Coiled-tube heat exchanger for High-Pressure Metal Hydride hydrogen storage systems – Part 1. Experimental study

This two-part study explores the development and thermal performance of a coiled-tube heat exchanger for hydrogen fuel cell storage systems utilizing High-Pressure Metal Hydride (HPMH). The primary purpose of this heat exchanger is to tackle the large amounts of heat released from the exothermic hydriding reaction that occurs when the hydrogen is charged into the storage vessel and is absorbed by the HPMH. The performance of heat exchanger was tested using 4 kg of Ti_1.1CrMn at pressures up to 280 bar. Tests were performed to assess the influence of different operating conditions on the effectiveness of the heat exchanger at removing the heat in a practical fill time (time required to complete 90% of the hydriding reaction). It is shown that distance of metal hydride particles from the coolant tube has the most dominant influence on hydriding rate, with particles closer to the tube completing their hydriding reaction sooner. Faster fill times were achieved by reducing coolant temperature and to a lesser extent by increasing pressurization rate. By comparing tests with and without coolant flow, it is shown that the heat exchanger reduces fill time by 75% while occupying only 7% of the storage pressure vessel volume. The second part of this study will present a 3D computational heat transfer model of the storage vessel and heat exchanger, and compare the model predictions to the experimental data.     

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger were investigated. Metal hydride beds were filled with AB₂ type hydrogen-storage alloy’s particles, Ti_0.42Zr_0.58Cr_0.78Fe_0.57Ni_0.2Mn_0.39Cu_0.03, with a storage capacity of 0.92 wt.%. Heat transfer model in the metal hydride bed based on the heat transfer mechanism for packed bed proposed by Kunii and co-workers is presented. The time-dependent hydrogen absorption/desorption rate and pressure in the metal hydride vessel calculated by the model were compared with the experimental results. During the hydriding, calculated hydrogen absorption rates agreed with measured ones. Calculated thermal equilibrium hydrogen pressures were slightly lower than the measured hydrogen pressures at the inlet of metal hydride vessel. Taking account of the pressure gradient between the inlet of metal hydride vessel and the metal hydride bed, it is considered that this discrepancy is reasonable. During the dehydriding, there were big differences between the calculated hydrogen desorption rates and measured ones. As calculated hydrogen desorption rates were lower than measured ones, there were big differences between the calculated thermal equilibrium hydrogen pressures and the measured hydrogen pressures at the inlet of metal hydride vessel. It is considered that those differences are due to the differences of the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity between the assumed and actual ones. It is important to obtain the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity both during the hydriding and dehydriding to design a metal hydride vessel.     

Hydrogen absorption by magnesium chemically deposited from a homogeneous phase: new strategies for the preparation of Mg-containing hydrogen storage materials

The use of a solution of magnesium in liquid ammonia enables the preparation of novel Mg-containing hydrogen storage materials. By using the solvent power of ammonia, magnesium particles highly dispersed on active carbon (AC) were prepared. By the use of AC on which catalytically active nickel had been previously supported, the preparation of samples was extended to include Mg---Ni binary systems. These samples were very active toward hydrogen absorption. For the Mg---Ni samples an increase in Ni loading (0.1–5 wt%) markedly enhanced the hydriding activity of the host magnesium.     

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.     

Kinetic study of the hydration of magnesium oxide for a chemical heat pump

A kinetic study of the hydration of magnesium oxide was performed to test the possibility of developing a magnesium oxide/water chemical heat pump. The hydration rate of magnesium oxide was measured by a gravimetric analysis with a sample of average particle size 10 μm for the reaction temperature 373–423 K and the reaction vapor pressure 12.3–47.4 kPa. It was a reasonable hypothesis that the reactant magnesium oxide had four reaction regimes. An empirical rate equation based on this hypothesis was proposed with parameters determined from experimentally measured values. The performance of the heat pump was estimated numerically using the rate equation. The heat output rate was large enough in comparison with other common heat pumps. It was shown that the reaction system would be applicable to a chemical heat pump system.     

