"John Bailey" <john_bailey@[EMAIL PROTECTED]
> wrote in message
news:bghbq399oh18qptvgfu5ol58ga55v2inpu@[EMAIL PROTECTED]
| 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.
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 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.
If you look at figure 46 of that do***ent 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.
|
