20. Mobility and partially confined space: A hydrogen city bus driving in a tunnel

Limits to scaling of electric vehicles and also concerns (environmental and others) regarding batteries make hydrogen use in urban transport an attractive option. Hydrogen can be a key player in decarbonisation of transport sector with fuel cell heavy-duty vehicles, like buses and trucks. However, hydrogen mobility raises specific safety concerns that need to be addressed for its widespread use.

The United Nations Global Technical Regulation No. 13 (GTR #13) (UNECE, 1998[1]) for Hydrogen and Fuel Cell Vehicles, which occurred within the World Forum for Harmonization of Vehicle Regulations of the Inland Transport Committee (ITC) of UNECE, defines vehicle requirements for hydrogen FCEVs that can achieve equivalent (or higher) levels of safety as those for conventional gasoline powered vehicles. It includes specifications on the allowable hydrogen levels in vehicle enclosures during in-use and post-crash conditions and on the allowable hydrogen emissions levels in vehicle exhaust during certain modes of normal operation. GTR can be applied globally; however, the regulatory bodies in each country decide its incorporation into national regulations.

The use of hydrogen powered vehicles inside tunnels and other confined spaces is constrained by some national regulations. For instance, in Japan the passage of vehicles that transport hydrogen is prohibited or restricted in long tunnels (over 5 km long) and underwater/waterfront tunnels, while there are no specific restrictions for FCEV entering tunnels in several countries revealing the need to develop relevant regulations, standards, and codes.

As the number of hydrogen powered vehicles increases, their impact on various road infrastructures, such as tunnels and other confined spaces (e.g. garages and repair shops) must be considered. Associated risks from hydrogen vehicles driving inside tunnels are that in the event of accidental leak, hydrogen can be trapped and accumulated on the ceiling or other cavities at high concentration levels that could lead to a severe explosion. Compared with urban environments where blast waves decay quicker an overpressure can maintain its strength for long distances inside the tunnel due to the high level of confinement (Venetsanos et al., 2008[2]).

Scientific numerical studies (Venetsanos et al., 2008[2]), (Middha and Hansen, 2009[3]) suggest that the worst case scenario for a hydrogen powered bus incident inside a tunnel, i.e. release of the entire hydrogen volume, is the formation of nearly-stoichiometric mixture in air and ignition when the maximum flammable volume inside the tunnel is reached. This can lead to unacceptably high overpressures. When more realistic scenarios are considered, the explosion pressure is reduced to levels that correspond to the eardrum rupture threshold and moderate building damage, or even lower.

Recent experiments performed by CEA (Bouix et al., 2021[4]) with a 50L-tank hydrogen rupture of 4.7 Mpa inside a real scale horseshoe tunnel1 showed that an overpressure of around 12.5 kPa was developed at the region close the explosion (threshold for people injured by flying glass and debris and moderate structural damage), which decays to about 6.6 kPa at 205 m from explosion.

The work of (Kim et al., 2021[5]) examined three Independent Protection Layers (IPLs) to reduce the risk from a release within a tunnel involving a hydrogen powered bus. These comprised of Temperature-Pressure relief device (TPRD) activation, ventilation and leak detection with safety shutdown. Three initiating events were considered: battery fire, bus fire, and hydrogen leak fire.2 When applying these protection measures, a battery fire case with TPRD activation failure was considered as a non-negligible risk, with an outcome frequency in 10-5 events per year. The bus fire case with TPRD activation failure was considered as a moderate risk level with its frequency approaching 10-6 event per year and for the hydrogen leak fire case, all possible cases,3 4 resulted in the non-negligible risk range (i.e., outcome frequency > 10 6 events per year).Therefore, additional IPLs in the current hydrogen-powered electric city bus design were recommended by (Kim et al., 2021[5]).

Another risk assessment (LaFleur et al., 2017[6]), (Ehrhart et al., 2019[7]) that focused on hydrogen vehicles incidents inside tunnels suggests that the most likely consequence of a crash is that there will be no additional hazard from the hydrogen fuel (98.1–99.9% probability). If the hydrogen does ignite, it is most likely to result in a jet flame from the pressure relief device released due to a hydrocarbon fire (0.03–1.8% probability).

Finally, based on the scientific findings of the HyTunnel-CS5 project the TPRD size in vehicles should be as small as possible (<1 mm). TPRD orientation in buses could be on the top, while in cars an oblique orientation at 45o degrees backwards is preferred.

  • Design of vehicles

    • Design hydrogen vehicles based on United Nations Global Technical Regulation No. 13 (GTR #13).

    • Consider the use of new technologies, like TPRD-less (leak-no-burst) tank that would not release hydrogen through TPRD in extreme conditions, like engulfing fire in hydrogen tank. However, the TPRD-less technology should be considered along with the fire resistance of the tank.

    • Use multiple TPRDs to prevent the leak of the total mass of the tank in localized fires.

    • Hydrogen powered vehicles should be fitted with warning signs to alert emergency services.

