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Metallurgical-grade Silicon Making Inroads in PV

The persistent shortage of polysilicon is creating a push towards metallurgical-grade silicon as an alternative raw material.

In 2006, Dow Corning Corporation announced the development of PV 1101 SoG Silicon, the first commercially available metallurgical feedstock produced using large scale manufacturing processes. Dow announced it was not a replacement for traditional silicon, but needed to be blended with polysilicon to be suitable for PV cells. Now, Q-Cells has announced the use of non-blended metallurgical silicon that would be used almost exclusively at their new 300 MW Malaysian plant.

Silicon is extracted from silica in electric furnaces using carbon electrodes at high temperatures. During the process of production, liquid silicon is collected at the bottom of the furnace. Drained and cooled liquid silicon is called metallurgical-grade silicon and this sort of silicon is at least 98% pure. Prior to the recent announcements, metallurgical-grade silicon produced in the arc furnace could not be used in solar cells or semiconductors without significant purification. Now, several cell plants have announced plans to use upgraded metallurgical grade (UMG) silicon as a replacement feedstock to solar grade silicon.

Q-Cells recently signed an agreement signed with Becancour Silicon, a division of Timminco, to supply purified metallurgical silicon for its new 300MW solar cell facility, Line VII in Malaysia. The Malaysian facility is expected to start production early in 2009. Q-Cells also said its production facility in Thalheim, Germany, which had a 20MW capacity for using the metallurgical silicon, would be expanded to 80MW in 2008.

Timminco, whose stock has climbed to over 30, up from 70 cents last year, has claimed to have developed a low cost high volume method for creating solar cell grade silicon directly from metallurgical grade silicon. Their process reportedly can be carried out on 5-to-10 ton batches of molten silicon. In response to industry skepticism, the company hired Photon Consulting for a report on their metallurgical silicon production processes. “Operations and processes have potential for massive growth and, possibly, for reshaping the silicon industry”, declared Michael Rogol Managing Director of Photon Consulting. “The (Timminco) equipment is very impressive, very low-cost, beyond poly-scale. In interviews, several customers have reported cell efficiencies above 14% and some above 15% utilizing 100% (unblended) solar grade silicon from Bécancour.”

Timminco has also announced supply agreements with Canadian Solar and Solar Power Industries. Canadian Solar has said that it has converted a solar cell production line to use 100 percent UMG silicon for an expected output of 30-40MW of its low-cost PV ‘e-Modules’ in 2008, primarily for the European and U.S. markets. The UMG line became operational in April with approximately 1MW so far produced.

Other companies are also working on processes to achieve the mass conversion of tonnages of upgraded metallurgical silicon for solar use.

In April, Ontario-based startup 6N Silicon Inc. has raised up to $20 million in a second round of financing. The company's goal is to be the lowest-cost provider of solar-grade silicon that doesn't need to be blended with high-purity silicon. They claim to have come up with a proprietary, low-energy process, which can be inexpensively scaled up, for upgrading standard metallurgical-grade silicon into solar grade silicon. 6N wants to be one of the industry's leading suppliers of solar-grade silicon within three to five years.

SST reports that JFE Steel is providing polysilicon made by a metallurgical process that drives off the impurities in the silicon (first with an electron beam in a vacuum, then with a plasma torch in an argon atmosphere) and refines and cools the melt in a continuous process similar to steelmaking. These metallurgical polysilicon furnaces can be brought online in small increments relatively quickly and cheaply, though they do turn out product of only "six nines" (99.9999%) purity -- less than the eleven nines of the conventional Siemens process, but apparently sufficient for at least current-generation solar cells.

ARISE Technology Corporation, also from Canada, has announced silicon feedstock development project, partially funded by Sustainable Development Technology Canada (SDTC). The project is developing a new approach being used for refining high-purity silicon needed for high-efficiency PV (photovoltaic) cells. The proprietary process that ARISE is developing is intended to produce 7N+ high-purity (99.99999 percent) silicon for PV applications using a simplified chemical vapor deposition process. ARISE believes that its ability to produce 7N+ high-purity silicon will enable it to meet its target of more than 20 percent efficiency for PV cells.

LDK Solar is also sampling metallurgical silicon as well as building its own polysilicon plant in China.

The global shortage of solar grade silicon has severely constrained solar market growth, and had other impacts such as the spurring of thin film technology development. Silicon will also remain the most significant cost component of silicon-based solar cells. If UMG silicon proves to be a viable alternative to silicon grade feedstock and can ramp quickly, big changes in the PV market can be expected.