Wind Shear Characteristics at 

Plains Tall Towers

Marc Schwartz and Dennis Ell

National Renewable Energy Labo

Windpower 06 Conference

June 5, 2006 Pittsburgh, PA

NREL/PR-500-39989
Objectives

• Analyze wind shear characteristics at ta

for diverse areas in the central plains (TDakota)

– Turbines hub heights are now 70-100 m ab– Wind measurements at 70-100+ m have be

• Present conclusions about wind shear 

for prime wind energy development reg
Background

• Tall tower measurements on existing c

towers established during past 5 years 

– U.S. DOE State Energy Program and Wind 

America

– State/university initiatives– Other research programs

• NREL obtains time series data from a v

sources

• 13 tall towers were used in the study

– 11 tall towers had highest anemometer at10– 2 tall towers the highest anemometer was a

Texas - 2 

Oklahoma Kansas - 6 Colorado South DakNorth Dak

Power C2 L Clas

6 H Clas2 L Clas2 H Clas1 L Clas
Technical Approach

• Create “clean” tall tower data sets

• Use wind speed shear exponent 

" from 

for wind shear characteristics

– Shear exponent does not tell all about shea

is easy to use in comparative analysis

• General analysis methodology

– Anemometer data from levels at or near 50 

• 2 or 3 measurement levels per tower

– Calculate wind shear statistics by averaging 

from individual measurements between lev

at least 3.0 m/s at same time

– Average data exclusion is 9%
Wind Shear Characterist

• Annual average• Diurnal variability

• Seasonal variability• Shear variation by prevailing wind direc

• Investigate wind shear variation by heig
Tall Tower Data Sets

• Create “clean” data sets for analysis (o

– Use data from set of anemometers on sam

judged by: 

• Overall data quality• Data recovery rates• Minimal tower effects

• Tower effects on the quality of data pos

challenge in creating data sets for analanticipated

– Wind speed and frequency by direction ana

identify tower effects

– Subjective decisions made to determine “le

used for analysis


Tower Effect Summary

• 8 out of 13 towers had at least one leve• Tower effects are common, but sometim

and changes to measured speed not e

– Tend to lower measured speed– Does not necessarily affect all levels on a to– Quite sensitive to wind direction
Analysis Features

• Statistics based on short periods of rec

years) and variable data quality

•

" is very sensitive to tower effects

– Example: if 50 m speed = 7.0 m/s and 80 m 

then 

" =.175

– If tower effect is + or – 0.1 m/s then 

" could 

to .223

• Tower effects made it difficult to determ

values for measurement layers

– Values judged to be within 0.05 range of m– Despite tower effects, trends in seasonal, d

direction shear variability still can be discer
Central Plains Tall Tower Locations Use

Site Name Anemometer Heights From To Lamar, CO (3) 52 113 10-05-2001 09-Ellsworth, KS 50 (80) 110 04-18-2003 09-Kearny, KS 50 80 (110) 04-29-2003 09-Sumner, KS 50 80 (110) 06-11-2003 09-Jewell, KS 50 (80) 110 04-23-2003 09-Ness, KS 50 (80) 110 06-04-2003 09-Logan, KS 50 80 (110) 05-01-2003 09-Hobart, OK 40 70 (100) 04-01-2002 12-Elk City, OK (10) 40 70 (100) 10-30-2003 08-Sweetwater, TX 50 (75) 100 05-17-2003 03-Washburn, TX 50 75 (100) 09-05-2003 10-Crow Lake, SD 50 70 12-26-2001 12-W. Finley, ND (10, 41) 56 85 08-07-2003 04-Anemometer heights in bold-face were used in shear analysis. 









Shear Climate - Overall and D

• Annual average " between 0.15 and 0

• Greater variation of annual wind shear 

towers within a region than between thnorthern plains

• Limited data in Kansas indicate annual 

consistent with height

• Diurnal shear pattern similar throughou

– Daytime " is 0.05-0.1– Nighttime " between 0.25-0.40

– Some seasonal variations among towers
Shear Climate - Directional and 

• Winds from the south exhibited higher 

winds from the north

– South winds generally had " values 0.2 – 0

0.1 - 0.2

• Some differences in the seasonal patte

shear

– Northern Texas, Oklahoma, and Kansas ex

distributions

– Central Texas, Colorado, and the Dakotas 

shears from Jul. to Oct. and lowest shears 
Lessons Learned and Conclu

• Tower effects are common 

– Attempt to minimize effects but unlikely to e

• Do not accept wind shear information f

at face value

• Need accurate wind direction data to h

possible tower effects

– Only 60% of the measurement levels acros

had high-quality direction data

• Thank you to all organizations who hav

tower data

• Long live tall towers!
Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36-Department of Energy (the “DOE”). The United States Government (the “Government”) retains athe work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrepublish or reproduce the published form of this work, or allow others to do so, for Government puNeither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employeesor implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness product, or process disclosed, or represents that its use would not infringe any privately owned rig

specific commercial product, process, or service by trade name, trademark, manufacturer, or otheimply its endorsement, recommendation, or favoring by the Government or any agency thereof. Tauthors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the Dagency thereof.