Distributed hydrogen generation from liquid hydrocarbon fuels to supply portable fuel cells presents an attractive, high energy density alternative to current battery technology. Traditional unit operation reactor design for hydrogen generation becomes inadequate with decrease in scale because of the unique challenges of size and weight minimization. To address the challenge of reactor scale-down, the concept of multifunctional reactors has emerged, in which synergistic combination of different unit operations is explored to achieve improved performance. The direct droplet impingement reactor (DDIR) studied here is based on this approach in which the liquid feed is atomized using a regularly spaced array of droplet generators with unparalleled control over droplet characteristics, followed by vaporization and reaction directly on the catalyst surface. Considering each droplet generator in the array as a unit cell, a comprehensive, first-principles model of the DDIR has been developed by considering the intimately coupled processes of 1) droplet transport, heating, evaporation, and impingement on the catalyst surface, 2) liquid reagent film formation, capillary penetration, and vaporization within the catalyst layer, and 3) gas phase heat and mass transfer and catalytic reactions. Simulations are performed to investigate the effect of reactor operating parameters on performance. Experimental validation of the model is carried out by visualizing droplet impingement and liquid film accumulation while simultaneously monitoring reaction product composition over a range of operating conditions. Results suggest an optimal unit cell shape for reaction selectivity based on a balance between reagent back diffusion and catalyst bed thermal resistance. Further, achieving a target throughput is best accomplished by adding together a larger number unit cells with optimized geometry and lower throughput (per unit cell) to more effectively spread heat and avoid hotspots at the catalyst interface. At the same time, conditions must be satisfied for ensuring droplet impingement on the catalyst surface, which become more stringent as unit cell throughput is decreased.

First- principles modeling is used to predict hydrogen permeability through Palladium (Pd)-rich binary alloy membranes as a function of temperature and H2 pressure. We introduce a simplified model that incorporates only a few factors and yields quantitative prediction. This model is used to predict hydrogen permeability in a wide range of binary alloy membranes and to find promising alloys that have high hydrogen permeability. We show how our efficient Density Functional Theory (DFT)-based model predicts the chemical stability and proton conductivity of doped barium zirconate (BaZrO3), barium stannate (BaSnO3), and barium hafnate (BaHfO3). Our data is also used to explore the physical origins of the trends in chemical stability and proton conductivity among different dopants. We also study potassium tantalate (KTaO3), which is a prototype perovskite, to examine the characteristics of undoped perovskites. Specifically, we study the impacts of isotope effects, tunneling effects, and native point defects on proton mobility in KTaO3. It is important to find and develop solid-state Li-ion electrolyte materials that are chemically stable and have high ionic conductivities for high performance Li-ion batteries. We show how we predict the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions.

Thesis (Ph.D.)--University of Melbourne, Dept. of Mechanical and Manufacturing Engineering, 2006. / Typescript. Includes bibliographical references (p. 251-276).

Thesis (Ph.D.)--University of Melbourne, Dept. of Mechanical Engineering, 2009. / Typescript. Includes bibliographical references (leaves 231-245)

