Quantitative risk assessment using a Japanese hydrogen refueling station model

Although hydrogen refueling stations (HRSs) are becoming widespread across Japan and are essential for the operation of fuel cell vehicles, they present potential hazards. A large number of accidents such as explosions or fires have been reported, rendering it necessary to conduct a number of qualitative and quantitative risk assessments for HRSs. Current safety codes and technical standards related to Japanese HRSs have been established based on the results of a qualitative risk assessment and quantitative effectiveness validation of safety measures over ten years ago. In the last decade, there has been much development in the technologies of the components or facilities used in domestic HRSs and much operational experience as well as knowledge to use hydrogen in HRSs safely have been gained through years of commercial operation. The purpose of the present study is to conduct a quantitative risk assessment (QRA) of the latest HRS model representing Japanese HRSs with the most current information and to identify the most significant scenarios that pose the greatest risks to the physical surroundings in the HRS model. The results of the QRA show that the risk contours of 10^?3 and 10^?4 per year were confined within the HRS boundaries, whereas the risk contours of 10^?5 and 10^?6 per year are still present outside the HRS. Comparing the breakdown of the individual risks (IRs) at the risk ranking points, we conclude that the risk of jet fire demonstrates the highest contribution to the risks at all of the risk ranking points and outside the station. To reduce these risks and confine the risk contour of 10^?6 per year within the HRS boundaries, it is necessary to consider risk mitigation measures for jet fires.     

Exact algorithms for incremental deployment of hydrogen refuelling stations

Given the large investments required to establish hydrogen refuelling stations (HRSs) and the difficulty in forecasting the sales of fuel cell electric vehicles, incremental HRS deployment offers an efficient method of establishing hydrogen infrastructure with a sufficient load factor and low financial risk. Considering that some HRSs are already in use, this study assumed that the optimal location of a new HRS maximises its distance from existing HRSs and minimises its distance from customer demand points. Accordingly, a multi-objective location model and efficient exact solution methods were proposed to determine the optimal location of one or two new HRSs. As a case study, the solution methods were applied to supply hydrogen to an increasing captive fleet of taxis in a large metropolis such as Paris with fixed demand points. The methods can be widely applied to effectively install one or two HRSs incrementally.     

Tube-trailer consolidation strategy for reducing hydrogen refueling station costs

The rollout of hydrogen fuel cell electric vehicles (FCEVs) requires the initial deployment of an adequate network of hydrogen refueling stations (HRSs). Such deployment has proven to be challenging because of the high initial capital investment, the risk associated with such an investment, and the underutilization of HRSs in early FCEV markets. Because the compression system at an HRS represents about half of the station's initial capital cost, novel concepts that would reduce the cost of compression are needed. Argonne National Laboratory with support from the U.S. Department of Energy's (DOE) Fuel Cell Technologies Office (FCTO) has evaluated the potential for delivering hydrogen in high-pressure tube-trailers as a way of reducing HRS compression and capital costs. This paper describes a consolidation strategy for a high-pressure (250-bar) tube-trailer capable of reducing the compression cost at an HRS by about 60% and the station's initial capital investment by about 40%. The consolidation of tube-trailers at pressures higher than 250 bar (e.g., 500 bar) can offer even greater HRS cost-reduction benefits. For a typical hourly fueling-demand profile and for a given compression capacity, consolidating hydrogen within the pressure vessels of a tube-trailer can triple the station's capacity for fueling FCEVs. The high-pressure tube-trailer consolidation concept could play a major role in enabling the early, widespread deployment of HRSs because it lowers the required HRS capital investment and distributes the investment risk among the market segments of hydrogen production, delivery, and refueling.     

