From sequential to coordinated discharge: Cascaded latent heat thermal energy storage systems

Cascaded latent heat thermal energy storage systems place phase change materials with different phase change temperatures along the heat transfer fluid flow path. During discharging, the stage wise thermal driving head, that is the material to fluid temperature difference, decreases along the flow path and limits heat extraction from downstream stages. This study examines these discharge side limitations in a three stage cascaded system and evaluates how a temperature matched arrangement restructures the discharge behavior, an aspect that has received little attention compared with charging analyses. Three dimensional transient conjugate heat transfer simulations, validated against discharge experiments for the multi material configuration with a maximum relative deviation of approximately 3.0%, were used to resolve stage wise solidification and heat transfer behavior. Four configurations were compared: three single material configurations and one multi material configuration in which materials with phase change temperatures near 54, 58 and 64 °C were arranged sequentially to match the evolving fluid temperature profile. In the single material cases, upstream heat exchange raised the fluid temperature entering downstream stages, delaying solidification and producing a sequential, stage by stage discharge. By contrast, the temperature matched arrangement preserved the stage wise temperature difference and enabled concurrent heat release, achieving the highest average heat discharge rate, 10.6% higher than the best single material case, while the largest total discharged energy was obtained in the single material case with the highest storage capacity. These results demonstrate that the arrangement of phase change materials is a governing design parameter that can shift cascaded storage systems from a sequential response toward a coordinated system level discharge under the tested configuration and operating conditions.