An analytical heat transfer model was established for a borehole heat exchanger (BHE) with single U-tube in stratified soils based on the moving finite line heat source model

taking the factors such as soil stratifications and groundwater advection into account. This model was validated by the data obtained in an on-site soil thermal response test. The soil temperature responses to the BHE heat injection were obtained by using this model and homogeneous model. Comparison of these two sets of results indicates that the soil temperature around the BHE along axial direction is not uniformly distributed due to the characteristics of soil stratification. It takes different length of time for the borehole wall temperature to reach stable and to maintain at a different stable temperature in every layer

due to the difference of physical properties of soil in each layer. Three cases are calculated and analyzed for homogeneous soil

stratified soil with groundwater advection in whole soil domain

and stratified soil with groundwater advection in some soil layer. It shows that the heat transfer efficiency coefficient is underestimated up to 6.3% if ignoring the soil stratification and the influence of groundwater advection. The thermal effect distance deviation in each soil layer may be up to 43% for cases with and without groundwater advection. In order to assess the soil temperature distribution characteristics around the BHE with the influence of groundwater advection

it is suggested that the stratified soil model is applied to calculate the thermal effect distance in each soil layer and their maximum value is chosen as the borehole spacing. Otherwise

the borehole spacing may be too small for the design of multiple BHEs.