The Volkswagen Group announced that it is choosing CO2 as the future low global warming potential (GWP) refrigerant for its mobile air conditioning (MAC) systems. The announcement follows press reports from the Geneva Motor Show that Daimler, BMW and Volkswagen would pursue CO2 (R744) as the refrigerant for MACs instead of R-1234yf (developed by Honeywell and DuPont). (Earlier post.)
Volkswagen said it would roll out CO2 MAC systems progressively over its entire vehicle fleet. With a GWP (Global Warming Potential) value of 1, R744 is 99.3% below the EU’s now-specified GWP limit of 150 for MAC systems. R134a, the current widespread MAC refrigerant, has a GWP of 1,300, according to the US Environmental Protection Agency (EPA).
While last year it appeared that the auto industry as a whole would shift to R-1234yf, in September 2012, Daimler said that internal testing had identified safety (flammability) issues under certain conditions, and that it would not use R-1234yf in its products. BMW and Volkswagen followed suit.
Daimler’s decision launched a new round of investigation by SAE International on the safety aspects of R-1234yf (earlier post). In February, the SAE Cooperative Research Project (CRP1234-4) team reported that the high level of confidence that R1234yf can be used safely in automotive applications continues to grow.
CO2 as a MAC refrigerant. 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, noted Germany’s Umweltbundesamt in a 2009 publication on CO2 systems.
(R134a replaced CFC-12 as the MAC refrigerant in the early 1990s due to the negative impact of CFCs on the Earth’s ozone layer. According to a paper by Delphi engineers investigating another refrigerant (R152a), R134a was chosen for its combination of refrigerating properties and safety characteristics, despite indications that its global warming potential (GWP) and resulting climate impact could one day become a concern.)
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.
For these reasons, Germany’s Federal Environment Agency (Umweltbundesamt, 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.
R152a (GWP = 124) is very similar to R-134a in terms of pressure, refrigerating characteristics, and compatibility with currently used system components and materials, making it easier and less expensive to implement. It is classified as an A2 refrigerant: minimal toxicity and moderate flammability. Compared to 1234yf, it ignites more readily and requires additional mitigation measures not required for 1234yf.
R1234yf (GWP = 4) (earlier post) is also a class A2 refrigerant. However, in 2009, a two-year Cooperative Research Program conducted through SAE International to investigate the safety and environmental performance of R1234yf can be safely accommodated through established industry standards and practices for vehicle design, engineering, manufacturing, and service, and can be used as the global replacement refrigerant in future mobile air conditioning systems.
That was the report is the third SAE report to evaluate the new refrigerant. The current work following Daimler’s balk in September 2012 is the fourth.
The performance-based standards for low GWP refrigerants in Europe and the United States allow automakers to choose a replacement for HFC-134a. However, other concerns including cost, feasibility, and safety can limit these options. HFC-152a, HFO-1234yf, and carbon dioxide are alternatives that may substantially reduce climate impacts and meet safety standards.
...A transition to a new refrigerant is not easy. Automakers must change the design of the refrigeration system and the vehicle, expect new training of service technicians, explore changes to design standards, and seek regulatory approval in the United States and Europe.—Minjares, ICCT Working paper
Ray Minjares (2011) Refrigerants for light-duty passenger vehicle air conditioning systems: Technical assessment of alternatives to HFC-134a (ICCT Working paper 2011-3)
Natural refrigerants: CO2-based air conditioning system put to practical testing (Umweltbundesamt, 2009)
Mahmoud Ghodbane, Timothy D. Craig, and James A. Baker (2007) Demonstration of an Energy-Efficient Secondary Loop HFC-152a Mobile Air Conditioning System