Simulation Study On Matching Characteristics Of Cascaded Phase Change Heat Storage Units For Solar Energy In The Severe Cold Region Of Inner Mongolia

Solar energy is one of the most commonly used renewable energy because of its rich reserves,safety and reliability.However,solar energy has intermittence and fluctuation,developing heat storage technology can effectively solve this problem.Among them,cascade phase change heat storage technology is one of the research hotpots in the field of solar heat storage technology because of its high heat storage efficiency and wider heat storage temperature range.The severe cold area of Inner Mongolia is rich in solar energy resources.If the cascade phase change heat storage technology is applied to solar energy storage,it is expected to enrich the clean heating mode in Inner Mongolia.By investigating the climate characteristics of the severe cold area in Inner Mongolia,this thesis selects Baotou area as a typical area,calculates the solar radiation on a typical day in Baotou area,and uses the change of inlet velocity to fit the solar radiation intensity at each time of a typical day.The physical model and mathematical model of cascade phase change heat storage unit are established.Analysis of cascade phase change thermal storage unit combination in different materials,different casing radius ratio and different heat storage unit length ratio under the matching characteristics of heat storage time and storage quantity of heat,heat storage efficiency and melting solidification effect,obtained the suitable for cold region of Inner Mongolia solar material combination of cascade phase change thermal storage unit,and the size of the heat storage units at all levels.According to the numerical simulation results,the inlet temperature and inlet flow velocity of the cascade phase change heat storage unit are optimized,and the best operation condition suitable for the solar cascade phase change heat storage system in Inner Mongolia is explored.The results show that under the same heat storage and release conditions,the total heat storage per unit mass of the combination of palmitic acid and polyethylene glycol reaches highest,the melting and solidification process of the two-stage materials is more synchronous,and the setting of temperature gradient is more reasonable.The decreasing trend of heat flux during solidification is more gentle.Comprehensive comparison shows that the material combination of palmitic acid and polyethylene glycol is more suitable for the solar cascade heat storage system in the severe cold area of Inner Mongolia.When the casing radius ratio is 1:1.6,the liquid phase ratio of the two-stage phase change materials is more synchronous during melting,so 1:1.6 is the optimal casing radius ratio of the cascade phase change heat storage unit.When the length ratio of heat storage unit is 1:2.5,the two-stage phase change materials complete the melting process at the same time.In the exothermic stage,the solidification process of primary and secondary phase change heat storage units is also relatively synchronous,so 1:2.5 is the optimal heat storage unit ratio of cascade phase change heat storage units.Judging from the synchronization and melting rate of the melting process of step-by-step phase change materials,increasing the inlet temperature can shorten the complete melting time of phase change materials.For the mode of optimal casing radius ratio,when the inlet temperature rises to 363 k,the melting rate of phase change materials reaches the maximum,and the melting process of step-by-step phase change materials is more synchronous;For the mode of optimal length ratio of heat storage unit,when the inlet temperature rises to 358 k,the melting rate of phase change material increases,and the melting process of step phase change material remains synchronous.Under the same solar irradiation conditions,setting step change materials in the radial sleeve can better promote the synchronous melting of step heat storage materials.