Automotive hydrogen storage system using cryo-adsorption on activated carbon

An integrated model of a sorbent-based cryogenic compressed hydrogen system is used to assess the prospect of meeting the near-term targets of 36 kg-H₂/m³ volumetric and 4.5 wt% gravimetric capacity for hydrogen-fueled vehicles. The model includes the thermodynamics of H₂ sorption, heat transfer during adsorption and desorption, sorption dynamics, energetics of cryogenic tank cooling, and containment of H₂ in geodesically wound carbon fiber tanks. The results from the model show that recoverable hydrogen, rather than excess or absolute adsorption, is a determining measure of whether a sorbent is a good candidate material for on-board storage of H₂. A temperature swing is needed to recover >80% of the sorption capacity of the superactivated carbon sorbent at 100 K and 100 bar as the tank is depressurized to 3–8 bar. The storage pressure at which the system needs to operate in order to approach the system capacity targets has been determined and compared with the breakeven pressure above which the storage tank is more compact if H₂ is stored only as a cryo-compressed gas. The amount of liquid N₂ needed to cool the hydrogen dispensed to the vehicle to 100 K and to remove the heat of adsorption during refueling has been estimated. The electrical energy needed to produce the requisite liquid N₂ by air liquefaction is compared with the electrical energy needed to liquefy the same amount of H₂ at a central plant. The alternate option of adiabatically refueling the sorbent tank with liquid H₂ has been evaluated to determine the relationship between the storage temperature and the sustainable temperature swing. Finally, simulations have been run to estimate the increase in specific surface area and bulk density of medium needed to satisfy the system capacity targets with H₂ storage at 100 bar.     

Three-dimensional modeling and simulation of hydrogen absorption in metal hydride hydrogen storage vessels

In this paper, a three-dimensional hydrogen absorption model is developed to precisely study the hydrogen absorption reaction and resultant heat and mass transport phenomena in metal hydride hydrogen storage vessels. The 3D model is first experimentally validated against the temperature evolution data available in the literature. In addition to model validation, the detailed 3D simulation results show that at the initial absorption stage, the vessel temperature and H/M ratio distributions are uniform throughout the entire vessel, indicating that hydrogen absorption is very efficient early during the hydriding process; thus, the local cooling effect is not influential. On the other hand, non-uniform distributions are predicted at the subsequent absorption stage, which is mainly due to differential degrees of cooling between the vessel wall and core regions. In addition, a parametric study is carried out for various designs and hydrogen feed pressures. This numerical study provides a fundamental understanding of the detailed heat and mass transfer phenomena during the hydrogen absorption process and further indicates that efficient design of the storage vessel and cooling system is critical to achieve rapid hydrogen charging performance.     

Simulation of heat and mass transfer in activated carbon tank for hydrogen storage

The charging process of hydrogen storage tank based on bed of activated carbon in a steel container at room temperature (295 K) and medium storage pressure (10 MPa) is simulated with an axisymmetric geometry model using the finite volume commercial solver Fluent. The mass flux profile at the entrance is established using user-defined functions (UDFs). The heat and mass transfer processes in the cylindrical steel tank packed with activated carbon are discussed considering the influence of viscous resistance and inertial resistance of the porous media. The velocity distribution and its effect on the temperature distribution are analyzed. The effects of the flow rate at the inlet and of the adsorption factor on the charging process are studied. A computational fluid dynamics (CFD) approach based on finite volume simulations is used. Results show that the temperature near the bottom of the tank is higher than that at the entrance, temperature in the center of the tank is higher than that near the wall and rises somewhat faster along the axial compared to the radial direction. The highest hydrogen absolute adsorption occurs at the entrance of the tank. A good agreement is found between the simulation results and the available experimental data. The maximum magnitude of the axial velocity is much higher than that of the radial component, resulting in more heat energy transfer along the axial direction than radial direction. In addition, the pressure reaches equilibrium earlier when the mass flow is higher, and the temperature reaches a maximum value faster.     

Optimization of hydrogen storage in metal-hydride tanks

The storage time of hydrogen in metal-hydride tanks (MHTs for short) is strongly influenced by the rate at which heat can be removed from the reaction bed. In the present work a two-dimensional mathematical model is developed and validated against experimental results. This model is used, first, to evaluate the impact of the tank wall thermal mass on the hydriding process. Walls in steel and in brass are tested and the obtained results show that there is no significant effect on hydrogen storage time. Then, the established model is used to study the dynamic behaviour inside various designs of MHTs: i) a cylindrical tank, ii) a cylindrical tank with external fins, iii) a cylindrical tank with a concentric tube filled with flowing cooling fluid and iv) a cylindrical tank with a concentric tube equipped with fins. Optimization results indicate that almost 80% improvement of the storage time can be achieved over the case where the tank is not optimized.     

