In late November, in a quiet corner of Devon, England, workers began adding a secret, light brown powder to water as they mixed up a special fluid that can store energy. They blended it with the utmost care, like some kind of giant protein shake, over the course of multiple weeks. Their goal was to achieve a mixture 2.5 times denser than water.
“It’s quite a hands-on process. At bigger scales, we would automate it,” says Stephen Crosher, chief executive and co-founder of RheEnergise, a British energy-storage company. He emphasizes that the mineral-based fluid must flow easily. “You want it to be really runny.”
That’s because, in the company’s demonstration system,