Power Plant Types
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The three major types of power plants today are the dry steam, flash, and binary systems. Dry steam uses steam to directly turn a turbine which fires the generator to make electricity. According to the U.S. Department of Energy (DOE) this is the oldest of the geothermal power plants, having been first used in Italy in the year 1904. A flash plant takes liquids at 680 degrees Fahrenheit or hotter, and converts it to vapor within a flash tank. The vapor then turns the turbine. A binary system uses a second liquid along with the geothermal liquid, and runs them through a heat exchanger to create vapor, which turns the turbine. The DOE states that most future systems will be binary. The advantage of these three closed-loop systems is no resources are actually extracted to produce heat.
Potential
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A 2006 Massachusetts Institute of Technology study finds that the United States has the potential for to use 100 giga-watts of geothermal power in the next 50 years.
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