Research On High Uniformity Fresnel Concentrator For Concentrating Photovoltaic Applications

With the problems of oil and natural gas being exhausted and the greenhouse effect becoming increasingly serious,it is imperative to develop and utilize sustainable,renewable energy and green,pollution-free new energy.Solar energy occupies an irreplaceable position in the new energy structure due to its advantages such as cleanliness and non-greenhouse effect.The effective utilization of solar energy is considered to be one of the fundamental ways to solve energy problems.Concentrating photovoltaic(CPV)power generation is one of the important way to use solar energy,and it has showed great potential in the future renewable energy field.The CPV systems mainly consist of multi-junction solar cells and concentrators.Multi-junction cells are the solar cells with the highest photoelectric conversion efficiency,which can reach more than 80% in theory.Many companies and laboratories have produced multijunction cells,whose photoelectric conversion efficiency is nearly 50%,while that of crystalline silicon solar cells is only about 20%.This reveals that multi-junction cells can convert solar energy into more electrical energy per unit area and improve the utilization rate of solar energy.However,the high cost of multi-junction solar cells hinders their large-scale application.Therefore,the concentrator becomes an important component of the CPV system.The use of inexpensive concentrators can increase the irradiation density of incident light,focusing large areas of sunlight onto small multijunction solar cells,thereby reducing the use areas of expensive multi-junction cells to greatly reduce power generation costs.The design of the concentrator is an important research content and research direction in CPV power generation technology,and has very important research significance and practical value in the application of new energy.The most commonly used concentrator is the Fresnel concentrator,but most of the traditional Fresnel concentrators have the following problems.First,the low concentrating uniformity causes the local temperature of the solar cell to be too high,thereby reducing the photoelectric conversion efficiency,and even burning the cell to make it scrap.Besides,the mismatch of the circular concentrating spot to the square surface of solar cells reduces the light utilization efficiency of the system.Moreover,concentrators designed with the single-wavelength cannot overcome the dispersion problem caused by the ultra-wide solar spectrum.Aiming at the existing problems,a new design method of Fresnel concentrator is proposed in this dissertation,both theoretical and experimental researches on the concentrating characteristics of the Fresnel concentrator are carried on.The main research contents and innovative achievements of this dissertation can be listed as follows:(1)Based on the theory of non-imaging optics,the effect of different incident lights on the concentrating spots of Fresnel concentrators are studied.Moreover,the factors that need to be considered in designing the Fresnel concentrator are analyzed in detail,which provides a theoretical basis for the optimal design of the Fresnel concentrator.(2)A optimal design method of the Fresnel concentrator with high uniformity is proposed,where the disadvantages of large dispersion,narrow response band,low uniformity of the traditional Fresnel concentrator are effectively overcomed.This method adopts the design of multi-focus multi-wavelength and off-axis non-rotationally symmetric design method,and it can form a uniform square spot on the receiving surface.Simulation analyses about the Fresnel concentrator are carried out based on the Monte Carlo ray-tracing method.Simulation results show that the square spot with a uniformity up to 82.5% can be obtained within the ultra-wide solar spectrum band from 300 nm to 1800 nm.Besides,the Fresnel concentrator designed by this method can achieve relatively uniform spots in different response bands of Ga In P/Ga As/Ga In As P/Ga In As four-junction solar cell,thus improving the photoelectric conversion efficiency of each sub-cell.(3)A method of non-linear focus distribution design is proposed to improve the concentrating uniformity.First,the performance parameters of the novel Fresnel concentrator are analyzed in detail,and the influences of sawtooth parameters,focal length and light acceptance angle of the novel Fresnel concentrator on the concentrating effect are explored.Then,the relationship among focal distribution,sawtooth arc area and concentrating energy at each focal point is studied,and a design method of nonlinear focus distribution is proposed based on it.This method adjusts the distance between the focal points according to the energy at each focal point,thus improving the energy distribution on the focal plane,and enhancing the uniformity of the concentrating spot.Finally,the method is verified by simulation.The results show that the novel Fresnel concentrator with non-linear focus distribution can obtain a concentrating spot with a uniformity of 84.7%,which can effectively solve the problem of uneven energy distribution of the spot.(4)A processing error correction model and the experimental research platform of the novel Fresnel concentrator are established.First,the processing size of the processed novel Fresnel concentrator is detected,and a processing error correction model is established based on the detection results.The simulation results of the correction model show that the processing error has great influence on the light utilization efficiency,focal length and transmittance of the concentrator.Then,the novel Fresnel concentrator is studied experimentally,and the result shows that the concentrating uniformity,the transmittance,the light utilization efficiency and the light acceptance angle of the Fresnel concentrator can reach 81.4%,73.1% and 0.48?,respectively,which are consistent with the results of theoretical design and simulation analysis,and satisfies the demand of practical applications.The experimental results verify the effectiveness of the optimal design method of the novel Fresnel concentrator proposed in this dissertation.The related design method of the novel Fresnel concentrator proposed in this dissertation is not only applicable to design the concentrator in the CPV system,but also has a broad guiding significance for other lenses with uniform spot based on nonimaging theory.