CPV industry
for new entrants in the cell market, reliability is your ticket to entry,” says Hartsoch. Decisions by SolFocus on the selection of cell suppliers are heavily influenced by the cost of the device and its performance. “You can have really low cost and lower efficiency, and that’s OK, or you can have really high efficiency and high cost, and that’s OK. The real issue is how the cost of the cells and the related performance affects the overall cost of energy from the system,” says Hartsoch.
Lerchenmüller hold similar views. However, he believes that efficiency is the key metric, arguing that one of the benefits of higher efficiency at the system level is a reduction in the number of cells and modules needed to deliver a given output power. He also thinks that it doesn’t necessarily follow that a cell must be more expensive, if it is more efficient.
Any company that is trying to win orders with CPV system manufacturers may have to begin with a qualification period, which can take several months. SolFocus and Soitec both have IEC-certified product. So if they are to adopt different cells, they have to build modules with these cells and then send them out to test labs to pass IEC certification. “It can take six months,” says Lerchenmüller. CPV system makers will only go with a start-up if it can scale its manufacturing capacity. “This industry is going to grow rapidly,” says Hartsoch,
“so you want to partner with the guys that have a technology that can go to volume manufacturing and increase quickly, and investors that are willing to take the companies there.” She points out that the capital costs for CPV system makers to build a 100 MW factory can be as low as just 12 cents per Watt, and depend on the CPV system design. “ For the cell guys, to increase their capacity, it’s a lot of capital investment.”
If a start-up increases its capacity, it will then be competing with the incumbents that are unlikely to need any in-house investment to cater for the predicted hike in orders over the next few years. “We have at least four cell manufacturers that are readily available to increase product at the right quality and the right cost,” says Lerchenmüller. He explains that these suppliers have a “huge buffer zone” in their manufacturing capacity, which is needed to accommodate the volatility of orders for cells for space applications. It is imperative to hit the deadlines for these aerospace contracts, as the knock- on effects of a delay would be disastrous. “Just converting this overcapacity [into production of cells for CPV] will serve the business for the next two to three years.”
Crunch time
The ease at which global triple- junction cell capacity can
Soitec and SolFocus: Adopting different approaches
The CPV system manufacturers Soitec and SolFocus have adopted different approaches to focusing sunlight onto triple-junction cells: the former uses a single lens, while the latter employs reflecting mirrors.
Soitec prides itself on the simplicity and robustness of its module. Its top part consists of a glass sheet, which
underneath it has a 70 µm-thick silicone film that is embossed and includes the Fresnel lens. The bottom section also features a sheet of glass, which is the foundation for mounting metal heat distributors and the III-V cell.
Engineers at the European outfit have extensively tested silicone for its stability under UV radiation. “We started with just normal UV bulbs, and we didn’t see anything,” says Hansjörg Lerchenmüller, Senior VP of the Customer Group of the
Solar Energy Business Unit at Soitec. So they added more lamps, but were still unable to detect any changes to the silicone. “So we then went down the brute force method and found a guy with a UV laser, and saw the first signs of degradation at UV doses equivalent to something like 3000 years.”
The modules that Soitec makes operate at concentrations of 500 suns. This factor could be increased to 800 or even 1000, but higher concentration requires the addition of a secondary focusing element.
According to Lerchenmüller, it is not clear whether savings resulting from using less semiconductor material outweigh the combination of the additional cost of a secondary optical element; reductions in overall efficiency that stem from additional reflections; increased risk to reliability; and lower manufacturing yields
that result from a more complex production process.
Modules made by Soitec use just one Fresnel lens made from silicone to focus the sun’s rays onto triple-junction cells.
With SolFocus’ reflector-based systems, which currently operate at 650 suns, the case for going to higher concentrations is much stronger. Regardless of the focusing technology, increases in concentration pay the penalty of a reduction in the acceptance angle of the incoming light.
However, according to SolFocus, the acceptance angle for its reflective system – which is over 1° for today’s modules – is two to four times higher than that for most Fresnel-based equivalents.
August / September 2011
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