Numerical study on hydrogen and thermal storage performance of a sandwich reaction bed filled with metal hydride and thermochemical material

Hydrogen storage and release process of metal hydride (MH) accompany with large amount of reaction heat. The thermal management is very important to improve the comprehensive performance of hydrogen storage unit. In present paper, thermochemical material (TCM) is used to storage and release the reaction heat, and a new sandwich configuration reaction bed of MH-TCM system was proposed and its superior hydrogen and thermal storage performance were numerically validated. Firstly, the optimum TCM distribution with a volume ratio (TCM in inner layer to total) of 0.4 was derived for the sandwich bed. Then, comparisons between the sandwich reaction bed and the traditional reaction bed were performed. The results show that the sandwich MH-TCM system has faster heat transfer and reaction rate due to its larger heat transfer area and smaller thermal resistance, which results in the hydrogen storage time is shortened by 61.1%. The heat transfer in the reaction beds have significant effects on performance of MH-TCM systems. Increasing the thermal conductivity of the reaction beds can further reduce the hydrogen storage time. Moreover, improving the hydrogen inflation pressure can result in higher equilibrium temperature, which is beneficial for the enhancing heat transfer and hydrogen absorption rates.     

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.     

Effect of pressure and temperature on cluster and particle heat transfer in a pressurized circulating fluidized bed

The present work reports the influence of pressure and bed temperature on particle‐to‐wall heat transfer in a pressurized circulating fluidized bed (PCFB). The particle convection heat transfer plays a dominant role in determining the bed‐to‐wall heat transfer coefficient. So far, no information is reported on the effect of pressure and bed temperature on particle‐to‐wall heat transfer in a PCFB in the published literature. The present investigation reports some information in this direction. The effect of system pressure and bed temperature are investigated to study their influence on cluster and particle heat transfer. The particle convection heat transfer coefficient increases with system pressure and bed temperature due to higher cluster thermal conductivity. The increase in particle concentration (suspension density) results in greater cluster solid fraction and also the particle concentration near the wall is enhanced. This results in higher cluster and particle convection heat transfer between the bed and the wall. Higher particle convection heat transfer coefficient results in enhanced heat transfer between the bed and the wall. The results will also help to understand the bed‐to‐wall heat transfer mechanism in a better way in a PCFB. Copyright © 2001 John Wiley & Sons, Ltd.     

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 resistance effect on methanol-steam reforming performance in micro-scale reformers

We numerically investigate hydrogen production based on methanol-steam reforming (MSR) using a micro-scale cylindrical packed bed reformer. The reformer wall is included in the physical model. The heat required for the reforming reaction is supplied either internally using a heating rod placed along the center of the reformer or externally by a heat flux applied at the reformer outer wall. Our results show that the thermal resistance from the heat source to the reformer environment plays an important role in the reformer performance. This thermal resistance depends on the reformer geometry, wall material and heat transfer coefficients inside the catalyst bed and outside the reformer. Based on our numerical results, it is suggested that better methanol conversion and hydrogen yield can be obtained using reformer wall material with low thermal conductivity and thin thickness. For both internal and external heating under the same heat rate supply, no significant difference in reformer performance was found.A water gas shift (WGS) reaction model was included in the present numerical model. In the reformer low-temperature zone the forward WGS reaction was clearly demonstrated, resulting in a decrease in carbon monoxide (CO) selectivity. In the high temperature zone the backward WGS reaction was also clearly demonstrated in which CO selectivity increases with the increase in temperature. For both internal and external heating under the same heat rate supply, our results indicated that CO selectivity is about thirty times lower when the WGS reaction is neglected.     

Thermal response of a heat pipe with axially “Ω”-shaped microgrooves

A transient model of capillary flow and heat transfer in a heat pipe with axially “Ω”-shaped microgrooves is developed and numerically analyzed to predict the thermal response characteristics. The transient distributions of the axial capillary radius and solid wall temperature, the evaporating mass rate, the time constant and instantaneous effective thermal conductivity are all investigated and discussed. The results indicate that the rise rate of wall temperature during the initial period is relatively larger, and the coordination of solid wall temperature response among the evaporator, adiabatic and condenser section is realized during the whole startup process. When the input power is increased/decreased, the evaporator temperatures start rising/dropping immediately. In particular, the time constant and instantaneous effective thermal conductivity in the startup process are larger than those in the shutdown process. Additionally, the accuracy of the present model is verified by experimental data obtained in this paper.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

