fuel_cell - Fuel Cells: Introduction and Current Status...

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Unformatted text preview: Fuel Cells: Introduction and Current Status Andre Benard Department of Mechanical Engineering Michigan State University East Lansing, MI 48824 Why all this excitement about fuel cells? Can have zero emissions Are highly efficient Can be used as long duration portable power sources Can use various hydrocarbon fuels (with reformers) Are practically noise-free Are potential replacements for IC engines (everyone has an opinion on this topic!) In this talk Introduce the basics of fuel cells Introduction to various types of fuel cells (PEM in particular) Discuss their potential applications Current status Issues/problems with fuel cells Modeling of fuel cells What are fuel cells? Use an electrochemical process to convert hydrogen and oxygen into electricity No combustion required Produces a DC current Two electrodes (cathode and anode) are separated by an electrolyte Reactants are stored externally and operates continuously What is a fuel cell? Sandwich structure two electrodes one electrolyte Generates electricity, water, and heat Fuels: H 2 , natural gas, methanol, gasoline, etc. Schematic of a fuel cell system Load 2e- H 2 O Electrolyte (Ion conductor) Positive ions or Negative ions O 2 H 2 O Fuel in H 2 Depleted fuel and product gases out Anode Oxidant in Depleted Oxidant and Product Gases out Cathode What are fuel cells? (cont.) Numerous types of fuel cells Often categorized by the electrolyte used Polymer Electrolyte Membrane (PEM) Alkaline fuel cells (AFC) Phosphoric acid (PAFC) Molten carbonate (MCFC) Solid oxide (SOFC) The Very Basics of Fuel Cells Two separate reactions : an oxidation half-reaction at the anode a reduction half-reaction at the cathode Oxidation half-reaction: 2H 2 4H + + 2e- Reduction half-reaction: O 2 +4H + +4e - 2H 2 O _____________________________________________ Cell reaction: 2H 2 +O 2 2H 2 O Catalysts are needed at both anode and cathode to increase the rate of each half-reaction. Pt is the best catalyst, a very expensive material. Comparison of fuel cells with other energy sources Heat Engines Combustion Pollution, noise, losses,... Gasoline, Diesel,... Air Fuel Cell Stack Electrochemical reactions Less losses, low emissions H 2 , CH 4 , CH 3 OH,... Fuel cells versus heat engines A heat engine cannot convert all of its energy supplied into mechanical energy (some of the heat is rejected) even for the ideal Carnot cycle Max Efficiency = (T HIGH- T LOW )/T HIGH IC engines use heat from T HIGH source, converts part of the energy into mechanical energy and rejects the rest into a sink at low temperature Fuel cells convert chemical energy directly into electrical energy, without conversion of heat to mechanical energy (fuel cells can exceed Carnot efficiency) Components of a fuel cell system Electric Motor Emissions Fuel: Hydrocarbon fuel Hydrogen Fuel Processor (except for H 2 and methanol) Appliance Fuel Cell Battery Five Popular Fuel Cells...
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fuel_cell - Fuel Cells: Introduction and Current Status...

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