This could only be meaningfully used for braking.
There's a pretty fundamental set of physical laws about the conservation of energy which explains the fundamental problem of perpetual motion machines. The physicists are not just a bunch of people in white coats being spoil sports, but are describing how the world really works. Look up "perpetual motion" on Google to see why there are problems.
*Your fan is extracting some useful energy from the wind but is also losing some energy in the form of friction (heat) and sound.
* The alternator and the drive mechanism are also less than 100% efficient (heat and sound etc), so the electricity coming out of the alternator has less energy than the fan harvests.
* Then, the battery storage efficiency is less than 100%.
* Then the motor driving the car is less than 100% efficient.
So with all these these inefficiencies you'd be lucky to get 50% or so through the system.
Now where is the wind coming from? If you're going down hill and using the system to harvest excess energy (which your regular brakes dump as heat), then you could use this mechanism to harvest energy.
However on the flat the system only harvests less energy than you need to move the car along In other words it won't give you free energy. It won't even help to reduce the gas consumption of the car because the extra drag cause by the fan is more than the energy you're getting out. That means the gas engine will have to work harder than if the system was not there at all.
As stated earlier, you could use this for harvesting braking energy on downhills (or when stopping at a traffic light). But there is still no free ride: you'll have to use gas to get to the top of the hill (though this system might help you get some of the way up).
Written in June 2008