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Posted: February 12, 2008

Emissions from photovoltaic life cycles

(Nanowerk News) New photovoltaic technologies, such as the recent introduction of thin-film cadmium–telluride (CdTe) materials, have nearly doubled the efficiency of solar cells within the past few years. But the methods of making the materials used for photovoltaic cells, whether from silicon, metal, or other material, have raised doubts about the environmental friendliness of these passive energy collectors. Purifying and producing silicon uses a lot of water and energy, and refining zinc and copper ores to get Cd, Te, and other elements creates metal emissions and an energy sink—all of which increase the technology's environmental footprint.
A new life-cycle assessment (LCA) of some of the leading photovoltaic technologies, published in ES&T ("Emissions from Photovoltaic Life Cycles"), shows that some may be better than others, particularly when it comes to emissions over their lifetimes. Overall, however, replacing traditional electricity grids fueled by gas, coal, and other means with photovoltaics would cut emissions of greenhouse gases, particulate matter, and other pollutants by 89–98%. Rooftop panels could further reduce emissions because of the resulting decrease in transmission lines and other infrastructure. But each form of photovoltaics has a different LCA profile, specific to heavy-metal emissions and electricity use in particular, the new analysis shows.
Led by Vasilis Fthenakis of Brookhaven National Laboratory and Columbia University, the LCA includes information from databases of more than a dozen active solar companies and provides a complex snapshot of the state of the solar industry up to 2006. Fthenakis and co-workers compared data from companies that make single-crystal, multicrystal, and ribbon silicon solar cells, all of which have different efficiencies in converting sunlight into electricity. They also compared these products with the thin-film CdTe photovoltaic systems manufactured by fast-growing Arizona-based First Solar.
The analysis took into account frames, cables, and other necessary support materials, as well as the energy required for manufacturing under three scenarios, each with a different proportion of electricity coming from coal, natural gas, or other sources. The team based their assumptions on ground-mounted systems under southern European light conditions, over 30-year lifetimes.
In the end, the CdTe photovoltaics came out on top. With more efficient energy conversion and the lowest cost, the technology used less energy and had fewer emissions overall, despite some Cd emissions during the manufacturing process. However, emissions from fossil-fuel-powered electricity dwarfed those Cd emissions by orders of magnitude.
The new assessment is "incredibly useful," says Corinne Reich-Weiser, a graduate student in mechanical engineering at the University of California Berkeley who works part-time for solar manufacturer SolFocus in San Jose, Calif. The work is unique in that it uses up-to-date processing data, she says. And because the assumptions are the same across the board with regard to yearly available sunlight, performance, and energy grids, "you can easily compare" all of the technologies, she adds.
But the origin of the electricity used to manufacture solar cells varies from place to place, Reich-Weiser points out. The current assessment, based on idealized European and U.S. grids, "is not telling you exactly what your impact is if you were to buy them." For example, impacts from components manufactured in China, where the electricity grid is often powered by coal, will differ from those impacts produced by components made in the U.S. or EU. She also notes that emissions from the transportation of those components before production and assembly, such as by rail or truck, are only partly considered. "Depending on the amount of goods transported throughout the supply chain, including every transportation leg may increase estimated greenhouse gas emissions by 30–50%," she says.
Ken Zweibel, president of Colorado-based PrimeStar Solar, notes that even if China were to adopt photovoltaics wholesale, produced entirely with coal-powered electricity, new solar materials would allow products with 30-year lifetimes to make up for those emissions in several years. Plus future technologies could further shift emissions: "The field is changing fast," adds Zweibel, who recently coauthored a "solar grand plan" with Fthenakis in Scientific American.
One component missing from the current analysis, says Fthenakis, is end-of-life and recycling data. "Those studies are not yet completed," he says, but "it's a safe assumption . . . that recycling will make the emissions profile better, [and] the feasibility of recycling is here."
First Solar, whose growth over the past few years has outpaced silicon manufacturers' with its CdTe approach, recently revealed some of the inner workings of its program, which includes investments for collection and recycling whenever a unit is sold. Lisa Krueger, vice president of sustainability for First Solar, says that recycling makes photovoltaics a "truly sustainable energy solution."
Source: Environmental Science & Technology (Naomi Lubick)
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