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Comprehensive risk assessment on hydrogen fuelled vehicles using tunnels in Hong Kong

By the Electrical and Mechanical Services Department, AECOM Asia Co. Ltd., and DNV AS

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The fight against climate change is a critical global concern. In alignment with our country’s goals to reach peak carbon emissions before 2030 and achieve carbon neutrality before 2060, Hong Kong has committed itself to reducing its carbon emissions by half (relative to 2005 levels) before 2035, with the aim of achieving carbon neutrality by 2050. To reach these ambitious targets, the HKSAR Government is actively adopting low-carbon energy sources.

 

Hydrogen has been recognised as an ultimate clean energy source and a secondary energy carrier with a wide range of potential uses for the future. Particularly, in fuel cell applications, hydrogen is combined with oxygen to generate electricity, emitting only water vapour and no carbon dioxide. As motor vehicles are the major contributors of local carbon emission on road and contribute to about 20% of the total carbon emission, the use of hydrogen fuelled vehicles (HFVs) is considered one of the promising measures for carbon reduction in Hong Kong.

 

The Environment and Ecology Bureau of the HKSAR Government published The Strategy of Hydrogen Development in Hong Kong (the Strategy) in June 2024, to set out the four major strategies of improving legislations, establishing standards, aligning with the market, and advancing prudently towards the creation of an environment conducive to the development of hydrogen energy in Hong Kong in an orderly manner. The Strategy was designed to enable Hong Kong to capitalise on the environmental and economic opportunities brought about by the recent developments of hydrogen energy in different parts of the world, our country in particular. It can also help Hong Kong broaden its co-operation with the Guangdong-Hong Kong- Macao Greater Bay Area (GBA) and even with the world, integrate into the country’s overall development, and develop new quality productive forces.

 

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Figure 1: The Strategy of Hydrogen Development in Hong Kong

 

However, as hydrogen is a highly flammable gas, the use of HFVs in tunnels poses safety concerns to the public despite the range of safety design features possessed by advanced hydrogen fuel system. The HKSAR Government has always attached great importance to hydrogen safety in Hong Kong. In this regard, the Electrical and Mechanical Services Department (EMSD) has appointed AECOM Asia Co. Ltd. (AECOM) and DNV AS (DNV) to carry out a comprehensive risk assessment study of HFVs with tunnels.

 

Risk-Based Approach

 

In the late 1990s, when the scheme for introducing liquefied petroleum gas (LPG) vehicles in Hong Kong to replace conventional diesel taxis and minibuses was being contemplated and not yet implemented, the HKSAR Government conducted a comprehensive risk assessment study to critically assess LPG vehicles’ road risk. One of the primary concerns of the risk assessment study was whether it is advisable to allow LPG vehicles to pass through tunnels, given the risks associated with potential LPG leaks and the possibility of subsequent ignition. Given the comparability of risks between LPG vehicles in tunnels and other fossil fuel vehicles in tunnels, the study concluded that the risk levels posed by LPG vehicles with fuel cylinder storage of less than 130 L were acceptable both on roads and in tunnels. Therefore, there was no restriction on LPG taxis and public light buses going through tunnels. Over the past 20 years and more, the safety records of LPG vehicles in Hong Kong have been very good.

 

Following the similar approach taken for the LPG vehicle scheme in the 1990s, a comprehensive risk assessment study was conducted to evaluate the risks associated with the fuel systems of HFVs operating within tunnels and to determine the acceptability of these risks.

 

 

A comprehensive risk assessment study was conducted to evaluate the risks associated with the fuel systems of HFVs operating within tunnels and to determine the acceptability of these risks.

 

HFVs incident records

 

A comprehensive review of past incidents of HFVs worldwide was conducted as a start. The review shows that, so far, there have been no reported incidents of hydrogen leakage or fires in the fuel system of HFVs, whether operating on roads or in tunnels. While acknowledging the world’s limited experience in the use of HFVs, this is a piece of evidence that the safety track record has been excellent to date.

 

Review on statutory requirements worldwide

 

Secondly, a literature review was conducted on the tunnel safety regulations or requirements applicable to HFVs worldwide. It was observed that, at present, none of the major countries in which HFVs are in operation, such as Mainland China, selected countries in Europe, Japan, and Korea, explicitly prohibit HFVs from tunnel use or impose safety restrictions on that particular use.

