A comprehensive study of the phase segregation of a urea-based phase change material tested under thermal cycling conditions

• Published in: Journal of energy storage 2023-04, Vol.60, p.106621, Article 106621
• Main Authors: Quant, Laura, Diarce, Gonzalo, Bouzas, Lourdes, García-Romero, Ana
• Format: Article
• Language: English
• Subjects: Eutectic; Material characterization; Phase change material PCM; Phase segregation; Thermal energy storage TES
• ISSN: 2352-152X
• DOI: 10.1016/j.est.2023.106621

Eutectic mixtures are used as PCMs due to the possibility to tailor the melting temperatures and mainly because the phase transition occurs at a unique temperature. Eutectic mixtures are assumed to be congruent melting and solidification materials. Compositional segregation has rarely been reported, when have been researched for its use as PCM. However, the previous premise does not always match the observed facts. The presented work aims to deepen the knowledge regarding the use of eutectic mixtures as PCMs and to determine the influence of operation parameters in the eutectic PCMs potential phase segregation. 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. Nevertheless, the eutectic mixture showed an unforeseen segregation phenomenon when it was exposed to repeated melting-solidification cycles using volumes in the scale of grams. As a result, the phenomenon was studied to determine the potential causes. An experimental campaign was performed to study the urea and sodium nitrate eutectic mixture under different conditions: consisting of thermal cycling using representative masses, and subsequently, the segregated materials and obtained samples were analyzed by different techniques (including XRD, HTXRD, and DSC); and the production of samples under different cooling conditions that were analyzed using microscopy (PLM and SEM). The results established a relationship between the operation conditions, with the resulting crystal structures, which explain the phase segregation in the eutectic mixture. A mitigation measure was determined consisting of mechanical stirring. •The urea and sodium nitrate eutectic mixture suffers phase segregation after repeated melting-and-solidification cycles.•The microstructural observation of the system shows that not all the eutectic mixtures behave as congruent materials.•The phase segregation of the eutectic is directly related to the non-reciprocal nucleation, shown in all the studied conditions.•Higher cooling rates showed smaller segregated volume than slower ones. Although, the complete prevention seems unfeasible.•The use of this material as a PCM is feasible if a proper stirring procedure is implemented in the thermal storage system.