Geothermal heating is the use of heat from inside the Earth to warm buildings, water, greenhouses, fish farms, pavements, and industrial processes. It is one of the oldest ways people have used energy. Hot springs were used for bathing long before written history, and some towns have piped natural hot water into homes for centuries. Today the same basic idea heats most homes in Iceland, tens of thousands of apartments around Paris, the Idaho State Capitol, and millions of individual houses fitted with ground-source heat pumps.
The term covers two quite different technologies, and it helps to keep them apart from the start:
- Direct use takes naturally hot water or steam from the ground and uses its heat directly, usually through pipes and heat exchangers. It needs a hot resource, which is common only in certain regions.
- Ground-source heat pumps (also called geothermal heat pumps or geoexchange systems) use the ordinary, mild temperature of shallow ground as a source of heat in winter and a place to dump heat in summer. An electric heat pump does the work of raising the temperature. These systems can be installed almost anywhere.
Both are distinct from geothermal power, which uses very hot resources to generate electricity. This article covers heating. It explains where the Earth's heat comes from, the history of its use, how each kind of system works, how much is used worldwide, what it costs, its environmental effects and risks, and some common misconceptions.
Where the heat comes from
The Earth's interior is hot. Part of that heat is left over from the planet's formation, and part is produced continuously by the decay of radioactive elements in rocks. Heat flows slowly outward toward the surface. In most places the temperature rises with depth, and in volcanic regions and near the edges of tectonic plates, hot rock and water come much closer to the surface.
Very close to the surface, the ground's temperature is controlled mostly by the sun and the climate, not by heat from below. A few meters down, the daily and seasonal swings of air temperature fade out. Below about 6 meters the ground stays close to the average annual air temperature of the location, and below about 10 meters seasonal variation essentially disappears. The U.S. Department of Energy describes shallow ground temperatures in the United States as roughly 40 to 70 degrees Fahrenheit (about 4.5 to 21 degrees Celsius), depending on latitude.
That gives two kinds of geothermal heating resource:
| Resource | Typical temperature | Where found | How it is used |
|---|---|---|---|
| Hot water or steam from deep wells or springs | Often 50 to 150 °C for direct use | Volcanic areas, plate boundaries, deep sedimentary basins | Piped directly or through heat exchangers to buildings and industry |
| Shallow ground or groundwater | Roughly the local average air temperature, often around 10 °C | Almost everywhere | Used with a heat pump that raises the temperature for heating |
Above about 150 °C, resources are usually more valuable for generating electricity, although the hot water left over after power generation can then be used for heating.
A short history
Ancient and medieval use
People have bathed in hot springs since at least the Paleolithic. In China, a stone pool was built at Mount Li in the 3rd century BC, during the Qin dynasty, at a site later used for the Huaqing palace. The Romans built public baths at Aquae Sulis, now Bath in England, in the 1st century AD, and used warm water for baths and underfloor heating. In Iceland, one of the island's original settlers is said to have built a hot pool about a thousand years ago.
The town of Chaudes-Aigues in south-central France has what is generally considered the oldest working geothermal district heating system in the world. Records indicate that in the 14th century water at about 82 °C from local springs was being piped to heat houses in the town, and the system is still in use.
Industrial beginnings
In 1827, at Larderello in Tuscany, steam from natural vents began to be used to extract boric acid from volcanic mud, the earliest known industrial use of geothermal heat. Larderello later became the site of the world's first geothermal power generation.
The first American district heating
In 1892 Boise, Idaho, opened what is described as the first district heating system in the United States powered by geothermal energy. The Warm Springs Avenue system, now the Boise Warm Springs Water District, was built to heat large homes along Warm Springs Avenue. According to John Lund of the Geo-Heat Center at the Oregon Institute of Technology, it heated about 250 large residences and is still operating. Klamath Falls, Oregon, copied the Boise approach around 1900.
In 1926 a deep geothermal well was used to heat greenhouses in Boise, and around the same time natural hot water was used to heat greenhouses in Iceland and Tuscany. In 1930 Charlie Lieb developed the first downhole heat exchanger, a loop of pipe placed in a well, to heat his house in Klamath Falls.
Reykjavík
Iceland's capital began using geothermal water for space heating in the first half of the twentieth century, and from the 1940s steam and hot water were heating large numbers of homes in Reykjavík. After the oil price shocks of the 1970s, the Icelandic government deliberately replaced oil with geothermal water in district heating across the country. This sharply lowered heating costs, and today at least 90 percent of Icelandic homes are heated with geothermal energy.
