Geothermal energy is produced by the heat of Earth’s molten interior. This energy is harnessed to generate electricity when water is injected deep underground and returns as steam (or hot water, which is later converted to steam) to drive a turbine on an electric power generator.
Moderate- to low-temperature geothermal resources are also used to heat buildings directly and to provide space heating through district heating systems in which heat is distributed to residences and commercial buildings from a central source. Most U.S. geothermal reservoirs are located in the western states, Hawaii, and Alaska, where the boundaries between massive plates in Earth’s crust provide a concentration of geological activity that traps the heat generated by nuclear decay of radioactive elements.
Most U.S. geothermal reservoirs are located in the western states, Hawaii, and Alaska, where the boundaries between massive plates in Earth’s crust provide a concentration of geological activity.
Although the United States generates more electricity from geothermal energy than any other country in the world, in 2015 it accounted for a small fraction of 1% of our total energy supply. That fraction is expected to grow at about 4.9% per year between now and 2040, when it is projected to make up as much as 4.6% of electricity generation. But its total contribution to U.S. energy consumption will remain less than 1% for the near future.
Conventional geothermal generation, which relies on heat sources within 3 kilometers (km) of the surface, is a mature technology with a limited resource base. A study of the western United States found that 13 gigawatts of electrical power capacity exists in identified geothermal resources in this region. Greatly expanding that base will require enhanced geothermal systems to mine heat down to a depth of 10 km. Such systems, however, face many technical challenges and are not being built presently.
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