Performance of a full-scale energy pile for underground solar energy storage

This study presents a field test to investigate the thermal injection performance of a full-scale energy pile for underground solar energy storage (USES). The tested energy comprises a full-scale bridge pile foundation and a spiral-shaped pipe. Numerical modeling was carried out to provide complementary results. Sensitivity analyses of the pipe configuration and operation mode on the thermal injection performance of the energy pile were performed based on the numerical modeling. The results showed that 84% of the injected thermal energy could be transferred to the surrounding soil by the energy pile, and the total amount of the thermal energy stored by a single energy pile was −3.2 GJ over 29.5 days. The maximum average value of the thermal injection rate of the energy pile was equal to −129 W/m, which was 2–3 times larger than the thermal injection rate of the geothermal boreholes. Compared to the continuous USES operation of the energy pile, the intermittent mode of the energy pile was more efficient for USES. Increasing the length of the heat exchange pipe of the energy pile was more effective in enhancing the thermal injection performance of the energy pile for USES than changing the pipe shape.