I want to make a low power refrigerator to liquify oxygen from the atmosphere. I have a source of electricity, and I know that at night I can expose things to the sky 1, 2 for a cold source and martial regolith or rock can be used as a heat source relative to the night sky.
Can I build some kind of electric powered refrigerator to liquify oxygen? Based on simple thermodynamics, roughly how many Joules would I need per gram of oxygen liquified? For simplicity lets ignore the problem of high rate of dry ice buildup on my condenser.