Electricity in the air

Jul 03, 2012 By Bob Silberg
The system developed at Langley flies a kite in a figure-8 pattern to power a generator on the ground.

(Phys.org) -- The faster a wind turbine's blade spins, the more energy you can get from it. And the farther you get from the hub, the faster that part of the blade is traveling. So the tips of the blades generate most of the turbine's power—as much as 90 percent, according to David North, an engineer at NASA's Langley Research Center in Virginia.

"What if I had a machine that was just the tip of the blade?" North said. "That's really the idea of airborne wind energy—get rid of 400 tons of tower and concrete, and just fly the tip. Basically, it's flying kites to create power."

A wind turbine that's flying at the end of a tether instead of fixed to a concrete foundation has big advantages. For one, it's highly portable—an attractive feature for potential users such as the military, which is eyeing the technology for war-zone bases where importing fuel comes with great risk and expense.

Another huge advantage is that an airborne system can go much higher, up to altitudes where the wind blows faster and more steadily. And with greater speed comes much greater energy. Moving twice as fast produces eight times the power. Moving three times as fast produces 27 times the power.

Aiming for the energy sweet spot

According to North, most tower turbines are about 80 to 100 meters (roughly 300 feet) high, which he says is "pathetically down in the boundary layer of Earth." The boundary layer is where friction from Earth's surface keeps the wind relatively slow and turbulent.

The sweet spot for wind energy starts around 2000 feet up. To use wind at that altitude to generate electricity, you’d have to build a turbine tower taller than the Empire State Building. Or you can fly a kite.

There are two basic types of airborne wind-energy systems. One, known as "flygen," is literally a flying generator, with turbines built into the kite. The resulting electricity travels by tether to a storage or distribution device on the ground.

In the other kind of system, the generator sits on the ground, powered by the reeling out of the tether as the wind catches the kite. By maneuvering the kite like a sailboat tacking upwind, the periodic reeling-in phase can take only about 10 percent of the energy produced by the reeling-out phase, for a 90 percent net gain.

Several private companies are trying to get airborne wind energy ready for market. NASA's contribution focuses on two aspects of the technology: autonomous flight control and aerodynamics.

"A lot of the systems that are flying have pretty cruddy aerodynamics," North said. He explained that companies under deadline pressure from investors aren’t able to spend much time on the difficult challenge of optimizing the kite’s efficiency. "Here at NASA," he said, "we have the luxury of focusing very specifically on problems and not have to worry about getting a commercial product fielded by a certain date."

Autonomy—the ability to set it and forget it for long stretches of time—is crucial to the airborne wind industry. It's fun to fly a kite manually, but 24/7 for months at a time is a little much to ask of a human operator, even if he or she could manage the precise maneuvers that are required over and over again. And the likelihood that airborne wind farms would be located far offshore, where air traffic tends to soar high above the altitude where these kites would fly, makes autonomy all the more desirable.

Fast, cheap and under control

The companies that have demonstrated autonomous flight so far have relied on sophisticated onboard electronics and flight-control systems, comparable to autopilot systems for commercial aircraft, according to North. "Our goal is to simplify the whole thing," he said, "especially if we are only flying at 2,000 feet, which is in most cases below the clouds."
On March 1, 2012, North and his colleagues at Langley achieved the world's first sustained autonomous flight using only ground-based sensors. "The breakthrough we've made," North said, "is we are basically using a cheapo digital webcam tied into a laptop computer (on the ground) to track the motion of the kite and keep it flying autonomously."

Langley's system operates much like Microsoft’s Kinect gaming system, which tracks the body movement of players. "It's pattern recognition software," North said. "The software is basically determining where the kite is, how the kite is oriented and how fast the kite is going, and using all that data to feed into the flight-control system."

The Langley prototype was small, with a wingspan of about 10 feet. But the devices the industry ultimately produces are likely to be much bigger. "Some people are talking very large, like wings the size of Boeing 747 airliners," North said.

Ironically, the biggest challenge the Langley team faces is having their test flights limited to low altitudes, to avoid interfering with aircraft. They are currently trying to work out a deal that would enable them to fly at 2,000 feet for long periods of time in the restricted airspace reserved for NASA above Wallops Island, Virginia.

Given a chance to develop this technology, who knows? We might see a day when those who scoff at green energy alternatives could be given this friendly piece of advice: Go fly a !

Explore further: Scientists invent award winning 2-in-1 motor for electric cars

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User comments : 12

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Jeddy_Mctedder
1.9 / 5 (7) Jul 03, 2012
there have been already a few attempts at fielding autonomously guided kites or wings.....

i'm still unconvinced our software guidance systems are up to the task of doing this profitably.

remember , when it comes to energy utilities, almost like with a weapons system---#1 you need reliability. if it is not reliable , it will NOT be profitable. everything comes after reliability.
Silverhill
1.8 / 5 (5) Jul 03, 2012
Moving twice as fast produces eight times the power. Moving three times as fast produces 27 times the power.
Breaking news! Kinetic energy has been redefined in terms of the *cube* of the velocity!
Lurker2358
4 / 5 (8) Jul 03, 2012
Moving twice as fast produces eight times the power. Moving three times as fast produces 27 times the power.
Breaking news! Kinetic energy has been redefined in terms of the *cube* of the velocity!


