SODAR Shear Measurements at Brewster Massachusetts

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					          SODAR Shear Measurements at
                  Brewster, Massachusetts

                                    FSRP0040-B




                                  June 6, 2008

                                     Prepared for:

                         Massachusetts Technology Collaborative
                                    75 North Drive
                                Westborough, MA 01581




1809 7th Avenue, Suite 900
Seattle, Washington 98101 USA
Phone: (206) 387-4200
Fax: (206) 387-4201
www.globalenergyconcepts.com
www.dnv.com
SODAR Shear Measurements at Brewster, Massachusetts                                   FSRP0040-B




                                           Approvals




                                                                  June 6, 2008
           Prepared by Anthony L. Rogers                          Date




                                                                  June 6, 2008
           Reviewed by Gordon Randall                             Date




   Version Block
     Version           Release Date       Summary of Changes
         A             May 23, 2008       Original
                                          Incorporated changes based on client comments.
         B             June 6, 2008       Updated entity name to reflect DNV’s June 1, 2008,
                                          acquisition of GEC.




DNV Global Energy Concepts Inc.                       i                                 June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                                           FSRP0040-B




                                                  Table of Contents

EXECUTIVE SUMMARY .......................................................................................................... 1
INTRODUCTION......................................................................................................................... 2
SITE DESCRIPTION................................................................................................................... 2
SODAR OPERATION AND DATA COLLECTION SUMMARY......................................... 4
   OVERVIEW OF SODAR OPERATION AND DATA PROCESSING AND VALIDATION ......................... 4
   SODAR DATA COLLECTION AT BREWSTER ................................................................................ 4
     Data Validation Statistics........................................................................................................ 5
WIND SPEED AND WIND SHEAR RESULTS ....................................................................... 8
   WIND SHEAR CHARACTERIZATION .............................................................................................. 8
   TOWER DATA............................................................................................................................... 9
   SODAR DATA ........................................................................................................................... 10
   COMPARISON OF SODAR AND TOWER DATA............................................................................ 13
   ESTIMATED 80-M MEAN ANNUAL WIND SPEEDS....................................................................... 15
   LONG-TERM ADJUSTMENTS ....................................................................................................... 15
DISCUSSION AND CONCLUSIONS ...................................................................................... 15
APPENDIX A – SODAR TECHNOLOGY OVERVIEW ........................................................ 1
   SODAR TECHNOLOGY ................................................................................................................ 1
   SODAR DATA FILTERING ........................................................................................................... 1
     Pre-processing SODAR Filtering ........................................................................................... 1
     Echo Rejection Algorithm ...................................................................................................... 2
     Post-processing SODAR Filtering.......................................................................................... 2
     SODAR Operation during Times of Precipitation.................................................................. 4
   DATA VALIDATION ...................................................................................................................... 4
   BIAS CORRECTIONS ..................................................................................................................... 4
     Volume Averaging.................................................................................................................. 4
     Scalar versus Vector Averages ............................................................................................... 5




DNV Global Energy Concepts Inc.                                      ii                                                         June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                                        FSRP0040-B



                                                     List of Figures
Figure 1. Aerial Photo of Brewster Site.......................................................................................... 3
Figure 2. DNV-GEC SODAR at Brewster Site .............................................................................. 3
Figure 3. Percentage of Valid Data Returns ................................................................................... 6
Figure 4. Breakdown of data removed by each filter...................................................................... 7
Figure 5. Averages of the Wind Speeds Measured at Each Height ................................................ 7
Figure 6. Correlation Coefficients versus Height ........................................................................... 8
Figure 7. Shear Coefficients Based on the 38-m and 49-m Tower Sensors ................................... 9
Figure 8. Average Turbulence Intensity Model (black line) and Measured Turbulence Intensity
     as a Function of Wind Speed, based on Tower Data (red crosses)....................................... 10
Figure 9. Time Series of SODAR Data from Three Heights........................................................ 11
Figure 10. Diurnal Shear Coefficients over Two Height Ranges using SODAR Data ................ 11
Figure 11. Shear Coefficients Determined from 40-m and 50-m SODAR Data and over the
     Range of 50-m to 80-m, using SODAR Data ....................................................................... 13
Figure 12. Wintertime Shear Coefficient Estimates based on SODAR and Tower Data............. 14
Figure 13. Wintertime Wind Speed Frequency Distributions of Tower and SODAR Data......... 14




                                                      List of Tables
Table 1. Changes of SODAR Operational Settings over the Period of Data Collection ................ 5
Table 2. Percentages of Data Removed by Filtering ...................................................................... 6
Table 3. Annual Averages of Tower Data ...................................................................................... 9
Table 4. Shear Coefficients Estimated from SODAR Data using 40-m and 50-m Data and 50-m
    and 80-m Data....................................................................................................................... 12
Table 5. Estimates of long-term mean wind speed at 80m at the Brewster site ........................... 15
Table 6. Average shear coefficients from tower and SODAR data.............................................. 16




DNV Global Energy Concepts Inc.                                     iii                                                      June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                    FSRP0040-B




                                  Executive Summary

The Massachusetts Technology Collaborative requested that DNV Global Energy Concepts Inc.
(DNV-GEC) deploy a SODAR at the Captains Golf Course in Brewster, Massachusetts, to
confirm wind shear estimates based on data collected at a former meteorological tower at the
same site. The tower data covered a period that spanned 2006 to 2007. The power law shear
coefficient estimated from the tower data (using a displacement height of 7 m to account for the
effect of the trees) is about 0.41. The DNV-GEC SODAR was placed at the same location as the
tower. It collected data from January 5 until March 11, 2008.

