Low-GWP Refrigerants: How Close Are We?

The automotive air-conditioning industry seemed to be on a smooth, albeit more expensive, path to a new low global-warming-potential refrigerant with the introduction of R-1234yf a few years ago. This was a single-compound chemical requiring a relatively low amount of system re-engineering, with the same pressure-temperature curve as R-134a. And it appeared to be the industry’s choice.

However, last September Daimler announced the results of its own flammability tests, and made it clear that it wouldn’t use R-1234yf, putting itself in opposition to the worldwide coalition (of which it was a member) that had agreed on this chemical.

Then, the new SAE International Cooperative Research Group (CRP-1234-4) reviewed the Daimler data, and reported that it backed the original CRP that had approved R-1234yf. The German carmakers pulled out of the new CRP on the grounds their safety concerns were not adequately addressed. The coalition of 10 other manufacturers remains nominally intact in support of R-1234yf.

The MAC Refrigerant Blend Cooperative Research Program (MRB CRP) is administered by SAE and is comprised of the following companies: Behr, Bosch, Chrysler, Cinetic Filling, Denso, Doowan, General Motors Company, Halla Visteon Climate Control, Hyundai, Jaguar Land Rover, Mexichem, Nissan, PSA, Renault, SAIC Motors, Sanden, Schrader International, Texa and Volvo Cars.

The only vehicles on sale in the U.S. with R-1234yf are the Cadillac XTS and Honda Fit electric vehicle (EV). A Cadillac ATS installation was changed to R-134a when a noise problem surfaced on the 2.0-L turbo, caused by compressor vibration resulting from the expansion valve setting selected for R-1234yf and proximity of the compressor mount to the front engine mount.

Nowadays, has the low-GWP refrigerant train – with R-1234yf in it – left the station?

Apparently there is enough belief it hasn’t, because, for one thing, it is expensive and for another, its efficiency is measurably lower than that of R-134a. And it’s still flammable, though only mildly so. However, its GWP is just 4.0, vs. 1430 for R-134a.

The German Federal Motor Transport Authority, KBA, has stated it will perform specific safety tests for the controversial air conditioning refrigerant 1234yt this coming July. According to the report, there will be a two-stage test procedure in which the KBA first determines the point at which the air conditioning of the model is damaged and the second step will prepare a vehicle of the same type with these defects. These tests will form the benchmark for standards of different driving profiles measuring the fire probability. However, no agreement with the SAE and KBA has been reached on how the test should be performed.

Meanwhile, another SAE CRP continues to work on AC6, a Mexichem-proposed blend of 85% R-1234ze, 9% R-134a, and 6% R-744 (with carbon dioxide used as a refrigerant). The blend is designated R445A by the ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers).

The GWP number of AC6 (R445A) is about 135, which is within the European Union (EU) regulatory limit of 150; the latter figure is generally accepted worldwide as the top number for a low-GWP refrigerant. Although the U.S. EPA doesn’t require a low-GWP refrigerant at this time, it does provide CAFE (Corporate Average Fuel Economy) credits for its use.

The R-744 content of AC6 (R445A) raises the question of selective leakage from the system, and as the highest-pressure ingredient in the blend, it has the highest potential leak rate. A leakage model in the CRP data shows there would be just a 5% performance loss if the R-744 level dropped from 6% to 2%. If the CRP identifies sufficiently improved hoses, it believes a seven-year service life would hold, and that would be a satisfactory period for the blend to be acceptable.

The automotive aftermarket and OE assembly lines have never had to work with a zeotropic blend, i.e., one in which the composition of the ingredients changes during the refrigeration cycle, as does AC6 (R445A). The CRP has had to consider the possible ways in which a system could accurately be charged. And because R-744 is so different from the other ingredients, a separate injection during the refrigerant charge event is being evaluated.

Part of the blend’s appeal is its performance as a heat pump refrigerant in a vehicle in which the refrigeration system cycle would be reversed. A heat pump is used on the newly introduced EV from Renault, called the ZOE, to reduce use of resistance heating, which has a greater effect on vehicle range.

