Anyone who has ever flown a kite in a strong breeze knows how much force the wind can blow. With this child’s game as a model, some companies want to generate energy in the future: They plan to let kites circle through the air at a height of several hundred meters in order to produce electricity. Up there, the wind blows much stronger and more evenly than in the lower layers. That’s why conventional wind turbines shoot up into the sky. New turbines on land today reach an average height of a good 200 meters, measured up to the tip of the rotor blades. However, the scope for further growth is not all that great. In contrast, the dragon power plants could go much higher for them.
The forces are particularly strong when the missiles do not stand still in the wind, but swing back and forth. “Kitesurfers know this: If you ride the sail in figure eights, it pulls like crazy. The sail literally shoots through the air. We use this principle to generate electricity,” says Florian Bauer, co-founder and managing director by Kitekraft from Oberpfaffenhofen. The spin-off from the Technical University of Munich has developed a kite made of aluminum and carbon, in which several mini wind turbines are integrated. When the plant glides through the air, they generate electricity. Conductors in the guide rope transport the energy to the ground. In order to get the kite airborne and to land it again, the Kitekraft designers designed the system’s electrical unit in such a way that the rotors can also be used as propellers.
If possible this year, the company wants to put a kite with a wingspan of 2.5 meters into continuous operation in order to prove the robustness and safety of the concept. In two years’ time, a ten-meter system is to come onto the market that will achieve an output of 100 kilowatts – a fortieth of the output of typical wind turbines on land. “In the future, we are aiming for spans of well over 20 meters. That would make power in the megawatt range possible,” says Bauer.
The electricity can be generated at height or on the ground
Kitekraft’s technology competes with another concept used by companies such as Skysails from Hamburg, Enerkíte from Brandenburg and Kitepower from the Netherlands: instead of generating electricity at high altitude, it uses a generator installed on the ground. It is driven by the unwinding of the rope as the kite climbs – which also moves in figure-of-eight lanes. Once the missile has reached its maximum flight altitude, it automatically assumes a position with low wind resistance, so that it can be pulled back close to the ground with little expenditure of energy. Then the cycle can begin again.
For Sarah Barber, Head of the Wind Energy Department at the Eastern Swiss University of Applied Sciences, kite power plants are particularly appealing because they allow high wind speeds to be used. “The output increases with the speed to the power of three. Doubling the wind speed means eight times the output. That’s why it’s generally important to go up in wind energy,” says the scientist.
However, she is far from certain that the concept will actually catch on – there are still several hurdles in the way. “The manufacturers first have to prove that the systems can withstand the changing mechanical loads over the long term,” explains Barber. There is still a lot to do with the controls. After all, keeping the kites on course and launching and landing them safely is anything but trivial, says Barber. According to the researcher, the greatest challenge is obtaining permits for the operation of the energy kites. So far, there are no sets of rules according to which authorities could give the green light.
If these tasks can be solved, the question remains: are kite power plants even necessary for the energy transition, given that wind turbines and solar parks are already supplying climate-neutral electricity at unbeatably low prices? Kitekraft boss Bauer is convinced that systems with an output of 100 kilowatts will in future be able to generate energy as cheaply as conventional wind turbines, partly because of the low material requirements. But they also have two unique advantages: “On the one hand, the flying wind turbines are almost invisible,” says Bauer. “This makes it possible to use locations for wind energy where conventional systems are out of the question for reasons of landscape protection.” And secondly, they could be transported to the installation site without heavy transport.
But it’s also possible without it, says Philip Bechtle, who researches kite power plants at the Physics Institute of the University of Bonn, among other things. “We don’t necessarily need the flying systems for the energy transition, the necessary technologies have been around for a long time.” In some niches, however, they could be a useful addition, says Bechtle. He cites coupling with solar parks as a possible field of application. There they could generate electricity when the photovoltaic system weakens due to thick clouds or darkness.
“In this way, the connection to the power grid could be better utilized. That makes the whole system more economical,” says the scientist. In the short term, he sees potential where expensive, dirty diesel generators are still generating electricity today, for example on islands or in remote locations. “This will be the entry-level market for system manufacturers,” says Bechtle. It should then also be easier to obtain a permit for the operation of the energy kites there.