Would a wind turbine on a moving car generate energy efficiently?

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A miniature wind turbine mounted on a moving car would not generate usable energy efficiently due to the laws of physics, particularly the conservation of energy. The turbine would create drag, requiring the car's engine to expend more energy to maintain speed, resulting in greater fuel consumption than the energy produced by the turbine. Even if the turbine were optimized for aerodynamics, it would still not produce enough energy to offset the additional drag it creates. The discussion also highlights that any energy generated would ultimately come from the car's kinetic energy, reinforcing that the concept resembles a perpetual motion machine, which is impossible. Overall, the consensus is that a wind turbine on a car would be counterproductive.
Xtensity
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I was talking on another forum about an idea someone proposed. They said if you put a miniature wind turbine as a Hood Ornament on a car, as the car drives, it would produce extra usable energy.


I said this would not work as if the wind is not blowing at the correct angle and velocity then the wind turbine on the car would actually slow it down to a minute degree. Meaning, the friction between the air and the wind turbine would slow down the car(to a very small degree) and the car would have to speed up to compensate for the loss of kinetic energy. The energy used to speed up the car to make up for the loss would be greater than the energy produced by the turbine meaning the energy consumption is being increased even more.

Who is correct and can you explain why? I've already tried to explain that the energy has to come from somewhere to move the wheel, and if it isn't coming from the non-moving air then it must be coming from the car.
 
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You have correctly explained it yourself, in useful terminology. Although elaboration is possible, it certainly isn't necessary. You're right; they're wrong. (Feel free to quote the Laws of Thermodynamics if you feel so obliged.)
 
Yes but I need someone to explain it in clearer terms. I have tried to explain it to the person (same as I did above) and they didn't understand me. I'm going to be linking them to this topic so if anyone else has a clearer explanation, feel free.
 
Tanstaafl!
 
Xtensity said:
Yes but I need someone to explain it in clearer terms. I have tried to explain it to the person (same as I did above) and they didn't understand me. I'm going to be linking them to this topic so if anyone else has a clearer explanation, feel free.

You could build a model.
Take the motor-generator I built this winter for example.

pf_motor_generator.jpg


On the left is the generator. Driving the generator via a fan belt is the motor.
When the fan belt was removed, the motor consumed 200 watts.
When the fan belt was installed, the motor consumed 300 watts.
This was just drag from the idling generator and belt.
If the generator was then hooked to a 40 watt load, the power consumed by the motor would probably go up another 100 watts.
The same thing is happening in your wind driven generators.
For every watt they generate, your vehicles primary engine is going to consume more than that watt. (Unless of course you turn off the car while driving or going down a hill and turn off the car, which would be unwise as power steering and power brakes would fail and you would probably die trying this experiment)

I was trying to explain the concept of regenerative braking to someone once, and they could not get it into their head that a generator under electrical load exerts a torque. Unless that torque is countered, the generator will quickly stop rotating.

The torque required to keep your friends little hood ornament fans rotating under load is called wind drag. That wind drag has to be countered by your vehicles primary engine.
 
Any energy you got from the generator could more efficiently be subtracted from the wind drag of the vehicle by increasing its aerodynamics. I.e. I think removing the hood ornament altogether would generate convert more energy into forward motion than it would deliver as electricity. If you want more electricity, why not just increase the output of your alternator and take the energy directly from the engine instead of first converting it into forward momentum and then getting it back from the wind drag?
 
Xtensity said:
Yes but I need someone to explain it in clearer terms. I have tried to explain it to the person (same as I did above) and they didn't understand me. I'm going to be linking them to this topic so if anyone else has a clearer explanation, feel free.

Wind turbines have a rather phenomenal amount of drag, and the amount of drag produced by the turbine will always outweigh the amount of energy you can get out of it.
 
Just quote the http://en.wikipedia.org/wiki/Conservation_of_energy" . When the car is standing still the turbine isn't moving. When the car is moving the turbine will move. So the energy for the turbine's movement comes from the car's movement. And because of the law of conservation of energy either the car will slow down a bit or will use more gas. If we would live in a frictionless and dragless world and you had a 100% efficient generator attached to the turbine you would still only get back the same amount of energy the car lost (in speed or gas). Now we don't live in such a world, so a portion of that energy will for example be lost to heat due to friction with the air and friction inside the turbine. So the turbine could never generate extra energy.
 
