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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 [[plate]]s, 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 [http://www.merriam-webster.com/concise/continental%20shield continental shields], which are characterized by lower-than-average geothermal gradient.

==See also==

==See also==