Jason Karp an electrical engineering Ph.D. student at the University of California, San Diego and his colleagues at the UC San Diego Jacobs School of Engineering are developing an inexpensive optical concentrator and assembly to offset the high cost of very efficient solar cells. The development should lead to solar concentrators that are less expensive and require fewer photovoltaic cells than existing solar concentrators or photovoltaic solar cell panels.

The design is an optical innovation. After the surface lenses focuses the light with a two-dimensional lens array a secondary optic, multimode slab waveguide is used as a secondary to collect and homogenize the sunlight.

Reflective facets fabricated on the backside of the waveguide act as fold mirrors to couple sunlight into the waveguide at angles, which exceed the critical angle for total internal reflection. These facets occupy a small fraction of the total waveguide surface and enable high geometric concentrations despite decoupling loss if light strikes a subsequent coupling region.

This geometry yields a thin, flat profile for moderate concentration systems that may be fabricated by low-cost roll manufacturing. The analyses of tradeoffs show optimized designs can achieve 90% and 82% optical efficiency at 73x and 300x concentration, respectively.

Karp and his group may be in the money – for concentrator photovoltaic (CPV) to be cost-effective, the complete cost of the optics, assembly and mechanical tracking must not exceed the cost savings gained from using small area PV cells.  The team gets it; the place to shave expense is in the collection of the light, saving a big share with reduced photovoltaic cell counts.

Sunlight collected by each aperture of the arrayed primary collector is coupled into a common slab waveguide using localized injection features such as prisms, gratings or scattering surfaces. Rays that exceed the critical angle defined by Snell’s Law propagate via total internal reflection (TIR) within the waveguide to the exit aperture, typically at the edge of the slab.  TIR is a complete reflection with negligible spectral or polarization-dependent losses, which enables long propagation lifetimes.  The waveguide transports sunlight collected over the entire input aperture to a single PV cell placed at the waveguide edge. PV alignment becomes trivial since comparatively large cells are cemented to the waveguide edge(s).  Fewer PV cells reduce connection complexity and allow one heat sink to manage the entire system output.

Planar MicroOptic Solar Concentrator. Click image for more info.

As illustrated, the innovation is going beyond a lens that concentrate an area to a PV, Karp’s waveguide collects several lenses to one or more PVs.  This has to dramatically cut costs and allow budgeting for extremely efficient PVs.

The goal was to design a concentrator optic, which could be fabricated at an extremely low cost per unit area. Constraining the design to be compatible with a continuous roll process-manufacturing platform, as opposed to injection molded and assembled elements, maximizes the cost advantage of CPV.  Roll processing can perform a range of functions on rigid or flexible substrates such as embossing of refractive or diffractive structures, dielectric and metallic deposition and the joining of multiple processed layers.

The team’s paper, in the January 2010 issue of the journal Optics Express, available in a pdf download, covers in detail concentrator geometry, coupling the waveguide, optimizing the system, and the building of a prototype.  The paper also discusses the method Karp and his team use to self align the concentrator during fabrication. At 12 pages and lucid for the non-optic expert, it’s a worthwhile read.

Prototype Planar MicroOptic Solar Concentrator. Click image for more info.

The team took their prototype outdoors for testing to find the prototype system reached 90% of its maximum optical efficiency with  ± 1° angular acceptance.  The optical efficiency of the prototype system was significantly lower than the optimized simulations using custom optical elements.  Despite its relative inefficiency the experimental measurements were in close agreement with the optical model and support the notion that optimized designs would also perform with high efficiency. The team is currently pursuing variations of the basic structure to increase both concentration and optical efficiency.

The team has demonstrated self-aligned fabrication using off-the-shelf components to create a 37.5x prototype concentrator with 32.4% optical efficiency. Systems with greater than 80% efficiency are expected when using a custom lens array with a 100% fill factor and minimal aberrations.  A CPV with multimode waveguides opens a new design space for large-scale concentrator optics with the added benefits of flux uniformity and fewer PV cells in a thin, planar geometry.

OK.  That’s all real technical.  But it works, the lens array and the waveguide beneath can be roll to roll process manufactured.  Mount some photovoltaic cells along the selected edge and you have a low cost high efficiency solar panel.  One has to like this, especially if the savings can justify the cost of a solar tracking system to keep the panel squarely facing the sun.

Something has to be done about solar panel costs – it looks like Karp and his team have a very good shot at helping build a much larger market.


22 Comments so far

  1. Building a Much Lower Cost Solar Cell | New Energy and Fuel | Future Of Solar Panels on May 5, 2010 1:56 AM

    […] the original post: Building a Much Lower Cost Solar Cell | New Energy and Fuel Share and […]

  2. TomPier on May 6, 2010 6:58 AM

    great post as usual!

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  4. Lowering Solar Cell Costs | Build Photovoltaic Solar Panels on May 10, 2010 4:32 AM

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  8. eric on July 28, 2010 10:53 PM

    A lot of these new solar panel labs shouldtake the time to colaborate to improve effiency, because the idea is to drive down cost & get their new product out to the market. The idea that companies are investing in average low power output devices for their energy needs is screwy and a waste of money when these new inventions blow old panel technology out of the water! It seems a measure of patience until new products are made available is in order!

  9.  Fire Grate on August 19, 2010 11:50 AM

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  18. Predrag Raos on November 6, 2011 1:41 AM

    The Rossi’s problem is that his invention is basically unpatentable or at least easily copy-catiable. You can’t patent a reaction, and forbid the use of the host of similar ones to boot. So he tries to skim as much money as possible till the competition rolls over him. If there is a scheme, then this is a bussiness scheme, and very reasonable at that.

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