A New Dimension for Solar Energy
By Massachusetts Institute of TechnologyTuesday, March 27, 2012 Get the
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Intensive research around the world has focused on improving the performance
of solar photovoltaic cells and bringing down their cost. But very little
attention has been paid to the best ways of arranging those cells, which are
typically placed flat on a rooftop or other surface, or sometimes attached
to motorized structures that keep the cells pointed toward the sun as it
crosses the sky.
Now, a team of MIT researchers has come up with a very different approach:
building cubes or towers that extend the solar cells upward in
three-dimensional configurations. Amazingly, the results from the structures
they've tested show power output ranging from double to more than 20 times
that of fixed flat panels with the same base area.
The biggest boosts in power were seen in the situations where improvements
are most needed: in locations far from the equator, in winter months and on
cloudier days. The new findings, based on both computer modeling and outdoor
testing of real modules, have been published in the journal Energy and
Environmental Science.
"I think this concept could become an important part of the future of
photovoltaics," says the paper's senior author, Jeffrey Grossman, the Carl
Richard Soderberg Career Development Associate Professor of Power
Engineering at MIT.
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The MIT team initially used a computer algorithm to explore an enormous
variety of possible configurations, and developed analytic software that can
test any given configuration under a whole range of latitudes, seasons and
weather. Then, to confirm their model's predictions, they built and tested
three different arrangements of solar cells on the roof of an MIT laboratory
building for several weeks.
While the cost of a given amount of energy generated by such 3D modules
exceeds that of ordinary flat panels, the expense is partially balanced by a
much higher energy output for a given footprint, as well as much more
uniform power output over the course of a day, over the seasons of the year,
and in the face of blockage from clouds or shadows. These improvements make
power output more predictable and uniform, which could make integration with
the power grid easier than with conventional systems, the authors say.
The basic physical reason for the improvement in power output — and for the
more uniform output over time — is that the 3D structures' vertical surfaces
can collect much more sunlight during mornings, evenings and winters, when
the sun is closer to the horizon, says co-author Marco Bernardi, a graduate
student in MIT's Department of Materials Science and Engineering (DMSE).
The time is ripe for such an innovation, Grossman adds, because solar cells
have become less expensive than accompanying support structures, wiring and
installation. As the cost of the cells themselves continues to decline more
quickly than these other costs, they say, the advantages of 3D systems will
grow accordingly.
"Even 10 years ago, this idea wouldn't have been economically justified
because the modules cost so much," Grossman says. But now, he adds, "the
cost for silicon cells is a fraction of the total cost, a trend that will
continue downward in the near future." Currently, up to 65 percent of the
cost of photovoltaic (PV) energy is associated with installation, permission
for use of land and other components besides the cells themselves.
Although computer modeling by Grossman and his colleagues showed that the
biggest advantage would come from complex shapes — such as a cube where each
face is dimpled inward — these would be difficult to manufacture, says
co-author Nicola Ferralis, a research scientist in DMSE. The algorithms can
also be used to optimize and simplify shapes with little loss of energy. It
turns out the difference in power output between such optimized shapes and a
simpler cube is only about 10 to 15 percent — a difference that is dwarfed
by the greatly improved performance of 3D shapes in general, he says. The
team analyzed both simpler cubic and more complex accordion-like shapes in
their rooftop experimental tests.
At first, the researchers were distressed when almost two weeks went by
without a clear, sunny day for their tests. But then, looking at the data,
they realized they had learned important lessons from the cloudy days, which
showed a huge improvement in power output over conventional flat panels.
For an accordion-like tower — the tallest structure the team tested — the
idea was to simulate a tower that "you could ship flat, and then could
unfold at the site," Grossman says. Such a tower could be installed in a
parking lot to provide a charging station for electric vehicles, he says.
So far, the team has modeled individual 3D modules. A next step is to study
a collection of such towers, accounting for the shadows that one tower would
cast on others at different times of day. In general, 3D shapes could have a
big advantage in any location where space is limited, such as flat-rooftop
installations or in urban environments, they say. Such shapes could also be
used in larger-scale applications, such as solar farms, once shading effects
between towers are carefully minimized.
A few other efforts — including even a middle-school science-fair project
last year — have attempted 3D arrangements of solar cells. But, Grossman
says, "our study is different in nature, since it is the first to approach
the problem with a systematic and predictive analysis."
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