Queuing-based approach for optimal dispenser allocation to hydrogen refueling stations

Dispenser allocation to hydrogen refueling stations aims at minimizing the number of dispensers while ensuring satisfactory performance of vehicle queues during the peak hour of a peak day. A queuing model is developed in this study to evaluate the queuing performance at such stations by incorporating the statistical and thermodynamic characteristics of refueling. An optimization framework is proposed to determine the minimal number of dispensers required to meet the upper limits imposed on two important performance measures: mean waiting time and mean queue length. Reasonable upper limits are provided for 70 MPa stations according to the effects of dispenser allocation and station capacity on queuing performance. Server (dispenser nozzle) utilization under the optimal dispenser allocation generally increases with an increase in station size and tends to exceed 50% for large stations. The proposed approach can offer significant performance improvements for small stations and considerable savings in the number of dispensers for large ones.     

The cavity profile of a diaphragm compressor for a hydrogen refueling station

A low flow rate and short diaphragm life are the two disadvantages of diaphragm compressors when applied in hydrogen refueling stations. A new generatrix of the cavity profile of a diaphragm compressor was developed in this study to increase the cavity volume and decrease the diaphragm radial stress. A reduction in the diaphragm radial stress that resulted from the new design was validated by experiment and numerical simulation. The volumes of the cavities with different generatrices and the radial stress distribution of the diaphragm were investigated under various design conditions. The results indicated that the volume of the cavity with the new generatrix was approximately 10% larger than that with a traditional generatrix at the same allowable stress and cavity radius. At a similar cavity volume and radius, the radial stress values of the diaphragm in the cavity with the new generatrix were low. The decrease rate of the maximal radial stress of the diaphragm in the cavity with the new generatrix reached 13.8%. In the diaphragm centric region, where additional stress was induced by discharge holes, the maximal radial stress decrease rate reached 19.6%.     

Design proposal for hydrogen refueling infrastructure deployment in the Northeastern United States

Fuel cell vehicles (FCVs) are expected to be commercially available on the world market in 2015, therefore, introducing hydrogen-refueling stations is an urgent issue to be addressed. This paper proposes deployment plan of hydrogen infrastructure for the success of their market penetration in the Northeastern United States. The plan consists of three-timeline stages from 2013 to 2025 and divides the designated region into urban area, suburban area and area adjacent to expressway, so that easy to access to hydrogen stations can be realized. Station is chosen from four types of stations: off-site station, urban-type on-site station, suburban-type on-site station and portable station, associated with growing demand. In addition, on-site station is used as hydrogen production factory for off-site station to save total investment. This deployment plan shows that 83% of urban residents can reach station within 10 min in 2025, and that more than 90% people especially in four major cities: Boston, New York City, Philadelphia, and Washington, D.C. can get to station within 10 min by Geographic Information System (GIS) calculation.     

An optimized control method for a high utilization ratio and fast filling speed in hydrogen refueling stations

A well-designed control system with a high utilization ratio of hydrogen and a fast filling speed are two critical objectives to ensure the reduction of cost and time required in the refueling process. In this paper, the popular three-stage refueling process is modeled with the aim to address both objectives. Using the real gas law of hydrogen, the utilization ratio of hydrogen filling is analyzed and the filling flowrate and time of each stage are evaluated. A multi-objective iterative optimization model is established and an optimization algorithm for the filling process is proposed to achieve both fast refueling and high utilization. Numerical results can be applied to the optimization of an actual hydrogen filling process. Besides, the tests show that an optimized control method can significantly improve the utilization ratio and allow refueling in a widely acceptable time.     

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.     

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.     

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.     

Effect of wind condition on unintended hydrogen release in a hydrogen refueling station

Ambient condition, especially the wind condition, is an important factor to determine the behavior of hydrogen diffusion during hydrogen release. However, only few studies aim at the quantitative study of the hydrogen diffusion in a wind-exist condition. And very little researches aiming at the variable wind condition have been done. In this paper, the hydrogen diffusion in different wind condition which including the constant wind velocity and the variable wind velocity is investigated numerically. When considering the variable wind velocity, the UDF (user defined function) is compiled. Characteristics of the FGC (flammable gas cloud) and the HMF (hydrogen mass fraction) are analyzed in different wind condition and comparisons are made with the no-wind condition. Results indicate that the constant wind velocity and the variable wind velocity have totally different effect for the determination of hydrogen diffusion. Comparisons between the constant wind velocity and the variable wind velocity indicate that the variable wind velocity may cause a more dangerous situation since there has a larger FGC volume. More importantly, the wind condition has a non-negligible effect when considering the HMF along the radial direction. As the wind velocity increases, the distribution of the HMF along the radial direction is not Gaussian anymore when the distance between the release hole and the observation line exceeds to a critical value. This work can be a supplement of the research on the hydrogen release and diffusion and a valuable reference for the researchers.     

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.     

