Novel Na2SO4-NaCl-ceramic composites as high temperature phase change materials for solar thermal power plants (Part I)

The eutectic Na2SO4-NaCl salt was investigated as a novel high temperature phase change material for solar thermal energy storage due to its low cost and melting point (626.0 degrees C) interested. The measured melting point and heat of fusion of the eutectic salt are 625.3 degrees C and 263.3 J/g, which are in agreement with their theoretical values. The eutectic salt shows good thermal stability after 100 melting-freezing thermal cycles in the range of 550-680 degrees C in air. Three types of the eutectic Na2SO4-NaCl-ceramic composites have been designed and evaluated to address the salt' corrosion issues with containment materials. Results show that eutectic Na2SO4-NaCl/both alpha-alumina and mullite composites with a mass ratio of salt to ceramics of 1:1 (wt%) have good thermal stability without cracking as well as with weight losses of 0.50% and 0.74% and the decrease of latent heat by 1% and 2% respectively after 20 thermal cycles, where no phase separation and chemical reaction occur. Their measured melting temperature and latent heat are consistent with their theoretical values. Addition of alpha-alumina and mullite to the eutectic Na2SO4-NaCl salt can reduce the eutectic Na2SO4-NaCl salt' supercooling. In contrast, the sample of the Na2SO4-NaCl/diatornite composite cracked after the cycles.