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One the most fundamental laws of physics states that this won't work. You can't make energy, you can just convert it from one form to another...and every time you do so, you WILL lose some. What your friend is proposing is a perpetual motion machine: something that creates more energy than it consumes. It doesn't work and it will never work. If he doesn't understand this then it's going to be very difficult to explain.

The main reason a car uses fuel in the first place is because of air resistance. If it wasn't there, you could just accelerate to speed and practically coast most of the way. A wind turbine on a car will increase air resistance, requiring more fuel. What's more, it will ALWAYS increase the air resistance by such an amount, that the car will require to burn more fuel than the amount of energy the turbine puts back in. Always. No matter what elaborate scheme you come up with...you can use turbines, magnets, slopes, gears, levers, thermocouples, you can split atoms and enlist all the forces of nature, and you will ALWAYS lose.
 
  • #10
There is one practical application—a turbine mounted on a stationary vehicle in the presence of a significant breeze. You could, for instance, stick one on the roof of a parked RV to power your TV or computer. In that case, there would be no parasitic losses to the vehicle.
 
  • #11
The total wind power incident on a stationary horizontal axis wind turbine (HAWT) is P = ½ρAv3 (Newton-meters/sec), where ρ is the air density, A is the frontal area of the HAWT, and v is the wind velocity. The maximum theoretical efficiency of converting this wind power to turbine power is 16/27 = 59% (called the Betz limit). because the air needs sufficient residual energy (velocity) to move away from behind the turbine after passing through the blades. See

http://www.wind-power-program.com/betz.htm

Actual HAWT maximum efficiencies are ~40 to 45%. So putting a HAWT on the hood of a car will produce more drag force (F = ½ρCdAv2, Cd is drag coefficient) than it produces in electrical power. See

http://en.wikipedia.org/wiki/Drag_(physics )

Bob S
 
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  • #12
What if the wind turbines were placed in such a way as to reduce drag by redirecting the flow of air, such as at the back of a vehicle. If the turbines shifted the air flow so that it reduced the low pressure behind the vehicle, couldn't the vehicle theoretically move forward more easily while the turbine motion also generated electricity?
 
  • #13
brainstorm said:
What if the wind turbines were placed in such a way as to reduce drag by redirecting the flow of air, such as at the back of a vehicle. If the turbines shifted the air flow so that it reduced the low pressure behind the vehicle, couldn't the vehicle theoretically move forward more easily while the turbine motion also generated electricity?
What you are describing is a fan, not a turbine. A turbine impedes airflow.
 
  • #14
brainstorm said:
What if the wind turbines were placed in such a way as to reduce drag by redirecting the flow of air, such as at the back of a vehicle. If the turbines shifted the air flow so that it reduced the low pressure behind the vehicle, couldn't the vehicle theoretically move forward more easily while the turbine motion also generated electricity?
This is a very interesting point. The power loss due to air drag by a vehicle is P = ½ρACvv3, where Cv is the coefficient of drag, and A is the frontal area of the vehicle. The object of streamlining a vehicle is to reduce Cv. If a wind turbine could be placed in a location such that the reduction in the vehicle air drag power loss was >= the output power of the turbine, then you could theoretically have turbine "efficiency" over 100%. Any comments?

Bob S
 
  • #15
Bob S said:
This is a very interesting point. The power loss due to air drag by a vehicle is P = ½ρACvv3, where Cv is the coefficient of drag, and A is the frontal area of the vehicle. The object of streamlining a vehicle is to reduce Cv. If a wind turbine could be placed in a location such that the reduction in the vehicle air drag power loss was >= the output power of the turbine, then you could theoretically have turbine "efficiency" over 100%. Any comments?

