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Regenerative Braking and Kinetic Energy PDF Print E-mail
Written by Greg Allen   
Sunday, 13 January 2008

Recently there has been a lot of focus on regenerative breaking and kinetic energy in hybrid electric vehicles.  A basic explanation of regenerative breaking is converting the cars motion into useable electric power.

In the conventional brake systems on cars now, friction is used to counterreact the forward motion of the Toyota Priusmoving car.  When you want to stop the car and press on the brake, the brake pads rub against the wheel or disc and the car slows down. There is a lot of heat that is created from this and it dissipates into the air wasting around 31% of the car's generated power. Currently, over time, the cycle of friction and wasted heat reduces the car's fuel efficiency and more energy is required from the engine to replace the lost energy from braking.

With the new hybrid cars, as the wheels turn, the generated energy is fed into an ultracapacitor for storage and recharging the battery.  The vehicle will use the stored energy when going up a hill or passing another car. Today's hybrids capture only about half the energy produced this way.  Ultracapacitors can help absorb a much higher percentage of this regenerative braking energy.

As an example, let's look at the Toyota Prius.  It allows its engine to operate at its most efficient power and torque by adding the driving force of an electric motor when more power is needed.  The excess power from the engine can charge the battery.  The electric motor compensates for peeks and dips in performance.  When you take your foot off of the accelerator, the engine switches off and when you press the accelerator, it comes back on instantly.  As you press the brakes, the engine shuts off and the kinetic energy from the car is converted by the motor that now acts like a generator to charge the battery.  When you put an ultracapacitor as a buffer between the battery and the generator, you can get hundreds of thousands of charge/discharge cycles extending this regenerative braking process.  A battery cannot withstand this sort of cycle like an ultracapacitor.

I must give credit to my friend Martin Lepselter for giving me a little nudge and suggesting I write about this.  He is an icon in the industry and I am privileged to talk to him and get emails on a regular basis. 

Readers have left 2 comments.
 No.1  Untitled
Take a look at AFS Trinity's implementation of this. Their entire design is based on ultracaps.

http://gristmill.grist.org/story/2008/1/13/101713/845
Guest (Unregistered) • 2008-01-14 15:46:58
 No.2  Untitled
I want to use a super cap and lithium ion battery along with regenerative braking in a product that uses significantly less energy than a car and has approximately 50% duty cycles of braking action and 50% duty cycle of electricly powering the motor.
I am speaking of DC motors in the 1 hp range.
Do you know of sources that would have controls for this type of regenerative system?
Thank you, Mike Wintermute, Indiana
Mike Wintermute (Unregistered) • 2008-12-21 09:51:35
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