Heat transfer characteristics of expanded graphite matrices in metal hydride beds

The thermal conductivity of metal hydride powders and the heat transfer coefficients between metal hydride powder beds and the walls of reaction beds are usually low. When metal hydride powders are used in practical applications, heat transfer enhancement techniques have to be generally applied. Here, expanded natural graphite/metal hydride compacts are described, possessing a high-effective thermal conductivity. Furthermore, the heat transfer coefficient between expanded natural graphite pellets and the inner surface of the surrounding steel tube has been investigated.     

Effect of a varying effective thermal conductivity term on heat conduction through a physical model of a hydride bed

This paper investigates the effect of implementing a varying effective thermal conductivity term on heat transfer simulations within a metal hydride (MH) reactor. The work presented utilises a 1D transient heat transfer analysis comparing simulations either with a fixed effective thermal conductivity or a varying effective thermal conductivity. This builds on previously published work in the literature that has investigated varying effective thermal conductivity for metal hydrides at a single point. The results of these simulations are compared to experimental work for validation.     

Simulation of Effective Thermal Conductivity of Metal Hydride Packed Beds

Several mathematical models are available for estimation of effective thermal conductivity of nonreactive packed beds. Keeping in view the salient differences between metal hydride beds in which chemisorption of hydrogen takes place and conventional nonreactive packed beds, modified models are proposed here to predict the effective thermal conductivity. Variation in properties such as solid thermal conductivity and porosity during hydrogen absorption and desorption processes are incorporated. These extended models have been applied to simulate the effective thermal conductivity of the MmNi_4.5Al_0.5 hydride bed and are compared with the experimental results. Applicability of the extended models for estimation of the effective thermal conductivity at different operating conditions such as pressure, temperature, and hydrogen concentration is discussed.     

In situ characterization of metal hydride thermal transport properties

Thermal property characterization of a metal hydride was conducted in a high pressure hydrogen environment with a transient plane source apparatus. This apparatus was integrated with a pressure vessel to measure the effective thermal conductivity and the thermal diffusivity of non-reactive and activated metal hydrides in hydrogen pressures up to 275 bar and 253 bar, respectively. In a non-reactive oxidized condition, the effective thermal conductivity of Ti_1.1CrMn powder increased with hydrogen pressure from 0.8 to 1.6 W/m·K. For activated powder, effective thermal conductivity increased from 0.3 to 0.7 W/m·K with hydrogen pressure. It is postulated that the smaller particle size associated with the reactive condition caused the reduction in effective thermal conductivity. Also, the derived specific heat of the activated Ti_1.1CrMn increased with reaction progress by a factor of two, with a maximum value at the fully hydrided state.     

Drastic enhancement of effective thermal conductivity of a metal hydride packed bed by direct synthesis of single-walled carbon nanotubes

Low heat conductivity restricts the rate of hydrogen absorption into a metal hydride, and this leads to a mismatch of the required absorption rate. The use of fin systems is standard in such cases, and the use of several different materials has been attempted. This includes high thermal conductivity carbon brushes and carbon nanotube. Unfortunately, such efforts have not been effective because the boundary thermal resistance has not been addressed. In this study, we focused on the direct synthesis of a single-walled carbon nanotube (SWCNT), which has high thermal conductivity, on particles in a packed bed, for reducing boundary thermal resistance and estimated effective thermal conductivity. Referring to Raman spectra, we succeeded in growing SWCNT on a metal hydride and effective thermal conductivity was estimated as a function of the filling ratios of the metal hydride and the SWCNT. Consequently, the effective thermal conductivity can satisfy the required value.     

Performance analysis of LaNi5 added with expanded natural graphite for hydrogen storage system

This paper presents a comparative study of two cases of metal hydride hydrogen storage units working on (i) LaNi₅ (ii) Compacts of LaNi₅ incorporated with expanded natural graphite (ENG). It has been observed from the previous studies that the hydriding/dehydriding reactions eventually causes large strain changes, due to which the hydride forming metal alloys disintegrate and form a powder bed. Such reactor beds usually have a low thermal conductivity which minimizes the heat transfer phenomenon occurring during the absorption of hydrogen gas. Therefore, there is a need to implement heat augmentation methods to significantly enhance the thermal conductivity. The objective of this research is to present a 2-D numerical model using Finite Volume Method (FVM) and estimate the hydrogen storage performance of a cylindrical metal hydride bed for both the cases, i.e. powdered metal hydride bed and ENG compacts-based reactor bed at different values of inlet pressure and heat transfer fluid temperature. In this study, a detailed investigation on the absorption process reveals that reactor beds with compacted disks of LaNi₅ and ENG demonstrate an enhanced effective thermal conductivity and efficient mass transfer. The simulation results show that a remarkable improvement in the heat transfer and hydrogen storage capacity with reduced absorption time can be achieved by using LaNi₅ and ENG compacts. It was observed that the average reactor bed temperature dropped from 345.13 K to 337.37 K when the ENG based compacted disks was introduced into the reactor bed. Moreover, for supply pressure of 24 bar and fluid temperature of 293 K, the time taken to absorb hydrogen into the rector to achieve stabilized hydrogen storage capacity was estimated to be 446s and 232 s for the case of metal hydride and ENG compacts-based bed, respectively.     

