The Thermal Cycle Stability Of Three Hydrated Salt Phase Change Materials Is Improved

In this paper,the supercooling and phase separation of CaCl₂?2H₂O,Na₂HPO₄·12H₂O groups and the hydrated salt composite phase change materials of MgSO₄?7H₂O and NH₄Al（SO₄）₂?12H₂O were mainly studied to improve the thermal storage performance and recycling stability of the materials.The research of these three hydrated salt phase change materials is of great significance for energy conservation and emission reduction,low carbon environmental protection,and solar energy utilization.Among them,the phase change material based on CaCl₂?2H₂O whose melting temperature are20-30°C.Studies have shown that when the mass fraction of CaCl₂?2H₂O,purified water,Mg Cl₂?6H₂O is 56.86%,42.22%,0.92%,the crystallization temperature of the composite phase change material is 20.7°C,the water bath fastly melt at 25-30°C,and the supercooling degree is 2°C.The results of this material test showed that total endothermic enthalpy reducts 33%after 150 hydartion-dehydration cycles,and there is no phase separation phenomenon.The supercooling degree of this material temperature maintains between 1.9 and 5.3°C,and cooling phase change crystallization reduced from 20.7°C to 17.4°C.The XRD analysis results shown that the composite material only contains the diffraction peak of CaCl₂?2H₂O and Mg Cl₂?6H₂O.The composite phase change material,Na₂HPO₄·12H₂O as the main material and KOH as the base material,whose melting temperature is 40-50°C.The each of mass fractions of Na₂HPO₄·12H₂O,KOH and water are 76.93%,7.69%and 15.38%.Its supercooling degree maintained at 0.7-5.4°C,cooling phase transition temperature is 42°C and total endothermic enthalpy value reduced by 6.6%after 100 hydartion-dehydration cycles.XRD characterization showed that the composite material was composed of Na₂HPO₄·12H₂O,KOH,K₂HPO_4,and Na OH.During the experiment,with the increased of hydroxypropyl methylcellulose（HPMC）from 0 to 0.3g,its supercooling degree decreased from 13.6°C to 3.2°C,and the content of distilled water and KOH can solve the phase separation problem of this material.MgSO₄?7H₂O and NH₄Al（SO₄）₂?12H₂O are used as composite systems,and the melting temperature is 70-80°C.MgSO₄?7H₂O and NH₄Al（SO₄）₂?12H₂O ratio of 5:5 when the composite material subcooling degree is the lowest,close to 0°C,there isn’t phase separation phenomenon arising,phase change crystallization temperature is 70.5°C,20-200°C total phase change enthalpy is 962.77k J/kg.The XRD results shown that this material only contains the diffraction peak of MgSO₄?7H₂O and NH₄Al（SO₄）₂?12H₂O.After the material underwent 150solidification-melting cycles,there is no phase delamination,the phase change crystallization temperature is still around 70.5°C,the supercooling degree is 0.7°C at the end of the 150th cycle,the total enthalpy of the phase change at 20-200°C dropp to 680.61 k J/kg,and the total enthalpy is reduce by 29.3%.The three composite phase change materials that have never been seen before.The phase change materials base on CaCl₂?2H₂O can be apply in greenhouses,and preliminary tests have been conducted for the application of materials in greenhouses.The phase change material base on Na₂HPO₄·12H₂O can be selected for northern winter soil kang heating,floor radiant heating.The phase change materials with MgSO₄?7H₂O and NH₄Al（SO₄）₂?12H₂O composite systems can be used in living water tanks,solar collectors.It is great significance for energy conservation and emission reduction,low carbon and environmental protection.