Sensitivity study of alanate hydride storage system

A successful metal hydride application is closely related to an optimized design of the storage hydrogen system. In previous studies, Hardy and Anton developed scoping and numerical models describing phenomena occurring during the loading process in an alanate storage system having the configuration of a cylindrical shell, tube and fin heat exchanger. In this paper, the numerical tool is used to evaluate the influence of varying the fin thickness and the number of heat exchanger tubes on both the loading and discharging processes. The objective is to evaluate the influence of the geometric parameters of these heat exchangers on the management of heat to be removed/supplied during the sorption process and thus optimize the loading/discharging times; while having the maximum possible volume for containing the hydride and the lightest weight of the storage system. Results showed that equipping the storage system with fins fitted to the heat exchanger tubes is the best design for efficient use of the hydride bed. In the absence of fins, a number of optimal tubes is determined, however, the hydrogen uptake rate is still lower than one obtained for the finned case and there is a reduction of volumetric and gravimetric storage capacities by comparison to the finned system.     

A novel multilayer fin structure for heat transfer enhancement in hydride‐based hydrogen storage reactor

Metal hydrides are promising means for compact hydrogen storage. However, the poor heat transfer in the tank packed with metal hydride powders often hinders the system from charging or discharging hydrogen effectively. In this investigation, a tube‐fin heat exchanger is supposed to be inserted to the tank, and an optimization problem accounting for both heat transfer enhancement and cost is formulated. We solve the problem with approximate analytical methods, and the influences of fin geometry are discussed. The comparison results support using quadratic curve‐shaped fins, whose effectiveness is also proved by the numerical simulation results. Furthermore, a novel multilayer fin structure with varying width is proposed, and the key parameters of it are discussed, including the number and the arrangement of fins. This paper is expected to provide new insights for the heat transfer enhancement design of hydride‐based hydrogen storage system.     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

Enhancement of heat transfer in hydrogen storage tank with hydrogen absorbing alloy (optimum fin layout)

Optimization of the fin layout in a metal hydride (MH) bed has been sought to enhance poor heat transmission in a hydrogen storage tank, and to obtain a maximum hydrogen absorption rate with a smaller volume of fins. Two different fin configurations, radial and circular fins, in a vertical cylindrical reactor vessel were tested with a La‐Ni‐based AB5 type hydrogen storage alloy. A two‐dimensional transient heat conduction analysis, coupled with predicted temperature and concentration of absorbed hydrogen in the bed for the exothermic hydride reaction, was used to evaluate enhancement of the hydrogen absorption time. The estimated temperature and concentration agreed within 6 K and 8.5%, respectively, with our experimental results. The effect of thickness and the spacing and shape of fins on the hydrogen absorption time were analytically evaluated, so that the optimum range of the each fin layout was obtained by the trade off between absorption time and reduction in the MH volume due to the volume occupied by fins. The hydrogen absorption time for the recommended layout of circular fins was reduced to approximately one‐third of that without fins. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 165–183, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20195     

Bio-inspired leaf-vein type fins for performance enhancement of metal hydride reactors

Hydrogenation of metals is an exothermic and reversible process. Thus, metal hydride reactors/devices become essentially heat-driven. Excellent heat control in the MH reactor is required to develop metal hydride devices such as H2 storage systems successfully. Few attempts at nature-inspired designs have proven to have good heat transfer capabilities. Based on this idea, the present study investigates novel bio-inspired leaf-vein type fins for the metal hydride reactor. Two reactor designs are proposed for heat transfer fluid flow, namely (i) central straight tube and (ii) narrow trapezoidal channels with 10 kg of LaNi₅ as a sample alloy. Compared to longitudinal finned single tube reactors (LFSTR), these designs provided better heat transmission and temperature uniformity. For LFSTR, Case-1, and Case-2, 90% storage capacity was reached in 210, 145, and 80 s. Different fin configurations, such as parallel, inclined fins, and fins of different thicknesses, are investigated further in the design with narrow trapezoidal channels. The inclined fin configuration shows better performance, and it is further optimized by varying the inclination angle from 3 to 9° and the fin number from 2 to 4. The optimized design with a 7° inclination angle and four fins required 57 s to attain 90% storage capacity and reduced absorption time by 73% compared to LFSTR. The influence of operating parameters such as hydrogen supply pressure, inlet temperature, and velocity of the heat transfer fluid on the performance is evaluated for the optimized design.     

