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Friday, May 23, 2008

 

The plug-in hydrogen hybrid, part 2

I've been trying to do a few calculations to size a hydrogen generator for a car, mostly because it appears that nobody ever tries figuring out what size system they'd actually need. In my previous post, I tried sizing a hydrogen generator for a thirsty SUV. The calculation was that it needed to break down 2.2 gallons of water into hydrogen per hour, in order to replace a gallon of gas burned in an hour. If you've looked at some of the pages offering blueprints for a hydrogen generator, you may already suspect that this is going to end up being several hundred times larger than what you've seen advertised on the Internet - most of them claim you'd only fill a two liter tank every couple of months.

Now I'm going to consider the electrical requirements of this thing. You're making 0.93 kg of hydrogen in an hour. Remember when I noted hydrogen's energy density is 142 megajoules per kilogram? You'll need to feed this thing 132 megajoules of energy in an hour if it were 100% efficient. Since power is energy per unit time, this works out to needing 36.7 kilowatts to drive the generator. Converting that to horsepower, by the way, indicates it would take 49 horsepower to drive this hydrogen generator. See why this thing isn't driven off the alternator? You'd be wasting a lot of the engine's power, need a much larger alternator, and probably have to replace your serpentine belt with a chain drive to boot. If this thing were running off 12 volt batteries, you'd need to supply it with 2500 amps. Far more current than a starter draws.

It's obvious this design is starting to get into trouble. In further posts, I'll size up a battery pack for this thing (it's going to be huge) and see if there are, in fact, ways this hydrogen hybrid idea might be salvagable.

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