The heat pump
The heat pump, a machine that moves heat from a cooler place to a warmer one using work, was described by Lord Kelvin in 1852. In 1912 the Swiss engineer Heinrich Zoelly patented the idea of using one to draw heat from the ground. Practical ground-source systems came in the 1940s. In 1946 J. Donald Kroeker demonstrated the first commercial geothermal heat pump, heating the Commonwealth Building in Portland, Oregon. In 1948 Professor Carl Nielsen of Ohio State University built an early residential open-loop system in his home. Robert C. Webber is also credited with an early homemade system in the late 1940s, though sources differ on dates.
Heat pumps became popular in Sweden after the 1973 oil crisis. Later improvements in plastic piping made buried loops cheaper and more durable, which helped the technology spread.
How direct-use heating works
The basic system
A direct-use system has three parts:
- A heat source. Usually one or more wells that bring hot water to the surface. In many modern systems, a second well returns the cooled water underground.
- A distribution system. Insulated pipes carry hot water, or heat from it, to customers.
- Equipment in buildings. Heat exchangers, radiators, or floor heating transfer the heat into the building. Meters measure how much heat each customer uses.
In some systems the geothermal water itself flows through the pipes and radiators. More often, a heat exchanger transfers the heat to a separate, clean water loop. This protects equipment from minerals and gases dissolved in geothermal water, which can corrode or clog pipes.
Production and injection wells
Many modern systems use a pair of wells called a doublet: one well produces hot water, and the other injects the cooled water back into the same rock layer, some distance away. Reinjection has several benefits. It maintains pressure in the underground reservoir, so the wells do not run dry. It disposes of water that may contain dissolved salts or gases. And it allows the rock to reheat the water over time. The two wells must be far enough apart that cold injected water does not reach the production well too quickly.
Peaking and hybrid systems
Heating demand peaks on the coldest days. Building a geothermal system large enough for the single coldest day would leave it partly idle most of the year. So many systems use geothermal heat for the steady base load and add a conventional boiler or heat pump for the peaks. Some systems also use heat pumps to boost the temperature of lukewarm geothermal water.
District heating
District heating distributes heat from a central source to many buildings through a network of pipes. Geothermal district heating has economies of scale: one set of wells and one heat exchanger station can serve a whole neighborhood or city. Lund lists the major components as the well field with its production wells, injection wells, and a peaking station; the transmission and distribution network; and the central pumping station together with in-building meters, heat exchangers, and circulation pumps.
Cascading
Geothermal water cools as its heat is used. A well-designed system can use the same water several times at falling temperatures, a technique called cascading. Hot water might first heat buildings, then warm a greenhouse, then heat a fish pond or melt snow on sidewalks, before being reinjected. Some newer projects first use moderately hot water to generate electricity in a binary power plant and then use the leftover heat for direct heating. Lund notes that water as cool as 74 °C has been used for binary power generation at Chena Hot Springs in Alaska.
Case studies in direct use
Iceland
Iceland is the clearest example of a country built around geothermal heat. In 2020, according to figures compiled for the country's geothermal update, direct use included about 23,094 terajoules (TJ) for space heating, 3,628 TJ for heated swimming pools, 2,036 TJ for melting snow, 1,404 TJ for fish farming, 429 TJ for greenhouses, and 393 TJ for industry. In Reykjavík, water that has already been used to heat homes is circulated at about 30 °C through plastic pipes under streets and sidewalks to keep them clear of snow and ice.
Some of Iceland's hot water comes from combined power and heat plants. The Nesjavellir plant, with 120 megawatts of electric capacity, sends hot water by pipeline to Reykjavík. These plants also release hydrogen sulfide, a gas with a rotten-egg smell, and a 2012 University of Iceland study expected concentrations to exceed the 24-hour health limit about twice a year on average.
The Paris basin
Beneath the Paris region lies a layer of limestone called the Dogger, about 1,500 to 2,000 meters deep, containing water at around 60 to 80 °C. The first geothermal heating well into the Dogger was drilled at Melun, south of Paris, in 1969. Since then the region has built around fifty geothermal heating networks, supplying the equivalent of about 250,000 homes, according to the industry news site ThinkGeoEnergy. Nearly all of these use the doublet design: water is pumped up, its heat is passed to the district heating network through a heat exchanger, and the cooled water, at about 40 °C, is injected back into the aquifer. The Paris basin is important because it shows that geothermal district heating does not require volcanoes. A deep, warm, water-bearing rock layer under a dense city is enough.