The SODAR data were subjected to a number of quality checks to help ensure that no data were
affected by ambient noise or unwanted echoes. The data were then validated by comparison with
measurements from an anemometer on the SODAR. Finally, the SODAR data were adjusted to
account for known biases. The results were then analyzed and compared to the tower data. Power
law shear coefficients were determined from each data set for wind speeds greater than 4 m/s and
after adjusting for the height of the nearby trees.

A comparison of the SODAR and tower shear coefficients, based on data from similar times of
the year and at similar heights, highlights the uncertainty that arises when comparing data
collected at different time periods and with different instruments. In spite of the differences
between the results from the two data sets, it appears that 1) power law shear coefficients based
on the tower measurements provide a reasonable estimate of the shear above the tower and
2) there is no evidence that the wind speeds or shear coefficients above the tower decrease
markedly from that which would be expected using the shear behavior determined by the tower
measurements.

Based on the data sets, two estimates of the long-term wind speed have been provided in this
report as guidance. These suggest that a reasonable estimate of the long-term 80-m mean wind
speed at the site is 7.0 m/s.




DNV Global Energy Concepts Inc.                       1                                  June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                     FSRP0040-B




                                         Introduction

The Massachusetts Technology Collaborative requested that DNV Global Energy Concepts Inc.
(DNV-GEC) deploy a SODAR at the Captains Golf Course in Brewster, Massachusetts, to
confirm wind shear estimates based on a former meteorological (met) tower at the same site.
Wind shear refers to the change of wind speed with height above the ground. In this report the
wind shear is characterized by a “power law” which is often used in the wind industry to
describe the increase of wind speed as the height above the ground increases. The wind shear in
Brewster will significantly affect the estimated financial performance of the project that is being
considered for the site. The met tower provided wind speed measurements at 20, 38 and 49 m
above the ground. Based on these measurements, the average power law wind shear coefficient
at the site was determined to be about 0.41. The DNV-GEC SODAR was used to determine
whether the tower shear measurements could reliably be used to estimate the wind speeds above
the tower up to a proposed turbine hub height of 80 m.

This report describes the site of the SODAR data collection, the results of the analysis of the
tower and SODAR data, and the details of the wind shear analysis.



                                      Site Description

The DNV-GEC SODAR was placed at the Captains Golf Course in Brewster, Massachusetts, on
January 4, 2008, and collected data until the end of March 11, 2008. The SODAR was placed at
the same location as a former met tower which had been maintained by the Renewable Energy
Research Laboratory (RERL) of the University of Massachusetts. According to RERL the met
tower was located at 41° 44’ 08.84’’ N by 70° 01’ 12.69’’ W. An aerial map showing this location is
indicated in Figure 1.

On January 4, 2008, DNV-GEC brought our SODAR trailer unit to the same site in Brewster and
began collecting data. The SODAR was placed at 41° 44’ 8.56” N, 70° 1’ 11.76” W, using the
WGS84 datum. The SODAR set-up included an R. M. Young anemometer on a 3-m pole that is
used for data validation. The first day of data was checked to make sure that the SODAR was
operating correctly. Data collection for this project started at 2:30 PM EST on January 5, 2008.
Data collection ceased on March 11, 2008, at Midnight. Figure 2 shows the DNV-GEC SODAR
installed at the Brewster SODAR site.

The SODAR was located just off the end of the golf course parking lot. There were rows of 10-m
high trees about 35 m to the north and south of the SODAR but no nearby obstacles to the east
and west. The SODAR beams were oriented to the west and north (where the distance to the
trees was slightly greater than to the south). There were few nearby houses but a busy two-lane
highway is located about 215 m west of the site. There is a tall FM radio tower about 800 m to
the east of the site.



DNV Global Energy Concepts Inc.                       2                                   June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                         FSRP0040-B




                            Met Tower Location




                                  Figure 1. Aerial Photo of Brewster Site




                             Figure 2. DNV-GEC SODAR at Brewster Site




DNV Global Energy Concepts Inc.                       3                      June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                    FSRP0040-B




            SODAR Operation and Data Collection Summary

Overview of SODAR Operation and Data Processing and Validation
A SODAR is an acoustic instrument that measures wind speed and direction at multiple heights
above the ground without the need for an instrumented tower. The SODAR determines the
vertical wind speed, designated W, and the horizontal wind speeds in two perpendicular
directions, typically east-west and north-south, depending on the orientation of the SODAR.
These measurements are designated U and V. From these three measurements, the wind speed
and direction are obtained.

DNV-GEC used multiple levels of data validation. The data were first quality checked by the
SODAR data collection system. DNV-GEC applied additional filtering. Finally, the
measurements were validated by a visual inspection of graphs of the data. The SODAR rejected
data collected during periods of precipitation or data with a low signal-to-noise ratio (SNR).
DNV-GEC’s data filtering rejected 10-minute data averages for which the U, V, or W
measurements had too much scatter (large standard deviations), for which the W wind speed was
too high or low, and when there was evidence of ambient noise or anomalous shear coefficients.
Once filtering was complete, the data and data statistics were checked in a final validation step.

In the final step of the SODAR data processing, corrections were applied to the validated data to
account for the systematic bias in SODAR measurements due to volume averaging and to
account for the differences between anemometer measurements (“scalar averages”) and SODAR
measurements (“vector averages”).

The details of each of these data processing and validation steps are included in Appendix A.

