Operational control methods for a parallel system combining ground source and air source heat pumps in a warm region

In warmer regions where cooling loads dominate, the number of parallel systems utilizing ground source heat pumps (GSHPs) and air source heat pumps (ASHPs) is increasing. However, the optimal operational control of GSHPs and ASHPs remains uncertain, and there are numerous examples of indiscriminately prioritizing GSHPs, which results in long-term ground temperature increases and system efficiency declines. This study aims to use transient system simulation program (TRNSYS) simulations to determine robust and effective operational control methods that can improve the energy performance of a parallel system consisting of GSHPs and modular ASHPs in a warm region of Japan. This study proposed the use of cumulative stop (CS) control for the GSHP during the summer period, taking advantage of the characteristic that the cooling load is superior to the heating load. This addresses the problem of ground temperature increases over time and successfully ensures a consistent and stable ground temperature over 10 years. In addition to the CS control, four optional strategies were proposed. These strategies utilize easily monitored ambient temperature and ground temperature data to deactivate the GSHP at the appropriate time. The performance of the parallel system was improved by each of the optional strategies in comparison to the CS control alone. The control methods proposed in this study can be seamlessly incorporated into both operating and retrofit systems, thereby greatly contributing to the widespread use of renewable energy and improving facility management.