Of course. That's very common approach and there are many such power plants ("storage"). But as you can imagine, this cannot be built anywhere, cheaply enough a

Of course. That's very common approach and there are many such power plants ("storage"). But as you can imagine, this cannot be built anywhere, cheaply enough and in large enough quantities. (http://upload.wikimedia.org/wikipedia/commons/3/33/Dlouhe_strane_horni_nadrz.jpg http://upload.wikimedia.org/wikipedia/commons/3/33/Dlouhe_st..., 2 × 325 MW for 6 hours when full, ~250 millions EUR to build)