On the other hand though, if the alternator were big and heavy enough (like 40-50lbs spinning), it might just be a decent enough acting flywheel at that low rpm. It's difficult to say.
Another nice thing would be that it would be almost silent running. Great for watching TV without headphones. This is important to me.
I've been doing some experimenting around with my big flywheel idea and it sure does seem to help a lot in getting more power with a 'semi-casual' physical effort.
I am not a work-out freak or something---but I would like a nice 'semi-casual' 30 minute workout every few days primarily for excercise AND to get some usuable power on a gloomy, no wind day.
That is my objective. My goal (challenge;hope;dream;etc) is to average 100watts during this 30 minute workout. (total= 50 watthours)
I want to start out with 120watts for 10min ; then reduce down to 100watts for 10 minutes ; then reduce down to 90 watts for the last 'easy' 10 minutes. (total=30min workout)
Anyway, that is my goal, I don't know if I can acheive it yet though, nor if its similar to your idea of 'a workout' or what.
I am still working on my design but maybe you might get an idea from it.
Obviously I have a very efficient giant AC servo-motor (only 1/4th ohm coils)as the generator. (it failed with the VAWT sadly--so now it will find life as a big ped-gen)
Here is a couple pictures of my test with the big 30" dia. particle board flywheel.(one equals 16 lbs. // I intend to attach a second one to make a 32 lb flywheel plus the approximate 40 lbs of rotor spinning inside the motor. (total about 72 lbs of 'flywheel' spinning)


Keep in mind, this is testing.
For now, I have to hold the 10-speed (in it's 10th fastest gear) bicicle to the big 30" flywheel with one hand while pedaling with the other hand.
So pedaling with only one hand I am getting a fairly easy 5 amps (14volts at the 12Vattery) which is about 70 watts going into the battery.
But it is difficult to hold it while pedaling though. It keeps slipping off the edge of the big flywheel since I don't have the other second piece of 30"dia. particle board attached yet. Sometimes it goes to 6amps.
I am very certain that I can achieve my 'semi-casual' pedaling at 7 or possibly even 8 amps. So I think I can do it. Of course, pedaling hard for a short time I expect it will go way higher.
Anyway, it is a fun and interesting challenge to try and get the most efficient Watthours you can get in the least time!
-Good luck with your testing and design. Look forward to whatever you come up with.