Prediction of the thermal conductivity of metal hydrides – The inverse problem

With sustainability as an important and driving theme, not merely of research, but that of our existence itself, the effort in developing sustainable systems takes many directions. One of these directions is in the transport sector, particularly personal transport using hydrogen as fuel, which logically leads on to the problem of hydrogen storage. This paper deals with the prediction of the effective conductivity of beds of metal hydride for hydrogen storage. To enable modeling of the effective thermal conductivity of these systems, it is necessary to arrive at the functional dependence of the thermal conductivity of the solid hydride on its hydrogen concentration or content. This is the inverse problem in thermal conductivity of multiphase materials. Inverse methods in general are those where we start from known consequences in order to find unknown causes. Using published and known data of the effective thermal conductivity of the hydride–hydrogen assemblage, we arrive at the unknown hydride conductivity by analysis. Among the models available in the literature for determination of the effective conductivity of the bed from the properties of the constituent phases, the model of Raghavan and Martin is chosen for the analysis as it combines simplicity and physical rigor. The result is expected to be useful for predicting the thermal conductivity of hydride particles and determining the optimum heat transfer rates governing the absorption and desorption rates of hydrogen in the storage system.     

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.     

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.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

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.     

保温材料热物性测试的实验及数值研究

针对保温材料导热系数低，采用常规的导热系数测试方法难以获得准确结果的问题，根据瞬态法导热系数测试原理，对常功率平面热源法进行了研究。建立传热的二维瞬态数学模型，借助FLUENT有限体积软件对常功率平面热源法中试样的温度分布和热量传递规律进行数值模拟，开发了一套保温材料导热系数测试装置。测试结果与文献数据能较好的吻合，最大误差不超过4％。测试结果可靠，测试精度较高。     

Optimal analysis of the exothermic process of a Mg/MgH2 thermochemical heat storage system

基于镁/氢化镁热化学储热系统,建立了二维非稳态数学模型.对吸氢放热过程中的传热传质现象进行了数值模拟,主要研究了壁面温度和反应床当量导热系数对系统反应速率的影响.结果表明,放热过程中存在最佳的壁面温度使反应速率达到最快,过高或者过低的壁面温度都将使反应床的温度偏离理论上的最佳值,从而降低反应速率.针对不同当量导热系数的反应床,最佳壁面温度也不相同;反应床的当量导热系数并非越大越好,应该根据具体的边界温度以及氢气压力情况进行合理的选择以获得最佳的反应速率.     

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.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of insulators,fluids and conductors using the transient plane source (TPS) technique

Simultaneous measurements of thermal conductivity and thermal diffusivity of composite red-sand bricks, glycerine and mercury have been made at room temperature by the recently developed transient plane source (TPS) technique. This paper describes, in brief, the theory and the experimental conditions for the simultaneous measurements of thermal conductivity and thermal diffusivity of insulators, fluids and metals. The source of heat is a hot disc made out of bifilar spirals. The disc also serves as a sensor of temperature increase in the sample. The measured values of the thermal conductivity and thermal diffusivity of these samples are in agreement with the values reported earlier using other methods. The advantage of the TPS technique is the simplicity of the equipment, simultaneous information on thermal conductivity and thermal diffusivity, and also the applicability of the technique to insulators, fluids and metals.     

