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Fuel Cell Technology Sustainable Energy Science and Engineering Center Fuel Cell Technology 1. Technology overview 2. Fuel cell performance 3. Fuel cell systems 4. Sample calculations 5. Experiment using PEM cell Goal: To provide a better understanding of the fuel cell technology, its benefits and systems issues that influence its application. Reference: Fuel Cell Handbook, 5th Edition, US DOE, October 2000. Sustainable Energy Science and Engineering Center Fuel Cell Technology Overview Fuel cell: A device for directly converting the chemical energy of a fuel into electrical energy in a constant temperature process. Besides offering high theoretical efficiency, especially at low temperatures, fuel cells emit low or zero levels of pollutants. They can run on wide variety of fuels ranging from gaseous fuels such as hydrogen and natural gas to liquid fuels such as methanol and gasoline. Main applications: Stationary power generation Transportation Battery replacement. Sustainable Energy Science and Engineering Center Ballard 250 kW PEMFC power system Sustainable Energy Science and Engineering Center Vehicle Fuel Cell System Sustainable Energy Science and Engineering Center Solar Hydrogen System Sustainable Energy Science and Engineering Center Operation of a Solid Polymer Fuel Cell (SPFC) Under load a single cell produces about 0.7 volts. Sustainable Energy Science and Engineering Center Fuel Cell Stack In order to achieve a useful output power individual cells are connected together in a ‘stack’ using an interconnect called bipolar plate. MEA: Membrane Electrode Assembly: A combination of electrolyte and electrodes Sustainable Energy Science and Engineering Center Fuel Cell Efficiency Carnot efficiency: The maximum efficiency of a heat engine is subject to the Carnot efficiency limitation, which defines the maximum efficiency that any heat engine can have if its temperature extremes are known. TL ηC =1− TH Where TH and TL are the absolute high and low temperatures respectively. Fuel cell efficiency: The theoretical efficiency of a fuel cell is related to the ratio of two thermodynamic properties, namely the chemical energy, represented by Gibbs Free Energy (∆Go) and the total heat energy, represented by the Enthalpy (∆Ho) of the fuel. ΔGo η = FC ΔH o Sustainable Energy Science and Engineering Center Fuel Cell Efficiency Efficiency vs Temperature Fuel cell operating at low temperatures emit low levels of pollutants such as SOx and NOx when using methanol and natural gas. Comparison of theoretical efficiency of a fuel cell running on hydrogen as a function of temperature with that of the Carnot efficiency at the same o temperature, assuming TL = 25 C Sustainable Energy Science and Engineering Center Fuel Cell Efficiency Under Load Sustainable Energy Science and Engineering Center Fuel Cell Types Two classes: Low temperature High temperature Sustainable Energy Science and Engineering Center Fuel Cell Types PEFC: Polymer electrolyte fuel cell; AFC: Alkaline fuel cell; PAFC: Phosphoric acid fuel cell; MCFC: molten carbonate fuel cell; ITSOFC: Intermediate temperature solid oxide fuel cell; TSOFC: Tubular solid oxide fuel cell Sustainable Energy Science and Engineering Center Fuel Cell Characteristics Low temperature fuel cell characteristics: Incorporation of precious metal electrocatalysts to improve performance Fast dynamic response and short start-up times Available commercially and near commercialization Pure supply of hydrogen as a fuel (e.g. catalysts are poisoned by CO) High temperature fuel cell characteristics: Fuel flexibility - use of hydrocarbon fuels Reduces the need for expensive electrocatalysts Generate useful ‘waste’ heat and well suited for cogeneration and combined cycle applications Long start-up times and are sensitive to thermal transients Expensive construction materials - reliability and durability is of concern At the demonstration stage Sustainable Energy Science and Engineering Center Fuel Cell Fuel All fuel cell can run on hydrogen as a fuel. High temperature fuel cells can also run directly on other fuels, especially hydrogen rich gases such as methane. Low temperature systems can run on specific liquid fuels such as methanol. The application of the fuel cell system often determines on which fuel it will run, and whether that fuel first needs to be processed into hydrogen-rich ‘reformate’, possibly also containing carbon dioxide, nitrogen or other non- detrimental products. Sustainable Energy Science and Engineering Center Fuels for Stationary Power Generation Common fuel: Natural gas - reformed by separate steam reformer before resulting hydrogen is fed into the fuel cell stack. High temperature fuel cells are able to operate directly to reform fuels such as methane directly