This idea and many of the comments just cannot grasp that the power generated will never be enough to overcome the power needed to negate the drag added by those little fans.
Electricity-generating windmills churn out free energy as long as the wind is blowing. So strapping one to the roof of your car where there's always a breeze as long as you're driving just seems obvious. The Transport Turbine—seen here as a 3D printed proof of concept—puts four small wind turbines on the roof of your car that generate electricity.
When available later this year, they'll come in a simpler 25 watt version, and a more industrial 1,000 watt model which can be used to keep a battery
perpetually charging in an electric car when it's parked. The extra power can also be used to run in-vehicle electronics like a Wi-Fi hotspot or an elaborate entertainment center, and just like their larger counterparts these turbines don't mind a little bad weather. At least anything below an F1 tornado or category one hurricane.
Update: The original headline to this post was Strap This Wind Turbine To Your Electric Car And You Can Drive Forever. That is not possible! Laws of physics and such. We've toned down the tongue-in-cheekiness (which admittedly did not read as such) and clarified the use case in the body of the post.