Geothermal direct use and ground-source heat pumps: Efficiency
How a heat pump delivers four units of heat for one
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Direct use delivers geothermal heat with no conversion to electricity, so the relevant measure is how fully plants are used. Ground-source heat pumps are measured by their coefficient of performance (COP): units of heat delivered per unit of electricity, typically about 4 in the field.
Chart: Efficiency ranges
In serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Carnot limit, heat pump, 10 °C ground to 35 °C floor heating | COP 12.31 | Theoretical limit | Theoretical maximum, 308 K / 25 K (calculated; shallow ground is 4.5-21 °C) |
| Ground-source heat pumps, German field test (average) | COP 4.32 | In service | Average annual seasonal performance factor in existing homes, Fraunhofer ISE four-year test |
| All heat pumps in the same test (77 units) | COP 2.6 to 5.47 | In service | Range of annual seasonal performance factors, air-source and ground-source |
| Air-source heat pumps, same test (comparison) | COP 3.42 | In service | Average annual seasonal performance factor, air-to-water units |
| All direct-use geothermal, capacity factor | 30%3 | In service | Heat delivered against installed thermal capacity, 2019 world average (Lund and Toth) |
| Ground-source heat pumps, capacity factor | 24.5%3 | In service | Same basis, 2019 world average |
Where the energy is lost
| Step | Loss |
|---|---|
| Temperature lift between ground and heating system | field COPs are about a third of the Carnot limit (calculated)12 |
| Seasonal idling of heat plant | direct-use plants run at full output about 2,628 hours a year3 |
A COP of 4.3 means 1 kWh of electricity moves 3.3 kWh of ground heat, delivering 4.3 kWh. Performance is best with low-temperature emitters such as underfloor heating and in well-insulated homes, because the smaller the temperature lift, the less work the compressor must do.
Sources
- Geothermal Heat Pumps (Energy Saver) US Department of Energy, 2024
- Studie: Wärmepumpen heizen auch im Altbau effizient (results of Fraunhofer ISE project WP-QS im Bestand) ZfK Zeitung für kommunale Wirtschaft, 2025
- Direct utilization of geothermal energy 2020 worldwide review (Lund and Toth, Geothermics 90, 101915) Elsevier, 2021
- Fraunhofer ISE research project completed: heat pumps provide climate-friendly heating in existing buildings Fraunhofer ISE, 2025