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Geothermal resources throughout the world have been used for various purposes for centuries. Hot springs were used for bathing and medicinal purposes in ancient Greece, Rome, Babylonia, and Japan, among many other places. Medicinal spas are still popular resorts throughout the world.

Geothermal resources throughout the world have been used for various purposes for centuries. Hot springs were used for bathing and medicinal purposes in ancient Greece, Rome, Babylonia, and Japan, among many other places. Medicinal spas are still popular resorts throughout the world.

Geothermal hot water has been used directly for space and district heating, for aquacultural and agricultural purposes, and for a variety of industrial applications. Much of Reykjavik, the capital of Iceland, located on the Mid-Atlantic spreading ridge, has been heated since the 1930s with geothermal hot water. Many other countries, such as Japan, New Zealand, Hungary, and the United States (Boise, Idaho, and Klamath Falls, Oregon, for example), use geothermal energy for space heating. Such direct use of geothermal energy, in fact, is possible virtually anywhere in the world.

Geothermal hot water has been used directly for space and district heating, for aquacultural and agricultural purposes, and for a variety of industrial applications. Much of Reykjavik, the capital of Iceland, located on the [https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Mid-Atlantic_Ridge.html Mid-Atlantic spreading ridge], has been heated since the 1930s with geothermal hot water. Many other countries, such as Japan, New Zealand, Hungary, and the United States (Boise, Idaho, and Klamath Falls, Oregon, for example), use geothermal energy for space heating. Such direct use of geothermal energy, in fact, is possible virtually anywhere in the world.

Direct use of geothermal energy, although considerable now and possibly a sizable source of heat in the future, is not included in the following discussion. The discussion is restricted to the use of geothermal energy for the generation of electricity, which was accomplished for the first time at [http://geoheat.oit.edu/bulletin/bull25-3/art2.pdf Larderello], Italy, in 1904. Although the first generator produced only enough electricity for five light bulbs, the Larderello geothermal field has been generating electricity on a commercial scale since 1913.

Direct use of geothermal energy, although considerable now and possibly a sizable source of heat in the future, is not included in the following discussion. The discussion is restricted to the use of geothermal energy for the generation of electricity, which was accomplished for the first time at [http://geoheat.oit.edu/bulletin/bull25-3/art2.pdf Larderello], Italy, in 1904. Although the first generator produced only enough electricity for five light bulbs, the Larderello geothermal field has been generating electricity on a commercial scale since 1913.

[[file:NesjavellirPowerPlant_edit2.jpg|thumb|400px|The Nesjavellir Geothermal Power Plant in Þingvellir, Iceland. By Gretar Ívarsson. Courtesy [https://en.wikipedia.org/wiki/Renewable_energy Wikipedia].]]

[[file:NesjavellirPowerPlant_edit2.jpg|thumb|400px|The Nesjavellir Geothermal Power Plant in Þingvellir, Iceland. By Gretar Ívarsson. Courtesy [https://en.wikipedia.org/wiki/Renewable_energy Wikipedia].]]

Economically significant concentrations of geothermal energy occur now where high temperatures (40° to more than 380°C; 104° to more than 716°F) are found in porous and permeable rocks at shallow depths (less than 3000 m; about 10,000 ft). The geothermal energy is stored in both the solid rock and the water or steam-filling pores and fractures. The steam or hot water are used mainly as the fuel for the operation of electricity-generating turbines or for space heating.

Economically significant concentrations of geothermal energy occur now where high temperatures (40° to more than 380°C; 104° to more than 716°F) are found in [[Porosity|porous]] and [[Permeability|permeable]] rocks at shallow depths (less than 3000 m; about 10,000 ft). The geothermal energy is stored in both the solid rock and the water or steam-filling pores and [[fracture]]s. The steam or hot water are used mainly as the fuel for the operation of electricity-generating turbines or for space heating.

[[file:St54Figure48.JPG|thumb|300px|World geothermal electricity generation.<ref name=Salvador_2005>Salvador, Amos, 2005, Energy-A historical perspective and 21st century forecast: AAPG Studies in Geology 54, 208 p.</ref>]]

[[file:St54Figure48.JPG|thumb|300px|World geothermal electricity generation.<ref name=Salvador_2005>Salvador, Amos, 2005, Energy-A historical perspective and 21st century forecast: AAPG Studies in Geology 54, 208 p.</ref>]]

There are, therefore, three main requirements for the commercial development of geothermal resources: shallow high temperatures, rocks with good permeability, and sufficient volumes of water.