Control of pollutants and emissions has become a major factor in the design of modern combustion systems. The “Liquid Hydrogen Fueled Aircraft - System Analysis” project funded in 2000 by the European Commission can be seen as such an initiative. Within the framework of this project, the Aachen University of Applied Sciences developed experimentally the “Micromix” hydrogen combustion principle and implemented it successfully in the Honeywell APU GTCP 36-300 gas turbine engine. Lowering the reaction temperature, eliminating hot spots from the reaction zone and keeping the time available for the formation of NOx to a minimum are the prime drivers towards NOx reduction. The “Micromix” hydrogen combustion principle meets those requirements by minimizing the flame temperature working at small equivalence ratios, improving the mixing by means of Jets In Cross-Flow and reducing the residence time in adopting a combustor geometry that provides a very large number of very small diffusion flames. In terms of pollutant emissions, compared to the unconverted APU, an essential reduction in emitted NOx was observed, stressing the potential of this innovative burning principle.<p>The objective of this thesis is to investigate the “Micromix” hydrogen combustion principle with the ultimate goal of an improved prediction during the design process. Due to the complex interrelation of chemical kinetics and flow dynamics, the “Micromixing” was analyzed first. Stereoscopic Particle Image Velocimetry was used to provide insight into the mixing process. A “simplified” set-up, that allowed to investigate the flow characteristics in great detail while retaining the same local characteristics of its “real” counterparts, was considered. The driving vortical structures were identified. To further investigate the physics involved and to extend the experimental results, numerical computations were carried out on the same “simplified” set-up as on a literature test case. In general, a number of physical issues were clarified. In particular, the interaction between the different vortical structures was looked into, and a kinematically consistent vortex model is proposed. After demonstrating the development of the mixing, the “cold flow” study was extended to a single injector. The double backward-facing step injector geometry was addressed experimentally and numerically. At design geometry, the flow appeared to behave single backward-facing like, with respect to the first gradation. In terms of varying step configurations, the flow was seen to be dependent on the periodic perturbation arising from the graded series of backward-facing steps. During the second part of the investigation, the “hot flow” was analyzed. Considering combustor similar operating conditions, a test burner was experimented on an atmospheric test rig. NOx emissions were traced by exhaust gas analysis for different working conditions. Particular flame patterns, such as a regular attached flame as well as lifted flames were observed. In parallel with the experimental work, numerical computations on a pair of opposite injectors, permitted to classify the combustion regime and the main factors involved in the NOx formation. Accordingly, NOx emission enhancing design changes are proposed. Finally, the demanding computational effort, worthy of acceptance for academic purposes, is found not agreeable as future design tool and improvements to speed up the design process are projected.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Indiana University-Purdue University Indianapolis (IUPUI) / An integrated micro PEM fuel cell system with self-regulated hydrogen generation from ammonia borane is reported to power portable electronics. Hydrogen is generated via catalytic hydrolysis reaction of ammonia borane solution in microchannels with nanoporous platinum catalyst electroplated inside the microchannels. The self-regulation of the ammonia borane solution is achieved by using directional growth and selective venting of hydrogen bubbles in microchannels, which leads to agitation and addition of fresh solution without power consumption. The device is fabricated on combination of polystyrene sheets cut by graphic cutter, a stainless steel layer cut using wire electrical discharge machining and bonding layers with double-sided polyimide tape. Due to the seamless bonding between the hydrogen generator and the micro fuel cell, the dead volume in the gas connection loops can be significantly reduced and the response time of self-regulation is reduced.

Call number: LD2668 .T4 1985 S335 / Master of Science

M. Tech. (Department of Electronic Engineering, Faculty of Engineering and Technology) --Vaal University of Technology| / Global warming is of increasing concern due to several greenhouse gases. The combustion of fossil fuels is the major contributor to the greenhouse effect. To minimalise this effect, alternative energy sources have to be considered. Alternative energy sources should not only be environmentally friendly, but also renewable and/or sustainable. Two such alternative energy sources are hydrogen and solar energy. The regenerative fuel cell, commonly known as a hydrogen generator, is used to produce hydrogen. The current solar/hydrogen system at the Vaal University of Technology’s Telkom Centre of Excellence makes use of PV array to supply power to an inverter and the inverter is connected to the hydrogen generator. The inverter provides the hydrogen generator with 220VAC. The hydrogen generator has its own power supply unit to convert the AC power back to DC power. This reduces the efficiency of the system because there will be power loss when converting DC power to AC power and back to DC power. The hydrogen generator, however, could be powered directly from a PV array. However, the hydrogen generator needs specific input parameters in order to operate. Three different input voltages with their own current rating are required by the hydrogen generator to operate properly. Thus, a DC-DC power supply unit needs to be designed to be able to output these parameters to the hydrogen generator. It is also important to note that current PV panel efficiency is very low; therefore, the DC-DC power supply unit also needs to extract the maximum available power from the PV array. In order for the DC-DC power supply unit to be able to extract this maximum power, a maximum power point tracking algorithm needs to be implemented into the design. The DC-DC power supply is designed as a switch mode power supply unit. The reason for this is that the efficiency of a switch mode power supply is higher than that of a linear power supply. To reach the objective the following methodology was followed. The first part of the research provided an introduction to PV energy, charge controllers and hydrogen generators. The problem statement is included as well as the purpose of this research and how this research was to be carried out. The second part is the literature review. This includes the background study of algorithms implemented in MPPT’s; it also explains in detail how to design the MPPT DC-DC SMPS. The third part was divided into two sections. The first section is the design, programming and manufacturing of the MPPT DC-DC SMPS. The second section is the simulation of the system as a whole which is the simulation of the PV array connected to the MPPT DC-DC SMPS and the hydrogen generator. The fourth part in the research compared the results obtained in the simulation and practical setup. The last part of the research provided a conclusion along with recommendation made for further research. The simulation results showed that the system works with an efficiency of 40,84%. This is lower than expected but the design can be optimised to increase efficiency. The practical results showed the efficiency to be 38%. The reason for the lower efficiency is the simulation used ideal components and parameters, whereas the practical design has power losses due to the components not being ideal. The design of the DC-DC switch mode power supply, however, indicated that the hydrogen generator could be powered from a PV array without using an inverter, with great success.

