27 Apr 2012

Faster than wind wind powered vehicle! Violates Thermodynamics?

Engineers (even CEans) are leary of engines that seem to violate the laws of thermodynamics. Here is an example of a working wind driven vehicle that travels down wind faster than the wind driving it. I understand that it was actually demonstrated I am unable to explain this. I am sure that there is some rational explanation.

Here it is without any comments:
https://www.instructables.com/id/Downwind-Faster-Than-The-Wind-Cart/

Video-

7 years ago
bioramani : I had covered that vehicle long time ago. But I need to understand the question better before I can think of answers. Why should a wind powered vehicle necessarily run at speeds equal to (?!) or lower than the wind speed?

I mean, I'm not very clear how the speed of the wind (~ Kinetic Energy) is being related to the K.E. of the cart.

On the similar lines; I've always wondered the following -

Let's say like in hydro power plants; we setup a mechanism to rotate the turbine through water stream falling from height X. Then through gear train, we transfer the energy to the generator that can power a motor to pump the water back to height X.

I was told that the energy lost in the gear train & turbine would never be able to match the effort required to lift the water to height X.
7 years ago
The_Big_K
bioramani : I had covered that vehicle long time ago. But I need to understand the question better before I can think of answers. Why should a wind powered vehicle necessarily run at speeds equal to (?!) or lower than the wind speed?

I mean, I'm not very clear how the speed of the wind (~ Kinetic Energy) is being related to the K.E. of the cart.

On the similar lines; I've always wondered the following -

Let's say like in hydro power plants; we setup a mechanism to rotate the turbine through water stream falling from height X. Then through gear train, we transfer the energy to the generator that can power a motor to pump the water back to height X.

I was told that the energy lost in the gear train & turbine would never be able to match the effort required to lift the water to height X.
I usually check CE before initiating a post. The apparent thermodynamics violation issue side tracked me.

If wind is pushing the cart, the cart moves because of the pressure of wind on the cart/sail. When the sail moves downwind the wind pressure decreases because the differential speed decreases. When the two match there is no net velocity difference; hence no wind pressure. If the vehicle moves faster, then the vehilcle pushes against the wind, which becomes a head wind. Hence my confusion.
The other case is quite straight forward. Any hydro electric generator has less than unity efficiency (say, e1) Likewise any pump (say, e2). The overall efficiency is the producct of the two efficiencies (e1 x e2). The net result is that for every litre of water exiting the turbine only (e1 x e2) litre can be pumped back.

Notwithstanding what the article claims, I still feel that there is a fly in the ointment; dal me kaala hai.
7 years ago
Agreed.

I'd believe the claim that the cart moves faster than wind only when I try it on my own. Of course, I don't intend to question the measurements in the above experiment but it'd still be something that just cannot be ignored.

Now as I'm writing this, if the cart is moving at speed great than the speed of the wind, then the windmill should stop as the two speeds match ( 😨 ) differential speed => zero.
7 years ago
The_Big_K
Agreed.

I'd believe the claim that the cart moves faster than wind only when I try it on my own. Of course, I don't intend to question the measurements in the above experiment but it'd still be something that just cannot be ignored.

Now as I'm writing this, if the cart is moving at speed great than the speed of the wind, then the windmill should stop as the two speeds match ( 😨 ) differential speed => zero.
I guess that the following is a stupid explanation. However, if it is true, all is explained.

The wind monitoring gadgets seem to be mounted on the car that is following the wind cart. Since this car is moving, it will sense a smaller wind velocity. Once the cart picks up steady state speed, it is likely that it is travelling faster than the wind velocity indicated on the pilot car.
Pretty weak explanation.
ISHAN TOPRE

ISHAN TOPRE

Branch Unspecified
7 years ago
bioramani
I guess that the following is a stupid explanation. However, if it is true, all is explained.

The wind monitoring gadgets seem to be mounted on the car that is following the wind cart. Since this car is moving, it will sense a smaller wind velocity. Once the cart picks up steady state speed, it is likely that it is travelling faster than the wind velocity indicated on the pilot car.
Pretty weak explanation.
Sir, it is a trick. I agree that no generator has power greater than 100% efficiency. What if the designers are using two of them? or will the overall efficiency still be the product of inefficiencies of product of two generators?
Gurjap

Gurjap

Branch Unspecified
7 years ago
Here's what I think the designers have done. They've done away with the sails; using those we simply can't get a velocity downstream that's more than the wind. They are using a wind turbine, which turns using the (down) wind. You'll notice the wheels are very small in diameter. I am speculating a high reduction ratio, implying a high wheel speed at low wind turbine speeds.

