Don’t build downwind, chump

Because wind turbines take energy from the air that flows over their blades, there is the obvious possibility of wake effects, i.e. where downwind turbines experience lower wind speeds. (The power at the turbine is related to the wind speed by a cubic function, so a small decrease in wind speed could lead to a large loss of power.)

Bearing in mind the incipient industrialisation of the North Sea, I wondered what the scale of such effects might be. Instinct said that they would be negligible. Are they?

Wind turbines are well spaced within farms. The wind farm visible from the beach at Gt Yarmouth (Scroby Sands) is fairly old and small. Its turbines are 60 m tall and have 40 m blades. Their files, as you look from shore, are 350 m apart, and the ranks are 500 m apart. These distances, a casual observer might surmise, are chosen to minimise or eliminate interference between turbines, and that the layout is chosen to favour performance from the prevailing wind direction. So you might think that if the wind blows directly from shore along the line of two turbines 500 m apart (about 6 turbine diameters), that the second turbine would suffer no performance loss.

Searching Google Scholar for an answer to this question I came across Nygaard et al 2020. The first thing these authors present is a within-farm wake model. The previous standard apparently represented a wake as an expanding regular cone, whereas a better fit to real-world data was obtained by Nygaard et al using a model where wake expansion slowed down over distance. Anyway the relevant observation for me was that in the real world (Westermost Rough wind farm), second and subsequent turbines only had 80% of the power of the foremost turbine if the wind was blowing directly along the line (ranks at Westermost Rough, like at Scroby Sands, are 6 turbine diameters apart, which for turbines with a 154 m diameter, means they are 900 m apart).

That sounds quite a large drop in performance, but the wind is not likely to be blowing exactly perpendicular to the ranks or files of turbines. What about cluster wakes: the effect of one wind farm on a completely different wind farm? One might naively think that if the power loss at 6 turbine diameters (c. 900 m in the case of Westermost Rough) was 20%, that when you start putting clear blue water between them the effect will end up being negligible.

Well, Nygaard et al have data on this too. There is another wind farm fairly close (15 km) to Westermost Rough: Humber Gateway, a set of 73 turbines with a rotor diameter of 112 m. That makes Westermost Rough 130 (15000/112) turbine diameters from Humber Gateway. What happened when the wind blew through Humber Gateway before it reached Westermost Rough?

This is the surprising (to me at least) result:

Figure 3 from Nygaard et al, with original legend

Humber Gateway is to the south, & Westermost Rough is the square with some turbines missing from the centre. The graph shows the power ratio between the two end turbines as the wind direction changes. Real world data is orange dots, their emulations are the lines. Of particular interest is where one and not the other is in the Humber Gateway’s cluster wake: a 30% loss of power results.

Nygaard et al go on to model blockage (the upstream effect of wind turbines on wind speed), which seems to be a minor problem compared to wakes, & then they combine the two effects to try to get a better estimate of wind speeds at turbines. They point out that neglecting wind turbine interactions, and even interactions between different wind farms, will lead to overestimation of energy production.

Conclusion

Presumably as the North Sea fills up with bird munchers, everyone will want their farms to upwind of their competitors.

A snap of a casualty on Gt Yarmouth North Denes taken about a year and a half ago. Scroby Sands wind farm is barely visible on the horizon. It is, however, close enough that the juxtaposition of the two were rather suspicious, in my eyes, at least. I think it’s a female sparrowhawk, but I didn’t flip it over to check.

Reference

Nygaard et al is available here.

42 Comments

  1. Funnily enough, I can’t remember anyone in Government, mainstream media, wind turbine companies, “environmental” pressure groups or anyone else pushing wind turbines at us, mentioning Nygaard et al, and the potential negative implications for the great claims being made on behalf of offshore wind.

    I’m sure the wind turbine companies themselves must be very well aware of it, on the other hand, yet the seem to be keeping quiet. Funny that.