On the selection of metal foam volume fraction for hydriding time minimization of metal hydride reactors

Here we examine how the hydriding time of a metal hydride reactor (MHR) varies with the volume fraction, φ_mf, of a metal foam installed in the reactor. Technically, an experimentally validated mathematical model accounting for the hydrogen absorption kinetics of LaNi₅ is used to compute the heat and mass transport in a cylindrical MHR. We then demonstrate that, with a fixed amount of metal hydride powder sealed in the reactor, saving a relatively small fraction (say, 1%) of the MHR internal volume to accommodate a metal foam usually suffices to substantially facilitate heat removal from the reactor, thereby greatly shortening the MHR hydriding time. However, for a metal foam of fixed apparent size, increasing φ_mf would reduce the metal hydride content, and hence the maximum hydrogen storage capacity, of the MHR. Consequently, if a prescribed amount of hydrogen is to be stored in the MHR, the hydriding time would decrease with increasing φ_mf at first (due to heat conduction augmentation), reach a minimum at an “optimal” φ_mf value, and then increase drastically due to metal hydride underpacking.     

“Hysteresis” in interaction of nanocrystalline magnesium with hydrogen

The kinetics of hydriding/dehydriding of magnesium has been significantly improved during the recent years by the introduction of new methods of fabrication of nanocrystalline magnesium-catalyst composites. However, the equilibrium hydrogen pressure for decomposition of nanocrystalline MgH₂ was found to be somewhat lower than for conventional magnesium hydride. Moreover, the essential difference in equilibrium hydrogen pressure for absorption and for desorption of hydrogen by nanocrystalline magnesium was reported by many authors. This difference called “hysteresis” is a common phenomenon for hydrides, but it is not observed for magnesium powders with an ordinary particle size (larger than 1 μm). The aim of the present work was to elucidate why the hysteresis arises in magnesium–hydrogen system with a decrease in particle size. It is shown that the “hysteresis” observed for nanocrystalline magnesium is an apparent phenomenon which is due to a hindered nucleation during hydriding at a pressure close to equilibrium. The pressures measured for desorption represent the real equilibrium. The plateau pressure of Mg + H₂ ↔ MgH₂ equilibrium is lower for nanocrystalline magnesium than that for conventional magnesium. This result is explained in terms of smaller surface energy of magnesium hydride in comparison with magnesium.     

Combustion of fine aluminum and magnesium powders in water

Micron-sized metal powders carried by a nitrogen flow were fed along the axis of a cylindrical hydrogen/oxygen diffusion flame. The particles ignited and burned in the water vapor at approximately 2500 K. Experiments were performed at atmospheric pressure. The environment in which particles burned was characterized in detail using computational fluid dynamics. The computations confirmed that the metal powders burned in water while the effect of oxygen and other oxidizing species could be neglected. Combustion was characterized experimentally for micron-sized powders of both aluminum and magnesium. Particle size distributions were measured using low-angle laser light scattering. Optical emission of the burning particles was recorded using filtered photomultiplier tubes. Measured durations of individual particle emission pulses were assumed to represent their burn times; these data were classified into logarithmically spaced time bins. The distribution of the particle burn times was correlated with their size distributions assuming that larger size particles burned longer. It was observed that correlation between the burn times, t, and particle diameters, D, can be approximately described as t ∼ D^0.64 and t ∼ D^0.68 for aluminum and magnesium powders, respectively. The results were compared to previous reports and possible reasons for discrepancies between the present and earlier results were discussed.     

