Heat transfer of composite phase change material modules containing a eutectic carbonate salt for medium and high temperature thermal energy storage applications

This paper concerns the heat transfer behaviour of Composite Phase Change Material (CPCM) modules made of a eutectic carbonate salt of NaLiCO₃ (phase change material, PCM), MgO (ceramic skeleton material, CSM) and graphite flakes (thermal conductivity enhancement material, TCEM). The CPCM has a melting point around 500 °C and is suitable for medium and high temperature thermal energy storage applications including peak shaving of power grids, effective use of curtailed wind energy, and concentrated solar power generation. Disk-like CPCM modules were fabricated for the work. The effects of TCEM loading and surface cooling conditions on the heat transfer were experimentally investigated and analysed. The results showed that the use of TCEM not only significantly enhanced heat transfer of the CPCM modules, but also reduced the temperature difference and hence the thermal resistance between heater surface and CPCM module surface, leading to a significant extent of enhancement of overall heat transfer. Temperature measurements of a flat surface of the CPCM modules as well as that within the modules showed a non-uniform temperature distribution perpendicular to heat transfer direction, suggesting the effect of CPCM microstructure on heat transfer. This microstructural effect was further investigated using a scanning electron microscope with energy dispersive spectrometry. The results indicated salt migration, particle breakage and particle redistribution in the interior of the CPCM modules during thermal cycling, leading to a more homogenous distribution of ingredients and more uniform heat transfer within CPCM modules.