A novel PCM/expanded graphite composite sphere with high thermal conductivity and excellent shape stability used for a packed-bed thermal energy system

In this research, a novel phase change material (NPCM) sphere was developed and its application in a packed -bed thermal energy storage (PBTES) system during thermal energy storage was studied. Paraffin wax (PW) with a melting point of 40 degrees C was selected as phase change material (PCM), expanded graphite (EG) as thermal conductivity enhancement material, and epoxy resin (ER) as a surface coating. The results show that the threedimensional heat conduction network constructed by EG foam improves the thermal conductivity of PW/EG. The thermal conductivity of NPCM is anisotropic. The thermal conductivities perpendicular and parallel to the EG compression direction are 9.03 (W/m & sdot; K) and 5.58 W/(m & sdot; K). NPCM has excellent shape stability with a leakage rate of only 2.95 %. Then, a PBTE system was built to record the temperature evolution of NPCM spheres at various inlet temperatures and volume flow rates during thermal energy storage, and thermodynamic analysis was performed. The results are as follows. (1) The temperature of the NPCM sphere near the inlet rises more rapidly and the melting time of PCM is shorter. (2) Increasing the inlet temperature improves the heat transfer and shortens the melting time of PCM while increasing the temperature inhomogeneity of the NPCM spheres. Increasing the inlet temperature increases the energy storage capacity. However, higher inlet temperature leads to lower energy efficiency and exergy efficiency. The energy efficiency and exergy efficiency are 49.66 % and 67.64 % for the inlet temperature of 60 degrees C, respectively. When the inlet temperature increases from 60 degrees C to 80 degrees C, the energy efficiency and exergy efficiency decrease by 16.13 % and 16.22 %, respectively. (3) Higher inlet volume flow rate also promotes heat transfer and reduces the melting time of the PCM, but has little effect on the temperature inhomogeneity of the NPCM spheres. Increasing the inlet volume flow rate does not increase the energy storage capacity. Similarly, increasing the inlet volume flow rate also results in a decrease in energy efficiency and exergy efficiency. At the inlet volume flow rate of 19.28 m 3 /h, the energy efficiency and exergy efficiency are 41.65 % and 56.67 %, respectively. When the inlet volume flow rate increases from 19.28 m 3 /h to 25.16 m 3 /h, the energy efficiency reduces by 7.80 % while the exergy efficiency reduces by 7.41 %. This research provides a foundation for the application of the NPCM sphere.