Preparation And Performance Of Shapestabilized Hydrated Salt Phase Change Materials

Phase change materials(PCMs)can store and release a large amount of thermal energy at a specific temperature and change from one phase state(solid,liquid)to another phase(liquid,solid).They can store the excess energy and release during peak energy demand.Hydrated salts have a series of advantages such as high enthalpy,high energy storage density,low cost,high thermal conductivity,non-toxic,non-flammable and so on.It can be utilized for thermal management systems in buildings,lithium-ion batteries,and solar power generation.However,hydrated salts also have several disadvantages when it was used.On the one hand,like organic PCMs,they tend to leak when heated above melting temperature.Hydrated salt has serious supercooling,which means when the temperature of molten hydrated salt is lower than its melting point but no crystallization occurs.The difference between the actual crystallization temperature and the melting temperature is called supercooling degree.Secondly,hydrated salt suffered a problem of phase separation.Due to the inconsistent melting process of hydrated salt,it dehydrated in steps.The salt with lower hydration numbers are precipitated at the bottom of water due to the difference in density.As a result,the enthalpy will be reset to zero after several heating and cooling cycle.Poor thermal stability is also a fatal disadvantage of hydrated salt.The water of hydrated salt is very easy to volatilize during use.Once the water of hydrated salt volatilized,the enthalpy of it will decrease accordingly.In addition,hydrated salt can corrode containers severely.Current researches of shape stabilized PCMs are all focused on how to increase the thermal conductivity.But there are few relative studies on its flexibility,which largely limited the further application of shape stabilized PCMs in heat storage management systems.At present,these problems must be effectively solved before the use of hydrated salts.In this thesis,the shape stabilized hydrated salt PCMs are studied.Hydrated salt microcapsules,shape stabilized hydrated salt/paraffin PCMs and flexible shape stabilized hydrated salt/polyurethane PCMs are prepared by inverse emulsion template method.For the prepared hydrated salt phase change microcapsules,the endothermic value of it reached to 136.1 J/g,and the supercooling degree was reduced from 24.2 ℃ to 14.5 ℃.For the hydrated salt/paraffin composite PCMs shape stabilized by cellulose sponge,thermal stability was significantly improved.The water in hydrated salt will not volatilize before 80 ℃.The paraffin also provided the nucleation sites to promote the crystallization of hydrated salt and reduced the supercooling degree from 34.2 ℃ to 19.5 ℃.The endothermic value of the shape stabilized PCMs was 227.3 J/g.For the flexible hydrated salt/polyurethane shape stabilized phase change material,polyurethane promoted the crystallization of the hydrated salt by providing nucleation sites and reduced the supercooling degree from 34.2 ℃ to 16.6 ℃.The thermal stability of the hydrated salt was improved very well.