Steam rising from the Nesjavellir Geothermal Power Station in Iceland
The Imperial Valley Geothermal Project near the Salton Sea, California.

Geothermal energy is the thermal energy in the Earth's crust which originates from the formation of the planet and from radioactive decay of materials in currently uncertain[1] but possibly roughly equal[2] proportions. The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in parts of the mantle convecting upward since it is lighter than the surrounding rock and temperatures at the core–mantle boundary can reach over 4000 °C (7200 °F).[3]

Geothermal heating, for example using water from hot springs has been used for bathing since Paleolithic times and for space heating since ancient Roman times, however more recently geothermal power, the term used for generation of electricity from geothermal energy, has gained in importance. It is estimated that the earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, although only a very small fraction is currently being profitably exploited, often in areas near tectonic plate boundaries.

As a result of government assisted research and industry experience, the cost of generating geothermal power decreased by 25% over the 1980s and 1990s.[4] More recent technological advances have dramatically reduced costs and thereby expanded the range and size of viable resource and in 2021 the U.S. Department of Energy estimates that geothermal energy from a power plant "built today" costs about $0.05/kWh.[5]

Worldwide, 13,900 megawatts (MW) of geothermal power was available in 2019.[6] An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications as of 2010.[7]

Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, plate boundary movement and interest rates. Pilot programs like EWEB's customer opt in Green Power Program[8] show that customers would be willing to pay a little more for a renewable energy source like geothermal. About 100 thousand people are employed in the industry.[9] The adjective geothermal originates from the Greek roots γῆ (), meaning Earth, and θερμός (thermós), meaning hot.

History

The oldest known pool fed by a hot spring, built in the Qin dynasty in the 3rd century BCE
The oldest known pool fed by a hot spring, built in the Qin dynasty in the 3rd century BCE

Hot springs have been used for bathing at least since Paleolithic times.[10] The oldest known spa is a stone pool on China's Lisan mountain built in the Qin Dynasty in the 3rd century BCE, at the same site where the Huaqing Chi palace was later built. In the first century CE, Romans conquered Aquae Sulis, now Bath, Somerset, England, and used the hot springs there to feed public baths and underfloor heating. The admission fees for these baths probably represent the first commercial use of geothermal power. The world's oldest geothermal district heating system in Chaudes-Aigues, France, has been operating since the 15th century.[11] The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.

In 1892, America's first district heating system in Boise, Idaho was powered directly by geothermal energy, and was copied in Klamath Falls, Oregon in 1900. The first known building in the world to utilize geothermal energy as its primary heat source was the Hot Lake Hotel in Union County, Oregon, whose construction was completed in 1907.[12] A deep geothermal well was used to heat greenhouses in Boise in 1926, and geysers were used to heat greenhouses in Iceland and Tuscany at about the same time.[13] Charlie Lieb developed the first downhole heat exchanger in 1930 to heat his house. Steam and hot water from geysers began heating homes in Iceland starting in 1943.

Global geothermal electric capacity. Upper red line is installed capacity;[14] lower green line is realized production.[7]
Global geothermal electric capacity. Upper red line is installed capacity;[14] lower green line is realized production.[7]

In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904, at the same Larderello dry steam field where geothermal acid extraction began. It successfully lit four light bulbs.[15] Later, in 1911, the world's first commercial geothermal power plant was built there. It was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958. In 2012, it produced some 594 megawatts.[16]

In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California.[17] The original turbine lasted for more than 30 years and produced 11 MW net power.[18]

The binary cycle power plant was first demonstrated in 1967 in the USSR and later introduced to the US in 1981.[17] This technology allows the generation of electricity from much lower temperature resources than previously. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low fluid temperature of 57 °C (135 °F).[19]

Resources

Enhanced geothermal system 1:Reservoir 2:Pump house 3:Heat exchanger 4:Turbine hall 5:Production well 6:Injection well 7:Hot water to district heating 8:Porous sediments 9:Observation well 10:Crystalline bedrock
Enhanced geothermal system 1:Reservoir 2:Pump house 3:Heat exchanger 4:Turbine hall 5:Production well 6:Injection well 7:Hot water to district heating 8:Porous sediments 9:Observation well 10:Crystalline bedrock

The Earth's internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts (TW),[20] and is replenished by radioactive decay of minerals at a rate of 30 TW.[21] These power rates are more than double humanity's current energy consumption from all primary sources, but most of this energy flow is not recoverable. In addition to the internal heat flows, the top layer of the surface to a depth of 10 m (33 ft) is heated by solar energy during the summer, and releases that energy and cools during the winter.

