Geothermal energy · Glossary

Geothermal energy is the technical use of heat from the subsurface. Below the uppermost 10 to 20 metres, this heat comes mainly from the heat flow from the Earth's interior. A large part of it is fed by the decay of natural radioactive elements in the rock. In the uppermost metres, by contrast, solar radiation, air temperature and seeping water shape the temperature of the ground. In the energy system, geothermal energy mainly supplies space heating, hot water and process heat. It can also be used for cooling. Electricity can be generated economically only where the extracted thermal water or the developed rock is hot enough.

In everyday language, the German words Geothermie and Erdwärme are often used synonymously. Technically, Erdwärme (earth heat) is the natural heat source. Geothermie (geothermal energy) is its technical development with boreholes, probes, wells, heat exchangers, pumps, power plants or heat networks. The German Geothermal Acceleration Act (Geothermie-Beschleunigungsgesetz) describes earth heat as energy stored in the form of heat beneath the solid surface of the Earth.

Shallow, medium-deep and deep geothermal energy

In Central Europe, the temperature in the subsurface rises on average by about 3 °C per 100 metres of depth. This increase is called the geothermal gradient. It deviates considerably from region to region. In the Upper Rhine Graben, for example, it is higher. According to the common rule of thumb, the temperature at a depth of 5,000 metres is about 160 °C. In Germany, the 400-metre limit is the relevant threshold: up to 400 metres one speaks of shallow geothermal energy, below that of deep geothermal energy. The GeoBG uses the same threshold. In practice, finer distinctions are often made. Boreholes between 400 and about 1,000 to 2,500 metres are regarded as medium-deep geothermal energy with roughly 20 to 60 °C. Deep geothermal energy in the narrower sense usually refers to depths from about 1,000 metres and temperatures above 60 °C.

Shallow geothermal energy uses ground heat collectors, borehole heat exchangers or groundwater wells. At a depth of around 100 metres, the ground is about 10 to 12 °C warm all year round. Down to 400 metres, the temperature rises to a maximum of about 25 °C. For this heat to be sufficient for heating, a heat pump raises it to the required temperature level. How well a system works depends on soil type, groundwater, available area, drilling depth, spacing of the probes and on the building. If a poorly insulated house has to be heated with a high flow temperature, the heat pump needs more electricity than in a building with surface heating. In summer, the ground can be used for passive cooling. The heat released in the process also helps to regenerate the soil.

Deep geothermal energy requires more infrastructure. The most common form is hydrothermal geothermal energy. It usually works with a doublet consisting of a production well and an injection well. Hot thermal water is extracted from a water-bearing rock layer, releases heat via a heat exchanger and is pumped back, cooled, into the same layer. The reinjection supports the pressure in the reservoir and limits the intervention in the water balance. Hydrothermal use taps thermal water of about 40 to well above 100 °C. In Bavaria, typical deposits at depths of 2,000 to 4,000 metres have temperatures of about 100 to 150 °C. There are also closed systems such as deep borehole heat exchangers. There are also petrothermal methods, in which hot rock of low permeability is artificially developed. If the water is not hot enough, large-scale heat pumps raise the temperature to the level of a heat network.

Key parameters and units

The thermal output of a system is stated in kilowatts or megawatts thermal. It indicates how much heat the system can deliver at a given moment. The quantity of heat delivered over a period of time is measured in kilowatt-hours, megawatt-hours or gigawatt-hours. In electricity generation, electrical output and the electrical energy generated are added. The thermal output of a hydrothermal system results mainly from two variables: the production rate and the temperature difference between the extracted and the reinjected water. This is why a low return temperature in the connected network is so valuable. A well with a high temperature but little water can deliver less than a site with a moderate temperature and a permanently high flow rate. For economic viability, the annual operating hours and the electricity demand of the pumps also count.

Significance for heat supply and the electricity system

In Germany, the greatest potential of geothermal energy lies in the heat sector. The Bundesverband Geothermie, an industry association, estimates the potential of deep geothermal energy at at least 100 to 300 TWh per year. That would be more than a quarter of annual heat demand. Such potential figures are not a forecast of actual expansion. If geothermal heat replaces gas or oil, their consumption falls. With shallow geothermal energy, electricity consumption does rise because of the heat pump. However, the heat delivered clearly exceeds the electrical energy used. The ratio over one year is called the seasonal performance factor. It is more favourable the lower the required flow temperature and the more stable the temperature of the source. In winter, the ground is warmer than the outside air. This is the efficiency advantage over air-source heat pumps.

Geothermal energy also plays a role for flexibility. Ground-source heat pumps can be operated with a time shift using a heat storage unit. Deep geothermal plants in a heat network operate most economically when they run as many hours per year as possible. They then cover the steady base heat. Storage units and boilers for peak load cover cold days and daily fluctuations.

Geothermal electricity is often described as baseload-capable. The word baseload is misleading if it is understood to mean that a plant must run constantly at full output regardless of demand. Geothermal energy does not depend on the weather and can produce very steadily. How it is sensibly operated, however, depends on electricity prices, heat sales, maintenance, pumping power and technical limits. At moderate temperatures, as are usual in Germany, the electrical efficiency is low. A plant with combined heat and power then distributes the heat between electricity generation and the heat network. A plant is not beneficial to the system simply because it runs steadily. It becomes so when its operation fits demand, grid bottlenecks, storage and market rules.

