Safety studies on high-pressure hydrogen vehicle refuelling stations: Releases into a simulated high-pressure dispensing area

If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogen with the same degree of confidence, and with comparable risk, as conventional liquid and gaseous fuels. The hazards associated with jet releases from leaks in a vehicle-refuelling environment must be considered if hydrogen is stored and used as a high-pressure gas since a jet release in a confined or congested area can create an explosion hazard. As there was insufficient knowledge of the explosion hazards, a study was initiated to gain a better understanding of the potential explosion hazard consequences associated with high-pressure leaks from hydrogen vehicle refuelling systems. This paper describes the experiments with a dummy vehicle and dispenser units to represent refuelling station congestion. Experiments with ignition of premixed 5.4 m × 6.0 m × 2.5 m hydrogen–air clouds and hydrogen jet releases up to 40 MPa (400 bar) pressure are described. The results are discussed in terms of the conditions leading to the greatest overpressures and overall conclusions are made from these.     

Experimental study on hydrogen explosions in a full-scale hydrogen filling station model

In order for fuel cell vehicles to develop a widespread role in society, it is essential that hydrogen refuelling stations become established. For this to happen, there is a need to demonstrate the safety of the refuelling stations. The work described in this paper was carried out to provide experimental information on hydrogen outflow, dispersion and explosion behaviour. In the first phase, homogeneous hydrogen–air mixtures of a known concentration were introduced into an explosion chamber and the resulting flame speed and overpressures were measured. Hydrogen concentration was the dominant factor influencing the flame speed and overpressure. Secondly, high-pressure hydrogen releases were initiated in a storage room to study the accumulation of hydrogen. For a steady release with a constant driving pressure, the hydrogen concentration varied as the inlet airflow changed, depending on the ventilation area of the room, the external wind conditions and also the buoyancy induced flows generated by the accumulating hydrogen. Having obtained this basic data, the realistic dispersion and explosion experiments were executed at full-scale in the hydrogen station model. High-pressure hydrogen was released from 0.8 to 8.0 mm nozzle at the dispenser position and inside the storage room in the full-scale model of the refuelling station. Also the hydrogen releases were ignited to study the overpressures that can be generated by such releases. The results showed that overpressures that were generated following releases at the dispenser location had a clear correlation with the time of ignition, distance from ignition point.     

A computational fluid dynamics evaluation of unconfined hydrogen explosions in high pressure applications

Over the last few decades, the demand for hydrogen has significantly grown. Its high-energy content and relatively small environmental effect make it an ideal energy source and chemical feedstock. However, the perceived high risk of hydrogen in the eyes of society is a key challenge that has to be addressed before any future widespread utilization of hydrogen can be achieved. In this study, the consequences of unconfined hydrogen releases are evaluated using a computational fluid dynamics simulation software, FLACS, to determine the potential to explode. In addition, the study includes the analysis of parameters that can promote hydrogen vapor cloud explosion, e.g., initial pressure, time to ignition, and leak height position.The results conclude that high-pressure hydrogen has the potential to build up a large vapor cloud and may explode even without confinement when the leak source is close to the ground. The highest overpressure produced in the simulation was 0.71 barg, which resulted from igniting a hydrogen gas cloud from a 207 bar hydrogen source leaking at 1 m height. The high overpressure suggests that hazard studies for hydrogen leaks near the ground should not assume a free flow jet release. This study also gives a recommended distance from a high-pressure hydrogen processing unit to nearby occupied buildings to use in conjunction with industrial spacing tables for fire hazards.     

Numerical modelling of release of subsonic and sonic hydrogen jets

For the general public to use hydrogen as a vehicle fuel, they must be able to handle hydrogen with the same degree of confidence as conventional liquid and gaseous fuels. For refuelling hydrogen cars, hydrogen is stored at high pressures up to 700 bar. The hazards associated with jet releases from accidental leaks of such highly pressurized storage must be considered since a jet release and dispersion can result in a fire or explosion. Therefore, it is essential to understand the dispersion characteristics of hydrogen to determine the extent of the flammable cloud when released from a high-pressure source. These parameters are very important in the establishment of the safety distances and sizes of hazardous zones. This paper describes the work done by us in modelling of dispersion of accidental releases of hydrogen, using the FRED (Fire Explosion Release Dispersion) software. The dispersion module in FRED is validated against experimental data available in the open literature for steady release and dispersion of cold and ambient hydrogen gas. The validation is performed for a wide range of hole sizes (0.5–4 mm), pressure (1.7–400 bar) and temperature (50–298 K).The model predictions of hydrogen gas jet velocity, concentration decay as a function of distance as well as radial concentration distribution are in good agreement with experiments. Overall, it is concluded that FRED can accurately model accidental release and dispersion of hydrogen in unconfined and open configurations.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 1: Effects of increase in congestion

