Theory questions!

[James1986]

Hi All,

I am doing some research into regenerative braking and was wondering if you guys could proof read my theory, and tell me if anything is missing! I'm assuming that everything is 100% efficient.

Basically, we have a biker on top of a hill (100m). The combined weight of the biker and the bike is 85 kg.

So, the Potential Energy (P.E.) of the biker is :

P.E.= M x g x h = 85 x 9.81 x 100 = 83385joules


Does that mean that this is the energy that a motor must overcome (provided no pedal assistance) to climb the 100m hill? Is this also the amount of energy that could be recaptured on the way back down?

So to raise or lower the biker by 1 metre using gravity, 834 joules must be spent.
Does this mean that for every 1 metre travelled in a second (i.e. travelling at 1m/s) the motor would regen 834watts per second? (With 100% efficiency)

If the motor is a 24v motor, is 24v the voltage directly given off in regen mode?

And if that is the case, then W= V x I , I = W/V = 834/24 = 34.74A

So the current given back is 34.74Amps?

And finally,

I'm not sure if any of the P.E. is used up in motion, as kenetic energy? Or is it that P.E. is transfered to K.E. and then into Electrical energy?


Thanks for your help guys!

James

 

[tillson]

Hi All,

I am doing some research into regenerative braking and was wondering if you guys could proof read my theory, and tell me if anything is missing! I'm assuming that everything is 100% efficient.

Basically, we have a biker on top of a hill (100m). The combined weight of the biker and the bike is 85 kg.

So, the Potential Energy (P.E.) of the biker is :

P.E.= M x g x h = 85 x 9.1 x 100 = 77350joules


Does that mean that this is the energy that a motor must overcome (provided no pedal assistance) to climb the 100m hill? Is this also the amount of energy that could be recaptured on the way back down?

So to raise or lower the biker by 1 metre using gravity, 773.50 joules must be spent.
Does this mean that for every 1 metre travelled in a second (i.e. travelling at 1m/s) the motor would regen 773.5watts per second? (With 100% efficiency)

If the motor is a 24v motor, is 24v the voltage directly given off in regen mode?

And if that is the case, then W= V x I , I = W/V = 773/24 = 32.2A

So the current given back is 32.2Amps?

And finally,

I'm not sure if any of the P.E. is used up in motion, as kenetic energy? Or is it that P.E. is transfered to K.E. and then into Electrical energy?


Thanks for your help guys!

James

James

Your calculation of PE looks OK to me although the acceleration due to gravity is 9.81 m/ss.

Assuming that everything is 100% efficient (which it isn't and is far from it in certain instances) and also that energy can neither be created or destroyed (only transformed from one form to another), then the rest of your calcs look OK to me. Although it is 20 years since I graduated Mechanical Engineering and haven't practiced it much since!

The P.E in reality will end up as heat (friction in moving parts, wind resistance etc) as you come down the hill if no regen braking takes place. If you assume 100% efficency then you can neglect friction etc so all the PE would end up as electrical energy (assuming 100% efficiency in the generation and storage process). Of course in the 100% efficient world without the regenerative braking the bike would continue in a straight line to infinity! Or until it came to another hill or a fixed object!

 

[HarryB]

Maybe the difficulty is where to put that energy on the way down. I wouldn't have thought that a battery would like such a large charge in such a short time, so much of the energy would end up wasted anyway. You really need some super-capacitors that can absorb the energy and then give it up to the batteries more gradually. Re-gen braking on an electric bike is seldom worth the hassle (lack of proper freewheel on the motor etc.) for such little gain. I am not sure how the Prius controls the re-gen braking - might go and find out.

 

[James1986]

Nice, thanks!

I knew I shouldn't have relied on my memory for the gravity acceleration constant =(

I'll edit the post above!

Next on my list is how much energy I can get from a normal flat braking environment!

It seems like there is a lot of energy available to reclaim! Even taking into account the 98% efficiency of gears, followed by the roughly 78% efficiency of most motors!

