Shulou(Shulou.com)11/24 Report--

Every child who has ever flown a kite should know this lesson: once you can get the kite off the ground and into the air, you are more likely to find a steady breeze to keep the kite high in the air.

A burgeoning wind power industry has learned from this experience. Some companies fly heavy kites to an altitude of 200 meters or more, using the wind that keeps the kites stable in the air to generate electricity.

At least ten companies in Europe and the United States are developing a variety of such kite power generation systems. If they succeed, it will be possible to use kites to build wind farms in places where the wind is not strong enough to support traditional wind turbine towers. Kites may also be a better choice for offshore wind power and could one day even replace at least some of the anchored wind towers currently in use.

"the kite power generation system has lower production and transportation costs and higher efficiency." Florian Florian Bauer, co-chief executive and chief technology officer of Kitekraft, a Munich company that is developing a kite power system. Kite power generation also has a lower carbon footprint, Bauer said. "if you have so many advantages, why build traditional wind turbines?"

But before it can become a widely used source of electricity, "aerial wind power" (the term is another name for a kite power generation system) needs to overcome a considerable number of technical and commercial challenges. In addition, people need to prove that the technology is safe, will not harm wildlife, and will not cause unbearable noise or visual disturbance to nearby residents.

At present, kite power generation is still in the new stage. Most companies are committed to relatively small verification kite power generation devices, and no one has expanded their power generation technology to a megawatt level comparable to traditional wind turbines. But this small version of the power generation system has entered the market.

In 2021, Tianfan Electric Power (SkySails Power) of Hamburg became the first company to offer commercial products. Its product model includes a controllable kite with an area of 180 square meters. The kite is connected to a ground station in a container by an 800-meter-long rope.

The track of a kite flying in the sky is painted in a huge graceful "8" shape and drives generators on the ground, producing an average output of 80 kilowatts, which is enough to power about 60 average American homes. Although this is tiny compared to a typical 2.75 megawatt wind turbine, its power is comparable to that of many portable industrial diesel generators. Such power generation units are designed to use electricity in remote areas away from the power grid.

Ultimately, these companies aim to build kites that can generate electricity on the order of megawatts. They envisioned hundreds of kites flying in groups in wind power plants to provide power to the grid.

This table shows the comparison of the output power of various power generation methods: existing kite "Skysail" (80 kW, which can power 60 homes); wind turbine (2.75MW, 2160 homes); and expected next generation kites (3.5MW, power supply 2800 homes) Small nuclear reactors (582MW, with 465600 homes) use high-speed winds close to the ground to be slowed by friction from trees, buildings, hills and the ground itself. So the higher the wind is, the greater the wind speed is-on average, the wind speed at 500 meters is 30.7 kilometers per hour faster than at 100 meters. Over the past few decades, a number of schemes have been proposed to develop and utilize these faster high-altitude winds, including installing turbine generators on lighter-than-air aircraft or hanging them from kites that are still in the air. But most companies are looking for a way to gain more energy by using maneuverable, computer-controlled kites to fly in the air in a specific pattern.

Airborne wind power systems use two methods to generate electricity. One method is to generate electricity on the ground, using the "pump" mode to run the generator on the ground. The ground end of the kite-flying rope is wound around a pulley, and when the kite flies in the wind, it pulls the rope loose from the pulley, which drives the generator to rotate to generate electricity. Then the kite hovered in the air, retracted the rope and wound it around the pulley, and the next cycle began.

Another way is to generate electricity from a kite. The method of generating electricity carried by kites requires kites to be rigid, similar to aircraft wings, and used to install small wind turbines. When a kite is flying, the wind drives the turbine to generate electricity and transmits it to the ground station through a cable.

Bauer's company uses kites to generate electricity, which makes the turbine blades dual-purpose. During the launch and landing phases, the blades are powered by engines and become propellers that allow kites to fly and hover like aerial drones. Once the kite reaches the right height, the turbine switches to power generation mode to get energy from the wind.

Air wind kites generate electricity in two ways. The "pump" mode uses the pull of the kite to rotate a drum on the ground to drive the generator (power generation, yellow lightning); when the rope wound around the drum reaches the end, the kite is pulled back and starts the next cycle (consuming a small amount of electricity, red lightning). Airborne power generation is generated by turbines installed on kites, which requires kites to be rigid. Building a kite power system has a potential advantage over today's wind towers in terms of materials used. The construction of wind turbine tower must have a solid foundation and steel structure, which can be used to set up the wind turbine to the right height. These sturdy supporting structures are no longer needed in kite-based power generation systems, and are replaced by relatively small ground stations and a light rope. A study by the European Air Wind Association (Airborne Wind Europe), an European trade association, found that a kite-based wind farm with an installed capacity of 50 megawatts consumes 913 tons of construction materials over a 20-year working life, compared with 2868 tons for a typical wind tower. Less raw material consumption makes the kite power generation system more environmentally friendly and cheaper.

