Quantitative risk assessment of the interior of a hydrogen refueling station considering safety barrier systems

A quantitative risk assessment of human life during the operation of a hydrogen refueling station (HRS) is conducted. We calculate the risks for three accident scenarios: a hydrogen leak from the external piping surrounding a dispenser, a hydrogen leak from an accumulator connection piping and a hydrogen leak from a compressor/connection piping in the HRS. We first calculate the probability of accident by multiplying the estimated leak frequency with the incident occurrence probability considering the ignition probability and failure probability of the safety barrier systems obtained through event tree analysis for each scenario. We next simulate the blast and flame effects of the ignition of concentration fields formed by hydrogen leakage. We then use existing probit functions to estimate the consequences of eardrum rupture, fatalities due to displacement by the blast wave, fatalities due to head injuries, first-degree burns, second-degree burns, and fatal burn injuries by accident scenario, leak size, and incident event, and we estimate the risk distribution in 1-m cells. We finally assess the risk reduction effects of barrier placement and the distance to the dispenser and quantify the risk level that HRSs can achieve under existing law. Quantitative risk assessment reveals that the risk for a leak near the dispenser is less than 10⁻⁶ per year outside a distance of 6 m to the dispenser. The risk for a leak near the accumulators and compressors exceeds 10⁻⁴ per year within a distance of 10 m from the ignition point. A separation of 6 m to the dispenser and a barrier height of 3 m keep the fatal risk from burns to the workers, consumers and residents and passersby below the acceptable level of risk. Our results therefore show that current laws sufficiently mitigate the risks posed by HRSs and open up the possibility for a regulatory review.     

The demonstration of incredibility of failure in structural integrity safety cases

A safety case must be produced to justify the operation of nuclear plant. Wherever possible, such cases seek to demonstrate that sufficient physical layers of protection exist to ensure the safety of plant operators and the public. This is the principle of Defence in Depth. However, there are some components, notably pressure vessels and other pressure boundary components, where the provision of multiple physical layers is not possible. For these components, safety cases seek to justify that failure of the component is an ‘Incredible’ event with a frequency of occurrence of less than 10⁻⁷ per year. This frequency is two orders of magnitude lower than the demonstrated failure frequency (10⁻⁵) of conventional pressure vessels.This paper presents the findings of The Technical Advisory Group on the Structural Integrity of Nuclear Plant (TAGSI), who have considered what key principles should be employed in producing an ‘Incredibility of Failure’ safety case. TAGSI favour the multi-legged, multi-element graphical structure, with legs chosen so that they contain conceptually different arguments, and elements making up the discrete arguments in each leg. A properly constructed safety case will exhibit ‘Conceptual Defence in Depth’, with each leg providing a conceptually different contribution to the whole. The provision of Conceptual Defence in Depth is necessary to demonstrate the low failure frequency of nuclear pressure vessels.The quantitative assessment of the overall failure frequency, or the probability of failure per year, requires, firstly, that separate estimates are made of the probability of failure of each leg using only the information provided in the leg, and, secondly, the combination of these separate estimates into an overall value. Assessments can be made quantitatively through a probabilistic assessment, but it is more usual for a deterministic assessment to be carried out — e.g. using partial safety factors to ensure a high safety margin. The concept of ‘Worth’ provides one method of assessing the ‘strength’ of each leg in the context of the whole safety case. The strength of the overall case can be gauged by adding up the separate values of Worth. A value of Worth of more than seven would correspond to a failure frequency less than 10⁻⁷ per year. Hence, it would demonstrate Incredibility of Failure.     

High-density automotive hydrogen storage with cryogenic capable pressure vessels

LLNL is developing cryogenic capable pressure vessels with thermal endurance 5–10 times greater than conventional liquid hydrogen (LH₂) tanks that can eliminate evaporative losses in routine usage of (L)H₂ automobiles. In a joint effort BMW is working on a proof of concept for a first automotive cryo-compressed hydrogen storage system that can fulfill automotive requirements on system performance, life cycle, safety and cost. Cryogenic pressure vessels can be fueled with ambient temperature compressed gaseous hydrogen (CGH₂), LH₂ or cryogenic hydrogen at elevated supercritical pressure (cryo-compressed hydrogen, CcH₂). When filled with LH₂ or CcH₂, these vessels contain 2–3 times more fuel than conventional ambient temperature compressed H₂ vessels. LLNL has demonstrated fueling with LH₂ onboard two vehicles. The generation 2 vessel, installed onboard an H₂-powered Toyota Prius and fueled with LH₂ demonstrated the longest unrefueled driving distance and the longest cryogenic H₂ hold time without evaporative losses. A third generation vessel will be installed, reducing weight and volume by minimizing insulation thickness while still providing acceptable thermal endurance. Based on its long experience with cryogenic hydrogen storage, BMW has developed its cryo-compressed hydrogen storage concept, which is now undergoing a thorough system and component validation to prove compliance with automotive requirements before it can be demonstrated in a BMW test vehicle.     

Review on optimization design,failure analysis and non-destructive testing of composite hydrogen storage vessel

Composite hydrogen storage vessels have been increasingly applied to hydrogen fuel cell vehicles. This review focuses on optimization design, failure analysis and nondestructive testing for enhancing the safety of composites hydrogen storage vessels in service. The optimization designs of the composite vessel components help to improve the durability and strength of composite vessels subjected to burst pressure and fatigue loads. In complex service environments, composite vessels may suffer from various failure forms (burst failure, fatigue failure and impact failure) which involve different damage processes and influence factors. More importantly, this review discusses the applications of acoustic emission, digital image correlation, optical fiber in studying the residual performance (burst pressure and fatigue life) and damage modes of the composite vessel. It is expected that the combination of nondestructive testing techniques plays an increasingly important role in developing the composite vessel for structural health monitoring.     

A mixed electronic and protonic conducting hydrogen separation membrane with asymmetric structure

In this work, highly doped ceria with lanthanum, La_0.5Ce_0.5O_2−δ (LDC), are developed as hydrogen separation membrane material. LDC presents a mixed electronic and protonic conductivity in reducing atmosphere and good stability in moist CO₂ environment. LDC separation membranes with asymmetrical structure are fabricated by a cost-saving co-pressing method, using NiO + LDC + corn starch mixture as substrate and LDC as top membrane layer. Hydrogen permeation properties are systemically studied, including the influence of operating temperature, hydrogen partial pressure in feed stream and water vapor in both sides of the membrane on hydrogen permeating fluxes. Hydrogen permeability increases as the increasing of temperature and hydrogen partial pressure in feed gas. Using 20% H₂/N₂ (with 3% of H₂O) as feed gas and dry high purity argon as sweep gas, an acceptable flux of 2.6 × 10⁻⁸ mol cm⁻² s⁻¹ is achieved at 900 °C. The existing of water in both sides of membrane has significant effect on hydrogen permeation and the corresponding reasons are analyzed and discussed.     

Safety approach for composite pressure vessels for road transport of hydrogen. Part 2: Safety factors and test requirements

Composite pressure vessels for transporting hydrogen on roads are a promising and efficient solution for supplying refueling stations. The safety factors of current ISO design standards are perceived as being restrictive for design. In this paper, new safety factors are calculated based on a probabilistic approach and by extending the methods used in the DNV Offshore standard DNV-OS-C501 “Composite Components”. Short-term and long-term conditions are addressed.     

Safety criteria for the transport of hydrogen in permanently mounted composite pressure vessels

The recent growth of the net of hydrogen fuelling stations increases the demands to transport compressed hydrogen on road by battery vehicles or tube-trailers, both in composite pressure vessels. As a transport regulation, the ADR is applicable in Europe and adjoined regions, and is used for national transport in the EU. This regulation provides requirements based on the behaviour of each individual pressure vessel, regardless of the pressure of the transported hydrogen and relevant consequences resulting from generally possible worst case scenarios such as sudden rupture. In 2012, the BAM (German Federal Institute for Materials Research and Testing) introduced consequence-dependent requirements and established them in national transport requirements concerning the “UN service life checks” etc. to consider the transported volume and pressure of gases. This results in a requirement that becomes more restrictive as the product of pressure and volume increases. In the studies presented here, the safety measures for hydrogen road transport are identified and reviewed through a number of safety measures from countries including Japan, the USA and China. Subsequently, the failure consequences of using trailer vehicles, the related risk and the chance are evaluated. A benefit-related risk criterion is suggested to add to regulations and to be defined as a safety goal in standards for hydrogen transport vehicles and for mounted pressure vessels. Finally, an idea is given for generating probabilistic safety data and for highly efficient evaluation without a significant increase of effort.     

