Phase change behaviour study of PCM tanks partially filled with graphite foam

The aim of this paper is to shorten the melting/solidification time of a high-temperature phase change material (PCM) using graphite foam inserts. Specifically, the phase change behaviour of a two-dimensional rectangular thermal storage tank, containing PCM with graphite foam insert, fully or partially filling the tank, was studied. The composite enclosure was designed assuming it was heated or cooled from the left side wall for charging or discharging, respectively, while the other three walls were perfectly insulated. First, the effect of foam porosity (0.8, 0.85, 0.9, and 0.95), under fully-filled scenarios, was numerically investigated. Then the phase change behaviour of four partially-filled scenarios, with averaged 0.9 porosity, was carried out. The 0.9 porosity foam case showed an excellent cycle performance. With this case, it only takes 68.2 and 65.1 min for entire melting and solidification, respectively. For a tank with no insert, it will take 164/856 min, respectively, to entirely melt/solidify the same mass of PCM as that of the 0.9-porosity-case. As expected, lower porosity values lead to higher heat transfer through conduction. However, our results show that with a fixed mass of foam, it is preferred to increase the foam porosity to fully fill the tank as opposed to a design with a lower porosity foam that only partly fills the tank. Finally, given the high graphite to PCM thermal conductivity ratio, the heat transfer through the foam is mainly due to conduction. Based on this assumption, a theoretical model is presented in parallel to numerical results. Our analysis for a foam-saturated PCM storage tank shows that the dimensionless time taken for completely melting the PCM, expressed as Fourier number, scales with rho(PCM,s)/rho(PCM,l) k(eff)/k(PCM,l) epsilon/Ste.

Automated summary

This study examines the use of graphite foam inserts to improve the melting/solidification efficiency of high-temperature phase change materials. By evaluating different porosities and filling scenarios, it was found that higher foam porosity significantly reduces the overall melting/solidification time, making it a preferred design choice.