【作者】 刘阳；

【导师】 陈则韶； 胡芃；

【作者基本信息】 中国科学技术大学 ， 制冷及低温工程， 2014， 博士

【摘要】 太阳能是重要的可再生能源,其高效利用己成为重要的研究课题。传统上,太阳能利用方式主要有光伏转换和光热转换。这两种方式均受制于较低的太阳能能流密度。采用聚光方式提高太阳能的辐射能流密度,光热利用时可提高集热温度,提高效率；光伏转换时则可减少光伏电池用量,降低成本。而且聚光电池具有比平板光伏电池更高的光电转换效率。因此,太阳能的聚光利用具有良好的前景。然而某些类型的聚光系统如碟式系统、槽式系统的焦斑能流分布不均,影响光伏电池转换效率。因此研制具有均匀焦斑的低成本、高性能的新型聚光器在聚光光伏应用中具有重要意义。聚光光伏发电的聚光倍数为数十至数百,且由于光伏电池仅能转换特定光谱区间的太阳辐射能,其余谱段的辐射能在电池内耗散为废热,造成电池严重的热负荷。对于光伏电池废热的利用,主要有光伏-热水系统等传统型电热联用系统,利用冷却介质对光伏电池进行冷却,而冷却流体则被加热成热流体以资利用。这种系统提高了太阳能的综合利用效率,但由于冷却介质温度受限于光伏电池工作温度,热利用效率很低。光谱分频技术可实现对不同光谱的太阳辐射能的分配,因此在聚光光伏利用中可将能高效光伏转换谱段的太阳辐射输送至光伏电池,将其余谱段的太阳辐射能集热回收。光谱分频技术不仅从源头上降低了光伏电池的热负荷,还实现光伏转换过程与光热转换过程的相互独立,使得热利用系统可获取比传统电热联用系统更高的集热温度。本文在传统线性菲涅尔聚光器的基础上,提出了一种改进型的聚光器,在此聚光器平台上构建了一种基于光谱分频技术的新型聚光光伏/光热综合利用系统,并围绕这一新型电热联用系统展开理论分析和实验研究。首先,提出了一种集光平板倾斜布置的改进型线性菲涅尔反射式(LFR)聚光器设计方案。以此聚光器为平台,提出了一种基于光谱分频技术的新型太阳能聚光光伏/光热综合利用系统方案。描述了该聚光分频利用系统中各部件之间的几何关系,并以几何聚光比和面积利用率为目标函数,对该系统的结构参数进行了优化计算。在考虑主要光学误差的前提下,利用光学仿真软件对该系统进行了光学仿真。仿真结果显示焦斑位置能流密度分布均匀,适于光伏电池使用；且在未使用再反射镜时热接收器已可收集约86%的光谱分频器反射能流。其次,以优化计算获得的结构参数构建了上述改进型LFR聚光器,并在此聚光器上搭建了太阳能聚光光伏/光热综合利用的实验装置。利用CCD测试法对实际聚光器的焦斑能流分布进行了测试,测试结果与光学仿真结果一致。利用该实验装置,测试了单晶硅电池和砷化镓电池在不同聚光倍数的全光谱太阳辐射下的I-V特性曲线,分析了各光伏电池性能的优劣。同时还测试了等光强的不同单色光光照条件下单晶硅电池的I-V曲线。根据曲线特征,得出在高辐照能流密度下,光伏电池的开路电压同低辐照能流密度下相似,具有微弱的光谱依赖特性。对影响光伏电池转换效率的内外部各因素展开深入分析,描述了光伏电池在单纯聚光以及聚光分频两种工作条件下的开路电压、短路电流以及填充因子三要素的表达式,根据转换效率的定义式即可计算出相应条件下光伏电池的转换效率。同时还构建了聚光条件下光伏电池的转换效率模型,并检验了该模型的可靠性。根据效率模型,分析了光伏电池聚光利用时存在最大转换效率并对应最佳聚光倍数。再次,建立了中高温太阳能热利用系统性能分析模型。对黑体表面和不同截止波长的理想选择性吸收表面的热接收器进行热力学分析,得出中高温太阳能光热利用效率与热接收器表面性质、入射能流密度及集热温度的关系。分析了传统的单纯聚光光热利用系统和采用聚光分频系统中的热接收器表面涂层的特性,确定了这两类系统中热接收器的最佳工作温度、系统热功转换效率与入射能流密度之间的关系。数值计算表明,单纯聚光光热利用系统中的中高温热接收器的最佳截止波长为1.6～2.0μa；而采用硅电池或砷化镓电池的聚光分频系统中的热接收器选择性吸收涂层的最佳截止波长为2.2～2.6μm。同时给出了对带有实际选择性吸收涂层的热接收器的最佳工作温度及对应的最大效率的分析计算方法。最后,针对聚光分频系统,提出通过比较光伏电池的光谱转换效率和设定工作温度下热利用子系统的最大效率的大小,来确定太阳光谱最佳分频位置。计算了采用不同光伏电池的聚光分频利用系统的总效率。并以采用单晶硅电池和采用特定截止波长的热接收器的热利用子系统构成的CPV/T分频利用系统为案例,根据聚光分频利用系统的结构及各部件的光学特性,计算了各环节的能量分布及系统总效率。计算结果表明,相同工作条件下聚光分频利用系统比单纯聚光系统具有更高的效率,且CPV/T分频利用系统的整体效率受温度影响低于相同条件下的CPV系统。因此,太阳能聚光分频利用系统具有良好的应用前景。更多还原

