This is a technological stretch but also promising. Solar cells are with us and getting efficiency up to almost fifty percent woukd make them highly useful and susceptable to continously dropping costs as well.
The real benefit of solar cells are that they are immediate and
justifies a structure providing shade. This sets the stage for the
higly efficient Eden machine that then draws water from the
atmosphere to feed an adjacent tree in the desert.
It is obviously not our only energy solution but it happens to be a
good one easily accepted.
New Technology
Could Boost Solar Cell Efficiency By 30 Percent
July 25th, 2014 | by
Ker Than, Inside Science
Scientists looking to
boost the efficiency of solar panels are taking a fresh look at an
exotic physics phenomenon first observed nearly 50 years ago in
glowing crystals.
Called singlet
fission, the process can enable a single photon of light to generate
two electrons instead of just one. This one-to-two conversion, as the
process is known, has the potential to boost solar cell efficiency by
as much as 30 percent above current levels, according to a
new review paper published in the Journal of Physical Chemistry
Letters.
Singlet fission “was
originally proposed to explain some weird results that were observed
in fluorescent organic crystals,” said the study’s first
author Christopher Bardeen, a chemist at the University of
California, Riverside. “It received a lot of attention in the 1960s
and 1970s, but then it was mostly forgotten.”
But beginning around
2006, Bardeen and other scientists exploring new ways to boost the
solar-energy conversion rates of photovoltaic panels began taking a
renewed interest in singlet fission. In recent years, experiments
conducted by Bardeen’s group not only helped confirm that the
phenomenon is real, but also that it can be highly efficient in a
variety of materials. The hope is that singlet fission materials can
be incorporated into solar panels to increase their energy conversion
efficiency–the ratio of electrons produced to the amount of photons
absorbed–beyond the current theoretical ceiling of approximately 32
percent, which is called the “Shockley-Queisser Limit.”
"The efficiency
of most commercial-grade PV panels, like the ones you would install
on your house, are around 20 to 25 percent," Bardeen said.
Engineers have managed
to overcome the Shockley-Queisser Limit through clever engineering to
boost the efficiency of photovoltaic, or PV, panels up to 50 percent
– for example, one technology, called multi-junction solar cells,
involves combining two or more semiconductor panels. But such
technologies are currently limited mostly to military and space
applications due to their high costs.
"It may be
possible to find a way to make [multi-junction cells] cheaply …
Some companies are trying to do this, but without much impact so
far," Bardeen said.
Many scientists
believe the only way the next wave, or “third generation,” of
photovoltaic technology will surpass the Shockley-Queisser Limit
while remaining inexpensive is if they make use of new physical
processes such as singlet fission.
"First generation
solar cells were based on silicon, and they were efficient but
expensive. The second generation cost much less and was based on
thin-film technology. The goal of the third generation is to keep
cost down but get efficiency as high as possible," Bardeen said.
Currently, solar cells
work by absorbing a photon and generating an exciton — a bound
electron with a negative charge and a positively charged “hole” —
which subsequently separates into an electron-hole pair. The
electrons are then harnessed as electricity. In singlet fission,
however, some photons — those with higher energy — get converted
into two excitons, each of which can split to yield two electrons.
Bardeen’s team estimates that singlet fission can boost efficiency
of solar cells by up to 30 percent, resulting in a maximum efficiency
of above 40 percent instead of the current 32 percent.
Experts predict that
it could be another 5 to 10 years before solar panels based on
singlet fission technology are ready for commercial use. Before that
can happen, scientists will need to gain a much better understanding
of how singlet fission works, said Josef Michl, a photophysicist
at the University of Colorado, Boulder, who helped revive interest in
singlet fission several years ago. At the moment, the main challenge
for researchers trying to create a singlet fission solar panel is “a
thorough understanding of the underlying physics that should allow
chemists to come up with more practical materials than the few that
we now know to work well in the laboratory,” said Michl, who was
not involved in the study.
Michl called Bardeen’s
group a “key player” in the worldwide effort to develop the
technology, and said that his team’s experimental work has helped
singlet fission shed its “reputation of an obscure and inefficient
phenomenon.”
The other primary
hurdle toward a functional singlet fission solar panel will be one of
engineering, Bardeen said. Once more materials that can undergo
singlet fission are developed, they will still need to be
incorporated into photovoltaic cells to convert solar energy into
electricity. Researchers led by Marc Baldo at the Massachusetts
Institute of Technology recently reported that they had proven that
it was possible to create a solar panel that uses singlet fission,
but the efficiency of their device was only 2 to 3 percent.
"Baldo’s group
showed that it could be done," Bardeen said, "but nobody’s
going to be putting those on rooftops tomorrow."
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