3-D Designs From An MIT Team Offers A New Dimension For Solar Energy

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 modelling
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 modelling 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 analysed 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 modelled 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."