Experimental studies were conducted with the objective of gaining a better understanding of the potential explosion hazard consequences that could be associated with a high-pressure leak from a hydrogen vehicle refuelling system. The first part of the study, described in this paper, was a series of experiments designed to establish hydrogen–air explosion overpressures in a well-defined and well understood 3 m × 3 m x 2 m (high) repeated pipe congestion. The results of the experiments are discussed in terms of the conditions leading to the greatest overpressures. It is concluded from the study that stoichiometric ratio in the range of 1.2–1.3 gives highest overpressure. Moreover, it was observed that increasing the congestion from 4-gate to 9-gate congestion leads to significant increase in the overpressure. In addition, it was concluded that, explosion in a hydrogen-air mixture is significantly more severe than the explosion in an ethane-air, methane-air or propane-air mixtures. This is attributed to higher laminar flame speed of hydrogen-air mixtures.     

Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 2: Numerical study

Safety studies for hydrogen retail stations involve identification of possible accidental scenarios, modelling of consequences and measures to mitigate associated hazards with it. Accidental release of hydrogen during its handling and storage can lead to formation of ignitable mixture in a very short time. Ignition of such a mixture can lead to generation of overpressure affecting structure and people. Understanding of the possible overpressures generated is critical in designing the system safe from explosion hazards. In the present study, the worst-case scenario where high-pressure hydrogen storage cylinders are enveloped by a premixed hydrogen-air cloud is numerically simulated. The computational domain mimics the setup for premixed hydrogen cloud in a mock hydrogen cylinder storage congestion environment experimentally studied by Shirvill et al. [1]. Large Eddy Simulations (LES) are performed using OpenFOAM CFD toolbox solver. The Flame Surface Wrinkling Model in LES context is used for modelling deflagrations [2]. Numerical simulation results are compared against experiments. Simulations are able to predict experimental flame arrival and overpressure reasonably well. The effects of ignition location, congestion and confining walls on the turbulent deflagrations in particular on explosion overpressure are discussed. It was concluded that explosion overpressure increases with increase in confinement.     

Analysis of jet flames and unignited jets from unintended releases of hydrogen

A combined experimental and modeling program is being carried out at Sandia National Laboratories to characterize and predict the behavior of unintended hydrogen releases. In the case where the hydrogen leak remains unignited, knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited, a classic turbulent jet flame forms. Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. Depending on the effective diameter of the leak and the tank source pressure, free jet flames can be extensive in length and pose significant radiation and impingement hazard, resulting in consequence distances that are unacceptably large. One possible mitigation strategy to potentially reduce the exposure to jet flames is to incorporate barriers around hydrogen storage equipment. The reasoning is that walls will reduce the extent of unacceptable consequences due to jet releases resulting from accidents involving high-pressure equipment. While reducing the jet extent, the walls may introduce other hazards if not configured properly. The goal of this work is to provide guidance on configuration and placement of these walls to minimize overall hazards using a quantitative risk assessment approach. The program includes detailed CFD calculations of jet flames and unignited jets to predict how hydrogen leaks and jet flames interact with barriers, complemented by an experimental validation program that considers the interaction of jet flames and unignited jets with barriers.     

Consequences of catastrophic releases of ignited and unignited hydrogen jet releases

The possibility of using a risk based approach for the safe installation and siting of stationary fuel cell systems depends upon the availability of normative data and guidance on potential hazards, and the probabilities of their occurrence. Such guidance data is readily available for most common hydrocarbon fuels. For hydrogen, however, data is still required on the hazards associated with different release scenarios. This data can then be related to the probability of different types of scenarios, from historical fault data, to allow safety distances to be defined and controlled using different techniques. Some data on releases has started to appear but this data generally relates to hydrogen vehicle refuelling systems that are designed for larger throughput, higher pressures, and the general use of larger pipe diameters than are likely to be used for small fuel cell systems.The aim of this paper is to report on work that is providing data for informing safety distances for high-pressure components/fuel cell systems and associated fuel storage. Using high-pressure release scenarios, the extent of the clouds, jets and, following ignition, fires and explosions were investigated.     

