Winds at high altitudes are strong and have enormous potential for generating energy. But so far they have not been tapped. A Dutch start-up is set to change that.
Accelerating the transition to renewable energies is key to fight the global climate crisis. Photovoltaic systems and wind turbines are important building blocks for a sustainable future, but they also have their limitations: Solar energy is only available during daylight, photovoltaic power stations come with high installation costs and substantial space requirements, and conventional wind turbines are transport cost intensive and need massive foundations. Dutch start-up Kitepower offers a mobile wind energy solution that is easy to trans- port, easy to install, and only requires small ground space. It consists of a ground station, a tether, a kite, and a kite control unit hanging just underneath the kite.
The system is operated in cycles. In each of these cycles, energy is generated during the reel-out phase, when the kite is flown in a cross-wind figure-of-eight pattern to achieve a high pulling force. In phase two, when the maximum tether length is reached, the kite’s profile is adjusted in order to reel in the tether, consuming a small fraction of the previously produced energy.
THE BEAM met up with Roland Schmehl, co-founder of Kitepower and leading academic in the emerging field.
THE BEAM: Mr. Schmehl, Kitepower has developed a system called Falcon which generates electricity using a kite. Can you explain the steps that led to this innovation? When did you start developing it?
Roland Schmehl: I have been involved in the development of airborne wind energy since 2009, when I was hired as a research group leader. With the university team we initially developed a 20 kilowatt system that used a robotic kite control unit and a tether that is reeled in and out between 200 and 600 meters. In 2014, an angel investor approached us with the aim to commercially develop the technology. A year later, we were awarded an EU funding of about 3.7 million euros and in 2016 we founded Kitepower as a spin-off from Delft University of Technology in the Netherlands. With the start of the commercial development, we have been working on a 100-kilowatt system called Falcon scaling up from 25 m2 to 60 m2 wing area.
Airborne wind energy systems are much more variable
At what height does wind power actually turn into airborne wind power? Good question. Conventional blade tips of towered wind turbines reach heights of up to about 200 meters, with the largest turbines reaching even 260 meters. Although these turbines are getting bigger and bigger there is a physical limit for towered systems. Airborne wind energy systems are much more variable, i.e. can be operated with tether lengths of up to currently 600 meters. Technically it would be possible to harvest energy even higher, however, this gets much more complex because the kite is further away, the visual contact is gone, the tether dynamics become more complex, and there is also a higher aerodynamic drag on the tether; all of this has a negative effect on performance.
In addition, the airspace for normal air traffic at our first test site near to Leiden, the Netherlands, begins at a height of 500 meters; To operate the kite in this airspace, there are not only technical but also increased legal hurdles requiring different permits. That is why Kitepower operates in the height range between 150 and 450 meters.
What are the technical challenges of a kite system for generating electricity?
There are a number of challenges. For example in flight control. In contrast to a wind turbine, a kite power system has an almost infinite number of degrees of freedom. To say it simple; the turbine can only rotate around the rotor axis, whereas a kite can fly in all directions.
There have been attempts to develop a kite that flies circular or figure-eight-shaped patterns all by itself without any control. This is theoretically possible, but difficult to implement in practice because the wind always fluctuates in magnitude and direction and varies in altitude. Therefore, all current prototypes have an automatic flight control. We developed and demonstrated automatic flight in Delft back in 2012.
Are there also innovations in terms of material?
Absolutely yes. We work with suppliers who provide us with high-performance materials. For example a tether made from a high-performance fibre called Dyneema. This is a polyethylene-based plastic in which the molecular chains are straightened by a special manufacturing process, giving this fibre a breaking strength that is 15 times stronger than steel. The material is also used in other areas, such as the military, because it is bulletproof and can be used in helmets, for example. For the kite itself, we use nylon and dacron, textile fibres one might know from backpacks and camping equipment. The latest generations of kites also contain reinforcements made of carbon fibre composites. And even Dyneema can now be used as a textile material, for heavily loaded parts. Actually, these high-performance kites should be called a hybrid of an airplane and a kite. And in order to advance into the megawatt production of electricity in the future, larger and even more powerful kites will be needed.
