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.     

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.     

Enhancement of hydrogen charging in metal hydride-based storage systems using heat pipe

Heat transfer in metal hydride bed significantly affects the performance of metal hydride reactors (MHRs). Enhancing heat transfer within the reaction bed improves the hydriding rate. This study presents performance analysis in terms of storage capacity and time of three different cylindrical MHR configurations using storage media LaNi₅: a) reactor cooled with natural convection, b) reactor with a heat pipe on the central axis, c) reactor with finned heat pipe. This study shows the impact of using heat pipes and fins for enhancing heat transfer in MHRs at varying hydrogen supply pressures (2–15 bar). At any absorption temperature, hydrogen absorption rate and hydrogen storage capacity increase with the supply pressure. Results show that using a heat pipe improves hydrogen absorption rate. It was found that finned heat pipe has a significant effect on the hydrogen charge time, which reduced by approximately 75% at 10 bar hydrogen supply pressure.     

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.     

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.     

Heat and mass transfer studies on plate fin-and-elliptical tube type metal hydride reactors

In the present work a two-dimensional transient model to study the heat and mass transfer characteristics of plate fin-and-elliptical tube type metal hydride reactors is presented. The relevant governing equations are solved numerically. The heat transfer coefficient and pressure drop on external fin side are estimated using available correlations. Effects of external fluid flow rate and temperature on the fin-and-tube metal hydride reactor are studied. Results show that the use of elliptical metal hydride tubes in place of the standard circular tubes give rise to compact systems in addition to considerably lower fan power consumption, with very little change in the average heat and hydrogen transfer rates. Even though the performance of the reactor depends very much on the fin-and-tube arrangement, for all the arrangements considered here, the reactors with elliptical tubes were found to perform better in terms of compactness, weight and fan power consumption. Considering the aspects of mean hydride bed thickness, tube wall thickness, tube mass, compactness, heat and hydrogen transfer rates and fan power consumption, elliptical tubes of 0.6 eccentricity appear to offer the best solution for the given application.     

The comprehensive review for development of heat exchanger configuration design in metal hydride bed

An optimal hydrogen storage reactor should have a higher chemical reaction rate by which the heat can be exchanged as fast as possible. The configuration of heat exchanger structure design plays a crucial role in improving heat and mass transfer effect in metal hydride beds. Consequently, a variety of different metal hydride bed configurations have been investigated in experimental and simulation works for the improvement of absorption/desorption rate. In this work, the development of metal hydride bed design in recent decades has been reviewed to help the readers summarize and optimize the reactor configuration. The summarization and review of metal hydrides design can be broadly classified into five distinct categories, which are: 1) design of cooling tubes, 2) design of fins, 3) increasing and arrangement of cooling tubes, 4) other geometric design, and 5) utilization of phase change material. This work is concentrated on assessing the heat and mass transfer effectiveness of various reactor structure configurations. The superiority and weakness of different configurations are summarized to give a comparison of the heat exchange effects. Moreover, the structural parameter analysis for each configuration is also reviewed from the heat and mass transfer aspect. Finally, some recommendations are provided for future metal hydride bed structural designs.     

Optimal design of a metal hydride hydrogen storage bed using a helical coil heat exchanger along with a central return tube during the absorption process

Metal-hydride (MH) reactors are one of the most promising approaches for hydrogen storage because of their low operating pressure, high storage volumetric density and high security. However, the heat transfer performance of the MH reactor for high hydrogenation rate is inferior. In this study, the heat transfer and hydrogen absorption process of metal hydride tank performance in Mg₂Ni bed is analyzed numerically using commercial ANSYS-FLUENT software. The MH reactor is considered a cylindrical bed including a helical tube along with a central straight return tube for the cooling fluid. The effects of geometrical parameters including the tube diameter, the pitch size and the coil diameter as well as operational parameters on the heat exchanged and hydrogen absorption reactive time are evaluated comprehensively. The results showed that the helical heat exchanger along with central return tube could effectively improve heat exchanged between the cooling fluid and the metal alloy and reduce the temperature of the bed results in a higher rate of hydrogen absorption. For a proper configuration and geometry of the helical coil heat exchanger with a central return tube, the absorption reaction time is reduced by 24% to reach 90% of the storage capacity. After the optimization study of the geometrical parameters, a system with the heat exchanger tube diameter of 5 mm, coil diameter of 18 mm and the coil pitch value of 10 mm is recommended to have lower hydrogen absorption time and higher hydrogen storage capacity. The presented MH reactor can be applied for improvement of heat exchange and absorption process in industrial MH reactors.     

