text: The air conditioner fan utilizes power such that air flows through the duct work and flows into the space to cool you off. You now want to use that air coming out of the vent to turn a fan which turns a shaft which generates power to drive a servo. In order to drive the servo fan, there will be a significant pressure drop in the a/c duct. That will de-rate the a/c fan. Additionally the servo fan will have mechanical losses due to friction. Say you got one tenth of a horsepower out to drive the servo. The a/c fan would supply one tenth horsepower plus mechanical losses. Additionally, since the a/c fan is de-rated it will function less efficiently. And you be hot. Plus the servo will generate heat and warm off the room more.  If you wanted to set the servo generator to the a/c exhaust the same thing. The exhaust fan would be de-rated and the a/c would not function efficiently as the amount of heat driven off would be less. Just plug your servo into the wall. There is no free lunch.