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.     

Energetic modeling,simulation and experimental of hydrogen desorption in a hydride tank

This paper presents a zero-dimensional (0D) model of hydride tank. The model aims to study the dynamic heat and mass transfers during desorption process in order to investigate the thermal-fluidic behaviors of this hydride tank. This proposed model has been validated experimentally thanks to a tailor-made developed test bench. This test bench allows the hydride characterization at tank scale and also the energetic characterization. The simulation results of the heat exchanges and mass transfer in and between the coupled reaction bed, show good agreement with the experimental ones. It is shown that the heat produced by a Proton Exchange Membrane Fuel Cell (PEMFC) (estimated starting from an electrical model) is enough to heat the metal alloy (FeTi) and therefore release the hydrogen with a sufficient mass flow rate to supply the PEMFC. Furthermore, the obtained results highlight the importance of the developed model for energy management of the coupling of fuel cell and hydride tank system.     

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.     

Hydrogen storage in metal hydride tanks equipped with metal foam heat exchanger

This paper presents a two-dimensional mathematical model to evaluate transient heat and mass transfer in a metal hydride tank (hereinafter MHT) with metal foam heat exchanger. The model is validated by comparison with experimental data. A good agreement is obtained.     

Study of a metal hydride based thermal energy storage system using multi-phase heat exchange for the application of concentrated solar power system

Thermal energy storage system is of great significance for the concentrated solar power system to keep the balance between power generation and demand. Metal hydride based thermal energy storage system is regarded as a promising method due to its good reversibility, low cost, and no by-product. Multi-phase heat exchange has much higher heat transfer coefficient than single-phase fluid heat exchange, thus facilitating the steam generation. In this study, a two-dimensional model of the metal hydride reactor using multi-phase heat exchange is proposed to estimate the performance and its feasibility of application in the concentrated solar power system. The results show that the velocity of the heat transfer fluid should match well with the thermal conductivity of the metal hydride bed to maintain the heat flux at a relatively constant value. The match of thermal conductivity of 3 or 5 W/(m·K) and fluid velocity of 0.0050 m/s results in the heat flux up to about 19 kW/m², which is increased by 3 orders of magnitude than single-phase heat exchange. In the thermal energy storage system, the reheating cycle is recommended to improve the utilization of the thermal energy. The efficiency of the system could be improved from 18.4% to 30.8% using the reheating cycle. The increased efficiency is comparable to the previously reported efficiency of 39.2%. Besides, the operation strategy of raising the steam temperature by increasing the hydrogen pressure or the superheater temperature is suggested for the system to obtain higher efficiency.     

Prediction of hydrogen desorption performance of Mg2Ni hydride reactors

This work performs the simulation of hydrogen desorption processes with Mg₂Ni hydrogen storage alloy to investigate the canister designs. Reaction rates and equilibrium pressures of Mg₂Ni alloy were calculated by fitting experimental data in literature using least squares regression. The obtained reaction kinetics was used to model the thermalfluid behavior of hydrogen desorption. Since the alloy powders will expand and shrink during the absorption and desorption cycle, the canisters considered are comprised of expansion volume atop the metal bed. In order to enhance the heat transfer performance of the canister, an air pipe is equipped at the canister centre line with/without internal fins. Detailed equations that describe the force convection of the heat exchange pipe and the natural convection at the reactor wall are carefully incorporated in the model. Simulation results show that the bare cylindrical canister can not complete the desorption process in 2.8 h, while the canister equipped with the concentric heat exchanger pipe and fins can complete desorption within 1.7 h.Results also demonstrate that the reaction rates can be further increased by increasing the pipe flow velocity and/or increasing the fin volume.     

MgH2 intermediate scale tank tests under various experimental conditions

Magnesium hydrogenation being an exothermic reaction, the loading time of a tank is limited by heat extraction. The compaction of ball-milled MgH₂ associated with Expanded Natural Graphite was used to improve the thermal conductivity of the resulting compacts. Taking advantage of these compacts, an intermediate scale tank (1.8 kg MgH₂) cooled down by forced air circulation was designed. The absorption is initiated at 90 °C. Since the intrinsic kinetic is not the limiting factor, the hydrogen pressure does not affect the loading process. The loading time is strongly dependent on the cooling efficiency. However, beyond a given air flow rate it doesn’t decrease any more, the heat transfer being limited by the thermal conductivity of the compacted disks. During desorption, the maximum hydrogen flow (25 Nl/mn) is directly proportional to the thermal power of the heating system. The tank absorbs 1170 Nl, has a specific-energy of 270 W h/kg and a system volumetric-density of 42 gr/l.     

Spatially resolved hydrogen desorption from aluminum hydride observed by magnetic resonance imaging

Light metal hydrides represent a promising class of materials for hydrogen storage. However, there are several technical challenges to overcome before their potential can be realized. Key among these is the often adverse absorption and desorption kinetics, which are a function of intrinsic reaction rates and practical operating temperatures. Modifying and controlling these kinetics require a thorough understanding of the hydrogen absorption and desorption processes. In this study, we have investigated the thermal decomposition of aluminum hydride, AlH₃, with Magnetic Resonance Imaging in order to visualize spatially the progress and extent of the reaction.     