How does a kite compare to wind turbines and solar power systems in terms of energy generation?
Comparing kites with wind turbines and solar power systems is difficult because the technologies function quite differently, using different resources. In fact, we want our kite power systems to complement the existing renewable energy sources. Wind turbines are operated close to the ground, up to about 200m. However, if you go higher, the wind becomes stronger and more constant. And the idea of airborne wind is that you can still generate electricity even when the wind turbines are stationary on the ground or have a poor yield. We see kite power as a useful addition to currently operated wind turbines. And it‘s similar with solar energy. Nothing is produced there at night. With airborne wind energy, however, you can also produce electricity at night. The problem of the base load, i.e. being able to supply electricity continuously, not only temporarily or under suitable conditions, could thus be defused. And that is the key advantage.
Are there any other innovations, for example in the ground station?
Yes. The ground station is the most expensive component of the kite power system, the generator is also located there, but in terms of new technological developments, the ground station is not the highlight. The aircraft, i.e. the kite, and the control of the kite are more the focus of innovation. I would say that the automatic flight control of the kite is the key enabling innovation for airborne wind energy.
Can kite power be used in all weather conditions?
Basically yes. Rain and snow, for example, are easy to handle. In theory, even strong winds can hardly harm the kite because the flight maneuvers can be stopped which decreases the aerodynamic load on the kite. One problem, however, are thunderstorms. Lightning strikes are a problem for the system, especially if the tether is wet and conducts electricity. Therefore it is better to haul in the kite during thunderstorms.
Is kite power particularly maintenance-intensive, since it is exposed to wind and weather?
Yes, that is another challenge for the future. The materials used, Dyneema, Nylon and Dacron, are subject to dynamic bending stress during the entire period of use, because the tether is constantly bent when it is rolled up. There is also UV radiation. And the combination of both ensures that both the tether and the kite have to be replaced quite often. We are working on improving this in the future. The installation of the 100 kW system, however, is straightforward. The system comes in a normal shipping container. Everything is in there. You can then put it anywhere and everything is set up within a few hours.
Is the system also suitable for metropolitan areas?
The use of airborne wind energy systems is primarily intended for remote areas that are difficult to reach, at least for now. Basically, a kite power system by itself is a kind of microgrid. It has its own rechargeable battery and supercapacitors to buffer and store the generated electricity. And then you can adapt the module with inverters to the respective local grid and feed in the electricity.
We are currently researching how to place several systems next to one another. The tethers shouldn’t tangle with each other and the kites should not collide. Plans for entire kite parks have yet to be worked out, except for theoretical studies.
Large-scale production is still a long way off
The company tested the Falcon in the Caribbean. What happened there?
We operated the Falcon on the island of Aruba which is part of the Kingdom of the Netherlands. The Dutch military approached Kitepower in April 2021 with the proposal to participate in a joint exercise of the engineering corps on Aruba in October 2021. They see the mobile system of Kitepower as the ideal solution to avoid being dependent on diesel fuel in their camps. They already use solar cells, but Kitepower has the decisive advantage of being more compact and also being able to be put into operation more quickly. At that military exercise, the goal was to provide electricity from the Kitepower system. It worked.
How much have the development costs been so far?
There was public funding from the EU and European national governments respectively of around 55 million euros. But it was not only for Kitepower, but also for several other companies in that sector. As far as Kitepower itself is concerned, there was the funding of 3.7 million euros from the EU in 2015, but this financing was shared with supplier companies and the university, who were project partners. In spring 2021 there was a private investment of 3 million euros. And then there are the smaller projects with the military, but I cannot say what sums were involved here.
How much will the unit costs be once the product is ready for the market?
That is a more difficult question. The Kitepower CEO always says he would like to get under 100,000 euros with the Falcon. However, we are talking about small series here. Large-scale production is still a long way off.
When will the Falcon be launched on the market?
There are already some power plant operators who are very interested in airborne wind energy. If all goes well, there will be first pilot plants in 2022.
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