Experimental investigation on mitigating the thermal contact resistance at the soil - energy geo-structure interface

Abstract

Abstract

Previous studies demonstrated the effectiveness of thermally enhanced backfill materials in improving borehole heat exchanger performance; however, limited experimental research studies exist on investigating the potential benefits of placing thermally enhanced soils at the soil-energy geo-structure interface to address and mitigate the thermal contact resistance. This paper presents results of a comprehensive experimental investigation on the use of thermally enhanced sands placed at the interface of energy geo-structures (EGS) such as piles and walls to improve heat transfer between energy geo-structures and the surrounding soils. The effectiveness of using of two different conductive additives named graphTHERM and carbon fibre in enhancing the thermal properties of sands was first evaluated. Subsequently, a series of tests using a relatively large-scale testing rig was performed to assess the influence of the thermally enhanced sand placed at the soil-EGS interface on the thermal performance of energy piles and walls during cooling. The results indicated that a 10% addition of graphTHERM and carbon fibres was an optimum level of additive improving the sand thermal conductivity by 62% and 35 % respectively. Placing a 100 mm of thermally enhanced sands by graphTHERM and carbon fibres at the interface between energy pile and surrounding soils was found highly effective in enhancing the heat transfer rate by 81 % and 54 % respectively. Furthermore, placing a 200 mm layer of thermally enhanced sands by graphTHERM and carbon fibres at the interface of energy walls increased the heat transfer rate by 108 % and 80 % respectively. The results demonstrated that the use of thermally enhanced sands at the soil-EGS interface was effective in expanding the thermally active soil zone without causing any increase in the lateral earth pressure acting on the EGS above those measured when the EGS was constructed in pure sand. The findings demonstrated the potential benefits of using thermally enhanced sands at the soil-EGS interface to improve heat exchange performance and contribute to the development of more efficient geo-energy systems.