Wind Farms in Complex Terrain: Numerical Simulation
of Wind and Wakes for Optimized Micrositing
S. Jafari, N. Chokani, R.S. Abhari
ETH Zürich, Switzerland
EWEA February 4-7, 2013
Vienna, Austria
Overview

Motivation

Objectives

Modeling Approach: Immersed Wind Turbine Model

Validation and Results
 Single Wake
 Multiple Wake
 Wind farm in complex terrain

Summary and Conclusion
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Motivation

Wind turbines operating in wakes of upwind wind turbines may have
 30-40 % power losses compared to upstream turbine
 up to 80% larger fatigue loads than upstream turbine
 Most models fail in predictions when more complex inflow or ground features
are present
 Interaction of topography with wake and wind flow not addressed satisfactorily
to date
 Microscale wind and wake flow must be simultaneously simulated to be able to:
 Account for change in inflow wind due to terrain effects
 Assess effect of elevated turbulence on wake’s evolution
 Investigate interaction of wake with adverse/favorable pressure gradients caused by
topography
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Objectives

Develop computationally efficient wake model to be embedded in RANS solver
used for microscale wind simulations with comparable grid requirements

Perform simultaneous simulations of microscale wind and wind turbine wake

Validate and evaluate predictions of flow field and power performance in wind
farms
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Numerical Approach

Turbines modeled using immersed wind turbine model (IWTM), embedded in
LEC’s RANS solver, MULTI3

Turbine represented as streamtube defined based on turbine operating point

Near wake modeled, velocity and turbulent field mapped at the end of inviscid
expansion of wake, but far wake resolved on computational grid

Boundary conditions imposed on Cartesian grid using immersed boundary
method
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Validation: Microscale Wind

Prediction of wind speed compared with field measurements over Askervein
(moderate terrain, Jafari et al., 2011) and Bolund Hill (complex terrain, Jafari et al., 2012)

Good agreement observed for both cases, up- and downstream of hill
Bolund Hill, 270o
Askervein Hill
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Mean Flow of Single Wake

Predicted evolution of wake in good agreement with wind tunnel experiments,
(Hassan,1992)

Maximum 12% difference between predicted and measured wind speed
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Turbulence Intensity in Single Wake

At x=2.5D, two peaks observed in turbulence intensity profile as expected

Evolution of turbulence intensity captured well both qualitatively and qualitatively
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Single Wake: Full-scale Measurement

Predictions compared
with full-scale
measurements at
Sexbierum wind farm

5.4 MW farm
consisting of 18
turbines, D=30 m

Maximum deficit
underestimated by
20%

Wake width predicted
well
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Validation: Multiple Wakes

Interactions of multiple wakes examined for offshore wind farms
 Horns Rev (offshore, Denmark), 80 Vestas V2.8-80
 Lillgrund (offshore, Sweden), 48 Siemens SWT-2.3-93
30

28
27
Power loss in array and sensitivity to wind direction captured for all cases
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Wind Farm in Complex Terrain

23.7 MW Mont Crosin wind farm located in Jura region, Switzerland (complex
terrain) consisting of 16 turbines with hub heights of 45 and 95 m

SCADA data collected and analyzed over one and a half years period
270o
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Simulation Set-Up

Upstream conditions of wind speed and turbulence and specified based on:

Long-term mesoscale [Weather Research Forecast Model (WRF)] simulations
performed over Switzerland, Jafari et al., 2012

Measurements using LEC’s nacelle mounted probe, Mansour et al., 2013
Computational grid for wind direction 170o
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Dominant wind direction from south-west quadrant
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Mont Crosin Wind Farm: Results

Impact of terrain on local wind evident

Performance of turbine 14 decreases
260o
relative to turbine 13 up to 65%
90 m AGL
260o
230o
45 m AGL
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Summary & Future Work

Simultaneous simulation of microscale wind and wakes accomplished with
computationally efficient wind turbine model

Model evaluated with broad range of test cases including wind tunnel/field
experiments, onshore/offshore, and flat/complex terrain

IWTM brings grid requirements for wake simulations closer to microscale wind
and facilitates use of Computational Fluid Dynamics for micrositing in complex
terrain
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Thank you.
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