Thermal cycling performance of a Shell-and-tube latent heat thermal energy storage system with paraffin/graphite matrix composite

In this paper, thermal cycling behavior of paraffin/graphite matrix (PGM) composite in shell-and-tube configuration is investigated experimentally. It is aimed to investigate the effects of various graphite matrix bulk densities (50 g/L and 75 g/L) and compaction speeds (12 mm/min, 90 mm/min, 120 mm/min) on the leakage characteristics, dimensional changes, structural distortions, and thermal performance (specifically, meltingsolidification durations). A total of 120 cycles of charge-discharge experiments are conducted, during which the aforementioned parameters are continuously monitored and recorded. The findings indicate that PGM composites exhibit no notable leakage. However, when comparing different scenarios, it is seen that a compaction speed of 120 mm/min leads to a greater leakage of PCM compared to the others. The dimensional changes of all samples are attributed to the thermal expansion and contraction of paraffin. Following the initial cycles, the samples shrink 2.5 % in diameter. The extent of structural deformation is influenced by two factors: the bulk density of the graphite matrix and the cycle count. For a bulk density of 50 g/L, the surface exhibits obvious cracks after 48 cycles, which progressively expand as the cycle number increases. However, when the bulk density reaches 75 g/L, no evidence of cracking is observed after 120 cycles. The results indicate that the composite with a bulk density of 75 g/L is less affected by cycling due to its higher thermal conductivity in comparison to the composite with a bulk density of 50 g/L. Additionally, the temperature distribution in the radial direction is more uniform in the former composite. While certain samples exhibit structural deformations, it is important to note that the thermal performance and structural integrity are preserved among all the samples tested.