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9 November 2010 Free-form Köhler nonimaging optics for photovoltaic concentration
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Concentration Photovoltaics (CPV) is one of the most promising areas for competitive solar electricity production. This promise relies upon the use of high-efficiency triple-junction solar cells (which already have proven efficiencies over 41%) and upon advanced optics designs, which allow for high concentration concurrent with high manufacturing tolerances, both key elements for low cost mass production. In this paper we will present the progress in the development of the most advanced CPV optical designs at present. These are based on free-form optics using Köhler homogenization. The degree of freedom of using free-forms allows the introduction of multiple functionalities in a few optical elements, which provide the required concentration with high tolerance and excellent light homogenization. Different families are presented. The first group uses a Fresnel lens as a primary optic (called the FK concentrator and the F-RXI concentrator) and a second group using mirrors as primaries (the XR and the XXR). How they compare among them and also with classical designs will be discussed. The FK is in the process of being brought to market and has already experimentally proven module electrical (DC) efficiencies over 30% (equivalent to over 32% with correction to Tcell=25ºC) with no AR coatings at a concentration of 625x with high tolerance angle (over ±1.2º).
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Pablo Benítez, Juan C. Miñano, Pablo Zamora, Marina Buljan, Aleksandra Cvetković, Maikel Hernandez, Rubén Mohedano, Julio Chaves, and Oliver Dross "Free-form Köhler nonimaging optics for photovoltaic concentration", Proc. SPIE 7849, Optical Design and Testing IV, 78490K (9 November 2010);

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