Safety

Hydrogen Fuel Cell Electric Vehicles (HFC EVs) represent an alternative to replace current internal combustion engine vehicles. The use of these vehicles with storage of compressed gaseous hydrogen (CGH2) or cryogenic liquid hydrogen (LH2) in confined spaces such as tunnels underground car parks etc. creates new challenges to ensure the protection of people and property and to keep the risk at an acceptable level. Several studies have shown that confinement or congestion can lead to severe accidental consequences compared to accidents in an open atmosphere. It is therefore necessary to develop validated hazard and risk assessment tools for the behaviour of hydrogen in tunnels. The HYTUNNEL-CS project sponsored by the FCH-JU pursues this objective. Among the experiments carried out in support of the validation of the hydrogen safety tools the CEA conducted tests on large-scale jet fires in a full-scale tunnel geometry.<br/>The tests were performed in a decommissioned road tunnel in two campaigns. The first one with 50 liters type II tanks under a pressure of 20 MPa and the second one with 78 liters type IV tanks under 70 MPa. In both cases a flate plate was used to simulate the vehicle. Downward and upward gas discharges to simulate a rollover have been investigated with various release diameters. For the downward discharge the orientation varied from normal to the road to a 45° rearward inclination. The first campaign took place under a concrete vault while the second under a rocky vault. Additional tests with the presence of a propane fire simulating a hydrocarbon powered vehicle fire were performed to study the interaction between the two reactive zones.<br/>In the paper all the results obtained during the second campaign for the evolution of the hydrogen jet-fire size the radiated heat fluxes and the temperature of the hot gases released in the tunnel are reported. Comparisons with the classical correlations from open field tests used in engineering models are also presented and conclusions are given as to their applicability.

This paper presents a systematic investigation that encompasses the safety assessment of a fuel preparation room (FPR) intended for a hydrogen-fueled ship. The primary objective is to determine the appropriate ventilation strategy to mitigate the risks associated with potential hydrogen leakage. The study focuses on a case involving an FPR measuring 10.2 m × 5.3 m × 2.65 m which is part of a 750 DWT hydrogen-powered fishing vessel. To identify the potential events leading to hydrogen dispersion an event tree analysis is conducted. Additionally existing regulations and guidelines related to the safety assessments of hydrogen leakage in enclosed areas are summarized and analyzed. Computational fluid dynamics FLACS-CFD are utilized for the consequence analysis in order to evaluate the impact of ventilation on hydrogen dispersion and concentration within the FPR. The research findings indicate significant effects of ventilation on the hazards and safety assessments of FPRs and high-pressure fuel gas supply systems. The study highlights that hydrogen vapor tends to accumulate at the ceiling and in the corners and spaces created by the equipment. The position and size of ventilation openings greatly influence the dispersion of hydrogen leakage. Proper ventilation design including top inlet ventilation and outlet ventilation on the opposite side helps to maintain a safe FPR by facilitating the efficient dispersion of hydrogen vapor. Moreover locating inlet ventilation on the same side as the outlet ventilation is found to hinder dispersion while the cross-ventilation achieved by placing inlets and outlets on opposite sides enhances airflow and dispersion. Consequently it is recommended to prioritize the structural design of FPRs and implement enhanced safety measures. Additionally updating the relevant regulations to address these concerns is strongly advised.

The accurate determination of the potential impact radius is crucial for the design and risk assessment of hydrogen pipelines. The existing methodologies employ a single point source model to estimate radiation and the potential impact radius. However these approaches overlook the jet fire shape resulting from high-pressure leaks leading to discrepancies between the calculated values and real-world incidents. This study proposes models that account for both the mass release rate while considering the pressure drop during hydrogen pipeline leakage and the radiation while incorporating the flame shape. The analysis encompasses 60 cases that are representative of hydrogen pipeline scenarios. A simplified model for the potential impact radius is subsequently correlated and its validity is confirmed through comparison with actual cases. The proposed model for the potential impact radius of hydrogen pipelines serves as a valuable reference for the enhancement of the precision of hydrogen pipeline design and risk assessment.

