Let's say we have a helium balloon carrying total weight of 1 kg. We set up 2 stations. Pumping station on the ground and deflating station at certain height. We pump the ballon until it start to float up. We catch it at deflating station at height H. Now the energy we gain is potential energy of the falling balloon which is mGH. The energy require for pumping and deflating a ballon is set! If any. The height H can stretch as high as you want to! => mGH is a linear increasing function.