Effects of permeability anisotropy and groundwater seepage on the seasonal performance of geothermal heat exchangers

Groundwater seepage significantly affects the thermal performance of ground heat exchanger (GHE) and the seasonal thermal balance of soil. However, most studies assumed isotropic soil conditions, neglecting effects of permeability anisotropy. To fill this gap, this study developed a novel three-dimensional thermal-hydraulic model that explicitly incorporates permeability anisotropy, validated against the in-situ thermal response test. Based on the validated model, the influence of groundwater seepage on ground temperature distribution and GHE thermal performance in winter and summer conditions was investigated. Furthermore, effects of permeability anisotropy, seepage velocity and seepage direction on heat transfer performance of GHE and ground temperature were investigated. The results indicated that groundwater seepage significantly enhances GHE thermal performance by up to 86.5% compared with the non-seepage condition, while effectively mitigating seasonal-scale heat/cold accumulation around the borehole. As seepage velocity increases from 1.4×10 -7 m·s -1 to 1.4×10 -5 m·s -1 , the heat transfer is improved by 80.59%-157.98% in winter and 80.9% -158.1% in summer. Considering the permeability anisotropy, horizontal seepage leads to the most favorable soil heat recovery and heat transfer efficiency compared with oblique seepage directions. These findings highlight the critical role of seepage anisotropy in GHE thermal behavior and provide insights for optimizing GHE design.