New Airlock Concept

In article ,
Jake McGuire wrote:
The air in the airlock (at STP, or some approximation thereof) is
expanded through a turbine or a reciprocating "expander" down to
something like Martian surface pressure in an external holding tank,
and the shaft work thereby obtained is used to drive the compressor of
a refrigeration system used to progressively liquefy the (cooled
during expansion) air from the holding tank...

Venting an airlock into a low-pressure holding tank, which is in turn
cleared by a scavenging system, has certainly been proposed before.
(Although for rover applications, remember that the holding tank's volume
has to be many times the airlock's air volume, i.e. the holding tank will
be big.) But I don't recall anyone suggesting liquefying it.

Bear in mind that you *cannot* liquefy a gas below its triple-point
pressure. Below that pressure, there is no liquid phase, only gas and
solid. (This is why dry ice doesn't melt, but goes directly to gas --
there is no liquid CO2 at 1atm.) Unfortunately, nitrogen's triple-point
pressure is about 125mbar, many times Martian surface pressure. Oxygen's
is much lower, and you could liquefy oxygen at Mars pressure, but not
nitrogen.

Whether this matters will depend on the cabin atmosphere, but a long-stay
expedition is going to have at least some nitrogen in the air for health
reasons, and preferably quite a lot to make air-cooled electronics
possible. Cabin pressure might still be somewhat lower than Earth's to
reduce prebreathing requirements, especially if the suit pressure is
relatively low.

...It looks like the work available from the depressurization is
approximately equal to the work needed to liquefy the air...

Note that even aside from the triple-point issue, at a low pressure you'll
need to cool it down somewhat farther to liquefy it.

Is this insane?

Aside from the triple-point problem, it's not ridiculous... but it's not
obvious what advantage it would have. It is more complex and probably
needs more power than just compressing the holding-tank air for injection
back into the cabin as a gas.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |