Effect of variable capsule size distribution for unstructured packed bed encapsulated phase change material system

Abstract

Packed bed encapsulated phase change material (PCM) energy storage systems have considerable potential for energy storage applications involved in renewable energy systems. In this paper, a 3D unstructured packed bed system consisting of spherical encapsulated phase change material with varying sizes of spheres is modelled. The main novelty of the work is that the geometry of the capsules and the flow between the capsules are resolved, even with unequal capsule sizes arranged in an irregular manner. The size of the spheres is varied by changing the radius of the spheres following specified normal distribution curves. Then, by changing the mean and standard deviation values, different size distributions of spheres are generated which are arranged in unstructured packed bed form. The model incorporates the flow of heat transfer fluid (HTF) through the gaps between the capsules, the heat transfer in both the HTF and the PCM capsules, the melting of PCM in each capsule, and the natural convection in the melted PCM. Based on the simulations, important parameters such as the melting time and energy storage rates are compared for different size distributions. It is found that for a normal distribution of capsule radius, increasing the mean radius by 2 times while keeping the standard deviation constant increases the melting rate considerably with an increase in melting time by 29.4%. The effect of standard deviation of radius was found to be significantly less as compared to that of the mean radius.