Influence of granite powder addition on thermal conductivity of bentonite-based backfill material in ground source heat pump

Granite powder was added to the backfill material of a bentonite-based source heat pump, and the associated effects of granite powder content, dry density, and water content and saturation on thermal conductivity were analyzed. The thermal conductivity improved with increasing water content and dry density, for granite powder contents below 25%. The influence of granite powder on maximum thermal conductivity decreased as water content increased. Thermal conductivity increased with increasing saturation, and it increased faster for saturation below 5%. The results provide a basis for improving the efficiency of ground-source heat pumps and suggest an alternative use for granite powder.