PUSH-IT: demonstration and development of seasonal underground heat storage systems

The EU aims to become carbon neutral by 2050. Large-scale seasonal underground heat storage has an important part to play in meeting this objective, because heating and cooling account for approximately half of Europe’s energy demand. The demonstration, testing and development of sustainable sources for storage systems, such as geothermal energy and solar energy, can therefore have a considerable impact. This is the main reason for the establishment of the PUSH-IT programme, which is supported by Horizon Europe, the European Commission’s scientific research funding programme.

The project’s outcomes are expected to promote the use of sustainable energy. They will also contribute to creating a balanced system for sharing the benefits and burdens of the heating transition. To this end, the PUSH-IT consortium consists of 19 very diverse partners, including heat suppliers, drilling companies, geological services and academic institutions. TU Delft is the coordinator of the programme. KWR is contributing to the optimal design, water quality control, and drilling method development.

Scope of the research

Sustainable sources like geothermal energy and solar energy supply a great deal of heat in the summer, while it is in the winter that we need it. The project will closely examine various technologies for the seasonal underground storage of heat. The goal is to lower the environmental impact of these technologies and to improve their performance through the simulation and monitoring of heat losses in the underground. The optimisation of the design and storage management will also limit these losses.  The aim is to use machine learning control techniques to better integrate the heat storage in the systems. Moreover, account will also be taken of factors that are non-technical, though crucial to a project’s chance of success, namely: citizen motivation and perceptions regarding heat storage. This also encompasses the research of policy measures and business models that engage citizens in decision-making about urban heating systems, including underground storage.

The project partners in PUSH-IT will develop, deploy and test the technologies for various configurations of heat sources, heat storage technologies, geological conditions and distribution systems. The involvement of stakeholders from various market and legal contexts makes the project an excellent opportunity for testing, integration, advancement and demonstration in the field of seasonal heat storage. The results will be monitored and evaluated for subsequent operational implementation all over Europe.

Three different technologies

PUSH-IT is a demonstration project focussed on large-scale seasonal heat storage up to 90 °C in geothermal reservoirs. Three different technologies are involved: Aquifer, Borehole and Mine Thermal Energy Storage (ATES, BTES, MTES).


Aquifer Thermal Energy Storage (ATES) is the storage and recovery of thermal energy in aquifers, which are permeable underground layers containing groundwater. By means of two or more groundwater wells, groundwater is simultaneously extracted and injected to carry heat to or from the aquifer. By using the groundwater as a heat carrier, the heat can be extracted from it in the winter, while in the summer it can be injected. Worldwide there are over 3000 ATES systems operating, and most of these operate at a low temperature level, combined with a heat pump. Heat storage at high temperatures (HT) is also very promising, but the technology still needs further development.



Borehole Thermal Energy Storage (BTES) is an underground heat exchanger that works on the principle of heat conduction to exchange the heat with the underground. The system consists of vertical tubes placed in a borehole. In the summer warm water is circulated in the tubes, which heats up the surrounding soil material. The colder water that is circulated through the system in the winter then extracts this heat from the underground. BTES systems have a lower capacity than ATES systems and, like them, are also frequently implemented at low temperatures in combination with a heat pump. Worldwide there are more than 1,000,000 BTES systems in operation. The use of such systems at higher temperatures is still under development.



Mine Thermal Energy Storage (MTES) makes use of groundwater present in abandoned mines. As in the case of the ATES system, the groundwater functions as a carrier to transport heat to and from the mine. This technology is still under development, and worldwide fewer than five systems are operational.


Delft as the Dutch demo site

Each of the three technologies will be demonstrated, tested and developed: ATES in the Netherlands (Delft), BTES and MTES in Germany (Darmstadt and Bochum, respectively). A follower site has also been designated for each technology (see illustration), where it will be tested and developed, but without a full demonstration. Together, these sites are representative of the geological conditions that are generally found in Europe.

Schematic illustration of demo sites – Delft, Darmstadt, Bochum – and follower sites – Berlin, Litomerice (Czech Republic) and United Downs (Cornwall) – for ATES, BTES and MTES, respectively. System integration, technology development and societal aspects are all important components for the achievement of the PUSH-IT project results.

The ATES demo site in the Netherlands involves a geothermal well at the TU Delft campus, which has yet to be realised. Once it is operational, the geothermal well will also produce heat in the summer, when the demand is low. This heat is to be stored in the aquifer located under the campus, at a depth of about 200 meters. In the winter, this heat will be extracted as a means of meeting the peak demand in a sustainable manner, so as to reduce the CO2 emissions from heating the campus.

In the future this system will also be able to supply the city of Delft with clean heat through combinations of geothermal energy and ATES. This approach has never before been implemented in the built environment. TU Delft is thus a pioneer, in part because the application is embedded in scientific research and education taking place at this location. Important innovations developed at the Delft site within PUSH-IT concern the drilling, completion and design of the wells, but also the monitoring and testing of the geothermal well and the water quality control and system integration/control.

Role of KWR

KWR is contributing to PUSH-IT in three important ways. The first is focussed on the HT-ATES well design. In this work KWR will evaluate the performance of the heat distribution of the Delft test site and the PUSH-PULL tests in Delft and Berlin. This information will be used to assess and improve the current views on optimal design methods, with regard to heat losses to the surrounding environment and the well extraction temperature.

Secondly, KWR will be investigating the water quality at the sites of the ATES and MTES systems. Groundwater is naturally used in both systems as a heat carrier. Large fluctuations in temperature change the physical properties and the chemical and microbiological conditions, which can result in precipitation or biofilm growth, potentially leading to the clogging of the well, pump and/or heat exchangers.

Lastly, KWR is involved in the further development of the Expanded Diameter Gravel Well (EDGW), which was developed a few years ago together with Haitjema and Vitens, and for which a pilot was also carried out.