You really need some super-capacitors that can absorb the energy and then give it up to the batteries more gradually. Re-gen braking on an electric bike is seldom worth the hassle (lack of proper freewheel on the motor etc.) for such little gain.

And there you have it! My final year project, summed up in 1! =D

 

[flecc]

Getting away from the theory for a moment, in practice regeneration has been a failure on electric assist bikes, the relative low mass of bike and rider coupled with the low speeds making it very unproductive. There have been a number of attempts and only one has showed any promise at all to date, that of the BionX bicycle motor, originally developed by Chrysler as part of research following an earlier oil supply crisis. That has several switched motor power modes with one position switching in regeneration instead of drive, primarily for downhill use but not specifically for braking. A few owners report it as worthwhile but they seem to be a minority.

The US Birkestrand Corporation tried regen in their "Motorised Wheel", but that wasn't too successful, the motor later being licenced to Sanyo who used it in their home market. Later, Giant Bicycle used that Sanyo motor in their Twist 1 and 2 models with regeneration, again not for braking, but these were widely considered a failure with little to no regen detectable in practice. Both those have been quickly discontinued and now re-released under different model names with motor freewheels and no regeneration.

Now Panasonic have stepped in with a new model with regenerative front wheel braking, nothing much known of it yet. Being Panasonic I'd expect it to be as good as it's possible to be, but I'm still doubtful whether it's worth it in practice or just worth it as a marketing gimmick to gain sales.

"Professor Pivot" writing a tech article on regeneration in the A to B magazine some while ago went through the subject of regen on bikes in some depth, the conclusion being that it's not worthwhile for the reasons I opened with.
.

 

[James1986]

Hi Flecc,

Those are some interesting points! Do you have some direct links you could post so that I can do some further reading; they would suit very well for my introduction in my report.

With regards to the hub motors available, I am actually designing my own hub motor with a planetary gear system which will have regenerative braking as standard. Depending on the gear you are in, you will be able to choose whether you get lots of regen, and increased slowdown speed, OR little regen, and small freewheel resistance. This will also give the option of flat speed, or hill climb. It does mean that there is no actual free freewheel, but I'm hoping with the gear ratios that I can make this minimal so as to reduce the effort for the rider.

Back on the technical side, with a 24v motor, what happens to the voltage when you variate the speed when used as a generator. Does the voltage given stay the same, but the current provided fluctuates?

Thanks

---Edit---

I just read prof pivots bit on e-bikes and hes quite right that only 15% of the energy goes back into the battery on the regen side. However, the point of introducing the supercap into the equation is that 100% of this energy can be saved. Which, from my calculations above, seems to be quite considerable!

 

[tillson]

introducing the supercap into the equation is that 100% of this energy can be saved. Which, from my calculations above, seems to be quite considerable!

But if that were the case, you would end up pedalling downhill and the resistance to pedalling down hill would be similar to pedalling along on level ground. This would make for a miserable cycling experience.

 

[Tiberius]

Hi All,

I am doing some research into regenerative braking and was wondering if you guys could proof read my theory, and tell me if anything is missing! I'm assuming that everything is 100% efficient.

Basically, we have a biker on top of a hill (100m). The combined weight of the biker and the bike is 85 kg.

So, the Potential Energy (P.E.) of the biker is :

P.E.= M x g x h = 85 x 9.81 x 100 = 83385joules


Does that mean that this is the energy that a motor must overcome (provided no pedal assistance) to climb the 100m hill? Is this also the amount of energy that could be recaptured on the way back down?

So to raise or lower the biker by 1 metre using gravity, 834 joules must be spent.
......

Hi James, you are ok up to there. And after that, you are not wrong, but bringing the speed into it complicates matters. Also the voltage out of the motor on regen is proportional to speed, so you need some fancy circuitry to make it work that way.

With a bike, the main power loss on the flat is aerodynamic drag. The relationship between the aero drag and the energy stored as KE is very different to that of a car. Basically the KE stored in a moving bike is not very much, so the amount you could recover each time you stop is not a lot. If you want to do the sums, assume it takes, say, 500 W to travel at 20 mph, then see how much distance you could go on the energy recovered at a stop. You'll find its not worth it unless you are doing stop-start all the time.

