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).     

Hydrogen storage in metal hydride tanks equipped with metal foam heat exchanger

This paper presents a two-dimensional mathematical model to evaluate transient heat and mass transfer in a metal hydride tank (hereinafter MHT) with metal foam heat exchanger. The model is validated by comparison with experimental data. A good agreement is obtained.     

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.     

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.     

Experimental and numerical study of a magnesium hydride tank

A small-scale experimental magnesium hydride tank was designed and tested to illustrate the feasibility of hydrogen storage in magnesium hydride. A prototype of the tank was filled with 123 g of previously ball-milled and doped MgH₂. About 80 nl of hydrogen can be reversibly stored at a pressure less than 1 MPa. However, owing to the fact that the heat of a reaction limits the absorption and desorption processes, these latter are slowed down. To have a better understanding of the heat and mass transfers in the tank, a numerical model was developed using the Fluent software. The numerical simulations of hydrogen sorption are found to be in good agreement with the experimental results. During desorption, as an example, the reaction occurs locally and progresses from the tank walls towards the core.     

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.     

Accelerating hydrogen absorption in a metal hydride storage tank by physical mixing

A novel method to improve the hydrogen absorption rate in a metal hydride tank is proposed by introducing physical mixing of the metal hydride powder to promote heat removal and accelerate the kinetics of the hydriding process. Experiments were conducted with and without mixing to demonstrate that the hydrogen absorption rate can be improved significantly by mixing. Mixing was achieved by tilting the cylindrical metal hydride tank back and forth by 90° during charging. A mathematical model was also developed to simulate the effects of physical mixing. The model results indicate that physical mixing enhances heat transfer by redistributing the hydride powder from the hot core to the boundary and facilitates heat removal by convection at the tank walls. After validating the model against experimental results, the effect of physical mixing on accelerating hydrogen storage was explored by changing the mixing rate and the convection coefficient at the tank wall, and by increasing the thermal conductivity of the hydride bed by adding aluminum foam. It was found that while higher mixing rates generally improve the absorption rate, the benefits of mixing are reduced for higher convection coefficients, and for higher weight fractions of Al foam. Simulations were also conducted with and without mixing as a function of tank size. The results show that the benefit of physical mixing increases with tank size.     

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.     

Mass transfer characteristics of the hydrophilic membrane with the isolation of heat transfer effect by isothermal bath

The efficiency and lifetime of a proton exchange membrane fuel cell (PEMFC) system is critically affected by the humidity of incoming gas which should be maintained properly for normal operating conditions. But the experimental characteristics of the humidifier are rarely reported. Water transport through the hydrophilic membrane is a coupled phenomenon of heat and mass transport. In this study, a laboratory scale test bench is designed to investigate the characteristics of water transport through the hydrophilic membrane. The mass transfer capability of the hydrophilic membrane is evaluated over various flow rates, temperature, pressure, and flow arrangements. In the experiment, the test bench is submerged in a constant temperature bath in order to isolate the effect of temperature variation between dry air and humid air. The results show the water transport of the hydrophilic membrane is significantly affected by operating temperature and operating pressure. Additionally, the flow arrangement demonstrates a minor effect but it should be considered along with the heat transfer effect.     

Numerical analysis of heat and mass transfer during absorption of hydrogen in metal hydride based hydrogen storage tanks

In this paper, hydriding in a cylindrical metal hydride hydrogen storage tank containing HWT5800 (Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5) is numerically studied with a two-dimensional mathematical model. The heat and mass transfer of this model is computed by finite difference method. The effects of supply pressure, cooling fluid temperature, overall heat transfer coefficient and height to the radius ratio of the tank (H/R) on the hydriding in the hydrogen storage tank are studied. It is found that hydride formation initially takes place uniformly all over the bed and hydriding processes take place at a slower rate at the core region. Supply pressure, cooling fluid temperature and overall heat transfer coefficient play significant roles during the absorption of hydrogen. At the H/R = 2 both maximum bed temperature and the average bed temperature are the highest, and the hydride bed takes the longest time to saturate.     

Analysis of hydrogen storage in metal hydride tanks introducing an induced phase transformation

Hydrogen absorption in a metal hydride tank is generally studied based on a heat and mass transfer analysis. The originality of this investigation is that the phase transformation from a solid (α phase) to hydride (β phase) solution is included in the hydrogen absorption mechanism. Toward this end, a modelling of the equilibrium pressure, composition (absorbed or desorbed hydrogen atoms per metal atoms), and isothermal curves of a LaNi₅ alloy is performed. Moreover, a kinetic model is developed taking into account the steps of hydrogen absorption and desorption (i.e., physisorption, chemisorption, surface penetration, nucleation and growth of the hydride phase and diffusion).     

