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Solar Power Moves into the Fast Lane

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Solar Power Moves into the Fast Lane

Copyright: EADS Astrium, A. Ribeiro Gamma
The photovoltaic (PV) industry is looking for ways to find ways of making solar energy competitive. One possibility is to bundle the sunlight and focus it on highly efficient solar cells.
The lens array is found on the glass pane's underside. Each field contains a Fresnel lens.

Flat Design, High Concentration

A leading international producer of photovoltaic systems, SolarTec International AG (based in Aschheim, near Munich), has developed marketable, highly efficient and cost-effective concentrator modules.
In these modules, a checkerboard array of silicone Fresnel lenses, located directly under the glass front panel, concentrates the incident sunlight. The design was developed in close cooperation with the well-known Ioffe Physio-Technical Institute in St. Petersburg, Russia.
The lens system is cast directly onto the back of a glass plate. Together, the glass and silicone form a laminate. Each Fresnel lens focuses sunlight onto a solar cell positioned exactly beneath it. In general, a concentrator module can only produce electricity if the incident light falls directly, that is vertically, onto the lens.
Matthias Sturm, head of R&D at SolarTec International, checks a concentrator module

Gearing Up for Mass Production

Extensive know-how is needed to cast the flat, delicately structured lens as precisely and reproducibly as possible – as well as an appropriate, pourable silicone.
SolarTec International decided to use WACKER's two-component, liquid silicone rubber soon to be released under the name ELASTOSIL® Solar 3214.
The Fresnel lens structure is cast
Matthias Sturm, head of R&D at SolarTec International, explains his choice: “It pours well, is crystal clear and elastic after curing, and accurately assumes the shape defined by the mold.”
Other characteristics typical of silicones, such as their chemical and thermal stability, predestine them for photovoltaic applications.
SolarTec International is currently preparing to manufacture flat concentrator modules in series. The final module-design details are now being perfected, and the production line has been set up.
Further Information

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Concentrator Module Design

A concentrator module from SolarTec International has a square 50 x 50 cm surface area and is about 8 cm thick. The checkerboard array of 144 silicone Fresnel lenses is clearly visible through the front glass cover. The lenses were cast directly on the back side of the glass sheet.

This primary optical device concentrates incident sunlight by a factor of 450. At each focal point there is a stack cell composed of three semiconducting layers (gallium indium phosphide and gallium indium arsenide on a germanium substrate). To maximize the module’s efficiency, each of these cells must be positioned exactly beneath the lens to within 20 µm of its focal center.

The cells are mounted on metal heat sinks required for dissipating excess heat. Directly above each cell is a secondary optical device that increases the module’s tolerance to tracking errors.

From Thick Lenses to Fresnel Lenses

A Fresnel lens is an especially flat optical lens that features a characteristic stepped structure. The idea behind this stepped structure is that, for any given lens material, the degree of refraction depends not on the thickness of the layer the light ray passes through, but on its angle of incidence.

To picture this stepped structure, mentally divide a conventional, convex lens into concentric rings. Replace the curved edges with flat ones, cut away the cylinders beneath these rings. Finally, fold the remaining structure together like a telescope so that the rings lie on a common baseline.

This stepped structure allows for a dramatic reduction in thickness and weight, particularly in lenses with a short focal length, compared to conventional optical lenses. CPV systems require strongly bundled light. The focal point should be kept to a minimum, and the incident light distributed as homogeneously as possible over the tiny cell surface.

The greater the number of concentric rings and the sharper the edges of the structures, the better the effect is. The delicate structures required for the Fresnel lens system constituting the primary optical device cannot be made from glass with sufficient precision, but only from liquid silicone rubber or plastics such as acrylic glass and polycarbonate.