Executive Summary of Accuracy for WINDCUBE 200S

Transcription

1 Executive Summary of Accuracy for WINDCUBE 200S The potential of offshore wind energy has gained significant interest due to consistent and strong winds, resulting in very high capacity factors compared to onshore wind farms. One of the major restrictions with offshore wind resource assessment studies is the use of met masts at high CAPEX expense. Alternative methods of measuring wind speed and direction at remote locations are feasible with scanning Doppler lidar. LEOSPHERE and DTU Wind Energy have investigated the ability of the WINDCUBE scanning Doppler lidar to measure wind speed and direction at remote areas offshore from an onshore location, up to several kilometers and at various heights above sea level. To assess the accuracy of the wind speeds retrieved from a sector scanning lidar, a study, comparing the measurements of a WINDCUBE 200S to a high quality met mast, was conducted in collaboration with the Technical University of Denmark (DTU). The scanning lidar was installed at DTU s test site for large wind turbines (Høvsøre, Western Denmark), at a distance of about 1.6km from a 116.5m high met mast, instrumented with cup anemometers and vanes at multiple heights (including 80 m, 100 m and m) above ground level (see picture to the left). A three months measurement campaign was conducted from June to August 2013 during which various sector scanning strategies were performed Figure 2 Picture of DTU s test station for large wind turbines, Høvsøre. View from WLS7 200S scanning head position. The top of the mast is shown with a black circle. evaluate: Figure 1 Picture of WLS7 200S scanning Lidar staring at the top of the met mast. over the met mast in order to define the best possible scanning scenarios and retrieval algorithms for wind speed and direction retrieval. DTU Wind Energy performed the analysis of the data. The objectives of this campaign were to 1. The accuracy of radial velocity measurements, by performing measurements with the beam staring towards the top of tower, and comparing to the projected radial velocity from the cup anemometer. 2. The accuracy of the reconstruction of the horizontal wind speed from the sector scans. This took place in three phases: a. The first phase focused on identifying the best possible sector scanning configuration and algorithm to retrieve wind speeds and directions at a given point within 10 minutes. The scanning scenarios considered were Planned Position Indicator (PPI) scans of 45 and 30 centered on the met mast, with a scanning speed of 2 /s and 3 /s. b. The second phase was to evaluate the impact on the wind speed accuracy when retrieving the horizontal wind speed at various locations, for example as would be necessary to assess the horizontal spatial variation of the wind speed at a potential wind farm site. c. The final phase of the study focused on applying the most suitable scan pattern at multiple heights in order to study the feasibility of measuring the wind speed profile, and potentially at different locations. The objective here is to define the number of acceptable measurement heights and locations in terms of data accuracy and availability. 1

2 3. The stability of the system pointing by hitting the met mast with the laser beam once an hour throughout the whole measurement campaign. The high CNR returned by the hard target observed by the lidar provides an indication of the constancy of the pointing accuracy. Results of the campaign are extracts issued from the DTU report, Comparison test of WLS200S-22 (Final) DTU Wind Energy LC I-046 (EN), that can be made available upon request :- 1. Radial velocity accuracy (extract from paragraph 6.3 of the DTU report) The 10 min mean lidar radial wind speed compared very well with the cup anemometer wind projected on the LOS direction. Figure 3 Lidar radial wind speed at 1616m range versus cup anemometer wind speed projected on the LOS 2. Accuracy of the reconstructed wind speed (paragraph 8.1 of the DTU report) a. Wind speed accuracy The revised wind speed and direction estimates provided by LEOSPHERE compared fairly well to the met mast measurements. The PPI scan with the largest sector (45 ) and a scanning speed of 3 /s provided the best comparison [to the top cup anemometer] with a deviation of 0.8% on average. Figure 4: Lidar reconstructed horizontal wind speed (with revised algorithm) from sector scan versus cup anemometer wind speed at 116.5m; for the 45 sector scan at 3 /sec. Every black dot is a 10 minute data, the plain red line in the result of the 2-parametric linear regression and the dashed line is the 1:1 line. 2

