Research On Cycling Stability Characterization Of Hydrated Salt As Phase Change Storage Material

At present,energy shortage and environmental pollution are becoming increasingly serious,and it is need to strike a balance between economic development,environmental protection and energy supply.Phase change energy storage materials can effectively solve the contradiction between energy supply and demand in time and space by storing heat,which attracted widespread attention.In the practical engineering applications,phase change energy storage materials often require good cyclostability to stand thousands of melting/solidification cycles,which is a key factor in the successful application of phase change materials.Inorganic hydrated salt is a typical phase change energy storage material suitable for the temperature range of 0-150?.It has the advantages of abundant source,low cost and large energy density,but there are supercooling and phase separation phenomena,which seriously affect its heat storage stability and service life.Therefore,the purpose of this study is to improve the cycling stability of disodium phosphate dodecahydrate by different means,in order to obtain the low degree of subcooling,the phase separation is effectively improved,the high latent heat is maintained and the long variable energy storage system.The main contents and achievements of this study are as follows:First,the inorganic-inorganic composite phase change materials were made by using diastric acid disodium dodecahydrate as the main phase change material and expanded graphite as supporting material.With the increase of the mass fraction of expanded graphite,the phase change temperature and latent heat of the composites decreased,the degree of supercooling decreased first and then increased.When the mass fraction of expanded graphite was 9%,the supercooling can be effectively suppressed,The composite phase change material was melted by 300 times melting/Solidification thermal cycle,the results showed that a little changed of the phase change temperature after 300 cycles,the degree of subcooling has been reduced,compared with pure dodecahydrate disodium hydrogen phosphate,composite material latent heat attenuation significantly slowed down.When the expansion graphite content was 11% and the sodium silicate nonahydrate was 4%,the system got the highest stability,and the latent heat attenuation rate was 26.13% after 300 cycles.In addition,the experimental results showed that with the increase of the amount of expanded graphite,the latent heat decay was slower and the stability was higher.Then phase separation and supercooling were improved by thickening agent and adding nucleating agent,it was found that the addition of 1% sodium polyacrylate could effectively inhibit the phase separation of disodium phosphate dodecahydrate through a lot of experiments,and sodium phosphate,titanium dioxide and polyethylene glycol as a nucleating agent can effectively inhibit its supercooling.The three long-term melting / solidification cycle tests of the three nucleating agents of sodium dodecahydrate were carried out,when 1% sodium polyacrylate as the thickener,3% titanium dioxide as the nucleating agent,the phase transition latent heat of the disodium dodecahydrate phosphate system was higher,the undercooling was smaller and can kept stability for a long time.According to the evaluation results of the thermal stability of each system and the size of the solid crystalline particles,it can be seen that the cyclic heat storage stability of the hydrated salt system is related to the size of the solid crystal particles.The smaller the average particle size of the phase change material,the better the thermal storage stability,and the worse.Last,selected the 1% sodium polyacrylate as thickener,3% titanium dioxide as nucleating agent,dodecahydrate disodium hydrogen phosphate phase change material as a thermal insulation agent,studied the effect of temperature control in the EPS incubator,through the temperature changes in the environment of-10? in the incubator to reflect the insulation effect of the phase change material.The results showed that the temperature in the EPS incubator was above 30? after 12 h in the constant temperature environment of-10?,the heat preservation effect was good.The innovation of this study is to carry out cyclic stabilization test of disodium dodecahydrate phosphate containing thickener and nucleating agent.It is found that the stability of circulating heat storage is related to the size of crystal particles.The smaller the crystalline particles of the phase change material,the better the thermal storage stability,and the worse.Therefore,it is possible to obtain a system with good thermal cycling stability by preparing the crystal of fine particles and keeping the size of the particles in the process of circulation to provide the idea of the stability of other hydrated salts.