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geothermal heating
Geothermal heating is the direct use of geothermal energy for heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. As of 2007, 28 GW of geothermal heating capacity is installed around the world, satisfying 0.07% of global primary energy consumption.〔 Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter.
Geothermal energy originates from the heat retained within the Earth since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application. However, even cold ground contains heat, below the undisturbed ground temperature is consistently at the Mean Annual Air Temperature〔(Mean Annual Air Temperature )〕 and it may be extracted with a heat pump.
== Applications ==
There are a wide variety of applications for cheap geothermal heat. In 2004 more than half of direct geothermal heat was used for space heating, and a third was used for spas. The remainder was used for a variety of industrial processes, desalination, domestic hot water, and agricultural applications. The cities of Reykjavík and Akureyri pipe hot water from geothermal plants under roads and pavements to melt snow. Geothermal desalination has been demonstrated.
Geothermal systems tend to benefit from economies of scale, so space heating power is often distributed to multiple buildings, sometimes whole communities. This technique, long practiced throughout the world in locations such as Reykjavík, Iceland,〔University of Rochester - History of the utilization of geothermal sources of energy in Iceland, ''http://www.energy.rochester.edu/is/reyk/history.htm''.〕 Boise, Idaho,〔District Heating Systems in Idaho, ''http://www.idwr.state.id.us/energy/alternative_fuels/geothermal/detailed_district.htm''.〕 and Klamath Falls, Oregon〔(Klamath Falls Geothermal District Heating Systems )〕 is known as district heating.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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