Constructal entransy dissipation rate minimization of a disc

Disc cooling problem is optimized by taking entransy dissipation rate minimization as optimization objective. The non-dimensional mean temperature difference of the disc cooling model with radial high conducting fins inserted is deduced. The effects of the fin geometry, the fin aspect ratio, the ratio between the high conductivity and low conductivity, the relative amount of high conductivity material and the number of high conducting fins on the entransy dissipation rate of disc cooling are analyzed. The optimization results show that the high conducting fin should be extended to the centre of circle as the heat transfer effect of the high conducting fins is improved, and there exists an optimal fin aspect ratio corresponding to minimum entransy dissipation rate for different high conducting effects of the fin, and the number of high conducting fins has a slight effect on the entransy dissipation rate. Comparison with those for maximum temperature difference minimization shows that the constructs based on entransy dissipation rate minimization are different from those based on maximum temperature difference minimization, but the optimal constructal shape changing potentials of the number of fins and the relative amount of high conductivity material are similar.     

Numerical modelling and heat transfer optimization of large-scale multi-tubular metal hydride reactors

Heat management during the absorption/desorption process is a key aspect in improving the performance of large-scale hydrogen storage systems. In this article, the absorption and desorption performance of a multi-tubular hydride reactor is numerically investigated and optimized for 60 kg mass of LaNi₅ alloy. The 90% absorption with 7, 14, and 19 tubes is achieved in 985, 404, and 317 s with an overall reactor weight of 78.46, 88, and 88.2 kg, respectively. The 14-tube reactor performance is investigated by introducing the longitudinal fins inside the tubes. The reactor performance is enhanced by allocating fins into different pairs of half and full fins constrained by overall fin volume. A thermal resistance network model is presented to investigate the effect of fin distribution and coolant velocity on equivalent resistance of the metal hydride reactor. Storage performance obtained from numerical model validates the thermal resistance analysis from heat transfer viewpoint. With six full fins, 90% hydrogen absorption is achieved in 76 s. However, tubes with 6, 8, and 12 pairs of half and full fins require 74, 58, and 54 s, respectively. The 14-tube reactor with 8 pairs of half and full fins is used for quantifying the augmentation in the absorption performance in response to operating conditions (supply pressure and heat transfer fluid temperature). A design methodology is outlined for the development of a large-scale multi-tubular hydride reactor based on a heat transfer optimization strategy.     

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.     

Hydrogen absorption performance of a novel cylindrical MH reactor with combined loop-type finned tube and cooling jacket heat exchanger

A novel cylindrical metal hydride (MH) reactor with loop-type finned tube and jacket heat exchanger was proposed in this work. This MH reactor is expected to possess high performance due to the enhanced heat transfer, compact structure and good gas tightness. A three-dimensional multi-physical model for hydrogen absorption was presented to investigate the evolutions of temperature and concentration in the MH bed, as well as the mean reaction rate of hydrogen absorption process. The effects of different fin configurations on the performance of the proposed MH reactor were also examined. It was indicated that the evolution curve of the mean reaction rate for the whole hydrogen absorption process can be divided into two stages. The reaction rate in the first stage is mainly dependent on the initial conditions (i.e., temperature and gas pressure) of MH bed, whereas the second stage is mainly influenced by the heat dissipation from MH bed to cooling fluid. For the proposed MH reactor, the total charging time for reaching 90% hydrogen saturation can be decreased by 56.8% and 81.9% as compared with that for cylindrical MH reactor with finned double U-shape tube heat exchanger and cylindrical MH reactor with finned single-tube heat exchanger, respectively. Also, it was found that the interlaced layout design of inner and outer fins can improve the uniformity of the temperature distribution inside the MH bed as compared with the parallel layout configuration. Besides, it was showed that increasing the number of fins with keeping the total fin volume constant, the absorption performance of the reactor can be improved.     

