It doesn’t look like much, but Finland recently flipped the switch on the world’s largest sand-based battery. Yes, sand. A sand battery is a type of thermal energy storage system that uses sand or crushed rock to store heat. Electricity — typically from renewable sources — is used to heat the sand. That stored heat can later be used for various ends, including to warm buildings. The economics are compelling, and it’s hard to get any cheaper than the crushed soapstone now housed inside an insulat...