There are, therefore, three main requirements for the commercial development of geothermal resources: shallow high temperatures, rocks with good permeability, and sufficient volumes of water.

At present, there are two major types of commercial geothermal systems for the generation of electricity: water-dominated (hot-water) systems and vapor-dominated (dry-steam) systems. Hot-water systems are much more common than dry-steam systems, of which only four commercial fields are known at present: The Geysers, 115 km (70 mi) north of San Francisco, California; Larderello, in central Italy; Matsukawa in Japan; and possibly Kawah Kamojang in western Java, Indonesia.

At present, there are two major types of commercial geothermal systems for the generation of electricity: water-dominated (''hot-water'') systems and vapor-dominated (''dry-steam'') systems. Hot-water systems are much more common than dry-steam systems, of which only four commercial fields are known at present: The Geysers, 115 km (70 mi) north of San Francisco, California; Larderello, in central Italy; Matsukawa in Japan; and possibly Kawah Kamojang in western Java, Indonesia.

A third type, the hot-dry-rock system (HDR), is designed to recover heat from impermeable hot dry rocks at depths of 1500–2000 m (5000–6500 ft) by drilling two closely located wells into them, artificially fracturing the rocks, and injecting water through one well and recovering it from the other after having been heated by the hot rocks. Experimental attempts to recover geothermal heat in this way have so far failed to demonstrate that hot-dry-rock systems are economically viable. In these systems, energy recharge is only by thermal conduction and, because of the slowness of this process, their geothermal energy should be considered exhaustible.<ref name=Stefansson_2000>Stefansson, V., 2000, The renewability of geothermal energy: Proceedings of the World Geothermal Congress 2000 (Japan), p. 883-888.</ref> Some experts, however, believe that the long-range future of geothermal energy may depend on HDR systems becoming a technological and economic reality.<ref name=Worldenergycouncil_2001>World Energy Council, 2001, [http://www.worldenergy.org/documents/ser_sept2001.pdf Survey of energy resources].</ref>

A third type, the ''hot-dry-rock system'' (HDR), is designed to recover heat from impermeable hot dry rocks at depths of 1500–2000 m (5000–6500 ft) by drilling two closely located wells into them, artificially fracturing the rocks, and injecting water through one well and recovering it from the other after having been heated by the hot rocks. Experimental attempts to recover geothermal heat in this way have so far failed to demonstrate that hot-dry-rock systems are economically viable. In these systems, energy recharge is only by thermal conduction and, because of the slowness of this process, their geothermal energy should be considered exhaustible.<ref name=Stefansson_2000>Stefansson, V., 2000, The renewability of geothermal energy: Proceedings of the World Geothermal Congress 2000 (Japan), p. 883-888.</ref> Some experts, however, believe that the long-range future of geothermal energy may depend on HDR systems becoming a technological and economic reality.<ref name=Worldenergycouncil_2001>World Energy Council, 2001, [http://www.worldenergy.org/documents/ser_sept2001.pdf Survey of energy resources].</ref>

The occurrence of favorable geothermal conditions for the commercial generation of electricity is limited geographically and geologically. Regions with geothermal potential are mainly located along belts of active magmatism, mountain building, and faulting principally localized along the boundaries of major Earth crustal plates, belts where either new material from the [[mantle]] is being added to the crust (spreading ridges) or where crustal material is being dragged downward and consumed in the mantle ([[subduction]] zones). In both cases, molten rock is generated and moved upward into the crust and near the surface of the Earth. Geothermal energy for the commercial generation of electricity is absent in the stable continental shields, which are characterized by lower-than-average geothermal gradient.

The occurrence of favorable geothermal conditions for the commercial generation of electricity is limited geographically and geologically. Regions with geothermal potential are mainly located along belts of active magmatism, mountain building, and faulting principally localized along the boundaries of major Earth crustal plates, belts where either new material from the [[mantle]] is being added to the crust (spreading ridges) or where crustal material is being dragged downward and consumed in the mantle ([[subduction]] zones). In both cases, molten rock is generated and moved upward into the crust and near the surface of the Earth. Geothermal energy for the commercial generation of electricity is absent in the stable continental shields, which are characterized by lower-than-average geothermal gradient.