Analysis of heat conducting enhancement measures on the composite for hydrogen storage by incorporation of activated carbon with MOFs

Experiments and numerical simulations were conducted for evaluating measures for enhancing adsorption capacity and heat conducting of an on board MOFs hydrogen storage system by cryo-adsorption. Solvothermal method was employed to synthesize MIL-101(Cr) composite by incorporating activated carbon. The composite was undergone structure characterization, structural morphology observation, thermal conductivity measurement and measurement of isotherm of hydrogen adsorption at 77.15 K within 0–6 MPa. Effect of adding expanded natural graphite (ENG) and equipping a honeycomb heat exchanging device (HHED) on mitigating the thermal effect on a 0.5 L hydrogen storage vessel packed with composite was investigated within a flow rate of hydrogen required by a ship's power unit. It shows that the sample incorporated by 1 wt% activated carbon respectively obtained about 14.5%, 26.2% and 5.7% increment in specific surface area, micropore volume and the maximum excess adsorption amount. Results also reveal that, within the flow rate 5 L·min^?1-25 L min^?1, the mean relative error between the experimental data and those from simulations is less than 1.61%, and the reduction in temperature fluctuation of the storage system is about 5 °C and 4 °C on charge and discharge process while equipping the HHED, which accordingly brought about 17%, 24.3%, 18.5% increment in accumulated amount of charge and discharge as well as the useable capacity ratio (UCR) of the system. It suggests that equipping a HHED is a more promising method for weakening the thermal effect on MOFs-hydrogen storage systems.     

A numerical investigation of hydrogen desorption phenomena in ZrCo based hydrogen storage beds

In this paper, a three-dimensional (3D) hydrogen desorption model is applied to the thin double layered annulus ZrCo based hydrogen storage bed to precisely study the hydrogen desorption reaction and resultant heat and mass transport phenomena inside the bed. The 3D hydrogen desorption simulations are carried out and calculated results are compared with the experimental data measured by Kang et al. [1]. The present model reasonably captures the bed temperature evolution behavior and the hydrogen discharging time for 90% desorption. In addition, the thin double layered annulus metal hydride bed (MHB) design is numerically evaluated by comparing with a simple cylindrical MHB. More uniform distributions in the bed temperature and H/M atomic ratio and resultant superior hydrogen desorption performance are achieved with the thin double layered annulus bed owing to its high external surface to volume ratio and thus more efficient heating. This numerical study indicates that efficient design of the metal hydride bed is key to achieve rapid hydrogen discharging performance and the present 3D hydrogen desorption model is a useful tool for the optimization of bed design and operating conditions.     

Experimental and numerical analysis of dynamic metal hydride hydrogen storage systems

This paper examines the dynamics of metal hydride storage systems by experimentation and numerical modelling. A specially designed and instrumented metal hydride tank is used to gather data for a cyclic external hydrogen load. Thermocouples provide temperature measurements at various radial and axial locations in the metal hydride bed. This data is used to validate a two-dimensional mathematical model previously developed by the authors. The model is then used to perform a parametric study on some of the key variables describing metal hydride systems. These variables are the equilibrium pressure, where the tails and concentration dependence are investigated, and the effective thermal conductivity of the metal hydride bed, where the pressure and concentration dependence are analyzed. Including tails on the equilibrium pressure curves was found to be important particularly for the accuracy of the initial cycles. Introducing a concentration dependence for the plateau region of the equilibrium pressure curve was found to be important for both pressure and temperature results. Effective thermal conductivity was found to be important, and the inclusion of pressure and concentration dependence produced more precise modelling results.     

Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

Improvement of heat transfer model for adsorptive hydrogen storage system

Hydrogen adsorption on high surface area activated carbon is an effective solution of hydrogen storage. Improvement is necessary for the heat transfer model of adsorptive hydrogen storage system. Distributed and lumped parameter models are implemented by the Comsol software and Matlab/Simulink software respectively. The evolution of pressure and temperature during charge and discharge processes is investigated. We adopted following measures for a further improvement on the model: (1) Wall temperature is improved by varying heat transfer coefficient; (2) A more realistic geometry with insert tube improves near inlet temperature; (3) Lumped parameter model is improved by considering thermal conductivity; (4) Distributed and lumped parameter models are well validated by experiments; (5) Heat transfer is modeled under conditions of air cooling and water cooling. The water cooling condition is better than air cooling condition in decreasing the temperature of the storage tank and improving the storage capacity.     

Exergy analysis of a hybrid solar hydrogen system with activated carbon storage

A proposed hybrid solar hydrogen system with activated carbon storage for residential power generation is assessed using exergy analysis. Energy and exergy balances are applied to determine exergy flows and efficiencies for individual devices and the overall system. A ‘base case’ analysis considers the proposed system without modification, while a ‘modified case’ extends the base case by considering the possibility of multiple product outputs. It is determined that solar photovoltaic-based sub-systems have the lowest exergy efficiencies (14-18%) and offer the most potential for improvement. A comparison of these two scenarios shows that the additional outputs raise the exergy efficiency of the modified case (11%) relative to the base case (4.0%). An investigation of the energy and exergy efficiencies of separate devices illustrates how energy analyses can be misleading. The hybrid system is expected to have several environmental benefits, which may offset to some degree economic barriers to implementation.     

