Determination of the phase diagram and main thermophysical properties of the erythritol–urea eutectic mixture for its use as a phase change material

Abstract The present article studies the binary mixture comprised by erythritol and urea for its application as a phase change material (PCM) in latent hat thermal energy storage systems for heating and domestic hot water applications. A sequential methodology was employed to evaluate the suitability of the mixture as a PCM. First, a preliminary evaluation was performed by means of the modeling of the phase diagram and by hot–stage microscopy. Afterwards, the results were refined with more precise techniques, such as Differential scanning calorimetry, the synthetic visual method and X-ray diffraction. The results showed a eutectic composition formed by 54.9% (w/w) erythritol and 45.1% (w/w) urea, with a melting temperature of 81.13 °C and a latent melting enthalpy of 248 J/g. The specific weight of the eutectic composition was measured at different temperatures. The density of the liquid PCM at 95 °C presented a value of 1.282 g/cm 3 and thus, the proposed eutectic mixture shows a volumetric storage capacity of 318 MJ/m 3 . These properties are suitable for the applications aimed. The current market price of the mixture is nearby 2.5€/kg, which can be regarded as reasonable. Since urea undergoes thermal degradation upon heating, a preliminary accelerated thermal cycling test was performed. After 85 thermal cycles, the enthalpy of the eutectic mixture was reduced by 3.6%. In contrast, pure urea showed a decrease of 75% of its initial storage ability. According to the results, the eutectic mixture formed by erythritol and urea can be a good candidate for use as a PCM in medium temperature range applications. Further research is required to evaluate the stability of the mixture on longer periods of use. The methodology employed has proven to be adequate for the development of new eutectic PCMs.