Effect of natural convection on thermal response test conducted in saturated porous formation: Comparison of gravel-backfilled and cement-grouted borehole heat exchangers

Thermal response tests (TRTs) have been conducted to evaluate two design parameters of borehole heat exchangers (BHEs): effective thermal conductivity and borehole thermal resistance. The effect of natural convection on groundwater-filled BHE performance has been reported mainly from northern Europe. Even in a backfilled or grouted BHE, if the formation is saturated and composed of porous medium, the estimation may depend on the heat injection rate. In this study, we experimentally examined the effect of natural convection on TRTs conducted in saturated porous formation. TRTs were conducted with two BHEs having the same geometry but different backfill materials: one was cement-grouted and the other was gravel-backfilled. TRTs were conducted for each BHE at two different heat injection rates (approximately 45 W/m and 90 W/m). The TRT data were analyzed by a parameter estimation method using a temporal superposition-applied infinite line source model. The results show that when the heat rate was almost doubled, the borehole thermal resistances of the gravel-backfilled and cement-grouted BHEs decreased by 9.8% and 8.7%, respectively. Based on the results, discussions on existing design methods related to typical practices in TRTs and advantages of backfilled BHEs from the perspectives of performance and constructability are presented.