Numerical investigation of the leakage and explosion scenarios in China's first liquid hydrogen refueling station

Promoting fuel cells has been one of China's ambitious hydrogen policies in the past few years. Currently, several hydrogen fueling stations (HRSs) are under construction in China to fuel hydrogen-driven vehicles. In this regard, it is necessary to assess the risks of hydrogen leakage in HRSs. Aiming at conducting a comprehensive consequence assessment of liquid hydrogen (LH2) leakage on China's first liquid hydrogen refueling station (LHRS) in Pinghu, a pseudo-source model is established in the present study to simulate the LH2 leakage using a commercial CFD tool, FLACS. The effects of the layout of the LHRS, leakage parameters, and local meteorological conditions on the LH2 leakage consequence has been assessed from the perspectives of low-temperature hazards and explosion hazards. The obtained results reveal that considering the prevailing southeast wind in Pinghu city, the farthest low-temperature hazard distance and lower flammable limit (LFL) -distance occurs in the leakage scenario along the north direction. It is found that the trailer parking location in the current layout of the LHRS will worsen the explosion consequences of the LH2 leakage. Moreover, the explosion will completely destroy the control room and endanger people on the adjacent road when the leakage equivalent diameter is 25.4 mm. The performed analyses reveal that as the wind speed increases, the explosion hazard decreases.     

Two-dimensional thermal analysis of liquid hydrogen tank insulation

Liquid hydrogen (LH₂) storage has the advantage of high volumetric energy density, while boil-off losses constitute a major disadvantage. To minimize the losses, complicated insulation techniques are necessary. In general, Multi Layer Insulation (MLI) and a Vapor-Cooled Shield (VCS) are used together in LH₂ tanks. In the design of an LH₂ tank with VCS, the main goal is to find the optimum location for the VCS in order to minimize heat leakage. In this study, a 2D thermal model is developed by considering the temperature dependencies of the thermal conductivity and heat capacity of hydrogen gas. The developed model is used to analyze the effects of model considerations on heat leakage predictions. Furthermore, heat leakage in insulation of LH₂ tanks with single and double VCS is analyzed for an automobile application, and the optimum locations of the VCS for minimization of heat leakage are determined for both cases.     

Assessment of offshore liquid hydrogen production from wind power for ship refueling

Green hydrogen from electrolysis has become the most attractive energy carrier for making the transition from fossil fuels to carbon-free energy sources possible. Especially in the naval sector, hydrogen has the potential to address environmental targets due to the lack of low-carbon fuel options. This study aims at investigating an offshore liquefied green hydrogen production plant for ship refueling. The plant comprises a wind farm for renewable electricity generation, an electrolyzer stack for hydrogen production, a water treatment unit for demineralized water production, and a hydrogen liquefaction plant for hydrogen storage and distribution to ships. A pre-feasibility study is addressed to find the optimal capacities of the plant that minimize the payback time. The model results show that the electrolyzer capacity shall be set equal to a value between 80% and 90% of the wind farm capacity to achieve the minimum payback times. Additionally, the wind farm capacity shall be higher than about 150 MW to limit the payback time to values lower than 11 years for a fixed hydrogen price of 6 €/kg. The Levelized Cost of Hydrogen results to be below 4 €/kg for a wide range of plant capacities for a lifetime of the plant of 25 years. Thus, the model shows that this plant is economically feasible and can be reproduced similarly for different locations by rescaling the different selected technologies. In this way, the naval sector can be decarbonized thanks to a new infrastructure for the production and refueling of liquified green hydrogen directly provided on the sea.     

Energetic evaluation of hydrogen refueling stations with liquid or gaseous stored hydrogen

The future success of fuel cell electric vehicles requires a corresponding infrastructure. In this study, two different refueling station concepts for fuel cell passenger cars with 70 MPa technology were evaluated energetically. In the first option, the input of the refueling station is gaseous hydrogen which is compressed to final pressure, remaining in gaseous state. In the second option, the input is liquid hydrogen which is cryo-compressed directly from the liquid phase to the target pressure. In the first case, the target temperature of −33 °C to −40 °C [1] is achieved by cooling down. In the second option, gaseous deep-cold hydrogen coming from the pump is heated up to target temperature. A dynamic simulation model considering real gas behavior to evaluate both types of fueling stations from an energetic perspective was created. The dynamic model allows the simulation of boil-off losses (liquid stations) and standby energy losses caused by the precooling system (gaseous station) dependent on fueling profiles. The functionality of the model was demonstrated with a sequence of three refueling processes within a short time period (high station utilization). The liquid station consumed 0.37 kWh/kg compared to 2.43 kWh/kg of the gaseous station. Rough estimations indicated that the energy consumption of the entire pathway is higher for liquid hydrogen. The analysis showed the high influence of the high-pressure storage system design on the energy consumption of the station. For future research work the refueling station model can be applied to analyze the energy consumption dependent on factors like utilization, component sizing and ambient temperature.     

