An inversion method to estimate the thermal properties of heterogeneous soil for a large-scale borehole thermal energy storage system

Abstract

Borehole thermal energy storage (BTES) is a technology in which the thermal energy generated over summer may be collected and stored in soil for extraction in winter. The difficulty of evaluating the thermal conductivity and volumetric heat capacity of heterogeneous soil introduces considerable uncertainty in BTES system design and operation. This study proposes a method to estimate the thermal properties of heterogeneous soil in a BTES system based on the modified duct ground heat storage model (DST) model. The inversion estimation method was verified using long-term monitoring data of soil temperature distribution and fluid temperatures from a large-scale BTES project in Chifeng, China. Results show that the layered thermal properties of heterogeneous soil led to the irregular distribution of soil temperature at different depths. The cumulative heat storage prediction error decreased from 27.8% to 3.7% when utilizing the proposed method relative to a thermal response test (TRT) - based parameter estimation. The influence of inaccurate estimates of soil thermal properties on the scaled expansion of BTES systems was also examined, and with results emphasizing the importance of correctly estimating soil thermal properties for BTES system design.