Accurate simplified dynamic model of a metal hydride tank

As proton exchange membrane fuel cell technology advances, the need for hydrogen storage intensifies. Metal hydride alloys offer one potential solution. However, for metal hydride tanks to become a viable hydrogen storage option, the dynamic performance of practical tank geometries and configurations must be understood and incorporated into fuel cell system analyses. A dynamic, axially-symmetric, multi-nodal metal hydride tank model has been created in Matlab–Simulink^® as an initial means of providing insight and analysis capabilities for the dynamic performance of commercially available metal hydride systems. Following the original work of Mayer et al. [Mayer U, Groll M, Supper W. Heat and mass transfer in metal hydride reaction beds: experimental and theoretical results. Journal of the Less-Common Metals 1987;131:235–44], this model employs first principles heat transfer and fluid flow mechanisms together with empirically derived reaction kinetics. Energy and mass balances are solved in cylindrical polar coordinates for a cylindrically shaped tank. The model tank temperature, heat release, and storage volume have been correlated to an actual metal hydride tank for static and transient absorption and desorption processes. A sensitivity analysis of the model was accomplished to identify governing physics and to identify techniques to lessen the computational burden for ease of use in a larger system model. The sensitivity analysis reveals the basis and justification for model simplifications that are selected. Results show that the detailed and simplified models both well predict observed stand-alone metal hydride tank dynamics, and an example of a reversible fuel cell system model incorporating each tank demonstrates the need for model simplification.     

Dynamic modeling and simulation of hydrogen supply capacity from a metal hydride tank

The current study presents a modeling of a LaNi₅ metal hydride-based hydrogen storage tank to simulate and control the dynamic processes of hydrogen discharge from a metal hydride tank in various operating conditions. The metal hydride takes a partial volume in the tank and, therefore, hydrogen discharge through the exit of the tank was driven by two factors; one factor is compressibility of pressurized gaseous hydrogen in the tank, i.e. the pressure difference between the interior and the exit of the tank makes hydrogen released. The other factor is desorption of hydrogen from the metal hydride, which is subsequently released through the tank exit. The duration of a supposed full load supply is evaluated, which depends on the initial tank pressure, the circulation water temperature, and the metal hydride volume fraction in the tank. In the high pressure regime, the duration of full load supply is increased with increasing circulation water temperature while, in the low pressure regime where the initial amount of hydrogen absorbed in the metal hydride varies sensitively with the metal hydride temperature, the duration of full load supply is increased and then decreased with increasing circulation water temperature. PID control logic was implemented in the hydrogen supply system to simulate a representative scenario of hydrogen consumption demand for a fuel cell system. The demanded hydrogen consumption rate was controlled adequately by manipulating the discharge valve of the tank at a circulation water temperature not less than a certain limit, which is increased with an increase in the tank exit pressure.     

Impacts of external heat transfer enhancements on metal hydride storage tanks

The rate at which hydrogen can be drawn from a metal hydride tank is strongly influenced by the rate at which heat can be transferred to the reaction zone. In this work, the impacts of external convection resistance on thermodynamic behaviour inside the metal hydride tank are examined. A one-dimensional resistive analysis and two-dimensional transient model are used to determine the impact of external fins on the ability of a metal hydride tank to deliver hydrogen at a specified flow rate. For the particular metal hydride alloy (LaNi₅) and tank geometry studied, it was found that the fins have a large impact on the pressure of the hydrogen gas within the tank when a periodic hydrogen demand is imposed. Model results suggest that the metal hydride alloy at the centre of the tank can be removed to reduce weight and cost, without detrimental effects to the performance of the system.     

