Study Of A Concentrating PV/Thermal System Based On Spectrum Splitting And An Improved LFR Concentrator

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.