9 September 2008 Maximum-performance photovoltaic concentration with unfolded aplanatic optics
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Some current high-concentration photovoltaic designs are based on folded mirror optics, i.e., the optical path is reversed via reflection in order to achieve minimal aspect ratio. One apparently overlooked class of aplanats is the unfolded dualmirror aplanat explored in this presentation. The confluence of practical constraints presented by many highconcentration photovoltaic systems creates a niche for unfolded aplanatic optics that permit the elimination of optical bonds between dielectric elements and solar cells while achieving an angular tolerance near the fundamental limit. The long-term material integrity of the optical bonds - especially at high irradiance - remains to be established and has been viewed as potentially precarious. Furthermore, the unfolded optic and the target can be decoupled. The disadvantage of unfolded optics is depth: concentrator aspect ratios far larger than the fundamental compactness bound realizable with folded optics. However, with high-concentration photovoltaics evolving to millimeter-scale cells, the corresponding depth of the concentrator would be only a few centimeters: compatible with precision large-volume fabrication techniques. We also show how a lens at the center of the aplanat's glazing permits a noticeable reduction in concentrator depth. A fringe benefit is the paucity of hot spots on the optical elements, creating the option of coated polymeric reflectors, which reduces mass and cost.
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Jeffrey M. Gordon, Jeffrey M. Gordon, Daniel Feuermann, Daniel Feuermann, Pete Young, Pete Young, } "Maximum-performance photovoltaic concentration with unfolded aplanatic optics", Proc. SPIE 7043, High and Low Concentration for Solar Electric Applications III, 70430D (9 September 2008); doi: 10.1117/12.792229; https://doi.org/10.1117/12.792229

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