Geothermal energy is heat taken from the Earth's crust. It comes from the Earth's formation and radioactive decay. People have used geothermal energy for heat and electricity for thousands of years.

Geothermal heating, such as using water from hot springs, was used for bathing since Paleolithic times and for warming buildings since Roman times. Geothermal power, which produces electricity from geothermal energy, has been used since the 20th century. Geothermal power plants generate electricity consistently, regardless of weather conditions. Geothermal resources could supply all of humanity's energy needs. Most geothermal energy is found near tectonic plate boundaries.

The cost of generating geothermal power dropped by 25% during the 1980s and 1990s. New technology continued to lower costs, making more resources usable. In 2021, the U.S. Department of Energy estimated that electricity from a new geothermal plant costs about $0.05 per kilowatt-hour.

In 2019, 13,900 megawatts (MW) of geothermal power was available worldwide. An additional 28 gigawatts was used for heating, industrial processes, and other purposes as of 2010. In 2019, the geothermal industry employed about 100,000 people.

The word "geothermal" comes from Greek words meaning "Earth" and "hot."

History

Hot springs have been used for bathing since at least Paleolithic times. The oldest known spa is located at the site of the Huaqing Chi palace. In the first century CE, the Romans took control of Aquae Sulis, now known as Bath, Somerset, England, and used the hot springs there to supply public baths and underfloor heating. The admission fees for these baths likely 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 use of geothermal energy began in 1827, when geyser steam was used to extract boric acid from volcanic mud in Larderello, Italy.

In 1892, the first district heating system in the United States was built in Boise, Idaho, and powered by geothermal energy. A similar system was later built in Klamath Falls, Oregon, in 1900. The world's first known building to use geothermal energy as its main heat source was the Hot Lake Hotel in Union County, Oregon, starting 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 around the same time. In 1930, Charles Lieb created the first downhole heat exchanger to heat his home. Geyser steam and water began heating homes in Iceland in 1943.

During the 20th century, geothermal energy was used to generate electricity. Prince Piero Ginori Conti tested the first geothermal power generator on July 4, 1904, at the Larderello steam field. It successfully lit four light bulbs. In 1911, the first commercial geothermal power plant was built there. It remained the only industrial producer of geothermal power until New Zealand built a plant in 1958. By 2012, it produced about 594 megawatts of electricity.

In 1960, Pacific Gas and Electric began operating the first U.S. geothermal power plant at The Geysers in California. The original turbine at this plant lasted more than 30 years and produced 11 megawatts of net power. An organic fluid-based binary cycle power station was first demonstrated in 1967 in the USSR and later introduced in the United States in 1981. This technology allows the use of heat sources as low as 81°C (178°F). In 2006, a binary cycle plant in Chena Hot Springs, Alaska, began operating, producing electricity from a record low temperature of 57°C (135°F).

Resources

The Earth has an internal heat content of 10 joules (30 terawatt-hours). About 20% of this heat is leftover from when Earth formed; the rest comes from radioactive decay of naturally found elements. For example, a 5,275-meter-deep hole drilled in the United Downs Deep Geothermal Power Project in Cornwall, England, found granite with very high thorium levels. Radioactive decay of this thorium is believed to cause the rock's high temperature.

Earth's interior is hot and pressurized enough to melt some rock and make the solid mantle behave like soft plastic. Parts of the mantle move upward because they are lighter than the surrounding rock. Temperatures at the boundary between Earth's core and mantle can reach over 4,000°C (7,230°F).

Earth's internal thermal energy moves to the surface through conduction at a rate of 44.2 terawatts (TW). This energy is replaced by radioactive decay of minerals at a rate of 30 TW. These energy rates are more than twice the total energy used by humans worldwide, but most of this energy cannot be used. In addition to internal heat, the top 10 meters (33 feet) of Earth's surface is heated by sunlight in summer and cools in winter.

Outside of seasonal changes, the temperature increase through Earth's crust is about 25–30°C (77–86°F) for every kilometer of depth in most areas. The average conductive heat flow is 0.1 megawatts per square kilometer. These values are much higher near tectonic plate boundaries, where the crust is thinner. Heat flow may also increase due to fluid movement through magma channels, hot springs, or hydrothermal circulation.

The ability to generate electricity from Earth's heat depends strongly on temperature. The easiest source of heat is a hot spring. The next best option is drilling into a hot underground water reservoir. Artificial reservoirs can be created by injecting water to break apart bedrock. These systems are called enhanced geothermal systems.

