Solidification performance improvement of phase change materials for latent heat thermal energy storage using novel branch-structured fins and nanoparticles

In the present study, we propose the combination of novel branch-structured fins and Al₂O₃ nanoparticles to enhance the performance of a phase change material (PCM) during the solidification process in a triple-tube heat exchanger. The inevitable drawback of PCMs is their lower heat conductivity, which can result in a long response time during the phase change process in latent heat thermal storage systems. Therefore, any serious improvement strategy needs an optimized phase change process. A mathematical model for a two-dimensional structure composed of a PCM with paraffin RT82 and Al₂O₃ nanoparticles that considers the thermal conduction in metal fins, Brownian motion of nanoparticles, and natural convection in a liquid phase PCM is proposed and verified based on experimental results. The impact of various volume fractions and fin layouts on the solidification process is discussed, involving the evolution and deformation of solid–liquid interfaces and distribution of isotherms and average temperature and liquid fraction curves. The results imply that the solidification behaviour can be significantly enhanced by the application of nanoparticles and metal fins. Compared with the inherent structure of the heat exchanger, the solidification time is decreased by 8.5%, 9.3%, and 10.3% for Al₂O₃ nanoparticles (at 2%, 5%, and 8%, respectively) only and by 83.0%, 80.7%, 80.8%, and 82.9%, respectively, for various fin layouts only. This is attributed to increased heat transfer by thermal conduction and natural convection. It can be concluded that the impact of the use of fins is preferable compared to that for nanoparticles, and the benefit of nanoparticles is limited.