The blog below was written by student intern Eric Miller.
Those of us who peruse SVTC’s blogs regularly know well the litany of hazardous compounds used or created as side products in the fabrication of solar panels – these include aerosolized silicon dust, silane gas, sulfur hexafluoride, sulfur dioxide, and trichloroethane, to name a few. But by far one of the most troubling of these substances is silicon tetrachloride, SiCl4, which is used in the production of polysilicon and which has proven to be one of the most environmentally damaging byproducts in the manufacture of photovoltaics.
Many might remember the oft-cited 2008 Washington Post article, which chronicles the plight of Li Gengxuan’s village in the Henan Province of China. Li remembers watching representatives from the local “green” polysilicon plant wordlessly dump buckets of bubbling SiCl4 onto the ground, leaving the villagers to deal with fields rendered infertile by the toxic waste, and with air turned acrid from the fumes. Within weeks, crops began to wilt, covered with a white, powdery chemical, and at times, visible clouds of noxious gas up to three feet high would blanket the area around the dump sites.
To provide some perspective, environmentalchemistry.com states that even a small spillage of silicon tetrachloride, exposed to water in the open air, warrants the immediate isolation of an area 100 feet in all directions around the spill. Tellingly, the same distance is recommended for the spillage of weaponized mustard gas, made infamous in WWI, and VX, the most potent nerve toxin ever synthesized. Now, granted, these numbers are as much a function of the mobility of a compound (whether it can be spread by wind, etc.) as its toxicity, but still this data, coupled with the anecdotal evidence, convincingly portrays silicon tetrachloride as a hazard to the environment and the humans living in it.
There are a considerable number of recycling methods that can be taken to alleviate this problem, though. For instance, heated to 1800 degrees Fahrenheit, SiCl4 breaks down into innocuous compounds that can safely be released or reused. However, this represents a huge investment of time, money, and energy, and the reality is that in China’s booming “green” movement there are a number of polysilicon plants, like the one near Li’s village, that simply aren’t making the effort. The Washington Post reports that using environmental protection technology, the cost of producing one ton of polysilicon should be around $84,000; in contrast, many businesses are turning it out at costs of $54,000 or even as low as $21,000 per ton, indicating that corners are being cut.
The potential for pollution, and the temptation to dump compounds like silicon tetrachloride, is a well-known problem within the industry. On many fronts, research is currently being conducted on alternative, more cost-effective methods of contaminant disposal and neutralization. One promising study by scientists at the Wuhan Institute of Technology in Wuhan, China, found that SiCl4 can be decomposed much more cheaply using plasma jets – accelerated streams of electrons that can be collided with toxic molecules to break them down.
While research such as this is a step in the right direction, a marginally cheaper method still won’t incentivize companies to adopt truly green practices. There must be a corresponding governmental and regulatory effort enforcing clean practices – in China, yes, but on the broader scale, as well. Photovoltaics have tremendous potential to revolutionize the way we harness energy and to make our existence on this planet more sustainable, but if we continue to use unsustainable methods to manufacture them, we will merely be paying lip service to our environmental ideals – the anecdotes of SiCl4 being dumped indiscriminately by “green” polysilicon plants in China speaks loudly to that.