A novel heat dissipation fin design applicable for natural convection augmentation

In this study, a comparative study of heat sink having various fin assembly under natural convection is investigated. The fin pattern includes a rectangular, a trapezoidal and an inverted trapezoidal configuration. Tests were performed in a well controlled environmental chamber having a heat load ranging from 3 to 20 W. From the test results, the heat transfer coefficient of the conventional rectangular fins is higher than that of the trapezoidal fins while the heat transfer coefficient of the inverted trapezoidal fins is higher than the trapezoidal one by approximately 25%, and it exceeds that of convectional rectangular fin by about 10%. The heat transfer improvements of the inverted trapezoidal fin are mainly associated with a larger temperature difference and inducing more air flow into the heat sink.     

Design of the metal hydride reactors – A review on the key technical issues

The metal hydride reactors are widely used in many industrial applications, for example, hydrogen storage, heat pump, thermal compression, gas separation, etc. The performance of the reactor is greatly affected by its design, which deserves careful study. Given the complicated nature of the hydride formation/decomposition processes, a series of technical issues are involved in the design of metal hydride reactors, such as primary configuration, thermal management, hydrogen transfer and mechanistic strength. These issues should be well addressed to fulfil the requirement of specific application. In this paper, the representative achievements with regards to the design issues so far were reviewed in detail, and some comments were made accordingly. It was concluded that an optimized reactor design comes from integrated considerations of numerous factors, particularly requirements for the applications and characteristics of the metal hydride system. The analytic hierarchy process was recommended for use in the selection of the optimum reactor scheme.     

Optimization of heat exchanger designs in metal hydride based hydrogen storage systems

Design of the heat exchanger in a metal hydride based hydrogen storage system influences the storage capacity, gravimetric hydrogen storage density, and refueling time for automotive on-board hydrogen storage systems. The choice of a storage bed design incorporating the heat exchanger and the corresponding geometrical design parameters is not obvious. A systematic study is presented to optimize the heat exchanger design using computational fluid dynamics (CFD) modeling. Three different shell and tube heat exchanger designs are chosen. In the first design, metal hydride is present in the shell and heat transfer fluid flows through straight parallel cooling tubes placed inside the bed. The cooling tubes are interconnected by conducting fins. In the second design, heat transfer fluid flows through helical tubes in the bed. The helical tube design permits use of a specific maximum distance between the metal hydride and the coolant for removing heat during refueling. In the third design, the metal hydride is present in the tubes and the fluid flows through the shell. An automated tool is generated using COMSOL-MATLAB integration to arrive at the optimal geometric parameters for each design type. Using sodium alanate as the reference storage material, the relative merits of each design are analyzed and a comparison of the gravimetric and volumetric hydrogen storage densities for the three designs is presented.     

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.     

Modeling and numerical simulation of a 5 kg LaNi5-based hydrogen storage reactor with internal conical fins

Hydrogen absorption by ~5 kg LaNi₅ in a metal hydride reactor is simulated. A cylindrical reactor (OD 88.9 mm, Sch- 40s, SS 316) with internal conical copper fins and cooling tubes (1/4^”, SS 316) carrying water at 1 m s⁻¹ and 293 K (inlet) is considered. Designs with 10, 13 and 19 equi-spaced fins and 2, 4 and 6 cooling tubes are explored. Hydrogen (15 atm) is supplied through a coaxial metal filter (OD 12 mm, SS 316). Conical fins offer enhanced heat transfer through higher surface area and funnelling effect for efficient loading of metal hydride powder. 19 fins + 6 tubes design requires 290 and 375 s for 80% and 90% hydrogen saturation level, respectively. The fins near the water inlet regions are more effective as the water temperature is lower in these regions. Trade-off exists between times taken for saturation and the mass of metal hydride.     

Enhancement of natural convection heat transfer from a vertical heated plate using inclined fins

An enhancement technique is developed for natural convection heat transfer from a vertical heated plate with inclined fins, attached on the vertical heated plate to isolate a hot air flow from a cold air flow. Experiments are performed in air for inclination angles of the inclined fins in the range of 30° to 90° as measured from a horizontal plane, with a height of 25 to 50 mm, and a fin pitch of 20 to 60 mm. The convective heat transfer rate for the vertical heated plate with inclined fins at an inclination angle of 60° is found to be 19% higher than that for a vertical heated plate with vertical fins. A dimensionless equation on the natural convection heat transfer of a vertical heated plate with inclined fins is presented. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 334–344, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20168     

