Safety

As part of the UK Government’s Net Zero targets to tackle Climate Change the Health and Safety Executive (HSE) aims to reach an authoritative view on the safety of using 100% hydrogen for heating across the UK to feed into Government policy decisions by the mid-2020s. This paper describes the background and process of a programme of work led by HSE in support of the Department for Energy Security and Net Zero (formerly BEIS) that will inform strategic policy decisions by 2026. The strategic framework of HSE’s programme of work was defined between BEIS and HSE. HSE’s programme of work follows on from a previous project which engaged with HSE policy regulatory and scientific colleagues working with industry stakeholders identifying knowledge gaps for the safe distribution storage and use of hydrogen gas in domestic industrial and commercial premises. These knowledge gaps were subsequently used in discussions with stakeholders to prioritise research projects and evidence gathering exercises. To review this scientific evidence HSE developed a review framework and convened Evidence Review Groups (ERGs) to cover all evidence areas encompassing topics such as quantified risk assessment material compatibility and operational procedures. These ERGs include representation from relevant divisions across HSE (policy regulation and science). The paper explains the structure of HSE’s input into the hydrogen for heating programme the ERG process and timelines along with the proposed outputs. Additional activities have been undertaken by HSE within the programme to highlight specific issues in support of the review process which will also be discussed.

In a future energy generation market the storage of energy is going to become increasingly important. Besides classic ways of storage like pumped storage hydro power stations etc the production of hydrogen will play an important role as an energy storage system. Hydrogen may be stored as a liquefied gas (LH2) on a long term base as well as for short term supply of fuel stations to ensure a so called “green” mobility. The handling with LH2 has been subject to several recent safety studies. In this context reliable simulation tools are necessary to predict the spill and spreading of LH2 during an accidental release. This paper deals with the different boiling regimes: film boiling transition boiling and nucleation boiling after a release and their modeling by means of an inhouse-code for wall evaporation which is implemented in the commercial CFD code ANSYS CFX. The paper will describe the model its implementation and validation against experimental data such as the HSL LH2 spill experiments.

Hydrogen–gasoline hybrid refueling stations can minimize construction and management costs and save land resources and are gradually becoming one of the primary modes for hydrogen refueling stations. However catastrophic consequences may be caused as both hydrogen and gasoline are flammable and explosive. It is crucial to perform an effective risk assessment to prevent fire and explosion accidents at hybrid refueling stations. This study conducted a risk assessment of the refueling area of a hydrogen–gasoline hybrid refueling station based on the improved Accident Risk Assessment Method for Industrial Systems (ARAMIS). An improved probabilistic failure model was used to make ARAMIS more applicable to hydrogen infrastructure. Additionally the accident consequences i.e. jet fires and explosions were simulated using Computational Fluid Dynamics (CFD) methods replacing the traditional empirical model. The results showed that the risk levels at the station house and the road near the refueling area were 5.80 × 10−5 and 3.37 × 10−4 respectively and both were within the acceptable range. Furthermore the hydrogen dispenser leaked and caused a jet fire and the flame ignited the exposed gasoline causing a secondary accident considered the most hazardous accident scenario. A case study was conducted to demonstrate the practicability of the methodology. This method is believed to provide trustworthy decisions for establishing safe distances from dispensers and optimizing the arrangement of the refueling area.

To mitigate the risk of hydrogen leakage in ship fuel systems powered by internal combustion engines a Bayesian network model was developed to evaluate the risk of hydrogen fuel leakage. In conjunction with the Bow-tie model fuzzy set theory and the Noisy-OR Gate model an in-depth analysis was also conducted to examine both the causal factors and potential consequences of such incidents. The Bayesian network model estimates the likelihood of hydrogen leakage at approximately 4.73 × 10−4 and identifies key risk factors contributing to such events including improper maintenance procedures inadequate operational protocols and insufficient operator training. The Bowtie model is employed to visualize the causal relationships between risk factors and their potential consequences providing a clear structure for understanding the events leading to hydrogen leakage. Fuzzy set theory is used to address the uncertainties in expert judgments regarding system parameters enhancing the robustness of the risk analysis. To mitigate the subjectivity inherent in root node probabilities and conditional probability tables the NoisyOR Gate model is introduced simplifying the determination of conditional probabilities and improving the accuracy of the evaluation. The probabilities of flash or pool fires jet fires and vapor cloud explosions following a leakage are calculated as 4.84 × 10−5 5.15 × 10−5 and 4.89 × 10−7 respectively. These findings highlight the importance of strengthening operator training and enforcing stringent maintenance protocols to mitigate the risks of hydrogen leakage. The model provides a valuable framework for safety evaluation and leakage risk management in hydrogen-powered ship fuel systems.

This study explores the integration and optimization of a hydrogen-based energy system emphasizing the use of metal hydride (MH) storage coupled with Proton Exchange Membrane Fuel Cell Micro Combined Heat and Power (PEMFC MCHP) system for residential applications. MH storage coupled to a heat pump operates at charging and discharging pressures of 10 bar. COMSOL model in 6.1 version using heat transfer in solids and fluids in brinkman equations modules is validated by experimental data and uses machine learning (Feedforward Neural Networks) for predictive modeling of MH dynamics. Smaller 500 NL tanks were found to have high mass-specific heat demand but faster hydrogen gas kinetics reaching (~77 % capacity in one hour) whereas larger 6500 NL (~57 %/hour) absorb hydrogen gas more gradually but reduce thermal management intensities. Using 13 × 500 NL tanks reach ~25 % discharge in 1 h but require ~2170 Wh heating whereas one 6500 NL tank only attains ~48.5 % discharge yet uses ~1750 Wh illustrating a trade-off between faster kinetics and lower thermal load. A genetic algorithm identified an optimal configuration of two 6500 NL tanks that covered ~68 % of total hydrogen gas consumption and 65 % of production at a maximum of 2.4 kW heating and 2.45 kW cooling. Additional comparisons with 170 bar compressed storage revealed lower instantaneous thermal requirements for high-pressure gas tanks. Adding a 170 bar compressed H2 alongside the 10 bar MH system hydrogen gas coverage rose from ~70 % to ~97 % when storage expanded to 200 Nm3 but at the cost of higher compression energy. The proposed MH-based approach especially at moderate pressures with carefully planned tank geometries achieves enhanced operational flexibility for a residential 120 m2 building’s space heating and hot water while machine learning optimizations further refine charge–discharge performance.

