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.     

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.     

CFD simulations of filling and emptying of hydrogen tanks

During the filling of hydrogen tanks high temperatures can be generated inside the vessel because of the gas compression while during the emptying low temperatures can be reached because of the gas expansion. The design temperature range goes from ?40 °C to 85 °C. Temperatures outside that range could affect the mechanical properties of the tank materials. CFD analyses of the filling and emptying processes have been performed in the HyTransfer project. To assess the accuracy of the CFD model the simulation results have been compared with new experimental data for different filling and emptying strategies. The comparison between experiments and simulations is shown for the temperatures of the gas inside the tank, for the temperatures at the interface between the liner and the composite material, and for the temperatures on the external surface of the vessel.     

H2 refueling assessment of composite storage tank for fuel cell vehicle

Hydrogen as compressed gas is a promising option for zero-emission fuel cell vehicle. The fast and efficient refueling of high pressure hydrogen can provide a convenient platform for fuel cell vehicles to compete with conventional gasoline vehicles. This paper reports the finding of adiabatic simulation of the refueling process for Type IV tank at nominal working pressure of 70 MPa with considering the station refueling conditions. The overall heat transfer involved in refueling process was investigated by heat capacity model based on MC method defined by SAE J2601. The simulation results are validated against experimental data of European Commission's Gas Tank Testing Facility at Joint Research Centre (GasTef JRC), Netherlands. The results confirmed that end temperature and state of charge significantly depends on refueling parameters mainly supply hydrogen temperature and filling rate.     

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.     

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.     

Assessment of CFD models for hydrogen fast filling simulations

High injection pressures are used during the re-fueling process of vehicle tanks with compressed hydrogen, and consequently high temperatures are generated in the tank, potentially jeopardizing the system safety. Computational Fluid Dynamics (CFD) tools can help in predicting the temperature rise within vehicle tanks, providing complete and detailed 3D information on flow features and temperature distribution. In this framework, CFD simulations of hydrogen fast filling at different working conditions are performed and the accuracy of the numerical models is assessed against experimental data for a type 4 tank up to 70 MPa.     

Modelling of fast fueling of pressurized hydrogen tanks for maritime applications

This paper studies fast fueling of gaseous hydrogen into large hydrogen (H2) tanks suitable for maritime applications. Three modeling methods have been developed and evaluated: (1) Two-dimensional computational fluid dynamic (CFD) modeling, (2) One-dimensional wall discretized modeling, and (3) Zero-dimensional modeling. A detailed 2D CFD simulation of a small H2-tank was performed and validated with data from literature and then used to simulate a large H2-tank. Results from the 2D-model show non-uniform temperature distribution inside the large tank, but not in the small H2-tank. The 1D-model can predict the mean temperature in small H2-tanks, but not the inhomogeneous temperature field in large H2-tanks. The 0D-model is suitable as a screening tool to obtain rough estimates. Results from the modeling of the large H2-tank show that the heat transfer to the wall during fast filling is inhibited by heat conduction in the wall which leads to an unacceptably high mean hydrogen temperature.     

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.     

Towards fast and safe hydrogen filling for the fuel vehicle: A variable mass flow filling strategy based on a real-time thermodynamic model

Dealing with the conflict between the temperature/pressure rise and the total mass of hydrogen is a key challenge for rapid hydrogen filling of the hydrogen storage tank (HST). The temperature/pressure rise and total mass of hydrogen cause safety risks because of the former and limited cruise as the result of the latter. Therefore, safe hydrogen filling strategy is essential for the promotion of hydrogen fuel cell vehicles (FCVs). The existing thermodynamic model of the hydrogen storage tank is simplified either in the hydrogen state or the heat conduction of the HST wall, which can be hardly used as the real-time and accurate references for developing the filling strategy. To solve this problem, this paper works out the mathematical expression of a HST thermodynamic model. With the proposed HST thermodynamic model, a variable mass flow hydrogen filling strategy is developed. The results show that at the mass flow (12  g/s), the errors of the thermodynamic model are 7.1% and 6.8% for the temperature and pressure rise, compared with the computational fluid dynamics (CFD) model. At the mass flow (4.84  g/s), the thermodynamic model errors are 8.3% and 7.1% for the temperature and pressure rise, compared with the experimental data. Also, compared with the rule-based hydrogen filling strategies, the final state of charge (SoC) with the new filling strategy improve by 3%, 3.7%, and 2.7% at different initial temperatures, different volumes, and initial SoCs, respectively.     

