A windmill is a tall, slender tower with three blades that turn in the wind. However?
Several companies are now challenging the classic image of the offshore wind turbine – including Sweden’s SeaTwirl, which simply reverses the technological concept.
Where a traditional windmill resembles the nose of a propeller plane, SeaTwirl’s ‘vertical axis windmill’ is like a whisk. This has great advantages, because while current offshore wind turbines have some serious limitations, the potential for vertical axis wind turbines is almost endless and they can play a crucial role in making Europe CO2 neutral within the next 30 years.
Garde is a French invention
The basic concept of reels like the SeaTwirl is not new. The technology is based on the Savonius mill, developed in the 1920s by the Finnish engineer Sigurd Johannes Savonius, and especially on [Darrieusmolen] by the Frenchman Georges Jean Marie Darrieus, who patented it in 1931.
The blades are on rods that protrude from the rotating mill tower. The generator, which converts the movement into electricity, is not, as usual, at the top of the tower, but at the bottom.
Darrieus described his invention as a ‘turbine with the rotating shaft across the current’.
This current here refers to the wind moving along the sea or land surface and striking the rotating shaft, and therefore the blades, at an angle of 90 degrees – in contrast to the much more common horizontal axis wind turbines where the wind is straight. on the shaft and the shovels are.
Darrieus turbines are scattered around the world – in California 5 percent of turbines are of this type – but the vertical axis wind turbine is not nearly as popular as its horizontal cousin.
This is mainly due to a phenomenon called efficiency, a measure of the amount of energy from the wind that is converted into electrical energy by the rotation of the turbine. The efficiency of vertical axis wind turbines was always a maximum of 40 percent, and about 50 percent for horizontal axis wind turbines.
The difference is due, among other things, to the basic design of the wind turbines. In a traditional wind turbine, all blades are constantly affected equally by the wind, but in a vertical-axis wind turbine, the turbine tower partially shields the rear blade from the wind, which affects the efficiency negatively.
The limit to a wind turbine’s efficiency is theoretically determined by Betz’s law – named after the German physicist Albert Betz – which states that a wind turbine can capture a maximum of 59.3 percent of the wind’s kinetic energy.
And we will have to wring as much energy out of the wind as possible.
Wind turbines continue to grow
Europe must in 2050 [klimaatneutraal] to be. Wind energy plays a decisive role in this, because the researchers estimate that wind turbines will account for at least half of the green electricity production by that time.
Ambitious climate targets – and the increasing need for sustainable energy – give offshore wind turbines a big boost. For example, the Danish company Vestas announced a 15 megawatt offshore wind turbine that could be on the market as early as 2024: a 280-meter-tall giant with wings longer than a football pitch, which [20.000 huishoudens] can supply power.
Denmark also has plans for a test center for wind turbines up to 450 meters high.
At these heights, the wind is stronger and more stable, which the wings of the hams can take advantage of. The power of a wind turbine increases with the wind speed to the third power, so a doubling of the wind speed produces eight times as much power. Tall turbines with large blades also increase the rotor area, so each turbine can capture more wind and convert it into electricity.
It is therefore advantageous to make the turbines high.
Now that traditional wind turbines are developing at a breakneck pace and getting taller and taller, you may be surprised why they suddenly need to be turned upside down.
But according to Swedish start-up SeaTwirl, vertical axis wind turbines have major advantages, giving them the opportunity to take on the giants of Vestas.
Test turbine resistant to hurricane
SeaTwirl installed its first turbine, the S1, in the waves off Lysekil on the Swedish west coast in 2015. Here he is still spinning in the wind. But 13 meters above sea level, the turbine is a small hole compared to the large offshore wind turbines that exist.
Its capacity of 30 kilowatts is 500 times smaller than Vestas’ future giant wind turbine. But the purpose of S1 was only to test if the concept really works and if the turbine is reliable enough to deliver the power despite high wind speeds and salt impact.
For example, in December 2015 the S1 had to deal with the forces of the Helga storm with wind speeds of up to 35 meters per second, but the test turbine withstood the [probleemloos].
In 2023, SeaTwirl expects to install the first turbine of the new generation, the S2x, with a height of 55 meters above sea level and a capacity of 1 megawatt, which is more than 30 times its predecessor.
A model of up to 10 megawatts is still a long way off, but according to the company it would have a diameter of approximately 135 meters and a wingspan of no less than 110 meters – almost the giant Vestas.
And that’s not all. Theoretically, the high-placed turbines could carry a wing length of up to 250 meters and deliver as much as 30 megawatts. The advanced design therefore has some clear advantages that SeaTwirl wants to exploit to conquer a place in the wind energy market.
Vertical axis provides advantages
First, a vertical axis wind turbine always faces the wind, regardless of which way it is coming. This is not the case with traditional wind turbines, which must have the wind at their backs before the blades start turning and generating power.
For that movement, called pitching, the wind turbine must have a built-in pitching gear, where a wind blade senses the wind direction and a motor turns the top of the turbine with gearbox and blades against the wind. Traditional turbines therefore need a lot of mechanics to function properly.
The vertical axis wind turbine also has the advantage that it does not have to stand on a pedestal on the seabed.
In a traditional wind turbine, all heavy components, such as the generator, gearbox and crank gear, are located in the machine housing, called the nacelle. Because it is high in the sky, it needs a solid foundation. Wind turbine manufacturers are toying with the idea of floating foundations, but currently wind turbines are limited to approx. [50 meter] sea depth. If the sea is deeper than this, the foundation for the mill will be too expensive to build.
In a vertical axis wind turbine, the heavy components are just above the water table and the center of gravity is therefore lower. Because the turbine floats in the water and only needs to be anchored to the seabed with cables, it can even be installed kilometers deep in the sea.
In many countries with a deep sea, where offshore wind turbines were never an option, the amount of wind energy can be suddenly increased with offshore farms with vertical axis wind turbines.
In addition, vertical axis wind turbines are more accessible because the main machine parts are located directly above the sea. A study by Sandia National Laboratories, which provides knowledge to the US Department of Energy, sees this as a major advantage of the vertical axis, because as traditional turbines get larger, the generator and other machine components also get taller.
For example, advanced turbines like SeaTwirl can conquer the world’s oceans and change our fixed ideas about what wind turbines should look like.