Browsing by Subject "fuel cell"
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Item A feasibility study of internal evaporative cooling for proton exchange membrane fuel cells(Texas A&M University, 2006-04-12) Snyder, Loren EAn investigation was conducted to determine the feasibility of using the technique of ultrasonic nebulization of water into the anode gas stream for evaporative cooling of a Proton Exchange Membrane (PEM) fuel cell. The basic concept of this form of internal evaporative cooling of the PEM fuel cell is to introduce finely atomized liquid water into the anode gas stream, so that the finely atomized liquid water adsorbs onto the anode and then moves to the cathode via electro-osmotic drag, where this water then evaporates into the relatively dry cathode gas stream, carrying with it the waste thermal energy generated within the fuel cell. The thermal and electrical performance of a 50 cm2 PEM fuel cell utilizing this technique was compared to the performance obtained with conventional water management. Both techniques were compared over a range of humidification chamber temperatures for both the anode and cathode gas streams so as to determine the robustness of the proposed method. The proposed method produced only meager levels of evaporative cooling (at best 2 watts, for which a minimum of 30 watts was required for adequate cooling), but the average cell voltage increased considerably (as much as a 10% gain), and the technique increased the fault tolerance of the fuel cell (the Nafion?? membrane did not dry out even if cell temperature went well in excess of 70?? C despite both anode and cathode humidification temperatures of 55?? C). An interesting phenomena was also observed wherein the fuel cell voltage oscillated regularly with a period of tens of seconds, and that the amplitude of this oscillation corresponded inversely with the level of humidification received by the fuel cell.Item An advanced fuel cell simulator(Texas A&M University, 2005-11-01) Acharya, Prabha RamchandraFuel cell power generation systems provide a clean alternative to the conventional fossil fuel based systems. Fuel cell systems have a high efficiency and use easily available hydrocarbons like methane. Moreover, since the by-product is water, they have a very low environmental impact. The fuel cell system consists of several subsystems requiring a lot of effort from engineers in diverse areas. Fuel cell simulators can provide a convenient and economic alternative for testing the electrical subsystems such as converters and inverters. This thesis proposes a low-cost and an easy-to-use fuel cell simulator using a programmable DC supply along with a control module written in LabVIEW. This simulator reproduces the electrical characteristics of a 5kW solid oxide fuel cell (SOFC) stack under various operating conditions. The experimental results indicate that the proposed simulator closely matches the voltage-current characteristic of the SOFC system under varying load conditions. Effects of non-electrical parameters like hydrogen flow rate are also modeled and these parameters are taken as dynamic inputs from the user. The simulator is customizable through a graphical user interface and allows the user to model other types of fuel cells with the respective voltage-current data. The simulator provides an inexpensive and accurate representation of a solid oxide fuel cell under steady state and transient conditions and can replace an actual fuel cell during testing of power conditioning equipment.Item Analysis and design of high frequency link power conversion systems for fuel cell power conditioning(Texas A&M University, 2005-11-01) Song, Yu JinIn this dissertation, new high frequency link power conversion systems for the fuel cell power conditioning are proposed to improve the performance and optimize the cost, size, and weight of the power conversion systems. The first study proposes a new soft switching technique for the phase-shift controlled bi-directional dc-dc converter. The described dc-dc converter employs a low profile high frequency transformer and two active full-bridge converters for bidirectional power flow capability. The proposed new soft switching technique guarantees soft switching over wide range from no load to full load without any additional circuit components. The load range for proposed soft switching technique is analyzed by mathematical approach with equivalent circuits and verified by experiments. The second study describes a boost converter cascaded high frequency link direct dc-ac converter suitable for fuel cell power sources. A new multi-loop control for a boost converter to reduce the low frequency input current harmonics drawn from the fuel cell is proposed, and a new PWM technique for the cycloconverter at the secondary to reject the low order harmonics in the output voltages is presented. The performance of the proposed scheme is verified by the various simulations and experiments, and their trade-offs are described in detail using mathematical evaluation approach. The third study proposes a current-fed high frequency link direct dc-ac converter suitable