Study On The Characteristics And Application Of Cold Storage Devices With Sub Zero Phase Change Materials

On the background of energy crisis,the cold storage technology develops rapidly.The application of cold storage technology can relief the contradiction between the demand of energy in the area of time,space and intensity,then the energy efficiency can be improved.At present,water storage and ice storage are widely used in cold storage technology,while there are few researches that focus on low temperature field.Therefore,it is necessary to do some research on the long-term stability of phase change materials,the cold storage characteristics of low-temperature cold storage devices and developing the application of cold storage devices.In this paper,the long-term stability of phase change materials,the cold storage characteristics of low-temperature cold storage devices and the using of the low-temperature cold storage in alternate temperature process are studied.The main contents of the research include the following aspects:Firstly,the effects of additives（1%sodium tetraborate,0.5%citric acid,1%guargum）on the freezing point and the subcooling degree of low temperature phase change materials and the long-term stability of Phase Change Materials were tested.The result shows that,after we add the additives to the solution,the freezing point of Ca Cl₂solution,Mg Cl₂solution,Na Br solution decrease,while the mixed salt solution（NH₄Cl and Na Cl）displays two freezing point.The subcooling degree of Ca Cl₂solution,Cu Cl₂solution rise while the others decrease.After100 cycles,the freezing point and the subcooling degree of solution of Ca Cl₂（without additive）,Ca Cl₂（with additive）,Na Br（without additive）and the mixed salt（NH₄Cl and Na Cl without additive）only change a little.We can see that they have good stability.However,the subcooling degree of solution of Mg Cl₂（with additive）,Mg Cl₂（without additive）and Na Br（with additive）change a lot after the 100 cycles so their stability is relatively poor.Secondly,the author built a system to test the cold storage performance of three cold storage units under several working conditions.The result shows that,when we use the lower temperature cooling liquid to cooling the cold storage unit,the average cold storage rate is increasing.Besides,increasing the mass flow rate of the cooling liquid is also benefit to increase the average cold storage rate.In addition,increasing the heat transfer area of the fins in the cold storage unit is also good to increase the heat transfer rate.Thirdly,the author analyzed the cold storage characteristic of the low temperature phase change sphere packed bed by simulation.The result shows that,under the same packed bed volume,decreasing the diameter of the cold storage sphere and increasing the mass flow of the air can improve the cold storage rate of the packed bed.At the same time,the pressure drop of the air is increased.If we reduce the ratio of height to diameter,both of the cold storage rate and the pressure drop are decreased.In addition,for the whole cold storage process,changing the flow direction of the air periodically will not improve the cold storage performance of the packed bed.Finally,the author designed a two-temperature cold storage system for a practical alternate temperature process,including the design of whole system and the heat exchange components.With the new design,the low efficiency problem of part-load can be avoided.The power of the compressor is decreased.