[LINK] NASA Power Kites

[stephen at melbpc.org.au]

Good old NASA. Here they are looking at wind power-generation with kites.

Lots of advantages. Eg cost (and flying at 2000 feet) efficiency, visual
and sub-sonic pollution. Thus, simple, cheap, almost-base-load renewable
energy, which would be almost free after you buy some $10-20,000 kites :) 

By Bob Silberg, NASA Jet Propulsion Laboratory

<http://climate.nasa.gov/EnergyInnovations/index.cfm?
FuseAction=ShowEnergy&EiID=727>


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 blade 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.

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.

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 kite!
--

Cheers,
Stephen