Electric car with a windmill
Sam suggests that he can make his electric car run a greater distance by installing a windmill on top of it. His theory is that when the car is running, the air blowing against it will help the windmill generate electricity and send it back to the battery.
If it's a calm day and there is no wind around, then will his theory work?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
7 solutions
if angle is right, the air going across the blades will make them spin. just like a helicopter rotor spinning backwards as if falls.
I don't get "There is no energy to be harvested from the air, since the air is static." The air is static wrt to the ground and is moving opposite to the car wrt to the car.
The air exerts a force and rotates the windmill and the windmill converts the kinetic energy of the air into the electrical energy, isn't it?
Log in to reply
I suppose an analogy would be a normal car which moves because the ground pushes on its tires. Even so, the energy of motion ultimately comes from the fuel stores within the car, and not from the ground.
Log in to reply
Well, the ground does not have any energy to give to the car, but the wind blowing wrt to the car can hit the blades of the windmill and rotate them and pass their energy to the windmill.
Log in to reply
@Rohit Gupta – I think we're getting at an interesting question here. We know that Galilean Relativity applies to motion, but does it apply to energy as well? Could a stationary object harvest this "wind energy"? If we conclude that a stationary object cannot harvest it, I'd say it's a fair bet that it's not the wind's energy at all. I think you actually explained to me once (with the normal car example) that since the instantaneous speed of the bottom of a car's wheel is zero relative to the ground, the ground is doing no work on the car. Hence even though the ground supplies the force which moves the car, it's actually the energy from the fuel stores which ends up as the car's kinetic energy. Seems paradoxical.
Log in to reply
@Steven Chase – I agree that the ground is not doing any work on the car and not supplying any energy to the car because in the ground frame the point of application of force that is the bottom most point of the tire is momentarily at rest.
About the wind energy, which is a relative term, is certainly non-zero in the car's frame of reference, and hence has the ability to do work on the blades and pass its energy to the windmill.
In the car's frame, the car is at rest and the wind is moving from its front and hits the blades. The blades are designed in a way that they deflect the incoming air and experience a force due to which they rotate. The rotational kinetic energy of the blades is gained by the kinetic energy of the air and this energy is passed to the battery.
Log in to reply
@Rohit Gupta – If the relativity principle applies to energy, yes. I'm not saying you're wrong, but I want to poke at that idea a bit. Let's say you have two objects traveling in the same direction. One has a speed of 10 and the other has a speed of 30 (relative to empty space). If you were to calculate the total kinetic energy using space as a reference, you would get something proportional to 10^2 + 30^2 = 1000. If you were to take the v = 10 object as reference, you would get something proportional to 0^2 + 20^2 = 400. If you were to take the v = 30 object as reference, you would again get 400. And if the one object sped up from 10 to 15 (relative to space), the total kinetic energy would increase if measured with respect to space, and decrease if measured in the relative frame. What do we make of this?
Log in to reply
@Steven Chase – I don't get what you are trying to convey here. I understand that the kinetic energy and work done are frame dependent.
Log in to reply
@Rohit Gupta – That was basically just a an example of said frame dependence.
So clearly in a scenario where the car is cruising the whole way, the windmill is a net loss, due to friction, etc. However, if we're considering reality anyway (by excluding frictionless equipment), cars usually have to accelerate and decelerate (or even stop and go) in real world travel. What if the windmill had controllable pitch, so that it could convert kinetic energy to electric energy when he needed to slow down anyway (instead of using brakes) and be feathered for minimal drag and no power generation the rest of the time? Such a design would be far too complicated to be practical, but there are simpler designs to efficiently convert kinetic energy to electric to slow a car down, save on brakes, and store a portion of the energy for the re- acceleration (some trains use them and there's been talk for years of putting them on cars). If the windmill could be put to that use with sufficient efficiency, the car would go further than a car without the windmill driving the same profile.
Log in to reply
Indeed. I believe that's called "regenerative braking". I think Ford has a truck that uses that technology.
How is the car "The car is just converting part of its own kinetic energy into electrical energy".? Can you please explain?
Log in to reply
What would happen if the car just turned off its engine and coasted (assuming that it started with some speed)?
Log in to reply
It would stop.....eventually...
Log in to reply
@Usman Khan – And the windmill would sap away some of the energy, acting as a load.
Log in to reply
@Steven Chase – Thanks for the answer, Sir. Appreciate it. Assalamu Alaikum
@Steven Chase – Lol. I l remember my question again....Yes, the windmill would sap away some of the kinetic energy of the car since it is acting as a load..That will slow down the car and hence it will travel a shorter distance....... But how is the car converting its own kinetic energy into electrical energy of the battery..?
Log in to reply
@Usman Khan – Some of the car's kinetic energy turns into the windmill's kinetic energy, via fluid forces. Then the windmill turns a magnet near some coils of wire, acting as an electric generator. An electric motor consumes the electric power from the generator, and turns it back into kinetic energy of the car. Of course, I'm leaving out exactly how each step works, but those are the broad points.
Log in to reply
@Steven Chase – Ok ok, you've given me very useful information...Thank you again.