Boise, Idaho
Boise has more than one geothermal system. Besides the 1892 Warm Springs district, the city of Boise began operating its own geothermal district heating system in 1983, helped by a 6.76 million dollar grant from the U.S. Department of Energy. It pumps geothermal water at about 77 °C (170 °F) from a reservoir under the city. The city system now heats more than 90 downtown buildings, including City Hall, the Idaho State Capitol, and several Boise State University buildings, according to local reporting.
Klamath Falls, Oregon
Klamath Falls has a long history of private geothermal wells and a city district heating system. A feasibility study began in 1977, and the system was built in 1979 and 1980 with a 2.58 million dollar federal grant covering 65 percent of the cost. It draws water at about 100 to 104 °C from two wells 112 and 274 meters deep. According to Lund, it heats 24 buildings totaling about 400,000 square feet, melts snow on about 14,000 square meters of sidewalks, and supplies a brewery and a greenhouse growing tree seedlings. Its capacity is about 8.5 megawatts thermal (MWt).
Elko, Nevada
Elko has had geothermal district heating since 1982. Lund reports that it serves 19 residential and commercial customers with water at about 81 °C, supplying space heating and hot water to about 28,000 square meters of buildings, melting snow on sidewalks, and heating a commercial laundry and a sewage plant's digesters. Heat was sold at 1.50 dollars per 1,000 gallons, which Lund calculated as about 2.1 cents per kilowatt-hour, or roughly 30 percent of the cost of the equivalent heat from natural gas.
U.S. systems overall
Lund counted at least 19 geothermal district heating systems in the United States, using water from about 59 to 103 °C. Together they had about 82 MWt of capacity, used about 840 TJ of heat a year, and had about 72 kilometers of pipelines. Besides Boise and Klamath Falls, older systems include Ketchum, Idaho (1929), the Oregon Institute of Technology campus (1962), and Midland, South Dakota (1964). Most others are less than thirty years old.
Europe
The European Geothermal Energy Council counted 280 geothermal district heating plants in Europe in 2016, with a total capacity of about 4.9 gigawatts thermal.
How ground-source heat pumps work
The principle
A heat pump works like a refrigerator. A refrigerator takes heat from inside the box and releases it into the kitchen through coils at the back. A heat pump in heating mode takes heat from a cool source outside the building and releases it inside at a higher temperature. In cooling mode it runs in reverse, taking heat from the building and releasing it outside.
Air-source heat pumps use outdoor air as their source. Ground-source heat pumps use the ground or groundwater instead. Because the ground a few meters down is warmer than winter air and cooler than summer air, a ground-source heat pump has an easier job in both seasons. The U.S. Department of Energy says geothermal heat pumps can reduce energy use by 25 to 50 percent compared with air-source heat pump systems.
Loop types
The part of the system in the ground is called the loop or ground heat exchanger. The Department of Energy describes four main types:
| Loop type | Description | When it is used |
|---|---|---|
| Horizontal closed loop | Pipes buried in trenches at least about four feet (1.2 m) deep | Usually the cheapest for houses with enough land, especially new construction |
| Vertical closed loop | Pipes inserted in drilled boreholes, typically about 15 to 120 m deep | Larger buildings, small lots, or shallow soil over rock |
| Pond or lake closed loop | Coils placed at least eight feet (2.4 m) under the surface of a suitable body of water | Where a large enough pond or lake is available; can be the cheapest option |
| Open loop | Well or surface water pumped directly through the heat pump and then returned | Where there is enough clean water and local rules allow discharge |
In closed loops, a fluid, usually water or water with antifreeze, circulates through sealed plastic pipes and never mixes with groundwater.
Efficiency
Heat pump efficiency is measured by the coefficient of performance (COP): the amount of heat delivered divided by the electrical energy used. An electric resistance heater has a COP of 1, delivering one unit of heat for each unit of electricity. The Department of Energy says geothermal heat pumps reach efficiencies of 300 to 600 percent on the coldest winter nights, meaning a COP of about 3 to 6, and that they move three to five times as much energy as they consume. Cooling efficiency is measured by a different number, the energy efficiency ratio (EER).
