Thermal stability of sodium nitrate microcapsules for high-temperature thermal energy storage

The NaNO3 microcapsules (MCP-NaNO3.2) with good thermal stability were prepared by twice repeating the process of microencapsulation. The, thermal stability of MCP-NaNO3-2 was tested by thermal cycles and high temperature heating in the furnace. The thermal properties, such as latent heat, melting point, supercooling degree, were determined by thermogravimetric analyzer (TG) and differential scanning calorimetry (DSC). The microcapsule morphologies were observed by optical microscope and scanning electron microscope. After 80 thermal cycles mainly between 250 and 350 degrees C, the latent heat of MCP-NaNO3-2 only had a small reduction of about 3.6%, the melting point of MCP-NaNO3-2 only decreased by about 1 degrees C, and the supercooling degree of MCP-NaNO3-2 slightly fluctuated within about 2 degrees C. Additionally, the thermal properties of the MCP-NaNO3-2 heated for 5 h at 350, 450, 500, and 550 degrees C showed that the latent heat of MCP-NaNO3-2 was relatively stable until 500 degrees C, from 136.1 to 130.3 J/g, but sharply decreased to 110.2 J/g at 550 degrees C. It is concluded that MCPNaNO3-2 has good thermal stability in thermal cycles and at the high temperature up to 500 degrees C and has a good potential for high-temperature thermal energy storage.