Zahir Dehouche

Influence of cycling on the thermodynamic and structure properties of nanocrystalline magnesium based hydride

Abstract We have investigated the effect of prolonged cycling on the hydriding/dehydriding properties and on the structure of nanocrystalline MgH 2 –V composite produced by high-energy ball milling. The hydrogen charge and discharge kinetics of the nanocomposite hydride were tested at 300°C using up to 2000 cycles. Pressure composition isotherms at 300°C were also carried out. The nanocomposite exhibits good reversibility in its hydrogenation/dehydrogenation curves after 2000 cycles. The results show some improvements in hydrogen capacity during cycling; this enhanced H-solubility is believed to be the result of structural relaxation. The sample resistance to hydrogen decrepitation was also evaluated via additional experiments involving SEM, BET specific surface area and X-ray crystal structure characterisations. These observations indicate that the nanostructured Mg-based composite does not decrepitate much upon cycling. However, a slight deterioration in the discharge rate of the nanocrystalline magnesium hydride is observed, apparently related to the crystal growth during cycling.

The catalytic effect of single-wall carbon nanotubes on the hydrogen sorption properties of sodium alanates

Single-wall carbon nanotubes (SWNTs) were examined as catalysts for improving the hydrogen absorption and desorption properties of Ti/Zr-doped NaAlH4 hydride, proposed as a reversible hydrogen storage material. We studied the hydrogen charge and discharge characteristics and stability of sodium aluminium composites ball milled with carbon additives such as SWNTs, graphite or activated carbon (AX-21). The SWNT–NaAlH4 system was tested at 160 °C for up to 200 cycles, and the sorption kinetics were enhanced by a factor of four. Also, the catalyzed NaAlH4 hydride with graphite and activated carbon additives shows fast absorption and desorption kinetics. Our results indicate that by creating new hydrogen transition sites, the structure of carbon in the composites plays an important role in enhancing the hydrogen absorption and release rates.

Moisture effect on hydrogen storage properties of nanostructured MgH2–V–Ti composite

Abstract In this paper, we present our recent results on the effect of moisture during prolonged cycling on hydrogen storage properties of nanostructured MgH 2 hydride with V and Ti catalysts additions synthesized by ball milling. The hydrogen charge and discharge stability of the nanocomposite hydride has been tested at 300° C for up to 1000 cycles under hydrogen containing 101 ppm moisture. Between the first and the 500th or 1000th cycle, an increase of hydrogen storage capacity by about 5% is observed in dynamic and PCT measurements. This could be due to structural relaxations. While absorption kinetics remain fast, the results show a significant and systematic slow-down of the desorption rates by about a factor of two during cycling. The X-ray diffraction patterns of nanocomposite performed before and after 1000 cycles reveal that the peak shape for magnesium remains unchanged indicating that the crystal growth is negligible. This microstructural stability during cycling suggests that the decreasing desorption rate of the nanocrystalline magnesium-based composite is not induced by any internal structural modification. On the other hand, the presence of moisture in the hydrogen gas during cycling induces surface effects which most likely cause the decrease of the hydrogen discharge flow rate.

Evaluation techniques of cycling effect on thermodynamic and crystal structure properties of Mg2Ni alloy

Abstract Hydrogen storage using reversible lightweight metal hydrides can open the way towards hydrogen utilization for energy applications like car fuel. Magnesium-nickel alloys are the most promising lightweight materials which offer a favorable volumetric hydrogen storage density. For a vehicular application, a prime consideration is the stability of the hydrogen absorbing/desorbing capacity of the hydride when it is subjected to a large number of absorption/desorption cycles. In order to estimate if magnesium-nickel alloys would be attractive candidates for practical hydrogen storage systems, a new and rapid experimental device was designed for fast continuous evaluation of the cyclic charge and discharge stability of the intermetallic compound Mg 2 Ni over a relatively large number of cycles. Repetitive hydriding and dehydriding of a metal alloy is simply performed by cycling the sample from the high-pressure circuit used for absorption to the low pressure circuit having enough volume to receive the hydrogen discharge. Simultaneously the sample holder containing the hydride may be alternated from the low temperature furnace for absorption to the high temperature furnace for desorption. The cycling stability of commercial crystalline Mg 2 Ni alloy was tested at 300°C over up to 2700 cycles. Periodically during the cycling experiments, we have also performed measurements of the dynamic hydrogen absorption/desorption; these were done at the reaction temperatures of 250°C and 300°C, respectively. Moreover, pressure composition isotherms at 300°C measurements were also carried out. The cycling effect was also evaluated via additional experiments involving SEM, Brunauer-Emmett-Teller (BET) specific surface area, specific heat and X-ray crystal structure analysis.

