The sand battery is the idea of two Finnish engineers, Markku Ylönen and Tommi Eronen. It is simplicity itself. Make a really big pile of sand. Heat it with excess renewable electricity to around 500º C (932º F), then use that heat later to heat homes, factories, even swimming pools. They say the sand can stay hot for 3 months or more. Together they founded Polar Night Energy, which has completed its first installation — 100 tons of sand inside what looks very much like a silo — in the town of Kankaanpää.