Finite element simulation of heat and mass transfer in activated carbon hydrogen storage tank

A mathematical model of heat and mass transfer in activated carbon (AC) tank for hydrogen storage is proposed based on a set of partial differential equations (PDEs) controlling the balances or conservations of mass, momentum and energy in the tank. These PDEs are numerically solved by means of the finite element method using Comsol Multiphysics^TM. The objective of this paper is to establish a correct set of PDEs describing the physical system and appropriate parameters for simulating the hydrogen storage process. In this paper, we establish an axisymmetric model of hydrogen storage by adsorption on activated carbon, considering heat and mass transfer of hydrogen in storage tank during the charging process at room temperature (295 K) and the pressure of 10 MPa. To simulate the hydrogen storage process accurately, the heat capacity of adsorbed phase, the contact thermal resistance between the AC bed and the steel wall and the inertial resistance of high speed charging hydrogen gas are all taken into account in the model. The governing equations describing the hydrogen storage process by adsorption are solved to obtain the pressure changes, temperature distributions and adsorption dynamics in the storage tank. The pressure reaches a maximum value of 10 MPa at about 240 s. A small downward trend appears in the later stage of the charging process, which lasts 700 s. The temperature distribution is highest in the center of the tank. The temperature history exhibits a rapid increase initially, followed by a steady decline. A modified Dubinin–Astakhov (D–A) model is used to represent the hydrogen adsorption isotherms. The highest hydrogen uptake is 10 mol H₂/kg AC, at the entrance of hydrogen storage tank, where the temperature is lowest. The adsorption distribution at a given time is mainly determined by the temperature distribution, because the pressure is almost uniform in the tank. The adsorption history, however, is dominated by the pressure history because the pressure change is much larger than temperature change during the charging process of hydrogen storage.     

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.     

Simulation of hydrogen storage tank packed with metal-organic framework

Hydrogen adsorption in high surface metal-organic framework (MOF) has generated significant interest over the past decade. We studied hydrogen storage processes of MOF-5 hydrogen storage systems with adsorbents of both the MOF-5 powder (0.13 g/cm³) and its compacted tablet (0.30 g/cm³). The charge–discharge cycles of the two MOF-5 adsorbents were simulated and compared with activated carbon. The physical model is based on mass, momentum and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. The adsorption process was modeled using a modified Dubinin–Astakov (D–A) adsorption isotherm and its associated variational heat of adsorption. The model was implemented by means of finite element analysis software Comsol Multiphysics™, and the system simulation platform Matlab/Simulink™. The thermal average temperature from Comsol simulation is used to fill the gap between the system model and the multi-dimensional models. The heat and mass transfer feature of the model was validated by the experiments of activated carbon, the simulated pressure and temperatures are in good agreement with the experimental results. The model was further validated by the metal-organic framework of Cu-BTC and is being extended its application to MOF-5 in this study. The maximum pressure in the powder MOF-5 tank is much higher than that in the activated carbon tank due to the lower adsorbent density of MOF-5 and resulting lower hydrogen adsorption. The maximum pressure in the compacted MOF-5 tank is a little bit lower than that in the activated carbon tank due to the higher adsorbent density and resulting higher hydrogen adsorption. The temperature swings during the charge–discharge cycle of both MOF-5 tanks are higher than that of the activated carbon tank. These are caused mainly by pressure work in the powder MOF-5 tank and by adsorption heat in the compacted MOF-5 tank. For both MOF-5 hydrogen storage systems, the lumped parameter models implemented by Simulink agree well with experimental pressures and with pressures and thermal average temperatures from Comsol simulation.     

Neural network based optimization for cascade filling process of on-board hydrogen tank

Compressed hydrogen storage is widely used in hydrogen fuel cell vehicles (HFCVs). Cascade filling systems can provide different pressure levels associated with various source tanks allowing for a variable mass flow rate. To meet refueling performance objectives, safe and fast filling processes must be available to HFCVs. The main objective of this paper is to establish an optimization methodology to determine the initial thermodynamic conditions of the filling system that leads to the lowest final temperature of hydrogen in the on-board storage tank with minimal energy consumption. First, a zero-dimensional lumped parameter model is established. This simplified model, implemented in Matlab/Simulink, is then used to simulate the flow of hydrogen from cascade pressure tanks to an on-board hydrogen storage tank. A neural network is then trained with model calculation results and experimental data for multi-objective optimization. It is found to have good prediction, allowing the determination of optimal filling parameters. The study shows that a cascade filling system can well refuel the on-board storage tank with constant average pressure ramp rate (APRR). Furthermore, a strong pre-cooling system can effectively lower the final temperature at a cost of larger energy consumption. By using the proposed neural network, for charging times less than 183s, the optimization procedure predicts that the inlet temperature is 259.99–266.58 K, which can effectively reduce energy consumption by about 2.5%.     