This paper presents work undertaken by the HSE as part of the Hytunnel-CS project a consortium investigating safety considerations for fuel cell hydrogen (FCH) vehicles in tunnels and similar confined spaces.<br/>Hydrogen vehicles typically have a Thermally activated Pressure Release Device (TPRD) providing protection to the on-board storage of the vehicle. Upon activation the content of the vessel is released in a blowdown. The release of this hydrogen gas poses a significant hazard of ignition. The consequences of such an ignition could also be compounded by confinement or congestion.<br/>HSE undertook a series of experiments investigating the consequences of these events by releasing hydrogen into a tunnel and causing ignitions. A sub-section of these tests involved steel structures providing congestion in the tunnel. The mass of hydrogen released into the tunnel prior to ignition was varied by storage pressure (up to 59 MPa) release diameter and ignition delay. The ignition delays were set based on the expected worst-case predicted by pre-simulation models. To assess the consequences overpressure measurements were made down the tunnel walls and for the tests with congestion at the face and rear of the congestion structures. The flame arrival time was also measured using exposed-tip thermocouples resulting in an estimate for flame speed down the tunnel. The measured overpressure and flame extent results are presented and compared against overpressure levels of concern.

Hydrogen Fuel Cell Electric Vehicles (HFC EVs) represent an alternative to replace current internal combustion engine vehicles. The use of these vehicles with storage of compressed gaseous hydrogen (CGH2) in confined spaces such as tunnels underground car parks etc. creates new challenges to ensure the protection of people and property and to keep the risk at an acceptable level. The HYTUNNEL-CS project sponsored by the FCH-JU was launched to develop validated hazard and risk assessment tools for the behavior of hydrogen leaks in tunnels. Among the experiments carried out in support of the validation tools the CEA has conducted tests on gas dispersion in a full-scale tunnel geometry. In the tests carried out hydrogen is replaced by helium under a pressure of 70 MPa in a 78 liter tank. The car is simulated by a flat plate called chassis and the discharges are made either downwards under the chassis or upwards to take into account a rollover of the car during the accident. Different thermally activated pressure relief device (TPRD) diameters are examined as well as different orientations of the discharge. Finally the mixing transient of helium with air is measured for distances between -50 and +50m from the release. Performing CFD simulations of such an under-expanded jet in an environment as large as a road tunnel demands a compressible flow solver and so a large computational cost. To optimize this cost a notional nozzle approach is generally used to replace the under-expanded jet by a subsonic jet that has the same concentration dilution behavior. The physics at the injection point is then not resolved and a model of these boundary conditions has to be implemented. This article first reviews the main experimental results. Then a model of boundary conditions is proposed to have a subsonic hydrogen jet that matches the dilution characteristics of an under-expanded jet. Furthermore this model is implemented in the TRUST LES computer code and in the Neptune-CFD RANS computer code in order to simulate some helium dispersion experiments. Finally results from the CFD simulations are compared to the experimental results and the effect of the exact shape of the tunnel is also assessed by comparing simulations with idealized flat walls and real scanned walls.

This paper presents work undertaken by the HSE as part of the Hytunnel-CS project a consortium investigating safety considerations for fuel cell hydrogen (FCH) vehicles in tunnels and similar confined spaces. The test programme investigating hydrogen dispersion in tunnels involved simulating releases analogous to Thermally activated Pressure Relief Devices (TPRDs) typically found on hydrogen vehicles into the HSE Tunnel facility. The releases were scaled and based upon four scenarios: cars buses and two different train designs. The basis for this scaling was the size of the tunnel and the expected initial mass flow rates of the releases scenarios. The results of the 12 tests completed have been analysed in two ways: the initial mass flow rates of the tests were calculated based upon facility measurements and the Able-Noble equations of state for comparison to the intended initial flow rate; and observations of the hydrogen dispersion in the tunnel were made based on 15 hydrogen sensors arrayed along the tunnel. The calculated mass flow rates showed reasonable agreement with the intended initial conditions showing that the scaling methodology can be used to interpret the data based on the full-scale tunnel of interest. Observations of the hydrogen dispersion show an initial turbulent mixing followed by a movement of the mixed hydrogen/air cloud down the tunnel. No vertical stratification of the cloud was observed but this effect could be possible in longer tunnels or tunnels with larger diameters. Higher ventilation rates in the tunnel resulted in a reduction of the residence time of the hydrogen and a slight increase in the dilution.