SODAR Data Collection at Brewster
During the two-month period of data collection, a few adjustments were made to the SODAR
operation and hardware. The SODAR operation started with the pulse power level set at 80% and
the pulse length set at 80 milliseconds (ms). These values are slightly below the typical power
level of 100% and pulse length of 100 ms. These lower initial power settings were used to reduce
the potential for objectionable sound levels at neighboring residences. As it became clear that the
SODAR was not audible at the closest residence, the power level and pulse length were
increased to improve the amount of valid data. Shortly after deployment, on about January 19,
unwanted noise was detected in the input signal for the vertical wind speed. It was determined
that the cause was a faulty electronic circuit board, which was replaced on January 24. Until it
was confirmed that the noise source had been eliminated, the SODAR signal-to-noise (SNR)
ratio was increased to make sure that the data were not affected. Some additional noise in the
signals was later determined to be due to a nearby FM radio transmitter. Efforts were made to
reduce its effect. DNV-GEC also filtered all data after they were quality checked by the SODAR
operating system. Table 1 summarizes the main events that occurred at the SODAR site in
Brewster.




DNV Global Energy Concepts Inc.                       4                                  June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                            FSRP0040-B


   Table 1. Changes of SODAR Operational Settings over the Period of Data Collection
        Date and Time                          Action                            Purpose
                                  SODAR installed and test data
     Fri 1/4/2008 4:00 PM         collection started, 80% power,              SODAR set-up
                                        80 ms pulse length
                                  Adjusted settings: 90% power,
    Sat 1/5/2008, 2:30 PM                                                 Start of data collection
                                        90 ms pulse length
                                   Echo rejection enabled on all
   Fri 1/11/2008, 12:23 PM                                                In case it was needed
                                             three axes
                                  SNR limit set to 12 dB, power     Concern that W noise might affect
   Sat 1/19/2008, 9:20 AM
                                         increased to 90%                        results
    Wed 1/23/2008 2:30 to
                                     Replaced circuit boards           To reduce noise in W signal
          6:00PM
                                  Increased pulse length to 100
    Fri 1/24/2008 7:45 AM                                               To maximize data recovery
                                     ms and power to 100%
                                                                     To maximize data recovery as W
    Thu 1/24/2008 5:00 PM           Reduced pulse SNR to 8
                                                                               noise was gone
                                                                     Test - to make sure there was no
    Fri 1/25/2008 1:30 PM         Set the frequency up to 4704
                                                                            resonance in system
                                  Transmit frequency returned to     Test - to make sure there was no
    Fri 1/25/2008 4:34 PM
                                               4504                         resonance in system
                                                                   To ensure radio noise does not affect
    Sat 1/26/2008 8:50 AM              SNR ratio set to 9.0
                                                                                    data
                                  SODAR trailer grounding rods
    Fri 2/8/2008 11:40 AM                                            To reduce effect of radio station
                                            installed
                                  Echo rejection disabled on all
     Fri 2/8/2008 4:55 PM                                                      Not needed
                                              axes
                                  Reduce transmit frequency to
   Mon 3/10/2008 9:35 AM                                             To reduce effect of radio station
                                              4303
  Wed. 3/12/2008, 12:00 AM           Data collection ceased               End of data collection



Data Validation Statistics
The percentage of valid data at each height is shown graphically in Figure 3. More than 21% of
the data were missing due to precipitation. The rest of the data that were removed from the data
set were removed primarily due to low SNR. At higher heights, as the signal strength decreased,
increasingly more data were removed due to poor SNR. Some data were eliminated by the
standard deviation filters. These data may have been affected by the circuit board noise or the
FM interference. Table 2 provides details regarding the data removed from the data set. Figure 4
presents a graph of these data. The amount of data invalidated by the SODAR for low SNR and
by DNV-GEC filters for high standard deviations is typical of SODAR operation.




DNV Global Energy Concepts Inc.                       5                                         June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                                 FSRP0040-B



                                            100



              Percent Acceptable Averages
                                                                     Percent of Valid Data
                                            80                   Post-Processing Filter Results


                                            60


                                            40


                                            20


                                             0
                                                          40       60          80      100        120       140
                                                                        Height above SODAR
                                                     Figure 3. Percentage of Valid Data Returns


                                                  Table 2. Percentages of Data Removed by Filtering
                                                 W           U, V         High W       Low W            Ambient    Low
 Height     Rain                                                                                                          SNR
                                              Std. Dev.    Std. Dev.    Wind Speed   Wind Speed          Noise    Shear
    m         %                                  %            %             %            %                %         %      %
    30       21.1                                0.6          7.8           0.9          0.2               0        2.5    1.4
    40       21.1                                0.8          4.4           0.1          0.2              0.1       2.4    1.6
    50       21.1                                0.9          2.9           0.2          0.1              0.1       2.1    1.6
    60       21.1                                0.8          2.2           0.2          0.1              0.1       2.7    1.9
    70       21.1                                 1            2            0.2          0.2              0.2       2.7    2.4
    80       21.1                                1.3          1.7           0.3          0.1              0.4       2.6    4.1
    90       21.1                                1.4          1.8           0.4          0.1              0.6       2.4    6.8
   100       21.1                                 2           2.2           0.5          0.1              0.8       2.6    9.5
   110       21.1                                2.3          2.6           0.6          0.1              1.2       2.9   12.3
   120       21.1                                2.8          3.2           0.8          0.1              1.5       2.6   17.2
   130       21.1                                2.7          3.8           0.8           0               1.5       2.7   24.5
   140       21.1                                2.5          3.3           0.9          0.1              1.8       2.7   33.6
   150       21.1                                2.2          2.7           0.9           0               1.4       2.3   42.4
Note: In this table, U and V are the vector components of the horizontal wind speed and W is the vertical wind speed.




DNV Global Energy Concepts Inc.                                               6                                      June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                                              FSRP0040-B




                                                40

                                                         Roof Closed          W Std. Dev.
                          Percentage Removed
                                                         UV Std. Dev.         High W Speed
                                                30       Low W Speed          Ambient Noise
                                                         Low Shear            Low Signal-to-Noise Ratio


                                                20



                                                10



                                                 0
                                                          40            60          80       100           120        140
                                                                                   Height, m
                                                     Figure 4. Breakdown of data removed by each filter


Figure 5 shows the averages of the wind speed data collected at each height. In this case, the
results do not exactly indicate the mean shear at the site as the averages at higher heights include
fewer 10-minute averages.

