E. A. Grens wrote:
As Bob notes, the "second law" says you can't win; the "third law" says you
can't break even. It is very hard to break these laws. Here, the energy
required to change buoyancy, considered in the underwater version, is
apparently ignored. Just another "perpetual motion" machine.

the wiley dirigible designer would hang some solar cells on the back of
that beast, and be able to pump air in and out to his heart's content.
You could heat and cool the air using the day/night cycle to give you
the positive and negative bouyancy. You could probably figure out some
way to make it switchable between reflecting and absorbing to allow
multiple positive and negative bouyancy cycles in a day.

The things big enough that you might be able to use the differential
wind speeds on its surfaces for energy, ala dynamic soaring.

The point is there is plenty of energy swirling around out there for the
grabbing, so no laws of physics need be violated.