Has the train already left the station? It seemed that the industry choice for a new low-global-warming (LGW) air-conditioning refrigerant—R-1234yf—was well on its way. All needed SAE International standards were completed, a manufacturing plant was coming on stream, and all the testing had produced satisfactory results. But a new refrigerant, in fact from a choice of two, is looking for a seat on that same train, and it has a lot of support.
The two refrigerants are AC-5 and AC-6, and they were introduced by a large chemical company with a history that traces its roots to R-134a. You may not recognize the name Mexichem Fluor, but this Mexican company now includes INEOS Fluor, an English chemical company that absorbed ICI Klea, among the first major producers of R-134a. The new refrigerants were developed in England. Presentations were made at the annual SAE Alternative Refrigerant and System Efficiency symposium in Scottsdale, AZ.
Although Mexichem has not publicly disclosed the formulas, they have been released to companies that signed nondisclosure agreements. The company has stated that both are below the European Community regulated global warming limit of 150.
The refrigerants are being evaluated by a Cooperative Research Program (CRP) under SAE rules. The CRP, which hopes to complete its feasibility study by mid-2012, is chaired by Enrique Peral-Antunez of Renault, and includes representatives of General Motors, Volvo, Hyundai and Kia, Jaguar/Land Rover, PSA Peugeot Citroën, Renault, Shanghai Automotive Industry Corp., Sanden, Denso, Visteon, Doowon Climate Control, and of course Mexichem Fluor.
New options are zeotropic blends
Both refrigerants are zeotropic blends. It’s well known that the auto A/C industry prefers a single compound refrigerant (CFC-12, R-134a, R-1234yf). But an azeotropic mixture would be almost comparably acceptable—that is, one that maintains the same composition as a liquid or a vapor, at least under operating conditions within the system. By comparison, a zeotropic mixture’s composition is constantly changing during vaporization and condensation, and the change in the vaporization temperature in particular affects thermodynamic performance in the evaporator. The degrees of change are called glide, and although stationary A/C systems using blends are designed to deal with small amounts of glide, AC-6 reportedly has a measurably greater amount of glide than refrigerants in present use, according to Peral-Antunez’s CRP report.
At this time, AC-5 matches the LCCP (life cycle climate performance, a measure of environmental impact) of R-1234yf, and both are comparably very low in flammability. AC-6 needs some improvement in efficiency to reach LCCP equality. However, engineering approaches to reach this point are believed to have been identified, and if the glide also proves to be manageable, AC-6 would be the preferred blend because of its far lower potential flammability than either R-1234yf or AC-5.
Although public information on the two blends is limited, the CRP report said that AC-5 has a pressure-temperature curve and cooling efficiency close to both R-134a and R-1234yf, that its flammability is similar to R-1234yf, and that the glide is similar to zeotropic blends in stationary cooling. AC-6 has a slightly higher working pressure, but compared with R-1234yf, this blend has potentially higher cooling efficiency with heat exchanger redesigns. Further, in addition to its overall reduced flammability, AC-6 is nonflammable at the temperatures encountered in transportation and handling, and it has a possible use as a heat pump fluid. Overall toxicity was described in promising terms and a preliminary toxicology estimate for the Occupational Exposure Limit, assigned by the CRP, shows both are higher than the 500 ppm of R-1234yf and close to the 1000 ppm for R-134a—800 ppm for AC-5, 900 ppm for AC-6.
There is a service issue with recovery and recycling the blends, legally required for all automotive A/C refrigerants in the U.S. The machines that can do this for R-134a are moderately priced and in use in most automotive service shops that work on A/C systems, and similar though more expensive machines have been developed for R-1234yf. A change in blend refrigerant composition from selective leakage of higher pressure components, determined by use of a refrigerant identifier, could require that a blend refrigerant be collected and shipped offsite for reformulation while only virgin or reformulated blend refrigerant would be installed in the system. This would add expense and complexity to its use, but there are ways that the issue could possibly be handled in a service shop, if the composition change from leakage is shown to be consistently predictable. The CRP is working to prove that in-shop recovery and reuse is feasible.