In situ activation with Mo of Ni–Co alloys for hydrogen evolution reaction

In this study Ni–Co alloys have been activated during hydrogen electrochemical production by adding Mo ions into the alkaline electrolyte. After dissolving different amounts of sodium molybdate in the Na(OH) electrolyte, Ni–Co alloys were used as cathodes for hydrogen evolution reaction. Afterwards a comparison between hydrogen overvoltage measured on Ni–Co alloys with and without in situ activation has been made. The in situ activation clearly shows an improvement of electrocatalytic properties of Ni–Co alloys for hydrogen evolution reaction. Depending on the alloy the best conditions are reached with different amounts of sodium molybdate in the electrolyte. The values of exchange current density for Ni–Co alloys without Mo, are an average of about 4.1 10⁻⁶ A/cm², while by using in situ activation, these values are about 3.5·10⁻⁴ A/cm². Therefore, exchange current density presents a value nearly one hundred-fold higher when molybdate ions are present in solution. Moreover, two Tafel slope values have been determined for HER on Ni–Co alloys with and without Mo in situ activation. Those Tafel slope values are different, so as their range of both overvoltage and current density, probably highlighting a different kinetic mechanism.     

Thermodynamic analysis of the para-to-ortho hydrogen conversion in cryo-compressed hydrogen vessels for automotive applications

Cryo-compressed hydrogen storage has potential applications in fuel-cell vehicles due to its large storing density and thermal endurance. The dormancy of storage can be extended when considering the endothermic conversion of para-to-ortho hydrogen. In present study, a thermodynamic model is established to analyze the effect of the conversion in a cryogenic pressure vessel. The influence of the parameters such as the filling density, initial temperature and initial ortho hydrogen fraction is studied. It is demonstrated that different “transition pressures” for the vessels exist for different filling densities. The conversion can carry out sufficiently and the dormancy can be extended significantly when the designed release pressure of the vessel matches with the transition pressure. The heat of absorption increases with the initial o-H₂ fraction, whereas the peak of conversion rate occurs earlier for the vessel with a large initial o-H₂ fraction. The dormancy can be extended by 163% for the vessel with filling density of 70 kg/m³. The investigations on the effect of the para-to-ortho hydrogen conversion can provide useful guideline for the design of cryo-compressed hydrogen vessels.     

Novel non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation

This study presents a new non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation. It shows that palladium and ruthenium can be deposited on an aluminum-oxide-modified porous Hastalloy by using our new EDTA-free plating bath at room temperature and 358 K, respectively. A 6.8 μm thick non-alloy Ru/Pd membrane film could be plated and helium leak test confirmed that the membrane was free of defects. Hydrogen permeation test showed that the membrane had a hydrogen permeation flux of 4.5 × 10⁻¹ mol m⁻² s⁻¹ at a temperature of 773 K and a pressure difference of 100 kPa. The hydrogen permeability normalized value with thickness of the membrane was 1.4 times higher than our pure Pd membrane having similar structure. The EDX profiles of the front and back side membrane, cross-sectional EDX line scanning and XRD profile show that there was no alloying progress between the palladium and ruthenium layer after hydrogen permeation test at 773 K.     

Concept risk assessment of a hydrogen driven high speed passenger ferry

A concept risk assessment of a hydrogen and fuel cell driven high speed passenger ferry has been performed. The study focused on fatality risk related to the hydrogen systems on the vessel, both during operation and while moored in harbour overnight. The main objective with the study was to evaluate whether the risk related to the hydrogen systems is equivalent to that of conventionally fuelled vessels and can be considered acceptable according to the requirements of the IGF-code (International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels). Since hydrogen behaves differently than other flammable gases, some adjustments to existing models and vulnerability criteria have been proposed. The conclusion of the study is that the estimated risk related to hydrogen systems is relatively low, and much lower than the expected acceptable risk tolerance level of 0.5–1.0 fatalities per 10⁹ passenger km. Furthermore, for the overnight mooring in harbour the estimated risks are well within acceptable limits. The work presented is part of a maritime case study performed within MoZEES, a Norwegian research centre for environmentally friendly technology and zero emission transport.     

Modeling of sudden hydrogen expansion from cryogenic pressure vessel failure

We have modeled sudden hydrogen expansion from a cryogenic pressure vessel. This model considers real gas equations of state, single and two-phase flow, and the specific “vessel within vessel” geometry of cryogenic vessels. The model can solve sudden hydrogen expansion for initial pressures up to 1210 bar and for initial temperatures ranging from 27 to 400 K. For practical reasons, our study focuses on hydrogen release from 345 bar, with temperatures between 62 K and 300 K. The pressure vessel internal volume is 151 L. The results indicate that cryogenic pressure vessels may offer a safety advantage with respect to compressed hydrogen vessels because i) the vacuum jacket protects the pressure vessel from environmental damage, ii) hydrogen, when released, discharges first into an intermediate chamber before reaching the outside environment, and iii) working temperature is typically much lower and thus the hydrogen has less energy. Results indicate that key expansion parameters such as pressure, rate of energy release, and thrust are all considerably lower for a cryogenic vessel within vessel geometry as compared to ambient temperature compressed gas vessels. Future work will focus on taking advantage of these favorable conditions to attempt fail-safe cryogenic vessel designs that do not harm people or property even after catastrophic failure of the inner pressure vessel.     