Transport and storage of hydrogen as a liquid (LH2) is being widely investigated as a solution for scaling up the supply infrastructure and addressing the growth of hydrogen demand worldwide. While there is a relatively wellestablished knowledge and understanding of hazards and associated risks for gaseous hydrogen at ambient temperature several knowledge gaps are yet open regarding the behaviour in incident scenarios of cryogenic hydrogen including LH2. This paper aims at presenting the models and tools that can be used to close relevant knowledge gaps for hydrogen safety engineering of LH2 systems and infrastructure. Analytical studies and computational fluid dynamics (CFD) modelling are used complementarily to assess relevant incident scenarios and compare the consequences and hazard distances for hydrogen systems at ambient and cryogenic temperature. The research encompasses the main phenomena characterising an incident scenario: release and dispersion ignition and combustion. Experimental tests on cryogenic hydrogen systems are used for the validation of correlations and numerical models. It is observed that engineering tools originally developed for hydrogen at ambient temperature are yet applicable to the cryogenic temperature field. For a same storage pressure and nozzle diameter the decrease of hydrogen temperature from ambient to cryogenic 80 K may lead to longer hazard distances associated to unignited and ignited hydrogen releases. The potential for ignition by spark discharge or spontaneous ignition mechanism is seen to decrease with the decrease of hydrogen temperature. CFD modelling is used to give insights into the pressure dynamics created by LH2 vessels rupture in a fire using experimental data from literature.

Hydrogen tanks used in transportation are equipped with thermal pressure relief devices to prevent a tank rapture in case of fire exposure. The opening of the pressure relief valve in such a scenario would likely result in an impinging and (semi-) confined hydrogen jet fire. Therefore twelve largescale experiments of hydrogen jet fires and one large-scale propane reference experiment have been conducted with various degrees of confinement orientations of the jet and distances from the nozzle to the impinging surface. Infrared and visible light videos temperatures heat fluxes and mass flow rate of hydrogen or propane were recorded in each experiment. It was found that the hydrogen flame can be visible under certain conditions. The main difference between an open impinging jet and an enclosed impinging jet fire is the extent of the high-temperature region in the steel target. During the impinging jet fire test 51% of the exposed target area exceeded 400C while 80% of the comparable area exceeded 400C during the confined jet fire test. A comparison was also made to an enclosed propane jet fire. The temperature distribution during the propane fire was more uniform than during the hydrogen jet fire and the localized hot spot in the impact region as seen in the hydrogen jet fires was not recorded.

Hydrogen has the potential to make countries energetically self-sufficient and independent in the long term. Nevertheless its extreme combustion properties and its capability of permeating and embrittling most metallic materials produce significant safety concerns. The Hydrogen Incidents and Accidents Database 2.0 (HIAD 2.0) is a public repository that collects data on hydrogen-related undesired events mainly occurred in chemical and process industry. This study conducts an analysis of the HIAD 2.0 database mining information systematically through a computer science approach known as Business Analytics. Moreover several hydrogen-induced ma terial failures are investigated to understand their root causes. As a result a deficiency in planning effective inspection and maintenance activities is highlighted as the common cause of the most severe accidents. The lessons learned from HIAD 2.0 could help to promote a safety culture to improve the abnormal and normal events management and to stimulate a widespread rollout of hydrogen technologies.

Hydrogen is a promising candidate for addressing environmental challenges in aviation yet its use in structural validation tests for Wing Structure-Integrated highpressure Hydrogen Tanks (SWITHs) remains underexplored. To the best of the authors’ knowledge this study represents the first attempt to assess the feasibility of conducting such tests with hydrogen at aircraft scales. It first introduces hydrogen’s general properties followed by a detailed exploration of the potential hazards associated with its use substantiated by experimental and simulation results. Key factors triggering risks such as ignition and detonation are identified and methods to mitigate these risks are presented. While the findings affirm that hydrogen can be used safely in aviation if responsibly managed they caution against immediate large-scale experimental testing of SWITHs due to current knowledge and technology limitations. To address this a roadmap with two long-term objectives is outlined as follows: first enabling structural validation tests at scales equivalent to large aircraft for certification; second advancing simulation techniques to complement and eventually reduce reliance on costly experiments while ensuring sufficient accuracy for SWITH certification. This roadmap begins with smaller-scale experimental and numerical studies as an initial step.

The Research Priorities Workshop (RPW) brought together experts from academia industry and government to identify and prioritise future research directions with regard to hydrogen safety. Over two days participants engaged in presentations and discussions covering key areas such as transportation and storage ignition phenomena cryogenic hydrogen risk assessment methodologies and others. A critical component of the workshop was the prioritisation exercise during which attendees voted on the most urgent and impactful areas for future research. This document summarises the workshop’s activities including the prioritisation results which will serve as input to guide global hydrogen safety research efforts. The combined rankings from industry and non-industry stakeholders highlighted Quantitative Risk Assessment (QRA) and Reliability Data as the top priority followed closely by Mitigation Sensors and Hazard Prevention and Phenomena Understanding and Modelling. Regulations Codes and Standards followed immediately with a particularly high ranking from the industry representatives. These priorities reflect a strong collective focus on those topics to ensure hydrogen’s safe and scalable adoption. The insights and recommendations gathered during the RPW are important for shaping the strategic research priorities necessary to support the safe commercialisation of hydrogen technologies.

