Mobile air conditioning is part of the vehicle control system that provides cooling, demisting, humidity control, as well as heating, defrosting and air-filtering. Directive 2006/40/EC on mobile air conditioning (MAC) required all new passenger cars (and light goods vehicles) from 1 January 2017 to be filled with a refrigerant with a global warming potential (GWP) no higher than 150. It should be noted that the Directive does not prescribe any specific refrigerant/system to fulfil this obligation, but following extensive evaluation for performance and safety HFO-1234yf was selected by almost all vehicle manufacturers as the refrigerant to replace HFC-134a.
Almost 100% of new passenger cars sold in the EU use HFO-1234yf as refrigerant (1). HFO-1234yf maintains comparable energy consumption to the HFC-134a systems it replaced, but considerably reduces the global warming impact of refrigerant emissions due to its very low GWP of 4 (IPCC Fourth Assessment Report value referenced in the F-Gas Regulation 517/2014, more recently revised to <1, the IPCC AR5 value). In comparison, HFC-134a has a GWP of 1430 (IPCC AR4 value referenced in the F-Gas Regulation, revised to 1300, the AR5 value).
HFO-1234yf has very similar thermophysical properties to HFC-134a allowing the use of systems similar to the replaced HFC-134a systems. Some system changes compared to HFC-134a can be introduced, to optimize the energy efficiency of HFO-1234yf systems, with an internal heat exchanger (IHX) being an option to improve energy efficiency. The decision to select HFO-1234yf was based on assessments of its environmental effects, objective comparisons of its technical performance and an evaluation of its safety in use as it is a mildly flammable refrigerant.
The environmental effects of HFO-1234yf have been thoroughly investigated, including its use in the EU for mobile air-conditioning. It has an extremely low GWP (<1), less than the GWP of CO2, and a very short atmospheric lifetime (10.5 days). HFO-1234yf breaks down in the atmosphere to form TFA, which is removed by dry deposition or rained out as it is very water soluble. It is well established that TFA is a ubiquitous natural component in rivers, lakes, and other surface water bodies. More than 95% of the salts of TFA found in the oceans are naturally produced. These salts are inert and not of toxicological or environmental concern in the small concentrations that are present in the oceans, playas, and lakes(2). The recent study on Future emissions and atmospheric fate of HFC-1234yf from mobile air conditioners in Europe(3), reported on the expected emissions of TFA after a complete conversion of the European vehicle fleet. Further information about TFA and its emissions is on our website see the article in the September 2018 EFCTC newsletter, and the EFCTC_Learn_about_TFA_from_HFCs_HFOs.pdf
A cooperative study by US EPA, Mobile Air Conditioning Society (MACS), and Japan Automobile Manufacturers Association (JAMA), for the Life Cycle Climate Performance (LCCP) found that for air-conditioning, HFO-1234yf, with a GWP <1, was the best performing option in all four selected climate regions: Frankfurt, Tokyo, Athens and Phoenix. With the very low GWP of HFO 1234yf, the overall carbon footprint of vehicle air-conditioning is dominated by the energy efficiency of the system(4)(5), The relative LCCP for CO2 systems was improved under lower ambient temperature conditions but did not provide comparable energy efficient cooling in hotter climates where air conditioning demand is most stringent. The 2018 report commissioned by AFCE(6) also states that studies such as (Papasavva, 2014) have shown that, in a global context, R-1234yf is a better alternative than CO2, when considering all direct and indirect emissions during the life cycle (LCCP) of the vehicle and the refrigerant, even if CO2 showed slightly better performance in cold or temperate climate. The AFCE report also noted that one of the issues for the potential use of CO2 systems is reliability due to the high pressure and the increased risk of leakage at the rotating compressor drive shaft.
The automotive industry is a global industry, and overall global environmental performance in a range of climates is an important factor when selecting mobile air-conditioning systems. The LCCP (Life Cycle Climate Performance) is used as an objective comparison for refrigerants for a range of climatic conditions and drive cycles(7).
