How sunburn inspired a new way to store energy

How sunburn inspired a new way to store energy Chris BaraniukTechnology ReporterJeff Liang, UCSBCalifornia's strong sun inspired Grace Han's energy storage researchThe sun does shine, sometimes, in Boston – but not like this.When chemistry professor Grace Han first visited southern California from Boston some years ago, she noticed the difference. How her skin would tingle with the first signs of irritation after just a few hours outside.Last year, she moved to take a job at the University of California, Santa Barbara, and regularly began wearing a large-brimmed hat, sunglasses and plenty of sun cream. Being a chemistry professor, she had already done her research."I was just reading about DNA photochemistry – for leisure," she recalls.That's when she realised that DNA molecules in people's skin that get damaged by sunburn could help her.

Those molecules change shape when irradiated by the sun, flexing into a strained version of their regular form.For decades, scientists have sought out molecules that can twist their shape, storing energy in the process, and then be prompted to revert to their original shape, releasing the stored energy on demand.A bit like setting and later triggering a mousetrap. It's known as molecular solar thermal (Most) energy storage and is a potentially very cheap and emissions-free way of supplying heat. These Most systems could store energy for many months or even years.Researchers have previously had limited success with the technology, but, thanks to the California sun, Han knew what to try next.It's important to activate the shape-shifting of the energy-storing molecules in a smooth, repeatable way.Luckily, millions of years of evolution has perfected this process when it occurs in certain plants and animals.Living things are all chemistry labs, in a sense, and some organisms have evolved so that they can repair sun-contorted molecules with the help of an enzyme called photolyase.Han realised that such molecules were therefore perfect candidates for an energy storage system.

"They are very, very small," she explains. "And can store a massive amount of energy per mass."Han PQ Nguyen, UCSBMolecules created at UCSB trapped enough energy to boil off a small amount of waterIn a paper published in February, she and colleagues described the most promising energy storage system of this kind to date, at least in terms of its energy density. It was powerful enough to cause a "very tiny kettle" in a vial to boil off a small amount of water rapidly, says Han.Her students, who carried out that part of the study, rushed to tell her how it went.

"When I actually saw the video and saw how quickly the entire solution was boiling, that was really remarkable," Han recalls.She emphasises that computer analyses predicting how the molecule would perform, made by her collaborator Kendall Houk at the University of California, Los Angeles, and his team, were crucial to the work.Fellow Most experimenter Kasper Moth-Poulsen, who leads research teams at the Polytechnic University of Barcelona in Spain and other institutions, was not involved in the study but was impressed by the results."I think our best systems were one megajoule [of energy per kilogram]. They had, I think, 1.6, which is really amazing," he says, referring to the energy density Han and her colleagues achieved.The 1.65 megajoules per kilogram recorded in their February paper is significantly greater than the energy density of lithium-ion batteries, currently the most popular type of battery for phones and electric cars.John Griffin/Lancaster UniversityJohn Griffin is working on solid versions of molecular storageThe Most system that Han and her colleagues came up with does have some limitations. For one thing, the wavelength of light that causes molecules at the heart of the setup to change shape is 300 nanometres – a form of "very harsh UV [ultraviolet] light", says John Griffin at Lancaster University.

"That does come from the sun to us but only in very small quantities."Plus, the trigger used to reverse the shape of the contorted molecule in order to release its energy was hydrochloric acid – a highly corrosive substance that must be neutralised after use. "Not the most ideal choice," admits Han.She says she is hopeful that it will be possible to improve the system's responsiveness to natural light, and also to trigger the energy release without requiring a toxic chemical.The ultimate goal of work like this is to decarbonise heating, which is notoriously difficult.The world still relies largely on fossil fuels for heating applications. Molecular solar thermal systems and fossil fuels are actually both forms of chemical energy storage.

But the Most technology "operates without burning anything" stresses Moth-Poulsen.Plus, Most could be made available anywhere on Earth, unlike fossil fuels, which are concentrated in some locations. That is why the blockade of the Strait of Hormuz has caused such problems recently, he points out. The fuels produced in that part of the world can't get to where people need them.Moth-Poulsen says that a Most energy storage system could also store energy long-term, even for multiple decades.