Images of burning electric cars keep making the rounds and unsettling motorists. In Germany, individual underground car parks have already been closed to electric cars due to the high fire load in the event of a fire. Graz University of Technology has now used fire tests to determine how fire-proof Austrian tunnels are in such a case.
More and more electric cars are also using tunnels. This increases the risk that one of them will start to burn while driving through – not because electric cars burn particularly often, but simply because there are more. And if the worst comes to the worst, electric vehicles pose different dangers than combustion engines.
What happens if the battery of an electric vehicle catches fire? How hot does it get, which gases are produced and what risks are emergency services or other people in the tunnel exposed to? At the “Zentrum am Berg” tunnel research center at the Montanuniversität Leoben, experts under the leadership of Graz University of Technology used large-scale fire experiments to investigate the effects of electric vehicle fires on the safety of tunnel users and the infrastructure.
The researchers set fire to battery modules and three lithium-ion battery technology and two diesel-powered cars and vans in the test tunnel. Extinguishing attempts were started after a fire time of ten minutes.
Austrian tunnels are fit enough
The good news after the experimental and numerical investigations in which experts from Graz University of Technology, the Montanuni Leoben, the Federal Fire Brigade Association and ILF Consulting Engineers Austria were involved: The hazard potential is not to be assessed much more critically than in the case of fires in cars with conventional internal combustion engines. “Austrian tunnels are fit enough for the challenges that come with burning electric vehicles,” says Peter Sturm from the Institute for Internal Combustion Engines and Thermodynamics at Graz University of Technology.
The heat release of the burning electric vehicles in the tunnel, measured by more than 30 sensors, was 6 to 7 megawatts, slightly higher than that of comparable diesel-powered vehicles (5 MW). According to the experts, however, this does not entail any new risks or dangers. In comparison, the fire load of a conventional truck is around 30 MW. “When the electric vehicles fire, it gets a little hotter, but it is not fundamentally more dangerous in the tunnel. The measured temperatures in the escape area are below the 60 degree limit for all fire tests. It’s not a pleasant temperature, but it is still possible to escape and fight fires, ”summarizes Sturm.
Low risk of toxic gases
A fire releases dangerous gases, especially hydrogen fluoride and carbon monoxide. “However, the thermally induced smoke gas stratification in the tunnel means that these highly concentrated fire gases mainly collect in the upper areas of the tunnel and thus outside the area relevant to humans. This means that the escape routes are not affected, ”explained the expert.
No all-clear for parking garages
According to Sturm, the ventilation systems play a large part in the comparatively low risk in road tunnels: “They do not exist in parking garages, for example, or at least not to a comparable extent. This means that fires in electric vehicles in garages are on a different sheet of paper and urgently need to be investigated more closely. In any case, our measurement results indicate a serious risk potential, ”warns the Graz researcher.
Type of drive makes a difference to some extent
According to the experts, the type of drive of the burning vehicle does not make any relevant difference with regard to damage to the structure and materials of the tunnel: Concrete damage from spalling is to be expected in both vehicle categories in the event of commercial vehicle fires. The damage pattern is roughly the same.
Among the extinguishing methods, fire fighting with water worked best. “However, experience shows that lithium-ion batteries are only successful when the water can reach the inside of the battery,” says Stefan Krausbar from the Austrian Federal Fire Brigade Association. In any case, the duration of extinguishing and the need for extinguishing agents would increase. The use of extinguishing lances has proven to be effective, but the emergency services must be specially trained for this. After use, the extinguishing water was heavily contaminated with heavy metals – especially nickel. This increases disposal costs, as Günter Rattei from Asfinag explains.
“Further tests urgently needed”
Project manager Sturm emphasized the importance of further investigations: The fire effects of battery-powered commercial vehicles – buses and trucks – have only been scaled up using numerical simulation based on assumptions about fire development, duration and pollutant release. Comprehensive fire experiments in large-scale trials would therefore significantly improve the quality of the information. The same applies to the specific risk situation in the event of e-vehicle fires in parking garages. “With all the joy about the advance of alternative drive systems, such safety-related ‘homework’ must not be neglected.”