Estimates from 2010 suggest that geothermal energy could generate between 0.035 and 2 terawatts of electricity, depending on how much money is invested. Some estimates assume wells as deep as 10 kilometers (6 miles), though wells in the 20th century rarely reached more than 3 kilometers (2 miles). Wells of this depth are common in the petroleum industry.

Geothermal power

Geothermal power is electricity made from geothermal energy. Dry steam, flash steam, and binary cycle power plants are used to produce this electricity. In 2010, geothermal electricity was generated in 26 countries.

By 2019, the total geothermal power capacity worldwide was 15.4 gigawatts (GW). Of this, 23.86%, or 3.68 GW, was in the United States.

Geothermal energy provides a large part of the electricity used in Iceland, El Salvador, Kenya, the Philippines, and New Zealand.

Geothermal power is considered a renewable energy source because the amount of heat taken from the Earth is very small compared to the Earth's total heat. On average, geothermal power plants release 45 grams of carbon dioxide for every kilowatt-hour of electricity produced, which is less than 5% of the emissions from coal-fired power plants.

Geothermal power plants are usually built near the edges of tectonic plates, where high-temperature geothermal resources are close to the surface. These plants are often located on land with high underground temperatures, good permeability, and access to large water reserves for the working fluid. Improvements in technology, such as binary cycle power plants and better drilling methods, have allowed geothermal energy to be used in more areas. Demonstration projects are active in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France. A project in Basel, Switzerland, was stopped after it caused earthquakes. Other demonstration projects are being built in Australia, the United Kingdom, and the United States. In Myanmar, over 39 locations have the potential for geothermal power production, including some near Yangon.

Geothermal heating

Geothermal heating uses heat from the Earth to warm buildings and provide hot water for people. Humans have used this method since ancient times. In 2004, about 70 countries used a total of 270 petajoules of geothermal energy for heating. By 2007, 28 gigawatts of geothermal heating met 0.07% of the world's total energy needs. This method is efficient because it does not require changing the form of energy, but it often has low capacity factors (around 20%) because heat is usually needed most during winter.

Even ground that feels cold contains heat. Below 6 meters (20 feet), the ground's temperature remains steady at the average yearly air temperature. This heat can be used with a ground source heat pump.

Types

Hydrothermal systems generate geothermal energy by using naturally occurring hydrothermal reservoirs. These systems can be either vapor-dominated or liquid-dominated.

Larderello and The Geysers are examples of vapor-dominated systems. These sites have temperatures between 240 and 300 °C, which produce superheated steam.

Liquid-dominated reservoirs (LDRs) are more common and have temperatures above 200 °C (392 °F). They are often found near volcanoes in or around the Pacific Ocean, as well as in rift zones and hot spots. Flash plants are typically used to generate electricity from these reservoirs. Steam from the well is enough to power the plant. Most wells produce 2–10 MW of electricity. Steam is separated from liquid using cyclone separators, and this steam drives electric generators. The condensed liquid is sent back down the well for reheating and reuse. As of 2013, the largest liquid system was Cerro Prieto in Mexico, which produces 750 MW of electricity from temperatures reaching 350 °C (662 °F).

Lower-temperature LDRs (120–200 °C or 248–392 °F) require pumping to move the water. These systems are common in extensional terrains, where heat is created by deep water circulation along faults, such as in the Western US and Turkey. Water passes through a heat exchanger in a Rankine cycle binary plant. This process turns water into vapor, which then vaporizes an organic working fluid to drive a turbine. Binary plants were first developed in the Soviet Union in the late 1960s and are now the most common type of new plants. Binary plants do not produce emissions.

An engineered geothermal system is a system created or improved by engineers. These systems are used in geothermal reservoirs with hot rocks but not enough natural reservoir quality, such as insufficient geofluid quantity, rock permeability, or rock porosity to function as natural hydrothermal systems. Types of engineered geothermal systems include enhanced geothermal systems, closed-loop or advanced geothermal systems, and some superhot rock geothermal systems.

Enhanced geothermal systems (EGS) use water injected into wells to be heated and then pumped back out. Water is injected under high pressure to expand existing rock cracks, allowing water to flow freely. This method was adapted from oil and gas fracking techniques. The rock formations used are deeper, and no toxic chemicals are used, which reduces environmental risks. Instead, materials like sand or ceramic particles are used to keep cracks open and ensure good flow rates. Drillers can use directional drilling to increase reservoir size.