Outside of the seasonal variations, the geothermal gradient of temperatures through the crust is 25–30 °C (77–86 °F) per km of depth in most of the world. The conductive heat flux averages 0.1 MW/km2. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation or a combination of these.

The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. The most demanding applications receive the greatest benefit from a high natural heat flux, ideally from using a hot spring. The next best option is to drill a well into a hot aquifer. If no adequate aquifer is available, an artificial one may be built by injecting water to hydraulically fracture the bedrock. This last approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America. Much greater potential may be available from this approach than from conventional tapping of natural aquifers.[22]

Estimates of the potential for electricity generation from geothermal energy vary sixfold, from 0.035to2TW depending on the scale of investments.[7] Upper estimates of geothermal resources assume enhanced geothermal wells as deep as 10 kilometres (6 mi), whereas existing geothermal wells are rarely more than 3 kilometres (2 mi) deep.[7] Wells of this depth are now common in the petroleum industry.[23] The deepest research well in the world, the Kola superdeep borehole, is 12 kilometres (7 mi) deep.[24]

Geothermal power

Main article: Geothermal power

Installed geothermal energy capacity, 2019
Installed geothermal energy capacity, 2019

Geothermal power is electrical power generated from geothermal energy. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. Geothermal electricity generation is currently used in 26 countries,[25][26] while geothermal heating is in use in 70 countries.[27]

As of 2019, worldwide geothermal power capacity amounts to 15.4 gigawatts (GW), of which 23.86 percent or 3.68 GW are installed in the United States.[28] International markets grew at an average annual rate of 5 percent over the three years to 2015, and global geothermal power capacity is expected to reach 14.5–17.6 GW by 2020.[29] Based on current geologic knowledge and technology the GEA publicly discloses, the Geothermal Energy Association (GEA) estimates that only 6.9 percent of total global potential has been tapped so far, while the IPCC reported geothermal power potential to be in the range of 35 GW to 2 TW.[7] Countries generating more than 15 percent of their electricity from geothermal sources include El Salvador, Kenya, the Philippines, Iceland, New Zealand,[30] and Costa Rica.

Geothermal power is considered to be a sustainable, renewable source of energy because the heat extraction is small compared with the Earth's heat content.[21] The greenhouse gas emissions of geothermal electric stations are on average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5 percent of that of conventional coal-fired plants.[27]

As a source of renewable energy for both power and heating, geothermal has the potential to meet 3-5% of global demand by 2050. With economic incentives, it is estimated that by 2100 it will be possible to meet 10% of global demand.[30]

Direct use data 2015
Country Capacity (MW) 2015[31]
United States 17,415.91
Philippines 3.30
Indonesia 2.30
Mexico 155.82
Italy 1,014.00
New Zealand 487.45
Iceland 2,040.00
Japan 2,186.17
Iran 81.50
El Salvador 3.36
Kenya 22.40
Costa Rica 1.00
Russia 308.20
Turkey 2,886.30
Papua-New Guinea 0.10
Guatemala 2.31
Portugal 35.20
China 17,870.00
France 2,346.90
Ethiopia 2.20
Germany 2,848.60
Austria 903.40
Australia 16.09
Thailand 128.51
Installed geothermal electric capacity
Country Capacity (MW)
2007[14]
Capacity (MW)
2010[32]
Capacity (MW)
2020[33]
% of national
electricity
production
% of global
geothermal
production
United States 2687 3086 3714 0.3 29
Philippines 1969.7 1904 1918 27 18
Indonesia 992 1197 2133 3.7 11
Mexico 953 958 962.7 3 9
Italy 810.5 843 944 1.5 8
New Zealand 471.6 628 1005 10 6
Iceland 421.2 575 755 30 5
Japan 535.2 536 603 0.1 5
Iran 250 250
El Salvador 204.2 204 n/a 25
Kenya 128.8 167 961 11.2
Costa Rica 162.5 166 n/a 14
Nicaragua 87.4 88 n/a 10
Russia 79 82
Turkey 38 82 1526
Papua-New Guinea 56 56
Guatemala 53 52
Portugal 23 29
China 27.8 24
France 14.7 16
Ethiopia 7.3 7.3
Germany 8.4 6.6
Austria 1.1 1.4
Australia 0.2 1.1
Thailand 0.3 0.3
Total 9,981.9 10,959.7 15,608

Geothermal electric plants were traditionally built exclusively on the edges of tectonic plates where high-temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a much greater geographical range.[22] Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel, Switzerland, was shut down after it triggered earthquakes. Other demonstration projects are under construction in Australia, the United Kingdom, and the United States of America.[34] In Myanmar over 39 locations capable of geothermal power production and some of these hydrothermal reservoirs lie quite close to Yangon which is a significant underutilized resource.[35]

Geothermal heating

Main article: Geothermal heating

Geothermal heating is the direct use of geothermal energy for some heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. As of 2007, 28 GW of geothermal heating capacity is installed around the world, satisfying 0.07% of global primary energy consumption.[7] Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter.

