Document Sample

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 DNV Global Energy Concepts Inc. 7 June 6, 2008 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

DOCUMENT INFO

Shared By:

Categories:

Tags:
correlation coefficients, Concepts Inc, turbulence intensity, shear behavior, data collection, Brewster, Massachusetts, wind speed, wind shear, Energy Concepts, Global Energy

Stats:

views: | 2 |

posted: | 4/4/2011 |

language: | English |

pages: | 26 |

OTHER DOCS BY nikeborome

How are you planning on using Docstoc?
BUSINESS
PERSONAL

By registering with docstoc.com you agree to our
privacy policy and
terms of service, and to receive content and offer notifications.

Docstoc is the premier online destination to start and grow small businesses. It hosts the best quality and widest selection of professional documents (over 20 million) and resources including expert videos, articles and productivity tools to make every small business better.

Search or Browse for any specific document or resource you need for your business. Or explore our curated resources for Starting a Business, Growing a Business or for Professional Development.

Feel free to Contact Us with any questions you might have.