efficiency of regenerative braking?

What is the efficiency of regenerative braking in electric
vehicles? How much kinetic energy is recovered?


--
Rich

"RichD" > wrote in message oups.com...
| What is the efficiency of regenerative braking in electric
| vehicles?

High.

How much kinetic energy is recovered?

Almost all. Losses are in heating only, brakes heat a lot,
cables, motors and batteries very little.

> | 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

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.

Graham

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?

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


Bret Cahill

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.

> RichD wrote:
> > What is the efficiency of regenerative braking in electric
> > vehicles? How much kinetic energy is recovered?
=============================================
Lets say you have a 20 HP/15KW EV with a 144V battery pack cruising at
50% pwm. The motor would have about 72V applied and we can assume if
the controller was disconnected and the generated voltage read while
coasting, it might be 50 or 60V... certainly less than the applied
voltage and less than the battery stack. If you just attach this to a
cap bank, it will be like 'shorting' the armature, so the braking
effect will be whatever torque the motor puts out at that V and I. I
thought you could control this braking effect by putting a bike pedal
toe cup on the accelerator and pulling back on the accelerator to
control the regen pwm 0-100%. Of course, the braking effect decreases
as the motor and car slows and as the cap bank charges. After braking,
an dc to dc step up can charge pump the braking capture caps up to
144+V to go back in the batteries. My main question remains.... if the
motor is generating 70V, should the caps in the regen capture bank have
the same voltage rating?

Bret Cahill wrote:

> 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.

The motors don't run "backwards" but are used in reverse current flow
to charge the HV battery via an ac/dc converter that is managed by the
HV ECU. Regeneration occurs whenever the various ECUs communicate to
the HV ECU that charging is the correct strategy, be it during braking,
extended battery-only operation, startup, or whatever else I'm
forgetting.

Toyota MDT in MO

On Sun, 17 Dec 2006, RichD wrote:

> What is the efficiency of regenerative braking in electric vehicles?
> How much kinetic energy is recovered?

The new stock being planned for the shallow lines of the Lodnon
Underground will have regenerative braking; the press release [1] says:

"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."

tom

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

--
THE DRUMMER FROM DEF LEPPARD'S ONLY GOT ONE ARM!

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. Shows how it can make a big difference in stop and
go city driving.

Bruce