Efficiency improvement of a PEMFC system by applying a turbocharger

A novel proton exchange membrane fuel cell (PEMFC) system that employs a turbocharger to reuse waste energy is proposed. This study is focused on optimal placement of the PEMFC turbocharger, using either the hydrogen feed stream or the stack outlet as the motive force. A one-dimensional isothermal two-phase model for a PEMFC and an isenthalpic model for the turbocharger are developed for system analysis. To find a better energy source, the system efficiency, power generation, and interaction with the balance of plant (BOP) are considered. To evaluate the feasibility of both systems, an exergy analysis is also performed. The results show that the system efficiency is higher when the turbocharger is installed after the hydrogen tank and that the amount of power generation is also larger when the turbocharger is located in the stack outlet. For the exergy analysis, the case for the turbocharger installed after the hydrogen tank shows higher performance. Therefore, it is preferable to install the turbocharger after the hydrogen tank.     

Active disturbance rejection control of metal hydride hydrogen storage

Metal hydride (MH) hydrogen storage is used in both mobile and stationary applications. MH tanks can connect directly to high-pressure electrolyzers for on-demand charging, saving compression costs. To prevent high hydrogen pressure during charging, hydrogen generation needs to be controlled with consideration for unknown disturbances and time-varying dynamics. This work presents a robust control system to determine the appropriate mass flow rate of hydrogen, which the water electrolyzer should produce, to maintain the gaseous hydrogen pressure in the tank for the hydriding reaction. A control-oriented model is developed for MH hydrogen storage for control system design purposes. A proportional-integral (PI) and an active disturbance rejection control (ADRC) feedback controllers are investigated, and their performance is compared. Simulation results show that both the PI and ADRC controllers can reject both noises from the output measurements and unknown disturbances associated with the heat exchanger. ADRC excels in eliminating disturbances produced by the input mass flow rate, maintaining the pressure of the tank at the charging pressure with little oscillations. Additionally, the parameters estimated by the ADRC's extended state observer was used to predict the state-of-charge (SOC) of the MH.     

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.     

Role of heat pipes in improving the hydrogen charging rate in a metal hydride storage tank

Metal hydrides can store hydrogen at high volumetric efficiencies. As the process of charging hydrogen into a metal powder to form its hydride is exothermic, the heat released must be removed quickly to maintain a rapid charging rate. An effective heat removal method is to incorporate a heat exchanger such as a heat pipe within the metal hydride bed. In this paper, we describe a two-dimensional numerical study to predict the transient heat and mass transfer in a cylindrical metal hydride tank embedded with one or more heat pipes. Results from a parametric study of hydrogen storage efficiency are presented as a function of storage tank size, water jacket temperature and its convective heat transfer coefficient, and heat pipe radius and its convective heat transfer coefficient. The effect of enhancing the thermal conductivity of the metal hydride by adding aluminum foam is also investigated. The study reveals that the cooling water jacket temperature and the heat pipe's heat transfer coefficient are most influential in determining the heat removal rate. The addition of aluminum foam reduces the filling time as expected. For larger tanks, more than one heat pipe is necessary for rapid charging. It was found that using more heat pipes of smaller radii is better than using fewer heat pipes with larger radii. The optimal distribution of multiple heat pipes was also determined and it is shown that their relative position within the tank scales with the tank size.     

Research of heat transfer processes during pre-launch chilldown of PS consumption lines of upper-stage LV

