Underground solar energy storage via energy piles

Abstract

Conventional piles embedded with geothermal loops, referred to as energy piles, have been successfully used as heat exchangers for the ground source heat pump system. For heating-dominated regions, it is crucial for the ground source heat pump system to keep the ground thermal balance in the long run. Solar energy is the most feasible source to charge the ground manually. In this study, thermal performance of an energy pile-solar collector coupled system for underground solar energy storage was investigated using numerical modeling. The results suggested that a lower flow rate should be adopted for the energy pile-solar collector coupled system to save the operational cost of the circulation pump. For the case with a pile length of 30 m, the decrease in the rate of solar energy storage was about 2% when the mass flow rate was reduced from 0.3 to 0.05 kg/s. Throughout a year, the maximum daily average rate of solar energy storage reached 150 W/m. It was also found that to increase the length and the diameter of the pile improved the thermal performance of the system by keeping its temperature relatively lower. In addition, the effects of the pile-pile thermal interference on reducing the rate of solar energy storage after a one-year operation were quantified to be within 10 W/m for groups with the pile-pile spacing of 3 times the pile diameter.