A metal hydride–polymer composite for hydrogen storage applications

To address the issue of the breakdown into fine powders that occurs in the practical use of metal hydrides, the possibility of using a polymeric material as a matrix that contains the active metal particles was experimentally assessed. A ball milling approach in the tumbling mode was used to develop a metal hydride–polymer composite with a high metal to polymer weight ratio. The alloy powder was blended with the polymer and a coating of the metal particles was obtained. The composite was consolidated by hot pressing and the pellets were characterized in terms of their hydriding–dehydriding properties. The materials did not show significant losses in either loading capacity or kinetic properties. The polymeric matrix resulted as being stable under hydrogen cycling. Further, from SEM observation it was confirmed that the metal powders remained embedded in the polymeric matrix even after a number of cycles and that the overall dimensional integrity was retained.     

AB5/ABS composite material for hydrogen storage

AB5 metal hydride (MH) particles were polymer dispersed in order to entrap the micro and nanoparticles produced by repeated fragmentations of the metal phase during the hydrogen charging/discharging cycles. Acrylonitrile-butadiene-styrene copolymer (ABS) was selected as a matrix on the basis of its physical and chemical properties. AB5/ABS composite pellets were obtained by using a dry mechanical particle coating approach in a tumbling-mill apparatus and successive consolidation by uniaxial hot pressing. A number of characterization techniques were used to assess the morphological, chemical and structural properties of the composites. High pressure DSC measurements, conducted at different pressure values, were used to assess the H₂ absorption properties and profile the Van't Hoff plots of the material. The overall results indicated that the AB5/ABS composite well tolerated the hydriding effects on metal particles, with no losses in hydriding kinetics. The material characteristics were found to be compatible with its application in developing MH-based H₂ storage devices.     

A hybrid method for hydrogen storage and generation from water

This communication describes a new hybrid method for storing hydrogen in solid inorganic hydride materials as well as producing it from water based on the reaction between LiOH/LiOH·H₂O and LiH. As a hydrogen storage method, the release and uptake of hydrogen in this method are accomplished via a series of simple reactions with good kinetics within a practically reasonable temperature range. The reversible hydrogen storage capacity of the material system is 6–8.8 wt.% at <350 °C. This capacity is one of the highest among all other metal hydrides known to date in the same temperature range. As a hydrogen production method, 100% of hydrogen generated by this method comes from water by its reaction with alkali metal oxides. This method is also an environmentally friendly alternative to the current commercial processes for hydrogen production. The preliminary thermodynamic calculation on energy required for complete regeneration shows that the current system is energetically favorable.     

Finite element-based simulation of a metal hydride-based hydrogen storage tank

In this paper, a novel 3D flexible tool for simulation of metal hydrides-based (LaNi₅) hydrogen storage tanks is presented. The model is Finite Element-Based and considers coupled heat and mass transfer resistance through a non-uniform pressure and temperature metal hydride reactor. The governing equations were implemented and solved using the COMSOL Multiphysics simulation environment. A cylindrical reactor with different cooling system designs was simulated. The shortest reactor fill time (15 min) was obtained for a cooling design configuration consisting of twelve inner cooling tubes and an external cooling jacket. Additional simulations demonstrated that an increase of the hydride thermal conductivity can further improve the reactor dynamic performance, provided that the absorbent bed is sufficiently permeable to hydrogen.     

