Experimental Comparison of the Effect of Natural Convection Between a Gravel Backfilled and a Cement Grouted Vertical Borehole Heat Exchanger

Abstract

The thermal properties of the ground must be considered in the design of borehole heat exchangers (BHEs) with a vertical closed loop. Generally, conduction is assumed to be the only process transferring heat in the ground. However, natural convection induced by density differences can also be an important process transferring heat in a porous medium, as in saturated ground with high porosity and no groundwater flow. In this study, the site-specific thermal properties of the ground were estimated by in situ thermal response tests (TRTs) and the heat transfer by natural convection inside and around the BHE was experimentally analyzed. Two different configurations of BHEs (one backfilled with gravel and another grouted with cement) were installed in a saturated formation composed of sandy soil. The TRT data were interpreted using an infinite line source (ILS) model with the quasi-Newton technique to consider the effect of disturbance from the external environment. Based on the four TRTs at different heat injection rates, we found that natural convection inside and around the BHE can significantly affect its thermal performance.