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North Sea offshore wind is waking up to a yield problem

Wind is a renewable resource, right? Of course. But it can also be ‘stolen’.

There’s only so much wind energy in a given location that can be extracted by turbines before it affects the yield of neighbouring installations. This is what’s known as ‘wake effects’ – the phenomenon of upwind turbines reducing the resource available downwind.

Wind wakes occur when the wind blows through a turbine, creating turbulence and reducing the wind speed beyond it. Effects can be acute when multiple turbines are closely spaced within a site, affecting the efficiency and power generation of the downwind machines. Densely-packed turbine layouts can also affect yield of the front row of turbines by creating a ‘blockage’ effect.

Wake and blockage problems are not new and are well understood by developers, who design their turbine layouts to minimise yield losses occurring within the boundaries of each site. But these effects don’t just occur within individual projects – they can spread out several kilometres and impact neighbouring developments too.

There have been numerous instances over the years of onshore wind developers objecting to proposed neighbouring sites that would ‘steal’ their wind resource. As capacity ramps up across the North Sea, the potential for wind wakes to be felt offshore is growing – to the extent that there is now a very real likelihood of negative effects crossing international boundaries.

The UK North Sea is already becoming crowded, as examined by E-FWD in relation to overlap between wind farms and carbon storage licences. But the wake effect poses a more widespread challenge that will probably affect vast swathes of maritime space.

“We want more wind farms regardless of the wake effect. That said, the wakes cannot be avoided,” said Nicolai Nygaard, senior lead specialist at Ørsted, speaking on a recent webinar on the topic hosted by Offshore Renewable Energy Catapult.

Gone with the wind

Wake effects pose a significant risk to decarbonisation. Modelling by Ørsted shows that generation can drop by as much as 30% when the wind direction changes and wake effects move across the first row of turbines in a project located downwind from another installation.

All of this highlights the importance of having robust, data-driven marine spatial planning and getting that right.

Tim Pick, former UK government offshore wind champion

Loss of generation from wake effects would need to be replaced by alternative sources, which could require dispatch of gas or coal-fired power plants. The extent of the problem will depend on how well governments and regulators across numerous jurisdictions take proactive measures in their marine spatial planning to mitigate the worst effects.

In the absence of strategic cooperation, “the best you can do in terms of planning your future business case is to use an unbiased wake model, one that you validated against the data, take into account the neighbours in terms of what’s [likely to be] built,” said Nygaard.

Wake effects reduce with distance. Ørsted found the impact on generation in front row turbines from a neighbouring upwind installation fall from 30% at 10km separation to 10% at 30km, and 5% at 50km or more. This implies that regulators should introduce a buffer zone between developments in marine spatial plans.

Lessons from the oil ‘gold rush’

However, unless buffer zones are coordinated between countries, there is a risk of prime acreage going unexploited. Professor KK DuVivier from the University of Denver Sturm School of Law and author of ‘Preventing Wind Waste’ cited the US onshore petroleum sector as a historical reference.

In the early days of onshore oil drilling, competitive leases and the ‘rule of capture’ created resource waste. The rule of capture asserts that the first party to exploit a resource owns it, triggering a rush to develop sites before neighbours could draw from the same pool, DuVivier said. The net effect was inefficient well drilling resulting in a reduction in the overall amount of recoverable resource.

You could turn this into an opportunity, if you collaborate not just on a cross-country level but also governments with developers.

Nicolai Nygaard, senior lead specialist at Ørsted

Governments intervened with forced well spacing, resource ‘pooling’ and unitisation agreements – whereby each party agrees that a resource area is aggregated as a ‘unit’, with each owner receiving a percentage interest in the unit.

Without a similar such agreement for transboundary wind resources, the North Sea could witness a ‘race to the water’ where states race to deploy first in the best spots before they are impacted by neighbours, said Eirik Finseraas, PhD candidate in offshore wind regulation at the University of Bergen.

Legal grey area

Racing to install turbines risks undermining adherence to environmental and spatial best practice, or even breaches of regional agreements such as the United Nations Convention of the Law of the Seas (UNCLOS).

Drafted in 1982, UNCLOS establishes rights and obligations on resource-specific basis. Its authors were clearly “forward-thinking”, Finseraas said, because they included a line asserting rights in relation exploitation from “water, currents and winds” at a time when utility-scale offshore renewables were merely a pipedream.

UNCLOS signatories must respect the principle of ‘good neighbourliness’ and respect others’ rights. In the context of wind wakes, this means they must consult neighbours where development is likely to result in wake effects, the researcher added.

All if this highlights the importance of having “robust, data-driven marine spatial planning and getting that right”, said Tim Pick, former UK government offshore wind champion, and chair of the UK’s Offshore Wind Growth Partnership.