Thermal cyclic charge and discharge stability of nanocrystalline Mg2Ni alloy

Abstract The hydrogen storage properties and the thermal cyclic charge and discharge capability of the nanocrystalline Mg 2 Ni alloy powder prepared by high energy ball milling are examined to evaluate the potential use of these materials in vehicular applications. Repetitive hydriding and dehydriding of nanocrystalline Mg 2 Ni alloy is performed at different cycling temperatures and pressures of absorption and desorption. It was found that the cycling did not affect the kinetic rates of absorption/desorption. X-ray crystal structure analysis and pressure concentration isotherms of the alloy, after initial activation and after 2100 cycles, reveal significant changes in the dynamic phase structure and the thermodynamic properties. In addition to these experiments, we have also performed scanning electron microscopy, Brunauer Emmett-Teller (BET) specific surface area and specific heat characterizations. These have indicated that the changes in volume during the 2100 hydriding/dehydriding cycles do not produce severe decrepitation. However, inherent deterioration in the charge capacity of the alloy, apparently related to the formation of the MgNi 2 phase, was observed.

Modelling and experimental study of key parameters of absorption on wetted sphere contactor

Abstract Liquid spray curtains can be used to contain the effect of accidental release of toxic gases; they work by a combination of dilution and absorption. In this work, we have investigated the possibility of adding chemicals to the liquid spray which may react with the toxic gases and enhance the absorption capacity of the spray. The work is aimed at a fundamental understanding of the chemistry and the mass transfer behavior of toxic gas — reactive liquid systems. We have performed continuous mass transfer rate experiments on one sphere absorber (the model systems considered being CO 2 Na 2 CO 3 and O 2 Na 2 SO 3 ), and we have interpreted these in terms of mass transfer with chemical reaction theory. The system was modelled as a series of a sphere gas-liquid reactor (SGLR) and a continuous-stirred tank (CST). A numerical technique was used to predict both the unsteady state liquid reactant concentration and the specific absorption rate of gas in the system. A systemic approach was employed to determine values of key parameters that quantify a given mechanism. The results obtained, together with pilot plant results on the hydrodynamics of liquid sprays, may form the basis for a predictive model of chemical liquid spray behavior in environmental control.

Trombe walls with nanoporous aerogel insulation applied to UK housing refurbishments

There is an opportunity to improve the efficiency of passive Trombe walls and active solar air collectors by replacing their conventional glass covers with lightweight polycarbonate panels filled with nanoporous aerogel insulation. This study investigates the thermal performance, energy savings, and financial payback period of passive Aerogel Trombe walls applied to the existing UK housing stock. Using parametric modeling, a series of design guidance tables have been generated, providing estimates of the energy savings and overheating risk associated with applying areas of Trombe wall to four different house types across the UK built to six notional construction standards. Calculated energy savings range from 183 kWh/m2/year for an 8 m2 system retrofitted to a solid walled detached house to 62 kWh/m2/year for a 32 m2 system retrofitted to a super insulated flat. Predicted energy savings from Trombe walls up to 24 m2 are found to exceed the energy savings from external insulation across all house types and constructions. Small areas of Trombe wall can provide a useful energy contribution without creating a significant overheating risk. If larger areas are to be installed, then detailed calculations would be recommended to assess and mitigate potential overheating issues.

Catalyzed Light Hydride Nanomaterials Embedded in a Micro-Channels Hydrogen Storage Container

Activated alloys synthesized by arc-melting were examined as catalysts for improving the hydrogen sorption characteristics of nanostructured magnesium hydride, proposed as a reversible hydrogen storage material. The MgH(2)-catalyst absorbing materials were prepared by ball milling of pure MgH(2) with hydrided Zr(47)Ni(53), Zr(9)Ni(11), and other alloys investigated. The nanostructured MgH(2)-intermetallic systems were tested at 250 degrees C and catalyst addition of eutectoid Zr(47)Ni(53) resulted in the fastest desorption time and highest initial desorption rate. The catalyzed Mg-hydride with activated Zr(9)Ni(11) and Zr(7)Ni(10) phases showed fast desorption kinetics. Moreover, the results demonstrated that the composition of dispersed Zr(x)Ni(y)catalysts has a strong influence on the amount of accumulated hydrogen and desorption rate of Mg-nanocomposite. Part two covers advanced micro-channels hydrogen storage module design based on the results of semi-empirical computer simulations of heat and mass transfers in the container. The micro-channels reservoir concept offers many advantages over the conventional metal hydride hydrogen storage system. It is a micro-structured system that can pack a lot of power into a small space and dissipate effectively the heat of the sorption reactions. This review summarizes recent patents related to CNTS.

Numerical simulation of catalytic inert membrane reactor

Abstract We are developping a new reactor model able to perform catalytic activation and separation simultaneously: the catalytic membrane reactor. This paper analyses the influence of gas diffusion through the membrane on mass transfer in the catalytic reactor with an inert membrane wall. A mathematical description of these reactors often leads to partial differential equations (PDE) with complex boundary conditions. Two efficient techniques are used to solve these equations. The method implies an orthogonal collocation on finite elements in the spacial dimension followed by a global orthogonal collocation. Theoretical comparisons are made between the concentration profiles in the radial direction obtained for both reactors. The effect of radial dispersion on the performance of the inert membrane reactor is examined. The applicability of approximate methods for membrane transport model is also discussed.

Enhancing the thermal conductivity of ethylene-vinyl acetate (EVA) in a photovoltaic thermal collector

This work was sponsored by ChapmanBDSP, London, UK and the Engineering and Physical Research Council, UK.