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An Experimental Study on the Effects of Wake Interference on the Performance of Wind Turbines over Flat and Complex Terrains. Advanced Flow Diagnostics and Experimental Aerodynamics Laboratory Department of Aerospace Engineering Ahmet Ozbay, Wei Tian and Hui Hu

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  1. An Experimental Study on the Effects of Wake Interference on the Performance of Wind Turbines over Flat and Complex Terrains Advanced Flow Diagnostics and Experimental Aerodynamics LaboratoryDepartment of Aerospace Engineering Ahmet Ozbay, Wei Tian and Hui Hu Iowa State University2242 Howe Hall, Ames, Iowa 50011-2271Email: aozbay85@iastate.edu

  2. OUTLINE • Motivation/Objectives • Introduction • Experimental set-up and procedure • Investigation of wake interference effects • Wake interference of wind turbines with different spacing • Wake interference within an array of turbines in a line • Multiple wake interactions in wind farms with different layouts • Upstream turbine operating(yaw) conditions • Terrain (2D-Ridge) effects on the wake interferences among multiple wind turbines • Summary

  3. Objectives of the Present Study The most important aerodynamic aspects in the design of wind farms: Velocity deficit: The velocity deficit is linked to the amount of power can be extracted from the flow. The power output could lose up to 40% when the wind turbine placed in the wake. Enhancement of turbulence intensity: Enhanced turbulence intensity in the wake is associated directly with the fatigue loads and failure of the wind turbine components. (Barthelmie et al., 2007) How to reduce the wake induced effects over flat and complex terrains? (1) Investigating the effects of the array spacing and wind turbines layout on the wake interference and performance of multiple wind turbines sited in a wind farm for higher total power yield and better durability. (2) Investigating the effects of topography (complex terrains -2D Ridge) on the wind turbine performance as well as on the wake interaction (3) Investigating the effects of upstream turbine operating (yaw, pitch) conditions on the efficiency of wind farm

  4. Objectives of the Present Study Comparison of onshore and offshore winds: • Offshore wind farms: • Wind turbines sitting on flat ocean surface • Near neutral atmospheric boundary layer winds • High wind speed with relatively low ambient turbulence level • Suffers from ‘deep array effect’ • Onshore wind farms: • Wind turbines sitting over complex terrains. • Atmospheric stability is rarely close to near-neutral (highly convective – unstable during the day time and highly stable nocturnal conditions with high shear at night time) • Much higher ambient turbulence level (4) Different characteristics of atmospheric boundary layer winds were simulated to compare the performances of wind turbines sited in onshore and offshore wind farms.

  5. INTRODUCTION – BOUNDARY LAYER • LOG-WIND PROFILE • POWER LAW PROFILE

  6. INTRODUCTION – WIND POWER POWER AVAILABLE IN THE WIND WIND POWER DENSITY (WPD)(watts/m2) is used to classify the winds • Wind speed plays a crucial role in the wind power • 10% increase in the wind speed leads to 33% increase in the wind power density • Wind resource assessment is important for wind turbine siting

  7. INTRODUCTION – WIND RESOURCE ASSESSMENT • WIND MEASUREMENT TOOLS • METEOROLOGICAL TOWERS • ANEMOMETERS ( WIND SPEED) • WIND VANES ( WIND DIRECTION) • SENSORS (TEMPERATURE, PRESSURE) • Weibull /Rayleigh probability function • Using probability density function over a wide range of wind speed to estimate the mean power from a turbine

  8. INTRODUCTION – WIND RESOURCE ASSESSMENT (2) Weibull /Rayleigh probability function • Shape factor , k , shape of the curve depending on the standard deviation of the wind speed (σu) • As k increases, mean velocity tends to increase and wind speed variations (σu) fall down http://www.wind-power-program.com/wind_statistics.htm

  9. INTRODUCTION – WIND TURBINE POWER • Limitations for wind turbine power • Betz limit – theoretical limit (Cp=0.59) • Cut in and cut out speeds • Power losses (wake, environmental, electrical, etc.) Optimum range (0.2 - 0.35)

  10. INTRODUCTION – WIND ENERGY IN U.S. • One of the fastest growth in terms of electric resource capacity, in GW, every year since 2005 • US policy suggest that wind energy will continue to play dominant role in needs of new electric resources in the world, US, Midwest, & Iowa • Wind turbine technology is in its infancy – needs to develop along multiple dimensions over the next 40 years • According to Department of Energy (DOE) recent report, US wind power can reach 300GW by 2030, with on-shore (land-based) wind capacity being a major contributor • Other predictions suggest as much as 600 GW by 2035

