'Staggering' wake losses pose grave threat to offshore wind sector, warn experts

IN DEPTH | Developers and governments are playing catch-up with science on wind wakes, say experts, with potentially devastating consequences for buildouts

“Frightening” new data is showing that wind wakes between offshore wind farms have been massively underestimated, putting projects in peril and setting the stage for future disputes over “wind theft,” experts have told Recharge.

Last year, a UK examining authority considering an application to build an offshore wind farm in the Irish Sea issued a quietly groundbreaking decision that may serve as a sign of things to come for the sector.

For the first time ever, the authority blocked the construction of a wind farm – the 500MW-plus Awel y Mor array off the coast of Wales – until its developer RWE assessed how much wind it would take from an existing neighbour and mitigate against this.

This was already a landmark case. To the knowledge of DLA Piper legal director Michael Greig, who acted for that neighbour, the 90MW Rhyl Flats wind farm, it was the first time two developers had fought over “wind theft,” as it is sometimes known.

But as more and more projects are packed into areas like the North Sea, where nine nations are targeting 300GW of capacity by 2050, it could be the first of a flood of similar disputes to break out between developers.

More cases are on the horizon. Orsted has intervened in planning applications for major UK projects raising concern over wake effects and citing the Awel y Mor ruling. That includes a 1.5GW project being developed by TotalEnergies and Corio Generation that Orsted said could impact its huge Hornsea projects.

In each of those interventions, its position is clear. If it is shown there will be wake effects on its wind farms, it wants mitigation, money or both.

Old wind wake models ‘do not work for offshore’

Wind wakes are like the dappled shadows cast by trees on a sunny day. Just as trees catch some sunlight and let the rest through, so turbines catch some of the wind, lowering its intensity behind them.

Engineers have long considered the wakes between turbines within wind farms when considering their layout. But attention has increasingly turned from wakes cast turbine-to-turbine to those cast over vastly longer distances, from wind farm to wind farm.

The problem is that the three models the majority of industry players still use to model wind wakes are “very simple” and “very limited” when it comes to estimating losses over longer distances, said Carla Ribeiro, former head of renewables analytics at consultancy DNV.

In fact, these “engineering” models, as they are known, assume that wakes dissipate around 2km downstream, said Ribeiro, now an independent consultant. And that is not correct, not even close.
Those models were built a long time ago based on smaller onshore turbines, and without much understanding of the atmosphere, according to Mike Optis, founder of wind power meteorology consultancy Veer Renewables.

They have "persisted to now when we’re building multi-gigawatt offshore clusters and expecting the same methods to hold,” he said. “We're finding they definitely do not.”
Wind wakes last longer at sea because there are no obstacles – like hills, trees and buildings – to help remix the wind flows behind turbines. The largely uniform surface temperature of oceans also means there is less vertical mixing of air.

How long the wakes trail is hard to determine and depends on factors including the size and layout of the wind farm and atmospheric conditions. But Optis and other experts told Recharge they have either modelled or observed wakes 100km long.

Sven Utermöhlen, offshore wind chief at RWE, has said that large clusters of developments could have "far-reaching wind-shadowing effects" significantly higher than industry-standard models.

Wakes can cause ‘staggering’ losses

The industry at large was arguably first alerted to this issue in 2019, when Orsted cut power production forecasts for projects after admitting its models for wind wakes and another phenomenon called blockage were “not sophisticated enough”, sending its share price tumbling.

That was a “wake-up call for the existence of unknown unknowns” in the science of wind energy assessments, said Ribeiro. Wind resource experts had known of these uncertainties but, until then, “not the wider industry or financial institutions.”

That year, Science declared that improving understanding of atmospheric and wind farm flow physics was one of the sector’s “three grand challenges”. Orsted for its part has since developed its own ‘TurbOPark’ model to avoid repeating past mistakes.

