Investigation on thermal and moisture migration performance in sand combined with graphite

The thermal and moisture diffusion behaviors in backfill materials play a key role on the long-term operation performance for the ground source heat pump systems. The thermal and moisture migration characteristics in sand based on a lab-scale setup were determined in terms of the heating temperature (35-45 °C), initial moisture content (10-20%) and the additive (5 wt% graphite). The thermo-physical properties of the sand, and sand/graphite blend were also evaluated. The effect of the anionic polyacrylamide (PAM) on the water retention ability of sand was examined. The values of the thermal diffusivity for the dry sand, and dry sand/graphite blend were 3.745 × 10−7 and 4.396 × 10−7 m2 s−1, respectively, which increased linearly with the initial moisture content from 0 to 10%, and then decreased slightly with the initial moisture content from 10% up to 20%. The thermal and moisture diffusion ranges enlarged with the increasing initial moisture content and heating temperature, as well as the addition of the graphite. In comparison with raw sand, the average value for the degree of the thermal diffusion of the sand/graphite blend increased by about 25%. PAM additive in sand/graphite was favorable to ensure long-term operation performance of the GSHP system.