So let's simulate this using our heads. The wind comes from behind the vehicle. The turbine turns, giving the vehicle a spurt of speed. If it were a sail, soon the low differential speed between the wind and the sail coupled with the high drag coefficient of such a vehicle would cause the vehicle to obtain a steady state at which its velocity is lower than the wind. But this being a turbine, it is designed to turn when there is any wind movement at all on the windward face (here exposed to the downwind) exists. As the velocity of the vehicle increases, this wind movement decreases, and this vehicle would therefore reach its own steady state, when the turbine can shale out only enough power to overcome friction. This steady state, with a relevant turbine blade design, would get the vehicle at a velocity probably higher than that of the wind.

Summing up, I think replacing the sail with a wind turbine does not only reduce drag, it also increases the range of the useful relative wind speed.

Or maybe it's a hoax.
7 years ago
Gurjap - Interesting. But still not 100% convinced. By the way, is there a way to simulate this in the labs with miniature models? I'm not sure whether the designer has made all the vehicle details available.
ISHAN TOPRE

ISHAN TOPRE

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7 years ago
Gurjap: True, but even if it is a turbine, it is hard to exceed speed of wind. Even in case of utmost efficiency, it cannot equal speed of wind.
I believe their is some trick and the designers have fitted some extra power source in the vehicle.
I oppose your logic in post #7. Because let us say, the vehicle gains a steady state and the turbine produces energy to counter drag& friction, it will only equal the speed of wind (sans any friction in parts).
7 years ago
The_Big_K
bioramani : I had covered that vehicle long time ago. But I need to understand the question better before I can think of answers. Why should a wind powered vehicle necessarily run at speeds equal to (?!) or lower than the wind speed?

I mean, I'm not very clear how the speed of the wind (~ Kinetic Energy) is being related to the K.E. of the cart.
You cannot generate infinite electricity by connecting a generator and a motor, which is powered by the generator, which is rotated by the same motor, which is powering it. One is the energy loss due to the frictional components present in the machine. And another is the laws of physics stops it!
Gurjap

Gurjap

Branch Unspecified
7 years ago
By the way, is there a way to simulate this in the labs with miniature models? I'm not sure whether the designer has made all the vehicle details available.
You'll be surprised by the amount of miniaturizing-and-running that is done in generating data in Fluid Mechanics.....well, maybe not. The thing is, making scale models of things and testing them out in labs is done for any new design of any kind of fluid machinery. The people who do that are called "big boys with big toys" in the industry.
Gurjap

Gurjap

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7 years ago
True, but even if it is a turbine, it is hard to exceed speed of wind. Even in case of utmost efficiency, it cannot equal speed of wind.
You are thinking sails, my friend. What we have here is a wind turbine. And a transmission system to transfer its power directly to the wheels. Which, as I said, has a high reduction ratio. Come to think of it, this transfer of power strategy will have a better efficiency than a sail.

I oppose your logic in post #7.
I welcome your opposition. This being an internet forum for engineers, the point is not to be proven right, but to be proven wrong and hence learn something.

Because let us say, the vehicle gains a steady state and the turbine produces energy to counter drag& friction, it will only equal the speed of wind (sans any friction in parts).
I am unable to follow your sequence of logic that you are using to contradict me. You are saying that the steady state is when the wind velocity on the windward face of the turbine blade drops to the point at which the vehicle has only enough power to overcome drag and friction, and hence cannot accelerate, and hence maintains velocity? That's exactly what I am saying.
ISHAN TOPRE

ISHAN TOPRE

Branch Unspecified
7 years ago
Gurjap
I am unable to follow your sequence of logic that you are using to contradict me. You are saying that the steady state is when the wind velocity on the windward face of the turbine blade drops to the point at which the vehicle has only enough power to overcome drag and friction, and hence cannot accelerate, and hence maintains velocity? That's exactly what I am saying.
Don't get be wrong. I am not opposing you. I am unable to understand your logic. 😀

You said that there is a wind turbine (I accept it). You said that it is powering the vehicle to certain steady state (velocity is still less that wind), I agree. Next you say once it reaches steady state, the turbine has only one work to do (to overcome drag and friction). This makes its velocity equal to that of wind.

But in no situation, can the velocity of vehicle exceed the velocity of wind. If the vehicle and turbine is 100% it will only equal the wind but never exceed the wind.

This is what makes me say- Why this Kolaveri Di? ☕
Gurjap

Gurjap

Branch Unspecified
7 years ago
You said that there is a wind turbine (I accept it). You said that it is powering the vehicle to certain steady state (velocity is still less that wind), I agree. Next you say once it reaches steady state, the turbine has only one work to do (to overcome drag and friction). This makes its velocity equal to that of wind.

But in no situation, can the velocity of vehicle exceed the velocity of wind. If the vehicle and turbine is 100% it will only equal the wind but never exceed the wind.
Hmmmm..... maybe I should have said it in this way: You should have a look at the blade profile. You'll see it's not a straight plank of wood. It has two faces: one face is exposed to a high-pressure wind, the other to low pressure wind. That is what causes the blade to move, as you can see.