A COP of 4 does not mean "400 percent efficient" in the sense of creating energy. The heat pump is not making heat from nothing. It is moving heat that is already in the ground and adding the energy of its own compressor.
Hypothetical worked example: comparing heating costs
The following example uses invented numbers to show the arithmetic. It does not describe any real house or price.
Suppose a house needs 20,000 kilowatt-hours (kWh) of heat in a winter, and electricity costs 15 cents per kWh.
- Electric resistance heating (COP 1): 20,000 kWh of electricity, costing 3,000 dollars.
- Air-source heat pump with an average seasonal COP of 2.5: 8,000 kWh, costing 1,200 dollars.
- Ground-source heat pump with an average seasonal COP of 4: 5,000 kWh, costing 750 dollars.
In this example the ground-source system saves 450 dollars a year compared with the air-source heat pump. Whether that is worth it depends on the extra installation cost. If the ground loop added 10,000 dollars, simple payback would take more than twenty years. If it added 3,000 dollars, payback would take under seven. This is why the economics of ground-source heat pumps depend so heavily on drilling and trenching costs, local energy prices, and any subsidies.
Costs and lifespan
The Department of Energy's consumer guide, published in 2021, says installing a geothermal heat pump costs more than an air-source system of the same capacity, but that the extra cost can often be recovered through energy savings in 5 to 10 years. It gave an example: an average system cost about 2,500 dollars per ton of capacity, so a typical 3-ton home unit would cost about 7,500 dollars plus installation and drilling, compared with about 4,000 dollars for a comparable air-source system with air conditioning. Prices vary widely by region and site, and these figures should be taken as illustrations rather than current quotes.
The same guide says the indoor heat pump typically lasts 20 years or more, and the underground loop 25 to 50 years. Ground-source systems are also quieter than air-source units, since there is no outdoor fan, and need little maintenance. Some U.S. utilities have offered special lower rates for geothermal customers. The U.S. Environmental Protection Agency has estimated that geoexchange systems can save homeowners 30 to 70 percent on heating and 20 to 50 percent on cooling compared with conventional systems.
Is a heat pump "really" geothermal?
There is a long-running argument over terminology. Strictly, most of the heat a shallow ground loop collects comes from the sun warming the surface and from the air, not from the Earth's deep interior. For that reason some engineers prefer the terms "ground-source heat pump" or "geoexchange" and reserve "geothermal" for direct use and power. Others, including the U.S. Department of Energy, use "geothermal heat pump" because the system uses heat stored in the ground. Either way, global statistics on geothermal direct use usually include ground-source heat pumps, and they make up most of the total.
Practical considerations for a home system
For a homeowner, the decision to install a ground-source heat pump usually comes down to a few questions. The Department of Energy's guidance points to several of them.
- Timing. Ground-source systems are a particularly good fit for new homes or major renovations, when the heating and cooling system is being replaced anyway and the ground loop can be installed before landscaping.
- Land and soil. A horizontal loop needs enough open land for trenches. A small lot, or shallow soil over bedrock, usually means vertical boreholes, which cost more but use little land and disturb the site less.
- Water. An open-loop system needs a reliable supply of reasonably clean water and must meet local rules on groundwater discharge. A pond loop needs a body of water of adequate size, depth, and quality.
- Budget. A dual-source heat pump, which combines an air-source unit with a ground loop, costs less to install than a full ground-source system and is more efficient than an air-source system alone, though less efficient than a full geothermal unit.
- Ratings. Buyers can compare models by their coefficient of performance for heating and their energy efficiency ratio for cooling, along with any efficiency certification.
A good installer will also size the system to the building. An oversized system costs more and may cycle on and off; an undersized ground loop can cool the surrounding ground over successive winters and lose efficiency. Sizing depends on the building's heat loss, which is why improving insulation before installing any heat pump often makes the system smaller and cheaper.
Deeper and newer technologies
Researchers are also exploring ways to use heat from deeper or hotter rock than conventional wells reach. Since 2000, studies have looked at using carbon dioxide instead of water as the fluid that carries heat in enhanced geothermal systems, which create or enlarge fractures in hot, dry rock. In Iceland, the Iceland Deep Drilling Project accidentally struck magma at about 2,100 meters depth. Its IDDP-1 well was used to produce high-temperature steam, an experiment described as the world's first magma-enhanced geothermal system. These projects are mainly aimed at electricity, but any hot resource can also supply heat, and combined heat and power from such wells is one possible future for geothermal district heating.
