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.     

Comparisons of hazard distances and accident durations between hydrogen vehicles and CNG vehicles

A comparison study is conducted to reveal the differences of hazard distances and accident durations between hydrogen vehicles and CNG vehicles during a representative accident in an open environment, i.e gas release from thermally-activated pressure relief device (TPRD). The analysis is performed for the scenario of impinging jet fires released from 4.2 mm TPRD diameter, with release inventory assumption on the basis of similar driving range: 4 kg hydrogen storage at 35 MPa and 20 kg methane storage at 25 MPa. Results show that the release duration for CNG vehicle is over two times longer than that for hydrogen vehicle, indicating that CNG vehicle jet fire accident is more time-consuming and firefighters have to wait a longer time before they can safely approach the vehicle. For both hydrogen vehicle and CNG vehicle, the longest hazard distance near the ground occur at a few seconds after the initiation of the TPRD. Afterwards the flames will shrink and the hazard distances will decrease. For firefighters with bunker gear, they must stand at least 6 m and 14 m away from the hydrogen vehicle and CNG vehicle, respectively. For general public, a perimeter of 12 m and 29 m should be set around the accident scene for hydrogen vehicle and CNG vehicle, respectively.     

Review on studies of the emptying process of compressed hydrogen tanks

Compressed hydrogen tanks are now widely used for onboard hydrogen storage in fuel cell vehicles (FCVs). However, because of the high storage pressure and the low thermal conductivity of carbon fibre reinforced polymer (CFRP), the emptying of such tanks during driving or emergency release can cause a significant temperature decrease and result in an in-tank gas temperature below the low safety temperature limit of ?40 °C even in warm weather. Once the gas temperature within the tank is lower than ?40 °C, the sealing elements at the boss of the tank may fail, and glass transition of the polymer liner of the type IV tank may occur; both can cause hydrogen leakage and severe safety problems. In this paper, the heat transfer correlations, thermodynamic analyses, computational fluid dynamics (CFD) simulations, experimental studies, and thermal management methods associated with the emptying process of compressed hydrogen tanks are comprehensively reviewed. Future research directions on this topic are suggested.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

Effects of tank heating on hydrogen release from metal hydride system in VoltaFCEV Fuel Cell Electric Vehicle

Metal hydride (MH) storage is known as a safe storage method because it does not require complex processes like high pressure or very low temperature. However, it is necessary to use a heat exchanger due to the endothermic and exothermic reactions occurring during the charging and discharging processes of the MH tanks. The performance of the MH is adversely affected by the lack of a heat exchanger or a suitable temperature range and it causes non-stable hydrogen supply to the fuel cell systems. In this study, effect of the tank surface temperature on hydrogen flow and hydrogen consumption performance were investigated for the MH hydrogen storage system of a hydrogen Fuel Cell Electric Vehicle (FCEV). Different temperature values were arranged using an external heat circulator device and a heat exchanger inside the MH tank. The fuel cell (FC) was operated at three different power levels (200 W, 400 W, and 600 W) and its performance was determined depending on the temperature and discharge flow rate of the MH tank. When the heat exchanger temperature (HET) was set to 40 °C, the discharge performance of the MH tank increased compared to lower temperatures. For example, when the FC power was set to 200 W and the HET of the system was at 40 °C, 1600 L hydrogen was supplied to the FC and 2000 Wh electrical energy was obtained. The results show that the amount of hydrogen supplied from the MH tank decreases significantly by increasing the flow rate in the system and rapid temperature changes occur in the MH tank.     

Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank

In high temperature proton exchange membrane (HT-PEM) fuel cells, waste heat at approximately 160 °C is produced, which can be used for thermal integration of solid state hydrogen storage systems. In the present study, an HT-PEM fuel cell stack (400 W) with direct liquid cooling is characterized and coupled to a separately characterized sodium alanate storage tank (300 g material). The coupled system is studied in steady state for 20 min operation and all relevant heat flows are determined. Even though heat losses at that specific power and temperature level cannot be completely avoided, it is demonstrated that the amount of heat transferred from the fuel cell stack to the cooling liquid circuit is sufficient to desorb the necessary amount of hydrogen from the storage tank. Furthermore, it is shown that the reaction rate of the sodium alanate at 160 °C and 1.7 bar is adequate to provide the hydrogen to the fuel cell stack. Based on these experimental investigations, a set of recommendations is given for the future design and layout of similar coupled systems.     

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.     