Thermal modeling and performance analysis of industrial-scale metal hydride based hydrogen storage container

In this paper, a performance analysis of a metal hydride based hydrogen storage container with embedded cooling tubes during absorption of hydrogen is presented. A 2-D mathematical model in cylindrical coordinates is developed using the commercial software COMSOL Multiphysics 4.2. Numerical results obtained are found in good agreement with experimental data available in the literature. Different container geometries, depending upon the number and arrangement of cooling tubes inside the hydride bed, are considered to obtain an optimum geometry. For this optimum geometry, the effects of various operating parameters viz. supply pressure, cooling fluid temperature and overall heat transfer coefficient on the hydriding characteristics of MmNi_4.6Al_0.4 are presented. Industrial-scale hydrogen storage container with the capacity of about 150 kg of alloy mass is also modeled. In summary, this paper demonstrates the modeling and the selection of optimum geometry of a metal hydride based hydrogen storage container (MHHSC) based on minimum absorption time and easy manufacturing aspects.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

Bed geometries,fueling strategies and optimization of heat exchanger designs in metal hydride storage systems for automotive applications: A review

This review presents recent developments for effective heat management systems to be integrated in metal hydride storage tanks, and investigates the performance improvements and limitations of each particular solution. High pressures and high temperatures metal hydrides can lead to different design considerations, which are discussed in the paper. Studies analyzing design procedures based upon different geometrical solutions and/or operation strategies are considered, and their related advantages are explained. Restrictions to the validity of particular results are also evaluated.     

Analysis of a metal hydride reactor for hydrogen storage

In this paper a two-dimensional model of an annular cylindrical reactor filled with metal hydride suitable for hydrogen storage is presented. Comparison of the computed bed temperatures with published experimental data shows a reasonably good agreement except for the initial period. Effects of hydrogen pressure and external fluid temperatures on heat transfer and entropy generation are obtained. Results show that the time required for hydrogen charging and discharging is higher when the thermal capacity of the reactor wall is considered. The time required for absorption and desorption can be reduced significantly by varying the hydrogen gas pressure and external fluid temperatures. However, along with reduction in time the entropy generated during hydrogen storage and discharge increases significantly. Results also show that for the given input conditions, heat transfer between the external fluid and hydride bed is the main source of entropy generation.     

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.     

Large scale magnesium hydride tank coupled with an external heat source

The experimental study of a large scale magnesium hydride tank (10 kg) is presented. In order to enhance thermal exchanges and improve storage time, the MgH₂ powder has been compacted with 10 wt.% of Expanded Natural Graphite. An efficient heat exchanger has been designed to transfer the heat from the endo/exothermic sorption reactions to an external heat source/sink through a high temperature heat transfer fluid. An improvement of the loading and discharging time along the first hydrogenation cycles is observed owing to material thermal conductivity modifications. The properties of the cycled material have been studied in order to understand this evolution of the tank behavior.     

Numerical simulation of heat and mass transfer during the absorption of hydrogen in a magnesium hydride

This paper presents a numerical work aiming at the prediction of the characteristics of an industrial tank filled with hydrides for hydrogen storage. A validation of the method is given and is followed by the resolution of an example which shows the importance of achieving a three-dimensional modelling for the design of an industrial tank. Finally, recent results obtained on a magnesium hydride laboratory tank are given.     

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.     

Design of a large-scale metal hydride based hydrogen storage reactor: Simulation and heat transfer optimization

An optimized design for a 210 kg alloy, TiMn alloy based hydrogen storage system for stationary application is presented. A majority of the studies on metal hydride hydrogen systems reported in literature are based on system scale less than 10 kg, leaving questions on the design and performance of large-scale systems unanswered. On the basis of sensitivity to various design and operating parameters such as thermal conductivity, porosity, heat transfer coefficient etc., a comprehensive design methodology is suggested. Following a series of performance analyses, a multi-tubular shell and tube type storage system is selected for the present application which completes the absorption process in 900 s and the desorption process in 2000 s at a system gravimetric capacity of 0.7% which is a vast improvement over similar studies. The study also indicates that after fifty percent reaction completion, heat transfer ceases to be the major controlling factor in the reaction. This could help prevent over-designing systems on the basis of heat transfer, and ensure optimum system weight.     

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

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

The effect of copper coated metal hydride at different ratios on the reaction kinetics

In this study, the process parameters that affect the improvement of hydrogen storage material properties were investigated. In order to accelerate the hydrogen charge/discharge processes and to obtain the required hydrogen at the desired flow rates in a short time, the thermal conductivity of the storage materials has been improved, and density analyses have been made. The ideal grinding time has been determined for the LaNi₅ material. Within the scope of the experimental studies, the thermal conductivity coefficients of LaNi₅ coated with copper and LaNi₅ ground for 5 h and coated with copper were increased by 500–750%, and the copper plating ratios were optimized. The materials obtained were characterized by XRD, SEM, and their density was measured with the Helium Pyknometer device and their thermal conductivity coefficients with the Hot Disk Thermal Constants Analyzer. In addition, the hydrogen storage of materials with increased thermal conductivity was investigated experimentally in the metal hydride reactor at the determined pressure. In the study, it was seen that the storage material coated with copper increases the heat transfer, reduces the hydrogen charging time in the metal hydride reactor, and increases the stable discharge time.