Geothermal energy

Geothermal energy is thermal energy in the Earth's crust. It combines energy from the formation of the planet and from radioactive decay. Geothermal energy has been exploited as a source of heat and/or electric power for millennia.

This article is about thermal energy generated and stored deep in the earth. For information about heat pumps used to extract heat from up to about 300 meters/yards from the surface, see ground source heat pump.
Steam rising from the Nesjavellir Geothermal Power Station in Iceland
The Imperial Valley Geothermal Project near the Salton Sea, California.
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Renewable energy

Geothermal heating, using water from hot springs, for example, has been used for bathing since Paleolithic times and for space heating since Roman times. Geothermal power, (generation of electricity from geothermal energy), has been used since the 20th century. Unlike wind and solar energy, geothermal plants produce power at a constant rate, without regard to weather conditions. Geothermal resources are theoretically more than adequate to supply humanity's energy needs. Most extraction occurs in areas near tectonic plate boundaries.

The cost of generating geothermal power decreased by 25% during the 1980s and 1990s.[1] Technological advances continued to reduce costs and thereby expand the amount of viable resources. In 2021, the U.S. Department of Energy estimated that power from a plant "built today" costs about $0.05/kWh.[2]

In 2019, 13,900 megawatts (MW) of geothermal power was available worldwide.[3] An additional 28 gigawatts provided heat for district heating, space heating, spas, industrial processes, desalination, and agricultural applications as of 2010.[4]

As of 2019 the industry employed about 100 thousand people.[5] Pilot programs like EWEB's customer opt-in Green Power Program suggest that customers would be willing to pay a little more for renewable energy.[6]

The adjective geothermal originates from the Greek roots γῆ (gê), 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

Hot springs have been used for bathing since at least Paleolithic times.[7] The oldest known spa is at the site of the Huaqing Chi palace. In the first century CE, Romans conquered Aquae Sulis, now Bath, Somerset, England, and used the hot springs there to supply public baths and underfloor heating. The admission fees for these baths probably represent the first commercial use of geothermal energy. The world's oldest geothermal district heating system, in Chaudes-Aigues, France, has been operating since the 15th century.[8] 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, the US's first district heating system in Boise, Idaho was powered by geothermal energy. It was copied in Klamath Falls, Oregon in 1900. The world's first known building to utilize geothermal energy as its primary heat source was the Hot Lake Hotel in Union County, Oregon, beginning in 1907.[9] A 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.[10] Charles Lieb developed the first downhole heat exchanger in 1930 to heat his house. Geyser steam and water began heating homes in Iceland in 1943.

Global geothermal electric capacity. Upper red line is installed capacity;[11] lower green line is realized production.[4]

In the 20th century, geothermal energy came into use as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904, at the Larderello steam field. It successfully lit four light bulbs.[12] In 1911, the world's first commercial geothermal power plant was built there. It was the only industrial producer of geothermal power until New Zealand built a plant in 1958. In 2012, it produced some 594 megawatts.[13]

In 1960, Pacific Gas and Electric began operation of the first US geothermal power plant at The Geysers in California.[14] The original turbine lasted for more than 30 years and produced 11 MW net power.[15]

A binary cycle power plant was first demonstrated in 1967 in the USSR and introduced to the US in 1981.[14] 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 temperature of 57 °C (135 °F).[16]

In 2021 Quaise Energy announced the idea of using a gyrotron as a boring machine to drill a hole 20 kilometers in depth. The technique used frequencies of 30-300 GHz to transfer energy to rock 1012 (1 trillion) times more efficiently than a laser. Lasers would be disrupted by the vaporized rock, which would effect the gyrotron's longer-wavelength much less. Drilling rates of 70 meters/hour were claimed to be possible with a 1 MW gyrotron.[17]

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

The Earth has an internal heat content of 1031 joules (3·1015 TWh), About 20% of this is residual heat from planetary accretion; the remainder is attributed to past and current radioactive decay of naturally occurring isotopes.[18] 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.[19]

Earth's interior temperature and pressure are high enough to cause some rock to melt and the solid mantle to behave plastically. Parts of the mantle convect upward since it is lighter than the surrounding rock. Temperatures at the core–mantle boundary can reach over 4000 °C (7200 °F).[20]

The Earth's internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts (TW),[21] and is replenished by radioactive decay of minerals at a rate of 30 TW.[22] These power rates are more than double humanity's current energy consumption from all primary sources, but most of this energy flux 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 cools during the winter.

Outside of the seasonal variations, the geothermal gradient of temperatures through the crust is 25–30 °C (45–54 °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 combinations of fluid circulation, either through magma conduits, hot springs, hydrothermal circulation.

