MAC Systems

The Reduced Global Warming Contribution of New Generation MAC Systems

Phasing out HFC-134a as a MAC Refrigerant

Directive 2006/40/EC on mobile air conditioning (MAC) stipulates that, from 1 January 2011, MAC of new types of vehicles have to be filled with a refrigerant with low impact on climate.

More specifically, the Directive prohibits MAC systems designed to contain fluorinated greenhouse gases with a global warming potential (GWP) higher than 150. It should be noted that the Directive does not prescribe any specific refrigerant/system to fulfil this obligation.

Prior to 2011, fluorocarbon refrigerant HFO-1234yf had been identified for use in MAC systems o fulfil the obligations of this directive. However, in light of the exceptional circumstances concerning problems encountered by the producers in supplying this gas, the Commission accepted to refrain from launching infringement procedures in cases where vehicle production continued with HFC-134a until 31 December 2012.

It is likely that HFC-134a will be replaced under different schedules in the various regions of the world. For example in the USA, in 2014, the EPA proposed prohibitions under SNAP on the use of HFC-134a in light duty motor vehicle air-conditioning from model year 2021.

Selecting a refrigerant to replace HFC-134a

From 2004, the automotive industry evaluated a number of fluorocarbon refrigerants as alternatives to conventional HFC-134a systems, in addition to continuing the evaluation of trans-critical CO2 systems. The evaluations were undertaken through a range of co-operative research programmes (CRPs), such as SAE-CRPs1 and the EU funded B-COOL project2.

 

It is necessary for any new refrigerant and system to meet industry requirements for efficiency, environmental impact, reliability, safety and cost. Extensive testing and evaluation is necessary to ensure that overall performance, safety and reliability are fully characterized prior to any decisions being taken to introduce new technology. This was not a simple task for the automotive industry given that the air-conditioning system has to perform in a wide range of climatic conditions. Environmental performance, in comparison to HFC-134a systems, can vary considerably for different climatic conditions. The automotive industry is a global industry, and overall global environmental performance may be different to regional environmental performance due to differences in average climatic conditions.

Car manufacturers still need to consider other technical developments that may impact on the MAC system. For example the development of electric vehicles that might require battery cooling and the provision of additional passenger heating. Heat pump systems are being considered to provide heating and cooling, but demisting capability needs to be considered. However, the system becomes more complex, as do environmental and performance comparisons.

 

In addition, improvements to the thermal management of the passenger compartment reduce the thermal load, so that less energy is required for the MAC system, potentially resulting in a smaller MAC system, and reduced fuel consumption associated with the operation of the MAC system.

1 A relevant example is the International MAC Refrigerant Blend Cooperative Research Program consortium, comprising a group of leading global vehicle OEMs plus Tier One suppliers, see http://www.sae.org/

2 B-COOL project: the identification of agreed methods to assess performance, fuel annual consumption and environmental impact as a first step for a new EU standard for low cost and high efficiency CO2 mobile air conditioning system for lower segment cars.

The Switch to Low GWP Refrigerant HFO-1234yf

The replacement of HFC-134a in new vehicles by fluorocarbon refrigerant HFO-1234yf maintains comparable energy consumption, but considerably reduces the global warming impact of refrigerant emissions due to its very low GWP (<1)3. In comparison, HFC-134a has a GWP of 1430 (IPCC Fourth Assessment Report value referenced in the F-Gas Regulation 517/2014).

 

HFO-1234yf has very similar thermophysical properties to HFC-134a allowing the use of modified HFC-134a air-conditioning systems with comparable energy efficiency in warm and hot climates, in contrast to CO2 (R-744). The energy efficiency of the trans-critical CO2 cycle is very sensitive to ambient temperature. Therefore the energy efficiency of HFO based systems is expected to be better in warm climates than CO2. With the lower GWP offered by use of HFOs, the overall carbon footprint of vehicle air-conditioning will be dominated by the energy efficiency of the system1. The SAE International MAC Refrigerant Blend Cooperative Research Program consortium (MRB CRP) has thoroughly investigated the safe use of HFO-1234yf for car air-conditioning (see also EFCTC position on HFOs and HFO Fact sheet).

According to the Technical and Economic Assessment Panel of the Montreal Protocol, HFO-1234yf, even if slightly flammable4, is seen as a global solution for the car industry, and it is now being implemented in several models, especially in Europe5. At the end of 2014, three million cars were assumed to be on the road using HFO-1234yf.6 A German car manufacturer in 2012 raised again the issue of HFO-1234yf flammability and postponed its introduction in its assembly lines, until a separate safety assessment would have been conducted. Since then, two separate studies have been completed.

 

Another Cooperative Research Program by the SAE confirmed previous conclusions that HFO-1234yf could be used safely for automotive air conditioning. Germany’s motor vehicle department KBA concluded that while HFO-1234yf was inherently more dangerous than HFC-134a in severe conditions, there was not enough evidence for an immediate action under the European product safety law.7 The results have also been assessed by the EU’s Joint Research Centre8 which concluded that “the results as such with the vehicles tested under the conditions as described (…) provided no evidence of a serious risk.”

3Please note that for HFO-1234yf, the GWP value of 4 listed in F-Gas Regulation 517/2014 has been revised, see  “Global warming potentials and radiative efficiencies of halocarbons and related compounds: A comprehensive review”.

4The flammability of HFO-1234yf is rated by ASHRAE Standard 34 as flammability class 2L (Low flammable refrigerant)

5TEAP report October 2014

6TEAP Update XXVI/9 Task Force Report September 2015, page 27

At the end of 2014, three million cars were assumed to be on the road using HFO-1234yf.

