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Photovoltaics

Concentrator Photovoltaics (cpv): Frequently Asked Questions

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1. What is the optimum concentration level for your solar cells?

A: While there is no clear answer to this question, concentration in the range of 500 to 1000 suns is a good place to start. At lower concentration ratios, silicon solar cells probably make better economic sense. At significantly higher concentration ratios (>1000x), systems can run into issues with cooling and series resistance. At very high concentration (>2000x) multijunction cells may run into issues with stability of the tunnel junctions.


2. Are your terrestrial cells the same as your space solar cells?

A: While there are similarities between a space solar cell and a terrestrial CPV cell, there are intentional differences incorporated into the cell structure to account for the different operating conditions on earth versus in space and the higher current under concentration for CPV cells.


3. Do you have standard cell sizes or standard configurations?

A: Our most common cell sizes are the CDO-100 and CDO-030. The CDO-100 is a concentrator cell with dual busbars (the two metal bars on the edge of the solar cell), and an area of 1 cm². Similarly, the CDO-030 has dual busbars and an aperture area of 0.3 cm² (5.5×5.5 mm aperture). These cells can be purchased in two configurations:

  1. Bare cell in waffle trays
  2. Whole undiced wafers (“processed wafers”) (CDO-030 only)

We also supply both types of cells for prototyping purposes as a Concentrator Cell Assembly (CCA): cell soldered to a ceramic substrate with a bypass diode and conformal coating

4. What is the output of the solar cell?

A: To find the power output of the cell, you need to know the area, the incoming irradiance, and the efficiency. The efficiency is a function of the operating temperature — the cell loses approximately 0.05% absolute efficiency for every °C operation above the nominal 25°C. So if the efficiency at your operating temperature is 38%, and the incoming irradiance is 100 W/cm² (i.e., 1000x effective concentration), and the cell is a CDO-030, the expected power output is 0.38 × 100 × 0.3 = 11.4 Watts, and the remaining irradiance is lost to heat.


5. Can I Have Samples of Your High Concentrator Cells?

A: Based on the large volume of product sample requests that we receive on a daily basis and the cost of the cells, we are unable to offer product samples.


6. Can I visit your site?

A: Our Sylmar, California facility does not have a showroom. As our facility manufactures products for a number of different programs, access to the facility is restricted.


7. What is the Minimum Number of Cells I Can Purchase?

A: Spectrolab has a minimum order value of $20,000 for new production. However, we do accept smaller orders for residual products in inventory, and for orders under $20,000 we add a $1,195 charge to cover our administrative costs.


8. For budgetary purposes, what is the price of the cells in large volume?

A: Higher cell efficiency and reliability is critical for terrestrial CPV systems since it is the one component that leverages the cost effectiveness of the whole CPV system. Spectrolab's current price for a tested solar cell on a processed wafer, 1 sq. cm. in area, is about $0.20/Watt assuming an incident illumination intensity of 1000 suns, and measured at 25 degrees Celsius. Longer term we are working to achieve further cost reductions through performance improvements (efficiency), raw materials cost reductions, and productivity.


9. Can you export CPV solar cells outside the United States?

A: Yes. Currently, concentrator photovoltaic (CPV) solar cells for terrestrial purposes (i.e., not for space) are controlled by the U.S. Department of Commerce under a commodity jurisdiction classification of EAR 99. This classification allows for the export of CPV solar cells to most destinations unless the receiving party is on the Denied Parties List or there is a country embargo in place.


10. What types of CPV solar cells can I buy?

A: Please refer to the current data sheets for our CPV products located on our web page at http://www.spectrolab.com/dataSheets.htm We continue to invest research and development to bring the most robust and high efficiency products to the CPV industry, and the data sheets will continue to reflect the most up-to-date listing of our available products.


11. Do you sell lenses, optics, trackers or related concentrator housing materials?

A: Spectrolab sells only bare cells or wafers in volume, and CCAs in prototype quantities.


12. Do you offer cells for dense arrays?

A. Yes. We have existing cell designs suitable for dense arrays and can provide customized solutions on request.


13. Do you offer information on bypass diodes, solder types, or thermal adhesive to use?

A: Generally, yes, but some information related to our processes is proprietary and can be made available only after a cooperative agreement is established with a customer.