for residential fuel cell power systems. The high frequency full-bridge inverter at the primary generates sinusoidally PWM modulated current pulses with zero current switching (ZCS), and the cycloconverter at the secondary which consists of only two bidirectional switches and output filter capacitors produces sinusoidally modulated 60Hz split single phase output voltage waveforms with near zero current switching. The active harmonic filter connected to the input terminal compensates the low order input current harmonics drawn from the fuel cell without long-term energy storage devices such as batteries and super capacitors.Item Analysis and design of power conditioning systems(2009-05-15) Harfman Todorovic, MajaA combination of high prices of fossil fuels and the increased awareness of their negative environmental impact has influenced the development of new cleaner energy sources. Among various viable technologies, fuel cells have emerged as one of the most promising sources for both portable and stationary applications. Fuel cell stacks produce DC voltage with a 2:1 variation in output voltage from no load to full load conditions. Hence, to increase the utilization efficiency and system stability, a power conditioner consisting of DC-DC and DC-AC converters is required for load interface. The design of power conditioners is driven by the application. This dissertation presents several different solutions for applications ranging from low-power portable sources for small electronics and laptop computers to megawatt-power applications for fuel cell power plants. The design and analysis for each power conditioner is presented in detail and the performance is verified using simulations and prototypes. Special consideration is given to the role of supercapacitors who act as the additional energy storage elements. It is shown that the supercapacitor connected at the terminals of a fuel cell can contribute to increased steady state stability when powering constant power loads, improved transient stability against load transients, and increased fuel efficiency (i.e. reduced hydrogen consumption).Item Design of a Control Strategy for a Fuel Cell/Battery Hybrid Power Supply(2010-01-14) Smith, Richard C.The purpose of this thesis is to design hardware and a control strategy for a fuel cell/battery hybrid power supply. Modern fuel cell/battery hybrid power supplies can have 2 DC/DC converters: one converter for the battery and one for the fuel cell. The hardware for the power supply proposed in this thesis consists of a single DC/DC buck converter at the output terminals of the fuel cell. The battery does not have a DC/DC converter, and it is therefore passive in the system. The use of one single converter is attractive, because it reduces the cost of this power supply. This thesis proposes a method of controlling the fuel cell's DC/DC buck converter to act as a current source instead of a voltage source. This thesis will explain why using the fuel cell's buck converter to act as a current source is most appropriate. The proposed design techniques for the buck converter are also based on stiff systems theory. Combining a fuel cell and a battery in one power supply allows exploitation of the advantages of both devices and undermines their disadvantages. The fuel cell has a slow dynamic response time, and the battery has a fast dynamic response time to fluctuations in a load. A fuel cell has high energy density, and a battery has high power density. And the performance of the hybrid power supply exploits these advantages of the fuel cell and the battery. The controller designed in this thesis allows the fuel cell to operate in its most efficient region: even under dynamic load conditions. The passive battery inherits all load dynamic behavior, and it is therefore used for peaking power delivery, while the fuel cell delivers base or average power. Simulations will be provided using MATLAB/Simulink based models. And the results conclude that one can successfully control a hybrid fuel cell/battery power supply that decouples fluctuations in a load from the fuel cell with extremely limited hardware. The results also show that one can successfully control the fuel cell to operate in its most efficient region.Item Increasing the CO tolerance of PEM fuel cells via current pulsing and self-oxidation(Texas A&M University, 2004-09-30) Thomason, Arthur HughAn investigation was conducted to determine and compare the effect of cell current pulsing and "self-oxidation" in increasing the CO tolerance of a PEM fuel cell. The most effective pulsing parameter values were also determined. Current pulsing involves periodically demanding positive current pulses from the fuel cell to create an anode over-potential, while "self-oxidation" or sustained potential oscillations is achieved when the anode catalyst becomes so saturated with CO that the anode over-potential increases to a value at which CO is oxidized from the catalyst surface. The CO tolerance of a fuel cell system with a Pt-Ru anode was tested using 50 and 496 ppm CO in the anode fuel. The performance of the system declined with an increase in CO