Development of high temperature phase change materials and composites for thermal energy storage

Abstract

The development of cost-effective and reliable high temperature phase change materials (PCMs) is key to successful utilization of solar thermal energy for concentrated solar thermal application. Inorganic eutectic salts relying on their advantages such as low cost, high both melting temperature and heat of fusion, have become good high temperature PCMs candidates. However, their low thermal conductivity and thermal stability as well as their highly corrosive nature on containment materials at high temperatures have been problematic for their applications. In order to solve the problems, development of high temperature salt-ceramic composites is a potential solution for use as high temperature PCMs, where high temperature molten salts as PCMs, are encapsulated by ceramic materials to avoid their corrosion issues with containment materials as well as improve their thermal stability.In this dissertation, novel eutectic Na2CO3-NaCl (59.45: 40.55, wt%) and Na2SO4-NaCl (68.05: 31.95, wt%) salts were firstly designed and investigated as potential PCMs candidates. Their thermophysical properties were measured, decomposition behaviours and long-term thermal stability were conducted to evaluate their performances. The results show that eutectic Na2CO3-NaCl salt is more thermally and chemically stable in a CO2 atmosphere than in a N2 atmosphere as the evolution of CO2 has been found in a N2 atmosphere. Moreover, eutectic Na2SO4-NaCl salt is thermally stable in a N2 atmosphere without weight loss up to 700 degrees Celsius.Three types of cost-effective ceramics which include α-alumina, diatomite and mullite, are investigated as supporting materials to encapsulate eutectic Na2CO3-NaCl and Na2SO4-NaCl salts to prevent their corrosion with containment materials and to improve the thermal conductivity. The results demonstrate that the eutectic Na2SO4-NaCl salt based α-alumina and mullite composites in an optimized weight proportion of 45: 55 are the promising high temperature PCMs for use in solar thermal power plants.