 

In contrast, more stringent regulations or rules are typically applied to tube trailers transporting hydrogen in bulk, owing to the potentially severe consequences of accidental hydrogen releases inside tunnels. All vehicles conveying dangerous goods, including LPG and petrol or diesel, are currently prohibited from entering tunnels in Hong Kong. It is therefore recommended that the same prohibition be applied to hydrogen tube trailers on grounds of safety. Tube trailers should use alternative routes or dangerous goods ferries to avoid passing through tunnels.

 

Risk acceptability criteria

 

Currently, there are no internationally recognised sets of criteria specifically for assessing the risks associated with the movement of gas fuelled vehicles through tunnels. In the previous tunnel studies for the LPG vehicle scheme, the acceptability of risk was determined by a comparison of LPG vehicles’ potential loss of life (PLL) with that of diesel vehicles and normal traffic incidents. (A given object’s PLL is calculated by the summation of the failure frequency multiplied by the resulting potential fatalities from a representative range of failure scenarios.) Similarly, in the comprehensive risk assessment study for HFVs passing tunnel, the calculated PLL of HFVs are compared with those of LPG vehicles and therefore other fossil fuel vehicles, which serves as the basis for the acceptability criteria.

 

Methodology for comprehensive risk assessment

 

A comprehensive benchmarking study of the fuel system of commercially used HFVs worldwide reveals that the standard working pressure of HFVs in both Mainland China and overseas is set at 350 barg, with multiple hydrogen fuel cylinders linked together and each cylinder’s capacity not exceeding 200 L generally. There is a growing interest in the industry to explore the use of 700 barg hydrogen cylinders for the HFVs. The expectation is that such a transition to 700 barg will cater to the foreseeable future’s market demands on longer range journeys, while hydrogen cylinders of 200 L each are considered practical for those HFVs with the flexibility to vary the number of cylinders to meet each HFV’s specific energy requirements. Taking into account the present situation and anticipated technology advancements, the study has evaluated using hydrogen fuel system of both 350 barg and 700 barg with 200-L cylinder, a size practical for HFVs. Such a design was adopted in Hong Kong’s first HFC double-decked bus, which was the study bus model for establishing the failure cases and for Computational Fluid Dynamics (CFD) simulation.

 

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Figure 2: Hydrogen fuel system of Hong Kong’s first HFC double-decked bus

 

Various technical workshops on methodologies of Hazard Identification (HAZID), Structured What-IF Technique (SWIFT), and Failure Modes and Effects Analysis (FMEA) were held among the HFV manufacturer and hydrogen equipment supplier, the EMSD, tunnel operators and risk specialists, to systematically identify all the possible failure scenarios and failure modes of the fuel system, using as the basis the actual design and configuration of the HFVs and the safety features available.

 

The identified potential causes of hydrogen leaks are classified into the following: (i) spontaneous leaks originating from fuel systems, covering all leaks that are not impact-induced or fire-induced; (ii) impact-induced leaks resulting from road traffic accidents; and (iii) fire-induced leaks caused by fire-related incidents. The failure frequencies of each fuel system component considered in the comprehensive risk assessment, such as cylinders, pipes, joints and valves, were determined with reference to internationally recognised historical failure databases, including that of the Compressed Gas Association to Sandia1 and analysis of the Hydrocarbon Release Database (HCRD) published by the International Association of Oil & Gas Producers (IOGP).

 

Then, a CFD software simulated the failed fuel system’s hydrogen dispersion to ascertain the size of flammable vapour cloud so as to determine explosion overpressure for consequence assessment.

 

The 1.9-km-long submerged Cross-Harbour Tunnel was selected as the representative tunnel in the assessment. Its relatively smaller cross-sectional area (at approximately 38m2) and high traffic volume2 would yield a more conservative risk assessment result for benchmarking purposes.

 

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Figure 3: 3D model of the Cross-Harbour Tunnel

 

The following hazardous outcomes of an accidental hydrogen leak were considered in the comprehensive risk assessment. Scenarios with hydrogen dispersion from a leak, and subsequent late ignitions with explosions, were also modelled with CFD.

