Thermal Conductivity of Sand-Bentonite Mixtures as a Backfill Material of Geothermal Boreholes

Abstract

Backfill materials play an important role in improving the thermal performance of geothermal heat exchangers. In the present work, thermal probe tests were conducted to measure the thermal conductivity of sand-bentonite mixtures under different conditions. Based on microscopic observations, the mechanism of bentonite affecting heat conduction between the sand grains was analyzed. Experimental results showed that the thermal conductivities of sand-bentonite mixtures increased gradually with increasing saturation and varied weakly when the saturation was over 80%. At nearly saturated state, the thermal conductivity of sand-bentonite mixtures first increased with increasing percentage of bentonite by dry mass, then reached a peak at the range of 10-12%, beyond which the thermal conductivity decreased quickly. The present work can extend the results recommended by ASHRAE and provide some useful guidelines for the design of ground source heat pump systems.