Application of granular materials for void space reduction within packed bed thermal energy storage system filled with macro-encapsulated phase change materials

Abstract

In the paper, various granular materials as potential fillers for the void space in between macro-encapsulated phase change materials of packed bed thermal energy storage are studied. Once filled with suitable granular materials, void space of the packed bed storage, which is the main drawback of this type of storage can be significantly reduced while providing improved storage capacity per designed volume. Thus, four different materials, namely, quartzite pebbles, aluminum particles, alumina particles and micro-encapsulated n-decane are considered as potential granular fillers for packed beds. Their effect on the charge and discharge time, storage efficiency and overall storage capacity are evaluated using four parametric analyses: different macro-encapsulated phase change materials, encapsulation sizes, types of granular materials and heat transfer fluid flow rates. Among the granular materials, alumina particles ensured the highest overall increment on the amount of stored thermal energy due to their high density and heat capacity, while micro-encapsulated n-decane particles provided the highest overall enhancement on the cyclic efficiency of the packed bed storage. It was noticed that the granular materials operate at their highest capacity when combined with in-house-developed phase change materials with ultra-low phase change temperature. When coupled with other phase change materials with higher phase change temperature or latent heat capacity, the advantages of filling the void space with the granular materials diminished. Moreover, void space reduction with the granular materials helps maintain the storage efficiency while allowing increase in phase change material encapsulation size to reduce its overall cost.