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regenerative braking

Discussion in 'Misc Electronics' started by RichD, Feb 3, 2008.

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  1. RichD

    RichD Guest

    What is the efficiency of regenerative brakes?


    Let's say we have a 3000 lb. vehicle, traveling 30 mph.
    It hits a red lght. That's 1350000 lb-(mi/hr)^2
    kinetic energy, dissipated through the disc brakes.

    Now assume it's a Prius - how much is recovered
    into the batteries?

    I'm not looking for a theoretical discussion, just a
    number. Anybody know the number?
     
  2. Androcles

    Androcles Guest

    | What is the efficiency of regenerative brakes?
    |
    |
    | Let's say we have a 3000 lb. vehicle, traveling 30 mph.
    | It hits a red lght. That's 1350000 lb-(mi/hr)^2
    | kinetic energy, dissipated through the disc brakes.
    |
    | Now assume it's a Prius - how much is recovered
    | into the batteries?
    |
    | I'm not looking for a theoretical discussion, just a
    | number. Anybody know the number?
    |

    80%. Friction with air still causes some braking but most
    of the KE is recovered and stored as useful energy to get
    going again. Remember that no energy is required to continue
    in a vacuum, so all the energy a car uses is wasted as heat
    directly through the exhaust pipe and radiator and the rest
    warms the atmosphere through friction.
     
  3. Androcles

    Androcles Guest

    | On Sun, 03 Feb 2008 11:52:51 GMT, "Androcles"
    |
    | >
    | >| >| What is the efficiency of regenerative brakes?
    | >|
    | >|
    | >| Let's say we have a 3000 lb. vehicle, traveling 30 mph.
    | >| It hits a red lght. That's 1350000 lb-(mi/hr)^2
    | >| kinetic energy, dissipated through the disc brakes.
    | >|
    | >| Now assume it's a Prius - how much is recovered
    | >| into the batteries?
    | >|
    | >| I'm not looking for a theoretical discussion, just a
    | >| number. Anybody know the number?
    | >|
    | >
    | >80%. Friction with air still causes some braking but most
    | >of the KE is recovered and stored as useful energy to get
    | >going again. Remember that no energy is required to continue
    | >in a vacuum, so all the energy a car uses is wasted as heat
    | >directly through the exhaust pipe and radiator and the rest
    | >warms the atmosphere through friction.
    | >
    | Is that a fact and reason based answer or just a guess? A battery is
    | not as efficient as a capacitor and there is a theorem from sophomore
    | EE that "proves" no more than 1/2 the energy stored in a capacitor can
    | be recovered.


    Sounds like a sophomore's proof.

    What you will not be able to recover is
    a) the heat lost to resistance.
    b) radiated energy.

    I mention the second because it is less obvious than the first.
    http://www.androcles01.pwp.blueyonder.co.uk/AC/oscillator.JPG
    http://www.androcles01.pwp.blueyonder.co.uk/AC/AC.gif

    One improves the efficiency of radiation by adding the correct length
    of antenna to the circuit above and then you call it a transmitter.


    More personal research would be needed to recover the
    | proof, but that would imply something like 40% of the KE absorbed by
    | the regenerative brakes could be recovered.

    If the objective is to burn fuel then cars are 100% efficient.
    If the objective is to convert chemical energy to mechanical
    energy then cars are 18% efficient, measured as lifting its
    own weight (and that of its passengers) against gravity.
    By driving the car off a cliff most of that energy can be
    recovered. If the car is used as a pile driver that would
    be useful work.
    A brake does not assist the car in doing useful work.
    The question asked was:
    " What is the efficiency of regenerative brakes? "