Experimental study of ignited unsteady hydrogen jets into air

In order to simulate an accidental hydrogen release from the low pressure pipe system of a hydrogen vehicle a systematic study on the nature of transient hydrogen jets into air and their combustion behaviour was performed at the FZK hydrogen test site HYKA. Horizontal unsteady hydrogen jets with an amount of hydrogen up to 60 STP dm³ and initial pressures of 5 and 16 bar have been investigated. The hydrogen jets were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen-air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

Simulation of high-pressure liquid hydrogen releases

Sandia National Laboratories is working with stakeholders to develop scientific data for use by standards development organizations to create hydrogen codes and standards for the safe use of liquid hydrogen. Knowledge of the concentration field and flammability envelope for high-pressure hydrogen leaks is an issue of importance for the safe use of liquid hydrogen. Sandia National Laboratories is engaged in an experimental and analytical program to characterize and predict the behavior of liquid hydrogen releases. This paper presents a model for computing hydrogen dilution distances for cold hydrogen releases. Model validation is presented for leaks of room temperature and 80 K high-pressure hydrogen gas. The model accounts for a series of transitions that occurs from a stagnate location in the tank to a point in the leak jet where the concentration of hydrogen in air at the jet centerline has dropped to 4% by volume. The leaking hydrogen is assumed to be a simple compressible substance with thermodynamic equilibrium between hydrogen vapor, hydrogen liquid and air. For the multi-phase portions of the jet near the leak location the REFPROP equation of state models developed by NIST are used to account for the thermodynamics. Further downstream, the jet develops into an atmospheric gas jet where the thermodynamics are described as a mixture of ideal gases (hydrogen–air mixture). Simulations are presented for dilution distances in under-expanded high-pressure leaks from the saturated vapor and saturated liquid portions of a liquid hydrogen storage tank at 10.34 barg (150 PSIG).     

Numerical study of hydrogen explosions in a refuelling environment and in a model storage room

Numerical simulations have been carried out for large scale hydrogen explosions in a refuelling environment and in a model storage room. For the first scenario, a high pressure hydrogen jet released in a congested refuelling environment was ignited and the subsequent explosion analysed. The computational domain mimics the experimental set up for a vertical downwards release in a vehicle refuelling environment experimentally tested by Shirvill et al. [6]. For completeness of the analysis, an analytical model has also been developed to provide the transient pressure conditions at nozzle exit. The numerical study is based on the traditional computational fluid dynamics (CFD) techniques solving Reynolds averaged Navier-Stokes equations. The Pseudo diameter approach is used to bypass the shock-laden flow structure in the immediate vicinity of the nozzle. For combustion, the Turbulent Flame Closure (TFC) model is used while the shear stress transport (SST) model is used for turbulence. In the second scenario, premixed hydrogen-air clouds with different hydrogen concentrations from 15% to 60% in volume were ignited in a model storage room. Analysis was carried out to derive the dependence of overpressure on hydrogen concentrations for safety considerations.     

Detailed simulations of the transient hydrogen mixing,leakage and flammability in air in simple geometries

During an accidental release, hydrogen disperses very quickly in air due to a relatively high density difference. A comprehensive understanding of the transient behavior of hydrogen mixing and the associated flammability limits in air is essential to support the fire safety and prevention guidelines. In this study, a buoyancy diffusion computational model is developed to simultaneously solve for the complete set of equations governing the unsteady flow of hydrogen. A simple vertical cylinder is considered to investigate the transient behavior of hydrogen mixing, especially at relatively short times, for different release scenarios: (i) the sudden release of hydrogen at the cylinder bottom into air with open, partially open, and closed tops, and (ii) small hydrogen jet leaks at the bottom into a closed geometry. Other cases involving the hydrogen releases/leaks at the cylinder top are also explored to quantify the relative roles of buoyancy and diffusion in the mixing process. The numerical simulations display the spatial and temporal distributions of hydrogen for all the configurations studied. The complex flow patterns demonstrate the fast formation of flammable zones with implications in the safe and efficient use of hydrogen in various applications.     

