Effect of the heat load distribution on thermal performance predictions of ground heat exchangers in a stratified subsurface

Abstract

Thermal performance predictions of ground heat exchangers (GHEs) can be largely affected by the ground stratification and the heat distributions. Based on a validated laboratory device, predictions from numerical models with variable (Qv) and constant (Qc) heat transfer rates along the depth of GHEs were compared against the experimental data. The model with Qv gave a more accurate prediction on the ground temperature, while the Qc model weakened the ground stratification effect and overpredicted the clay temperature by up to 76.6%, especially at the late stage. These models predicted different ground temperature distributions at various locations and time. Specifically, for a two-layered structure in this study, the sand temperature predicted by the model with Qv was higher than the clay one, while the Qc model witnessed an opposite trend and more severe thermal interference in both layers at the end of the 24 h operating period. Furthermore, a comparison of the models with layered and equivalent material reveals that the effect of axial heat load distribution could be more significant on the heat transfer of a stratified subsurface. Therefore, it is recommended to apply the variable axial thermal exchange distributions for heat transfer predictions of GHEs in a layered ground.