Insights into the thermophysical properties and heat conduction enhancement of NaCl-Al2O3 composite phase change material by molecular dynamics simulation

Abstract

In concentrated solar power (CSP) applications, the thermal energy storage (TES) system with molten salt is widely utilized. However, molten salts usually suffer from relatively low thermal conductivity. In this regard, adding nanoparticles has been considered as an effective way to improve the thermal conduction of molten salt. In the present work, the thermophysical properties of NaCl-Al2O3 composite phase change material (PCM), including the melting point, specific heat capacity, melting enthalpy, self-diffusion coefficient as well as thermal conductivity, are first examined by using molecular dynamics simulations. Simulation results show that the thermal conductivity of base salt NaCl can be significantly improved by adding Al2O3 nanoparticles. An enhancement of 17.94% has been observed with 5.565 wt% addition of Al2O3 nanoparticles. The results also suggest the doped nanoparticles can reduce the melting point and melting enthalpy but increase the specific heat capacity of base salt. The increase in self-diffusion coefficient of composite PCM with temperature indicates the existence of micro-convection around nanoparticles in base salt. Furthermore, the microscopic mechanism of thermal performance enhancement by adding nanoparticles into molten salt has been revealed. The results indicate that the interfacial thermal resistance is greatly related to the type of base salt and nanoparticles. Based on energy analysis, it is concluded that the variation of ion collision frequency in base salt due to the addition of nanoparticles plays a major role in the enhancement of thermal conductivity of NaCl-Al2O3 composite phase change material.