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.

12 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 1 person

  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.

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  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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  9. “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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  10. 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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  11. 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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