Finland has switched on the world’s largest sand battery, a revolutionary system that stores renewable energy as heat inside ordinary sand. Designed for long-term storage, it can hold energy for weeks or even months with very low losses — solving one of clean energy’s hardest problems. • The battery uses crushed sand heated by excess wind and solar power. • Heat is stored at high temperatures inside an insulated steel silo. • Energy loss is extremely low, even over long storage periods. • Stored