Nano-PCM filled energy storage system for solar-thermal applications

In this paper, a nano-PCM filled enclosure, which is a representative geometry of a thermal energy storage (TES) system, is investigated using scale analysis, numerical simulation, and experimental analysis. The enclosure is assumed to be square in shape. It is also assumed that one vertical wall of the enclosure is actively participating in absorbing energy from a source while the remaining walls are insulated. The thermal boundary condition at the active wall is treated as 'constant heat flux boundary condition' in this paper. The energy absorbing material, i.e., the nano-PCM, is CuO nanoparticles dispersed in coconut oil PCM. The influence of the volume fraction of nanoparticles (0≤φ≤5%) is investigated on the flow and thermal fields, heat transfer rate, energy stored and liquid fraction during the melting process of nano-PCM at different values of Rayleigh number based on base PCM (104≤Raφ=0%≤108). The Rayleigh number is adjusted by adjusting the size of the enclosure (i.e., higher Ra represents the larger enclosure). In addition to the isothermal lines and velocity vectors, heatlines are utilized to exhibit the energy flow patterns inside the enclosure during the melting process. Besides the numerical calculations, scale analysis is presented to demonstrate the different stages of melting process of nano-PCM. The detailed scale analysis assists to identify relationship of Nusselt number and solid-liquid interface location as a function of well established dimensionless numbers: Stefan number (Ste), Fourier number (Fo), and Rayleigh number (Raφ=0%). Finally, an experimental setup is developed to visualize the melting process of nano-PCM inside a prototype enclosure. Experiments are conducted to illustrate the impact of adding nanoparticles into PCM on the melting process. The numerical and experimental results show the significant improvement of the melting process by adding nanoparticles to PCM.