Effect of forced thermal recharging on the thermal behaviour of a field scale geothermal energy pile

Many applications require ground source heat pumps coupled with energy piles to operate for a given number of hours per day. During the times when the heat pumps are switched off, the ground temperatures normally recover naturally. The ground can be forcefully recharged during the rest periods using solar heaters or cooling towers combined with energy piles. The effect of solar thermal recharging on energy extracted, ground and pile temperatures, and pile thermal strains and stresses are presented in this paper. Forced ground thermal recovery is experimentally investigated and included for 8 hours operation with 16 hours heating and 16 hours operation with 8 hours heating. The longer recharge duration gives higher benefits in terms of energy extracted and ground temperature recoveries with larger thermal loads on the pile. The pile temperatures, thermal strains and stresses undergo cyclic response for both operating modes.