[mlwiese]

You can use CO2 as a refrigerant. Non flammable and enables heat pumps to be practical at lower temperatures. Lots of products on the market. See https://r744.com

[tgsovlerkhgsel]

I assume using it results in a less efficient system though, right?

[GuB-42]

I don't know about efficiency but the pressure is much higher, making the system much more expensive.

[userbinator]

People experimented with CO2 refrigerant around the end of the 19th century, when refrigeration had just been invented and most other refrigerants hadn't been invented yet; it was called "carbonic acid" and required much heavier machinery due to the high pressures. (Typically over 1000PSI for CO2, compared to 100-200PSI for HC, CFC, and HCFC.)

[namibj]

Runs at high pressure, can be quite efficient though, iirc.