Improving hydrogen refueling stations to achieve minimum refueling costs for small bus fleets

Fuel cell vehicles using green hydrogen as fuel can contribute to the mitigation of climate change. The increasing utilization of those vehicles creates the need for cost efficient hydrogen refueling stations. This study investigates how to build the most cost efficient refueling stations to fuel small fleet sizes of 2, 4, 8, 16 and 32 fuel cell busses. A detailed physical model of a hydrogen refueling station was built to determine the necessary hydrogen storage size as well as energy demand for compression and precooling of hydrogen. These results are used to determine the refueling costs for different station configurations that vary the number of storage banks, their volume and compressor capacity.It was found that increasing the number of storage banks will decrease the necessary total station storage volume as well as energy demand for compression and precooling. However, the benefit of adding storage banks decreases with each additional bank. Hence the cost for piping and instrumentation to add banks starts to outweigh the benefits when too many banks are used. Investigating the influence of the compressor mass flow found that when fueling fleets of 2 or 4 busses the lowest cost can be reached by using a compressor with the minimal mass flow necessary to refill all storage banks within 24 h. For fleets of 8, 16 and 32 busses, using the compressor with the maximum investigated mass flow of 54 kg/h leads to the lowest costs.     

Dynamic simulation for optimal hydrogen refueling method to Fuel Cell Vehicle tanks

A dynamic simulation approach to investigate an optimal hydrogen refueling method is proposed. The proposed approach simulates a transient temperature, pressure and mass flow rate of hydrogen flowing inside filling equipment in an actual station during the refueling process to an Fuel Cell Vehicle (FCV) tank. The simulation model is the same as in an actual hydrogen refueling station (HRS), and consists of a Break-Away, a hose, a nozzle, pipes and an FCV tank. Therefore, we can set actual configurations and thermal properties to the simulation model, and then simulate the temperature, pressure and mass flow rate of hydrogen passing through each position based on the supply conditions (temperature and pressure) at the Break-Away. In this study, the simulated temperature, pressure and mass flow rate are compared with the corresponding experimental data. Therefore, we show that the dynamic simulation approach can accurately obtain those values at each position during the refueling process and is an effective step in proposing the optimal refueling method.     

Liquid pump-enabled hydrogen refueling system for medium and heavy duty fuel cell vehicles: Station design and technoeconomic assessment

Recent progress in submerged liquid hydrogen (LH₂) cryopump technology development offers improved hydrogen fueling performance at a reduced cost in medium- and heavy-duty (MDV and HDV) fuel cell vehicle refueling applications at 35 MPa pressure, compared to fueling via gas compression. In this paper, we evaluate the fueling cost associated with cryopump-based refueling stations for different MDV and HDV hydrogen demand profiles. We adapt the Heavy Duty Refueling Station Analysis Model (HDRSAM) tool to analyze the submerged cryopump case, and compare the estimated fuel dispensing costs of stations supplied with LH₂ for fueling Class 4 delivery van (MDV), public transit bus (HDV), and Class 8 truck (HDV) fleets using cryopumps relative to station designs. A sensitivity analysis around upstream costs illustrates the trade-offs associated with H₂ production from onsite electrolysis versus central LH₂ production and delivery. Our results indicate that LH₂ cryopump-based stations become more economically attractive as the total station capacity (kg dispensed per day) and hourly demand (vehicles per hour) increase. Depending on the use case, savings relative to next best options range from about 5% up to 44% in dispensed costs, with more favorable economics at larger stations with high utilization.     

