On Sun, 3 Feb 2008 02:50:27 -0800 (PST), RichD
<r_delaney2001@[EMAIL PROTECTED]
> wrote:
>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?
20%
"Regenerative braking can be extremely powerful. According to Craig
Van Batenburg, who teaches Honda and Toyota hybrid service at
Automotive Career Development Center in Worcester, MA, no more than 17
percent of its capability is used in these cars to ‘avoid putting
people into the wind****eld.’ Even at that low level of use, in a
typical mixture of highway and around-town driving, regenerative
braking can recover about 20 percent of the energy normally wasted as
brake heat. This reduces the drawdown of the battery charge, extends
the overall life of the battery pack and reduces fuel consumption."
quoted from: Regenerative Braking Charges Ahead by Jacques Gordon in
Motor Age
http://www.search-autoparts.com/searchautoparts/article/articleDetail.jsp?id=68244
Don't believe anything from the web without citations, data, and
reasonable logic.
Checking further, it is somewhat suprising that regenerative braking
can even work much of the time. Batteries are not happy with large
surges of current, preferring a metered trickle, preferably controlled
by a microprocessor. The high tech solution to that is to add an
ultracapacitor.
from the same article:
"An ultracapacitor has a surface area that is several orders of
magnitude greater than conventional types, and the separation is less
than 10 angstroms (one angstrom is one ten-billionth of a meter). It
can hold a pretty big charge, and it’s voltage output and discharge
rate can be controlled with external circuitry. For instance, large
ultracapacitors are often used to provide hours of backup power for
computer systems after a general power failure. That is a relatively
long and slow discharge when compared to capacitors used to start a
motor. However, ultracapactors also can be used to deliver very high
voltage for a shorter period of time."
That then runs into Newburgh's theorems:
http://www.iop.org/EJ/abstract/0031-9120/40/4/008
Two theorems on dissipative energy losses in capacitor systems
Ronald Newburgh 2005 Phys. Educ. 40 370-372
doi:10.1088/0031-9120/40/4/008
"Abstract. This article examines energy losses in charge motion in two
capacitor systems. In the first charge is transferred from a charged
capacitor to an uncharged one through a resistor. In the second a
battery charges an originally uncharged capacitor through a
resistance. Analysis leads to two surprising general theorems. In
the first case the fraction of energy dissipated in the resistor
depends solely on the ratio of the two capacitances. The values of the
original charge and the resistance play no role. In the second case
half of the energy supplied by the battery is dissipated and half is
stored in the capacitor. The values of the battery emf and the
resistance play no role."
I seem to recall that Newburgh's theorems can be bypassed by using a
large inductance to limit the charging current. Perhaps a clever
engineer can figure out how to use the inherent inductance of one of
the motors to provide that ballast.
John
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