Experimental and numerical investigation of a long-duration Thermal Response Test: Borehole Heat Exchanger behaviour and thermal plume in the heterogeneous rock mass

This paper presents in-situ measurements of a long-duration Thermal Response Test (TRT) (heating phase of 7 months), conducted in a heterogeneous bedrock of conduction dominated heat transfer. The in-situ test was simulated by 3D numerical modelling, by assuming homogeneous and isotropic ground conditions considering the TRT data of the first few days. Based on the analysis of the experimental and numerical results, the behaviour of the Borehole Heat Exchanger for longer heating and recovery periods can be predicted based on the typical-duration TRT results. However, this behaviour is sensitive to the heat input variations, indicating the need for an accurate estimation of the energy needs of the building and the variable thermal loading during the operation of the system. Critical factors for the prediction of the temperature field evolution in the surrounding ground were detected based on the analysis of high-resolution temperature profiles. They include the distance to the heating source, borehole bottom end effects, bedrock heterogeneity and air temperature variations. Anisotropic effects are not detected, despite the expected anisotropic behaviour of the bedrock.