Quantitative risk assessment of a hydrogen refueling station by using a dynamic physical model based on multi-physics system-level modeling

Numerous accidents in HRSs have been reported worldwide in accident databases; therefore, many researchers have performed quantitative risk assessments (QRAs) of HRSs to enable risk-informed decision making in determining the safety distances or risk mitigation measures. The HRSs, located in urban areas such as Tokyo in Japan, are situated in congested areas with tall buildings and high population density; thus, they have relatively narrow station areas. However, the QRAs are generally suitable for large plants such as nuclear power plants or chemical plants; therefore, relatively small plants or installations, such as HRSs, have not yet been considered as QRA objects. Hence, it is necessary to conduct detailed QRAs with risk analyses and reduce the applied uncertainties for relatively small plants or installations. We applied a model-based approach of risk assessment to model the HRS process using multi-physics system-level modeling and simulated a target system using Modelica—an equation-based, object-oriented modeling language that allows acausal modeling of complex cyber-physical systems The primary aim of this study was to conduct a QRA of an HRS based on multi-physics system-level modeling. First, we modeled the HRS components and physical relationships between the components using basic physical equations. Then, we elucidate a QRA based on the constructed model. The difference in the leakage rates due to the leak positions and dynamic behavior of the model parameters were calculated using the constructed model. Finally, we estimated the individual risks of all the scenarios and compared the resulting risk contours based on the constructed model that includes the hydrogen-fuel dynamic behavior with those based on the traditional model. These results indicate that it is possible to assess whether the risks around the station boundary are acceptable based on the scenario information obtained by evaluating the risks near the station.     

Does risk information change the acceptance of hydrogen refueling stations in the general Japanese population?

Hydrogen refueling stations (HRSs) are inevitable infrastructure for the utility of fuel cell vehicles, but they can raise people's safety concerns. We analyzed whether information on the risk/safety measures changed people's acceptance of HRSs. Respondents were provided those information and asked to rate their acceptance of an HRS placement either beside their home or at the gas station closest to their home. The respondents' perception of the risk of HRSs and their attitudes on environmental issues are analyzed by factor analyses. The results show that provision of the quantitative risk information and risk acceptance criteria increased the acceptability of HRS in proximity to the homes of respondents (P < 0.1) but decreased the acceptability of HRS at the nearest gas station. Factor analyses suggest that risk information on HRS alleviates the respondents' feelings of dread or uncertainty, leading to better acceptance. Our study should promote improved risk communication prior to HRS installation.     

Hydrogen refuelling stations in the Netherlands: An intercomparison of quantitative risk assessments used for permitting

As of 2003, 15 hydrogen refuelling stations (HRSs) have been deployed in the Netherlands. To become established, the HRS has to go through a permitting procedure. An important document of the permitting dossier is the quantitative risk assessment (QRA) as it assesses the risks of the HRS associated to people and buildings in the vicinity of the HRS. In the Netherlands, a generic prescribed approach exists on how to perform a QRA, however specific guidelines for HRSs do not exist. An intercomparison among the QRAs of permitted HRSs has revealed significant inconsistencies on various aspects of the QRA: namely the inclusion of HRS sub-systems and components, the HRS sub-system and component considerations as predefined components, the application of failure scenarios, the determination of failure frequencies, the application of input parameters, the consideration of preventive and mitigation measures as well as information provided regarding the HRS surroundings and the societal risk. It is therefore recommended to develop specific QRA guidelines for HRSs.     

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.     

Construction of a structural equation model to identify public acceptance factors for hydrogen refueling stations under the provision of risk and safety information

Hydrogen refueling stations (HRSs) are an inevitable infrastructure for the utility of fuel cell vehicles; however, they can raise public safety concerns. The aim of this study is to establish a framework for public acceptance of HRSs in Japan upon the provision of risk and/or safety measure information on HRSs. We executed an in-person interview survey asking the respondents about their acceptance of HRSs and then constructed a structural equation model on HRS acceptance with four endogenous factors. The common factors to determine acceptability were “Dread” and “Independent”. “Balance” was added to the factors for the risk-informed group. If risk information was provided, people tended to judge based on their inherent sense of “Balance”; however, if it was not provided, their judgment was based on their intuitive “Dread” of HRSs or hydrogen. This study reveals risk perception characteristics and attempts to promote improved risk communication prior to HRS installation.     

