# A new thought about Regenerative Braking



## HDS (Aug 11, 2008)

Hi all,
I am working on theory and I do not know if it has ever been tried before. 

The trouble I have with regenerative braking conceptually is that if you are braking from 60 mph to 0, you are applying the brakes for about 5 seconds. Batteries can only absorb electricity at a certain rate which is going to vary depending on their state of charge at the time you hit the brakes. If you have just left your home (and hence your charger) the batteries are not going to be very "thirsty" and will not take on very much charge in that 5 seconds. When they talk about recovering 10% of the energy of braking, I think that that is very optimistic. I bet that reality is that 1% will be closer to the correct average. That means that 99% of the kinetic energy of bringing that mass to a stop is wasted. 

Here is my solution. You have a second motor (or motors) that drives a flywheel (or flywheels). The total mass of the flywheel(s) will depend on the vehicle mass being stopped and started. When you bring a vehicle to a stop you start off with a high amount of brake pressure which tapers off as you come to a stop. This matches the inertia requirements of getting the stationary flywheels up to speed, which needs a high amount of current draw when the brakes are first applied to get them spinning and tapers off as they come up to speed. For rapid braking, a lot of current can be dumped into the motor driving the flywheels to maximum sped in a hsotr time.

A 75lb flywheel with a 24" diameter spinning at about 9K RPM has about 100 horsepower of energy in it. I am not talking about storing energy on Friday afternoon to use on Monday morning. I am talking about using the energy of the flywheel to get the vehicle up and running when the light turns green.

I would accomplish this by using a second controller with a 5K ohm throttle circuit attached to the brake pedal. I would use the pedal travel range between the spot when your foot first hits the pedal to the spot where mechanical brakes engage. (You still have to have the mechanical brakes ) This throttle would accelerate the flywheel drive motor just like it would a traction motor. While waiting at the light, the flywheel would sit and spin. Once the light turns green, the flywheel motor becomes a generator and sends its' energy to the traction motor.

It is natural to ask, "Why not just have 75lbs. of more batteries?"

Because, batteries are nasty, expensive, require due diligence to preserve their lifespan, are difficult to recycle, must be charged very carefully, and have a limited life. A flywheel is a simple device and will last forever.

Picture this. You are getting ready to leave and hit a button on your keyring. The flywheels rev up to maximum speed on household A/C. You start out of your garage to the end of your driveway on flywheel power, starting at 10K and dropping to 9K by the end of your driveway. You slow down bringing the flywheel to 9.5K. You drive to the end of your street dropping from 9.5K to 3K on your flywheel, achieving say, 25MPH. You bring it to a stop bringing the flywheel to 7K. You take off down the road and by the time you hit 35MPH your flywheel has stopped. Your are a mile from your house, riding at 35MPH and you have not hit your batteries yet.

I think that this has tremendous potential. Has it ever been tried? Are there pitfalls that I have not taken into consideration?

I'd love to hear your thoughts.
HDS


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## e_canuck (May 8, 2008)

HDS said:


> A 75lb flywheel with a 24" diameter spinning at about 9K RPM has about 100 horsepower of energy in it. I am not talking about storing energy on Friday afternoon to use on Monday morning.
> HDS


Hi HDS.

I admit you lost me a bit with the details of the concept and the numbers. But to me the important part is 75Lbs ( 34 Kg ) spinning at 9000 rpm.

The material of the flywheel would have to be very strong. Without doing any calculation I would say that it should be very well balanced and made of a very strong material. Should the wheel break apart the shrapnel would go right through the "engine compartment" and be dangerous. A flywheel of that magnitude would then not be cheap and easy. The bearings for one would have to endure for a long time without over heating, so more engineering and preventive maintenance. Like on a train, every bearings are inspected all the time.

Another thing I wonder about is the gyroscopic effect. The flywheels on a bicycle ( 2 of them ) are very light, are turning at moderate speeds and yet even at low speed are strong enough to hold the bicicle upright.

The gyroscopic effect would in my opinion make you vehicule very hard to steer.

If it is mounted with the rotation axis inline with the vehicule. The vehicule would have a tendancy to roll on its back accellerating or decelerating.

If the axis is perpendicular to the vehicule turning would make the vehicule pitch up or down affecting traction ( or skidding ).

Mounting the axis verticaly also make the vehicule want to yaw ( turn ).

Perhaps storing energy from a conveyor belt with the flywheel stationnary might work.

I suspect the scheme to be difficult to build and dificult to manage in a moving vehicule.

DP


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## TheSGC (Nov 15, 2007)

It's not a new idea, just one that hasn't been used much since a freewheeling flywheel is extremely dangerous, and if in an accident, can turn to a deadly weapon.

