Optimisation of metastable supercooled liquid phase change material for long-term heat energy accumulation

This research study investigates sodium acetate trihydrate as a metastable supercooled liquid phase change material for long-term heat energy storage and is an efficient evaluation of various sodium acetate trihydrate-to-water ratios and heat exchanger geometries to enhance storage efficiency. Experimental modules with spiral and toroid squiggle heat exchangers were developed to assess energy retention during liquefaction, sensible heat discharge, and latent heat discharge phases. Experimental outcomes indicate that the toroid squiggle design extends latent heat discharge duration by up to 35 min compared to the spiral exchanger, reaching a maximum of 29 min. The optimal sodium acetate trihydrate-to-water ratio was 92 %, balancing high theoretical latent heat capacity (93.6 Wh) and low theoretical to real latent heat capacity ratio with phase stability. The toroid squiggle exchanger with the mentioned accumulation substance demonstrated better heat transfer, maintaining energy output above 100 W for 11 min and above 50 W for 35 min, while the spiral design showed lower values, retaining above 100  W for only 7 min and above 50 W for 26 min. Furthermore, specific heat capacity measurements showed that sodium acetate trihydrate-to-water 92:8 ratio (SAT 92) exhibited specific heat values of 2.1 kJ/kg·K in the solid phase and 5.0 kJ/kg·K in the liquid phase, confirming its strong thermal storage potential with minimal phase instability. The conclusions have highlighted the importance of optimizing heat exchanger geometry and sodium acetate trihydrate composition for sustainable energy storage and these significant insights will contribute to improving seasonal heat accumulation technologies, particularly in synergy with renewable energy systems.