Finland installed the world’s first functional sand battery this month. It heats up the sand, which can retain that heat for months at a time, according to the developers. This stored heat is energy that can be tapped later. All over the world, electric grid operators are looking to store more renewable energy from wind and solar and turning to big batteries, sometimes acres of them. Here in the U.S., more than 90% of this storage relies on lithium-ion batteries, according to the government, thanks to their efficiency and relatively low cost. Marketplace’s Kimberly Adams spoke with Jason Burwen, vice president of energy storage for the American Clean Power Association, an industry group. She asked him what other kinds of battery innovations are getting him excited.