[C3H8]

that brings up a good point. assuming one was actually able to get all this to work properly. lets say for arguments sake you could spin an alternator at 40amps, or 70amps. the number doesn't matter. what i am now asking is, how would you prevent the alternator from over revving and blowing its self up under high heat and low electrical load conditions. example would be cruising down the highway at 70mph during a cool summer day with no a/c on, no headlights or running lights, no real load on the alternator. (worse would be climbing a mountain pass ). the turbine alternator would need some form of mechanical slow down apparatus.

the first thing i would think of would be a waste gate like a turbo has. but turbo waste gate doesn't prevent the turbo from over-revving. it only prevents the motor from seeing excessive boost. proper sizing of a turbo is what generally prevents it from over revving.

you would need some sort of governor or braking system for the alternator to keep it from spinning too fast. i would post a link however i don't have 10 posts under my belt yet.
daviddarling.info/encyclopedia/W/AE_wind_turbine_braking.html
that is a "link" to a website that explains how wind turbines avoid over revving. they either make the blades inefficient during high wind conditions to slow it down, or dump excess power to a bank of resistors.

sense this steam powered alternator in a very basic sense, works along the same principle as a wind turbine, i would think that those 2 ways would also work for the alternator. and i would suggest throwing out the resistor idea. that puts undue load and stress on the alternator not to mention wearing out the brushes quicker then normal. best bet i can see would be some sort of dynamic braking via an adjustable turbine. when things get going too quickly, maybe a spring loaded centrifugal flap could extend and slow things down by creating wind resistance.

although i am still skeptical about this working on an ecomodder's exhaust, i would like to see this succeed. if it ever does i'll be the first one to purchase a unit and install it on my car.

another idea i had was to purchase a small gt series ball bearing turbo and remove the compressor housing and compressor blade. if one were to then install a small pulley on the turbo shaft where the compressor use to be, one could drive a small alternator or generator via a belt off this pulley and shaft. i have a few smaller turbo'd sitting in my garage. none of which, are ball bearing though. :/
i would ot recommend a turbo that uses bushings. they are not designed for radial loads like that which would be experienced with a belt and pulley. they are designed for small axial loads only.