Energetic modeling,simulation and experimental of hydrogen desorption in a hydride tank

This paper presents a zero-dimensional (0D) model of hydride tank. The model aims to study the dynamic heat and mass transfers during desorption process in order to investigate the thermal-fluidic behaviors of this hydride tank. This proposed model has been validated experimentally thanks to a tailor-made developed test bench. This test bench allows the hydride characterization at tank scale and also the energetic characterization. The simulation results of the heat exchanges and mass transfer in and between the coupled reaction bed, show good agreement with the experimental ones. It is shown that the heat produced by a Proton Exchange Membrane Fuel Cell (PEMFC) (estimated starting from an electrical model) is enough to heat the metal alloy (FeTi) and therefore release the hydrogen with a sufficient mass flow rate to supply the PEMFC. Furthermore, the obtained results highlight the importance of the developed model for energy management of the coupling of fuel cell and hydride tank system.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

TiCrVMo alloys with high dissociation pressure for high-pressure MH tank

High-pressure metal hydride (MH) tank is a possible hydrogen storage system for fuel cell vehicles. The merit of the high-pressure MH tank system is improved by the use of a metal hydride with high dissociation pressure. In this study, TiCrV and TiCrVMo alloys with BCC structure has been developed for the high-pressure MH tank system. The developed TiCrVMo alloy shows 2.4 mass% of effective hydrogen capacity between 0.1 MPa and 33 MPa at 298 K, which has a dissociation pressure of 2.3 MPa at 298 K. By investigating the dissociation pressures of the synthesized metal hydrides, it is found that Mo has a special effect to increase dissociation pressure of the metal hydrides. This effect is probably attributed to the large bulk modulus of Mo compared to other elements.     

Hydride bed control: Understanding of hydride bed using tank efflux

Control of dehydriding (desorption of hydrogen or hydrogen isotope) rate from a hydride bed in fusion fuel cycle is one important design point to estimate a real supplying amount of hydrogen from the hydride bed. In a real system tens of batch-type hydride beds are to be utilized for supplying a certain amount of hydrogen isotope at the same time. A study on efflux time from a hydraulic water tank was applied as a fundamental similarity test of the gas fueling system. As a result, liquid efflux from a tank shows a similar behavior with the desorption pattern of the depleted uranium hydride bed system. As much important as keeping a hydride buffer vessel pressure in a hydride bed system, similar tendency was studied in the tank efflux system; i.e., to keep the secondary vessel height there needs a certain amount of liquid flow from the upper tank and the tank height difference. From one tank with connected another tank flow with understanding of tank efflux model a complicated multi-tanks behavior could be understood by simulating its complex efflux characteristics, and it is likely to be applied to the multi-hydride beds system.     

小型PEMFC电源系统热设计实验研究

通过实验研究了利用燃料电池产生的废热以强制对流传热的方式给金属氢化物储氢器加热的可行性与具体的设计方案,与目前已报道的国内外便携式PEMFC系统相比,该方案无任何附属设备,使系统保持较高的整体效率,提高了金属氢化物储氢器的放氢性能.通过正交实验和实验数据的方差分析得知该方案在保证金属氢化物储氢器持续放氢的同时,对PEMFC无明显负面影响.     

Hydrogen storage systems based on hydride materials with enhanced thermal conductivity

The reaction of hydrogen gas with a metal to form a metal hydride is exothermic. If the heat released is not removed from the system, the resulting temperature rise of the hydride will reduce the hydrogen absorption rate. Hence, hydrogen storage systems based on hydride materials must include a way to remove the heat generated during the absorption process. The heat removal rate can be increased by (i) increasing the effective thermal conductivity of the metal hydride by mixing it with high-conductivity materials such as aluminum foam or graphite, (ii) optimizing the shape of the tank, and (iii) introducing an active cooling environment instead of relying on natural convection. This paper presents a parametric study of hydrogen storage efficiency that explores quantitatively the influence of these parameters. An axisymmetric mathematical model was formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank, and to predict the transient temperatures and mass of hydrogen stored as a function of the thermal conductivity of the enhanced hydride material, aspect ratio of the cylindrical tank, and thermal boundary conditions. The model was validated by comparing the transient temperature at selected locations within the storage tank with concurrent experiments conducted with LaNi₅ material. The parametric study revealed that the aspect ratio of the tank has a stronger influence when the effective thermal conductivity of the metal hydride bed is low or when the heat removal rate from the tank surface is high (active cooling). It was also found that for a hydrogen filling time of 3 min, adding 30% aluminum foam to the metal hydride maximizes hydrogen absorption under natural convection, whereas the addition of only 10% aluminum foam maximizes the hydrogen content under active cooling. For filling times beyond 3 min, the amount of aluminum foam required to maximize hydrogen content can be reduced for both natural convection and active cooling. This study should prove useful in the design of practical metal hydride-based hydrogen storage systems.     