Performance assessment of an electrochemical hydrogen production and storage system for solar hydrogen refueling station

This paper investigates the performance of a hydrogen refueling system that consists of a polymer electrolyte membrane electrolyzer integrated with photovoltaic arrays, and an electrochemical compressor to increase the hydrogen pressure. The energetic and exergetic performance of the hydrogen refueling station is analyzed at different working conditions. The exergy cost of hydrogen production is studied in three different case scenarios; that consist of i) off-grid station with the photovoltaic system and a battery bank to supply the required electric power, ii) on-grid station but the required power is supplied by the electric grid only when solar energy is not available and iii) on-grid station without energy storage. The efficiency of the station significantly increases when the electric grid empowers the system. The maximum energy and exergy efficiencies of the photovoltaic system at solar irradiation of 850 W m-2 are 13.57% and 14.51%, respectively. The exergy cost of hydrogen production in the on-grid station with energy storage is almost 30% higher than the off-grid station. Moreover, the exergy cost of hydrogen in the on-grid station without energy storage is almost 4 times higher than the off-grid station and the energy and exergy efficiencies are considerably higher.     

Risk analysis on mobile hydrogen refueling stations in Shanghai

To better understand the hazards and risks associated with the mobile hydrogen refueling stations, a risk analysis was preformed to improve the safety of the operation. The risks to the station personnel and to the public were discussed separately. Results show that the stationary risks of the mobile stations to the personnel and refueling customers are lower than the risk acceptance criteria over an order of magnitude, so the occupational risks and the risks to customers are completely acceptable. The third party risks can be acceptable as long as the appropriate mitigation measures, especially well designed parking area and operation time, are implemented. Leak from booster compressors is the main risk contributor to the stationary risks due to the highest failure rates according to the generic data and the worst harm effects based on the consequence evaluations. However, the failure of the tube storages will result in the largest financial loss, though the likelihood of this scenario is much less than that of failure from booster compressors. As for the road risks of the mobile stations, they can be acceptable as long as the appropriate mitigation measures, especially well-planned itinerary and transport time, are implemented.     

A sieverts apparatus for measuring high-pressure hydrogen isotherms on porous materials

A high-pressure Sieverts apparatus, specifically designed to investigate carbon and other low-density materials for hydrogen storage, has been constructed and used to investigate potential storage materials for which the volume of the sample is uncertain or difficult to define. The apparatus can be managed from a computer via a graphical interface and used to measure gas sorption isotherms from 77 K to 873 K, utilising a computer driven piston pressure booster to compress the gas to 340 bar. Based on measurements of activated carbons and graphene-like samples, this article demonstrates the low sensitivity of the apparatus to uncertainty in the sample volume and the quality of data obtained for H₂ adsorption isotherms measured at 296 K and 77 K at pressures up to 300 bar.     

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.     

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.     

Analysis of the economy of scale and estimation of the future hydrogen production costs at on-site hydrogen refueling stations in Korea

This paper deals with the analysis of the economy of scale at on-site hydrogen refueling stations which produce hydrogen through steam methane reforming or water electrolysis, in order to identify the optimum energy mix as well as the total construction cost of hydrogen refueling stations in Korea. To assess the economy of scale at on-site hydrogen stations, the unit hydrogen costs at hydrogen stations with capacities of 30 Nm³/h, 100 Nm³/h, 300 Nm³/h, and 700 Nm³/h were estimated. Due to the relatively high price of natural gas compared to the cost of electricity in Korea, water electrolysis is more economical than steam methane reforming if the hydrogen production capacity is small. It seems to be the best strategy for Korea to construct small water electrolysis hydrogen stations with production capacities of 100 Nm³/h or less until 2020, and to construct steam methane reforming hydrogen stations with production capacities of 300 Nm³/h or more after 2025.     

Analyzing the levelized cost of hydrogen in refueling stations with on-site hydrogen production via water electrolysis in the Italian scenario

Hydrogen refueling infrastructures with on-site production from renewable sources are an interesting solution for assuring green hydrogen with zero CO₂ emissions. The main problem of these stations development is the hydrogen cost that depends on both the plant size (hydrogen production capacity) and on the renewable source.In this study, a techno-economic assessment of on-site hydrogen refueling stations (HRS), based on grid-connected PV plants integrated with electrolysis units, has been performed. Different plant configurations, in terms of hydrogen production capacity (50 kg/day, 100 kg/day, 200 kg/day) and the electricity mix (different sharing of electricity supply between the grid and the PV plant), have been analyzed in terms of electric energy demands and costs.The study has been performed by considering the Italian scenario in terms of economic streams (i.e. electricity prices) and solar irradiation conditions.The levelized cost of hydrogen (LCOH), that is the more important indicator among the economic evaluation indexes, has been calculated for all configurations by estimating the investment costs, the operational and maintenance costs and the replacement costs.Results highlighted that the investment costs increase proportionally as the electricity mix changes from Full Grid operation (100% Grid) to Low Grid supply (25% Grid) and as the hydrogen production capacity grows, because of the increasing in the sizes of the PV plant and the HRS units. The operational and maintenance costs are the main contributor to the LCOH due to the annual cost of the electricity purchased from the grid.The calculated LCOH values range from 9.29 €/kg (200 kg/day, 50% Grid) to 12.48 €/kg (50 kg/day, 100% Grid).     

