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.     

Estimation of final hydrogen temperature from refueling parameters

Compressed hydrogen storage is currently widely used in fuel cell vehicles due to its simplicity in tank structure and refueling process. For safety reason, the final gas temperature in the hydrogen tank during vehicle refueling must be maintained under a certain limit, e.g., 85 °C. Many experiments have been performed to find the relations between the final gas temperature in the hydrogen tank and refueling conditions. The analytical solution of the hydrogen temperature in the tank can be obtained from the simplified thermodynamic model of a compressed hydrogen storage tank, and it serves as function formula to fit experimental temperatures. From the analytical solution, the final hydrogen temperature can be expressed as a weighted average form of initial temperature, inflow temperature and ambient temperature inspired by the rule of mixtures. The weighted factors are related to other refueling parameters, such as initial mass, initial pressure, refueling time, refueling mass rate, average pressure ramp rate (APRR), final mass, final pressure, etc. The function formula coming from the analytical solution of the thermodynamic model is more meaningful physically and more efficient mathematically in fitting experimental temperatures. The simple uniform formula, inspired by the concept of the rule of mixture and its weighted factors obtained from the analytical solution of lumped parameter thermodynamics model, is representatively used to fit the experimental and simulated results in publication. Estimation of final hydrogen temperature from refueling parameters based on the rule of mixtures is simple and practical for controlling the maximum temperature and for ensuring hydrogen safety during fast filling process.     

Experimental studies on temperature rise within a hydrogen cylinder during refueling

In this research, experiments were performed to investigate the thermal behaviors such as temperature rise and distributions inside 35 MPa, 150 L hydrogen storage cylinders during its refueling. The main factors affecting the temperature rise in the fast fill process such as the mass filling rate and initial pressure in the cylinder were considered. The experimental results show that the mass filling rate is a constant when the ratio of the pressure in the tank to the cylinder is higher than 1.7, and the mass filling rate decreases when the ratio is lower than 1.7; the temperature inside the cylinder increases nonlinearly in the filling process and the maximum value of temperature rise at the interface of the cylinder exists in the caudal region; the temperature rise reaches a larger value with a lower initial pressure in the cylinder or a higher mass filling rate. Furthermore, the limit of mass filling rate in the case of different ambient temperature was obtained.     

Impact of refueling parameters on storage density of compressed hydrogen storage Tank

Hydrogen fuel cell vehicle (HFCV) is one of the key contributors to sustainable development of the society. For commercial deployment and market acceptability of fuel cell vehicles, efficient storage of hydrogen with an optimum refueling is one of the important challenge. Compressed hydrogen storage in Type IV tanks is a mature and promising technology for on-board application. The fast refueling of the storage tank without overheating and overfilling is an essential requirement defined by SAE J2601. In this regard, station parameters such as hydrogen supply temperature, filling rate and vehicle tank parameters such as filling time strongly influences the storage capacity of the tank, affecting driving range of the fuel cell vehicle. This paper investigates the impact of these parameters on storage density of the tank defined in terms of state of charge. For this, refueling simulation based on SAE J2601 protocol has been performed using computational fluid dynamic approach to investigate the influence of station parameters on storage density of the tank. Further, the root cause analysis was carried out to investigate the contribution of station and vehicle tank parameters for enhancing the storage density of the tank. Finally, the regression model based on these refueling parameters was developed to predict the density attained at different filling conditions. The results confirmed the strong contribution of pressure, filling time, supply temperature and least contribution of temperature, filling rates in enhancing the storage density of the tank. The results can provide new insight into refueling behavior of the Type IV tank for fuel cell vehicle.     

Numerical simulation of temperature rise within hydrogen vehicle cylinder during refueling

Basing on the Spallart-Allmaras turbulence model and the real gas equation of state, a numerical model is proposed in this paper to study the mechanism of temperature rise within hydrogen vehicle cylinder during refueling. The model is validated by comparing calculated results with experimental data. With the validated model, the effect of mass filling rate, initial pressure within cylinder and ambient temperature on the maximum temperature rise during refueling are investigated. The study shows that the maximum temperature rise increases with the growth in mass filling rate and ambient temperature, while it descends as the initial pressure increases. Finally, an empirical formula is obtained by fitting numerical results and effective methods for temperature control is given.     

