On 11/15/2013 12:15 AM, Chris de Morsella wrote:

So the energy would be stored in the potential energy of the heavy piston and the water would just be a working medium? Why not put the weight on a cable and use a purely mechanical system? I'd think that could be more efficient that a water turbine.

Other engineering friends of mine have levelled the same criticism of that idea. I guess the only counter argument could be that the very heavy piston's weight would be born directly by the compressed water column,

Water doesn't compress. You're just using it to move the weight up and down. As far as energetics go, it's no different than a cable hoist.

and that for very large masses the cable systems could present serious problems with stresses from wear -- have you ever seen when an elevator cable is un-torqued; it literally whips around as the stresses are released. Wondering about the stresses that repeated winding and unwinding of the support cables would induce.

The main problem is just size. I considered designing an energy storage system to be used with home, rooftop PVs. I looked around for something heavy that could be lifted up to store energy. My car. It weighs about 2600lb. Suppose I lifted it ten feet (high as the roof). I've stored 26,000ft*lb = 10Wh :-(


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