Improvement of thermal performance of spiral heat exchanger on hydrogen storage by adding copper fins

The reaction time of hydrogen in metal-hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure of hydrogen in the case of absorption and desorption) has been discussed and obtained results show that a good choice of these parameters is important.     

Experimental formula for estimating porosity in a metal hydride packed bed

An experimental formula for estimating porosity in a metal hydride packed bed is presented. The formula was developed by direct observation of the volume changes of a metal hydride packed bed under free expansion in a vessel. The experimental results showed that the cycles of expansion and contraction were repeated at large porosities above 60% after a rapid state change caused by early particle breakup. The formula for porosity was expressed as a function of the reacted fraction and as a function of the cycle number. The function formula of the reacted fraction can be used to compute different values of porosity for expansion by absorption and for contraction by desorption. The coefficients assuming 100% hydrogen storage based on the experiments with LaNi₅ were an expansion ratio of 16.7% and a contraction ratio of 8.4%, on average. This experimental porosity formula is useful for effective thermal conductivity calculations and for numerical simulations of metal hydride packed bed behavior.     

Effect of design parameters on enhancement of hydrogen charging in metal hydride reactors

The effects of heat transfer mechanisms on the charging process in metal hydride reactors are studied under various charging pressures. Three different cylindrical reactors with the same base dimensions are designed and manufactured. The first one is a closed cylinder cooled with natural convection, the fins are manufactured around the second reactor and the third reactor is cooled with water circulating around the reactor. The temperatures of the reactor at several locations are measured during charging with a range of pressure of 1–10 bar. The third reactor shows the lowest temperature increase with the fastest charging time under all charging pressures investigated. The effective heat transfer coefficients of the reactors are also calculated according to the experimental results and they are found to be 5.5 ± 1 W m⁻² K⁻¹, 35 ± 2 W m⁻² K⁻¹ and 113 ± 1 W m⁻² K⁻¹, respectively. The experimental results showed that the charging of hydride reactors is mainly heat transfer dependent and the reactor with better cooling exhibits the fastest charging characteristics.     

Predictive calculation of the effective thermal conductivity in a metal hydride packed bed

The effective thermal conductivity of a metal hydride packed bed was calculated by considering the influence of expansion during hydrogen absorption and contraction during hydrogen desorption. The porosity was calculated using an experimental formula developed by direct observation, which was used in combination with other referenced methods. However, none of the methods could express the reported experimental value response to pressure change using only the experimental porosity formula. The area contact model was modified so that the porosity and the contact area changed with expansion and contraction. The contact area change was calculated by assuming a simple geometrical deformation caused the difference between the particle expansion and the packed bed expansion. The calculation results of the improved area contact model with the deformed factor and the shape factor were in good agreement with the reported experimental data. This calculation method of the effective thermal conductivity with the influence of expansion and contraction is expected to be useful for designing of heat transfer enhancement of a hydrogen storage tank.     

Simulation of transient heat and mass transfer during hydrogen sorption in cylindrical metal hydride beds

A CFD analysis of heat and mass transfer in cylindrical metal hydride beds is carried out using the commercial code Fluent 6.2. The effect of bulk diffusion is considered for mass transfer in the solid phase. Temporal and spatial variations of temperature and concentration in hydride bed are plotted. Emphasis is given to monitor the motion of hydrogen within the bed and to the influence of the L/D$L/D$ ratio and porosity. It is observed that a concentration variation in the bed is the driving force for hydrogen flow in hydride beds. The gas movement is observed to be from saturated cooler peripheral region towards the unsaturated hotter core region of the bed.     

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.     

Performance simulation of metal hydride hydrogen storage device with embedded filters and heat exchanger tubes

A practical metal hydride based hydrogen storage device would consist of many filters to distribute hydrogen gas and heat exchanger tubes to cool or heat the hydride bed depending on whether hydrogen is being absorbed or desorbed. This paper presents the simulation of such a device with LaNi₅ as the hydriding alloy. A study of the geometric and operating parameters has been carried out to identify their influence in the hydriding performance of the storage device.     

