Influence of cascaded graphite foams on thermal performance of high temperature phase change material storage systems

In this study, a numerical analysis is used to compare the effects of uniform and cascaded graphite foams on the charge-discharge cycle performance of thermal energy storage systems where a phase change material (PCM) with a phase change temperature of 855 K is used as the storage media. Firstly, for a constant PCM mass, the insertion of single graphite foams with respective porosities of 80%, 85%, 90% & 95% in the storage tank resulted to an exponential increase in the charge-discharge time. It was therefore concluded that a graphite foam with a porosity higher than 90% is less beneficial because the gradient of the porosity versus charge-discharge time graph becomes steeper. Secondly, for a constant PCM mass, graphite foams of porosity 80%, 85% and 90% were combined to obtain a cascaded porosity structure. The three foam structures were cascaded in such a way that the average porosity was either 85% or 89%. Overall, results obtained for cascaded porosity structures showed that combinations having a low porosity graphite foam near the cooling/heating wall performed better than their counterparts having a higher porosity foam near the heating/cooling wall. Particularly, the best combination at an average porosity of 85% and 89% accelerated the charge-discharge time by 5% and 4%, respectively, as compared to their respective single graphite foam cases.