Optimal design of thermal energy storage systems using multiple phase change materials: Effects of material arrangement in concentric cylinders

Abstract

Design guidelines for sequencing multiple phase change materials (PCMs) in latent heat thermal energy storage (LHTES) systems remain insufficiently established. This study numerically investigates melting in concentric multi-PCM LHTES systems, focusing on how radial PCM arrangement governs phase-change progression under gravity. Six multi-PCM arrangements (all permutations of 20, 30, and 40 °C) and a single-PCM baseline are analyzed using a two-dimensional enthalpy–porosity model. As the liquid layer thickens, buoyancy-driven convection intensifies with increasing Rayleigh number, producing asymmetric melting fronts, thermal stratification, and vortex-dominated laminar flow, with heat transfer enhanced in the upper regions by convective circulation. PCM spatial ordering emerges as the governing design parameter controlling internal thermal resistance and total melting time. The monotonic arrangement (40–30–20 °C) is optimal and achieves the shortest melting time, whereas the single-PCM baseline (20–20–20 °C) takes 1.60 times longer and the non-monotonic arrangement (20–30–40 °C) takes 14.00 times longer. Non-monotonic arrangements that place the highest-melting PCM in the middle or inner layer also make melting time longer than in the optimal arrangement. The findings are limited to melting (charging); solidification was not investigated. These results establish a quantitative design guideline for PCM sequencing to improve charging rate and power density in LHTES systems.