With that aside: http://www.greencarcongress.com/2007/10/flybrid-flywhee.html

The flywheel idea has been something I always wanted to try on a bicycle, for real. My friend and I did try something back in elementary school (yeah, we're geeks with equally geeky engineering dads...) with a bike and a flywheel made from a heavy wheel barrow tire. It wasn't the best setup and only had an on/off setting, but it did move the bike after gliding down a hill. It was just for fun, and dangerous since it engaged when you let off the brakes (yeah, that was dumb), but wicked fun.


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## madderscience (Jun 28, 2008)

I don't know any details, but the british expiremented with flywheel-hybrid transit buses many years ago. They used exotic materials that would be less likely to explode like a bomb if the flywheel failed.

Another issue with the flywheel thing is that a big spinning disk in the vehicle will cause strange handling behavior (ever tried tilting a spinning top?) With the amount of energy stored in the flywheel this could get weird fast.

Why not either 1) just don't quite charge the batteries fully, so that there is room for regen when leaving home (with LiFePO4 you generally want to charge them to not much more than 95% usually anyway) or 2) use a bank of ultracapacitors to capture the regen energy?

And I think the 10% benefit for regen applies to overall range. The actual amount of kinetic energy being recovered is probably around 50% or so. (if your motor is 80% efficient as a generator, and your transfer electronics are 80% efficient, and your batteries are 80% efficient at charging, you get 48% or so)


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## JRP3 (Mar 7, 2008)

Regen works well as is, no fancy engineering necessary. With Lithium batteries they are not, or should not be, charged to absolute maximum capacity because that preserves their life, so you should always have some room to take some regen.


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## Jimdear2 (Oct 12, 2008)

To all answering,

I've heard or it was inferred that part of the programing fo the new OEM EV and Hybrid EV vehicle controllers with regen, take your known driving patterns into consideration. They recharge the battery pack considering things like you are known to drive mostly downhill in the morning and put X Kw into the batt with regen so the pack is only charged to Y State Of Charge to compensate.

Not a perfect system but a start on a better way.

The idea of mechanical storage of regen power brings to mind an 2 alternate fairly well developed green power sources. 


The first idea id still being fully develope, but instead of a motor/generator and flywheel, or expensive supercaps or batteries left in a low SOC, subsitute a air motor/compressor and HP storage tank.
The second idea is a fully mature system, almost off the shelf except for software. Go to hydraulic storage when the batteries are at full SOC. Use a small Pump/Motor and an accumulator as an intragrated part of the regen braking system. You can even keep regen braking after the accumulator and batteries are full by wasting the pump pressure against an orafice and dumpinig it as waste heat.
I see all of the positives mentioned, with only one real MAJOR negitive. Is the weight and added complecity worth it?

Interesting thought exercises though.

Dream a little


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## JRP3 (Mar 7, 2008)

There is no need to add weight and complexity to a working, simple system. Look at the Tesla Roadster. To preserve the life of the pack it is never fully charged, and the benefit of that is there is always room for regen. Any lithium system should be set up the same way. As soon as you start driving you are pulling energy from the pack which because of losses will always be more than you put back in from regen. Ford did develop a hydraulic recovery and boost system which they abandoned.


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## Jimdear2 (Oct 12, 2008)

JRP3 said:


> There is no need to add weight and complexity to a working, simple system. Look at the Tesla Roadster. To preserve the life of the pack it is never fully charged, and the benefit of that is there is always room for regen. Any lithium system should be set up the same way. As soon as you start driving you are pulling energy from the pack which because of losses will always be more than you put back in from regen. Ford did develop a hydraulic recovery and boost system which they abandoned.


JR,

Believe me when I say I am a firm believer in KISS. 

But it's a lot of _*fun*_ to throw ideas around, keeps the mind sharp (at my age you have to keep something sharp). 

You never know, someone a lot smarter them me  might just come along and see something in a wild idea that changes the world.


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## etischer (Jun 16, 2008)

HDS said:


> A 75lb flywheel with a 24" diameter spinning at about 9K RPM has about 100 horsepower of energy in it.


100 hp for how many milliseconds? 

This system would basically be like down shifting a car, but instead of compression from the engine slowing you down, you just have a flywheel spinning up. I think the inertia of the car will far exceed the inertia of your flywheel. wouldn't you need a motor and controller capable of rengerating 100hp just for a few seconds?

I think most AC drives are around 90% efficient in regen. They can regen all the way down to zero speed, and at a torque equal to the torque during acceleration. I don't think a flywheel will match the regen torque of the traction motor. If the pack voltage is exceeded during regen, you can always programatically limit the regen torque and begin using friction brakes.


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## JRP3 (Mar 7, 2008)

Sure it's great to throw ideas around, but they should have some fundamental merit to them otherwise you're just complicating things for no reason. In this case it's a solution in search of a problem. One of the things I like about EV's is the potential for simplicity. As you add other elements the cost of construction and maintenance goes up as do the possible failure modes.


