Geothermal energy

thumb|upright=1.3|Steam rising from the [[Nesjavellir Geothermal Power Station in Iceland]]

upright=1.3|thumb|The [[Imperial Valley Geothermal Project near the Salton Sea, California]]

Geothermal energy is thermal energy extracted from 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.

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, 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. 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

thumb|right|The oldest known pool fed by a hot spring, built in the [[Qin dynasty in the 3rd century BC]]

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.

[[File:geothermal capacity.svg|thumb|left|Global geothermal electric capacity. Upper red line is installed capacity; lower green line is realized production. 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

Resources

thumb|left|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. 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 the 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 of 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, 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.

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.

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.

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. In Myanmar over 39 locations are capable of geothermal power production, some of which are near Yangon.

{| class="wikitable sortable floatright" style="text-align:right;"

|+ Direct use data 2015

!scope="col"| Country

!scope="col"| Capacity (MW) 2015

!scope="row"| United States

|17,415

!scope="row"| Philippines

!scope="row"| Indonesia

!scope="row"| Mexico

|155

!scope="row"| Italy

|1,014

!scope="row"| New Zealand

|487

!scope="row"| Iceland

|2,040

!scope="row"| Japan

|2,186

!scope="row"| Iran

|81

!scope="row"| El Salvador

!scope="row"| Kenya

|22

!scope="row"| Costa Rica

!scope="row"| Russia

|308

!scope="row"| Turkey

|2,886

!scope="row"| Papua New Guinea

|0.10

!scope="row"| Guatemala

!scope="row"| Portugal

|35

!scope="row"| China

|17,870

!scope="row"| France

|2,346

!scope="row"| Ethiopia

!scope="row"| Germany

|2,848

!scope="row"| Austria

|903

!scope="row"| Australia

|16

!scope="row"| Thailand

|128

{| class="sortable wikitable" style="text-align:right;"

|+ Installed geothermal electric capacity

!scope="col"| Country

!scope="col"| Capacity (MW) (2024)

!scope="col"| % of national electricity production

(2024)