The U.S. Department of Energy (DOE) developed a vehicle Framework model to simulate fuel cell-based light-duty vehicle operation for various hydrogen storage systems. This transient model simulates ...the performance of the storage system, fuel cell, and vehicle for comparison to Technical Targets established by DOE for four drive cycles/profiles. Chemical hydrogen storage models have been developed for the Framework for both exothermic and endothermic materials. Despite the utility of such models, they require that material researchers input system design specifications that cannot be estimated easily. To address this challenge, a design tool has been developed that allows researchers to directly enter kinetic and thermodynamic chemical hydrogen storage material properties into a simple sizing module that then estimates system parameters required to run the storage system model. Additionally, the design tool can be used as a standalone executable file to estimate the storage system mass and volume outside of the Framework model. These models will be explained and exercised with the representative hydrogen storage materials exothermic ammonia borane (NH3BH3) and endothermic alane (AlH3).
•A design tool was developed for chemical hydrogen storage materials researchers.•The tool sizes the storage system components using material properties as inputs.•It can be used as a standalone model or within the vehicle Framework model.•Material properties and storage system component dimensions were correlated.
Two detailed, unit cell models, a transverse fin design and a longitudinal fin design, of a combined hydride bed and heat exchanger are developed in COMSOL
® Multiphysics incorporating and accounting ...for heat transfer and reaction kinetic limitations. MatLab
® scripts for autonomous model generation are developed and incorporated into (1) a grid-based and (2) a systematic optimization routine based on the Nelder-Mead downhill simplex method to determine the geometrical parameters that lead to the optimal structure for each fin design that maximizes the hydrogen stored within the hydride.
The optimal designs for both the transverse and longitudinal fin designs point toward closely-spaced, small cooling fluid tubes. Under the hydrogen feed conditions studied (50 bar), a 25 times improvement or better in the hydrogen storage kinetics will be required to simultaneously meet the Department of Energy technical targets for gravimetric capacity and fill time. These models and methodology can be rapidly applied to other hydrogen storage materials, such as other metal hydrides or to cryoadsorbents, in future work.
► Detailed, on-the-fly, H
2 storage bed models constructed in COMSOL
® Multiphysics. ► Models account for limitations in heat transfer and reaction kinetics simultaneously. ► Systematic optimization of bed gravimetric capacity by linking to MatLab
® routines. ► Methodology applies to any simulation-based engineering system, not solely H
2 storage.
The design and evaluation of media-based hydrogen storage systems requires the use of detailed numerical models and experimental studies, with significant amount of time and monetary investment. Thus ...a scoping tool, referred to as the Acceptability Envelope, was developed to screen preliminary candidate media and storage vessel designs, identifying the range of chemical, physical and geometrical parameters for the coupled media and storage vessel system that allow it to meet performance targets. The model which underpins the analysis allows simplifying the storage system, thus resulting in one input-one output scheme, by grouping of selected quantities.
Two cases have been analyzed and results are presented here. In the first application the DOE technical targets (Year 2010, Year 2015 and Ultimate) are used to determine the range of parameters required for the metal hydride media and storage vessel. In the second case the most promising metal hydrides available are compared, highlighting the potential of storage systems, utilizing them, to achieve 40% of the 2010 DOE technical target. Results show that systems based on Li–Mg media have the best potential to attain these performance targets.
► Steady state model of the storage bed set up to evaluate the acceptability envelope for metal hydrides. ► An easy and compact expression of the acceptability envelope developed. ► The model is applicable for different materials and under different operating conditions. ► Acceptability envelope model applied with the DOE technical targets as inputs. ► Acceptability envelope model used to compare different promising materials.
The control of an axisymmetric free jet (
Re
U
e
=
6600
) using a single synthetic jet was investigated experimentally. The interaction was examined for a range of momentum coefficients, Strouhal ...numbers, and synthetic jet orientations (with respect to the main jet). To better explore the complex flow field resulting from the interaction, a rendering technique was used where three-dimensional flow fields were calculated from multiple two-dimensional measurement planes. The synthetic jet deflects the majority of the main jet flow away from it, while drawing some of the flow back toward it. Also, the synthetic jet is shown to appreciably raise the main jet’s turbulent quantities, suggesting that mixing has been enhanced. Using triple decomposition, it was shown that the random and coherent motions have similar contributions to the turbulent stresses near the interaction region; whereas the coherent motions prevail farther downstream (and along the shear layers). Measurements of the streamwise vorticity showed that the interaction results in the formation of counter-rotating streamwise vortices, similar to the effect of passive tabs. The size and strength of these structures can be controlled by changing the synthetic jet’s momentum coefficient, actuation frequency, or orientation. At low momentum coefficients, the largest effect is obtained for a Strouhal number of 0.32; while at higher momentum coefficients saturation is obtained due to the high excitation level. A steady control jet, which only utilizes the direct impact mechanism, results in vectoring and a deep penetration into the main jet. However, it yields decreased spreading compared to a synthetic jet with the same momentum coefficient.