Preparation of Li-Mg-N-H hydrogen storage materials for an auxiliary power unit

Solid hydrogen storage materials as H₂ supply for PEM fuel cells have been attempted over the past decades because of their high efficiencies in H₂ storage. However, most investigations were focused on the stage of tank design for the storage materials. The Li-Mg-N-H hydrogen storage system was for the first time integrated into a HT-PEM fuel cell stack for a prototype auxiliary power unit, the maximum working temperature being 200 °C. With a designed output of 1 kW, a few kilograms of storage materials are needed. By using commercially available raw materials, an up-scaled preparation of the storage material was performed using laboratory facilities. Preparation conditions were established with the aid of FTIR, TG-DSC and x-ray diffraction to ensure the desired quality of materials. Prior to power the fuel cell stack, the storage materials need to go through an exothermic metathesis, and severe temperature overshooting is expected, which may cause deterioration in material performance and safety issue. Operation conditions were tested and the temperature overshooting could be effectively prevented under adequate conditions.     

Artificial neural networks and neuro-fuzzy inference systems as virtual sensors for hydrogen safety prediction

Hydrogen is increasingly investigated as an alternative fuel to petroleum products in running internal combustion engines and as powering remote area power systems using generators. The safety issues related to hydrogen gas are further exasperated by expensive instrumentation required to measure the percentage of explosive limits, flow rates and production pressure. This paper investigates the use of model based virtual sensors (rather than expensive physical sensors) in connection with hydrogen production with a Hogen^®20 electrolyzer system. The virtual sensors are used to predict relevant hydrogen safety parameters, such as the percentage of lower explosive limit, hydrogen pressure and hydrogen flow rate as a function of different input conditions of power supplied (voltage and current), the feed of de-ionized water and Hogen^®20 electrolyzer system parameters. The virtual sensors are developed by means of the application of various Artificial Intelligent techniques. To train and appraise the neural network models as virtual sensors, the Hogen^®20 electrolyzer is instrumented with necessary sensors to gather experimental data which together with MATLAB neural networks toolbox and tailor made adaptive neuro-fuzzy inference systems (ANFIS) were used as predictive tools to estimate hydrogen safety parameters. It was shown that using the neural networks hydrogen safety parameters were predicted to less than 3% of percentage average root mean square error. The most accurate prediction was achieved by using ANFIS.     

Safe,long range,inexpensive and rapidly refuelable hydrogen vehicles with cryogenic pressure vessels

Hydrogen storage is often cited as the greatest obstacle to achieving a hydrogen economy free of environmental pollution and dependence on foreign oil. A compact high-pressure cryogenic storage system has promising features to the storage challenge associated with hydrogen-powered vehicles. Cryogenic pressure vessels consist of an inner vessel designed for high pressure (350 bar) insulated with reflective sheets of metalized plastic and enclosed within an outer metallic vacuum jacket. When filled with pressurized liquid hydrogen, cryogenic pressure vessels become the most compact form of hydrogen storage available. A recent prototype is the only automotive hydrogen vessel meeting both Department of Energy's 2017 weight and volume targets. When installed onboard an experimental vehicle, a cryogenic pressure vessel demonstrated the longest driving distance with a single H₂ tank (1050 km). In a subsequent experiment, the vessel demonstrated unprecedented thermal endurance: 8 days parking with no evaporative losses, extending to a month if the vehicle is driven as little as 8 km per day. Calculations indicate that cryogenic vessels offer compelling safety advantages and the lowest total ownership cost of hydrogen storage technologies. Long-term (∼10 years) vacuum stability (necessary for high performance thermal insulation) is the key outstanding technical challenge. Testing continues to establish technical feasibility and safety.     