  11. INTRODUCTION – WIND ENERGY IN U.S. • A target of 20% of US electricity from wind energy by 2030 has been set up by the U.S. Department of Energy (DOE). • Iowa is second in the nation in installed wind energy capacity and it has the highest density of wind power generation capacity with 29.9 kW/km2 • According to the Energy Information Administration (EIA), Iowa has reached the milestone of 20% of the state’s electricity, supplying the state with a full one fifth of its energy needs. • Top Wind Energy Production States: • Texas: 10,377 MW • Iowa: 4,322 MW • California: 3,927 MW • Illinois: 2,743 MW • Minnesota: 2,733 MW • Washington: 2,573 MW • Oregon: 2,513 MW • (The data as of Feb 28, 2012)

  12. EXPERIMENTAL SET-UP AND PROCEDURE Cobra probe JR3 Force/Moment Transducer • Measured parameters: • Dynamic wind loads • Power output and rotational frequency of wind turbine models • Detailed flow field (mean velocity and turbulence) measurements with cobra probe 1:350 scaled model to simulate a 2MW wind turbine with 90m rotor blades Optical tachometer

  13. EXPERIMENTAL SET-UP AND PROCEDURE 127 mm ERS-100 prototype of wind turbine blade developed by TPI

  14. Simulation of Incoming Flow with Different Turbulence Levels • Wind speed profile of in atmospheric boundary layer (ABL): • POWER LAW PROFILE Onshore wind farm  =0.15 Offshore wind farm  =0.11 Low turbulence intensity case (10% at hub height) High turbulence intensity case (18% at hub height)

  15. Simulation of Incoming Flow with Different Turbulence Levels Low turbulence intensity case (10% at hub height) High turbulence intensity case (18% at hub height)

  16. Simulation of Incoming Flow with Different Turbulence Levels Wind speed distribution at the hub height • Weibull dimensionless shape factor (k) – breadth of the wind speed distribution • The variation of the shape parameter with the incoming flow turbulence (σ/Um) • As the incoming flow turbulence level decreases, the shape of the distribution tends to be tight – less variation in the wind speed (offshore)

  17. Investigation of wake interference effects – A Effect of spacing

  18. Investigation of wake interference effects – A Wake effects – PIV results Phase locked PIV results Ensemble averaged (free-run)PIV results

  19. Investigation of wake interference effects – A Wake effects – PIV results Low turbulence High turbulence The evolution of the wake vortex structure (phase-locked measurements) Power spectrum of the velocity fluctuations (u’) at the top-tip height (x/D=0.5)

  20. Investigation of wake interference effects – B 5 Turbines in a line Low turbulence intensity case (10% at hub height) High turbulence intensity case (18% at hub height) 6D 6D 6D 6D pos 1 pos 3 pos 4 pos 5 pos 2 (Barthelmie et al., 2007)

  21. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts (b) staggered wind farm with stream-wise spacing 3D (a) aligned wind farm with stream-wise spacing 3D (c) aligned wind farm with stream-wise spacing 6D

  22. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  23. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  24. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  25. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  26. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  27. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  28. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  29. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts Aligned 3D-spacing Staggered 3D-spacing Aligned 6D-spacing 0.222 0.145 0.145 Standard Deviation of Thrust Force

  30. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts

  31. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts Aligned 3D-spacing Staggered 3D-spacing Aligned 6D-spacing 0.277 0.201 0.190 Standard Deviation of Thrust Force

  32. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts Wind farm efficiency : CPtotal : The total power output of wind farm CPalone : Power output of single wind turbine under incoming flow n :The number of wind turbine in the wind farm

  33. Investigation of wake interference effects – C Multiple wake interactions in wind farms with different layouts 3D Aligned wind farm • In staggered wind farm, the velocity deficit and added turbulence is much lower compared with the aligned wind farm.   • Staggered wind farm is much more efficient than the aligned wind farm with similar stream-wise and span-wise turbine spacing. • The turbulence level of atmospheric boundary layer wind is effective on the wind farm efficiency. • The improvement in the wind farm efficiency due to the incoming flow turbulence is more pronounced in the staggered wind farm. 3D Staggered wind farm

