efficiency of regenerative braking?

Bret Cahill wrote:
> > | What is the efficiency of regenerative braking in electric
> > | vehicles?
>
> > High.
>
> Isn't regenerative braking just the DC traction motors running
> backwards?
>
> An alternator that worked over a broad rpm range touted 70% efficiency
> in a recent patent.
>
> It would be interesting if they could beat that.
>
> Bret Cahill

As others here have pointed out, the motors don't run backwards, the
electrical current does. When DC electric motors run they also act as
generators. The voltage produced by the generator, sometimes called
the "back EMF" is proportional to the rpm. As the rpm goes up this
back EMF goes up and limits how high the rpm will go with no load on
the motor.

One way to make the motor charge the battery is to use a motor with
field windings and control the back EMF by controling the field
current. This same idea is used for the "voltage regulator" in many
automotive alternators.

Actually I think the most efficient systems in use today are varialbe
frequency AC systems. Many of the same ideas work and some are easier
to impliment with AC rather than DC.

Bruce

Bret Cahill wrote:

> Eeyore wrote:
> > Bret Cahill wrote:
> >
> > > > | What is the efficiency of regenerative braking in electric
> > > > | vehicles?
> > >
> > > > High.
> > >
> > > Isn't regenerative braking just the DC traction motors running
> > > backwards?
> >
> > No.
>
> How is the electricity being regenerated? By another component, say,
> an alternator or DC generator mounted on the same shaft as the traction
> motor?

It's called motor-generator action.


> Alternators generally aren't all that efficient off the design point rpm.

Traction motors aren't alternators. What gave you that idea about the rpm anyway
?

Graham

> What gave you that idea about the rpm anyway?

You know how to brake at just one rpm?


Bret Cahill

In article .com>, " > writes:
>
>R.H. Allen wrote:
>> RichD wrote:
>> > What is the efficiency of regenerative braking in electric
>> > vehicles? How much kinetic energy is recovered?
>>
>> According to Wikipedia, it's just a bit over 30%.
>>
>> http://en.wikipedia.org/wiki/Regenerative_braking
>>
>> Some other sources appear to quote higher values, but I think the
>> figures you find will depend on whether you're looking solely at the
>> efficiency of the motor during regenerative braking, the amount of
>> energy that ends up the batteries, or the amount of *recoverable* energy
>> that ends up in the batteries -- the figures for each will all be
>> different. It isn't clear to me which the Wikipedia article refers to.
>
>The answer depends on the details of the specific application.
>
>Figure the motor/generator itself is 90% to 95% efficient. That would
>mean that you would lose 5 to 10% putting the energy into storage, then
>another 5 to 10% converting it back to mechanical energy. If the
>battery pack is used to store the energy, it has internal resistance so
>some of the energy is lost as heat. Batteries are not very efficient
>for rapid charge and discharge cycles. As a guess let's say you get
>60% of what you put in back out, with the remainder going to heat. So
>you could be looking at 90%*90%*60%=48.6%
>
>>From that you could subtract maybe another 10% for loses in the
>controler, bringing it down to just under 40%. There could be a bit
>more lost if a DC to DC voltage converter is used.
>
>Using capacitors eliminates the battery losses, but they mean added
>complexity, cost and weight.
>
>My guestimate would be that a good capacitor system should be able to
>recover 70%, a good battery system 50% and any decent system at least
>30%.
>
>Something to keep in mind is the energy of a moving vehicle is
>proportional to the square of its velocity. So a car doing 30 mph with
>a 50% recovery system could get back up to about 21 mph on the
>recovered energy.

Well, you should keep in mind that the amount of kinetic energy
present at any moment is a very small fraction of the amount of energy
used overall. If you take a car with a mass of 1000kg going at 15m/s
(about right for city driving) its kinetic energy is a tad over 110
kJ, the equivalent of 2.5g of gas. That's thermal, of course,
factoring in efficiencies it may be the equivalent of 7-8g of gas (in
terms of delivered mechanical energy). And now you recoup about 30%
of this. Unless you spend all your driving accelerating like hell
from the light, then slamming on the breaks before next light, I doubt
you'll ever recoup more than a small fraction of what the system
costs. But, it makes a good selling point:-)

> Shows how it can make a big difference in stop and
>go city driving.
>
Some difference, hardly big.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

> Figure the motor/generator itself is 90% to 95% efficient.

Is it 90 - 95% efficient acting as a DC generator over a broad range of
rpms?


Bret Cahill

> > Isn't regenerative braking just the DC traction motors running
> > backwards?

> Simply put, yes. In the current hybrid system common to several
> manufactures, there are two motors in the transaxle. The larger motor
> is designated for providing the majority of motive torque. The smaller
> motor is used as a "starter" as it is attached directly to the engine
> output, and also functions to react to the larger motor to effectively
> create infinitely variable output gearing ("shifting"). To get to the
> point, both motors are either energized or tapped by the HV ECU to
> create torque or recharge the HV battery, respectively.

What's the efficiency of the motor-generator in generator mode?

Is it the same at any rpm?


Bret Cahill

> "The new trains will also have regenerative braking systems, which recycle
> energy that, on current sub-surface lines trains, is lost in braking.

> This will save about 20-25 per cent of that energy, making a real
> contribution towards tackling CO2 emissions."

Energy lost to friction braking is probably more than wind and rolling
resistance combined for a commuter train.

Still, it seems they could do a little better than 20 - 25% considering
economies of scale.

> [1] http://www.tfl.gov.uk/tfl/press-cent...t.asp?prID=961


Bret Cahill

Bret Cahill wrote:

> > What gave you that idea about the rpm anyway?
>
> You know how to brake at just one rpm?

What *are* you talking about ?

Have you gone mad ?

Graham

Bret Cahill wrote:

> What's the efficiency of the motor-generator in generator mode?

Probably in the same region as its efficiency as a motor but I have to say I'm
not totally sure.

> Is it the same at any rpm?

The same principle applies.

Graham

" wrote:

> Batteries are not very efficient
> for rapid charge and discharge cycles. As a guess let's say you get
> 60% of what you put in back out, with the remainder going to heat.

That's why super-caps are used to store the regenerated energy. No such loss.

Graham