Aromatic amines as proton carriers for catalytic enhancement of hydrogen evolution reaction on copper in acid solutions

The catalytic effect of some aromatic amines towards hydrogen evolution reaction on copper in diluted sulfuric acid solution has been studied. Since amines facilitate the transport of protons from the solution bulk to the interface in the cathodic hydrogen evolution reaction, they are known as proton carriers. The catalytic effect of aniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline, N,N-diethylaniline, o-toluidine, m-toluidine and p-toluidine has been highlighted by linear sweep voltammetry. The kinetic parameters for the hydrogen evolution reaction (cathodic transfer coefficient 1-α and exchange current density i_o) in the presence of the studied aromatic amines were derived from the Tafel plots. It has been found that the catalytic effect of amines is active even at low concentration. Thus, in 0.5 mol L⁻¹ H₂SO₄ solution the exchange current density increases by two orders of magnitude, from 2.01⋅10⁻⁵ A m⁻² in the absence of aniline to 2.85⋅10⁻³ A m⁻² in the presence of 10⁻⁴ mol L⁻¹ aniline. The influence of amines concentration on the catalytic effect is described in detail for the case of m-toluidine. The results obtained by voltammetry have been compared with electrochemical impedance spectroscopy data. Furthermore, the kinetic parameters for the hydrogen evolution reaction have been determined as a function of temperature and amines concentration.     

Chemical hydrogen storage using polynuclear borane anion salts

The polynuclear borane anions nido-B₁₁H₁₄⁻, closo-B₁₀H₁₀²⁻ and closo-B₁₂H₁₂²⁻ undergo heterogeneous transition metal-catalyzed hydrolysis to generate hydrogen. The rate of hydrolysis is dependent upon the concentration of polynuclear borane anions and surface area of the metallic catalyst. The aqueous solutions of polynuclear borane anions, especially the closo-B₁₀H₁₀²⁻ and closo-B₁₂H₁₂²⁻ dianions, are very stable for an extended period of time (years) in the absence of catalyst and spontaneously generate hydrogen in the presence of a rhodium metal catalyst. These polynuclear borane anions have a high potential for use in portable hydrogen storage systems without requiring high pH aqueous media for storage as in the case of NaBH₄.     

A planar microfabricated electrolyzer for hydrogen and oxygen generation

We present the design, fabrication and testing of a microfabricated planar reactor for the hydrogen evolution reaction (HER) using thin film Pt electrodes and polydimethylsiloxane (PDMS) fluidic chamber. The reactor is designed to separate gases by flow dynamics and reactor flow is analyzed by three-dimensional finite element analysis. The planar geometry is scalable, compact and stackable. Using KOH 28 wt% electrolyte, we have achieved a hydrogen generation density of 0.23 kg h⁻¹ m⁻³ and an efficiency of 48% with a flow rate of 10 ml min⁻¹ and cell voltage of 3 V.     

Dynamic Bayesian network based approach for risk analysis of hydrogen generation unit leakage

Hydrogen leakage is a crucial risk for the hydrogen generation unit, which would lead to the potential fire and explosion accidents. Hydrogen leakage risk analysis is the essential alternative to ensure the safety of the hydrogen generation process. This paper presents a dynamic risk analysis methodology regarding the hydrogen leakage in the hydrogen generation unit by using the dynamic Bayesian network, which is employed to address the potential uncertainty and dynamic nature underlying the leakage risk of the hydrogen generation unit. A case study of hydrogen generation unit is carried out to demonstrate the applicability and advantage of the proposed methodology. Results indicate that the leakage probability of hydrogen generation unit can be significantly decreased within one year through equipment repair. Furthermore, the failure and repair rates of overflow alarm and pressure sensor are the most contributory factors to the hydrogen generation unit leakage. Finally, some active mitigative suggestions are presented to further reduce the leakage risk of the hydrogen generation unit.     

Electrochemical hydrogen discharge of high-strength low alloy steel for high-pressure gaseous hydrogen storage tank: Effect of discharging temperature

Hydrogen discharge technique of high-strength low alloy steel for high-pressure gaseous hydrogen storage tank was developed by using an electrochemical technique. The electrochemical hydrogen discharge of high-strength low alloy steel were investigated in a deaerated borate buffer solution (0.3 M H₃BO₃ + 0.074 M N₂B₄O₇, pH = 8.4). By applying a potential of +630 mV_SCE which is higher than the hydrogen equilibrium potentials and lower than the pitting potential, the oxidation reaction of metal (Fe → Fe²⁺ + 2e⁻) is limited and oxidation reaction of the hydrogen (H₂ + 2OH⁻ → 2H₂O + 2e⁻) was induced simultaneously. Thus, the pre-charged hydrogen inside the specimen was eliminated effectively without any damage to the specimen. The electrochemical hydrogen discharge method was performed at 25 °C, 50 °C and 75 °C. The efficiency of hydrogen discharge was accelerated with increasing temperature because the exchange current density of hydrogen is increased with temperature.