The automotive air-conditioning industry seemed to be on a smooth albeit more expensive path to a new low global-warming-potential (GWP) refrigerant, R-1234yf. It's a single-compound chemical, requiring a comparatively lower amount of system re-engineering, with the same pressure-temperature curve as R-134a. And it appeared to be the industry's choice.
However, when Daimler announced new flammability tests last September, it also made clear it wouldn't use R-1234yf, putting itself in opposition to the worldwide coalition that had agreed on this chemical. A new SAE International Cooperative Research Group (CRP-1234-4) reviewed the Daimler data and last month reported that it backed the original CRP that had approved R-1234yf. The German carmakers then 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 for R-1234yf.
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.
The commitment to R-1234yf isn't rock solid, but it came down to "no practical alternatives had been demonstrated." The question is, "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 (estimated at $40-45 lb/$88-99 kg). For another, its efficiency is measurably lower than that of R-134a. And though mildly flammable, it's still flammable. However, its GWP is just 4.0 vs. 1430 for R-134a.
Work on AC6 blend
Another SAE CRP continues to work on AC6, a Mexichem-proposed blend of 85% R-1234ze, 9% R-134a, and 6% R-744 (carbon dioxide used as a refrigerant). The blend is designated R445A by ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers).
R-1234ze, a hydrofluoro-olefin like R-1234yf, is in production as an aerosol propellant and foam blowing agent. Evaluation of industry available data indicates a materials cost of under $10 /lb ($22/kg), and CRP data that performance is equal to R-134a and it has lower flammability than R-1234yf. There are patent questions that have been raised, but Mexichem reportedly is "working through" them.
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 the 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 Daimler decision to eschew R-1234yf creates a problem within the EU, as its new A-body cars fit the "new type" description that requires a low-GWP refrigerant. Once the GWP regulation enforcement began January 1, 2013, EU regulators told member nations not to register cars required to have a low-GWP refrigerant, but not so equipped.
UBA, a German-government chartered environmental advisory group, recently stepped into the dispute, proposing that any carmaker that commits to installing R-744 A/C by the end of 2015 receive an exemption. A reaction to UBA's move from Daimler and other German carmakers is likely to require study and is not expected immediately.
However, this puts R-744 back into the picture, despite pre-2010 SAE CRP testing that—among several issues—had shown lower life cycle climate performance (LCCP) efficiency vs. other alternatives to R-134a in warm climates. However, Volkswagen Group Chairman Ferdinand Piech recently has called R-744 the "natural choice." It would be permitted in the U.S., but because it's an asphyxiant it must meet EPA conditions that say the 15-min passenger cabin concentration may not reach 4%. Test results of a small car with the system in recirculation (no outside air), adding human respiration and an evaporator leak, could produce results that would force installing the system entirely underhood, which would further reduce efficiency.
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, and 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 system (see page 29 of the Oct. 23, 2012 edition of AEI in print or online at http://www.nxtbook.com/nxtbooks/sae/12AEID1023/index.php).