System simulation modeling and heat transfer in sodium alanate based hydrogen storage systems

In this paper, we examine the feasibility of an on-board hydrogen storage system using sodium alanate as the hydrogen storage material. A two-dimensional model is used for evaluating refueling dynamics as well as heat transfer coefficients for the system level model. A parametric study is conducted to understand the influence of different operating parameters on the refueling time. System level performance of this storage system during driving conditions is evaluated using a simulation model developed in Matlab/Simulink platform.     

Behavior of scaled-up sodium alanate hydrogen storage tanks during sorption

Sodium alanate is being experimentally tested in scaled-up quantities. For this purpose, several tanks have been designed and constructed. The tank functionality during absorption and desorption of hydrogen was demonstrated in a scale of 8 kg of alanate, with a peak technical absorption time below 10 min. The absorption and desorption data show good reproducibility. Neutron radiography was used in another tank to show the powder’s physical behavior during sorption, showing conservation of the macroscopic structure during cycling.     

Cycling and engineering properties of highly compacted sodium alanate pellets

Sodium alanate powder comprised of NaH and Al was doped with 3 mol% titanium chloride (TiCl₃) and pelletized into highly compacted cylindrical pellets. The pelletization process was performed to improve thermal conductivity and volumetric hydrogen capacity of the metal hydride, compared to loose or tapped powder, which are vital requirements for on-board hydrogen storage applications. The pelletization process was performed over a range of 69–345 MPa (10–50 kPSI) with a 95% increase in density and improvement in thermal conductivity 18 times greater compared to powder at the maximum pelletization pressure (1.60 g/cm³ and 0.82 g/cm³; 9.09 W/m K and 0.50 W/m K, respectively). Hydrogen cycling capacities and kinetics were not adversely affected by the pelletization process although 10 cycles are required to obtain full hydrogen capacity. Pellet cycling capacity maintained a stable 4 wt% H₂ over 50 cycles. Ti-doped NaH + Al pellets exhibited similar thermal cycling expansion as with the loose powder; within 30 cycles there was a 50% loss in pellet density and by 50 cycles the loss in pellet structural integrity made handling problematic.     

Enhanced volumetric hydrogen density in sodium alanate by compaction

Powder compaction is a potential process for the enhancement of the volumetric and gravimetric capacities of hydrogen storage systems based on metal hydrides. This paper presents the hydrogen absorption and desorption behaviour of compacts of sodium alanate material prepared under different levels of compaction pressure. It is shown that even at high compaction levels and low initial porosities, hydrogen absorption and desorption kinetics can proceed comparatively fast in compacted material. Furthermore, experimental hydrogen weight capacities of compacted material are higher than the experimental values obtained in case of loose powder. It is demonstrated that the kinetic behaviour of the compacted material during cycling is directly associated to the volumetric expansion of the compact, which is quantitatively measured and analyzed during both hydrogen absorption and desorption processes. The cycling behaviour and dimensional changes of compacted sodium alanate material are a key consideration point if it is used as hydrogen storage materials in practical tank systems.     

Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage: (I) absorption

The Lithium–Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB₂/2 LiBH₄ + MgH₂) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy ΔH and entropy ΔS are determined to amount to −34 ± 2 kJ∙mol H₂⁻¹ and −70 ± 3 J∙K⁻¹∙mol H₂⁻¹, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 °C and 412 °C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 ± 3 kJ∙mol H₂⁻¹. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations.     

Parallel FTIR-ATR and gravimetrical in-situ measurements on sodium alanate powder samples during hydrogen desorption

An experimental set-up has been developed to carry out in-situ measurements on hydrogen storage materials. It has been used to perform FTIR–ATR measurements on sodium alanate samples during hydrogen desorption and, in parallel, to perform gravimetrical measurements on the same sample. The ATR spectra showed the typical broad Al–H vibrational features, changing according to the phase change the material undergoes during the load cycle. It is shown that the absorbance of the material at these bands and the hydrogen content correspond to each other in a reproducible way. This behavior may be used to measure the hydrogen mass released by an alanate hydrogen reservoir by measuring the optical absorbance at defined wavelengths. In this paper the results of these experiments that may give rise to the realization of a level sensor in future hydrogen storage applications are presented.     