Next, you must appreciate that given the peculiar shape of the blade, the wind on the high-pressure (windward) side of the blade is by no means the same wind that is blowing from the rear of the vehicle. It has turned at some angle, and hence its velocity has changed.

This is what I meant when I said "replacing the sail with a wind turbine does not only reduce drag, it also increases the range of the useful relative wind speed." See my point?
ISHAN TOPRE

ISHAN TOPRE

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7 years ago
Gurjap: Understood 😀 I need to study the topic a bit. Nice explanation (I hope so) 😀
zaveri

zaveri

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7 years ago
The_Big_K
bioramani : I had covered that vehicle long time ago. But I need to understand the question better before I can think of answers. Why should a wind powered vehicle necessarily run at speeds equal to (?!) or lower than the wind speed?

.
that is exactly what bioramani is trying to say, when he says that this vehicle violates thermodynamics.

according to the laws of thermodynamics, energy outputed from a system or a machine is always lesser than the energy that is inputed to it. the difference between the input and output energy results in transmission losses, heat losses etc.

thus if this WPV runs at speed greater than wind speed, then the output exceeds the input and this is a violation of thermodynamics
spork

spork

Branch Unspecified
7 years ago
Here it is without any comments:
https://www.instructables.com/id/Downwind-Faster-Than-The-Wind-Cart/
Without any comments? There are over 150 comments - about half of them mine.

As the guy who designed and built the cart (and who's head is poking out of it in the above video) I'll be happy to answer any questions. But let me say that it's not a hoax - just a brain-teaser. We take the physical laws very seriously where I come from, so we never violate them.

By the way, is there a way to simulate this in the labs with miniature models?
There sure is. That's precisely what the above link shows. It's a series of detailed build videos that will show you how to build a small working model for yourself at minimal cost. If you don't put any batteries or motors in it, you'll know it's not a hoax 😀

I'm not sure whether the designer has made all the vehicle details available.
We've made all the details available in ludicrous detail. You can check out our project blog at:
www.fasterthanthewind.org

And you can look at our reports, the observer reports, and the data from the world record run at www.NALSA.org
spork

spork

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7 years ago
Gurjap
You are thinking sails, my friend. What we have here is a wind turbine. And a transmission system to transfer its power directly to the wheels.
Actually, it's not a turbine - but a propeller. The wheels are in fact turning the propeller, not the other way around. In the next couple of months I hope to demonstrate the vehicle going directly upwind faster than the wind. For that task we will in fact replace the propeller blades with turbine blades.

This being an internet forum for engineers, the point is not to be proven right, but to be proven wrong and hence learn something.
That's exactly the right attitude. I originally conceived of this vehicle only as a brain-teaser. We only built it because people were clearly not prepared to accept the answer I offered.
Gurjap

Gurjap

Branch Unspecified
7 years ago
Actually, it's not a turbine - but a propeller. The wheels are in fact turning the propeller, not the other way around.
Wait up. First off, I thought propellers....well, they propelled vehicles by using external power sources, like motors and gas turbines (like on a turboprop). And the wheels are turning the propeller? So, what's turning the wheels?

Edit: after viewing several videos, I think I get the basic picture. A propeller is geared to the wheels. A slight push/gust of wind pushes the vehicle forward. The wheels rotate, causing the propeller to turn. Because of the blade design of the propeller, the exit velocity of the wind relative to the propeller is still enough to propel the vehicle, even though the vehicle velocity itself is greater than the wind.
spork

spork

Branch Unspecified
7 years ago
Gurjap
Edit: after viewing several videos, I think I get the basic picture...
Yup. You do have the basic idea. It's a bit like a feedback loop - and therefore sounds impossible to most folks. But it's like an audio feedback loop - unplug the amp and it quickly stops. In this case it's the wind that feeds our loop.

Keep in mind, the vehicle is moving over the ground faster than it's moving through the air (remember it has a tailwind - even though it's going faster than the wind). Thus, given that energy = force x distance, you can see that you can harness more energy at the wheels than is needed to produce equal force (i.e. thrust) at the prop.

Also, keep in mind, the spinning propeller is not being pushed along by the tailwind after we're above wind speed. Like the prop on a Cessna, it's pulling its way through the new air in front.
hans magne

hans magne

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6 years ago
Are you guys discussing the possibility of this being a perpetual machine?
spork

spork

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6 years ago
hans magne
Are you guys discussing the possibility of this being a perpetual machine?
It's no more a perpetual motion machine than a sailboat is.
Gurjap

Gurjap

Branch Unspecified
6 years ago
spork you didn't do any bond graph modeling for this, did you? It would put off the perpetual motion machine-related questions.

And it's too bad we can't use this on the sailboats. Or maybe we can! Why not hook up the (water) propeller inside the water to the airscrew out in the open? Same principle.

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