Geothermal energy is one of the less known renewable energy sources and is stored beneath the Earth's surface as heat, and is linked to volcanoes, hot springs, fumaroles and geysers.
So, unlike other renewable energies whose source is solar radiation, whether directly as solar thermal and photovoltaic or indirectly as wind, hydro and biomass energy, geothermal energy comes from the heat inside the Earth; it is heat that feeds on the one hand, from the decay of radioactive isotopes, and on the other hand, from differential movements among the various layers that make up the Earth and from the latent heat of the outer core crystallisation.
Considering the whole Earth's surface, the total geothermal power that comes from the inside is 4.2 x 1012 J. It is an immense amount of energy, but only a fraction of it can be used by humankind.
Geothermal energy is, in its broadest sense, the earth heat energy transmitted from the inner layers to the outermost part of the Earth's crust.
Thus, the portion of the heat detached from the inside of the earth, which can be exploited by man in technical and economic conditions, is called geothermal resource. The latter are classified according to the temperature of the geothermal fluid, which will determine its uses and applications. Therefore, the aim of geothermal energy is the exploitation of this heat energy from the inside of the earth.
The high temperature geothermal resources are used mainly for production of electricity when it comes to high temperature reservoirs (higher than 100-150 °C). When the reservoir’s temperature is not enough to produce electricity, its main uses are thermal in industrial, residential and service sectors. For temperatures below 100 °C, a direct use or through a geothermal heat pump (heating and cooling) can be made. When it comes to very low-temperature resources (below 25 ºC) the possibilities of use lie with air conditioning and hot water provision.
So far, the use of this energy in the world has been limited to areas where geological conditions are very favourable. But current technological advances for equipment and improvements in exploration and drilling allow nowadays geothermal energy to count on technology for electricity production from geothermal resources at significantly lower temperatures as needed years ago, and the artificial generation of "stimulated reservoirs" (EGS), which requires direct human intervention to create the reservoir; this adds great future potential for high-temperature geothermal energy.
Geothermal energy is a form of sustainable energy for a sustainable use currently and future, both from the standpoint of highly-securised energy supply, and from the thermal point of view, as an alternative high-efficient energy as opposed to conventional heating and cooling systems.
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