Airborne systems have some key advantages, says Lorenzo Fagiano, an engineer at the Polytechnic University of Milan and board member of the industry association Airborne Wind Europe, founded in 2019. In some countries, land suitable for wind farms are becoming scarce: Farms typically need 71 acres to generate a megawatt, compared to 12 acres for a fossil fuel plant, and ideal locations will eventually run out. “Early farms are in the best places, and the best places are limited,” says Cristina Archer, director of the Center for Research in Wind (CReW) at the University of Delaware.

Also, in general, the higher you go, the faster the wind. “For a double increase in wind speed, that’s eight times the power,” says Fagiano. An overhead system can reach up to 800 meters high (half a mile), well above the 200-300 meter peak of the tallest wind turbines. The theoretical global limit for wind power at high altitudes has been estimated to be about 4.5 times greater than what could be harvested at ground level.

It’s relatively cheap and easy to bring a kite to a remote location, adds Fagiano; these systems come in a container and can be dropped off wherever there is a road or dock. They can also be attached to a barge anchored in deep water where a traditional wind turbine cannot hold firmly. Their height is adaptable, so they can be moved up or down where the wind is blowing the strongest, which often changes with the seasons. “That’s such a good idea,” Archer acknowledges. “The appeal is its simplicity in terms of materials and cost.”

“It will not replace conventional wind,” adds Archer. But proponents envision wind farms hosting hundreds of kites floating on barges in the deep waters offshore, while single wings – or smaller arrays – could unfurl to help power remote islands, facilities temporary military or mining operations in the mountains.

These ideas have been around for decades, but the road to using kites, wings or drones to capture wind energy has been a bumpy one. In 2020, for example, an airborne wind energy company acquired by Google shot to fame after engineers couldn’t run their system economically. But others looking for lighter, simpler versions of the technology, like SkySails, are now going commercial. A 2021 U.S. Department of Energy report to Congress concluded the idea has a lot of potential, with airborne systems likely capable of harvesting the same order of magnitude of energy as ground-based wind systems in the U.S. . But, they add, the technology has a long way to go before it becomes an important part of the country’s energy solution.

SkySails actually started in 2001 with a different goal: to build flexible kite wings to pull ships at sea. The shipping industry has traditionally relied on a crude, dirty fossil fuel called bunker fuel, and the idea was that a wing could, like sails of old, help to greatly reduce a ship’s fuel requirements. It was a concept ahead of its time. SkySails expected oil prices to continue to rise, making their product more attractive. Instead, oil prices crashed in 2009 (and again in 2014 and 2020). Now, with stricter requirements from the UN’s International Maritime Organization for ships to reduce emissions, other companies, including an Airbus spin-off, are making fenders to tow massive ships. But in 2015, SkySails focused on power generation with SkySails Power.

Their system – like several others in development – relies on a parachute-like wing of around 150 square meters to ride on the wind. There are no turbines in the air and the tether is not an electric wire. Instead, power is generated on the ground, from the tug on the line. “Winch breakage generates electricity,” says Fagiano. The software flies the kite autonomously in a figure-eight pattern to get the strongest possible pull to generate power. The system then changes the wing’s flight pattern so that it can be pulled with minimal resistance, expending some energy to pull it up. This pattern repeats itself, creating far more energy than it consumes.

It sounds simple and the power generation system is pretty standard. But Stephan Brabeck, director of technology at SkySails, explains that it took the team around 7 years to perfect the flight software, in particular so that the wing could safely land and take off autonomously. They have now made and sold five units, Brabeck says, with Maurice’s being the first to be operational. They estimate that the wing will have to land around 14 times a year due to heavy rain, unsuitable winds or thunderstorms. Occasional hurricanes, which an overhead system can contend with hidden on the ground, are what make the island unsuitable for traditional wind turbines, says aerospace engineer Brabeck.

The sails are less intrusive on the horizon than traditional turbines and quieter too, Brabeck says. And they make economic sense, he says, for anyone currently paying more than 30 cents per kilowatt hour from diesel generators. But there are challenges. Wind turbines can kill or injure migratory birds, and how birds will react to these kites “has not yet been studied very well,” Fagiano says. SkySails has ongoing studies. Tethering such a system, Archer notes, could theoretically trigger drones or even small planes. And if a tether breaks or the guidance system fails, a system can crash to the ground.

This might not be a big deal for a soft wing, but other companies are looking for stiff wings more like a hang glider than a paraglider. These may be more efficient and have better control, but crashes may be a bigger problem, making them a better bet for offshore use. “Essentially, they are airplanes,” Fagiano explains. “They will have to achieve a level of reliability close to civilian aircraft.”

A third, more ambitious option is to build a hard-winged drone that packs heavy wind turbines and generators and sends electricity down the cable. This option would produce more constant energy (without needing to cycle between energy production and energy expenditure), but it’s a difficult problem to solve.

“We’re talking about a completely new technology with a lot of aspects,” Fagiano says. “New wind turbines. Brand new. Google took over one such project, led by Makani Technologies, in 2013. They had a few successful trials, but the saving didn’t add up, and in 2020 the Makani project folded. Google released a YouTube movie about the experiment and made all of Makani’s R&D and patents available for free.

Many other companies are now in the race to pick up where Google left off or find a better solution. This includes Netherlands-based Kitepower, which has a project in the Caribbean, and Norway-based Kitemill, which aims to manufacture megawatt-scale systems. Others are even designing similar systems that work on the same principle, but underwater rather than in the air, using ocean currents instead of the wind to drive a figure-of-eight underwater glider. SkySails plans to test the concept of an airborne wind farm in the US Midwest before moving offshore. “You need a lot of space,” says Brabeck.

As commercial activity ramps up, Fagiano says, one of the biggest hurdles is regulatory: Airspace rules aren’t designed to accommodate these wings. “It’s chicken and egg,” he says. “As long as there are no technologies, they do not regulate. Without regulation, it is difficult for companies to raise funds.

With the first commercial pilot products now available, “in remote areas, the costs are already quite competitive,” says Fagiano. If aerial wind systems begin to be mass-produced, he says, there is no doubt that they will produce affordable energy. “The question,” he says, “is whether we ever reach mass production.”