The Swiss Federal Materials Testing and Research Laboratory (Empa), based in Dübendorf, has questioned the consequences of an electric car fire in an underground car park or tunnel.
The study conducted by Empa, and the conclusions of which have just been released, was to answer this general question: “What happens if an electric car catches fire in a road tunnel or in an underground car park? “. In particular, it was a question of assessing whether the situation is then more dangerous than with thermal models.
To find out, the Swiss laboratory, together with tunnel safety expert Lars Derek Mellert, carried out several simulations in the Hagerbach test gallery. This site has been used since 1970 to carry out testing, research and development work in the field of tunnel construction under real conditions.
The soot and combustion gases, but also the extinguishing water, were carefully examined.
Fire in an enclosed space
The researchers considered 3 different scenarios. The first was to observe the impact, in an enclosed space (28 x 28 m, with a ceiling height of 2.5 m) and without ventilation, of a lithium-ion battery fire with an energy capacity of 32 kWh. Empa has calculated an air volume of 2,000 m3 in such a parking area.
For the experiment, the decor has been reduced to 1 / 8th while keeping the proportions. Thus a 4 kWh pack was set on fire in a room with a volume of 250 m3.
Following the combustion of cells, the researchers looked at 3 points in particular. First of all, they wanted to find out how soot settles on tunnel walls, surfaces, and on the suits worn by firefighters. They then looked at the toxicity of the tailings and the precautions to be taken to clean up the site after the unfortunate event.
No more dangerous than a thermal
As a result of this first experience, Lars Derek Mellert estimated that “a burning electric car is no more thermally dangerous than a burning car with conventional propulsion”.
What about the polluting emissions resulting from the incident? Whatever the propulsive energy of the vehicle, “they have always been dangerous and possibly fatal”, the report concludes.
Highly corrosive and toxic, hydrofluoric acid is considered a specific hazard for lithium-ion batteries on fire. In the tests carried out in the Hagerbach tunnel, the concentrations remained “below the critical range”, put forward the editors. Therefore, excluding electric cars from underground parking lots and tunnels is neither useful nor necessary.
In contrast, the study emphasizes that: “Regardless of the type of training or energy storage system, the primary objective should be to ensure that all people can leave the danger zone as quickly as possible.”
Fire in a space equipped with sprinklers
The second experiment differs from the previous one by adding a fire extinguishing system with sprinklers. This device did not prevent the complete destruction of the battery by the flames.
When a lithium-ion pack ignites, a copious sprinkling with water only makes it possible to cool the whole and the immediate surroundings, in order to prevent the propagation of the flames to other cells still intact or to vehicles. parked nearby. A phenomenon that firefighters are now very familiar with.
What is modified with the presence of nozzles is a very different diffusion of soot and smoke. The walls and the ceiling will be less affected. It is mainly the ground that will be soiled then.
With this scenario, the researchers wanted to examine in particular the elements present in the water after the fire.
Toxic water to treat
A major difference with the fire of an electric car with lithium-ion battery, compared to a gasoline or diesel vehicle, it is the level of toxicity of the liquid which will have been used to sprinkle the pack in flame or to clean the walls and ceiling after the disaster. “It is important that this highly contaminated water does not flow into the sewer system without adequate pre-treatment,” the researchers warn.
According to them, “the chemical load of the extinguishing water exceeds the Swiss limit values for industrial wastewater by a factor of 70, and the cooling water is even up to 100 times higher than the limit value”.
Lars Derek Mellert warned underground car park operators: “Don’t try to clean up soot and dirt yourself. Soot contains large amounts of cobalt oxide, nickel oxide and manganese oxide. These heavy metals cause serious allergic reactions on unprotected skin ”. His advice after an electric car fire: Call in professionals equipped accordingly.
Fire in a ventilated tunnel
With the latter scenario, it was a question of measuring the usefulness of a ventilation system in the event of a lithium-ion battery fire and the impact of soot and smoke on it.
The experiment was carried out with an extraction device which evacuated the whole at a constant speed of 1.5 meters per second in a hose measuring 160 m. The suction outlet was placed 50, 100 and 150 m from the flames.
The researchers wanted to define the risk of corrosion over the long term of the metal parts that make up the assembly.
No lasting deterioration
The first positive point observed by the researchers was the efficiency of the tunnel ventilation systems in evacuating the fumes from a burning lithium-ion battery. No noticeable difference with those generated by a combustion engine car.
In addition, as in underground car parks, a disaster with an electric vehicle does not result in a long deterioration of the affected area. No corrosion was caused to the ventilation system or tunnel equipment. The staff usually working in these spaces would have nothing to fear in particular.
The main task of the study was to reassure parking lot operators, who were increasingly likely to ask themselves a few questions: “What to do if an electric car catches fire? What are the health risks for my employees? What are the effects of such a fire on the operation of my complex? “. Logical, with an increasingly important presence of connected cars in their establishments.
When LPG-powered cars were sometimes banned from parking in underground parking lots, it was mainly due to the blast of a potential explosion that could suddenly entrap people and weaken the structure of the buildings involved. With, as a result, potentially considerable damage.
Nothing like this with an electric car! There are a few videos circulating the web showing EVs going up in flames in these types of buildings. The fire was violent, however, with temperatures rising very quickly. The equipment in detection and video surveillance systems enables emergency services to be alerted quickly. An important point that allows us to consider that the disaster does not spread endlessly to the vehicles parked nearby.
Empa has noted the opacity in the tunnels of combustion fumes escaping from burning lithium-ion batteries. They quickly hide the light, until they are exhausted by the exhaust fans. A system set up independently of the development of EVs, and which works very well in principle.
Note that the observations were carried out on batteries alone, without the rest of the car going around.
If bans on access to car parks and road tunnels were required for electric vehicles, there is no doubt that they would constitute a new obstacle to their development. If only because of the charging stations present, which, for many electric motorists, have become essential for the daily use of these devices. A scenario that would have occurred while the other black or gray dots of electric mobility are gradually fading.