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Thermal Management in the Measurement of Metal Hydride Kinetics

Many metal hydride nanopowders are currently being investigated as a potential hydrogen storage media. The kinetic properties of hydrogen absorption in TiCrMn, a metal hydride of interest, are largely unknown. This study will use coupled thermal and kinetic modeling to analyze a combination of novel and well-established techniques which can be used to experimentally determine these parameters. Since these measurements must be taken at isothermal conditions and the metal hydride absorption reaction is highly exothermal, specific thermal considerations must be made in these models. Typical instruments available for kinetics measurements suspend the samples in a small chamber, effectively thermally isolating them from the cooling or heating system designed to control sample temperature. The design modeled herein will eliminate that convective resistance layer, thereby increasing the amount of heat that can be rapidly diffused out of the sample. Additionally, an electronically controlled active temperature control system will be modeled as a method of maintaining “quasi-isothermal” conditions in the metal hydride during measurements.© 2008 ASME

Methods for quantifying the influences of pressure and temperature variation on metal hydride reaction rates measured under isochoric conditions

Analysis techniques for determining gas-solid reaction rates from gas sorption measurements obtained under non-constant pressure and temperature conditions often neglect temporal variations in these quantities. Depending on the materials in question, this can lead to significant variations in the measured reaction rates. In this work, we present two new analysis techniques for comparison between various kinetic models and isochoric gas measurement data obtained under varying temperature and pressure conditions in a high pressure Sievert system. We introduce the integral pressure dependence method and the temperature dependence factor as means of correcting for experimental variations, improving model-measurement fidelity, and quantifying the effect that such variations can have on measured reaction rates. We use measurements of hydrogen absorption in LaNi5 and TiCrMn to demonstrate the effect of each of these methods and show that their use can provide quantitative improvements in interpretation of kinetics measurements.

Performance of thermal enhancement materials in high pressure metal hydride storage systems

Over the past two years, key issues associated with the development of realistic metal hydride storage systems have been identified and studied at Purdue University’s Hydrogen Systems Laboratory, part of the Energy Center at Discovery Park. Ongoing research projects are aimed at the demonstration of a prototype large-scale metal hydride tank that achieves fill and release rates compatible with current automotive use. The large-scale storage system is a prototype with multiple pressure vessels compatible with 350 bar operation. Tests are conducted at the Hydrogen Systems Lab in a 1000 ft2 laboratory space comprised of two test cells and a control room that has been upgraded for hydrogen service compatibility. The infrastructure and associated data acquisition and control systems allow for remote testing with several kilograms of high-pressure reversible metal hydride powder. Managing the large amount of heat generated during hydrogen loading directly affects the refueling time. However, the thermal management of hydride systems is problematic because of the low thermal conductivity of the metal hydrides (∼ 1 W/m-K). Current efforts are aimed at optimizing the filling-dependent thermal performance of the metal hydride storage system to minimize the refueling time of a practical system. Combined heat conduction within the metal hydride and the enhancing material particles, across the contacts of particles and within the hydrogen gas between non-contacted particles plays a critical role in dissipating heat to sustain high reaction rates during refueling. Methods to increase the effective thermal conductivity of metal hydride powders include using additives with substantially higher thermal conductivity such as aluminum, graphite, metal foams and carbon nanotubes. This paper presents the results of experimental studies in which various thermal enhancement materials are added to the metal hydride powder in an effort to maximize the effective thermal conductivity of the test bed. The size, aspect ratio, and intrinsic thermal conductivity of the enhancement materials are taken into account to adapt heat conduction models through composite nanoporous media. Thermal conductivity and density of the composite materials are measured and enhancement metrics are calculated to rate performance of composites. Experimental results of the hydriding process of thermally enhanced metal hydride powder are compared to un-enhanced metal hydride powder and to model predictions. The development of the Hydrogen Systems Laboratory is also discussed in light of the lessons learned in managing large quantities of metal hydride and high pressure hydrogen gas.Copyright

System level permeability modeling of porous hydrogen storage materials.

