Numerical Study On Optical Properties And Heat Transfer Characteristics Of Dish-type Solar Heat Absorber

Solar photothermal power generation technologies have alleviated contradictions among exhaustion of non-renewable resources,ecological damage and rising energy demands of human beings.Solar photothermal power generation technologies include trough,tower,and dish solar photothermal power generation technology.Among three of them,dish solar photothermal power generation technology which is flexible layouts,modularity,and suitable for distributed energy systems is favored by governments and scholars all over the world.As a key component of the dish solar photothermal power generation system,shapes of heat absorber cavities,the optical loss and the heat exchange loss during operations will seriously affect the performance of whole system.Therefore,heat absorbers are the research focus of solar photothermal power generation systems,and research on heat absorbers is of great significance.In this thesis,an optical tracking model based on Monte-Carlo method and a three-dimensional heat transfer model based on finite volume method are established.The optical properties and heat transfer performance of the heat absorber of compound truncated cone cavity are discussed by TracePro and Fluent.Firstly,optical properties of the heat absorber of compound truncated cone cavity and the spherical heat absorber cavity are compared.Then,the heat absorber of compound truncated cone which has better optical performance is regarded as the research object,influences of the absorption rate of inner walls of the heat absorber,the reflectivity of the concentrating mirrors,solar irradiance on it is studied.At the same time,effects of the cone angle of the reflection cone,the thickness of the optical window and the distance between the light aperture and the focal plane of concentrating mirror on the optical properties of the heat absorber of compound truncated cone is obtained.Next,according to the uneven distribution of energy flow on the inner wall of the heat absorber,energy flow distribution of the endothermic tube wall is obtained by TracePro,and it is used as the thermal origin of the heat transfer model.Considering the thermal conduction,thermal convection,and thermal radiation among the heat absorber,the working fluid molten salt and the ambient air,the heat loss,thermal efficiency and system light-heat conversion efficiency of the heat absorber of compound truncated cone are compared by Fluent under different inclinations of the heat absorber.Finally,effects of inlet speeds,inlet temperatures of molten salt,and irradiance on the heat transfer performance of the heat absorber of compound truncated cone are analyzed.Results show that,compared with the spherical heat absorber cavity,the heat absorber of compound truncated cone cavity has better optical performance,and shapes of heat absorbers have little influence on the system optical efficiency,but energy flux distributions of inner walls of cavities will be different greatly.When the absorption rate of inner walls and the reflectivity of concentrating mirrors increase,the total luminous flux and the system optical efficiency of the heat absorber of compound truncated cone cavity increase.When irradiation intensities increase,the total luminous flux of the heat absorber of compound truncated cone cavity increases,but the system optical efficiency does not change.Cone angles of the reflecting cone affect the amount of light in the cavity,luminous flux of the cavity and the system optical efficiency.The heat absorber of compound truncated cone has the best optical performance when the cone angle of the reflecting cone is 160°.When the distance between the light aperture and the focal plane of concentrating mirror changes from-100 mm to 100 mm,the luminous flux of tube surfaces and the system optical efficiency are normally distributed,when the distance is between-30 mm and 0mm,the thickness of the optical window has little effect on the luminous flux of the tube surface and the system optical efficiency.After the heat absorber of compound truncated cone is installed with a optical window,average thermal flux of tube walls decreases,but the distribution law has little change.With the augment of the inclination of the heat absorber,the heat loss decreases gradually,and the convective heat loss is sensitive to the change of inclination angles,after the optical window is installed,the heat loss decreases slowly.The optical window makes the thermal efficiency of the heat absorber improved,but the system light-heat conversion efficiency declined to some extent.Molten salt outlet temperature,the average temperature of each tube front wall and various heat losses gradually decrease with the augment of the inlet velocity of molten salt,and they rise as the inlet temperature of molten salt and irradiance rise.Thermal efficiency of the heat absorber rises when the inlet velocity of molten salt rises,and it decreases with the augment of the inlet temperature of molten salt.Under different molten salt inlet velocities,when the irradiance rise,thermal efficiency increases firstly and then decreases,and the greater the molten salt inlet velocity,the greater the irradiance at which the thermal efficiency begins to decrease.