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.     

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.     

Experiments on solid state hydrogen storage device with a finned tube heat exchanger

The absorption and desorption performances of a solid state (metal hydride) hydrogen storage device with a finned tube heat exchanger are experimentally investigated. The heat exchanger design consists of two “U” shaped cooling tubes and perforated annular copper fins. Copper flakes are also inserted in between the fins to increase the overall effective thermal conductivity of the metal hydride bed. Experiments are performed on the storage device containing 1 kg of hydriding alloy LaNi₅, at various hydrogen supply pressures. Water is used as the heat transfer fluid. The performance of the storage device is investigated for different operating parameters such as hydrogen supply pressure, cooling fluid temperature and heating fluid temperature. The shortest charging time found is 490 s for the absorption capacity of 1.2 wt% at a supply pressure of 15 bar and cooling fluid temperature and velocity of 288 K and 1 m/s respectively. The effect of copper flakes on absorption performance is also investigated and compared with a similar storage device without copper flakes.     

A new algorithm for solving transient heat and mass transfer in metal–hydrogen reactor

A new algorithm based on the lattice Boltzmann method (LBM) is proposed as a potential solver for two-dimensional and dynamic heat and mass transfer in metal–hydrogen reactor. To check the validity of this algorithm, computational results have been compared with the experimental data and a good agreement is obtained. The advantages of the proposed numerical approach include, among others, simple implementation on a computer, accurate CPU time, and capability of stable simulation.     

Experimental investigations and a simple balance model of a metal hydride reactor

The metal hydride reactor filled with 5 kg of the AB₅-type (LaFe_0.5Mn_0.3Ni_4.8) alloy was investigated with respect to the hydrogen discharge rates classified using C-rate value, which is discharge of the maximum hydrogen capacity 750 st L within 1 h. The reactor cannot be fully discharged with a constant flow rate, for each temperature of hot water and flow rate there exists a moment of crisis at which the hydrogen flow drops under the constant value. The nominal capacity of the reactor reaches 80% of maximum capacity if sufficient heat transfer is provided. The simple balance model of a metal hydride reactor is developed based on the assumption of uniform temperature and pressure inside a metal hydride bed. The model permits to predict behavior of the metal hydride reactor in different operation regimes, quantitative agreement is obtained for low C-rates (less than 4) and sub-critical modes.     

Complete and reduced models for metal hydride reactor with coiled-tube heat exchanger

Thermal effects during hydriding/dehydriding have a significant influence on the performance of metal hydride hydrogen storage system. The heat exchanger is widely used in the metal hydride reactor in order to improve the efficiency of system. In this work, based on mass balance, momentum balance, energy balance equations, equation of reaction kinetics and equilibrium pressure equation, a two dimensional axisymmetric model of metal hydride reactor packed with LaNi₅ is developed on Comsol platform. The model is validated by comparing its simulation results with the experiment data and the simulation results from other works. Then, the straight pipe heat exchanger and the coiled-tube heat exchanger are taken into consideration in order to improve heat transfer from metal hydride reactor to ambient environment. The complete three dimensional model is developed for the metal hydride reactor equipped with the coiled-tube heat exchanger. The case with coiled-tube heat exchanger shows better efficiency than the other. In general, the temperature in central area is higher than others. In order to cool central area effectively, two designs of heat exchangers, including the combination of coiled-tube heat exchanger and straight pipe heat exchanger and the concentric dual coiled-tube heat exchanger, are studied. The results show that it is an effective method to improve the efficiency of metal hydride reactor by equipping dual coiled-tube heat exchangers. Reduced two dimensional model is applied to metal hydride reactor with coiled-tube heat exchanger to reduce computing time. The simulation results of reduced model generally agree with those of complete three dimensional model.     

