[en] The purpose of this work is to study the effect of non constant 2-dimensional free convection in hydromagnetic flows during the motion of a viscous incompressible electrically conducting fluid through a highly porous medium. The porous medium is bounded by a vertical plane surface of constant temperature. This surface absorbs the fluid with a constant velocity and the velocity vibrates about a mean constant value. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved subject to the relevant boundary conditions with the assumptions that the solution consists of a mean part and a perturbed part.An analytical solution for the velocity field is derived. The effects of the Hartmann parameter, permeability parameter, frequency parameter and Grashof parameter on the velocity field are discussed

[en] Fuel cell systems are attractive for their high efficiency (i.e., electric power generated per weight/volume of fuel,) and lower emissions. These systems are being developed for applications that include transportation (propulsion and auxiliary), remote stationary, and portable. Where these systems use on-board fuel processing of available fuels, the fuel processor requires high-purity water. For utility applications, this water may be available on-site, but for most applications, the process water must be recovered from the fuel cell system exhaust gas. For such applications, it is critically important that the fuel cell system be a net water-producing device. A variety of environmental conditions (e.g., ambient temperature, pressure), fuel cell system design, and operating conditions determine whether the fuel cell system is water-producing or water-consuming. This paper will review and discuss the conditions that determine the net-water balance of a generic fuel cell system and identify some options that will help meet the water needs of the fuel processor

[en] A novel approach for the fabrication and assembly of a solid oxide fuel cell system is described which enables effective scaling of the fuel delivery, manifold, and fuel cell stack components for applications in miniature and microscale energy conversion. Electrode materials for solid oxide fuel cells are developed using sputter deposition techniques. A thin film anode is formed by co-deposition of nickel and yttria-stabilized zirconia (YSZ). This approach provides a mixed conducting inter-facial layer between the nickel electrode and electrolyte layer. Similarly, a thin film cathode is formed by co-deposition of silver and yttria-stabilized zirconia. Additionally, sputter deposition of yttria-stabilized zirconia thin film electrolyte enables high quality, continuous films to be formed having thicknesses on the order of 1-2(micro)m. This will effectively lower the temperature of operation for the fuel cell stack significantly below the traditional ranges at which solid oxide electrolyte systems are operated (600-1000 C), thereby rendering this fuel cell system suitable for miniaturization, Scaling towards miniaturization is accomplished by utilizing novel micromachining approaches which allow manifold channels and fuel delivery system to be formed within the substrate which the thin film fuel cell stack is fabricated on, thereby circumventing the need for bulky manifold components which are not directly scalable. Methods to synthesize anodes for thin film solid-oxide fuel cells (TFSOFCs) from the electrolyte and a conductive material are developed using photolithographic patterning and physical vapor deposition. The anode layer must enable combination of the reactive gases, be conductive to pass the electric current, and provide mechanical support to the electrolyte and cathode layers. The microstructure and morphology desired for the anode layer should facilitate generation of maximum current density from the fuel cell. For these purposes, the parameters of the deposition process and post-deposition patterning are developed to optimize a continuous porosity in the anode layer. The fuel cell microstructure is examined using scanning electron microscopy and the power output generated is characterized through current-voltage measurement. Results demonstrating the generation of electrical current in the temperature range of 200-400 C for a thin film solid oxide fuel cell stack fabricated on a silicon wafer will be presented

No abstract available

[en] One carried out comparison analysis of electrode operation in plasma power facilities (PF) of various types magnetohydrodynamic generators and thermal emission converters. Paper contains the results of experimental verification of the universal electrophysical technique of fluctuation diagnostics to control electric arc processes, as well as, to detect and to prevent emergencies in PF operation. It is shown that fluctuation diagnostics techniques may be efficiently used when designing real-time control devices and devices to control various plasma PFs to improve their reliability and service life

[en] In a solid oxide fuel cell (SOFC), the cathode processes account for a majority of the overall electrochemical losses. A composite cathode comprising a mixture of ion-conducting electrolyte and electron-conducting electro-catalyst can help minimize cathode losses provided microstructural parameters such as particle-size, composition, and porosity are optimized. The cost of composite cathode research can be greatly reduced by incorporating mathematical models into the development cycle. Incorporated with reliable experimental data, it is possible to conduct a parametric study using a model and the predicted results can be used as guides for component design. Many electrode models treat the cathode process simplistically by considering only the charge-transfer reaction for low overpotentials or the gas-diffusion at high overpotentials. Further, in these models an average property of the cathode internal microstructure is assumed. This paper will outline the development of a 1-dimensional SOFC composite cathode micro-model and the experimental procedures for obtaining accurate parameter estimates. The micro-model considers the details of the cathode microstructure such as porosity, composition and particle-size of the ionic and electronic phases, and their interrelationship to the charge-transfer reaction and mass transport processes. The micro-model will be validated against experimental data to determine its usefulness for performance prediction. (author)