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  2. There’s the memorable image of the wake at Vattenfall’s Horns Rev Offshore Wind Farm, Denmark:

    Liked by 1 person

  3. I always thought that there needed to be 10D spacing to stabilise the flow downstream of obstructions. That is what it is for orifice plates in compressible fluids. The data you analysed indicates that this rule of thumb seems correct.

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  4. It seems to me that any rule governing the required spacing of wind turbines not only has to offset the downwind turbulence caused by upstream turbines but also the loss of energy within the wind; energy extracted from the wind by the upstream turbines. Or is the turbulence in the wake caused by this extraction? I seem to recall that no more than a third of the energy within the wind intersected by the turbine blades can be extracted.

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  5. Alan, the Betz limit is at the maximum value of 4a(1-a)^2 where a is the fractional decrease in wind velocity at the turbine. It maxes out at 0.59 ish when a = 1/3, which is when the wind speed has been reduced by 2/3. But I don’t know how well this works in the real world.

    Chris, I suppose the spacing used is a compromise between the available area and the wake effect. If there are fewer turbines in the same space, each will perform better, but additional turbines might still be profitable.

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  6. “Long-Range Wake Losses Offshore Much Greater Than Expected, New Study Shows”

    https://www.offshorewind.biz/2022/08/18/long-range-wake-losses-offshore-much-greater-than-expected-new-study-shows/

    “Commonly used engineering wake models vastly underpredict energy losses due to external wakes, the US-based technical services provider ArcVera Renewables has found in a new study.

    According to ArcVera Renewables, the study confirms the severe under-prediction of long-range wake losses by engineering wake loss models in common use and investigates long-range wake loss potential at the New York Bight offshore development sites.

    Velocity deficits, as high as 1 m/s or 10 per cent, persist up to or greater than 100 kilometres downwind of large offshore arrays, leading to long-range energy deficits much greater than expected by most subject matter experts in the industry, ArcVera said.

    ArcVera used the Weather Research and Forecasting (WRF), a high-fidelity numerical weather prediction model, to carry out the research. The Wind Farm Parameterization (WFP) was added to the model to account for the effects of the wind turbines in the domain.

    ”This new study provides an important cautionary lesson as the wind industry proceeds to ever-larger wind turbine models with greater farm density across the globe,” said Greg Poulos, CEO of ArcVera Renewables.

    ”WRF-WFP’s results here show that engineering wake or WFAI models currently under-predict long-range wake losses by a significant margin. Unexpected losses are likely to accrue from wind farms once thought to be too far away to be material to project performance.”…”.

    Liked by 2 people

  7. Mark, that is an extraordinary stat if true: 10% at 100 km. That would make the turbines capable of local climate change on their own.

    Liked by 1 person

  8. Yes, if true. It relies on models, and we here tend to be sceptical of models. However, it’s definitely worth a read, IMO.

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  10. “Offshore wind farms are projected to impact primary production and bottom water deoxygenation in the North Sea”

    https://www.nature.com/articles/s43247-022-00625-0

    Abstract
    The wind wake effect of offshore wind farms affects the hydrodynamical conditions in the ocean, which has been hypothesized to impact marine primary production. So far only little is known about the ecosystem response to wind wakes under the premisses of large offshore wind farm clusters. Here we show, via numerical modeling, that the associated wind wakes in the North Sea provoke large-scale changes in annual primary production with local changes of up to ±10% not only at the offshore wind farm clusters, but also distributed over a wider region. The model also projects an increase in sediment carbon in deeper areas of the southern North Sea due to reduced current velocities, and decreased dissolved oxygen inside an area with already low oxygen concentration. Our results provide evidence that the ongoing offshore wind farm developments can have a substantial impact on the structuring of coastal marine ecosystems on basin scales.

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  11. Mark, presumably the hypothesis is that the lower wind speed causes less mixing in the surface layer. There are already huge natural diurnal and seasonal changes in North Sea chemistry. My “gut feeling” would be that the turbine wakes would not cause measurable changes. But that is arguing from ignorance. I’ll read the paper when I have a mo. I really hope these guys are wrong, because even if they’re right, it won’t result in the build out of wind being curtailed.

