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Here are some interesting thermoelectric generator apps: http://www.hi-z.com/Hi-Z.Brochure.2006.pdf

Here is the link to the guy I was talking about. http://www.hi-z.com/websit07.htm

I thought that originally, too. But at some point they say the tailpipe doesnt got hot. I suspect that he is using a large thermoelectric generator on the exhaust or something is just pumping the heat elsewhere. He talked about there being a large heavy module near the cab on a semi, probably like this.

My problem is that he claims to go from %35 to %97 efficiency. I doubt he'll be able to effectively generate say 300hp (~225 kW) in electricity. Also one problem is in order to carry that much power effectively he'd have to do it at a high voltage, but electrolysis is more efficient at lower voltages.

....when the thermoelectric effect is used in the other direction.

Like here

Though Peltier cooling does have it's advantages/niches.

run it from a thermoelectric generator... peltier device working backwards the current/voltage characteristic is a perfect match...

You do realize that you've just described something rather like a perpertual motion machine? It's not quite, but to use a hot-to-warm difference to create an equal warm-to-cold difference is pushing it a bit.

It just might work with the "cold" - really warm - end of the generator cooled by airflow through a honkin' set of cooling fins, while the car's moving. There would be some wide-reaching redesigns - generally the electronics keeps well away from the hot parts of the exhaust. But the catalytic convertor is notoriously hot.

I easily found a commercial 19 W module that runs between a 450F/230C hot side and an 85F/30C cold side. The graphs on the spec sheet show lower efficiency with a cold side as warm as 100C. 3 inches square. And they take PayPal!!! $154 US. I'm actually tempted to buy one and mount it on my muffler just for geek value.

I'd expect you'd be lucky to get 50% total efficiency. That may be in the commercial domain, ask an economist.

Thermocouples are solid state and will generate electricity from differences in temperature. A company called Hi-Z produces some nice looking modules. Some interesting applications and specs are available at their site.

I found a link to stuff you're talking about. Click here to see a company in the real world trying to make a buck peddling thermoelectric exhaust energy scavenging equipment.