Experimental investigation and phase separation elimination of Na2SO4·10H2O and KAl(SO4)2·12H2O mixtures for thermal energy storage

Phase change materials (PCMs) have become a new strategy to deal with energy crisis because of their thermal storage properties. As an important inorganic PCMs, hydrated salts have advantages including high latent heat, moderate thermal conductivity, and non-flammability. Na2SO4·10H2O and KAl(SO4)2·12H2O have great potential in thermal energy storage fields including solar energy utilization, building energy conservation and waste heat recovery. However, single hydrated salts have defects including fixed phase change points and phase separation, and the related mechanisms on phase separation elimination of hydrated salts are still lacking. In this work, the melting and solidification process of several Na2SO4·10H2O and KAl(SO4)2·12H2O mixtures was investigated to solve this problem. Phase change points could be adjusted by the compositions change of hydrated salts, and phase separation could be eliminated intrinsically when mass ratio of Na2SO4·10H2O is low than 50%. Besides, the morphology and structure characterization confirmed that Na2SO4·7H2O and anhydrous Na2SO4 are not coexist in the change of phase process, and the formation of anhydrous Na2SO4 is the cause of phase separation. Furthermore, the mechanism of phase separation elimination was interpreted as that Na2SO4·7H2O formed during the cooling process can not only fixed lots of water, but also provide the thickening effect to avoid the phase separation. When the mass ratio Na2SO4·10H2O is 30%, the hydrated salt mixtures have three melting points at 36.0 °C, 75.4 °C, and 82.9 °C, high latent heat of 256.2 J/g. This hydrated salt mixtures have great potential to apply in thermal energy storage fields, and the formation of metastable substance could provide novel methods to solve phase separation of hydrated salts.