As for recovering the PE as you go down a hill, that is a bit better. But it comes back to aero drag. If you want to recover the PE by regen, then you have to lower yourself down slowly, or have a very steep hill.

Regen is, however, not that difficult to implement - at the design stage of the controller it is almost zero cost to add. Even though the rewards are small for most routes, its a little disappointing that its not seen more often.

There are also practical difficulties. It can't be done with a hub motor with a freewheel, and I expect anything driving through derailleur gears would object too. It would work on direct drive hub motors, but they are seen more on homebuilt specials than production bikes.

Nick

 

[flecc]

Hi Flecc,

Those are some interesting points! Do you have some direct links you could post so that I can do some further reading; they would suit very well for my introduction in my report.

I'm sorry I don't have any links for you James, just reporting an accumulation of past knowledge.

There's an important point that's often missed on both regeneration and regenerative braking on bikes, and that's a key difference with cycling. As a long experienced cyclist, I avoid using the brakes since it's a silly waste of expended energy. Instead I use the kinetic energy to enhance my progress which is much more efficient than any energy conversion process. To use this to best effect entails knowing the conditions ahead, but since most cycling takes place on familiar ground that's not difficult.

For example, on one route returning home from the south I meet a short but steep downhill. I let the bike have it's head and reach 30 to 37 mph depending on what I'm riding, and this often carries me nearly half a mile along the following flat road before speed drops to motor drive speed.

Apart from the relatively loss free energy conversion, I get a rest from pedalling refreshing my following performance, and the fast downhill bit and early following speed is exhilarating which is also conducive to mood and performance, gains which are unique to cycling. The technical theory is great for larger vehicles like the Prius, but these human factors in cycling can't easily be factored into equations.
.

 

[frank9755]

As Flecc says, cyclists try to avoid braking; I know I anlways have and I was reading the other day that Chris Boardman apparently won a tour de france stage without ever touching his brakes!

However I agree there would be a little more scope for a utility electric cyclist in a hilly area.

 

[Blew it]

James.

Hi,

You may find a recent thread on the EndlessSphere forum interesting. It's about a cross Canada E-bike trek by Justin-le. Regenerative braking was just one of the many groundbreaking ideas being Beta tested on that journey.

Be aware, It's a very long thread running to 41 pages, I'm sure you will enjoy reading it at the same time gathering useful information.

Here is the LINK

All the best

Bob

 

[James1986]

Thanks! Am looking into it now.

From what everyone is saying, it is seeming like I have a bit more to think about. I'll be honest in saying that the only time I cycled as a kid was to school and back on a mountain bike, so I'm not very savvy with technique!

I think the main benefit of adding regen to my design is for the inner city cyclists who have to stop start all the time between traffic, or worse, pavement crossings. But yet again, I'm still at Uni and have never had to commute on bicycle. If, however, it is relatively simple to add the circuitry then I might as well add it, and concentrate on clever hub design to try and maximise other benefits, such as hill climbing, freewheeling, and other more desirable features.

What do you all think?

 

[MaryinScotland]

Maybe the difficulty is where to put that energy on the way down. I wouldn't have thought that a battery would like such a large charge in such a short time,

Maybe someone could come up with a mechanical energy store, equivalent to a clock spring (or even a rubber band!) that could be "wound up" on the descent, and release the stored energy at a more acceptable rate to the battery. Or, in a landscape that's all up-and-down, release the mechanical energy straight back to the wheel and bypass the electrics altogether.

Theories are fun. But for the more elaborate ones, I doubt whether anyone will invest the time and money needed, not only to make them work, but to make them light and cheap enough for cycling.

Mary

 

[tillson]

Maybe someone could come up with a mechanical energy store,

I think F1 teams are experimenting with this type of KE recovery using fly wheels or capacitors. I think it might even start to appear next season.