Thesis (M. Tech. - (Engineering: Electrical, Department: Electronic Engineering, Faculty of Engineering and Technology)) -- Vaal University of Technology, 2011. / “Never before in peacetime have we faced such serious and widespread shortage of energy” according to John Emerson, an economist and power expert for Chase Manhattan Bank. Many analysts believe that the problem will be temporary, but others believe the energy gap will limit economic growth for years to come. A possible solution to this problem can be fuel cell technology. Fuel cells (FCs) are energy conversion devices that generate electricity from a fuel like hydrogen. The FC however, could also be used in the reverse or regenerative mode to produce hydrogen. The reversible fuel cell (RFC) can produce hydrogen and oxygen by introducing water to the anode electrode chamber, and applying a potential across the anode and cathode. This will cause the decomposition of the water to produce oxygen at the anode side and hydrogen at the cathode side. In order to make this process as efficient as possible several aspects need to be optimised, for example, the operation temperature of the RFC, water management inside the RFC and supply voltage to the RFC. A three cell RFC and its components were constructed. The three cell RFC was chosen owing to technical reasons. The design factors that were taken into consideration were the different types of membranes, electrocatalysts, bipolar plates and flow topologies. A water trap was also designed and constructed to eliminate the water from the hydrogen water mixture due to water crossover within the MEA. In order to optimise the operation of the RFC a number of experiments were done on the RFC. These experiments included the optimal operating voltage, the effect that the temperature has on the production rate of hydrogen, and the effect that the water flow through the RFC has on the production rate of hydrogen. It was found that there is no need to control the water flow through the RFC because it had no effect on the production rate of hydrogen. The results also showed that if the operating temperature of the RFC were increased, the energy it consumes to warm the RFC significantly decreases the efficiency of the whole system. Thus the RFC need not be heated because it consumes significantly more energy to heat the RFC compared to the energy available from the hydrogen produced for later use. The optimised operating voltage for the three cell RFC was found to be 5.05 V. If the voltage were to be increased or decreased the RFC efficiency would decrease.

Hydrogen has been shown to be a promising replacement for fossil fuels for use in light duty vehicles because it is a clean, renewable and plentiful resource with a high gravimetric energy density. However, in order to obtain an acceptable volumetric energy density, densification of the hydrogen is required. Adsorptive materials have been shown in the literature to increase volumetric and gravimetric storage densities. A major issue with adsorptive storage is that the adsorption process generates heat and optimal storage conditions are at temperatures below 100 K at pressures up to 50 atm. There is a need to develop heat exchanging architecture that enables adsorbents to be a viable method for hydrogen storage by managing the thermal environment of the storage tank. Based on previous modeling efforts to determine an acceptable bed module height for removal of heat via microchannel cooling plates, a thermal management system has been designed and tested capable of removing the heat of adsorption within adsorbent-filled hydrogen storage tanks. The system uses liquid nitrogen cooling to maintain tank temperatures of below 80 K at 50 atm. System studies show that the microchannel architecture offers a high cooling capacity with about a 6% displacement volume. Simulations and experiments have been conducted to evaluate the design for the cooling capacity, pressure drop, and flow distribution between and across the cooling plates, stress due to the pressurized environment, and thermal stress. Cost models have been developed to demonstrate that the system can be manufactured for a reasonable cost at high production volumes. Experimental results show that the modular system offers an acceptable cooling capacity and pressure drop with good flow distribution while adequately managing thermal stresses during operation. / Graduation date: 2013