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## JRP3 (Mar 7, 2008)

etischer said:


> If the pack voltage is exceeded during regen, you can always programatically limit the regen torque and begin using friction brakes.


Or a resistance element could be switched in to drain off voltage momentarily as heat, keeping the same braking feel. I may implement just that on my system and have my electric water heater element kick temporarily on if pack voltage gets near the cutoff. The cutoff is for the controller limit, not the battery limit, which will be higher than the controller.


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## Jimdear2 (Oct 12, 2008)

JRP3 said:


> Sure it's great to throw ideas around, but they should have some fundamental merit to them otherwise you're just complicating things for no reason. In this case it's a solution in search of a problem. One of the things I like about EV's is the potential for simplicity. As you add other elements the cost of construction and maintenance goes up as do the possible failure modes.


JRP3

Hey sounds like a lot of fun, I'll keep coming up with insane, off the wall, over complicated Rube Goldberg ideas and you can keep slapping my silly ideas down. 

Never know something usefull may come out of it and we will sure keep everthing on the lighter side and everyone entertained. 

Have a good one


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## Jimdear2 (Oct 12, 2008)

JRP3 said:


> Or a resistance element could be switched in to drain off voltage momentarily as heat, keeping the same braking feel. I may implement just that on my system and have my electric water heater element kick temporarily on if pack voltage gets near the cutoff. The cutoff is for the controller limit, not the battery limit, which will be higher than the controller.


JRP3,

Damn, the water heater idea to dump waste current/heat is a really good one for winter driving and would be pretty simple to implement as well.

Too bad Pelitier(spelling?) devices aren't more developed. You could reduce the pack loads for heat in the winter and cooling the summer using waste heat from braking loads.


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## JRP3 (Mar 7, 2008)

Well it won't be a large amount of energy, just a momentary thing, so I doubt it would contribute any meaningful amount of heating or cooling. Once you've driven even a short distance your pack voltage will be down enough that all regen will go back into it. The only time I can see the need to dump voltage is taking off with a charge fresh off the charger and heading immediately down a very long steep hill.


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## Jimdear2 (Oct 12, 2008)

JRP3 said:


> Well it won't be a large amount of energy, just a momentary thing, so I doubt it would contribute any meaningful amount of heating or cooling. Once you've driven even a short distance your pack voltage will be down enough that all regen will go back into it. The only time I can see the need to dump voltage is taking off with a charge fresh off the charger and heading immediately down a very long steep hill.


JRP3,

It may not be a big recovery but it is a recovery and for no major increase of complexity. Every watt you save instead of throwing away, no matter how inefficient the recovery is work you can use.

I think engineers call that elegant.


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## Voltswagen (Nov 13, 2008)

This idea may not have much merit right now due to cost....but in the future if the component prices dropped........

Instead of using a flywheel with its inheritant dangers why not use Regen to charge Supercaps? They accept a faster rate of charge compared to batteries and can discharge that energy back into the system much quicker.

Go head....slam me.


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## JRP3 (Mar 7, 2008)

No slamming necessary, the idea is sound, other than the cost issue. However, by the time the cost comes down they may be unnecessary. A123, PHET, LIFEbatt, Altairnano, can all handle high discharge/recharge rates, so there would be no point to carrying around empty caps just on the chance that you might need some regen. Frankly even the Tesla and the EMini using commodity cells can handle all the regen available, what's the point of adding anything else?


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## 1clue (Jul 21, 2008)

This was actually looked at some years back. From what I remember, they were going to put a flywheel into an evacuated (pure vacuum) chamber to reduce friction, and then turn it at some ungodly RPM.

The idea is sound, the economics of the whole thing is what you have to look at. Find out how much energy you can store in a system that _you can make_ and then figure how much that will cost you, and decide if it's worth the effort.

If I remember correctly, they were talking about turning the flywheel so that the wheel is horizontal, like a top. That way no matter which direction you turn the flywheel doesn't exert any weird forces on the car.

Personally I think going with a super-efficient motor and controller would get you more bang for your buck, but it might be worth a try to build this flywheel.


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## judebert (Apr 16, 2008)

Supposedly the Formula-I racers were going to start using it as a regenerative braking system starting this year.


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## Anaerin (Feb 4, 2009)

judebert said:


> Supposedly the Formula-I racers were going to start using it as a regenerative braking system starting this year.


They're using KERS (*K*inetic *E*nergy *R*ecovery *S*ystem), which is essentially a high-speed capacitor pack and an electric motor/generator. So the Capacitors are acting as an "Electrical Flywheel", storing and releasing the power soaked up through Regen.


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## NeilBlanchard (Feb 11, 2009)

Hi,

Would a capacitor(s) work better to quickly absorb power, and then be able to slowly charge the batteries using power from the capacitor?


