Tibor Fabian

Development of portable fuel cell arrays with printed-circuit technology

Abstract Portable hydrogen/oxygen fuel cell power sources were constructed using printed-circuit board (PCB) technology. Multiple iterations of miniature planar fuel cell devices were prototyped, demonstrating fast cycle innovation and dramatic power density improvements in 200xa0W. The lightweight laminate PCB technology allows the best prototypes to achieve >700xa0mW/cm 2 area power density and >400xa0mW/cm 3 volumetric power density. PCB technology offers an intriguing platform for portable fuel cell development below 1xa0kW. Possibilities for on board diagnostics/control and further power density improvements are envisioned.

Direct extraction of photosynthetic electrons from single algal cells by nanoprobing system.

There are numerous sources of bioenergy that are generated by photosynthetic processes, for example, lipids, alcohols, hydrogen, and polysaccharides. However, generally only a small fraction of solar energy absorbed by photosynthetic organisms is converted to a form of energy that can be readily exploited. To more efficiently use the solar energy harvested by photosynthetic organisms, we evaluated the feasibility of generating bioelectricity by directly extracting electrons from the photosynthetic electron transport chain before they are used to fix CO(2) into sugars and polysaccharides. From a living algal cell, Chlamydomonas reinhardtii, photosynthetic electrons (1.2 pA at 6000 mA/m(2)) were directly extracted without a mediator electron carrier by inserting a nanoelectrode into the algal chloroplast and applying an overvoltage. This result may represent an initial step in generating high efficiency bioelectricity by directly harvesting high energy photosynthetic electrons.

Active Water Management for PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells require humidified gases to maintain proper membrane humidification, but this often results in a problematic accumulation of liquid water. Typically, excessive air flow rates and serpentine channel designs are used to mitigate flooding at the cost of system efficiency. In this paper, we present an active water management system that decouples water removal from oxidant delivery. The system uses a porous carbon flow field plate as an integrated wick that can passively redistribute water within the fuel cell. The system also employs an external electro-osmotic (EO) pump that actively removes excess water from the channels and gas diffusion layer. For a 25 cm 2 fuel cell with 23 parallel air channels, we demonstrate a 60% increase in maximum power density over a standard graphite plate with a low air stoichiometry of 1.3. EO pumping represents a negligible parasitic load, consuming typically less than 0.5% of the fuel cell power. Experimental and modeling results show that simple passive water transport through the porous carbon alone can prevent flooding at certain operating conditions and flow field dimensions. However, active water management with EO pumping facilitates robust operation with a high volumetric power density across all operating conditions.

A robust capacitive angular speed sensor

This paper presents a contactless capacitive angular speed sensor for automotive applications. The sensor is based on a passive rotating electrode placed between two mechanically static and electrically active electrodes. The different characteristics of the charge transfer at various sensor positions is utilized as an input for the calculation of the rotational speed. The main advantages of this low cost system are its capability to operate at high temperatures and humidity as well as its insensitivity to vibrations, dirt, dew and moisture deposited on the three sensor electrodes. The mathematical model of the sensor further enables the optimization of the sensor characteristics for specific applications. Experimental results from a prototype designed for the speed-measurement of a steering-wheel show a relative speed error of /spl plusmn/4% at a resolution better than 1/spl deg//s.

Lateral Ionic Conduction in Planar Array Fuel Cells

A performance degradation phenomenon is observed in planar array fuel cells. This effect occurs when multiple cells sharing the same electrolyte membrane are connected in series to build voltage. The open circuit voltage (OCV) and low current behavior of such a series connected planar stack is lower than should be expected. The flow of ionic cross currents between cells in the array, dubbed membrane cross-conduction, is proposed as the likely cause for this loss phenomenon. This hypothesis is confirmed by experimental observations. An equivalent circuit model for a planar double cell is developed which takes into account membrane cross conduction. This model is shown to predict the observed current-voltage behavior of an experimental planar double cell while a simple series model does not. The validated model is used to investigate the impact of various fuel cell parameters on the membrane cross-conduction effect. Design rules are extracted to minimize membrane cross-conduction losses for a linear fuel cell array. It is concluded that the membrane cross-conduction phenomenon primarily affects the OCV and low current density behavior of planar fuel cell arrays. Losses due to membrane cross conduction are minimal for conservative cell spacing, but can be significant for densely packed fuel cell arrays.

Mobility and Power Feasibility of a Microbot Team System for Extraterrestrial Cave Exploration

Planetary scientists are greatly interested in the caves present on the Moon and Mars, however these areas present major challenges to current space robots. A new space robotics concept, microbots, is presented and a possible reference mission to Mars is discussed. The feasibility of the mobility and power systems of the microbot are analyzed within the context of the reference mission. The results of this analysis are that the microbot system is a feasible concept for a development timeline of approximately 10 years.