Experimental results of an air-cooled lab-scale H2 storage tank based on sodium alanate

One possibility to store hydrogen in fuel-cell driven automobiles is the storage in solid state hydrides. Sodium alanate (NaAlH₄) is a well-known hydride desorbing up to 5 wt.% H₂ with reasonable rates at temperatures above 120 °C. Therefore a high temperature PEM fuel cell (HT-PEM FC) system with exhaust temperatures of about 180 °C can be used to provide the required enthalpy of reaction. In this study, the absorption and desorption behaviour of a lab-scale tank containing 304 g cerium-doped NaAlH₄ is studied using (exhaust) air as heat transfer medium. For absorption reactions an optimal temperature for maximal reaction rates is identified. Additionally, the importance of an adapted heat management is shown for the present tank. For desorption experiments different operation procedures are used and the constraints in temperature and air-flow given by the HT-PEM are considered. For all 25 experiments a good cycling stability has been measured with a stabilised material capacity of more than 3.7 wt.% H₂.     

Controlled degradation of highly compacted sodium alanate pellets

Compaction of sodium alanate doped with 3 mol% titanium chloride (TiCl₃) into rigid cylindrical pellets improves thermal conductivity, density and volumetric hydrogen capacity of a traditionally poorly conductive material. However, hydrogen cycling of alanate pellets results in significant expansion which counteracts the advantages of compaction. Restricting the area in which pellets can expand into minimizes these losses with no adverse effect to cycling capacity. Confined pellets had a 50% less decrease in density over 30 cycles, 5 times greater thermal conductivity within 10 cycles and maintain structural integrity through 50 cycles compared to free pellets. In addition, pellets within mechanical confinement fused into a rigid stack within the first few hydrogen cycles thereby reducing surface contact resistance between pellets by 3.5 times. Improved thermal conductivity and heat transfer through a pellet bed of materials such as complex metal hydrides, is a key aspect for on-board storage applications.     

Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank

In high temperature proton exchange membrane (HT-PEM) fuel cells, waste heat at approximately 160 °C is produced, which can be used for thermal integration of solid state hydrogen storage systems. In the present study, an HT-PEM fuel cell stack (400 W) with direct liquid cooling is characterized and coupled to a separately characterized sodium alanate storage tank (300 g material). The coupled system is studied in steady state for 20 min operation and all relevant heat flows are determined. Even though heat losses at that specific power and temperature level cannot be completely avoided, it is demonstrated that the amount of heat transferred from the fuel cell stack to the cooling liquid circuit is sufficient to desorb the necessary amount of hydrogen from the storage tank. Furthermore, it is shown that the reaction rate of the sodium alanate at 160 °C and 1.7 bar is adequate to provide the hydrogen to the fuel cell stack. Based on these experimental investigations, a set of recommendations is given for the future design and layout of similar coupled systems.     

Experimental study of powder bed behavior of sodium alanate in a lab-scale H2 storage tank with flow-through mode

Chemical hydrogen storage in complex hydrides offers the potential of high gravimetric storage densities compared to intermetallic hydrides, and is therefore a promising technology for mobile applications. The main challenge for mobile application is still the required high refuelling rate of the hydrogen storage tanks. Since hydrogen is bonded by an exothermal chemical reaction in complex hydrides, appropriate storage tanks require high heat transfer rates for the cooling of the tank. Hydride tanks that are state of the art rely on an indirect cooling and are additionally equipped with e.g. finns, foams, etc. to improve the heat transfer rate. For the present study, an improved laboratory tank, which allows for indirect as well as direct cooling by excess H₂ gas (flow-through mode), has been designed and built. This laboratory tank is filled with 87 g of NaAlH₄ (doped with 2 mol% CeCl₃) and equipped with 8 thermocouples as well as two pressure sensors. Experimental results presented in this paper show a significant influence of the cooling by gaseous excess H₂ on the flow-directional temperature profiles at the part of the reaction bed close to the inlet. Considering the overall conversion, this influence is rather small due to the low heat capacity flux (ρc_p)_H2. Furthermore, it is shown that changes in material properties, attributed to the effects of heat and mass transport as well as intrinsic reaction kinetics, can be measured and assessed by the temperature and pressure sensors. After about 10 complete charging and discharging cycles, the initial permeability K of the bed has decreased by 50% to 1.6·10⁻¹² m². In the same time, the initial thermal conductivity has increased by a factor of 1.3 to values reported in literature (0.67 Wm⁻¹ K⁻¹) and remains constant during further cycles. Additionally, it is observed that the reaction rate of the second absorption step improves, even after a total of 36 cycles.     

