fuel cells an electrochemical conversion device chemical reactions cause electrons (current) to flow...
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Fuel cells
• An electrochemical conversion device• Chemical reactions cause electrons (current)
to flow• Requires a fuel, an oxidant and an electrolyte (
a substance that contains free ions and acts as a conductor)
• Typical type of fuel cell is called a proton exchange membrane fuel cell (PEMFC)
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Hydrogen Fuel Cells
• Clean-only emission is water• Expensive to produce• Highly efficient-in an automobile, efficiencies
of converting fuel energy to mechanical energy of 60% could be achieved, almost double the current efficiencies
• Hydrogen itself has issues as a fuel source
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Issues with Hydrogen
• Abundant in nature, but not a freely available fuel
• Must be unbound from compounds• Currently obtained via steam reforming– Steam and a nickel catalyst react, producing H– Need steam at very high temperatures, 1600F
• In the future, H is anticipated to be produced by the electrolysis of water, requiring large amount of water and electricity
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Electrolysis• Pass an electrical current through water and
obtain H• Pass a direct current from a battery or other DC
power supply through a cup of water (salt water solution increases the reaction intensity making it easier to observe).
• Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode.
• Choice of the electrode is critical, you do not want a metal that will react with oxygen
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Issues with Hydrogen
• Storage-occurs in gas form at room temperature, hard to contain
• As a liquid, it can be stored, but needs temperatures of -253 C.– As a liquid, its energy density increases 1000 times– In principle, could replace gasoline as a liquid fuel, but
not practical at this time• One solution is to store it as a metallic hydride
(the negative ion of Hydrogen in a compound with another element) at room T.
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Issues with H
• Highly explosive– Forms a volatile mixture with air
• A mixture of 4-75% of H in air is explosive, compared with natural gas which is only explosive in a range of 5-15% concentration in air
• Ignition energy is small, needing only 2 x 10-5 J (basically a spark of static electricity can ignite H)
• Only good news is its low density means if there is a H leak, it disperses quickly
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Hydrogen• Hindenburg disaster• Hindenburg was a
German passenger airship (zeppelins) built for transatlantic air flight.
• Filled with Hydrogen• Something caused
ignition of the Hydrogen-cause is debatable
• 36 fatalities out of 79 people onboard
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Alchohol• Use methanol or ethanol as a fuel– Already gone over ethanol
• Methanol is already in use at Indy 500 race– Proven that no significant loss of performance is
experienced (though they are in the process of switching to ethanol)
• About ½ the energy content of gasoline• Produces only CO2 and water– Some nitrogen oxides produced in the engine
• Can be manufactured from re-newable sources (biomass for example)
• Technologies exist now.
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Disadvantages
• Very dangerous– Burns with no visible flame-needs a colorant
added– Fumes are toxic
• CO2 is a greenhouse gas• Currently made mostly from natural gas-a
non-renewable fossil fuel• Possibly more corrosive than ethanol to
engine parts
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Use in liquid fuel cells• Another use is as a input to a
liquid feed fuel cell• In these cells, Methanol
replaces hydrogen• Methanol has a much higher
energy density and is easier to store than H
• Current methanol fuel cells produce power too low for vehicles, but can be used in cell phones, laptops etc
• Advantage is that they store lots of power in a small space, which they over a long period of time
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Environmental effects of energy production
• All of our energy producing mechanisms have some effect on the environment– Production of waste products pollutes air, water
and ground– Disruptions to local ecosystems
• Our job is to understand and mitigate these effects to the best of our ability
• Philosophy : If it hurts (the environment) when you do that, don’t do that!
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Air pollution• If its in the air, its in your body• Components of the Earth’s Atmosphere:– Nitrogen 78.08%– Oxygen 20.95%– Argon 0.93%– Also small amounts of Neon, Helium, Krypton,& Hydrogen
• In addition, there are compounds whose concentrations vary with height: water vapor, carbon dioxide, methane, carbon monoxide, ozone, ammonia
• These are naturally occurring concentrations, any additional influx or destruction of these compounds via human beings alters the system.
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Profile of the Earth’s atmosphere