1. Wall Effects in Offshore Wind Farms
Karol Mitraszewski
Co-authors:
Kurt S. Hansen (DTU Wind)
Nicolai Gayle Nygaard (Dong Energy)
Pierre-Elouan Réthoré (DTU Wind)

2. Let’s introduce the name: WALL EFFECTS
1. Motivation
A generic offshore wind farm
Wind coming from the open sea
COMMON MODEL PREDICTIONS:
𝑃1≅𝑃2≅… ≅𝑃7
OBSERVATIONS FROM A NUMBER OF OFFSHORE WIND FARMS:
𝑃1≠𝑃2≠… ≠𝑃7
Power outputs of turbines located in the outer edges of the farm differ significantly, order of magnitude of these differences: ≈10% !
What are the causes of these effects?
How often do they occur? Are they at all significant?
Are they important in terms of AEP?
Let’s introduce the name: WALL EFFECTS

3. 2. Approach
Quantify the wall effects by analyzing power outputs of turbines located in outer edges of the Horns Rev 1 farm (10 min. SCADA data) depending on wind direction, wind speed, turbulence intensity and stability for wake-free flow cases.
„Ambient” wind signal derived from two „leader” turbines located in the analyzed edge, towards the center of the cluster (looking downwind):
Wind speed derived based on power & pitch curves
Wind direction based on nacelle position
Turbulence intensity based on std. deviation of electric power measurement
Leading turbine choice flow case dependent

4. 3. Presentation of results
Wind direction impact, western (onshore) winds
Color of arrow heads = power output relative to the mean of the highlighted row
Basic statistics of the data base query
Maximum power output in the outer edge = red cross
Minimum power output in the outer edge = green cross
Direction of arrows = „ambient” wind direction
COAST app.13 km away
OPEN SEA

5. 3. Presentation of results
Wind direction impact, western (onshore) winds

6. 3. Presentation of results
Wind direction impact, western (onshore) winds

7. 3. Presentation of results
Wind direction impact, western (onshore) winds

8. 3. Presentation of results
Wind direction impact, western (onshore) winds

9. 3. Presentation of results
Wind direction impact, western (onshore) winds

10. 3. Presentation of results
Wind direction impact, western (onshore) winds

11. 3. Presentation of results
Wind direction impact, western (onshore) winds

12. 3. Presentation of results
Wind direction impact, western (onshore) winds

13. 3. Presentation of results
Wind direction impact, western (onshore) winds

14. 3. Presentation of results
Wind direction impact, western (onshore) winds

15. 3. Presentation of results
Wind direction impact, western (onshore) winds

16. 3. Presentation of results
Wind direction impact, western (onshore) winds

17. 3. Presentation of results
Wind direction impact, western (onshore) winds

18. 3. Presentation of results
Wind direction impact, western (onshore) winds

19. 3. Presentation of results
Wind direction impact, western (onshore) winds

20. 3. Presentation of results
Wind direction impact, western (onshore) winds

21. 3. Presentation of results
Wind direction impact, western (onshore) winds

22. 3. Presentation of results
Wind direction impact, eastern (offshore) winds
COAST app.13 km away
OPEN SEA

23. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

24. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

25. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

26. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

27. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

28. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

29. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

30. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

31. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

32. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

33. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

34. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

35. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

36. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

37. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

38. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

39. 3. Presentation of results
Wind direction impact, eastern (offshore) winds

40. 3. Presentation of results
Wind direction impact, observed trends
For western winds the speed-up zone occurs consequently on the left side of the cluster (looking downwind) despite changing orientation of the wind with respect to the layout and coast.
For eastern winds the speed-up zone occurs consequently „further out offshore” despite changing orientation of the wind with respect to the layout and shore.
Explain well what the X-Y figures contain
The observed wall effects are both qualitatively as well as quantitatively different for eastern and western winds, hence:
The driving mechanism of wall effects is different for eastern and western winds
Shore vicinity has an impact on wall effects

41. 3. Presentation of results
Wind speed, turbulence and stability impacts
Wall effects decrease with wind speed
Wall effects decrease with turbulence intensity
Wall effects are most profound under stable atmospheric conditions
For more detailed information please consult the full text of the paper

42. 4. Results interpretation
Physical wall effect driving mechanisms, two regimes
Two regimes
„Coriolis regime” under western winds.
„Coastal regime” under eastern winds
The division is not a
"clear cut"

43. 4. Results interpretation
Physical wall effect driving mechanisms, Coriolis regime
Why is the speed-up region occuring always on the left of the cluster (looking downwind) ?
What’s the source of asymmetry?
The streamline convergence mechanism is belived to be causing wall effects under western winds
Inspiration:
Coriolis force driven streamline convergence/divergence is a documented phenomenon in coastal meteorology
Streamline convergence/divergence causes acceleration/decceleration of the flow
Although this is a hypothesis, observations supporting it can be found in wind farm related literature and coastal meteorology papers (see the full text of the paper)

44. 4. Results interpretation
Physical wall effect driving mechanisms, Coastal regime
Gradual increase in wind speed at hub height with distance to shore under offshore winds is induced by the roughness change at the coast and the corresponding vertical profile change.
This mechanism is causing the speed-up zone to appear „further-out offshore” under eastern winds at HR1

45. 5. Results interpretation
The impact of wall effects on AEP
Coastal effects are a site inherent feature
Coriolis force is by definition always perpendicular to air particle velocity
Hence the work done by Coriolis forces on a control volume of air is 0.
Coriolis effects are believed not to have an impact on AEP
𝑈 𝑤𝑖𝑛𝑑
𝐹 𝐶
𝐹 𝑃
Only the wind direction is altered locally resulting in streamline convergence
Wall effects are probably an „AEP neutral” phenomenon, although this hypothesis requires further scientific validation
However, the Coriolis induced wake turning is not included in the present wake models

46. 6. Summary of findings
Wall effects are a frequently occuring phenomenon
They are caused by Coriolis forces and coastal effects
They decrease with wind speed and turbulence intensity
The Coriolis induced wall effects at HR1 usually do not induce power output variations greater than 10%
The Coastal induced wall effects are more significant (up to 17%) (although this is very site specific)
Wall effects are probably AEP neutral
Further research: confirmation of the above presented hypotheses with the use of a meso-scale flow modeling tool. This task is being carried out in collaboration with:

47. Thank you.