What is Geothermal Energy?
Key facts
- The ground can act as a heat source for heating and a heat sink for cooling.
- Ground-source heat pumps normally use shallow ground loops, boreholes or groundwater.
- Geothermal heat-pump systems can also extend into medium-depth and deeper boreholes.
- Heat pumps require electricity to move heat and raise it to the required delivery temperature.
- Heating and cooling performance depends on both the building and subsurface system.
- Shared borefields can serve multiple buildings through campus or district geo-exchange networks.
- Depth alone does not determine whether a system is a GSHP, medium-depth BHE, deep BHE or CLGS.
- Hydrothermal geothermal systems can also use large heat pumps, but they access geothermal heat differently.
What is a geothermal heat pump?
A ground-source heat pump exchanges heat with the ground or groundwater and uses an electrically driven heat-pump cycle to provide useful heating or cooling.
The term geothermal heat pump is widely used as a synonym. For technical clarity, ThinkGeoEnergy generally uses GSHP for conventional shallow systems, while this guide uses geothermal heat pump more broadly when discussing the growing continuum of ground-coupled systems at greater depths.
Most established GSHP installations use shallow geothermal energy.
Daily and seasonal temperature changes at the surface diminish with depth. Below roughly 10–15 metres (33–49 feet), ground temperatures are typically much more stable than surface air temperatures, although geology, groundwater, latitude, climate and surface conditions influence the exact thermal profile.
This thermal stability makes the subsurface an effective heat source during heating periods and a heat sink during cooling periods.
But the principle does not necessarily stop at conventional shallow borehole depths.
A growing class of medium-depth and deeper borehole heat-exchange systems combines deeper closed wells with heat pumps. These systems can access warmer ground, provide more heat from individual drilling locations and serve much larger thermal loads.
Geothermal Resources
Direct Use of Geothermal Energy
U.S. Department of Energy, Geothermal Heating & Cooling
How do geothermal heat pumps work?
Ground-source systems use the same basic refrigeration cycle as other heat pumps.
The four principal components are the evaporator, compressor, condenser and expansion valve.
A separate ground loop, borehole or groundwater circuit connects the heat pump to the subsurface.
Heating mode
In heating mode, fluid circulating through the ground system absorbs heat from the subsurface. In the heat pump's evaporator, this energy is transferred to the refrigerant. The compressor raises the refrigerant's pressure and temperature. It then passes through the condenser, where heat is transferred into the building's heating system. The refrigerant subsequently passes through the expansion valve, reducing its pressure and temperature before returning to the evaporator.Cooling mode
Many geothermal heat pumps are reversible. In cooling mode, heat is removed from the building and transferred into the ground. Because subsurface temperatures are generally more stable than summer outdoor air temperatures, the ground can provide an effective heat sink. Some systems can also provide passive or free cooling, using relatively cool ground temperatures with little or no compressor operation.European Commission JRC, Heat Pumps in the European Union – 2024 Status Report (pdf)
What types of geothermal heat-pump systems are there?
System configuration depends on available space, geology, groundwater conditions, thermal demand and project scale.
Horizontal closed-loop systems
Horizontal systems place sealed pipes relatively close to the surface. Fluid circulates through the buried loops and exchanges heat with the surrounding ground. These systems reduce the need for deeper drilling but require sufficient land for excavation.Vertical borehole systems
Vertical systems use drilled borehole heat exchangers (BHEs). They require less surface area than horizontal systems and are therefore suitable for urban sites, larger buildings and locations where available land is limited. Building-scale boreholes are commonly around 50–300 metres (164–984 feet) deep, although actual depths vary substantially with geology, thermal conductivity, groundwater, drilling conditions, thermal demand and local regulation. Several boreholes can be combined into a borefield. For larger projects, a thermal response test (TRT) can help determine the effective thermal conductivity of the ground and thermal behaviour of the installed borehole before finalising the borefield design.Open-loop groundwater systems
Open-loop systems use groundwater itself as the thermal source or sink. Water is pumped from an aquifer, exchanges heat with the heat-pump system and is then discharged or reinjected according to local hydrogeological conditions and regulation. This is an important exception to the simple distinction between systems that do and do not produce subsurface fluid. Shallow groundwater is generally close to the surrounding ground temperature and cannot normally meet a building's heating demand without temperature upgrading. The heat pump remains the core technology providing useful heating or cooling. ThinkGeoEnergy therefore treats these systems as open-loop groundwater heat pumps, rather than as hydrothermal direct-use systems.Shared borefields
A borefield does not need to serve only one building. Several buildings can share underground heat-exchange infrastructure, reducing the need for each property to develop a separate ground system. Shared borefields can also form the basis of larger campus and district geo-exchange networks.Other configurations
Some projects integrate ground heat exchange directly into foundations through energy piles or other thermo-active structural elements. Lake or pond loops can also be used where suitable water bodies and environmental conditions allow.How efficient are geothermal heat pumps?