A heat pump also could help gasoline and diesel engines – whose efficiency has been improving, so there is less waste heat for cabin comfort and window defogging/defrosting. Concept testing AC6 (R445A) in heating mode showed it has higher capacity than R-134a and would provide heat at lower ambient temperatures.

“The other issue at hand is, if you go back and look at our MAC service reports, we used to have to service vehicles up to twice per year because of A/C leaks. As time went on, we learned more and more about the environmental issue and how we could improve the systems,” explains MACS technical advisor Ward Atkinson. Atkinson was chairman of the SAE control standards committee for some 40 years, during which time that SAE committee wrote over 50 standards. He retired this past March and is now a technical advisor to MACS.

MACs is currently running a 2013 field service update which has revealed vehicle emission rates are now down to about 15 grams a year. “We are now looking at not having to go in and service those systems for anywhere from five to nine years. This is a big gain in itself. Plus the fact that the charge amounts have been greatly reduced. We are now in the area of less than 1000 grams and more in the 600- to 800-gram area. So we have tightened the systems up, and we have reduced the emissions from the systems, but we now get into the argument of what is really taking place,” says Atkinson.

“Even if the refrigerant has a big GWP number, the refrigerant is now less than 5% of the lifetime usage of the car. But if you use a less efficient refrigerant, and these other refrigerants we are looking at are less efficient, what happens is that when you run the system, the tailpipe emissions go up,” explains Atkinson.

“You don’t get anything for nothing. So the question at hand is, when you look at the lifetime of the car and the tailpipe emissions when using the A/C system, it far exceeds what the direct benefit is of the refrigerant. So here we have done all these things to improve the efficiencies, but we are still considered the bad guy because we have a bad GWP number,” he adds.

As if the question of the right refrigerant weren’t enough, OE makers also face a system efficiency challenge in AC17, the EPA’s new drive cycle for A/C fuel consumption, required from 2017-2025. The AC17 regulation prescribes a lengthy test as of 2017, although some engineering analysis is permitted in lieu of baseline test data. For 2020-2025, AC17 test data must demonstrate reduced fuel consumption to receive EPA CAFE credits.

EPA’s A/C-related credits cover CAFE credits for solar/thermal upgrades from glass, reflective paint, parked-car ventilation in hot soak, and a list of vehicle technologies. They include: reduced A/C reheat, default to recirculated air in ambients above 75°F/24°C, improved evaporators and condensers, more efficient blower motors and compressors, and compressor oil separators to reduce energy from oil circulation through the refrigeration.

It is clear that there is much more research in the offing before we get to a refrigerant that significantly reduces global warming potential safely, efficiently and economically. But that Holy Grail appears to be getting closer with every summer that passes.

SIDEBAR

SAE International MRB CRP Announces Progress on New Alternative Refrigerant

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The SAE International (SAE) MAC Refrigerant Blend Cooperative Research Program (MRB CRP), comprising a group of leading global vehicle OEMs plus Tier One suppliers, has continued its assessment of the operating, technical, and safety performance of low-GWP blended refrigerants (AC5 and AC6) developed by Mexichem, and based upon R-1234zeE.

At meetings held in Paris on 3rd-6th June 2013 the team gave extensive review to Phase 3 of its detailed fault-tree analysis, which is focusing on blended refrigerant AC6. MRB CRP has already reported on the reduced flammability of AC6 as compared with R-1234yf. To refine this fault-tree analysis, two additional programs of experimental work have been commissioned for execution over the next three months, including trials to extend the understanding of the real-world flammability characteristics of AC5, AC6 and R-1234yf.

The MRB CRP team also confirms its intention to issue a technical White Paper for these blends, based upon the information previously reported at SAE Thermal Management System Symposium (TMSS) in October 2012, and more recently updated at the UNEP Round Table on New Technology in Refrigeration and Air Conditioning, held in Milan on June 7, 2013. This White Paper will be available on the SAE website this month.

SAE International is a global association of more than 138,000 engineers and related technical experts in the aerospace, automotive, and commercial-vehicle industries.