Thermal Properties of Backfill Material for Underground Heat Exchange Applications

Abstract

We are investigating the effect of the thermal properties of backfill material on the efficiency of underground thermal energy systems. We have used aluminum to increase the thermal properties of clay-bentonite and to investigate its potential as a backfill material in underground heat exchange applications. The measurements of thermal properties were made at room temperature using the Transient Plane Source (TPS) technique. The measurements were based on an infinite homogeneous medium using different composites of aluminum and clays in amounts of aluminum concentration ranging from 6 to 25 percent of the sample’s weight. The results confirmed an overall relative increase of 170% and 135% in the thermal conductivity and diffusivity, respectively as the Al content increased by 25%. The estimated average of the thermal transmittance (U-value ) of these samples was in the range from 0.40 to 1.1 W/m2K, which was directly proportional to the relative increase in the measured properties. Our findings can be applied to underground thermal energy systems, ground source heat pumps, and solid dielectric underground transmission lines.