Impossible-Seeming Wind Vehicles Might One Day Produce Abundant Energy

By Eric Wolff | June 10, 2010 2:01 am

Intuitively, there just shouldn’t be any way for something wind-powered to move directly downwind faster than the wind itself. It’s impossible: Release a balloon, and the wind blows the balloon as fast as the wind is moving, and that’s as fast as any wind-powered object can go, before the wind. Sure, sailboats can win a race against the balloon by moving diagonally across the wind, but moving in a straight line down a 10 kph wind, and the balloon moves at 10 kph. End of story.

Or, start of story.

Rick Cavallaro and John Borton have built a cart that moves 2.86 times the speed of the wind, moving straight downwind. That may seem impossible, but after a year of tinkering and some financial assistance from Google and Joby Energy,  they did it. Don’t believe me? Check out the video. Keep a weather eye out for the green flag at 0:35. Notice how it’s blowing the exact opposite direction of the orange wind socks on the cart? That’s because the cart is going faster than the wind.

How is it possible?

That’s the mind bending part, and even Borton and Cavallaro admit that it’s kind of a brain twister to grasp the physics involved. But after a half hour on the phone, they managed to teach me the rudiments of what’s going on.

The key to it all is the difference between relative air speed and actual ground speed. Let’s go back to the balloon. The balloon propelled by the 10 kph wind is moving at 10 kph along the ground, but it’s relative air speed is 0 kph. It’s moving the same speed as the wind.

Ok, now bring the cart back.

BUFC_5
The Blackbird, lined up at
New Jerusalem runway in Tracy, Calif.

The cart is quite aerodynamic, so it makes a crappy sail, but it’s still a sail. The wind gives it a push, and the car starts to roll, however slowly. As it moves along the ground, the wheels turn. At 0 kph the cart has an air speed (relative to the 10 kph wind) of -10 kph. As it gets rolling, it will catch up to the wind in velocity: at 5 kph ground speed, it has a -5 kph relative air speed, and at 10 kph ground speed, it has a relative air speed of 0 kph. At 10 kph ground speed, the cart is just like the balloon, and would not beat the balloon in a race.

But unlike the balloon, the cart has a 17-foot propeller linked by a complicated drive train to the wheels. And it’s the wheels that provide the work to turn the propeller. Remember that: The wheels turn the prop. Not the wind. Not magic pixie dust. The wheels turn the propeller. That’s important.

At 0 kph air speed, the propeller, sections of which have already been pulling on the car, really begins to bite on the air. It pulls the car forward exactly as a propeller pulls an airplane forward. The ground speed of the car increases, turning the wheels faster, which turn the propeller even faster, adding yet more acceleration. And now the whole project seems ridiculous,because everyone knows a perpetual motion machine is impossible.

But the wind never stops adding power to the system. Come back to the difference between the relative air speed and the ground speed. In the example, the cart reaches a ground speed of 10 kph, and relative air speed of 0 kph. The propeller kicks in and the cart accelerates: Ground speed rises to 20 kph, with relative air speed of 10 kph; then 30 kph ground speed with relative air speed of 20 kph, then it finally reaches a top speed of 28.6 kph, with a relative air speed of 18.6 kph (meaning, going 18.6 kph faster than the wind). There’s some loss to friction and to the drive train, but generally the wheels are always doing 10 kph-worth more work then the propeller, because the propeller is pulling through  air that’s already moving of its own accord (Cavallaro and Borton like to compare this to how a boat driving  down river moves faster than a boat in still water). That difference in work is where the extra energy enters the system, allowing the cart to move faster than the wind directly before the wind.

If that was totally baffling, don’t worry, you’re in good company. Cavallaro and Borton have worked with some of the finest minds in aerodynamics to develop the cart, and they’ve been put down as morons by other, equally fine minds. But the cart works.

The two men got into the project as an academic exercise, a kind of proof of concept with no real application. But they’ve since realized they designed and built a  device that extracts an extraordinary amount of energy from the wind, indeed, far more than any stationary wind turbine currently dotting Texas or the seas north of Germany. Borton said their cart derives 23 horsepower at top speed, roughly four times the theoretical maximum and seven times the amount of work a traditional wind turbine gets. They’ve formed a company, called Thin Air Designs, to try and tap the commercial potential of their cart.

Tapping that power will be a trick–the cart has to move, after all, and as cool as the visual would be of carts zipping along the salt flats to power Las Vegas, the high voltage power lines they’d need to transmit the power into the grid would be no joke. But it’s a fascinating application from something everyone said should be impossible.

Image and video courtesy of Thin Air Designs.

