There is a renewable energy available night and day. It is invisible. And it can be found in the sea. As already anticipated in the article’s title, it is offshore wind energy. This green power is one of the fastest growing in recent years. The latest demonstration is the auction of coastal water rights held by the state of California in the USA. It is estimated that the area, which covers 135,000 hectares, could power 1.5 million homes. Much of this area is deep water that will require new approaches, such as floating offshore wind.
This article covers topics like:
What is offshore wind energy
First, a basic definition: offshore wind energy uses wind turbines in coastal waters that can be attached to the bottom, both with fixed or floating structures. The most common models feature blades and a horizontal axis, i.e., the modern version of the traditional Dutch or Don Quixote’s windmills.
This type of energy could overtake onshore wind by 2030 according to IDAE estimates. With an increase of 48.5 % in the last decade, the speed of implementation of offshore wind energy and its contribution to renewable energies have been impressive. However, it is a relatively young type of energy. Offshore wind is barely thirty years old since the first offshore wind farm was installed in 1991 in Vindeby, Denmark. Today, almost half of the world’s installed capacity is in China.
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Advantages of offshore wind
There are several explanations for this warm welcome. Offshore wind combines many of the advantages of renewable energies. Firstly, in terms of sustainability or reduction in the use of fossil fuels. Secondly, it shares the characteristics of onshore wind, such as its high energy conversion capacity, which ranges from 20 to 40 %. However, it also offers some specific advantages:
- Offshore winds are usually stronger than onshore winds. In the case of wind energy, small speed increases translate into higher energy yields.
- Offshore winds are more uniform and continuous, allowing for a more stable energy supply.
- The installation of offshore wind farms generates less visual impact than on land.
- As a rule of thumb, the most populated areas tend to be on the coast, which opens up the possibility of generating energy where it is most needed.
Of course, this energy is not without its challenges, such as installing the turbines in a more hostile environment like the sea, often at great depths. But, as we will see next, even these barriers are being overcome thanks to the latest technological breakthroughs.
As young as this renewable energy is, plenty of approaches have been tested over the past three decades. This includes the type of turbine, but especially how the structure is attached to the seafloor. There are two broad categories:
Fixed offshore wind
Until recently, offshore wind turbine platforms (FOWP) were always installed with a foundation fixed to the seabed up to 196-feet deep. This required various types of foundations, summarized as follows:
- Gravity. Similar to their land-based counterparts, these wind turbines are installed without the need for special drilling. The ballast and the weight of the structure are sufficient. They are normally used in shallow waters, between 30 and 65 feet.
- Monopile. This is the most common design in fixed offshore wind and is based on a tubular structure fixed to the bottom by drilling. It is usually used at depths of up to 82 feet.
- Tripod and tripile. It is usually used for heavy turbines and is based on a central column supported by three sleeves and diagonal reinforcements to ensure the rigidity of the assembly. In the case of the tripile model, a transition piece is added above the surface. In both cases, depths of up to 164 feet can be reached.
- Jacket or lattice. It resorts to gravity foundations but using three or four main legs that are joined by diagonal pieces forming a lattice. This system allows installations at depths below 164 feet, which is close to the limit of current fixed platforms.
Floating offshore wind
While offshore wind energy is highly efficient, much of the world’s coastal waters are deeper than advisable for a fixed foundation. As a reference, the deepest installation at present is a 190-feet-deep jacket platform in Scottish waters. How to overcome this limit?
Since 2007, some prototypes of floating offshore wind turbines have been installed, allowing them to operate at much greater depths. You can read a more detailed explanation here, both of its typology and its advantages, but for the moment, the following primary modalities have been proposed:
- Spar buoy. Basically, it is a floating buoy with ballast at the bottom. The structure is anchored to the bottom by catenary or tensioned lines.
- Semi-submersible platform or barge. This approach is based on a horizontal float anchored to the bottom with tensioned cables.
- TLP (Tension Leg Platform). As the name suggests, this model uses several tensioned steel legs connected to a submarine base. Some estimates show that these offshore wind turbines could reach a depth of 650 feet.
Offshore wind power is not the only renewable energy that can be obtained from the seas. If you want to learn about other alternatives, you can read this article on wave and tidal energy or this one on blue or salinity gradient energy, which leverages the power released by the convergence of fresh and saltwater.