Heat-pump performance is commonly expressed using the Coefficient of Performance (COP).
For heating, COP compares useful heat delivered with the electrical energy consumed by the heat pump at a defined operating condition.
A COP of 4 means that four units of useful heat are delivered for each unit of electricity consumed within the defined measurement boundary.
Ground-source heat pumps often achieve COP values broadly around 3–5 under favourable conditions, but this is an illustrative range rather than a guaranteed level of performance.
Climate, source temperature, ground conditions, system design, required delivery temperature and the measurement boundary all matter.
COP, SCOP and SPF
COP describes performance at a particular operating point. Seasonal Coefficient of Performance (SCOP) considers operation across standardised seasonal conditions. Seasonal Performance Factor (SPF) is commonly used for measured performance over a longer period under actual operating conditions. The system boundary is important. Depending on the definition used, an SPF may include electricity consumed by circulation pumps, auxiliary heaters and other equipment in addition to the heat pump. Cooling performance can be expressed using a cooling COP, while seasonal cooling efficiency may use metrics such as SEER or a seasonal performance factor.Why source and supply temperatures matter
The temperature difference between the heat source and the required delivery temperature has a major influence on heat-pump performance. The greater this temperature lift, the more electricity the heat pump generally requires. Ground-source systems therefore work particularly well with lower-temperature heating systems such as underfloor heating, appropriately designed radiators and low-temperature thermal networks. This also affects retrofits. Buildings designed around high-temperature heating systems may require emitter upgrades, energy-efficiency improvements or other measures. Deeper borehole systems can provide warmer and stable source temperatures. This can reduce the required temperature lift and improve heat-pump performance. The benefit must be balanced against greater drilling cost and well complexity. Greater depth does not automatically mean a more efficient or more economical overall system.What determines geothermal heat-pump performance?
The building and underground heat-exchange system need to be considered together.
Ground temperature
Ground temperature defines the thermal conditions available to the heat pump. Below the shallow zone strongly affected by seasonal weather, temperatures become progressively more stable. At greater depths, the geothermal gradient increasingly influences the available temperature.Thermal conductivity
Soil and rock differ in their ability to transfer heat. More conductive formations can support effective heat exchange, while less conductive ground may require additional loop length or a larger borefield.Groundwater
Groundwater can significantly affect underground heat transport. In open-loop systems, groundwater provides the source or sink directly. In closed-loop systems, groundwater movement can still influence heat transfer around boreholes.Borehole spacing and thermal interference
Closely spaced boreholes can influence one another thermally. Borefield design therefore needs to consider the behaviour of the complete underground system rather than simply multiplying the expected output of one borehole.Building load and annual heat balance
The relationship between annual heating and cooling demand influences long-term ground conditions. A system that continually extracts more heat than it returns can gradually cool the surrounding ground. Cooling-dominated systems can create the opposite effect. Over many years, this imbalance can lead to long-term thermal drift and reduced performance if it is not considered in system design.Heating, cooling and seasonal thermal storage
Ground-source systems can move heat in both directions, allowing the subsurface to interact with seasonal energy demand.
Heat rejected during summer can warm the ground, and part of that energy may subsequently be recovered during heating periods.
Large borefields can therefore provide a degree of seasonal thermal storage.
Borehole Thermal Energy Storage (BTES) develops this principle more deliberately, using arrays of boreholes to store thermal energy over periods of months.
Another related concept is Aquifer Thermal Energy Storage (ATES), where groundwater-bearing formations are used to store and recover heat and cold. ATES has its own hydrogeological requirements and should not be confused with a standard open-loop groundwater heat pump.
Seasonal storage systems can also integrate solar thermal energy, recovered industrial heat or other energy sources.
