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Geothermal
Geothermal Resources: Where and How Much
The geothermal resource challenge is finding water and/or steam that is both close to the surface, hot enough, and available in sufficient quantities to make it worth exploiting. There are many locations with water that is both accessible and hot enough to support direct-use. Geothermal resources that can support electricity generation, however, are much less common. There are only a relatively small number of known locations in the world with geothermal resources sufficient for electricity generation; many of these are located in the Western United States.
Geothermal electricity generation requires large quantities of steam and/or water that is at least 100 degrees C (and preferably much hotter), close to the surface, and not easily tapped out. Locating these resources is similar to oil and gas exploration: it's possible to locate areas that are promising by examining the local geological conditions, but ultimately one has to drill an exploratory well to see what's really down there.
Even then, it's not clear how long that resource will last. Output at the world's largest geothermal complex, The Geysers, has dropped due to geothermal resource depletion. However, that depletion is currently being remedied by reinjecting treated wastewater from Santa Rose to recharge the aquifer. Production has increased since the inception of that project. Modern reservoir modeling techniques are now used to estimate the level of electric production compatible with sustaining the geothermal resource.
Ultimately, the decision of whether a specific site can support electricity generation is one of economics, educated guesswork, and risk preference.
There's little doubt that there remains a technical potential for increased geothermal electricity generation in the Western United States. One estimate puts the known, accessible potential at more than 20 GW. How much of this potential makes economic sense to develop is not clear, and depends in large part on prevailing power prices and future market conditions.
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