Formations of wind turbines arranged in a flock, a delta or linear rows of tens of wind turbines installed offshore, out at sea — these are the geometries of offshore wind.
"Impressive." That is the adjective used by the British electricity industry's central authority, the Central Electricity Generating Board – Cegb, to describe the potential of offshore wind energy. For years, the new frontier of wind energy across the world has been the open sea: offshore wind. It is no coincidence that the United States has its own offshore wind farm.
Offshore installations is the name given to formations arranged in a flock, a delta or linear rows, made up of tens of wind turbines installed offshore, out in the open sea.
With a view to large-scale exploitation of wind energy, installing wind turbines at sea offers enormous advantages, without question.
There is space at sea — plenty of it.
There is better wind quantity and quality: more consistent and more intense.
No landscape constraints, no visual reservations.
Set against all this, there are slightly higher construction costs.
The first offshore wind farm, comprising 11 wind turbines (with a total of 5 MW), was built in the Baltic Sea off Vindeby, in Denmark, in the early 1990s — where today, installations ranging from 3 MW to several GW are already in operation everywhere across the North Sea.
On offshore wind, Germany started from zero in 2002, with ambitions to reach 15% of renewable energy from offshore within twenty years: the Deutsche Energie-Agentur (Dena) stated that offshore wind is an energy source "fundamental to Germany's energy and climate future, entering a highly dynamic phase," inaugurating installations on the island of Borkum and tasking Prokon Nord Energiesystems GmbH with building 200 of them by 2010.
The first and largest formation of the new millennium was inaugurated in 2005 in British waters, at Kentish Flats, and generates 90 MW on its own. To return to the study that prompted the British exclamation of amazement, even excluding critical sea areas (those within 5 km of the coast; those with seabeds shallower than 10 metres or deeper than 50; those with heavy shipping traffic; military zones…) their offshore wind energy potential amounts to around 230 TWh per year — enough to cover the total annual consumption of the United Kingdom's population.
On this front, the British always think big. In August 2016 they approved the construction of what will be the world's largest offshore wind farm, comprising 300 wind turbines spread across an area of 480 square kilometres in the North Sea, approximately 89 kilometres off the Yorkshire coast. It will supply up to 1,800 megawatts of electricity to around 1.8 million British homes. The Hornsea Project Two by Danish company Dong Energy is expected to create around 2,000 jobs during construction and 580 during the operational and maintenance phase.
In neighbouring Scotland there are plans to build offshore wind installations of 7.5 GW (East Anglia Offshore Windfarm) or even 18 GW, and at the Burbo Bank offshore wind farm off Liverpool, 8-megawatt wind turbines have begun generating electricity.
Italy has as much as 11,700 square kilometres of sea surface suitable for offshore wind. The ideal areas are found primarily in central and southern Italy, with Puglia leading the way.
As things stand, Italy has no offshore wind installations in prospect, despite more than 15 projects having been submitted for facilities off the Italian coast, all of which have been met with indifference or outright opposition.
Given that in Germany, Denmark and the Netherlands, there is already barely any space left for new onshore installations at the windiest sites, the choice to go offshore will be an obligatory one.
Well, another attraction is wind speed, which is on average greater offshore than onshore. Turbulence is lower, meaning turbine rotors are subjected to less stress. Furthermore, offshore turbines are designed to withstand wave motion and wind/wave interaction.
Offshore turbines can also be installed in very deep sea locations, harnessing coastal winds, using the same construction technologies as oil platforms. Floating turbines, on the other hand, are placed out in the open sea and typically the design uses a three-point anchoring system with steel cables secured to the seabed.
On the other hand, additional costly works are required compared to onshore installations, for the foundations, and the turbines must be "marinised" — that is, protected from the corrosive effects of salt and water (marinising the turbines raises overall costs by 1–2%). The cost of marine cables and connections can be higher, though not always, as can the costs of installation and maintenance (increases that are nonetheless so negligible that they have not yet been factored in at the major offshore wind sites of Vindeby and Tunø Knob, for example, or at semi-offshore installations in coastal or near-sea locations). These are costs that can in any case be offset by not having to build access roads, not having to engage in disputes with local communities, nor carry out ecological restoration or reforestation works, nor commission studies to demonstrate that… fish don't swim into them. The blades turn in the air.
A number of studies on various wind farms in various countries are beginning to emerge. The offshore positioning of large wind installations reduces the problems of aesthetic and acoustic impact, since the wind turbines are situated beyond the visible horizon, at least 3 km from the coast, and also reduces the issues linked to the feared danger to birds — raptors and migratory species in particular — and to bats, which are associated with onshore wind installations. Offshore wind farms are a viable solution for densely populated coastal countries with heavily urbanised territories. Some researchers even argue that the creation of platforms and systems of pylons and underwater cables could, over time, generate zones of repopulation and biodiversity on the seabed, as occurs around the piles and anchorings of oil platforms.
For example, a 2012 study by Environmental Research Letters, "Short-term ecological effects of an offshore wind farm in the Dutch coastal zone", demonstrated that offshore wind installations in the North Sea have had negligible impacts on natural habitat and fauna. Other studies have shown that wind turbines do not disturb birds.
A monitoring programme was carried out at the Egmond aan Zee offshore wind farm, Owez, which supplies energy to 100,000 homes, analysing its short-term ecological effects on a number of faunal groups.
Negative effects may be caused by the rotation of the moving blades and their underwater noise. The results show no short-term effects on the benthos or in the sandy area between the generators, while the new hard substrate has actually led to the establishment of new species and new faunal communities. The composition of bivalve organisms was not affected by the wind farm. Certain types of fish, such as cod, appear to find shelter within the complex. Some bird species seem to avoid the wind farm, while others are indifferent and have remained stable or are even attracted to it: cormorant numbers have increased in the vicinity of the turbine blades. The study states that a large offshore wind installation has an impact on an extremely variable ecosystem and concludes that "overall, the wind farm creates a new type of ecological balance within the pre-existing habitat".