How much geothermal heat is used
The most comprehensive global figures come from reviews compiled every five years for the World Geothermal Congress by John Lund and colleagues.
Growth over time
| Year or review | Installed direct-use capacity | Notes |
|---|---|---|
| 2005 | about 28 GWt | About 72 countries; figures summarized in the Wikipedia article on geothermal heating |
| End of 2009 | 50,583 MWt | WGC 2010 review |
| WGC 2015 review | 70,329 MWt | Previous five-year review |
| End of 2019 | 107,727 MWt | WGC 2020 review by Lund and Toth |
The 2020 review by Lund and Aniko Toth reported that direct use at the end of 2019 had an installed capacity of 107,727 MWt, up 52 percent from five years earlier, and annual use of about 1,020,887 TJ, or 283,580 gigawatt-hours. Its summary covered 88 countries. The average capacity factor was about 0.30, meaning that over a year the installed equipment delivered about 30 percent of what it could if it ran at full output all the time. This is low because heating demand is seasonal.
By type of use
According to the same review:
| Use | Installed capacity (MWt) | Annual energy (TJ/yr) |
|---|---|---|
| Ground-source heat pumps | 77,547 (71.6% of capacity) | 599,981 (59.2% of energy) |
| Space heating (of which about 91% of capacity is district heating) | 12,768 | 162,979 |
| Greenhouse and covered ground heating | 2,459 | 35,826 |
| Aquaculture pond and raceway heating | 950 | 13,573 |
| Industrial process heat | 852 | 16,390 |
Other uses not listed in the table include bathing and swimming, agricultural drying, snow melting, and cooling. An earlier breakdown for 2015, summarized on Wikipedia, gave annual use in gigawatt-hours as: geothermal heat pumps 90,293; bathing and swimming 33,164; space heating 24,508; greenhouse heating 7,407; aquaculture 3,322; industrial uses 2,904; cooling and snow melting 722; agricultural drying 564; and others 403.
Leading countries
At the end of 2019, the countries with the largest direct-use capacity including heat pumps were:
| Country | Installed capacity (MWt) | Annual use (TJ/yr) |
|---|---|---|
| China | 40,610 | 443,492 |
| United States | 20,713 | 152,810 |
| Sweden | 6,680 | 62,400 |
| Germany | 4,806 | not listed in summary |
| Turkey | 3,488 | 54,584 |
The pattern differs by country. In the United States and Sweden, almost all of the capacity is ground-source heat pumps. China has large heat pump capacity but also major direct use, including bathing and district heating. Turkey and Iceland rely heavily on district heating from hot water.
Uses by temperature
Different uses need different temperatures, which helps explain why cascading works.
| Use | Typical temperature of geothermal fluid |
|---|---|
| Space heating | about 50 to 100 °C |
| Agriculture, including greenhouses and fish farming | about 25 to 90 °C |
| Bathing and swimming | warm water, often blended down to comfortable temperatures |
| Snow melting | low temperatures, often using water already used for heating |
| Ground-source heat pumps | shallow ground, often around 10 °C |
| Electricity generation | usually above about 150 °C, though binary plants use lower |
Advantages
- Steady supply. Unlike solar and wind, geothermal heat is available day and night in all weather.
- High efficiency in direct use. Using hot water for heat involves no conversion to electricity and back, so little energy is lost.
- Low operating cost. Once wells and pipes are built, the heat itself costs nothing. The Elko example sold heat at about 30 percent of the cost of natural gas.
- Low emissions. Direct use produces far fewer greenhouse gas emissions than burning fossil fuels for heat. Heat pumps' emissions depend on how the electricity they use is generated.
- Local resource. Heat cannot be imported or exported easily, so it is not affected by fuel price shocks. Iceland's shift away from oil after the 1970s shows the value of this.
- Long life. Ground loops and wells can last for decades.
Limitations and risks
High upfront cost and drilling risk
Geothermal systems cost a lot to build and little to run. Deep wells are expensive, and there is a risk that a well will not find water as hot or as plentiful as expected. That exploration risk makes financing harder, especially for small communities. For heat pumps, the cost of drilling boreholes or digging trenches is the main reason they cost more than air-source units.