concentration. Current pulses of various amplitude, frequency, and duty cycle were applied to the cell while CO was present in the anode fuel. With 50 ppm CO in the anode fuel, the most effective pulse in increasing CO tolerance while maintaining normal cell operation was 1.0 A/cm2, 0.25 Hz, and a 5% duty cycle. A pulse (120 Hz, 50% duty cycle) similar to the ripple current often generated when converting DC to single-phase 60 Hz AC had a positive effect on the CO tolerance of the system, but at frequencies that high, the pulse duration was not long enough to completely oxidize the CO from the catalyst surface. With 496 ppm CO in the anode fuel, a pulse of 1.0 A/cm2, 0.5 Hz, and a 20% duty cycle proved most effective. When the cell was exposed to 496 ppm CO, without employing pulsing, "self-oxidation" occurred and CO was periodically oxidized from the catalyst surface. However, pulsing allowed the cell to operate at the desired voltage and power a higher percentage of the time than "self-oxidation"; hence, pulsing was more effective.Item Multiport Converter Topologies for Distributed Energy System Applications(2014-07-28) Hawke, JoshuaIn the midst of a present-day global energy renaissance, power electronics has evolved into a top-tier technology discriminator in distributed energy resource (DER) systems. Faced with the formidable task of integrating various types of DER technologies into singular systems, there is a growing appetite for multiport converter (MPC) design. In response, three unique DER MPC topologies are presented: the power sharing converter (PSC), the multi-level nine switch converter (ML9SC), and the modular fuel cell hybrid energy storage (MFC+HES) converter. First, low-voltage and medium-voltage PSC architectures are shown to decouple series-connected source currents and enable independent control. Multidimensional modeling and analysis is then discussed. Next, three system designs are discussed: single-zone, dual-zone, and multi-zone. Each implements PSC technology and high-frequency isolated full-bridge converters to interface multiple fuel cell sources to a medium voltage grid via a single multilevel neutral point clamped inverter interface. A 1-MW simulation and a reduced-scale hardware prototype offer collaborative insight into the inherit benefits of the proposed PSC systems: increased output power, operational flexibility, thermal balancing, source availability, and cost-effectiveness. Secondly, the ML9SC is presented as a component-minimized multi-port converter with low cost, high efficiency, high power quality, and low noise. The multiport characteristic of the ML9SC can be effectively employed in uninterruptible power systems, six-phase wind generators, and doubly-fed induction wind generators. Next, operating constraints and modulation index limits are analyzed at different operating conditions. Loss breakdown is analyzed and compared with the conventional back-to-back multi-level converter. Finally, simulation results are included as proof of concept. Lastly, the proposed MFC+HES converter integrates energy-dense MFC technology with power-dense storage technology. System modularization and hybridization are discussed initially, followed by a selection between supercapacitors and lithium-ion batteries (LIBs). Next, system topology and design is discussed, and the MFC and LIBs are electrically modeled such that Middlebrook?s Extra Element Theorem can mitigate unwanted system resonance and optimize system design. Simulation and hardware results for a 100W MFC+HES system realizes a 300% boost current response capability as well as the following system benefits: limp-home capability, evenly distributed heat/aging, and maximized output power.Item Wide input range DC-DC converter with digital control scheme(Texas A&M University, 2006-04-12) Harfman Todorovic, MajaIn this thesis analysis and design of a wide input range DC-DC converter is proposed along with a robust power control scheme. The proposed converter and its control is designed to be compatible to a fuel cell power source, which exhibits 2:1 voltage variation as well as a slow transient response. The proposed approach consists of two stages: a primary three-level boost converter stage cascaded with a high frequency, isolated boost converter topology, which provides a higher voltage gain and isolation from the input source. The function of the first boost converter stage is to maintain a constant voltage at the input of the cascaded DC-DC converter to ensure optimal performance characteristics with high efficiency. At the output of the first boost converter a battery or ultracapacitor energy storage is connected to take care of the fuel cell slow transient response (200 watts/min). The robust features of the proposed control system ensure a constant output DC voltage for a variety of load fluctuations, thus limiting the power being delivered by the fuel cell during a load transient. Moreover, the proposed configuration simplifies the power control management and can interact with the fuel cell controller. The simulation results and the experimental results confirm the feasibility of the proposed system.