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One can transport and store oxygen more efficiently (volume and weight of tank) if it is liquid under the order of 1 atmosphere. Liquid oxygen (LOX) is kept cryogenic in insulated tanks for short periods of time as a propellant in rockets, and for weeks just about anywhere in well insulated but heavier tanks (see below). I believe oxygen is transported from earth to the ISS as LOX, as suggested in this question.

On the ISS is there a (at least some) supply of oxygen that is stored as LOX? Is the "cold of space" when the sun isn't shining ever used to keep it cold, or just good insulation and a refrigerator? Writing this answer got me thinking about this. LOX boils at about 54K at 1 atmosphere. If heat leaks into the tank through insulation or the liquid access connection or boil-off vent, it is removed by the boil-off process. However if you need to make it last a long time, supplemental cooling might be necessary.

below: "Portable container for transport of liquid oxygen".

enter image description here

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  • $\begingroup$ The cold of space is only available in permanenty-shaded areas. Providing those would put additional constraints on the ISS attitude, IDK how feasible that would be. $\endgroup$ – Hobbes May 18 at 10:33
  • $\begingroup$ @Hobbes I wrote the "cold of space" when the sun isn't shining to allow for reflective/insulating doors that would open when the sun isn't shining to cool the contents, then close when the Sun is out again. Not practical of course, and it turns out there isn't any (or at least much) LOX anyway. $\endgroup$ – uhoh May 18 at 13:18
  • $\begingroup$ @Hobbes you can also get fairly cold with a double-sided one of these, in the ballpark of 130 down to 100 K even exposed broadside to the Sun (colder if edge-on), but again not so practical. $\endgroup$ – uhoh May 18 at 13:20
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Hard to prove a negative, but no, there is no provision for storing LOX on the ISS. The coldest thing up there is the MELFI, the Minus Eighty Degree Laboratory Freezer (for) ISS.

Oxygen (and nitrogen) is stored as a high pressure gas in the tanks fitted to the outside of the US airlock. (The dog-house shaped objects around the airlock in the picture below). These tanks were brought up full by the shuttle, for example in this writeup from the STS-129 press kit.

...a High Pressure Gas Tank (HPGT) full of oxygen from ELC-2 to a spot on the outside of the Quest airlock, placing it amidst the other dog house-shaped tanks that are used to replenish atmosphere lost when spacewalkers enter and exit the station.

(ELC-2 was an External Logistics Carrier)

enter image description here

Now that the shuttle logistics path isn't available they have to use NORS instead. The NORS tanks are internal to the station.

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