Detailed treatment of these combinations belongs in the Hybrid Geothermal Systems guide.
Campus and district geo-exchange systems
Geothermal heat pumps can serve groups of buildings as well as individual properties.
A large borefield or several distributed borefields can connect buildings through an ambient-temperature thermal network.
Each building uses its own heat pump to raise or lower the network temperature to what it requires for heating, cooling or domestic hot water.
This differs from conventional district heating, where useful higher-temperature heat is normally generated centrally and distributed directly to customers.
Ambient networks can also allow buildings with different thermal profiles to exchange heat indirectly.
A building rejecting heat while cooling can contribute energy to the network while another building requires heating.
This can help balance heating and cooling demand across a campus, neighbourhood or development.
Some networks of this type are described as fifth-generation district heating and cooling, although not every district geo-exchange system fits that definition.
Medium-depth and deep geothermal heat-pump systems
A growing technology segment extends the geothermal heat-pump principle considerably deeper than conventional building-scale boreholes. Terms used in technical literature include medium-depth borehole heat exchanger (MDBHE), deep borehole heat exchanger (DBHE), medium-deep geothermal heat-pump system and deep coaxial borehole heat exchanger. The terminology is not standardised. Some technical literature uses DBHE for boreholes deeper than about 500 metres (1,640 feet), but this is a literature convention rather than a universal technology boundary.Medium-depth and deep borehole heat exchangers
At greater depths, coaxial borehole configurations become increasingly important. In a typical coaxial arrangement, fluid moves through one part of a sealed borehole and returns through another after exchanging heat with the surrounding rock. An insulated inner pipe can reduce unwanted heat exchange between the downward and upward flows. Deeper wells can access warmer rock and potentially deliver more heat from each drilling location. This can be attractive where thermal demand is large or available surface space is limited. The trade-off is higher drilling cost and greater technical complexity. Pressure losses, hydraulic resistance, heat transfer, well construction and long-term thermal behaviour all become increasingly important.Heating, cooling and seasonal subsurface thermal storage
Some medium-depth and deep BHE systems are designed for bidirectional operation. Heat is extracted during heating periods and can be returned to the subsurface during cooling periods or when another source of surplus heat is available. This allows some systems to combine heating, cooling and seasonal subsurface thermal storage. Storage performance remains site-specific and depends on geology, borehole configuration, operating temperatures and the balance between heat extraction and injection.Examples of deeper geothermal heat-pump systems
Finland-based QHeat provides one example of this deeper geothermal heat-pump segment. The company describes purpose-designed coaxial wells typically 1–2 kilometres (3,280–6,560 feet) deep, combined with heat pumps for heating, cooling and seasonal subsurface thermal storage. QHeat's wells extend to depths also encountered by some other closed-loop geothermal concepts. They fit within the geothermal heat-pump continuum discussed here because the heat pump remains integral to reaching useful delivery temperatures. Depth alone does not change the technology category. Norway-based GTML describes closed-loop systems around 150–650 metres (492–2,133 feet) deep for heating, hot water and cooling with heat pumps. The company also describes the possibility of returning heat to the wells for seasonal storage and offers deeper concepts extending towards 1,500 metres (4,920 feet). GTML illustrates how the transition from conventional vertical BHEs to medium-depth systems is gradual rather than defined by one fixed depth. Some developers use terms such as energy well for systems of this type. This is useful descriptive or commercial terminology, but ThinkGeoEnergy does not treat energy well as a formal technology category. These examples illustrate different parts of the emerging medium/deep heat-pump segment. Their individual depth ranges and configurations are company-specific rather than universal industry definitions.Where deeper BHE systems meet CLGS
Medium-depth and deep BHE systems share some characteristics with Closed-Loop Geothermal Systems (CLGS). Both can circulate a working fluid through a sealed underground system without producing naturally occurring formation fluid. The distinction therefore cannot be based simply on whether the system is “closed loop,” nor can it be defined by depth alone. For ThinkGeoEnergy, medium and deep BHE systems remain within the geothermal heat-pump family where the subsurface installation functions primarily as a ground-coupled heat source or sink and the heat pump remains essential to reaching useful delivery temperatures. CLGS is treated as a separate technology family where the sealed subsurface circuit is developed primarily as a geothermal heat-extraction system in its own right, potentially using more complex well architectures and delivering heat or power without depending on a building-coupled heat-pump cycle. The boundary is evolving, and individual technologies may not fit neatly into one category. ThinkGeoEnergy does not use Advanced Geothermal Systems (AGS) as a synonym for CLGS. Advanced geothermal is a broader and inconsistently used term.Hydrothermal geothermal combined with heat pumps
Large heat pumps are also increasingly used with hydrothermal geothermal resources.
