Windturbine-type structures on moving vehicles?

[apratim.ankur]

why don't we see wind harnessing structures (like tiny fan-like structures) mounted on vehicles to capture the relative motion of the wind against the moving vehicle?
I had one guess that mounting such structures would increase the surface area of the vehicle and thus the air drag acting on it, leading to a greater rate of consumption of the fuel, such that there would be no net gain of energy. How good is this guess?
But, what if such structures could be built without increasing the frontal surface area of the vehicle (on which the wind is acting)? Like for instance, what if instead of a rigid/fixed frontal part of a vehicle (say the windscreen of a motor-bike), we have some moving parts there (like small wind-turbines, enhanced with piezoelectric material blades etc.) ; won't this produce a little extra energy? or maybe significant energy at high speeds??
I guess this is not violating any known physical laws, as the kinetic energy of the moving air relative to the vehicle that was previously being converted into heat on collision with the vehicle's surface is now instead being used to drive the tiny turbines and strike the piezoelectric material to produce electrical power. Is this correct? Or am I wrong somewhere?

 

[russ_watters]

There are places on a car where blocking airflow would improve fuel efficiency(such as air flowing through the engine compartment), but you would always gain more fuel efficiency with a flat plate to completely block the airflow than you would gain with a wind turbine (in drag reduction and energy production).

Blocking airflow toward the windshield wouldn't help because it doesn't actually change anything for the better.

 

[apratim.ankur]

...you would always gain more fuel efficiency with a flat plate to completely block the airflow than you would gain with a wind turbine (in drag reduction and energy production).

why?
as the rate consumption of the fuel depends only on the air resistance (which depends on the frontal area of the vehicle) and friction from the ground; why would fuel efficiency be greater if both these aspects are kept same?

 

[Jobrag]

"as the rate consumption of the fuel depends only on the air resistance (which depends on the frontal area of the vehicle) "
And the nature of the surface smooth is better than rough.

 

[apratim.ankur]

yes perhaps, because air resistance would be greater for a rough surface as compared to a smooth one.
but isn't this fact irrelevant to the case being discussed?

 

[russ_watters]

No. There is significant airflow through the engine compartment and a lot of drag associated with it. A few cars even have shutters to block some of this air when it is cold outside, which increases fuel economy notably.

 

[xxChrisxx]

It may be better to think of this as, the amount the air gets chewed up by the car/object moving through it. The less 'messy' the air is behind the car, the less energy we have lost in moving it. You can imagine a little windmill, churning up the flow as the wind flows across its blades will create a larer disruption than a plate moving through the air.

In engineering speak.
The Cd of a moving turbine blade would be the same or higher than a plate of the same swept area.

If you are inclined can actually use windturbine calculations for this for the power generated by a turbine vs the power lost by pushing a circular plate through the air. This holds becuase all the matters is the relative air speed.

If the air is moving past a static turbine, it's doing the work.
If you move a turbine thought the air, you are doing the work.




Now a duct with a flap on the end that lead to a turbine, would give a benefit. As you could close the flap for low drag, then on overrun you could open it and let air flow o recapture energy. This would never be used for two reasons.

1. Regenerative braking can capture more energy.
2. Ducting the air to a low pressure region behind the car to reduce drag would be more beneficial.

 

[sophiecentaur]

There really isn't any juice in this particular lemon. Any system for 'harvesting energy' in order to provide more motive energy smacks of 'perpetual motion machines', which are a non starter.
The best thing to do, as always, is to try to make the body shape as efficient as possible.
I guess it may be possible to reduce drag by strategically introducing some 'active airflow' around the vehicle. But the energy for this would be far better obtained directly from the fuel and not from a turbine system which would only introduce inefficiency.

 

[berkeman]

And just to remind the OP, PMM mechanism discussions are not allowed on the PF. Here is the quote from the Rules link at the top of the page, along with links for more reading that you can do if you want to learn more about the subject. This thread is closed.

Pseudoscience, such as (but not limited to):

Perpetual motion and "free energy" discussions
http://wiki.4hv.org/index.php/Free_Energy_Debunking
http://en.wikipedia.org/wiki/Perpetual_motion
http://www.skepdic.com/freeenergy.html
http://www.skepdic.com/perpetual.html