Design And Experimental Investigations On Optical System For High-Concentration Photovoltaics

With the rapid growth of the global economy and the continuing improvement on the level of industrialization,the demand for energy by the human being will grow constantly.Due to continuous consumption on traditional energy which is nonrenewable resource,so the development of renewable energy is becoming imperative.Photovoltaic power generation can be classified into crystalline silicon solar power generation and concentrated photovoltaic power generation.Compared with the crystalline silicon solar power generation technology,the concentrated photovoltaic technology has the following advantages:(1)The multi-junction solar cells used by the concentrated photovoltaic have higher conversion efficiency and the conversion efficiency of triple-junction cell in commercial mass production can reach 40%,while the theoretical conversion efficiency of crystal silicon and thin film solar cells is only about 28%.(2)There are rooms for further improving the efficiency of multi-junction solar cells.Solar cells made by Azure,Sharp and Fraunhofer ISE achieved today's champion efficiencies of 44.4%and 46.0%for triple-and four-junction solar cells,respectively.The theoretical conversion efficiency of multijunction solar cells can exceed 60%and the efficiency has increased by 0.9%absolute per year.(3)As for the high conversion efficiency of the multi-junction solar cells,the power generation per unit area is large and the floor space of the system is smaller than that of crystalline silicon cell.At the same time,the tracking systems of concentrated photovoltaic have brackets to support and keep away from the ground,so the land can also continue to grow lawn,low shrubs and other comprehensive utilization.Although the concentrated photovoltaic has high efficiency and room for further improvement,the energy cost of concentrated photovoltaic is still higher than that of other power generation technologies,which is still the main factor restricting its development of applications.Based on the theory of non-imaging optical design and indoor and outdoor testing data and production experience,the design,manufacturing process and performance testing of the concentrated photovoltaic system are investigated in this dissertation.The specific research topics covered in this paper are as follows:1.Theoretical analysis on the four main factors that affect the efficiency of the concentrated photovoltaic systems:spectral matching,uniformity of irradiance distribution,temperature dependence and tracking tolerances of the system,which is essential for the design of the concentrated photovoltaic systems.2.Based on the theory of non-imaging optics,design and experimental verification on the primary optical element and secondary optical element for transmitted concentrating system are carried out.An annular-divided Fresnel lens was designed as primary optical element for the concentrating optical system and the uniformity of irradiance distribution on solar cell can be improved by 15.9%.Theoretical study on the design of the secondary optical elements and two kinds of concentrating optical systems with ball lens as secondary optical element are proposed,which include the adhesive packaging system with truncated ball lens as secondary optical element and the mechanical mounting with ball lens as secondary optical element.The design,ray tracing analysis and experimental tests on these two different concentrating optical systems are carried out respectively.The experimental results show that both systems can achieve power generation efficiency of over 30%and acceptance angle of greater than 0.7°.The proposed solutions using ball lens as secondary optical elements can also reduce the risk of failure of long term reliability.3.The combined thermal and electrical power generating system with a divided freeform surface mirrors is investigated experimentally.A prototype concentrator was manufactured and evaluated and the tests included intensity distribution of the concentrating spot,temperature field distribution on the surface of receiver assembly,photo-electric conversion and the photo-thermal conversion efficiency of the system.The test results show that photoelectric conversion efficiency of 7.9%and photo-thermal conversion efficiency of 51.2%can be obtained.At the same time,an aspherical mirror was proposed and designed to produce a square focused spot with uniform distribution,so as to reduce the requirements for the manufacturing process of the mirror.4.The processing technology and error source of SOG Fresnel lens are studied.The theoretical model of SOG Fresnel lens is corrected by testing the lens tooth profile.Then an experiment was conducted to verify the choice of the size of solar cell in theory.Finally,based on the principle of total internal reflection,the effects of silicone spillage occurring on the lower-bottom sidewalls of the SOEs during the bonding process on the optical efficiency of the receiver assembly are studied.Three different types of refractive SOEs are selected to simulate and analyze the effects of silicone spillage height on optical efficiency,acceptance angle and temperature variations.5.According to the industrial standard IEC62108 for concentrated photovoltaic and IEC60904 for solar simulator,the indoor power rating and outdoor electrical performance testing methods are investigated to provide methods for the performance evaluation of concentrated photovoltaic modules.The main innovations of this paper include:1.Based on the ball lens as sencondary optical element,optical systems with two kinds of packaging process are proposed,which can not only allow us to obtain the power generation efficiency of 30%and the acceptance angle of 0.7°,but also to effectively improve the reliability of the concentrated photovoltaic system.2.The effects of silicone spillage occurring on the lower-bottom sidewalls of the secondary optical elements during the bonding process on the optical efficiency of the receiver assembly are studied.It can provide a theoretical basis for the receiver packaging process.The theoretical model of Fresnel lens is modified based on experimental tests,and the simulation results are in good consistence with the experimental data.3.Based on the theory of non-imaging optical design and combined with considerable test results,design and experimental investigations on optical system for high-concentration photovoltaics are performed from many aspects such as optical design,manufacturing process and test,which is essential for the design of concentrated photovoltaic systems.