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Competitive, abundant and renewable: adjectives that perfectly describe wind energy. Wind energy is one of the driving forces behind the broader societal green energy transition, but its potential is far from fully realized, especially as far as offshore wind farms are concerned. When the sea water gets too deep, the conventional monopile structures that support the turbines are no longer an option. However, most of the wind potential is in the middle of the oceans, where the strongest currents blow but the waters are deeper.
That's where floating wind turbines come in. This technology allows wind farms to be installed farther offshore, where they don't take up acres of usable land and benefit from stronger currents. You're probably wondering how they manage not to sink; good point. Before we delve deeper into the specifics of these turbines, we need to understand the basics of floating.
How do objects stay afloat?
Objects such as balls, inflatables or empty bottles float on the surface of water because they are less dense than water. They all have hollow spaces with lots of air, less dense than water. You may wonder how huge objects manage not to sink. Although they have lots of hollow spaces, the shape also plays a big role. When a larger part of an object's surface - the outside - is in contact with water, it has more buoyancy or stays afloat better. When a body floats on water, it pushes the water out of the way - what Archimedes theorized as the displacement theory. In turn, it pushes the water back. The larger the body on the water, the greater the amount of water it displaces, keeping it afloat, which is also facilitated by the shape of the hull.
These simple concepts are the starting point for designing larger objects such as wind turbines.
How do wind turbines float?
Although their actual operation is no different from that of other wind turbines - the wind pushes on the blades, causing them to spin and then drive a generator that produces electricity - the magic happens at their feet. Instead of anchoring a tower into the ground, floating wind farms sit on a platform with multi-lines, such as chains and ropes. The trick is in designing an efficient platform that keeps the turbine stable even in the heaviest winds and storms. The electricity produced is transported via cables to a nearby substation, which raises the voltage and then sends energy to shore.
The turbines are installed on floating offshore wind platforms (FWOPs), which are made of concrete, steel or a hybrid of the two and provide buoyancy and stability. FWOPs are then anchored and secured to the seabed using a system of steel cables and anchors that distribute the weight of the wind turbine so that the generator remains stable under the harshest conditions. There are different types of floating platforms for different scenarios.
Choosing the best platform
Choosing one type of platform over another depends on several factors. First, the sea and seabed, then the wind in the area, the size of the turbine and the depth of the port. Therefore, some platforms resemble the concepts behind boats, with lifting plates; others are semi-submersible - mimicking the behavior of a sphere - and still others place weight at the lowest possible point to improve stability. A more complicated but innovative concept is the tensioned leg platform. This idea minimizes the space the platform takes up on the sea surface and has a star-shaped geometry with arms attached to the sea floor by cables, just like the other platforms.
Floating platforms can support wind turbines that can produce up to ten megawatts of power. This is similar to conventional offshore generators, but several times more than onshore generators.
The advantages of floating wind turbines
Unlike fixed structures, floating wind turbines offer new opportunities and possibilities. The fact that turbines can be exposed to stronger currents means that rotors can produce more power and be more efficient over time, harnessing more wind than onshore or conventional offshore solutions. In addition, they have a lower impact on the environment because they do not require huge concrete foundations on the ground or seabed to operate.
Moreover, their visual impact is minimal because they are many miles offshore. Since they are more efficient than onshore generators, it may not be necessary to install onshore turbines in the long run. Another important advantage is that floating turbines can be assembled in port and towed to the wind farm site. However, all the advantages also have disadvantages.
Challenges to overcome
First and foremost are the technical challenges that must be solved. Although different platform designs exist, further research needs to be done on the forces acting on the platforms. Their movement could cause heavier forces on the tower and blades, with more aerodynamic factors to consider.
If the technology becomes commercially available, cost is another concern. Because they operate in deep water, hundreds of meters of mooring lines and electrical cabling will require significant upfront investments.
Although Northern California's continental shelf has great wind energy potential, it is rapidly declining, making it nearly impossible to install conventional offshore turbines. That's why the federal government auctioned off five lease areas 20 miles offshore and received bids worth more than $757 million. The need for clean energy is being felt there as the recurring drought reduces hydroelectric production.
Iberdrola - a Spanish energy company - is working on Spain's first floating wind farm. The utility said it would invest a billion euros in the venture and expects it could be operational by 2026. Currently, the company is exploring different location options for the park.
With more research and engineering needed, floating offshore wind is on its way to becoming one of the ways to power our society in a green way.