A Numerical Investigation of Heat Transfer Mechanisms in Gas-Solid Fluidized Beds Using the Two-Fluid Granular Temperature Model

In this work, the two-fluid granular temperature model is used to investigate the heat exchanged between a heated wall and a gas-solid fluidized bed. Numerical simulations were performed in 2-D and 3-D fluidized beds using a solid phase effective thermal conductivity correlation based on the granular temperature. The heat exchange in the bubbles' wake is investigated by tracking the train of bubbles that rises along the heated wall. Large heat transfer coefficients were obtained in the rear wake region of bubbles due to relatively larger granular temperature there and intense particle circulation.     

Conduction-combined forced and natural convection in lid-driven enclosures divided by a vertical solid partition

Numerical simulations of the conduction-combined forced and natural convection (mixed convection) heat transfer and fluid flow have been performed for 2-D lid-driven square enclosure divided by a partition with a finite thickness and finite conductivity. Left vertical wall of enclosure has two different orientations in positive or negative vertical coordinate. Buoyancy forces are taken into account in the system. Horizontal walls are adiabatic while two vertical walls are maintained isothermal temperature but the temperature of the left moving wall is higher than that of the right stationary wall. Thus, heat transfer regime between moving lid and partition is mixed convection. Conduction occurs along the partition. And, pure natural convection is formed between the partition and the right vertical wall. This investigation covers a wide range of Richardson number which is changed from 0.1 to 10, thermal conductivity ratio varies from 0.001 to 10. It is observed that higher heat transfer was formed for higher Richardson number for upward moving wall for all values of thermal conductivity ratio. When forced convection becomes effective, the orientation of moving lid becomes insignificant. Heat transfer is a decreasing function of increasing thermal conductivity ratio for all cases and Richardson numbers.     

The effects of hydrogen addition,inlet temperature and wall thermal conductivity on the flame-wall thermal coupling of premixed propane/air mixtures in meso-scale tubes

The effects of hydrogen addition, inlet temperature, wall thermal conductivity and wall thickness on the flame-wall coupling of the propane/air flames in a meso-scale tube are numerically investigated using a two dimensional model along with the detailed chemical mechanism. Higher wall thermal conductivity can result in preheating the fresh mixture uniformly in strongly flame-wall coupled system, which is vital to enhance the burning rate of fuel mixture. With the increase of wall thermal conductivity or hydrogen addition, the leading edge of the flame shifts from the wall to the axis, meanwhile the flame is more convex towards the unburned side near the leading edge. As the hydrogen addition and inlet temperature increase, the flame propagation speed increases significantly, while the maximum temperature and maximum total enthalpy decrease due to the reduced heat recirculation power. The flame propagation speed has a negative correlation with heat loss. The chemical reactions in preheat zone are enhanced at low wall thermal conductivity due to the higher inner wall temperature. Thinner combustor wall leads to higher flame speed and higher heat loss simultaneously. Results have implications on the choice of solid wall material and heat recirculation design in a stable meso-scale combustor for different fuels.     

水平管壳式相变蓄热装置的强化传热研究

基于焓法模型对水平管壳式相变蓄热装置热性能的增强进行研究,首先分析蓄热过程中传统管壳式装置内材料的传热及流动机理;然后引入椭圆元素并对比椭圆内管及外壳的强化传热效果;最后对热源温度、相变材料导热系数及初始温度对装置热性能的作用规律进行探讨。结果显示,椭圆外壳的强化传热效果优于内管,同等条件下,长短轴之比为2的椭圆外壳可使蓄热时间缩短53.5%。热源温度升高,椭圆外壳的强化传热效果进一步增强,相变材料的导热系数及初始温度对装置热性能的影响较小。     

Effects of wall thermal conductivity on entropy generation and exergy losses in a H2-air premixed flame microcombustor

