Effects of pressure levels in three-cascade storage system on the overall energy consumption in the hydrogen refueling station

Studies show that compared with the one-buffer system, the cascade storage system has lower energy consumption in high-pressure hydrogen refueling stations. In the present study, practical dynamic models of the whole hydrogen refueling process are established to evaluate the energy consumption. Accordingly, the filling performance of the three-cascade storage system and single tank storage system are analyzed. Moreover, the impact of the three pressure levels and the charging sequence of the three tanks on the energy consumption are investigated. The obtained results show that changing from one buffer to three tanks gives a total energy saving of approximate 34%. For the three-cascade storage system, the total energy consumption increases approximately linearly with the increase of the pressure of the high-pressure tank. Whereas it shows concave curve shape trends with the increase of low-pressure level and the medium-pressure level. Furthermore, the charging sequence from the low-pressure buffer to the high one decreases the total operation energy consumption to a value slightly lower than the adverse charge sequence.     

Development of efficient hydrogen refueling station by process optimization and control

Development of efficient hydrogen refueling station (HRS) is highly desirable to reduce the hydrogen cost and hence the life cycle expense of fuel cell vehicles (FCVs), which is hindering the large scale application of hydrogen mobility. In this work, we demonstrate the optimization of gaseous HRS process and control method to perform fast and efficient refueling, with reduced energy consumption and increased daily fueling capacity. The HRS was modeled with thermodynamics using a numerical integration method and the accuracy for hydrogen refueling simulation was confirmed by experimental data, showing only 2 °C of temperature rise deviation. The refueling protocols for heavy duty FCVs were first optimized, demonstrating an average fueling rate of 2 kg/min and pre-cooling demand of less than 7 kW for 35 MPa type III tanks. Fast refueling of type IV tanks results in more significant temperature rise, and the required pre-cooling temperature is lowered by 20 K to achieve comparable fueling rate. The station process was also optimized to improve the daily fueling capacity. It is revealed that the hydrogen storage amount is cost-effective to be 25–30% that of the nominal daily refueling capacity, to enhance the refueling performance at peak time and minimize the start and stop cycles of compressor. A novel control method for cascade replenishment was developed by switching among the three banks in the order of decreased pressure, and results show that the daily refueling capacity of HRS is increased by 5%. Therefore, the refueling and station process optimization is effective to promote the efficiency of gaseous HRS.     

Single-tank storage versus multi-tank cascade system in hydrogen refueling stations for fuel cell buses

Many countries in Europe are investing in fuel cell bus technology with the expected mobilization of more than 1200 buses across Europe in the following years. The scaling-up will make indispensable a more effective design and management of hydrogen refueling stations to improve the refueling phase in terms of refueling time and dispensed quantity while containing the investment and operation costs. In the present study, a previously developed dynamic lumped model of a hydrogen refueling process, developed in MATLAB, is used to analyze tank-to-tank fuel cell buses (30–40 kg_H2 at 350 bar) refueling operations comparing a single-tank storage with a multi-tank cascade system. The new-built Aalborg (DK) hydrogen refueling station serves as a case study for the cascade design. In general, a cascading refueling approach from multiple storage tanks at different pressure levels provides the opportunity for a more optimized management of the station storage, reducing the pressure differential between the refueling and refueled tanks throughout the whole refueling process, thus reducing compression energy. This study demonstrates the validity of these aspects for heavy-duty applications through the technical evaluation of the refueling time, gas heating, compression energy consumption and hydrogen utilization, filling the literature gap on cascade versus single tank refueling comparison. Furthermore, a simplified calculation of the capital and operating expenditures is conducted, denoting the cost-effectiveness of the cascade configuration under study. Finally, the effect of different pressure switching points between the storage tanks is investigated, showing that a lower medium pressure usage reduces the compression energy consumption and increases the station flexibility.     

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.     

