Achieving a mitigation of ground thermal imbalance and size-reduction of borehole heat exchangers by connecting a hybrid heat source in series

Abstract

Recent studies have highlighted the potential of hybrid ground source heat pump (HyGSHP) systems for enhancing the coefficients of performance (COPs) of heat pumps and reducing the total required length of borehole heat exchangers (BHEs) when compared with ground source heat pump (GSHP) systems. However, previous studies were predominantly focused on the advantages of parallel-connected HyGSHP systems; the detailed analysis and quantitative interpretation of series-connected HyGSHP systems have not been reported owing to the dearth of analysis methods available for such a configuration. To address this, this study proposes the implementation of serial HyGSHP systems with permanent-dewatering (PD) modules covering 10% of the building peak load as a hybrid heat source. To evaluate the long-term energy efficiency of the proposed HyGSHP systems and the reduction in the total required length of BHEs, an optimization method coupled with a comprehensive simulation tool is employed. The results show that although the total required length of BHEs is reduced by half, the proposed serial-connected HyGSHP cases effectively fulfill the building load demand, similar to the conventional GSHP cases, even with higher COPs in certain cases. This improvement can be attributed to the decreasing temperature of the source-side heat carrier fluid facilitated by the serial-connected PD module with BHEs, which considerably mitigate the imbalance between the ground heat injection and extraction.