The numbers for the compressor heads come out at about 23%-24% derate, based on some online stuff from a compressor manufacturer. Your windmill will be about the same derate.
If you use a brushed DC motor, you will need special brushes, as most are mentioning. Has to do with the Dielectric properties of air. Believe it or not, air is an electrical insulator. It's much better at sea level than at 10,000'. I guess what happens is there is more arcing between the brushes and the commutator due to the air gap's involved. Basically, eats itself up quicker. Even with the special carbon brushes, life may not be all that great.
One company mentioned that with a standard brushed DC motor, could be all over with in a few hours from firing it up.
Absolutely the best way to go DC with these small motors, at any elevation, is brushless DC (BLDC). That's what I would do. I would want my motor to last a long darn time without worries or much maintenance.