14. I am a potential consumer and I wish to buy something for my home. What can I buy from you?

A: Spectrolab does not offer products intended for residential applications. Our products are components incorporated into CPV commercial utility systems. We are not a CPV system provider.


15. Do you manufacture custom CPV solar cells?

A: We prefer to standardize our solar cells. This ensures the lowest prices and the shortest delivery times. However, custom solar cells may be available for large-volume purchases. The customer may pay for non-recurring engineering involved in development and may be responsible for qualifying the assembly to meet unique requirements. There is a minimum requirement of approximately US $40,000 for non-recurring engineering. Delivery time for a non-standard cell is subject to engineering time commitments. Pricing and lead time can be furnished upon request.


16. What is the lead time for your products?

A: LLead times vary, depending on the configuration and quantity of cells ordered. Standard bare cells require about six weeks, but lead time may be shorter or longer depending on factory loads.


17. What is the maximum temperature a concentrator solar cell can handle?

A: During assembly, the cells can be exposed to temperatures as high as 300° C for a few minutes. During operation, we suggest maintaining a temperature below 110°C. Higher operating temperatures will reduce the cell efficiency and may jeopardize the lifetime of the cells in the field. Cell performance is a function of temperature. For every 10° C rise in cell temperature, the cell efficiency drops by about 0.5% absolute efficiency. That is, a 40% cell at 25° C will be a 39% cell at 45° C.


18. Do you sell reject or surplus CPV solar cells?

A: Yes, subject to availability.


19. How can I attach CPV cells on cooling plates or ceramic sub-mounts?

A: Assembly details are specific to the CPV system being developed. Generally, cells are attached with either solder or conductive epoxy. In either case it is very important to ensure that the attachment is void-free or nearly so, in order to avoid overheating under high concentration.


20. Is active cooling of solar cells required?

A: The question of how to cool the cells is up to the CPV system designer. Typically dense-array modules are actively cooled, whereas point focus systems are passively cooled.


21. What kinds of optics are best for concentrating light on your cells?

A: Our solar cells work with reflective or refractive optics. For refractive optics, we recommend the use of a light homogenizer to smooth the flux on the cells and to avoid issues of chromatic aberration. The multijunction cell consists of three subcells in series, each of which absorbs a different segment of the solar spectrum. If the optics fail to transmit certain wavelengths, one subcell may limit the current output of the entire multijunction cell.


22. Does uniformity of light make a difference in terms of cell performance?

A: Yes, with highly non-uniform flux, the efficiency will drop and temperature variation could cause reliability issues.


23. What is the highest-efficiency cell you have produced?

A: Spectrolab has the highest efficiency production cells in the industry, and in 2009 achieved a new world record (at the time) for highest confirmed efficiency at 41.6% which was confirmed by the National Renewable Energy Laboratory of the US Department of Energy. Please note that our production cell efficiency is optimized for use with a glass secondary (or coverglass) but the efficiency we quote is for a measurement in air. Thus, in a real system, there is a “glassing gain” of 1–2% relative (or 0.4–0.8% absolute) in the cell efficiency.


24. How do I solder the solar cells onto ceramic substrates?

A: Achieving a good solder joint between the cells and the next level substrate is critical for the robustness of the cells under continuous illumination. A solder joint that has large voids will result in high rates of cell infant mortality due to thermal runaway. We use a vacuum solder process and fluxless solder to meet this requirement.


25. What warranty is offered for CPV cells?

A: Warranty clauses are discussed in our terms and conditions for sale. There is an initial acceptance period during which the defective cells are replaced if they do not meet the agreed-upon specifications. For large volume customers we offer a 25-year cell warranty, provided use of the cell is in strict compliance with operating environment limits.


26. Where can I buy a concentrator system?

A: WPlease contact concentrator PV system manufacturers directly. A web search on "CPV" will yield some answers. As Spectrolab does not represent customers, you are encouraged to contact them directly.


27. Do you provide technical support?

A: Our prices do not include engineering support. When technical assistance is required, it is available on a time and materials basis (8 hour minimum). Please contact us if you require such assistance.


 

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