Hydrogen absorption in Ni-catalyzed Mg: A model for measurements in the low temperature range

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this article we continued the work presented in our previous paper by modeling the hydrogen absorption in Ni-catalyzed magnesium in the range of pressures of 500 kPa–5000 kPa and temperatures from 423 K to 468 K. A new model based in the Ginstling–Brounshtein diffusion equation was proposed for the hydrogen absorption kinetics. It adds the contribution of the pressure of the gaseous phase and the enthalpy of reaction to the previously mentioned diffusive model. An activation energy for the process was estimated and the value obtained (112 kJ/mol) was concordant with previous values reported in the literature.     

Hydrogen release properties of lithium alanate for application to fuel cell propulsion systems

In this paper the results of an experimental study on LiAlH₄ (lithium alanate) as hydrogen source for fuel cell propulsion systems are reported. The compound examined in this work was selected as reference material for light metal hydrides, because of its high hydrogen content (10.5 wt.%) and interesting desorption kinetic properties at moderate temperatures. Thermal dynamic and kinetic of hydrogen release from this hydride were investigated using a fixed bed reactor to evaluate the effect of heating procedure, carrier gas flow rate and sample form. The aim of this study was to characterize the lithium alanate decomposition through the reaction steps leading to the formation of Li₃AlH₆ and LiH. A hydrogen tank was designed and realized to contain pellets of lithium alanate as feeding for a fuel cell propulsion system based on a 2-kW Polymeric Electrolyte Fuel Cell (PEFC) stack. The fuel cell system was integrated into the power train comprising DC-DC converter, energy storage systems and electric drive for moped applications (3 kW). The experiments on the power train were conducted on a test bench able to simulate the vehicle behaviour and road characteristics on specific driving cycles. In particular the efficiencies of individual components and overall power train were analyzed evidencing the energy requirements of the hydrogen storage material.     

Regeneration of sodium borohydride from sodium metaborate,and isolation of intermediate compounds

A method for the regeneration of sodium borohydride from its hydrolysis product, sodium borate, is described. The dried borate salt was converted to a series of sodium tetraalkoxyborates prior to reaction with NaAlH₄ in refluxing diglyme to regenerate sodium borohydride. At room temperature in tetrahydrofuran, the same reactants formed the partial hydrides NaBH₃(OR) (R = C1-4 alkyl). Products were characterised by multinuclear ¹H, ¹³C, ¹¹B and ²⁷Al NMR studies. The intermediate alkoxyborate products were confirmed by single crystal X-ray structural studies.     

Honeycomb metallic structure for improving heat exchange in hydrogen storage system

A two-dimensional model for predicting heat and mass transfer in an alanate hydride reactor with metallic honeycomb structure (MHCS) heat exchanger has been developed. Using this model, a numerical study was performed to examine the influence of the MHCS’s cell size on the profiles of temperature, concentrations of the formed species, and hydrogen charging rate. The obtained results showed that the reduction of the MHCS’s cell size combined with an external cooling design configuration permits better use of the storage system. Based on this model, a comparison of the operating performance of various reactor designs was carried out. It was found that equipping the reactor with hexagonal cooling tubes clearly improved the performance of the charging process without further loss in the gravimetric and volumetric capacities of the hydrogen storage system.     

Numerical modeling of hydrogen absorption measurements in Ni-catalyzed Mg

Magnesium has been deeply studied as a possible hydrogen storage material for both, mobile and static applications. In this work, hydrogen absorption in Ni-catalyzed magnesium was measured in a wide range of pressure (500 kPa–5000 kPa) and temperature (498 K–573 K). Using this information, a model for the absorption kinetics and thermal behavior of the hydrogen storage system was proposed. This model could be used in the design of Ni-catalyzed magnesium storage tanks and other applications. It considers the independent contribution of three variables: temperature, pressure and reacted fraction to estimate the hydrogen absorption rate. An activation energy for the process was estimated and the value obtained (92 kJ/mol) was concordant with previous values reported in the literature.     