  • Design of tunnels6

    • Provide mechanical ventilation inside tunnels (1-2 m/s) to reduce the hydrogen vapour concentration in the event of a leakage.

    • Ensure sufficient distance of main tunnel and fittings and equipment, like dust collectors and exhaust fans that can trap hydrogen in flammable concentrations.

    • Avoid roof obstructions inside the tunnel, because they pose a potential risk in respect to possible fast deflagration or transition to detonation.

    • The design of future tunnels should include appropriate cross section design to avoid flammable mixture accumulating in the tunnel ceiling.

    • Set larger safety distances between vehicles when driving inside tunnels.

  • Safety devices

    • The TPRD size should be reduced to avoid a flammable mixture at the tunnel ceiling in the event of a leak. The TPRD orientation in buses should be at the top of the vehicle.

    • Systems involving more than one PRDs should be designed to avoid simultaneous opening of all PRDs.

    • Additional protection could be provided by:

      • a battery fire suppression system within the battery pack,

      • a fire barrier between the battery pack area and the hydrogen tank,

      • increasing the tank integrity/fire resistance to thermal threats (minimum of 1 hour), and

      • a fire resisting deck to protect the upper deck area.

  • Practices

    • Risk-based categorisation of tunnels to define which ones allow or not H2 powered vehicles to enter.

    • Emergency responders should receive training for reaction to incidents that involve hydrogen vehicles. Some key elements are presented below:

      • Emergency responders should remain at least 2 min before approaching damaged vehicles following activation of TPRD.

      • If there’s no sign of hydrogen release, first responders should stand at least 6 m away from the vehicle.

  • Controls

    • Perform frequent safety checks on vehicle integrity by an independent, competent engineer.


[4] Bouix, D. et al. (2021), Full-scale tunnel experiments for fuel cell hydrogen vehicles: jet fire and explosions, 9th International Conference on Hydrogen Safety (ICHS 2021), 21-24 September 2021, ID42 1197-1210.

[7] Ehrhart, B. et al. (2019), “Ehrhart, B., Brooks, D., & Muna, A., Lafleur, C. (2019). Risk assessment of hydrogen fuel cell electric vehicles in tunnels.”, Fire Technology, Vol. 56.

[5] Kim, E. et al. (2021), Minimum fire size for hydrogen storage tank fire test protocol for hydorgen-powered electric city bus determined via risk-based approach, 9th International Conference on Hydrogen Safety (ICHS 2021), 21-24 September 2021, ID175 1187-1196.

[6] LaFleur, C. et al. (2017), Hydrogen fuel cell electric vehicle tunnel safety study.

[3] Middha, P. and O. Hansen (2009), “CFD simulation study to investigate the risk from hydrogen vehicles in tunnels”, International Journal of Hydrogen Energy, Vol. 34/14, pp. 5875–5886.

[1] UNECE (1998), Global Technical Regulations (GTRs): 1998 Agreement on Global Technical Regulations (GTRs), https://unece.org/transport/standards/transport/vehicle-regulations-wp29/global-technical-regulations-gtrs.

[2] Venetsanos, A. et al. (2008), “CFD modelling of hydrogen release, dispersion and combustion for automotive scenarios”, Journal of Loss Preventation in the Process Industries, Vol. 21/2, pp. 162-184.


← 1. Tunnel du Mortier located in the commune of Autrans in the Vercors, France.

← 2. It should be noted that for the initiating event frequencies related to some of the system components, such as battery, and for the conditional probabilities applied for the protection layers, data from industry were used.

← 3. Case 1 – hydrogen leak occurs from the compressed hydrogen storage system and is detected followed by a safety shutdown procedure; Case 2 – hydrogen\leak occurs; however, detection and safety shutdown fails followed by ignition; therefore, a flame jet occurs but TPRD is activated successfully to omit any possible catastrophic tank explosion; Case 3 – hydrogen leak occurs and detection & safety shutdown fails followed by ignition; therefore, a flame jet occurs and TPRD fails to operate properly; Case 4 – hydrogen leak occurs and detection & safety shutdown fails; however, ignition does not occur resulting in safe release of hydrogen through venting.

← 4. For this conclusion it was also assumed that TPRD activation may not occur due to the jet flames not being able to reach and heat up the TPRD. Thus, the Independent Protection Layer (IPL) offered by TPRD activation (case 2 and case 3 braches) was bypassed resulting in tank rupture with outcome frequency 2.9E-06 event per year. Multiplied by a factor of 2.5 to account for having 5 identical hydrogen tanks placed within a bus with 2 TPRDs for each tank the total outcome frequency was 7.25E-06 event per year.

← 5. Hy-Tunnel-CS is an EU-funded project with pre-normative research for safety of hydrogen driven vehicles and transport through tunnels and similar confined spaces, https://hytunnel.net/.

← 6. Proper design of vehicles is generally preferred over design of tunnels, as several recommendations for tunnel design can only be applied to newly built tunnels and not to existing infrastructure.

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