Sodium nitrite-sodium nitrate eutectic based composite phase change material (CPCM) for medium-temperature thermal energy storage (TES): Exploring the relationship between microstructures and thermal properties

This paper is concerned with a novel medium-temperature composite phase change material (CPCM). More specifically, the CPCM contains a sodium nitrite‑sodium nitrate phase change material for latent and sensible heat storage, magnesium oxide as a ceramic matrix material for shape-stabilisation and sensible heat storage, and expanded graphite as a thermal conductivity enhancer material (TCEM) to improve heat transfer. The focus is on understanding the relationship between the CPCM microstructures and their thermal properties. This CPCM was found to be unique, exhibiting both a reversible solid-solid transition (≈175 °C) and a reversible solid-liquid transition (≈223 °C), with a total latent heat of fusion of ∼120 J/g. In-depth microstructural characterisation of the CPCM indicated adequate thermal stability during thermal cycling in air up to 350 °C, which was also confirmed by their stable latent heat and thermal conductivity. The results highlighted a correlation between microstructures of the CPCMs and their thermal properties, as well as potential thermal decomposition reactions in high temperature industrial applications of molten salts such as concentrating solar power. • Relationship between microstructures and thermal properties of inorganic salt CPCM explored. • Effect of thermal cycling on material performance investigated. • Solid-solid phase change of inorganic salt compounds found credible for TES. • Greater latent heat of CPCM observed than that of solar salt based PCM. • Thermal decomposition mechanisms of NaNO 2 in an open system elucidated.