Enhanced thermal performance of borehole heat exchangers using a microencapsulated phase-change material composite: Experimental and numerical analysis

Phase change materials (PCMs) are regarded as an effective backfill material with thermal regulation capability for borehole based shallow geothermal systems. However, the proportion of PCMs needs to be customized according to the thermal stability design of underground systems, which remains challenging. In this study, microencapsulated PCMs (MPCM) were synthesized via the sol–gel method using capric acid and lauric acid in a molar ratio of 13:7. The MPCM exhibited a phase change temperature of 19.39 °C, a latent heat of 141.2 J/g, and a microcapsule coating rate of 71.45 %. The MPCM was incorporated into backfill material to form a composite (MPCM sands) for borehole heat exchangers (BHEs). Its properties were characterized by DSC, multi-channel temperature tests, and 1 HNMR. Experimental results indicated that the composite with 10 wt% MPCM showed the smallest temperature variation under the same heat absorption. A numerical model of a single borehole under intermittent cooling operation was established, revealing that the 10 % MPCM sands composite exhibited superior overall thermal performance. Both experimental and simulation results suggest that the design of PCM-based backfill should comprehensively consider thermal conductivity, phase change temperature, phase change rate, and volume constraints to determine the optimal PCM proportion. The optimal 10 wt% MPCM composite thus achieved a coefficient of performance (COP) of 5.38 and a heat exchange rate of 98.44 W/m, effectively minimizing the potential for underground instability associated with material leakage.