EWEA February 4-7, 2013Vienna, Austria
Wind Farms in Complex Terrain: Numerical Simulation of Wind and Wakes for Optimized Micrositing
S. Jafari, N. Chokani, R.S. AbhariETH Zürich, Switzerland
February 6, 2013 Samira Jafari 2
Overview Motivation
Objectives
Modeling Approach: Immersed Wind Turbine Model
Validation and Results
Single Wake
Multiple Wake
Wind farm in complex terrain
Summary and Conclusion
February 6, 2013 Samira Jafari 3
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
February 6, 2013 Samira Jafari 4
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
February 6, 2013 Samira Jafari 5
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
February 6, 2013 Samira Jafari 6
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 hillAskervein Hill Bolund Hill, 270o
February 6, 2013 Samira Jafari 7
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
February 6, 2013 Samira Jafari 8
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
February 6, 2013 Samira Jafari 9
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
February 6, 2013 Samira Jafari 10
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
28 2730
Power loss in array and sensitivity to wind direction captured for all cases
February 6, 2013 Samira Jafari 11
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
February 6, 2013 Samira Jafari 12
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 Dominant wind direction from south-west quadrant
February 6, 2013 Samira Jafari 13
Mont Crosin Wind Farm: Results Impact of terrain on local wind evident
Performance of turbine 14 decreases
relative to turbine 13 up to 65%
260o
260o
230o
90 m AGL
45 m AGL
February 6, 2013 Samira Jafari 14
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
February 6, 2013 Samira Jafari 15
Thank you.