Dunno why this occurred to me.
But here goes.
If you have an airship, full of helium lets say, in half inflated gas bags, it can lift a substantial weight to an altitude where the gas has expanded to fill its gas bags, and cant expand any more, more or less IIRC.
Now, at that point, with respect to the earth's surface that weight has considerable potential energy. mgh in fact. And yet that can be released by compressing all the gas out of the bags into say gas cylinders, and release all that potential energy as a nice freely falling airship.. and repeat the process over and over..now please don't tell me that the energy to compress the gas (not to mention the energy you might get out of its expansion) matches the potential energy in the weight at a couple of miles altitude...
So, what has LOST energy..in this system?
I cant puzzle it out.