Engineers calculate the
ultimate potential of next-generation solar panels
December 18, 2019
Purdue University
Most of today's solar panels
capture sunlight and convert it to electricity only from the side facing the
sky. If the dark underside of a solar panel could also convert sunlight
reflected off the ground, even more electricity might be generated.
Most of today's solar panels
capture sunlight and convert it to electricity only from the side facing the
sky. If the dark underside of a solar panel could also convert sunlight
reflected off the ground, even more electricity might be generated.
Double-sided solar cells are
already enabling panels to sit vertically on land or rooftops and even
horizontally as the canopy of a gas station, but it hasn't been known exactly
how much electricity these panels could ultimately generate or the money they could
save.
A new thermodynamic formula
reveals that the bifacial cells making up double-sided panels generate on
average 15% to 20% more sunlight to electricity than the monofacial cells of
today's one-sided solar panels, taking into consideration different terrain
such as grass, sand, concrete and dirt.
The formula, developed by two
Purdue University physicists, can be used for calculating in minutes the most
electricity that bifacial solar cells could generate in a variety of
environments, as defined by a thermodynamic limit.
"The formula involves
just a simple triangle, but distilling the extremely complicated physics
problem to this elegantly simple formulation required years of modeling and
research. This triangle will help companies make better decisions on
investments in next-generation solar cells and figure out how to design them to
be more efficient," said Muhammad "Ashraf" Alam, Purdue's Jai N.
Gupta Professor of Electrical and Computer Engineering.
In a paper published in
the Proceedings of the National Academy of Sciences, Alam and coauthor
Ryyan Khan, now an assistant professor at East West University in Bangladesh,
also show how the formula can be used to calculate the thermodynamic limits of
all solar cells developed in the last 50 years. These results can be
generalized to technology likely to be developed over the next 20 to 30 years.
The hope is that these
calculations would help solar farms to take full advantage of bifacial cells
earlier in their use.
"It took almost 50 years
for monofacial cells to show up in the field in a cost-effective way,"
Alam said. "The technology has been remarkably successful, but we know now
that we can't significantly increase their efficiency anymore or reduce the
cost. Our formula will guide and accelerate the development of bifacial
technology on a faster time scale."
The paper might have gotten
the math settled just in time: experts estimate that by 2030, bifacial solar
cells will account for nearly half of the market share for solar panels
worldwide.
Alam's approach is called the
"Shockley-Queisser triangle," since it builds upon predictions made
by researchers William Shockley and Hans-Joachim Queisser on the maximum
theoretical efficiency of a monofacial solar cell. This maximum point, or the
thermodynamic limit, can be identified on a downward sloping line graph that
forms a triangle shape.
The formula shows that the
efficiency gain of bifacial solar cells increases with light reflected from a
surface. Significantly more power would be converted from light reflected off
of concrete, for example, compared to a surface with vegetation.
The researchers use the
formula to recommend better bifacial designs for panels on farmland and the
windows of buildings in densely-populated cities. Transparent, double-sided panels
allow solar power to be generated on farmland without casting shadows that
would block crop production. Meanwhile, creating bifacial windows for buildings
would help cities to use more renewable energy.
The paper also recommends ways
to maximize the potential of bifacial cells by manipulating the number of
boundaries between semiconductor materials, called junctions, that facilitate
the flow of electricity. Bifacial cells with single junctions provide the
largest efficiency gain relative to monofacial cells.
"The relative gain is
small, but the absolute gain is significant. You lose the initial relative
benefit as you increase the number of junctions, but the absolute gain
continues to rise," Khan said.
The formula, detailed in the
paper, has been thoroughly validated and is ready for companies to use as they
decide how to design bifacial cells.
This research was partially
supported by the National Science Foundation under award 1724728.
Story Source:
Materials provided by Purdue University. Original
written by Kayla Wiles. Note: Content may be edited for style and length.
Journal Reference:
Muhammad A. Alam, M. Ryyan
Khan. Shockley–Queisser triangle predicts the thermodynamic efficiency
limits of arbitrarily complex multijunction bifacial solar cells. Proceedings
of the National Academy of Sciences, 2019; 116 (48): 23966 DOI: 10.1073/pnas.1910745116
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