Heat from the hot core of the earth can be extracted and used in various ways (including ground source heat pumps described separately)
Hot dry rocks
One form of geothermal energy is produced using hot rocks, a few kilometers beneath the earth’s surface. Water is pumped into this hot, crystalline rock using an injection well. As it flows through fractures in the rock the water heats up and is returned to the surface through another well, known as a production well. At the surface the heat is extracted from the water and using a steam turbine, generates electricity. The water is then recalculated to mine more heat.
Geothermal energy not only has little impact on climate or the environment but it is also clean, quiet and virtually inexhaustible. One cubic kilometer of hot granite at 250 degrees centigrade is said to have the stored energy equivalent of 40 million barrels of oil.
Hot dry rock (HDR) energy is increasingly becoming an important source of energy but only a small number of locations currently have the right conditions for cost effective production. The granite rock must be no deeper than 5 kilometers, the current capability of drilling equipment, and it must be covered by around 3 kilometers of insulating rock which prevents the heat escaping to the surface.
Unlike hot dry rocks, where water is pumped through the underground rocks, an aquifer is an underground layer of permeable rock in which water already naturally occurs. As the hot water flows through the rock it can be extracted using a borehole.
Water temperatures between 50 and 150 degrees centigrade can be used for heating and higher temperatures are used to produce electricity. Those areas with the best geological conditions for producing electricity usually occur close to crustal plate margins.
After the heat has been extracted the cooled water is then pumped back into the ground. Whilst geothermal aquifers are not completely renewable as heat is usually extracted at a rate quicker than it is replenished by the surrounding rocks, geothermal energy is much less environmentally damaging than fossil fuels.
Geothermal power plants emit 1000-2000 times less carbon dioxide than fossil fuel power plants and take up a much smaller area of land.