@Steven Chase – Also since the car is moving and the fan is moving with the car, the fan exerts a force on the air molecules which in turn apply a force on the fan which should over the fan to generate some extra electricity... Wouldn't that help?
@Usman Khan – drag from windmill, weight and drag from support structure.
Based on the information given "Sam suggests that he can make his electric car run a greater distance by installing a windmill on top of it." So it should already be assumed that Sam's car already has an internal power source i.e. Engine, solar panels batteries etc and thus would be increasing the efficiency of the existing engine. As a result then driving the car a further distance think of a combine cycle power plant. How ever if the wind mill is the only power source then yes it would be a shorter distance
The description is incomplete... ditching friction and having perfect storage of energy, the car will drive the same distance.....
Log in to reply
The drag forces can not be neglected in this scenario.
While added weight and drag are factors, there is lift generated by the fan and pull generated by the pressure differential along the blades' edges that compensate. It could also act as a flywheel which would tend to improve efficiency. A lot would depend on the design, which is why I said it had no net effect.
Log in to reply
When the car moves in a stationary air, the blades of the windmill hits the air molecules and accelerates them. The air molecules get the energy from the car, thus the windmill is actually leaking the energy of the car to the air.
This is not possible to answer because there isn't enough information provided to solve it. Reasoning: It depends on the efficiency of what is being used to convert the kinetic energy of the fan to the battery and whether it overcomes the drag created by the fan. In other words regardless if the air is static or as hard as a hurricane if the power plant can't efficiently covert that power to charge a battery it doesn't matter.
Thinking practically the whole system (car + windmill)have increased mass ie they would need more kinetic energy , secondly due to installation of windmill there would be a greater drag resistance offered while moving so as a whole energy would be lost against the wind ressistance (due to windmill) and due to increased mass of the system
If the windmill mass is small relative to the car then the drag component would be reduced
Log in to reply
Well, there are two energies to compare, 1) the energy lost by windmill due to the air drag, 2) the electrical energy produced by the windmill.
Won't we need to establish which is greater to ascertain if the car will travel a greater distance or not?
Log in to reply
What if the blades could bend to maximise the torque per cross sectional drag area?
We do need to establish which is greater, and that would be the energy that is lost: You cannot have a system which, solely under its own power, produces more energy than it uses. That leaves us with either the same distance or less. As no sytem is 100% efficient, the windmill will be losing energy as waste so will produce less energy than is used to turn it. This results in a net loss.
The fan would capture only a portion of the energy of the drag against the air it would create, and even this energy would be used with less than perfect efficiency. So net effect would be a loss of energy, not a gain.
What if the wind is also blowing?
Log in to reply
The problem should have probably specified that this is not the case. However, wind would generally be less intense than the air movement due to cars own movement, and it would be erratic in direction, so while helping part of the time, it would be counterproductive just as often.
Log in to reply
Marta, assuming the wind is in fact blowing, the car will actually go further. See https://www.youtube.com/watch?v=s4j-h4vVABo for an example of a wind powered car diving directly into the wind. "His theory is that when the car is running, the air blowing against it will help the windmill generate electricity and send it back to the battery." -> this statement is ambiguous about where the air is coming from. @Rohit Gupta
Log in to reply
@Wouter Steenbergen – Do you supposed it was his theory that started this problem in the first place?
Log in to reply
@Marta Reece – I have mentioned that the wind is not blowing with respect to the ground.
Log in to reply
@Rohit Gupta – So you have and it is appreciated. What if discussion can be entertaining but is not pertaining.
It seems like we are missing one thing. The car will travel the same distance, with or without the windmill, depending on how much gasoline is available. The car is powered to move forward by gasoline, not by the battery.
Log in to reply
It is an electric car, so it is powered by a battery.
Now that is a good understandable answer.
Out put is always less than Input
In this case, car is running with the help of fuel & fan move due to moving of car, generate electricity and feed back to battery, during each steps there is loss of energy.
But, the windmill is also generating the extra energy due to the air moving against it? Even if some of the generated energy is lost, still we will gain some energy from the windmill, isn't it?
Since there is no independent wind, the wind mill that has added mass to the car system is driven by the the car battery in essence. The inherent losses in the wind mill further drain the battery
There is no wind wrt to the ground but the air is moving opposite to the car with respect to the car.
i guess if the windmill is at 90 degrees wrt car then the drag would be comparatively less than its originally shown in the picture....
But then the blades of the windmill won't rotate to produce electricity effectively.
I understand that this will not improve efficiency while the car is accelerating or at constant motion. However, would it be possible to only allow the windmill to work while the car is braking? (Eg withdraw it the rest of the time). And if you do this, would you not take some of the car's kinetic energy and turn it into useful energy in the battery? In doing this you don't waste all the energy being converted into heat in the brakes, and therefore increase efficiency overall.
Nice, this seems to be the most efficient way to use the windmill. The windmill can be used to bring the car to a halt more quickly, and it can generate energy to power the car later. It cannot completely substitute regular brakes though, it becomes ineffective when the car is moving slowly.
But if the car is traveling, won't it's motion create enough wind or spin for the windmill?
The car is just converting part of its own kinetic energy into electrical energy (with losses), and then converting the electrical energy back into kinetic energy (with losses). There is no energy to be harvested from the air, since the air is static.