Numerical simulation of the hydrogen storage with reaction heat recovery using metal hydride in the totalized hydrogen energy utilization system

Numerical simulation of a hydrogen storage tank of a Totalized Hydrogen Energy Utilization System (THEUS) for application to commercial buildings was done to verify the practicality of THEUS. THEUS consists of a fuel cell, water electrolyzer, hydrogen storage tank and their auxiliary machinery. The hydrogen storage tanks with metal hydrides for load leveling have been previously experimentally investigated as an important element of THEUS. A hydrogen storage tank with 50 kg AB5 type metal hydride was assembled to investigate the hydrogen-absorbing/desorbing process, which is exothermic/endothermic process. The goal of this tank is to recover the cold heat of the endothermic process for air conditioning, and thus improve the efficiency of THEUS. To verify the practical effectiveness of this improved system, we developed a numerical simulation code of hydrogen storage tank with metal hydride. The code was validated by comparing its results with experimental results. In this code the specific heat value of the upper and lower flanges of the hydrogen storage tank was adjusted to be equal to the thermal capacity of the entire tank. The simulation results reproduce well the experimental results.     

Optimization of heat transfer device and analysis of heat & mass transfer on the finned multi-tubular metal hydride tank

Based on the previous studies on heat and mass transfer characteristics of hydride tank, whether the reaction heat of hydride bed can be removed quickly is a determinant factor of the reaction rate. As the core part of reaction system, the heat transfer optimization in the tank can significantly enhance the reaction rate. In this paper, the optimization of heat transfer fins for a finned multi-tubular metal hydride tank is presented, and the heat transfer equations of tank with various configuration fins (radius, thickness and number) are derived. By analyzing the effects of fin configurations on the heat transfer device, we found that the thermal resistance of reaction system reduces with the increase of the fin radius, thickness and number. In order to study transient reaction process inside the hydride tank with various configuration and operation conditions, a 3-D mathematical model is developed and validated based on the experimental data from literature. Through simulation and optimization on hydride tank with different configurations, we got that the fin number has the most significant positive effect on the absorption reaction process. The numerical simulation results show that the hydrogen absorption rate is proportional to hydrogen pressure, heat transfer coefficient and fluid flow velocity, and the hydrogen pressure has the most remarkable impact among these factors. The H₂ absorption is accomplished in 1720 s at 1 MPa, and the absorption reaction is completed within 2000 s at the H₂ pressure of 0.8 MPa. Moreover, the maximum difference in absorption completion time is only 190 s under different heat transfer coefficients and fluid flow velocities.     

Stress measurement of MlNi4.5Cr0·45Mn0.05 alloy during hydrogen absorption-desorption process in a cylindrical reactor

Hydrogen stored in a solid state form of metal hydrides offers a safe and efficient storage technique for hydrogen application. In a closed metal hydride tank, stresses may occur on the tank wall due to hydride expansion during hydrogen absorption process. In the present investigation, a novel testing system for stress evolution of MlNi_4.5Cr_0.45Mn_0.05 alloy in a closed cylindrical reactor during hydrogen absorption-desorption process was built. The results show that considerable swelling stress is developed on the inner reactor wall during activation process though a high free space of 45% is presented. Increasing hydrogen charging pressure and alloy loading fraction increase the as-generated swelling stress. The metal hydride particle expansion caused by hydrogen absorption is the intrinsic factor for swelling stress evolution. The presence of particle agglomerate in a closed tank in which its expansion is constrained is responsible for the observed swelling stress accumulation.     