  34. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions • Two turbines in tandem arrangement with 2D spacing • Upstream turbine is installed on a turn table and yawed up to 50˚ with an increment of 10˚ • Test cases: • Flow measurements in the wake of the upstream turbine with yaw angle from 0˚ to 50˚ • The power output and dynamics forces for both upstream and downstream turbine • Flow field measurements in the near wake of the downstream turbine with the yaw angle of upstream turbine changing from 0˚ to 50˚ Experimental set-up for wind tunnel testing g U∞ g 2D

  35. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions Previous studies on upstream turbine operating (yaw) conditions Adaramola & Krogstad (2011) Effect of upstream turbine yaw angle on the downstream turbine performance Effect of yaw angle on the performance of upstream turbine

  36. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions Previous studies on upstream turbine operating (yaw) conditions Effect of yaw angle on the wind turbine performance Pri Mamidipudi (2011) Yaw control: The forgotten controls problem • It was found that there is a cos3 dependency between loss of power and yaw angle especially between -20ᵒ and +20ᵒ for a scaled wind turbine model.

  37. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions Uhub = 5.1 m/s – high turbulence inflow Uhub = 6.1 m/s – low turbulence inflow P (g) ≈ 0.5Cp⍴ AUeff3 P (g) ≈ 0.5Cp ⍴ AU∞3cos3(g) P (g) ≈ cos3(g)P0_yaw F(g) ≈ 0.5Cp⍴ AUeff2 g U∞ g 2D F(g) ≈ 0.5Cp⍴ AU∞2cos2(g) F(g) ≈ cos2(g) F0_yaw

  38. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions g U∞ g 2D

  39. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions Vertical velocity profile at x/D =2 downstream: Low turbulence g U∞ g 2D High turbulence • Wake is deflected sideways by yawing the upstream turbine which results in reduced velocity deficit in the wake. • Effect of yawing the upstream turbine is less pronounced in the wake for the high turbulence flow due to the highly turbulent nature of the flow – turbulent mixing.

  40. Investigation of wake interference effects – D Upstream Turbine operating (yaw) conditions The overall efficiency of the wind farm (2 turbines): • Effects of upstream turbine yaw angle • Decrease the upstream wind turbine power output with a cos3 dependency between loss of power and yaw angle • Increase the power output of downstream wind turbine • For the low turbulence inflow, the increase of overall power output is up to 6% at an appropriate upstream turbine (α=10˚) yaw angle • For the high turbulent inflow, yawing the upstream turbine does not make any improvement on the overall wind farm power output Low turbulence High turbulence

  41. Investigation of wake interference effects – E Complex terrain (2D-Ridge) effects CASE 1 – Moderate slope (H/L = 0.22), slope = 12˚ CASE 2 – High slope (H/L = 0.41), slope = 22˚ • Separation on the lee side (effect of the slope) • (reduced mean speed and higher turbulence levels) • Speed-up effects • (Higher wind speeds, great potential for energy production) • According to Arya (1988), the largest speed-ups are observed over three-dimensional hills of moderate slope. • 3D Hills are found to produce lower wind speed increases than 2D Ridges

  42. Investigation of wake interference effects – E Complex terrain (2D-Ridge) effects Gaussian curve: h/2 h Moderate slope 2D-Ridge Slope =12° 3D 3D 3D 3D h/2 h High slope 2D-Ridge Slope =22° 3D 3D 3D 3D D Flat surface 3D 3D 3D 3D pos2 pos4 pos3 pos1 pos5

  43. Investigation of wake interference effects – E Complex terrain (2D-Ridge) effects Mean velocity and turbulence intensity profile

  44. Investigation of wake interference effects – E Complex terrain (2D-Ridge) effects Moderate slope 2D-Ridges 6D 6D 6D 6D • Measuring the characteristics of surface winds over complex terrains (hill and valley ) • The performances of single wind turbine sited over different locations over complex terrains

  45. Investigation of wake interference effects – E Complex terrain (2D-Ridge) effects Moderate slope 2D-Ridges 6D 6D 6D 6D

  46. SUMMARY • Factors affecting the complex dynamics of the wind farms were investigated in detail; • Turbine spacing • Wind farm layout (aligned and staggered) • Upstream turbine operating (yaw) conditions • Terrain effects (flat and complex terrain) • Incoming flow character and its interaction with different wind farm layouts

  47. Thank you!

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