Experimental and numerical study of modified expanded graphite/hydrated salt phase change material for solar energy storage

Abstract

Solar thermal storage system has strict requirements on the problems of hydrated salt phase change materials (PCMs) such as supercooling, phase separation, low thermal conductivity and cyclic stability. In addition, the long-term storage/release heat cycle of PCMs lead to the loss of crystallization water and shorten the service life. Herein, we demonstrate an improved model to predict the thermal conductivity of modified carbon-based matrix form-stable PCMs based on the relationship between porosity (ε) and modified expanded graphite (MEG) content. The proposed new model may predict the thermal conductivity of carbon-based matrix composites with relatively high precision. Furthermore, the hydrophilic and hydrophobic groups on the surface of EG sheet have been rearranged via titanate coupling agent, and then MEG is introduced as high thermal conductivity porous matrix. The modified aluminum potassium sulfate dodecahydrate (MAPSD) is used as the PCM. Five different EG contents of MEG/APSD composite PCM samples with the same density have been prepared by the method of “melting blend-solidification and form-stable”. In addition, 80 times storage/release heat experiments are carried out on the samples and the heat transfer process of the three-dimensional (3D) model is numerically simulated. It results show that the weight loss of the composites with MEG samples decreased by only 7.85%. The simulation results suggest that the temperature field and phase change interface vary uniformly. Moreover, the composites samples with MEG has the superior heat transfer performance and much higher heat storage/release rate. With sample density 900 kg/m3 and the MEG content of 20 wt%, the MEG/APSD composites has relatively good thermal performance and high cost performance. Therefore, it can be applied for industrial waste heat energy recovery applications, the solar domestic hot water system and mid-temperature thermal energy storage system.