The heat content of the Earth is approximately 10 joules (Annex: Orders of magnitude (energy)").[16]​ This heat naturally flows to the surface by conduction at a rate of 44.2 TW[17]​ and is replenished at a rate of 30 TW by radioactive decay.[4]​ These rates are more than double humanity's current primary energy consumption, but most of it is too diffuse—approximately 0.1 W/m² on average— enough to take advantage of. The Earth's crust acts as a thick insulating layer that must be pierced by pipes—or magma, water, etc.—to allow heat to be emitted from within.

The generation of electricity requires high temperatures that can only come from deep areas. Heat must be transported to the surface through magmatic conduits, hot springs, hydrothermal circulation, oil wells, water wells, or a combination of all of these. This circulation can occur naturally when the crust is thin: magmatic conduits transport heat to the crust and hot springs bring heat to the surface. If hot springs do not exist, a well must be drilled above a hot water table. Away from plate tectonic confluence zones, the geothermal gradient is 25-30 °C per kilometer of depth in most of the world, so wells would need to be several kilometers deep to allow for electricity generation.[16] The quantity and quality of recoverable resources increases with drilling depth and proximity to plate boundaries.

In soil that is hot but dry, or where water pressure is not adequate, fluid injection can stimulate production. Developers drill two holes at a candidate mining site and break up the rock using explosives or high-pressure water. They then pump water or liquefied carbon dioxide through one well and it comes out the other in gaseous form.[11] This technique is known as hot dry rock") (HDR) in Europe and enhanced geothermal systems") (EGS) in North America. Through it, a much greater energy potential would be available than with the conventional use of natural water tables.[11]​.

Estimates of potential geothermal electricity generation capacity range from 35 to 2000 GW, depending on the scale of investments.[16] This does not include geothermal energy that is not transformed into electricity, such as through cogeneration, geothermal heat pumps or other direct uses. A 2006 report from the Massachusetts Institute of Technology, which included the potential of EGS systems, estimated that an investment of $1 billion for research and development over the next 15 years would enable the creation of 100 GW of geothermal electrical capacity by 2050, in the United States alone.[11] This report also estimated that more than 200 zettajoules "Joule (unit)") (ZJ) would be extractable, with the potential to increase this figure to more than 2000 ZJ through technological improvements, enough to supply all of the world's current energy demand for several millennia.[11]​.

Currently, geothermal wells rarely exceed 3 km in depth.[16] Deeper estimates assume the existence of wells up to 10 km. Drilling to this depth is possible today thanks to the oil industry, although it is an expensive process. The deepest research well in the world is the Kola superdeep well, 12.3 km long.[18] This record has recently been equaled by commercial wells such as the Z-12 of ExxonMobil's Sakhalin-I project in Chayvo.[19] Wells longer than 4 km generally involve drilling costs in the tens of millions of dollars.[20] The technological challenges are drilling wide wells at low cost and breaking large volumes of rocks.

Geothermal electricity is considered sustainable since the extraction rate is small compared to the heat content of the Earth, but it must still be monitored to avoid local depletion.[4] Although geothermal locations can provide power for many decades, individual wells can cool down or run out of water. The three oldest extraction sites – Larderello, Wairakei and The Geysers – have reduced production from their respective peaks. It is not clear if this is due to a higher rate of extraction than replacement from greater depths or if the aquifers are being emptied. If production is reduced and water is reinjected, these wells could theoretically recover their full potential. These strategies have already been implemented in some places. The long-term sustainability of geothermal energy has been demonstrated at Larderello since 1913, at Wairakei since 1958,[21]​ and at The Geysers since 1960.[22]​.

Contenido

Las centrales geotermoeléctricas son similares a otras centrales termoeléctricas de turbina: el calor de una fuente de energía —en el caso de la geotérmica, el calor del interior de la Tierra— se utiliza para calentar agua u otro fluido de trabajo. Dicho fluido hace girar la turbina de un generador, produciendo electricidad. Posteriormente, el fluido se enfría y es devuelto a la fuente de calor.

