Two small-scale versions of 3D photovoltaic arrays were among those tested by Jeffrey Grossman and his team on an MIT rooftop to measure their actual electrical output throughout the day. Image: Allegra Boverman |
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 Massachusetts Institute of Technology (MIT) researchers has come up with
a very different approach: Building cubes or towers that extend the solar cells
upward in 3D 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 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.
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% 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%—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.”