Improvement of heat transfer through fins: A brief review of recent developments

Fins or extended surfaces are generally used in heat exchangers to enhance heat transfer between the main surface and ambient fluid. Various types of simple‐shaped fins, namely, rectangular, square, annular, cylindrical, and tapered, have been used with different geometrical combinations. To satisfy industrial demand, different trials have also been carried out for designing optimized fins. The optimization of fins can be performed either by enhancing heat dissipation at an exact fin weight or by diminishing the weight of the fin by precise heat dissipation. Recently a notable amount of work on some typical fins, like, porous fins and perforated fins, has also been carried out. This paper presents a brief review on heat transfer enhancement using fins of different types considering variable thermophysical and geometric parameters, which will also be useful for future use of geometrical modifications of extended surfaces, based on the cost and availability of space.     

Heat management on rectangular metal hydride tanks for green building applications

A numerical study fully validated with solid experimental results is presented and analysed, regarding the hydrogenation process of rectangular metal hydride tanks for green building applications. Based on a previous study conducted by the authors, where the effective heat management of rectangular tanks by using plain embedded cooling tubes was analysed, in the current work the importance of using extended surfaces to enhance the thermal properties and the hydrogenation kinetics is analysed. The studied extended surfaces (fins) were of rectangular shape; and several combinations regarding the number of fins and the fin thickness were examined and analysed. The values for fin thickness were 2-3-5 and 8 mm and the number of fins studied were 10-14-18 and 20. To evaluate the effect of the heat management process, a modified version of a variable named as Non-Dimensional Conductance (NDC) is introduced and studied. A novel AB₂-Laves phase intermetallic was considered as the metal hydride for the study. The results of the hydrogenation behaviour for the introduced parameters (fin number and thickness) showed that the rectangular tank equipped with the cooling tubes in combination with 14 fins of 5 mm fin thickness has the capability of storing hydrogen over 90% of its theoretical capacity in less than 30 min.     

Thermofluidynamic modelling of hydrogen absorption in metal hydride beds by using multiphysics software

In this study, a performance analysis of metal hydride reactors (MHRs) based hydrogen storage during absorption process is presented. The study shows the effect of using heat pipe and fins for enhancing heat transfer inside MHRs at various hydrogen supply pressures. Three different cylindrical MHR configurations using LaNi₅ as a storage media were adopted including: i) reactor cooled by means of natural convection, ii) reactor equipped with a heat pipe along its central axis, iii) reactor equipped with finned heat pipe. A 3-D mathematical model is developed and utilized to simulate the thermofluidynamic behaviour of a metal hydride bed. The simulation study is conducted by solving simultaneously the energy, mass, momentum, and kinetic differential equations of conservation by using COMSOL multiphysics 5.2a software. Parameters such as hydrogen stored capacity, internal temperature distribution for the reactor, and their duration have been optimized. The model was validated against experimental result which have been previously published by the authors. The obtained results confirmed that the simulation and experimental results reasonably match where the maximum error vlaue was less than 8% at 10-bar hydrogen supply pressure, which proves that the model has efficiently captured the key experimental trends. On the other hand, the MHR design, which is equipped with a finned heat pipe is shown a superior performance as compared to all the other tested configurations in terms of charging time and storage capacity. Therefore, the model can be used as a helpful tool in the optimization of the MHR designs and performance.     

