You could get one of those little generators that rub on the tire, like you had on your bicycle when you were a kid....or you could install a wind mill....or, like Bud says, install a high output rotor and stator. I think the 45 AMP unit is the largest offered. You may have to do a little research to find out for sure what all it will be necessary to change.
I don't think it will be a cheap fix.