Analysis of the effect of helical tube and U-tube wrapped with SSPCM: a numerical comparison

Abstract Borehole heat exchangers (BHEs) are currently one of the most widely employed technologies for shallow geothermal energy utilization. Recent studies have explored various tube configurations to enhance the thermal performance of BHEs, with particular attention to helical tubes and U-tubes. In parallel, shape-stabilized phase change materials (SSPCM), characterized by high thermal conductivity and latent heat storage capacity, have been increasingly utilized as backfill materials to further improve thermal efficiency. Diverse SSPCM backfill methods, including direct backfilling and externally wrapped backfilling, have been investigated. However, existing research predominantly focuses on the direct backfill method and U-tube configurations, while studies on the externally wrapped backfill method and helical tubes remain limited. To address this gap, the present study investigates the thermal performance of SSPCM as an externally wrapped backfill in both helical tube and U-tube BHE configurations. SSPCM prepared from graphite adsorbed paraffin was used as backfill material, which was applied externally to BHEs, forming a novel borehole heat exchanger (NBHE). Numerical models for both helical and U-tube configurations were developed, with model validation conducted based on previous experimental data. The results indicate that SSPCMs can perform better in helical tube forms because of the property of SSPCM to moderate the temperature fluctuation. The findings show that, after 8 hours of operation, heat transfer capacity increased by 13.9% and 10.74% for the helical tube and U-tube configurations, respectively, under equivalent flow rates. The corresponding heat transfer gains were 615 kJ and 368 kJ, respectively. Notably, despite the complete melting of the SSPCM within the first hour of operation, the BHEs with SSPCM backfill maintained a significant heat transfer temperature differential during extended operation periods.