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Glossary · Energy industry & energy transition

Aquifer thermal energy storage

Aquifer thermal energy storage thermal storage Geology seasonal storage district heating heat

An aquifer thermal energy storage system is an underground thermal energy storage. It uses a natural water-bearing rock layer, a groundwater aquifer, as the storage medium. Groundwater is extracted via wells, heated or cooled, and returned to the subsurface. Later it is extracted again and the heat or cold is used. The abbreviation ATES stands for Aquifer Thermal Energy Storage. It does not refer to a single machine but to a system made up of geology, wells, pumps, heat exchangers, measurement and control technology, and a connected building, district or heating network. An aquifer thermal energy storage system usually serves as seasonal storage: it absorbs heat in summer and releases it in winter. With cold, the process works in the opposite direction.

How it works

In its basic form, an aquifer thermal energy storage system consists of one or more well pairs, also called doublets. One well taps the warm zone in the aquifer, the other the cold zone. In summer, water is extracted from the cold well. It absorbs heat via a heat exchanger, for example from building cooling, solar collectors or waste heat, and is fed into the warm well. In winter the pumping direction is reversed: the warm water is extracted, releases its heat, often with the help of a heat pump, and flows back, cooled, into the cold zone. There it is available as a cold source the following summer. This seasonal reversal of the pumping direction distinguishes aquifer thermal energy storage from the direct use of geothermal energy. To put it graphically: underground, two large, unbounded "water bubbles" lie side by side, one warmer, one colder. Water is pumped back and forth between them in an annual rhythm.

What is stored is thermal energy. It is stated in kilowatt hours, megawatt hours or, for large systems, gigawatt hours. This is to be distinguished from thermal output in kilowatts or megawatts, which indicates how quickly heat can be charged into or discharged from storage. An example shows the difference: for a planned high-temperature aquifer storage in Berlin-Adlershof, a storage capacity of about 30 GWh at a thermal output of about 10 MW is cited. The output depends above all on how productive the aquifer is, what extraction rate is permissible, how large the temperature difference is and how many wells there are.

Low-temperature and high-temperature storage

The most important distinction is based on storage temperature. Low-temperature systems (LT-ATES) operate at no more than about 25 °C, in isolated cases up to about 40 °C. They heat and cool mainly large office, commercial and public buildings, almost always together with a heat pump. The heat source is often the waste heat or cold of the supplied building itself. High-temperature systems (HT-ATES) store heat from about 50 °C; some sources already set the threshold at around 40 °C. They are usually charged with external heat, such as industrial waste heat, solar thermal energy, or heat from geothermal energy or combined heat and power. Typical consumers are a heating network or large building complexes. Higher temperatures broaden the range of possible applications. At the same time, however, the technical and permitting risks increase.

Prevalence and examples from Germany

According to a review paper from 2025, around 3,500 aquifer thermal energy storage systems are installed worldwide, about 85 percent of them in the Netherlands. The authors attribute this to favourable subsurface conditions and suitable legal frameworks there. The great majority are low-temperature systems. In Germany, by contrast, the technology is a niche: according to the same paper, only two systems were fully in operation, although more than half of the country's land area with near-surface porous aquifers is said to be suitable in principle.

  • Berlin, parliamentary buildings (since 1999): A cold storage at a depth of about 60 m with 14 wells supplies water at 10 to 11 °C for free cooling. A heat storage at a depth of about 320 m was charged with around 70 °C from combined heat and power plants and achieved recovery rates of just under 80 percent. According to a 2021 report, heat storage operation was provisionally suspended due to technical problems.

  • Neubrandenburg: From 2005, surplus heat from a combined-cycle power plant was stored at a depth of around 1,200 m. At the cold well, temperatures around 30 °C promoted sulfate-reducing bacteria, which led to corrosion of the pumps. At the beginning of 2019 the storage was shut down because strongly fluctuating heat demand and heat supply did not allow efficient seasonal use. Since then the municipal utility has relied on a short-term storage.

  • Rostock-Brinckmanshöhe (since 2000): Solar heat is stored at about 50 °C at a depth of around 20 m and used in winter via a heat pump. According to the 2021 report, the system ran largely trouble-free for over 20 years.

  • Bonn, "Bonner Bogen" (since 2009): A low-temperature system with six wells down to just under 30 m depth supplies around 60,000 m² of usable floor space. Because significantly more cooling than heating takes place over the course of the year, more heat enters the aquifer than is withdrawn from it. This shows how important balanced operation is.

There are also numerous research and planning projects, for example in Berlin-Adlershof, Hamburg, Kiel, Leipzig and Mannheim. According to the 2025 overview, most of them are still at an early stage of development. Connection to district heating networks is a focus here.

