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.
Kabasa et al. (Tue,) studied this question.