Figure 6 shows the correlation coefficients of the SODAR data with respect to the R. M. Young
wind speed data and also with respect to the 80-m SODAR data. In all cases the correlation
coefficients decrease as expected as the distances between the heights of the two data sets
increases.

                                               160

                                               140
             Height Above SODAR, m




                                                                                                Mean Wind Speeds
                                               120

                                               100

                                               80

                                               60

                                               40                                                              SODAR
                                                                                                               R.M. Young
                                               20

                                                0

                                                     0             5                 10                   15                20
                                                                             Mean Wind Speed, m/s
                                      Figure 5. Averages of the Wind Speeds Measured at Each Height




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SODAR Shear Measurements at Brewster, Massachusetts                                                      FSRP0040-B




                                     140


             Height above SODAR, m   120
                                                                   SODAR-RMYoung Corr. Coeff
                                     100

                                     80

                                     60
                                                                                       RMYoung
                                     40                                                SODAR


                                           1.0       0.8        0.6         0.4        0.2       0.0
                                                             Correlation Coefficient
                                             Figure 6. Correlation Coefficients versus Height




                                           Wind Speed and Wind Shear Results

Wind Shear Characterization
Wind shear refers to the change of wind speed with height above the ground. In this report the
wind shear is characterized by a “power law” which is often used in the wind industry to
describe the increase of wind speed as the height above the ground increases. It can be described
mathematically as:

                                                                             α
                                                                   ⎛ h   ⎞
                                                            V = V0 ⎜
                                                                   ⎜h    ⎟
                                                                         ⎟                             (Equation 1)
                                                                   ⎝ o   ⎠

Here V is the wind speed at a specific height of interest, h, V0 is the wind speed at a known
reference height, h0, and α is the shear coefficient which characterized the degree of wind speed
change as the height increases. For higher values of α, the wind speeds increase more rapidly
with height. At a site like Brewster, where there are a number of trees around the site, a
displacement height, d, is often used to correct for the effect of the trees on the flow. In this case
the shear characterization that is used is:

                                                                                 α
                                                                  ⎛ h−d ⎞
                                                                  ⎜h −d ⎟
                                                           V = V0 ⎜     ⎟                              (Equation 2)
                                                                  ⎝ o   ⎠

In this report all characterizations of shear use equation 2 with a displacement height of 7 m.



DNV Global Energy Concepts Inc.                                      8                                    June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                FSRP0040-B


Tower Data
Before proceeding, it is useful to look at the previously collected tower data. A year of data from
February 1, 2006, until January 31, 2007, was analyzed for comparison with the SODAR wind
speed measurements. Measurements were collected at 20 m, 38 m, and 49 m on the tower. The
annual averages of the data are shown in Table 3.

                                                     Table 3. Annual Averages of Tower Data
                                                                  Mean Wind         Data
                                                        Height    Speed, m/s     Recovery
                                                         49 m        5.60          99.8%
                                                         38 m        4.99          99.9%
                                                         20 m        3.57         100.0%



The shear coefficient based on the annual averages at 38 m and 49 m is 0.38. The average shear
coefficient, derived from the individual 10-minute 38 m to 49 m shear coefficients is 0.41, for all
conditions when the 38 m wind speed was greater than 4 m/s.

Figure 7 shows diurnal averages of the shear coefficients (based on the 38-m and 49-m tower
measurements) for conditions when the 38-m wind speeds exceeded 4 m/s. The figure also
shows diurnal averages for the concurrent period of SODAR data. Because the beginning and
end of the tower data span this period, the data includes data from February 1, 2006, until March
11, 2006, and from January 5, 2007, until January 31, 2007. The graph shows that the average
winter diurnal shear coefficients are typically lower than the overall annual diurnal shear
coefficients during the nighttime and are greater than the annual diurnal shear coefficients during
the daytime hours.

                                           0.6
                                                                  Shear From Tower Data
             Power Law Shear Coefficient




                                           0.5                   Based on 38 and 49 m Data


                                           0.4

                                           0.3

                                           0.2

                                           0.1                                         One Year
                                                                                       Winter

                                           0.0
                                                 0        5           10          15          20
                                                                      Hour of Day
          Figure 7. Shear Coefficients Based on the 38-m and 49-m Tower Sensors



DNV Global Energy Concepts Inc.                                          9                          June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                    FSRP0040-B


The tower data were also used to characterize the turbulence at the site. The SODAR does not
measure turbulence intensity but turbulence intensity is needed to estimate scalar averaged wind
speeds from the SODAR data. Therefore, the Brewster tower data were used to develop a
relationship between wind speed and standard deviation of the wind speed at the site. This
relationship, based on 49-m data, was assumed to apply to all heights. This was then used with
the SODAR data to estimate the turbulence intensity at the site and thereby, the correction to be
applied to the SODAR data to correctly estimate the scalar-averaged wind speeds. Figure 8
shows the relationship between turbulence intensity and wind speed (the black curve) that was
derived from the data and used to estimate scalar wind speed. For comparison, the graph also
shows the 49-m turbulence intensity data collected at the tower.