The paper deals with heat transfer processes during pre-launch chilldown of consumption lines of the upper stage propulsion systems (PS) by a cryogenic component. Propulsion systems operating on high-efficiency cryogenic propellant components may be used at the first as well as at the upper stages of launch vehicles to deliver spacecrafts into final orbits. Cryogenic PS produce high specific impulse being environmentally friendly, which may provide a growing interest towards their application. Low temperature of cryogenic propellant components may pose considerable challenges when chilling down, fueling and storing components as well as draining fuel tanks and starting PS, especially under zero gravity conditions in long-term space flights. Complex experimental facilities (EF) along with test methods as well as physical and mathematical models for calculating nonsteady multiple-phase heat transfer processes with intense phase transformations are being developed to overcome these challenges. The methods for mathematical modelling of the chilldown and cryogenic component flow processes have been proposed considering mass - and energy-conservation equations in relation to supply lines, tank gas blanket and tank volume filled with cryogen liquid. Closing dependency ratios have been specified according to results of bench and flight testing of PS to solve equations for heat transfer and friction in the cryogenic supply system components of test stand (TS) and PS.Also, the paper studies heat transfer processes during pre-launch chilldown of consumption lines (CL) of 12KRB LV PS with the lox/liquid hydrogen engine KVD-1 considering results of bench and flight testing of the block.Findings on chilldown and fueling of the 12KRB upper stage may be used to verify calculation models while designing and developing the KVTK advanced upper stage with the lox/liquid hydrogen engine RD0146 of the Angara-A5V heavy class launch vehicle.     

Aluminium alloy based hydrogen storage tank operated with sodium aluminium hexahydride Na3AlH6

Here we present the development of an aluminium alloy based hydrogen storage tank, charged with Ti-doped sodium aluminium hexahydride Na₃AlH₆. This hydride has a theoretical hydrogen storage capacity of 3 mass-% and can be operated at lower pressure compared to sodium alanate NaAlH₄. The tank was made of aluminium alloy EN AW 6082 T6. The heat transfer was realised through an oil flow in a bayonet heat exchanger, manufactured by extrusion moulding from aluminium alloy EN AW 6060 T6. Na₃AlH₆ is prepared from 4 mol-% TiCl₃ doped sodium aluminium tetrahydride NaAlH₄ by addition of two moles of sodium hydride NaH in ball milling process. The hydrogen storage tank was filled with 213 g of doped Na₃AlH₆ in dehydrogenated state. Maximum of 3.6 g (1.7 mass-% of the hydride mass) of hydrogen was released from the hydride at approximately 450 K and the same hydrogen mass was consumed at 2.5 MPa hydrogenation pressure. 45 cycle tests (rehydrogenation and dehydrogenation) were carried out without any failure of the tank or its components. Operation of the tank under real conditions indicated the possibility for applications with stationary HT-PEM fuel cell systems.     

Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger

Thermal effects during hydriding/dehydriding have a significant influence on the performance of metal hydride hydrogen storage system. The heat exchanger is widely used in the metal hydride reactor in order to improve the efficiency of system. In this work, based on mass balance, momentum balance, energy balance equations, equation of reaction kinetics and equilibrium pressure equation, a two dimensional axisymmetric model of metal hydride reactor packed with LaNi₅ is developed on Comsol platform. The model is validated by comparing its simulation results with the experiment data and the simulation results from other works. Then, the straight pipe heat exchanger and the coiled-tube heat exchanger are taken into consideration in order to improve heat transfer from metal hydride reactor to ambient environment. The complete three dimensional model is developed for the metal hydride reactor equipped with the coiled-tube heat exchanger. The case with coiled-tube heat exchanger shows better efficiency than the other. In general, the temperature in central area is higher than others. In order to cool central area effectively, two designs of heat exchangers, including the combination of coiled-tube heat exchanger and straight pipe heat exchanger and the concentric dual coiled-tube heat exchanger, are studied. The results show that it is an effective method to improve the efficiency of metal hydride reactor by equipping dual coiled-tube heat exchangers. Reduced two dimensional model is applied to metal hydride reactor with coiled-tube heat exchanger to reduce computing time. The simulation results of reduced model generally agree with those of complete three dimensional model.     

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.     

Semi-empirical evaluation of PEMFC electro-catalytic activity

The investigation of the performance of small single PEMFC was carried out by employing a purposely designed test bench with complete control of the operational parameters. A MEA preparation method was also developed constituting the base for testing new electrocatalytic materials or improved electrode assembling techniques. Beyond determining the polarization curves, other tests have been carried out like electrical resistance measurement by the current interruption method and voltammetric characterization. A semi-empirical approach based on a simplified mathematical model has been used to fit the experimental polarization data, providing useful parameters for evaluating the electrocatalytic activity together with other significant data of the MEA performance. The analysis of polarization curves includes the determination of the OCV by considering the mixed potential at the oxygen electrode by fitting the initial data with an arbitrary oxidation reaction. This can provide an estimate of hydrogen crossover. Possible change of ohmic resistance with current intensity is discussed. The preliminary results, demonstrating the feasibility of the method are reported.