Computational study on metal hydride based three-stage hydrogen compressor

A mathematical model for predicting the performances of a three-stage metal hydride based hydrogen compressor (MHHC) is presented. The performance of the MHHC is predicted by solving the unsteady heat and mass transfer characteristics of the coupled metal hydride beds of cylindrical configuration. The governing equations for energy, momentum and mass conservations, and reaction kinetic equations are solved simultaneously using the finite volume method. Metal hydrides chosen for a three-stage MHHC are LaNi₅, MmNi_4.6Al_0.4 and Ti_0.99Zr_0.01V_0.43Fe_0.99Cr_0.05Mn_1.5. Numerical results obtained for a single-stage MHHC using MmNi_4.6Al_0.4 are in good agreement with the experimental data reported in the literature. Using three-stage compression, a maximum pressure ratio of 28 is achieved for the supply conditions of 20 °C absorption temperature and 2.5 bar supply pressure. A maximum delivery pressure of 100 bar is obtained for the operating conditions of 20 °C absorption temperature and 120 °C desorption temperature.     

Hydrogen storage system based on hydride materials incorporating a helical-coil heat exchanger

Metal hydrides offer the potential to store hydrogen at modest pressures and temperatures with high volumetric efficiencies. The process of charging hydrogen into a metal powder to form the hydride is exothermic. The heat released by the reaction must be removed quickly in order to maintain a rapid charging rate. An effective method for heat removal is to embed a heat exchanger within the metal hydride bed. Here, we investigate the effectiveness of a helical coil heat exchanger tube to remove the heat generated during the absorption process. This paper presents a three-dimensional mathematical model formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank embedded with a helical-coil cooling tube. We present results from a parametric study of hydrogen storage efficiency as a function of helical coil pitch and convective heat transfer coefficient (h) within the cooling tube. We also explore the effect of adding aluminum foam to enhance the thermal conductivity of the metal hydride. The parametric study reveals that the mass of stored hydrogen is less sensitive to the coil pitch when aluminum foam is added. It is also found that the absorption rate increases with h as expected, although the rate of improvement diminishes at high values of h. Results were examined at filling times of 3 and 6 min to draw conclusions about the overall effectiveness of this hydrogen storage system. At 3 min, it is found that the addition of 5% Al foam is optimal, and h = 1000 W/m²-K is sufficient to bring the metal hydride to saturation; under these conditions a non-dimensional pitch of 0.5 maximizes the hydrogen absorption. Adding Al foam beyond 5% does not improve volumetric efficiency as the Al foam begins to displace the active hydrogen-absorbing material.     

Hydrogen release from a metal hydride tank with phase change material jacket and coiled-tube heat exchanger

Hydrogen fuel cells are received increasingly wide attention in order to develop green ships and reduce greenhouse gas emissions in the field of waterway transportation. Metal hydrides (MHs) can be used to store hydrogen for green ships due to their high volumetric storage capacity and safety. Various measures should be considered in the design and manufacture process of the MH reactor to strengthen its performance of heat and mass transfer and obtain an acceptable hydrogen storage capacity. In this work, LaNi₅ hydride is used as the hydrogen storage material and packed in the reactor. A basic axisymmetric numerical model for the hydrogen storage system without a heat exchanger has been developed and proved to be effective through the comparison between its simulation results and the published data during dehydriding. A hybrid heat exchanger, which is consisted of a phase change material (PCM) jacket and a coiled-tube, has been applied into the hydrogen storage system to relieve the thermal effect of MH in the dehydriding process on system performance. Effects of the heat transfer coefficient between the circulating heating water in the coil-tube and the MH bed, the temperature of circulating heating water and the pressure at the outlet on the dehydriding performance have been investigated. Based on parametric study, the relationships among the average dehydriding rate, the heat transfer coefficient, the heating water temperature and the outlet pressure have been found and fitted as simple equations. These fitted equations can be considered as a reference, which provides an important method to effectively control the dehydriding rate in order to satisfy the fuel requirement of the power unit and ensure the safe navigation of green ships in the future.