Experimental study of hydrogen storage with reaction heat recovery using metal hydride in a totalized hydrogen energy utilization system

Experimental results for hydrogen storage tanks with metal hydrides used for load leveling of electricity in commercial buildings are described. Variability in electricity demand due to air conditioning of commercial buildings necessitates installation of on-site energy storage. Here, we propose a totalized hydrogen energy utilization system (THEUS) as an on-site energy storage system, present feasibility test results for this system with a metal hydride tank, and discuss the energy efficiency of the system. This system uses a water electrolyzer to store electricity energy via hydrogen at night and uses fuel cells to generate power during the day. The system also utilizes the cold heat of reaction heat during the hydrogen desorption process for air conditioning. The storage tank has a shell-like structure and tube heat exchangers and contains 50 kg of metal hydride. Experimental conditions were specifically designed to regulate the pressure and temperature range. Absorption and desorption of 5,400 NL of hydrogen was successfully attained when the absorption rate was 10 NL/min and desorption rate was 6.9 NL/min. A 24-h cycle experiment emulating hydrogen generation at night and power generation during the day revealed that the system achieved a ratio of recovered thermal energy to the entire reaction heat of the hydrogen storage system of 43.2% without heat loss.     

Metal hydride hydrogen storage tank for light fuel cell vehicle

We describe a metal hydride (MH) hydrogen storage tank for light fuel cell vehicle application developed at HySA Systems. A multi-component AB₂-type hydrogen storage alloy was produced by vacuum induction melting (10 kg per a load) at our industrial-scale facility. The MH alloy has acceptable H sorption performance, including reversible H storage capacity up to ∼170 NL/kg (1.5 wt% H). The cassette-type MH tank was made up of 2 cylindrical aluminium canisters with transversal internal copper fins and external aluminium fins for improving the heat exchange between the heating medium and the MH tank. Heat supply and removal was provided from the outside using air at T = 15–25 °C. The MH tank was tested at the conditions of natural or forced (velocity ∼2 m/s) air convection. The tests included H₂ charge of the tank at P = 15–40 bar and its discharge at P = 1 bar. The tank in the H₂ discharge mode was also tested together with open cathode low-temperature proton exchange membrane fuel cell (LT PEMFC).     

Design of a solar hydrogen refuelling station following the development of the first Croatian fuel cell powered bicycle to boost hydrogen urban mobility

Hydrogen refuelling stations are important for achieving sustainable hydrogen economy in low carbon transport and fuel cell electric vehicles. The solution presented in this paper provides us with a technology for producing carbon dioxide free hydrogen, which is an approach that goes beyond the existing large-scale hydrogen production technologies that use fossil fuel reforming. Hence, the main goal of this work was to design a hydrogen refuelling station to secure the autonomy of a hydrogen powered bicycle. The bicycle hydrogen system is equipped with a proton exchange membrane fuel cell stack of 300 W, a DC/DC converter, and a metal hydride storage tank of 350 NL of hydrogen. The hydrogen power system was made of readily available commercial components. The hydrogen station was designed as an off-grid system in which the installed proton exchange membrane electrolyzer is supplied with electric energy by direct conversion using photovoltaic cells. With the hydrogen flow rate of 2000 cc min⁻¹ the hydrogen station is expected to supply at least 5 bicycles to be used in 20 km long city tourist routes.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

The design and optimization of a cryogenic compressed hydrogen refueling process

Cryo-compressed hydrogen storage has excellent volume and mass hydrogen storage density, which is the most likely way to meet the storage requirements proposed by United States Department of Energy（DOE）. This paper contributes to propose and analyze a new cryogenic compressed hydrogen refueling station. The new type of low temperature and high-pressure hydrogenation station system can effectively reduce the problems such as too high liquefaction work when using liquid hydrogen as the gas source, the need to heat and regenerate to release hydrogen, and the damage of thermal stress on the storage tank during the filling process, so as to reduce the release of hydrogen and ensure the non-destructive filling of hydrogen. This paper focuses on the study of precooling process in filling. By establishing a heat transfer model, the dynamic trend of tank temperature with time in the precooling process of low-temperature and high-pressure hydrogen storage tank under constant pressure is studied. Two analysis methods are used to provide theoretical basis for the selection of inlet diameter of hydrogen storage tank. Through comparative analysis of the advantages and disadvantages of the two analysis methods, it is concluded that the analysis method of constant mass flow is more suitable for the selection in practical applications. According to it, the recommended diameter of the storage tank at the initial temperature of 300 K, 200 K and 100 K is selected, which are all 15 mm. It is further proved that the calculation method can meet the different storage tank states of hydrogen fuel cell vehicles when selecting the pipe diameter.     

