Thermophysical properties of a subsoil drill core for geothermal energy applications

A simple, invisible, and seasonally operated solution to avoid urban heat islands is the harvesting of solar urban excess heat from building surfaces, sidewalks, streets and squares and transport into a geothermal borehole heat exchange (BHE) field as seasonal thermal energy storage. However, since temperatures in urban surfaces are sometimes very high and the temperature of the subsoil of a city is already higher than the climatically induced temperatures, the thermophysical properties of the subsoil for calculation and designing of these systems is crucial to avoid thermal heating and cooling plumes around the BHE fields and thus negative thermal effects. In this work a geothermal borehole heat exchanger for testing purposes was installed in Vienna. The 80 m long drill core was used for thermophysical investigations of the subsoil. Samples of this drill core at different depths were prepared and effective thermal conductivity and specific heat capacity were conducted. The Heat Flow Meter (HFM) method was applied for stationary effective thermal conductivity measurements, Differential Scanning Calorimetry (DSC) for evaluation of the specific heat capacity. The samples to be measured were measured twice, once in their original moisture state and then again after a defined drying process. The results of the effective thermal conductivity and specific heat capacity show a strong dependency of the moisture content of the observed subsoil sample. In case of the samples at 80 m depth and a temperature of 25 °C, effective thermal conductivity of the moist sample was approx. 2 W m-1K-1 in the moist and approx. 0.63 W m-1K-1 in the dry state. Specific heat capacity at the same depth and temperature resulted in 1.61 J g-1K-1 in the moist and 0.91 J g-1K-1 in the dry state.