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Unit 10: Energy Challenges // Section 9: Geothermal Energy

Geothermal power systems tap our planet's natural radioactive energy and the fact that temperature and pressure inside Earth increase with depth. Earth's geothermal gradient is steeper in some regions than others, generally because of volcanic activity or large natural deposits of naturally radioactive material in granitic rocks. Energy companies can drill a mile or more to tap underground reserves of steam and hot water, much in the same way as they drill for oil and natural gas.

Early geothermal plants used steam pumped directly from underground. Today, however, most geothermal power plants pump water down into wells, use subsurface heat to warm it, and return it to the surface to form steam, which drives electric turbines to generate electricity. Geothermal power has been an established technology since the early 20th century and is economically viable in geologically suitable sites, such as in the Geysers field in northern California or in Iceland, which produces most of its energy in this way.

Geothermal energy is considered a renewable resource because it draws from the essentially unlimited heat in the Earth's interior. Where resources are good, it produces reliable power with virtually no atmospheric pollutants or greenhouse gas emissions. In the United States, most of the best geothermal resources are located west of the Mississippi River (Fig. 17). It has proven difficult to extend this technology to areas with great demand for electricity, such as the eastern United States and much of Europe, because the local geology does not provide sufficiently high subsurface temperatures. Thus, geothermal power is a minor component of energy supply in most parts of the world today. In the future, it may prove viable in areas of tremendous geothermal potential to use excess geothermal energy to produce more transportable forms of energy—for example, by extracting hydrogen from sea water.

U.S. geothermal resources (estimated temperature at 6 kilometers depth)

Figure 17. U.S. geothermal resources (estimated temperature at 6 kilometers depth)
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Source: © 2006. United States Department of Energy. Energy Efficiency and Renewable Energy.

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