John Bailey wrote:
> On Sun, 03 Feb 2008 11:52:51 GMT, "Androcles"
> <Headmaster@[EMAIL PROTECTED]
> wrote:
>
>> "RichD" <r_delaney2001@[EMAIL PROTECTED]
> wrote in message
>>
news:fc536e3e-6d54-45fc-a36a-31c432e32c58@[EMAIL PROTECTED]
>> | 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.
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
> proof, but that would imply something like 40% of the KE absorbed by
> the regenerative brakes could be recovered.
>
> An Analysis of Hybrid Electric Propulsion Systems for Transit Buses
> Milestone Completion Re****t 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.
|
