XRD investigation of phase separation stability of supercooled sodium acetate trihydrate composites for thermal energy storage

Abstract

Sodium acetate trihydrate (SAT) is a promising phase change material for thermal energy storage, utilizing its stable supercooling properties. However, long-term supercooling stability and heat of fusion are compromised by phase separation, which remains a significant challenge. While numerous studies have explored additives to mitigate phase separation, these efforts have largely focused on fresh, lab-grade SAT over short durations. The long-term behavior of non-lab grade SAT, particularly in storage application with additives, remains underexplored. This study addresses this gap by investigating the phase separation stability of supercooled food-grade SAT (FGSAT) composites. Using X-ray diffraction (XRD) analysis, the stability of these composites over a period of three years has been tracked. The evolution of crystal structures in expired food-grade SAT (FGSAT) was analyzed by adding water, reducing sample size, and incorporating additives. The findings revealed that crystal orientation was the key indicator for distinguishing phase separation in FGSAT. Neither the addition of water nor the reduction of sample height altered the crystallographic orientation of either unexpired or expired FGSAT, which remained as position (−402). However, when thickeners such as carboxymethyl cellulose (CMC) or a polymeric solution was added, the preferred orientations of expired FGSAT shifted to (−402) and (110), respectively. Samples taken after cyclic testing in a thermal storage prototype demonstrated that the combined preferred orientation (110) of FGSAT/HD310 composites remained stable. This study offers a promising approach to track SAT phase separation during supercooled state by focusing on crystallization orientation, providing valuable insights for long-term heat storage and optimizing SAT composites. • The phase separation stability during stable supercooling of food-grade SAT composites was investigated • The crystal orientation of the solidified material was found as a reliable indicator for identifying suitable SAT composites • 19 material categories have been used – including SAT composites with extra water and stabilizing additives • Different stabilization effects in food grate SAT composites with thickening agents and liquid polymeric solutions

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