A novel type of borehole heat exchanger wrapped with shape-stabilized phase change material

Abstract

Phase change materials (PCMs) are often used in borehole heat exchangers (BHEs) to improve their performance. However, the most commonly used method of full borehole backfill still suffers from low efficiency of PCM application and environmental pollution. This study proposes a novel backfill method: wrapping shape-stabilized phase change material (SSPCM) directly around heat exchange tubes to create a novel borehole heat exchanger (NBHE). NBHE improves the application efficiency of SSPCM compared to conventional BHE. In addition, NBHE can completely physically isolate the SSPCM from the soil and avoid PCM leakage. We evaluated the performance of the NBHE using a numerical model validated with experimental data. Based on summer conditions, the performance of the NBHE was compared with that of a BHE with SSPCM in the conventional backfill method. Results showed that, compared to the conventional BHE, the novel backfill method and conventional backfill method improved heat exchange by 33.8 % and 18.3 % on the first day with SSPCM, respectively. After one week of the cycle, the novel backfill method can increase the heat exchange capacity by 113,340 kJ using the conventional backfill method, corresponding to an increase of 11.71 %. In addition, after a one-week cycle, SSPCM recovered consistently in both approaches. Compared with the conventional backfill method, the heat exchange per unit depth of the novel backfill method was enhanced by 26.3 % and 32.2 % when the soil thermal conductivity was 2.48 W/(m·K) and 1.5 W/(m·K), respectively. Furthermore, the heat exchange per unit depth decreases as the initial soil temperature increases.