Laminar free convection underneath a downward facing inclined hot fin array

A combined theoretical, experimental and numerical study was conducted to investigate the problem of laminar free convection underneath a hot isothermal and inclined fin array. The influence of inclination on the location where the flow stagnates, and splits, was examined. Heat transfer rates were calculated for different fin array geometries and temperatures. The results show that for small inclination angles the cooling rate is essentially constant. Beyond a certain angle, the tilting of the fin array enhances substantially the heat transfer rate. Sensitivity analyses indicate that the heat transfer coefficient increases at higher fin temperatures and larger fin spacing, but is of a lesser sensitivity to fin height changes. Additionally, it was discovered that the array optimal fin spacing do not depend on the inclination angle. In the theoretical part, a semi empirical model was developed for the heat transfer coefficient of horizontal and slightly inclined arrays that have large fin spacing. In effect it constitutes the necessary modeling addition to the previously developed model for moderately and tightly spaced fins of slightly tilted arrays. Together, they provide analytical expressions for the heat transfer coefficient of slightly inclined arrays, for any fin spacing.     

Entropy generation and its relation to heating and cooling requirements of a metal hydride hydrogen storage reactor

In this paper a metal hydride hydrogen storage reactor is analyzed from heat and mass transfer and entropy generation points of view. A transient two dimensional energy equation along with suitable reaction kinetics and entropy balance equation is solved numerically. Results are obtained keeping hydrogen flow rates constant during absorption and desorption. For a fixed mass of metal hydride in the reactor the amount of hydrogen transferred and the time in which the transfer takes place are kept fixed. Using the mathematical model the entropy generated during the process and the external cooling and heating fluid requirements are obtained. Results show how improvement in the design and/or operating conditions leads to reduced cooling and heating requirements and lower entropy generation. For the system considered in the study the internal heat transfer characteristics of the hydride bed are seen to influence the reactor performance significantly. With improved bed heat transfer the required heat transfer fluid temperature during desorption can be reduced and that during absorption can be increased significantly. This automatically leads to lower entropy generation and a more economic system operation. It is expected that the methodology proposed and the results presented in this study will be useful in the optimal design of metal hydride reactors for a variety of practical applications, including hydrogen storage.     

Designs of metal hydride reactors

As metal hydride beds have extremely poor heat transfer characteristics, the configurations of hydride reactors will have a marked effect on the performance and cost effectiveness of these metal-hydrogen systems. Therefore, it is of great significance to develop these reactors into the most suitable configurations for optimum heat transfer performance. The results of comparative studies of one-dimensional reactors and two-dimensional reactors can be used as guides to design reactors optimally and to augment heat and mass transfer processes in metal hydride beds. The comparative study of the three elementary reactors reveals the fact that the 011 reactor has an excellent heat and mass transfer performance and reactors with similar configurations may be recommended to be used in practical metal-hydrogen systems.     

Optimization of heat transfer device and analysis of heat & mass transfer on the finned multi-tubular metal hydride tank

Based on the previous studies on heat and mass transfer characteristics of hydride tank, whether the reaction heat of hydride bed can be removed quickly is a determinant factor of the reaction rate. As the core part of reaction system, the heat transfer optimization in the tank can significantly enhance the reaction rate. In this paper, the optimization of heat transfer fins for a finned multi-tubular metal hydride tank is presented, and the heat transfer equations of tank with various configuration fins (radius, thickness and number) are derived. By analyzing the effects of fin configurations on the heat transfer device, we found that the thermal resistance of reaction system reduces with the increase of the fin radius, thickness and number. In order to study transient reaction process inside the hydride tank with various configuration and operation conditions, a 3-D mathematical model is developed and validated based on the experimental data from literature. Through simulation and optimization on hydride tank with different configurations, we got that the fin number has the most significant positive effect on the absorption reaction process. The numerical simulation results show that the hydrogen absorption rate is proportional to hydrogen pressure, heat transfer coefficient and fluid flow velocity, and the hydrogen pressure has the most remarkable impact among these factors. The H₂ absorption is accomplished in 1720 s at 1 MPa, and the absorption reaction is completed within 2000 s at the H₂ pressure of 0.8 MPa. Moreover, the maximum difference in absorption completion time is only 190 s under different heat transfer coefficients and fluid flow velocities.     

Heat and mass transfer studies on elliptical metal hydride tubes and tube banks

A transient, two-dimensional model to predict the heat and mass transfer of elliptical metal hydride tubes and tube banks is presented. Results are obtained by solving the set of governing equations using suitable numerical methods. Results obtained on elliptical hydride tube banks are compared with standard circular tube banks. It is shown that compared to a circular tube of same internal hydride volume, the use of elliptical hydride tube can provide more compact reactors with lower ratio of fan work to heat transfer. Hence they may be suitable for applications such as storage of hydrogen for automobiles, etc. It is also seen that for elliptical tubes there is noticeable difference in temperatures and concentrations between the major and minor axes. The model presented and the results obtained may be used for optimal design of elliptical hydride tube bank reactors.     

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.