Geothermal energy is heat (thermal) derived from the earth (geo). It is the thermal energy contained in the rock and fluid (that fills the fractures and pores within the rock) in the earth's crust. In most areas, this heat reaches the surface in a very diffuse state. However, due to a variety of geological processes, some areas, including substantial portions of the Western United States, are underlain by relatively shallow geothermal resources. Below the crust of the earth, the top layer of the mantle is hot, liquid rock called magma. The crust of the earth floats on this liquid magma mantle. When magma breaks through the surface of the earth in a volcano, it is called lava.

For every 330 ft (100 m) you go below ground, the temperature of the rock increases about 5.5°F (3°C). Deep under the surface, water sometimes makes its way close to the hot rock and turns into hot water or into steam. The hot water can reach temperatures of more than 300°F (148°C). This is higher than the boiling point for water; it is only because the water is contained under pressure that it does not turn to steam. When this hot water comes up through a crack in the earth, it immediately flashes to steam and we call it a geyser or hot spring, the most famous of which is the Geysers in California. Sometimes, people use the hot water in swimming pools or in health spas.

The current production of geothermal energy from all uses places third among renewables, following hydroelec-tricity and biomass, and ahead of solar and wind. Geothermal resources can be classified as low temperature, less than 194°F (90°C), moderate temperature, 194 to 302°F (90 to 150°C), and high temperature, greater than 302°F (150°C). The application of these resource is influenced by temperature, pressure, location, and accessibility. The highest temperature resources are generally used only for electric power generation. Current U.S. geothermal electric power generation totals more than 2,200 MW Uses for low and moderate temperature resources can be divided into two categories: direct use and ground-source heat pumps.

• Direct use, as the name implies, involves using the heat in the water directly (without a heat pump or power plant) for such applications as heating of buildings, industrial processes, greenhouses, aquaculture (growing of fish), and resorts. Direct-use projects generally operate at temperatures between 100 to 300°F (38 to 149°C) from hot water found near the earth's surface. Current U.S. installed capacity of direct use systems totals more than 1,600 MMBtu/h (470 MW), or enough to heat 40,000 average-sized houses.

• Using resource temperatures of 40 to 100°F (4 to 38°C), the geothermal heat pump, a device which moves heat from one place to another with the addition of mechanical energy, uses the relatively constant temperature of soil or surface (ground) water as a heat source and sink for the heat pump, which provides heating and/or cooling for buildings.

Historically, the extraction and use of geothermal energy for electric generation systems has focused on dry or vapor-dominated and wet or liquid-dominated (partially flashed steam from hot water at or near saturation temperature) sources. Bottom-hole temperatures as high as 800°F (427°C) have been reported at a depth of 8,100 ft (2,469 m).

Under-developed resource types of geothermal energy include hot dry rock and geopressured zones. The hot dry rock category, which is believed to offer much promise as a useful energy source, is a geological formation that possesses a high heat content, but no meteoric or magmatic waters to provide a heat-transport medium. Hence, the injection of water is required in order to carry heat to the surface. In these areas, the thermal gradient exceeds 45°F per mile (15°C per kilometer), so very deep drilling is not required. While the thermal conductivity of rock may limit energy transfer, there remains vast untapped energy stored in this form throughout many mountain ranges in the country. Geopressed zones, which are found at depths of 5,000 to 20,000 ft (1,500 to 6,100 m), contain water at temperatures ranging from 140 to 375°F (60 to 190°C) at fluid pressures from 3,000 to 14,000 psi (207 to 966 bar). There is a high degree of potential energy value available in this high-pressure fluid that can be applied to electric generation. The potential of this resource is enormous, but so far, no technology exists to extract energy in a commercially useable way.

Note that geothermal energy does pollute the environment with moderate amounts of sulfur and carbon emissions. However, it is believed that since the tapped sources are natural, much of this would occur anyway.

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