High-Temperature Activated AB2 Nanopowders for Metal Hydride Hydrogen Compression

A reliable process for compressing hydrogen and for removing all contaminants is that of the metal hydride thermal compression. The use of metal hydride technology in hydrogen compression applications though, requires thorough structural characterization of the alloys and investigation of their sorption properties. The samples have been synthesized by induction - levitation melting and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns. Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be reached from the selected alloys using water of 90oC. Experimental evidence shows that the maximum hydrogen uptake is low since all the alloys are consisted of Laves phases, but it is of minor importance if they have fast kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost absent while all the alloys release nearly all the absorbed hydrogen during desorption. Due to hardware restrictions, the maximum hydrogen pressure for the measurements was limited at 100 bars. Practically, the maximum pressure that can be reached from the last alloy is more than 150 bars.Comment: 9 figures. arXiv admin note: text overlap with arXiv:1207.354

Efficient hydrogen storage in up-scale metal hydride tanks as possible metal hydride compression agents equipped with aluminium extended surfaces

In the current work, a three-dimensional computational study regarding coupled heat and mass transfer during both the hydrogenation and dehydrogenation process in upscale cylindrical metal hydride reactors is presented, analysed and optimized. Three different heat management scenarios were examined at the degree to which they provide improved system performance. The three scenarios were: 1) plain embedded cooling/heating tubes, 2) transverse finned tubes and 3) longitudinal finned tubes. A detailed optimization study was presented leading to the selection of the optimized geometries. In addition, two different types of hydrides, LaNi5 and an AB2-type intermetallic were studied as possible candidate materials for using as the first stage alloys in a two-stage metal hydride hydrogen compression system. As extracted from the above results, it is clear that the case of using a vessel equipped with 16 longitudinal finned tubes is the most efficient way to enhance the hydrogenation kinetics when using both LaNi5 and the AB2-alloy as the hydride agents. When using LaNi5 as the operating hydride the case of the vessel equipped with 60 embedded cooling tubes presents the same kinetic behaviour with the case of the vessel equipped with 12 longitudinal finned tubes, so in that way, by using extended surfaces to enhance the heat exchange can reduce the total number of tubes from 60 to 12. For the case of using the AB2-type material as the operating hydride the performance of the extended surfaces is more dominant and effective compared to the case of using the embedded tubes, especially for the case of the longitudinal extended surfaces

Numerical study on a two-stage metal hydride hydrogen compression system

A multistage Metal Hydride Hydrogen Compression (MHHC) system uses a combination of hydride materials in order to increase the total compression ratio, whilst maximizing the hydrogenation rate from the supply pressure at each stage. By solving the coupled heat, mass and momentum conservation equations simultaneously the performance of a MHHC system can be predicted. In the current work a numerical model is proposed to describe the operation of a complete compression cycle. Four different MHHC systems are examined in terms of maximum compression ratio, cycle time and energy consumption and it was found that the maximum compression ratio achieved was 22:1 when operating LaNi5 (AB5-type) and a Zr–V–Mn–Nb (AB2-type intermetallic) as the first and second stage alloys respectively in the temperature range of 20°C (hydrogenation) to 130°C (dehydrogenation)

Efficient hydrogen storage and compressors by using metal hydrides.

Hydrogen gas is considered as the key "green" technology for its endless production-consumption procedure, known better as the "water-cycle". In this cycle, hydrogen is directly produced from water electrolysis and while forms water again, when combusted with oxygen, releasing energy. However, as the most lightweight gas, the volumetric density of hydrogen makes it inappropriate for use in most fuel applications. Hydrogen storage technologies that promote the compression of the gas, along with a solid state form of storage, based on the consumer needs, are a critical component to further development of a hydrogen fuel-based economy. Among the breakthrough hydrogen technologies, metal hydrides have drawn special attention due to their multiple properties. Their ability to react with hydrogen at various pressure and temperature conditions opened a whole new universe for storing energy in the form of hydrogen. Moreover, their unique capability to operate as thermal machines in order to compress hydrogen is of major importance and it will probably affect future hydrogen technologies. The research described in this thesis tries to reveal new metal hydrides with enhanced properties or to enrich knowledge on already investigated compounds. The research conducted was multidisciplinary in nature and all the involved detailed investigations tried to identify correlations between micro-structural and chemical properties with the thermodynamic properties of the metal hydrides. The research undertaken focused in three key research areas: (i) structural characterization by means of the X-Ray Diffraction (XRD) technique and Rietveld analysis, (ii) microstructural observation and microchemical characterization by SEM/EDX analysis and (iii) thermodynamic properties investigation by hydrogen absorption/desorption measurements. The interactions between these different properties can be complex, and they are not always resulting to data that can be easily exploited. In a more extensive manner, several Ti- or Zr-based intermetallic alloys have been synthesized from pure elements either using the arc-melting or the induction-levitation melting method. Both techniques are characterized as "Rapid Solidification Processes" with crucial influence on the crystal structure of the investigated compounds. The word "crucial" has been used in the last phrase since the rapid solidification of the liquid compounds result in fine, well-structured microstructures that are, as well as with the chemical properties, the key factor of their thermodynamic properties. All the compounds have been fully characterized by XRD and SEM/EDX, and they are mostly exhibiting the hexagonal C14 or the cubic C15 type Laves phases. More specifically, Zr-based compounds with Laves phase structures are considered advantageous hydrides for the large span of plateau pressures (0 - 1000 bar) they can exhibit when forming reversible metal hydrides. Since these intermetallic compounds are considered as low temperature hydrides, all hydrogen Pressure-Composition Isotherms (PCI) have been conducted in the range of 0 to ~100 oC. It has been shown that small compositional changes can affect the atomic structure that has a direct effect, in respect, on the thermodynamic properties. Thus, compounds with really close composition can have a very large difference in the equilibrium pressures in some given temperatures. Finally, the metal hydride hydrogen compression technology is presented in terms that metal hydrides with successive equilibrium pressures can be used in order to increase hydrogen pressure using only waste heat as driving force

Investigation of ZrFe2-type materials for metal hydride hydrogen compressor systems by substituting Fe with Cr or V

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Structural, microchemistry, and hydrogenation properties of TiMn0.4Fe0.2V0.4, TiMn0.1Fe0.2V0.7 and Ti0.4Zr0.6Mn0.4Fe0.2V0.4 metal hydrides

In this work, TiFe-based alloys have been developed according to the stoichiometry Ti(1-x)A(x)Fe(1-y)B(y) (A Zr; B Mn, V). The hydrogen solubility properties have been investigated to develop dynamic hydrides of Ti-based alloys for hydrogen storage applications. The hydrogenation behavior of these alloys has been studied, and their hydrogen storage capacities and kinetics have been evaluated. Several activation modes, including activation at high temperatures under hydrogen pressure, have been attempted for the as-milled powders. In order to clarify the structural/microstructural characteristics, and chemical composition before and after hydrogenation, X-Ray Diffraction (XRD), EDAX-Mapping Analysis and Scanning Electron Microscopy (SEM), have been carried out for the samples. Modeling of the isotherms has been performed by using MATLAB programming. The maximum gravimetric density of 4.3 wt%, has been obtained on the sample with the BCC main phase. The calculated enthalpy of reaction (Delta H) is found to be about 4 kJ/mol