Explosions of combustible gases under ambient turbulence exhibit complex flame propagation and overpressure evolution characteristics posing challenges to explosion safety assessments. In this study explosion behaviors of hydrogen-doped natural gas under various wind speeds were investigated using a small-scale experimental system. The results show that when the wind speed does not exceed 2 m/s ambient turbulence promotes flame acceleration and overpressure enhancement with the maximum overpressure increased by 20.7% compared to the no-wind condition. However when the wind speed exceeds 2 m/s turbulence suppresses flame propagation leading to a reduction in maximum overpressure by up to 50.5%. Under early-stage turbulent disturbances the flame front exhibits instability from the ignition stage resulting in a continuous transition from laminar to turbulent combustion without a distinct critical instability radius. Furthermore a modified overpressure prediction model is proposed by incorporating a flame wrinkling factor into the Thomas model and adopting a dimensionless distance treatment from the TNO multi-energy model. The proposed model achieves a root mean square error of 0.140 kPa under various wind speed conditions demonstrating good predictive accuracy.

Green hydrogen presents a promising solution for decarbonisation but its widespread adoption faces significant challenges. To meet Europe’s 2030 targets a 250-fold increase in electrolyser capacity is required necessitating an investment of €170-240 billion. This involves constructing 20-40 pioneering megaprojects each with a 1-5 GW capacity. Historically pioneering energy projects have seen capital costs double or triple from initial estimates with over 50% failing to meet production goals at startup due to new technology introductions site-specific characteristics and project complexity. Additionally megaprojects costing more than €1 billion frequently succumb to the "iron law" which states they are often over budget take longer than anticipated and yield fewer benefits than expected mainly because key players consistently underestimate costs and risks. Pursuing multiple pioneering megaprojects simultaneously restricts opportunities for iterative learning which raises risks related to untested technologies and infrastructure demands. This vision paper introduces a novel risk assessment framework that combines insights from pioneering and megaprojects with technology readiness evaluations and comparative CO2 reduction analyses to tackle these challenges. The framework aims to guide investment decisions and risk mitigation strategies such as staged scaling and limiting the introduction of new technology. The analysis highlights that using green ammonia for fertiliser production can reduce CO2 emissions by 51 tons of CO2 per ton of hydrogen significantly outperforming hydrogen use in transportation and heating. This structured approach considers risks and environmental benefits while promoting equitable risk distribution between developed and developing nations.

Hydrogen is a potential key component of a carbon-neutral energy carrier and an input to marine industrial processes. This study examines the consequences of coupled hydrogen release and marine environmental factors during floating photovoltaic hydrogen production (FPHP) system failures. A validated three-dimensional numerical model of FPHP comprehensively characterizes hydrogen leakage dynamics under varied rupture diameters (25 50 100 mm) transient release duration dispersion patterns and wind intensity effects (0–20 m/s sea-level velocities) on hydrogen–air vapor clouds. FLACS-generated data establish the concentration–dispersion distance relationship with numerical validation confirming predictive accuracy for hydrogen storage tank failures. The results indicate that the wind velocity and rupture size significantly influence the explosion risk; 100 mm ruptures elevate the explosion risk producing vapor clouds that are 40–65% larger than 25 mm and 50 mm cases. Meanwhile increased wind velocities (>10 m/s) accelerate hydrogen dilution reducing the high-concentration cloud volume by 70–84%. Hydrogen jet orientation governs the spatial overpressure distribution in unconfined spaces leading to considerable shockwave consequence variability. Photovoltaic modules and inverters of FPHP demonstrate maximum vulnerability to overpressure effects; these key findings can be used in the design of offshore platform safety. This study reveals fundamental accident characteristics for FPHP reliability assessment and provides critical insights for safety reinforcement strategies in maritime hydrogen applications.

Hydrogen energy is considered the most promising clean energy in the 21st century so hydrogen refuelling stations (HRSs) are crucial facilities for storage and supply. HRSs might experience hydrogen leakage (HL) incidents during their operation. Hydrogen-producing and refuelling integrated stations (HPRISs) could make thermal risks even more prominent than those of HRSs. Considering HL as the target in the HPRIS through the method of fault tree analysis (FTA) and analytic hierarchy process (AHP) the importance degree and probability importance were appraised to obtain indicators for the weight of accident level. In addition the influence of HL from storage tanks under ambient wind conditions was analysed using the specific model. Based upon risk analysis of FTA AHP and ALOHA preventive measures were obtained. Through an evaluation of importance degree and probability importance it was concluded that misoperation material ageing inadequate maintenance and improper design were four dominant factors contributing to accidents. Furthermore four crucial factors contributing to accidents were identified by the analysis of the weight of the HL event with AHP: heat misoperation inadequate maintenance and valve failure. Combining the causal analysis of FTA with the expert weights from AHP enables the identification of additional crucial factors in risk. The extent of the hazard increased with wind speed and yet wind direction did not distinctly affect the extent of the risk. However this did affect the direction in which the risk spreads. It is extremely vital to rationally plan upwind and downwind buildings or structures equipment and facilities. The available findings of the research could provide theoretical guidance for the applications and promotion of hydrogen energy in China as well as for the proactive safety and feasible emergency management of HPRISs.

