Performance of a hybrid ground source heat pump system with open-loop and closed-loop ground heat exchangers

Ground source heat pump (GSHP) systems have gained attention as high-efficiency heat sources because they utilize relatively stable ground temperatures through ground heat exchangers (GHX). However, closed-loop and open-loop ground heat exchangers have their own limitations, such as the need for many boreholes, dependence on groundwater, and long-term accumulation of ground heat. For this reason, hybrid ground source heat pump systems using closed-loop and open-loop ground heat exchangers have been proposed. In this study, three cases were compared through simulation: a closed-loop system, an open-loop system, and a hybrid system. Case 1 used only closed-loop ground heat exchangers. Case 2 used only open-loop ground heat exchangers. Case 3 used a hybrid configuration with both closed-loop and open-loop ground heat exchangers. Simulations were conducted for each case to evaluate outlet fluid temperature. Simulation results showed that, under the same building conditions, the system using open-loop ground heat exchangers could meet the required capacity with fewer boreholes than the system using closed-loop ground heat exchangers. Specifically, only two standing column well ground heat exchangers were sufficient to maintain the outlet fluid temperature range of the boreholes compared to twenty vertical closed-loop ground heat exchangers. Furthermore, in the hybrid system, the temperature fluctuations during heating and cooling were reduced, and the thermal stability of the ground heat source was enhanced compared with that of the closed-loop system. These results show the applicability of a hybrid system incorporating closed-loop and open-loop ground heat exchangers and suggest that additional studies, including optimization of capacity, operating conditions, and borehole sizing, are needed to improve the design and long-term performance.