On the effect of process temperature on the performance of activated carbon bed hydrogen storage tank

A numerical model is used to investigate hydrogen charging in packed bed storage tank as function of the charging process temperature. The model is based on the solution of the 2D transport equations for mass, momentum and energy in porous media. The equation system is characterized by the existence of adsorption source terms in both mass and energy equations along with very low Mach number flow condition. It was solved using a pressure splitting technique. Results showed that heating effects due to mechanical energy dissipation and to the exothermal character of the adsorption reaction are enhanced at low temperature. They result in a significant tank capacity reduction. This reduction reaches 25% of the total storage capacity as predicted assuming isothermal charging process. Unlike what has been suggested in several previous studies the major part of the tank capacity decrease is mainly due to the decrease of the gas density. Using recently developed activated carbon monolith with a conductivity exceeding 10 W m^?1 K^?1 can help in limiting the heating effect and reducing this capacity limitation. It makes possible the development of packed bed storage tanks that fulfill the DOE recommendations.     

潜艇燃料电池AIP氢燃料活性炭低温吸附储存

设计利用潜艇液氧冷量的燃料电池(FC)-AIP活性炭低温吸附储氢系统,在模拟潜艇航行中晃动和振动的平台上,测试氢在活性炭上的吸附等温线和储氢系统在为质子交换膜燃料电池(PEMFC)供气时的特性。结果表明,吸附等温线受平台晃振的影响小;温度为113K、压力为6MPa时,比表面积为1450m2.g-1的SAC-02活性炭储氢系统的质量储氢密度可超过当前艇用储氢合金的质量储氢密度;在2kW PEMFC电堆典型工况所需的氢气量(质量流率21.44L.min-1)下,通过充气过程的液氧预冷和放气过程的循环介质加热,可使储罐中心和壁面在整个过程中的最大温差小于5℃。活性炭低温吸附储氢系统的质量密度和储放氢特性能满足艇用FC-AIP系统的要求。     

Conditioning of hydrogen storage by continuous solar adsorption in activated carbon AX-21 with simultaneous production

The capacity of hydrogen storage by solar adsorption in activated carbon AX-21 and filling rate with simultaneous production have been conditioned under a minimum pressure, to nullify the cost of energy supplied to compressor. A gas accumulator tank connected to electrolyzer and continuous adsorption beds have been proposed in the process scheme. Minimum pressure required for the tank at an ambient filling temperature fixed to 25 °C is only 2 bar. While at atmospheric filling pressure the corresponding value of filling temperature is found to be 5 °C. However, a cooling fluid at low temperature for adsorbent bed during the adsorption process will be an efficient way for increasing the stored amount of hydrogen. Almost 4.5 kg of hydrogen can be stored in an adsorbent mass of 200 kg. The adsorption flow rate has been also modelled to be controlled for being adapted to production rate.     

CFD model for charge and discharge cycle of adsorptive hydrogen storage on activated carbon

High surface area activated carbons and other microporous adsorbents have generated a significant amount of interest over the past decade as storage media for hydrogen and natural gas, due to their high storage capacity at low temperatures and their use in gas purification processes. This paper uses computational fluid dynamics (CFD) to simulate the charging and discharging of a sorption-based hydrogen storage system. The CFD model is based on the mass, momentum and energy conservation equations of a system formed of gaseous and adsorbed hydrogen, an activated carbon bed and steel tank walls. The adsorption process is modeled using the Dubinin–Astakov adsorption isotherms extended to the supercritical regime. The model is implemented using Fluent. In our study, we can obtain accuracy peak temperature of simulation due to a non-constant isosteric heat of adsorption is used, derived from the model isotherms. We adopt piecewise heat capacity to consider the heat capacity of the adsorbed phase of hydrogen. We can make a conclusion that the simulated temperatures without consideration of heat capacity for hydrogen in adsorbed phase (c_pa), rise faster and reach higher peaks than the simulated temperatures with consideration of c_pa, and diverge more from experimental results. Also, we study the changes of temperature, pressure and adsorption during the charging and discharging processes as well as when the system is idle (which we define as dormancy) in the case of room temperature water cooling. The results are compared with experimental data from a storage unit cooled with room temperature water. The simulated pressure is in a good agreement with the experimental values. The simulated temperature profiles are also generally in good agreement with the experimental values, except close to the inlet and the wall. In addition, we have studied the effect of quality of the mesh on the accuracy and stability of the numerical computation and the influence of the mass flow rates on temperature and adsorption capacity.     