Structural analysis of metal hydride-based hybrid hydrogen storage systems

Hybrid hydrogen storage systems, which see the adoption of metal hydride materials charged at high pressure, can be a viable method to reach good gravimetric and volumetric capacities under selected conditions, since hydrogen is stored both as element bound to the hydride and as high pressure gas. A general structural model, which can simulate high pressure hybrid storage tanks, has been developed, with the aim of describing the performance of the system under various operating conditions. A baseline case has been simulated, comparing tanks composed of SS316 and IM6 graphite fiber reinforced epoxy composite that contain metal hydride materials that can store weight fractions of bound hydrogen ranging from 2% to 8%. Sensitivity analyses were performed for the baseline studies with the aim of determining the operating conditions that maximize gravimetric and volumetric capacities. Results show that high pressure systems are optimal (in terms of gravimetric and volumetric capacity) for tank materials having low density and a high allowable stress, while a low operating pressure is preferable for high density tank materials, especially when coupled with metal hydrides capable of storing a high weight fraction of bound hydrogen.     

Dynamic first principles model of a complete reversible fuel cell system

A dynamic model of a discrete reversible fuel cell (RFC) system has been developed in a Matlab Simulink^® environment. The model incorporates first principles dynamic component models of a proton exchange membrane (PEM) fuel cell, a PEM electrolyzer, a metal hydride hydrogen storage tank, and a cooling system radiator, as well as empirical models of balance of plant components. Dynamic simulations show unique charging and discharging control issues and highlight factors contributing to overall system efficiency.     

Enhancement of hydrogen storage performance in shell and tube metal hydride tank for fuel cell electric forklift

Novel metal hydride (MH) hydrogen storage tanks for fuel cell electric forklifts have been presented in this paper. The tanks comprise a shell side equipped with 6 baffles and a tube side filled with 120 kg AB₅ alloy and 10 copper fins. The alloy manufactured by vacuum induction melting has good hydrogen storage performance, with high storage capacity of 1.6 wt% and low equilibrium pressure of 4 MPa at ambient temperature. Two types of copper fins, including disk fins and corrugated fins, and three kinds of baffles, including segmental baffles, diagonal baffles and hole baffles, were applied to enhance the heat transfer in metal hydride tanks. We used the finite element method to simulate the hydrogen refueling process in MH tanks. It was found that the optimized tank with corrugated fins only took 630 s to reach 1.5 wt% saturation level. The intensification on the tube side of tanks is an effective method to improve hydrogen storage performance. Moreover, the shell side flow field and hydrogen refueling time in MH tanks with different baffles were compared, and the simulated refueling time is in good agreement with the experimental data. The metal hydride tank with diagonal baffles shows the shortest hydrogen refueling time because of the highest velocity of cooling water. Finally, correlations regarding the effect of cooling water flow rate on the refueling time in metal hydride tanks were proposed for future industrial design.     

New dynamic modeling of a real embedded metal hydride hydrogen storage system

In this work, the dynamic responses of the on board hydrogen storage system with commercially available metal hydride tanks are investigated based on the database collected from fuel cell electric vehicles operation experiments. A mathematical model considering the heat transfer measured by the temperature control system is developed to analyze the absorption and desorption reaction during operation. As a seal container filled with unknown metal hydride, the practical used hydrogen storage tank is a gray box in the embedded storage system. Without the information about characteristics of material, current operation state and degradation degree, the parameters of the model are uncertain. Particle swarm optimization (PSO) algorithm is applied to search for the optimal parameter set. The simulation results of the hydrogen storage system model using these parameters show great agreements with the real operating data under different temperature conditions; the maximal error is lower than 9%.     

