Thermal cycling aging of encapsulated phase change material - Compressed expanded natural graphite composite

A paraffin-Compressed Expanded Natural Graphite (CENG) composite encapsulated in aluminium shell has been thermally aged in a packed bed heat storage tank. A hydraulic test bed has been settled in order to impose 8500 thermal cycles where the material underwent a phase change at each cycle. The PCM phase change provokes an expansion of the composite structure that might affect the effective thermal conductivity and then the heat transfer rate of the Thermal Energy Storage (TES). This performance has been monitored all along the cycling test showing a non-significant change in heat transfer rate, despite a delamination of the composite within the capsule. This phenomenon has been explained by the delamination that affects mainly the thermal conductivity in the axial direction while the heat transfer from the PCM to the aluminium shell occurs essentially on the planar direction. Indeed, the thermal conductivity in planar direction is approximatively six times higher than on the axial direction. A DSC analysis on new and aged samples showed that the phase change temperatures remained the same and the latent heat magnitude decreased by less than 10%. A FTIR analysis confirms the chemical stability of the PCM-CENG composite after 8500 cycles.