Do you know how offshore wind farms work?

#wind power #renewable energy #Iberdrola projects

Offshore wind energy is the clean and renewable energy obtained by taking advantage of the force of the wind that is produced on the high seas, where it reaches a higher and more constant speed than on land due to the absence of barriers. In order to make the most of this resource, mega-structures are installed that are seated on the seabed and equipped with the latest technical innovations. Discover what these real sea giants are like and how they work.


  • Offshore wind energy is renewable, unlimited and non-polluting.
  • There are more wind resources offshore than onshore (up to twice as much as in a medium onshore wind farm).
  • When located offshore, the visual and acoustic impact is very small, so much larger areas can be used. Thanks to this, offshore wind farms typically have several hundred megawatts of installed capacity.
  • The ease of maritime transport, which has few limitations with regard to cargo and dimensions in comparison with land transportation, has made it possible for offshore wind turbines to reach much larger unit capacities and sizes than onshore wind turbines.


Currently, offshore wind farms are located in shallow waters (up to 60 metres deep) and away from the coast, marine traffic routes, strategic naval installations and spaces of ecological interest.

According to the latest report from WindEurope, the European wind energy Association, Offshore wind in Europe: trends and key statistics 2018, published in February 2019, European farms have an average depth of 27.1 metres (only slightly less than the year before) and are at an average distance of 33 km from the coast, as opposed to the 41 km average recorded in the 2017 report. The United Kingdom is the country with the highest installed capacity in Europe, with a total of 44% of all offshore wind energy installations (in MW). It is followed by Germany (34%), Denmark (7%), Belgium (6.4%) and Holland (6%).


How does an offshore wind farm work?
How does
an offshore wind farm work?
The force of the wind turns the blades.
The blades are attached to the nacelle through the hub.
The low-speed shaft spins at the same speed as the blades (7 - 12 turns per minute).
The gearbox increases this speed more than 100 times and transfers it to the high-speed shaft.
The high-speed shaft (+1,500 revolutions per minute) transmits this speed to the generator.*
The generator transforms the kinetic energy it receives into electricity.
The electricity produced by the generator is fed down through the inside of the tower.
The converter converts the direct current into alternating current.
The transformer raises the voltage (33 kV - 66 kV) in order to transport it across the wind farm.
The electricity is transmitted via underwater cables to the substation.
At the substation, the electricity is converted to high voltage current (+150 kV).
Electricity is transported through the distribution network until homes.
(*) Some technologies use low-speed generators coupled directly to the low-speed shaft.


 SEE INFOGRAPHIC: How does an offshore wind farm work? [PDF]

Know the process in detail

Electrical energy is produced in the wind turbine, a mammoth structure that is fixed to the seabed using different types of supports. It has a controller that starts and stops the turbine depending on the weather conditions, as well as a mechanism that determines the wind direction which allows it to be orientated correctly. The structure, whose height depends on the sea surface topography, is equipped with a beaconing system with specific lights and colours that make it very visible to maritime and air traffic to ensure maximum safety.

The force of the wind turns the blades, which are designed to capture the maximum kinetic energy: they can move even in very light winds, down to 11 kilometres per hour. The blades are connected to the turbine through the hub, which in turn is connected to the low-speed shaft, which rotates at the same speed as the blades (between 7 and 12 revolutions per minute). A gearbox increases this speed more than 100 times and transfers it to the high-speed shaft, which moves at more than 1,500 revolutions per minute and transmits this force to the generator (some technologies use low-speed generators coupled directly to the low-speed shaft). This is where the kinetic energy is transformed into electricity.

The electricity is fed down the inside of the tower to the base, where a converter transforms it into alternating current. It is then transported via underwater cables to a transformer where the voltage is raised (to between 33 and 66 kV) so that it can be transported across the wind farm. From there, it goes to the substation, where the electricity is converted to high voltage current (more than 150 kV) and transported through the distribution network.

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Turbine capacity on the high seas has increased considerably in the last decade, according to the WindEurope Offshore wind in Europe: trends and key statistics 2018 report, and this year wind turbines with capacities of almost 9 MW began to be implemented. The study highlights that the average capacity of offshore wind farms under construction in Europe is 561 MW, while in 2018 the average capacity per wind turbine was 6.8 MW, 15% more than in 2017. Between 2007 and 2017 alone, the power of the turbines increased by 102%.

This evolution can be clearly seen in the offshore wind projects developed by the Iberdrola group: West of Duddon Sands, Wikinger, East Anglia ONE, Saint-Brieuc, Vineyard Wind and Baltic Eagle.

Evolution of unitary capacity

and rotor of our offshore wind turbines

120 m 3.6 MW

West of Duddon sands

Irish Sea
(United Kingdom)
In operation since 2014
135 m 5 MW


Baltic Sea
In operation since 2017
154 m 7 MW

East Anglia One

North Sea
(United Kingdom)
In operation since 2020
8 MW 167 m


Saint-Brieuc Bay
Under construction
174 m 9.5 MW

Baltic Eagle

Baltic Sea
Under construction
220 m 13 MW

Vineyard Wind 1

Under construction


Find out how the power of the turbines has increased

West of Duddon Sands was the first wind farm of its kind launched by the company in 2014. Located in the Irish Sea, off the British coast, it has 108 wind turbines which provide a total of 388.8 MW of power, 3.6 MW each. The circumference traced by the blades of each turbine (also called the rotor) reaches 120 metres.