Energy management of a thermally coupled fuel cell system and metal hydride tank

Being produced from renewable energy, hydrogen is one of the most efficient energy carriers of the future. Using metal alloys, hydrogen can be stored and transported at a low cost, in a safe and effective manner. However, most metals react with hydrogen to form a compound called metal hydride (MH). This reaction is an exothermic process, and as a result releases heat. With sufficient heat supply, hydrogen can be released from the as-formed metal hydride. In this work, we propose an integrated power system of a proton exchange membrane fuel cell (PEMFC) together with a hydride tank designed for vehicle use. We investigate different aspects for developing metal hydride tanks and their integration in the PEMFC, using water as the thermal fluid and a FeTi intermetallic compound as the hydrogen storage material. Ground truth simulations show that the annular metal hydride tank meets the hydrogen requirements of the fuel cell, but to the detriment of the operating temperature of the fuel cell (FC).     

Experimental study of a metal hydride vessel based on a finned spiral heat exchanger

The reaction time of hydrogen in metal hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure, and hydrogen tank volume) has been discussed and obtained results show that a good choice of these parameters is important.     

Numerical investigation of hydrogen absorption in a stackable metal hydride reactor utilizing compartmentalization

In this paper, a three-dimensional model for hydrogen absorption in a metal alloy has been developed, validated against the experimental data in the literature, and then applied to a novel design for a hydrogen storage unit. The proposed design is similar to the fuel cell stack, but here the Membrane Electrode Assembly (MEA) has been replaced by a metal hydride (MH) reactor placed between the flow-field plates. These are stacked together to achieve the required amount of hydrogen storage. The flow-field plates have channels engraved on one side for hydrogen supply and on the other, for coolant/heating medium. It is known that the effectiveness of a hydrogen storage unit is directly related to its heat transfer area, and therefore, the choice of its geometry is very important. The larger the size, the more the resistance to heat transfer. Although, the internal tubular heat exchangers have proven to be effective in heat transfer, they pose severe challenges such as cooling/heating medium leakage due to tube erosion, stresses generated, etc. and they displace the active metal hydride from the tank. The present stacked MH reactor configuration helps to overcome these challenges by stacking small MH reactors together and there is no chance of the cooling/heating medium leaking into the metal hydride. Numerical simulations were performed to investigate the effect of coolant flow rate and percentage of flow-field plate rib area exposed to the MH reactor on temperature evolution and the amount of hydrogen stored. Further, a detailed study was carried out to understand the effect of compartmentalization of the MH reactor on temperature distribution. The results revealed that compartmentalization substantially helps to uniformly distribute the temperature in the metal bed, which is very important to maintain uniform utilization of the metal powder. Consequently, the uniform metal powder density for repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities.     

Thermal–fluid behavior of the hydriding and dehydriding processes in a metal hydride hydrogen storage canister

A study of the hydrogen absorption and desorption processes using LaNi₅ metal hydride is presented for investigation on the influences of expansion volume and heat convection. The hydrogen storage canister comprises a cylindrical metal bed and a void of expansion volume atop the metal. The expansion volume is considered as a domain of pure hydrogen gas. The gas motion in the metal hydride bed is treated as porous medium flow. Concepts of mass and energy conservation are incorporated in the model to depict the thermally coupled hydrogen absorption and desorption reactions. Simulation results show the expansion volume reduces the reaction rates by increasing thermal resistance to the heat transfer from the outside cooling/heating bath. The assumption usually adopted in simulating heat transfer in a metal hydride tank that heat convection in the reaction bed may be ignored is not valid when expansion volume is used because heat convection dominates the heat transfer through the expansion volume as well as the metal bed. The details of the thermal flow pattern are demonstrated. It is found that, due to the action of thermal buoyancy, circulations are likely to happen in the expansion volume. The hydrogen gas accordingly, instead of going directly between the inlet/outlet and the metal bed, tends to move with the circulation along the boundary of the expansion volume.