Improving efficiency (and driving down the cost per watt) is the holy grail of photovoltaic (PV) panel makers. MIT's new design doesn't look at the panels as much as how they're arranged. Last year a kid looked at mimicking the way a tree positions its leaves as a more efficient way of arranging panels. Now researchers at MIT are stacking them in a way that produces twenty times the power from the same square footage of land (or building roof).
From
MIT News:
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.”