Hydrogen release from sodium borohydrides at low temperature by the addition of zinc fluoride

The major challenge of complex hydride for hydrogen storage is to release hydrogen at a moderate temperature with rapid kinetics. This paper reports a new composite system (NaBH₄/ZnF₂) to generate hydrogen at a low temperature below 100 °C. The NaBH₄ and ZnF₂ mixture was prepared by a mechanical milling in a 2:1 M ratio. Dehydrogenation of the mixture during a heating process shows that the initial dehydrogenation temperature was below 100 °C with favorable kinetics. The structural analysis confirmed the formation of NaBF₄ and Zn in the reaction products. The study concerning the reaction mechanism reveals that ZnF₂ plays a role of reagent rather than catalyst. The produced intermediate of NaZnF₃, however, affects the decomposition of the emitted B₂H₆. This work suggests that the extension of the decomposition mechanism of NaBH₄/ZnF₂ composite system would favor the development of new catalysts for complex hydride systems.     

Dynamics of hydrogen powered CAES based gas turbine plant using sodium alanate storage system

Typical compressed air energy storage (CAES) based gas turbine plant operates on natural gas or fuel oils as fuel for its operation. However, the use of hydro-carbon fuels will contribute to carbon emissions leading to pollution of the environment. On the other hand, the use of hydrogen as fuel for the gas turbine will eliminate the carbon emissions leading to a cleaner environment. Hydrogen can be produced using renewable energy sources like wind, solar etc. Storage of hydrogen is a bottleneck for such a system. A high capacity sodium alanate metal hydride bed is used in this study to store the hydrogen. The dynamics of the CAES based gas turbine plant operating with hydrogen fuel is presented along with discharge dynamics of the metal hydride bed. The heat required for desorbing the hydrogen from the metal hydride bed is provided partly by the hot flue gas exiting from the low pressure turbine and partly by external heating. Thus some of the heat from the flue gas is extracted. A novel multiple bed strategy is employed for efficient desorption. Each bed consists of a shell and tube, with alanate in the shell and heating fluid flowing through the helical coiled tube. Hydrogen combustor is modeled using a simplified Continuous Stirred Tank Reactor (CSTR) assumption in CANTERA. The NO_x emissions in the low pressure turbine exhaust stream are presented.     

A reactor model for hydrogen generation from sodium borohydride and water vapor

This paper reports new data on the production of hydrogen from water vapor plus NaBH₄, or NaBH₄ + 10% CoCl₂. Data were collected with the aid of an isothermal semi-batch reactor with in-situ H₂ rate measurement. The reaction of NaBH₄ to generate H₂ proceeds via three steps: deliquescence, dissolution and reaction. The deliquescence regime of NaBH₄ in the presence of 10 weight percent CoCl₂ is defined. The H₂ yield is quantified at various reaction conditions (reaction temperature 70–120 °C, relative humidity 31–69%). CoCl₂ significantly accelerates the rate of H₂ production compared to deliquescence + reaction of pure NaBH₄. It is also found that a combination of high temperature and high relative humidity contributes to high H₂ rate and yield, and either of the two factors dominates the reaction at different conditions. A two-part reactor model accounting for the mechanism of the steam hydrolysis by NaBH₄ is developed. The model captures the dissolution + reaction step as well as reaction-only step and was validated by experimental data.     

Hydrogen generation and storage from hydrolysis of sodium borohydride in batch reactors

The catalytic hydrolysis of alkaline sodium borohydride (NaBH₄) solution was studied using a non-noble; nickel-based powered catalyst exhibiting strong activity even after long time storage. This easy-to-prepare catalyst showed an enhanced activity after being recovered from previous use. The effects of temperature, NaBH₄ concentration, NaOH concentration and pressure on the hydrogen generation rate were investigated. Particular importance has the effect of pressure, since the maximum reached pressure of hydrogen is always substantially lower than predictions (considering 100% conversion) due to solubility effects. The solubility of hydrogen is greatly enhanced by the rising pressure during reaction, leading to storage of hydrogen in the liquid phase. This effect can induce new ways of using this type of catalyst and reactor for the construction of hydrogen generators and even containers for portable and in situ applications.