Modelling and experimental results of heat transfer in a metal hydride store during hydrogen charge and discharge

A numerical model for the transient hydrogen charge/discharge rates and thermal behaviour of metal hydride stores was developed and verified against experiments using a cylindrical reactor filled with AB₅-type metal hydride. The model assumes local thermal equilibrium between the gas and solid phases, and incorporates the pressure and temperature-dependent hydrogen reaction rates, as well as heat transfer in the porous metal hydride bed. The model was verified through experimental data. The experiments were performed using a unit with hydrogen storage capacity of 130 Nl H₂; the store was submerged in an isothermal water bath. Experiments at different water bath temperatures and charge/discharge hydrogen pressures indicated a relation between charge/discharge time and these parameters. The reactor's ability to deliver a constant hydrogen flow at different water bath temperatures was experimentally investigated. During simulations it was found that the model applied is sensitive to perturbations of some of its parameters; activation energy of absorption, effective conductivity and heat of reaction were found to be the most important ones. The charge and discharge performances of the store are controlled by the reaction rate in the first half-part of the H absorption/desorption experiments and by a heat transfer in the second half-part of charge/discharge.     

DESIGN AND PERFORMANCE ANALYSIS OF A METAL HYDRIDE AIR-CONDITIONER

The design and performance aspects of a 3⋅5 kW (1 ton) cooling capacity metal hydride air-conditioner working with a ZrMnFe/MmNi_4⋅5Al_0⋅5 pair are presented. The analysis is based on the heat transfer and reaction kinetics of coupled beds containing ZrMnFe alloy on the hot side and MmNi_4⋅5Al_0⋅5 on the cold side. The effects of important design and operating parameters, viz. cycle and delay times, bed thickness, effective thermal conductivity, air velocity and operating temperatures, on system performance are studied. The performance of the system is characterized by the mass of the alloys required and the COP. The results show that the initial and running costs of the system depend mainly on the internal and external heat transfer characteristics of the hydride heat exchangers. It is shown that a 1 cm ID tube, a cycle time of 3 minutes, an effective thermal conductivity of about 2⋅5 W m⁻¹ K⁻¹ and air velocity of about 3 m s⁻¹ result in optimum performance in terms of alloy inventory and COP. © 1997 by John Wiley & Sons, Ltd.     

Experimental and theoretical study of the hot-wire method applied to low-density thermal insulators

The use of the hot wire method for thermal conductivity measurement has recently known a significant increase. However, this method is theoretically not applicable to materials where radiative heat transfer is not negligible such as low-density thermal insulators. In order to better understand the influence of radiative contribution, we developed a two dimensional simulation of transient coupled heat transfer and made hot-wire measurements on low-density Expanded PolyStyrene (EPS) foams. The analysis of theoretical and experimental results shows that classical hot-wire apparatus are poorly adapted to low-density insulators. However, if an appropriate hot-wire apparatus is used, the estimated equivalent thermal conductivity is in close agreement with that estimated by the guarded hot-plate method.     

Measurement of thermal conductivity, thermal diffusivity and heat capacity of highly porous building materials using transient plane source technique

This paper presents both experimental and theoretical works concerning evaluation of the thermal conductivity, thermal diffusivity and heat capacity of wood composites. Moreover, the aim of this study is to show that the transient plane source technique originally used for measuring thermal properties of isotropic materials can be spread worthy of heat capacity, thermal conductivity and thermal diffusivity measurements of highly porous materials. Measurements of the thermal conductivity, thermal diffusivity and heat capacity have been performed at room temperature (20 ± 0.5°C) and normal pressure. An attempt has been made to predict the thermal diffusivity of wood composites from the predicted values of thermal conductivity using a Verma et al's model based on Ohms law and the calculated values of heat capacity using the enthalpy concept. The predicted values by the proposed model are compared with the values of the thermal diffusivity measured using the TPS method. A comparison shows a good agreement.     

Thermal design of a hydrogen storage system using La(Ce)Ni5

Hydrogen storage within a metal hydride involves exothermic and endothermic processes for hydrogen absorption and desorption, respectively. In addition, the thermal conductivity of the particulate metal hydride (i.e., powder) after repeated absorption processes is extremely low compared to its bulk phase. Low heat conduction through the metal hydride powder makes the hydrogen charging slow; thus, appropriate thermal management is necessary to achieve the fast charging time with the maximum energy density. In this work, we propose a thermal design of a portable hydrogen storage system made of a 300-mL vessel by balancing the internal and external thermal resistances. A copper-mesh structure is employed inside the vessel for enhancing the effective thermal conductivity of metal hydride powder (i.e., reducing the internal thermal resistance). On the other hand, a compact fan is used for enhancing the forced convection heat transfer from the vessel (i.e., reducing the external thermal resistance). Consequently, a copper-mesh structure sacrificing 4.3% of the internal vessel volume was manufactured by following the thermal design. In addition, the effect of the proposed thermal design was confirmed by actual hydrogen-charging experiments that showed 73.5% reduction of the charging time.