Analysis of a borehole ground heat exchanger with shape-stabilized phase change material backfill

Abstract

GSHP (ground source heat pump) is a popular green building technology that utilizes renewable energy sources, but its efficiency can decrease significantly due to heat accumulation in soil. The borehole ground heat exchanger (BGHE) with shape-stabilized phase change material (SSPCM) backfill is a novel method to alleviate temperature fluctuation owing to better thermophysical properties. However, relevant experimental studies are still in scarcity and the impacts of backfill material on the soil temperature remain to be explored. This study conducted an experimental measurement on the soil temperature of a BGHE with SSPCM (expand graphite adsorbed paraffin) and sand backfill. A numerical model is established accordingly to be validated by the experimental results, in order to provide additional simulations on the sand-SSPCM mixture to identify the system performance and cost-effectiveness under different mass ratios of SSPCM. The experimental results show that SSPCM can effectively mitigate temperature fluctuations, especially closer to the heat source. The excess temperature difference is up to 10 °C near the heater wall. Compared to sand backfill, the sand-SSPCM backfill area's outside also maintains a steady temperature for a longer time during the cooling process. The numerical model couples the line heat source model of the BGHE with the validated phase change process. The simulation results found that the heat influence radius under the novel backfill was reduced by 16.5% after one operating cycle. Furthermore, after a 14 h shutdown, the novel backfill temperature is higher than the traditional backfill in the backfill area. The impact of various SSPCM mass mixing ratios indicates that a higher mass ratio can help reduce temperature variations, but the larger the mass ratio the less cost-effective it becomes. The optimum mass ratios are dependent on different parameters used as objectives.