Final hydrogen temperature and mass estimated from refueling parameters

The final temperature and mass of compressed hydrogen in a tank after a refueling process can be estimated using the analytical solutions of a lumped parameter thermodynamic model of high pressure compressed hydrogen storage system. The effects of three single refueling parameters (ambient temperature, initial pressure and mass flow rate) and three pairs of the refueling parameters on the final hydrogen temperature are studied, for both 35 MPa and 70 MPa tanks. Overall expressions for the final hydrogen temperature, expressed as a function of the three factors, are obtained. The formulae for the final hydrogen temperature provide an excellent representation of the reference data. The effects of the refueling parameters (mass flow rate, initial pressure and inflow temperature) on the final hydrogen mass are determined from the physical model. An overall expression of the final hydrogen mass is also obtained. The final hydrogen temperature can be controlled by reducing the ambient temperature or the mass flow rate, or increasing the initial pressure. The final hydrogen mass can be maximized by reducing the mass flow rate or the inflow temperature, or increasing the initial pressure. This study provides simple engineering formulae to assist in establishing refueling protocols for gaseous hydrogen vehicles.     

Hydrogen refueling stations and fuel cell buses four year operational analysis under real-world conditions

Worldwide about 550 hydrogen refueling stations (HRS) were in operation in 2021, of which 38% were in Europe. With their number expected to grow even further, the collection and investigation of real-world station operative data are fundamental to tracking their activity in terms of safety issues, performances, maintenance, reliability, and energy use. This paper analyses the parameters that characterize the refueling of 350 bar fuel cell buses (FCB) in five HRS within the 3Emotion project. The HRS are characterized by different refueling capacities, hydrogen supply schemes, storage volumes and pressures, and operational strategies. The FCB operate over various duty cycles circulating on urban and extra-urban routes. From data logs provided by the operators, a dataset of four years of operation has been created. The results show a similar hydrogen amount per fill distribution but quite different refueling times among the stations. The average daily mass per bus and refueling time are around 14.62 kg and 10.28 min. About 50% of the total amount of hydrogen is dispensed overnight, and the refueling events per bus are typically every 24 h. On average, the buses' time spent in service is 10 h per day. The hydrogen consumption is approximately 7 kg/100 km, a rather effective result reached by the technology. The station utilization is below 30% for all sites, the buses availability hardly exceeds 80%.     

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.     

Hydrogen refueling station compression and storage optimization with tube-trailer deliveries

Hydrogen refueling stations require high capital investment, with compression and storage comprising more than half of the installed cost of refueling equipment. Refueling station configurations and operation strategies can reduce capital investment while improving equipment utilization. Argonne National Laboratory developed a refueling model to evaluate the impact of various refueling compression and storage configurations and tube trailer operating strategies on the cost of hydrogen refueling. The modeling results revealed that a number of strategies can be employed to reduce fueling costs. Proper sizing of the high-pressure buffer storage reduces the compression requirement considerably, thus reducing refueling costs. Employing a tube trailer to initially fill the vehicle's tank also reduces the compression and storage requirements, further reducing refueling costs. Reducing the cut-off pressure of the tube trailer for initial vehicle fills can also significantly reduce the refueling costs. Finally, increasing the trailer's return pressure can cut refueling costs, especially for delivery distances less than 100 km, and in early markets, when refueling stations will be grossly underutilized.     

Determining correlations between final hydrogen temperature and refueling parameters from experimental and numerical data

During the hydrogen filling process, the excessive temperature rise may cause the hydrogen storage tank to fail. Therefore, preventing the temperature from rising too high is an important guarantee for the safety of the hydrogen storage cylinder. The analytical solution of a single-zone thermodynamic model for hydrogen refueling is obtained. Based on the analytical solution of the final hydrogen temperature derived from the hydrogen filling theoretical model, the relationship among the final hydrogen temperature and the initial temperature and the inlet temperature and the ambient temperature is obtained. The model is used to achieve correlations coefficients among the above parameters. Data of Type III 40L tank and Type IV 29L tank used in the model are from the experiment, and data of Type III 25L tank and Type IV 174L tank are from the simulation. The results show that our analytical solution is applicable for determining correlations between final hydrogen temperature and refueling parameters from experimental and numerical data. Our analytical solution is more accurate than the reduced model reported in reference. At the same time, the effects of the initial temperature and the inlet temperature on the final temperature are stronger in Type IV tank than in the Type III tank. This study may provide guides for improving hydrogen refueling standards.     

