Environmental Science & Technology
February 6, 2008
http://pubs.acs.org/subscribe/journals/esthag-w/2008/feb/science/nl_pvlifecycle.html
New photovoltaics change solar costs
A new life-cycle assessment of photovoltaic technologies shows that
some are better than others.
By NAOMI LUBICK
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 <
http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/asap/abs/es071763q.html
>, 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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