Sanyo and SunPower quarrel over who has the cells with the highest efficiencies
The trend toward higher PV module efficiencies was much in evidence at this year’s Intersolar Europe trade fair. Sanyo and SunPower bumped heads over who holds the record for crystalline silicon cells. Though there are a number of different methods for measuring cell efficiencies, SunPower is the clear winner.
It is almost midday at the New Munich Trade Fair Centre, the site of Intersolar Europe 2011. In Hall A3, one of more than a dozen large exhibition rooms at the sprawling conference space in Munich, Germany, a woman at stand 540 tries to catch the attention of visitors streaming by. A few of them stop and listen. Without batting an eye, she delivers her brief speech into the microphone. She says that Japan’s Sanyo Electric Co. Ltd. has boosted the efficiency of its Heterojunction with Intrinsic Thin Layer (HIT) cells by adding a third busbar, by improving the electrical properties between the conductive layers and by using a sophisticated pyramid surface structure. The new HIT N240 module on display next to her is described as having a cell efficiency of 21.6 percent – the “highest in the world” according to the aggressive advertising.
Some in the audience are taken aback by these words, for only two halls down, SunPower Corp., headquartered in San Jose, California, also boasts of an efficiency record for crystalline silicon (c-Si) solar cells. SunPower is Sanyo’s biggest competitor in the high-performance module segment. Two SunPower employees strolling past the Sanyo stand confidently dismiss Sanyo’s claim. “That’s a lot of baloney,” one of them says, adding, “Ours is more efficient.”
No universal standard
The Californians at SunPower’s trade booth are obviously a little surprised by Sanyo’s sales promotion. A few days earlier, SunPower received an award from Guinness World Records for having the highest efficiency modules obtainable on the market, says spokeswoman Ingrid Ekstrom. The company initially unveiled modules that break the 20-percent barrier at Intersolar Europe: a 333 W module and a 327 W module, each with 96 cells and efficiencies of 20.4 percent and 20.1 percent, respectively. According to the datasheet, their cell efficiencies are 22.9 percent and 22.5 percent. Since 2007, SunPower has made cells with an average efficiency of 22.4 percent, says Ekstrom. So why does Sanyo claim to be the world champion in cell efficiency?
Christian Comes, a European sales manager for Sanyo, does not provide a conclusive answer. The best HIT module at Sanyo’s Intersolar stand achieves an efficiency of 19 percent. “It‘s a neck-and-neck race,” he says, arguing that SunPower makes its modules so big that the frame plays a smaller role in relation to the cell surface. This increases the module efficiency. But this doesn’t explain Sanyo’s advertising campaign, which boasts of the company’s record-setting cell efficiencies.
What at first sight appears to be a petty quarrel reveals a deeper issue: efficiency can be measured in a number of very different ways. The two companies apparently mistrust each other, although both have their products tested by internationally recognized institutions. SunPower primarily relies on the US-based National Renewable Energy Laboratory (NREL) and Sanyo uses Japan’s National Institute of Advanced Industrial Science and Technology (AIST).
Just as Sanyo accuses SunPower of using larger module sizes to its advantage, SunPower reproaches Sanyo for deriving its cell efficiency from the module aperture area. This is the total laminated cell area, including the spaces between the cells. During testing, light is reflected from these spaces and scattered diffusely onto the cells, increasing the measured efficiencies. This is not the usual standard for determining cell efficiency, and thus it can be misleading to consumers, according to SunPower. Sanyo could not be reached to discuss this practice.
And the winner is… SunPower
Wilhelm Warta, who handles silicon cell characterization, quality assurance and measuring technology development at the Fraunhofer Institute for Solar Energy Systems (ISE) in Germany, does not generally regard measuring the efficiencies of laminated cells as a problem. “From a calibration perspective, conditions must be defined under which a statement can be understood,” says Warta. He notes that using different standards »gives rise to additional areas of uncertainty and sets the stage for endless disputes.
According to Warta, it makes more sense for Sanyo to test samples in the laminated state – regardless of the effect of the interspace reflection – since HIT cells can also collect light reflected from their rear-side foil. This effect can increase the measured output by as much as 1 percentage point, thereby boosting efficiency, says Warta. Thus, the existing standard – laying cells on a nonreflecting black measuring block to be calibrated – favors SunPower.
Apart from this, there are still no valid international measuring standards, according to Warta. This is the case for conventional c-Si cells and also for rear-contact cells made by the Californian manufacturer. With the conductors on the rear of the cells, it is difficult to achieve an even contact to the base in direct-current sun simulators, in order to reach a standard measurement temperature of 25 °C. This is by no means a trivial problem, and, to the best of Warta’s knowledge, it has only been resolved to a certain extent at the ISE. “For a correct comparison, both cell types with rear-side foil should be encapsulated, as in a module. The correct evaluation of the environment can then also be carried out during a single cell measurement in a so-called minimodule.”
Nevertheless, as far as publicly available and independently confirmed test results are concerned, SunPower is the clear winner of the contest, with a 1.3-percentage-point difference between the efficiencies of its cells and those of its rivals.
SunPower has taken steps to protect its claim on the efficiency record. In December, it called on Sanyo to refrain from advertising its cells as having the highest efficiencies, and the Japanese company immediately acceded to the request, says Ekstrom.
But apparently the Sanyo marketing department could not resist boasting at the trade fair. With so many visitors at Intersolar Europe, companies will do anything to attract attention.
California-based SunPower broke the 20-percent module efficiency barrier just as French oil giant Total SA acquired a majority stake in the company.
At Intersolar Europe, Sanyo advertised its HIT modules – falsely – as having the world’s highest cell efficiencies.
Source: Photon International
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