Tough choices: HFO-1234yf vs. R-744

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Life-cycle performance comparisons of R-134a vs. HFO-1234yf and R-744 (with orifice tube with bypass) show HFO-1234yf to be the best environmental choice, assuming all new vehicles use HFO-1234yf or R-744 by 2017.

The last barrier to adoption of HFO-1234yf as the low-global-warming replacement for R-134a auto A/C refrigerant was lowered with the satisfactory completion of key toxicity tests by DuPont and Honeywell. All vehicle platforms sold in the European Community that are new as of Jan. 1, 2011 must use an A/C refrigerant with a global warming potential (GWP) number of under 150, and HFO-1234yf is just 4.

However, some activist environmental non-governmental organizations (NGOs) are strongly in favor of R-744 (carbon dioxide as a refrigerant) and have been pressuring carmakers, so limited use of R-744 remains possible. The status of the refrigerant selection process was assessed by participants at a December 2008 meeting called by the U.S. EPA to determine industry readiness for a changeover.

Life-cycle study boosts HFO-1234yf

HFO-1234yf is close to R-134a in all key characteristics, is a virtual replacement, and would provide equal cooling performance and efficiency. Although the substance is mildly flammable and requires safety mitigation, it is the leading refrigerant choice due to its Life Cycle Climate Performance (LCCP), an internationally peer-reviewed method of evaluating the total global warming impact of mobile A/C systems. LCCP includes the effect of the refrigerant itself if it leaks out, the far greater impact of lifetime energy to run the A/C, plus manufacturing and other inputs, including system end-of-life. LCCP was developed by General Motors in cooperation with JAMA (Japanese Auto Manufacturers Association) and an SAE work group. The model ("Green MAC LCCP"), in spreadsheet form, is at www.epa.gov/cppd/mac.

HFO-1234yf also has been shown to have excellent compatibility with plastics and synthetic rubbers used in automotive A/C and an even lower permeation rate from hoses than R-134a. R-744 cannot use rubber/plastic hose materials on the high-pressure side of the system; it requires metal hoses and special seals, major contributors to high system cost.

NGOs such as­ ­Greenpeace and Deutsche Umwelthilfe oppose HFO-1234yf because it is not a natural refrigerant. Although R-744 has a GWP number of just one, it requires complete re-engineering of automotive A/C. R-744 poses considerable challenges not solved to date, and tests performed outside cool areas of Europe show that it doesn't match the combination of cooling performance and fuel efficiency of R-134a or HFO-1234yf. JAMA fleet tests indicate it degrades performance and fuel economy significantly on small cars. And according to General Motors, R-744 would increase system cost for parts alone by $350. As important, the LCCP study shows R-744, despite a lower GWP number for the refrigerant itself, is 10 to 15% poorer than HFO-1234yf in overall environmental performance. Furthermore, at high concentrations, R-744/carbon dioxide is an asphyxiant, and as a result of human respiration, carbon dioxide builds up naturally in a passenger cabin. Therefore, as a refrigerant, it also requires safety mitigation.

Most OEM prefer HFO-1234yf

To date, vehicle manufacturers representing 40 brands, equaling 70% of European sales, have made it clear they want HFO-1234yf, and AEI was told they will gradually issue announcements during 2009. Arkema, a leading European refrigerant producer, has said it will produce HFO-1234yf, and DuPont and Honeywell also are certain to do so, as potential customers make their decisions.

Currently, only BMW, Daimler, VW, Audi, and Porsche—all members of German auto manufacturers' association VDA—have expressed a preference for R-744 air conditioning. Like the NGOs, German government environmental officials have said they would like R-744 because it is natural. However, even the German manufacturers have identified R-744 development issues that remain. Further, reports indicate not all R-744 system components, for which GM sought production quantity price bids from the limited group of suppliers, currently meet GM's durability standards. The term "new platform" is not well-defined, and it is possible that there legally could be delays in implementation of the R-134a phaseout.

Flammability, toxicity dangers disputed

The NGOs have raised the specter of flammability and the possibility of poison gas (hydrogen fluoride), but the world automotive majority has forcefully responded, describing the videos that have been produced as flawed. The flames they show are oil, not refrigerant, explained Ward Atkinson of Sun Test Engineering, chair of the SAE Interior Climate Control Committee, who added they were produced at temperatures higher than seen under conditions cited in the videos. Seven research laboratories around the world, plus individual vehicle manufacturers, have evaluated HFO-1234yf for toxicity and flammability and determined it is safe, explained GM's William Hill, HVAC integration engineer and chair of two SAE Cooperative Research Groups that evaluated HFO-1234yf for materials compatibility, toxicity, flammability, and LCCP. CRP1234-1 and CRP1234-2 endorsed the new refrigerant as the best choice to replace R-134a.

Hydrogen fluoride is formed when a gas with fluorine is exposed to a flame. The issue of hydrogen fluoride was addressed by the SAE CRP1234-1, which cited experiments that showed the amount of hydrogen fluoride formed "is extremely low" when a flame was applied to what would be equivalent to an accidental release. The amount was the same for R-134a and HFO-1234yf, and there are no known published medical reports of any documented injuries in the 16 years of R-134a use, a CRP1234-1 report said.

HFO-1234yf difficult to ignite

Despite mild flammability, HFO-1234yf is difficult to ignite, with an auto-ignition temperature of 405ºC (761ºF), which means the air-refrigerant mixture must be at that temperature for combustion, explained DuPont technical fellow Barbara Minor. She said a test was made by heating a plate to 800ºC (1472ºF) from behind with a propane/oxygen torch and spraying refrigerant vapor (at a flammable concentration) onto the plate. There was no ignition, she said. Not until the plate temperature was raised to 900ºC (1652ºF) and oil was added to the refrigerant vapors did ignition occur (apparently of the oil) with both R-134a and HFO-1234yf, indicating no real-world difference between them.

HFO-1234yf requires high minimum ignition energy (approximately 10,000 milliJoules), so it is difficult to ignite with a spark. This was confirmed, Minor explained, with tests made with a 12-volt high-capacity battery, shorted to produce a high-amperage spark in an air-refrigerant mixture with concentrations of over 8% HFO-1234yf (the minimum flammable concentration is 6.2%). No ignition occurred, with repeated tests made at temperatures of 20ºC (68ºF), 60ºC (140ºF), and 80ºC (176ºF).

The Hyundai-Kia Technical Center ran flammability tests with a plate heated to 505ºC (941ºF) to simulate extreme underhood conditions. Performing tests with R-134a, HFO-1234yf, and R-744, it injected mixtures of refrigerant vapor and two different PAG (poly alkylene glycol) oils onto the plate. The oils, separately tested for flammability, have ignition temperatures of 417ºC (783ºF) and 427ºC (801ºF). They ignited with R-134a and HFO-1234yf, but the flame lasted less than one second and did not propagate to either refrigerant. With R-744, one oil did not produce a visible flame and the other did, but inasmuch as carbon dioxide is a fire extinguisher, unsurprisingly the flame also did not propagate.­­

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