Geothermal energy originates from the heat retained within the Earth since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface.[36] Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application. However, even cold ground contains heat, below 6 metres (20 ft) the undisturbed ground temperature is consistently at the Mean Annual Air Temperature[37] and it may be extracted with a ground source heat pump.

Types

Geothermal energy comes in either vapor-dominated or liquid-dominated forms. Larderello and The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240 to 300 °C that produce superheated steam.

Liquid-dominated plants

Liquid-dominated reservoirs (LDRs) are more common with temperatures greater than 200 °C (392 °F) and are found near young volcanoes surrounding the Pacific Ocean and in rift zones and hot spots. Flash plants are the common way to generate electricity from these sources. Pumps are generally not required, powered instead when the water turns to steam. Most wells generate 2–10 MW of electricity. Steam is separated from a liquid via cyclone separators, while the liquid is returned to the reservoir for reheating/reuse. As of 2013, the largest liquid system is Cerro Prieto in Mexico, which generates 750 MW of electricity from temperatures reaching 350 °C (662 °F). The Salton Sea field in Southern California offers the potential of generating 2000 MW of electricity.[16]

Lower-temperature LDRs (120–200 °C) require pumping. They are common in extensional terrains, where heating takes place via deep circulation along faults, such as in the Western US and Turkey. Water passes through a heat exchanger in a Rankine cycle binary plant. The water vaporizes an organic working fluid that drives a turbine. These binary plants originated in the Soviet Union in the late 1960s and predominate in new US plants. Binary plants have no emissions.[16][38]

Enhanced geothermal systems

Main article: Enhanced geothermal system

Enhanced geothermal systems (EGS) actively inject water into wells to be heated and pumped back out. The water is injected under high pressure to expand existing rock fissures to enable the water to freely flow in and out. The technique was adapted from oil and gas extraction techniques. However, the geologic formations are deeper and no toxic chemicals are used, reducing the possibility of environmental damage. Drillers can employ directional drilling to expand the size of the reservoir.[16]

Small-scale EGS have been installed in the Rhine Graben at Soultz-sous-Forêts in France and at Landau and Insheim in Germany.[16]

Economics

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Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations. However, capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate.[39]

A power plant at The Geysers
A power plant at The Geysers

As noted above, drilling cost is a major component of a geothermal power plant's budget and is one of the key barriers to wider development of geothermal resources. A power plant must have production wells to bring the hot fluid (steam or hot water) to the surface and must also have injection wells to pump the liquid back into the reservoir after it has passed through the power plant. Drilling geothermal wells is more expensive than drilling oil and gas wells of comparable depth for several reasons:

In total, electrical plant construction and well drilling cost about €2–5 million per MW of electrical capacity, while the break-even price is 0.04–0.10 € per kW·h.[14] Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and break-even above $0.054 per kW·h in 2007.[41] The capital cost of one such district heating system in Bavaria was estimated at somewhat over 1 million € per MW.[42] Direct systems of any size are much simpler than electric generators and have lower maintenance costs per kW·h, but they must consume electricity to run pumps and compressors. Some governments subsidize geothermal projects.

Geothermal power is highly scalable: from a rural village to an entire city,[43] making it a vital part of the renewable energy transition.[citation needed]

The most developed geothermal field in the United States is The Geysers in Northern California.[44]

Geothermal projects have several stages of development. Each phase has associated risks. At the early stages of reconnaissance and geophysical surveys, many projects are canceled, making that phase unsuitable for traditional lending. Projects moving forward from the identification, exploration and exploratory drilling often trade equity for financing.[45]

Renewability and sustainability

Geothermal power is considered to be renewable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 1031 joules (3·1015 TWh), approximately 100 billion times the 2010 worldwide annual energy consumption.[7] About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes.[46] For example, a 5275 m deep borehole in United Downs Deep Geothermal Power Project in Cornwall, England, found granite with very high thorium content, whose radioactive decay is believed to power the high temperature of the rock.[47]

Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it. According to most official descriptions of geothermal energy use, it is currently called renewable and sustainable because it returns an equal volume of water to the area that the heat extraction takes place, but at a somewhat lower temperature. For instance, the water leaving the ground is 300 degrees, and the water returning is 200 degrees, the energy obtained is the difference in heat that is extracted. Current research estimates of impact on the heat loss from the Earth's core are based on a studies done up through 2012. However, if household and industrial uses of this energy source were to expand dramatically over coming years, based on a diminishing fossil fuel supply and a growing world population that is rapidly industrializing requiring additional energy sources, then the estimates on the impact on the Earth's cooling rate would need to be re-evaluated.

