Experimental Investigation On Discharging Characteristics Of Supercooled Salt Hydrate Phase Change Materials Induced By Local Cooling

Based on the supercooled characteristics of salt hydrate phase change materials,in this paper,two kinds of salt hydrates.CH3COONa·3H2O(SAT)and Na2S2O3·5H2O(STP),were experimentally investigated with a flexible and reliable triggering method of local cooling by semiconductor refrigerator.First,by changing the experimental parameters of water content,mass,input power,cooling temperature and CMC content,a series of experiments were conducted to compare and analyze triggering and discharging characteristics of the two salt hydrates in small stainless-steel containers(0.352 L).The results show that the local cooling is an effective method to trigger solidification for heat release of supercooled SAT and STP.Especially,adding extra water and CMC both can alleviate the phase separation problem of SAT,and the water content of 44%is the ideal system ratio with the induction time being of 2 min and maximum discharging temperature of 52.8?,while the two methods both inhibit the induction process of STP with poor modification.The probability of heterogeneous nucleation increases along with the increase of sample mass,the induction time is shorter with higher discharging temperature and longer heat release time.The higher the input power(P)is,the faster the unit is cooled,the induction time is shorter.The higher the cooling temperature,the greater the sensible heat of PCMs,the higher the exothermic temperature but the longer the induction time.Then,combined with phase separation and discharging characteristics of SAT and STP,supercooled mixtures of SAT-STP with different mass ratios were prepared and examined according to complementary principle.The results indicate that the phase separation problem of SAT has been significantly alleviated with the addition of STP.With the increase of SAT content in mixtures,the induction time period tends to firstly increase and then decrease.The trend of maximum discharging temperature curve roughly corresponds with the binary phase diagram.Furthermore,the binary mixtures of salt hydrates expand the phase change temperature range of PCMs and are potential PCMs in supercooled thermal energy storage.In addition,a stainless steel rounded-rectangular thermal energy storage unit(1.83 L)was used as an experimental object to study the practicability of pure SAT,STP and their eutectic mixture(without external additive)induced by semiconductor cooler.The results demonstrate that the triggering method of local cooling can successfully activate solidification for energy release of single salt hydrates in the large-capacity thermal storage unit.With the increasing of the input power of the semiconductor cooler,the overall crystallization induction period decrease.However,the poorer the crystallization rate,the slower the response of temperature measurements,especially for the one located far away from the operation position.And the discharging temperature in the unit center is higher and lasts longer than in other positions.But the eutectic mixture in the large-capacity storage unit is less likely to be successfully induced.