[en] 'Full text:' Introduction The Proton Exchange Membrane Fuel Cell (PEMFC) technology is considered the most promising candidate for mobile applications, due to its high power density, short start-up times and immediate response to changes in power demand. PEMFC systems tend, however, to become rather complex in order to provide for optimum water and thermal management, and facilitate stable operation. Auxiliary components add to cost and volume, and may reduce reliability. Pressurized operation may increase system power density, but to the sacrifice of efficiency. The atmospheric systems are inferior to pressurized systems with respect to water self-sufficiency and usually demand voluminous water condenser systems. At high power densities the amount of waste heat becomes considerable, and for larger systems liquid cooling is usually inevitable. But even for smaller, air-cooled systems, thermal management is challenging because of the relatively small temperature difference between the fuel cell and the surroundings. Over more than a decade there has been a trend towards simpler PEMFC systems holding a minimum number of auxiliary components, operating at atmospheric pressure and utilizing various self-humidifying techniques. However, due to the complexity of PEMFC operation, the degree of simplification becomes a trade-off between system cost and volume, and controllability. Experimental In the present work a small 10 cell PEMFC stack for demonstrational purposes was designed, assembled and tested. Commercial MEAs (Gore) and GDLs (E-TEK) were used. Thermocouples were inserted into the cathode air channels. Based on a total of 300 temperature measurements a semi-3-dimensional temperature distribution in the stack was obtained. Cell performance was characterized by obtaining polarization curves for each cell and measuring the steady state temperature distribution at a current density of 0.10 A/cm². Results and Discussion Stable performance was obtained at 0.10 A/cm². over a period of 4 hours (<10mV loss in single cell voltages). Stack operation was limited to current densities below 0.4 A/cm², primarily due to uncontrollable internal temperature increase. The temperature distribution plots show a somewhat lower temperature close to the air inlet compared to the middle of the stack and at the air outlet. Temperature gradients of up to 10^oC (measured in the air channels) were measured indicating substantial overall temperature gradients in the system. Substantial cooling of cells 1-3 (due to air fan blowing towards adjacent aluminum end-plate) did not influence cell performance significantly. Temperature contour plots indicate that some cells seem to operate at higher temperatures than the other cells (probably due to higher inner resistance). Conclusions The simplicity of the PEMFC system with few auxiliary components limits the possibility for adequate thermal management and thereby the capability of high current density operation. Operation at current densities >0.2 A/cm². was not possible for longer periods due to insufficient cooling of the fuel cell stack, and the danger of creating hot spots. Polarization curves (Figure 2) show very similar single cell performance despite substantial temperature gradients. (author)

[en] CO poisoning is a major issue when reformate is used as a fuel in PEM fuel cells. Normally it is necessary to reduce the CO to very low levels (∼5 ppm) and CO tolerant catalysts, such as Pt-Ru, are often employed. As an alternative approach, we have studied the use of pulsed oxidation for the regeneration of CO poisoned cells. Results are presented for the regeneration of Pt and Pt-Ru anodes in a PEM fuel cell fed with CO concentrations as high as 10,000 ppm. The results show periodic removal of CO from the catalyst surface by pulsed oxidation can increase the average cell potential and increase overall efficiency. A method for enhancing the performance of a fuel cell stack using a microprocessor-based Fuel Cell Health Manager (FCHM) has been developed. The results of a cost/benefit analysis for the use of a FCHM on a 4 kW residential fuel cell system are presented. (author)

[en] 'Full text:' An innovative gas infusion polymer electrolyte membrane (PEM) fuel cell prototype has been developed. PEM Fuel cell is operated under pressurized hydrogen gas and super oxygenated water. Gas infusion delivers super oxygenated water to the PEM fuel cell cathode at concentrations between 50 and 200 ppm, which overwhelms the limitation of 7 ppm concentration due to Henry's Law of the water within the cell. With adequate mixing, the water adjacent to the catalyst sites will have a higher concentration of oxygen than would be experienced in a cell running with the conventional method. The membrane is fully hydrated since the stack is constantly being fed with a liquid stream, which is recycled. Thermal management can be easily accomplished by liquid stream feed. The simple design of stack components, i.e. bipolar plate, gas diffusion layer (GDL) is studied. Application of gas infusion concepts in PEM fuel cell may alleviate cathode flooding and membrane dehydration problems in the absence of a humidifier. (author)

[en] 'Full text:' The 5 kW SOFC system has been under development at Fuel Cell Technologies Ltd. (FCT) since 2001. The power output and thermal capacity were chosen after considering typical load data for residential, remote and small commercial applications. A specification for a system was developed in conjunction with target market customers, and much useful feedback into the operability, reliability and maintainability of the system was incorporated. A single prototype was designed and built in 2002 utilizing tubular cells manufactured by Siemens Westinghouse Power Corporation (SWPC). FCT was responsible for system integration and Balance of Plant while SWPC was responsible for the generator which incorporated the cell stack, recuperator, insulation systems and reformer. This prototype was successfully tested at FCT in 2002 and was rapidly followed by four first generation or 'Alpha' demonstration units. The design methodology for the 5 kW system will be described, including the testing carried out to verify performance and reliability of the various subsystems. Results of safety and reliability analyses will also be included leading to an overall description of the design. In the presentation, the performance and design of these first generation units will be described. We will also present some site operating results. As this program was being completed, the second generation or 'Beta' unit was being designed and developed at FCT, with the goal of incorporating the lessons learned on site. This improved unit will be described and the results of early demonstration results of the Beta units will be incorporated into the presentation. It is anticipated that this will show the improved performance of a highly reliable and safe combined heat and power SOFC system. (author)