Just when the pundits declared coal companies dead in the water, along comes another idea. Why not use the vast underground network as a pumped storage system?
Stay tuned to see what the real issues prove to be. The questions that will emerge are the realities that coal mines are not located near load centers. The actual power flows on the distribution system as this storage is used must be considered to be sure the source/sink nature to this does not create an overload. Also, I am sure someone will find a critter in these mines that will now lose their habitat and tie this up in the courts for years.
Things like this never seem to bother the Germans. I love German engineers. They get things done when they know they are good ideas.