The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. Applications receive the greatest benefit from a high natural heat flux most easily from a hot spring. The next best option is to drill a well into a hot aquifer. An artificial one may be built by injecting water to hydraulically fracture bedrock. This last approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America.[23]

2010 estimates of the potential for electricity generation from geothermal energy vary sixfold, from 0.035to2TW depending on the scale of investments.[4] Upper estimates of geothermal resources assume wells as deep as 10 kilometres (6 mi), although 20th century wells rarely reached more than 3 kilometres (2 mi) deep.[4] Wells of this depth are common in the petroleum industry.[24]

Geothermal power
Main article: Geothermal power
Installed geothermal energy capacity, 2022[25]

Geothermal power is electrical power generated from geothermal energy. Dry steam,, flash steam, and binary cycle power stations have been used for this purpose. As of 2010 geothermal electricity was generated in 26 countries.[26][27]

As of 2019, worldwide geothermal power capacity amounted to 15.4 gigawatts (GW), of which 23.86 percent or 3.68 GW were in the United States.[28]

Geothermal energy supplies a significant share of the electrical power in Iceland, El Salvador, Kenya, the Philippines and New Zealand.[29]

Geothermal power is considered to be renewable energy because the heat extraction is insignificant compared with the Earth's heat content.[22] 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 coal-fired plants.[30]

Direct use data 2015
Country Capacity (MW) 2015[31]
United States17,415
Philippines3
Indonesia2
Mexico155
Italy1,014
New Zealand487
Iceland2,040
Japan2,186
Iran81
El Salvador3
Kenya22
Costa Rica1
Russia308
Turkey2,886
Papua New Guinea0.10
Guatemala2
Portugal35
China17,870
France2,346
Ethiopia2
Germany2,848
Austria903
Australia16
Thailand128
Installed geothermal electric capacity
Country Capacity (MW)
2021[32]		  % of national
electricity
production		  % of global
geothermal
production (2021)
United States38890.324.8
Indonesia22773.714.5
Philippines19282712.3
Turkey167610.7
New Zealand984106.3
Mexico97636.2
Kenya86311.25.5
Italy8021.55.1
Iceland756304.8
Japan4810.13
Costa Rica262141.7
Iran
El Salvador204251.3
Nicaragua153101
Russia81
Papua New Guinea56
Guatemala49
Germany40
Chile40
Honduras39
Portugal29
China
France16
Guadeloupe15
Croatia10
Ethiopia7
Austria1
Australia0
Total15,644

Geothermal electric plants were traditionally built on the edges of tectonic plates where high-temperature geothermal resources approach the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a greater geographical range.[23] 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 US.[33] In Myanmar over 39 locations are capable of geothermal power production, some of which are near Yangon.[34]

Geothermal heating
Main article: Geothermal heating

Geothermal heating is the use of geothermal energy to heat buildings and water for human use. Humans have done this 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 satisfied 0.07% of global primary energy consumption.[4] 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.

Even cold ground contains heat: below 6 metres (20 ft) the undisturbed ground temperature is consistently at the Mean Annual Air Temperature[35] that 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 volcanoes in/around the Pacific Ocean and in rift zones and hot spots. Flash plants are the common way to generate electricity from these sources. Steam from the well is sufficient to power the plant. Most wells generate 2–10 MW of electricity. Steam is separated from liquid via cyclone separators and drives electric generators. Condensed liquid returns down the well for reheating/reuse. As of 2013, the largest liquid system was Cerro Prieto in Mexico, which generates 750 MW of electricity from temperatures reaching 350 °C (662 °F).

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 plants. Binary plants have no emissions.[13][36]

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 flow freely. The technique was adapted from oil and gas fracking techniques. 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 reservoir size.[13]

Small-scale EGS have been installed in the Rhine Graben at Soultz-sous-Forêts in France and at Landau and Insheim in Germany.[13]

Economics

As with wind and solar energy, geothermal power has minimal operating costs; capital costs dominate. Drilling accounts for over half the costs, and not all wells produce an exploitable resources. For example, a typical well pair (one for extraction and one for injection) in Nevada can produce 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate, making the average cost of a successful well $50 million.[37]

A power plant at The Geysers

Drilling geothermal wells is more expensive than drilling oil and gas wells of comparable depth for several reasons:

