Investigation of heat exchange efficiency and thermal migration of Deeply Buried Pipe Energy Pile group under the groundwater seepage

Abstract

The seepage has a significantly positive effect on the heat transfer of underground energy structures. However, the effect of thermal migration caused by seepage is ignored. Previous studies mainly focused on the Ground Source Heat Pump (GSHP) and the Inside Buried Pipe Energy Pile (IBP-EP). In this study, a novel underground energy structure named Deeply Buried Pipe Energy Pile (DBP-EP) was proposed and thermal-seepage coupled models of DBP-EP groups with three different layouts was developed. Meanwhile, the variation of heat exchange efficiency and the effect of thermal migration caused by seepage were analyzed. The results showed that seepage could eliminate heat accumulation around the DBP-EP group and enhance the heat exchange efficiency. However, the thermal migration transferred the heat from the upstream of DBP-EPs to the downstream, and resulted in the thermal disturbance for the downstream. When the layout of DBP-EP group was horizontal parallel, the heat exchange efficiency of the downstream DBP-EPs was 4 %-6% lower than the upstream. The staggered layout could decease the influence of thermal migration on the downstream DBP-EPs and further reduce unnecessary heat exchange loss. Besides, the appropriate spacing of the DBP-EP group, which achieved the optimum utilization of land space, was related to the seepage velocity. When the seepage velocity increased from 60 m/a to 120 m/a, the minimum spacing of the DBP-EP group with the staggered layout could be reduced by about 1 m. The heat exchange efficiency of the DBP-EP could be improved by 9 %-10 % than GSHP. This study will have certain engineering guiding significance for the design method of DBP-EP project site selection, operation and DBP-EP group layout.