Phase Change Materials (PCMs) are substances that store and release thermal energy through reversible solid-liquid phase transitions. In recent years, PCMs have received increased attention as effective components in thermal energy storage systems. Using PCMs across different technologies can improve thermal energy storage, reduce dependence on conventional energy sources, lower greenhouse gas emissions, and support global sustainability and energy efficiency objectives. These materials can maintain a stable temperature during phase transitions while absorbing or releasing latent heat. This characteristic enables PCMs to regulate temperature changes, making them suitable for applications such as passive thermal storage, solar energy systems, and building energy optimization. In building envelopes, they improve thermal inertia, reducing indoor temperature variations and heating or cooling demands. The same goes for solar energy systems; PCMs store excess solar heat during peak periods and release it when energy demand rises, improving overall system efficiency.
PCMs are classified into three main categories: organic, inorganic, and eutectic materials. Organic PCMs, like paraffins and fatty acids, are characterized by their chemical stability; they are non-corrosive and have congruent melting points, but they have low thermal conductivity and low latent heat in certain high-demanding applications. Inorganic PCMs, especially hydrated salts, offer high latent heat storage capacity and improved thermal conductivity. This particular aspect makes them attractive for medium- or high-temperature energy storage. Aside from these advantages, inorganic PCMs often have drawbacks such as phase separation, supercooling, or corrosive behavior. Eutectic PCMs combine two or more components to achieve specific melting temperatures and improved thermal stability, effectively merging the advantages of both organic and inorganic materials.
In this paper, a comparative study is presented about the most common studied inorganic hydrated salts, such as: magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), lithium nitrate trihydrate (LiNO₃·3H₂O), sodium sulfate decahydrate (Na₂SO₄·10H₂O), calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O), calcium chloride hexahydrate (CaCl₂·6H₂O), magnesium chloride hexahydrate (MgCl₂·6H₂O), sodium acetate trihydrate (CH₃COONa·3H₂O) and their eutectic mixtures. These materials exhibit melting temperatures ranging from 29 °C to over 100 °C, making them suitable for a wide range of thermal applications. For example, calcium chloride hexahydrate melts around 29 °C with a latent heat of approximately 190 kJ/kg but presents corrosion and supercooling issues. Sodium sulfate decahydrate offers a higher latent heat (about 230 J/g) and a melting range of 30–35 °C, though it is exposed to phase separation and leakage. Sodium acetate trihydrate has a melting temperature of approximately 58 °C and a high latent heat of 264 kJ/kg, but can exhibit supercooling and segregation phenomena. Magnesium-based hydrated salts, such as magnesium nitrate and magnesium chloride hexahydrates, operate over a wider temperature range and provide improved thermal storage capacity, although supercooling remains a significant limitation.
Improving property stabilization is a common method used for phase change material systems. Expanded graphite used as an additive was reported to improve thermal stability and heat transfer performance. Microencapsulation is a method used to prevent leakage during phase transitions and to improve thermal reliability.
To establish the properties of the considered hydrate salts, this paper analyzed: DSC for determining melting temperatures and latent heat values; TGA for thermal stability and degradation behavior; SEM for structural and morphological analysis; and XRD and FTIR to confirm chemical composition and compatibility between the PCM cores and the encapsulated materials.
In conclusion, inorganic hydrated salts and their eutectics have significant potential for thermal energy storage and energy efficiency applications due to their high latent heat and favorable thermal properties. Single or eutectic formulations presented in this paper represent an important research focus for enhancing their performance and enabling the implementation of PCMs in sustainable energy systems.
