*2.2. Geothermal System*

The geothermal system is built of a single double U-tube borehole heat exchanger (BHE), utilized as heat sink of the cold water circuit (summer) and heat source to supply the GCHP system (winter) as shown in Figure 2. With respect to the geothermal system itself, Figure 3 shows the structure of the soil at the drilling location and the design of the BHE. The thermistor string of an additional reference BHE (Ref) of the same type is used to analyze the impact of utilizing geothermal energy on the surrounding soil.

The soil primarily consists of micaceous clay. Nevertheless, for the first 18 m below ground surface fine, medium and coarse sands are present. A layer of till and silt is located underneath. There are no relevant ground water flows at the drilling location in general. Thermal conductivity of the grouting material is λ = 2 W ·m<sup>−</sup><sup>1</sup> ·K−1.

**Figure 3.** Structure of the soil and design of the borehole heat exchangers.
