According to the researchers, the large-scale use of solar energy will require significant advances in storage technologies to meet the demands of an energy-hungry population. Other than liquid fuels, they say, existing energy-storage materials do not offer the necessary mix of high energy density, high stability, easy handling, transportability and low cost.
But it looks like that could change with the use of "photoswitching" molecules. The molecules comprise a set of innovative materials that absorb energy, store it, and then release it when needed.
“Some molecules, known as photoswitches, can assume either of two different shapes, as if they had a hinge in the middle,” said the MIT researchers. “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 release that energy for use, the molecules simply need to be exposed to a small amount of light, heat or electricity, making them switch back to their other shape and radiate 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.
At the heart of the research is a photoswitching chemical compound called azobenzene, which they attached to substrates of carbon nanotubes. When packed together in correct proportions, the azobenzene molecules formed “teeth” on the nanotubes, which gripped teeth on nearby nanotubes, resulting in a mass able to create a viable amount of energy storage.
If the technology can be brought out of the lab and into the commercial world, the storage would most likely take a liquid form, which would be easy to transport, said Timothy Kucharski, the paper’s lead author.
“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 wrote. “That way one could stockpile the charged material for use when the sun's not shining.”
The scientists say the new technology could be put to great use in places where inhabitants burn wood or dung for cooking, which can lead to deleterious indoor air quality as well as contributing to deforestation and climate change.
“For solar cooking, one would leave the device out in the sun during the day,” said 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.”
The research team is now looking into 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.”
Here’s to putting the sun to good use, 24 hours a day.
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