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Solar & Alternative Energy

A micro-optic solar concentrator

Lens arrays couple sunlight into a common slab waveguide to create large, inexpensive optics for solar photovoltaics.
26 April 2010, SPIE Newsroom. DOI: 10.1117/2.1201004.002673

Silicon-based photovoltaics (PVs) convert less than 20% of incident sunlight into electrical energy, yet account for almost all solar-power generation. High-efficiency solar cells developed for the space industry have demonstrated more than 41% conversion efficiency by layering multiple semiconductor junctions that capture large portions of the solar spectrum.1 Fabrication and material costs limit these cells to only a few square centimeters, making them impractical for flat-panel installations. Concentrator photovoltaics (CPV) incorporate large-area optics that collect and deposit energy onto small, efficient solar cells with the promise of reducing electricity-generation costs compared to silicon-based PVs.

Optics for solar concentration typically consist of lenses or mirrors focusing onto secondary elements that eliminate intensity variations at the PV cell. A common approach places dozens of lenses into a shared tracking platform, each focusing onto independent secondary optics and solar cells.2 The large quantity of components increases mounting, alignment, and electrical-connection costs. Our proposed concentrator design replaces discrete optics with a 2D lens array and a common slab waveguide. Sunlight collected by the array focuses onto localized mirrors positioned to reflect light at angles that exceed the critical angle for total internal reflection and, therefore, couple into the waveguide. Coupled light is homogenized as it propagates towards the exit aperture at the slab edge(s) (see Figure 1). The PV cell and heatsink mount directly to the output edge. The coupling mirrors are fabricated using simple lithography techniques that make the design compatible with large-scale manufacturing, including roll processing.

Figure 1. Micro-optic concentrators combine a lens array and slab waveguide. At each focus, 120° mirrored prisms couple light into the waveguide (inset). PV: Photovoltaic.

The amount of focusing provided by solar concentrators is defined by the geometric-concentration ratio, which describes the ratio of input to output apertures. Optical efficiency is the fraction of light reaching the PV cell. It accounts for surface reflections, material absorption, and losses associated with propagation within the waveguide. The micro-optic concentrator uses 120°-apex prisms placed at each focus that symmetrically reflect and couple sunlight into the waveguide. Since the thickness remains uniform, guided light may strike a subsequent coupler and reflect out of the system as loss.3 The geometric-concentration ratio becomes the waveguide length divided by twice its thickness, with no dependence on width. The lens and acceptance angle determine the size of each coupling prism as well as impact losses within the waveguide. Propagation losses scale with waveguide length, leading to a tradeoff between geometric-concentration ratio and optical efficiency.

We simulated the design using Zemax nonsequential ray tracing. An optimized system used 2.4mm-diameter, F/2.45 lenses (F: Focal ratio) focusing into a 1mm-thick BK7 glass slab. Prism couplers covered 0.11% of the back surface. At 100× geometric concentration (200mm waveguide), 89% of the light reached the PV cell. At 300× concentration (600mm waveguide), we calculated 81.9% efficiency. Propagation losses scale with the optical path to the PV cell, causing reduced efficiency at higher concentrations (see Figure 2). For comparison, Fresnel-lens concentrators are 78–87% efficient when considering F number, draft angle, and corner rounding.4

Figure 2. Optical efficiency versus geometric concentration (Cgeo) for simulated F (focal ratio)/2.45 lenses and a 1mm-thick waveguide.

The lens array must be laterally and rotationally aligned to the waveguide. We used a self-aligned UV-lithography technique where prisms were molded in SU-8 photopolymer over the entire waveguide surface. The mounted lens array acted as a mask to selectively cross link polymer regions to ensure proper size and position of each coupler.5

We constructed a proof-of-concept concentrator using commercially available components and self-aligned lithography. The system consisted of F/1.1 acrylic lenses focusing into a 75mm-long by 1mm-thick glass slab. Simulations predicted only 44.8% efficiency because of strong lens aberrations and low array fill factor. We experimentally measured 32.43% optical efficiency with ±1.0° angular acceptance. Figure 3 shows images of the fabricated prisms and testing.

Figure 3. (a) Scanning-electron-microscope image of waveguide surface and coupling prisms. (b) Output edge of the waveguide. (c) Outdoor testing under sun (c).

In summary, solar concentration using waveguides offers a new design approach for large-scale optics compatible with volume manufacture and assembly. CPV requires inexpensive optics to offset high-efficiency solar-cell costs. Micro-optic concentrators use a slab waveguide to transport and homogenize sunlight collected by a 2D lens array. The planar geometry reduces the number of individual components and can be fabricated using simple lithography techniques. We demonstrated coupling using a proof-of-concept prototype and are building revised systems with near-diffraction-limited F/3 lenses to improve efficiency and begin testing with PV cells. For higher concentrations (>500×), we are researching orthogonal-coupling methods for concentration along the slab width.

Jason H. Karp, Joseph E. Ford
University of California at San Diego
La Jolla, CA

Jason Karp is a photonics PhD student. His research interests include the design and fabrication of optical systems for solar concentration, imaging, and free-space communication.