This is a different system architecture.
A hydrothermal production well brings naturally heated formation water to the surface. Heat is transferred into a district-heating or other thermal system, usually through a heat exchanger, and the cooled geothermal fluid is reinjected.
A large heat pump can extract additional useful heat from the geothermal fluid or raise the temperature supplied to the network.
The heat pump may therefore be a major, or even essential, part of the heating plant without turning the project into a ground-source heat-pump system.
Danish geothermal developer Innargi provides a useful example.
Its geothermal heating concept uses production and injection wells connected to a surface heating plant. A heat exchanger keeps geothermal fluid separate from the district-heating circuit, while a heat pump helps achieve the required network temperature and extracts additional heat before the geothermal water is reinjected.
ThinkGeoEnergy classifies this primarily as hydrothermal geothermal direct use with heat-pump integration.
Its primary Knowledge Centre home is therefore Direct Use of Geothermal Energy. The wider integration of large heat pumps with geothermal resources will also be covered under Hybrid Geothermal Systems.
Why the distinction matters
Terminology can become confusing because hydrothermal operators sometimes describe production and reinjection as a closed loop. This is not the same as a Closed-Loop Geothermal System. A hydrothermal system produces naturally occurring reservoir fluid and subsequently reinjects it. A CLGS or closed borehole heat exchanger circulates a purpose-installed working fluid within a sealed underground circuit and exchanges heat through the well structure without producing formation fluid. Open-loop shallow groundwater heat pumps are another distinct case. They do abstract groundwater, but at temperatures where a heat pump remains the essential mechanism for useful heating or cooling. The combination of resource type, access method and role of the heat pump is therefore more useful for classification than depth or the words “open” and “closed” alone.Benefits and limitations of geothermal heat pumps
Benefits
- Heating and cooling from one system. Many geothermal heat-pump systems provide both.
- Stable source temperatures. The subsurface experiences smaller temperature fluctuations than outdoor air.
- High energy performance. Well-designed systems can deliver several units of useful heat for each unit of electricity consumed.
- Reduced local combustion. Buildings and thermal networks can be heated without on-site fossil-fuel combustion.
- Low surface impact. Most ground heat-exchange infrastructure is underground.
- Long-lived underground infrastructure. Properly designed ground loops and boreholes can remain in service longer than the heat-pump equipment connected to them.
- Flexible project scale. Applications range from individual homes to large buildings, campuses, shared borefields and deeper thermal networks.
- Seasonal storage potential. Some configurations can return heat to the subsurface and recover it later.
Limitations
- Upfront cost. Drilling or excavation can require significant initial investment.
- Site requirements. Horizontal systems need available land, while vertical systems require drilling access.
- Increasing cost and complexity with depth. Deeper wells may provide more heat from each location, but individual wells become more demanding and costly.
- Subsurface uncertainty. Geology, groundwater and thermal properties influence design and performance.
- Permitting and groundwater regulation. Requirements can be significant for drilling and open-loop systems.
- Electricity dependence. Heat pumps, circulation pumps and controls require electrical power.
- Urban installation complexity. Existing utilities, access restrictions and drilling logistics can complicate dense urban projects.
- Retrofit challenges. Buildings with high-temperature heating systems may require building or emitter upgrades.
- Specialist capability. Successful systems require coordination between geothermal, drilling, heat-pump and building-services expertise.
Geothermal heat pumps around the world
Geothermal heat-pump systems are deployed in homes, commercial buildings, institutional facilities, campuses and shared thermal networks across many countries.
Deployment is also extending into medium-depth and deeper borehole systems.
Global statistics are difficult to compare directly.
Markets may report the number of installed heat pumps, thermal capacity, useful heat delivered, shallow-geothermal capacity or all heat-pump technologies together.
Medium-depth and deeper BHE systems can also be classified differently between countries and datasets.
Current market data should therefore be provided through dynamic ThinkGeoEnergy datasets rather than fixed into evergreen article text.