Review and comparison of worldwide hydrogen activities in the rail sector with special focus on on-board storage and refueling technologies

This paper investigates hydrogen storage and refueling technologies that were used in rail vehicles over the past 20 years as well as planned activities as part of demonstration projects or feasibility studies. Presented are details of the currently available technology and its vehicle integration, market availability as well as standardization and research and development activities. A total of 80 international studies, corporate announcements as well as vehicle and refueling demonstration projects were evaluated with regard to storage and refueling technology, pressure level, hydrogen amount and installation concepts inside rolling stock. Furthermore, current hydrogen storage systems of worldwide manufacturers were analyzed in terms of technical data.We found that large fleets of hydrogen-fueled passenger railcars are currently being commissioned or are about to enter service along with many more vehicles on order worldwide. 35 MPa compressed gaseous storage system technology currently dominates in implementation projects. In terms of hydrogen storage requirements for railcars, sufficient energy content and range are not a major barrier at present (assuming enough installation space is available). For this reason, also hydrogen refueling stations required for 35 MPa vehicle operation are currently being set up worldwide.A wide variety of hydrogen demonstration and retrofit projects are currently underway for freight locomotive applications around the world, in addition to completed and ongoing feasibility studies. Up to now, no prevailing hydrogen storage technology emerged, especially because line-haul locomotives are required to carry significantly more energy than passenger trains. The 35 MPa compressed storage systems commonly used in passenger trains offer too little energy density for mainline locomotive operation - alternative storage technologies are not yet established. Energy tender solutions could be an option to increase hydrogen storage capacity here.     

Review on equipment configuration and operation process optimization of hydrogen refueling station

The construction of hydrogenation infrastructure is important to promote the large-scale development of hydrogen energy industry. The technical performance of hydrogen refueling station (HRS) largely determines the refueling efficiency and cost of hydrogen fuel cell vehicles. This paper systematically lists the hydrogen refueling process and the key equipment applicable in the HRS. It comprehensively reviews the key equipment configuration from the hydrogen supply, compression, storage and refueling of the HRS. On the basis of the parameter selection and quantity configuration method, the process optimization technology related to the equipment utilization efficiency and construction cost was quantitatively evaluated. Besides, the existing problems and prospects are put forward, which lays the foundation for further research on the technical economy of HRSs.     

Generalized model development for a cryo-adsorber and 1-D results for the isobaric refueling period

We have developed 3-D model equations for a cryo-adsorption hydrogen storage tank, where the energy balance accommodates the temperature and pressure variation of all the thermodynamic properties. We then reduce the 3-D model to the 1-D isobaric system and study the isobaric refueling period, for simplified geometry and charging conditions. The hydrogen capacity evolution predicted by the 1-D axial bed model is significantly different than that predicted by the lumped-parameter model because of the presence of sharp temperature gradients during refueling. The 1-D model predicts a higher hydrogen capacity than the lumped-parameter model. This observation can be rationalized by the fact that a bed with temperature gradients on equilibration should desorb gas, whenever the adsorbed phase entropy is lower than the gas phase entropy. The 1-D analysis of the isobaric refueling period does not show any significant difference in hydrogen capacity evolution among the axial, single and multicartridge annular bed designs. Hence, a multicartridge annular design, though giving a slightly lower pressure drop, does not offer any heat and mass transfer enhancement over the single cartridge design. And, the single cartridge annular design appears to be optimal.     

Hydrogen energy storage method selection using fuzzy axiomatic design and analytic hierarchy process

This paper presents an integrated Fuzzy Analytical Hierarchy Process (fAHP) and Weighted Fuzzy Axiomatic Design (wfAD) methodology for a strategic level problem of hydrogen energy storage (HES) method selection for Turkey. Considering alternatives of tank, metal hydride and chemical storage, we design our decision problem with respect to five assessment criteria as weight, capacity, storage loss and leak, reliability, and total system cost. In the solution methodology, we first implement fAHP to determine criteria weights by incorporating linguistic judgement and vague information. Next, we define functional requirements and design parameters for the selection problem and use wfAD to compare alternatives. After implementing the proposed integrated approach, we perform additional sensitivity analysis experiments to test the robustness of the solution with respect to changes in functional requirements.     

