Thermal performance assessment of coupled soil stratification and groundwater advection effects on vertical borehole heat exchanger

Abstract

The growing interest in geothermal energy as a low-carbon alternative underscores the need for precise thermal characterization of subsurface environments. To enhance the design accuracy and operational reliability of ground source heat pump systems under complex geological conditions, this study builds an improved model that simultaneously accounts for soil stratification and groundwater advection to evaluate the thermal performance of borehole heat exchangers. A three-dimensional numerical model was established in ANSYS to simulate coupled heat transfer within multi-layered soil under summer-mode operation and was validated against in-situ thermal response test data, demonstrating errors below 5 %. Results indicate that neglecting soil stratification can lead to deviations of up to 18.6 % in thermal influence distance and 8.4 % in heat transfer efficiency, potentially risking underestimation of borehole spacing and system performance. Moreover, incorporating groundwater advection in the 3rd soil layer enhances convective heat transport, lowering outlet water temperatures by 0.6 °C and improving heat transfer efficiency by 40.96 %, while the proposed groundwater advection influence rate reached a maximum of 10.97 %. These findings demonstrate that both stratification and advection affect temperature distribution and thermal performance of soil, offering crucial insights for optimizing borehole design, spacing, and long-term GSHP operations in heterogeneous aquifer environments. This work provides critical insights for optimizing ground source heat pump systems under complex geological conditions, enabling more accurate heat exchange predictions and efficient multi-borehole layouts in practical applications.