                                    1.0


                                    0.8
             Turbluence Intensity




                                    0.6


                                    0.4


                                    0.2


                                    0.0

                                          0   2   4    6      8       10   12   14
                                                      Wind Speed, m/s
   Figure 8. Average Turbulence Intensity Model (black line) and Measured Turbulence
       Intensity as a Function of Wind Speed, based on Tower Data (red crosses)



SODAR Data
Time series of the corrected SODAR data from three heights are shown in Figure 9. During the
two-and-a-half-month period of data collection, the wind speeds ranged from close to zero to
over 20 m/s at 90 m above the ground. These data were used to determine the wind shear at the
site. Mean shear as a function of hour of the day was determined from corrected SODAR data. In
the calculation, the power law shear coefficients were determined only when the wind speed at
40 m was greater than 4 m/s.

Power law shear coefficients were determined over two height ranges. Shear coefficients were
determined using the 40-m and the 50-m SODAR data, as this estimate of wind shear coefficient
most closely reflects the shear that is determined using the 38-m and the 50-m met tower data.
Shear coefficients were also determined for heights between 50 m and 80 m as this reflects the
shear above the 49-m sensor on the tower. A graph of the power law shear coefficients over the
40-m to 50-m range and the 50-m to 80-m range is shown in Figure 10. The figure shows that the


DNV Global Energy Concepts Inc.                           10                            June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                          FSRP0040-B


shear coefficient used to extrapolate from 50 m to 80 m is higher than the shear coefficient
measured using the 40-m and 50-m data. This is not typical of relatively flat sites surrounded by
trees. Table 4 includes the data that are shown in Figure 10.

Figure 11 shows the ratio of the diurnal shear coefficients estimated from the SODAR data over
the 50-m to 80-m range to the diurnal shear coefficients estimated from the SODAR data over
the 40-m to 50-m range.

                                           25

                                                                                              30 m
                                           20                                                 60 m
                                                                                              90 m
                     Wind Speed, m/s




                                           15


                                           10


                                            5


                                            0
                                                     10    20       30        40       50     60        70
                                                                         Day of Year
                                            Figure 9. Time Series of SODAR Data from Three Heights




                                           0.7
                                                          Shear from corrected SODAR data
                                                           over two different height ranges
             Power Law Shear Coefficient




                                           0.6

                                           0.5

                                           0.4

                                           0.3

                                           0.2

                                           0.1                                                       50_80
                                                                                                     40_50
                                           0.0
                                                 0         5             10          15        20
                                                                         Hour of Day
     Figure 10. Diurnal Shear Coefficients over Two Height Ranges using SODAR Data



DNV Global Energy Concepts Inc.                                            11                                 June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                       FSRP0040-B


 Table 4. Shear Coefficients Estimated from SODAR Data using 40-m and 50-m Data and
                                  50-m and 80-m Data
                                                Mean Shear coefficient
                                  Hour of Day
                                                 40_50        50_80
                                      0           0.38         0.43
                                      1           0.35         0.44
                                      2           0.39         0.43
                                      3           0.32         0.44
                                      4           0.37         0.41
                                      5           0.35         0.42
                                      6           0.32         0.43
                                      7           0.33         0.38
                                      8           0.28         0.36
                                      9           0.25         0.31
                                      10          0.22         0.26
                                      11          0.21         0.27
                                      12          0.20         0.25
                                      13          0.19         0.27
                                      14          0.19         0.26
                                      15          0.26         0.28
                                      16          0.25         0.36
                                      17          0.28         0.39
                                      18          0.33         0.45
                                      19          0.36         0.47
                                      20          0.36         0.47
                                      21          0.35         0.44
                                      22          0.35         0.45
                                      23          0.35         0.44




DNV Global Energy Concepts Inc.                   12                       June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                    FSRP0040-B



                     2.0



                     1.5
             Ratio


                     1.0



                     0.5          Ratio of mean shear from corrected SODAR data
                                                   50_80 / 40_50


                     0.0
                           0            5          10          15         20
                                                   Hour of Day
Figure 11. Shear Coefficients Determined from 40-m and 50-m SODAR Data and over the
                       Range of 50-m to 80-m, using SODAR Data



Comparison of SODAR and Tower Data
Wintertime power law shear coefficients based on the SODAR and tower are shown in Figure
12. All of these data are based on data for which the 40 m SODAR wind speed or 38 m tower
wind speed is greater than 4 m/s. These are some of the same data shown in Figures 7 and 10.
The wintertime diurnal shear coefficients based on the tower data are greater than comparable
shear coefficients based on the 40 m and 50 m SODAR measurements (the green lines in Figure
12) but have a similar diurnal pattern. As discussed above, the shear coefficients as measured by
the SODAR between 50 m and 80 m are greater than those between 40 and 50 m. Finally, it can
also be seen that the SODAR shear coefficients between 50 m and 80 m are similar to those
based on the tower data between 38 m and 49 m sensors.

The reason for the different shear coefficients over the 38-m to 50-m range as determined by the
tower data and the shear coefficients over the 40-m to 50-m range as determined from the
SODAR data is unclear. Possible sources of the differences include effects related to
measurement with two different technologies, different shear behavior during the two different
wintertime periods or selective sampling of low shear periods by the SODAR. Errors in the
determination of the measurement heights on the tower, differences in the calibrations of the two
anemometers on the tower or other sources of systematic differences between the SODAR and
anemometers measurements might contribute to the observed difference. The data were also
collected during two totally different time periods, although each includes data from January 5
through March 11. While it seems unlikely, the shear behavior might have been different during
these two periods. Finally, the SODAR data has many more gaps over that period than the tower
data. It could be possible that coincidentally, the data measured by the SODAR might have
included only the periods of low shear. On the other hand, the wind speed distributions during




DNV Global Energy Concepts Inc.                      13                                 June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                       FSRP0040-B


the two winter periods are similar (see Figure 13), suggesting that the SODAR results are not
based on a selective sampling of data from certain wind speeds or atmospheric conditions.