Active disturbance rejection control of metal hydride hydrogen storage

Metal hydride (MH) hydrogen storage is used in both mobile and stationary applications. MH tanks can connect directly to high-pressure electrolyzers for on-demand charging, saving compression costs. To prevent high hydrogen pressure during charging, hydrogen generation needs to be controlled with consideration for unknown disturbances and time-varying dynamics. This work presents a robust control system to determine the appropriate mass flow rate of hydrogen, which the water electrolyzer should produce, to maintain the gaseous hydrogen pressure in the tank for the hydriding reaction. A control-oriented model is developed for MH hydrogen storage for control system design purposes. A proportional-integral (PI) and an active disturbance rejection control (ADRC) feedback controllers are investigated, and their performance is compared. Simulation results show that both the PI and ADRC controllers can reject both noises from the output measurements and unknown disturbances associated with the heat exchanger. ADRC excels in eliminating disturbances produced by the input mass flow rate, maintaining the pressure of the tank at the charging pressure with little oscillations. Additionally, the parameters estimated by the ADRC's extended state observer was used to predict the state-of-charge (SOC) of the MH.     

On-board and Off-board performance of hydrogen storage options for light-duty vehicles

Leading physical and materials-based hydrogen storage options are evaluated for their potential to meet the vehicular targets for gravimetric and volumetric capacity, cost, efficiency, durability and operability, fuel purity, and environmental health and safety. Our analyses show that hydrogen stored as a compressed gas at 350–700 bar in Type III or Type IV tanks cannot meet the near-term volumetric target of 28 g/L. The problems of dormancy and hydrogen loss with conventional liquid H₂ storage can be mitigated by deploying pressure-bearing insulated tanks. Alane (AlH₃) is an attractive hydrogen carrier if it can be prepared and used as a slurry with >50% solids loading and an appropriate volume-exchange tank is developed. Regenerating AlH₃ is a major problem, however, since it is metastable and it cannot be directly formed by reacting the spent Al with H₂. We have evaluated two sorption-based hydrogen storage systems, one using AX-21, a high surface-area superactivated carbon, and the other using MOF-177, a metal-organic framework material. Releasing hydrogen by hydrolysis of sodium borohydride presents difficult chemical, thermal and water management issues, and regenerating NaBH₄ by converting B–O bonds is energy intensive. We have evaluated the option of using organic liquid carriers, such as n-ethylcarbazole, which can be dehydrogenated thermolytically on-board a vehicle and rehydrogenated efficiently in a central plant by established methods and processes. While ammonia borane has a high hydrogen content, a solvent that keeps it in a liquid state needs to be found, and developing an AB regeneration scheme that is practical, economical and efficient remains a major challenge.     

Comments on solid state hydrogen storage systems design for fuel cell vehicles

In recent years, significant research and development efforts were spent on hydrogen storage technologies with the goal of realizing a breakthrough for fuel cell vehicle applications. This article scrutinizes design targets and material screening criteria for solid state hydrogen storage. Adopting an automotive engineering point of view, four important, but often neglected, issues are discussed: 1) volumetric storage capacity, 2) heat transfer for desorption, 3) recharging at low temperatures and 4) cold start of the vehicle. The article shall help to understand the requirements and support the research community when screening new materials.     

An approach to design single BCC Mg-containing high entropy alloys for hydrogen storage applications

Mg is a lightweight element that can increase the gravimetric hydrogen storage capacity of high entropy alloys (HEAs). This work presents a new approach to design single-phase Mg-containing HEAs with attractive hydrogen storage properties. The design method is based on four calculated parameters (?, VEC, $\overline{\Delta {\text{H}}_{\infty }}$ and $\overline{\Delta {\text{H}}_{\text{f}}^{\text{o}}}$) that allow us to find alloy compositions that form single body-centered cubic (BCC) solid solutions with high hydrogen affinity. The method was tested in the Mg–Al–Ti–Mn–Nb system and the Mg₁₂Al₁₁Ti₃₃Mn₁₁Nb₃₃ alloy was selected among 1326 calculated compositions. This alloy was produced by high energy ball milling resulting in a homogeneous single-phase BCC alloy that absorbed 1.7 wt.% of H by forming a BCC monohydride. Despite its H uptake being H/M = 1, the gravimetric capacity of the lightweight Mg₁₂Al₁₁Ti₃₃Mn₁₁Nb₃₃ alloy was comparable to refractory BCC-HEAs with H uptake of H/M = 2.     

