Surely you could have a motor keeping the drum moving (wasting energy I know) but when you run out of power, you turn the motor off and use the drum's kinetic energy. I guess it's just a larger, inefficient battery.,(reply>), This was my thought, as you could run a flywheel from a motor/generator, but I'd imagine it would be very inefficient! I'd say that we just need enough renewable energy to cover any outage, or just burn rubbish in the power stations!