As you may be aware, 134a, which is a commonly used ozone friendly automotive refrigerant, still has 1700 times the GWP of CO2. Propane is a suitable non-global warming refrigerant. I submitted the very first proposal for a NGWP refrigeration system under the EPA SNAP program almost two decades ago, and there is now an international standard based on this. Tecumseh makes DC compressors under the Masterflux brand name, and which have been widely used in military applications. I hope that the A/C system that is chosen does not add to man made global warming, which, after all, is a primary reason for the designs' existence.