Price a key factor to consider
Several factors are driving consideration of the blends. Mexichem hasn’t given an estimated price, but it has more than hinted that either blend would be far lower priced than R-1234yf. With the price for R-1234yf somewhere in the $45-$60 OE price per pound ($100-$130 per kg), a low-cost refrigerant could save car manufacturers billions per year worldwide. The long-used R-134a is perhaps 10% of the price of R-1234yf.
Patent issues, including lawsuits in Europe and the U.S., have kept all but Honeywell and DuPont, who have a joint venture, from building a plant to produce R-1234yf. That has restricted potential supplies and inhibited the anticipated price reductions from a competitive environment. Even with competition, R-1234yf is believed to be much more expensive to make than R-134a, so dramatic price cuts are not necessarily in the cards. At the least, the Mexichem blends create a potentially competitive atmosphere.
Much of the comparative testing has been on AC-6. Its performance and efficiency is similar to R-134a at high cooling loads. AC-6 efficiency is lower at low loads, but a study by Creative Thermal Solutions (CTS), a thermal laboratory in Urbana, IL, indicated that the cause was maldistribution of refrigerant in the R-134a evaporator that was used, not considered an unusual problem for a non-optimized system.
Xinzhong Li, Engineering Manager of CTS, told the SAE meeting that a 2°C (3.6°F) increase in average evaporating temperature would raise COP (coefficient of performance) by 6.6%. “A new design for the evaporator has a big potential,” he said. Development of a suitable evaporator configuration for AC-6 is on the CRP work plan.
The difference in efficiency between AC-6 and R-1234yf even at this early stage of AC-6 system development is equal to 50 W, which translates into just 1 to 2 L (0.26 to 0.53 gal) of fuel per year, it was claimed in a presentation at the SAE meeting by Renault’s JM L’Huillier and Peral-Antunez. Renault data was developed with Valeo Thermal Systems.
CTS studies also showed that high discharge pressure from the compressor could be reduced, and system efficiency increased, with a new condenser design optimized for the refrigerant. Li said that CTS will be testing new condensers with different flow path configurations.
Although R-1234yf is only very mildly flammable, the idea of an even-less-flammable refrigerant is very appealing. It also could produce savings in shipping and handling as well as design of A/C components and service equipment.
Materials that are compatible for hoses and seals have been identified for the blends. Some compressor oil issues remain to be solved but no insurmountable problems were raised. Both subjects are under CRP study for optimization.
A zeotropic blend is not the automatic “no no” anymore. It does require special component designs, but with today’s very tight systems, leakage is minimal. That means that high leakage, even if it results in a blend composition change that affects performance, is not the issue it was in the days of the now-discontinued CFC-12, when all systems might leak as much as 0.5 to 1 lb (0.23 to 0.45 kg) per year in normal operation and require annual recharges.
Today, a normal system may leak an average of perhaps 10 g (0.35 oz)/year, and even that number is coming down. The commercial and residential A/C industries have used blend refrigerants, but their systems do not require flexible hoses, from which seepage may occur. And the refrigerant line joints are not subjected to the deterioration (and therefore leakage) from vehicle operating vibrations and temperature.
The effect of blend composition change on flammability from a worst-case result of refrigerant leakage showed that AC-6 still had a lower burning velocity and had a narrower range of flammability even at 60°C (140°F)/50% relative humidity. No data was released for a worst-case effect on system performance and possibly durability, although it reportedly is available to those who signed the nondisclosure agreement.
An LGW refrigerant will have great importance in the 2017-2025 period, when automakers face toughening regulations measured in g/CO2 mi. The LGW credits are substantially higher than a manufacturer can earn by A/C anti-leak improvements. And although there was minor tweaking to the 2017-2025 allocation of credits between cars and trucks for improved system efficiency, the overall total is essentially the same as for the 2012-2016 period, explained Brian Nelson, U.S. EPA engineer with the Assessment and Standards Division. The maximum credits for an LGW refrigerant and improved efficiency are 18.8 g/mi for cars and 24.4 g/mi for trucks.