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## JRP3 (Mar 7, 2008)

Only if the regen rate was greater than the battery could safely handle.


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## 1clue (Jul 21, 2008)

Search on capacitor driven cars. There are a couple out there, including at least one small car which is driven ONLY by caps.

The problem there is that the caps are so expensive and bulky that your car costs more for a 1/4 mile run than an equivalent EV would cost. You can build for normal driving, or you can build for acceleration, or if you have a few tens of thousands of dollars laying around doing nothing, you might be able to build one car that does both fairly well.


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## Frank (Dec 6, 2008)

Anaerin said:


> They're using KERS (*K*inetic *E*nergy *R*ecovery *S*ystem), which is essentially a high-speed capacitor pack and an electric motor/generator. So the Capacitors are acting as an "Electrical Flywheel", storing and releasing the power soaked up through Regen.


At one time there are also mechanical systems being considered:

http://www.motorauthority.com/brake-energy-regeneration-in-f1-by-2009.html

but I understand there's a move afoot to try to standardize them to keep costs reasonable. Anyone else have updated info?


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## HDS (Aug 11, 2008)

Hi all:
This is my first time on since I posted this thread. Between working 2 jobs and having 3 year old triplets, I don't have the time to pursue my EV "dream" like I wish I could. Thanks to all for the responses. Thanks SGC for that link, it was encouraging to see where others have pursued this. 

I have a friend that worked for a company called Beacon Power. They use carbon fiber flywheel cylinders that spin at 30K RPM and are used in large scale UPS systems. They have now found a niche in the utility power field where they speed up the cylinders when the load on the grid is low and put the energy pack onto the grid when demand is high. These flywheels are suspended by magnets for frictionless bearings; not practical for EV's. He had told me that if the flywheels are kept at 10K RPM or lower, than you can save a ton of money is the design. (That's where I got the 10K). It may be entirely possible that a maximum speed of 8K is more practical to be within the tolerances of lessor (and cheaper) materials. After all, every motorcycle or automobile has a flywheel. My 2 stroke Kawasaki would rev to 8K before it would "redline". 

I shared my thoughts with another friend who told me about his college professor that believed that the flywheel was an underused and under-appreciated device. He had taken an ICE and put a Briggs & Stratton engine in the trunk. It would come on automatically and its' only purpose was to bring a 100 lb flywheel with a 4' diameter (mounted like a turntable under the vehicle) up to some predetermined RPM. This flywheel powered a generator that drove the traction motor for the wheels. He couldn't tell me if regenerative braking was put back into the flywheel or not. The professor claimed to get 100 mpg out of it. Considering this friend is my age (about 50) that had to be some time ago. The professor was ahead of his time.

I didn't know that Lithium batteries were generally charged to 95% of maximum charge. Just another great tidbit of info that comes from this site. Ultra caps sound great but I understand that heat and cooling is an issue with them.

EESTOR batteries sound promising too; perhaps these have already been discussed in another thread. 

Anyway, thanks again to all for the responses. 
HDS


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## JRP3 (Mar 7, 2008)

HDS said:


> He had taken an ICE and put a Briggs & Stratton engine in the trunk. It would come on automatically and its' only purpose was to bring a 100 lb flywheel with a 4' diameter (mounted like a turntable under the vehicle) up to some predetermined RPM. This flywheel powered a generator that drove the traction motor for the wheels. He couldn't tell me if regenerative braking was put back into the flywheel or not. The professor claimed to get 100 mpg out of it. Considering this friend is my age (about 50) that had to be some time ago. The professor was ahead of his time.


I don't think so. Unless there is some significant information missing I don't see how using a Briggs and Stratton ICE to turn a flywheel to then turn a generator which then powers an electric motor could in anyway be an efficient system. You have power conversion losses all over the place.


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## Astronomer (Aug 7, 2008)

JRP3 said:


> I don't think so. Unless there is some significant information missing I don't see how using a Briggs and Stratton ICE to turn a flywheel to then turn a generator which then powers an electric motor could in anyway be an efficient system. You have power conversion losses all over the place.


To be fair, one advantage of converting ICE to electric is the torque conversion, which can more than offset the energy conversion loss. Diesel locomotives make the best use of this (can't get a train going by revving up the diesel engine and popping the clutch!  ), and even serial hybrid cars take sufficient advantage of an electric motor's max torque at 0-RPM to offset the conversion losses. ICE power plants, for all their bluster, are just so inefficient at getting a vehicle moving. In the described setup, I imagine that the flywheel serves much the same purpose as the batteries would to help moderate the flow of energy, though I'm having some trouble picturing exactly how that would work.

One huge advantage I see with the flywheel idea is this: Spin up the flywheel, put a car jack underneath one corner of the car, jack up the car, and all four tires lift up off the ground! 

HDS, I enjoy reading and thinking about your ideas. Enjoy your kids while they're young.


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