Increasing regen braking efficiency; DIY flywheel batteries?

[vanilla ice]

The little regen reading I've done here at ES has led me to believe that max C charge rate is the limiting factor to get a good amount of the braking/downhill power back in to the chemical batteries. I've also read about using resistors as a load to eliminate mechanical rear brakes. I'm sure it works well for its purpose but it seems a waste.

I was thinking feeding that power in to a small gen motor connected to a flywheel would be ideal for getting that 10% regen efficiency up to 50% or so. After braking or downhill runs, you could gradually feed the chem bats with the flywheel at a rate they could absorb the power well. I know it'd take some fancy clutch work, but it seems doable.

Obviously this is gonna be a lot easier in something big like an EV car compared with the lighter stuff like ebikes. But who knows, maybe a scaled down version could even work on a bike. I'm not really up on the latest and greatest flywheel tech. It seems pretty simple but I know it takes a lot of rpm to store a good amount of energy. Do you guys think the flywheel is a DIYable approach to regen?

 

[fechter]

One problem is the voltage across the flywheel generator/motor will be a function of flywheel rpm, so it ends up being sort of like a capacitor.

It will be challenging to devise a controller scheme that will allow efficient input and output over a wide speed range for the flywheel. Not impossible. The other thing is you'd want to do the math to see how big this thing would need to be for a given amount of power storage.

 

[GGoodrum]

Just how much re-gen current are you talking about? a123 cells can be recharged at rates up to at least 8-10C, so with even a "minimum" 4p/9.2Ah configuration, that would be 74-92A.

-- Gary

 

[fechter]

I pounded my old SLA's with high regen currents with no problems.

The problem is if you are fully charged and decide to go downhill. You need to either disable the regen when the battery voltage reaches max or divert the power into a big dump resistor.

 

[vanilla ice]

I'm not sure. I had i mind steep hills and full speed to stop lights. Does that fall within the a123's range?

The wiki article on flywheels brings up protection from breakage and gyroscopic effects. Both could be problems for vehicles because of weight and handling. Respectively.

One novel benefit would be spinning up the wheel from the wall outlet, and it could get you up to speed that first time with no drain from the chem bat!

I was thinking for a DIY test rig you could have a bicycle wheel with weights in place of the tire. Maybe you could tuck in a 20" composite wheel mounted on a common extended rear axle of the bicycle with a 26" rear wheel. If mounted inboard enough it should allow enough clearance for cornering. The larger diameter of the wheel based flywheel allows you to get more MOI per pound than a small diameter flywheel. Maybe with a bullet proof Kevlar fender to protect your butt from any flywheel shrapnel. For sure this setup wouldn't be pretty but this is just some brainstorming on how to try to pull this off shade tree style.

Actually a bicycle trailer mounted flywheel would eliminate a lot of the issues. Thats probably the better way to go.

 

[TylerDurden]

The problem I see is mass vs. velocity:

You don't want a lot of mass to pull around, so you would need to get a flywheel going so fast the bearings couldn't take it.

:?

 

[vanilla ice]

Well, thats why I thought that getting a smaller amount of mass further away from the axle would give it more inertia, getting by that problem. Less rpm, less mass, but get the mass you do have really far away from the center of rotation. But I have no idea if it would be enough.

It'd be interesting test out how much energy per pound you could store with a weighted 700c flywheel at normal bicycle wheel rpm. On a test stand. If it turned out good enough you could plop it on a trailer. I do realize the charging circuit and clutch mechanisms would be a PITA to build.. even if it turned out it could store a worthwhile amount of energy. Which it may not be able to.

 

[TylerDurden]

The further the mass is from the axis, the more pronounced the precession will be. That could get interesting.

:?

 

[vanilla ice]

I imagined a vertically mounted wheel so it'd be aligned like the regular ole wheels of the car or trike/bike.. but I'm not familiar with how that effect you mentioned works. I guess you could choose rotation direction depending on if there was some favorable effect on traction or weight transfer during the braking/accel. And turning, well.. I can't really visualize what all goes on. Like you say it could get interesting negotiating a low speed 90 degree turn with the flywheel going at full tilt.

The high tech flywheel energy storage devices described below use vacuum cans and floating bearings to reduce friction. Thats out of the window for a DIY project, thats why I had in mind regen. The need here is merely to spread out the time period the batteries have to absorb the power from that short strong braking event or steep down hill run. It would not need to spin for lengthy periods of time.

http://en.wikipedia.org/wiki/Flywheel_energy_storage

Maybe use of this is only possible with vehicles much larger than bicycles...

http://en.wikipedia.org/wiki/Gyrobus