Wind Energy: Swapping Turbines for Kites
The ideas to harness wind energy from a kite, or other highflying tethered wind energy collectors are pretty numerous, however most are economically unsustainable even though beautiful, spirit lifting, and fun to watch. Kites are a blast. Can they make power?
The founders of Berlin-based wind energy developer NTS GmbH are sending stunt kites into the skies to harness the wind and convert the kinetic energy generated into electricity.
Stunt kites are used for kite surfing, a fashionable sporting activity, with the number of enthusiasts participating in this cross between wind surfing and stunt kite flying growing at a tremendous pace. When the wind catches the kite, the surfer is carried meters into the air; and the greater the leap, the bigger the thrill. But a modern stunt kite is capable of being much more than a mere piece of sporting equipment – it has the potential to become a useful energy producer.
Fraunhofer Wind Energy Gathering Stunt Kites Tethered to Generator Cart on Test Track.
A stunt kite’s aerial movements can be used to drive a generator, which in turn converts this kinetic energy into electricity. The NTS GmbH folks brought in the Fraunhofer Institute for Manufacturing Engineering and Automation IPA (FIMEA) in Stuttgart. The project partners intend to use their new method to harness the power of strong winds at altitudes of up to 500 meters.
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Joachim Montnacher, an engineer at FIMEA, explains how a “kite power station” works. “The kites fly at a height of 300 to 500 meters, perfectly positioned to be caught by strong winds. Cables, about 700 meters in length, tether the kites to vehicles and pull them around a circuit on rails. A generator then converts the kinetic energy of the vehicles into electricity. The control and measuring technology is positioned on the vehicles.”
Compared to conventional wind turbine technology that relies on rotors, kite technology offers a wide range of advantages. Wind speeds at ground level tend to be low, but the speed increases dramatically the higher you go. At a height of 100 meters wind speeds are around 15 meters a second; at 500 meters they exceed 20 meters a second.
Montnacher points out, “The energy yield of a kite far exceeds that of a wind turbine, whose rotor tips turn at a maximum height of 200 meters. Doubling the wind speed (higher up) results in eight times the energy. Depending on wind conditions, eight kites with a combined surface area of up to 300 square meters can equate to 20 conventional 1-megawatt wind turbines.”
The altitude advantage of kites solves in part the struggle with the consistancy of the wind that hinders wind turbines, because the higher you go, the windier things get. Figures in Germany for the past year show that at a height of 10 meters, there is only about a 35 percent chance of wind speeds reaching 5 meters a second, but at 500 meters that likelihood goes up to 70 percent.
Another advantage is that it costs considerably less to build a system that, among other things, does not require towers weighing hundreds of tons apiece.
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The project partners have divided up responsibilities: NTS GmbH will design the kites and construct the high-altitude wind farm, and the researchers from FIMEA will be in charge of developing the control and measuring technology, which includes the cable winching mechanism and cable storage.
One of the jobs of the control unit is to transmit the measuring signals to the cable control and kite regulation mechanisms. A horizontal and vertical angle sensor located in each cable line and a force sensor within the cable distributor guarantee precise control of the kite’s movements as it follows either a figure-of-eight or sine-wave flight path up above. These flight maneuvers generate a high pulling power of up to 10 kilonewtons (kN) – meaning that a 20-square-meter kite (about 24 square yards or a bit less than a 15 foot square) has the capacity to pull one metric ton. Each vehicle would be pulled by a different flight system.
At a test site in Mecklenburg-West Pomerania, FIMEA researchers and NTS GmbH have been able to send a kite on its maiden voyage along a 400-meter-long straight track. A remote control similar to those used to fly model planes was used to manually control the kite. The team now wants to reconfigure the test track making it into a loop. Computers will eventually be used to achieve fully automatic control of the kites.
Guido Lütsch, managing director of NTS GmbH said, “According to our simulations, we could use an NTS track running a total of 24 kites to generate 120 gigawatt hours a year (GWh/year). To put this into perspective, a 2-megawatt wind turbine produces around 4 GWh/year. So an NTS system could replace 30 2-megawatt turbines and supply power to around 30,000 homes.”
With the successful test flights on the demonstration track, the project partners are confident that their computer simulations will hold up in reality. The first investors are already on board.
Sounds very good. There are a couple troublesome questions. One is the amount of land area involved and the other is the matter of re flying downed kites. Then there’s the matter of 24 of them becalmed and coming down all at once.
The vast advantage is that kites could work in far more places than the wind turbine friendly regions gathering the energy much closer to where it’s needed and used. The build cost advantage may well over take the land area matter and the skills to re fly a set of kites can’t be too exacting.
The kite idea has seen a lot of very innovative, costly and high power producing ideas. This might be the one that goes commercial. It could get very productive when scale, optimization and experience are added in. Let ‘em fly!
By. Brian Westenhaus