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SunPower Corp.

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Project Objective:  Identify the critical optical issues, critical system integration issues, and to assess the likely ultimate potential of lens-based concentrator module technology. 

Approach/Background:  Studies have shown that the concentrator could be the ultimate low-cost photovoltaic (PV) device.  In order to accomplish the long-term program goal of brining multijunction concentrators to 33% efficiency, the potential optical systems must be fully researched and those with greatest probability of success identified.  The effect of the balance of the module system, such as optical alignment and tolerancing, must also be identified. 

Status/Accomplishments: During this subcontract SunPower performed fundamental research on Fresnel lenses, secondary optical elements, and module system integration.  Various optical elements were designed using SunPower software and tested on a commercial ray-tracing program.  Additionally, an outdoor test facility to test optical systems was designed and fabricated.  A detailed cost analysis was done on a variety of lens-based concentrator modules using commercial SunPower cells.  Two different secondary optical elements (SOEs) were fabricated using an experimental low-cost material. 

Preliminary outdoor lens studies show that the optimum operation focal length for a Fresnel lens may not be at the maximum optical transmission point due to mal distribution of light at that length.  However, ray trace studies show that it is possible to design an SOE with both good transmission and flux distribution.  There appears to be a tradeoff between uniform flux distribution and acceptance angle.

The costing study was done in two phases.  The first phase looked at the general effect of aperture size, concentration ratio, and similar factors.  The result indicates that the most cost-effective lens size would be about 11 inches square and the heat dissipater size should also be about 11 inches square.  This design point used a SunPower cell with an active area of 1.21 square centimeters.

The second phase of the costing study compared specific designs.  It was found that the SunPower micro-concentrator was lower relative cost than the competitive designs studied due to various factors.

The types of designs studied were:

1. A large integrated type design where the modules are self supporting structures and also part of a tracker structure
2. Modules with sheet metal housing where the housing is the heat dissipater 
3. Modules with sheet metal housing and separate heat dissipaters 
4. Modules with plastic housing and separate heat dissipaters 
5. The SunPower micro concentrator  

The SunPower micro concentrator would make an ideal candidate for a module using III-V concentrator cells.  A very high concentration ratio is possible.  The cells are tiny, providing high wafer yield.  The III-V cells would not suffer from edge degradation effects like silicon cells display.  The shipping costs are low compared with bulky traditional Fresnel modules.  The primary lens uses a combination of a solid aspheric center and a Fresnel reflective/refractive outer portion.  An SOE is used on top of the cell to improve acceptance angle.  The total height of the micro concentrator is less than one inch.  The module has wide customer acceptance because it looks somewhat like existing flat-plate product. 

Several candidate materials were tested for low-cost SOEs, which include acrylic, polycarbonate, and exotic high temperature plastics.  It was discovered that one candidate material that has the potential for molding low-cost SOEs.  Additional testing is necessary, but the preliminary results are encouraging.  The cost of these SOEs could be on the order of 10 cents, whereas the molded glass SOEs are close to 6 dollars.  The material can be easily molded to any shape and also used to encapsulate the receiver in the same molding. 

SunPower High-Performance PV Publications:
"Lens-Based Concentrator Modules: Exploring Critical Optical and System Integration Issues," National Center for Photovoltaics Program Review Meeting, Oct. 14-17, 2001 (Lakewood, CO). (PDF 34 KB)

N. Kaminar, S. Daroczi, L. Pavani, P. Verlinden