An orthogonal investigation in pore size effect of EG on thermal properties and crystallinity of EG/CaCl2·6H2O composite

Abstract

Porous expanded graphite (EG), a typical carbon-based porous material, is an effective encapsulating material for hydrated salts. Recently, the EG/hydrated salts composites have emerged as promising hybrid PCMs for energy storage and temperature regulation. However, the pore size effect of EG on the thermal properties of the EG/hydrated salts has not been thoroughly investigated. In this work, an orthogonal analysis is carried out to investigate the pore size effect of EG on the absorption of hydrated salts, and the thermal properties as well as crystallization properties of the EG/CaCl2·6H2O composites. The mesh number is determined as the primary factor that affecting the enthalpy and crystal matching rate of the EG/CaCl2·6H2O composites. The EG with larger mesh has smaller pore size, which accounts for greater surface tension and capillary force to absorb the CaCl2·6H2O into the pores of EG. The EG/CaCl2·6H2O with lager EG mesh number exhibits larger latent heat, higher thermal conductivity, and improved crystallization behavior. The optimal EG/CaCl2·6H2O composites with EG (100 mesh, 700 W, 30 s) possesses the largest enthalpy of 167.4 J g−1 and highest thermal conductivity of 3.886 W/(m·K). The thermal conductivity of the optimum EG/CaCl2·6H2O is 6.2 times higher than that of CaCl2·6H2O. Moreover, the subcooling of CaCl2·6H2O was inhibited when it was combined with EG. The phase change gypsum board with 5 wt% optimum EG/CaCl2·6H2O composite shows remarkable temperature-delay performance and high stability after 100 thermal cycles, demonstrating great potential for using as heat storage material.