Multi-hub hydrogen refueling station with on-site and centralized production

In recent decades, the consequences of climate changes due to greenhouse gas (GHG) emissions have become ever more impactful, forcing international authorities to find green solutions for sustainable economic development. In this regard, one of the global targets is the reduction of fossil fuels utilization in the transport sector to encourage the diffusion of more environmentally friendly alternatives. Among them, hydrogen is emerging as a viable candidate since it is a potentially emission-free fuel when produced by exploiting renewable energy sources (RES). Nevertheless, to allow widespread use of this gas in the transport sector, several technoeconomic barriers, including production cost, and lack of distribution and storage infrastructure, have to be overcome. Distributed hydrogen production via renewable energy-powered electrolysis could be an effective solution to reduce cost and lead to economies of scale. In this study a multi-hub configuration with on-site production from PV-powered electrolysis and centralized production from steam methane reforming (SMR) is proposed. In particular, an infrastructure network for a bus refueling station located in Lazio is considered as a case study. First, each hub, composed of PV panels, an electrolyzer, a compression system, high-pressure and low-pressure storages, and hydrogen dispensers with chiller, is modeled in a Matlab/Simulink environment. Then, a design perturbation analysis is carried out to determine the impact of the configuration on the refueling station performance in terms of carbon emissions levels and the Levelized Cost of hydrogen (LCOH). The results show a significant influence of the station size on the economic performance highlighting significant benefits (reduction up to 40% in the LCOH) for a 80 bus HUB with a saturating trend towards larger sizes. CO₂ emissions per unit mass of hydrogen are kept limited for all the stations thanks to the synergistic effects of SMR and Electrolyzer. Interconnecting more than one station each other further benefits can be achieved from the environmental perspective (savings up to 5 tons of CO₂ are demonstrated for a typical summer case study).     

Temperature rise of hydrogen storage cylinders by thermal radiation from fire at hydrogen-gasoline hybrid refueling stations

This study focuses on two types of hydrogen-gasoline hybrid refueling stations, and a risk assessment study on thermal radiation is carried out with a fire at each hybrid station. One of the hybrid stations has bare hydrogen storage cylinders, and the other has container walls around the cylinders. We calculate radiative flux to the cylinders from the fire occurring at the gasoline refueling machines in each hybrid station. Additionally, we calculate the temperature rise of the cylinders based on the obtained radiative flux. To evaluate a dangerous case for hybrid stations, we calculate the radiative flux and temperature rise using a large scale and high temperature fire. Based on our analysis, we find that the container walls can greatly insulate the radiative flux. Therefore, we show that we are able to keep the temperature of the cylinders below the hazardous temperature of 358 K by installing container walls around them.     

Optimal operation of a photovoltaic generation-powered hydrogen production system at a hydrogen refueling station

As the popularity of fuel cell vehicles continues to rise in the global market, production and supply of low-carbon hydrogen are important to mitigate CO₂ emissions. We propose a design for a hydrogen refueling station with a proton exchange membrane electrolyzer (PEM-EL)-based electrolysis system (EL-System) and photovoltaic generation (PV) to supply low-carbon hydrogen. Hydrogen is produced by the EL-System using electricity from PV and the power grid. The system was formulated as a mixed integer linear programming (MILP) model to allow analysis of optimal operational strategies. Case studies with different objective functions, CO₂ emission targets, and capacity utilization of the EL-System were evaluated. Efficiency characteristics of the EL-System were obtained through measurements. The optimized operational strategies were evaluated with reference to three evaluation indices: CO₂ emissions, capacity utilization, and operational cost of the system. The results were as follows: 1) Regardless of the objective function, the EL-System generally operated in highest efficiency state, and optimal operation depended on the efficiency characteristics of the EL-System; 2) mitigation of CO₂ emissions and increase in capacity utilization of the EL-System required trade-offs; and 3) increased capacity utilization of the EL-System showed two opposing effects on hydrogen retail price.     

Thermodynamic modeling of hydrogen refueling for heavy-duty fuel cell buses and comparison with aggregated real data

The foreseen uptake of hydrogen mobility is a fundamental step towards the decarbonization of the transport sector. Under such premises, both refueling infrastructure and vehicles should be deployed together with improved refueling protocols. Several studies focus on refueling the light-duty vehicles with 10 kg_H2 up to 700 bar, however less known effort is reported for refueling heavy-duty vehicles with 30–40 kg_H2 at 350 bar. The present study illustrates the application of a lumped model to a fuel cell bus tank-to-tank refueling event, tailored upon the real data acquired in the 3Emotion Project. The evolution of the main refueling quantities, such as pressure, temperature, and mass flow, are predicted dynamically throughout the refueling process, as a function of the operating parameters, within the safety limits imposed by SAE J2601/2 technical standard. The results show to refuel the vehicle tank from half to full capacity with an Average Pressure Ramp Rate (APRR) equal to 0.03 MPa/s are needed about 10 min. Furthermore, it is found that the effect of varying the initial vehicle tank pressure is more significant than changing the ambient temperature on the refueling performances. In conclusion, the analysis of the effect of different APRR, from 0.03 to 0.1 MPa/s, indicate that is possible to safely reduce the duration of half-to-full refueling by 62% increasing the APRR value from 0.03 to 0.08 MPa/s.     