  • Geothermal reservoirs are usually in igneous or metamorphic rock, which is harder to penetrate than the sedimentary rock of typical hydrocarbon reservoirs.
  • The rock is often fractured, which causes vibrations that damage bits and other drilling tools.
  • The rock is often abrasive, with high quartz content, and sometimes contains highly corrosive fluids.
  • The rock is hot, which limits use of downhole electronics.
  • Well casing must be cemented from top to bottom, to resist the casing's tendency to expand and contract with temperature changes. Oil and gas wells are usually cemented only at the bottom.
  • Well diameters are considerably larger than typical oil and gas wells.[38]

As of 2007 plant construction and well drilling cost about €2–5 million per MW of electrical capacity, while the break-even price was 0.04–0.10 € per kW·h.[11] 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.[39]

Heating systems are much simpler than electric generators and have lower maintenance costs per kW·h, but they consume electricity to run pumps and compressors.

Development

Geothermal projects have several stages of development. Each phase has associated risks. Many projects are canceled during the stages of reconnaissance and geophysical surveys, which are unsuitable for traditional lending. At later stages can often be equity-financed.[40]

Sustainability

Geothermal energy is considered to be sustainable because the heat extracted is so small compared to the Earth's heat content, which is approximately 100 billion times 2010 worldwide annual energy consumption.[4] Earth's heat flows are not in equilibrium; the planet is cooling on geologic timescales. Anthropic heat extraction typically does not accelerate the cooling process.

Wells can further be considered renewable because they return the extracted water to the borehole for reheating and re-extraction, albeit at a lower temperature.

Replacing material use with energy has reduced the human environmental footprint in many applications. Geothermal has the potential to allow further reductions. For example, Iceland has sufficient geothermal energy to eliminate fossil fuels for electricity production and to heat Reykjavik sidewalks and eliminate the need for gritting.[41]

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

However, local effects of heat extraction must be considered.[22] Over the course of decades, individual wells draw down local temperatures and water levels. The three oldest sites, at Larderello, Wairakei, and the Geysers experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. Reducing production and injecting additional water could allow these wells to recover their original capacity. Such strategies have been implemented at some sites. These sites continue to provide significant energy.[42][43]

The Wairakei power station was 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 it was down-rated to intermediate pressure and the output to 157 MW. In 2005 two 8 MW isopentane systems were added, boosting output by about 14 MW. Detailed data were lost due to re-organisations.

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

Fluids drawn from underground carry a mixture of gasses, notably carbon dioxide (CO
2), hydrogen sulfide (H
2S), 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 fossil fuel plants.[44] A few plants emit more pollutants than gas-fired power, at least in the first few years, such as some geothermal power in Turkey.[45] Plants that experience high levels of acids and volatile chemicals are typically equipped with emission-control systems to reduce the exhaust.

Water from geothermal sources may hold in solution trace amounts of toxic elements such as mercury, arsenic, boron, and antimony.[46] These chemicals precipitate as the water cools, and can damage surroundings if released. The modern practice of returning geothermal fluids into the Earth to stimulate production has the side benefit of reducing this environmental impact.

Construction can adversely affect land stability. Subsidence occurred in the Wairakei field.[8] In Staufen im Breisgau, Germany, tectonic uplift occurred instead. A previously isolated anhydrite layer came in contact with water and turned it into gypsum, doubling its volume.[47][48][49] Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. A 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.[50]

Geothermal power production 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.[8] 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.[8]

Production

Philippines

The Philippines began geothermal research in 1962 when the Philippine Institute of Volcanology and Seismology inspected the geothermal region in Tiwi, Albay.[51] The first geothermal power plant in the Philippines was built in 1977, located in Tongonan, Leyte.[51] The New Zealand government contracted with the Philippines to build the plant in 1972.[52] The Tongonan Geothermal Field (TGF) added the Upper Mahiao, Matlibog, and South Sambaloran plants, which resulted in a 508 MV capacity.[53]

The first geothermal power plant in the Tiwi region opened in 1979, while two other plants followed in 1980 and 1982.[51] The Tiwi geothermal field is located about 450 km from Manila.[54] The three geothermal power plants in the Tiwi region produce 330 MWe, putting the Philippines behind the United States and Mexico in geothermal growth.[55] The Philippines has 7 geothermal fields and continues to exploit geothermal energy by creating the Philippine Energy Plan 2012-2030 that aims to produce 70% of the country's energy by 2030.[56][57]

United States

According to the Geothermal Energy Association (GEA) installed geothermal capacity in the United States grew by 5%, or 147.05 MW, in 2013. This increase came from seven geothermal projects that began production in 2012. GEA revised its 2011 estimate of installed capacity upward by 128 MW, bringing installed U.S. geothermal capacity to 3,386 MW.[58]

See also

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