Small-scale EGS systems have been built in the Rhine Graben at Soultz-sous-Forêts in France, and at Landau and Insheim in Germany.

Closed-loop geothermal systems, sometimes called Advanced Geothermal Systems (AGS), are engineered systems that use a working fluid heated in hot rock reservoirs without direct contact with rock pores or fractures. The working fluid stays inside a closed loop of deeply buried pipes that transfer Earth’s heat. Advantages of closed-loop systems include: (1) no need for geofluid, (2) no need for the hot rock to be permeable or porous, and (3) all working fluid can be reused without loss. Eavor, a Canadian geothermal company, tested its closed-loop system in shallow soft rock in Alberta, Canada. The area is part of a sedimentary basin with a geothermal gradient too low for electricity generation. However, the system produced about 11,000 MWh of thermal energy during its first two years of operation.

Economics

Geothermal power, like wind and solar energy, has low costs for operating the system, but the initial costs are high. Drilling wells is the biggest part of these costs, and not all wells can be used for energy production. For example, in Nevada in 2009, a pair of wells (one for taking energy out and one for putting water back in) could produce 4.5 megawatts (MW) of electricity. Drilling such a pair cost about $10 million, but 20% of wells failed, making the average cost for a successful well about $50 million.

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

• Geothermal energy is often found in hard igneous or metamorphic rock, which is harder to drill than the softer sedimentary rock where oil and gas are usually found.
• The rock is often broken, causing vibrations that wear out drilling tools.
• The rock may contain sharp quartz and corrosive fluids that damage equipment.
• The rock is very hot, which limits the use of electronic tools deep in the well.
• Geothermal wells must be fully cemented from top to bottom to handle temperature changes, unlike oil and gas wells, which are usually cemented only at the bottom.
• Geothermal wells are much larger in diameter than typical oil and gas wells.

In 2007, building a geothermal power plant and drilling wells cost about €2–5 million per MW of electricity produced. The cost to make the energy profitable was between €0.04–0.10 per kilowatt-hour (kW·h). Enhanced geothermal systems, which use more advanced technology, had higher costs, with initial expenses above $4 million per MW and a profit point above $0.054 per kW·h.

From 2013 to 2020, most funding for renewable energy came from private companies, making up about 75% of all money spent. The balance between private and public funding depends on how attractive and ready a technology is for use. In 2020, only 32% of geothermal energy investments came from private sources.

In January 2024, the Energy Sector Management Assistance Program (ESMAP) published a report titled "Socioeconomic Impacts of Geothermal Energy Development." The report showed that geothermal energy creates more jobs than wind or solar energy, with about 34 jobs created per megawatt of electricity produced. These jobs are in many areas, such as construction, engineering, and maintenance. The report explains that geothermal projects help train workers through hands-on experience and formal education, improving local skills and employment. It also notes that geothermal projects often work closely with communities, leading to better roads, training programs, and shared income. These improvements can help farmers grow more food and ensure a stable food supply. The report also highlights efforts to support gender equality and include underrepresented groups by offering jobs, education, and training. These actions help grow the economy, increase government income, and create more stable and diverse national economies. They also improve health, education, and community unity.

Development

Geothermal projects go through several steps to develop. Each step has risks. Many projects are stopped during early steps like exploration and surveying the ground, which are not suitable for getting loans. Later steps are often funded by investors.

A common problem happens when geothermal systems are in areas with rock rich in carbonate. In these cases, hot water moving upward from underground can dissolve parts of the rock. As the water cools near the surface, minerals in the water form solid deposits called calcium scale. This calcium scale can slow down water flow and cause the system to stop working so it can be fixed.

Sustainability

Geothermal energy is considered sustainable because the heat taken from the Earth is very small compared to the Earth's total heat, which is about 100 billion times greater than the world's energy use in 2010. The Earth's heat is not in balance; the planet is slowly cooling over very long periods of time. Human activities taking heat from the Earth usually do not speed up this cooling process.

Wells can also be seen as renewable because the water taken from them is often returned to the well to be heated again and used later, though at a lower temperature.