The simulation and analysis of leakage and explosion at a renewable hydrogen refuelling station

The number of hydrogen refuelling stations (HRSs) is steadily growing worldwide. In China, the first renewable hydrogen refuelling station has been built in Dalian for nearly 3 years. FLACS software based on computational fluid dynamics approach is used in this paper for simulation and analysis on the leakage and explosion of hydrogen storage system in this renewable hydrogen refuelling station. The effects of wind speed, leakage direction and wind direction on the consequences of the accident are analyzed. The harmful area, lethal area, the farthest harmful distance and the longest lethal distance in explosion accident of different accident scenarios are calculated. Harmful areas after explosion of different equipments in hydrogen storage system are compared. The results show that leakage accident of the 90 MPa hydrogen storage tank cause the greatest harm in hydrogen explosion. The farthest harmful distance caused by explosion is 35.7 m and the farthest lethal distance is 18.8 m in case of the same direction of wind and leakage. Moreover, it is recommended that the hydrogen tube trailer should not be parked in the hydrogen refuelling station when the amount of hydrogen is sufficient.     

Hydrogen explosions in 20′ ISO container

This paper describes a series of explosion experiments in inhomogeneous hydrogen air clouds in a standard 20′ ISO container. Test parameter variations included nozzle configuration, jet direction, reservoir back pressure, time of ignition after release and degree of obstacles. The paper presents the experimental setup, resulting pressure records and high speed videos. The explosion pressures from the experiments without obstacles were in the range of 0.4–7 kPa. In the experiments with obstacles the gas exploded more violently producing pressures in order of 100 kPa.     

Comparison of the explosion characteristics of hydrogen,propane, and methane clouds at the stoichiometric concentrations

Numerical simulations were performed to study explosion characteristics of the unconfined clouds. The examined cloud volume was 4 m × 4 m × 2 m. The build-in obstruction inside the cloud was the 8 × 8 × 4 perpendicular rod array. The obstacle volume blockage ratio was 0.74. Three gases were considered: hydrogen/air at the stoichiometric concentrations, propane/air at the stoichiometric concentrations, and methane/air at the stoichiometric concentrations. The hydrogen/air cloud explosion has higher peak overpressure and the overpressure rises locally at the nearby region of the cloud boundary. The explosion overpressures of both methane/air and propane/air are lower, compared with the hydrogen/air, and decreases with distance. The maximum peak dynamic pressure is reached beyond the original cloud, which is clearly different from the explosion peak overpressure tends. Furthermore, dynamic pressure of a cloud explosion is of the same order as overpressure. The explosion flame region for the hydrogen/air cloud is approximately 1.25 times of the original width of the cloud. The explosion flame regions for propane/air or methane/air clouds are approximately 1.4 times of the original width of the cloud. Unlike the explosion overpressures, the explosion temperatures have little difference between the three mixture examined in this study. The higher energy of explosive mixture generates a high temperature hazard effect, but the higher energy of explosive mixture may not generate a larger overpressure hazard effect in a gas explosion accident.     

Refuelling scenarios of a light urban fuel cell vehicle with metal hydride hydrogen storage. Comparison with compressed hydrogen storage counterpart

The high price of hydrogen fuel in the fuel cell vehicle refuelling market is highly dependent on the one hand from the production costs of hydrogen and on the other from the capital cost of a hydrogen refuelling station's components to support a safe and adequate refuelling process of contemporary fuel cell vehicles. The hydrogen storage technology dominated in the vehicle sector is currently based on high-pressure compressed hydrogen tanks to extend as much as possible the driving range of the vehicles. However, this technology mandates the use of large hydrogen compression and cooling systems as part of the refuelling infrastructure that consequently increase the final cost of the fuel. This study investigated the prospects of lowering the refuelling cost of small urban hydrogen vehicles through the utilisation of metal hydride hydrogen storage. The results showed that for low compression hydrogen storage, metal hydride storage is in favour in terms of the dispensed hydrogen fuel price, while its weight is highly comparable to the one of a compressed hydrogen tank. The final refuelling cost from the consumer's perspective however was found to be higher than the compressed gas due to the increased hydrogen quantity required to be stored in fully empty metal hydride tanks to meet the same demand.     