Kites may also have the opportunity to display their talents in offshore wind power in deep water. Today, if offshore wind power is built in places where the water is too deep to lay foundations, wind turbines float on huge barge-like structures that must be able to withstand the weight of turbines and remain stable. Because kites are much lighter than wind turbines, they only need to be fixed with lighter and cheaper barges.

But the price of these advantages is the increase in system complexity. If kite power generation systems are to bring real economic benefits, they must be able to operate for a long time with little or no one monitoring. This requirement presents a computer control challenge, said Chris Vermillion, director of the Renewable Energy and Energy efficiency Control and Optimization Laboratory at North Carolina State University and a consultant to a kite power technology company.

Kites for power generation do not float passively in the air. On the contrary, kites use their own aerodynamic characteristics to fly in a "crosswind" mode, which is a bit like the technique of "zigzag" traveling back and forth in the wind in sailing. When kites fly perpendicular to the wind, their wings generate lift and pull the rope with greater force. This extra and lift can be converted into additional speed, which can either generate a greater force pulling the cable for ground-based power generation, or into a higher air velocity to drive airborne turbines faster. Regardless of the mode of power generation, the amount of electricity generated is at least one order of magnitude higher than that of not following the "crosswind" mode.

Flying in the "crosswind" mode increases the relative speed between the kite and the air, so the kite can harvest potential energy from the wind. This picture shows the experimental results of the KM1 prototype of Kite Mill (Kitemill), and you can see the increase in air speed. The blue part of the curve shows the low air speed in the take-off and landing segments of the kite, while the yellow part shows that the relative speed with the air can reach a very high level when the kite is circling in crosswind mode at high altitude. But this high-tech operation requires continuous adjustment and control of kites, which requires pilots or computers to complete. Rigid kites are controlled by adjusting steering components such as flaps and rudders, in the same way as aircraft flight control. Soft kites are controlled by adjusting the length of the steering line, similar to the way parachutes turn.

At this stage, the most advanced kite systems can fly continuously for hours or days at a time under the control of a computer. they use a computer carried by a kite or a computer on the ground to achieve continuous steering correction. When the wind stays steady, they tend to work very well, says Mr Vermilin.

But in order to promote this technology, kites must have the reliable ability to cope with sudden and unpredictable changes in the state of the wind, such as sudden gusts. They also need to be able to take off and land automatically so that they can land in bad weather and lift off when the wind direction and speed are right.

"people still have a lot of work to do in order to increase the operating life of the kite power generation system to the order of several years or decades, rather than requiring the kite power generation system to operate continuously for a few days or weeks, as in the technical verification phase," Fermilin said. "

The choice of kite size is also a problem. Small kites are easy to develop and have low manufacturing costs. However, because the gravity and tension of the rope increase with the kite flying height, small kites do not work well at an altitude of more than 300 meters, where the wind is generally strongest. Some companies hope to develop larger and more efficient kites that can fly higher and produce megawatts of output. But it is followed by higher risks and costs.

Big kites bring risk of failure Makani, which once had leading technology, now faces a series of problems, including difficulties in controlling kites and several crashes. In a report to the public, Markani CEO Ford Felke said the company intends to accelerate its progress, often increasing the size of kites too quickly without resolving previous technical problems.

The final obstacle is to ensure that the social and environmental costs of the technology are acceptable. Traditional wind farms often face opposition from residents because they worry that the turbines of wind turbines will produce noise, interfere with the line of sight and affect beauty. Advocates of kite power generation estimate that kites have less visual and noise impact than traditional wind towers. However, according to the published research analysis, there is no experimental evidence to prove that this is true. Although kites are lighter than wind towers, the authors of the study believe that their diving movements and noise may still cause as much or more interruptions as wind towers.

The impact of kites on birds is also unclear, the report said. Similarly, advocates say kites may be safer for birds than wind turbines because kites usually fly higher than birds. However, the rope itself moves quickly and is difficult to find, and it may be difficult for birds to avoid them.

The scope of application of kite power generation technology is still unknown. Advocates, including Bauer, are confident that they can solve engineering and computer problems and make kites efficient.

But the U.S. Department of Energy reported to Congress in 2021 that kite power generation is "an immature and unproven technology that will take a long time to develop before it can be deployed to a meaningful scale at the national level." The cost of traditional wind power continues to fall, making it harder for kite power to show its advantage, the report said.

Fermilin agrees, saying: "I don't think airborne wind power is an alternative to existing land-based traditional wind power." But he thinks kites will find opportunities to develop their talents outside the military and the power grid over the next decade, which is also a potential application listed for him by the company he consulted. In addition, he believes that kites may be an advantageous option for deepwater wind power, and its lighter weight has a huge advantage over wind turbine towers.

Author: Kurt Kleiner

Translation: there is a good future.

Revision: cloud opening and leaves falling

Original link: Could High-flying Kites Power Your Home?

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Kurt Kleiner

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