Evaluation of thermal-mechanical properties of quartz sand-bentonite-carbon fiber mixtures as the borehole backfilling material in ground source heat pump

Ground source heat pumps (GSHPs) use sustainable technology and renewable geothermal energy to exchange heat between the ground and the interior of residential, industrial, and commercial buildings. The borehole grouting material is a key contributor to the stability and thermal transmission properties of GSHPs. To investigate the thermomechanical properties of quartz sand-bentonite-carbon fiber mixtures to assess their potential as a borehole backfilling material, the thermal needle and unconfined compressive strength (UCS) tests were conducted. The thermal conductivity of quartz sand-bentonite-carbon fiber mixtures increased with the sand particle size, and the thermal conductivity showed a parabolic relationship with bentonite fraction for all sand particle sizes. The thermal conductivity of the mixtures increased with increasing moisture content and degree of saturation, and increased linearly with increasing dry density. The optimal percentage of bentonite, by dry mass, was approximately 10-12%. A relatively high thermal conductivity (1.90-1.98 W/m center dot K) and an unconfined compressive strength of 124-200 kPa were considered acceptable for borehole backfilling material used for GSHP. The practicality of quartz sand-bentonite-carbon fiber mixtures as borehole backfilling material for GSHPs was verified using the TICA GSHP Design Program. This research confirms that quartz sand-bentonite-carbon fiber mixtures are viable new borehole backfilling material that enhance the thermal efficiency and reduce the costs of GSHPs. (c) 2019 Elsevier B.V. All rights reserved.