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Unformatted text preview: ENGRI 111
FUEL CELLS 6CO2 + 6H2O C6H12O6 + 6O2
Carbohydrates: high-calorie food
~ 1% conversion http://www.sinauer.com/ www.tribuneindia.com Grtzel Cell Different species involved in chargecarrier generation and transport Dye plays role of chlorophyll Excited electron is transferred to TiO2 TiO2 plays role of electron relay mechanism carrying away electron Charge generation and transport separated reduces chance of recombination I2 in liquid electrolyte completes reaction c v 2 Ru2+ Ru3+ + e I2 + e I- I- + Ru3+ Ru2+ + I2 www.grundutbildning.kemi.uu.se Fuel Cells Fuel cells are under intense development as cleaner, more efficient energy sources efficient energy Fuel cells could reduce dependence on imported oil (fossil fuels) If H2 comes from electrolysis of water, then H2 adds no greenhouse gases to the environment Distributed production hydrogen can be produced anywhere there is water and electricity Fuel Cells High temperature fuel cells High power, highenergy fixed installations Ambient (or near ambient) temperature fuel cells Portable or mobile applications Fuel Cell Applications
www.gm.com www.pcworld.com www.theregister.co.uk Working of a Fuel Cell HYDROGEN + OXYGEN WATER + ELECTRICAL ENERGY www.qsinano.com/apps_fuelcell.php Fuel Cells: Fuel/Byproducts O2 and H2 are the fuel H2O is the byproduct fuel cells eliminate pollutants such as nitrogen oxides (NOx) and greenhouse gases O2 is available in air H2 must be provided electrolysis of water electricity can be provided from renewable energy sources reformed from traditional energy sources natural gas, oil, methanol, ethanol... biomass, sun, wind, water Reforming Methanol
CH3OH H2O + CO CO + 2H2 CO2 + H2 The process starts by first vaporizing methanol and water Water splits into hydrogen and oxygen Oxygen combines with CO to produce CO2 Uses a catalyst Reforming Natural Gas
CH4 + H2O CO + 3H2 CO2 + H2 H2O + CO Water splits into hydrogen and oxygen Oxygen combines with CO to produce CO2 Issues No reaction is perfect. Methane, methanol, carbon monoxide can pass through CO poisons catalysts used in the fuel cell CO2 is a pollutant Fuel Cells: Challenges Materials Hydrogen storage plastic membrane (electrolyte) electrodes, catalysts compressed gas, liquid, metal hydride cost is down (10x in three years) even if mass produced today, fuel cells will be 10x too expensive Transition from an oilbased economy to hydrogen Cost Infrastructure Polymer Membranes High conductivity Low permeability High strength Nafion
Polymers: Plastics Membrane Requirements High proton conductivity Good barrier properties (no gas/fuel crossover) Water retention above 100 C Mechanical strength Low cost (DOE transportation target is $10/kW-hr) Hydrogen Storage Most vehicles can carry 400 miles worth of fuel Safe Easy to replenish H2 storage Metal Hydrides Compressed Storage Liquid Storage Carbon Nanotubes Metal Hydrides Some metals absorb hydrogen and release it at a higher temperature Challenges Very high temperature needed to extract H2 Energy is required to store and release hydrogen Weight of metal hydride Compressed Storage Strong, lightweight tanks to store hydrogen gas can be made out of lightweight carbon fiber composites Current tanks can carry 5,000 psi of H2 Challenges Double capacity to improve vehicle range For safety reasons tanks are made out of materials that are either too expensive or too heavy Tanks are quite large packaging problems Safety perception Liquid Storage Hydrogen becomes a liquid at 20K, 253 C Challenges Substantial amount of energy required to chill hydrogen and refrigerate 34 % per day will "boiloff" Mechanism to capture or release safely Hydrogen Storage Carbon Nanotubes Hollow tubes that can be filled with hydrogen Challenges Reliable, inexpensive way of making nanotubes not available Small capacity (0.7 vol.%) ...
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- Spring '07