Location
Hot water resources are common only in some regions. District heating also needs enough buildings close together to justify the pipe network. A geothermal resource far from a town is of little use for heating, because heat is hard to transport over long distances without losses.
Seasonal demand
Heating is needed mainly in winter, so equipment sits partly idle in summer. This lowers the capacity factor and raises the cost per unit of heat delivered. Using heat for year-round purposes, such as fish farming, greenhouses, or industry, improves the economics.
Water chemistry and gases
Geothermal water often contains dissolved minerals and gases. These can corrode or clog pipes and must be handled carefully. Some, like hydrogen sulfide, can affect air quality, as in the Icelandic case. Reinjection reduces surface discharge of these substances.
Ground disturbance: the Staufen case
Drilling can cause serious problems if local geology is poorly understood. The best-known example is Staufen im Breisgau, a historic town in southwestern Germany. In September 2007, seven boreholes were drilled for a ground-source heat pump system to heat the 16th-century town hall. According to a 2010 study by Ingo Sass and Ulrich Burbaum in Acta Carsologica, the drilling pierced layers of anhydrite, a mineral that swells when it absorbs water and turns into gypsum. Groundwater under pressure reached the anhydrite, and the ground began to rise.
Some weeks after drilling ended, the town center started to lift. The uplift reached about 10 millimeters a month at its peak and by March 2010 totaled about 26 centimeters. Around 250 buildings had been damaged by then, and a later report put the number of affected buildings at 269. Remedial work, including grouting, slowed the maximum uplift rate to less than 4 millimeters a month by May 2012, but did not stop it entirely.
Staufen is a reminder that shallow geothermal systems, though usually safe, require proper geological surveys before drilling. Other risks noted in the scientific literature include connecting separate aquifers, which can affect groundwater quantity or quality; leaks of antifreeze or drilling fluids; and, in rare cases, worsening of landslides or cave development. Site-specific geological and hydrogeological information can reduce these risks.
Resource depletion
If heat or water is taken out faster than it is replenished, a geothermal reservoir can cool or its pressure can fall. Reinjection and careful management of the rate of extraction help prevent this. The Paris basin has been in use since 1969, and French geological researchers have modeled how the cold injected water spreads around each doublet so that new wells can be placed without interfering with old ones.
Common misconceptions
Geothermal heating only works near volcanoes. Hot water for direct use is most common in volcanic areas, but it also exists in deep sedimentary basins far from volcanoes, as under Paris. Ground-source heat pumps work almost anywhere.
A ground-source heat pump runs on free energy. It needs electricity to run its compressor and pumps. It is efficient because it moves several units of heat for each unit of electricity, not because it uses none.
Geothermal heating and geothermal power are the same thing. Power plants use very hot resources to generate electricity. Heating uses lower-temperature resources directly or through heat pumps. The two can be combined, with heating using the warm water left after power generation.
The heat in a shallow ground loop comes from the Earth's core. Most of the heat in the top few meters comes from the sun and the atmosphere. Deep heat from the interior matters for deeper wells.
Geothermal heat is unlimited at every site. The Earth's heat is enormous overall, but any single reservoir can be cooled or depleted if it is used too fast without reinjection or recovery time.
A COP of 4 means the system is 400 percent efficient and breaks the laws of physics. The heat pump moves heat that already exists. Counting the ground heat as an input, no energy is created.
How good is the evidence?
The global figures in this article come from the World Geothermal Congress reviews, which compile country reports submitted by national experts. They are the standard reference, but they have limits. Countries estimate their heat pump numbers differently, and figures for small and private systems can be rough. Small differences between reviews may reflect better reporting rather than real change.
Statistics on the costs and savings of heat pumps come mainly from government agencies, such as the U.S. Department of Energy and the Environmental Protection Agency, and from industry. They are reasonable averages, but actual costs depend heavily on the site, the building's insulation, local energy prices, and installer quality.
Case studies such as Boise, Klamath Falls, Elko, and the Paris basin are well documented in technical literature from the Geo-Heat Center at the Oregon Institute of Technology and from French geological agencies. The Staufen case has been studied in detail in peer-reviewed geology journals.
Frequently asked questions
What is geothermal heating? The use of heat from the ground to warm buildings, water, greenhouses, and other things, either directly from hot water underground or through ground-source heat pumps.