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

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

Effects of geometry and inconstant mass flow rate on temperatures within a pressurized hydrogen cylinder during refueling

Higher refueling rate leads to higher temperature rise within the cylinder. Excessive temperature should be avoided during the refueling progress. In this paper, we studied the effective methods to control the temperature rise by simulations based on the Computational Fluid Dynamics (CFD). Cylinders of different length to diameter ratios and different inlet diameters were simulated. We found that smaller radio of length to diameter can boost for temperature control and temperature distribution. Larger inlet diameter can restrain temperature rise. Comparing the simulation results with constant, increasing and decreasing mass flow rate, the refueling with increasing flow rate obtains the lowest temperature rise.     

Optimization of hydrogen vehicle refuelling requirements

The requirements regarding the refuelling process in order to prevent over-heating and over-filling significantly influence hydrogen fuelling station design and have a strong impact on potential fuelling performance. Consequently, refuelling station costs, reliability, and performance can be substantially improved by working on the way these requirements are formulated, in order to achieve shorter fuelling duration with a simpler process and less cooling. Two potential optimization opportunities were extensively investigated in the course of the EU funded HyTransfer project: (i) Application of the temperature limits to the tank material rather than to the gas inside the tank, (ii) Specification of the average delivery temperature rather than of the delivery temperature profile. Multiple research activities were carried out to this end. New models of various types were developed for predicting both the gas and material temperatures inside a vessel during filling and defueling. An experimental programme involving 82 filling and emptying tests of instrumented Type 4 and Type 3 vessels was performed for validating these models. New methods were developed and applied for determining the value of the gas-to-wall heat transfer coefficient from the temperature measurements. The balance of heat transferred from the gas to the liner and to the bosses in a type 4 vessel was reconstructed. CFD simulations were performed for analysing temperature disparities, and the thermal stratification observed in certain filling conditions reproduced. Criteria on gas injection conditions were identified for ensuring gas temperature homogeneity, a key assumption made by fuelling protocols. The temperature variations in the wall material were studied for future investigation of less conservative definitions of the maximum acceptable temperature in Hot Case situations. The effect of changing the delivery temperature profiles without changing the average delivery temperature was also analysed.     

Refueling hydrogen fuel cell vehicles with 68 proposed refueling stations in California: Measuring deviations from daily travel patterns

This paper examines the deviation of refueling a hydrogen fuel cell vehicle with limited opportunity provided by the 68 proposed stations in California. A refueling trip is inserted to reported travel patterns in early hydrogen adoption community clusters and the best and worst case insertions are analyzed. Based on these results, the 68 refueling stations provide an average of 2.5 and 9.6 min deviation for the best and the worst cases. These numbers are comparable to currently observed gasoline station deviation, and we conclude that these stations provide sufficient accessibility to residents in the target areas.     

On optimal investment strategies for a hydrogen refueling station

The uncertainty and cost of changing from a fossil-fuel-based society to a hydrogen-based society are considered to be extensive obstacles to the introduction of fuel cell vehicles (FCVs). The absence of existing profitable refueling stations has been shown to be one of the major barriers. This paper investigates methods for calculating an optimal transition from a gasoline refueling station to future methane and hydrogen combined use with an on site small-scale reformer for methane. In particular, we look into the problem of matching the hydrogen capacity of a single refueling station to an increasing demand. Based on an assumed future development scenario, optimal investment strategies are calculated. First, a constant utilization of the hydrogen reformer is assumed in order to find the minimum hydrogen production cost. Second, when considerations such as periodic maintenance are taken into account, optimal control is used to concurrently find both a short term equipment variable utilization for one week and a long term strategy. The result is a minimum hydrogen production cost of $4–6/kg, depending on the number of reinvestments during a 20 year period. The solution is shown to yield minimum hydrogen production cost for the individual refueling station, but the solution is sensitive to variations in the scenario parameters.     