                                                 0.6
                   Power Law Shear Coefficient

                                                 0.5                      Wintertime Shear


                                                 0.4

                                                 0.3

                                                 0.2
                                                            Tower 38_50
                                                            SODAR 50_80
                                                 0.1
                                                            SODAR 40_50

                                                 0.0
                                                        0      5           10          15           20
                                                                           Hour of Day
   Figure 12. Wintertime Shear Coefficient Estimates based on SODAR and Tower Data



                                                 0.25                                        SODAR 50 m
                                                                                             Tower 50 m
             Fraction of Occurrences




                                                 0.20


                                                 0.15


                                                 0.10


                                                 0.05


                                                 0.00
                                                        0          5             10               15
                                                                          Wind Speed, m/s
  Figure 13. Wintertime Wind Speed Frequency Distributions of Tower and SODAR Data


Despite the uncertainty raised by the differences between the shear coefficients as estimated
using both the tower and the SODAR, it appears that that 1) shear coefficients based on the tower
measurements provide a reasonable estimate of the shear above the tower and 2) there is no
evidence that the wind speeds or shear coefficients above the tower decrease markedly from that
which would be expected using the shear behavior determined by the tower measurements.



DNV Global Energy Concepts Inc.                                              14                            June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                     FSRP0040-B


Estimated 80-m Mean Annual Wind Speeds
Two estimates of the 80-m annual mean wind speed were determined using the SODAR and
tower data:
    1. The diurnal shear coefficients determined from the 38-m and 49-m tower measurements
       were applied to the tower data as a function of time of day (using the data depicted by the
       solid blue line in Figure 7). Using this approach, the estimated 80-m annual average is
       7.04 m/s.
    2. The estimate of the 50-m to 80-m shear determined from SODAR data can be applied to
       the tower data on an hourly basis (using the data depicted by the solid blue line in Figure
       12). This approach results in an estimate of 6.90 m/s. This approach assumes that the
       diurnal shear patterns measured in the wintertime are similar to those encountered
       throughout the year.


Long-Term Adjustments
Finally, these averages were adjusted to reflect long-term trends. The average wind speed at
Logan Airport during the 11-year period including 1997 through 2007 was 5.00 m/s, 1.6%
greater than during the period of data collection at Brewster. A correction has been applied to the
1-year estimates determined above to provide an estimated long-term average wind speed at the
Brewster site at 80 m. The results are provided in Table 5. These values are provided as
guidance. The Logan data used in this analysis have not been validated by DNV-GEC, nor have
any corrections been made for possible changes in sensor technology used at the site since 1997.
These estimates also do not include ranges of uncertainty. It appears that a reasonable estimate of
the 80-m long-term mean wind speed at Brewster would be at least 7.00 m/s.

        Table 5. Estimates of long-term mean wind speed at 80m at the Brewster site
                                                         Long-Term 80-m
                                     Approach              Wind Speed
                             1     Tower Diurnal Shear        7.2
                             2    SODAR Diurnal Shear         7.0




                              Discussion and Conclusions

SODAR and anemometer data have been collected at the Captains Golf Course in Brewster,
Massachusetts. The SODAR data have been used to better understand the wind shear at the site
above the highest tower measurement of 49 m. Both data sets have been used to estimate the
long-term mean wind speed at 80 m at the Brewster site. The average of the power law shear
coefficients during conditions when the 38-m tower or 40-m SODAR wind speed was greater
than 4 m/s is tabulated in Table 6.




DNV Global Energy Concepts Inc.                    15                                    June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                      FSRP0040-B


                Table 6. Average shear coefficients from tower and SODAR data
                                                           Power Law Shear
                                       Data Source           Coefficient
                                  One year of Tower Data         0.41
                                    Winter Tower Data            0.38
                                   SODAR 40 m / 50 m             0.31
                                   SODAR 50 m / 80 m             0.38

A number of conclusions can be drawn from the analysis of the tower and SODAR data:
    1. The wind shear coefficient that describes the wind shear above the highest measurement
       height on the tower seems to be greater than that over the heights of the tower
       measurements.
    2. The wind shear measured by the SODAR in early winter 2008 over the range from 40 m
       to 50 m is lower than that measured during a similar period using the tower
       measurements at 38 m and 49 m.
    3. The 50- to 80-m wind shear measured by the SODAR in 2008 is similar but generally
       less than that measured by the tower in 2006-2007 from 38 m to 49 m.

These three observations are somewhat contradictory but may be explained by a number of
possible factors. The apparent contradictions highlight the uncertainty in the results and the
difficulties of comparing measurements collected over different time periods with different
technologies.

Nevertheless, at the very least, the SODAR data do provide some important information:
    1. There is no evidence that the wind speeds or shear coefficients above the tower decrease
       markedly from that which would be expected using the shear behavior determined by the
       tower measurements.
    2. Shear coefficients based on the tower measurements provide a reasonable and possibly
       conservative estimate of the shear above the tower.

Based on the two data sets available, two approaches were used to estimate the long-term 80-m
average wind speed at the Brewster site. It appears that a reasonable estimate of the 80-m long-
term mean wind speed at Brewster would be at least 7.0 m/s.




DNV Global Energy Concepts Inc.                      16                                   June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                        FSRP0040-B



                Appendix A – SODAR Technology Overview

SODAR Technology
The SODAR trailer unit owned by Global Energy Concepts is an Atmospheric Research and
Technology (ART) Model VT-1 SODAR. It measures wind speed acoustically. High frequency
chirps (~4500 Hz) are emitted from the SODAR in three consecutive directions: one in the
vertical direction (W) and two in orthogonal directions approximately 17° from vertical. The
horizontal wind speed components, U and V, are calculated from the two orthogonal tilted
beams. After each signal is emitted, a portion of the acoustic energy is reflected back to the
SODAR at some shifted frequency. The amount of this apparent frequency shift (the Doppler
shift) is directly related to the velocity of the reflector (the wind). After every chirp, the SODAR
calculates the wind speed in the direction of the beam at each specified height (range gate). The
default range gate heights are from 30 m to 160 m at 10-m increments. The wind speeds are then
averaged in each direction (U, V and W) over a 10-minute interval and the average vector wind
speed and wind direction are determined at each range gate.