Hydrogen (H2 ) is positioned as a key solution to the decarbonization challenge in both the energy and transportation sectors. While hydrogen is a clean and versatile energy carrier it poses significant safety risks due to its wide flammability range and high detonation potential. Hydrogen leaks can occur throughout the hydrogen value chain including production storage transportation and utilization. Thus effective leak detection systems are essential for the safe handling storage and transportation of hydrogen. This review aims to survey relevant codes and standards governing hydrogen-leak detection and evaluate various sensing technologies based on their working principles and effectiveness. Our analysis highlights the strengths and limitations of the current detection technologies emphasizing the challenges in achieving sensitive and specific hydrogen detection. The results of this review provide critical insights into the existing technologies and regulatory frameworks informing future advancements in hydrogen safety protocols.

No countermeasures exist for accidents that might occur in hydrogen-based facilities (leaks fires explosions etc.). In South Korea discussions are underway regarding measures to ensure safety from such accidents such as the construction of underground hydrogen storage tank facilities. However explosion vents with a minimum ventilation area are required in such facilities to minimize damage to buildings and other structures due to accidental explosions. These explosion vents allow the generated overpressure and flames to be safely dispersed outside; however a safe separation distance must be secured to minimize damage to humans. This study aimed to determine the safe separation distance to minimize human damage after analyzing the dispersed overpressure and flame behavior following a vent explosion. Explosion experiments were conducted to investigate the influence of the ignition source location on internal and external overpressure and external flame behavior using a cuboid concrete structure with a volume of 20.33 m3 filled with a hydrogen-air mixture (29.0 vol.%). The impact on overpressure and flame was increased with the increasing distance of the ignition source from the vent. Importantly depending on the ignition location the incident pressure was up to 24.4 times higher while the reflected pressure was 8.7 times higher. Additionally a maximum external overpressure of 30.01 kPa was measured at a distance of 2.4 m from the vent predicting damage to humans at the “Injury” level (1 % fatality probability). Whereas no significant damage would occur at a distance of 7.4 m or more from the vent.

The development of hydrogen production technologies as well as new uses represents an opportunity both to accelerate the ecological transition and to create an industrial sector. However the risks associated with the use of hydrogen must not be overlooked. The mitigation of a hydrogen explosion in an enclosure is partly based on prevention strategies such as detection and ventilation but also on protection strategies such as explosion venting. However in several situations such as in highly constrained urban environments the discharge of the explosion through blast walls could generate significant overpressure effects outside the containment which are unacceptable. Thus having alternative mitigation solutions can make the effects of the explosion acceptable by reducing the flame speed and the overpressure loading or suppressing the secondary explosion. The objective of this paper is to present the experimental study of the mitigation of hydrogen-air deflagration in a 4 m3 vented enclosure by injection of aqueous foam. After a description of the experimental set-up the main experimental results are presented showing the influence of aqueous foam on flame propagation (Fig. 1). Different foam expansion ratios were investigated. An interpretation of the mitigating effect of foam on the explosion effects is proposed based on the work of Kichatov [5] and Zamashchikov [2].

The interaction of a shock wave with a specific angle or concave wall due to its reflection and focusing is a way to onset the detonation provided sufficiently strong shock wave. In this work we present numerical simulation results of the detonation initiation due to the shock reflection and focusing in a 90-degree wedge for mixtures of H2 and air. The code used was ddtFoam [1–3] that is a component of the larger OpenFOAM open-source CFD package of density-based code for solving the unsteady compressible Navier-Stokes equations. The numerical model represents the 2-D geometry of the experiments performed by Rudy [4]. The numerical results revealed three potential scenarios in the corner after reflection: shock wave reflection without ignition deflagrative ignition with intermediate transient regimes with a delayed transition to detonation in lagging combustion zone at around 1.8 mm from the apex of the wedge and ignition with an instantaneous transition to detonation with the formation of the detonation wave in the corner tip. In the experimental investigation the transition velocity for the stoichiometric mixture was approximately 715 m/s while in the numerical simulation the transition velocity for the stoichiometric mixture was 675.65 m/s 5.5% decrease in velocity.

Cryogenic liquid is often stored in a vacuum insulated Dewar vessel for a high efficiency of thermal insulation. Multi-layer insulation (MLI) can be further applied in the double-walled vacuum space to reduce the heat transfer from the environment to the stored cryogenic fluid. However in loss-of-vacuum accident (LOVA) scenarios heat flux across the MLI will raise to orders of magnitudes larger than with an intact vacuum shield. The cryogenic liquid will boil intensively and pressurize the vessel due to the heat ingress. The pressurization endangers the integrity of the vessel and poses an extra catastrophic risk if the vapor is flammable e.g. hydrogen. Therefore safety valves have to be designed and installed appropriately to make sure the pressure is limited to acceptable levels. In this work the dynamic process of the heat and mass transfers in the LOVA scenarios is studied theoretically. The mass deposition - desublimation of gaseous nitrogen on cryogenic surfaces is modeled as it provides the dominant contribution of the thermal load to the cryogenic fluid. The conventional heat convection and radiation are modeled too although they play only secondary roles as realized in the course of the study. The temperature dependent thermal properties of e.g. gaseous and solid nitrogen and stainless steel are used to improve the accuracy of calculation in the cryogenic temperature range. Presented methodology enabling the computation of thermodynamic parameters in the cryogenic storage system during LOVA scenarios provides further support for the future risk assessment and safety system design.