A generalized cryo-adsorber model and 2-D refueling results

Our earlier work [Int J Hyd Energy 2010; 35: 3598-3609], describes a 3-D model for the cryo-adsorber and the 1-D results for the isobaric refueling period using quasi-static adsorption approximation. Herein the isobaric constraint is relaxed by solving the Darcy equation for computing the pressure drop in the bed, the quasi-static approximation is relaxed by introducing the Langmuir adsorption kinetics, and the 2-D refueling results are compared with the 1-D quasi-static isobaric refueling results. In spite of the significant differences in formulation, the two results compare well with each other. The 2-D refueling results show that the pressure transients equilibrate quickly and a nearly steady pressure profile gets established in the bed. This observation justifies the isobaric approximation used earlier. The Langmuir kinetics used here has a desorption rate constant which could vary depending on the diffusional resistance at adsorbent particle level. A sensitivity analysis of this parameter shows that the refueling rate varies negligibly while this parameter is varied over many orders of magnitude. This observation shows that as long as the pellets are small enough, refueling is controlled by macroscopic processes like simultaneous cooling/adsorption in the bed and the movement of the adsorption front out of the bed, rather than the molecular processes like sorption at an adsorbent site or diffusion through the adsorbent lattice. This observation justifies the quasi-static approximation used earlier. The above two approximations offer significant computational advantage to the design and optimization of cryo-adsorber beds with complex geometry.     

Hydrogen refueling process from the buffer and the cascade storage banks to HV cylinder

The focus of this research is on refueling process from a buffer and a cascade storage bank. A thermodynamic analysis is developed to investigate the filling process of fuel transmission from a storage bank to hydrogen cylinder. Refueling Process from Buffer and Cascade Storage Banks is the subject of this research. Filling the hydrogen cylinder to the required final condition is influenced by the volume and pressure of storage bank. For both buffer and cascade storage banks, ambient temperature is also an important parameter that affects the initial condition, the final condition and the refueling process. Comparison of buffer and cascade storage banks showed that refueling time using buffer storage bank is 200 s less than the cascade storage bank. However, the energy required for gas storage is higher in buffer storage system. As shown by the study, reduction in the final temperature of the filling process can be achieved by controlling the ambient temperature, the initial pressure and the fuel charging rate.     

A thermodynamic analysis of refueling of a hydrogen tank

A thermodynamic analysis of refueling of a gaseous hydrogen fuel tank is described. This study may lend itself to the applications of refueling a hydrogen storage tank onboard a hydrogen fuel-cell vehicle. The gaseous hydrogen is treated as an ideal or a non-ideal gas. The refueling process is analyzed based on adiabatic, isothermal, or diathermal condition of the tank. A constant feed-rate is assumed in the analysis. The thermodynamic state of the feed stream also remains constant during refueling. Ideal-gas assumption results in simple closed-form expressions for tank temperature, pressure, and other parameters. The non-ideal behavior of high-pressure gaseous hydrogen is addressed using the newly developed equation of state for normal hydrogen, which is based on the reduced Helmholtz free energy formulation. Sample calculations are presented using initial tank and feed stream conditions commensurate to practical vehicular applications. Comparing to the non-ideal analysis, the ideal-gas assumption always results in under-prediction of the tank temperature and pressure irrespective of the filling condition. For a given target tank pressure, the refueling time is the shortest under adiabatic condition and is the longest under isothermal condition with the tank being maintained at the initial tank temperature. The adiabatic and isothermal conditions can be viewed, respectively, as the lower and upper bounds of the refueling time for a given final target tank pressure.     

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.     

Development of correlation equations on hydrogen properties for hydrogen refueling process by machine learning approach