New systems have better energy efficiency due to improved heat exchanger and compressor performance. Improvements to the thermal management of the passenger compartment can reduce the thermal load resulting in reduced MAC energy consumption. Engine compartment air-flow to remove heat from the condenser is are also important factor for energy consumption, particularly when a vehicle is at very low speed.
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. 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(8). The results have also been assessed by the EU’s Joint Research Centre(9) which concluded that “the results as such with the vehicles tested under the conditions as described (…) provided no evidence of a serious risk.”
Hybrid electric cars typically use an electric compressor instead of a compressor driven from the engine. Plug-in hybrids and all electric vehicles also need temperature control for the batteries, in particular, cooling during the battery charging phase. For these systems, the MAC system can be configured to also provide cooling to the battery, which may be via a chiller loop or by a direct expansion evaporator. For all electric vehicles, there is no combustion engine waste heat to provide heating which instead can be provided by a combined air-conditioner/heat-pump. The balance of cooling and heating requirements will depend on the climatic conditions and can have a significant impact on the vehicle range.
Heavy-duty trucks use MAC systems that are similar to those used for light duty vehicles, but with larger refrigerant charge. The choice of refrigerant (for trucks and buses) is not regulated by the Directive 2006/40/EC on mobile air conditioning (MAC), but some measures in the F-Gas Regulation 517/2014 will apply. Typically, at present heavy-duty trucks use HFC-134a. Where air-conditioning is required while the truck is parked, an auxillary air-conditioning system may be installed.
Buses and Coaches have larger air-conditioning systems with refrigerant charge that may be ≥ 5kg depending on the size of the bus. Conventional engine driven vehicles use belt driven compressors and typically use HFC-134a or R-407C. At least one bus air-conditioner manufacturer in the EU has approved the use of non-flammable R-513A and R-450A as lower GWP alternatives to R-134a in its bus air conditioning systems. With the lower GWP alternative refrigerants, the system delivers a virtually identical performance and have the same safety classification as R-134a (Class A1).
All electric buses require electrically driven systems, and depending on climatic conditions, some systems offer AC and heat pump functions to avoid the use of direct electrical heating. Typically, self-contained hermetic roof units containing non-flammable HFC-134a or R-407C are currently used. Battery cooling may also be required to ensure optimum performance, and this may be provided by a direct refrigerant circuit or by a secondary glycol loop.
1) From 2017, some German car manufacturers have introduced trans-critical CO2 systems for some premium models. 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) see October 2014 TEAP XXV/5 Task Force Report.
2) UNEP Ozone Secretariat, Ecological Issues on the feasibility of managing HFCs: Focus on TFA Inter-sessional informal meeting, 12-13 June 2015 Informal Brief on Ecological Issues on HFCs June 2015 see EFCTC_Learn_about_TFA_from_HFCs_HFOs.pdf
3)Stephan Henne, Dudley E. Shallcross, Stefan Reimann, Ping Xiao, Dominik Brunner, Simon O’Doherty, and Brigitte Buchmann, Future Emissions and Atmospheric Fate of HFC-1234yf from Mobile Air Conditioners in Europe, Environ. Sci. Technol., 2012, 46 (3), pp 1650–1658 DOI: 10.1021/es2034608.
4) TEAP 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
5) 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
6) Etat des lieux sur l’efficacite energetique des fluides et systemes a faible PRP disponibles. Etude commanditée par l’AFCE et réalisée par le Cemafroid, le CITEPA et EReIE. (Energy Efficient State of the Art of Available Low-GWP Refrigerants and Systems) Final Report September 2018 TEAP Report October 2014ttp://www.afce.asso.fr/wp-content/uploads/2018/10/Final-rapport-energy-efficiency-GWP-2018.pdf
7) A well-established LCCP comparison tool is the GREEN-MAC-LCCP Model™, and guidance is available in a standard SAE J2766 Life Cycle Analysis to Estimate the CO2 – Equivalent Emissions from MAC Operation.
8) October 2014 TEAP XXV/5 Task Force Report page 17
9) JRC: Ref. Ares(2014)573175 – 04/03/2014.