Thermal conductivities and characteristics of ternary eutectic chloride/expanded graphite thermal energy storage composites

Ternary eutectic chloride (NaCl-CaCl2-MgCl2)/expanded graphite (EG) composites were prepared for thermal energy storage applications at a solar thermal power plant. Heat capacity and latent heat thermal energy storage (LHTES) characteristics of the composites including the melting temperature and latent heat capacity were investigated using a differential scanning calorimetry (DSC) technique, and the effects of EG additives in the composite on thermal conductivities were evaluated using a hot disk analyzer. The ternary eutectic chloride/EG composites with expanded volumes ranging from 150 to 250 g/L and the EG mass fractions ranging from 0 to 5 wt% were prepared by absorbing liquid chlorides into the EG at high temperature. Experimental results indicated that the specific heat capacity of solid composites decreased with EG mass fraction and temperature. In the liquid state, the effect of the EG loading on the specific heat was not uniform. Specific heat capacity reached maximum at 1 wt% of EG loading and the specific heat capacities of all samples rose with temperature. The melting temperatures of the composites were the same as the pure ternary eutectic chloride, but the phase change latent heat decreased with the mass percentages of EG in the composites. The thermal conductivities of the composites were 1.35-1.78 times higher than that of the pure ternary eutectic chloride.