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  12. Mark; it’s models all the way down…as the saying goes.
    There have been some very large wind farms in the N. Sea for some years so a bit of real-life sampling should have been possible. That would show clearly whether the models have any validity. Of course, the authors may not want to face such testing!

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  13. “New Understanding of Wind Farm Efficiency”

    https://www.trinity.ox.ac.uk/news/new-understanding-wind-farm-efficiency

    Andrew Kirby says: ‘The commonly observation reduction in power is due to two different effects: firstly, there is a wake behind every turbine with a reduced wind speed. When these wakes affect other turbines, it can reduce the farm power by up to 40%. Secondly, the farm power can be reduced by the farm interacting with the atmosphere and slowing the wind. Both industry and academia struggle to model these two effects accurately. This uncertainty makes wind farm projects more expensive to finance.’

    ‘Our paper proposes new concepts to better understand the impact of these two effects. We found that for large wind farms the interaction with the atmosphere was the most important. This is contrary to the approach taken by the wind energy industry and suggests that large wind farms should be designed differently in the future.’

    https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/twoscale-interaction-of-wake-and-blockage-effects-in-large-wind-farms/89DDB33925C5A6F4A3E0F387587FDE0A

    Liked by 1 person

  14. “Scroby Sands wind turbine fire off Norfolk coast self extinguishes”

    https://www.bbc.co.uk/news/uk-england-norfolk-66510193

    A fire on an offshore wind turbine has self extinguished, its owner has said.

    A plume of black smoke was seen billowing from the Scroby Sands wind farm, 1.5 miles (2.5km) off the coast of Great Yarmouth, Norfolk.

    HM Coastguard said the alarm was raised at 10:50 BST on Tuesday and that all personnel had been accounted for following an evacuation.

    RWE, the German company that owns the wind farm, said no-one was on board the turbine when the fire broke out.

    The company hoped to have the wind farm back operational on Wednesday….

    Fair play to the BBC for reporting this, and on the front page of its website too. However, no marks for finishing with the usual piece of propaganda from the German wind farm company about how much installed capacity the wind farm has (as opposed to how much electricity it actually generates) and how many households it can power (as opposed to how many it does power, when, and how intermittently):

    RWE said its 30 turbines had an installed capacity of 60 megawatts and was able to power more than 48,000 households.

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  15. Well, that’s very environmentally friendly:

    “Scroby Sands: Investigation into wind turbine fire ‘may take weeks'”

    https://www.bbc.co.uk/news/uk-england-norfolk-66525400

    …RWE, the German company that owns the site, said a team had already started removing fire debris at low tide…

    …”When something like this happens, you can only watch as the fire burns out.”..

    …It said the majority of damage was caused to the nacelle – the enclosure at the top of the tower which housed the generating components – and that it may take weeks to determine the cause of the blaze.

    Investigators could not assess the damage until it was safe to do so, the company added.

    “An electrical fault is the most likely cause,” Mr Smith added. “There’s a lot of electrical components in those things and electrical items are the most likely things to generate heat.”..

    …The Maritime and Coastguard Agency (MCA) also said it was liaising with RWE, the local authority and environmental stakeholders, to ensure an appropriate clean-up operation was carried out.

    It added that a local navigation warning had also been issued to keep vessels away from the affected area….

    As usual, the article concludes with the usual advert for renewables, quoting installed capacity, rather than actual output:

    RWE said its 30 turbines at Scroby Sands had an installed capacity of 60 megawatts and was able to power more than 48,000 households.

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  16. “Talks needed over density of offshore windfarms in Europe, warn experts
    Turbines can ‘steal’ energy from those downwind and neighbouring projects could lead to conflicts of interest”

    https://www.theguardian.com/news/2023/nov/10/talks-needed-over-density-of-offshore-windfarms-in-europe-warn-experts

    …too dense a forest of turbines could become an economic problem. As the blades of each individual turbine go round, they take strength out of the wind, researchers have found. The blades harvest energy that is then converted into electricity. The net result is that downwind blades will not be turning as fast; the energy otherwise available from the wind can be cut 20% as far as 50km away.