Improving chiller performance and energy efficiency in hydrogen station operation by tuning the auxiliary cooling

In a hydrogen station that operates with direct fueling through the use of a 700 bar boost compressor, the outlet hydrogen temperature can significantly increase, stressing the chiller system. This paper evaluates improvements that can be made to the auxiliary cooling system integrated with the compressor. Both theoretical modeling and experiments were performed at Cal State LA Hydrogen Research and Fueling Facility. The findings suggest that adjusting the auxiliary closed-loop cooling system from 15 °C to 10 °C reduced the station energy consumption and decreased the demand on the station chiller that needed to provide ?20 °C hydrogen at the hose. The overall energy consumption for a single fueling reduced by between 2.86 and 9.43% for the set of experiments conducted. After the temperature of the closed-loop cooling system was reduced by 5 °C, the boost compressor outlet temperature dropped from 46-50 °C–40 °C and consequently at the hose the hydrogen temperature declined by 3 °C. Results were scaled up with a forecast on the number of daily refueling events. With a low number of daily fuelings, the proposed set-up showed a minor influence on the overall station energy consumption. However, the benefits were more pronounced for a connector station with sales at 180 kg/day, where the energy efficiency improved by between 1.4 and 5.5%, and even more so for a higher capacity station at 360 kg/day, where the improvement was between 2.9 and 8%.     

Effect of cascade storage system topology on the cooling energy consumption in fueling stations for hydrogen vehicles

One of the main obstacles of the diffusion of fuel cell electric vehicles (FCEV) is the refueling system. The new stations follow the refueling protocol from the Society of Automotive Engineers where the way to reach the target pressure is not explained. This work analyzes the thermodynamics of a hydrogen fueling station in order to study the effects of the cascade storage system topology on the energy consumption for the cooling facility. It is found that the energy consumption for cooling increases, expanding the total volume of the cascade storage system. Comparing the optimal and the worst volume configurations of the cascade storage tanks at different ambient temperatures, the energy saving is approximately 12% when the average ambient temperature is 20 °C and around 20% when the average ambient temperature is 30 °C. The energy consumption for cooling is significantly influenced by the topology of the cascade storage system and it is particularly relevant in the case of low daily-dispensed amount of hydrogen.     

Numerical investigation on temperature-rise of on-bus gaseous hydrogen storage cylinder with different thickness of liner and wrapping material

Gaseous hydrogen stored in high-pressure cylinder is a proper solution for the application of hydrogen fuel cell buses (HFCB). As far as the on-bus hydrogen storage system (OBHSS) is concerned, the filling of hydrogen gas needs to be finished in an acceptable time, which unavoidably brings the increase of temperature of hydrogen gas in OBHSS. And excessive temperature of hydrogen gas is unfavorable to mechanical properties of wrapping material and even the service life of the storage cylinder, so it is urgent to work out effective strategies on the temperature-rise in the storage cylinder. It is noticed that the studies on the relationship between the temperature-rise and the geometrical parameters of on-bus gaseous hydrogen storage cylinder (OBGHSC), e.g. thickness of liner and fiber/epoxy composite laminate, are still not deep enough. Motivated by this fact, this research is therefore devoted to studying the relationship between the temperature-rises of both hydrogen gas and solid materials in OBGHSC and the geometrical parameters of wrapping material and liner of OBGHSC, and to developing several temperature-rise correlations. To do so, a 2-dimensional (2D) axisymmetric computational fluid dynamics (CFD) model is applied for the simulation of fast filling process and holding process of 70 MPa OBGHSC. The simulation results show that the temperature distribution during the filling is different for different type III storage cylinders, while the highest temperature is always in the head dome junction region for type IV storage cylinders. For the carbon fiber/epoxy composite laminate (CFEC), the temperature varying tendencies are not the same for different type III storage cylinders, while the temperature in type IV storage cylinder decreases with the increase of thickness of CFEC. At last, based on the obtained numerical data, the correlations for highest value of mass-averaged temperature-rise of hydrogen gas and the correlations for maximum temperature-rise of CFEC that account for the effects of dimensionless parameters are proposed. The correlations reveal the relationship between the temperature-rise and the structure of hydrogen storage cylinder and can be used to direct the fast filling process for OBHSS in this research.     