Influence of obstacle morphology on safety of nuclear hydrogen production system

The safety of the nuclear hydrogen production system is a key issue that cannot be ignored in the future commercialization. The reasonable arrangement of obstacles can effectively improve the safety of the system under accident conditions. In this paper, based on the high-pressure hydrogen storage tank model of a nuclear hydrogenation plant, two mechanisms by which obstacles can effectively improve safety are analyzed. In order to determine the obstacle design scheme with outstanding effect, the response surface methodology is used to study the influence of the structural parameters and spatial position parameters of obstacles of different shapes, and the comprehensive performance of different obstacles is compared by the TNO multi-energy method. The results show that the semi-cylindrical surface can control the separation distance within 141 m at a lower processing cost, which has good engineering utilization value. This study can provide a valuable reference for the design of nuclear hydrogen production systems.     

Towards fast and safe hydrogen filling for the fuel vehicle: A variable mass flow filling strategy based on a real-time thermodynamic model

Dealing with the conflict between the temperature/pressure rise and the total mass of hydrogen is a key challenge for rapid hydrogen filling of the hydrogen storage tank (HST). The temperature/pressure rise and total mass of hydrogen cause safety risks because of the former and limited cruise as the result of the latter. Therefore, safe hydrogen filling strategy is essential for the promotion of hydrogen fuel cell vehicles (FCVs). The existing thermodynamic model of the hydrogen storage tank is simplified either in the hydrogen state or the heat conduction of the HST wall, which can be hardly used as the real-time and accurate references for developing the filling strategy. To solve this problem, this paper works out the mathematical expression of a HST thermodynamic model. With the proposed HST thermodynamic model, a variable mass flow hydrogen filling strategy is developed. The results show that at the mass flow (12  g/s), the errors of the thermodynamic model are 7.1% and 6.8% for the temperature and pressure rise, compared with the computational fluid dynamics (CFD) model. At the mass flow (4.84  g/s), the thermodynamic model errors are 8.3% and 7.1% for the temperature and pressure rise, compared with the experimental data. Also, compared with the rule-based hydrogen filling strategies, the final state of charge (SoC) with the new filling strategy improve by 3%, 3.7%, and 2.7% at different initial temperatures, different volumes, and initial SoCs, respectively.     

Aluminium alloy based hydrogen storage tank operated with sodium aluminium hexahydride Na3AlH6

Here we present the development of an aluminium alloy based hydrogen storage tank, charged with Ti-doped sodium aluminium hexahydride Na₃AlH₆. This hydride has a theoretical hydrogen storage capacity of 3 mass-% and can be operated at lower pressure compared to sodium alanate NaAlH₄. The tank was made of aluminium alloy EN AW 6082 T6. The heat transfer was realised through an oil flow in a bayonet heat exchanger, manufactured by extrusion moulding from aluminium alloy EN AW 6060 T6. Na₃AlH₆ is prepared from 4 mol-% TiCl₃ doped sodium aluminium tetrahydride NaAlH₄ by addition of two moles of sodium hydride NaH in ball milling process. The hydrogen storage tank was filled with 213 g of doped Na₃AlH₆ in dehydrogenated state. Maximum of 3.6 g (1.7 mass-% of the hydride mass) of hydrogen was released from the hydride at approximately 450 K and the same hydrogen mass was consumed at 2.5 MPa hydrogenation pressure. 45 cycle tests (rehydrogenation and dehydrogenation) were carried out without any failure of the tank or its components. Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems.     

Effectiveness of a blower in reducing the hazard of hydrogen leaking from a hydrogen-fueled vehicle

To handle a hydrogen fuel cell vehicle (HFCV) safely after its involvement in an accident, it is necessary to provide appropriate emergency response information to the first responder. In the present study a forced wind of 10 m/s or faster with and without a duct was applied to a vehicle leaking hydrogen gas at a rate of 2000 NL/min. Then, hydrogen concentrations were measured around the vehicle and an ignition test was conducted to evaluate the effectiveness of forced winds and the safety of emergency response under forced wind conditions. The results: 1) Forced winds of 10 m/s or faster caused the hydrogen concentrations in the vicinity of the vehicle to decline to less than the lower flammability limit, and the hydrogen gas in the various sections of the vehicles were so diluted that even if ignition occurred the blast-wave pressure was moderate. 2) When the first responder had located the hydrogen leakage point in the vehicle, it was possible to lower the hydrogen concentrations around the vehicle by aiming the wind duct towards the leakage point and blowing winds at 10 m/s from the duct exit.     

