Experimental investigation on discharging characteristics of supercooled CH3COONa·3H2O-Na2S2O3·5H2O mixtures triggered by local cooling with Peltier effect

Abstract

Latent heat stored in the supercooled salt hydrate liquid can be released at the desired time by triggered crystallization. In this paper, the melted sodium acetate trihydrate (SAT, melting point Tm = 55.7 °C) and sodium thiosulfate pentahydrate (STP, Tm = 48.9 °C) are cooled by natural convection of room air with respective supercooling degrees of 33 °C and 28 °C. Experiments are performed on the effect of local cooling by semiconductor refrigerator on triggering crystallization of supercooled SAT, STP and their binary mixtures with different mass ratios. The results show that SAT has higher discharging temperature and longer discharging time than STP; but STP has less phase separation, better cycling stability and shorter induction period during the crystallization than SAT after repeated melting-freezing cycles. Moreover, the phase separation problem of SAT could be significantly alleviated with the addition of STP. With the increase of SAT content in the SAT-STP mixtures, the induction period during the crystallization tends to firstly increase and then decrease. The tested curve of the maximum discharging temperatures presents as a V-shaped trend and roughly corresponds with the binary phase diagram. The eutectic mixture of SAT-STP (mSAT:mSTP = 28:72, the eutectic temperature of 40.8 °C) can be triggered in 4 min and shows a discharging temperature peak measured at 39 °C. The discharging time of 3.3 h with temperature above 30 °C is longer than that of single-component salt hydrates. The obtained results are helpful for system design and practical application of solar energy storage based on supercooled phase change materials.