How Sunburn Inspired a New Method for Storing Solar Energy

The intense Californian sunshine has prompted a breakthrough in molecular energy storage technology, offering a potential new approach to decarbonising heating systems worldwide.

When chemistry professor Grace Han relocated from Boston to the University of California, Santa Barbara, she quickly noticed the difference in solar intensity. The relocation required her to adopt protective measures including wide-brimmed hats, sunglasses and substantial sun cream. Her academic background led her to research DNA photochemistry, which unexpectedly provided the foundation for innovative energy storage research.

Professor Han discovered that DNA molecules damaged by solar radiation undergo structural changes, flexing into strained configurations of their regular form. This observation became the basis for developing molecular solar thermal (Most) energy storage, a technology that scientists have pursued for decades. The concept involves molecules that twist their shape whilst storing energy, then revert to their original form to release that energy on demand.

Most systems offer several advantages over conventional energy storage. They can potentially retain energy for months or even years, providing a cheap and emissions-free method of supplying heat. Evolution has refined the process of repairing sun-damaged molecules in certain plants and animals through an enzyme called photolyase, making these molecules ideal candidates for energy storage applications.

The molecules identified by Professor Han possess exceptional characteristics. Their diminutive size allows them to store substantial amounts of energy per unit mass. In a paper published in February, Professor Han and her colleagues described what appears to be the most promising energy storage system of this type achieved to date, measured by energy density.

The system proved powerful enough to rapidly boil a small quantity of water in a miniature kettle contained within a vial. Computer analyses conducted by collaborator Kendall Houk at the University of California, Los Angeles, proved crucial in predicting molecular performance. The research team achieved an energy density of 1.65 megajoules per kilogram, significantly exceeding the energy density of lithium-ion batteries currently used in mobile phones and electric vehicles.

Kasper Moth-Poulsen, who leads Most research teams at the Polytechnic University of Barcelona and other institutions, expressed admiration for the results. His best systems had achieved one megajoule per kilogram, making the 1.6 megajoules per kilogram achieved by Professor Han’s team particularly noteworthy.

The system does face certain limitations, however. The wavelength of light required to trigger the molecular shape change is 300 nanometres, a form of harsh ultraviolet light that reaches Earth only in small quantities. The trigger used to reverse the molecular configuration and release stored energy was hydrochloric acid, a highly corrosive substance requiring neutralisation after use. Professor Han acknowledged this was not the ideal choice, though she remains optimistic about improving the system’s responsiveness to natural light and developing non-toxic methods for triggering energy release.

The broader objective of this research is to decarbonise heating, which remains heavily dependent on fossil fuels. Molecular solar thermal systems and fossil fuels both constitute forms of chemical energy storage, but Most technology operates without combustion, as Moth-Poulsen emphasised. Most systems could be deployed anywhere on Earth, unlike fossil fuels, which are concentrated in specific regions. Recent disruptions caused by the Strait of Hormuz blockade have highlighted the vulnerabilities in conventional fuel distribution networks.

Moth-Poulsen suggested that Most energy storage systems could retain energy for multiple decades, far exceeding the hours, days or months achievable with thermal energy stored as heat. Harry Hoster at the University of Duisberg-Essen, who also serves as scientific director of the hydrogen-focused ZBT Center for Fuel Cell Technology in Germany, raised additional considerations regarding practical implementation.

The light-sensitive molecules in a Most system must be distributed in relatively thin layers to ensure adequate light penetration. Hoster estimated that under optimistic conditions, the maximum practical thickness would be approximately 5 millimetres. Packaging molecules in liquid form necessitates pumping that liquid between different parts of the system, adding cost and complexity whilst increasing potential points of failure.

John Griffin at Lancaster University confirmed that he and colleagues are developing solid state versions of Most technology. Professor Han is also researching solid iterations, which could potentially take the form of transparent window coatings capable of releasing heat to prevent condensation or warm interior spaces.

Hoster expressed scepticism about Most technology’s ability to provide all heating requirements in buildings, suggesting more suitable applications might include warming temperature-sensitive components on satellites or aircraft. He praised the scientific achievement whilst acknowledging practical limitations.

The field remains relatively niche at present. Griffin recalled attending a Most technology conference last year with approximately 70 attendees, representing essentially the entire global community working on this technology. Nevertheless, ongoing innovations and research suggest continued development in this emerging area of energy storage.