Bob S

Are the moderators ahead of the send action ?
Just had a post disappear as I started to hit send. Anyhow I'll just say, warmer air flowing through a turbine of some sort, converting energy and pushing cold air into the draft area behind the vehicle will reduce drag to a point that the turbine should power the vehicle and have a surplus energy.

I'm sure I could have said it better, but that does not happen often.

Ron
 
  • #16
Bob S said:
This is a very interesting point. The power loss due to air drag by a vehicle is P = ½ρACvv3, where Cv is the coefficient of drag, and A is the frontal area of the vehicle. The object of streamlining a vehicle is to reduce Cv. If a wind turbine could be placed in a location such that the reduction in the vehicle air drag power loss was >= the output power of the turbine, then you could theoretically have turbine "efficiency" over 100%. Any comments?

Bob S

The situation we're talking about involves 0 air movement. Meaning all of the energy MUST be coming from the cars kinetic energy as it moves forward. Even if drag and friction on the turbine were minimized, the turbine still could never produce 100% of the energy that it takes to spin it to generate a select amount of energy. If it somehow manages to generate more than 100% of the energy which it took to spin it, then you just made a perpetual motion machine.
 
  • #17
RonL said:
Are the moderators ahead of the send action ?
Just had a post disappear as I started to hit send.
No...
Anyhow I'll just say, warmer air flowing through a turbine of some sort, converting energy and pushing cold air into the draft area behind the vehicle will reduce drag to a point that the turbine should power the vehicle and have a surplus energy.

I'm sure I could have said it better, but that does not happen often.
Well it happens all the time in a jet engine, but a car driving down the highway is not a jet engine. A turbine by itself doesn't do any pushing and even in a jet engine is removing energy from the exhaust that would otherwise be propelling the plane.
 
  • #18
Xtensity said:
The situation we're talking about involves 0 air movement. Meaning all of the energy MUST be coming from the cars kinetic energy as it moves forward. Even if drag and friction on the turbine were minimized, the turbine still could never produce 100% of the energy that it takes to spin it to generate a select amount of energy. If it somehow manages to generate more than 100% of the energy which it took to spin it, then you just made a perpetual motion machine.

Perpetual motion machine is an exaggerated claim. If the vacuum forming behind the vehicle is increasing the drag, then it is already taking unnecessary energy from the engine. All this turbine would be doing is getting that energy back from the drag and reducing the drag at the same time. Probably it would be more efficient to just reduce the drag without the turbine, though, I'm guessing.
 
  • #19
There is more than one thing going on in respect to the car and the hood ornament windmill.
Wind blowing straight at a stationary car at the speed of 50mph will produce the same exact drag friction as if the car is moving at 50mph. Drag is calculated using the square factor.
Now consider a windmill, it is stationary and if a wind of 50mph blows straight at it, the energy of the wind is calculated using a cube factor.

In my mind, energy in the air motion affects, at the expense of drag on the car shell, can be two different values. Proper ducting and turbine design can extract this energy to a high level of efficiency.

My understanding is that a volume of warm air flowing at some velocity and performing work on a turbine of flow through design, will discharge that same volume at a colder temperature.
Will energy required to move a volume of air, not be proportional and related to it's temperature ? if so then that amount of work (heat) has to be transferred into some action. I would propose on an electric car a Regen system.

Ron
 
  • #20
I'm not sure what the power formula for a turbine spinning in air is. But, I'm going to go ahead and guess that it's similar to the formula for how much power a car needs to push that turbine through the air. In fact, the formulas are probably damn near identical.

There's probably an air pressure, an area, and a V^3 in there.

That said, I just don't see how any air pressure/ air deflection/ strategic placement trick would result in the power extracted by the turbine being greater than the power used to get it to spin.
 
  • #21
Try this simple intuitive thought experiment:

1) build any vehicle with any configuration of wind-driven electric generators.
2) disengage the generators so they they rotate freely without the friction of generating current.
3) measure how much drag is reduced by disengaging the generators.
4) compare X (the reduction in drag) to Y (the amount of energy generated when the generators are engaged).
5) Is Y greater than or equal to X?
 
