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Geothermal energy: the Earth's heat as an energy resource
The geothermal energy is generated from Earth's heat. It is a renewable energy because the velocity of Earth's cooling is about 130 ºC to each 1000 million years, and this value is assumed to be unlimited at a human scale. It is also considered as a renewable resource of very low environmental impact which is present throughout the planet. Its direct use is the thermal use, which has been widely applied during the history, for example in ancient baths, spas or Provence wells, and is currently used in many countries for heating and air-conditioning.
The heat energy of the Earth is also used for electric power generation in places with high geothermal gradient values, where the groundwater reaches temperatures of above 100 ºC at depths and volumes that make profitable its exploitation. It should be noted that in regions where the geothermal gradient is normal, 3 ºC per 100m, wells must be drilled to depths of more than 4km to achieving enough temperature to produce electricity. The most promising technique for producing electricity in this context is called EGS (Enhanced Geothermal System), which generates high temperature reservoirs.
Direct exploitation of subsurface heat is mainly used in industrial processes that require hot water, as fish farms, climate control of greenhouses, climate control of buildings, or using a distribution network for an entire neighbourhood, or an entire city (district heating and cooling systems). Nowadays the term "geothermal energy" is currently applied to the process of using the energy from the near subsurface with geothermal heat pumps for buildings climate control.
