Finite element model for charge and discharge cycle of activated carbon hydrogen storage

One of the main challenges to introduce hydrogen on the energy market is to improve on-board hydrogen storage and develop more efficient distribution technologies to increase the amount of stored gas while lowering the storage pressure. The physisorption of hydrogen on activated carbons (AC) is being investigated as a possible route for hydrogen storage. The objective of this work is to study the performance of adsorption-based hydrogen storage units from a "systems" point of view. A realistic two-dimensional axisymmetric geometric model which couples mass, momentum and energy balances is established based on the thermodynamic conservation laws using finite element method as implemented in COMSOL Multiphysics™. We consider the charging and discharging of the storage unit at a rated pressure of 9 MPa, and at an initial temperature of 302 K. The results are compared with experimental data obtained at the Hydrogen Research Institute of the University of Quebec at Trois-Rivieres. The storage tank is cooled by ice water. Research results show that both the simulated variations of pressure and temperatures during charge and discharge processes are in good agreement with the experimental data. The temperatures in the central region of tank are higher than those at the entrance and near the wall at the end of charge time while they are lower than those at the entrance and near the wall at the end of discharge time. The velocities are largest at the entrance, and decrease gradually along the axis of the tank. Owing to thermal effects, the larger flow rates result in less amount of adsorption in the condition of the same charging pressure. Hence measures of increasing heat transfer should be adopted, such as increasing the thermal conductivity of the storage bed. From the point of view of storage capacity, it is therefore possible to realize rapid hydrogenation, which is conducive to the use of such systems for on-board hydrogen storage based on activated carbon adsorption.     

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.     

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.     

Hydrogen adsorption on modified activated carbon

The aim of this work is to investigate hydrogen adsorption on prepared super activated carbon (AC). Litchi trunk was activated by potassium hydroxide under N₂ or CO₂ atmosphere. Nanoparticles of palladium were impregnated in the prepared-AC. Hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77, 87, 90 and 303 K, up to 5 MPa. Experimental results revealed that specific surface area of the prepared-AC increased according to KOH/char ratio. The maximum specific surface area reached up to 3400 m²/g and total pore volume of 1.79 cm³/g. The maximum hydrogen adsorption capacity of 2.89 wt.% at 77 K and under 0.1 MPa, was obtained on these materials. The hydrogen adsorption capacity of the 10 wt.% Pd-AC was determined as 0.53 wt.% at 303 K and under 6 MPa. This amount is higher than that on the pristine AC (0.41 wt.%) under the same conditions.     

Copper-doped activated carbon from amorphous cellulose for hydrogen,methane and carbon dioxide storage

The transition away from fossil fuel and ultimately to a carbon-neutral energy sector requires new storage materials for hydrogen and methane as well as new solutions for carbon capture and storage. Among the investigated adsorbents, activated carbons are considered especially promising because they have a high specific surface area, are lightweight, thermally and chemically stable, and easy to produce. Moreover, their porosity can be tuned and they can be produced from inexpensive and environmentally friendly raw materials. This study reports on the development and characterization of activated carbons synthesized starting from amorphous cellulose with and without the inclusion of copper nanoparticles. The aim was to investigate how the presence of different concentrations of metal nanoparticles affects porosity and gas storage properties. Therefore, the research work focused on synthesis and characterization of physical and chemical properties of pristine and metal-doped activated carbons materials and on further investigation to analyze their hydrogen, methane and carbon dioxide adsorption capacity. For an optimized Cu content the microporosity is improved, resulting in a specific surface area increase of 25%, which leads to a H₂ uptake (at 77 K) higher than the theoretical value predicted by the Chahine Rule. For CH₄, the storage capacity is improved by the addition of Cu but less importantly because the size of the molecule hampers easy access of the smaller pores. For CO₂ a 26% increase in adsorption capacity compared to pure activated carbon was achieved, which translated with an absolute value of over 48 wt% at 298 K and 15 bar of pressure.     

Cryo-adsorptive hydrogen storage on activated carbon. I: Thermodynamic analysis of adsorption vessels and comparison with liquid and compressed gas hydrogen storage

This paper presents a thermodynamic analysis of cryo-adsorption vessels for hydrogen storage. The analysis is carried out with an unsteady lumped model and gives a global assessment of the behavior of the storage system during operation (discharge), dormancy and filling. The adsorbent used is superactivated carbon AX-21™. Cryogenic hydrogen storage, either by compression or adsorption, takes advantage of the effect of temperature on the storage density. In order to store 4.1 kg H₂ in 100 L, a pressure of 750 bar at 298 K is necessary, but only 150 bar at 77 K. The pressure is further reduced to 60 bar if the container is filled with pellets of activated carbon [7]. However, adsorption vessels are submitted to intrinsic thermal effects which considerably influence their dynamic behavior and due to which thermal management is required for smooth operation. In this analysis, among energy balances for filling and discharge processes, the influence of the intrinsic thermal effects during vessel operation is presented. Hydrogen losses during normal operation as well as during long periods of inactivity are also considered. The results are compared to those obtained in low-pressure and high-pressure insulated LH₂ and CH₂ tanks.     

Adsorption and compression contributions to hydrogen storage in activated anthracites

We prepared activated carbons (ACs) that are among the best adsorbents for hydrogen storage. These ACs were prepared from anthracites and have surface areas (S_BET) as high as 2772 m² g⁻¹. Anthracites activated with KOH presented the highest adsorption capacities with a maximum of 5.3 wt.% at 77 K and 4 MPa. Non-linearity between hydrogen uptake at 77 K and pore texture was confirmed, as soon as their S_BET exceeded the theoretical limiting value of (geometrical) surface area, i.e., S_BET > 2630 m² g⁻¹. We separated adsorption and compression contributions to total hydrogen storage. The amount of hydrogen stored is significantly increased by adsorption only at moderate pressure: 3 MPa and 0.15 MPa at 298 and 77 K, respectively. Hydrogen adsorption on ACs at high pressure, above 30 MPa at 298 K and 8 MPa at 77 K, has not interest because more gas can be stored by simply compression in the same tank volume.     