    Firstly we have to decide if stopping the car is efficient,
    because clearly if that is the only purpose then a friction
    brake is 100% efficient for succeeding or 0% efficient
    for wasting useful kinetic energy. Indeed, locking the brake
    will result in no heat loss at all, that will be transferred to the
    tyres skidding against the road surface, but few would
    call that efficient.
    A regenerative brake returns some of the energy to the
    battery without the corresponding heat loss and is therefore
    82% efficient, the other 18% being lost to air resistance.
    This can be improved upon by streamlining all cars to
    look like aircraft. Of the 82%, 1-2% will be lost heating
    the cables between the brake and the battery.
    The figure of 80% is necessarily approximate since
    an identical brake fitted to a different vehicle will change
    the overall efficiency.

    |
    | An Analysis of Hybrid Electric Propulsion Systems for Transit Buses
    | Milestone Completion Report by O'Keefe and Vertin of the
    | National Renewable Energy Laboratory
    | http://tinyurl.com/292xyt gives the efficiency of regenerative braking
    | as 49-50% at best and 39% as current practice.

    If you look at figure 46 of that document you'll see many variable
    parameters including vehicle dimensions, location, time of year etc.
    and we can argue the efficiency of brakes on roller coasters in the
    Swiss Alps or kids burning rubber and doing wheelies on motor
    cycles or the space shuttle deploying a parachute on landing.
    http://static.howstuffworks.com/gif/space-shuttle-landing3.jpg
    Regenerative braking on Airbus 308 would not be efficient, lifting
    batteries to 30,000 feet is a waste of fuel and it doesn't have
    electric turbo fans to recover the energy.

    A Prius is not a transit bus and a transit bus is only efficient when
    fully laden, it serves no purpose when driven around empty.
     
  4. tadchem

    tadchem Guest

    Hard data is ever harder to find. Back-of-the-envelope estimates seem
    to be running at up to 20%
    http://geekbuffet.wordpress.com/2007/06/10/getting-back-what-you-put-in/

    Tom Davidson
    Richmond, VA
     
  5. Sam Wormley

    Sam Wormley Guest

    Depends on the breaking rate... to fast and a large
    part is dissipated as heat.

    http://www.wired.com/cars/energy/news/2004/05/63541
     
  6. And another calculation gives an answer of 80%? Makes me very confident of
    the accuracy.
     
  7. And another calculation gives an answer of 80%? Makes me very confident of
    the accuracy.[/QUOTE]

    I had earlier heard of estimates in the 80% range (from theory), so
    this sounds good to me. I would also assume that it depends somewhat
    on driving style. Locking brakes does not provide good efficiency. I
    believe all the hybrids have hydraulic backup, don't they?
     
  8. Battery is storing energy chemically at almost constant voltage,
    therefore proof you are talking about does not apply. In fact
    a traditional 18650 Li-ion battery at low current rate (say C/10 rate)
    can be charged/discharged at 98% energy efficiency.
    Main energy loss is due to IR drop both during charge and discharge,
    which declines with I^2. E_loss = I^2*R

    Unfortunately during regenerative breaking currents are very high
    so batteries have to be designed with very low R (that is why
    only Panasonic and Sanyo can make suitable for HEV NiMH batteries).
    But even using the latest version of Panasonic NiMH batteries with
    further reduced R, 80% efficiency of both energy recovery + subsequent
    reuse (which both have losses) appears very optimistic. I would bet on
    60%, but I would also appreciate somebody posting the real number.

    Regards,
    Evgenij



    More personal research would be needed to recover the
     
  9. Sorry man, it might not be obvious, but this effect is true, easily
    provable based on the differential equation describing capacitor
    discharge and known to every electric engineer:
    http://www.smpstech.com/charge.htm

    As other people have stated earlier, this applies only for straight
    linear capacitor / resistor systems without inductance.

    Switching systems with inductance is a different story and they can
    be 90-95% efficient.

    It does not apply to batteries because they are governed by different
    differential equations. Their recharge can be approximated as as highly
    non-linear capacitors, and because of this non-linearity can be shown
    that they can be much more efficient than straight capacitors (mostly
    because they maintain almost constant voltage).

    Regards,
    Evgenij



    Even
     
  10. Guest

    It depends on how fast you decelerate.

    Energy from an emergency stop is not worth it.

