Geothermal direct use and ground-source heat pumps: Techniques

Boreholes, trenches, ponds and hot-water district networks

What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.

Ground-source heat pumps provide about 72% of the world's direct geothermal heating capacity; district heating, greenhouses, fish farms, bathing and drying use hotter water directly.

Comparison

Method	Efficiency	Cost	Life-cycle emissions	Maturity	Scale
District heating, greenhouses, fish farming, spas, drying³⁴	Capacity factor about 30% across all direct uses	No authoritative global figure found	No harmonised life-cycle figure found	mature	2019: space heating 12,768 MWt (91% district heating), greenhouses 2,459 MWt, aquaculture 950 MWt, industrial heat 852 MWt; Geretsried (Germany) closed loop will heat up to 20,000 homes
Ground-source heat pumps¹²³⁷	Average seasonal COP 4.3 in a German field test	Several times the installed cost of an air-source unit; extra cost repaid by savings in 5-10 years	Heat pumps of all types in the same test emitted 64% less CO2 than gas heating in 2024; depends on the electricity mix	mature	77,547 MWt and about 6.46 million units (12 kW equivalents) worldwide in 2019, led by China and the United States

Using hot water directly is the simplest form of geothermal heat, but suitable resources are local. Ground-source heat pumps work almost anywhere and beat air-source heat pumps in the field (seasonal COP 4.3 against 3.4) because the ground is warmer than winter air, but drilling or trenching makes them dearer to install. Emissions depend on the electricity supply.

How each method works

District heating, greenhouses, fish farming, spas, drying

Hot water from wells passes through heat exchangers that warm a district heating network, greenhouse pipes, fish ponds, pools or dryers. The cooled water is reinjected. No conversion to electricity is needed.³⁴

Ground-source heat pumps

A loop of plastic pipe is buried in trenches at least 4 ft (1.2 m) deep, in boreholes 100-400 ft (about 30-120 m) deep, or in a pond. A water-antifreeze mix picks up ground heat at 4.5-21 °C and a compressor-driven heat pump raises it to heating temperature; in summer the cycle reverses to cool.¹²³⁷

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
Innovative geothermal project powers up in Bavaria Energy Institute, New Energy World, 2025
Fraunhofer ISE research project completed: heat pumps provide climate-friendly heating in existing buildings Fraunhofer ISE, 2025