The island of Curacao bears all the hallmarks of a tropical paradise. A city-sized scrap of land in the Lesser Antilles, it boasts sandy beaches, coral reefs and an undersea cliff called “the blue edge” that attracts scuba divers from all over the world. The trade winds that once brought sailing ships (and, unhappily, slave traders) to this former Dutch colony also endow it with a balmy climate. Temperatures in December hover around 27 °C, and the sun shines, on average, 270 days a year.

Look closer, though, and Curacao has its challenges. Though richer than most of its Caribbean neighbours, and less isolated than many, the cost of living here is high. Curacao’s 150,000 residents import a lot of essential goods, including the fossil fuels that generate much of their electricity. Dependence on imported fuel is ironic as well as expensive. Thanks to a huge oil refinery near its main harbour, Curacao had, in 2014, the second-highest carbon emissions per capita of any country in the world – not a great look for a low-lying island threatened by rising sea levels.

To Johannes Peschel, the answer to Curacao’s carbon quandary lies in its greatest natural resource: the wind. Curacao was an early adopter of wind power, and by 2017 commercial wind farms were producing 30% of its energy. But Peschel, a German entrepreneur with a bushy beard and a surfer’s laid-back enthusiasm, wants to push that figure higher, and his plans do not involve adding more turbines to the coastline. Instead, he and his local partners are pinning their hopes on a technology that is both centuries old and entirely new: giant, tethered kites that generate energy as they soar through the blue Caribbean sky.

Let’s go fly a kite

For Peschel and others in the small but growing airborne wind energy (AWE) community, the future of wind power looks nothing like the familiar three-bladed turbines scattered across the hills and coasts of Europe. Researchers in this nascent field are working on a dizzying array of devices, including kites, wings, drones and even a set of spinning, sky-borne hoops that are being developed by a Scottish firm called Windswept and Interesting. In all cases, the goal is to harness high-altitude wind in a way that is cheaper and more flexible than erecting lofty columns of steel and concrete.

To residents of islands like Curacao, as well as communities in areas without conventional electrical grids, AWE offers substantial advantages. Peschel – whose company Kitepower spun out of a research group at TU Delft in the Netherlands – notes that its biggest kite, with an area of 100 m² and a generating capacity of 100 kW, fits into a large surf bag and can be launched by two people in 20 minutes. Fixed-wing AWE systems are bulkier, but not by much. Rolf Luchsinger, who leads a Swiss AWE firm called TwingTec, drew appreciative murmurs at a recent industry conference in Glasgow, UK, when he showed a photo of TwingTec’s drone and ground station (total mass: 1 tonne) sitting next to a standard turbine (20 tonnes). The two systems, Luchsinger observed, produce comparable amounts of energy – but only TwingTec’s will fit in a shipping container.

TwingTec, Kitepower and a clutch of similarly named companies (EnerKite, Kitemill, SkySails Power, Windlift and so on) are pursuing the same path to market that conventional renewables followed a generation ago. By offering small, 10–100 kW systems to customers in remote locations – where costs per kWhr are high and the main alternatives are dirty, noisy diesel generators – they aim to refine their technology and prove its worth before scaling it up.

Other firms are targeting large-scale power generation from the start. A second Netherlands-based start-up, Ampyx Power, is developing a tethered aircraft that can take off and land on floating platforms far out to sea. Its newest model has a rated power of 300 kW, and chief executive Richard Ruiterkamp says the company will begin flying it at a purpose-built test site in County Mayo, Ireland, sometime next year. In August the US-based firm Makani tested its 600 kW kite in a Norwegian fjord, collecting reams of data on how the kite-tether-buoy system (see photo above) behaves under real-world conditions. Both companies envision a future where large, multi-kite farms deliver hundreds of megawatts of low-carbon energy to the grid. “I’m really passionate about bringing renewable energy to more and more people around the globe,” says Doug McLeod, Makani’s technical programme manager. The August test was, he says, “an exciting moment”.

Up to the highest height

Kites have been used to pull loads since ancient times, but their power-generating potential was not fully appreciated until relatively recently. During the oil crisis of the late 1970s, Miles Loyd, an engineer at Lawrence Livermore National Laboratory in the US, began studying a phenomenon that novice kite-fliers learn the hard way: a kite moving perpendicular to the wind pulls much harder than a stationary kite with the wind behind it. This is because the kite’s aerodynamic lift force scales with the square of its apparent airspeed. In cross-wind flight, this apparent airspeed can be easily an order of magnitude higher than the wind speed relative to the ground.

Loyd’s contribution was to calculate that a kite flown in fast loops across the wind would produce enough lift not only to support itself, but also to generate a useful amount of power – hundreds of times more power, in fact, than a kite in static flight. Using realistic numbers for wing area, wind speed and lift-to-drag ratio, Loyd estimated that a single “crosswind” kite could produce as much as 45 MW. This was far higher than the turbines of Loyd’s day, and it remains competitive even now. As of 2019 the world’s most powerful commercial wind turbine, the Vestas V164, maxes out at 8.8 MW.