Microcombustion is a promising method for fulfilling the energy requirements of small-scale systems currently powered by portable batteries. However, its applications rely upon mitigation of heat losses, which adversely affect flame stability and performance. Heat losses in turn depend upon wall properties, especially thermal conductivity. It is thus necessary to systematically investigate the relationship between wall thermal conductivity and microcombustor performance using the exergy analysis. In this work, entropy generation rates of different irreversible processes in an annular microcombustor were computed for stoichiometric hydrogen-air mixture using CFD simulations of reactive flow for wall thermal conductivities in the range 0.1-325 W/m K. Chemical reaction, heat conduction, and mass diffusion were the dominant contributors to entropy generation, in the decreasing order. Irreversibilities due to combustion decreased as thermal conductivities increased. Diffusion contributions were most sensitive to the changes in thermal conductivity but chemical reaction and heat conduction contributions changed marginally. Results showed that walls did not contribute significantly to entropy generation, but increased wall heat losses at higher thermal conductivities adversely affected the exergetic performance of microcombustor through availability losses and by influencing the flow gradients. Based on the results of this study, wall thermal conductivity in the range 0.1-1.75 W/m K was found suitable in order to obtain uniform wall temperature profiles and high exergetic efficiencies.     

Investigation of boiling heat transfer for improved performance of metal hydride thermal energy storage

The inherent nature concerning the intermittency of concentrating solar power (CSP) plants can be overcome by the integration of efficient thermal energy storage (TES) systems. Current CSP plants employ molten salts as TES materials although metal hydrides (MH) have proven to be more efficient due to their increased operating temperatures. Nonetheless, the heat exchange between the MH bed and the heat transfer medium used to operate a heat engine is a critical factor in the overall efficiency of the TES system. In this work, a computational study is carried out to investigate the performance of a magnesium hydride TES packed bed using a multiphase (boiling) medium instead of single-phase heat absorption methods. The boiling heat transfer behaviour is simulated by using the Eulerian two-fluid framework. The simulations are conducted at a transient state using SST-k-ω Reynolds-Averaged Navier-Stokes equations. It is observed that, unlike the single-phase heat collection method, the multiphase heat absorption method maintains a constant temperature in the heat transfer fluid throughout the reactor. Consequently, a higher temperature gradient is realised between the MH bed and heat transfer fluid (HTF), leading to improvements in the overall reaction rate of the hydrogenation process.     

Optimization of methanol synthesis and cyclohexane dehydrogenation in a thermally coupled reactor using differential evolution (DE) method

This paper presents a study on optimization of a methanol synthesis and cyclohexane dehydrogenation in a thermally coupled reactor. A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methanol and benzene production in a thermally coupled reactor. Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes and reduces the size of the reactors. In this work, the catalytic methanol synthesis is coupled with the catalytic dehydrogenation of cyclohexane to benzene in a heat exchanger reactor formed of two fixed beds separated by a wall, where heat is transferred across the surface of tube. A steady-state heterogeneous model of the two fixed beds predicts the performance of this novel configuration. The co-current mode is investigated and the optimization results are compared with corresponding predictions for a conventional (industrial) methanol fixed bed reactor operated at the same feed conditions. The differential evolution (DE), an exceptionally simple evolution strategy, is applied to optimize methanol and benzene synthesis coupled reactor considering methanol and benzene mole fractions as the main objectives. The simulation results have been shown that there are optimum values of initial molar flow rate of endothermic stream and inlet temperature of exothermic and endothermic sides to maximize the objective function. The optimization method has enhanced the methanol mole fraction by 3.67%. The results suggest that optimal coupling of these reactions could be feasible and beneficial. Experimental proof-of-concept is needed to establish the validity and safe operation of the novel reactor.     

Parabolic modeling of the pultrusion process with thermal property variation

Pultrusion is a manufacturing method for fiber-reinforced composite with constant cross-section. In this process, a fiber creel is impregnated in a resin bath and passes through a heated die with a constant pulling force and the elevated die temperature induces the curing-resin process. At the present work the effect of variable properties (thermal conductivity and volumetric heat capacity) during the pultrusion process of thermosetting composite materials is numerically studied. The thermal properties are considered as a function of both temperature and degree of cure distributions inside the carbon/epoxy matrix composites. A two-dimensional parabolic model using the finite element method to solve the energy and degree of cure transport equations was used. These two equations are coupled by a source term from resin curing exothermic reaction. The resin cure kinetics and the properties that are temperature-dependent are both modeled by expressions obtained from the literature. The computational domain is discretized using an unstructured mesh with triangular elements and an adaptive refinement. Iterative algorithms are used to solve the algebraic equation system. Results showed that as the temperature and degree of cure along the die extension increase the volumetric heat capacity and the thermal conductivity also elevate. The influence of the pulling speed and the die temperature in the thermal property variation is also analyzed. It is verified that the temperature profile at the pultruded bar centerline for the variable property case is smoother than the constant one, similarly when the pulling speed is increased. The degree of cure development is delayed for the variable property simulation, requiring a larger die length to reach a suitable degree of cure design value. Moreover, the proper knowledge of these characteristics allows a better pultrusion process design.     

