Numerical investigation of cascaded phase-change material mass ratios for latent heat thermal energy storage in solar crop drying

The influence of individual cascaded phase change materials (PCMs) on heat transfer in latent heat thermal energy storage (LHTES) units is not well understood. Consequently, optimal PCM mass-distribution strategies remain an open design problem. This study investigates the effect of cascaded PCM mass distributions and porosity on the charging–discharging behaviour of a packed-bed LHTES unit. It assesses energy and exergy storage and recovery with extended night-time solar crop drying as the design objective. Numerical simulations were conducted using COMSOL Multiphysics 6.1. A total of 33 kg of three PCMs (melting temperatures 28 °C, 38 °C, and 48 °C) were encapsulated in 364 cylindrical capsules and arranged in a cylindrical tank. Mass distributions corresponding to bed porosities of 0.3, 0.525, and 0.75 were evaluated. PCM mass distributions and packed-bed porosity were observed to strongly influence the thermal behavior and performance of the LHTES unit. Configurations with larger fractions of low-temperature PCM appeared to charge faster, store more energy, and discharge longer, but delivered limited heat within the target drying range (40–50 °C). The uniform distribution and porosity achieved the highest round-trip energy efficiency (50.44%) and sustained the target temperature range for approximately 0.9 h. In contrast, the configuration with a higher fraction of high-temperature PCM exhibited the highest exergy storage (61.17%) and overall exergy efficiency (43.15%). Lower heat transfer fluid (HTF) flow rates prolonged both charging and discharge durations. The results suggest that the uniform distribution and porosity provide a balanced compromise, while non-uniform designs better meet specific energy or exergy targets. Optimization of mass distributions should be driven by specific performance objectives.