Neural network based optimization for cascade filling process of on-board hydrogen tank

Compressed hydrogen storage is widely used in hydrogen fuel cell vehicles (HFCVs). Cascade filling systems can provide different pressure levels associated with various source tanks allowing for a variable mass flow rate. To meet refueling performance objectives, safe and fast filling processes must be available to HFCVs. The main objective of this paper is to establish an optimization methodology to determine the initial thermodynamic conditions of the filling system that leads to the lowest final temperature of hydrogen in the on-board storage tank with minimal energy consumption. First, a zero-dimensional lumped parameter model is established. This simplified model, implemented in Matlab/Simulink, is then used to simulate the flow of hydrogen from cascade pressure tanks to an on-board hydrogen storage tank. A neural network is then trained with model calculation results and experimental data for multi-objective optimization. It is found to have good prediction, allowing the determination of optimal filling parameters. The study shows that a cascade filling system can well refuel the on-board storage tank with constant average pressure ramp rate (APRR). Furthermore, a strong pre-cooling system can effectively lower the final temperature at a cost of larger energy consumption. By using the proposed neural network, for charging times less than 183s, the optimization procedure predicts that the inlet temperature is 259.99–266.58 K, which can effectively reduce energy consumption by about 2.5%.     

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.     

Experimental and numerical study on temperature rise within a 70 MPa type III cylinder during fast refueling

The fast refueling of hydrogen results in a temperature rise, which may lead to the failure of the hydrogen storage cylinder. Hence, study of temperature rise during refueling is a significant concern regarding hydrogen safety. In this research, a well-design system for 70 MPa hydrogen refueling was developed. Several refueling experiments on a type III cylinder have been conducted to study the temperature rise during the refueling process on this system. The experimental results show that the gas in caudal region and the aft domes junction surface achieved the maximum temperature rise. A Computational fluid dynamics (CFD) model was also validated by the experimental results. Finally, effects of initial pressure and ambient temperature on temperature rise were studied using this model. The results show that with the increase of initial pressure and the decrease of the ambient temperature, the final gas temperature decreases approximately linearly. This pilot research can provide invaluable guidance in developing advanced refueling standard.     

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.     

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.     

Fast filling strategy of type III on-board hydrogen tank based on time-delayed method

In order to study the fast filling problem of the type III on-board hydrogen tank, a 3D computational fluid dynamics (CFD) simulation model is proposed. Several simulation calculations are completed to simulate the fast filling process under different initial conditions. In order to control the temperature rise during the fast filling process, the effects of different mass flow rates are studied. Based on the control of mass flow rate, various time-delayed filling strategies for different conditions are proposed to meet the requirement of shortening the filling time as much as possible without exceeding the maximum temperature limit. It is found that if the delay duration is determined, how the filling time is allocated has little effect on the final temperature rise. The proposed strategy can complete the filling within 155s in a general environment, which saves 62% of the time compared with the filling with constant mass flow rate. This research provides the theoretical basis and technical support for mass flow control strategies of fast filling in the hydrogen refueling stations and has guiding significance for the actual filling process of large-capacity hydrogen tanks.     

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.     

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.     

Numerical investigations on the fast filling of hydrogen tanks

High-pressure storage of hydrogen in tanks is a promising option to provide the necessary fuel for transportation purposes. The fill process of a high-pressure tank should be reasonably short but must be designed to avoid too high temperatures in the tank. The shorter the fill should be the higher the maximum temperature in the tank climbs. For safety reasons an upper temperature limit is included in the requirements for refillable hydrogen tanks (ISO 15869) which sets the limit for any fill optimization. It is crucial to understand the phenomena during a tank fill to stay within the safety margins.The paper describes the fast filling process of hydrogen tanks by simulations based on the Computational Fluid Dynamics (CFD) code CFX. The major result of the simulations is the local temperature distribution in the tank depending on the materials of liner and outer thermal insulation. Different material combinations (type III and IV) are investigated.Some measurements from literature are available and are used to validate the approach followed in CFX to simulate the fast filling of tanks. Validation has to be continued in the future to further improve the predictability of the calculations for arbitrary geometries and material combinations.     

Investigation on no-vent filling process of liquid hydrogen tank under microgravity condition

In order to investigate the no-vent filling performance under microgravity, the computational fluid dynamic (CFD) method is introduced to the study, where a model aiming at filling a liquid hydrogen (LH₂) receiver tank is especially established. In this model, the solid and fluid regions are considered together to predict the coupled heat transfer process. The phase change effect during the filling process is also taken into account by embedding a pair of mass and heat transfer models into the CFD software FLUENT, one of which involves liquid flash driven by pressure difference between the fluid saturated pressure and the tank pressure, and the other one indicates and calculates the evaporation–condensation process driven by temperature difference between fluid and its saturated state. This CFD model, verified by experimental data, could accurately simulate the no-vent filling process with good flexibility. Moreover, no-vent filling processes under different gravities are comparatively analyzed and the effects of four factors including inlet configuration, inlet liquid temperature, initial wall temperature and inlet flow rate, are discussed, respectively. Main conclusions could be made as follows: 1) Compared to the situations in normal gravity, the no-vent filling in microgravity experiences a more adequate liquid–vapor mix, which results in a more steady pressure response and better filling performance. 2) Inlet configuration seems to have negligible effect on the no-vent filling performance under microgravity since liquid could easily reach the tank wall and then cause a sufficient fluid-wall contact under any inlet condition. 3) Higher initial tank wall temperature may directly cause a higher pressure rise in the beginning, while this effect on the final pressure is not significant. Sufficient precooling and reasonable inlet liquid subcooled degree are suggested to guarantee the reliability and efficiency of the no-vent fill under microgravity.