Thoughts on a powerful bike generator

There is a closed thread that asked the question on uses for powerful bike generators.
I'm no electrical engineer so this is going to be a pretty rough outline, hopefully not requiring any unobtainium.

The design would consist of four dynamos, two in front and two in back, each pair positioned similarly to caliper brakes, alternately the back brakes would be completely replaced by the dynamo pair and the front brakes would remain as dedicated brakes.

Ideally the frame of the bicycle would be packed with sealed capacitors designed to fit the tubing, and there would be controllers to shift current around as needed depending upon what was happening.

Scenarios
-Going fast downhill, only using enough resistance in the rear dynamos to power Bright headlights and rear running lights.
-Going fast downhill and seeing a problem, applying more electrical braking the lights get much brighter, the capacitors charge up and controller drains capacitors to charge batteries.
-Now the bike is slowed, problem averted, can dim the lights back down to regular downhill running brightness and resume speed.
-From a brief stop, a stoplight or going up another hill hill the capacitors could start being dumped back into the dynamos to give a push heading uphill or for quicker acceleration, the controller holding back enough juice to continue steadily charging batteries...
-If slowing the bike down required more and more resistance I'm sure there are ways to just plain dump energy; horns, amplifiers, arc lights, radio transmission, Air Compressor?!

Compressing air, so long as your tank didn't blow up on you might be more efficient than converting to electricity. Other than the air powered car I haven't heard much about it.


For village use of bicycle power generation it seems like there aught to be a way to make a fairly large low tech but precise flywheel, maybe molded in the ground onsite out of concrete, rebar and steel cable, then jacked up and set on it's bearings in line with the earth's rotation, maybe in a containment pit built for it. The bearings and transmission could be the high tech part, the wheel would be slowly spun up and balanced over some time until it was fit for it's operating speed.
People would take turns charging the wheel and keeping it topped off, maybe set it up so that a single person could pedal in some energy while many people could also hook in and pedal together.
Then at peak use times there would be a lot of high load power available, light up the soccer field maybe?

It's a nice thought, and potentially workable. I think the difference between power density and energy density needs better understanding before we get much further.



Batteries have, generally, lots of energy but can't let it out that fast. Capacitors have much less energy per kg or square cm, but can output it really quickly, likewise they can charge quickly too. Electric bikes are becoming popular to give a boost when going uphill, but having enough capacitors to do the same thing isn't a 1-man show at the moment.

There's also mechanical capacity. If you want a generator that can sink the power required for an emergency stop in the time an emergency stop takes, then it'll be quite big, and consequently heavy, and you won't use 90% of its capacity for 95% of the time. Most people won't put up with that when the simpler rim or hub or disc brakes will do it for a fraction of the weight, complexity, cost and reliability.

There's plenty of discussion here about generators and hub dynamos in particular, they being the kind of things most cyclists are prepared to put up with and maintain speed/lightness.

Unobtainium.

I'll bet that there is a a happy medium somewhere.
Personally I'm not very satisfied with the constant drag of a dynamo on a bicycle. A system that could take surges and smooth them out for useful electrical power seems best, thus the dynamo as brake idea. Enough capacitor to keep a battery charging between surges would be great.
Having enough juice produced to actually boost back uphill might just be unfeasible until the next generation of batteries come out (nano engineered to charge like capacitors, they are coming soon).
Likewise having enough resistance in the dynamos to make them function meaningfully as caliper brakes might or might not work out (although rear brakes only have about 20% of the braking power of front brakes anyways).
Being able to generate a lot of light for long downhill routes, and having the ability to electronically slow if not stop the bike for long descents would still be very useful. People have burned up rims on long mountain roads, pretty hairy.

Thanks too jdp298 for the graphic.

If you look at the BionX system, you get an impression about the required size for a meaningful effect in braking. This may not be the perfect generator, being designed as a motor. It has a recuperation mode to charge the battery in descents, said to improve reach by some 10%. So, there's not too much to be gained. The recuperation is limited due to thermal reasons, using full-on generating mode on long descents in combination with small wheels has destroyed some units already.
But for recharging light batteries, I like the idea. How about a hub dynamo used as a roller on the tire, with cable actuation maybe?