Performance of a hydrogen refueling station in the early years of commercial fuel cell vehicle deployment

Since 2003, the National Fuel Cell Research Center at the University of California, Irvine (UCI) has operated the first U.S. publicly accessible hydrogen refueling station (HRS). During this period, the UCI HRS supported all manufacturers in the early, pre-commercialization years of the fuel cell electric vehicle (FCEV). This paper describes and analyzes the performance of the UCI HRS during the first five years of FCEV commercialization, over which time the station has dispensed the most hydrogen daily in the California network. The station performance is compared to aggregate data published by NREL for all U.S. HRSs. Using the Hydrogen Delivery Scenario Analysis Model, typical daily refueling profiles are analyzed to determine the effect on HRS design. The results show different daily refueling profiles could substantially affect HRS design and ultimately the cost of hydrogen. While technical issues have been reduced, the compressor, dispenser, and fueling rate are areas for improvement.     

Improved safety by crossanalyzing quantitative risk assessment of hydrogen refueling stations

With the goal of building 310 hydrogen refueling stations (HRSs) in Korea by 2022, restrictions, such as location restrictions and separation distances, are being eased, so developing ways to improve technology and safety. As HRSs contain major facilities such as compressors, storage tanks, dispenser, and priority control panels, and a leakage could result in a large fire or explosion caused by an ignition source. To perform quantitative risk assessment, programs, namely, Hy-KoRAM and Phast/Safeti were used in this study. It could determine the damage range and effect on radiant heat and flame length, as well as personal and societal risks, using these programs. The crossanalysis of the two programs also improves the facility's safety and the reliability of the results.     

The impact of hydrogen refueling station subsidy strategy on China's hydrogen fuel cell vehicle market diffusion

Lack of hydrogen refueling stations (HRSs) has hindered the diffusion of hydrogen fuel cell vehicles (HFCVs) in the Chinese transport market. By combining the agent-based model (ABM) and the experience weighted attraction (EWA) learning algorithm, this paper explores the impact of government subsidy strategy for HRSs on the market diffusion of HFCVs. The actions of the parties (government, HRS planning department and consumers) and their interactions are taken into account. The new model suggests dynamic subsidy mode based on EWA algorithm yields better results than static subsidy mode: HFCV purchases, HRS construction effort, total number of HRSs and expected HRS planning department profits all outperform static data by around 27%. In addition, choosing an appropriate initial subsidy strategy can increase the sales of HFCVs by nearly 40%. Early investment from government to establish initial HRSs can also increase market diffusion efficiency by more than 76.7%.     

Quantitative risk analysis of life safety and financial loss for road accident of fuel cell vehicle

Vehicular use of hydrogen is the first attempt to apply hydrogen energy in consumers’ environment in large scale and has raised safety concerns in both public authorities and private bodies such as fire services and insurance companies. This paper analyzes typical accident progressions of hydrogen fuel cell vehicles in a road collision accident. Major hydrogen consequences including impinging jet fires and catastrophic tank ruptures are evaluated separately in terms of accident duration and hazard distances. Results show that in a 70 MPa fuel cell car accident, the hazards associated with hydrogen releases would normally last for no more than 1.5 min due to the empty of the tank. For the safety of general public, a perimeter of 100 m is suggested in the accident scene if no hissing sound is heard. However, the perimeter can be reduced to 10 m once the hissing sound of hydrogen release is heard. Furthermore, risks of fatalities, injuries, and damages are all quantified in financial terms to assess the impacts of the accident. Results show that costs of fatalities and injuries contribute most to the overall financial loss, indicating that the insurance premium of fatalities and injuries should be set higher than that of property loss.     

Evaluating uncertainty in accident rate estimation at hydrogen refueling station using time correlation model

Hydrogen, as a future energy carrier, is receiving a significant amount of attention in Japan. From the viewpoint of safety, risk evaluation is required in order to increase the number of hydrogen refueling stations (HRSs) implemented in Japan. Collecting data about accidents in the past will provide a hint to understand the trend in the possibility of accidents occurrence by identifying its operation time However, in new technology; accident rate estimation can have a high degree of uncertainty due to absence of major accident direct data in the late operational period. The uncertainty in the estimation is proportional to the data unavailability, which increases over long operation period due to decrease in number of stations. In this paper, a suitable time correlation model is adopted in the estimation to reflect lack (due to the limited operation period of HRS) or abundance of accident data, which is not well supported by conventional approaches. The model adopted in this paper shows that the uncertainty in the estimation increases when the operation time is long owing to the decreasing data.     

