Preparation and thermal properties of a novel ternary molten salt/expanded graphite thermal storage material

In thermal energy storage, the use of phase change materials (PCM) is a very efficient energy storage method. In the field of medium temperature thermal storage, nitrate PCM have always been a research hotspot, but their relatively high melting point and relatively low latent heat of phase change severely limit their application in thermal energy storage. In this paper, LiNO3 and NaCl with higher latent heat value were selected to improve the thermal properties of nitrate. A novel ternary mixed molten salt of NaNO3-LiNO3-NaCl with high latent heat was prepared and studied. NaNO3-LiNO3-NaCl/EG composite phase change material (CPCM) was prepared by selecting expanded graphite (EG) with a large thermal conductivity as the carrier adsorption material for molten salt. The thermophysical properties, thermal stability, and heat transfer ability of PCM and CPCM were tested. The composition and microstructure of the materials were also characterized. The experimental results showed that compared with pure NaNO3, the phase change temperature of NaNO3-LiNO3-NaCl was reduced by 45.3 °C, and the latent heat of phase change was increased by 59.4 J/g. NaNO3, LiNO3, NaCl and EG had good compatibility, without chemical reaction between them. From the microscopic morphology of the composite material, it can be seen that the PCM was uniformly dispersed into the porous structure of EG without agglomeration. The ternary new molten salt had higher thermal decomposition temperature and better thermal stability. When the addition ratio of EG was 20 %, the thermal conductivity of the ternary molten salt increased from 1.350 to 5.017 W/m·K. EG greatly improved the heat transfer capacity of PCM. And the PCM added with EG still had a high latent heat of phase change. The results show that NaNO3-LiNO3-NaCl/EG has broad application prospects in the field of medium and high temperature heat storage.