Performance evaluation of innovative backfill materials containing phase change materials with low melting points for geothermal energy piles in cold regions

This study investigates the enhancement of geothermal energy pile (GEP) performance in cold regions by incorporating phase change materials (PCMs) with melting points near 6 °C, suitable for cold regions such as Edmonton, Canada. The cementitious backfill materials were developed using two PCM contents (3 and 5 wt.%) and two flake graphite ratios (6 and 12 wt.%). The developed backfill materials exhibited strong heat storage, with 5 wt.% PCM mixture achieving specific latent heat values up to 14.28 J/g and 8.5 J/g for 3 wt.% PCM. Additionally, flake graphite effectively mitigated the typical reduction in thermal conductivity associated with PCM incorporation, achieving values of 1.61–2.80 W/m·K in saturated conditions and 0.94–1.80 W/m·K under oven-dry conditions. Numerical modeling demonstrated that the developed backfill materials could enhance total heat extraction by up to 7%, reaching 20.83 MJ over 30 min, which improves GEP efficiency in cold climates. In conclusion, the developed backfill materials demonstrate excellent heat-transfer and heat-storage performance, enhancing the efficiency of GEPs and supporting their broader application in cold regions.