Novel structured catalysts configuration for intensification of steam reforming of methane

Structured catalysts, using highly conductive carriers, can improve the heat transfer along the catalytic bed, affording high performance with a flattened radial temperature gradient. The effect of thermal conductivity of structured carriers on highly endothermic Steam Reforming reaction is investigated. The performance of the structured catalysts, obtained on Cordierite and Silicon Carbide (SiC) monoliths, demonstrates the direct correlation between the thermal conductivity of the carrier, the methane conversion and the hydrogen productivity. The evaluation of the monolith configuration shows that the SiC “wall flow” guarantees a better axial and radial thermal distribution, with respect to the SiC “flow through”, resulting in better catalytic activity up to a temperature reaction of 750 °C. The comparison among the performance of the structured catalysts and the commercial 57-4MQ, provided by Katalco-JM, highlights the choice of structured catalysts, which require a lower temperature outside of the reactor, increasing the process efficiency.     

Thermal Modeling of Mg2Ni-Based Solid-State Hydrogen Storage Reactor

In this paper, a numerical study of coupled heat and hydrogen transfer characteristics in an annular cylindrical hydrogen storage reactor filled with Mg₂Ni is presented. An unsteady, two-dimensional (2-D) mathematical model of a metal hydride reaction bed of cylindrical configuration is developed for predicting the hydrogen storage capacity. The effect of volumetric radiation is accounted in the thermal model. Effects of hydride bed thickness, initial absorption temperature, hydride bed thermal conductivity, and hydrogen supply pressure on the hydrogen storage capacity are studied. A thinner hydride bed is found to enhance the hydriding rate, accomplishing a rapid reaction. At an operating condition of 20 bar supply pressure and 573 K initial absorption temperature, Mg₂Ni stores about 36.7 g hydrogen per kg alloy. For a given bed thickness and an overall heat transfer coefficient, there exists an optimum value of hydride bed thermal conductivity. The present numerical results are compared with the experimental data reported in the literature, and good agreement was observed.     

太阳能斯特林混凝土储热系统传热特性研究

设计碟式太阳能斯特林机混凝土储热系统,并对熔融盐及混凝土传热过程进行理论分析,对混凝土储释热过程进行模拟,运用多目标遗传算法进行优化,得到以下结论：在释热过程,选取290 ℃为流体出口的有效温度临界值,有效释热时间约2.1 h时,流体出口温度约为563 K,释热效率约为71%;高温混凝土和熔融盐沿着流程方向均存在一个温跃层区域,随着时间的延长,温跃层沿着流程方向逐渐向下游偏移,当温跃层移动到出口处时,熔融盐出口温度开始下降,温跃层占据的长度越小,储热系统效率越高;随着导热系数的增加,释热效率及有效释热时间提高。通过TOPSIS对解集进行重新排序分析,最优工况是蓄热量为2885 MJ、换热系数为672 W/（m·K）及储热效率为87%。     

循环流化床锅炉膜式水冷壁管与鳍片上的温度分布

研究了循环流化床锅炉膜式水冷壁管的传热，并通过采用二维传热分析方法，讨论了带有竖直鳍片和横向鳍片的水冷壁管上温度与热流分布。探讨了炉膛侧传热系数、水冷壁管水侧传热系数、水温、床温、水冷壁管材的导热率以及竖直鳍片部最高，然后逐渐下降，但在横向鳍处理的根部又会上升。为了验证传热分析的真实性，在1台6MWth循环流化床锅炉膜式水冷壁管的横裁面内安置了0.8mm的热电偶，测量子水管横截面上的一些点的温度。实际测量值符合得相当好。     

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed‐to‐wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross‐sectional solids concentration play a dominant role in better understanding of bed‐to‐wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed‐to‐wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed‐to‐wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed‐to‐wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.