I was thinking about how we currently store large quantities of energy - in the form of batteries, correct? - and then I got to thinking that wouldn't it be much more simple and efficient to just have turbines move buckets of sand into an elevated container and then when the energy is desired, allow the sand the fall and push turbines below it? Theoretically the efficiency would be at least in the 90% range if it was designed well, possibly even approaching 99%. I would have said doing this with water but in my opinion water is too valuable to store away as potential energy than is sand or pulverized rocks. What do you guys think of this idea?