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Solar Footprint

Page history last edited by PBworks 14 years, 3 months ago


Environmental Science & Technology

February 6, 2008




New photovoltaics change solar costs


A new life-cycle assessment of photovoltaic technologies shows that

some are better than others.




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 <


>, 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."


posted to ClimateConcern

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