edit: looks like you guys have found out most of what I found out already.
I just asked my father about CO2 phase change. He says it is indeed used, but it is very expensive. Take a look at R744 systems on the web.
Operating pressures of 1500psi to 2000psi are common, which means that tubing must be stronger, joints must be stronger/tighter, etc... I believe the pressure differential is also higher, but I'm not sure.
On a side note, CO2 has great potential to replace R-134a in cars, due to the greenhouse effects of R-134a (even if R-134a doesn't damage ozone, it is a great greenhouse gas).
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