Understanding the effects of variable permeability along the cross-section of packed beds of encapsulated phase change materials for latent heat thermal energy storage systems

Abstract

This work analyzes the impact of variable porosity along packed beds of encapsulated phase change materials for Latent Heat Thermal Energy Storage (LHTES) systems on their charging performance. A novel model encompassing variable permeability along the cylindrical bed’s cross-section due to geometrical constraints and a smooth phase change that is consistent with calorimetry measurements was adopted. A Finite Volumes code is used to solve the governing equations, while unknown phase change parameters are estimated using experimental data and the Levenberg-Marquardt inverse analysis algorithm. The aggregate result of the in situ estimation of the parameters related to the non-isothermal phase change and the proposed variable porosity model is a deviation of their results of at most 10% in comparisons involving temperature measurements for both the heat transfer fluid and phase change material. The significance of accounting for the variable permeability and the associated channeling effect is shown, as a considerable portion of the packed bed does not contact an enough amount of the hot heat transfer fluid to efficiently charge the LHTES system. The constant porosity model can overpredict the duration of the full conversion of heat input into heat stored by roughly 27 min and underestimate the full charging time by more than 200 min, which may lead to an erroneous sizing of the thermal energy storage unit. The usage of smaller capsules can enforce a more uniform heating of the packed bed, which allows for the heat transfer performance to approach the limit predicted by the constant porosity case.