wall effects in offshore wind farms

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)

1. Motivation

COMMON MODEL PREDICTIONS:

OBSERVATIONS FROM A NUMBER OF OFFSHORE WIND FARMS:

A generic offshore wind farm

Wind coming from the open sea

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

2. ApproachQuantify 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):

a) Wind speed derived based on power & pitch curvesb) Wind direction based on nacelle positionc) Turbulence intensity based on std. deviation of electric power measurementd) Leading turbine choice flow case dependent

3. Presentation of resultsWind direction impact, western (onshore) winds

OPEN SEACOAST app.13 km away

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

3. Presentation of resultsWind direction impact, western (onshore) winds

3. Presentation of resultsWind direction impact, eastern (offshore) winds

OPEN SEACOAST app.13 km away

3. Presentation of resultsWind direction impact, eastern (offshore) winds

3. Presentation of resultsWind 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.

The observed wall effects are both qualitatively as well as quantitatively different for eastern and western winds, hence:a) The driving mechanism of wall effects is different for eastern and western windsb) Shore vicinity has an impact on wall effects

1. Wall effects decrease with wind speed2. Wall effects decrease with turbulence intensity3. Wall effects are most profound under stable atmospheric

conditions4. For more detailed information please consult the full text of

the paper

3. Presentation of resultsWind speed, turbulence and stability impacts

4. Results interpretationPhysical 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"

4. Results interpretationPhysical wall effect driving mechanisms, Coriolis regime

Inspiration: Coriolis force driven streamline convergence/divergence is a documented phenomenon in coastal meteorology

The streamline convergence mechanism is belived to be causing wall effects under western winds

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)

Why is the speed-up region occuring always on the left of the cluster (looking downwind) ?

What’s the source of asymmetry?

4. Results interpretationPhysical 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

5. Results interpretationThe impact of wall effects on AEP

Wall effects are probably an „AEP neutral” phenomenon, although this hypothesis requires further scientific validation

Coastal effects are a site inherent feature

Coriolis effects are believed not to have an impact on AEP

Coriolis force is by definition always perpendicular to air particle velocity

�⃗�𝑤𝑖𝑛𝑑 �⃗�𝐶

�⃗�𝑃

Hence the work done by Coriolis forces on a control volume of air is 0.

Only the wind direction is altered locally resulting in streamline convergence

However, the Coriolis induced wake turning is not included in the present wake models

6. Summary of findings

1. Wall effects are a frequently occuring phenomenon2. They are caused by Coriolis forces and coastal effects3. They decrease with wind speed and turbulence intensity4. The Coriolis induced wall effects at HR1 usually do not induce

power output variations greater than 10%5. The Coastal induced wall effects are more significant (up to

17%) (although this is very site specific)6. Wall effects are probably AEP neutral 7. 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:

Thank you.

wall effects in offshore wind farms

Documents

ambient wind direction

western onshore winds3

wind speed

generic offshore wind

ambient wind signal

power output relative

power outputs of turbines

red crossminimum power