why don't we see wind harnessing structures (like tiny fan-like structures) mounted on vehicles to capture the relative motion of the wind against the moving vehicle? I had one guess that mounting such structures would increase the surface area of the vehicle and thus the air drag acting on it, leading to a greater rate of consumption of the fuel, such that there would be no net gain of energy. How good is this guess? But, what if such structures could be built without increasing the frontal surface area of the vehicle (on which the wind is acting)? Like for instance, what if instead of a rigid/fixed frontal part of a vehicle (say the windscreen of a motor-bike), we have some moving parts there (like small wind-turbines, enhanced with piezoelectric material blades etc.) ; won't this produce a little extra energy? or maybe significant energy at high speeds?? I guess this is not violating any known physical laws, as the kinetic energy of the moving air relative to the vehicle that was previously being converted into heat on collision with the vehicle's surface is now instead being used to drive the tiny turbines and strike the piezoelectric material to produce electrical power. Is this correct? Or am I wrong somewhere?