For a child, the simple act of flying a kite can be exhilarating; running and pulling at the line to send it ever higher into the sky where the winds dance to their own wild tune.
Those same winds that captivate young imaginations are also working their magic on a growing body of researchers intent on harvesting what are known as high-altitude winds. At a height of 200 meters (656 feet) and more, winds tend to blow stronger and more steadily than those closer to the ground.
These winds are so strong, in fact, that they could be used to generate more electricity than we need and significantly more than wind turbines on land can produce. A doubling of wind speed can theoretically generate up to eight times more power.
Moritz Diehl, who heads the Department of Microsystems Engineering at the University of Freiburg, said harvesting high-altitude winds is one of the "most promising" technologies for generating renewable energy in the future.
"You see all the sky above conventional turbines, and you think all this wind energy is just blowing there and it's not used," he said.
Stephan Wrage, CEO of the German wind power company SkySails-Power, wants to change that and make the "largest yet untapped source of renewable energy worldwide" suitable for mass use.
And he's not the only one. For years, engineers, various startups and international companies have been in a race to bring high-altitude winds down to Earth at low cost. Many have failed in their attempts and some have gone bankrupt. But others are on the verge of bringing their flying power plants to the market.
Flying wind turbines and other ideas
One of the first projects to attract attention was launched by the US energy company Altaeros in 2010. Their prototype was a generator attached to a helium balloon - in other words, a wind turbine without a heavy base and tower.
Tested in Alaska, it was connected to the ground by a cable. According to the company, it produced energy for about 50 households at an altitude of 600 meters.
Around the same time, the German company SkySails developed a high-altitude kite to pull entire container ships. The idea was to save diesel used to run the engine, by up to 10%.
Although the test with the kite worked, the shipping company went bankrupt and neither the kite nor the helium wind turbine conquered the market. But both prototypes pointed to one thing: harvesting high-altitude winds requires flying power plants.
Google investment generates hype, until crash
Enter Google. In 2013, the tech giant bought the US airborne wind energy company Makani for an undisclosed amount, triggering euphoria in the niche sector. Their flying power plant, a device about the size of a small aircraft, climbed to an altitude of around 300 meters where it circled in a continuous and automated loop.
The high speeds propelled small wind wheels on the wings, which generated electricity. At the time, Moritz Diehl thought it sounded "crazy" - but it worked. A single flying power plant made enough energy for 300 households, according to Makani.
It seemed to be the breakthrough everyone had been waiting for, until a device crashed into the sea during a test mission. Google's parent company Alphabet subsequently dropped the project, expressing doubts about the economic viability of the flying power-drone.
Not the end, but the beginning
The end of Makani didn't spell the end of airborne wind energy. A new wave of startups has kept working on increasingly small devices that use ever less material. Some have pursued Makani's approach, while others have attached their drone to a rope which tugs at a generator. Still others took the same approach but replaced the drone with a kite.
Among them is SkySails-Power - successor to the bankrupt German company behind the towing kites. Now specializing in energy generation, it has come up with a device that uses a "pumping cycle" to generate power. The kite takes off automatically, directs itself against the wind and unwinds a rope from a generator. It flies in a figure eight, constantly tugging at the rope and creating energy.
The kite is designed to remain airborne for hours, days and weeks. In bad weather or in dangerous conditions, it triggers an alarm and can be recovered.
Not designed to replace existing wind power
Though the sector still requires considerable investment and clarification of many regulatory questions around air traffic, Wrage said the technology could help the 1.4 billion people globally who live off-grid, and often use dirty diesel generators to power their homes.
According to a study by members of the wind industry itself, airborne wind energy could become significantly cheaper than diesel - and even cheaper than traditional wind energy.
SkySails-Power, currently leader in the sector, has sold a first unit to Mauritius. The company is looking to build a high-altitude wind hub in East Africa and operate offshore kite wind farms.
SkySails said a single one of its kites can create energy for up to 500 households, using 90% less material than traditional wind turbines. Other advantages include flexibility of location.
"You could also operate them above the forest. You could stop them operating and or even land if there's a swarm of birds passing by," said Diehl.
Rishikesh Joshi, an aerospace engineering researcher at the Delft University of Technology, said "it will still take a few years" before the technology makes a difference. "The wind industry also took around 40 years to develop to be this cheap," he said.
In the meantime, traditional turbines continue to turn. And even when the airborne wind energy sector is more advanced, the idea is not to replace existing turbines but to make greater use of the winds that blow high above the ground.
Source: DW - Deutsche Welle