 

  • Jet fire – resulting from a continuous leak that is ignited while being released (namely, immediate ignition), forming a flame jet.
  • Fireball – resulting from an instantaneous release of flammable gas that is ignited while being released. This may be due to a spontaneous release or fire-induced release. Hydrogen fireballs may involve blast overpressure and projectile, but the blast overpressure is assumed to predominate in tunnels.
  • Flash fire – resulting from a release that is ignited at some distance from the point of release (namely, a delayed ignition) but which does not produce any significant overpressure in the open air. In a tunnel, it is assumed that all late ignited gas releases also result in an overpressure due to the confinement.
  • Vapour cloud explosion (namely, deflagration) – resulting from a release that is ignited at some distance from the point of release, producing overpressure but no detonation.
  • Detonation – resulting from a release that is ignited at some distance from the point of release, with the flame front undergoing a Deflagration to Detonation Transition (DDT), producing a much larger overpressure.

 

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Figure 4: Simulation of typical temperature contour of hydrogen jet fire (image courtesy of DNV Digital Solutions)

 

Safety features of hydrogen fuel system

 

In Hong Kong’s first HFC double-decked bus, the fuel system comprises five 193-L Type IV hydrogen cylinders, which are connected to one another by rigid steel pipework and placed in a ventilated compartment at the bus’s rear. These hydrogen fuel cylinders adhere to the rigorous standards set forth in Regulation (EC) No. 79/2009 of European Environment Agency. They have passed a comprehensive type approval process that confirms their suitability for use. The testing regimen for these cylinders includes the following: (i) bonfire test, which ensures that, in the event of a fire, the cylinders will only vent hydrogen through the designed pressure relief devices and will not rupture under specified fire conditions; (ii) drop-tests, which verify the cylinders’ robustness and their ability to withstand severe impact forces similar to those that might occur during vehicle collisions; and (iii) bullet penetration tests, which demonstrate that any puncture or crack accidentally created in the cylinder wall would not widen and lead to a rupture.

 

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Figure 5: Bonfire test of hydrogen fuel cylinder (image courtesy of China Automotive Engineering Research Institute Co. Ltd.)

 

Each of the hydrogen fuel cylinders has also been fitted with Thermal Pressure Relief Device (TPRD). This device is designed to vent the entire contents of the cylinder in emergencies, such as when the cylinders are engulfed by fire. This prevents a fire induced rupture scenario. Moreover, the risk of fire damage is further minimised through the strategic placement of fire suppression devices within the compartment. These devices are positioned to quickly extinguish any fire that may spread across the cylinders or the associated pipework. Furthermore, recent research indicates that, for a Type IV hydrogen fuel cylinder, if the volume of hydrogen contained inside is less than 54%, the cylinder will not rupture in the event of a fire.3 Instead, it will only experience leakage. This illustrates the very low likelihood of a cylinder rupture during a fire scenario.

 

Each hydrogen fuel cylinder is fitted with a multi-function valve on top. A manifold system connects these cylinders. In the event of an unexpected gas leak, either at the cylinder valve or in the downstream system, the Excess Flow Valve (EFV) is activated almost instantaneously, and prevents further leakage by isolating the affected cylinders automatically. As a secondary protection, a Solenoid Valve (SOV) or automatic shut-off valve fitted downstream of the EFV will be triggered when hydrogen is detected by the hydrogen detectors in the fuel cylinder compartment.

 

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Figure 6: Typical multi-function valve of each cylinder consisting of TPRD, EFV and SOV (L); Typical hydrogen detector installed in fuel cylinder compartment (R)

 

Risk assessment results

 

The robust design and integration of several advanced safety devices not only substantially reduce the likelihood of major gas leaks, but also contain minor leaks rapidly and extinguish minor fire incident, preventing escalation into severer events. Moreover, the strategic placement of hydrogen fuel cylinders and their discharge orientation (upward position via TPRD) adds an additional layer of safety: It ensures that, in a fire, hydrogen is vented away from the HFV to reduce the risk of explosion. There is a relatively low chance of severe vehicle collisions within tunnel environment. This is because of the enforcement, within tunnels, of stringent traffic controls, including speed limits, prohibitions on overtaking and lane changes, and implementation of one-way traffic systems. Historical data have shown a much lower traffic incident rate within tunnels compared with open roads. All these factors contribute to the low frequencies of failure events resulting from vehicular damages, and the overall low-risk level.

