Phase change materials (PCMs) are attractive components of thermal energy storage systems due to their high energy densities and the relatively small temperature differences needed for effective charging and discharging. In this study, experimental data were examined in order to better understand energy exchange in packed beds and porous media comprised of solid/liquid phase change capsules. Air was used as the heat transfer fluid; hot air was injected in order to drive the melting process, while cold air was injected to accomplish freezing or solidification of the PCM.
Theory was developed to describe the temperature variations throughout the bed. Temperatures in the bed were found to vary exponentially near the phase change fronts. For cold air injection into a bed initially above the phase change temperature, a second wave was observed ahead of the phase change front that can be described as a broadening traveling thermal wave due to diffusion/dispersion. For hot air injection into a packed bed of solidified PCM capsules initially at a temperature below the phase change temperature, the thermal waves in the cold region showed isotherm velocity retardation due to incomplete thawing.
A shrinking core model of the melting or solidification of the PCM in the capsules was developed to document the internal heat transfer constraints within the capsules. The results of that study support the conclusion that slower wave velocities occur due to partial melting.