Overcharging of a cascaded packed bed thermal energy storage: Effects and solutions

Abstract

This paper focuses on the optimization of a cascaded latent heat thermal energy storage system that utilizes phase change materials in a packed bed form, to achieve a high volumetric storage capacity and high storage efficiency. In particular, the effect of overcharging as a phenomenon that limits the key performance indices in current systems is discussed. A novel solution by an optimized scaling of the storage capacities in each temperature region of a cascaded storage system is presented. High cyclic efficiency is achieved using different storage capacities in each temperature region. This ensure the charge times of each region remain close to one another and will eventually obtain a charge-to-discharge time ratio closer to one. In addition, this ratio is found to be inversely related to the cyclic efficiency. This paper also provides a generalized map, which functions as a unique and important tool to optimize the performance indices of packed bed systems. After the capacities of phase change materials are selected, it is possible to follow a guideline towards the optimal sizing and selection of the packed bed thermal energy storage system, prior to a start of the design process. The generalized map is also useful when the ideal performance indices are pre-selected and the optimal storage capacities of each region can be determined from the generalized map. This paper also discusses the effect of encapsulation diameter, and determines that a smaller diameter will result in high performance indices in a system but at a higher cost.