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Science & Technology

February 21, 2011
Volume 89, Number 8
pp. 37 - 38
Article Updated March 28, 2011, 1:00 PM
DOI:10.1021/CEN021511170038

Making Solar Panels Greener

Producing photovoltaic panels more sustainably will require reducing energy consumption, toxic substances

Sarah Everts

SUNCATCHERS Researchers try to make photovoltaic production more sustainable. Shutterstock
SUNCATCHERS Researchers try to make photovoltaic production more sustainable.
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With more than enough sunlight beaming down on Earth to supply humanity’s immense energy demands, there’s little doubt that photovoltaic technology will play a significant role in sating these appetites in a sustainable way.

But before solar-cell-laced landscapes spawn green utopian reveries, some argue it’s worth taking a measured look at the environmental impact of photovoltaic technology from cradle to grave. For example, consider crystalline silicon-based photovoltaic solar panels, which currently boast about 80% of the global market. Makers of these panels have borrowed much of their technology from the electronics industry, which relies on an abundance of chemicals and energy-intensive steps that pose risks to human and environmental health.

But if companies want to market their photovoltaic panels as a “green technology,” they must instead choose more environmentally friendly manufacturing processes and plan for the panels’ safe disposal, says Dustin Mulvaney, who examines life-cycle analyses of photovoltaic technology at the University of California, Berkeley, and who contributed to a solar-panel life-cycle analysis for the Silicon Valley Toxics Coalition.

Some in the industry are starting to pay heed. Photovoltaic companies are increasingly setting up programs that will collect and recycle panels after their 20–25-year lifespan, with some businesses committing to not sending their products to developing nations in which other electronic waste is processed unsafely. Most companies are looking to reduce the amount of energy required to produce photovoltaics, thereby improving the bottom line and environmental profile of manufacturing. Researchers in both academia and industry are also starting to scrutinize the photovoltaic production process to figure out which chemicals could and should be replaced.

Solar panels based on crystalline silicon must be in use about two years before the cumulative energy they supply to the grid balances the energy required to produce them—the so-called energy-payback time, says Holger Neuhaus, general manager of Germany’s SolarWorld Innovations, SolarWorld’s R&D company. A significant chunk of the energy required to make crystalline silicon photovoltaic panels originates from the high temperatures required to remove oxygen from quartz to make pure silicon crystals, explains Edwin Kroke, a chemist at the Technical University Bergakademie, in Freiberg, Germany.

Many scientists have tried developing more-energy-efficient ways to purify and crystallize silicon, Kroke says. But none of the fledgling processes outperform the industrial status quo.

Making matters worse, much of the crystallized silicon goes to waste when it is sliced into wafers. Half of the highly pure silicon is lost to the cutting slurry because the wire cutters are about the same thickness as the 180–200-μm-thick wafers, says Jochen Rentsch of the Fraunhofer Institute for Solar Energy, in Freiburg, Germany. Finding a way to recycle the lost silicon would reduce waste across the industry, he adds.

Turning a cut wafer into a usable solar panel involves a series of wet-chemical etching steps that clean the slurry chemicals off the wafer, texturize the surface to optimize conversion of light into electricity, apply layers of boron and phosphorus solutions to make the silicon semiconducting, and then clean up the semiconducting surfaces. For cleaning and etching, SolarWorld previously used SF6, “a very nasty gas in terms of greenhouse effects and ozone layer depletion,” Neuhaus says. The company now opts to do a lot of etching with strong acids. Several other companies are doing the same, Mulvaney says, or they are using NF3 gas, which has a better greenhouse gas profile than SF6.

Ideally, etching could be done with even more benign chemicals, Kroke says. His team is investigating the use of hydrogen peroxide as an etching agent, but it doesn’t yet perform to industry standards.

Berlin-based photovoltaic company Solon has committed to reducing harmful chemicals in their panel production by 3–5% each year. For example, Solon and other firms, including SolarWorld and Japan’s Mitsubishi Electric, are striving to reduce and even eliminate lead from photovoltaic products. In Europe, heavy metals, including lead, are not permitted in electronic components, but photovoltaic panels are currently exempt from this restriction, Neuhaus says. But with no economic incentive for companies to exclude lead from photovoltaic panels, regulations will be required to make lead-free panels the industry standard, he adds.

Even though crystalline silicon dominates the photovoltaic market, other technologies, particularly thin-film cadmium-telluride-based photovoltaic cells, are gaining a significant foothold. CdTe photovoltaic panels pay back their consumed energy in about a year, twice as fast as crystalline silicon photovoltaic cells. These photovoltaic devices don’t require as many chemical processing steps to deposit the CdTe semiconducting material on glass panels, and the cadmium is a by-product of zinc mining, but these devices don’t fulfill other green chemistry principles. Cadmium is a carcinogen that the U.S. Environmental Protection Agency lists as toxic. And tellurium is so rare that some people consider it unsustainable.

“There’s enough tellurium to supply our solar cells, but we do not comment on how or why we’ve drawn that conclusion,” says David J. Eaglesham, chief technology officer at First Solar, which produces CdTe photovoltaic panels. “We view cadmium as our main hazardous material,” he adds, noting that First Solar’s panels are “hermetically sealed” and that the company set aside funds for free recycling of its products.

SolarWorld helped initiate PV Cycle, a recycling association of some 80 companies that produce photovoltaic panels from a variety of materials. Crystalline silicon photovoltaic panels in particular will dominate the first serious wave of photovoltaic electronic waste in the next decade as their lifespan ends, says Jun-Ki Choi of Brookhaven National Laboratory, who studies recycling models.

As companies figure out how to establish effective recycling programs for retired photovoltaic panels, as well as how to make those panels in more sustainable ways, they will have to continue to balance priorities that are sometimes at odds. Improving the energy efficiency of cells will make the energy payback time shorter, “but increasing efficiency means your whole process has to be cleaner, therefore more etching steps with more chemistry and more rinsing, and therefore more water consumption,” Fraunhofer Institute’s Rentsch says. Resolving these challenges will ensure that photovoltaics don’t just produce renewable energy but are themselves renewably produced.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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