But unitising wind is not as straightforward as petroleum because the resource is spread out over a much larger area. Conventional oil and gas resources are clearly defined by the contours of the deposit. It is not clear at which point a wind unitisation area should be defined, said Pick.

Wind wakes modelling. Credit: University of Texas at Dallas
Wind wake modelling. Credit: University of Texas at Dallas

The landlord’s responsibility

Even when wake effects occur within a national maritime boundary, there is a strong need for proactive planning to avoid conflicts. In the UK, the Crown Estate is well placed to do this and recently commissioned Frazer Nash consultancy to assess wakes and blockage production losses for a number of generic wind farm configurations.

Some conflicts can be dealt with in the planning system, but this is not ideal because by the time a project is at the consents stake “somebody has already paid for a lease,” Pick said, adding that it is paramount to avoid two Crown Estate leaseholders in dispute over potential yield impacts arising from wake effects.

Given the sheer number of offshore wind projects planned across multiple littoral states of the North Sea, this is where transboundary effects are likely to be felt first. The good news is that this offers North Sea countries the opportunity to establish a global precedent for resolving such conflicts.

“You could turn this into an opportunity, if you collaborate not just on a cross-country level but also governments with developers, in terms of figuring out where future wind farms should be [located],” said Ørsted’s Nygaard.

Waking up to wake effects

“Planners and regulators should set offshore wind targets not in terms of nameplate capacity but rather in terms of actual generation (gigawatt hours), considering wake/blockage effects,” said Ken Kasriel, energy economist at ORE Catapult. Fortunately, the relevant authorities appear to be doing just that, he added.

The European Commission’s North Sea Energy Cooperation report in November recommended that wake solutions are explored in the development of marine spatial planning collaboration in the Southern North Seas for post-2030 developments. “I would like it to start a little before 2030, but you can’t have everything,” Kasriel said.

Germany’s Federal Ministry for Economic Affairs and Climate Action recently published a major study from Fraunhofer IWES into wind wake effects. And the Dutch government will publish in early 2024 public data from modelling a hypothetical 2050 energy capacity scenario for the North Sea which “can be used for detailed wake analysis studies and/or assessing requirements for the electricity grid of the future”.

The industry itself is also starting to think strategically. BWO, Germany’s offshore wind industry trade body, in October proposed reducing the capacity and delaying the commissioning date of two large wind park sites to avert negative impact on downwind projects. BWO said the N-9.4 and N-9.5 plots in the German Bight would reduce full-load hours of neighbouring plots by 5% unless they were cut by 50% to 1 GW each and delayed from the 2020s until after 2040.

Negotiated agreement

While these are encouraging developments, the industry is still operating in a “vacuum of regulation” around inter-project and transboundary wake effects, said Pick. This will remain the case until new guidance is drawn up and agreed upon.

In the meantime, some developers are taking matters into their own hands by entering into bespoke wake loss agreements that compensate a downwind operator from a new development that will impact its yield.

Stewart Gordon, managing associate at Addleshaw Goddard, said these bilateral agreements establish an up-front annual payment from the new project to the existing one based on an agreed baseline percentage impact on yield and increased operations and maintenance (O&M) costs arising from increased turbulence. The payment is then adjusted from the baseline according to actual real-world operational performance data from the existing wind farm.

Floating offshore wind allows us to plan more flexibility with a view to mitigating wake effects.

Eirik Finseraas, University of Bergen

Wake loss agreements are difficult and lengthy to negotiate, and must include indemnity and dispute resolution clauses since disagreements over data are likely, Gordon said. Moreover, they raise interesting legal questions around causation: to what extent can the lower yield of a downwind turbine be attributed to wave effects rather than, for example, the operator’s failure to maintain the machine adequately?

Floating a solution

Wake effects are not going away, but the evolution of offshore wind technology will alter the nature of wake effects – both for better and worse.

The push for larger turbines will reduce the total number of machines installed per site, which reduces wind wakes. But the scale of the wake effect is a function of rotor diameter, and larger machines have longer blades, which will increase the distance over which wake effects and turbulence are observed.

The move into deeper waters implies a shift to floating turbines, which raises interesting possibilities around the potential to change wind farm layouts if wake effects prove to be acute. Floating wind also opens the door to site-specific experimentation with differing configurations and in-field learning opportunities.

Floating wind offers a reason to be “optimistic” about wake effect, said Finseraas from the University of Bergen. “We aren’t necessarily fixed to shallow seabed areas and [this] allows us to plan more flexibility with a view to mitigating wake effects. There are reasonable ways in which we can mitigate this as an issue at large,” he added.

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