Have you ever flown a kite?
Have you ever flown a kite underwater? What? Flying a kite underwater, are you crazy?
A Swedish company not only flies kites underwater, but also uses kites to generate electricity, and a single "kite" can output 1.2 megawatts of electricity, which can meet the electricity needs of a town of 1,000 people.
Are you running out of brains?
The idea of challenging everyone's mind, comes from the Swedish company Minesto, whose founders noticed a phenomenon that few people care about: kites always seem to tend to take a figure 8 when flying at high altitudes. He thought it would allow the kite to gain more speed than the surrounding wind when hovering down or diving, allowing it to use to collect more energy.
Previous racing yachts have used this strategy, allowing the boat to sail at speeds of up to 60 mph in winds of up to 15 mph. And companies in Germany, the Netherlands and several other countries, including NASA in the United States, are testing flying kites high into the air to generate electricity, but in the water, can you really fly kites?
Minesto does not believe in this evil, they have found their own way to go off the beaten path, after years of research, they have developed the "Dragon" series of underwater power kites, the largest one is the Dragon-12, wingspan of 12 meters, weighs 28 tons, turbine diameter of 3.5 meters, can generate 1.2 megawatts of electricity, has been installed in February this year to the seabed of a waterway in the Faroe Islands between Iceland and Norway, where there are rich tidal resources, and has begun to transmit electricity to the local national grid.
You've probably dropped your glasses, but how exactly is this done? How do you fly a kite underwater?
Minesto's underwater kite looks like a yellow airplane, the structure is simple, and it is completely modular. The largest part is the fuselage and wing, which may be integrated, with a generator, gearbox and control system installed in front, and an elevator and rudder installed behind the left and right wings to control the up, down, left and right directions, which is the key to the underwater kite to walk the figure 8. The biggest difference with an airplane is that the propeller of an airplane is mounted on the head, and its propeller, that is, the turbine, is mounted on the tail.
Underneath the fuselage is a sturdy tether of 55 to 100 meters, which can hold the kite firmly to the bottom of the sea, allowing the kite to surf in the tidal currents to generate electricity, just as a kite in the air uses the wind to generate electricity. The on-board system controls the kite to move under the water in a predetermined figure-8 course, which can obtain a relative flow rate several times faster than the speed of the current, allowing the turbine to generate more power.
In other words, this turbine is based on the same principle as a wind turbine, except that it uses water that is nearly 1,000 times denser than air, and also uses the figure-8 route of an underwater kite to operate, which increases the efficiency several times and thus brings higher power generation efficiency. Just look at its size, it has a wingspan of only 12 meters and weighs 28 tons, while a wind turbine of the same level has a tower height of 85 meters, a rotor diameter of 62 meters, and a weight of nearly 100 tons.
Compared to tidal turbines fixed to the bottom of the sea, the Dragon-12 is also much more efficient. In 2019, Scotland's MeyGen project, which claims to be the world's largest tidal power station, installed the first four 1.5-megawatt turbines on the seabed, with rotors up to 18 meters in diameter and a base weighing hundreds of tons, but providing only about the same power as the Dragon-12.
Many people may know that the power generation of wind turbines mainly depends on the diameter of the rotor, which is proportional to the square of the diameter, so the wind turbine is designed to be larger and larger, and the world's largest 22 megawatt wind turbine launched by Mingyang, China, has reached an astonishing 310 meters in diameter.
Although the energy of air and water is not the same, why is it that the Scottish turbine with an 18-meter rotor can only produce about 3.5 meters of electricity compared to the Dragon-12?
This is another point of knowledge that you may not know, whether it is water or wind power, the power generation is proportional to the third power of the flow rate, isn't it amazing? As long as the speed is increased a little, the power can be greatly increased, no wonder Minesto came up with this way of using a kite to walk the figure 8, because it can produce several times more power than a stationary turbine.
Other advantages of Minesto include its small size and light weight compared to competing technologies, weighing only 1/15th of a megawatt, and the ability to generate electricity cost-effectively at low flow rates of 1.2 m/s. It is precisely because of its small size and beauty that it can be manufactured and installed at low cost, and can be easily towed to the shore for repair and maintenance, reducing the cost of offshore operations.
Then there's the underwater kite, which is completely submerged at the bottom of the sea, with little to no impact on the scenery and minimal impact on the environment, although a whale that accidentally hits its turbine may not necessarily agree. Finally, its power generation is highly predictable, as the tides and currents are strictly controlled by nature.
According to Minesto's analysis, the potential installed capacity of global tidal and ocean currents is as high as 600GW, which is larger than the current global installed capacity of nuclear power of 400GW, which is still incomplete statistics, so underwater kites have extremely promising prospects.
Minesto has been promoting their underwater kite technology in many countries and regions around the world, including Sweden, Wales, Northern Ireland, etc., and the first location in Asia is Taiwan, because of the abundant tidal and current resources. Minesto has set up a company in Taiwan to build Asia's first underwater kite farm in Kaohsiung.
When I was writing this article, there was actually a lingering question, why does the kite move in the air in the figure? This company did not say, and there was no simple and clear explanation on the Internet, just when I was about to give up, I actually found the research paper of Jiang Yiqi, Yin Zelong, and Huang Shuo of Beijing No. 2 Middle School in 2018, which was qualitatively and quantitatively analyzed and calculated, and experimentally verified.
To put it simply, the reason why the kite moves in the air is the result of the complex interaction between the kite's attempt to stabilize itself under different wind conditions and the aerodynamic forces, especially the deflection force model that provides a reliable explanation for the specific motion patterns observed.
The paper of the three students comprehensively explained that the "8" trajectory of the kite is generated by the need for stability and balance of the aerodynamic forces acting on the kite, and these aerodynamic forces are not symmetrically distributed during the flight, which leads to the continuous adjustment of the flight direction to form a figure-eight trajectory, rather than a simpler circular or elliptical trajectory.
Finally, they also found that in light wind conditions, because the wind speed is small, the kite is more likely to be in a stable floating state, so it is difficult to make a figure-8 trajectory. In order to let the kite enter the figure-8 trajectory, it is necessary to meet the conditions of fast wind speed, large kite line tension, and a large initial tangential velocity of the kite, because even if the wind is larger, but the initial tangential velocity of the kite is small, the kite will also rise directly and reach a balance point at a high place, which becomes our common stable kite flying situation.
So if you read this article and want to invest in underwater kite power generation, it is best to read this paper first, to avoid that the underwater kite is built, but it cannot be made to go 8, the power generation efficiency will be reduced several times, and the levelized cost will not be competitive.
Dissertation: "Research on the Origin of the Trajectory of the Kite Figure 8"
https://oss.linstitute.net/wechatimg/2018/07/qiuchengtong2017physicsbronze2.pdf