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SODAR and Extrapolated Tower Wind Shear Profile Comparison in Various Topographic Conditions

SODAR and Extrapolated Tower Wind Shear Profile Comparison in Various Topographic Conditions. Elizabeth Walls Niels LaWhite Second Wind Inc EWEC 2009 Marseille. Introduction. SODAR (Sonic Detection and Ranging):

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SODAR and Extrapolated Tower Wind Shear Profile Comparison in Various Topographic Conditions

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  1. SODAR and Extrapolated Tower Wind Shear Profile Comparison in Various Topographic Conditions Elizabeth Walls Niels LaWhite Second Wind Inc EWEC 2009 Marseille

  2. Introduction • SODAR (Sonic Detection and Ranging): • measure wind data by transmitting acoustic pulses and analyzing the frequency content of the returned signal • Triton Sonic Wind Profiler: • Low-power, monostatic, phased-array SODAR commercialized in early 2008 • Several Triton vs. Tower comparisons • Great correlation at anem. height • How do the extrapolated tower shear profiles compare to the measured Triton data? • How does the error in extrapolation translate to error in predicted power?

  3. Outline • Site and Data Set Description • 4 sites across the U.S. with varying topography • 2 months of concurrent tower and Triton data • Triton vs. Tower Data: Validation • Shear Exponent Estimation using Triton Data • Extrapolated Wind Shear Profile Comparison • Theoretical Power Output Comparison

  4. Site and Data Set Descriptions • Cranberry Bog in Massachusetts • Flat site surrounded by trees • 60 m met tower • Data Used for comparison: • May 15th – July 15th, 2008 • Open Field in Kansas • Flat and open terrain • 60 m met tower • Data Used for comparison: • Sept. 1st – Nov. 1st, 2008

  5. Site and Data Set Descriptions • Ridgeline in Washington State • Complex, hilly terrain • 50 m met tower • Data Included: August 15th – Oct. 15th, 2008 • Wind Farm in Washington State • Several wind turbines ~300 m from Triton • 60 m met tower • Data Included: Sept. 1st – Oct. 17th, 2008

  6. Dir. Sectors Included Anems Triton vs. Tower Data: Filters • Data Filtering for Correlation Study: • Triton Quality Factor > 90% • Quality: function of Signal-to-Noise Ratio (SNR) and the number of valid data points over ten-minutes • Triton Vertical Wind Speed < +/-1.5 m/s • Max Value of Two Anems Used • Reduces tower shadow effects • Data Filtering for Average Wind Speed Comparison • Triton Quality Factor > 95% • Triton Vertical Wind Speed < +/-1 m/s • Average Value of Two Anems Used • Ratio of Anems = 0.98 - 1.02 • Anem Wind Speed > 2 m/s • Direction Sectors 45º from boom with 30º width

  7. Triton vs. Tower Data: Cranberry Bog, MA • Data Interval: May 15th to July 15th, 2008 • Triton Operational Uptime = 98.4% • Corr. Coeff. = 0.968 • Valid Triton data (High Q) @ 60 m = 99.5% • % Diff. In Avg. Wind Speed = -1.1 %

  8. Triton vs. Tower Data: Open Field, KS • Data Interval: Sept. 1st to Nov. 1st, 2008 • Triton Operational Uptime = 99.3% • Corr. Coeff. = 0.976 • Valid Triton data (High Q) @ 60 m = 94.5% • % Diff. In Avg. Wind Speed = -0.55 %

  9. Triton vs. Tower Data: Ridgeline, WA • Data Interval: Aug. 15th to Oct. 15th, 2008 • Triton Operational Uptime = 94.9% • Corr. Coeff. = 0.988 • Valid Triton data (High Q) @ 50 m = 91.1% • % Diff. In Avg. Wind Speed = -7.6 % • Large diff. due to terrain and distance from tower

  10. Triton vs. Tower Data: Wind Farm, WA • Data Interval: Sept. 1st to Oct. 17th, 2008 • Triton Operational Uptime = 99.8% • Corr. Coeff. = 0.966 • Valid Triton data (High Q) @ 60 m = 97.4% • % Diff. In Avg. Wind Speed = -0.6 %

  11. Shear Exponent Estimation using Triton Data • Power Law Profile: • Use Triton Data from 40 m to 120 m • Plot ln(U/Ur) vs ln(z/zr) • Slope of best-fit = Power Law Exponent, Alpha

  12. Shear Exponent Estimation using Triton Data, cont’d • Alpha found for each Triton data set:

  13. Extrapolated Wind Shear Profile Comparison • For each data set, found: • Triton Alpha (using data from 40 to 120 m) • Tower Alpha (using data from 2 heights) • Tower data extrapolated using both Triton and Tower Alphas

  14. Extrapolated Wind Shear Profile Comparion, cont’d • Wind speed profile extrapolations from other two sites:

  15. Theoretical Power and Equivalent Wind Speed • How do varying wind shear profiles translate into theoretical power available in wind? • Power Produced: • Equivalent Hub Height Wind Speed:

  16. Theoretical Power Output Comparison • Assuming ideal turbine operation: Cp = 16/27 and 100% efficiency • % Difference = • With hub height = 80 m and rotor radius = 40 m, % difference in predicted power:

  17. Range of Uncertainty Radius = 40 m Hub Height = 80 m Power as function of Rotor Radius and Hub Height • Error increases with both rotor radius and hub height • +ve % diff. : Tower data leads to overprediction • -ve % diff. : Tower data leads to underprediction • With hub height of 100 m and a radius of 40 m, the percent difference ranged from -16.4% to 9.3%

  18. Summary • Analyzed two months of concurrent Triton and tower data from 4 different sites across the U.S. • At each site, showed excellent agreement between the tower and Triton data in terms of correlation (Ravg = 0.975) and average wind speed • Estimated alpha (power law exponent) using both the Triton and tower data • Used both alphas to generate extrapolated wind shear profiles • Calculated the theoretical power production with each wind shear profile and found the percent difference

  19. Conclusions • Extrapolating wind shear profiles, based on tower data, can lead to under or over estimation of wind speeds • Error in theoretical power increases with rotor radius and, more drastically, with hub height • SODARs (and other remote sensing devices) measure wind speed across the rotor diameter which reduces uncertainty in shear exponent estimation.

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