Hydrogen has the potential to be an important source of clean energy but the development of efficient and cost-effective methods for storing hydrogen is a key challenge that needs to be addressed in order to make widespread use of hydrogen as a possible energy sourc. There are different methods for storing hydrogen (i.e. compressed it at high pressures liquefied by cooling the hydrogen to a temperature of -253°C and stored with a chemical compound) each with its own advantages and disadvantages.<br/>MAST3RBoost (Maturing the Production Standards of Ultraporous Structures for High Density Hydrogen Storage Bank Operating on Swinging Temperatures and Low Compression) is a European project which aims to provide a solid benchmark of cold-adsorbed H2 storage (CAH2) at low compression (100 bar or below) by maturation of a new generation of ultraporous materials for mobility applications i.e. H2-powered vehicles including road and railway air-borne and waterborne transportation. Based on a new generation of Machine Learning-improved ultraporous materials – such as Activated Carbons (ACs) and high-density MOFs (Metal-organic Frameworks) – MAST3RBoost project will enable a disruptive path to meet the industry goals by developing the first worldwide adsorption-based demonstrator at the kg-scale.<br/>The design of the tank is supported by numerical investigation by mean of the use of Computational Fluid Dynamic (CFD) commercial code. In this a paper a preliminary analysis of the refilling of tank is presented focused on the effect of different tank configurations on the hydrogen temperature and on the hydrogen adsorption.

To reveal the influence mechanisms of seasonal climatic factors (wind speed wind direction temperature) and leakage direction on hydrogen dispersion and explosion behavior from single-source leaks at typical risk locations (hydrogen storage tanks compressors dispensers) in hydrogen refueling stations (HRSs) this work established a full-scale 1:1 three-dimensional numerical model using the FLACS v22.2 software based on the actual layout of an HRS in Xichang Sichuan Province. Through systematic simulations of 72 leakage scenarios (3 equipment types × 4 seasons × 6 leakage directions) the coupled effects of climatic conditions equipment layout and leakage direction on hydrogen dispersion patterns and explosion risks were quantitatively analyzed. The key findings indicate the following: (1) Downward leaks (−Z direction) from storage tanks tend to form large-area ground-hugging hydrogen clouds representing the highest explosion risk (overpressure peak: 0.25 barg; flame temperature: >2500 K). Leakage from compressors (±X/−Z directions) readily affects adjacent equipment. Dispenser leaks pose relatively lower risks but specific directions (−Y direction) coupled with wind fields may drive significant hydrogen dispersion toward station buildings. (2) Southeast/south winds during spring/summer promote outward migration of hydrogen clouds reducing overall station risk but causing localized accumulation near storage tanks. Conversely north/northwest winds in autumn/winter intensify hydrogen concentrations in compressor and station building areas. (3) An empirical formula integrating climatic parameters leakage conditions and spatial coordinates was proposed to predict hydrogen concentration (error < 20%). This model provides theoretical and data support for optimizing sensor placement dynamically adjusting ventilation strategies and enhancing safety design in HRSs.

Hydrogen is a promising environmentally friendly fuel with the potential for zero-carbon emissions particularly in maritime applications. However owing to its wide flammability range (4–75%) significant safety concerns persist. In confined spaces hydrogen leaks can lead to explosions posing a risk to both lives and assets. This study conducts a numerical analysis to investigate hydrogen flow within hydrogen storage rooms aboard ships with the goal of developing efficient ventilation strategies. Through simulations performed using ANSYS-CFX this research evaluates hydrogen diffusion stratification and ventilation performance. A vertex angle of 120◦ at the ceiling demonstrated superior ventilation efficiency compared to that at 177◦ while air inlets positioned on side-wall floors or mid-sections proved more effective than those located near the ceiling. The most efficient ventilation occurred at a velocity of 1.82 m/s achieving 20 air exchanges per hour. These findings provide valuable insights for the design of safer hydrogen vessel operations.

Combustion of hydrogen-enriched natural gas is a valuable short-term strategy for reducing CO2 emissions from high temperature industrial heating. This paper presents several experiments on combustion characteristics and the formation of nitrogen oxides. The experiments included hydrogen contents up to 100% and fuel heat inputs up to 75 kW. Water-cooled lances were used to influence the furnace temperature. The analysis includes the distribution of furnace temperatures the composition of flue gas the cooling capacity of the lances under steady-state operating conditions and OH*-chemiluminescence imaging of the near burner region. The presented results demonstrate the dependence of furnace conditions and NOX formation on various factors such as different air inlet fluxes furnace temperature and fuel composition for constant heat inputs. Efficiency increased by up to 5.5% and significant changes in flame shaped along with a maximum increase in NOX emissions when comparing natural gas to hydrogen was measured at 167%.

Accidental hydrogen releases from pipelines pose significant risks particularly with the expanding deployment of hydrogen infrastructure. Despite this there has been a lack of thorough investigation into hydrogen leakage from pipelines especially under complex real-world conditions. This study addresses this gap by modeling hydrogen gas dispersion jet fires and explosions based on practical scenarios. Various factors influencing accident consequences such as leak hole size wind speed wind direction and trench presence were systematically examined. The findings reveal that both hydrogen dispersion distance and jet flame thermal radiation distance increase with leak hole size and wind speed. Specifically the longest dispersion and radiation distances occur when the wind direction aligns with the trench which is 110 m where the hydrogen concentration is 4% and 76 m where the radiation is 15.8 kW/m2 in the case of a 325 mm leak hole and wind under 10 m/s. Meanwhile pipelines lacking trenching exhibit the shortest distances 0.17 m and 0.98 m at a hydrogen concentration of 4% and 15.8 kW/m2 radiation with a leak hole size of 3.25 mm and no wind. Moreover under relatively higher wind speeds hydrogen concentration stratification occurs. Notably the low congestion surrounding the pipeline results in an explosion overpressure too low to cause damage; namely the highest overpressure is 8 kPa but this lasts less than 0.2 s. This comprehensive numerical study of hydrogen pipeline leakage offers valuable quantitative insights serving as a vital reference for facility siting and design considerations to eliminate the risk of fire incidents.