Surface functionalization-enhanced spillover effect on hydrogen storage of Ni–B nanoalloy-doped activated carbon

Surface functionalization-enhanced spillover effect on hydrogen storage behaviors of Ni–B nanoalloy-doped activated carbon is investigated in comparison to the 3D graphene-based material. It is discovered that the hydrogen storage capacity of the activated carbon increases from Ni–B nanoalloy-doping but much less than that of graphene. After surface functionalization, although the specific surface area and micropore volume of the doped activated carbon decrease significantly, a hydrogen storage capacity is still almost as same as that of the unfunctionalized larger surface one, while showing a large desorption hysteresis. We argue that the surface functionalization greatly enhances the spillover process on carbon based adsorbents, thus playing an essential role in hydrogen storage capacity improvement.     

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.     

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.     

Optimization of activated carbons for hydrogen storage

Activated carbons (ACs) having hydrogen storage performances among the highest reported so far (i.e. 6.6 wt. % at 77 K and 4 MPa) are presented. These materials were prepared by chemical activation of anthracite with KOH. The effects of two experimental parameters: KOH/anthracite weight ratio (W) and activation temperature (T), on the hydrogen storage capacity were studied by application of central composite design and response surface methodology. A quadratic model was developed for correlating W and T to the hydrogen storage capacity. The analysis of variance showed that W the only significant parameter in the range of the experimental conditions tested. Our optimized AC showed higher hydrogen capacity in terms of absolute and excess storage properties than the well-known MAXSORB-3.     

Cryo-adsorptive hydrogen storage on activated carbon. II: Investigation of the thermal effects during filling at cryogenic temperatures

This paper presents an investigation of the thermal effects during high pressure filling of a cryo-adsorptive hydrogen storage tank. The adsorbent is powdered activated carbon. A two-dimensional model is formulated, which describes hydrodynamics, heat transfer and adsorption phenomena in cylindrical tanks. Experiments with a tank containing about 10 kg of adsorbent were carried out to parameterize and validate the model. Good agreement between experiments and simulations could be obtained. Numerical results are then presented concerning filling processes. Two cooling concepts are investigated: a LN₂ cooling jacket and a recirculation system which uses the hydrogen itself as the cooling fluid. The results show that short filling times can only be achieved with the recirculation system.     

Posidonia Oceanica and Wood chips activated carbon as interesting materials for hydrogen storage

The goal is to investigate the feasibility to use a local biomass (Posidonia Oceanica and Wood chips), as a raw precursor, to the production of activated carbons (AC) with a high surface area and remarkable hydrogen (H₂) adsorption properties.Biomasses (particle size of 0.3–0.4 mm) were pyrolyzed at 600 °C with a heating rate of 5 °C/min under an argon atmosphere. The biochar obtained from the carbonization step was chemically activated with KOH. The activation methodology induces a considerable improvement of the properties of the porous carbon in terms of carbon content (from 58 to 69 wt% to 93–96 wt%), surface area (from 41 to 425 m²/g to 2810–2835 m²/g) and H₂ adsorption in cryogenic condition (from 0,1 wt% to over 5 wt%).All porous carbons were characterized in terms of elemental analysis (CHNS–O), textural properties and H₂ adsorption measurements.     

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H₂ addition on CH₄ decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H₂ to CH₄ changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H₂ flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H₂, and when the H₂ flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H₂ to CH₄ and the catalyst deactivation is restrained. Filamentous carbon is obtained when H₂ is introduced into CH₄ reaction gas compared with the agglomerate carbon without H₂ addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.     

Impact of synthesis conditions of KOH activated carbons on their hydrogen storage capacities

39 activated carbons (ACs) were prepared by KOH activation of anthracite, using weight ratios KOH/anthracite (W) ranging from 1.5 to 7, activation temperatures (T) from 973 to 1073 K, and heating rates (Hr) from 1 to 5 K min⁻¹. ACs with high apparent surface areas (>3400 m² g⁻¹), high micropore volumes (>1 cm³ g⁻¹) and high hydrogen storage capacities (up to 6.6 wt. %) were obtained. A statistical study was carried out to clarify the impact of synthesis conditions on hydrogen storage capacities of the resultant ACs. Analysis of variance (ANOVA) showed that both W and T have a significant impact on hydrogen storage capacity, whereas Hr has not. A quadratic model was used to correlate W², W, T² and T to hydrogen storage capacities. The model adequately evaluated the impact of synthesis conditions on hydrogen storage capacities of the resultant ACs.     

Efficiency of low-temperature adsorptive hydrogen storage systems

This study presents a thermodynamic analysis of adsorptive hydrogen storage systems at cryogenic temperatures, focusing mainly on the efficiency of the storage system. Four different operation modes, differing in the process used to release the hydrogen and the availability of cooling during storage time, are examined. A sensitivity analysis was performed to determine the dependency of the efficiency on several input parameters including storage time, enthalpy of adsorption, energy required for cooling, and system size. It can be concluded that the energetic efficiency of the hydrogen storage system not considering production and usage steps ranges from 65 to 81% for short storage times, depending on the energy effort and hydrogen losses of the applied operation mode. The storage time has the most noticeable impact on efficiency and cryo-adsorptive hydrogen storage is not suitable for long storage times due to the increasing cooling effort or hydrogen losses. For short storage times e.g. hours, cooling during storage time may be omitted due to the low hydrogen losses.     

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.