You can also operate the CO2 in a 'trans-critical' mode which gets around some of the issues of the low critical temp. This is done by using a gas cooler to coo

You can also operate the CO2 in a 'trans-critical' mode which gets around some of the issues of the low critical temp. This is done by using a gas cooler to cool supercritical fluid in place of the typical condenser. There aren't really many good alternatives that are cheap, have very low GWP (less than 100), are non-toxic and non-flammable. HFOs are probably the closest to this, but even these are often mildly flammable and typically quite expensive (often > $50/kg).