In this second instalment of our blog series “navigating complexities offshore wind farm performance”, our focus shifts to the role that atmospheric stability plays in shaping energy production estimates for the IJmuiden Ver wind zone. Building upon insights from the Wind Energy Science (WES) journal publication, “Investigating energy production and wake losses of multi-gigawatt offshore wind farms with atmospheric large-eddy simulation,” we consider the interactions between wind speed, and prevailing atmospheric conditions, to bring you some interesting takeaways.
Offshore Atmospheric Stability: The Unseen Player
Atmospheric stability is a measure of the atmosphere’s tendency to resist vertical motions. Offshore, so-called unstable conditions occur when cold air is flowing over warmer sea water. In this case, rising and falling patches of air cause efficient vertical mixing. Conversely, stable conditions occur when warm air is flowing over colder sea water. As warmer air is lighter than colder air, vertical motions are suppressed in this scenario.
The above animation (simulated by the Whiffle LES model) clearly illustrates the difference between the two flow regimes. The vertical cross-section indicates potential temperature with blue colors refering to colder air and red colors refering to warmer air. In the left-hand panel, featuring unstable conditions, the air is warmed from the surface onwards, resulting in vigorous vertical mixing. In the right-hand panel the air is much warmer than the sea surface resulting is stable conditions. In this case the atmosphere is stratified and turbulence is weak.
Atmospheric stability has an important impact on wind farm performance: in unstable conditions turbine-induced wakes dissolve relatively fast, while in stable conditions turbine wakes extend much further. The figure below shows the wind speed at hub height for north-northeasterly wind directions for contrasting stability conditions. Clearly, in unstable conditions (the left panel) the reduction in wind speed is much smaller than in stably-stratified conditions (the right panel).
What’s more, the relevance of atmospheric stability for energy yield is not universally spread across all wind speeds; its impact is most prominent within a narrow range of wind speeds. Although the strongest atmospheric stability occurs at very low wind speeds, these are not relevant for energy production as the wind speed is lower or around the cut-in speed of the turbines anyway. On the other hand, for very strong winds, turbines will operate at rated power irrespective of any stability effects. As such, a region of intermediate wind speeds will exist, where the impact of stability is most relevant, and it’s within this specific range that its influence becomes especially fascinating.
Zooming in on the IJmuiden Ver Scenario
Turning to the IJmuiden Ver context, atmospheric stability’s sway becomes apparent, especially concerning neighboring wind farms. This insight gains further confirmation from another research study by Peter Baas et al., titled: “The impact of wakes from neighboring wind farms on the production of the IJmuiden Ver wind farm zone”.
Their findings indicate that on days marked by stable conditions and when wind originates from directions with upstream wind farms in the exact path, production deficits due to the presence of the upstream wind farms can notably escalate by up to 20%. These effects are particularly pronounced when wind speeds hover around 10 m/s, revealing a clear connection between wind speed and stability’s implications. Additionally, the study unveils a distinct disparity between stably stratified and unstable conditions, with the former exhibiting notably greater reductions in energy production.
Additionally, neighboring wind farms introduce dynamic impacts, contingent on wind direction and specific sectors. These insights show that the wakes from the IJmuiden Ver wind farm could affect the Nederwiek Zuid area, highlighting how wind patterns are connected. Given the anticipated growth of wind energy in the North Sea, off the coast of North America, and beyond, acknowledging the cluster effect is becoming increasingly imperative. The effect of upstream wind farms must be factored into annual energy production assessments for present-day and future wind farms.
Understanding wake losses
The interaction between atmospheric stability and wake losses is closely tied to variables like wind speed, wind farm layout, and location. The findings from these studies clearly demonstrate how stable stratification conditions impact wakes losses and the energy production in the context of the IJmuiden Ver wind farm scenario.
Next up… blockage effects
Stay tuned, in the next blog we explore the influence of blockage effects on wind farm performance.
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