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.     

Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications

The present study compares the dehydrogenation kinetics of (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ systems and their vulnerabilities to the NH₃ emission problem. The (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixture. On the basis of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system are discussed in light of the newly proposed reaction mechanism.     

System modeling methodology and analyses for materials-based hydrogen storage

In the global efforts to develop advanced materials-based hydrogen storage, the various on-board reversible hydrides, adsorbents and chemical storage candidate materials and systems each have their individual strengths and weaknesses. An overarching challenge in associated research and development is to devise material/system architectures which satisfy all requirements for viability in a particular application area, such as light-duty vehicular transportation. System modeling at the level which encompasses not only the storage material and vessel/reactor, but also integration with a fuel cell and balance-of-plant components, provides a more complete assessment of viability and guides options for improvement. The current work covers the methodology developed for conducting such system modeling consistently across multiple organizations and will present performance results from studies focused on reversible hydride systems. Connecting this high level modeling to more detailed finite element design simulations will be one aspect of our framework approach. The complex hydride NaAlH₄ is representative of novel materials under development and will be used as the basis for properties, such as temperature dependent kinetics, which influence the integrated system configurations and component sizing. While system charging is included through the sizing of certain components, emphasis is placed on hydrogen discharge by the storage system, interrogated through drive cycle transients. Comparisons of performance relative to requirements, including effective gravimetric capacity, effective volumetric density and energy utilization, are given for the baseline material and for a sensitivity study on material density.     

Thermodynamic simulation of hydrogen based thermochemical energy storage system

The increasing energy demand needs the attention for energy conservation as well as requires the utilisation of renewable sources. In this perspective, hydrogen provides an eco-friendly and regenerative solution toward this matter of concern. Thermochemical energy storage system working on gas-solid interaction is a useful technology for energy storage during the availability of renewable energy sources. It provides the same during unavailability of energy sources. This work presents a performance analysis of metal hydride based thermal energy storage system (MH-TES), which can transform the waste heat into useful high-grade heat output. This system opens new doors to look at renewable energy through better waste heat recovery systems. Experimentally measured PCIs of chosen metal hydride pairs, i.e. LaNi_4.6Al_0.4/La_0.9Ce_0.1Ni₅ (A-1/A-3; pair 1) and LaNi_4.7Al_0.3/La_0.9Ce_0.1Ni₅ (A-2/A-3; pair 2) are employed to estimate the thermodynamic performance of MH-TES at operating temperatures of 298 K, 373 K, 403 K and 423 K as atmospheric temperature (T_atm), waste heat input temperature (T_m), storage temperature (T_s) and upgraded/enhanced heat output temperature (T_h) respectively. It is observed that the system with alloy pair A-1/A-3 shows higher energy storage density of 121.83 kJ/kg with a higher COP of 0.48 as compared to A-2/A-3 pair. This is due to the favourable thermodynamic properties, and the pressure differential between coupled MH beds, which results in higher transferrable hydrogen. Besides, the effect of operating temperatures on COP is studied, which can help to select an optimum temperature range for a particular application.     

Environmental reactivity of solid-state hydrogen storage systems: Fundamental testing and evaluation

In order to enable the commercial acceptance of solid-state hydrogen storage materials and systems it is important to understand the risks associated with the environmental exposure of various materials. In some instances, these materials are sensitive to the environment surrounding the material and the behavior is unique and independent to each material. The development of testing procedures to evaluate a material’s behavior with different environmental exposures is a critical need. In some cases material modifications may be needed in order to reduce the risk of environmental exposure. We have redesigned two standardized UN tests for clarity and exactness; the burn rate and self-heating tests. The results of these and other UN tests are shown for ammonia borane, NH₃BH₃, and alane, AlH₃. The burn rate test showed a strong dependence on the preparation method of aluminum hydride as the particle size and trace amounts of solvent greatly influence the test results. The self-heating test for ammonia borane showed a failed test as low as 70 °C in a modified cylindrical form. Finally, gas phase calorimetry was performed and resulted in an exothermic behavior within an air and 30%RH environment.     

