Development and Characterization of Controlled Low-strength Materials as a Heat Transfer Medium for Horizontal Ground-Source Heat Pump System

In a horizontal ground-source heat pump system, thermal properties of the material backfilling the ground heat exchanger are the key factor that affects the performance of the system. The aim of this study is to evaluate the feasibility of steel-making slag-based controlled low-strength materials (CLSM) as a heat transfer medium for horizontal ground-source heat pump systems. Firstly, the engineering properties of CLSM mixtures were evaluated with different steel-making slag contents and steel-making slag types. Secondly, the effects of the GWT level drop on the thermal–hydraulic properties of different backfill materials and the performance of the system was investigated using a numerical analysis model of COMSOL Multiphysics based on measured water retention characteristics. The in-door thermal response test was conducted to validate the numerical analysis model and access the heat exchange rate of spiral-coil GHE backfilled with CLSM and natural sand. Finally, to access and compare the beneficial use of the CLSM over the typical soil, an economic analysis of 50 case studies for the horizontal GHE was performed. The results indicate that the suggested steel-making slag-based CLSM satisfied all requirements of common backfill material. Furthermore, using ground steel-making slag to replace the sand in the CLSM mixture can enhance the thermal conductivity up to 2.35 W/(mK), which is double that of the conventional grout (bentonite). Another advantage of CLSM is a high air-entry value, which can limit the influence of the GWT drop. Finally, as expected, the GHE backfilled with the CLSM shows a better economic performance compared to that of the GHE backfilled with an ordinary soil owing to its high heat transfer performance.