What is the difference between direct use and a ground-source heat pump? Direct use takes naturally hot water and uses its heat. A ground-source heat pump uses the mild, steady temperature of shallow ground, and an electric heat pump raises the temperature to heat a building.
Where is the oldest geothermal heating system? Chaudes-Aigues in France is generally considered the oldest working geothermal district heating system. It dates to the 14th century and uses water at about 82 °C.
Which country uses the most geothermal heat? At the end of 2019 China had the largest installed direct-use capacity, about 40,610 MWt, followed by the United States, Sweden, Germany, and Turkey. Iceland uses the most per person, heating at least 90 percent of its homes this way.
How efficient is a geothermal heat pump? The U.S. Department of Energy says geothermal heat pumps reach efficiencies of 300 to 600 percent on the coldest winter nights and can reduce energy use by 25 to 50 percent compared with air-source heat pumps.
How long do they last? According to the Department of Energy, the indoor heat pump typically lasts 20 years or more, and the ground loop 25 to 50 years.
Is geothermal heating safe? Usually, yes, but drilling must be done with good knowledge of local geology. The Staufen case in Germany showed that drilling through certain rock layers can cause the ground to swell and damage buildings.
Does a geothermal heat pump also cool? Yes. In summer it can run in reverse, taking heat from the building and putting it into the ground.
Why is the capacity factor of geothermal heating so low? Because heat is needed mostly in winter. Equipment sized for cold days sits partly idle in summer, so over a whole year it delivers only about 30 percent of its maximum possible output, according to the 2020 worldwide review. Adding year-round uses such as greenhouses, fish farming, and industrial heat raises the figure.
What happens to the water after its heat is used? In modern systems it is usually injected back into the same rock layer through a second well. This keeps up underground pressure, avoids discharging minerals and gases at the surface, and lets the rock reheat the water. Some older systems discharge used water to ponds or rivers after it has cooled, as in Elko, Nevada, where it passes through a cooling pond to wetlands.
Can a city use geothermal heat without volcanoes? Yes. The Paris region heats the equivalent of about 250,000 homes using water from a deep limestone layer, with no volcanic activity.
Key takeaways
- Geothermal heating includes both direct use of hot water and ground-source heat pumps that use shallow ground temperatures.
- People have used geothermal heat for thousands of years. Chaudes-Aigues has heated houses since the 14th century, and Boise opened America's first geothermal district heating in 1892.
- Global direct use reached about 107,727 MWt at the end of 2019, with ground-source heat pumps making up about 72 percent of capacity.
- Iceland heats at least 90 percent of its homes with geothermal energy, and the Paris basin shows that deep sedimentary aquifers can heat a major city.
- Heat pumps move several units of heat for each unit of electricity, but they cost more to install than air-source systems.
- The main limitations are high upfront cost, drilling risk, location, seasonal demand, and water chemistry. Staufen shows the importance of careful geological surveys.
Sources
- Geothermal heating (Wikipedia)
- J. W. Lund and A. N. Toth, "Direct utilization of geothermal energy 2020 worldwide review," Geothermics (2021)
- Global Geothermal Direct Use Update (WGC 2020 summary)
- reGlobal, "Over $22 billion have been invested in geothermal energy during 2015-2019" (summary of Lund and Toth)
- John W. Lund, "Geothermal District Heating," Geo-Heat Center, Oregon Institute of Technology (U.S. DOE)
- U.S. Department of Energy, Consumer Guide to Geothermal Heat Pumps (2021)
- U.S. Department of Energy, Energy Saver: Geothermal Heat Pumps
- Geothermal power in Iceland (Wikipedia)
- ThinkGeoEnergy, "Geothermal in the Greater Paris area making better and better use of enormous potential"
- Boise Warm Springs Water District
- KTVB, "How Boise's city setup allowed for geothermal energy"
- I. Sass and U. Burbaum, "Damage to the historic town of Staufen (Germany) caused by geothermal drillings through anhydrite-bearing formations," Acta Carsologica 39(2), 2010
- IGA/UNU geothermal training report referencing Chaudes-Aigues (Orkustofnun)
- European Geothermal Energy Council, Geothermal Market Report 2016 Key Findings
- R. Gordon Bloomquist, "Geothermal Heat Pumps, Four Plus Decades of Experience," Geo-Heat Center Quarterly Bulletin 20(4), 1999
- M. Zogg, "History of Heat Pumps: Swiss Contributions and International Milestones" (2008)