GridScale (IMAGE) Aarhus University Caption When there is a surplus of electricity from wind or solar, the energy storage is charged. This is done by a system of compressors and turbines pumping heat energy from one or more storage tanks filled with cool stones to a corresponding number of storage tanks filled with hot stones. This makes the stones in the cold tanks very cold, while it gets very hot in the hot tanks, up to 600 degrees. The heat can be stored in the stones for many days. When the power is needed again, the process runs the opposite way, so that the stones in the hot tanks get colder, while they get hotter in the cold tanks. Credit Claus Rye, Stiesdal Storage Technologies. Usage Restrictions May only be used with appropriate credits. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.