    This drop will be enough to make some windfarms uneconomical, especially with construction costs rising and profit margins being squeezed. Some offshore windfarm projects have already been postponed, but despite this a dozen countries are pressing ahead with projects close to the boundaries of their exclusive economic zones, hoping to make the most of the available winds.

    There are no international agreements at present on how to deal with potential conflict of interest when these planned windfarms are close to those of neighbours. The researchers suggest political cooperation is urgently needed to avoid international conflict over this resource that could otherwise disrupt Europe’s green energy transition….

    Liked by 1 person

  17. Thanks Mark. I’m firmly not an expert, but “don’t build downwind, chump” probably counts as a valid warning. Nice of the experts to catch up with the other experts who wrote the paper discussed in the OP.

    Liked by 1 person

  18. “As the blades of each individual turbine go round, they take strength out of the wind, researchers have found” – are the researchers school kids I wonder, as adult researchers must have thought about this?
    maybe they did, and can/could see subsidy money rolling in any way.

    Liked by 1 person

  20. STOP PRESS: ‘Wind turbines act as wind breaks!’

    Scientists have rediscovered the Law of Conservation of Energy as they realise that energy absorbed from a moving air mass by turbines in order to generate electricity is not miraculously replaced downwind.

    OMG – wind energy is not quite so ‘renewable’ as we were led to believe!

    Liked by 2 people

  21. There must be some tricky effects downwind of one of these beasts. As well as extracting some of the wind’s energy, the airflow probably ends up with a spiral motion. I wonder whether, for the prevailing wind direction, the arrays use alternate directions of rotation for the blades?

    Liked by 1 person

  22. Mike H. There is a wonderful aerial photograph of an array of wind turbines, each of which disturbs the air for hundreds of metres downwind by causing condensation clouds. It clearly demonstrates the need for spacing of the turbines both side to side and downwind. 
    There also is a formula, whose name I forget, that determines the maximum percentage of wind energy that may be extracted. I presume that a turbine downwind of another might be able to extract its own percentage of the depleted wind energy.

    Liked by 1 person

  23. Further to above

    I failed to find the photograph I mentioned but similar may be found from Google putting in “wind turbine turbulence, North Sea”

    the formula defines the Betz Limit which states that the maximum energy that theoretically can be extracted by a wind turbine is just under 60%.

    Liked by 1 person

  25. Mike H. Yes it is. Note that with this wind direction, disturbed air affects downwind turbines which for this wind direction appear to be far too close. Downwind turbines are extracting energy from air streams that must already be energy depleted from interaction with the upwind turbines. The downwind turbines must be operating at much lower efficiency.

    Liked by 2 people

  27. JIT. My apologies. Memory loss due to age (two years ago is a long long time) but no real excuse. Nevertheless the picture is worth a repeat.

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  28. “Switched on for the first time the largest wind turbine in history: It has caused an unexpected effect”

    https://www.eldiario24.com/en/switched-on-the-largest-wind-turbine/2206/

    Despite having a reputation for contributing to global warming and toxic emissions, China turned the tables and is now one of the nations going green. Mingyang Smart Energy installed a 20 MW offshore giant wind turbine in August 2024, which is the largest wind turbine in the world. With its 128-meter blades and 242-meter height, it can produce enough electricity each year to power 96,000 homes.

    However, the activation of the turbine has brought about an unforeseen consequence: changes in the microclimate surrounding the installation site. Although wind turbines typically change airflows, the structure’s unparalleled size is enhancing these impacts, leading researchers to investigate how such enormous installations can affect regional weather patterns….

    ...By reducing the amount of space needed for wind farms, the higher efficiency also makes it possible for fewer turbines to produce the same amount of energy. Notwithstanding the successes, the turbine’s enormous size has had unanticipated environmental effects. Studies have shown that turbines of this size modify the immediate microclimate and local wind patterns, resulting in changes to wind speeds and temperature distributions surrounding the installation....