Falling oil prices took some of the shine off Loyd’s ideas, and a few of his approximations would be right at home amid the frictionless surfaces and spherical cows of introductory physics courses. In estimating the maximum power output, Loyd ignored both the mass of the kite-tether system and the aerodynamic drag on the tether. Neither approximation is valid, and the second is particularly egregious. Luchsinger, a physicist by training, says that for small craft like TwingTec’s drone, the drag on the tether is greater than the drag on the wing. Even so, Loyd’s work remains influential. Most AWE companies are building kites that fly across the wind and generate electricity using one of the two methods Loyd proposed (see box).

Send it soaring

In his seminal paper (J. Energy 4 106), Miles Loyd proposed two ways of making crosswind kites do useful work. One method – which he termed “lift mode” – is to use the kite’s aerodynamic lift to pull a load on the ground, for example by winding the tether around a drum, allowing it to unspool, and using the drum’s rotation to drive an electric generator. The alternative – known as “drag mode” – is to attach turbines to the crosswind kite, generate electricity onboard and transmit it to the ground via a conducting tether.

Kites that operate in lift mode must periodically lose lift and be reeled in while the tether is slack. In effect, these kites act like giant, slow-motion, airborne yo-yos, generating a lot of energy on their way out and losing a little on their way in. (Kitepower’s Roland Schmehl, more prosaically, likens the “pumping” process to the thermodynamic cycle of a reciprocating engine.) The crosswind devices at (among others) Ampyx Power, EnerKite, Kitepower, Kitemill, SkySails Power and TwingTec all produce electricity in this fashion.

AWE systems that operate in drag mode fly in less complex patterns, and their onboard turbines can power motors that enable vertical take-off and landing. Makani’s kite generates energy as it flies in circles across the wind, and hovers before landing on a floating buoy called Havdrage-1 (the name means both “ocean kite” and “ocean dragon” in Norwegian). Another firm, Windlift, is developing a drag-mode AWE system with funding from the US military. Its chief executive, Rob Creighton, notes that drag-mode devices can remain aloft in powered flight if the wind slackens – a big advantage for customers who may not want to stop other activities to land a kite at short notice. The disadvantage is that conductive tethers have larger diameters, and so experience more drag.

If the power-generating potential of crosswind flying helped get AWE off the ground, other advantages may prove just as important in keeping it aloft. A key part of the technology’s appeal is that kites can harvest wind energy that conventional turbines cannot. A typical 1.5 MW turbine stands 100 m tall, and its net capacity factor – the actual energy produced, divided by the maximum possible – is usually less than 50% due to the intermittent nature of wind. Kites, in contrast, can fly at altitudes of up to 500 m, where winds are stronger and more consistent.

The greatest attraction of AWE systems, though, is their low mass. An energy kite is a turbine stripped back to its power-generating essence, with no need for weighty components such as a base and a tower. Kitepower’s co-founder Roland Schmehl, a researcher at TU Delft, compares it to a set of flying rotor tips. The drastic reduction in mass should, in principle, reduce costs, making wind energy more likely to be exploited. According to Cedric Philibert, a renewable-energy expert at the International Energy Agency (IEA), conventional wind turbines in European waters could generate 2600–6000 TWhr per year, or 80–180% of current EU demand – but only at a cost of €55–70/MWhr. The current price in France, Philibert notes, is €44/MWhr.

AWE advocates say that their devices will make it possible to harness much more of the Earth’s wind energy. A combined solar and AWE farm, where kites cast tiny, transient shadows as they circle high above the photovoltaic panels, is one possibility. Kitepower’s Curacao project, in which the company plans to operate three to five 100 kW kites on an existing conventional wind farm, is another. The most tantalizing prospect, though, is that AWE could unlock the “stranded resource” of wind in off-shore locations where the water is too deep to fix conventional turbine towers to the ocean floor. Compared to a floating turbine, a floating AWE platform would need a much less massive counterbalance to keep it upright in stormy seas.

Turbulence ahead

All these advantages come at a price, and Ruiterkamp of Ampyx Power sums it up well. “The threshold for getting a first series to work is way higher than in other areas of wind energy,” he told his colleagues in Glasgow. “We are working on intrinsically unstable systems. If something goes wrong, we are immediately in a situation that needs to be handled.”

With a device as complex as an energy kite, the list of things that can go wrong is extensive. In another talk at the Glasgow meeting, Kitemill technical manager Lode Carnel rattled off a litany of problems that his Norway-based team had overcome on their current, rigid-wing prototype. Weak links breaking. Electrical connectors not up to industrial standards. Tether and wing materials that proved too flimsy or heavy. Though minor in themselves, Carnel explained that these issues contribute to a vicious circle in which investors see AWE technology as immature, and thus not worth funding – meaning that companies struggle to get the days, weeks and months of flight experience they need to refine their systems.

