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Google-Backed Wind-Powered Car Goes Faster Than the Wind
sterlingda writes "A wind-powered car has been clocked in the US traveling downwind 2.85 times faster than the 13.5 mph wind. The definitive research by Rick Cavallaro of FasterThanTheWind.org is being funded by Google and Joby Energy. The run should now settle the DWFTTW (downwind faster than the wind) debate that has been raging for some time on the Internet about whether or not such a feat was possible."
Sailing vessels only go faster than the wind when they travel with the wind coming from the side. No matter how fast the vessel goes, the wind keeps blowing from the side and delivering energy to the vehicle. When you try to go faster than the wind in the direction of the wind, the relative motion to the air goes down to zero and then you start going against a head wind. Obviously the wind can not be the propelling force beyond the point where you go as fast as the wind in the conventional sailing sense, because at that speed there is no wind (motion is relative). The described device uses the sailing force to accelerate and then produces its own faster wind, so to speak, by driving a propeller via a transmission from the wheels.
Firstly, ignore that it's moving.
You have 0m/s ground, and a 10m/s wind.
You put up a wind turbine - it can extract power from this 10m/s difference.
The funky part of this idea is that this still works when you're moving faster than 10m/s.
For the moment - imagine that the turbine is a pure 'airscrew'.
It describes a helix in space - like the DNA molecule.
For every meter the air moves "forward" relative to it, it turns 1m clockwise.
Considering the air as completely rigid for the moment, the airscrew goes forward in a rigid helix, unchanged by load.
So - 10m/s wind - airscrew turns at 10m/s. Simple.
You can extract - say - 100N * 10m/s = 1kW of power.
Funky part coming up.
Now. You're moving at 20m/s. Twice as fast as the wind.
Of course this will slow you down - you can't use this to make power!
Well - not quite.
If you are moving at 20m/s in the direction of the wind - for a total speed with regards to the wind of
30m/s then the blades need to be spinning at 30m/s in order to keep up.
But, you can use gearing from the wheels so that the 'base' speed of this spin is 20m/s.
That is - when you push the car along on a windless day - the airscrew creates no drag - because it is spun at exactly the right speed by gearing from the wheels. It has effectively - by rotating at the right speed - cancelled out the movement of the car.
This cancellation then allows you to ignore the speed of the car, and instead work off the speed difference between the wind and ground!
In reality - it's very far from an airscrew, and turbines have a lot of drag. It's the same basic concept though.
Another beautiful and 'obvious' when you think of it bit of physics.
Boats can not go downwind faster than the wind. Rather than jump out and try to announce to the world how much smarter you are than the people who actually did stuff, maybe you should first go read and comprehend what they actually did.
The more I look at that description and think about it, the more skeptical I become. It reminds me of someone trying to sell a perpetual motion machine. You have this battery to start it, and it shines this light. And all around it are solar panels, that absorb the light and keep the battery charged. Of course this doesn't work, there's never any net gain, and since there's losses in the system, it fails.
Here, the wind speeds the vehicle up to say 15mph, same as the wind. The wheels rob the vehicle of some speed in exchange to spin the propeller backwards. (which I must admit is a very interesting, novel approach!) which provides a force on the wind blowing the vehicle, which by itself would appear to accelerate the vehicle faster than 15mph.
But I see no reason why the drag from the wheels isn't exactly canceling out the benefit of rotating the propeller. And then the losses of friction etc step in, and you end up with a vehicle traveling slightly slower than the speed of the wind.
Basically, you can't turn the propeller without investing energy, because you're turning it against a resistance, namely the wind blowing on it. The more you want to resist the wind, (the faster you want to go than the wind) the more energy is required on the prop. And so as this theoretical vehicle accelerates and more energy is available from the wheels, (and is being robbed from the vehicle speed) the more energy you have to invest in the propeller. It's the same as a perpetual motion machine. You can't get more out without putting more in, and you can't put more in until you've gotten more out.
But then of course everyone asks "but he proved it with his prototype. I would ask if this was a sustained speed. Here's a scenario where it could work for a short time only:
First it looks like the blades on the prop can be pitched. That makes sense for control anyway. Lets say the car is blown up to speed while the blades are pitched at 0 degrees. Power is drawn from the wheels to spin up the prop. This slows the vehicle initially but the wind is constant and eventually the prop is up to full speed (perhaps very fast!) and the wind continues to blow and brings the vehicle back up to about it's speed. (probably only close, due to various friction elements)
Then suddenly the blades on the prop are pitched heavily, and now there's a good wind blowing out the back.
The vehicle would surely lurch forward. This is spending the energy of the inertia of the prop to accelerate the vehicle. This will only last a short time. Yes, the vehicle is traveling faster now and the wheels are turning faster, but you can't rob power off the wheels to keep the prop up to speed because that would slow the vehicle down. Remember the perpetual motion machine above. Any energy you take from the wheels to spin the prop to keep it up to speed must provide equal or less energy in the end to the prop than you are taking from the wheels. And the vehicle's acceleration crests.
At this point the prop will be spinning slower but still backward, enough to reach equilibrium, such that the energy of pushing on the prop to accelerate the vehicle is equal to the energy being taken from the wheels.
And then it starts to slow down, slowly, due to drag. And during which the prop slows down, STOPS, and reverses direction.
How can it stop and even reverse direction while the wheels still turn? Good question! The prop is currently working against the wind. Energy must be invested in the motor, not merely to spin it in the direction you want, but to even resist being spup the other way. It's easier to understand if you look at a prop being spun freely by the wind. If you put a dead battery on the terminals, the battery will start to charge, but the blades will slow down. You are providing a load on the prop, and are withdrawing energy from the prop's speed and transferring it to the battery. Energy is always moved around, never created or destroyed. In the
I work for the Department of Redundancy Department.
But I see no reason why the drag from the wheels isn't exactly canceling out the benefit of rotating the propeller.
The Energy generated from the wheels has to match the Energy lost by the propeller. Thanks to gearing, the force is not the same.
Energy isn't the problem, a decent sized windmill can generate a megawatt of power. And it can generate the energy perpetually (assuming perpetual wind).
Consider if the vehicle was stationary, then we could easily generate the power from the wind: the force against the wheels wouldn't lose us any energy because E=mv^2 and so dE/dv=0 when v=0. Now imagine we are travelling at exactly the speed of the wind. Then our velocity relative to the wind is 0 so dE/dv=0. Thus we can push against wind without losing any energy, the same way a stationary windmill can push against the ground without losing energy. And so we can generate energy from the ground speed without losing kinetic energy (ignoring for the moment that the propeller doesn't have perfect grip on the air)
So we are currently travelling at wind speed, and generating energy from the ground. We now use that energy to push against the the wind to make us go even faster. Note that even a 50KW engine feels powerful when we are going slow and in first gear, and even a 200KWH engine can't burn rubber when we are going at 100KM/h. This comes back to E=mv^2, because Energy is proportional to the square of the Velocity, it takes more energy to speed up the faster we are going.
Note that we are still travelling faster relative to the ground than the air. Thus we can use the same trick as gears in an engine, we use a high gear relative the ground so have only a small force. We use a low gear relative to the air so we generate more force (for the same energy). We continue to speed up until the energy we gain from the different gearing ceases to make up for friction and other inefficiencies in the system (such as the propeller not having perfect grip on the air).