Thermal properties of a novel medium temperature thermal energy storage composite based on sodium nitrate as phase change material

Thermal energy storage (TES) has been one of the main research topics during the last decades. The increase of energy costs and the growth of awareness of environmental concerns have promoted the development of new technologies and new materials for energy storage. Developing new materials with high storage density, thermal stability and low cost would reduce energy cost and improve the energy efficiency in wellknown and established processes by recovering and storing heat. Moreover, it would solve the renewable intermittence. The aim of this study is to formulate a NaNO3/MgO/Cg composite for intermediate temperature TES applications. Several compositions with different amounts of NaNO3 as PCM, MgO as the ceramic matrix and graphite as thermal conductivity enhancer have been formulated. The main properties related to thermal energy storage applications have been characterized: melting point, latent heat, specific heat, thermal conductivity and thermal expansion coefficient. After being cycled the microstructure of the composite was characterized as well as their thermal properties in order to study the stability when they are charged and discharged at working conditions. The results show that the melting point of the PCM remains around the pure molten salt, the latent heat increases with the PCM content, as expected, as well as the energy density of the composite. Combining the effect of Cg addition and the insertion of the PCM in a ceramic matrix a novel TES material with enhanced thermal conductivity has been developed.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016