R-445A (AC6) as MAC Refrigerant

Even though it has been concluded that the slightly flammable HFO-1234yf can be safely applied, car industry OEMs continued to explore other alternatives to HFC-134a that may offer additional benefits particularly for safety risk assessments. This has resulted in the development of R-445A (AC6), a refrigerant blend based on HFO-1234zeE with a GWP of <150 as required by Directive 2006/40/EC.  The refrigerant was assessed by the SAE MAC Refrigerant Blend Cooperative Research Program consortium, over a period of 3 years, and the conclusion and main findings have been published at the SAE TMSS (Thermal Management Systems Symposia) in 2013 and 2014. It is classified as non-flammable for transport and handling, but like HFO-1234yf it is rated as flammability class 2L by ASHRAE Standard 34 and has similar energy efficiency to HFO-1234yf. Performance data for R-445A has been presented at the SAE TMSS 2014. As it is a blend refrigerant it has a temperature glide in the heat exchangers, but there were no significant issues in drop in conditions9. No other critical issues have been identified by the SAE MRB CRP.

Trans-Critical CO2 Systems

Until 2009, car companies and component manufacturers invested considerable research and development activity assessing the use of CO2 as the refrigerant for car MAC systems. The extremely high system pressures and the specific technical requirements for the efficient operation of a trans-critical CO2 system dictate that a completely new system had to be designed and tested. Several companies initially made announcements on the introduction of trans-critical CO2 systems. However there are challenging technical and reliability issues, and the systems developed to date deliver poor energy performance in hot climates in comparison to HFC-134a and other low-GWP fluorocarbon refrigerant options.

“The interest in and development of R-744 in this sub-sector declined significantly around 2009-2010, as car manufacturers and suppliers concentrated their efforts on HFO-1234yf. In 2013 there has been a renewed interest in R-744 MACs, with several German OEMs announcing their intention to develop such systems. However, to date no global OEM has installed R-744 for automotive air-conditioning. The main barriers for R-744 systems have been issues such as costs, reliability and servicing aspects… Some German OEMs work on introducing R-744 by 2017”.10

7 October 2014 TEAP XXV/5 Task Force Report page 17

8 JRC: Ref. Ares(2014)573175 – 04/03/2014.

9 Renault and Nissan Overview on R-445A (AC6) as MAC Refrigerant presentation at SAE TMSS (Thermal Management Systems Symposia) 2014

Environmental Comparison of refrigerants for mobile air-conditioning

The car companies and air-conditioning manufacturers have undertaken comparisons of a number of types of systems and refrigerants. Much of this work has been carried out in consortia such as the Alternative Refrigerant Cooperative Research Project, which allows a more rapid assessment of new technologies.

Factors that influence the comparison are the GWP and leakage rate for the refrigerant (direct effect), and energy efficiency in a range of climatic conditions (indirect or energy effect). The efficiency of trans-critical CO2 systems decreases much more rapidly in hot conditions. In its review of Options for Future Mobile Air Conditioning Systems the TEAP report comments ‘With GWPs less than 150 energy use dominates.’11

The LCCP (Life Cycle Climate Performance) is used as an objective comparison for refrigerants for a range of climatic conditions and drive cycles. A well established LCCP comparison tool is the GREEN-MAC-LCCP Model™ (available at http://www.epa.gov/cpd/mac/compare.htm), and guidance is available in a standard SAE J2766 Life Cycle Analysis to Estimate the CO2 – Equivalent Emissions from MAC Operation.

Comparisons have shown that the LCCP of trans-critical CO2 systems is better than that of the HFC-134a baseline only in cold climates despite the difference in GWP (1 by default for CO2 and 1430 for HFC‑134a)12 (3).

More recently a number of tests have been performed in hot and cold climates. R‑744 has been demonstrated to be similar in efficiency as best in class HFC-134a systems for some experimental fleet vehicles, except when the vehicle is idling and also under high temperature conditions (above 35°C).13

HFO 1234yf, with a GWP of <1, is better than trans‑critical CO2 in a very wide range of climates, and globally significantly better than trans-critical CO2. It is also supported by LCCP analysis, which shows that HFC-1234yf would be superior to R-744 for most ambient temperatures. R-445A performance is comparable to HFO-1234yf when used in the same equipment design in a wide range of climates as supported by the LCCP analysis carried out by the SAE MRB CRP.

10October 2014 TEAP XXV/5 Task Force Report
11TEAP Task Force Decision XX/8 Report May 2009 Assessment of Alternatives to HCFCs and HFCs and an Update of the TEAP 2005 Supplement Report Data page 46
12SAE International Industry Evaluation of low global warming potential refrigerant HFO-1234yf, Phase 3 Final Report completed October 2009 , which reports the Life Cycle Impact of HFO-1234yf in mobile air-conditioning systems and compares it to trans-critical CO2 systems
13October 2014 TEAP XXV/5 Task Force Report.

Further reading:

SAE International Industry Evaluation of low global warming potential refrigerant HFO-1234yf, Phase 3 Final Report completed October 2009, which reports the Life Cycle Impact of HFO-1234yf in mobile air-conditioning systems and compares it to trans-critical CO2 systems

See also

  • B-COOL Project – Ford Ka and Fiat Panda R-744 MAC Systems
  • DOI: 10.4271/2009-01-0967
  • Malvicino, C., Seccardini, R., Markowitz, M., Schuermanns, K. et al., “B-COOL Project – Ford Ka and Fiat Panda R-744 MAC Systems,” SAE Technical Paper 2009-01-0967, 2009, doi:10.4271/2009-01-0967.