Geothermal power is also considered to be sustainable thanks to its power to sustain the Earth's intricate ecosystems. By using geothermal sources of energy present generations of humans will not endanger the capability of future generations to use their own resources to the same amount that those energy sources are presently used.[48] Further, due to its low emissions geothermal energy is considered to have excellent potential for mitigation of global warming.

Electricity generation at Poihipi, New Zealand
Electricity generation at Poihipi, New Zealand
Electricity generation at Ohaaki, New Zealand
Electricity generation at Ohaaki, New Zealand
Electricity generation at Wairakei, New Zealand
Electricity generation at Wairakei, New Zealand

Even though geothermal power is globally sustainable, extraction must still be monitored to avoid local depletion.[21] Over the course of decades, individual wells draw down local temperatures and water levels until a new equilibrium is reached with natural flows. The three oldest sites, at Larderello, Wairakei, and the Geysers have experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. If production is reduced and water is reinjected, these wells could theoretically recover their full potential. Such mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958,[49] and at The Geysers field in California since 1960.[50]

Falling electricity production may be boosted through drilling additional supply boreholes, as at Poihipi and Ohaaki. The Wairakei power station has been running much longer, with its first unit commissioned in November 1958, and it attained its peak generation of 173 MW in 1965, but already the supply of high-pressure steam was faltering, in 1982 being derated to intermediate pressure and the station managing 157 MW. Around the start of the 21st century it was managing about 150 MW, then in 2005 two 8 MW isopentane systems were added, boosting the station's output by about 14 MW. Detailed data are unavailable, being lost due to re-organisations. One such re-organisation in 1996 causes the absence of early data for Poihipi (started 1996), and the gap in 1996/7 for Wairakei and Ohaaki; half-hourly data for Ohaaki's first few months of operation are also missing, as well as for most of Wairakei's history.

Environmental effects

Geothermal power station in the Philippines
Geothermal power station in the Philippines
Krafla Geothermal Station in northeast Iceland
Krafla Geothermal Station in northeast Iceland

Fluids drawn from the deep Earth carry a mixture of gases, notably carbon dioxide (CO
2
), hydrogen sulfide (H
2
S
), methane (CH
4
) and ammonia (NH
3
). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kilograms (269 lb) of CO
2
per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants.[51][needs update] But a few plants emit more than gas-fired power, at least in the first few years, such as some geothermal power in Turkey.[52] Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust.

In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic elements such as mercury, arsenic, boron, and antimony.[53] These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.

Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand.[11] In Staufen im Breisgau, Germany, tectonic uplift occurred instead, due to a previously isolated anhydrite layer coming in contact with water and turning into gypsum, doubling its volume.[54][55][56] Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. The project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection.[57]

Geothermal has minimal land and freshwater requirements. Geothermal plants use 3.5 square kilometres (1.4 sq mi) per gigawatt of electrical production (not capacity) versus 32 square kilometres (12 sq mi) and 12 square kilometres (4.6 sq mi) for coal facilities and wind farms respectively.[11] They use 20 litres (5.3 US gal) of freshwater per MW·h versus over 1,000 litres (260 US gal) per MW·h for nuclear, coal, or oil.[11]

Production

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According to the Geothermal Energy Association (GEA) installed geothermal capacity in the United States grew by 5%, or 147.05 MW, since the last annual survey in March 2012. This increase came from seven geothermal projects that began production in 2012. GEA also revised its 2011 estimate of installed capacity upward by 128 MW, bringing current installed U.S. geothermal capacity to 3,386 MW.[58]

Legal frameworks

Some of the legal issues raised by geothermal energy resources include questions of ownership and allocation of the resource, the grant of exploration permits, exploitation rights, royalties, and the extent to which geothermal energy issues have been recognized in existing planning and environmental laws. Other questions concern overlap between geothermal and mineral or petroleum tenements. Broader issues concern the extent to which the legal framework for encouragement of renewable energy assists in encouraging geothermal industry innovation and development.

See also

References

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Bibliography

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