CO₂ refrigerant for auto air conditioning, assumed to be a non-starter a few years ago, seems to be live technology once more. The SAE Interior Climate Control Committee (ICCC) has reopened a suite of five standards for auto A/C that would use R-744—CO₂ as a refrigerant.

The decision was made at the ICCC meeting during the 2016 SAE World Congress, after a presentation by Moritz Knotzele, HVAC project engineer at Porsche Engineering Services GmbH. Knotzele leads the service procedures working group for the system, under development by a German consortium. He noted considerable progress in the engineering of recovery/recharge equipment for R-744 systems, described work in leak detection and said that English translations of German DIN standards would be available shortly for the ICCC.

How things change in the A/C technology world! It was only six years ago that a German R-744 project was put on what seemed like permanent hold, despite its ultra-low global warming number (1.0), which easily surpassed environmental limits in Europe and the U.S. (150 or below). The German manufacturers that had spent so much time, money and effort on development, grudgingly have been installing R-1234yf (global warming number 4-10) to satisfy European Commission regulations.

However, there was distaste for using a refrigerant that was covered by a series of Honeywell patents whose validity they questioned, but with enough technical coverage to hold off production competition to date. With R-1234yf priced 15-20 times more than still-used R-134a (global warming number 1430), research on CO₂ as a refrigerant restarted after Daimler's well-publicized and disputed complaints about R-1234yf safety in late 2012.

A competitor to R-1234yf?

Soon after, the development consortium of OEMs and suppliers under the aegis of VDA (German Auto Manufacturers Association) was reconstituted and the first vehicles with R-744 (CO₂ used as a refrigerant) are being projected for 2017 introduction on two Mercedes models (E-Class and S-Class) and the Audi A8. If the R-744 program is successful, it eventually could put competitive pressure on pricing of R-1234yf.

The five SAE standards that had been mothballed in draft form and opened again for further development are: J2769—R-744 Service Hoses, Fittings and Couplers for Mobile A/C Systems Service Equipment; J2770—R-744 Service Standards for Mobile A/C Systems; J-2771 R-744 Refrigerant Removal and Charging Equipment for Mobile Refrigerant Systems; J2773—Refrigerant Risk Analysis for Mobile A/C Systems (covers R-744); J2774—R-744 Minimum Performance Criteria for Electronic Leak Detectors; and J2775—R-744 Ultraviolet Leak Detection Minimum Requirements for Mobile A/C Systems.

U.S. EPA normally requires recovery and recycling of all auto A/C refrigerants. But because CO₂ is a naturally-occurring gas, the service equipment need only remove any other substances, such as refrigerant oil, before safely venting it. Service equipment first would recover and weigh R-744 remaining in a system as a diagnostic aid to determine the size of a leak, and incorporate oil filtration/separation to remove any refrigeration oil that had been in circulation.

Knoetzele said that service equipment developed to date had met a 99% oil removal specification. Because CO₂ is an asphyxiant in high concentrations, and can affect driver performance even at relatively low levels, EPA has set U.S. conditions on its use as an auto A/C refrigerant, based on a direct refrigerant expansion evaporator in the cabin.

The EPA limits are based on the possibility of leakage into the passenger compartment and combined with CO₂ from normal human respiration. There is a short term (15 min) exposure limit of 3% (30,000 parts per million) for the passenger free space in the cabin, and an absolute limit of 4% (40,000 ppm) in the passenger breathing zone.

R-744 use challenges

SAE J2772 is a standard that covers analytic and physical tests to evaluate refrigerant concentration in the cabin in the early stages of vehicle development, and was developed to cover R-744 and R-1234yf. But it contained sufficiently general procedures for many refrigerants. Electronic CO₂ sensors developed in recent years are available for possible use today.

R-744 has been a challenging refrigerant to use in automotive A/C. The pressures in a refrigerant cycle may rise to 10 times those in an R-134a/R-1234yf system, so component durability has been an issue. Further, high-pressure pulsations have created noise problems. The refrigerant itself has a condensation temperature of 31°C (88°F). So the efficiency from a phase change (condensation to a liquid) normally does not occur in the front heat exchanger, which as a result is referred to as a gas cooler rather than a condenser. However, an IHX (Internal Heat Exchanger) in the system configuration, may produce some high-pressure side condensation and improved efficiency.

There has been no announcement when (or even if) R-744 systems would be sold in the U.S., as the automakers are likely to limit initial applications to a few models, and for the European market, where the performance and durability could be more easily monitored. But with SAE standards in place, any decision to export vehicles using CO₂ A/C to the U.S. would be simplified.