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How would a turbine that takes energy from the air current generated by a passing car decrease the energy of the car?
Not sure where you got that idea from, but how would that generate a meaningful amount of energy? It seems very unlikely that such a system would ever recover the energy spent on its construction.
What kind of junk energy is there to harvest from a car (in meaningful amounts)? I guess breaking is the obvious answer, but that’s already covered by regenerative breaking. Most car-based energy harvesting systems seem to employ speedbumps that clearly take useful (kinetic) energy away from the car (probably at a very poor efficiency).