Performance of novel Na2SO4-NaCl-ceramic composites as high temperature phase change materials for solar power plants (Part II)

Abstract

The performance of the eutectic Na2SO4-NaCl-α-alumina and-mullite composites (melting point of 626.0 °C) with different salt to ceramic ratios (45:55, 50:50, 55:45, 60:40, wt%) was investigated to evaluate their potential application as novel high temperature (> 600 °C) phase change materials for solar thermal energy storage. The results show that the thermal stability of these composites decreases with increased salt content. The eutectic Na2SO4-NaCl-α-alumina and- mullite (45:55, wt%) composites show better thermal performance than the composites with other mass ratios after 20 thermal heating-freezing cycles at the temperature range of 550–680 °C in air. 300 thermal cycle tests were therefore carried out with each of these two composites to evaluate their longer-term thermal stability. The results show that the cycled Na2SO4-NaCl/α-alumina (45:55, wt%) composite has a weight loss of 4.6% and its heat of fusion decreases from 117.8 J/g to 105.3 J/g while the cycled Na2SO4-NaCl/mullite (45:55, wt%) composite has a weight loss of 6.9% and its heat of fusion decreases from 117.6 J/g to 98.6 J/g. XRD and SEM-EDS analysis confirm that the eutectic Na2SO4-NaCl salt has good chemical compatibility with α-alumina and mullite ceramic materials, where no chemical reaction and phase separation occurs in the two composites during the 300 thermal cycling test. Addition of α-alumina and mullite (55 wt%) to the eutectic Na2SO4-NaCl salt can improve its thermal conductivity by more than 3 times with α-alumina having a better enhancement than mullite.