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Friday, June 20, 2008


The plug-in hydrogen hybrid, part 3

When I last blogged about the idea of a "plug in hydrogen hybrid" that had an onboard hydrogen generator powered by plug in batteries, things looked a bit crazy. The SUV needed 36.7 kilowatts to drive its hydrogen generator, and that assumes that it's a 100% efficient generator. I'd translated that to 2500 amps if the generator ran at 12 volts. In practice, it probably wouldn't - you'd use a higher input voltage and less current. But this figure helps because batteries are rated in amp-hours. Although these are over a 20 hour discharge period, we'll assume it can deliver the same number of amp-hours in a single hour. We need a total of 2500 amp-hours of battery.

Since these batteries are going to be drawn down very low and recharged, a good starting point might be to see what we'd need if we were buying Optima deep cycle batteries. From that chart, we see the D31A model puts out 75 amp hours. So we'd need 34 such batteries to run our generator for an hour. And 34 of these would cost $7818.30, and weigh 2033.2 pounds. Quite a lot of batteries to lug around.

Well, maybe our initial assumption, an SUV that got 20 miles to the gallon at a steady 60 mph, was a bit unrealistic. What if you have an economy car that got 50 mpg at the same speed? That's easy, multiply the number of amp hours you need by 0.4 and repeat the math. You come up with 1000 amp hours, 14 batteries, 837.2 pounds, and a tab of $3219.30.

And those assume that you're trying to get 50% better mileage with a 100% efficient generator, for just one hour. Realistically, you're not going to get a 100% efficient system. There's a good chance you may need a battery pack 50% to twice as heavy. Carrying around a a ton of batteries (literally) is going to drag that mileage back down, to say nothing for what it does to your acceleration.

At this point, it looks like the idea of a plug-in hydrogen hybrid can't be saved. Or can it? Here's a hint: This example is plugged in the wrong way.

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