    …However, preserving a balance between energy and local ecosystems will depend on making sure that such technologies don’t unintentionally disturb them. With the potential to smash records, this is the biggest wind turbine ever made. In actuality, its height is the least of its many advantages because energy efficiency is rapidly increasing and it has far more potential than the nuclear power plants that America wishes to “resurrect”.

    The concept is straightforward: decarbonise and, as we have shown in New York, do so over the sea if there is no room (Garcia, 2024). Environmentalists and scientists are investigating the potential effects of these shifts on coastal climates, bird migration patterns, and marine ecosystems. The turbine’s energy benefits are indisputable, but it also serves as a warning that advancements in renewable energy have their own set of drawbacks.

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  29. Mark; it’s hard to see why these effects should be news. The dramatic photos of Horn’s Reef windfarm generating a fogbank have been widely publicised.

    Also the statement: “By reducing the amount of space needed for wind farms, the higher efficiency also makes it possible for fewer turbines to produce the same amount of energy” is wrong, according to Prof David MacKay in his excellent book on Sustainable Energy. He showed that wind turbines need to be spaced at least 5 times the rotor diameter from each other to minimise losses. So bigger turbines will mean fewer units but the area required will be the same, whatever the machine size.

    Incidentally there’s a 26 MW machine in development, also in China.

    Liked by 1 person

  30. Slightly O/T, but this seems like as good a place as any to post this:

    “Climate Changing: Germany’s 30,000 Wind Turbines Causing Local Rainfall Droughts”

    https://stopthesethings.com/2023/06/09/climate-changing-germanys-30000-wind-turbines-causing-local-rainfall-droughts/

    ...There is already plenty of evidence to show that wind turbines are causing local nighttime temperatures to rise by as much as 1.5°C. Which would be no surprise to grape growers who use banks of large frost fans to stir up the air at nighttime, and thereby prevent frosts from forming and protect their crops.

    Now a German study suggests that its 30,000 industrial wind turbines are causing droughts of the conventional kind (as in a lack of precipitation, not a lack of wind)….

    Like

  31. “‘Catastrophic’ wake losses threaten existence of Orsted Irish Sea wind farms

    Danish developer owns 1.85GW of operational wind farms in Irish Sea but says wakes from planned projects could ‘shorten life’ of assets whose revenues would be hit”

    https://www.rechargenews.com/wind/-catastrophic-wake-losses-threaten-existence-of-orsted-irish-sea-wind-farms/2-1-1785911

    Orsted says that wake losses from offshore wind farms planned by EnBW, BP and RWE pose an existential threat to its Irish Sea assets, as a former project CFO at the Danish developer tells Recharge of the “catastrophic” impact they could have based on recent modelling.

    An assessment commissioned by Denmark’s Orsted has found that four planned wind farms in the Irish Sea could result in wake losses and a drop in annual energy production (AEP) of up to 5.34% for its operational assets nearby.

    Liked by 2 people

  33. dfhunter, I think in the end, the issue will affect pretty much all offshore wind farms, but sadly, not before they have trashed the seas and oceans.

    Like

  34. Not news to readers of Cliscep:

    “Blowout! Windfarms blocking each other…

    Researchers will investigate how nearby offshore windfarms can reduce each other’s energy output”

    https://www.energylivenews.com/2025/03/13/blowout-windfarms-blocking-each-other/

    Scientists are about to investigate a new phenomenon – how offshore windfarms block each other from generating power!

    The University of Manchester boffins have secured funding to tackle the growing challenge of how wind farms affect each other’s energy output as they expand.

    The research project, known as POUNDS (Prediction Of UnqualifieD losseS from offshore wind farm wakes), will run for 12 months to assess how energy production is impacted when wind farms are built closer together…..

    Aye, where there’s an acronym, there’s brass.

    Like

  35. “Wind theft’: The mysterious effect plaguing wind farms”

    https://www.bbc.com/future/article/20250506-renewable-energys-trouble-with-wind-theft

    …Simply put, as the spinning turbines of a wind farm take energy from the wind, they create a wake and slow the wind beyond the wind farm. This wake can stretch more than 100km (62 miles) for very large, dense offshore wind farms, under certain weather conditions. (Though more typically, the wakes extend for tens of kilometres, according to researchers). If the wind farm is built upwind of another wind farm, it can reduce the downwind producer’s energy output by as much as 10% or more, studies suggest….

