Mobile air conditioning with fluorinated refrigerants
An EU directive bans the use of fluorinated greenhouse gases with global warming potential exceeding 150 for mobile air conditioning systems in motor vehicles since January 2011. Thus the current refrigerant tetrafluoroethane (R134a) has to be replaced. Tests in laboratory and car showed that the new refrigerant 1234yf selected by car industry can lead to fires.
Mobile air conditioning with fluorinated refrigerants
The refrigerant that has been used in mobile air conditioning systems up to present is the fluorinated greenhouse gas tetrafluoroethane (R134a). In 2013, passenger cars in Germany alone leaked 2,400 tonnes of the greenhouse gas R134a, or 3.5 million tonnes of CO2 equivalents – the amount of CO2 that 1.6 million passenger cars emit with their exhaust per year.
Directive 2006/40/EC relating to emissions from motor vehicle air conditioning systems prohibits the use of this substance in new passenger cars and similar commercial vehicles as of 1 January 2011. After 1 January 2017 the ban extends to the air conditioning systems in all new vehicles in these classes. The automobile industry has considered carbon dioxide (CO2) and a fluorinated substance, 2,3,3,3‑Tetrafluoropropene (1234yf), as alternative refrigerants. For the sake of climate protection the Federal Environment Agency (UBA) favours CO2 as a refrigerant for mobile air conditioning systems.
Flammable fluorinated substitute refrigerant
1234yf as a refrigerant is a relatively new substance with specific properties that must be considered when used as a refrigerant for 1234yf is flammable and produces hydrogen fluoride (HF) upon combustion. 1234yf has a Global Warming Potential four times higher than CO2. What is even more important to note is the technical possibility of refilling 1234yf-run mobile air conditioning units with the even more climate-damaging R134a. In contrast, only CO2 can be refilled as the refrigerant in CO2-run systems – which is a great advantage considering that in some countries ozone-depleting chlorofluorocarbon (CFC) is still being put into mobile air conditioning systems.
Laboratory test of the ignition behaviour of the refrigerant 1234yf
In October 2009 the Federal Environment Agency commissioned the Federal Institute for Materials Research and Testing (BAM) to test the ignition behaviour of the refrigerant 1234yf in comparison to R134a which has been used up to now. Tests were conducted in such a way as to replicate as much as possible the use of this substance in mobile air conditioning systems.
A summary of the most significant outcome of the BAM tests, concluded in June 2010:
BAM determined an explosion limit range for the refrigerant 1234yf of 6.2 mol-% to 14.4 mol-%. The refrigerant 1234yf is not as easy to ignite as propane or petrol vapours. However, due to its low lower explosion limit, it must be labelled as highly flammable.
Below the lower explosion level concentration of ca. 6 mol-% 1234yf, no self-sustaining flame propagations could be observed; however, there were extreme stretches of typical blue flames.
When such a flame stretch was observed parts of the flame were detached. These flames can lead to fires depending on the surrounding of the flame detachments.
The ignition behaviour of 1234yf when exposed to ignition sources like sparks is erratic and unpredictable. It is strongly dependent on the circumstances in which the refrigerant is released (e.g. an accident, changes occurred in the vehicle due to accident, geometry of engine bay). As a result, precautionary risk management is complicated.
The minimum ignition temperature of 1234yf is 405°C. This is defined as the lowest temperature at which self-ignition can occur. In actuality, higher surface temperatures are usually needed for an ignition of a pure mixture of 1234yf and air.
Compressor oil also leaks with a 1234yf leak from a mobile air conditioning system. In an accident or in a fire caused by outside sources, there may be fuel vapours in the air, both of which decrease the ignition temperature of the lower explosion limit.
BAM points out that the additional risk of fire and explosion caused by 1234yf is relatively low in relation to the risk posed by other fuels and materials. The hazards of 1234yf consist in the decomposition products generated in a fire (note: There are about 30,000-40,000 vehicle fires a year in Germany).
BAM is very critical in its evaluation of the formation of hazardous quantities of hydrofluoric acid when 1234yf is exposed to ignition sources, e.g. open flame or hot surfaces. In virtually all of the tests in which the refrigerant was released in controlled circumstances the concentrations of hydrofluoric acid recorded exceeded those of the automobile industry acute exposure guideline level (AELG 2 level of 95 ppm for an exposure time of 10 min). Exceeding the AELG 2 level leads to irreversible health damage. This level was well exceeded in tests conducted inside a vehicle, too.
BAM conducted the tests in comparison to R134a. This refrigerant is non-flammable at atmospheric conditions but can nevertheless ignite at elevated temperature and/or pressure or in the presence of other fuels. Yet, R134a is not as reactive as 1234yf, which is why BAM assesses the hazard associated with R134a with respect to the formation of hydrofluoric acid as much less serious.
The BAM tests prove that there are risks associated with the use of 1234yf. The risks result from the hydrogen fluoride that is produced in a fire and, at high temperatures, is also released from the refrigerant 1234yf without the presence of fire. Some 30,000-40,000 vehicles catch fire every year in Germany. The formation of hydrogen fluoride can be expected in such an event. BAM did not quantify the additional risk as only automobile manufacturers do this for specific vehicles. However, it points out that a comprehensive risk analysis is required if 1234yf is used and many preventative measures must be taken, e.g. thorough isolation of hot surface in the engine bay, installation of an automatic extinguisher system in the engine bay, measures to prevent entry of hydrogen fluoride into passenger compartment, measures to prevent generation of sparks, also in accidents (including cutting electric power), as well as education of rescue workers.
The BAM investigations are an important part of the ongoing dialogue about the safety of using refrigerant 1234yf in mobile air conditioning systems. Automobile manufacturers are called upon to take the outcome into account in their actions and decisions.
Laboratory test: Ignition behavior of 1234yf on hot surface
The video shows a simulation of refrigerant release: 50 grams of 1234yf (tetrafluoropropene) blended with 3% refrigerant oil are released on a hot metal surface (600°C).
The UBA’s motto, For our environment (“Für Mensch und Umwelt”), sums up our mission pretty well, we feel. In this video we give an insight into our work.
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