Scaled-up production of a promising Mg-based hydride for hydrogen storage

The feasibility of scaling up the production of a Mg-based hydride as material for solid state hydrogen storage is demonstrated in the present work. Magnesium hydride, added with a Zr–Ni alloy as catalyst, was treated in an attritor-type ball mill, suitable to process a quantity of 0.5–1 kg of material. SEM–EDS examination showed that after milling the catalyst was well distributed among the magnesium hydride crystallites. Thermodynamic and kinetic properties determined by a Sievert's type apparatus showed that the semi-industrial product kept the main properties of the material prepared at the laboratory scale. The maximum amount of stored hydrogen reached values between 5.3 and 5.6 wt% and the hydriding and dehydriding times were of the order of few minutes at about 300 °C.     

Metal hydride material requirements for automotive hydrogen storage systems

The United States Department of Energy (DOE) has published a progression of technical targets to be satisfied by on-board rechargeable hydrogen storage systems in light-duty vehicles. By combining simplified storage system and vehicle models with interpolated data from metal hydride databases, we obtain material-level requirements for metal hydrides that can be assembled into systems that satisfy the DOE targets for 2017. We assume minimal balance-of-plant components for systems with and without a hydrogen combustion loop for supplemental heating. Tank weight and volume are driven by the stringent requirements for refueling time. The resulting requirements suggest that, at least for this specific application, no current on-board rechargeable metal hydride satisfies these requirements.     

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.     

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.     

Development of large MH tank system for renewable energy storage

Metal Hydrides (MH) can absorb large quantities of hydrogen at room temperature and ordinary pressure. Because MH can store hydrogen at a pressure less than 0.1 MPa safely and compactly, it is looked to as a method of storing hydrogen produced by electricity derived from renewable energy sources. To study this method of storing renewable energy, we made a MH tank system which could store hydrogen in the range of 1000 Nm³. A Mm-NiMnCo alloy was used for this MH tank system. MH becomes pulverized with absorbing and desorbing hydrogen, and this causes the problem of MH tank transformation owing to the partial distribution of the pulverized MH powders. Our MH material, named “Hydrage?,” was made using a technique to compose the MH powders with polymer materials without decreasing the hydrogen absorption and desorption rate. With this technique, the MH powders were immobilized, and strain on the MH tank was reduced. Furthermore, this technique enabled uniform dispersion of the MH powders, and high-density filling in MH tank was achieved relative to that attainable in a conventional MH tank. An MH tank system with a capacity of 1000 Nm³ is 1,800 mm in width, 3,150 mm in length, and 2,145 mm in height. The system for renewable energy storage consists of 9 tanks. About 7.2 tons of MH were used in this system. This system could work at temperatures from 25 to 35° C, and its maximum hydrogen absorption and desorption rate is 70 Nm³/h with a medium flow rate of 30 NL/min. This type of MH tank system, which can store a large amount of hydrogen safely and compactly, has the potential to become popular with various applications in the future.     

System simulation model for high-pressure metal hydride hydrogen storage systems

On-board hydrogen storage systems employing high-pressure metal hydrides promise advantages including high volumetric capacities and cold start capability. In this paper, we discuss the development of a system simulation model in Matlab/Simulink platform. Transient equations for mass balance and energy balance are presented. Appropriate kinetic expressions are used for the absorption/desorption reactions for the Ti_1.1CrMn metal hydride. During refueling, the bed is cooled by passing a coolant through tubes embedded within the bed while during driving, the bed is heated by pumping the radiator fluid through same set of tubes. The feasibility of using a high-pressure metal hydride storage system for automotive applications is discussed. Drive cycle simulations for a fuel cell vehicle are performed and detailed results are presented.