Meet 'photoswitches,' a breakthrough set of materials that
act as their own batteries, absorbing energy and releasing it on demand.
Andres Gutierrez/AP
The next big thing in solar energy could be microscopic.
Scientists at MIT and Harvard University have devised a way to store
solar energy in molecules that can then be tapped to heat homes, water
or used for cooking.
The best part: The molecules can store the heat forever and be
endlessly re-used while emitting absolutely no greenhouse gases.
Scientists remain a way’s off in building this perpetual heat machine
but they have succeeded in the laboratory at demonstrating the viability
of the phenomenon called photoswitching.
“Some molecules, known as photoswitches, can assume either of
two different shapes, as if they had a hinge in the middle,” MIT
researchers said in statement about the
paper published in the journal Nature Chemistry.
“Exposing them to sunlight causes them to absorb energy and jump from
one configuration to the other, which is then stable for long periods of
time.”
To liberate that energy all you have to do is expose the molecules to
a small amount of light, heat or electricity and when they switch back
to the other shape the emit heat. “In effect, they behave as
rechargeable thermal batteries: taking in energy from the sun, storing
it indefinitely, and then releasing it on demand,” the scientists said.
The researchers used a photoswitching substance called an
azobenzene,
attaching the molecules to substrates of carbon nanotubes. The
challenge: Packing the molecules closely enough together to achieve a
sufficient energy density to generate usable heat.
It appeared that the researchers had failed when they were only able
to pack fewer than half the number of molecules needed as indicated by
an earlier computer simulation of the experiment.
But instead of hitting a projected 30 percent increase in energy
density, they saw a 200 percent increase. It turned out that the key was
not so much packing azobenzene molecules tightly on individual carbon
nanotubes as packing the nanotubes close together. That’s because the
azobenzene molecules formed “teeth” on the carbon nanotubes, which
interlocked with teeth on adjacent nanotubes. The result was the mass
needed for a usable amount of energy storage.
That means different combinations of photoswitching molecules and
substrates might achieve the same or greater energy storage, according
to the researchers.
So how would molecular solar storage work if the technology can be
commercialized? Timothy Kucharski, the paper’s lead author and a postdoc
at MIT and Harvard, told The Atlantic that most likely the storage
would take a liquid form, which would be easy to transport.
“It would also enable charging by flowing the material from a storage
tank through a window or clear tube exposed to the sun and then to
another storage tank, where the material would remain until it's
needed,” Kucharski said in an email. “That way one could stockpile the
charged material for use when the sun's not shining.”
The paper’s authors envision the technology could be used in
countries where most people rely on burning wood or dung for cooking,
which creates dangerous levels of indoor air pollution, leads to
deforestation and contributes to climate change.
“For solar cooking, one would leave the device out in the sun during
the day,” says Kucharski. “One design we have for such an application is
purely gravity driven – the material flows from one tank to another.
The flow rate is restricted so that it's exposed to the sun long enough
that it gets fully charged. Then, when it's time to cook dinner, after
the sun is down, the flow direction is reversed, again driven by
gravity, and the opposite side of the setup is used as the cooking
surface.”
“As the material flows back to the first tank, it passes by an
immobilized catalyst which triggers the energy-releasing process,
heating the cooking surface up,” he adds.
Other versions of such device could be used to heat buildings.
Kucharski said the MIT and Harvard team is now investigating other
photoswitching molecules and substrates, “with the aim of designing a
system that absorbs more of the sun's energy and also can be more
practically scaled up.”
http://www.theatlantic.com/technology/archive/2014/04/scientists-discover-how-to-generate-solar-power-in-the-dark/360679/