Using energy instead of materials has helped reduce the human impact on the environment in many cases. Geothermal energy has the potential to help reduce this impact even more. For example, Iceland has enough geothermal energy to stop using fossil fuels for electricity and to heat sidewalks in Reykjavik, removing the need for salt to melt ice.

However, the local effects of taking heat must be considered. Over many years, individual wells can lower the temperature and water levels in their area. Three of the oldest geothermal sites—Larderello, Wairakei, and the Geysers—had reduced energy production because heat and water were taken out faster than they could be replaced. Lowering the amount of energy produced and adding more water to the wells may help these sites recover their original energy output. These methods have been used at some locations, and these sites still provide a lot of energy.

The Wairakei power station started operating in November 1958. It reached its highest energy production of 173 MW in 1965, but by then, the supply of high-pressure steam was already decreasing. In 1982, the station was adjusted to use intermediate pressure, reducing its output to 157 MW. In 2005, two 8 MW isopentane systems were added, increasing the total output by about 14 MW. Detailed information about the station was lost because of changes in how the organization was managed.

Environmental effects

Fluids taken from underground contain a mix of gases, including carbon dioxide (CO₂), hydrogen sulfide (H₂S), methane (CH₄), and ammonia (NH₃). These gases can cause global warming, acid rain, and bad smells if released into the air. Current geothermal power plants release about 122 kilograms (269 pounds) of CO₂ for every megawatt-hour (MW·h) of electricity produced, which is much less than emissions from fossil fuel plants. Some geothermal plants, like those in Turkey, may release more pollutants than gas-fired power plants during their first few years of operation. Plants that produce high amounts of acids and volatile chemicals usually have systems to reduce harmful emissions. New closed-loop technologies developed by Eavor may one day eliminate these emissions completely.

Water from geothermal sources can contain small amounts of harmful substances like mercury, arsenic, boron, and antimony. These chemicals form solid materials when the water cools, which can harm the environment if released. Modern methods that return geothermal fluids back into the ground help reduce this environmental risk.

Building geothermal plants can affect land stability. In the Wairakei field, the ground sank in some areas. In Staufen im Breisgau, Germany, the ground rose due to a layer of anhydrite turning into gypsum when it mixed with water, increasing its volume. Enhanced geothermal systems may cause earthquakes during hydraulic fracturing. A project in Basel, Switzerland, was stopped after more than 10,000 small earthquakes, some measuring up to 3.4 on the Richter Scale, occurred during the first six days of water injection.

Geothermal power uses much less land and freshwater compared to other energy sources. Geothermal plants need 3.5 square kilometers (1.4 square miles) per gigawatt of electricity produced, while coal plants and wind farms require 32 square kilometers (12 square miles) and 12 square kilometers (4.6 square miles), respectively. Geothermal plants use 20 liters (5.3 gallons) of freshwater per MW·h of electricity, compared to over 1,000 liters (260 gallons) per MW·h for nuclear, coal, or oil plants.

Production

The Philippines started studying geothermal energy in 1962 when the Philippine Institute of Volcanology and Seismology examined the geothermal area in Tiwi, Albay. The first geothermal power plant in the Philippines was built in 1977 in Tongonan, Leyte. The New Zealand government made an agreement with the Philippines to build the plant in 1972. The Tongonan Geothermal Field added three power plants—Upper Mahiao, Matlibog, and South Sambaloran—which together have a capacity of 508 megawatts.

The first geothermal power plant in the Tiwi region opened in 1979, and two more plants were built in 1980 and 1982. The Tiwi geothermal field is located about 450 kilometers from Manila. The three geothermal power plants in Tiwi produce 330 megawatts of electricity. This places the Philippines behind the United States and Mexico in geothermal energy growth. The Philippines has seven geothermal fields and continues to use geothermal energy through the Philippine Energy Plan 2012–2030, which aims to generate 70% of the country’s energy from geothermal sources by 2030.

According to the Geothermal Energy Association (GEA), geothermal energy capacity in the United States increased by 5%, or 147.05 megawatts, in 2013. This growth came from seven geothermal projects that began operating in 2012. The GEA updated its 2011 estimate of installed geothermal capacity by 128 megawatts, raising the total U.S. geothermal capacity to 3,386 megawatts.

The municipal government of Szeged is working to reduce its gas use by 50% by using geothermal energy for its district heating system. The Szeged geothermal power station has 27 wells, 16 heating plants, and 250 kilometers of pipes to distribute heat.