In large parts
of the developing world, people have abundant heat from the sun during the day,
but most cooking takes place later in the evening when the sun is down, using
fuel -- such as wood, brush or dung -- that is collected with significant time
and effort.
Now, a new
chemical composite developed by researchers at MIT could provide an
alternative. It could be used to store heat from the sun or any other source
during the day in a kind of thermal battery, and it could release the heat when
needed, for example for cooking or heating after dark.
A common
approach to thermal storage is to use what is known as a phase change material
(PCM), where input heat melts the material and its phase change -- from solid
to liquid -- stores energy. When the PCM is cooled back down below its melting
point, it turns back into a solid, at which point the stored energy is released
as heat. There are many examples of these materials, including waxes or fatty
acids used for low-temperature applications, and molten salts used at high
temperatures. But all current PCMs require a great deal of insulation, and they
pass through that phase change temperature uncontrollably, losing their stored
heat relatively rapidly.
Instead, the new
system uses molecular switches that change shape in response to light; when
integrated into the PCM, the phase-change temperature of the hybrid material
can be adjusted with light, allowing the thermal energy of the phase change to
be maintained even well below the melting point of the original material.
The new
findings, by MIT postdocs Grace Han and Huashan Li and Professor Jeffrey
Grossman, are reported this week in the journal Nature Communications.
"The
trouble with thermal energy is, it's hard to hold onto it," Grossman
explains. So his team developed what are essentially add-ons for traditional
phase change materials, or, "little molecules that undergo a structural
change when light shines on them." The trick was to find a way to
integrate these molecules with conventional PCM materials to release the stored
energy as heat, on demand. "There are so many applications where it would
be useful to store thermal energy in a way lets you trigger it when
needed," he says.
The researchers
accomplished this by combining the fatty acids with an organic compound that
responds to a pulse of light. With this arrangement, the light-sensitive
component alters the thermal properties of the other component, which stores
and releases its energy. The hybrid material melts when heated, and after being
exposed to ultraviolet light, it stays melted even when cooled back down. Next,
when triggered by another pulse of light, the material resolidifies and gives
back the thermal phase-change energy.
"By
integrating a light-activated molecule into the traditional picture of latent
heat, we add a new kind of control knob for properties such as melting,
solidification, and supercooling," says Grossman, who is the Morton and
Claire Goulder and Family Professor in Environmental Systems as well as
professor of materials science and engineering.
The system could
make use of any source of heat, not just solar, Han says. "The
availability of waste heat is widespread, from industrial processes, to solar
heat, and even the heat coming out of vehicles, and it's usually just
wasted." Harnessing some of that waste could provide a way of recycling
that heat for useful applications.
"What we
are doing technically," Han explains, "is installing a new energy
barrier, so the stored heat cannot be released immediately." In its
chemically stored form, the energy can remain for long periods until the
optical trigger is activated. In their initial small-scale lab versions, they
showed the stored heat can remain stable for at least 10 hours, whereas a
device of similar size storing heat directly would dissipate it within a few
minutes. And "there's no fundamental reason why it can't be tuned to go
higher," Han says.
In the initial
proof-of-concept system "the temperature change or supercooling that we
achieve for this thermal storage material can be up to 10 degrees C (18 F), and
we hope we can go higher," Grossman says.
Already, in this
version, "the energy density is quite significant, even though we're using
a conventional phase-change material," Han says. The material can store
about 200 joules per gram, which she says is "very good for any organic
phase-change material." And already, "people have shown interest in
using this for cooking in rural India," she says. Such systems could also
be used for drying agricultural crops or for space heating.
"Our
interest in this work was to show a proof of concept," Grossman says,
"but we believe there is a lot of potential for using light-activated
materials to hijack the thermal storage properties of phase change
materials."
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