The Frankfurter Rundschau reports that Daimler has developed prototype mobile air conditioning (MAC) systems using CO2 as the coolant and has begun lab testing. According to the report, the Volkswagen Group also is in development of CO2MAC systems, but has yet to begin testing.
Both automakers are looking to CO2 as an alternate low global warming potential coolant to R-1234yf, to which Daimler originally objected in 2012, citing safety concerns. (Earlier post.)
In March 2013, the Volkswagen Group announced that it was choosing CO2 as the future low global warming potential (GWP) refrigerant for its mobile air conditioning (MAC) systems. (Earlier post.)
Reducing the greenhouse gas emissions from MACs have been under investigation since the late 1990s; cars equipped with R134a (tetrafluoroethane) MAC systems produce emissions corresponding to 7 grams of CO2 per driven kilometer. Mobile air conditioning systems are the single most important source of fluorinated greenhouse gases, due mainly to high emissions during their operation and their large numbers, according to Germany’s Federal Environment Agency (Umweltbundesamt, UBA).
Industry essentially ended up with three alternative low GWP refrigerants from which to choose: R744 (CO2), R152a, and R1234yf.
CO2 is an A1 refrigerant, indicating minimal toxicity and non-flammability. With its GWP of 1, it is also the refrigerant that has the least impact on climate. It has a high cooling capacity and is available worldwide at low cost in the required qualities.
The UBA took the position in the late 2000s that R134a should be replaced by the natural refrigerant R744. However, R744 systems operate at pressures 5 to 10 times higher than R134a systems, necessitating development of high-pressure hoses, compressors and other components. This raises issues of cost, as well of durability.
In addition, a CO2 leak into the passenger compartment is a safety hazard since the gas can displace a significant amount of oxygen. There are several methods available for safeguarding passengers, both in direct expansion (primary loop) and secondary loop systems.