"Marc" wrote:

2) Where is the power source? Conservation of energy says that the plane
could not continue to ascend and descend without some input of energy.

A gas balloon can oscillate up and down by harnessing the
temperature differential with altitude. As it rises, it
cools, the gas contracts and the balloon then falls to
lower, warmer, altitudes, where the gas expands and the
cycle repeats. It could in theory have wings that allow it
to glide somewhat during each cycle.

I seriously doubt the practicality of such a craft.

A similar concept, alluded to in the text, is currently used
in extremely low powered ocean monitors that use a phase
change material for buoyancy control. The phase change
material has a larger volume when cold, so it rises when
submerged to a chilled depth. Winglike structures let it
"glide" towards the desired location to be sampled. They're
slow, but effective and can cruise for months. Electronics
are powered by solar energy at the surface during each
cycle.

The
initial ascent would be powered by the energy required to separate helium
from the atmosphere, which is accomplished on the ground, but it should not
be true that the plane would "always land with its tanks fully pressurized",
since theoretically the amount of energy generated by the turbines during
the descent could not be enough to compress the gas used for the ascent.
Esecially if the efficiency of the turbines is 20%.

Agreed.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
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