10 million AA batteries

There has been a lot of press about the UK power company EON's plans to create a giant battery? The two questions that come to my mind are: is it green, and does it make economic sense?

Batteries are not all that green. They have all sorts of nasty chemicals and their ability to store a charge slowly decreases with time. They are also expensive. It will be interesting to see how this plan can compete with hydrogen or compressed air as energy storage.