Experimental evaluation of heat transfer performance under natural and forced convection around a phase change material encapsulated in various shapes

Abstract

Cold thermal energy storage can reduce peak electricity use in electric air conditioning systems as well as improve free space cooling, through convective heat transfer between a surrounding fluid (heat transfer fluid or air) and encapsulated phase change material (PCM). In this work, heat transfer performances and phase change behaviours of six PCM capsules with equal-volumes but varying-shapes were studied experimentally, for use in cold latent heat storage. A tetra-n-butylammonium bromide-based PCM with an adjustable phase change temperature of 0–12 °C was injected into capsules of spherical, cylindrical (short and long), pyramidal, tetrahedral, and a biomimetic red blood cell shape. First, an infrared camera recorded the surface temperature variation during the melting process of each capsule in air under natural convection. Second, an experimental setup was built to investigate latent heat storage during the freezing and melting processes for each shape under forced convection, focusing on the effects of fluid temperature, flowrate and PCM capsule tilt angle on freezing and melting times. The results showed that the freezing and melting times were dependent on not only the surface area to volume ratio, but also on the distance from centroid to inner capsule surface. Importantly, the red blood cell shape was found to be the most favourable geometry because it yielded the most rapid freezing and melting process, which were further enhanced in a configuration where the freestream flow directly impinged onto its centroid. The work is a pioneering study on the effects of PCM capsule geometry, providing experimental data and recommendations for future studies on CTES and applications.