Effects of groundwater flow on the heat transfer performance of energy piles: Experimental and numerical analysis

The underground heat exchangers of ground source heat pumps pass through different strata. The physical properties of these strata, particularly groundwater flow, affect heat exchange efficiency. In this study, energy piles located in an ecological tourism resort near Binhai Lake, Tianjin, China were subjected to heat transfer performance testing. First, the comprehensive thermal conductivity of surrounding soils and heat exchange rate of each pile were determined using thermal response tests (TRT) and thermal performance tests (TPT), and previously undisturbed soil was drilled from the field and subjected to laboratory evaluations respectively, which indicates that effects of groundwater flow on the heat transfer performance of thermal physical properties, thermal conductivity, and moisture content. Finally, 3D finite element modeling was used to simulate heat exchanges in the underground heat exchanger at different groundwater flow velocities, thus verifying the positive effects of groundwater on the heat exchange performances of energy piles. Our calculations suggested that as the groundwater flow velocity increased, the average inlet and outlet water temperature decreased and the comprehensive thermal conductivity performance increased significantly. Therefore, even slight groundwater flow could have a significant effect on the heat exchange performance of the pile exchanger.