Advanced backfilling materials for enhancing the thermal efficiency of ground heat exchangers
Uxía Calviño, Javier P. Vallejo, Yulong Ding, Luis Lugo
Abstract
• Microencapsulated PCM slurries in water, sand and bentonite backfills are evaluated. • Melting and crystallization behaviours vary with PCM concentration and backfill type. • Transition temperature variations reached 4.8 K for melting and 5.1 K for crystallization. • Sand and bentonite composites showed up to 6 and 7 times higher latent heat. • Sand composites achieved up to 3.5 times higher specific heat capacity. Residential and industrial heating and cooling systems are among the largest consumers of energy. To meet the growing energy demand in this sector, sustainable solutions are being increasingly adopted, with geothermal heat pump systems emerging as a popular choice. The main component of these systems is the ground heat exchanger that comprises a borehole, typically filled with grout, and tubing that carries a heat transfer fluid, transferring thermal energy between the ground and the heat pump. To enhance the thermal performance of the backfilling material, the addition of phase change materials has been presented as a promising solution. This study examines the addition of a microencapsulated organic phase change material slurry into three commonly used geothermal boreholes backfilling materials (water, sand, and bentonite) at various concentrations. The phase change characteristics, including heating–cooling curves from 2 to 10 K·min −1 , were analysed by differential scanning calorimetry. Isobaric heat capacities within solid and liquid phases were also measured. Solid-liquid transition temperatures vary up to 4.8 K for melting and 5.1 K for crystallization of bentonite-based samples. Up to 7 times higher latent heat and up to 3.5 times higher isobaric heat capacity were noticed with increasing loading of microencapsulated phase change material. The designed materials show potential characteristics as backfilling materials for geothermal vertical boreholes, particularly those using bentonite and sand backfills.