Thermal and economic benefits of a coil-column ground heat exchanger with recycled concrete backfill in horizontal ground source heat pump applications

In this study, we constructed a full-scale in-situ thermal response test (TRT) bed to evaluate the heat exchange performance of a newly developed coil-column system (CCS) as a novel ground heat exchanger (GHE) for a horizontal ground source heat pump (GSHP) system. The TRT was also performed for the conventional GHE (U-type) to compare and investigate the feasibility of the CCS. This study also aimed to propose a recycled concrete-based backfill material (RCB) to improve the heat exchange ability of the horizontal GSHP system, reduce material costs, and mitigate environmental impact. Engineering properties and thermal conductivity were examined through laboratory tests. Furthermore, the in-situ heat transfer performance of the proposed materials was evaluated using the full-scale TRT. After that, a numerical analysis-based finite element code was used to compare the performance of full-scale CCS and other GHEs (U-type and spiral-coil type) and conduct economic analysis. The results showed that the proposed backfill material possesses high thermal conductivity and satisfies all the engineering property requirements for general backfill applications. The heat exchange capacity of the CCS is 1.2 times higher than that of the conventional U-type GHE, despite requiring 5.75 times less installation length. Thus, the CCS combined with the proposed backfill material is a highly feasible GHE compared to that of the U-type and spiral-coil type: 776% and 65% higher in net present value, 150% and 23% higher in internal rate of return, and 6 years and 5 years shorter in payback period, respectively.