Experimental and numerical investigations on the thermal performance of a borehole ground heat exchanger with PCM backfill

Borehole ground heat exchanger (BGHE) with phase change material (PCM) backfill is a new type of efficient geothermal energy utilization technology. In this paper, a simulation experimental system on the thermal performance of BGHE with PCM backfill has been built. The experimental performance investigations of the BGHE operated in summer and winter modes have been conducted. The results indicate that the soil thermal interference radius with PCM backfill are about 86.5% and 87.8% of that with soil backfill for summer and winter modes, respectively. At the same time, the PCM backfill can increase the heat exchange rate of BGHE and delay the variation of soil temperature due to the latent heat release of PCM during the phase change. The mass ratio of mixed acid to oleic acid content has great influence on the soil thermal interference radius and heat transfer rate of BGHE. The minimum soil thermal interference radius throughout the year is found to be 0.47 mat the mass ratio of 4:6 under the test conditions, and the corresponding total heat transfer of BGHE are 16146.09 and 4966.13 kJ for summer and winter mode, respectively. To further study the effects of thermal properties of PCM, running time and alternate cooling and heating operation, numerical simulation on the influences of phase change temperatures, latent heats, different time ratios of operation to off and alternate cooling and heating cycle on the soil temperature variation and energy storage performance of BGHE with PCM backfill have been undertaken. The results show that the heat exchange rate of the BGHE can be significantly enhanced by backfilling PCM with low and high phase change temperature for summer and winter modes, respectively. For improving energy storage effect and shortening soil thermal interference radius of the BGHE, a PCM with large latent heat should be used. During the alternate cooling and heating cycle, PCM backfill can reduce the peak value of soil temperature fluctuation and delay the recovery rate of soil temperature, and the daily total heat transfer amount of single borehole can be improved. Additionally, in order to balance soil temperature and PCM recovery, an optimal ratio of operation to recovery time should be found for a specific building. (C) 2019 Elsevier Ltd. All rights reserved.