A methodology was developed to determine the range of coupled material parameters and operating conditions that allow an adsorbent based hydrogen storage system to meet performance targets. The range ...of acceptable parameters forms a multi-dimensional volume, or envelope. For this reason, the methodology is referred to as the Adsorbent Acceptability Envelope. The model evaluates the performance of the overall storage tank, comprised of the adsorbent material, the heat transfer system and the pressure vessel. Two cases were analyzed, both based on the flow-through cooling approach providing the cooling power required to charge hydrogen, with results presented and discussed. The first application (the forward problem) analyzed the gravimetric and volumetric performance of MOF-5® based hydrogen storage beds, under various operating conditions. Results demonstrated that the system can reach a gravimetric capacity of approximately 4 wt% and volumetric capacity of about 20 g/L within 200 s during the absorption process. The second application (the inverse problem) identified the range of selected material parameters, required to meet the U.S. Department of Energy targets for gravimetric and volumetric capacity. Results showed that the most important parameters are the maximum capacity and the density of the material. Adsorbents having a density on the order of twice that of nominal powder form MOF-5® can meet the 2020 DOE targets (i.e. system gravimetric capacity of 0.045 kgH2/kgSystem and system volumetric capacity of 0.030 kgH2/LSystem). A density of about 3–4.5 times the nominal value is required to meet the DOE 2025 targets (i.e. system gravimetric capacity of 0.055 kgH2/kgSystem and system volumetric capacity of 0.040 kgH2/LSystem). Likewise, a material with a maximum adsorption capacity approximately equal to three times that of nominal MOF-5® can meet the 2020 DOE targets, while a maximum capacity about 4.5 times the nominal value is required to meet the 2025 DOE targets.
•Acceptability of adsorbents for H2 storage depends on relative parameter values.•Parameters are for both material characteristics and the storage system interface.•Trade-off between parameters precludes simple application of a table of values.•Methodology can be modified to suit range of system operations and isotherms.•Specific application to MOF-5 for DAR isotherm is used as a demonstration.
In order to enable the commercial acceptance of solid-state hydrogen storage materials and systems it is important to understand the risks associated with the environmental exposure of various ...materials. In some instances, these materials are sensitive to the environment surrounding the material and the behavior is unique and independent to each material. The development of testing procedures to evaluate a material’s behavior with different environmental exposures is a critical need. In some cases material modifications may be needed in order to reduce the risk of environmental exposure. We have redesigned two standardized UN tests for clarity and exactness; the burn rate and self-heating tests. The results of these and other UN tests are shown for ammonia borane, NH
3BH
3, and alane, AlH
3. The burn rate test showed a strong dependence on the preparation method of aluminum hydride as the particle size and trace amounts of solvent greatly influence the test results. The self-heating test for ammonia borane showed a failed test as low as 70 °C in a modified cylindrical form. Finally, gas phase calorimetry was performed and resulted in an exothermic behavior within an air and 30%RH environment.
► Dry and humid air exposure studies were conducted on potential hydrogen storage materials ammonia borane and alane to assess the risk of their exposure to the environment. ► Alane is susceptible to vigorous reaction in humid air. ► Synthesis techniques of alane affecting particle size impacted the environmental exposure results with smaller particles being more active. ► Ammonia borane is relatively stable at ambient conditions in air but subject to self-heating and hydrogen release if heated above 70 °C. ► Both alane and ammonia borane are susceptible to burning with subsequent release of hydrogen when exposed to an open flame.
The U.S. Department of Energy (DOE) has developed the Framework model to simulate fuel cell-based light-duty vehicle operation for various hydrogen storage systems. This transient model simulates the ...performance of the storage system, fuel cell, and vehicle for comparison to DOE's Technical Targets using four drive cycles. Metal hydride hydrogen storage models have been developed for the Framework model. Despite the utility of this model, it requires that material researchers input system design specifications that cannot be easily estimated. To address this challenge, a design tool has been developed that allows researchers to directly enter physical and thermodynamic metal hydride properties into a simple sizing module that then estimates the systems parameters required to run the storage system model. This design tool can also be used as a standalone MS Excel model to estimate the storage system mass and volume outside of Framework and compare it to the DOE Technical Targets. This model will be explained and exercised with existing hydrogen storage materials.
•A design tool has been developed to assist metal hydride materials researchers.•The tool sizes the storage system components using material properties as inputs.•It can be used as a standalone model or with the HSECoE vehicle Framework model.•Correlations were demonstrated between material properties and storage system size.
This work combines materials development with hydrogen storage technology advancements to address onboard hydrogen storage challenges in light-duty vehicle applications. These systems are comprised ...of the vehicle requirements design space, balance of plant requirements, storage system components, and materials engineering culminating in the development of an Adsorbent System Design Tool that serves as a preprocessor to the storage system and vehicle-level models created within the Hydrogen Storage Engineering Center of Excellence. Computational and experimental efforts were integrated to evaluate, design, analyze, and scale potential hydrogen storage systems and their supporting components against the Department of Energy 2020 and Ultimate Technical Targets for Hydrogen Storage Systems for Light Duty Vehicles. Ultimately, the Adsorbent System Design Tool was created to assist material developers in assessing initial design parameters that would be required to estimate the performance of the hydrogen storage system once integrated with the full fuel cell system.
Display omitted
•Model analysis of adsorption-based hydrogen system for fuel cell applications.•Evaluation of various engineering design options for tank and balance-of-plant.•Thermodynamic equations to predict charging/discharging of activated carbon and MOF-5.•Prototype validation and model sensitivity analysis for data comparison.•Full-vehicle level analysis compared to DOE standards to determine applicability.
Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H2 storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen ...storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H2 storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N2 cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.
Display omitted
•Hydrogen storage capacities of MOFs were studied at material and system level.•MOFs studied include MOF-5, IRMOF-20, SNU-70, UMCM-9, DUT-23 (Co) and NU-100.•Excess 77 K hydrogen adsorption decreases with uniaxial compaction of MOFs.•Powder packing efficiency of MOFs has a large effect on the system capacity.•The results highlight the material and system attributes which must be optimized.
PDF
