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.
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 3-D
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 3-D 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 3-D
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 3-D 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 3-D 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, 3-D 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 3-D 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."
David Gracias, an associate professor of chemical and biomolecular
engineering at Johns Hopkins University who was not involved in this research,
says that Grossman and his team "have demonstrated theoretical and
proof-of-concept evidence that 3-D photovoltaic elements could provide
significant benefits in terms of capturing light at different angles. The
challenge, however, is to mass produce these elements in a cost-effective
manner."
