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 °C) interested. The measured melting point and heat of fusion of the eutectic salt are 625.3 °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 °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 α-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 α-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/diatomite composite cracked after the cycles.