Qualitative risk analysis of the overhead hydrogen piping at the conceptual process design stage

Namie, which is a town in Fukushima prefecture in Japan, has started a green innovation project using hydrogen to eliminate CO₂ release by 2050 and establish a zero-carbon city. One idea is to develop an overhead hydrogen piping, which is a low-pressure hydrogen supply system, in public spaces. To develop and implement the piping in Namie and other areas, it is important to both develop the process and perform the risk analysis during a preliminary process design stage. The purpose of this study was to qualitatively analyze and identify the critical risks and effective safety measures for the piping. A total of 183 accidental scenarios were identified by using a hazard identification study, and the preventive and mitigation safety measures were summarized. Hydrogen dispersion simulations revealed that low-pressure hydrogen dispersion leaking from small holes and fractured piping in public spaces could result in concerning risks depending on the leak size, direction, position, obstacles, and ignition source. In addition, a shutdown system of the piping was evaluated with a multiphysics modeling simulation, and its importance was discussed to proceed with detailed investigations. Finally, we suggested risk reduction measures based on the concept of inherent safety for the development of low-pressure hydrogen systems, and the findings can be applied to cases such as Namie.     

Two-tier pressure consolidation operation method for hydrogen refueling station cost reduction

An operation strategy known as two-tier “pressure consolidation” of delivered tube-trailers (or equivalent supply storage) has been developed to maximize the throughput at gaseous hydrogen refueling stations (HRSs) for fuel cell electric vehicles (FCEVs). The high capital costs of HRSs and the consequent high investment risk are deterring growth of the infrastructure needed to promote the deployment of FCEVs. Stations supplied by gaseous hydrogen will be necessary for FCEV deployment in both the near and long term. The two-tier pressure consolidation method enhances gaseous HRSs in the following ways: (1) reduces the capital cost compared with conventional stations, as well as those operating according to the original pressure consolidation approach described by Elgowainy et al. (2014) [1], (2) minimizes pressure cycling of HRS supply storage relative to the original pressure consolidation approach; and (3) increases use of the station's supply storage (or delivered tube-trailers) while maintaining higher state-of-charge vehicle fills.     

Effectiveness of small barriers as means to reduce clearance distances

Hydrogen clearance or safety distance can be defined as the minimum distance between a hydrogen leak source and surrounding equipment, property or personnel beyond which the risk to the said recipients associated with existing hydrogen hazards is deemed acceptable. The same principal is applied to determine clearances to ignition sources and air intakes only the criteria in this case are the risk of ignition or the risk of intaking a flammable mixture. The study of effects of small barriers as means to reduce clearance distances for compressed hydrogen releases is important for the development of installation codes and risk mitigation requirements. In this paper, computational fluid dynamics (CFD) modeling techniques were applied to the numerical simulation of the effects of a protective wall of 1 m by 1 m on reducing the size of hydrogen cloud. The protective wall was 1 m away from a 70 MPa (700 bar) 60 L tank, from which an incidental hydrogen release impinged horizontally onto the wall, causing a complicated 3D dispersion of hydrogen cloud. In-house CFD codes first accurately estimated the non-linear hydrogen mass release rate decreasing with time. Then the effects of the wall on the propagation speed of the hydrogen cloud moving behind the wall were investigated using the PHOENICS software package, provided with both the ideal gas law and the real gas law expressed by the Abel-Nobel equation of state (AN-EOS). The distributions of lower flammability limit (LFL) and 50% of LFL hydrogen clouds were described in detail based on the numerical results. It was found that the 50% of LFL hydrogen clouds (2% vol) could propagate behind the wall in less than 0.2 s after the onset of the release. The horizontal extents corresponding to 50% of LFL hydrogen cloud on the central vertical plane are 9.6 m at 5 s when they are predicted using the ideal gas law. When using the real gas law, the predicted extents decrease to 6.3 m at 5 s. The ideal gas law significantly overestimates the hypothetical hydrogen cloud volumes for LFL, or fractions of LFL, for different release times at the current initial stagnation pressure level (700 bar). The current model codes and standards generally specify clearance distances for hydrogen based on the regulators’ experience in other flammable gases, like natural gas or propane, rather than on real hydrogen gas properties that particularly deviate from ideal gas law under high pressure. On the other hand, it is relatively more conservative to exploit the ideal gas law to predict the combustible hydrogen cloud extents than using the real gas law for industrial applications. The numerical results from the impingement release also confirm that a small protective wall, or a barrier, can reduce the hydrogen concentration behind the wall. The numerical results can be further applied for defining the zoning requirements for Canadian Electrical Code and clearance distances for Canadian Hydrogen Installation Code.     