Spontaneous ignition of hydrogen leaks: A review of postulated mechanisms

Over the last century, there have been reports of high pressure hydrogen leaks igniting for no apparent reason, and several ignition mechanisms have been proposed. Although many leaks have ignited, there are also reported leaks where no ignition has occurred. Investigations of ignitions where no apparent ignition source was present have often been superficial, with a mechanism postulated which, whilst appearing to satisfy the conditions prevailing at the time of the release, simply does not stand up to rigorous scientific analysis. Some of these proposed mechanisms have been simulated in a laboratory under superficially identical conditions and appear to be rigorous and scientific, but the simulated conditions often do not have the same large release rates or quantities, mainly because of physical constraints of a laboratory. Also, some of the release scenarios carried out or simulated in laboratories are totally divorced from the realistic situation of most actual leaks. Clearly there are gaps in the knowledge of the exact ignition mechanism for releases of hydrogen, particularly at the high pressures likely to be involved in future storage and use. Mechanisms which have been proposed in the past are the reverse Joule–Thomson effect, electrostatic charge generation, diffusion ignition, sudden adiabatic compression, and hot surface ignition. Of these, some have been characterised by means of computer simulation rather than by actual experiment, and hence are not validated. Consequently there are discrepancies between the theories, releases known to have ignited, and releases which are known to have not ignited. From this, postulated ignition mechanisms which are worthy of further study have been identified, and the gaps in information have been highlighted. As a result, the direction for future research into the potential for ignition of hydrogen escapes has been identified.     

Non-homogeneous hydrogen deflagrations in small scale enclosure. Experimental results

University of Pisa performed hydrogen releases and deflagrations in a 1.14 m³ test facility, which shape and dimensions resemble a gas cabinet. Tests were performed for the HySEA project, founded by the Fuel Cells and Hydrogen 2 Joint Undertaking with the aim to conduct pre-normative research on vented deflagrations in enclosures and containers used for hydrogen energy applications. The test facility, named Small Scale Enclosure (SSE), has a vent area of 0,42 m² which can host different types of vent; plastic sheet and commercial vent were tested. Realistic levels of congestion are obtained placing a number of gas bottles inside the enclosure. Releases are performed from a buffer tank of a known volume filled with hydrogen at a pressure ranging between 15 and 60 bar. Two nozzles of different diameter and three different release directions were tested, being the nozzle placed at a height where in a real application a leak has the highest probability to occur. Three different ignition locations were investigated as well. This paper is aimed to summarize the main features of the experimental campaign as well as to present its results.     

Research on the design of hydrogen supply system of 70 MPa hydrogen storage cylinder for vehicles

A hydrogen supply system of 70 MPa hydrogen storage cylinder on vehicles is designed, in which a compressor is proposed to use the new type of ion compressor. The system is simulated statically by Aspen Plus. Meanwhile, during the process of hydrogen charged from the third-stage high-pressure hydrogen storage tank to the hydrogen storage cylinder on vehicles, the dynamic variety of the third-stage high-pressure hydrogen storage tank is simulated dynamically by Aspen HYSYS Through the simulation, obtaining the results that there are difference between theoretical calculation and simulation for the volume of third-stage high-pressure hydrogen storage tank and the average volume flow of hydrogen in a third-stage high-pressure hydrogen storage tank varies with its pressure and volume. By comparing the results of Aspen Plus simulation and Aspen HYSYS simulation, there are some differences. The designed system can be applied to hydrogen stations and any operating conditions involving the supply hydrogen.     

Experiment study on the pressure and flame characteristics induced by high-pressure hydrogen spontaneous ignition

A series of experiments were conducted to study the pressure and combustion characteristics of the high-pressure hydrogen during the occurrence of spontaneous ignition and the conversion from spontaneous ignition to a jet fire and explosion. Different initial conditions including release pressure (4–10 MPa), tube diameter (10/15 mm), and tube length (0.3/0.7/1.2/1.7/2.2/3 m) were tested. The variation of the pressure and flame signal inside and outside of the tube and the development of the jet flame were recorded. The experimental results revealed that the minimum ignition pressure required for self-ignition of hydrogen at different tube diameters decreased first and then increased with the extension of tubes. The minimum ignition pressure for tubes diameters of 10 mm and 15 mm is no more than 4 MPa and the length of the tubes is L = 1.7 m. The minimum release pressure required for spontaneous ignition of a tube D = 15 mm is always lower than that of a tube D = 10 mm at the same tube length. When the spontaneous ignition occurred, it did not absolutely trigger the jet fire. The transition from spontaneous ignition to a jet fire must go through the specific stages.