 

The comprehensive risk assessment showed that the PLL of HFVs in tunnels is lower than the PLL of LPG taxis and minibuses as derived from the LPG tunnel study conducted in the late 1990s. Moreover, it is substantially below the estimated risk of conventional traffic accidents in tunnels. Furthermore, the PLL value was found to be dominated by small and medium leaks scenarios with relatively higher failure frequencies and relatively low explosion overpressure. In summary, the risk from HFVs in tunnels is considered acceptable within a maximum traffic volume and when the specific safety features are implemented.

 

The comprehensive risk assessment has also indicated that the HFVs risk levels are particularly sensitive to the amount of hydrogen stored in each individual fuel cylinder (namely, the hydrogen inventory) as that amount relates to both storage capacity and working pressure. These parameters significantly influence the potential size of the flammable vapour cloud that could be formed in a release or leakage, and consequently, the magnitude of any potential explosion. This factor is even more critical than the aggregated cylinder inventory, since simultaneous leakage from multiple cylinders is extremely rare. Thus, it is crucial to maintain the hydrogen inventory in terms of both storage capacity and working pressure of the individual cylinders at or below 200 L and 700 barg respectively.

 

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Figure 7: Contour of flammable vapour cloud in CFD model

 

 

 

Conclusion

 

A comprehensive study focused on the safety and risk associated with HFVs in tunnels has been conducted, utilising the design and construction of Hong Kong’s first HFC double-decked bus as the study model. This model aligns with the global practices adopted for HFVs: working pressure of 350 barg or 700 barg and hydrogen storage capacity of 200 L per cylinder.

 

By benchmarking against the risk profiles of LPG vehicles, the comprehensive risk assessment concludes that the risk levels associated with the planned number of HFVs using tunnels are acceptable.

 

 

The risk levels associated with the planned number of HFVs using tunnels are acceptable.

 

 

Because the development of HFVs falls within assumed study parameters and all the safety provisions are in place (including automatic shut-off valves, EFVs, TPRDs, vent pipe protruding vertically out of the vehicle ceiling, hydrogen gas detectors and emergency stops), a conclusion can be drawn: that an inherently safe operation of HFVs is achievable. From risk and safety perspectives, HFVs with hydrogen inventory in each cylinder at or below 700 barg and 200 L should be allowed to traverse in tunnels if they comply with the Code of Practice for Hydrogen Fuelled Vehicles and Maintenance Workshops published by the EMSD. This is in line with the international prevailing practice, where there are no developments of special rules or regulations for HFVs in tunnels in general.

 

The risk assessment findings concluded that the risk of HFVs in tunnels are comparable to those of LPG vehicles and other fossil fuel vehicles. On 14 July 2024, a milestone was achieved as Hong Kong’s first HFC double-decked bus successfully completed its inaugural cross-harbour trip between Kowloon to Hong Kong Island through tunnels, with key Government officials and Legislative Council members on board. The result of this study, and the successful operational demonstration, reinforce hydrogen’s potential to play a pivotal role in Hong Kong’s transition to sustainable green transportation.

 

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Figure 8: HFC double-decked bus successfully passing through road tunnels on 14 July 2024 (Photo courtesy of Citybus Ltd.)

 

References

 

  1. Sandia National Laboratories (2009). Analyses to Support Development of Risk-Informed Separation Distances for Hydrogen Codes and Standards. SAND 2008-0874.
  2. The Transport Department (2022). ‘Traffic Statistics - Cross-Harbour Tunnel’. Available at: https://www.td.gov.hk/mini_site/atd/2022/en/section4-10.html. (Accessed on 22 November 2024).
  3. Kashkarov S, Dadashzadeh M, Sivaraman S, and Molkov V (2022). ‘Quantitative Risk Assessment Methodology for Hydrogen Tank Rupture in a Tunnel Fire’. Hydrogen, 3(4), 512-530. doi: https://doi.org/10.3390/hydrogen3040033

 

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