A permeability model for hydrogen transport in a porous material is successfully applied to both laboratory-scale and vehicle-scale sodium alanate hydrogen storage systems. The use of a Knudsen number dependent relationship for permeability of the material in conjunction with a constant area fraction channeling model is shown to accurately predict hydrogen flow through the reactors. Generally applicable model parameters were obtained by numerically fitting experimental measurements from reactors of different sizes and aspect ratios. The degree of channeling was experimentally determined from the measurements and found to be 2.08% of total cross-sectional area. Use of this constant area channeling model and the Knudsen dependent Young & Todd permeability model allows for accurate prediction of the hydrogen uptake performance of full-scale sodium alanate and similar metal hydride systems.

Advanced Transient Plane Source Method for the Measurement of Thermal Properties of High Pressure Metal Hydrides

Metal hydrides are a promising material type for hydrogen storage in automotive applications, but thermal property data is needed to optimize the necessary heat exchangers. In the present work, the transient plane source method is integrated with a pressure vessel to measure these properties for metal hydride powder as a function of pressure during the hydrogenation process. The properties under investigation include effective thermal conductivity, thermal diffusivity, specific heat, and thermal contact resistance. The results of this work with oxidized Ti1.1 CrMn powder provide effective thermal conductivity values similar to data reported in literature for other metal hydride materials. The experimental measurements are also well modeled by the Zehner-Bauer-Schlunder interpretive model for packed beds as a function of gas pressure. Extending the test method and ZBS model to estimate the contact resistance provides values that were two orders of magnitude less than measurements previously reported for other hydride materials.Copyright

Metal Hydride Component Design (MHy-CoDe) Tool for the Selection of Hydrides in Thermal Systems

A system has been developed to enable the targeted down-selection of an extensive database of metal hydrides to identify the most promising materials for use in thermal systems. The materials’ database contains over 300 metal hydrides with various physical and thermodynamic properties included for each material. Submodels for equilibrium pressure, thermophysical data, and default properties are used to predict the behavior of each material within the given system. The application used at this time is a stationary combined heat and power system containing a hightemperature proton exchange membrane (PEM) fuel cell, a hot water tank, and two metal hydride beds used as a heat pump to increase the efficiency of a natural gas system. The targeted down-selection for this system focuses on the system’s coefficient of performance (COP) for each potential pair and the corresponding sensitivity of the COP and has been used to identify the top 20 pairs, with COPs >1.3, for use in this application.

Chemically B-N Modified Activated Carbon and its Thermal Stability and Desorption Enthalpy With Methanol

A chemical modification of activated carbon is demonstrated through boron and nitrogen incorporation via microwave-assisted heating. The surface modification of the activated carbon was imaged by scanning electron microscope. The crystallinity of the material was quantified by X–ray diffraction, and the chemical content as well as bonding environment were investigated using X-ray photoelectron and Raman microscopies. The observed increment in desorption enthalpy of modified activated carbon with methanol as measured through differential scanning calorimetry and its superior thermal stability in air as measured by thermogravimetric analysis suggest that the modified material is a promising candidate for efficient sorption processes in waste thermal and solar energy driven cycles.© 2012 ASME

Design Construction and Test of a Subscale Ammonia Borane Reactor

Two ammonia borane (AB or NH3 BH3 ) dehydrogenation routes, namely hydrolysis and thermolysis, have been described in the literature. The work done on design, construction and testing of a subscale AB reactor is reported herein along with a discussion on the results. In this work, an AB dehydrogenation reactor system capable of handling two grams of AB per batch was designed. Operational safety, material compatibility and manufacturability were the major design requirements. The reactor system consisted of a high pressure feeder, a cylindrical stainless steel reactor vessel, an evolved gas heat exchanger and an ammonia filter, and a hydrogen flow meter. The reactor was operated in a temperature range of 430 K to 445 K for a nominal batch reaction time of 30 minutes. Measurements of hydrogen yield rates, system storage capacity and analysis of the reaction kinetics were completed. Overall repeatability of hydrogen yields was confirmed. A few practical problems associated with byproduct formation and removal are discussed in this paper.Copyright