Efficiency of various grouting materials for borehole heat exchangers

In borehole heat exchangers (BHE), grouting material plays a significant role in the heat transfer between the ground and the heat carrier fluid in the pipes. To guarantee proper sealing capacity of the grouting materials, the grout must also fulfill suitable hydraulic and mechanical properties. This paper evaluates the performance of various grouting materials, through thermal, hydraulic and mechanical laboratory characterizations. In particular, the addition of graphite powder to improve the thermal properties of grouting material is tested. In parallel, the characteristics of two different widely used commercial grouting materials (i.e. bentonite-based and silica sand-based materials) are also investigated. Afterwards, the specific heat exchange rate and the borehole resistance of borehole heat exchangers are assessed experimentally in a 1 × 1 × 1 m3 sandbox under, successively, dry sand and fully water-saturated sand conditions. During the operations, the monitored temperatures in the sandbox are in good agreement with analytical predictions. This study demonstrates that the homemade admixture prepared with 5% natural flake graphite can be considered as an appropriate grout for BHEs regarding to its rheological and thermo-physical properties. Thermally-enhanced grouting can be of significant interest in a high thermal conductivity ground (such as saturated sand) because it minimizes the thermal resistance of the BHE.