Comparison of mixed convection effects in summer and winter on the energy efficiency of energy piles

In recent years, shallow geothermal energy as a renewable energy source has received considerable attention and studying the factors that impact the efficiency of this system, particularly groundwater flow, is crucial. Although previous research has shown that groundwater flow enhances the efficiency of shallow geothermal energy systems, the evaluation of the system performance under the mixed influence of groundwater flow and natural convection is still under investigation. In this study, an energy pile was modeled for a period of 90 days using COMSOL Multiphysics software to examine the effects of mixed convection flow on the energy efficiency of energy piles during different seasons. The finite element model accounts for the circulation of heat carrier fluid inside embedded U-shaped heat exchanger pipes. The developed model was validated by comparing it with experimental results. The results indicate that when both mixed convection and ground surface temperature are considered during the 90 days, natural convection can reduce the output power of the energy pile by 23 % in winter while in the summer the output power increases by 1.4 %, compared to the scenario where natural convection is not considered. Thus, the winter effects of natural convection are significant and should be carefully considered.