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17_hydrogen - UCSD Physics 12 The Hydrogen Economy What it...

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Unformatted text preview: UCSD Physics 12 The Hydrogen Economy What it is, and what it isn't UCSD Physics 12 The Transportation Problem About a third of our annual energy usage goes into transportation Gasoline makes this very convenient packs as much as 11 Calories per gram though heat-engine efficiency is just 20% Natural gas very bulky (and will run out) We've seen the problems with ethanol (from corn) Solar cars are impractical, at 12 horsepower Electric cars need batteries (but could at least use solar/wind as source of electricity) batteries store only 0.021 Calories per gram some gain in fact that conversion to mechanical is 90% efficient Desperately need a replacement for portable gasoline Spring 2010 2 Q 2 UCSD Physics 12 The Hydrogen Solution If we could find pockets of hydrogen gas, this would be great, as hydrogen carries about 32 Calories/gram through simple reaction: 2H2 + O2 2H2O Two big problems: No natural pockets of hydrogen gas Very bulky in gaseous form 11,000 times less energy-dense than gasoline 3 times more bulky than natural gas, energy-wise First point is more fundamentally significant: Hydrogen is not a fuel source for our future Spring 2010 3 UCSD Physics 12 Hydrogen Extraction But hydrogen can be extracted from naturally occurring substances (like H2O) This hydrogen is "post-reaction" hydrogen, meaning it's the end-product of an energetic reaction To get the hydrogen back, we have to put energy in, running the reaction backwards In essence, hydrogen becomes a way to store energy derived from other (traditional?) sources Spring 2010 4 UCSD Physics 12 Electrolysis One way hydrogen can be extracted is from water via electrolysis pass electric current through water and dissociate hydrogen from oxygen hydrogen forms on negative terminal, oxygen on positive terminal bubbles are collected for use twice as much H2 forms as O2 (can you figure out why?) Spring 2010 5 UCSD Physics 12 Defeating the purpose? Hydrogen burns very cleanly, producing little more than water this is great for the environment But if the hydrogen is produced by fossil-fuelpowered plants, all this advantage is lost coal puts out tons of CO2, plus SO2, and other nasties have to deal with plant efficiency (33%) times electrolysis efficiency (65%) times engine efficiency (optimistic 65% fuel cell??) = 14% better off burning fossil fuel directly in your car, and getting 20% efficiency that is, less CO2 would be emitted this way Spring 2010 Q 6 UCSD Physics 12 Sensible Solutions For cleanliness and efficiency, really want to use solar or wind power to run the electrolysis this way, it's zero-emission more importantly, it's not then diminishing a resource Downside is that we would need to greatly expand our electricity production far beyond today's levels because all transportation would have to come out of this resource exacerbated by inefficiencies of process Doesn't make sense to pursue hydrogen until we ramp up non-fossil electricity production Spring 2010 7 UCSD Physics 12 Hydrogen Fuel Cells Fuel cells essentially run electrolysis backwards, gaining electrical energy from the bonding of hydrogen gas with oxygen gas Theoretical efficiency is 83%, but practical efficiency may be <65% much better than heat engine at 2025% Spring 2010 8 UCSD Physics 12 Hydrogen Fuel Cell Car GM made a hydrogen fuel cell car prototype price tag: $1M but, it can be done, and would certainly be cheaper in mass-production--but not clear we'll take this path Spring 2010 9 UCSD Physics 12 Announcements and Assignments Extra Credit posted: up to 2% of grade start early: putting off until deadline will be ugly HW #8 posted, due 5/28 Spring 2010 10 ...
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