  • #22
From Bob S
This is a very interesting point. The power loss due to air drag by a vehicle is P = ½ρACvv3, where Cv is the coefficient of drag, and A is the frontal area of the vehicle. The object of streamlining a vehicle is to reduce Cv. If a wind turbine could be placed in a location such that the reduction in the vehicle air drag power loss was >= the output power of the turbine, then you could theoretically have turbine "efficiency" over 100%. Any comments?

Xtensity said:
The situation we're talking about involves 0 air movement. Meaning all of the energy MUST be coming from the cars kinetic energy as it moves forward. Even if drag and friction on the turbine were minimized, the turbine still could never produce 100% of the energy that it takes to spin it to generate a select amount of energy. If it somehow manages to generate more than 100% of the energy which it took to spin it, then you just made a perpetual motion machine.
A spoiler on the rear of an automobile with some wind turbines in it may be more efficient (lower overall drag coefficient) than no spoiler at all. But then, the spoiler without the turbines would be even better.

Bob S
 
  • #23
Bob S said:
A spoiler on the rear of an automobile with some wind turbines in it may be more efficient (lower overall drag coefficient) than no spoiler at all. But then, the spoiler without the turbines would be even better.
Especially if the spoiler had neon lighting and green flames painted on it. Sorry, couldn't resist:)
 
  • #24
A windmill on a car could be useful if you had a cross wind and wanted to take advantage of it to supplement the energy delivered by the motor. Conceivably it could be designed so as to almost eliminate drag. You could turn it to forward to supplement braking and recover some of the car's kinetic energy in a battery, say. A novel twist on dynamic braking. But you cannot use the virtual wind created by the car itself, for the reasons already given.

AM
 
  • #25
Using a windmill to convert the car's kinetic energy when braking seems kinda not optimal. Using a flywheel (if you want to use it to accelerate again) or some kind of spring for that is much better.
 
  • #26
brainstorm said:
Try this simple intuitive thought experiment:

1) build any vehicle with any configuration of wind-driven electric generators.
2) disengage the generators so they they rotate freely without the friction of generating current.
3) measure how much drag is reduced by disengaging the generators.
4) compare X (the reduction in drag) to Y (the amount of energy generated when the generators are engaged).
5) Is Y greater than or equal to X?
Let me give a real-world example that may help you out:

All cars have a radiator and grille in the front. In a lot of cars there is an air scoop to direct air to the radiator. Flow through the radiator (and then the engine compartment) is awfully draggy. Virtually any obstruction to that flow would would reduce drag. So if you placed a turbine in the airscoop, less air would get to the radiator, so the drag of the car would decrease. But a sheet of plywood across the airscoop would do a better job of blocking the airflow and would therefore decrease drag even more: and more importantly, the sheet of plywood would save more energy than the turbine could generate.

In other words, there is nothing you can do with a turbine to reduce the drag on a car that couldn't be done better another way.
 
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  • #27
Ducted fan design using tandem heat pump principle (or triple) sucks the car forward and at the same time pushes forward using a cold air discharge thrust.

One only needs to design for a air volume flow and BTU extraction equal to power needed to sustain motion. And as ErikD mentioned a flywheel has a place in the mix.

Later

Ron
 
  • #28
Sooo...use a heat pump to reduce the efficiency of a ducted fan in a sort-of inverse jet engine arrangement? Why would anyone want to waste energy in such a way?
 
  • #30
Om, there was that discussion but there could have been so much more:smile:

Russ, before I embarrass myself too much, let's see if what I think is right.

A 1500 watt heat strip will radiate 1500 watts of heat, giving a 1:1 ratio, now if a heat pump with a COP of 10 will receive a 1500 watt input and return a heating value of 15,000 watts (in principle) is this basically correct ?
If that heat pump draws heat from an atmospheric temperature of 90 or 100 degrees will it's efficiency be better or is that COP based on quantity of refrigerant and cycle time and system size ?

Is the 15,000 watt equivalent heat from the COP 10 heat pump the same as a 15,000 watt heat strip of 1:1 ratio ?

If I'm correct or even close, I'll say a little more about my comment above.

Thanks
Ron
 

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