【Abstract】 Solar energy is inexhaustible, clean and regarded as one of the most importantrenewable energy to replace limited and polluting fossil fuels. Its efficient conversioninto heat and electricity or other energy forms at low cost attracts worldwide concerns.The traditional methods of solar energy utilization are mainly photovoltaic powergeneration and solar thermal utilization, both of which are limited by the dilute solarenergy. High solar flux by a solar concentrator may lead to high temperatures and thushigh thermal efficiency for solar thermal utilization and achieve high powergeneration efficiency at low cost because numerous traditional PV panels are replacedby much less concentrator solar cells which are of high conversion efficiency. Thus,solar concentrating application shows a promising prospect. However, some solarconcentrators such as dish and trough concentrator, inherent poor uniformity of theconcentrated flux will deteriorate the PV conversion efficiency. Thus, development ofa solar concentrator with low cost, high efficiency and stability is of great importance.For a concentrating PV (CPV) system, flux density on the target ranges from dozensto hundreds of suns, which leading to heavy waste heat flux generation because oflimited spectral response of solar cells. Thus, a cooling system for solar cells isgenerally required in a CPV system. The removed heat by the cooling system can beused for heating purpose, but the temperature of the fluid is normally restricted to alow temperature to ensure efficient operation of solar cells. The beam splittingtechnique is a promising method to reduce the heat load of solar cells drastically andachieve higher temperature for thermal utilization by splitting the inefficient radiationto a thermal absorber. With the beam splitting technique, a CPV system combinedwith a thermal utilization cycle developing a concentrating PV/Thermal (CPV/T)system which allows the PV module and the thermal absorber to be organized in athermally decoupled way and operated at independent temperatures. In this thesis, anovel CPV/T hybrid solar system based on the beam splitting technique and animproved linear Fresnel reflector (LFR) concentrator was proposed. Relatedtheoretical analysis and experimental work were carried out as follows: Firstly, an improved linear Fresnel reflector (LFR) concentrator utilizing slopedpanels was proposed. Based on the concentrator and the beam splitting technique, anovel CPV/T hybrid solar system was developed. Relations between the structuralparameters and the optical performances of the hybrid system were investigated and optimized to pursue the highest geometrical concentration ratio and aperture utilization ratio. Ray-tracing simulations were carried out by considering the main optical errors and the simulated results show a good uniformity of about92%for the concentrated flux distribution on the solar cells and about86%energy reflected by the spectral beam splitter was intercepted by the thermal receiver without a secondary reflector.Secondly, an experimental installation of the CPV/T hybrid system with the beam splitting technique and the improved LFR concentrator constructed according to the optimized structural parameters was developed. Flux density distributions of the concentrated radiation were tested by using the CCD camera method which show a good uniformity and agree well with the simulated results. Also, the optical concentration ratio of the concentrator was achieved by measuring the direct solar radiation density and the concentrated flux density. The Ⅰ-Ⅴ characteristics of c-silicon solar cells and GaAs solar cells were tested under full-spectrum light at a variable intensity, and their performances were analyzed. Ⅰ-Ⅴ characteristics of c-Si solar cell under monochromatic lights at variable intensity were also tested. It shows that the open-circuit voltage of solar cell is slightly spectral dependence. Based on the analysis of internal and external factors that affect solar cells performance, the open-circuit voltage, short-circuit current and fill factor under both the full-spectrum concentrating condition and the split-spectrum concentrating condition were described to calculate the conversion efficiency. Furthermore, an efficiency model for solar cells under concentrated illumination was developed and validated. On the basis of the model, the highest conversion efficiency of solar cell and the corresponding optimal concentration ratio were analyzed.Thirdly, a model to analyze the thermodynamic performance of the medium-high temperature solar thermal system was developed. Thermodynamic analysis on thermal receivers with ideal selective coatings of different cutoff wavelengths and blackbody thermal receivers have been conducted to investigate the relations of the efficiency with the properties of selective coatings, the incident solar flux and the working temperature. The calculated results indicate that the cutoff wavelengths of selective coatings affect the thermodynamic performance of medium-high temperature solar thermal receivers greatly. For thermal utilization that converts the full solar spectrum, the optimal cutoff wavelengths for the medium-high temperature thermal receivers are in the range of1.6～2.0μm. And for thermal utilization in a CPV/T hybrid solar system with beam splitting technique which apply c-Si or GaAs solar cells as PV devices, the optimal cutoff wavelengths for thermal receivers are in the range of2.2～2.6μ. The relations of the optimal working temperature and the corresponding efficiency with the incident solar flux for both the hybrid system employing c-Si and GaAs solar cells were developed and applied for a medium-high temperature solar thermal receiver with actual selective coatings.Finally, a method to determine the optimal splitting waveband for CPV/T hybrid solar system was proposed by comparing the PV conversion efficiency of solar cells with the efficiency of thermal utilization under variable operating temperature.mThe overall efficiencies of CPV/T hybrid system employing different solar cells were investigated. Based on the experimental data of the components, thermodynamic analysis on the CPV/T hybrid system was carried out and shows more work production than the CPV system under the same conditions and less negative influence of the PV temperature. Thus, the beam splitting CPV/T hybrid system has a promising prospect.更多还原