Engineers are prone to trying to prove they're the smartest person in the room. I love your friend's independent analysis of the possible operation of a simple, common device that's been around for 100+ years with abundant supporting literature.

So anyway, since one can buy 12V, 6W bicycle dynamos, I'm guessing you're trying to charge a 24V battery? What bicycle borne application do you have that requires 24V battery power? On the other hand, if this is a stationary set-up, there are better ways to move electrons than little bottle dynamos.