New free-form optics design methods for solar concentrators and imaging applications

Student thesis: Doctoral Thesis


Concentrating photovoltaic (CPV) systems use optics to concentrate sunlight onto small area solar cells. A concentration ratio of 100 means, that only 1% of the original solar cell area is needed. Replacing expensive semiconductor material with less expensive mirrors or lenses provides a pathway to lower costs. High-efficiency multi-junction solar cells can boost the conversion efficiency of CPV modules beyond 30%, but their expense means that they require a high concentration ratio of several hundred suns to be economically viable. Achieving such levels of concentration normally requires dual-axis tracking of the sun's movement. Due to their size and bulkiness, these trackers are less adequate for small- to mid-scale installations like flat rooftops. In contrast, photovoltaic (PV) modules with more compact and flexible single-axis trackers are already in use on flat rooftops. However, CPV systems designed for single-axis trackers are limited to concentration ratios that may not be high enough to make economic use of multi-junction solar cells.

In this work, a new approach to CPV is proposed that integrates part of the external solar tracking functionality into the CPV module. The system consists of two laterally moving lens arrays, combining the concentration and steering of the incident sunlight. To unleash the full potential of the considered system, a free-form optics design is absolutely essential. However, the design and optimization of free-form optics is far from being trivial, given the very large number of parameters needed to describe such surfaces.

Direct free-form design methods, developed in recent years, allow a direct calculation of the optical surfaces without iterations. One particular method, the Simultaneous Multiple Surfaces (SMS) design method has served as a starting point. As it did not allow the direct design of moveable optics, an extended SMS design method has been developed to calculate the surfaces of the laterally moving free-form lenses. Even though this approach gave already promising results, this design method raised new questions about how to select certain initial values. To address this problem, a new analytic optics design method has been developed that is capable of calculating the surfaces of the moveable free-form lenses. Simulations showed that the obtained analytic free-form lens design outperforms all preceding optical designs. Based on this free-form optics design, it is possible to use more compact and flexible single-axis trackers without being forced to abandon high-efficiency multi-junction solar cells; key to open small to mid-scale installation markets for CPV.

Furthermore, it is shown that this new free-form optics design method has also potential for other application areas. For imaging systems with high aspect ratio, ray tracing simulations for calculated free-form lenses demonstrate superior imaging performance when compared to conventional rotational symmetric systems.
Date of AwardMar 2013
Original languageEnglish
SupervisorHugo Thienpont (Promotor), Youri Meuret (Promotor), Johan Stiens (Jury), Rik Pintelon (Jury), Erik Stijns (Jury), Peter Schelkens (Jury), Pablo Benitez (Jury), Alois Herkommer (Jury) & Kristiaan Neyts (Jury)


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