At Cop 15 a company named Hidro displayed sustainablepowerstation.com ,,they were only ones allowed, so we thought it had some merit but not sure?. Principle is that the amount of energy used to put a little air into a tank is less than the amount of floatation work that air tank lifts to the surface? Then on the way back down the air is first released and gravity does some work to go down. I believe it does not take as much energy to float the tank as the float ride up gives out? Maybe someone knows more. I know the rule is energy in equals energy out, but maybe there is an exception ? gravity ? floatation ?