Safety

Hydrogen fuel cell vehicles (HFCVs) represent an important breakthrough in the hydrogen energy industry. The safe utilization of hydrogen is critical for the sustainable and healthy development of hydrogen fuel cell vehicles. In this study risk factors and preventive measures are proposed for on-board hydrogen systems during the process of transportation storage and use of fuel cell vehicles. The relevant hydrogen safety standards in China are also analyzed and suggestions involving four safety strategies and three safety standards are proposed.

One of the significant safety concerns in large-scale storage and transportation of liquefied (cryogenic) hydrogen (LH2) is the formation of flammable hydrogen/air mixtures after leakages during storage or transportation. Especially in maritime transportation hydrogen accumulations could occur within large and congested geometries. The installation of passive auto-catalytic recombiners (PARs) is a suitable mitigation measure for local areas where venting is insufficient or even impossible. Numerical models describing the operational behavior of PARs are required to allow for optimizing the location and assessing the efficiency of the mitigation measure. In the present study the operational behavior of a PAR with a compact design has been experimentally investigated. In order to obtain data for model validation an experimental program has been performed in the REKO-4 facility a 5.5 m³ vessel. The test procedure includes two phases steady-state and dynamic. The results provide insights into the hydrogen recombination rates and catalyst temperatures under different boundary conditions.

In the present work we present the results of numerical simulations involving the dispersion and combustion of a hydrogen cloud released in an empty tunnel. The simulations were conducted with the use of ADREA-HF CFD code and the results are compared with measurements from experiments conducted by HSE in a tunnel with the exact same geometry. The length of the tunnel is equal to 70 m and the maximum height from the floor is equal to 3.25 m. Hydrogen release is considered to occur from a train containing pressurized hydrogen stored at 580 bars. The release diameter is equal to 4.7 mm and the release direction is upwards. Initially dispersion simulation was performed in order to define the initial conditions for the deflagration simulations. The effect of the initial wind speed and the effect of the ignition delay time were investigated. An extensive grid sensitivity study was conducted in order to achieve grid independent results. The CFD model takes into account the flame instabilities that are developed as the flame propagates inside the tunnel and turbulence that exists in front of the flame front. Pressure predictions are compared against experimental measurements revealing a very good performance of the CFD model.

The widespread use of fossil fuels in automobiles has become a concern particularly in light of recent frequent natural disasters prompting a shift towards eco-friendly vehicles to mitigate greenhouse gas emissions. This shift is evident in the rapidly increasing registration rates of hydrogen vehicles. However with the growing presence of hydrogen vehicles on roads a corresponding rise in related accidents is anticipated posing new challenges for first responders. In this study computational fluid dynamics analysis was performed to develop effective response strategies for first responders dealing with high-pressure hydrogen gas leaks in vehicle accidents. The analysis revealed that in the absence of blower intervention a vapor cloud explosion from leaked hydrogen gas could generate overpressure exceeding 13.8 kPa potentially causing direct harm to first responders. In the event of a hydrogen vehicle accident requiring urgent rescue activities the appropriate response strategy must be selected. The use of blowers can aid in developing a variety of strategies by reducing the risk of a vapor cloud explosion. Consequently this study offers a tailored response strategy for first responders in hydrogen vehicle leak scenarios emphasizing the importance of situational assessment at the incident site.

Well characterized experimental data for consequence model validation is important in progressing the use of liquid hydrogen as an energy carrier. In 2019 the Health and Safety Executive (HSE) undertook a series of liquid hydrogen dispersion and combustion experiments as a part of the Pre-normative Research for Safe Use of Liquid Hydrogen (PRESLHY) project. In partnership between the National Renewable Energy Laboratory (NREL) and HSE time and spatially varying hydrogen concentration measurements were made in 25 dispersion experiments and 23 congested ignition experiments associated with PRESLHY WP3 and WP5 respectively. These measurements were undertaken using the hydrogen wide area monitoring system developed by NREL. During the 23 congested ignition experiments high variability was observed in the measured explosion severity during experiments with similar initial conditions. This led to the conclusion that wind including localized gusts had a large influence on the dispersion of the hydrogen and therefore the quantity of hydrogen that was present in the congested region of the explosions. Using the hydrogen concentration measurements taken immediately prior to ignition the hydrogen clouds were visualized in an attempt to rationalize the variability in overpressure between the tests. Gaussian process regression was applied to quantify the variability of the measured hydrogen concentrations. This analysis could also be used to guide modifications in experimental designs for future research on hydrogen combustion behavior.

Ahead of a potential large-scale implementation of hydrogen as an energy carrier in society safety regulation systems should be in place to provide a systematic consideration of safety related concerns. Knowledge is essential for regulatory activities. At the same time it is challenging to obtain sufficient information when regulating emerging technologies – it may be difficult to address informational shortcomings in regulatory matters as analysts can be prone to under-communicate the significance of uncertainties. Furthermore Strength of Knowledge (SoK) has been developed to address the quality of background knowledge in risk analyses. An example of a SoK framework is based on the following four conditions that is used to assess whether knowledge can be considered weak or strong: the issue of simplifications availability and reliability of data consensus among experts and general understanding of the phenomena in question. In theory this concept seems relevant for the introduction of hydrogen as an energy carrier mainly because there is little historical data to develop sound analyses creating uncertainties. However there are no clear-cut guidelines as to how knowledge gaps should be handled in the development of regulatory requirements. In this paper we consider the relevance of a specific approach for SoK assessment in the context of safety and security regulation of hydrogen as an energy carrier in society. We conclude that there are some challenges with the proposed framework and argue that further research should be conducted to identify or develop a method for handling uncertainties in regulatory processes regarding hydrogen systems as energy carriers in societies.

