april 2016, -
solar storage has for too long been focused on converting photons to electricity, according to a massachusetts institute of technology (mit) project funded by bmw. for instance, if your purpose is to use the solar-to-electricity-battery process to later defrost your windshield, wouldn't it be much more efficient to cut out the middle-man and store the solar photons inside the transparent polymer inside every windshield, then later release it directly as phonons (heat) for defrosting?
that is the concept of mit's professor jeffrey grossman, postdoctoral researcher david zhitomirsky, and doctoral candidate eugene cho and their solar thermal fuel polymer--a transparent film that can be sandwiched between the two layers of glass on your cars windows (which is already done for safety--so they don't shatter). the transparent film stores the solar energy by varying its molecular configuration--a engineered reaction to light--then releases the heat on-demand as it relaxes back to its "normal" chemical configuration.
the layer-by-layer solar thermal fuel polymer film comprises three distinct layers (4 to 5 microns in thickness for each) which are cross-linked to enable building up films of tunable thickness. (source: mit)
"this material can be engineered to have many different specific properties, but in its current version is absorbs uv [ultra-violet] photons to change its shape into a high-energy form, then when it snaps back into its low-energy shape it releases phonons--vibrations--that instantly heat up their surroundings," zhitomirsky told ee times in an exclusive interview.
of course, there are many other applications of such as solar-to-solid-fuel conversions, such as in a memory that uses a laser to change the shape of the molecule from a 0 to a 1 and back again, but bmw funded the research at mit specifically because a great deal of electrical energy is wasted in an electric vehicle (ev) defrosting its windows, sometimes reducing the evs range as much as 30 percent. but by releasing the stored heat inside the windshield as phonons, the ice touching the windshield could be liquified, allowing the windshield wipers to easily remove any amount of ice or snow that had accumulated overnight.
at mit they of course targeted bmws application, but also can envision using the polymer in many other applications, such as weaved into clothing that can "charge up" while in sunlight then release the charge as heat when, say a skier, moves into the shade.
"you could even save money on you house's heating bill at home if your solar charged clothing kept you warm instead of your forced-air furnace," zhitomirsky told ee times.
in both cases, cutting out the solar-to-electricity-to-battery-to-heating-coil cycle greatly increased the efficiency of the entire process (of course only if you are storing the solar energy for later heating something up). both the low-energy and high-energy molecular configurations of the solar thermal fuel polymer are stable until triggered (which can be done either electrically or with a burst from a laser or a heat coil.
the platform for testing macroscopic heat release provides sufficient energy to trigger the solar thermal fuel materials (right) which release heat when changing into the uncharged film (left). (source: mit)
there are several different formulations of solar thermal fuels which snap into a high-energy, yet stable, configuration that can then jolted back into a low-energy configuration at which time they emit a heat generating phonon. most solar thermal fuels which absorb photons and emit phonons within the same material, including grossman's team's earlier work, were with liquids that perform similarly, but this is the first solid-state polymer film that they have found that is easy to manufacture on a large scale. at first they used its bulk form, but after much experimentation found that forming a thin-film from it made it applicable to a much wider range of applications.
after much experimentation, some of which used slurries of nanotubes, they settled on azobenzene--a well-known polymer--because of it's easy manufacturing as well as the huge amount of photons it could store is its chemical configuration, then release upon demand as heat-generating phonons. the researchers also modified azobenzene chemically so that it could be formed into multilayer thin films, each layer of which could store photons and could be triggered to release all their energy simultaneously as phonons.
a standard spin-coating process that enables deposition of the solar thermal fuel polymer which can then be readily charged into a variable thickness film layer-by-layer process. (source: mit)
one disadvantage to the azobenzene film today is that it has a slightly yellowish tint, which the researchers are confident they can change to complete transparency. they are also working to double its boost in temperature ambient the environment.
besides bmw this research was also supported by a nserc canada banting fellowship.
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