Development and CFD analysis for determining the optimal operating conditions of 250 kg/day hydrogen generation for an on-site hydrogen refueling station (HRS) using steam methane reforming

The objective of this study to develop and undertake a comprehensive CFD analysis of an effective state-of-the-art 250 kg/day hydrogen generation unit for an on-site hydrogen refueling station (HRS), an essential part of the infrastructure required for fuel cell vehicles and various aspects of hydrogen mobility. This design consists of twelve reforming tubes and one newly designed metal fiber burner to ensure superior emission standards and performance. Experimental and computational modeling steps are conducted to investigate the effects of various operating conditions, the excess air ratio (EAR) at the burner, the gas hourly space velocity (GHSV), the process gas inlet temperature, and the operating pressure on the hydrogen production rate and thermal efficiency. The results indicate that the performance of the steam methane reforming reactor increased significantly by improving the combustion characteristics and preventing local peak temperatures along the reforming tube. It is shown that EAR should be chosen appropriately to maximize the hydrogen production rate and lifetime operation of the reformer tube. It is found that high inlet process gas temperatures and low operating pressure are beneficial, but these parameters have to be chosen carefully to ensure proper efficiency. Also, a high GHSV shortens the residence time and provides unfavorable heat transfer in the bed, leading to decreased conversion efficiency. Thus, a moderate GHSV should be used. It is shown that heat transfer is an essential factor for obtaining increased hydrogen production. This study addresses the pressing need for the HRS to adopt such a compact system, whose processes can ensure greater hydrogen production rates as well as better durability, reliability, and convenience.     

Optimization on volume ratio of three-stage cascade storage system in hydrogen refueling stations

Three-stage cascade storage systems are widely adopted in hydrogen refueling stations. Their volume ratio has a remarkable impact on the performance of refueling systems. In this study, a thermodynamic model that considers the complete refueling–recovery process is developed. The effects of volume ratio on the utilization ratio and the specific energy consumption of the model is investigated, and the optimization of the volume ratio is explored and discussed. The utilization ratio decreases with the increase in the proportion of low-pressure stage volume (pLP), and a proper volume of medium-pressure stage improves the utilization ratio. The specific energy consumption decreases as pLP increases when the stationary storage capacity is relatively small. However, when the stationary storage capacity is relatively large, the specific energy consumption does not decrease monotonically, and a low specific energy consumption and a high utilization ratio can be simultaneously obtained at low pLP.     

Hydrogen generation and storage from hydrolysis of sodium borohydride in batch reactors

The catalytic hydrolysis of alkaline sodium borohydride (NaBH₄) solution was studied using a non-noble; nickel-based powered catalyst exhibiting strong activity even after long time storage. This easy-to-prepare catalyst showed an enhanced activity after being recovered from previous use. The effects of temperature, NaBH₄ concentration, NaOH concentration and pressure on the hydrogen generation rate were investigated. Particular importance has the effect of pressure, since the maximum reached pressure of hydrogen is always substantially lower than predictions (considering 100% conversion) due to solubility effects. The solubility of hydrogen is greatly enhanced by the rising pressure during reaction, leading to storage of hydrogen in the liquid phase. This effect can induce new ways of using this type of catalyst and reactor for the construction of hydrogen generators and even containers for portable and in situ applications.     

Hydrogen key technology to cover the energy storage needs of NEOM City

NEOM City is supposed to be a renewable-energy-only city in Saudi Arabia. The project has planned a huge capacity of non-dispatchable wind and solar photovoltaic but has not addressed yet the issue of a long time, large storage of energy. Battery energy storage is the only product off-the-shelf, and we know already only works for the storage of small amounts of energy over short time frames. The other solutions for energy storage are not off-the-shelf products, but in many cases, only nice ideas to be proven workable. The only other opportunity to make NEOM a truly renewable-energy-only City today is to use the extra wind and solar photovoltaic power to produce hydrogen through electrolyzers, and then partially use this hydrogen to produce the missing electricity to stabilize the grid, and export the excess hydrogen. Adopting extra wind and solar photovoltaic to make NEOM a hydrogen production hub in addition to a renewable-energy-only city is an even more attractive proposition. As NEOM has not fully acknowledged this issue, same as the scientific community, the most likely solution without an urgent debate within the scientific community will be to import electricity from the combustion of hydrocarbon fuels while paying carbon credits, with is inconsistent with the renewable-energy-only aspiration.