on the anode of the fuel cell (DIRECT). This process causes severe temperature gradients across the stack. Use a separate catalyst bed within the system to split the hydrogen and carbon (INDIRECT). Typical fuel cell type: Phosphoric Acid Fuel Cell (PAFC) Sustainable Energy Science and Engineering Center Fuels for Transportation Common fuel: Hydrogen - severe size, weight, cost and performance constraints Methanol is being suggested to be a good compromise fuel. It is easy to process into hydrogen using autothermal reforming. Reformer-based systems have longer response times and are less efficient and more polluting than direct hydrogen systems. Typical fuel cell type: Direct Methanol Fuel Cell (PAFC) Sustainable Energy Science and Engineering Center Fuels for Battery Replacement Common fuel: Propane or butane - severe size, weight, cost and performance constraints Fuel cell type: Solid Oxide Fuel cell (SPFC). The electrolyte is a solid, nonporous metal oxide, usually Y2O3-stabilized ZrO2. High power density for miniaturization. Also uses most easily handled electrolyte of the low temperature fuel cells. Sustainable Energy Science and Engineering Center Fuel Cell Stability Suitability of different fuel cell types for applications Sustainable Energy Science and Engineering Center Fuel Cell System Costs The low temperature fuel cells can cost as little as $30/kW for transport and $300/kW for stationary power in mass production. Fuel cell system for large scale power generation < $1,500/kW Automobile fuel cell system < $50/kW (1 kW = 1.341 hp) Sustainable Energy Science and Engineering Center Fuel Cell State of the Art 1. Battery replacement for mobile phones/ laptops 2. Portable power systems of several hindered watts for replacement of small diesel generators. 3. Small power or cogeneration systems 4. Transport based fuel cell systems Sustainable Energy Science and Engineering Center TSOFC with a Gas Turbine Engine Siemens Westinghouse Concept Sustainable Energy Science and Engineering Center Distributed Power Generating Systems 13.5 Sustainable Energy Science and Engineering Center System Performance Requirements Sustainable Energy Science and Engineering Center Subsystems and Components Sustainable Energy Science and Engineering Center Electrochemical reactions in fuel cells See LECTURES 18 &19 of EML 4450, FALL 2006 Reference: Fuel Cell Handbook, 5th Edition, US DOE, October 2000. Sustainable Energy Science and Engineering Center Nernst Equation The Nernst equation and open circuit: The electrochemical work, which is done by the movement of electrons through a difference in a electrical potential, is denoted as We or Wcell. In electrical terms, the work done by electrons with the charge nF (n is the number of electrons transferred per mole of fuel and F is the charge carried by a mole of electrons, which is Faraday’s constant - 96,485C/mole-1) moving through a potential difference, E ( voltage difference across electrodes) is RT W = nFE E = E o − lnQ e nF ΔG =−nFE Eo is the standard electrode potential ΔGo =−nFE o o We also assume here that a complete ΔG =ΔG + RT lnQ reversible oxidation of a mole of fuel ΔGo RT E = − lnQ Electrical work done = charge x voltage nF nF Sustainable Energy Science and Engineering Center Hydrogen Fuel Cell For a hydrogen-oxygen fuel cell, the overall reaction stoichiometry is 1 H + O → H O 2 2 2 2 The electrons transferred in this reaction, n = 2. Using the partial pressures of water, hydrogen and oxygen in the reaction coefficient, we then have RT P E = E o − ln H 2O nF P P1/2 H 2 O2 Diluting the reactant gases will lower the maximum voltage that the cell can produce. Sustainable Energy Science and Engineering Center Fuel Cell Reactions Fuel cell reactions and the corresponding Nernst equation a the cell reactions are obtained from the anode and cathode reactions shown in an earlier table. Sustainable Energy Science and Engineering Center Ideal Standard Potential Ideal Standard Potential, Eo : Standard conditions: one atmosphere and 25oC The ideal cell voltage for H2/O2 fuel cell Eo = 1.229V with liquid water as product Eo = 1.18V with gaseous water as product Eo is a strong function of the cell temperature Ideal voltage, E for the oxidation of hydrogen Sustainable Energy Science and Engineering Center Irreversible Losses Fuel cell Voltage/Current plot Activation Polarization: Related to the rates of electrochemical reactions at an electrode surface. Ohmic Polarization: Losses due to resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials Concentration Polarization: Due to inability of the surrounding reactant material to maintain the initial concentration. Cell voltage, V = E- Losses Sustainable Energy Science and Engineering Center Activation Polarization
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