    Liked by 1 person

  36. Mark – just as well I read your link before commenting on this “mysterious effect”.

    “Colloquially, the phenomenon is known as wind theft – though as Eirik Finserås, a Norwegian lawyer specialising in offshore wind energy, notes: “The term wind theft is a bit misleading because you can’t steal something that can’t be owned – and nobody owns the wind.”

    “We have seen wake effects for years, and knew they happen,” says Ouro. “The problem is that in order to achieve net zero, we need to deploy a given amount of offshore wind capacity. So for 2030, we need to have three times more capacity than we have now, which means that in less than five years, we need to deploy thousands more turbines,”

    “If operators or countries try to avoid these wake effects by securing the best spots for themselves, it can create another risk, he warns: wake effects may cause what’s known as “the ‘race to the water’ phenomenon, whereby states rush development in order to reap benefits from the best-yet available wind resources”. Rushing development could then increase the risk of ignoring other important aspects of wind farm planning, such as protecting the marine environment, he says.”

    I despair sometimes/often. Only one quote on “protecting the marine environment” I could find.

    Liked by 1 person

  37. “Miliband backs Orsted in wind theft row with BP and EnBW

    Energy secretary tells BP and EnBW to assess how wake losses from new offshore wind project will affect Orsted sites, although whether he will order mitigation or compensation remains to be seen”

    https://www.rechargenews.com/wind/miliband-backs-orsted-in-wind-theft-row-with-bp-and-enbw/2-1-1818415

    UK energy secretary Ed Miliband has sided with Orsted on a key issue in its wind theft row with BP and EnBW over a planned offshore wind farm in the Irish Sea, a decision with implications for numerous other disputes developers are fighting over wake losses.

    Orsted is currently involved in several disputes with fellow offshore wind developers over wind wakes that could stretch from new planned UK projects to existing assets owned by the Danish developer.

    Like

  38. “The ferocious wind wars being fought in the middle of the North Sea”

    All told, around 20 gigawatts (GW) of wind projects are now ensnared in wind theft disputes, according to the consultancy Tamarindo.

    It would be funny, if it wasn’t so stupid. Telegraph link.

    Liked by 1 person

  39. “Britain is running out of space for offshore wind, warns Miliband’s energy tsar”

    Unfortunately, we still have plenty of room for idiots.

    According to Dan McGrail, the interim chief of the quango that everyone was demanding be created prior to the election 2024, Great British Energy Off, every turbine put up post 2030 is going to have to be a floater.

    [It’s never going to happen. No populace is so stupid to acquiesce in their own destruction, surely?]

    Inevitably, the floaters will cost an arm and a leg, but never fear. We can just soak the public.

    Luke Clark, RenewableUK’s head of floating offshore wind, welcomed Mr McGrail’s comments.

    He said: “Floating wind is set to become one of the biggest global industries of the 21st century, and Britain is already an international leader in this innovative technology.“

    “In the longer term, this will enable us to generate vast amounts of cheap electricity further out to sea where wind speeds are even higher, ensuring that costs plummet while boosting Britain’s energy security.”

    Vast amounts of cheap electricity? Costs plummeting? Energy security?

    I’m sorry, no. It will enable some folks to make vast amounts of money, until the public wise up to just how stupid is the idea of erecting giant floating turbines out in the middle of a hostile environment.

    Telegraph link.

    Liked by 2 people

  40. It’s also worth noting that CfD AR6 produced a strike price (at 2012 prices) for floating offshore wind at £139.93 per MWh, which is almost three times higher than other wind farms. How increased reliance on one of the most expensive forms of UK electricity production will see “costs plummeting” is something of a mystery.

    Liked by 2 people

  41. Can’t wait for BBC Verify to expose this as UK nonsense/stupidity. Will have to wait until Trump drops down the agenda, may be a long wait.

    Like

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