Optis, who has worked with offshore heavyweights Equinor and Shell, said he has observed wind farms that are seeing up to 20% of their wind resource hoovered up by a neighbour, equating to “massive losses.”

Veer is currently involved in a joint research project with the US government’s National Renewable Energy Laboratory (NREL), where Optis used to work, and the National Offshore Wind Research & Development Consortium to model wind wakes in the US Atlantic.

The research Optis and his colleagues are carrying out uses newer wind wake models that account for atmospheric conditions. These are generating “frightening results” for wake losses, he said, “much higher than what the industry is estimating.”

On the US Atlantic coast, Optis and experts from NREL and elsewhere conducted a study that found external wake effects could cause a power deficit approaching 15% at wind farms, "which is staggering," he said, "it’s really got a lot of people concerned."

Offshore wind farms in the North Sea, where 300GW of capacity is being targeted for 2050, could he said end up generating up to 30% less power than expected. Orsted research based on 37 pairs of projects has found that a front row of turbines at a wind farm, when downwind of another, can see an almost 30% drop in resource, even when the sites are 20km apart.

Martin Dörenkämper, an atmospheric physicist at the Fraunhofer Institute, said wind farms that now have around 3,800 full load hours in the German Bight might see only 2,500 such hours in future expansion scenarios his team has modelled, a 34% reduction.

Ribeiro cautions that validation of models producing some of these worrying figures remains "scarce," and "until this validation becomes extensive, these models are unproven."

North Sea nations are now, perhaps unsurprisingly, paying more attention to the issue.

The Danish Energy Agency told Recharge it has commissioned a study using a newer atmospheric model on the effects of planned North Sea expansion. The Dutch and German governments have also commissioned similar studies for specific development zones.

Large, unexpected wake losses could ‘destroy’ projects
“The implications are going to be huge,” said Optis. Even losing 20% of the wind resource “could destroy the financials of your project.”

Jan-Peter Elffers, an offshore wind finance specialist at Dutch advisory Voltiq, said that energy yields from such projects are often “a bit lower” than expected, although he would be “surprised” if wake losses far exceed industry estimates.

But he agreed that even underestimating the wake effects for a wind farm by 10-20% could be catastrophic, resulting in the “postponing or cancelling” of projects not yet built.
For wind farms already up and running, investors would face a “significant hit on their equity return”.

Ribeiro said that “smart” developers or investors will have considered “a level of uncertainty” in their financial modelling that “can absorb some level of underperformance.

“Nevertheless, it all depends on the level of underestimation of losses, and that is only likely to become apparent once all wind farms are operating at capacity.”

“As is perhaps natural and expected,” there is always “some degree of optimism on the developer side,” she said. Some investors “may also be infected by that.”

“In that sense, maybe the industry has slightly put its head in the sand on this one.”

Who owns the wind and can it be stolen?

If you own a property with a beautiful view of the countryside and a towering apartment complex is built squarely in front of your window, you’d probably be quite upset by this.

In fact, often, you’d be due compensation, said Ignacio Herrera Anchustegui, an energy law professor at the University of Bergen in Norway. “Because your property decreases in value and you lose something – in this case, a nice view.”

Orsted's Hornsea wind farms have raised concerns over wind wakes in the planning process for the 1.5GW Outer Dowsing project being developed by TotalEnergies and Corio Generation. The distance between Outer Dowsing and Hornsea 2 is 20km, the same distance Orsted modelling shows one wind farm can have a dramatic impact on another's resource.

Awel y Mor gives a useful preview of arguments that may arise. In that case, the aggrieved incumbent Rhyl Flats cited a DNV estimate that the wake from Awel y Mor could cost it 2% of its wind resource and 26GWh of power generation over five years.

That would not threaten the viability of Rhyl Flats, countered Awel y Mor, which challenged the incumbent to carry out a more detailed assessment.

The two projects then clashed over who should pay for that assessment, with Awel y Mor arguing it should not have to as The Crown Estate – the seabed landlord for England and Wales – had set appropriate distances between projects in its leasing process.