Development of strategic hydrogen refueling station deployment plan for Korea

The Republic of Korea government has set yearly targets of hydrogen cars and buses and plans to install hydrogen refueling stations nationwide. This paper proposes a methodology for developing a strategic deployment plan with three mathematical models. For a given target, future refueling demand locations and amount from general road and expressway are systematically estimated. First, the required number of refueling stations to satisfy the target covering ratio of the total demand set by the government is determined by the Station number determination model. Next, the locations of the capacitated stations and the allocation of demand to the stations are determined by the second Max cover and the third p-median models. Since the max covering is more important than minimizing the travel time, the two models are used sequentially. The nationwide hydrogen station deployment plan for the years 2022–2040 obtained by the proposed methodology is reported.     

Fluid thermal stratification in a non-isothermal liquid hydrogen tank under sloshing excitation

To the safe space operation of cryogenic storage tank, it is significant to study fluid thermal stratification under external heat leaks. In the present paper, a numerical model is established to investigate the thermal performance in a cryogenic liquid hydrogen tank under sloshing excitation. The interface phase change and the external convection heat transfer are considered. To realize fluid sloshing, the dynamic mesh coupled the volume of fluid (VOF) method is used to predict the interface fluctuations. A sinusoidal excitation is implemented via customized user-defined function (UDF) and applied on tank wall. The grid sensitivity study and the experimental validation of the numerical mode are made. It turns out that the present numerical model can be used to simulate the unsteady process in a non-isothermal sloshing tank. Variations of tank pressure, liquid and vapor mass, fluid temperature and thermal stratification are numerically investigated respectively. The results show that the sinusoidal excitation has caused large influence on thermal performance in liquid hydrogen tank. Some valuable conclusions are arrived, which is important to the depth understanding of the non-isothermal performance in a sloshing liquid hydrogen tank and may supply some technique reference for the methods of sloshing suppression.     

Study on thermal stratification in liquid hydrogen tank under different gravity levels

In the present study, one CFD model is selected to research the effect of gravity scale on the thermal performance in liquid hydrogen tank. Four gravity levels (1g₀, 10^?1g₀, 10^?2g₀ and 10^?3g₀) are compared to recognize the influence of the reduced gravity on fluid thermal stratification. The results show that with the increasing of the gravity level, the vapor temperature distribution becomes more uniform, and the liquid stratum layer develops faster. Compared the CFD results with the results of two stratification theoretical models, the stratum thickness calculated by CFD model is close to the values of Tellep model. While the stratum temperature of CFD model is much closer to that of Reynolds model. With vortex occurring among two slosh baffles, the streamline in the liquid stratum likes a plume. Influenced by the surface tension in reduced gravity, liquid close to the tank wall moves up with the interface becoming curved. The interface area rises with the decrease of gravity. The gravity of 10^?1g₀ still plays the dominant role with the interface area of 10^?1g₀ being almost the same as that of 1g₀. While for others, the effect of the surface tension shows up gradually.     

A study of hydrogen leak and explosion in different regions of a hydrogen refueling station

The consequences of hydrogen leaks and explosions are predicted for the sake of the safety in hydrogen refueling stations. In this paper, the effect of wind speed on hydrogen leak and diffusion is analyzed in different regions of a hydrogen refueling station, and the influence of delayed ignition time on hydrogen explosion after an accidental hydrogen leak is further studied by numerical simulation. Results show that the effect of wind speed on the probability of hydrogen fires is distinctive in different regions of hydrogen refueling station. The size of combustible clouds in the trailer front region and the outer region increases in the low wind speed case, and the front of combustible clouds is formed in a spherical shape in the outer region, which can greatly increase the probability of hydrogen explosion. However, the high wind speed may cause an increase of the risk of accidents in the near ground region. Moreover, a non-linear correlation is shown between the rate of combustible cloud dissipation and wind speed after the hydrogen stops leaking. In addition, it is found that an increase in delayed ignition time may lead to an increase in explosion intensity, which is related with the larger high temperature area and stronger explosion overpressure. Two flame fronts and the reverse propagation of the explosion overpressure can be observed, when the delayed ignition time is larger.     

