The plastic tubes of borehole heat exchangers transport geothermal heat with the help of water and a heat pump. Antifreeze agents ensure smooth operation and the supply of heat in winter. The tubes are enclosed by mostly cement-based backfill material to prevent harmful substances from seeping in case the tubes are porous.

However, not all building materials are suitable for this kind of use. Both during the heating period in winter and while cooling in summer, the backfill material is exposed to moisture and large temperature fluctuations including frost when the borehole heat exchanger is in operation. The high thermal loads can lead to cracks in the building material or to the heat exchanger tubes detaching themselves. In this case, it might no longer be possible to prevent the antifreeze agents from leaking out and entering the groundwater.

In the OPTIMOG project, researchers from the Ostwestfalen-Lippe University of Applied Sciences and the Technical University Darmstadt seek to identify the most suitable materials in collaboration with their cooperation partners from industry. With help of geotechnical and thermophysical lab experiments, the hydraulic, thermal and mechanic properties of different backfill and heat exchanger materials are to be identified. Following that, numerical models will be developed, which will prepare the ground for the envisaged transfer of the test results into practice.

The experiments are being put to the test, enabling the building materials to be observed with respect to drilling, boreholes and the ground. This holistic approach is essential bearing in mind the attempt to find a long-term optimal solution: once backfill material is introduced, it can no longer be exchanged.