Novel instruments and methods to estimate depth-specific thermal properties in borehole heat exchangers

Abstract Standard thermal response tests (TRT) are typically carried out to evaluate subsurface thermal parameters for the design and performance evaluation of borehole heat exchangers (BHE). Typical interpretation methods apply analytical or numerical solutions, which assume that the ground is homogeneous, isotropic and infinite. However in reality, the underground is commonly stratified and heterogeneous, and therefore thermal properties might significantly vary with depth. Thus, novel instruments and methods are necessary to characterize thermo-physical properties along the BHE. In this study, two novel in-borehole temperature measurement instruments, Geoball and Geowire, are assessed during the performance of a distributed TRT (DTRT). The latter is evaluated in comparison to the widely used fiber optical thermometers. Our results suggest that both novel instruments have several advantages. For instance, both devices are able to instantaneously measure temperature with a higher spatial resolution. In addition, our study evaluates two methods to estimate depth-specific thermal conductivities: (1) a computer program based on infinite line source (ILS) approach and (2) a recently suggested inverse numerical procedure. For the latter less data is required, while demonstrating an accurate resolution to even detect thin conductive geological layers. Moreover, the average value of the depth-specific local effective estimates for both methods is significantly close to the effective subsurface conductivity of 3.20 W/m-K calculated based on standard TRT: 1.27 % below for the computer program and 0.28 % below for the numerical procedure.