Numerical simulation of hydrogen leakage diffusion in seaport hydrogen refueling station

Ningbo's seaport hydrogen refueling station was used as the research object. The effects of different leakage angles, wind direction, roof shape, leakage hole diameters, temperature, and humidity on the diffusion of hydrogen leakage were studied by numerical simulation. The influence of leakage angle on hydrogen leakage is mainly reflected in the presence or absence of obstacles. The volume of the flammable hydrogen cloud was reduced by 31.16%, and the volume of the hazardous hydrogen cloud was reduced by 63.22% when there was no obstacle. The wind direction can significantly impact hydrogen leakage, with downwind and sidewind accelerating hydrogen discharge and reducing the risk. At the same time, headwind significantly increases the volume of the flammable hydrogen cloud. Compared with no wind, the volume of the flammable hydrogen cloud increased by 71.73% when headwind, but the volume of the hazardous hydrogen cloud decreased by 24.00%. If hydrogen shows signs of accumulation under the roof, the sloping roof can effectively reduce the hydrogen concentration under the roof and accelerate the hydrogen discharge. When the leakage angle θ = 90°, the sloping roof reduced the volume of the flammable hydrogen cloud by 11.74%. The leakage process was similar for different leak hole diameters in the no wind condition. The inverse of the molar fraction of hydrogen on the jet centerline was linearly related to the dimensionless axial distance of the jet in different cases. Using a least squares fit, the decay rate was obtained as 0.0039. In contrast, temperature and humidity have almost no effect on hydrogen diffusion. Hydrogen tends to accumulate on the lower surface of the roof, near the roof pillars and the hydrogen dispenser. In this paper, a set of hydrogen detector layout schemes was developed, and the alarm success rate was verified to be 83.33%.     

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.     

Impact of hydrogen refueling configurations and market parameters on the refueling cost of hydrogen

The cost of hydrogen in early fuel cell electric vehicle (FCEV) markets is dominated by the cost of refueling stations, mainly due to the high cost of refueling equipment, small station capacities, lack of economies of scale, and low utilization of the installed refueling capacity. Using the hydrogen delivery scenario analysis model (HDSAM), this study estimates the impacts of these factors on the refueling cost for different refueling technologies and configurations, and quantifies the potential reduction in future hydrogen refueling cost compared to today's cost in the United States. The current hydrogen refueling station levelized cost, for a 200 kg/day dispensing capacity, is in the range of $6–$8/kg H₂ when supplied with gaseous hydrogen, and $8–$9/kg H₂ for stations supplied with liquid hydrogen. After adding the cost of hydrogen production, packaging, and transportation to the station's levelized cost, the current cost of hydrogen at dispensers for FCEVs in California is in the range of $13–$15/kg H₂. The refueling station capacity utilization strongly influences the hydrogen refueling cost. The underutilization of station capacity in early FCEV markets, such as in California, results in a levelized station cost that is approximately 40% higher than it would be in a scenario where the station had been fully utilized since it began operating. In future mature hydrogen FCEV markets, with a large demand for hydrogen, the refueling station's levelized cost can be reduced to $2/kg H₂ as a result of improved capacity utilization and reduced equipment cost via learning and economies of scale.     

Comparative risk assessment of liquefied and gaseous hydrogen refueling stations

Several countries are incentivizing the use of hydrogen (H₂) fuel cell vehicles, thereby increasing the number of H₂ refueling stations (HRSs), particularly in urban areas with high population density and heavy traffic. Therefore, it is necessary to assess the risks of gaseous H₂ refueling stations (GHRSs) and liquefied H₂ refueling stations (LHRSs). This study aimed to perform a quantitative risk assessment (QRA) of GHRSs and LHRSs. A comparative study is performed to enhance the decision-making of engineers in setting safety goals and defining design options. A systematic QRA approach is proposed to estimate the likelihood and consequences of hazardous events occurring at HRSs. Consequence analysis results indicate that catastrophic ruptures of tube trailer and liquid hydrogen storage tanks are the worst accidents, as they cause fires and explosions. An assessment of individual and societal risks indicates that LHRSs present a lower hazard risk than GHRSs. However, both station types require additional safety barrier devices for risk reduction, such as detachable couplings, hydrogen detection sensors, and automatic and manual emergency shutdown systems, which are required for risk acceptance.     

Research on sealing performance and self-acting valve reliability in high-pressure oil-free hydrogen compressors for hydrogen refueling stations

Piston ring sealing and valve design play an important role in high-pressure oil-free reciprocating compressors for hydrogen refueling stations. The severe non-uniformity of the pressure distribution was suggested to be the root cause of the premature failure of the sealing rings, and therefore a mathematical model was established to simulate the unsteady flow within the gaps of piston rings, based on which the pressure distribution was obtained and the mechanism of the non-uniform abrasion of the rings was disclosed. The method to equalize the pressure difference through each ring was proposed by re-distributing the cut size of each ring, and it was validated experimentally. Aiming at the problem that the self-acting valves in hydrogen compressors could be easily destroyed by severe impact, this paper investigated the motion and impact of valves theoretically and experimentally, based on which the methodology was explored to design the parameters of valves for hydrogen compressors.