Why air? If you are using solar to compress the air...why not take it a step further. Use the underground storage you propose to store hydrogen. Solar energy dr

Why air? If you are using solar to compress the air...why not take it a step further. Use the underground storage you propose to store hydrogen. Solar energy drives electrolysis, leaving you with hydrogen and oxygen, both of which can be safely stored and have vastly greater energy potential(when burned) than the stored energy of air. I love your idea of using solar to build a store of energy, but you could refine the idea a bit and really do something fun. Having a large quantity of oxygen(liquify it!) could be a boon to the backyard rocket engine enthusiasts :)