Unconstrained Melting Heat Transfer of Nano-Enhanced Phase-Change Materials in a Spherical Capsule for Latent Heat Storage: Effects of the Capsule Size

Toward accelerated latent heat storage, the unconstrained melting heat transfer in spherical capsules was revisited experimentally in the presence of nano-enhanced phase-change materials (NePCMs), with an emphasis on the influence of capsule size on the rates of melting, heat transfer, and latent heat storage. It was shown that increasing the size of the spherical capsule leads to two competing effects, i.e., thicker molten layer in the close-contact melting (CCM) region and stronger natural convection. However, the NePCM with a high loading (3 wt % graphite nanoplatelets (GNPs)) is not preferred for all capsule sizes as a result of the significantly deteriorated heat transfer in both CCM and natural convection, because the dramatic viscosity growth at such a high loading leads to increased thermal resistance across the molten layer and loss of natural convection that overweigh the increased thermal conductivity. The 1 wt % NePCM sample was exhibited to be able to facilitate latent heat storage for two cases, i.e., in the smallest capsule having a radius of 14.92 mm at a higher wall superheat of 30 °C and in the intermedium 24.85 mm capsule at a lower wall superheat of only 10 °C. It was suggested that a relatively low loading of a specific NePCM can cause a faster rate of latent heat storage over the baseline case of the matrix phase-change material (PCM), if the capsule size (and the wall superheat) can be adjusted properly to regulate the molten layer thickness and the intensity of natural convection.