    But if you only have to decelerate a bit to take a turn and you start
    breaking in time then you can get most of the energy back.

    My sail car design uses a 250 watt motor as electrical assist with
    cycling. It looks to me like my 4 wheeled bicycle is going to do a lot
    more breaking then a conventional bike. Hitting the regen-breaks would
    probably take some meters to stop the thing with such small motor.
    Most electric motors can take big surges over a short duration, should
    be the same when working as a generator. Then the batteries and the
    capacitors determine how much current one can store.

    http://wind-car.go-here.nl
     
  11. Dear John Bailey:

    ....
    But we don't have a resistor, we have an inductor adding or
    removing kinetic energy from the system. The dielectric of the
    capacitor has losses, and the conductors do too. But this can be
    brought to less than 20% losses without too much difficulty.

    http://www.ecass-forum.org/eng/faq/faq001.html
    https://www.electrochem.org/dl/ma/201/pdfs/0233.pdf

    The deal breakers will be charge / discharge rate (aka. current).

    David A. Smith
     
  12. RichD

    RichD Guest


    ***************************************
    Toyota engineer Dave Hermance said drivers who
    slowly roll through intersections using "California
    stops" are decreasing their mileage. "If you don't
    stop, you don't get the free energy of regenerative braking."
    ***************************************

    How did Toyota get to #1 with such numbskull
    engineers?
     
  13. RichD

    RichD Guest

    Dear David A. Smith,

    Inductors do not source or dissipate real power.
     
  14. RichD

    RichD Guest

    Interesting. It is a schoolboy level proof.

    But the automotive circuit does not conform
    to the models discussed. The car battery is not
    charging/discharging a capacitor, but an
    electric motor/generator, which resembles a
    R-L circuit with a back emf (i.e. another battery).

    However, it might still partially apply... how to
    model charging a non-ideal battery? Does it
    look capacitive?
     
  15. Sam Wormley

    Sam Wormley Guest

    Hopefully that engineer was joking!
     
  16. HLS

    HLS Guest

    Inductors do not source or dissipate real power.
     
  17. RichD

    RichD Guest

    "So how does my exam look, Doc?"
    "You're fine, so I decided to inject you with AIDS."
    "You did what?!?!"
    "Well, how do you expect to enjoy the benefits of
    modern medicine if you're healthy?"
     
  18. I would not downplay the generality of this relationship. It has
    nothing to do with complexity of the system.
    It generally applies to arbitrary systems that consists of (as many
    as you want, and arbitary connected) resistors and capacitors, and
    arbitrary path of discharge (e.g. it does not have to be constant current).
    As long as both capacitors and resistors are linear, you can not beat
    this rule.

    The only way to beat it is to add inductors to the mix, or to make
    capacitors non-linear (e.g. batteries).
    It is important to understand this conceptually to avoid some
    attempts to beat the rule by making some complex systems (but without
    inductors), which would be a bit like trying to beat the Karnot's
    efficiency law by using alcohol instead of water in a boiler.

    Regards,
    Evgenij
     
  19. Benj

    Benj Guest

    Great 'theorem"! Tell you what. I've got a real nice large HV
    capacitor. To prove your "theorem" I'll charge it up to a nice voltage
    and then use a resistor to remove half the energy stored in it. At
    that point we'll apply the terminals to your ass.

    Moron.

    And Guys! Please leave your liberal educations behind. In Science and
    Engineering Quantity Matters! No, Virginia, the world CANNOT be saved
    by windmills!

    One key parameter in regenerative braking is the rate at which energy
    produced and returned to the storage system. If you are going 60 and
    just slow down, you can put large percentage of the energy back into
    the system (except for the friction and other losses you are
    overcoming, natch) but if you come up to a read light and lock the
    wheels, you get next to nothing back. Even with rapid stops there is
    the question how rapidly and efficiently batteries can be charged at
    high currents even assuming the regenerate system is capable of those
    currents. Physicists trying to do engineering! This is a hoot!
     
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