Since then, the power of wind turbines has undergone a breakthrough. In the Wikinger offshore wind farm, located in the Baltic Sea off the coast of Germany and operational since late 2017, each of the 70 turbines provides 5 MW and has a diameter of 135 metres. This results in a total installed capacity of 350 MW, just 30 less than West of Duddon Sands but with 38 fewer wind turbines.

Even more significant are the improvements introduced in the East Anglia ONE, a large-scale offshore wind energy project in operation since 2020. With 102 turbines, each with a unit capacity of 7MW and a rotor diameter of 154 metres, East Anglia ONE is the largest offshore wind farm in the world, supplying 714 MW. This means that with six fewer turbines than West of Duddon Sands, East Anglia ONE provides almost twice the power.

At the Saint-Brieuc wind farm, the group's first major offshore wind project in Brittany, 8 MW turbines will be installed, each with a rotor diameter of 167 metres. This will result in a total installed capacity of 496 MW with only 62 turbines.

However, we will find the greatest unit power in Vineyard Wind 1 and Baltic Eagle. Vineyard Wind 1, the first offshore wind farm developed by the company in the USA, will have an installed capacity of 800 MW supplied by 13 MW wind turbines and 220-metre rotors. Baltic Eagle, meanwhile, will be built next to Wikinger, in Germany, and will have a capacity of 476 MW generated by 52 turbines with rotors of 174 metres and developing 9.5 MW.

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The development of new types of foundations that allow these installations to be located further away from the coast and the continuous evolution in the power and design of wind turbines are just some of the progress we will see in the coming years. These advances undoubtedly augur a long and prosperous future for offshore wind farms.

 Evolution of wind energy in Europe

 Utility of the future

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What is offshore wind energy?

Offshore wind energy is obtained by harnessing the power of the wind at sea, where the wind reaches a higher speed and is more constant because there are no barriers. In order to make the most of this resource, mega-structures are installed that are seated on the seabed and equipped with the latest technical innovations.

What are the benefits of offshore wind energy?

Because this is a type of renewable energy, there are numerous benefits because it is inexhaustible (wind is an unlimited resource) and does not pollute. (It is a source of energy that produces low greenhouse gas emissions (GHGs), which are the main culprits responsible for global warming).

The wind blows more at sea than it does on dry land and can produce up to twice the power obtained from onshore facilities. What's more, these plants have a lower visual impact and are less noisy which means that their installed power can be far higher than on land, reaching hundreds of megawatts. Likewise, sea transport is easy, allowing unit powers and sizes far higher than those possible on land.

What is the difference between offshore and onshore wind farms?

The main difference lies in the technological difficulty of building them because the structures and their maintenance are more complex due to the offshore environment, which is governed by strict safety requirements. The construction and operation of offshore wind farms requires the use of highly specialised logistical resources. On the other hand, the capacity to generate electricity is higher offshore, because the wind resource is superior and more regular than it is on dry land, which means a higher yield of power. It is easier to transport the components required to install a wind farm offshore, which is why wind turbines with unit power of over 10 MW - and even 15 MW - are feasible in an offshore setting. It is more difficult to transport them on land, where unit power of around 5 MW is the norm.

Due to access constraints during operation, wind farm elements require higher reliability and components are designed with higher levels of redundancy than onshore wind farms.

What is the environmental impact of an offshore wind farm?

To install an offshore wind farm a positive Environmental Impact Statement (EIS) must be obtained, as well as a favourable study showing the compatibility of the facility with other uses of the maritime space. For this, very rigorous and strict studies must be conducted in the years prior to the start of the project, including an analysis of the compatibility of the wind farm with navigation, marine fauna, avifauna, migration routes, sediment transport dynamics, etc. These studies are complemented by thorough monitoring of these aspects during the construction and operation phases of the farm.

To protect the environment on wind farm sites, the offshore power industry is using cutting edge, extremely innovative solutions. For example, Iberdrola has used advanced noise mitigation systems while building offshore wind farms such as the underwater bubble curtains used for the Wikinger project in the Baltic Sea. This system protects marine mammals from being affected during construction.

Types of offshore wind farms:

There are two types, distinguished by the type of anchorage used to secure the wind turbines:

Marine wind turbines with fixed foundations:

These are installed on a fixed support structure on the seabed. In turn, there are different types of foundations: monopile (the tower is installed on a large steel cylinder embedded in the seabed); gravity-supported (requires a high-mass, large-area concrete or steel platform resting directly on the prepared seabed); or using jackets (reticular steel structures with three or four anchor points on the seabed). Current fixed foundation technology permits structures to be installed up to 60 metres below the surface.

Offshore wind turbines on floating platforms:

This type of technology allows these wind generation farms to be built farther offshore in very deep waters. The floating bases make it possible to harness the enormous potential of the wind in huge offshore areas. With this type of technique, depth restrictions are determined by the laying of underwater electricity evacuation infrastructures, which can be placed hundreds of metres below sea level.

Depending on the system used to fasten the equipment to the sea bed, they are classified as: single floating columns or spars, semi-submersible platforms or tension-leg platforms.