Nanoparticle-enhanced phase change materials (i.e., nano-PCM) exhibit improved heat transfer and have been extensively investigated because of their potential application to latent heat thermal energy storage systems. However, the sedimentation of nanoparticles is a concern, as observed from numerous experimental investigations, which limits the application of nano-PCMs in an operation with many repetitive cycles. The studies of sedimentation of nanoparticles have been limited to qualitative observations, and the quantitative studies are primarily limited to the sedimentation of nanoparticles with time in nanofluids, with few studies on the sedimentation in nano-PCM. The different potential techniques to quantify the concentration of nanoparticles after each thermal cycle have been discussed. A novel image analysis technique was used to measure the concentration and non-uniformity in the dispersion of nanoparticles after each thermal cycle. The understanding of effect of different thermophysical properties of PCM and nanoparticles on the stability of the nano-PCM is essential for the development of stable nano-PCM for their practical applications. The present study analyzes the effect of size, density, and concentration of nanoparticles, along with the effect of viscosity, density, and difference in the density in solid and liquid phase of PCM on the stability of nano-PCM. Two different concentrations of copper oxide (<50 nm) and iron oxide (50-100 nm) nanoparticles in Rubitherm35 HC and CuO in coconut oil were studied. The larger density difference in the solid and liquid state of PCM leads to higher sedimentation. The effect of particle size dominated over nanoparticles density on sedimentation process.
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