Optimization of nanoparticles concentration inside dynamically adjusted latent heat thermal energy storage system to augment phase change processes

Abstract

This paper reports an extensive analytical study on the effects of incorporating nanoparticles on heat transfer during phase-change processes in Latent Heat Thermal Energy Storage Systems (LHTESS). In the first part, an analytical correlation to quantify the unidirectional phase change duration and average heat transfer rate inside a finite-dimensional geometry for a wide range of Stefan numbers (Ste) is developed. A new correlation is required as existing correlations are limited to smaller Ste values, and Ste increases steadily with the addition of nanoparticles. In the second part, an analytical expression is developed to quantify the effects of nanoparticles on unidirectional phase change processes, using the correlations developed in the first part, and validated using numerical results. In the third part, the correlations for optimal nanoparticles volume fractions, corresponding maximum enhancement in the heat transfer rate, and reduction in the time for unidirectional phase change processes are developed. The addition of the nanoparticles in Phase Change Materials (PCMs) reduces the volumetric heat capacity. Therefore, in the current study, the length of the LHTESS is adjusted dynamically to maintain the constant thermal capacity of the system, which increases the total volume of the system. Furthermore, the critical thermal conductivity ratios of nanoparticles to PCM necessary for heat transfer enhancement in 1-D phase change processes are obtained. The proposed correlations are applicable both for melting and solidification processes. Nanoparticles can theoretically enhance the mean heat transfer rate by up to 200% and reduce unidirectional phase change duration by up to 67%.