Constructal design of Y-shaped assembly of fins

This work relies on constructal design to perform the geometric optimization of the Y-shaped assembly of fins. It is shown numerically that the global thermal resistance of the Y-shaped assembly of fins can be minimized by geometric optimization subject to total volume and fin material constraints. A triple optimization showed the emergence of an optimal architecture that minimizes the global thermal resistance: an optimal external shape for the assembly, an internal optimal ratio of plate-fin thicknesses and an optimal angle between the tributary branches and the horizontal. Parametric study was performed to show the behavior of the minimized global thermal resistance. The results also show that the optimized Y-shaped structure performs better than the optimized T-shaped one.     

Numerical Study of Solid State Hydrogen Storage System with Finned Tube Heat Exchanger

Abstract

Absorption of hydrogen gas inside the metal hydride (MH)-based hydrogen storage system generates significant amount of heat. This heat must be removed rapidly to improve the performance of the system which can be accomplished by embedding a heat exchanger inside the MH bed. In this article, a tubular shape MH system, equipped with a heat exchanger consisting of copper tube and pin fin is presented. A detailed 3D mathematical model is developed using COMSOL Multiphysics 4.3b for the numerical study of absorption and desorption processes inside the storage system. Impact of various operating and geometric parameters on the charging time of the storage system has been examined. It is observed that these geometric and operating parameters influence the charging time of the storage system. In the last, the impact of heat exchanger material on the performance of the storage system is explored. It is found that aluminum made heat exchanger is the best for the storage systems. The absorption process is accomplished in 1152?s at the operating parameters of 15?bar, 298 K, and 6.75 lit/min. This numerical work suggests that the efficient design of storage system is very important for rapid absorption and desorption of hydrogen.     

Design and investigation of hydriding alloy based hydrogen storage reactor integrated with a pin fin tube heat exchanger

The reaction between metal hydride (MH) and hydrogen gas generates substantial amount of heat. It must be removed rapidly to sustain the reaction in the metal hydride hydrogen storage reactor. Previous studies indicate that the performance of the reactor can be improved by inserting an efficient heat exchanger design inside the metal hydride bed. In the present study, a cylindrical shaped metal hydride system containing LaNi₅, integrated with a finned tube heat exchanger assembly made of copper pin fins and tubes, is presented. A 3-D numerical model is formulated in COMSOL Multiphysics 4.4 to study the transient behavior of sorption process inside the reactor. Experimental data obtained from the literature is used to approve the legitimacy of the proposed model. Influence of various operating and geometric parameters on the total absorption time of the reactor has been investigated. It is found that hydrogen supply pressure is the most influencing factor to increase the absorption rate of hydrogen. Total absorption time of the reactor is found to be 636 s with maximum storage capacity of 1.4 wt% at the operating conditions of 15 bar H₂ gas supply pressure, heat transfer fluid temperature of 298 K and flow rate of 6.75 l/min.     

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

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

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

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

A metal hydride system for a forklift: Feasibility study on on-board chemical storage of hydrogen using numerical simulation

This paper describes a technical feasibility study of on-board metal hydride storage systems. The main advantages of these systems would be that of being able to replace counterweights with the weight of the storage system and using the heat emissions of fuel cells for energy, making forklifts a perfect use case. The main challenge is designing a system that supplies the required energy for a sufficiently long period. A first draft was set up and analyzed to provide a forklift based on a fuel cell with hydrogen from HydralloyC5 or FeTiMn. The primary design parameter was the required amount of stored hydrogen, which should provide energy equal to the energy capacity of a battery in an electric vehicle. To account for highly dynamic system requirements, the reactor design was optimized such that the storage was charged in a short time. Additionally, we investigated a system in which a fixed amount of hydrogen energy was required. For this purpose, we used a validated simulation model for the design concepts of metal hydride storage systems. The model includes all relevant terms and parameters to describe processes inside the system's particular reactions and the thermal conduction due to heat exchangers. We introduce an embedded fuel cell model to calculate the demand for hydrogen for a given power level. The resulting calculations provide the required time for charging or a full charge depending on the tank's diameter and, therefore, the necessary number of tanks. We conclude that the desired hydrogen supply times are given for some of the use cases. Accordingly, the simulated results suggest that using a metal hydride system could be highly practical in forklifts.     

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.