ac evaporator
Propane makes a good refrigerant too.
LOTS of things are "on the table" "sort of." Most people just don't realize that when the Freon family of refrigerants was invented and developed, engineers thought they had the world by the tale
A refrigerant should:
Operate well under the temperatures and pressures of the system. Hell, CO2 is a refrigerant, but the operating pressures and temp is extreme. Actually it IS used in some ?cascade? ultra low temp stuff
Mix with oil and not destroy seals, and internal parts
It would be handy if it wasn't explosive or flammable
It would be VERY handy if it wasn't toxic or poisonous.
"Before my time" they used methyl chloride and sulfur dioxide, both very nasty indeed. One of them, under adverse conditions, would remove copper from the tubing and deposit it on the cylinder walls of the compressor.........not really what you want.