Effects of tank heating on hydrogen release from metal hydride system in VoltaFCEV Fuel Cell Electric Vehicle

Metal hydride (MH) storage is known as a safe storage method because it does not require complex processes like high pressure or very low temperature. However, it is necessary to use a heat exchanger due to the endothermic and exothermic reactions occurring during the charging and discharging processes of the MH tanks. The performance of the MH is adversely affected by the lack of a heat exchanger or a suitable temperature range and it causes non-stable hydrogen supply to the fuel cell systems. In this study, effect of the tank surface temperature on hydrogen flow and hydrogen consumption performance were investigated for the MH hydrogen storage system of a hydrogen Fuel Cell Electric Vehicle (FCEV). Different temperature values were arranged using an external heat circulator device and a heat exchanger inside the MH tank. The fuel cell (FC) was operated at three different power levels (200 W, 400 W, and 600 W) and its performance was determined depending on the temperature and discharge flow rate of the MH tank. When the heat exchanger temperature (HET) was set to 40 °C, the discharge performance of the MH tank increased compared to lower temperatures. For example, when the FC power was set to 200 W and the HET of the system was at 40 °C, 1600 L hydrogen was supplied to the FC and 2000 Wh electrical energy was obtained. The results show that the amount of hydrogen supplied from the MH tank decreases significantly by increasing the flow rate in the system and rapid temperature changes occur in the MH tank.     

Modeling of adsorption storage of hydrogen on activated carbons

The storage of hydrogen on board vehicles is one of the most critical issues for the transition towards an hydrogen-based transportation system. An electric vehicle powered by a typical gasoline tank will require 3.1 kg of hydrogen (H₂) to achieve a range of 500 km. Compared to a typical gasoline tank, this would correspond to a hydrogen density of 65 kg/m³ (including the storage system) and 6.5 wt%. Presently, only liquid hydrogen (LH₂) systems with a density of 51 kg/m³ and 14 wt% is close to this target. However, LH₂ is costly and requires more complex refueling systems. The physical adsorption of hydrogen on activated carbon can reduce the pressure required to store compressed gases. Though an efficient adsorption-based storage system for vehicular use of natural gas can be achieved at room temperature, the application of this technology to hydrogen using activated carbon as the adsorbent requires its operation at cryogenic temperature. We present the results of a parametric and comparative study of adsorption and compressed gas storage of hydrogen as a function of temperature, pressure and adsorbent properties. In particular, the isothermal hydrogen storage and net storage densities for passive and active storage systems operating at 77, 150 and 293 K are compared and discussed.     

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.     

Hydrogen storage potential in MIL-101 at 200 K

Hydrogen adsorption isotherms for MIL-101 metal-organic framework are reported within a wide pressure range for temperatures between 77 and 295 K. Data modeling with the modified Dubinin-Astakhov equation shows a good fitting with the experimental results. The calculated absolute adsorption allowed the evaluation of the total hydrogen storage capacity for high pressure storage tank filled with MIL-101 as sorbent. The results show that the gravimetric and volumetric storage capacities at 198 K and 70 MPa are within the present-day accepted DOE targets, even if the storage capacity is slightly decreased by 3–6% as compared to the tank without sorbent. Moreover, the calculations reveal that the dormancy time is much increased, as compared to a tank without sorbent, exceeding the ultimate DOE target of 14 days. The MIL-101 assisted cold high-pressure hydrogen storage at ∼200 K and 70 MPa, brings about an additional advantage and seems promising for both mobile and stationary applications.     

