Performance evaluation of shell-tube latent heat storage unit using nanoparticles with cascaded concentration

The low thermal conductivities of phase change materials (PCM) and decreasing temperature gradient in heat flow direction during phase change process significantly limit the charging and discharging rate of latent heat storage (LHS) unit. In this study, multi-layer nanoparticles-enhanced PCMs (NEPCMs) with different nanoparticles concentrations arranged in the radial direction was proposed to overcome the above-mentioned drawback. Numerical simulation was conducted to examine the evolution of solid-liquid interface, temperature and velocity during the phase change process. Results show that using multi-layer NEPCMs with negative-gradient nanoparticle concentration provides the best melting and solidification performances, followed by the uniform and positive-gradient ones. Two-layer NEPCMs unit provides better the melting performance and nearly the same solidification performance compared with three-layer NEPCMs unit. Compared with single-layer NEPCMs unit, two-layer NEPCMs unit with the negative-gradient nanoparticle concentration reduces the melting and solidification times by 12.5% and 26.5% respectively. It is also found that the enhancement potential of the negative-gradient case increases with the increase of temperature difference between HTF and PCM melting point.