In a first-of-its-kind project for Alberta ATCO Gas and Pipelines Ltd. (ATCO) began delivering a 5% blend of hydrogen (H2) in natural gas into a subsection of the existing Fort Saskatchewan natural gas distribution system (approximately 2100 customers). The project was commissioned in October 2022 with the intention of increasing the blend to 20% H₂ in 2023. As part of project due diligence ATCO in partnership with DNV undertook Quantitative Risk Assessments (QRAs) to understand any risks associated with the introduction of blended gas into its existing distribution system and to its customers. This paper describes key findings from the QRAs through the comparison of risks associated with H2 blended natural gas at concentrations of 5% and 20% H₂ and the current natural gas configuration. The impact of operating pressure and hydrogen blend composition formed a sensitivity study completed as part of this work. To provide context and to help interpret the results an individual risk (IR) level of 1 × 10-6 per year was utilised as a reference threshold for the limit of the ‘broadly acceptable’ risk level and juxtaposed against comparable risk scenarios. Although adding hydrogen increases the IR of ignited releases from mains services meters regulators and end user appliances the ignited release IR was always well below the broadly acceptable reference criterion for all operating pressures and blend cases considered as part of the project. The IR associated with carbon monoxide poisoning dominates the overall IR and the results demonstrate that the reduction in carbon monoxide poisoning associated with the introduction of H₂ blended natural gas negates any incremental risk associated with ignited releases due to H₂ blended gas. The paper also explains how the results of the QRA were incorporated into Engineering Assessments as per the requirements of CSA Z662:19 [1] to justify the conversion of existing natural gas infrastructure to H₂ blended gas infrastructure.

For widespread adoption of hydrogen fuel cell powered vehicles such vehicles need to be able to provide similar transportation capabilities as their gasoline/diesel powered counterparts. Meeting this requirement in many regions will necessitate access to tunnels. Previous work completed at Sandia National Laboratories provided high-fidelity consequence modeling of hydrogen vehicle tunnel crashes for a specific fire scenario in selected Massachusetts tunnels. To consider additional tunnels a generalized tunnel safety analysis framework is being developed. This framework aims to be broader than specific fire scenarios in specific tunnels allowing it to be applied to a range of tunnel geometries vehicle types and crash scenarios. Initial steps in the development of the generalized framework are reported within this work. Representative tunnel characteristics are derived based on data for tunnels in the U.S. Tunnel dimensions shapes and traffic levels are among the many characteristics reported within the data that can be used to inform crash scenario specification. Various crash scenario parameters are varied using lower-fidelity consequence modeling to quantify the impact on resulting safety hazards for time-dependent releases. These lower-fidelity models consider the unignited dispersion of hydrogen gas the thermal effects of jet fires and potential impacts of overpressures. Different sizes/classes of vehicles are considered as the total amount of hydrogen onboard may greatly affect scenario-specific consequences. The generalized framework will allow safety assessments to be both more agile and consistent when applied to different types of tunnels.

Power-to-gas technology can be used to convert excess power from renewable energies to hydrogen by means of water electrolysis. This hydrogen can serve as “chemical energy storage” and be converted back to electricity or fed into the natural gas grid. In the presented study a leak in a household pipe in a single-family house with a 13 KW heating device was experimentally investigated. An admixture of up to 40% hydrogen was set up to produce a scenario of burning leakage. Due to the outflow and mixing conditions a lifted turbulent diffusion flame was formed. This led to an additional examination point and expanded the aim and novelty of the experimental investigation. In addition to the fire safety experimental simulation of a burning leakage the resulting complex properties of the flame namely the lift-off height flame length shape and thermal radiation have also been investigated. The obtained results of this show clearly that as a consequence of the hydrogen addition the main properties of the flame such as lifting height flame temperature thermal radiation and total heat flux densities along the flame have been changed. To supplement the measurements with thermocouples imaging methods based on the Sobel gradient were used to determine the lifting height and the flame length. In order to analyze the determined values a probability density function was created.

The use of liquid hydrogen in maritime applications is expected to grow in the coming years in order to meet the decarbonisation goals that EU countries and countries worldwide have set for 2050. In this context The Norwegian Public Roads Administration commissioned large-scale LH2 dispersion and explosion experiments both indoors and outdoors which were conducted by DNG GL in 2019 to better understand safety aspects of LH2 in the maritime sector. In this work the DNV unignited outdoor and indoor tests have been simulated and compared with the experiments with the aim to validate the ADREA-HF Computational Fluid Dynamics (CFD) code in maritime applications. Three tests two outdoors and one indoors were chosen for the validation. The outdoor tests (test 5 and 6) involved liquid hydrogen release vertically downwards and horizontal to simulate an accidental leakage during bunkering. The indoor test (test 9) involved liquid hydrogen release inside a closed room to simulate an accident inside a tank connection space (TCS) connected to a ventilation mast.