The idea is simple: Send kites or tethered drones hundreds of meters up in the sky to generate electricity from the persistent winds aloft. With such technologies, it might even be possible to produce wind energy around the clock. However, the engineering required to realize this vision is still very much a work in progress.
Dozens of companies and researchers devoted to developing technologies that produce wind power while adrift high in the sky gathered at a conference in Glasgow, Scotland last week. They presented studies, experiments, field tests, and simulations describing the efficiency and cost-effectiveness of various technologies collectively described as airborne wind energy (AWE).
In August, Alameda, Calif.-based Makani Technologies ran demonstration flights of its airborne wind turbines—which the company calls energy kites—in the North Sea, some 10 kilometers off the coast of Norway. According to Makani CEO Fort Felker, the North Sea tests consisted of a launch and “landing” test for the flyer followed by a flight test, in which the kite stayed aloft for an hour in “robust crosswind(s).” The flights were the first offshore tests of the company’s kite-and-buoy setup. The company has, however, been conducting onshore flights of various incarnations of their energy kites in California and Hawaii.
“In 2016, we started flying crosswind flight—our system’s power generating mode—with our 600-kW kite, the same model that we used for this summer’s test in Norway,” Felker says. (For comparison, the second-most-powerful wind energy kite being developed today only generates a maximum of 250 kilowatts per kite.) “Our test site in Hawai’i is focused on maturing the energy kite system for continuous hands-off operation.”
By contrast, the Norway tests showcase a strength of airborne wind energy. Makani’s 26-meter M600 prototype, created with support in part by Royal Dutch Shell Plc, requires only an anchored buoy to operate. A traditional wind turbine, experiencing far stronger forces from the wind pushing on its long turbine blades, must be firmly rooted to structures that extend all the way down to the sea floor. So the 220-meter-deep North Sea waters where Makani’s tests took place would simply not be feasible for traditional wind energy turbines, which typically can only operate in depths of less than 50 meters.
As technical program manager Doug McLeod explained at the Airborne Wind Energy Conference 2019, hundreds of millions of people who live near the ocean have no shallow waters nearby and thus no viable options for traditional offshore wind.
“Currently there is no available technology that can economically produce energy from the wind in these places,” McLeod says. “As a floating airborne wind energy technology, Makani believes that we can unlock this stranded resource.”
The buoy for the M600 test flyer, he said, was in fact made from existing oil and gas platform materials. The M600 flyer is an eight-propeller monoplane drone whose propellers loft the drone into the sky from its vertical resting position on the buoy. It looks a little like a traveling musician with a guitar slung over her back.
Once the flyer has achieved altitude—its tether currently extends 500 meters—then the propellers switch off and become mini-wind turbines. Roland Schmehl, co-organizer of AWEC2019 and associate professor of aerospace engineering at Delft University of Technology in the Netherlands, says the flyer’s eight 80-kW rotors make for an impressive system that will be tough for other companies to match.
“The idea is to demonstrate that you can really fly offshore with such a 600-kilowatt kite,” he says. “And the sheer size of the system is something that is very hard to even imagine for most other startups.”
Makani CEO Felker says the purpose of the August test flights in the North Sea was not to produce anything close to the rated generating capacity of the flyer. Rather it was to collect data that Makani engineers can now use to run many more simulated test flights to further develop their system.
“The successful flights validated that our models for launch, landing, and crosswind flight from a floating platform were indeed accurate,” he says. “This means we can confidently use our modeling tools to test system changes—flying thousands of flight hours in simulation to continue to de-risk our technology in advance of commercial operation.”
Types of airborne wind energy systems
Representatives from the world’s next-largest AWE system—the AP-3 tethered flyer from Ampyx Power, based in The Hague, in the Netherlands—also presented a progress report at AWEC2019.
“We want to really experience what it takes to install a commercial-size airborne wind energy system in our AP-3 project,” said Sören Sieberling, project manager at Ampyx Power.
Schmehl says that with a 250-kW flyer called AP-3, Ampyx is finalizing its code and prototype in order to test the system soon. According to an Ampyx representative, AP-3 is still being tested and developed at company headquarters in The Hague. The company is expected to subject AP-3 to test flights in Ireland next year.
Unlike Makani, whose M600 generates wind energy via spinning rotors at altitude—and then sends electricity down through the tether—Ampyx’s AP-3 uses the wind’s power to tug on the line. So the AP-3’s electric generator is on the ground. The kite must therefore spend some of its flight time reeling the line back in so it can tug the line back out again.
The AWE field, Schmehl says, is today active with many research programs and startup companies. Schmehl is an advisor to a company based in the city of Delft called Kitepower that is developing a 100-kW AWE system.
One German company, Kiteswarms, is attaching multiple drones to the end of a tether, hoping that strategy will add capacity and improve reliability as it scales up the technology. Norway’s Kitemill and Switzerland’s Twingtec use vertical take offs and landings to loft their flyers to altitudes where they can begin generating electricity.
Other companies use high-lofted parachute-like kites for generating plain-old propulsion. According to their website, Germany’s SkySails uses AWE “kites” to propel ships at sea, including their futuristic, solar-powered SkySails catamaran yacht.
The Dutch landscape artist Daan Roosegaarde developed a public art project called Windvogel that flew AWE kites at night with illuminated neon green tether ropes. It looked like giant light sabers battling it out in the sky.
Ultimately, the list of AWE projects will narrow, Schmehl predicts, down to the technologies that prove to solve real world problems. For now, though, the field is enjoying its rapid growth spurt.
“We as an industry now have to start earning money—and stop developing for the ultimate, perfect end-product,” Schmehl says. “We should… get our feet into the water and start selling and operating systems.”
This article appears in the December 2019 print issue as “Makani Demos Energy Kites Over The North Sea.”