Violation of conservation of energy thought experiment:
Let us imagine an air compressor is invented with incredible efficiency. This compressor will compress a gas and recover back 99% percent of that energy when the gas is released from the compression chamber. So far this violates no laws of physics.
Now imagine we were to take an enormous hydrogen balloon and lift a very large mass to a substantial height. The hydrogen balloon drops of the mass on some very high platform, then compresses its hydrogen to the point where it loses its buoyancy and it will descend. The balloon then reloads another mass, decompresses the hydrogen, and repeats. Were the 99% efficient compressor to waste less energy than the gravitation potential of the lifted masses, would we not have violated conservation of energy? So where is the flaw in this experiment?