A Case Study of a Long-Duration Thermal Response Test in Borehole Heat Exchangers

Shallow closed-loop geothermal systems are worldwide applied providing economical and environmental benefits. This paper presents an in-situ study of four Borehole Heat Exchangers of 100 m long, installed in an heterogeneous bedrock in the campus of the University of Liege (Liege, Belgium). A Thermal Response Test (TRT) of a heating phase of 7 months was conducted in one of the boreholes. During this test, temperature was measured at the pipe inlet and outlet, as well as along the four boreholes by the fiber optics. To further investigate the measured data, the test was simulated by 3D numerical modeling. The comparison of the measured data with the numerical results allowed to detect the critical parameters for the behavior of the BHE and for the temperature evolution in the surrounding rock mass. In this case study, the behavior of the BHE could be predicted based on the results of a typical-duration TRT (of a few days), considering the ground an homogenous and isotropic material. However, the thermal plume in the surrounding ground seems to be influenced by several factors, such as the bedrock heterogeneity, the distance to the heating source, air temperature variations and thermal effects at the borehole bottom end.