Entwicklung von photovoltaischen Konzentratormodulen mit Fresnel-Linsen und reflektiver Sekundäroptik

In concentrating photovoltaic (CPV) modules, a concentrating optics is used to concentrate the sunlight on solar cells with a. In the work presented here, Fresnel-lenses are used as concentrating optics to focus the sunlight on multijunction solar cells based on III-V Materials. To bring this technology one step closer to commercialization, an automated pilot production line for manufacturing the base plates of CPV-modules has been built in close cooperation with the company Concentrix Solar, a company founded in 2004 as a spin-off from Fraunhofer ISE in Freiburg, Germany. At the same time, new concepts to improve the module technology have been developed and realized in the pilot line. In order to achieve a high CPV system efficiency, a lot of research has been devoted to the optimization of the interaction between the components involved in the module in order to optimize the optical, mechanical, electrical and thermal performance. The peak irradiation in the center of the focus point of the Fresnel lenses used exceeds a concentration factor of 2000x. This leads to a high localized heat generation in the solar cell, necessitating efficient means to distribute the heat. To simulate the temperature and heat flux distribution in a solar cell assembly, a thermal model has been developed. To calibrate the model and to obtain realistic parameters of the energy flows within the modules, a thermal test module has been built and investigated during outdoor operation. The thermal model has been incorporated into a finite element method (FEM) simulation. Based on this simulation, the components of the module, particularly the thermal interfaces and the heat spreaders, have been redesigned for optimized performance and reduced material usage. Faults in the interface layers, such as voids in the solder layer below the solar cell, have been analyzed using X-ray imaging and included in the thermal FEM-model. When the solar energy is concentrated on the solar cell, part of the light is lost due to effects such as chromatic aberration of the lens, misalignment of the module to the sun or limited assembly accuracy. These effects lower the optical efficiency of a CPV module. To increase the optical efficiency, a secondary optics can be used which is located in close proximity to the solar cell and helps to concentrate the sunlight on the cell. It is the objective of the work described here to develop a reflective secondary optics relying on principles of 'design for manufacturing' in order to assure low production costs qualifying the optics for high volume manufacturing. The developed optics is manufactured in a fully automated deep-drawing and precision stamping process followed by chemical surface polishing. The effect of the secondary optics was larger than anticipated in the early phase of the development. This can be explained mainly through the increased usage of circumsolar radiation as well as the reduction of chromatic aberration of the Fresnel lens. Comparing the module with secondary optics to a reference module in a long term outdoor measurement, an increase of 1.3 %abs in efficiency could be attributed to the use of the newly developed secondary optics. At the beginning of the development work in 2005, the highest efficiency measured under outdoor conditions was 26.9 %. Using secondary optics, this value could be raised significantly. The best module with secondary optics built within the scope of the work presented here has reached 29.1 % efficiency in outdoor operation, which ranks among the highest efficiencies published for CPV modules worldwide