Frequently asked questions about geothermal heat pumps
What is a geothermal heat pump?
A geothermal heat pump uses the ground or groundwater as a heat source or sink and a heat pump to provide heating or cooling.Is a geothermal heat pump the same as a ground-source heat pump?
In common usage, yes. ThinkGeoEnergy generally uses GSHP for conventional shallow systems while using geothermal heat pump more broadly in this guide for related heat-pump-centred ground-exchange systems.How does a geothermal heat pump work?
It transfers heat between the ground and a building using a refrigeration cycle. In heating mode, heat is extracted from the ground. In cooling mode, building heat is transferred into the ground.Can geothermal heat pumps cool buildings?
Yes. Many systems are reversible, and some can provide passive cooling when ground temperatures allow.How efficient are geothermal heat pumps?
Ground-source heat pumps often achieve COP values broadly around 3-5 under favourable conditions. Actual performance depends on source and delivery temperatures, ground conditions, system design and how auxiliary electricity is counted.How deep are geothermal heat-pump boreholes?
Conventional building-scale vertical boreholes are often around 50-300 metres (164-984 feet) deep. Medium-depth and deep BHE systems can extend several hundred metres or more than a kilometre. There is no universal depth at which one technology category automatically becomes another.What is the difference between horizontal and vertical ground loops?
Horizontal systems use relatively shallow excavation and require more land. Vertical systems use drilled boreholes and require less surface area.What is an open-loop geothermal heat-pump system?
An open-loop system pumps groundwater from an aquifer, exchanges heat with it and then discharges or reinjects the water according to local hydrogeological and regulatory requirements.What is a geothermal borefield?
A borefield is a group of borehole heat exchangers designed to operate together as one underground thermal system.Can geothermal heat pumps use deeper wells?
Yes. Medium-depth and deep borehole heat-exchange systems can extend several hundred metres or more than a kilometre and can support larger heating, cooling and storage applications.Can geothermal heat pumps serve multiple buildings?
Yes. Shared borefields and campus or district geo-exchange networks can serve multiple buildings.Can deeper geothermal wells store heat?
Some medium-depth and deep BHE systems can return heat to the subsurface during periods of surplus or cooling demand and later recover thermal energy. Storage performance depends on geology, system design and operating strategy.Are deep geothermal heat-pump wells the same as CLGS?
Not necessarily. Medium-depth and deep BHE systems and CLGS can both use sealed subsurface circuits, but their system purpose and architecture can differ. Depth alone does not define the category.Can hydrothermal geothermal systems use heat pumps?
Yes. A hydrothermal geothermal project can use a large heat pump to extract additional heat or raise the temperature supplied to a district-heating network. Because naturally occurring geothermal reservoir fluid is still produced and reinjected, ThinkGeoEnergy classifies this primarily as hydrothermal direct use with heat-pump integration.How long do geothermal ground loops last?
Ground loops and boreholes can have long service lives when correctly designed and installed and commonly outlast the heat-pump equipment connected to them.Are geothermal heat pumps renewable?
They use renewable thermal energy from the ground together with electricity. Overall environmental performance depends on the electricity supply, system efficiency, drilling, materials and other lifecycle factors.How are geothermal heat pumps different from deep geothermal energy?
Conventional GSHPs generally use relatively shallow ground temperatures and an electrically driven heat pump. Between these systems and deeper geothermal technologies is a growing range of medium-depth and deep borehole heat exchangers. Hydrothermal geothermal systems differ because they produce naturally occurring geothermal reservoir fluid, while CLGS uses a purpose-installed sealed subsurface circulation system. Sources and further reading- European Commission Joint Research Centre Heat Pumps in the European Union – 2024 Status Report (pdf)
- U.S. Department of Energy - Geothermal Heating & Cooling
- National Renewable Energy Laboratory - Comprehensive Geothermal Heat Pump Package (pdf)
- IEA Heat Pumping Technologies Annex 52 – Long-term performance monitoring of GSHP systems
- Geothermal Energy: A comprehensive review of deep borehole heat exchangers: subsurface modelling studies and applications
- Renewable and Sustainable Energy Reviews Research status and future development of medium-deep geothermal heat pump systems
- QHeat Geothermal heating, cooling and thermal storage
- GTML Energy Geothermal energy well solutions
- Innargi Geothermal heating