[Objective] The global energy crisis is an urgent issue, highlighting the importance of shallow geothermal energy as a pivotal renewable energy source to support sustainable social development. Energy piles, a key method for harnessing shallow geothermal energy, hold enormous potential for growth and widespread application in global energy conservation and emission reduction projects. Research into the thermal performance of precast high-strength concrete (PHC) energy piles has advanced rapidly. Notably, phase change materials (PCMs) can absorb or release latent heat at a constant temperature during phase transitions, which makes PCMs an ideal backfill material to improve the heat transfer performance of PHC energy piles. However, a key drawback of PCMs is their low thermal conductivity, which could compromise the heat transfer performance of the PHC energy piles. To optimize the use of PCM in backfill materials, researchers have been exploring the addition of substances with high thermal conductivity. Graphite, in particular, has emerged as one of the most preferred fillers to enhance PCM thermal conductivity. Notwithstanding these advancements, there is limited research on how varying graphite volume fractions in PCM backfill materials affect the heat transfer performance of precast PHC energy piles. [Methods] We established an indoor model of PHC energy piles and conducted tests using different backfill materials. To calculate the thermal conductivity of the backfill materials with different graphite volume fractions, we proposed and validated a theoretical calculation model based on the Maxwell model. A finite numerical model was developed using COMSOL Multiphysics, underpinned by empirical validation. Leveraging these models and experimental investigations, we analyzed the effects of different graphite volume fractions on several parameters, including the temperature difference between the inlet and outlet of the PHC energy pile, pile temperature, and the phase change state of the backfill material. [Results] The temperature difference increases with the graphite volume fraction. During the heat transfer process, the temperature difference gradually decreases until it stabilizes. A higher graphite volume fraction accelerates the heat exchange process within the PHC energy pile system, thus contributing to improved heat transfer performance. We observed a strong linear correlation between the thermal conductivity of the backfill materials and the temperature difference. As the graphite volume fraction increases, the pile temperature rises rapidly during the heat transfer process. When the operation time of the PHC energy pile remains constant, the pile temperature increases with increasing graphite volume fraction, whereas the axial temperature increment of the pile shows no significant change and exhibits a relatively uniform distribution. The use of the backfill materials designed in the numerical models ensures the stable operation of the PHC energy piles. The rate of phase change can be accelerated, and the recovery time can be shortened by increasing the graphite volume fraction of the backfill materials. [Conclusions] Phase change backfill materials with a high graphite volume fraction can improve the heat transfer performance of PHC energy piles. This improvement is pivotal in meeting the high endurance energy requirements of buildings, thereby facilitating the efficient extraction of geothermal energy. © 2024 Tsinghua University. All rights reserved.