Boosting cell performance and fuel utilization efficiency in a solar assisted methanol microfluidic fuel cell

Solar generated hydrogen from an optimized P25 thin film of 3.2 mg/cm² with 0.25% of platinum as co-catalyst improves the peak power output of a methanol microfluidic fuel cell operated with a methanol to water ratio of 1:1 almost ninefold, from 22 mW/cm² to 213 mW/cm². Different methanol to water ratios in the fuel tank generate similar amounts of hydrogen, but the cell performance has large variations due to the different oxidation kinetics of hydrogen and methanol in the fuel breathing anode, resulting in a mixed-potential anodic performance. The trade-off between power output and fuel utilization diminishes in this system. The methanol utilization efficiency at peak power operation increases from 50% (for 0.2 V) to 78% (for 0.5 V) for methanol to water ratio of 1:1. The result indicates that in-situ generation of hydrogen by solar light can be applied to both portable and large-scale stationary fuel cells.     

Review on the research of hydrogen storage system fast refueling in fuel cell vehicle

A comprehensive review of the hydrogen storage systems and investigations performed in search for development of fast refueling technology for fuel cell vehicles are presented. Nowadays, hydrogen is considered as a good and promising energy carrier and can be stored in gaseous, liquid or solid state. Among the three ways, high pressure (such as 35 MPa or 70 MPa) appears to be the most suitable method for transportation due to its technical simplicity, high reliability, high energy efficiency and affordability. However, the refueling of high pressure hydrogen can cause a rapid increase of inner temperature of the storage cylinder, which may result not only in a decrease of the state of charge (SOC) but also in damages to the tank walls and finally to safety problems. In this paper, the theoretical analysis, experiments and simulations on the factors related to the fast refueling, such as initial pressure, initial temperature, filling rate and ambient temperature, are reviewed and analyzed. Understanding the potential relationships between these parameters and the temperature rise may shed a light in developing novel controlling strategies and innovative routes for hydrogen tank fast filling.     

Characteristics of hydrogen leakage sound from a fuel-cell vehicle by hearing

Fuel-cell vehicle, run on hydrogen, is known that it has better energy efficiency than existing gasoline cars. The vehicles are designed so that hydrogen leaks from the tank are stopped automatically upon detection of hydrogen leakage or detection of impact in a collision. However, we investigated the characteristics of hydrogen leakage sound from a hydrogen-leaking vehicle and the threshold of discrimination of hydrogen leakage from noise at a crossing with much traffic to examine a method to rescue people safely depending on the sense of hearing in the event of a continuous hydrogen leak. Here, in the discrimination threshold test, we conducted the test by using helium, which is alternative gas of hydrogen leakage sound. We clarified that hydrogen leakage sound from vehicles has directivity, height dependence, and distance dependence. Furthermore, we confirmed the threshold flow rate for distinguishing hydrogen gas when hydrogen leakage is heard at a distance of 5–10 m from the center of the hydrogen leaking vehicle in a 74 dB traffic noise environment.     

Dehydrogenation kinetics of MgH2-based hydrogen storage tank at different operating temperatures and mass flow rates

The most efficient electrical production from fuel cells integrated with hydride-based hydrogen storage tanks is obtained from suitable dehydrogenation kinetics, facilitating constant flow rate of hydrogen supply for a long period of time. Dehydrogenation kinetics of hydride-based tanks depends strongly on operating temperatures and system pressures, relating to hydrogen mass flow rate (H₂-FR) released from the tank. Since a cylindrical tank (96.2 mL) mounted with central heat exchanger containing TiF₄-MWCNT-MgH₂ (～45 g) shows excellent de/rehydrogenation and reversibility (5.4 wt % H₂ upon 20 cycles), the effects of different operating conditions (T = 300–340 °C and H₂-FR = 0.6–1.0 standard L/min) on kinetic properties of this MgH₂-based tank are of significant interest. Kinetic properties either rapid or steady-state desorption can be optimized by tuning the operating temperature and H₂-FR simultaneously. Moreover, the performances in the axial and radial directions of the tank as well as kinetic behaviors and rate-limiting step during dehydrogenation at different temperatures and H₂-FR of MgH₂-based tank are investigated.