Sand is a favored thermal energy storage media as it has very high thermal stability allowing it to cycle between ambient air temperature and over 1000°C. The wide temperature range increases energy storage density and system efficiency. By using sand as a low-cost, durable medium, engineers are unlocking new ways to store heat and supply consistent, round-the-clock clean energy. What Is Thermal Energy Storage?Four years ago, researchers at the National Renewable Energy Laboratory (NREL) won Department of Energy (DOE) ARPA-E funding to invent a new long-duration thermal energy storage technology able to discharge heat or power for 100 hours. A towering 23-foot silo packed with 100 tons of low-grade sand now stands as proof of concept for large-scale thermal batteries. These systems capture surplus. As the world explores novel ways to store renewable energy, one humble material has emerged as a surprising candidate: common sand.