Typical misconceptions

Geothermal energy is said to be equally available everywhere: Heat exists everywhere in the subsurface, but not everywhere in a technically and economically usable form. For deep geothermal energy, temperature, permeability of the rock, water-bearing capacity and the chemistry of the thermal water must fit together. Exploration and knowledge of the site are therefore the prerequisite of every project.
Geothermal energy is said to be risk-free because it is renewable: Wrongly planned shallow systems can damage groundwater, connect water-bearing layers or affect neighbouring systems. Deep geothermal energy can trigger perceptible seismic events, especially when pressure conditions in the subsurface change. This applies particularly to petrothermal methods with stimulation. Permitting, monitoring, drilling technology and operational management are therefore integral parts of the plant.
The heat is said to be free: The source costs nothing, but developing it costs a great deal. With deep geothermal energy, the high costs arise at the beginning, for exploration, drilling, tests, plant construction and network connection. In addition there is the discovery risk (Fündigkeitsrisiko): a large part of the investment is spent before it is established what output can be achieved permanently. If the project is successful, running costs are low. However, pumping electricity is required.
Geothermal energy is said to be automatically emission-free: In operation, CO₂ emissions are usually very low. However, the climate balance also includes drilling, building materials, pumping electricity, the refrigerant of the heat pump and gases that may be dissolved in the thermal water. With shallow geothermal energy, the balance additionally depends on the electricity mix and on the efficiency of the heat pump.

Legal framework in Germany (as of November 2026)

Under Section 3(3) no. 2b of the Federal Mining Act (Bundesberggesetz, BBergG), earth heat from a depth of 400 metres is a mining-law resource owned by the state (bergfreier Bodenschatz). It therefore does not belong to the owner of the land. Anyone who wants to explore for it generally needs a permit under Section 7 BBergG. Anyone who wants to extract it needs a licence under Section 8 BBergG. In addition there are operating plans and often seismic monitoring. Earth heat from lesser depths falls mainly under water law, as a rule examined by the lower water authority. Building-law requirements and requirements of the Länder also apply. Irrespective of the purpose, every borehole that goes deeper than 100 metres must be notified to the mining authority under Section 127 BBergG. The mining authority can require an operating plan in individual cases. The details of the procedures differ depending on the federal state.

The Geothermal Acceleration Act (Geothermie-Beschleunigungsgesetz, GeoBG) has applied since 23 December 2025. It covers the permitting of deep and shallow geothermal installations, heat pumps, heat storage units and heat pipelines. Under Section 4, the construction and operation of these installations are in the overriding public interest until the targeted greenhouse gas neutrality in 2045. In weighing of interests, they are to be given priority. Excepted is the weighing against concerns of national and alliance defence. The Act also facilitates early commencement of construction. Objection and action for annulment against permits have no suspensory effect. Actions concerning deep geothermal installations are decided at first instance by the higher administrative court (Oberverwaltungsgericht). Owners must tolerate seismic exploration with vibro-trucks and are compensated for it. The nature-conservation special rule for such measurements in Section 6 has applied since 22 June 2026. The Act implements requirements of the EU Renewable Energy Directive (EU) 2023/2413. It speeds up procedures but replaces neither the examination of groundwater protection nor the mining-law permit.

Municipal and economic context

For municipal heat planning, geothermal energy is interesting because it taps local heat sources. A heat network with geothermal feed-in can supply many buildings without every house needing its own borehole or heat pump. For this, heat demand in the network area must be dense enough. In loosely built-up areas, decentralised ground-source heat pumps are often more economical. In dense neighbourhoods and with large consumers such as hospitals, swimming pools or industry, deep geothermal energy can form the backbone of a network.

A comparison with other renewable heat sources also helps put things in perspective. Solar thermal energy varies strongly with the seasons. Biomass can be stored, but is limited by land and competing uses. Waste heat depends on industry, data centres or wastewater. Geothermal energy can be a steady local source. However, it is tied to the location, and heat cannot be transported over long distances with low losses like electricity. For one city, a geothermal project can be very valuable, while for the neighbouring region without a suitable network it hardly plays a role. Geothermal energy is therefore not a universal substitute technology but a family of methods. Its value results from the interplay of subsurface, demand, network, financing and operation.

Sources

Gesetz zur Beschleunigung des Ausbaus von Geothermieanlagen, Wärmepumpen, Wärmeleitungen und Wärmespeichern (Geothermie-Beschleunigungsgesetz – GeoBG) — Bundesministerium der Justiz / Bundesamt für Justiz (gesetze-im-internet.de); 2025-12-22. Fußnote zum Inkrafttreten; §§ 2–7, 9, 10
Geothermie — Regierungspräsidium Darmstadt (Bergbehörde Hessen); unbekannt. Abschnitt Was ist Geothermie und rechtliche Hinweise
UmweltWissen: Erdwärme – die Energiequelle aus der Tiefe — Bayerisches Landesamt für Umwelt; unbekannt. Abschnitte geothermischer Gradient und tiefe Geothermie
Tiefe Geothermie — Geologischer Dienst Nordrhein-Westfalen; unbekannt. Einleitung und Abschnitt Hydrothermale Nutzung
Wärmeversorgung mit Mitteltiefer und Tiefer Geothermie – Schritt für Schritt — Bundesverband Geothermie e. V.; 2026 (genaues Datum unbekannt). Einleitungsteil, Grafik zu Tiefenstufen

Related glossary entries

Edited by eCMS on 12.11.2026 05:05