You have an auto junkyard near you 
Electric engine fans run on 12 volt,get a scrap one or 2.They move pretty good CFM & should be cheap 
They're waterproof,so you could mount them outside & since they're DC,you can reverse the rotation if needed
Electric engine fans run on 12 volt,get a scrap one or 2.They move pretty good CFM & should be cheap They're waterproof,so you could mount them outside & since they're DC,you can reverse the rotation if needed
If they are brushless motors, you can not just reverse the polarity without letting the magic smoke out. Brushless motors are essentially AC motors with an integrated controller that handles the conversion (and starting) for you.
can confirm that not all DC-powered fans can opperate on reverse polarity. Not all will vaporize into forever-sleep, but some simply wont spin, or will spin strangely.
It must of been because I was working on older cars
92 chevy 454 SS pickup was just one that if connected backwards it would spin backwards,oh well,YMMV