Challenges of alternative fuel vehicles and their impact on safety in road tunnels

Statistics show that conventional cars are on the decline (gasoline, diesel), while half of all new cars in Austria are already powered by alternative fuels. Hybrid vehicles (HEVs), battery-powered vehicles (BEVs), as well as hydrogen-powered vehicles (HPVs) are considered as emerging vehicle types in the passenger car sector. Their increase is expected to have extensive effects on several parties that influence the safety of road tunnels. With each fuel having different material properties, their storage in automotive applications is correspondingly diverse. The focus of this thesis is to analyze the risks and potential impacts of these new energy carriers on road tunnel safety.
Battery electric storage, which is almost exclusively realized with lithium-ion batteries today, poses the risk of thermal runaway. During a BEV fire, this exothermic reaction results in a slightly higher maximum heat release than in conventional vehicles. In addition, the released smoke contains more hazardous substances, such as more carbon monoxide, a higher concentration of hydrofluoric acid, or various metals from inside the battery. However, field tests showed that natural stratification of the smoke helps to ensure that the concentrations that are directly hazardous to humans are not present at face level.
Extinguishing a battery fire is a major challenge for firefighters, as it is not possible to extinguish a battery fire itself, but rather to prevent the thermal runaway from propagating. New methods for fighting battery fires were outlined, such as an access point for firefighters, a fire container, an extinguishing lance, or a fire blanket. However, no clearly recommended measure was found; conventional cooling with plenty of water was still considered as the best practice.
Hydrogen-powered vehicles, currently mainly fuel cell electric vehicles (FCEV) with compressed gaseous storage, have significantly different scenarios compared to conventional vehicles in the event of an incident. On the one hand, the safety valve on the tank can release the fuel most likely in the form of jet flames, which are dangerous but only of relatively short duration. On the other hand, this safety mechanism can fail and might result in a tank rupture. If hydrogen escapes without ignition and accumulates, a vapor cloud explosion may occur. Both explosion scenarios, with their serious consequences for people and the tunnel structure, must be prevented as far as possible. For firefighters, the easy identification of vehicles with such energy carriers was determined to be an important step in incident management.
In a detailed quantitative risk assessment based on a 1.2 km unidirectional model tunnel, the human risk for BEVs and HPVs was investigated. The resulting total risk for battery electric vehicles was even slightly lower (–0.1 %) than for conventional vehicles, while it increased by about 2 % for hydrogen-powered vehicles. Thus, in the tunnel configuration considered, the expected growth of BEV and HPV passenger cars did not lead to a significant increase in the total risk. This may be slightly different when looking at heavy goods vehicles, but there is currently little data available for these vehicle categories.
A study of the use of battery-powered dump trucks in tunnel construction sites showed similar results. Despite the challenges of extinguishing such fires, the probability of a BEV fire was estimated to be lower than for diesel-powered dump trucks. Therefore, from a personal safety perspective, there is no additional risk to be expected from replacing diesel trucks with BEV trucks.