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Geothermal energy applications use heat from the earth (geo + thermal) for a variety of purposes, including electrical generation, process heat, greenhouses and aquaculture, melting of snow and ice on roads and sidewalks, and the heating and cooling of buildings. Geothermal heat consumes no fuel (it is driven by radioactive processes deep in the earth), produces little or no greenhouse gas and -- because the processes are fundamentally simple -- is extremely reliable, averaging 95% online availability.

Geothermal installations range from simple backyard wells to industrial-scale plants that drill hundreds or thousands of feet down to get to hot rock or steam. Because the cost of a plant is directly proportional to how deep the well must go to reach the rock or steam, most of the industrial plants in the U.S. are located in the western states, Alaska and Hawaii, where geothermal energy is closer to the surface. In contrast, geothermal heat for buildings can be tapped anywhere in the country, even New England.

Geothermal space heating utilizes heat-pump technology, which takes advantage of the fact that, a few feet underground, the earth remains at a relatively constant temperature year-round. A geothermal heat pump warms in winter by withdrawing the underground heat and concentrating it by means of a refrigerant, then cools in the summer by withdrawing the heat from a room via the refrigerant and sinking it into the relatively cool well. 

The initial barrier to heat pump technology, whether residential or industrial, is the high cost of the wells and equipment -- a cost which is soon paid back by the use of free energy from the earth's crust.