CATEGORIZED UNDER: Energy, Transportation

Comments (12)

  1. This is the most comprehensive explanation article written on the subject. Excellent job getting it right. Bravo!

  2. ridgerunner

    Faster than the wind directly downwnd?
    But that’s unpossible!

  3. ArtLab

    A thought experiment. On a day with zero wind, could this vehicle be push started, by a powered vehicle, up to a speed that the wheels powering the prop would provide enough thrust to maintain it’s ground speed? If not, why?

  4. Guest

    That would be a “perpetual motion machine”

  5. HK

    What would be the theoretical maximum speed of a wind-powered car going directly against the wind ?
    What would be possible in reality ?

  6. S.K.Graham

    If you are baffled by this, try reading this blog — you don’t need to be an aerodynamics expert or mathematical genius to understand: http://rightnice.blogspot.com/2010/08/racing-wind.html

    There is no “perpetual motion” going on here, no “free energy” it is just plane ordinary physics… that surprises us with something we didn’t expect.

  7. Bill

    Let me put my 2 cents in the pot. First I would like to start by offering my congratulations and thanks to Rick Cavallaro and John Borton for their great accomplishment. Thanks for the many hours I have spent thinking and analyzing what the Blackbird did and what it might mean. I can not remember when I have been so enthralled and obsessed about something. I first heard about the Blackbird when I read the article in Wired magazine. As a windsurfer I knew that you could not go straight down wind faster than the wind on a sailboard. I taught windsurfing at one time and was aware that when the wind speed doubled, the pull on the sail quadrupled. I had always felt, that in going faster than the wind I was some how getting more out than was being put in, even after knowing the theory behind it. I guess that was part of my being so intrigued by what the Blackbird did. If I would have read Rick’s explanation before hand, I would have been on his side. Another way I thought about it after it had been accomplished, helped me personally come to terms with it. It is just a little bit different way of thinking about it. The sails (props) of the Blackbird at no point in time sailed straight down wind , they were always sailing across the wind. The moment the Blackbird started to move, its sails (props) began sailing across the wind. This means that the sails never sailed straight down wind faster than the wind, always across. Every thing together went faster than the wind straight down wind. Can you imagine how fast the prop had to sail across the wind to do that? To get going 3 times the speed of the wind down wind (not straight down wind) on a sailboard I would probably need to be going at least 6 times the wind speed at a 45 degree angle off wind. Can this be done on ice? If not, then there is definitely more going on with the Blackbird than meets my eye.
    When the Blackbird began to move, it was be pushed by a tail wind. Both the wheels and prop begin to move as they are tied together. At some point the prop turns fast enough to provide propulsion. At the same time the tail wind is decreasing. Movement is depending more and more on prop propulsion and less on the tail wind. The fact that the tail wind reduces the head wind that would be there without it, is reducing drag.
    At some point the propulsion caused by the prop turning, added to the inertia of motion, enables the Blackbird to start running on its own power. Meaning that, it no longer requires the tail wind to push it. This might be happening before it reaches the speed of the actual wind. It might still need the tail wind to reduce the head wind. The fact that without a head wind, it does not take a whole lot of energy to keep an object moving, once it is moving, is helping. At the zero wind point (head and tail wind equal) the propulsion from the prop turning and the energy of inertia are the only energies available. From the time that the Blackbird reaches the speed of the wind and on, the only existing energies to move it faster are self created, the head wind and the prob wind. At this time the tail wind is only reducing the head wind. At this time the prop propulsion, added to the inertia of motion, allows the Blackbird to accelerate. Without the tail wind reducing the head wind, the head wind would be equal to the speed of the Blackbird and maybe not let it accelerate. With the tail wind reducing the head wind the Blackbird can accelerate to more than three times the speed of the wind. When the Blackbird goes twice the speed of the wind, the head wind simulates the condition of having a head wind not reduced by a tail wind, but also at this time it has twice the inertia of motion helping it. The question is, could the Blackbird become self propelling without this extra inertia of motion.
    I think maybe the reason this could be possible is, like I said before, that lift is not directly proportional to wind speed. It is exponential. It seems to me that the energy required to produce the wind is less than the energy provided by the lift. Until now no one knew how to take advantage of this.
    Think of it like this, using a spark to extract energy from gasoline. The energy it takes to create a spark is much less than the energy released by the gasoline. The reason being is that the energy is already in the gasoline. You are just using a spark to extract it. The wind might be like the spark. The wind is what is being used to extract energy from the atmospheric pressure in an amount greater than the energy needed to create the wind. Just a thought.
    To find out if the Blackbird could self propel, could be done by pulling or pushing it to say 20 mph on a windless day and letting go. If it is capable of self propulsion it will do so. What do you have to lose, Rick? Remember, few thought that the Blackbird could go down wind as fast as it does. I do not think that before hand, you even thought it would go as fast as it did. Maybe more is going on here than meets the eye.
    If it can self propel, I have already thought of how a windless windmill (atmospheric pressure engine) could be made. :-)

