Chemist Reports Highest Energy Density Yet in Molecular Solar Thermal Storage System

"I think our best systems were one megajoule [of energy per kilogram]. 6, which is really amazing," Moth-Poulsen said, referring to the energy density achieved by Han and her colleagues. 65 megajoules per kilogram.

That figure is greater than the energy density of lithium-ion batteries. The Most system that Han and her colleagues developed has limitations. The wavelength of light that causes the molecules to change shape is 300 nanometres.

John Griffin at Lancaster University noted that 300 nanometres is a form of very harsh UV light that reaches Earth from the sun in very small quantities. The trigger used to release the stored energy was hydrochloric acid. Hydrochloric acid is highly corrosive and must be neutralised after use.

"Not the most ideal choice," Han admitted. Griffin is working on solid versions of molecular storage. Han is researching solid iterations of Most that could take the form of transparent window coatings.

In a really optimistic scenario, the light-sensitive molecules in a Most system could be spread 5mm thick, according to Harry Hoster at the University of Duisberg-Essen. Packaging molecules in liquid form would require pumping them around a system, adding cost and complexity. "The moment you need to pump stuff around you have more things that can get broken," Hoster said.

Solid-state versions could release heat to prevent condensation or warm rooms, though Hoster is sceptical that Most will supply all the heat required in a building. Most technology operates without burning anything and could be made available anywhere on Earth, unlike fossil fuels concentrated in specific locations.

Moth-Poulsen noted that thermal energy stored as heat might last only a few hours, days or months at best, while Most systems could store energy for multiple decades.

The ultimate goal is to decarbonise heating, which still relies largely on fossil fuels. Hoster described the work as great science. "It's beautiful that they managed to get this functionality right," he said.

Griffin attended a conference last year on Most technology with roughly 70 attendees, representing the whole global community working on it. BBC News reported these developments.