Experiences with an Enhanced Thermal Response Test (ETRT) with high groundwater flow

With an enhanced thermal response test (ETRT) depth-specific effective thermal conductivities can be determined. For this, a correct determination of the specific heat load is crucial. With an ETRT the heat is injected along the depth of the borehole heat exchanger (BHE) by applying a defined voltage to an electrically conductive cable. Hereby, an uniform specific heat load along the entire length of the cable is assumed. However, the electrical resistance of the heating cable and therefore the specific heat load depend on temperature. In this study, an ETRT is conducted at a study site with high groundwater flow. The results are critically evaluated with regard to the specific heat load and depth-specific effective thermal conductivities. We show an inaccuracy of 12 % in the determination of effective thermal conductivities when assuming constant electrical resistance of the heating cable with time. When using temperature measurements from distributed temperature sensing (DTS) to adjust the specific heat load along the length of the heating cable, variations of the specific heat load along the BHE in the order of 3 % are observed. Depth-specific effective thermal conductivities are evaluated ranging between 3.3 W m^-1 K^-1 and 12 W m^-1 K^-1. For 60 % of the evaluated depth-intervals the results of the infinite line source (ILS) forward model do not converge according to the criterion of Δλ_eff/λ_eff < 0.05/20h. These depth intervals are characterised by high groundwater velocities (> 0.6 m d^-1) and a temperature increase for the evaluation with linear regression of ΔT < 0.6 K. From our results, we provide recommendations for the determination and adjustment of the specific heat load during an ETRT to account for temporal and spatial variations. Furthermore, we emphasise the need for higher specific heat loads in groundwater influenced depth intervals and elaborate difficulties associated with the increase of the specific heat load and the corresponding temperature increase along the heating cable.