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 sudden failure of a pressurised hydrogen vessel was identified as a scenario of concern due to the severity of the consequences associated with such an event. In order to investigate this scenario experimentally HSE designed a bespoke and reusable ‘sudden release’ vessel. This paper presents an overview of the vessel and the results of a series of 13 tests whereby hydrogen was released from the bespoke vessel into a tunnel at pressures up to 65 MPa. The starting pressure and the volume of hydrogen in the vessel were altered throughout the campaign. Four of the tests also included congestion in the tunnel. The tests reliably autoignited. Overpressure measurements and flame arrival times measured with exposed-tip thermocouples enabled analysis of the severity of the events. A high-pressure fast-acting pressure transducer in the body of the vessel showed the pressure decay in the vessel which shows that 90% of the hydrogen was evacuated in between 1.8 and 3.2 ms (depending on the hydrogen inventory). Schlieren flow imagery was also used at the release point of the hydrogen showing the progression of the shock front following initiation of the tests. An assessment of the footage shows an estimated initial velocity of Mach 3.9 at 0.4 m from the release point. Based on this an ignition mechanism is proposed based upon the temperature behind the initial shock front.

A quantitative risk assessment on a representative liquid hydrogen storage system was performed to identify the main drivers of individual risk and provide a technical basis for revised separation distances for bulk liquid hydrogen storage systems in regulations codes and standards requirements. The framework in the Hydrogen Plus Other Alternative Fuels Risk Assessment Models (HyRAM+) toolkit was used and multiple relevant inputs to the risk assessment (e.g. system pipe size ignition probabilities) were individually varied. For each set of risk assessment inputs the individual risk as a function of the distance away from the release point was determined and the risk-based separation distance was determined from an acceptable risk criterion. These risk-based distances were then converted to equivalent leak size using consequence models that would result in the same distance to selected hazard criteria (i.e. extent of flammable cloud heat flux and peak overpressure). The leak sizes were normalized to a fraction of the flow area of the source piping. The resulting equivalent fractional hole sizes for each sensitivity case were then used to inform selection of a conservative fractional flow area leak size of 5% that serves as the basis for consequence-based separation distance calculations. This work demonstrates a method for using a quantitative risk assessment sensitivity study to inform the selection of a basis for determining consequence-based separation distances.

Hydrogen could gradually replace fossil fuels mitigating the human impact on the environment. However equipment exposed to hydrogen is subjected to damaging effects due to H2 absorption and permeation through metals. Hence inspection activities are necessary to preserve the physical integrity of the containment systems and the risk-based (RBI) methodology is considered the most beneficial approach. This review aims to provide relevant information regarding hydrogen embrittlement its effect on materials’ properties and the synergistic interplay of the factors influencing its occurrence. Moreover an overview of predictive maintenance strategies is presented focusing on the RBI methodology. A systematic review was carried out to identify examples of the application of RBI to equipment exposed to hydrogenated environments and to identify the most active research groups. In conclusion a significant lack of knowledge has been highlighted along with difficulties in applying the RBI methodology for equipment operating in a pure hydrogen environment.

The measurement of hydrogen concentration in fuel cell systems is an important prerequisite for the development of a control strategy to enhance system performance reduce purge losses and minimize fuel cell aging effects. In this perspective paper the working principles of hydrogen sensors are analyzed and their requirements for hydrogen control in fuel cell systems are critically discussed. The wide measurement range absence of oxygen high humidity and limited space turn out to be most limiting. A perspective on the development of hydrogen sensors based on palladium as a gas-sensitive metal and based on the organic magnetic field effect in organic lightemitting devices is presented. The design of a test chamber where the sensor response can easily be analyzed under fuel cell-like conditions is proposed. This allows the generation of practical knowledge for further sensor development. The presented sensors could be integrated into the end plate to measure the hydrogen concentration at the anode in- and outlet. Further miniaturization is necessary to integrate them into the flow field of the fuel cell to avoid fuel starvation in each single cell. Compressed sensing methods are used for more efficient data analysis. By using a dynamical sensor model control algorithms are applied with high frequency to control the hydrogen concentration the purge process and the recirculation pump.

Hydrogen is one of the most promising alternative sources to relieve the energy crisis and environmental pollution. Hydrogen can be stored as cryogenic compressed hydrogen (CcH2) to achieve high volumetric energy densities. Reliable safety codes and standards are needed for hydrogen production delivery and storage to promote hydrogen commercialization. Unintended hydrogen releases from cryogenic storage systems are potential accident scenarios that are of great interest for updating safety codes and standards. This study investigated the behavior of CcH2 releases and dispersion. The extremely low-temperature CcH2 jets can cause condensation of the air components including water vapor nitrogen and oxygen. An integral model considering the condensation effects was developed to predict the CcH2 jet trajectories and concentration distributions. The thermophysical properties were obtained from the COOLPROP database. The model divides the CcH2 jet into the underexpanded initial entrainment and heating flow establishment and established flow zones. The condensation effects on the heat transfer and flow were included in the initial entrainment and heating zones. The empirical coefficients in the integral model were then modified based on measured concentration results. Finally the analytical model predictions are shown to compare well with measured data to verify the model accuracy. The present study can be used to develop quantitative risk assessment models and update safety codes and standards for cryogenic hydrogen facilities.

Hydrogen leakage and explosion accidents have obvious dangers ambiguity of accident information and urgency of decision-making time. These characteristics bring challenges to the optimization of emergency alternatives for such accidents. Effective emergency decision making is crucial to mitigating the consequences of accidents and minimizing losses and can provide a vital reference for emergency management in the field of hydrogen energy. An improved VIKOR emergency alternatives optimization method is proposed based on the combination of hesitant triangular fuzzy set (HTFS) and the cumulative prospect theory (CPT) termed the HTFS-CPT-VIKOR method. This method adopts the hesitant triangular fuzzy number to represent the decision information on the alternatives under the influence of multi-attributes constructs alternatives evaluation indicators and solves the indicator weights by using the deviation method. Based on CPT positive and negative ideal points were used as reference points to construct the prospect matrix which then utilized the VIKOR method to optimize the emergency alternatives for hydrogen leakage and explosion accidents. Taking an accident at a hydrogen refueling station as an example the effectiveness and rationality of the HTFS-CPT-VIKOR method were verified by comparing with the existing three methods and conducting parameter sensitivity analysis. Research results show that the HTFS-CPT-VIKOR method effectively captures the limited psychological behavior characteristics of decision makers and enhances their ability to identify filter and judge ambiguous information making the decisionmaking alternatives more in line with the actual environment which provided strong support for the optimization of emergency alternatives for hydrogen leakage and explosion accidents.

