Geothermal energy is thermal energy extracted from the Earth's crust. It combines energy from the planet's formation and from radioactive decay. Geothermal energy has been exploited as a source of heat and/or electric power for millennia.
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. Geothermal power plants produce power at a constant rate, regardless of 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. Technological advances continued to reduce costs and thereby expand the amount of viable resources. In 2021, the US Department of Energy estimated that power from a newly built plant costs about $0.05/kWh.
16 gigawatts (GW) of geothermal power was available worldwide in 2025, which was less than 1% of renewable power capacity. An additional 28 gigawatts provided heat for district heating, space heating, spas, industrial processes, desalination, and agricultural applications as of 2010. As of 2019 the industry employed about one hundred thousand people.
The adjective geothermal originates from the Greek roots (), meaning the Earth, and (), meaning hot.
History
Hot springs have been used for bathing since at least Paleolithic times. 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. 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. 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. 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.
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. 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.
In 1960, Pacific Gas and Electric began operation of the first US geothermal power plant at The Geysers in California. The original turbine lasted for more than 30 years and produced 11 MW net power.
An organic fluid-based binary cycle power station was first demonstrated in 1967 in the USSR and later introduced to the US in 1981.. This technology allows the use of temperature resources as low as . In 2006, a binary cycle plant in Chena Hot Springs, Alaska came online, producing electricity from a record low temperature of .
Resources
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. 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.
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 .
The Earth's internal thermal energy flows to the surface by conduction at a rate of 44.2 terawatts (TW), and is replenished by radioactive decay of minerals at a rate of 30 TW. 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 internal heat flows, the top layer of the surface to a depth of is heated by solar energy during the summer and cools during the winter.
Outside the seasonal variations, the geothermal gradient of temperatures through the crust is 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, or 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 hot water reservoir may be built by injecting water to hydraulically fracture bedrock. The systems in this last approach are called enhanced geothermal systems.
2010 estimates of the potential for electricity generation from geothermal energy vary widely, from depending on the scale of investments. Upper estimates of geothermal resources assume wells as deep as , although 20th century wells rarely reached more than deep. Wells of this depth are common in the petroleum industry.
Geothermal power
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.Geothermal Energy Association. Geothermal Energy: International Market Update May 2010, p. 4-6.
As of 2019, worldwide geothermal power capacity amounted to 15.4 gigawatts (GW), of which 23.86% or 3.68 GW were in the United States.
Geothermal energy supplies a significant share of the electrical power in Iceland, El Salvador, Kenya, the Philippines and New Zealand.
Geothermal power is considered to be a renewable energy because heat extraction rates are insignificant compared to the Earth's heat content. 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% of that of coal-fired plants.
Geothermal electric plants were traditionally built on the edges of tectonic plates where high-temperature geothermal resources approach the surface. These plants are usually built on land where the subsurface has high temperatures, is highly permeable, and is near a large water reserve as a source for a working fluid. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a greater geographical range. 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. In Myanmar over 39 locations are capable of geothermal power production, some of which are near Yangon.
+ Direct use data 2015 Country Capacity (MW) 2015 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) (2024) % of national electricity production (2024) % of globalgeothermal production (2024) Australia 00.0% 0.0% Austria 00.0% 0.0% Canada 60.0% 0.0% Chile 95 0.4% 0.6% China 260.0% 0.2% Taiwan 70.0% 0.0% Costa Rica 2638.3% 1.7% Croatia 100.0% 0.0% El Salvador 20911.2% 1.4% Ethiopia 70.1% 0.0% France 160.0% 0.1% Germany 44 0.0%0.3% Guadeloupe 156.6% 0.1% Guatemala 491.8% 0.3% Honduras 39 2.0% 0.3% Hungary 3 0.0% 0.0% Iceland 788 26.8% 5.1% Indonesia 2,688 18.8% 17.4% Italy 772 1.1%5.0% Japan 461 0.3% 3.0% Kenya 940 33.7% 6.1% Mexico 999 2.9%6.5% New Zealand 1,275 14.3% 8.3% Nicaragua 165 21.5% 1.1% Papua New Guinea 51 12.8%0.3% Philippines 1,952 21.0%12.7% Portugal 29 0.1%0.2% Romania 0 0.0%0.0%Russia810.1%0.5%Thailand00.0%0.0%Turkey1,7342.5%11.2%United States2,7030.6%17.5%Total16,738
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. 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 the undisturbed ground temperature is consistently at the Mean Annual Air Temperature that may be extracted with a ground source heat pump.
Types
Hydrothermal systems Hydrothermal systems produce geothermal energy by accessing naturally occurring hydrothermal reservoirs. Hydrothermal systems come in either vapor-dominated or liquid-dominated forms.
Vapor-dominated plants Larderello and The Geysers are vapor-dominated. Vapor-dominated sites offer temperatures from 240 to 300 °C that produce superheated steam.
In these systems, the steam is piped directly from underground wells to a turbine, which drives a generator that produces electricity.
Liquid-dominated plants Liquid-dominated reservoirs (LDRs) are more common with temperatures greater than 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 .
Lower-temperature LDRs () require pumping. They are common in extensional terrains, where heating occurs 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.
Engineered geothermal systems