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The occurrence of favorable geothermal conditions for the commercial generation of electricity is limited geographically and geologically. Regions with geothermal potential are mainly located along belts of active magmatism, mountain building, and faulting principally localized along the boundaries of major Earth crustal [[Plate tectonics|plate]]s, belts where either new material from the [[mantle]] is being added to the [[crust]] (spreading ridges) or where crustal material is being dragged downward and consumed in the mantle ([[subduction]] zones). In both cases, molten rock is generated and moved upward into the crust and near the surface of the Earth. Geothermal energy for the commercial generation of electricity is absent in the stable [http://www.merriam-webster.com/concise/continental%20shield continental shields], which are characterized by lower-than-average geothermal gradient.
The occurrence of favorable geothermal conditions for the commercial generation of electricity is limited geographically and geologically. Regions with geothermal potential are mainly located along belts of active magmatism, mountain building, and faulting principally localized along the boundaries of major Earth crustal [[Plate tectonics|plate]]s, belts where either new material from the [[mantle]] is being added to the [[crust]] (spreading ridges) or where crustal material is being dragged downward and consumed in the mantle ([[subduction]] zones). In both cases, molten rock is generated and moved upward into the crust and near the surface of the Earth. Geothermal energy for the commercial generation of electricity is absent in the stable [http://www.merriam-webster.com/concise/continental%20shield continental shields], which are characterized by lower-than-average geothermal gradient.
[[File:Figure 1|400px|thumbnail|right|Figure 1: Remaining heat and radioactive heat [http://www.mpoweruk.com/geothermal_energy.htm].]]
==Sources of internal heat==
# 1 Remaining heat and radioactive heat (Figure 1)
* '''Geothermal Gradient''' is called to the variation of temperatura according to depth rocks are.It is read in Celcius or Farenheit grades by the unit of vertical distance.The gradient in our planet is quite variable and it depends on different causes.
* '''Surface Heat Flow''' is the heat that continuosly comes up to the surface.It is read in Joules per Second.It is estimated that the average is 42x1012J/seg and has two different sources:
* '''Remaining heat release''':It is 34x1012J/seg. It comes florm the Mantle and the Nucleus and it is the remaining heat after the formation of the planet.
* '''Radioactive heat release''':It is 8x1012 J/seg .It comes from the Crust and it is produced by the fission of radiactive isotopes.
[[File:Figure 2|thumbnail|Figure 2: Igneos phenomena Strokkur Geyser,Iceland [http://www.stewartsmithphotography.co.uk/shop/uncategorized/strokkur-geysir-national-park-iceland/]. ]]
# 2 Igneos phenomena
In certain regions the geotermal gradient can be increased considerably due to magmatic process (Figure 2).Most of the áreas of goethermal interest are situated in volcanic regions.In many places of our planet,hydrotermal evidence can be seen on the surface provided that geologial conditions are favourable.
[[File:Figure 3|thumbnail|[http://www.stewartsmithphotography.co.uk/shop/uncategorized/strokkur-geysir-national-park-iceland/].]]
# 3 Plate Tectonics and heat distribution
The unequal distribution of the heat inside the Earth is what causes the movement of tectonic plates and all of endogenous phenomena (Figure 3).All the áreas having higher heat flows are related to plate borders or Hot Spots.These higher flows are related,besides,to other parameters such as conrtical width,rocks thermic conductivity ,permeability, fluids ´ presence.etc.
==Kinds of Geothermal Deposits==
# 1 Very low temperature deposits (T<30ºC) :It is generally considered that temperature seasonal variations are percived only over the first 10 meters f the soil. After that there is a interval where the temperature is almost constant and it is considered that below 20 meters, the temperature increases three grades every 100 meters.Therefore ,all the planet is a source of geotermal resources of low temperature.These resources have domestic use.
[[File:Figure 4|thumbnail|Figure 4: Goethermal heating district. City of Boise. EEUU. [http://energy.ltgovernors.com/geothermal-resource-descriptions.html].]]
# 2 Low temperature fields (30ºC<T<90ºC) :They are found in sedimentary basins with normal o hightly superior geotermal gradient.The sole condition is the existence of one or more aquifer horizons in adequate depths.Ej Paris Basin (France) ;Boise (EEUU) (Figure 4).
# 3 Medium temperature deposits (90ºC<T<150ºC): They are foundin various places of our planet.They usually belong to áreas with specific geologic characteristics such as áreas of thin crust,higher concentration of radioactive isotopes or áreas close to or near igneous bodies.They can also be found over regions with higher gradient but a lower depth.They generally need an igneous intrusión and it is usual that,in this kind of field,water comes up to the surface forming hot springs.
[[File:Figure 5|thumbnail|Simplified diagram of a geotermal power plant. [http://www.bgs.ac.uk/research/energy/geothermal/].]]
# 4 High temperature fields (T>150ºC): These deposits are found in áreas of higher igneous activity.They are related to extremely high gradients, corresponding to global tectonic process.They occur in plate borders, cortical thinnes áreas or Hot Spots. They need an igneous source an aquifer and a impermeable layer to keep up pressure. In many cases there is surface evidence suchs as geysers and fumaroles
# 5 Hot dry Rock Deposits or Geotermal Stimulated Systems
They are a kind of high temperature field but in wich the content off water and permeability is improved artificially by means of hydraulic fracture (the same method used in unconventional hydrocarbons resources).An example of this is the Soultz-sous-Forets Geothermal Field in France,where the heat source is a fractured granitic pluton
==Extraction and uses==
[[File:Figure 6|thumbnail|Figure 6. Simplified diagram of a geotermal plant [https://www.linkedin.com/pulse/smart-grid-energy-harvesting-martin-ma-mba-med-gdm-scpm-pmp].]]
A geotermal central Works similarly to a thermic one :The only variation is the way to heat water.Water vapour at high temperaturas (up to 600ºC) is directed from the inside the earth to the central, allowing the evaporation of water present in different pipes surrounding the boiler. The water vapour reches higher pressure,thus it is used to move a turbine connected to the generator .When the turbine turns,electricity is produced, wich goes from the generator to the transformers. These increase the tensión of electricty, to transpor this energy along electrical network to home users.The refrigeration system allows to condenses the water vapour,so that can be used again.
==Sources==
* Guia de la Energia Geotermica;2008;Fundacion de la Energia de la comunidad de Madridhttp://www.fenercom.com/pages/publicaciones/publicacion.php?id=50
* British Geological Survey pagehttp://www.bgs.ac.uk/research/energy/geothermal/
* Página del Instituto cartográfico y Geológico de Cataluña http://www.igc.cat/web/es/subsol_geotermia_jaciment.html
* “EnergiaGeotermica”.Publicacion de la Secretaria de Energia de la República Argentina. http://www.energia.gov.ar/contenidos/archivos/publicaciones/libro_energia_geotermica.pdf
==See also==
==See also==