    I like to always realize that things are going on at the sub atomic level, that no one knows anything about. Most of the time this has no visible affect at our level, that we know of. But every once in a while it is seen at our level. Like the Gecko lizard walking on the ceiling. Scientist believe that a sub atomic force is coming into play that enables this. The van der Waals force. a.http://en.wikipedia.org/wiki/Van_der_Waals_force. This is not to say that I think that something is going on with the Blackbird at the sub atomic level, I am only trying to make people realize how much there is that man does not know. Even at the Newtonian level man does not fully understand the forces of a vortex or cavitation. We should always keep an open mind and not be too quick in dismissing something as being impossible.
    At 70 I am still a dreamer and maybe losing it.

  8. Bill

    Upon further thought I realized that I would only need to sail 4.2 times the speed of the wind to achieve the equivalent of what the Blackbird did. That is probably achievable on ice, even though the Greenbird did not do it on land. It is still pretty amazing what the Blackbird accomplished. Nothing to lose, other than feeling stupid for trying, in trying to see if the Blackbird could self propel. It does not seem possible to me, but then again I would not have thought that the Blackbird could have gone 3 times the speed of the wind straight down wind. I say 3 times,rather than 2.8, as I believe Rick when he says it did it in some of the runs.

  9. Owen

    The cart can’t self-propel in zero wind, and it is not a perpetual-motion machine. The answer to both of these quandaries is that even though the cart is travelling faster than the wind, it is still extracting energy from the wind. The propeller pushes a stream of air backward to create the thrust that pushes the cart forward. If you picture this from the “whole system” point of view, there’s a stream of air in the path where the cart has travelled that is now moving more slowly than it was before the cart came along (because it has been slowed down by the cart propeller). The kinetic energy of the air in that stream has been reduced, and that’s the energy that has gone into the cart movement. In the “steady state” the cart is moving along continuously pulling energy out of the wind to balance the energy it is continuously losing to drag and friction. In the overall system (cart, air, ground) energy is still conserved and no laws of thermodynamics have been broken.

    So it isn’t a perpetual motion machine because it needs wind power. But it is very clever.

    It wouldn’t self-propel in zero wind because in that case the propeller would actually be increasing the kinetic energy of the air around it (by increasing it’s velocity rather than decreasing it) which, sadly, means it would only dissipate the energy of that initial push you gave it, turning it into air flow and a little warming here and there due to friction, until it came to rest again.

    Nuts.

    I think I went through the same mental contortions as most folks on this. It is counter-intuitive, but as stated in other posts above it is still consistent with the laws of physics that have held up to every other test so far. It makes you realize how hard it is to objectively evaluate something that just doesn’t seem “right”. Probably a bit like coming to see the world as spherical rather than flat. Kudos to these guys for thinking outside the box when all around them were trying to stuff them back into it.

  10. Bill

    Here is my take on going directly into the wind faster than the wind. Using Rick Cavallaro’s http://www.wired.com/magazine/2011/02/ff_fasterthanwind/3/ explanation for being able to do it down wind, I would have to ask, is it possible to tack upwind faster than the wind? To reach a point up wind faster than the wind on a sailboard, I would need to sail up wind at a 45 degree angle at over 1.41 times the speed of the wind in order to duplicate going directly into the wind faster than the wind. Let me explain this a little more. When you are sailing up wind across the wind there is an up wind element. This up wind element has a speed that would need to be faster than the wind in order to tack up wind faster than the wind. Not sure, but would say right off hand that it can not be done.
    Another thing to realize in going upwind faster than the wind, is that if it is possible, then this vehicle would be capable of self propulsion. It could go, until it wore out., without any wind upon reaching a given speed. That is not to say it is impossible, but things would have to be going on that we don’t know about. Like maybe a vortex for example. I was talking to my son about it, and he told me that I should read about Schauberger’s (http://en.wikipedia.org/wiki/Viktor_Schauberger) findings on the subject of vortexes. Schauberger had noticed that a trout could stay stationary in a swift stream using very little movement. Schauberger thought it had to do with a natural vortex caused by the shape of the trout. How ever the trout does it, it seems to be reducing the equivalent of a head wind. How that could be used in sailing up wind I do not know. It does seem to open a possibility. Remember too, that lift (vacuum in the atmospheric pressure) is exponential to wind speed.
    Anyway, I am not going to be too quick to say it is impossible to go directly into the wind faster than the wind. If it can be done, the ramifications are mind boggling.

  11. amy zappia

    what if you filled the tires and body of the vehicle with helium?

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