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.

This paper presents a comprehensive review of existing explosion mitigation techniques for tunnels and evaluates their applicability in scenarios of hydrogen tank rupture in a fire. The study provides an overview of the current state of the art in tunnel explosion mitigation and discusses the challenges associated with hydrogen explosions in the context of fire incidents. The review shows that there are several approaches available to decrease the effects of explosions including wrapping the tunnel with a flexible and compressible barrier and introducing energy-absorbing flexible honeycomb elements. However these methods are limited to the mitigation of the action and do not consider either the mitigation of the structural response or the effects on the occupants. The study highlights how the structural response is affected by the duration of the action and the natural period of the structural elements and how an accurate design of the element stiffness can be used in order to mitigate the structural vulnerability to the explosion. The review also presents various passive and active mitigation techniques aimed at mitigating the explosion effects on the occupants. Such techniques include tunnel branching ventilation openings evacuation lanes right-angled bends drop-down perforated plates or high-performance fibre-reinforced cementitious composite (HPFRCC) panels for blast shielding. While some of these techniques can be introduced during the tunnel's construction phase others require changes to the already working tunnels. To simulate the effect of blast wave propagation and evaluate the effectiveness of these mitigation techniques a CFD-FEM study is proposed for future analysis. The study also highlights the importance of considering these mitigation techniques to ensure the safety of the public and first responders. Finally the study identifies the need for more research to understand blast wave mitigation by existing structural elements in the application for potential accidents associated with hydrogen tank rupture in a tunnel.

This work formed part of the H21 programme whose objective is to reach the point whereby it is feasible to convert the existing natural gas (NG) distribution network to 100% hydrogen (H2) and provide a contribution to decarbonising the UK’s heat and power sectors with the focus on decarbonised fuel at point of use. Hydrogen has an ATEX Gas Group of IIC compared to IIA for natural gas which means further precautions are necessary to prevent the ignition of hydrogen during network operations. Both electrostatic and friction ignition risks were considered. Network operations considered include electrostatic precautions for polyethylene (PE) pipe and cutting and drilling of metallic pipes. As a result of the updated basis of safety from ignition considerations existing flow stopping methods were reviewed to see if they were compatible. Commonly used flow stopping methods were tested under laboratory conditions with hydrogen following the methodologies specified in the Gas Industry Standards (GIS). A new basis of safety for flow stopping has been proposed that looks at the flow past the secondary stop as double isolations are recommended for use with hydrogen.

Experiments and numerical simulations were conducted to study the flame acceleration (FA) in stoichiometric CH4/H2/air mixtures with various hydrogen blend ratios (i.e. Hbr = 0% 20% 50% 80% and 100%). In the experiments high-speed photography was used to record the FA process. In the calculations the two-dimensional fully-compressible reactive Navier-Stokes equations were solved using a high-order algorithm on a dynamically adapting mesh. The chemical reaction and diffusive transport of the mixtures were described by a calibrated chemical-diffusive model. The numerical predictions are in good agreement with the experimental measurements. The results show that the mechanism of FA is similar in all cases that is the flame is accelerated by the thermal expansion effects various fluid-dynamic instabilities flame-vortex interactions and the interactions of flame with pressure waves. The hydrogen blend ratio has a significant impact on the propagation speed and the morphological evolution of the flame during FA. A larger hydrogen blend ratio leads to a faster FA and the difference in FA mainly depends on the increase of flame surface area and the interactions between flame and pressure waves. In addition as the hydrogen blend ratio increases there are fewer pockets of the unburned funnels in the combustion products when the flame propagates to the end of the channel.

A growing share of renewable energy production in the energy supply systems is key to reaching the European political goal of zero CO2 emission in 2050 highlighted in the green deal. Linked to the irregular production of solar and wind energies which have the highest potential for development in Europe massive energy storage solutions are needed as energy buffers. The European project HyPSTER [1] (Hydrogen Pilot STorage for large Ecosystem Replication) granted by the Clean Hydrogen Partnership addresses this topic by demonstrating a cyclic test in an experimental salt cavern filled with hydrogen up to 3 tons using hydrogen that is produced onsite by a 1 MW electrolyser. One specific objective of the project is the assessment of the risks and environmental impacts of cyclic hydrogen storage in salt caverns and providing guidelines for safety regulations and standards. This paper highlights the first outcome of the task WP5.5 of the HyPSTER project addressing the regulatory and normative frameworks for the safety of hydrogen storage in salt caverns from some selected European Countries which is dedicated to defining recommendations for promoting the safe development of this industry within Europe.

Hydrogen refuelling is imperative for the emerging market of hydrogen vehicles. The pressure control valve (PCV) at the hydrogen refuelling station (HRS) plays a major role in ensuring that hydrogen delivery to the vehicle follows the prescribed refuelling protocols. A three-dimensional CFD model with a detailed resolution of PCV motion affecting heat and mass transfer is developed. The PCV motion controlling the mass flow rate is simulated using dynamic mesh. The CFD model captures refuelling from high-pressure tanks through entire HRS equipment to onboard tanks capturing pressure and temperature changes upstream and downstream of the PCV. The Joule-Thomson effect resulting in a hydrogen temperature increase at PCV is captured using the NIST real gas database. The model is validated against Test No.1 of NREL on refuelling through the entire equipment of HRS. The CFD model can be used to design HRS equipment parameters including PCV and develop efficient refuelling protocols.

