Wind farm blockage and wakes A coupled engineering model

VindKraftNet 29 October 2020

Nicolai Gayle Nygaard

Power variation among front-row turbines Nacelle lidar measurements in wind farm

– Wind speed reduction exceeds the standalone turbine induction

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Wind farm blockage clues

Mitraszewski, Hansen, Nygaard and Rethoré TORQUE 2012

“Wall effect”

Increasing power trend towards ends of front row

Nygaard and Brink, WESC 2017

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Measured flow around a single turbine (from above)

Minute by minute evolution

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Measured flow around a single turbine (from above)

Average over 30 minutes

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Turbine blockage zone

Blockage zone Wake recovery

– Based on work by Gribben and Hawkes @ Frazer-Nash in Offshore Wind Accelerator project

– Uses Rankine half-body (homogeneous flow + point source)

– Adjusted to match flow in 1D momentum theory

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Single turbine induction model

Wind direction

Single turbine induction model

Superposition Coupling with wake model

Ground effect

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Wind farm blockage model

Wind farm

blockage model

Wake model

CT

∆U

Axial induction

Strength given by inflow wind speed and thrust curve

Δ𝑈𝑖 =

𝑗≠𝑖

Δ𝑈𝑖𝑗Aggregated induction from all turbines

Wind direction

1. Solve for wakes from upwind to downwind

2. Use resulting CT values to calculate wind speed reduction from blockage Δ𝑈𝑖

3. Run wake model with 𝑈0𝑖 = U0 + Δ𝑈𝑖

Iterate until convergence

Only wake model applied inside Rankine half-body

Turbine self-induction ignored (this is accounted for in the power curve)

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Coupling wind farm blockage with wakes

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From single turbine blockage to global blockage

– Each turbine creates an upstream blockage and a downstream wake

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From single turbine blockage to global blockage

– Each turbine creates an upstream blockage and a downstream wake

– The aggregated blockage from all turbines creates a stronger wind speed reduction

– Seamless integration with wake model

– TORQUE paper:

– J. Phys.: Conf. Ser. 1618 062072 (2020)

– Uses earlier version of the blockage model

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Comparison with SCADA data

Compare power variation along front row

SCADA

– Filter on inflow at reference turbine

– Inflow wind speed 8±0.5 m/s

– Wind direction in 20° sector

Model

– Adjust freestream wind speed in model to match inflow at reference turbine

– Calculate model results at 1° resolution

– Frequency-weighted average across sector

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Comparison with SCADA data

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Complexity from background flow gradients

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Comparison with SCADA data

– Working on improvements to the model

– Images turbines aloft to simulate atmospheric boundary layer height

– Momentum conservation

– Other initiatives:

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Outlook

Offshore Wind Accelerator

SCADA data

https://orsted.com/en/our-business/offshore-wind/offshore-operational-data

Wind data

https://orsted.com/en/our-business/offshore-wind/wind-data

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Ørsted data sharing