Germany is embarking on an innovative project to turn a hard coal mine into a giant battery that can store surplus solar and wind energy and release it when supplies are lean. The Prosper-Haniel coal mine in the German state of North-Rhine Westphalia will be converted into a 200 megawatt pumped-storage hydroelectric reservoir that acts like a giant battery. The capacity is enough to power more than 400,000 homes, Governor Hannelore Kraft said, according to