On: Peter Thiel, Bill Gates, Khosla fund LightSail Ene...

[ck2]

Why not use excess energy to lift millions of tons of worthless rock off the ground and then when power source fails, use the winding down via gravity to turn generators?

Gravity is never going away and never going to run out.

[technotony → ck2]

I believe that the problem with this is that it's very weak, ie you need to lift a large amount of weight to a great height to store much energy.

More details: http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

[jswanson → ck2]

This is already being done, by pumping water uphill: http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

I'm sure there are a bunch of interesting tradeoffs between using water or a bunch of rocks though.

[DaniFong → ck2]

There's a company called Energy Cache attempting that. Major design and construction challenge to achieve low cost. Maybe possible.