Long-term assessment of the thermal stability of sodium nitrate-urea eutectic phase change material

The eutectic mixture formed by urea and sodium nitrate can be an interesting candidate for use as a phase change material for thermal energy storage in space heating and domestic hot water applications. It shows a melting point of 85 °C, a melting enthalpy of 172 J/g and a price around 0.9 €/kg. However, the thermal stability of the mixture is a great concern for this application. A preliminary evaluation of the thermal stability was performed and previously reported by the authors. It consisted of an accelerated thermal cycling test with 210 thermal cycles and the material showed a stable behavior. Nevertheless, the long-term stability of urea in the liquid state at temperatures below 100 °C is uncertain and requires a specific study. The main objective of the present work is to evaluate the long-term thermal stability of the mixture when it is exposed to long periods of use under conditions representative of actual applications, by means of analyzing the thermal and compositional behavior of samples remaining at 100 °C for several periods up to one year. A methodology is proposed, which intends to isolate the thermal degradation phenomenon from others, such as phase segregation, supercooling, and polymorphism, that can be introduced by thermal-cycling. It also aims to be more representative of the actual application than the accelerated thermal cycling approach.