Capacitive sensor for active tip clearance control in a palm-sized gas turbine generator

The efficiency of a gas turbine has an inverse relationship to the clearance between the rotor blades and the casing. Recent efforts in miniaturization of micro gas turbine engines have created a new challenge in blade tip clearance measurement. This paper describes the development of a capacitive tip clearance measurement system, based on a synchronous detection of a phase-modulated signal, for a palm-sized gas turbine engine with an integral ceramic rotor piece. A surface modification of the ceramic compressor and rotor with conductive coating is utilized to create a novel configuration of a tip clearance probe. The probe capacitance varies by approximately 120 fF for a 100-/spl mu/m blade displacement. Periodic autocalibration is used to reduce the effects of temperature drift on the sensor output. The remaining measurement error drift of 1.5 fF//spl deg/C was caused by the temperature drift of the probe parasitic capacitor. The random uncertainty was between 1.9 and 6.9 /spl mu/m depending on the tip clearance gap.

Measurement of Temperature and Reaction Species in the Cathode Diffusion Layer of a Free-Convection Fuel Cell

Micron- and millimeter-scale sensors were employed to acquire first-of-their-kind experimental measurements of the spatial and temporal distributions of temperature, oxygen partial pressure, and relative humidity in the mass transport layer immediately above the planar, horizontal cathode of polymer electrolyte membrane fuel cell PEMFC driven by natural convection. The sensors provide approximately 1 mm or better spatial resolution and 1 s temporal resolution. Substantial changes in temperature and reaction species concentrations were observed with increasing current density during a current-voltage I‐V scan. A linear decrease in oxygen partial pressure and a linear increase in water vapor partial pressure were observed with increasing current density, consistent with a flux balance analysis. Spatially resolved profiles normal to the surface indicate that thermal and reaction species gradients extend up to 6 mm above the horizontal cathode surface. Complementary horizontal profiles parallel to the surface reveal that the cell’s cathode rib structure visibly influences oxygen distribution. Most significantly, these data show that thermal and species concentration effects are not confined to the gas diffusion layer GDL, but extend well beyond the cathode surface, into the surrounding space. The measurements were used to estimate diffusion and/or convection mass transfer coefficients above the cathode surface. Transient data reveal substantial differences in the time constants associated with oxygen, water, and heat transport. The insights provided by this study should prove useful to inform and validate future physical models of air-breathing fuel cell systems.

Optimization of Passive Air Breathing Fuel Cell Cathodes

Air-breathing polymer electrolyte membrane fuel cells (ABFCs) use free convection airflow to supply oxygen to their cathodes. These cells are typically characterized by low output power densities compared to forced-convection fuel cells. Because ABFC designs rely on natural convection air delivery, cathode performance is often the bottleneck. This paper specifically examines the tradeoff between mass transport losses and ohmic electrical resistance losses for optimal ABFC cathode design. Optimization is non-trivial because the simultaneous requirements for excellent cell compression, current collection and gas access are often in contradiction. Simple scaling analysis and experimental observations suggest that the tradeoff between lateral mass transport resistance losses and cathode/gas diffusion layer (GDL) contact resistance losses determines optimal ABFC cathode design. In order to quantitatively study these effects, we have tested a series of different cathode geometries in a standardized ABFC. Using high frequency resistance measurements and fast-scan polarization measurements, we have been able to interrogate both the ohmic and mass transport losses associated with each cathode configuration. We have also used pressure sensitive foils to examine the pressure distribution for representative configurations, providing a quantitative link between pressure distribution and cell resistance. Finally, we have studied the effect of deploying a current collecting contact layer between the cathode and the GDL. Results indicate that the deployment of a sufficiently stiff yet highly porous contact layer significantly reduces contact resistance losses while imposing minimal additional mass transport losses. A stiff yet porous contact layer reduces the contact resistance losses by increasing total contact surface area and providing a more even distribution of pressure across the face of the cell. By minimizing contact resistance losses, this strategy enables the deployment of ABFC cathode structures with greater than 90% open area, thereby leading to enhanced ABFC performance, particularly at high current densities.Copyright

A measurement algorithm for capacitive speed encoder with a modified front-end topology

This paper presents an angular speed measurement algorithm for capacitive multiple plate angular encoders with disturbed rotational symmetry. The described sensor is aimed for automotive applications and therefore has to fulfill specific requirements. An important objective is fast replacement in the vehicle. It is realized with a new asymmetric sensing plate configuration resulting in increased measurement error due to the disturbed symmetry. This paper presents a simple model for the influence of the new sensing plate configuration on the capacitance between the fixed plates. A measurement algorithm based on a segment capacitance model for the asymmetric plate configuration is proposed. The main advantage of this algorithm is the suppression of steep slopes in the sensor angular measurement error and thus, the reduction of the relative angular speed error.