© Stadtwerke Lübeck energie

Seasonal heat storage systems: summer surpluses for cold winters

By the end of 2026, more than 20 different concepts are to be created for integrating seasonal heat storage systems. Researchers are investigating how cities and municipalities can store heat in the summer and integrate it into their year-round heat supply.

For example in Lübeck, where the municipal utilities are planning to merge up to six district heating networks close to the city centre. One completed solar thermal system, several air-source and river-water heat pumps, and possibly an additional solar or geothermal system are set to feed the required heat into the network in the future. A pit thermal energy storage can store surplus energy generated in the summer for several months, so that it can be released in winter when required. Such storage facilities are essentially large, sealed water basins with an area of at least two football pitches.

Researchers from the Chair of Energy Technology at the University of Duisburg-Essen are investigating, as part of the HANSE microproject, how large exactly the pit thermal energy storage system in Lübeck would need to be, where it could best be located, and how the network infrastructure should be designed. “The topic of seasonal storage systems on this scale for urban heating networks, when the space available on site is extremely limited, is something completely new and innovative and therefore the work is quite challenging,” says Christian Thommessen from the University of Duisburg-Essen, who is responsible for this case study.

URBAN ARENA funding programme supports knowledge exchange and transfer of results

The Federal Ministry for Economic Affairs and Energy is funding the work of the participating research institutions in the form of 23 microprojects as part of the URBAN ARENA Seasonal Heat Storage initiative. URBAN ARENA is a pilot activity within the European partnership Driving Urban Transitions.

The microprojects that have now been launched are feasibility studies examining cities and municipalities with very different conditions, starting with the type and size of the existing buildings and the available energy supply to local landscape and geological conditions. The project participants are investigating how, under such different conditions, a seasonal heat storage system could be integrated into the existing infrastructure in order to develop a practical and economically viable solution.

All participants from research and the heat suppliers are therefore in continuous dialogue. Existing knowledge is being specifically applied to local challenges and translated into results that are relevant to practical applications. In other words, URBAN ARENA links research with real applications and specifically helps bridge the gap between research and municipal implementation.

Feasibility of different seasonal heat storage technologies

In addition to pit thermal energy storage systems, researchers in URBAN ARENA are investigating two other key technologies that can be used to store heat seasonally: aquifer thermal energy storage and borehole thermal energy storage. An aquifer storage system could, for example, be used as an open underground storage system in Bremerhaven, where heat could potentially be stored at a depth of 600 to 700 metres. “This geological formation already has a certain temperature, which means it can also store heat at high temperatures,” explains Dr Nora Koltzer, responsible for the microproject at the Fraunhofer Research Institution for Energy Infrastructures and Geotechnologies IEG. Borehole thermal energy storage systems, on the other hand, could provide a possible solution for the town of Hilchenbach in the Siegerland region (North Rhine-Westphalia), where heat can be stored at shallower depths via multiple boreholes forming a borehole field.

All the results from these studies are expected to be published in spring 2027, giving heat suppliers that were not involved in the projects the opportunity to build on the findings. Ultimately, the aim is to develop various concepts for transforming heat supply systems in cities and municipalities.