Performance evaluation of a novel horizontal ground heat exchanger: Coil-column system

Abstract

Horizontal ground heat exchangers (GHEs), which are used for heating/cooling buildings, have garnered significant attention in recent years owing to their low construction cost and ease of installation. However, they require a large installation area due to their low efficiency. In this study, a new horizontal GHE type, named a coil-column system (CCS), is proposed, and its feasibility is investigated. CCS is expected to increase the total heat exchange rate of the GHE, thus reducing the required installation area. First, the heat exchange performance of the CCS is investigated and compared with existing exchangers using COMSOL Multiphysics. The numerical model was validated using an in-door thermal response test in a 5 m × 1 m × 1 m mock-up steel box. Afterward, a parametric study was conducted to evaluate the effect of coil pitch, installation depth, and space between the coil-column on the heat transfer efficiency of the CCS. Finally, the feasibility of the CCS was comprehensively evaluated by considering the heat transfer performance and economic parameters. From the results, it was found that the heat exchange rate of CCS was double and triple that of the spiral-coil and the straight-line types. The parametric study reveals that the pitch between the coil and the space between the coil-column affect the short- and long-term heat exchange performance of CCS, whereas the installation depth influences both short- and long-term performances. The economic analysis of the CCS indicates that at an installation depth of 4 m, a space between the coil-column of 0.2 m, and a pitch of 0.15–0.2 m have the highest internal rate of return because they offer a trade-off between the construction cost and heat transfer performance. Furthermore, using a controlled low-strength material as a thermal-enhanced grouting material increased the annual heat exchange rate by 25% and resulted in a notable increase in the internal rate of return (19.5% compared with 15.6% when using soil).