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NASA researchers aim to help get airborne wind power systems off the ground


July 4, 2012

The system developed at Langley flies a kite in a figure-8 pattern to power a generator on...

The system developed at Langley flies a kite in a figure-8 pattern to power a generator on the ground (Photo: NASA)

Currently, land-based tower wind turbines are the dominant source of wind power, but they take up a lot of space and generally need to be placed in high visibility areas, such as the tops of hills or ridges. They are also located close to the ground, where friction from the Earth’s surface slows the wind and increases its turbulence, negatively affecting the efficiency of the turbines. NASA engineers are looking at technologies that would help airborne wind power systems, capable of generating much more power, get off the ground.

There are two basic types of kite-based airborne wind-energy systems. “Flygen” systems see turbines built into the kite that generate the electricity and feed it via a tether to a storage or distribution device on the ground. The second features a ground-based generator that is powered by the reeling out of the tether as the kite catches the wind. By tacking the kite upwind like a sailboat, the periodic reeling-in phase takes much less energy – around 10 percent – than is produced by the reeling-out phase, resulting in a 90 percent net energy gain.

Both systems also rely on the aerodynamics of the kite and autonomous flight control. It is these two aspects of the technology that NASA researchers are looking to improve to help make airborne wind-energy systems a viable alternative to ground-based turbines.

"A lot of the systems that are flying have pretty cruddy aerodynamics," says David North, an engineer at NASA's Langley Research Center in Virginia. He points out 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."

North says that, while several companies attempting to bring airborne wind energy to market have demonstrated autonomous flight, they have relied on sophisticated onboard electronics and flight-control systems, comparable to autopilot systems used on commercial aircraft. "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."

North and his colleagues at Langley achieved the world’s first sustained autonomous flight using only ground-based sensors on March 1, 2012. "The breakthrough we've made 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,” North explained.

The system developed by the NASA researchers builds on the principle that the tips of a wind turbine’s blades generate as much as 90 percent of the turbine’s power because they are further from the hub and spin faster than the rest of the blade. In effect, placing a wind turbine at the end of a tether instead of it being attached to a concrete foundation allows the kite to act as a flying blade tip. It also allows the system to harness the much faster and steadier winds that can be found at higher altitudes.

The autonomous system functions in a similar way to Microsoft’s Kinect, with pattern recognition software determining where the kite is positioned, how it is oriented, and how fast it is moving. This data is fed into a flight-control system that keeps the kite in the air flying in a figure-8 pattern. The prototype kite only had a wingspan of about 10 feet (3 m), which is much smaller than the devices expected to be used in commercial applications. "Some people are talking very large, like wings the size of Boeing 747 airliners," North said

So far, the team’s test flights have been limited to low altitudes to avoid interfering with aircraft, but they are trying to gain permission to fly at 2,000 feet for long periods of time in the restricted airspace reserved for NASA above Wallops Island, Virginia. Above 2,000 feet is considered the sweet spot for airborne wind-energy systems.

While the research will benefit renewable energy generation on Earth, NASA says the airborne power-generation systems could also be put to use on neighboring worlds, such as Mars, Venus, and Titan.

"The funny thing about wind energy," North said, "is it goes by velocity cubed. So you can still extract significant amounts of energy even if the density is very low (like the thin atmosphere on Mars)."

Source: NASA

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag.   All articles by Darren Quick

"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."

Translation: Unlike the losers in the private sector, we can spend as much time as we want because we have an endless supply of taxpayer dollars and nobody is anticipating the results.

I am completely in favor of research and progress. But someone should teach the scientists a little tact and diplomacy, too.

Hanover Fist
5th July, 2012 @ 11:19 am PDT

Shouldn't you use 2 kites so that you can generate at a constant 90%.

5th July, 2012 @ 03:00 pm PDT

I'd be concerned about the durability of the tethers in such a system. The large dynamic stress reversals (that 90% figure) may cause early fatigue failure, even if tethers were made of advanced fibers like Vectran.

5th July, 2012 @ 04:12 pm PDT

re; Gadgeteer

Reducing the tension by 90% is unlikely to cause early fatigue. However it is vitally important to avoid shock loading when you bring the tension back up.

5th July, 2012 @ 05:37 pm PDT

We actually built some ground based steering systems at Makani, perhaps four years ago, so I don't think Langley were the first to do it. I also believe some of the european teams (Enerkite?) have done autonomous ground based steering. The reason we use kite based sensors is essentially to increase both system performance and reliability. Getting data about what is actually happening at the kite allows us to control using more complete information, so we both respond better to wind changes, and control the kite to better utilize power from gusts. That said, it's great to see national labs starting to think about this space.

5th July, 2012 @ 07:46 pm PDT


Stress reversal only applies to elastic rigid members..

A string can't have a stress reversal, because as soon as it goes to Zero tension, it is Just zero... You can't put a string in Compression...

Of course cyclic loading of a string is another factor....

But a Kite will not give Zero tension in the control lines.... (unless it is stalled)

So it will not be full cycle loading merely varying the tension...

More important is the UV reistance of the control lines and kite... Sunlight breaks these things down in fairly short times..

5th July, 2012 @ 08:16 pm PDT

What happens when there is no wind? Does the apparatus land and take off when there is wind again?

18th July, 2012 @ 03:58 pm PDT

"Translation: Unlike the losers in the private sector, we can spend as much time as we want because we have an endless supply of taxpayer dollars and nobody is anticipating the results."

Wrong. NASA has a very stringent budget, and has found itself on the hot seat many times, such as when the Hubble didn't work when it was first launched. You should work on your fact-checking skills.

Bill Wilson
1st August, 2012 @ 12:07 pm PDT

The main reason I've joined this forum is that the comments are somewhat recent. Other forums, including Makani, and Joby are already 2 years out-of-date.

I have several comments to make, so I will not include all my thoughts in this one; rather I will only list the areas of concern in this one:

1. There is precious little new information in this article. Only the NASA info seems recent. And it seems irrelevant to the main concerns having to do with airborne wind power generation. Kite control has been successfully demonstrated several ways, to all intents and purposes, except for the problem of collision avoidance in a multiple kite 'farm' circumstance.

2. transmission of 20 KW to 2 MW of power requires developments in materials science to reduce the weight of the tether, which at 2000' of kite altitude would need to be 3000 to 4000 feet in length, based on a reasonable tether angle of 30 to 45 degrees.

3. The "artificial" airspace constraint put on the kite systems by the FAA and the general aviation communities are totally choking off the full on development of efficient systems. I believe that monied interests are perhaps using these constraints to further the use of more traditional carbon-based energy sources, also forcing the entreprenurial startups to consider only grid-based development solutions.

15th August, 2012 @ 10:09 am PDT
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