In that case, the examining authority sided with Rhyl Flats, acknowledging that it “may be seen as creating a precedent” in the process, one that has already been leapt on by Orsted among others in similar objections to new projects.

But Rhyl Flat’s counsel Greig cautions that even within the UK, the laws governing such disputes vary onshore and offshore, as well as between England and Scotland.

“You've got the same technical issue applying everywhere… but you've got different legal regimes and different policy context in each part,” he said, with developers having to potentially soon familiarise themselves with such legal regimes the world over.

Herrera said developers might also argue whether it is the state or another relevant authority that has “not done a good job in dividing the areas or foreseeing possible conflicts.”

“I’m very happy. I’m producing a lot of power. And then the government says, well, I'm going to give a licence to someone else and we're going to put [a new wind farm] nearby. And they don't take into consideration the wind wake effect.”

"Could this be a breach of the terms of the license, or the legitimate expectations of the project developers to not see their wind currents disturbed by other parks as it was at the time they made the investment?"

Developers have ‘little to no legal protection’ from cross-border wakes

The wind has no regard for international borders, territorial waters or exclusive economic zones (EEZs), and neither do wind wakes.

But interpreting what international law says about wind wakes between nations takes us into truly uncharted waters, said Herrera Anchustegui.
“It becomes even more complex,” he said. And, worryingly for developers, “there's not a lot of protection.”

States in principle have sovereign rights to exploit the wind in their EEZ, said Herrera. "In principle, they can do whatever they want.”

Herrera Anchustegui recently co-authored a paper exploring this issue, taking the potential wake effects of Norway’s Sorlige Nordsjo 2 zone, only 22km from Denmark’s planned Nordsren III vest zone, as a case study.

Were wind wakes from Nordsren III to reduce the capacity factor of Sorlige Nordsjo 2 by 20%, that would imply an annual production loss of 7.9TWh, the study found, the average annual electricity consumption of two million EU households.

Should a loss like that result in a dispute, an aggrieved party could bring a claim under the United Nations Convention of the Law of the Sea. This stipulates that states must pay “due regard” to other interests, including across international borders. In other words, be a good neighbour.

But what that means in the context of wind wakes is unclear and untested.

Relying on this principle should a dispute break out might not be a strong case, said Herrera Anchustegui, but the only case.

Even worse for developers, they may have to persuade a state to bring a case for them, as companies cannot raise claims at the International Tribunal for the Law of the Sea, which adjudicates such matters.

Persuading a state to bring an international legal claim against a potentially friendly neighbour, all to protect a developer’s profit margins, appears a tough sell indeed.

Sorry for your loss

With so much legal jeopardy and uncertainty surrounding wind wakes, it is unsurprising developers are keen to find amicable solutions.

Increasingly, they are turning to wake loss agreements, under which one developer agrees to compensate another for losses caused by wake effects.

The rationale, said Addleshaw Goddard managing associate Stewart Gordon, who has worked on such agreements, is that this helps avoid an aggrieved developer raising planning objections or even legal challenges that could derail a project.

Striking a deal helps provides certainty to all parties, he said, legally and financially.

There is now a “general acceptance” among developers that they must "agree in good faith" to pay appropriate compensation, he said, with the main point to be decided usually if this is a fixed sum or based on data from the wind farm concerned.

More generally, the wind sector should "try to think proactively," said Herrera Anchustegui, trying to minimise wake effects and potential disputes they might cause through “good strategic planning.”
“Trying to come up with solutions that are win-win, not lose-lose.”

There will undoubtedly be desire among developers to resolve issues around wind wakes through collaboration, rather than fighting each other tooth and nail over every gust of wind.

Then again, Awel y Mor may serve as a precautionary tale. For RWE does not just own a controlling stake in that project. It also owns over half of Rhyl Flats. And if even two RWE projects can't amicably settle their differences over wind wakes, that does not bode well for the sector.