Coiled-tube heat exchanger for high-pressure metal hydride hydrogen storage systems – Part 2. Computational model

This second part of a two-part study presents a transient, three-dimensional numerical model for a high-pressure metal hydride (HPMH) hydrogen storage system that is cooled by a coiled-tube heat exchanger. The model uses the same geometry examined in the first part of the study and its predictions are compared to experimental results also discussed in the first part. The model involves solving coupled heat diffusion and hydriding reaction equations for Ti_1.1CrMn. These equations are solved to determine the spatial distribution of hydride temperature as a function of time over the entire duration of the hydriding reaction, which is shown to agree favorably with the experimental data. The model also serves as an effective means for tracking the detailed temporal variations of the heat exchanger’s key performance parameters for different hydride locations relative to the coolant tube. These variations can aid in determining optimum placement of the coolant tube relative the hydride powder. Like the experimental study, the model proves that coolant temperature has the greatest influence on the time needed to complete the hydriding reaction.     

Design methodology and thermal modelling of industrial scale reactor for solid state hydrogen storage

In this study, a novel set of comprehensive arithmetic correlations has been proposed to design an industrial scale cylindrical reactor with embedded cooling tubes (ECT) for metal hydride (MH) based hydrogen storage and thermal management applications. Based on ASME standards, different nominal pipe sizes were imparted into a cylindrical reactor design with ECT to accommodate 50 kg of LaNi_4.7Al_0.3 alloy. A three dimensional numerical model has been developed using COMSOL Multiphysics 4.3a to predict the hydriding performance of designed reactors, which was further experimentally validated as well. At an absorption condition of 30 bar supply pressure and 298 K absorption temperature with 60 lpm volumetric HTF flow rate, 6 inch reactor with 99 ECT portrayed better heat transfer characteristics. From the parametric investigation, it is observed that the variation of supply pressure has predominant effect followed by the variation of the HTF flow rate on hydriding (absorption) kinetics of the device. However, the variation of absorption temperature has minuscule influence on the hydriding performance. At a supply condition of 30 bar and 298 K with water flow rate of 30 lpm, a hydrogen storage capacity (HSC) of 1.29 wt% was achieved within 2060 s.     

Influence of metal powder particle's shape on the kinetics of hydriding

The aim of this paper is to study the influence of the metal powder particle's shape on the kinetics of isothermal hydriding under constant pressure. We present arguments that this influence is small and may be neglected. The mathematical model of hydriding is described and applied to two series of experimental curves (for uranium and magnesium) for three convenient symmetrical model shapes: sphere, long thin cylinder, and flat thin plate. The fitting shows that quality of the approximation of the experimental curves by the model ones is comparable for these three shapes (yet spheres and cylinders provide better results compared to a plate) for similar kinetic parameters. The reasons for the shape-independent behaviour are discussed using some theoretical arguments based on the suggested model.     

Models for metal hydride particle shape,packing, and heat transfer

A multiphysics modeling approach for heat conduction in metal hydride powders is presented, including particle shape distribution, size distribution, granular packing structure, and effective thermal conductivity. A statistical geometric model is presented that replicates features of particle size and shape distributions observed experimentally that result from cyclic hydride decrepitation. The quasi-static dense packing of a sample set of these particles is simulated via energy-based structural optimization methods. These particles jam (i.e., solidify) at a density (solid volume fraction) of 0.671 ± 0.009 – higher than prior experimental estimates. Effective thermal conductivity of the jammed system is simulated and found to follow the behavior predicted by granular effective medium theory. Finally, a theory is presented that links the properties of bi-porous cohesive powders to the present systems based on recent experimental observations of jammed packings of fine powder. This theory produces quantitative experimental agreement with metal hydride powders of various compositions.     