              Note : Vector Wind Speed =    (U   Speed ) + (V Speed )
                                                        2               2
                                                                                     (Equation A-1)
              where U Speed and V Speed are corrected for Vertical Speed (W )

In addition to wind speed, the SODAR also records wind direction, ambient temperature, the
presence of precipitation and the wind speed as measured by a R. M. Young anemometer
mounted on a 3-m high pole.

SODAR Data Filtering
This section describes the filtering that is applied to the data at both the pre-processed and post-
processed stages. The main function of these filters is to remove spurious data caused by high
levels of ambient or electrical noise and to ensure good quality data.

Pre-processing SODAR Filtering
The pre-processing filtering is implemented in the SODAR control software. When the SODAR
collects data, there are four initial criteria that must be met in order for the data to be considered
valid. First, the signal-to-noise ratio (SNR) is calculated at each height and if it is found to be
below the user-defined minimum then the data is discarded. Next, the amplitude of the signal is
calculated and the data is removed if it is below the minimum allowed amplitude. The third
criterion is called the consensus check. Once the 10-minute interval is complete, there will be
~150 data samples at each height. The average Doppler shift of each sample is calculated at each
height and if, over the averaging time interval, a data sample has a Doppler shift beyond the
range of the average Doppler shift plus or minus the “consensus” (default = 100 Hz), then the
data point is removed. Finally, if, over the 10-minute interval, there is less than the minimum
percent of valid data points (default = 15%) then the data for that 10-minute interval is
considered invalid and is removed from the data set.




DNV Global Energy Concepts Inc.                   A-1                                       June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                       FSRP0040-B


Echo Rejection Algorithm
In addition to the pre-processing SODAR data filtering described above, the manufacturer has
included an optional echo rejection algorithm which is designed to minimize the effect of echoes
caused by ground clutter. Ground clutter is defined as trees, buildings, bushes or any stationary
object surrounding the SODAR that could reflect the signal at a zero Doppler shift. When echoes
occur in this way, the measured wind speed is biased low since the SODAR will interpret the
zero Doppler shift as zero wind speed. Echoes from ground clutter affect the lower range gates
more significantly than the higher range gates.

Ideally, the SODAR would be situated in an area void of ground clutter. When this is not
possible, ART’s echo rejection algorithm can be employed. The echo rejection option is a built-
in function in the ART Model VT-1s control software and can be enabled at the user’s discretion.
The algorithm works by comparing the amount of spectral energy at the zero Doppler shift to
spectral energy at other frequencies. If there is sufficient energy at a frequency other than the
zero-shift, then the wind speed is calculated at this frequency and the energy at the zero-shift is
ignored. It has been found in previous data sets that the echo rejection option is very effective at
reducing the effects of ground clutter contamination.

Post-processing SODAR Filtering
Once the wind speeds have been measured by the SODAR, additional filters are applied to the
data by DNV-GEC. These filters were designed by comparing SODAR measurements to
anemometer readings and determining appropriate cut-offs for removing erroneous data. These
filters include the following:
    1. Maximum turbulence intensity filter. For this filter, turbulence intensity is defined as
       the W, U or V speed divided by the overall vector wind speed. The maximum W
       turbulence intensity limit used in the filtering was 0.4 and the maximum U and V
       turbulence intensity applied was 0.9. These values have been shown to remove invalid
       measurements while retaining the majority of good data.
    2. Minimum and maximum normalized W wind speed filter. This filter uses the W wind
       speed divided by vector wind speed. Minimum and maximum normalized W wind speed
       limits were also defined based on comparisons between SODAR and anemometry data.
       The minimum and maximum values used in the filtering algorithm were –0.12 and 0.16,
       respectively.
    3. Noise filter. There may be occurrences of extraneous noise entering the system which
       can contaminate the SODAR signal. The noise filter is designed to remove these
       erroneous data averages. The noise algorithm compares the calculated wind speed at each
       height to the wind speed measured by the anemometer (mounted on a 3-m pole). At each
       time step, the average difference between the SODAR (at each height) and the
       anemometer are calculated using the measured differences from the most recent five time
       steps. If the difference, at that time step, is greater than the average difference plus 4 m/s,
       then the data are discarded.




DNV Global Energy Concepts Inc.                  A-2                                        June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                                          FSRP0040-B



          ( Avg Difference)height = ⎛ (Diff t −5 + Diff t −4 + Diff t −3 + Diff t −2 + Diff t −1 ) ⎞
                                    ⎜                                                              ⎟
                                        ⎝                                 5                       ⎠ height (Equation A-2)
          If (Diff t )height > ( Avg Difference + 4)height Then Discard

    4. Shear filter. Finally, a shear filter is applied to the data to remove data affected by
       ground clutter. At sites where ground clutter is present, echoes tend to contaminate the
       signal and bias the wind speed low, particularly at lower range gate heights. This results
       in apparently very large wind shear at lower heights. The shear filter uses the 70-m wind
       speed as a reference. The average wind speed is calculated at 70 m, using wind speeds at
       60, 70 and 80 m. It then compares the wind speed at every height to the reference 70-m
       wind speed. The shear power law exponent, alpha, is calculated at each height using the
       reference 70-m wind speed as the datum. Equation A-3 shows the wind shear power law
       expression where U is wind speed [m/s], z is height, zr is the reference height and α is the
       power law exponent.

