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Where the High-Altitude Wind Blows
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This Makani airborne wind turbine resembles small, unmanned aircraft
This Makani airborne wind turbine resembles small, unmanned aircraft. A new study reveals the best locations for these and other types of high-altitude wind turbines. Andrea Dunlap/Google

Researchers have published a map identifying the optimal “sweet spots” for high-altitude wind turbines worldwide.

May 6, 2014—Imagine a world in which wind turbines are airborne, tethered to the ground by thin cords while they capture energy at altitudes at which the wind always blows, providing a constant supply of energy. That concept is much closer to reality than you might think: the technology already exists and the prototypes have been built, tested, and proved capable. Honing in on locations around the globe for such airborne wind farms and getting permits to install them is the challenge that the industry currently faces, according to Cristina Archer, Ph.D., an associate professor in the College of Earth, Ocean, and Environment at the University of Delaware.

Identifying areas across the globe for these high-altitude wind farms has now become slightly easier with last month’s publication of “Airborne Wind Energy: Optimal Locations and Variability” in the journal Renewable Energy. The research maps out the global “sweet spots” for high-altitude wind turbines within the first 3 km of atmosphere above the earth’s surface.

Traditional wind turbines are stationary and have a fixed height, that of the tower that supports hub, generator, and blades, according to Archer, the article’s lead author. High-altitude wind turbines, on the other hand, are not constrained in the same manner. “Airborne devices are amazing because they can move up and down [and] they can be controlled,” Archer explains. This allows them to capture much more energy than traditional, stationary turbines because they can adjust their altitude to stay within a steady stream of wind at all times. 

“For traditional wind turbines, you’re focusing on about 100 m from the ground, [and] there are a lot of good spots, but not everywhere is feasible,” Archer says. “Not everywhere will make sense, because some spots are just not windy enough.”

Airborne wind turbines are attached to the ground via tethers, which either transmit the wind-produced energy or turn ground-based generators

Airborne wind turbines are attached to the ground via tethers,
which either transmit the wind-produced energy or turn
ground-based generators. In this Makani design, the generators
are located on the turbine itself. Andrea Dunlap/Google

But higher altitudes are windier, creating more potential locations for the wind farms, Archer says. “If you can go to 500 [or] 600 m, pretty much anywhere on earth you can get enough wind to actually generate power,” she says. “So all of the sudden, it opens up this possibility of harnessing wind pretty much anywhere on earth, which is amazing.”

There are three general groups of high-altitude wind-turbine systems at the moment, Archer explains. These include soft, kitelike designs that are attached to heavy generators located on the ground; turbines that resemble small, unmanned aircraft that house individual generators in the sky, and simpler systems that make use of helium blimps to hold aloft turbines. (Read “Frozen Flier” about a balloon-based wind turbine that can float 1,000 ft above the ground in Civil Engineering, May 2014, page 40.) Each type is attached to the ground via tethers, which either transmit the energy down to the ground or turn the ground-based generators. Sophisticated control systems operated by computers control the airborne path of the devices, sending them aloft, maneuvering them horizontally in figure eights or circles as their designs require, and bringing them down again as necessary, she says.

While it may seem counterintuitive to place such expensive equipment in the air with only a tether between them and a freefall to the ground, at the heights that are being explored by Archer and her team “it’s actually very unlikely that you will go from enough wind to sustain a device to all of the sudden having no wind,” Archer explains. “It will never happen—if anything, there will be a gradual decreasing of the speed, and your control algorithm would be detecting that the speed is going down.” At that point, preparations could begin to bring the turbine back to the ground if the forecast was for a continued decrease in wind speed, she says. 

But selecting a location for a high-altitude wind farm that will maximize the chance of steady wind speeds while at the same time minimizing the necessary tether heights has proven tricky. Archer and her coauthors’ wind map, which focuses on the first 3 km of atmosphere, could aid in the selection process by providing valuable information about prime locations.

“We were wondering, do you really need to be as high as you can get?” she says. “Which intuitively is the way to go—the [longer] your tether, the higher you can go, the faster the wind. That’s the basic understanding.” However, that widely held belief isn’t necessarily correct, the researchers found.

Another view of the Makani airborne wind turbine

Sophisticated control systems operated by computers control the
airborne path of the high-altitude wind turbine devices, sending
them aloft, maneuvering them, and bringing them down again as
necessary. Andrea Dunlap/Google

“Sometimes the highest winds in the lowest 2 or 3 km [of the atmosphere] are not necessarily at 3 km,” she says. “They could actually be at 500 m or 600 m.” Sending devices past this “sweet spot,” as Archer dubs it, isn’t maximizing the potential wind harvest, and uses longer—and more expensive and heavier —tethers than necessary.

By identifying the level of the atmosphere that hosts the best wind speeds, “you get the best without needing to have these long tethers,” she says. “That’s the great story.”

Data released in the last few years by the National Center for Atmospheric Research (NCAR), of Boulder, Colorado, made the mapping possible. The researchers used the NCAR’s Climate Four-Dimensional Data Assimilation dataset, which covers 21 years—1985 to 2005—and uses a combination of simulations correlated with recorded observations.

The data were “very fine in terms of horizontal resolution and in terms of temporal resolution,” Archer says. “We had a 40 km horizontal resolution, which perhaps doesn’t sound that fine, but having the data at 40 km resolution for the entire globe is amazing—it is a gigantic data set.”

For their global map, the researchers analyzed data from 18 such vertical levels during consecutive two-hour periods over the course of two months—January and July—for each of the entire 21 years covered by the data. These two months were selected as representing the summer months in both hemispheres, when wind speed maxima tend to be at their peak, according to the article. Regions with wind speeds in excess of 10 m/s-1 occurring at least 15 percent of the time were identified and then mapped, according to the article.

But while the global wind map created by the researchers is a first, it does not have the fine-grained analysis necessary as a final step before building an airborne wind farm, Archer points out.

“If you’re investing in a new project, and you want to know exactly how much wind you’re going to get at location A, where A has a latitude and longitude, and it’s exactly this mountain, or wherever it is, you’ll…want to do a measurement campaign [in situ] so that you get exact values, for exactly the location that you’re interested in,” she says. While a resolution of 40 km on a global scale is extremely useful at a macro level, Archer says, it can miss specific topographic features¬—such as the precise location of coastlines—that can affect wind speeds, she says.

As finer resolution data are released in the future, Archer anticipates creating more specific maps that can identify even more localized wind speed maxima. But in the meantime, the researchers have identified several promising areas—including the Great Plains in the United States, certain oceanic regions near the Tropics, and specific locations offshore of the horn of Africa—and the heights at which their wind speed maxima occur.

Luca Delle Monache, Ph.D., a scientist with the NCAR, and Daran L. Rife, the head of mesoscale modeling at San Diego, California-based DNV GL – Energy, coauthored the article.


 

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