Hydrogen physisorption in high SSA microporous materials - A comparison between AX-21_33 and MOF-177 at cryogenic conditions

The two most promising materials for a hydrogen cryo-adsorption tank, activated carbon AX-21_33 and metal-organic framework MOF-177, have been investigated in the pressure range up to 2 MPa and at temperatures from 77 K to 125 K and at room temperature. The total hydrogen storage, including adsorbed hydrogen and gaseous hydrogen, has been determined for both samples. The results were evaluated with respect to the operating conditions of a tank system at cryogenic conditions, assuming a maximum tank pressure of 2 MPa and a minimum back pressure for the hydrogen consumer of 0.2 MPa. AX-21_33 shows a usable capacity of 3.5 wt.% in the case of isothermal operation at 77 K and 5.6 wt.%, if the tank is loaded at 77 K and the temperature is increased by 40 K during unloading. Under the same conditions, MOF-177 has a usable capacity of 6.1 wt.% and 7.4 wt.%, respectively. The results show that the heat of adsorption has a high impact on the amount of hydrogen remaining in a tank after unloading and that the heat management plays a crucial role for the design of a cryogenic tank system.     

Thermal effect of the adsorption heat on an adsorbed natural gas storage and transportation systems

This paper experimentally studies the thermal effect that results from the adsorption heat on both the charge and discharge performance of adsorbed natural gas (ANG) storage and transportation systems. Two storage tanks built with temperature systems and security control were used during the adsorption and desorption process. Temperature, flow rate and discharge amount were recorded experimentally at 2, 3 and 4 MPa adsorption pressures, using different activated carbon (AC) as an adsorbent bed. Results show that the central region of the adsorbent bed suffers from the severest temperature fluctuation of the charge and discharge process. It was observed that the best discharged amount was 4 MPa using, G1220 Extra AC as an absorbent bed. Conclusions detected that it is possible mitigate the temperature fluctuations with improved AC properties and the amount of NG desorbed is linearly proportional to the respective tank’s hydraulic volumes.     

Promising electrochemical hydrogen storage properties of nano biomass derived from walnut shell

Novel nano biomass (NBM) was synthesized using a general and simple synthetic approach. In this process, the walnut shell is used as a green carbon source. According to the transmission electron microscopy and dynamic light scattering results, the average particle size of the produced activated carbon was 2.25 nm. The surface area of the NBM was around 420.5 m²/g totally. High pore volume, high internal surface area, lightweight as well as easy availability are some features that attract research interests on activated carbon as a solid-state hydrogen storage medium. Nano biomass was deposited directly on a copper substrate by the slurry-coating method. The electrochemical properties of nano biomass were investigated in a three-electrode electrolytic cell with 6 M KOH as the electrolyte by galvanostatic charging and discharging. Several parameters such as the impact of the number of charge and discharge cycles and discharge time are studied. Different experimental results show that Cu-NBM has 1596 mAh/g discharge capacity (corresponding to a hydrogen storage capacity of 5.66 wt%) after 16 cycles at room temperature and atmospheric conditions. Due to porosity of NBM particles, the nano biomass showed reversible hydrogen storage capacities that were better than those of previously reported porous carbons.     

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.     

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.     

Experimental and numerical investigation of the thermal effects during hydrogen charging in packed bed storage tank

This paper reports on an experimental investigation and numerical simulations of the heating dynamics during hydrogen charging in activated carbon packed bed storage tank. Results showed that the experimentally observed heating dynamics is well predicted using a two-dimensional transport model that makes use of classical averaging rules usually adopted to describe flows in porous media and a linear driving force model to describe the adsorption kinetics. The contribution of the different phenomena to the overall temperature increase of the tank during the charging process was analysed both experimentally and using the developed numerical model. Results showed that the adsorption process is responsible for about 24% of the temperature increase even for moderately adsorbing activated carbon.     

Comparison of the expenses required for the on-board fuel storage systems of hydrogen powered vehicles

The on-board storage of hydrogen in vehicles requires sophisticated tank systems and appreciable energy expenditure. The energy requirement for the loading of liquid hydrogen in a cryogenic storage tank is substantially higher than the amount needed to charge a corresponding hydride vehicle tank. However, the hydride tank must provide a considerable part of its hydrogen fuel energy content in order to meet the energy requirements due to its own weight. This amount increases rapidly with increasing design range. Considering the total primary energy flow for both cases, it turns out that a clear break-even point exists. For design ranges higher than about 200 km the liquid hydrogen storage requires less energy expense than the hydride storage tank, the difference being very substantial (more than 100%) in the usual 400–500 km range. Similar results are found in the costs of the on-board storage of hydrogen for different design ranges. A comparison of these results with the storage costs studied earlier for large-scale stationary hydrogen storage facilities indicates that, from the specific capacity point of view, the break-even conditions are very similar in both cases, although due to different reasons.