geothermal power Resources

The heat content of the earth is 1031 Joules, enough to theoretically satisfy global energy demand for 10 billion years. This heat naturally flows up to the surface by conduction at a rate of 45 TW, or three times the rate of human consumption from all primary energy sources. However, the bulk of this natural flow is too geographically diffuse (0.1 W/m2 on average) to be recoverable. The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) in order to release the heat underneath.
In addition to heat emanating from deep within the Earth, the top 10 m of the ground accumulates solar energy (i.e. warms up) during the summer, and releases that energy (i.e. cools down) during the winter. The seasonal energy stored this way is much smaller in total scale and less dense, but the heat flow rates are much higher, more easily accessible, and evenly distributed around the world. A geothermal heat pump can extract enough heat from shallow ground anywhere in the world to provide wintertime home heating.
Electricity generation requires high temperature resources that can only come from deep underground. The heat must be carried to the surface by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation, oil wells, drilled water wells, or a combination of these. This circulation sometimes exists naturally in the most favorable areas where the crust is thin: magma conduits bring the heat close to the surface, and naturally occurring hot springs bridge the last gap to the surface. If no hot spring is available, a well must be drilled into a hot aquifer. Away from tectonic plate boundaries the geothermal gradient is 25-30°C per km of depth in most of the world, and wells would have to be drilled several kilometers deep to permit electricity generation. The quantity and quality of recoverable resources improves with drilling depth and proximity to tectonic plate boundaries.
In ground that is hot but dry, or where water pressure is inadequate, it is possible to inject a fluid to stimulate production. Two boreholes are bored into a candidate site, and the deep rock between them is fractured by explosives or high pressure water. Water is pumped down one borehole and steam comes up the other. Liquefied carbon dioxide may also be used. This concept is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America. A much greater resource potential may be available from this approach than from conventional tapping of natural aquifers.

Estimates of the electricity generating potential of geothermal energy vary greatly from 35 to 2000 GW, depending on the scale of financial investments in exploration and technology development. This does not include non-electric heat recovered by co-generation, geothermal heat pumps and other direct use. A 2006 report by MIT, that took into account the use of enhanced geothermal system, estimated that an investment of 1 billion US dollars in research and development over 15 years would permit the development of 100 GW of generating capacity by 2050 in the United States alone. The MIT report estimated that over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's present energy needs for several millennia.
At present, geothermal wells are rarely more than 3km deep. Upper estimates of geothermal resources assume wells as deep as 10 km. Drilling at this depth is now possible in the petroleum industry, although it is an expensive process. For example, Exxon has announced an 11-kilometre (7 mi) hole at the Chayvo field, Sakhalin, and a 12 km well has been reported on the Kola peninsula. Wells drilled to depths greater than 4 kilometres (2 mi) generally incur drilling costs in the tens of millions of dollars.[citation needed] The technological challenges are to drill wide bores at low cost and to break rock over larger volumes.
Geothermal power is considered to be sustainable because the heat extraction is small compared to the Earth's heat content, but extraction must still be monitored to avoid local depletion. Although geothermal sites are capable of providing heat for many decades, individual wells may cool down or run out of water. The three oldest sites, at Larderello, Wairakei, and the Geysers have all reduced production from their peaks. It is not clear whether these plants extracted energy faster than it was replenished from greater depths, or whether the aquifers supplying them are being depleted. If production is reduced, and water is reinjected, these wells could theoretically recover their full potential. These mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy production has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960.

geothermal power Development around the world

Geothermal electricity is generated in 24 countries around the world including the United States, Iceland, Italy, Germany, Turkey, France, The Netherlands[citation needed], Lithuania[citation needed], New Zealand, Mexico, El Salvador, Nicaragua, Costa Rica, Russia, the Philippines, Indonesia, the People's Republic of China, Japan and Saint Kitts and Nevis. During 2005, contracts were placed for an additional 0.5 GW of electrical capacity in the United States, while there were also plants under construction in 11 other countries. A number of potential sites are being developed or evaluated in South Australia that are several kilometres in depth. When direct use is included, geothermal power is used in over 70 countries.

Installed geothermal electric capacity as of 2007

country	Capacity (MW)
USA	2687
Philippines	1969,7
Indonesia	992
Mexiko	953
Italy	810,5
Japan	535,2
New Zealand	471,6
Iceland	421,2
El salvador	204,2
Costa Rica	162,5
Kenya	128,8
Nicaragua	87,4
Russia	79
Papua-New Guinea	56
Guatemala	53
Turkey	38
China	27,8
Portugal	23
France	14,7
Germany	8,4
Ethiopia	7,3
Austria	1,1
Thailand	0,3
Australia	0,2

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