Dry steam plants

Dry steam power stations (in English: Dry steam power stations) are the simplest and oldest design. They directly use geothermal steam at 150 °C or more to drive the turbines.[16]​.

Flash steam power plants

Flash steam power stations raise hot water at high pressure through wells and introduce it into low-pressure tanks. As its pressure decreases, part of the water vaporizes. This vapor is separated from the liquid and used to drive a turbine. Leftover liquid water and condensed steam can be injected into the reservoirs again, making the process potentially sustainable.[23]​ At The Geysers, California, 20 years of electrical production have depleted groundwater and operations have been greatly reduced. To restore part of its original capacity, a water injection system has been developed.[24]​.

Binary cycle power plants

Binary cycle power stations are the most recently developed, and can operate with fluid temperatures of only 57 °C.[10] Moderately hot water is passed alongside another fluid with a boiling point much lower than that of water. This causes the secondary fluid to vaporize and is used to drive the turbines. This is the most common type of geothermoelectric plant in projects currently under construction.[25] Both the Rankine cycle and the Kalina cycle are used. The thermal efficiency of these plants is approximately 10-13%.

Geothermal energy does not require fuel, so it is not affected by price fluctuations. However, investment costs tend to be high. Drilling accounts for approximately half of the total cost, and deep resource exploration involves significant risks.

In total, the construction of a geothermal power plant and the drilling costs amount to between €2 million for each MW of capacity, while the levelized cost of energy" is €0.04-0.10 per kWh.[26] Advanced geothermal systems usually exceed these costs, with investments of around €4 million per MW, and levelized costs of $0.054 per kW h in 2007.[32]​.

Geothermal electricity generation is very expandable: a small plant can supply a rural community.[33]​.

The largest geothermal power plant in the world is The Geysers, in California, with 1,517 MW installed and an average plant factor of 63% (955 MW).[34]​[35]​.

The heat content of the Earth is approximately 10 joules (Annex: Orders of magnitude (energy)").[16]​ This heat naturally flows to the surface by conduction at a rate of 44.2 TW[17]​ and is replenished at a rate of 30 TW by radioactive decay.[4]​ These rates are more than double humanity's current primary energy consumption, but most of it is too diffuse—approximately 0.1 W/m² on average— enough to take advantage of. The Earth's crust acts as a thick insulating layer that must be pierced by pipes—or magma, water, etc.—to allow heat to be emitted from within.

The generation of electricity requires high temperatures that can only come from deep areas. Heat must be transported to the surface through magmatic conduits, hot springs, hydrothermal circulation, oil wells, water wells, or a combination of all of these. This circulation can occur naturally when the crust is thin: magmatic conduits transport heat to the crust and hot springs bring heat to the surface. If hot springs do not exist, a well must be drilled above a hot water table. Away from plate tectonic confluence zones, the geothermal gradient is 25-30 °C per kilometer of depth in most of the world, so wells would need to be several kilometers deep to allow for electricity generation.[16] The quantity and quality of recoverable resources increases with drilling depth and proximity to plate boundaries.

In soil that is hot but dry, or where water pressure is not adequate, fluid injection can stimulate production. Developers drill two holes at a candidate mining site and break up the rock using explosives or high-pressure water. They then pump water or liquefied carbon dioxide through one well and it comes out the other in gaseous form.[11] This technique is known as hot dry rock") (HDR) in Europe and enhanced geothermal systems") (EGS) in North America. Through it, a much greater energy potential would be available than with the conventional use of natural water tables.[11]​.