Distinction from other technologies

Aquifer thermal energy storage systems are easily confused with geothermal energy. Geothermal energy harnesses natural heat from the earth. An aquifer thermal energy storage system, by contrast, temporarily stores heat or cold that was previously introduced, so the subsurface serves primarily as storage space. Overlaps nevertheless exist, for example when a decommissioned geothermal plant is converted into a storage, as in Neubrandenburg. Aquifer storage differs from borehole thermal energy storage in that the latter circulates a heat transfer medium in closed pipes, whereas here the groundwater itself is extracted. This enables large storage volumes with little space requirement at the surface. However, it places higher demands on water chemistry, hydraulics and monitoring. Above-ground or buried storages such as the pit thermal energy storage need a lot of land but are hardly dependent on geology. Mine and cavern storages use other underground cavities. An aquifer thermal energy storage system is not an electricity storage.

Benefits for the heat transition and the electricity system

Heat demand is high in winter and low in summer. Solar heat, by contrast, arises mainly in summer, and waste heat from industry, data centres or cooling systems often arises all year round. An aquifer thermal energy storage system can shift summer surpluses into the heating season. In this way it can relieve fossil peak-load boilers and allow heat pumps to run more evenly. If heat is stored in summer, a warmer source is available to the heat pump in winter. This improves its seasonal performance factor and reduces the electricity demand per kilowatt hour of heat. If heat pumps or power-to-heat plants generate heat mainly when a lot of wind and solar power is available, the storage can buffer this heat over time. In doing so it shifts heat, not electricity. How much a storage contributes at system level depends on the specific network: on flow and return temperature, load profile, heat sources and operating strategy. According to a review paper, aquifer storage can reduce greenhouse gas emissions by up to 74 percent compared with gas boilers and compression chillers. This is a maximum value and not the normal case.

Geological, technical and operational limits

Suitable aquifers are those that are sufficiently permeable and thick, lie at a suitable depth, and in which the groundwater flows only slowly. If it flows quickly, the stored heat drifts away with the groundwater flow. If the warm and cold zones lie too close together, a thermal short circuit threatens. Temperature changes can trigger precipitation, corrosion and microbial processes; wells can become clogged or lose performance. Neubrandenburg is an example of this. A storage never operates without losses: part of the heat disperses in the subsurface. Recovery rates of around 80 to 90 percent have been reported in individual cases, for example at the Reichstag and at a Hamburg test well, but they depend on the site. Operation is a further factor. If a storage is charged one-sidedly over the long term, as in Bonn, or if supply and demand fluctuate strongly, as in Neubrandenburg, the benefit decreases. Regulatory requirements can additionally restrict the room for manoeuvre considerably: in a demonstration project in Berlin-Mitte, the temperature in the aquifer may fluctuate by only 3 K, i.e. between about 10 and 16 °C. Exploration, hydrogeological modelling, trial operation and permanent monitoring are therefore indispensable.

Legal framework in Germany

For planning, construction and operation in Germany, two laws are primarily decisive: the Water Resources Act (Wasserhaushaltsgesetz, WHG) and, depending on depth and design, the Federal Mining Act (Bundesberggesetz, BBergG). How the permitting procedures proceed in detail is determined by the competent state authorities. The procedures therefore differ markedly from region to region. In addition, the Geothermal Acceleration Act (Geothermie-Beschleunigungsgesetz, GeoBG) has applied since 23 December 2025. It expressly covers heat storage systems as well. For the term, it refers to the Heat Planning Act (Wärmeplanungsgesetz): there, a heat storage is a device for the time-limited storage of heat, including the technology for charging and discharging. Under § 4 GeoBG, the construction and operation of such systems are in the overriding public interest until greenhouse gas neutrality is achieved in 2045. In weighing protected interests, they are to be given priority weight, except against interests of national and alliance defence. § 5 GeoBG also establishes a public interest in early commencement under the WHG and BBergG. Water, mining and environmental law assessments are not eliminated as a result. They are merely weighted differently and can proceed more quickly. For municipal heat planning, aquifer storage remains a site-dependent option, not a standard component.

Typical misconceptions

  • "The storage is renewable": The storage does not generate any energy itself. Its climate impact depends on what it is charged with and which generation it replaces. If it is charged with solar heat, unavoidable waste heat or efficient heat pump heat, it can contribute greatly to decarbonisation. With fossil-generated heat, it mainly improves operating processes.

  • "Suitable geology is enough": Without a suitable heat source, sufficient and predictable demand, a connected network and a long-term operator, geological suitability alone is of little use. The shutdown in Neubrandenburg and the project in Lüneburg that was not implemented show this.

  • "Large storage capacity means high output": Energy content and charging or discharging output are two separate design variables. The output is limited by the permissible extraction rates and the number of wells.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 14:26

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