The sodium nitrate–urea binary mixture as a phase change material for medium temperature thermal energy storage. Part I: Determination of the phase diagram and main thermal properties

This research studies a binary mixture composed of sodium nitrate and urea for use as a phase change material in latent thermal energy storage systems between 75 and 90 °C. This range of temperatures is suitable for heating and domestic hot water applications, such as solar thermal systems and cogeneration devices. The work is divided into two parts. In this Part I, the phase diagram of the system was experimentally determined via DSC, the synthetic visual method and X–ray diffraction. A eutectic composition was found at 71.25% (w/w) urea and 28.75% (w/w) sodium nitrate. The main thermal properties and density of this composition were studied in detail to evaluate its potential as a PCM. The results showed a melting temperature of 85 °C and a melting enthalpy of 172 J/g. These properties are suitable for the intended purpose. Currently, the mixture presents a market price below 1 €/kg, which is highly competitive. In Part II, the water uptake of the material was evaluated to gain knowledge of its hygroscopic behavior. In addition, because urea undergoes thermal degradation upon heating, a preliminary accelerated thermal cycling test was performed to study the thermal reliability of the mixture. According to the results, the eutectic mixture formed by sodium nitrate and urea has potential for use as a PCM in the intended temperature range. Further research focused on thermal degradation over longer periods of use is encouraged to establish whether the material has a proper lifespan for actual thermal energy storage applications.