It doesn’t help that most AWE devices, unlike turbines, need to land when wind conditions are poor. Landings and launches are hard to automate, and each one raises the risk of catastrophic failure. Lorenzo Fagiano – an associate professor at Italy’s Politecnico di Milano who did his PhD research on AWE – points out that even if an individual kite can fly for two days straight, a large-scale AWE farm with 300 such kites would still experience 54,000 take-off-and-landing events each year. “This phase has to be proven against any fault with an extremely high probability,” he warns.

In off-shore operations, the bar for safe launch and landing is even higher. Each contributing system must work in a remote and harsh environment, with minimal maintenance and generally without supervision. In this respect, the industry’s first offshore test was both an impressive milestone and a sign of how much work remains. Landing Makani’s kite on a moving buoy was, McLeod observes, “like trying to parallel park a car while the kerb is moving up and down and back and forth, and also rotating for good measure”. Although that landing was successful, the test ended with the loss of the kite.

Look out below

In a young industry with a new technology, such mishaps are inevitable. They are also the reason why AWE is, at present, confined to lightly populated areas. At the Glasgow conference, there was broad agreement among delegates that an accident – such as a kite colliding with an aircraft, or with a person on the ground – would harm the entire sector, regardless of which company was responsible. But the question of how to regulate AWE provoked a rare public dispute, centred on a question that members of this friendly, freewheeling community sometimes struggled to answer: just what are these strange new objects in the sky?

For Neal Rickner, Makani’s chief operations officer and a former F/A-18 pilot, the answer is simple. “We are talking about wind turbines, not aircraft,” he declared. “From a pilot’s point of view, you would not expect an energy kite to be in one part of the sky at one moment, and another part at the next.” A failure of a kite’s tether, Rickner argued, should be treated just like a failure in the blade of a conventional turbine. “We need to imagine a future where systems are highly reliable, and therefore operate under the same procedures as the [conventional wind] industry is using today,” he concluded.

Some of Rickner’s fellow panellists were less convinced. Michiel Kruijff, head of technology at Ampyx Power, explained that its kite stores energy on board so it can fly back to its platform if its tether breaks. That means it must follow aviation rules, although Kruijff said that Ampyx hopes to negotiate some adjustments. But in his view, even companies without these constraints need to face some facts. “We all acknowledge that we are primarily developing an energy-generation device,” he said. “But we cannot deny that there is an element in it that is not like a wind turbine.” Turning to Rickner, he asked rhetorically, “When you look at your beautiful system in the air, do you say, ‘Look how beautifully it is rotating’? Or do you say, ‘Look how beautifully it is flying’?”

Semantic and regulatory disputes aside, questions about how to keep energy kites safe may prove easier to answer than questions about how to make them reliable enough for commercial operations. Asked to name the hardest problem in AWE, Ruth Marsh, who spent 15 years working in conventional wind energy before joining Makani as its product and system lead, looks thoughtful. “There are a lot of hard problems,” she says. Then Marsh – a former aeronautical engineer at NASA, and a rare veteran woman in a field that skews young and male – offers an analogy.

In solar panels, she says, structural and power-generation components are separate. They can be optimized independently. In conventional turbines, structure and power interact, so they must be optimized together. But energy kites have lots of moving parts, all of which must work optimally together for the system to perform at its peak. As the technology matures, Marsh suggests that the most important numbers to watch will be power production, net capacity factor and levelized cost of energy – hard, commercial measures that will, in the end, determine whether the AWE industry takes off.

Flying high and growing big

Over the next few years, AWE developers hope to fly their devices for longer periods, with less human intervention. Kitepower’s mini-AWE park in Curacao will operate automatically under the watchful but remote eyes of technicians in Delft, who will use sensor data to decide when to land the kite for maintenance and re-launch by a local ground crew. Another company, SkySails Power, has already deployed its kite as an auxiliary generator on a catamaran operated by an environmental charity, Race for Water. In 2020 it plans to offer commercial, ground-based “plug and play” AWE units of up to 500 kW.

For companies at this stage of development, the state-of-the-art is days or weeks of automated flying. The long-term goal, especially for off-shore applications, is to push that to months or even years of complete autonomy. It’s an ambitious target, and it isn’t clear when (if ever) it will be met. Fagiano points out that self-driving cars and robotic systems, which face analogous challenges, are getting billions of euros in funding. For AWE, he says, that level of support “may never be there”.

The physics behind airborne wind energy is sound, and it has a rare ability to capture the imagination

Still, the physics behind AWE is sound, and it has a rare ability to capture the imagination. “People like kites, and they like green energy,” says Joep Breuer, Kitepower’s technical manager. “It’s a very likeable technology.” By 2027 the IEA predicts that wind will be the number-one source of the EU’s energy, but even that will not be enough to meet stringent emissions targets. Alexander Bormann, the chief executive of EnerKite, concurs. “Some people think that airborne wind is crazy stuff,” he says. “You know what I think is crazy? Installing less capacity in renewable energy. We need to save gigatonnes of carbon dioxide today, not tomorrow. We need to fly high and grow big.”