Effect of shape-stable phase change materials on heat transfer characteristics of ground source heat pump backfill materials

Ground source heat pump (GSHP) systems can utilize shallow geothermal energy to realize low-carbon buildings. The backfill material used in GSHPs directly affects the heat exchange between the ground heat exchanger and soil. Phase change materials (PCMs) can utilize the phase change process to realize both the storage and release of energy. Applying PCMs to GSHP backfill materials may be beneficial towards improving efficiency and saving energy. In this study, decanoic acid and lauric acid were used as composite PCM and graphite powder was added to enhance thermal conductivity. Expanded perlite was used as absorbent and silica sol was used to shape and encapsulate PCM. A shape-stable phase change material (SSPCM) was prepared for GSHP backfilling. Basic characteristics including thermal conductivity of SSPCMs were assessed and analyzed, and one-dimensional heat transfer experiments were carried out. The results showed that at a decanoic acid to lauric acid mass ratio of 7:3, the phase change temperature of binary organic acids is 19.9 °C, and the latent heat of phase change is 124.03 kJ/kg. At a 60% mass content percentage of PCMs, the amount of added graphite powder was 4%, the thermal conductivity of the SSPCM encapsulated by silica sol was 0.1431 W/(m·K), and the mass loss rate of SSPCM was less than 5% after 25 high and low temperature cycles. Materials with the above parameters have suitable comprehensive properties and can be used as preferred phase change backfill material for GSHP. The thermal conductivity of the backfill material is negatively correlated with the mixing ratio of SSPCM, and positively correlated with water content. At an intermittent ratio of 10:14, after 10 h of heating, the temperature range of the phase change backfill sample was 0.38–7.45 °C lower than that of the original soil backfill sample; after 14 h of cooling, the temperature of the phase change backfill sample was closer to the initial temperature. Therefore, adding SSPCM to the backfill material is beneficial to the long-term intermittent operation of GSHPs.