High temperature metal hydrides for energy systems Part B: Comparison between high and low temperature metal hydride reservoirs

A dynamic analysis model aimed at describing the hydrogen absorption and desorption phases of a metal hydride has been calibrated for magnesium hydride in Part A of the present work. We can make use of the estimate of the main kinetic parameters associated to this kind of hydride in order to study the behaviour of a metal hydride-based energy system.A metal hydride becomes the basis of an energy system when the enthalpy related to its hydriding/dehydriding reactions is used by an applicator. Therefore, magnesium hydride, which is a high-temperature hydride, can be virtually placed in an energy system thanks to the model and its main energy-related characteristics can be calculated. This allows us to get a glimpse of the performances of magnesium hydride in the field of energy production and to compare them to those of well-established low-temperature hydrides, such as LaNi₅ hydride.     

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.     

A chemical heat pump using hydration of mgo particles in a three-phase reactor

A chemical heat pump using hydration of magnesium oxide in a three-phase reactor is proposed. Magnesium oxide particles suspended in the triethylene glycol are hydrated exothermally by introducing water vapour. The hydration rate was measured under the temperatures ranging from 383 K to 523 K. It was found that the reaction rate was proportional to the amount of adsorbed water molecules, and correlated in an equation.     

尺寸效应和表面效应对纳米颗粒比热容的影响

从弹性介质假设出发，考虑内部和表面原子对比热容的贡献不同，建立起纳米颗粒比热容的理论模型。分析了尺寸效应、温度和表面原子振动软化对纳米颗粒比热容的影响，提出比热容与尺寸和温度之间的关系；以氧化铜颗粒为计算对象，具体计算考虑表面和尺寸效应的比热容，结果与实验数据符合较好。     

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.     

Mathematical modelling of hydrogen storage systems

A detailed mathematical model of a hydride bed has been developed to describe its behaviour under both hydriding and dehydriding conditions. In contrast to other hydride bed models previously reported in the literature, this model simulates an actual, commercially available containment vessel, rather than that of an abstract ideal situation. Thus the model provides a convenient means of predicting the time taken to release or absorb given amounts of hydrogen. These are calculated from the heat transfer characteristics and diffusion properties of particular metal alloys. Comparisons are given between the actual operating characteristics and those simulated by the model. A brief discussion of the reaction kinetics of hydriding certain metal alloys is also included.     

Correlation between hydrogen storage properties and textures induced in magnesium through ECAP and cold rolling

It is feasible to obtain a significant enhancement of the hydrogen storage capability in magnesium by selecting an appropriate sequence of mechanical processing. The Mg metal may be produced with different textures which will then give significant differences in the absorption/desorption kinetics and in the incubation times for hydrogenation. Using processing by equal-channel angular pressing (ECAP), different textures may be produced by changing both the numbers of passes through the ECAP die and the ram speed. Significant grain refinement is easily avoided by using commercial coarse-grained magnesium as the starting material. The use of cold rolling after ECAP further increases the preferential texture for hydrogenation. The results show that the hydriding properties are enhanced with a (002) texture where the improved kinetics lie mainly in the initial stages of hydrogenation. An incubation time is associated with the presence of a (101) texture and this is probably due to the magnesium oxide stability in this direction.     

Three-dimensional modeling and simulation of hydrogen absorption in metal hydride hydrogen storage vessels

In this paper, a three-dimensional hydrogen absorption model is developed to precisely study the hydrogen absorption reaction and resultant heat and mass transport phenomena in metal hydride hydrogen storage vessels. The 3D model is first experimentally validated against the temperature evolution data available in the literature. In addition to model validation, the detailed 3D simulation results show that at the initial absorption stage, the vessel temperature and H/M ratio distributions are uniform throughout the entire vessel, indicating that hydrogen absorption is very efficient early during the hydriding process; thus, the local cooling effect is not influential. On the other hand, non-uniform distributions are predicted at the subsequent absorption stage, which is mainly due to differential degrees of cooling between the vessel wall and core regions. In addition, a parametric study is carried out for various designs and hydrogen feed pressures. This numerical study provides a fundamental understanding of the detailed heat and mass transfer phenomena during the hydrogen absorption process and further indicates that efficient design of the storage vessel and cooling system is critical to achieve rapid hydrogen charging performance.