India, June 18 -- Finland has just switched on the world's largest sand battery, and it's already making waves in the energy world. The concept is refreshingly straightforward: when there's extra solar or wind power, the battery stores that energy as heat in a massive silo filled with sand. When the town needs heating, especially during those long Finnish winters, the stored heat is released and piped into homes, offices, and even the local swimming pool, cutting fossil fuels out of the equation.

Lithium batteries get all the attention, but they're costly and come with their own environmental baggage. The Finnish team at Polar Night Energy decided to go back to basics and use sand, which is often just industrial waste from fireplace fact...