Hydrogen as one of the most promising renewable clean energy sources holds significant strategic importance and vast application potential. However as a high-energy combustible gas hydrogen poses risks of fire and explosion in the event of a leakage. Hydrogen production plants typically feature large spatial volumes and complex obstacles which can significantly influence the diffusion pathways and localized accumulation of hydrogen during a short-term high-volume release further increasing the risk of accidents. Implementing effective hydrogen leakage monitoring measures can mitigate these risks ensuring the safety of personnel and the environment to the greatest extent possible. Therefore this paper uses CFD methods to simulate the hydrogen leakage process in a hydrogen production plant. The study examines the molar fraction distribution characteristics of hydrogen in the presence of obstacles by varying the ventilation speed of the plant and the directions of leakage. The main conclusions are as follows: enhancing ventilation can effectively prevent the rapid increase in hydrogen concentration with higher ventilation speeds yielding better suppression. After a hydrogen leak in a confined space hydrogen tends to diffuse along the walls and accumulate in corner areas indicating that hydrogen monitoring equipment should be placed in corner locations.

Hydrogen is commonly used as feedstock in industrial processes and is regarded as a potential future energy carrier. However its reactivity and low density make it difficult to handle and store safely. Indoor hydrogen dispersion can cause a fire or explosion hazard if encountering an ignition source. Safety practices often use time expensive modelling techniques to estimate risk associated with hydrogen. A neural network based surrogate model could efficiently replace Computational Fluid Dynamics (CFD) modelling in safety studies. To lower the dimensionality of this surrogate model a dimensional analysis based on Buckingham’s Pi-theorem is proposed. The dimensional analysis examines stratified filling and highlights the functional parameters involved in the process. Stratified filling occurs for buoyancy dominated releases and is characterized by layers of decreasing concentration starting at the ceiling of the enclosure and developing towards the bottom. The study involves four dimensional cases that were simulated using Computational Fluid Dynamics (CFD) to demonstrate the usefulness of the proposed dimensionless time and dimensionless volume. The setup considered in this paper consists of a parallelepiped enclosure with standard atmospheric conditions a single release source and one pressure outlet to ensure constant pressure during the release. The results of the CFD simulations show a distinct pattern in the relation of hydrogen molar fraction and dimensionless time. The pattern depends on the dimensionless height of the measurement location. A five-parameter logistic (5PL) function is proposed to fit the data from the CFD models. Overall the paper provides insights into the functional parameters involved in the evolution of hydrogen mass fractions during stratified filling. It provides a nondimensional surrogate model to compute the evolution of the local concentrations of hydrogen during the development of stratification layers.

Building safe and convenient fuelling stations is key to deploying the arrival of commercial/public-use fuel cell electric vehicles (FCEVs). As the most public-facing hydrogen applications second only to the FCEVs hydrogen stations are an efficient tool to educate the public about hydrogen safety and normalize its use to fill up our vehicles. However as an emerging technology it is the industry’s responsibility to ensure that fuelling infrastructures are designed and maintained in accordance with established safety standards and thus that the fuelling process is inherently safe for all users. On the other end it is essential that consumers have all the necessary information at reach to help them feel safe while fuelling their zero-emission vehicles.<br/>This paper will provide a snapshot of the safety systems used to help protect members of the public using hydrogen fueling stations as well as the information used to educate people using this equipment. This will cover the different processes involved in hydrogen fueling stations the dangers that are present to customers and members of the public at these sites and the engineering design choices and equipment used to mitigate these dangers or prevent them from happening. Finally this paper will discuss the crucial role of understanding the dangers of hydrogen at a public level and showing the importance of educating the public about hydrogen infrastructure so that people will feel comfortable using it in their everyday lives.

The aim of the H21 project is to undertake measurements analysis and field trials to support the safe repurposing of Great Britain’s natural gas distribution network for hydrogen. As part of this project work has been ongoing to identify aspects of existing natural gas procedures that will need to be modified for hydrogen and to support the development of new procedures. This has included a review of the scientific basis of current displacement purging practices analysis of the potential implications of switching from natural gas to hydrogen and experimental support work. The reduced density and viscosity of hydrogen means that minimum purging velocities should (in principle) be higher for hydrogen to avoid stratification and ensure adequate removal of the purged gas during pipeline purging operations. A complicating factor is the high molecular diffusivity of hydrogen (roughly three times that of natural gas) which causes hydrogen to mix over short distances more rapidly than natural gas. Current models for pipeline purging do not take into account the mixing effect related to molecular diffusion. The wider flammable limits lower ignition energy and greater potential for combustion to transition from deflagration to detonation with hydrogen means that indirect purging with nitrogen is currently being investigated for distribution pipelines. This paper reviews the ongoing analysis of hydrogen pipeline purging and discusses a potential future scientific programme of work aimed at developing a new pipeline purging model that accounts for molecular diffusion effects.