Construction And Performance Of Linear Fresnel Concentrating System Based On Novel Secondary Concentrator

Concentrated solar thermal power generation is equipped with a large-capacity heat storage system,which enables new energy sources to have the ability to undertake basic heat sources and power sources.The main reflector of the linear Fresnel concentrating heat collecting system is installed near the ground,and has many advantages such as strong wind resistance.It is a concentrating solar energy technology with development potential.Four of the first Nation Concentrating Solar Power（CSP）demonstration projects adopt linear Fresnel concentrating technology.Compared with the point focusing technology,the line focusing technology has a larger heat loss.In the heat loss of the linear Fresnel concentrating collecting system,with the increase of temperature the radiation heat loss dominates.Researchers mostly use the method of reducing the radiation coefficient of the selective absorption film on the surface of the heat sink to reduce the heat loss.However,the radiation coefficient of the selective absorption film increases with the increase of the operating temperature of the system,and its low radiation characteristics are not easy to maintain during long-term operation.In this paper,a method is proposed to suppress the heat radiation loss of the system by adding a spectrally selective infrared high-reflection film glass cover.On the basis,building a new secondary condenser with metal heat absorption pipe to replace the vacuum collector tube,which can significantly reduce the"gap loss"of the secondary concentrator,and effectively improve the optical efficiency of the system.The main research contents include the following:（1）A Si O₂/AZO（Al:Zn O）/Al₂O₃/Glass/Si O₂ spectral selective infrared high-reflection film with high visible light transmittance and high infrared reflectance was designed,and the theory simulation investigation of the spectral selective infrared film was carried out using TFCal film design software.By optimizing the thickness of each functional film,the film parameters with high transmittance in visible light and high reflectivity in infrared light were obtained.The preparation process of single-layer Al₂O₃,AZO and Si O₂ film was studied by magnetron sputtering method,and the optical constants of the materials used in the spectral selective infrared high-reflection film were calculated by reflection spectrum inversion,and the optimization was again carried out using TFCal thin film design software to complete the preparation of the spectral selective infrared high reflection film.The experimental results show that the film has high transmittance in the visible light range and high reflectivity in the infrared region（corresponding to low radiation rate）.The experimental results are basically consistent with the simulation results.（2）A new type of secondary concentrator was constructed based on the research of spectral selective infrared high reflection film.The geometrical optical efficiency simulation model of the secondary concentrator was established by self-programming in Matlab to analyze the effects of the gap,receiving half-angle and interception ratio on the geometrical optical efficiency of the newly constructed secondary concentrator.The preparation of the secondary concentrator was completed,and the geometrical optical efficiency is tested.The results are basically consistent with the simulation results,and the error is less than 2.8%.The geometrical optical efficiency of the secondary concentrator is more than 96%,which is10%-15%higher than that of the secondary concentrator composed of a vacuum heat collector and CPC.（3）The heat transfer analyses of the metal heat absorption tube,the vacuum heat collector tube and the secondary concentrator composed of the metal tube,the glass cover and the CPC were carried out,and a one-dimensional steady heat transfer model,a thermal resistance model and a heat transfer model between the components of the secondary concentrator were established.The main heat losses of seven kinds of secondary concentrators were compared and analyzed.The results show that the radiative heat loss can be reduced by increasing the thermal resistance and changing the radiation rate of the wall surface,and reducing the radiation of the glass sleeve and the surface of the glass cover is an effective way to reduce the radiative heat loss of the heat sink.It was verified that the addition of a glass cover coated with a film with high transmission in visible light and high reflection in mid-to-far infrared light at the opening position of the secondary concentrator can reduce the natural convection heat loss and greatly reduce the radiation heat loss,The novel constructed secondary concentrator has lower heat loss in this paper.（4）The new secondary condenser was applied in the linear Fresnel condenser system to realize the optimal design of the 21-column cylindrical primary mirror without shadow occlusion,and the calculation method of the design parameters for the unshaded mirror field was given and carried out.The cosine efficiency,end loss efficiency,shadow occlusion efficiency,and optical efficiency of the system were analyzed for the typical days.The results show that the optical efficiency increases gradually of the system from the winter solstice to the summer solstice,the instantaneous optical efficiency of the system reaches 65.25%;The optical efficiency of the system on the winter solstice day is the smallest;and the average optical efficiency of the system on the spring breeze day reaches 55.15%.