Wake Physics: Large Wind Farms Are Making Downstream Turbines Unprofitable

A Wake-Up Call

By Klimanachrichten

A virtual wake-up call for the wind power industry. Two companies are sounding the alarm as they risk losing a lot of money.

Although they are not making losses yet, they are earning less. It’s about the the wake effect on wind farms by other wind farms.

Windmesse.de

The expansion of offshore wind energy in the North Sea is a central component of the European energy transition. However, two of the biggest players in the industry are now warning of negative effects: Ørsted and Equinor have jointly calculated that the planned 1.5 gigawatt wind farm ‘Outer Dowsing’ could cause significant so-called wake losses. These are yield losses that occur when the wind is weakened by upstream wind farms, causing downstream turbines to produce less electricity.

The two companies estimate that their existing wind farm projects in the British part of the North Sea could lose up to 361 million pounds – the equivalent of around 422 million euros – in the long term as a result of the new wind farm. The wind farms already in operation, which are dependent on constant wind conditions in order to achieve their planned output and ensure profitability, will be particularly affected.”

The phenomenon is called the wake effect and it is by no means new, as you can see in the Sciencemediacenter, an article from 2012:

The existence of wind turbine wakes has been known for decades. For smaller wind turbines and onshore wind farms, it for a long time was not considered to be so important. With the increasing size of individual wind turbines (multi-MW turbines) and larger wind farms in recent years, the size and length of the wakes are increasing and becoming increasingly relevant. I pointed this out back in 2010 and developed a simple model that can be used to estimate the length of wind farm wakes. This model shows the dependence of the length of wakes on subsurface roughness and thermal stratification of the air. Wakes of tens of kilometers in length can be predicted for offshore wind farms with stable stratification. Ms. Lundquist’s working group already presented simulation results with the WRF flow model in 2012, some of which show even longer wakes.“

The topic is also covered by basicthinking, and the losses could be significantly higher than scientists first thought in 2012.

Based on the results of their study, the researchers estimate that offshore turbines take the wind away from each other. As a result, wind farms in the Atlantic lose more than 30 percent of their output. The scientists call this the “wake effect”.

It occurs when the wind flows through the turbines and the forward or upstream turbines draw energy from it. As a result, the wind speed behind the turbines slows down. The result: lower power generation.

The wake effect is particularly pronounced with offshore wind turbines, as there are no houses or trees to swirl the air. This would help to disperse the wakes. Under certain weather conditions, they can also reach up to 55 kilometers and affect other wind farms.

Especially on hot summer days, when the air above the cool sea surface remains stable, wakes can persist for longer and spread further.”

Last year, public broadcaster mdr had a very interesting article about this. It is interesting that this problem was not considered when the energy transition was being planned. However, it is not surprising.

Axel Kleidon sees offshore turbines, i.e. those at sea, as a particular challenge. These are often only just being built and many wind turbines are planned in relatively small spaces. And in an environment where, according to Martin Dörenkämper, losses are generally more pronounced. “Over the sea, the flatter surface leads to a more constant wind speed distribution with height and therefore a considerably less turbulent wind resource than at onshore locations.” In other words, the almost unbraked wind power hits the rotor blades, generating more power there, but then returns to its former strength less quickly due to the lack of ambient turbulence and corresponding mixing: as a result, the wake effects last longer and can still be felt over longer distances.

According to Axel Kleidon, this may lead to a legal problem, as the longer the distances, the more likely it is that wind farms operated by different operators will ‘steal the wind’ from each other. This makes it all the more important to coordinate, especially at national borders such as in the North Sea between Germany, Denmark and the Netherlands: ‘The slice of the North Sea that is managed by Germany is relatively small. The area of the Netherlands and Denmark is much larger. It would make sense to coordinate this instead of somehow focusing on a national level. If you want to expand 70 gigawatts in the North Sea, you can say that it doesn’t all have to be in the German sector, but can also be in the exclusive economic zone of Denmark or the Netherlands.'”

Source: Klimanachrichten.

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