You need power to make power, it doesn't matter what form you get it in, be it wind, hydro etc.
If you were to run an alternator from your A/C fan, the fan would either
- draw more power to keep up with the power the alt is making for you, or
- not have enough power to run the alt and stall out.
Power doesn't come from nothing so running the alt from your a/c fan would just make the a/c take more power. Its very inefficient, -if- it works at all.