Thermo-mechanical coupling performance analysis of phase change material and nanofluid synergized high-efficiency energy piles for shallow geothermal energy utilization

Energy piles, which integrate thermal energy exchange with structural load-bearing functions, represent a promising solution for ground source heat pump (GSHP) systems. However, traditional energy piles often suffer from limited heat transfer capacity and thermal-induced stress. To address these challenges, this study proposes a novel phase change material/nanofluid synergized energy pile (PCM/NFSEP). In this design, nanofluids are used as the working fluid within embedded heat exchange pipes to enhance internal convective heat transfer, while organic phase change materials (PCMs) are backfilled between the pipe and pile body to regulate external heat flow through latent heat storage. A sandbox-scale experimental system was constructed to evaluate thermal behavior, and a 3D coupled thermo-mechanical model was developed in COMSOL for comprehensive simulation. Results show that the heat transfer rate of the PCM/NFSEP increased from 75.4 W/m to 118.1 W/m, representing a 56.6 % improvement over traditional energy piles. The thermal impact distance was reduced from 0.61 m to 0.41 m, indicating effective suppression of soil heat accumulation. Mechanically, the pile-top displacement after 30 days of operation decreased from 0.66 mm to 0.19 mm, demonstrating a significant reduction in thermal expansion. Additionally, the system's performance was further enhanced by increasing inlet fluid temperature, nanofluid concentration, and selecting PCMs with higher thermal conductivity. This study confirms that PCM/NFSEP systems offer improved thermal efficiency, reduced environmental thermal disturbance, and better structural stability, providing valuable guidance for the design and application of high-performance energy piles in shallow geothermal systems.