A study of barrier walls for mitigation of unintended releases of hydrogen

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. An experimental and modeling program has been performed at Sandia National Laboratories to better characterize the effectiveness of barrier walls to reduce hazards. This paper describes the experimental and modeling program and presents results obtained for various barrier configurations. The experimental measurements include flame deflection using standard and infrared video and high-speed movies (500 fps) to study initial flame propagation from the ignition source. Measurements of the ignition overpressure, wall deflection, radiative heat flux, and wall and gas temperature were also made at strategic locations. The modeling effort includes three-dimensional calculations of jet flame deflection by the barriers, computations of the thermal radiation field around barriers, predicted overpressure from ignition, and the computation of the concentration field from deflected unignited hydrogen releases. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting jet flames in a desired direction and can help attenuate the effects of ignition overpressure and flame radiative heat flux.     

Dynamic planning and energy management strategy of integrated charging and hydrogen refueling at highway energy supply stations considering on-site green hydrogen production

The layout of electric vehicles charging stations and hydrogen refueling stations (HRSs) is more and more necessary with the development of electric vehicles (EVs) and progress in hydrogen energy storage technology. Due to the high costs of HRSs and the low demand for hydrogen, it is difficult for independent HRSs to make a profit. This study focuses on the dynamic planning of energy supply stations on highways in the medium and long term, considering the growth of EV charging demand and the change in the proportion of hydrogen fuel cell vehicles (HFCVs). Based on the perspective of renewable energy generators (REGs), this study seeks the dynamic optimal configuration and comprehensive benefits of adding HRS and battery to existing EVCS considering the travel rules of new energy vehicles (NEVs). The results show that (1) It is profitable for REGs to invest in HRSs; (2) The economy of investment in batteries by REGs depends on the source-load matching. It is feasible only when the output of renewable energy is difficult to meet the demand. (3) The business model of REGs producing hydrogen on-site and supplying both electricity and hydrogen is feasible.     

Analysis of transient supersonic hydrogen release,dispersion and combustion

A hydrogen leak from a facility, which uses highly compressed hydrogen gas (714 bar, 800 K) during operation was studied. The investigated scenario involves supersonic hydrogen release from a 10 cm² leak of the pressurized reservoir, turbulent hydrogen dispersion in the facility room, followed by an accidental ignition and burn-out of the resulting H₂-air cloud. The objective is to investigate the maximum possible flame velocity and overpressure in the facility room in case of a worst-case ignition. The pressure loads are needed for the structural analysis of the building wall response. The first two phases, namely unsteady supersonic release and subsequent turbulent hydrogen dispersion are simulated with GASFLOW-MPI. This is a well validated parallel, all-speed CFD code which solves the compressible Navier-Stokes equations and can model a broad range of flow Mach numbers. Details of the shock structures are resolved for the under-expanded supersonic jet and the sonic-subsonic transition in the release. The turbulent dispersion phase is simulated by LES. The evolution of the highly transient burnable H₂-air mixture in the room in terms of burnable mass, volume, and average H₂-concentration is evaluated with special sub-routines. For five different points in time the maximum turbulent flame speed and resulting overpressures are computed, using four published turbulent burning velocity correlations. The largest turbulent flame speed and overpressure is predicted for an early ignition event resulting in 35–71 m/s, and 0.13–0.27 bar, respectively.