Experimental investigation on triggering solidification of supercooled sodium acetate trihydrate with extra water and different thickening agents by local cooling

Abstract

Addition of extra water or thickening agents can reduce the phase segregation of salt hydrate phase change materials, but may influence the supercooling stability and crystallization of the mixture solution for supercooled thermal storage. Triggering solidification of supercooled sodium acetate trihydrate (SAT) is experimentally investigated by local cooling method (using a 72 W semiconductor refrigerator) with the addition of extra water as well as 7 thickening agents, i.e. carboxymethyl cellulose (CMC), gelatin, urea, starch, polyvinyl alcohol (PVA), polyacrylamide (PAM) and xanthan gum (XG). The water content in SAT water solution ranges from 40.01 % (only crystal water content, no extra water) to 50 % (9.99 % extra water). The mass ratio of thickening agents to SAT is between 0.25 % and 3 %. The results show that 44 % water content SAT solution can effectively alleviate the phase segregation and also has relatively short induction period for crystallization initialization with the discharging temperature as high as 51.8 °C. Among the 7 thickening agents, gelatin, CMC, starch and XG present better performances than urea, PVA and PAM for alleviation of phase segregation. Each additive has its suitable mass proportion considering the triggering and discharging performances. With the same local cooling power of 72 W, SAT samples containing 1 % gelatin present the highest discharging temperature (Tdis) of 55.8 °C as well as shorter induction period (τin) of 1.1 min; 2 % CMC samples have the discharging temperature as high as 55 °C and induction period of 11 min; the amount of starch has slight effect on triggering performances and the relatively better proportion is 0.25 % with Tdis = 52 °C and τin = 1.6 min; the suitable XG content is 0.5 % with Tdis = 49.2 °C and τin = 7.3 min under present conditions. The obtained results are helpful in selecting SAT additives for phase segregation reduction, better triggering and discharging performances based on supercooled thermal storage.