The energy transition which refers to shift of the energy system from fossil-based resources to renewable and sustainable energy sources becomes a global issue to mitigate the progression of climate change. Hydrogen can play an important role in long-term decarbonization of energy system and achievement of carbon neutrality. Currently, the utilization of hydrogen in the energy system is focused on a road transportation sector as a fuel in a vehicle fleet.Compressing gaseous hydrogen is the most well-established technology for storage in hydrogen-fueled vehicles. The refueling hydrogen requires short filling time while ensuring the safety of storage tanks in a vehicle. However, a fast filling of hydrogen in high pressure leads to a rapid temperature rise of hydrogen stored in tank. Therefore, many numerical and experimental studies have been carried out to analyze the filling process. Various thermo-physical properties of gaseous hydrogen such as density, viscosity, and thermal conductivity are required for the numerical studies and the accurate hydrogen properties are essential to obtain reliable results.In this work, a polynomial equation is proposed with respect to temperature and pressure in ranges of 223.15 K < T < 373.15 K and 0.1 MPa < P < 100.1 MPa to present various hydrogen thermo-physical properties by adopting different coefficients. The coefficients are determined by a machine learning method to regress the equation using a great number of reference data. The equation is trained, tested, and validated using different datasets for each property. The order of the equation has been changed from 2 to 5. Then, the accuracies are estimated and compared with respect to the order. The average relative errors (REs) of the 5th order equation are assessed to lower than 0.3% except for molar volume and entropy. The accuracy of the equation is also examined with experimental data and other correlation equations for density, viscosity, and thermal conductivity which are required for numerical simulations of hydrogen refueling. The proposed equation presents better accuracy for viscosity and thermal conductivity than literature equations. In density calculation, a literature equation shows better performance than the proposed equation, but the difference between their accuracies is not so significant. In calculation time comparison, it is revealed that the proposed equation rapidly responses adequate to computational fluid dynamics (CFD) simulations.Results of the study can provide accurate and reliable hydrogen property values in a fast and robust means specifically for simulation of hydrogen refueling process, but not restricted only to the process. Correlation equations proposed in the present work can aid in optimizing a hydrogen value chain including production, storage, and utilization by providing accurate hydrogen property.     

Eliciting preferences on the design of hydrogen refueling infrastructure

Hydrogen vehicles are already a reality, However, consumers will be reluctant to purchase hydrogen vehicles (or any other alternative fuel vehicle) if they do not perceive the existence of adequate refueling infrastructure that reduces the risk of running out of fuel regularly while commuting to acceptable levels. This fact leads to the need to study the minimum requirements in terms of fuel availability required by drivers to achieve a demand for hydrogen vehicles beyond potential early-adopters.This paper studies consumer preferences in relation to the design of urban hydrogen refueling infrastructure. To this end, the paper analyzes the results of a survey carried out in Andalusia, a region in southern Spain, on drivers' current refueling tendencies, their willingness to use hydrogen vehicles and their minimum requirements (maximum distance to be traveled to refuel and number of stations in the city) when establishing a network of hydrogen refueling stations in a city. The results show that consumers consider the existence in cities of an infrastructure with a number of refueling stations ranging from approximately 10 to 20% of the total number of conventional service stations as a requisite to trigger the switch to the use of hydrogen vehicles. In addition, these stations should be distributed in response to the drivers’ preferences to refuel close to home.     

Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

Comparative study of turbulence models performance for refueling of compressed hydrogen tanks

Compressed hydrogen gas is a popular mode of fuel storage for hydrogen powered vehicles. When hydrogen gas is filled at high pressure, the gas temperature increases. The maximum gas temperature should be within acceptable safety standards. Numerical studies can help optimize the filling process. There is a high level of turbulence in the flow as the high velocity inlet jet is penetrating the nearly stagnant gas in the tank. Selection of a suitable turbulence model is important for accurate simulation of flow and heat transfer during filling of hydrogen tanks. In the present work, a comparative study is performed to identify suitable turbulence model for compressed hydrogen tank filling problem. Numerical results obtained with different turbulence models are compared with available experimental data. Considering accuracy, convergence and the computational expenses, it is observed that the realizable k-ε model is the most suitable turbulence model for hydrogen tank filling problem.     

CFD model performance benchmark of fast filling simulations of hydrogen tanks with pre-cooling

High gas temperatures can be reached inside a hydrogen tank during the filling process because of the large pressure increase (up to 70–80 MPa) and because of the short time (∼3 min) of the process. High temperatures can potentially jeopardize the structural integrity of the storage system and one of the strategies to reduce the temperature increase is to pre-cool the hydrogen before injecting it into the tank. Computational Fluid Dynamics (CFD) tools have the capabilities of capturing the flow field and the temperature rise in the tank. The results of CFD simulations of fast filling with pre-cooling are shown and compared with experimental data to assess the accuracy of the CFD model.