No, but you're not thinking the problem through.

1) A mass of air proportionate to it's own velocity passes the wind turbine every second, therefore in the case of wind passing any given point, the MASS of the air is itself proportional to the velocity.

2) Apply kinetic energy formula as normal.

d = density of air mass
A = Cross sectional area of the wind turbine
m = A * d * V

Ek = 0.5mV^2

Plug in "m" above.

Ek = 0.5*A*d*V^3

Have fun...
Nanoparticler
5 / 5 (2) Jul 03, 2012
The article clearly states that the POWER available is a cubic function. Which is correct. Assuming equal cross sectional areas, the available power in a wind stream is related to the wind speed as a cubic function.

In 8th grade science terms: power and kinetic energy are not the same thing.
Lurker2358
2 / 5 (4) Jul 03, 2012
The article clearly states that the POWER available is a cubic function. Which is correct. Assuming equal cross sectional areas, the available power in a wind stream is related to the wind speed as a cubic function.

In 8th grade science terms: power and kinetic energy are not the same thing.


Not exactly. See above.

because the wind is a fluid of arbitrarily long proportions, for the purposes of which we are interested in it, the LENGTH of the air moving past the turbine increases proportional to the velocity, so that is one V component. Then you must apply the normal V^2 component from the kinetic energy formula. Therefore, the Kinetic Energy of "Wind moving past a turbine" is proportional to the cube of velocity.

The "Power" of wind is actually proportional to the kinetic energy differentiated with respect to time...
kochevnik
1 / 5 (4) Jul 03, 2012
i'm still unconvinced our software guidance systems are up to the task of doing this profitably.
It was also mathematically proven that a heavier-than-air craft is impossible to control. Only Arab goatherders with stolen mossad passports are capable of such acumen, and only by exceeding the aircraft it's rated airspeed by half.
mrlewish
1.5 / 5 (4) Jul 03, 2012
The authors are wrong. There is not that much energy in higher altitude winds. The reason it goes so fast up high is the lack of resistance up there lets it build up to high speeds over time. The actual "extra" energy that is added to the wind up there is negligible.
Silverhill
4.8 / 5 (4) Jul 03, 2012
@Lurker2358
the LENGTH of the air moving past the turbine increases proportional to the velocity, so that is one V component. Then you must apply the normal V^2 component from the kinetic energy formula. Therefore, the Kinetic Energy of "Wind moving past a turbine" is proportional to the cube of velocity.
Oops! (I suspected that there was something fairly simple that I was hastily overlooking, though it was *not* the "8th grade" difference between kinetic energy and power. Thanx, Lurker!)

@kochevnik
It was also mathematically proven that a heavier-than-air craft is impossible to control.
No, it was "proven" using an inadequate model of the conditions. ...Which should have alerted the "prover" that something was wrong in the calculations, not in the mechanism!
julianpenrod
1.7 / 5 (6) Jul 04, 2012
Among other things, the article seems to have a difficulty differentiating between energy and power. Too, among other things, it begins by differentiating between energy from different parts of the turbine blades because the tips are moving the fastest. But, it's not any point on the blade which is related to energy, it's the coil in a magnetic field inside the housing. That has one size. Too, it's the angular rate of speed which determines the energy output, and that is the same for every part of the turbine blades. In fact, the ends of turbine blades move because the solidity of the blqades carry them along. They can end up moving faster than the wind. And that could represent an energy loss to the system. It should be mentioned that there were designs which were more efficient at capturing wind energy and could work in any direction without re-aiming. They consisted of vertically mounted blades, curved like a pinwheel, that rotated as the wind moved.
unknownorgin
1.5 / 5 (4) Jul 04, 2012
If a one pound weight is lifted one foot in one second then one foot pound of work (power) is performed so if a one pound weight is lifted 10 feet in one second then 10 foot pounds of work is done. By this example it is clear that work performed for a unit of time (power) is directly porportional to velocity or rate of motion at a constant unit of force.
Husky
not rated yet Jul 04, 2012
i hope thin lightweight conducting graphene wire comes into reality, so that you can get up a kite at decent altitude without the wireweight penalty.
A-man
not rated yet Jul 11, 2012
remember , when it comes to energy utilities, almost like with a weapons system---#1 you need reliability. if it is not reliable , it will NOT be profitable. everything comes after reliability.


You are exactly right. That's why our company is developing a system that would be reliable, especially at take-off and landing. System needs to be fully automated.