Estimates of potential geothermal electricity generation capacity range from 35 to 2000 GW, depending on the scale of investments.[16] This does not include geothermal energy that is not transformed into electricity, such as through cogeneration, geothermal heat pumps or other direct uses. A 2006 report from the Massachusetts Institute of Technology, which included the potential of EGS systems, estimated that an investment of $1 billion for research and development over the next 15 years would enable the creation of 100 GW of geothermal electrical capacity by 2050, in the United States alone.[11] This report also estimated that more than 200 zettajoules "Joule (unit)") (ZJ) would be extractable, with the potential to increase this figure to more than 2000 ZJ through technological improvements, enough to supply all of the world's current energy demand for several millennia.[11]​.

Currently, geothermal wells rarely exceed 3 km in depth.[16] Deeper estimates assume the existence of wells up to 10 km. Drilling to this depth is possible today thanks to the oil industry, although it is an expensive process. The deepest research well in the world is the Kola superdeep well, 12.3 km long.[18] This record has recently been equaled by commercial wells such as the Z-12 of ExxonMobil's Sakhalin-I project in Chayvo.[19] Wells longer than 4 km generally involve drilling costs in the tens of millions of dollars.[20] The technological challenges are drilling wide wells at low cost and breaking large volumes of rocks.

Geothermal electricity is considered sustainable since the extraction rate is small compared to the heat content of the Earth, but it must still be monitored to avoid local depletion.[4] Although geothermal locations can provide power for many decades, individual wells can cool down or run out of water. The three oldest extraction sites – Larderello, Wairakei and The Geysers – have reduced production from their respective peaks. It is not clear if this is due to a higher rate of extraction than replacement from greater depths or if the aquifers are being emptied. If production is reduced and water is reinjected, these wells could theoretically recover their full potential. These strategies have already been implemented in some places. The long-term sustainability of geothermal energy has been demonstrated at Larderello since 1913, at Wairakei since 1958,[21]​ and at The Geysers since 1960.[22]​.

Contenido

Las centrales geotermoeléctricas son similares a otras centrales termoeléctricas de turbina: el calor de una fuente de energía —en el caso de la geotérmica, el calor del interior de la Tierra— se utiliza para calentar agua u otro fluido de trabajo. Dicho fluido hace girar la turbina de un generador, produciendo electricidad. Posteriormente, el fluido se enfría y es devuelto a la fuente de calor.

Dry steam plants

Dry steam power stations (in English: Dry steam power stations) are the simplest and oldest design. They directly use geothermal steam at 150 °C or more to drive the turbines.[16]​.

Flash steam power plants

Flash steam power stations raise hot water at high pressure through wells and introduce it into low-pressure tanks. As its pressure decreases, part of the water vaporizes. This vapor is separated from the liquid and used to drive a turbine. Leftover liquid water and condensed steam can be injected into the reservoirs again, making the process potentially sustainable.[23]​ At The Geysers, California, 20 years of electrical production have depleted groundwater and operations have been greatly reduced. To restore part of its original capacity, a water injection system has been developed.[24]​.

Binary cycle power plants

Binary cycle power stations are the most recently developed, and can operate with fluid temperatures of only 57 °C.[10] Moderately hot water is passed alongside another fluid with a boiling point much lower than that of water. This causes the secondary fluid to vaporize and is used to drive the turbines. This is the most common type of geothermoelectric plant in projects currently under construction.[25] Both the Rankine cycle and the Kalina cycle are used. The thermal efficiency of these plants is approximately 10-13%.

Geothermal energy does not require fuel, so it is not affected by price fluctuations. However, investment costs tend to be high. Drilling accounts for approximately half of the total cost, and deep resource exploration involves significant risks.

In total, the construction of a geothermal power plant and the drilling costs amount to between €2 million for each MW of capacity, while the levelized cost of energy" is €0.04-0.10 per kWh.[26] Advanced geothermal systems usually exceed these costs, with investments of around €4 million per MW, and levelized costs of $0.054 per kW h in 2007.[32]​.

Geothermal electricity generation is very expandable: a small plant can supply a rural community.[33]​.

The largest geothermal power plant in the world is The Geysers, in California, with 1,517 MW installed and an average plant factor of 63% (955 MW).[34]​[35]​.