Rock as Heat Storage

Between 2022 and 2023, three coaxial borehole heat exchangers, each 750 metres deep, were installed underground in the south-east of the Lichtwiese campus. Since then, the demonstrator has served as a large-scale test site for thermal modelling, long-term monitoring, and system optimisation. It is also part of the EU Horizon project ‘PUSH-IT’, which is researching high-temperature geothermal heat storage at various locations across Europe.

Existing systems operate at drilling depths of around one hundred metres. The system at the TU was developed to test an innovative form of seasonal heat storage that can still be operated economically at greater depths – although the drilling costs increase considerably. The drilling locations on the campus were specifically chosen to maximise energy efficiency. Numerical simulations showed that the system delivers the desired positive outcomes.

A central element of the research was a distributed geothermal response test. To that end, fibre-optic cables were installed in the wells to record the temperature curves over depth and time at high resolution, thus enabling the function of the system to be examined in detail. Coaxial borehole heat exchangers are closed systems: a heat transfer fluid circulates within a pipe system and releases heat to the surrounding rock or absorbs it – without the exchange of water or substances with the subsurface that occurs in open systems. This simplifies control and monitoring considerably.

The tests confirmed the excellent thermal conductivity of the surrounding crystalline rock in the deeper layers – a decisive factor for the efficient charging and discharging of the heat storage. At the same time, low internal heat losses of the borehole heat exchangers were observed: Warm and cold fluid flows in the same pipe system sometimes influence each other, which can lead to thermal short circuits and thus to a loss of efficiency. The knowledge gained in the experiments on the Lichtwiese is already being incorporated into the further development of insulation and pipe routing in order to minimise such losses in future systems.

The research should aim to determine whether the system can contribute technically and economically to supplying the university: In the long term, the MD-BTES system at TU Darmstadt could be expanded to significantly increase the underground storage volume. This would allow renewable heat – from solar thermal systems or process waste heat, for example from computer centres and laboratories – to be fed into the campus district heating network on a seasonal basis. “Seasonal“ means that, regardless of when the heat is generated, it can be temporarily stored underground and used in the cold season as required to supply heat – a contribution to sustainability and the energy transition.

The SKEWS project is funded by the Federal Ministry for Economic Affairs and Energy of the Federal Republic of Germany (Funding code: 03EE4030A).

Maximilian Schedl/sip