An important element of modern firefighting is sometimes the use of foam. After the use of extinguishing foam on vehicles or machinery operated by compressed gases it is conceivable that masses of foam were enriched by escaping fuel gas. Furthermore new foam creation enriched with a high level of fuel gas from the deposed foam solution becomes theoretically possible. The aim of this study was to carry out basic experimental investigations on the combustion of water-based H2/air foam. Ignition tests were carried out in a transparent and vertically oriented cylindrical tube (d = 0.09 m; 1.5 m length) and a rectangular thin layer channel (0.02 m × 0.2 m; 2 m length). Additionally results from larger scale tests performed inside a pool (0.30 m × 1 m × 2 m) are presented. All ducts are semi-confined and a foam generator fills the ducts from below with the defined foam. The foams vary in type and concentration of the foaming agent and hydrogen concentration. The expansion ratio of the combustible foam is in the range of 20 to 50 and the investigated H2-concentrations vary from 8 to 70% H2 in air. High-speed imaging is used to observe the combustion and determine flame velocities. The study shows that foam is flammable over a wide range of H2-concentrations from 9 to 65% H2 in air. For certain H2/air-mixtures an abrupt flame acceleration is observed. The velocity of combustion increases rapidly by an order of magnitude and reaches velocities of up to 80 m/s.

HyCARE project supported by the Clean Hydrogen Partnership of the European Union deals with a prototype hydrogen storage tank using a solid-state hydrogen carrier. Up to 40 kilograms of hydrogen are stored in 12 tanks at less than 50 barg and less than 100°C. The innovative design is based on a standard 20-foot container including 12 TiFe-based metal hydride (MH) hydrogen storage tanks coupled with a thermal energy storage in phase change materials (PCM). This article aims at showing the main risks related to hydrogen storage in a MH system and the safety barriers considered based on HyCARE’s specific risk analysis. Regarding the TiFe MH material used to store hydrogen experimental tests showed that the exposure of the MH to air or water did not cause spontaneous ignition. Furthermore an explosion within the solid MH cannot propagate due to internal pore size. Additionally in case of leakage the speed of hydrogen desorption from the MH is self-limited which is an important safety characteristic since it reduces the potential consequences from the hydrogen release. Regarding the integrated system the critical scenarios identified during the risk analysis were explosion due to release of hydrogen inside or outside the container internal explosion inside MH tanks due to accidental mix of hydrogen and air and asphyxiation due to inert gas accumulation in the container. The identification phase of risk analysis identified the most relevant safety barriers already in place and recommended additional ones if needed which were later implemented to further reduce the risk. The main safety barriers identified were material and component selection (including the MH selected) safety interlocks safety valves ventilation gas detection and safety distances. The risk management process based on risk identification and assessment contributed to coherently integrate inherently safe design features and safety barriers.

Among the new energy carriers aimed at reducing greenhouse gas emissions the use of hydrogen is expected to grow significantly in various applications and sectors (i.e. industrial commercial transportation etc.) due to its high energy content by weight and zero carbon emissions. The increasingly widespread use of hydrogen will require massive distribution from production sites to final consumers and the delivery by means of liquid hydrogen road tankers may be a suitable cost-effective option for market penetration in the short-medium term. Liquid hydrogen (LH2) presents different hazards compared to gaseous hydrogen and an accidental release in confined spaces such as road tunnels might lead to the formation of a flammable hydrogen cloud that might deflagrate or even detonate. Nevertheless the potential negative effects on users in the event of accidental leakage of liquid hydrogen from a tanker in road tunnels so far have not been sufficiently investigated. Therefore a 3D Computational Fluid Dynamics model for the release of LH2 and its dispersion within a road tunnel was developed in this study. The proposed model was validated by a comparison with certain experimental and numerical studies found in the literature. Such modeling is demanding for long tunnels. Therefore the results of the simulations (e.g. the amount of hydrogen contained within the cloud) were combined with established simplified consequence methods to estimate the overpressures generated from a potential hydrogen deflagration. This was then used to evaluate the effects on users while evacuating from the tunnel. The findings showed that the worst scenario is when the release is in the middle of the tunnel length and the ignition occurs 90 s after the leakage.

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (φ = 0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck flames with closed flame fronts are found to be stabilized inside a recirculation zone.

The MultHyFuel Project funded by the Clean Hydrogen Partnership aims to achieve the effective and safe deployment of hydrogen as a carbon-neutral fuel by developing a common strategy for implementing Hydrogen Refueling Stations (HRS) in a multifuel context. The project hopes to contribute to the harmonisation of existing regulations codes and standards (RCS) by generating practical theoretical and experimental data related to HRS.<br/>This paper presents how a set of safety critical scenarios have been identified from the initial preliminary as well as detailed risk analysis of three different hydrogen refueling station configurations. To achieve this a detailed examination of each potential hazardous phenomenon (DPh) or major accident event at or near the hydrogen dispenser was carried out. Particular attention is paid to the scenarios which could affect third parties external to the refueling station.<br/>The paper presents a methodology subdivided into the following steps:<br/>♦ determination of the consequence level and likelihood of each hazardous phenomenon<br/>♦ the classification of major hazard scenarios for the 3 HRS configurations specifically those arising on the dispensing forecourt;<br/>♦ proposal of example preventative control and/or mitigation barriers that could potentially reduce the probability of occurrence and/ or consequences of safety critical scenarios and hence reducing risks to a tolerable level or to as low as reasonably practicable.