[yorkie_chris]

You're taking energy out of the air stream, there has to be some pressure loss there. One thing says "it's negligible loss" but if it's negligible loss, it must be negligible output.

Just same as the RAT turbines on jets, they don't run them all the time to drive the cabin electrics and such because the drag outweighs the gain.


One way you could do it, the waste heat from the engine is adding energy to the flow, so try to make it act like a ramjet, and extract some of that energy that way.
Problem is ramjets don't really work until you're up into high supersonics because otherwise there is no compression of the flow.
So, with the speeds of cars (basically at a standstill) you'd need a compressor and basically run it on turboshaft engine principle.

No idea whether that would be applicable to the volume and quality of heat dumped from a car engine!