Now, coal-mine companies in the U.S. and Germany are stepping in with a retrograde solution: turn their old mines into enormous power-storage systems, using century-old technology to keep the lights on when the weather doesn’t cooperate.
The retrofitting, the first of its kind according to its developers, would use a simple technology called pumped storage, which requires little more than two pools of water, one at the surface, another deep underground.
.. the pump would use excess power on windy and sunny days to spurt water up from a reservoir almost 2,000 feet below ground to a pool at the surface. If there is a shortage of sun or wind, the water is released to allow gravity to bring it back toward the bottom of the mine, turning a turbine along the way to make electricity.