3 In the paragraph 8.1 of the report, the ability to measure wind speeds from directions orthogonal to the scanning sector is also mentioned and shown below in the graph where The gain and offset resulting from the linear regression are reasonably close to those obtained for the narrow wind sector , however the scatter (i.e. the coefficient of determination) is clearly larger. Figure 5: Lidar reconstructed horizontal wind speed (with revised algorithm) from sector scan vs cup anemometer wind speed at 116.5m; for SCAN4 for the wind directions orthogonal to the lidar beam. In each plot: every black dot is a 10 minute data, the plain red line in the result of the 2-parametric regression and the dashed black line is the 1:1 line. b. Capacity to retrieve the wind at multiple locations (paragraph 8.2) The analysis on sampled data sets, simulating the measurement resulting at one location if the Lidar was measuring sequentially at four locations over 10 minutes...showed that on average the wind speed and direction accuracy was not affected although the measurement uncertainty (scatter) increased as the data availability decreased Figure 6: Example of the 10 min mean wind speed from sector scanning lidar sampled at 4 different locations versus 10 min mean cup anemometer wind speed c. Wind Profile accuracy (paragraph 9 of the DTU report) For the results below, the met mast data were blind to LEOSPHERE when reconstructing the wind speed and direction of the lidar sector scan. The Lidar was configured to measure at three heights: 81, 101 and 117 m. The lidar retrieved wind speed compared reasonably well in comparison to the cup anemometer measurements, with an overestimation of 1.5% on average at 117m. The overestimation at lower heights was observed to be slightly higher and the reason for the 3

4 increased overestimation is not clear. Hourly shear estimates compare reasonably to cup averaged shear estimates. Figure 7 Left: Lidar reconstructed horizontal wind speed (with revised algorithm) from sector scan versus cup anemometer wind speed at 116.5m; for the 45 sector scan at 3 /sec. Right: Lidar reconstructed wind direction (with revised algorithm) from sector scan versus vane at 100 m; for the 45 sector scan at 3 /sec. Above: every black dot is a 10 minute data, the plain red line in the result of the 2-parametric linear regression and the dashed line is the 1:1 line. One third of the measurements from the first phase were used, since the lidar scans at sequentially at three heights. 3. Pitch and roll stability (paragraph 6.3) The laser beam elevation and azimuth angles were verified and remained consistent during the whole measurement campaign. The figure below shows the hard-target return time series for the entire period of the campaign. Figure 8 Time series of the CNR (displayed as color from dark blue for low CNR to red for very high CNR) for the line-of-sight (LOS) elevation angle of 3.5 and all azimuth angles between and 45. The above plot shows the measurements taken at the 1600m range, which coincides with the closest distance to the actual distance from the lidar to the met mast. The consistency of the time series is an illustration of the consistent pointing accuracy during the entire campaign. 4

5 Conclusions:- The experimental campaign carried out in collaboration between DTU and LEOSPHERE has shown the optimal scanning pattern and adequate algorithm for the retrieval of the horizontal wind speed and wind direction, in the range of a Long Range WINDCUBE Scanning Lidar. Indeed, as part of the algorithm that enables to retrieve the horizontal wind speed, this campaign has shown that a specific sector scanning scenario of 45 with a 3 /s scan speed allows the optimal wind speed retrieval. Under these recommended conditions, the algorithm developed by LEOSPHERE to process the data from radial to horizontal wind speed and wind direction, provides overall good results, where the mean wind speed error observed between the tower and the lidar was 0.8%. Providing that the system has been thoroughly tested in terms of measurement accuracy prior to deployment, the WINDCUBE scanning Lidar system from LEOSPHERE could be a suitable tool for offshore wind resource assessment applications, scanning offshore from the coast or an alternative to masts for power curve measurements where it is not possible to access the terrain at 2.5D around the wind turbine. As for future improvements, recommendations in terms of scanning Lidar measurement accuracy have been given by DTU following this campaign, that are included in the product roadmap for LEOSPHERE. Contact : David Langohr : dlangohr@leosphere.com 5