Ken (bcc) may be interested in my 'intuitive' description of the 
vehicle power-loss with speed formula. [Ignore the first part which 
deals with getting power from vehicles using roadside windmills].

_________


>> Taking power from cars while they are braking may make sense.
>> But, anything that generates power from a car's slipstream on a 
>> level
>> motorway is liable to be a bad idea overall.

>>> http://www.metropolismag.com/cda/story.php?artid=2466

> While I basically agree, I think there's a niche where you use 
> energy that
> otherwise gets wasted that doesn't have to be a bad idea. This 
> one -- if it
> works and if the energy to produce the generators isn't excessive --  
> seems
> to be in this category. You seem to think it's not?

My reasoning, possibly fallacious, is that anything that can derive 
useful power from a travelling vehicle is liable to increase the 
vehicle's energy consumption. While it *may* be that the energy is 
free and otherwise wasted, as "breeze" is necessarily generated by the 
vehicle regardless, Murphy says that if you effectively bring the 
airflow to a standstill, which is what an ideal wind turbine does (and 
a real one never will) then you are creating a greater air velocity 
gradient than otherwise exists and thereby taking  extra energy from 
the travelling vehicle. As this energy is provided (in most cases) 
from hydrocarbon fuel which is carried by the vehicle and converted 
with limited efficiency (about 20 to 30% perhaps) and as the wind 
turbines are also rather less than 100% efficient, then you are 
powering them in a way which is liable to be ecologically 
unproductive.

_________________

For interest.

    Drag = 0.5 x Rho x Cd x A x V^2.

Rho = air density = 1.3 kg/m^3.
A = frontal area m^2.
V = velocity m/s
Drag = force in Newton.
Cd = drag coefficient.

Power = force x distance.
=  Drag x V

= k x A x V^3

    k = prior constants combined.

This gives your power loss due to drag and may be used to play all 
sorts of interesting games.

The drag formula can be derived from first principles by working out 
the energy required to accelerate air from the path of the vehicle to 
vehicle speed as it is forced out of the way by the travelling 
vehicle. Nobody ever explains it that way, but plug in the figures and 
that's what you get :-). And it makes sense intuitively.


Cd is unity for a flat plate and suitably less as you produce a shape 
which moves the air aside without having to bring it up to vehicle 
speed.



            Russell


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