Experimental studies on LaNi4.25Al0.75 alloy for hydrogen and thermal energy storage applications

Reversible exothermic and endothermic reactions between metals/alloys and hydrogen gas provide great opportunity to utilize various thermal energy sources such as waste heat, industrial exhaust, and solar thermal energy. Metal hydrides with favourable properties to operate at medium temperature heat (about 150 °C) are limited, and studies on hydrides in this temperature range are scarce. Hence, the present study aims at experimental investigations on LaNi_4.25Al_0.75 alloy in the temperature range of 150 °C–200 °C. A novel cartridge type of reactor is employed to investigate the hydrogen storage characteristics and thermal storage performance of this alloy. LaNi_4.25Al_0.75 is found to have a hydrogen storage capacity of about 1.20 wt% at 10 bar and 25 °C. In addition, it can store a total thermal energy of 285.7 ${\mathrm{k}\mathrm{J}.\mathrm{k}\mathrm{g}}_{\mathrm{M}\mathrm{H}}^{-1}$ and can deliver heat at an average rate of 287.5 ${\mathrm{W}.\mathrm{k}\mathrm{g}}_{\mathrm{M}\mathrm{H}}^{-1}$ at an efficiency of 64.1%.     

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.     

Correlation of the volumetric heat transfer coefficient for air-rock bed thermal storage system - considering humid air

Rock bed thermal storage with humid air green house or regenerative air cooling application are becoming widely investigated nowadays. The effect of the mass transfer in the circulated air has not received much attention. In this experimental study, the effect of the mass in the operated air is studied and new correlation for calculating the coefficient of volumetric heat transfer for humid air is presented.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

Numerical study on hydrogen and thermal storage performance of a sandwich reaction bed filled with metal hydride and thermochemical material

Hydrogen storage and release process of metal hydride (MH) accompany with large amount of reaction heat. The thermal management is very important to improve the comprehensive performance of hydrogen storage unit. In present paper, thermochemical material (TCM) is used to storage and release the reaction heat, and a new sandwich configuration reaction bed of MH-TCM system was proposed and its superior hydrogen and thermal storage performance were numerically validated. Firstly, the optimum TCM distribution with a volume ratio (TCM in inner layer to total) of 0.4 was derived for the sandwich bed. Then, comparisons between the sandwich reaction bed and the traditional reaction bed were performed. The results show that the sandwich MH-TCM system has faster heat transfer and reaction rate due to its larger heat transfer area and smaller thermal resistance, which results in the hydrogen storage time is shortened by 61.1%. The heat transfer in the reaction beds have significant effects on performance of MH-TCM systems. Increasing the thermal conductivity of the reaction beds can further reduce the hydrogen storage time. Moreover, improving the hydrogen inflation pressure can result in higher equilibrium temperature, which is beneficial for the enhancing heat transfer and hydrogen absorption rates.     

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.     

Dynamic simulation of hydrogen-based off-grid zero energy buildings with hydrogen storage considering Fanger model thermal comfort

In this study, some locations with different climates, off-grid zero energy buildings with hydrogen energy storage systems are designed, and transient analysis is conducted. These considered buildings supply their electricity consumption without using the electrical grid and PV panels or wind turbines. Also, they supply thermal comfort to occupants by using a vapor compression chiller and humidifier. Domestic hot water of occupants is supplied using solar collectors. For analyzing building's performance and objectives achievement, TRNSYS software is used. Also, for evaluating occupant thermal comfort, the Fanger model is used. The considered building is a one-story building with a 150 m² area. Four occupants are considered. Both of them are seated at rest, and another is seated with light working such as typing. Using the Fanger model equation and MATLAB software, the thermal comfort of occupants is determined. For domestic hot water consumption, verified profiles that vary during 24 h of the day are considered. Achieved results show that for humid and cold cities, PV panels with an area of 73 and 76 m² can be supplied the required electricity of considered building with four occupants and battery state of charge is higher than 50% and 10%, respectively. Moreover, with a suitable air conditioner system, the predicted percentage of dissatisfied (PPD) can be lower than 12% and 8% for humid and cold cities. Therefore, the building can be converted to a zero-energy building using its rooftop area.     

Novel sandwich-type dimetallocenes: Toward promising candidate media for high-capacity hydrogen storage

With respect to first-principles calculations, the sandwich-type dinuclear organometallic compounds as (C₅H₅)₂TM₂ (M = Sc and Ti) can adsorb up to eight hydrogen molecules. The corresponding gravimetric hydrogen-storage capacity is 6.7 wt% for (C₅H₅)₂Ti₂ and 6.8 wt% for (C₅H₅)₂Sc₂. The multimetallocenes (e.g., CpTi₃Cp and CpTi₄Cp) complexes can further increase the H₂ adsorption capacity to 8.7 wt% and 10.4 wt%, respectively. These sandwich-type organometallocenes proposed in this work are favorable for reversible adsorption and desorption of hydrogen under ambient conditions. These predictions will likely provide a new route for developing novel high-capacity hydrogen-storage materials.