So like refrigerators and freezers and things like that use the Carnot Cycle. And I understand how that cools things down. But I’ve also heard that with conventional methods the biggest temperature difference you can make is about 100⁰C. So how do we get something like liquid nitrogen at -195⁰C? Are we putting a freezer inside a freezer inside a freezer? Or how do we get stuff that cold?
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Liquid nitroget temperatures can be reached via compression and expansion.
When gas is compressed with pressure it heats up. When gas expands due to lack of pressure it cools down.
So you take gas and compress it greatly. It gets hot. You let it cool down (or cool it with anoter refrigerator).
Now you let this cool pressurized gas expand back to normal pressure. It cools down.
With big enough pressure difference you can cool down enough to turn gas into liquid.
Fridges and AC units also do their cooling (and heating) with the same principle of compressing and expanding gases. The pressure differences they use are just much smaller.
Here is a video on making liquid nitrogen: [Veritasium: Making Liquid Nitrogen From Scratch!](https://youtu.be/dCXkaQa53QQ)
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