                                                                      α
                                               U ( z) ⎛ z         ⎞
                                                       =⎜         ⎟                                    (Equation A-3)
                                               U ( zr ) ⎜ zr
                                                        ⎝
                                                                  ⎟
                                                                  ⎠

        If the shear exponent is greater than the user-defined maximum allowable shear exponent
        then the data point is removed. If the shear exponent is less than the user-defined
        minimum shear exponent then the data point is removed. If the 60-m or 80-m data point
        has been deleted due to shear, then the 70-m data point is also removed.

        Shear filter limits have been determined based on tall tower data from a number of
        different types of terrain. The values shown in Table 7 are those shear coefficient cutoffs
        which would result in the elimination of no more than 2.5% of the highest shear and
        lowest shear data. As shown, for more complex and forested terrain, the range of
        acceptable alphas is relatively wide. Conversely, the range of acceptable alphas is much
        narrower for the offshore tower (Cape Wind). For the Brewster data set, the alpha limits
        based on the Hull WBZ tall tower were used in the shear filter.

                             Table A-1. Summary of Tall Tower Alpha Limits
                                                       Day Alpha              Day Alpha    Night Alpha     Night Alpha
 Site                 Site Description
                                                          Min                   Max            Min            Max
 Nantucket, MA        Coastal                              -0.2                  0.7           -0.3             0.8
 Hull WBZ, MA         Coastal/complex terrain              -0.5                  0.8           -0.3             0.9
 Hatfield, MN         Onshore: flat with no trees          -0.5                  0.9           -0.5             1.1
 Isabella, MN         Onshore: forested                    -0.3                  1             -0.5             1.2
 Cape Wind            Offshore                             -0.2                  0.6           -0.2             0.6




DNV Global Energy Concepts Inc.                            A-3                                                June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                  FSRP0040-B


SODAR Operation during Times of Precipitation
The vertical wind speed (W) is used in conjunction with the U and V wind speeds to calculate
the horizontal wind speed results in the SODAR. Since rain affects the vertical wind speed
measurements, the SODAR roof is programmed to close and the SODAR ceases to collect data
during periods of rain.

Data Validation
After the SODAR data processing and the application of the post-processing filters, the SODAR
data are further checked to make sure that the data appear to be good. These checks include:
    •   Studying the relationship between the SODAR data at each height and the R. M. Young
        measurements (and any nearby tower measurements, if they are available)
    •   Looking at correlation coefficients between the SODAR data at each height and the R. M.
        Young anemometer
    •   Looking at the change of signal amplitudes versus height
    •   Looking at the relationship between the wind direction as reported by the SODAR and
        the R. M. Young anemometer.

Bias Corrections
As part of the SODAR data processing, corrections are applied to the data to account for the
systematic bias in SODAR measurements due to volume averaging and to account for the
differences between anemometer measurements (“scalar averages”) and SODAR measurements
(“vector and averages”), as explained below.

Volume Averaging
Unlike anemometer measurements (measurements at a point in space), SODAR wind speed
measurements are based on the acoustic reflections from a volume of air that extends below and
above the nominal measurement height. Because of a variety of physical processes, the SODAR
may under-estimate the wind speed at lower heights due to the volume averaging. The degree of
underestimation depends on atmospheric conditions but is primarily a function of height and
wind shear. A correction based on these factors has been developed at DNV-GEC and has been
applied to the data. Figure A-1 shows a graph of the correction factors that were applied to the
SODAR data, as a function of the “apparent shear coefficient” which is that determined from the
raw SODAR data. It can be seen that, at 30 m and at high shear coefficients, the correction can
amount to over 4%. The correction factor quickly falls off with increasing height.




DNV Global Energy Concepts Inc.                  A-4                                   June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                              FSRP0040-B



                                               1.05



             Correction to SODAR Wind Speeds
                                                      30 m
                                                      40 m
                                               1.04   50 m
                                                      60 m
                                                      70 m
                                                      80 m
                                                      90 m
                                               1.03   100 m
                                                      110 m
                                                      120 m
                                                      130 m
                                               1.02   140 m
                                                      150 m
                                                      160 m

                                               1.01




                                                      0.1        0.2            0.3        0.4
                                                              Apparent Shear Coefficient
    Figure A-1. Correction for Volume Averaging as a Function of Shear Coefficient (as
             measured using the raw SODAR data) and Measurement Height



Scalar versus Vector Averages
Wind turbine power curves are based on anemometer measurements. Anemometers provide what
is known as a scalar wind speed. SODAR measurements provide what is known as a vector wind
speed. The two are not always the same. The SODAR measures the instantaneous wind speed
components and then averages them to determine the vector wind speed. Anemometers measure
the instantaneous wind speed (i.e., U and V components are indistinguishable) and the average
scalar wind speed is calculated. The scalar wind speed is typically 1 – 2 % higher than the vector
wind speed. The difference between the two is a function of turbulence intensity. The correction
that was applied to the data, as a function of turbulence intensity, is shown in Figure A-2.




DNV Global Energy Concepts Inc.                                      A-5                          June 6, 2008
SODAR Shear Measurements at Brewster, Massachusetts                                           FSRP0040-B



                                 1.08

                                              Conversion of Vector to Scalar Averages
             Correction Factor   1.06



                                 1.04



                                 1.02



                                 1.00
                                        0.0       0.1           0.2            0.3      0.4
                                                        Turbulence Intensity
        Figure A-2. Correction Factor Used to Convert Vector Averages to Equivalent
                                      Scalar Averages




DNV Global Energy Concepts Inc.                              A-6                               June 6, 2008

				
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