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Spotlight on Optics


  • August 2013

Optics InfoBase > Spotlight on Optics > Nonimaging achromatic shaped Fresnel lenses for ultrahigh solar concentration

Nonimaging achromatic shaped Fresnel lenses for ultrahigh solar concentration

Published in Optics Letters, Vol. 38 Issue 10, pp.1730-1732 (2013)
by Fabian Languy and Serge Habraken

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Spotlight summary: The amount of renewable solar energy making it into our power grids could see a substantial increase with the invention of cheap, simple and efficient designs for solar concentration. An efficient concentrator lens can significantly reduce the required area of photovoltaic (PV) cells per collected kilowatt-hour, thus driving down manufacturing costs and allowing more units to be installed for the same price.

Two primary factors currently limit the efficiency of standard solar concentrator optics. First, geometric aberrations prevent conventional concentrator lenses from accurately focusing the sun’s light to a small fixed spot as the sun arcs across the sky, even if it is tracked quite accurately. Second, chromatic aberrations additionally limit the localizability of solar radiation — it is challenging to force sunlight’s entire spectrum of colors into a small area. For example, a single Fresnel lens concentrator, which includes both geometric and chromatic aberrations, can achieve a solar concentration ratio of ~1000x. A recently proposed Fresnel doublet design, based upon fusing two different inexpensive lens materials together, can decrease chromatic aberration to improve this ratio by a factor of two. However, the theoretical solar concentration ratio limit of approximately 46,000x is still nowhere in sight, leaving much room for improvement.

Languy and Habraken’s most current work has given a new twist to the above-mentioned Fresnel doublet concentrator design, quite literally. By carefully analyzing the effect of shaping a Fresnel doublet into a dome-like curve, they have come up with an optimal free-form lens shape that can achieve much higher concentration ratios than those previously reported (~8500x). The lens’s curved shape primarily helps overcome the first issue discussed above — at off-center positions the sun’s energy can still be guided to a small PV cell’s fixed location. Additionally, the optimized shape improves conversion efficiency by distributing the sunlight’s flux more uniformly across the PV cell area.

Previous attempts at improving concentrator lenses have neglected the potential of the free-form Fresnel doublet, first, for fear of soiling — any dirt adhering to the lens will absorb radiation and thus decrease efficiency. Languy and Habraken’s proposed lens cleverly avoids soiling by ensuring its Fresnel grooves are facing downward. A second, more challenging issue confronting their design concerns its manufacturability. While Languy and Habraken argue that certain properties of their free-form lens should allow for easy construction, only an experimental test can truly identify the actual challenges and costs associated with fabrication of such an unconventional optical element. However, this absence of experimental realization certainly does not detract from the significance of this work’s smart optimization process and interesting conclusion. Hopefully, its clever modeling techniques will find their way into other areas of concentrator lens design, such as for linear concentrator layout, or even to modify parabolic reflectors. Regardless of such extensions, it appears the current free-form lens’s potential benefit to solar energy could be direct and significant — a greatly needed push to help renewable energy production gain some momentum.

--Roarke Horstmeyer

Technical Division: Optical Design and Instrumentation
ToC Category: Optical Design and Fabrication
OCIS Codes: (080.2740) Geometric optics : Geometric optical design
(220.1000) Optical design and fabrication : Aberration compensation
(220.1770) Optical design and fabrication : Concentrators
(350.6050) Other areas of optics : Solar energy
(220.4298) Optical design and fabrication : Nonimaging optics

Posted on August 27, 2013

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