Estimation of consequence and damage caused by an organic hydride hydrogen refueling station

Organic hydride hydrogen refueling stations are currently being developed in Japan. For these stations, we estimate the consequence and damage caused by explosions and heat radiation after a hydrogen leak, and the acute toxicity caused by the leakage and dispersion of methylcyclohexane and toluene energy carriers. First, the organic hydride hydrogen refueling station is defined, and an accident scenario for four leak sizes of hydrogen and chemical leak accidents is set. Next, simulations of the blast wave pressure and heat radiation after the hydrogen leak and of atmospheric dispersion for the evaporation after liquid methylcyclohexane and toluene leaks are performed. Probit functions or threshold values are created for each type of effects caused by the explosion, heat and the inhalation effect on humans of toluene acute toxicity. Population data for the area surrounding the station are created in a 10-m mesh. The consequence and damage are estimated for each leak size. The results show that although the explosion and chemical leak affects the area around the refueling station, the effects are small in all of the accident scenarios. In contrast, although the area of the heat effect is limited to inside the refueling station, the burn damage is large, and there is a need for conducting quantitative risk assessment.     

Three dimensional channel effect on the flow characteristics and the performance of hydrogen Knudsen compressors

As a new type of the micro fluidic device, Knudsen compressor can provide the potential utilizations on the hydrogen transport in the micro systems. Considering actual structure of the compressor is three-dimensional, flow characteristic studies are the key issue for the performance predictions. Firstly, the model of three-dimensional Knudsen compressor is built, and the validity of the model is proved by comparison with the experimental result. Secondly, the flow behaviors in the three-dimensional model is investigated, and the distributions of pressure and velocity are investigated. Also, the performance of the hydrogen Knudsen compressor in two-dimensional structure and three-dimensional structure are compared and discussed. Thirdly, the three-dimensional hydrogen Knudsen compressors with different width are analyzed, and the pressure increase in different cases of the hydrogen Knudsen compressors are studied.     

Techno-economics of novel refueling stations based on ammonia-to-hydrogen route and SOFC technology

This paper presents the economic assessment of novel refueling stations, in which through advanced and high efficiency technologies, the polygeneration of more energy services like hydrogen, electricity and heat is carried out on-site.The architecture of these polygeneration plants is realized with a modular structure, organized in more sections.The primary energy source is ammonia that represents an interesting fuel for producing more energy streams. The ammonia feeds directly the SOFC that is able to co-generate simultaneously electricity and hydrogen by coupling a high efficiency energy system with hydrogen chemical storage.Two system configurations have been proposed considering different design concepts: in the first case (Concept_1) the plant is sized for producing 100 kg/day of hydrogen and the power section is sized also for self-sustaining the plant electric power consumption, while in the second one (Concept_2) the plant is sized for producing 100 kg/day of hydrogen and the power section is sized for self-sustaining the plant electric power consumption and for generating 50 kW for the DC fast charging.The economic analysis has been carried out in the current and target scenarios, by evaluating, the levelized cost of hydrogen (LCOH), the levelized cost of electricity (LCOE), the Profitability Index (PI), Internal rate of Return (IRR) and the Discounted Payback Period (DPP).Results have highlighted that the values of the LCOH, for the proposed configurations and economic scenarios, are in the range 6–10 €/kg and the values of the LCOE range from 0.447 €/kWh to 0.242 €/kWh.In terms of PI and IRR, the best performance is achieved in the Concept_1 for the current scenario (1.89 and 8.0%, respectively). On the contrary, in the target scenario, thanks to a drastic costs reduction the co-production of hydrogen and electricity as useful outputs, becomes the best choice from all economic indexes and parameters considered.     

Effects of storage types and conditions on compressed hydrogen fuelling stations performance

In hydrogen fuelling stations hydrogen is usually stored in the high-pressure buffer or cascade storage systems. Buffer storage system includes a single pressure reservoir, while the cascade storage system is usually divided into three reservoirs at low, medium and high-pressure levels. In the present study, first and second laws of thermodynamics have been employed to analyze the filling process associated with these two storage systems. The important parameters such as filling time, filled mass and compressor input work have been examined in detail. Assuming the same final vehicle on-board in-cylinder pressure for both storage systems, the results reveal that filling time of the buffer storage system is much less than the cascade storage system. However, the filled mass related to the buffer system for the same conditions is approximately equal of the cascade system. Furthermore, the buffer system is accompanied with much higher entropy generation as compared to the cascade storage system, which directly reflects in the amount of required compressor input work. Entropy generation minimization has also been employed to determine the optimized low and medium-pressure reservoir pressures for the cascade storage system, which corresponds to the lowest required compressor input work for a specific high-pressure reservoir in the cascade systems.