friction => loss... pure and simple.
Best possible is helical gears, second best is probably a heavy duty timing belt (it sure is a lot cheaper than helical gears!). Expect to lose a few watts in your drivetrain anyway, and expect any cogging or friction in your alternator portion to be multplied by the same ratio that your gearring speeds it up, so unless you have an extremely low friction alternator you will see reduced low wind performance rather than increased (because it will take more wind to start the machine moving).