Meteorological Conditions at Lillgrund. Lillgrund Pilot Project

Meteorological Conditions at Lillgrund Lillgrund Pilot Project March 2009

Type of document Document identification Rev. No. Report date Project No. REPORT 6_2 LG Pilot Report 1.0 March 13, 2009 21858-1 Author Hans Bergström Uppsala University Project name Lillgrund Pilot Project Customer Vattenfall Vindkraft AB Reviewed by Approved by The Reference Group Distribution No. of pages No. of appendices The Swedish Energy Agency 25 0 PREFACE Vattenfall s Lillgrund project has been granted financial support from the Swedish Energy Agency and Vattenfall will therefore report and publish experiences and lessons learned from the project. This report is compiled in a series of open reports describing the experiences gained from the different aspects of the Lillgrund Wind Farm project, for example construction, installation, operation as well as environmental, public acceptance and legal issues. The majority of the report authors have been directly involved in the Lillgrund project implementation. The reports have been reviewed and commented by a reference group consisting of the Vattenfall representatives Sven-Erik Thor (chairman), Ingegerd Bills, Jan Norling, Göran Loman, Jimmy Hansson and Thomas Davy. The experiences from the Lillgrund project have been presented at two seminars held in Malmö (4 th of June 2008 and 3 rd of June 2009). In addition to those, Vattenfall has presented various topics from the Lillgrund project at different wind energy conferences in Sweden and throughout Europe. All reports are available on www.vattenfall.se/lillgrund. In addition to these background reports, a summary book has been published in Swedish in June 2009. An English version of the book is foreseen and is due late 2009. The Lillgrund book can be obtained by contacting Sven-Erik Thor at sven-erik.thor@vattenfall.com. Although the Lillgrund reports may tend to focus on problems and challenges, one should bear in mind that, as a whole, the planning and execution of the Lillgrund project has been a great success. The project was delivered on time and within budget and has, since December 2007, been providing 60 000 households with their yearly electricity demand. Sven-Erik Thor, Project Sponsor, Vattenfall Vindkraft AB September 2009 DISCLAIMER Information in this report may be used under the conditions that the following reference is used: "This information was obtained from the Lillgrund Wind Farm, owned and operated by Vattenfall." The views and judgment expressed in this report are those of the author(s) and do not necessarily reflect those of the Swedish Energy Agency or of Vattenfall. 1 (25)

Meteorological conditions at Lillgrund SUMMARY Meteorological measurements collected during the period 1 st of September 2003 to 28 th February 2006 taken on a tower located at Lillgrund in Öresund, have been analysed. The observed mean wind speed for the period was 8.4 m/s at 65 m height. The long-time corrected average was estimated to be 8.6 m/s with an uncertainty of ±0.35 m/s. The corresponding Weibull parameters are 2.41 as regards the shape parameter and 9.8 m/s as regards the scale parameter. The wind direction distribution showed a peak for winds from the west-southwestern sector with a secondary smaller maximum for winds from southeast. The daily variation showed a maximum in wind speed during late afternoon and a minimum during the morning hours. The amplitude was on the average being about 0.4 m/s at 65 m height. The turbulence intensity at 65 m height was found to increase from about 0.06 at 10 m/s to 0.09 on the average for the wind speed 25m/s. The average density at 65 m was 1.25 kg/m 3 during the measurement period. 2 (25)

SAMMANFATTNING Meteorologiska mätningar för perioden 1 september 2003 till 28 februari 2006 gjorda i en mast på Lillgrund i Öresund har analyserats. Den uppmätta medelvindhastigheten på 65 m höjd var 8.4 m/s. Det långtidskorrigerade medelvärdet har beräknats till 8.6 m/s med en osäkerhet på ±0.35 m/s. Detta motsvaras av en Weibullfördelning med formfaktor 2.41 och skalfaktor 9.8 m/s. Vindriktningsfördelningen uppvisade en topp för vindar från väst-sydväst med ett andra mindre maximum för vindar från sydost. Dygnsvariationen i vindhastighet uppvisade ett mindre maximum under sena eftermiddagen och ett minimum under förmiddagen. Amplituden var i medeltal 0.4 m/s på 65 m höjd. Turbulensintensiteten på 65 m höjd ökade i medeltal från ca 0.06 vid 10 m/s till 0.09 vid vindhastigheter på 25 m/s. Luftens medeldensitet på 65 m höjd var 1.25 kg/m 3 under mätperioden. 3(25)

TABLE OF CONTENTS PREFACE... 1 DISCLAIMER... 1 1 INTRODUCTION... 5 2 SITE AND MEASUREMENTS... 5 3 RESULTS... 7 4 COMMENTS AND CONCLUSIONS... 25 5 REFERENCES... 25 4(25)

1 INTRODUCTION Measurements taken on a tower located at Lillgrund in Öresund, about 10 km west of the Swedish coastline, have been analysed. Results on meteorological conditions will be presented here, using data taken from the period 1 st September 2003 to 28 th February 2006. 2 SITE AND MEASUREMENTS The measurement site at Lillgrund is located offshore in the Öresund area about 10 km west of Skåne. Figure 1 shows the location of the Lillgrund site (55 30 0.00 N, 12 45 36.00 E; RT90: x=6156480, y=1307418). The measurements are taken on a 65 m high offshore lattice tower. The measurement system is designed and constructed by the Wind Energy Department at Risø. The system has been in operation since the end of August 2003. A drawing of the tower is shown in Figure 2. Wind measurements were taken at 3 levels; 25, 40, and 65 (63) m above sea level, using calibrated Risø cup anemometers. At the 25 and 40 m heights anemometers were mounted on booms, one on a boom pointing towards north and the other one on a boom pointing towards south. Thus, measurements could more or less be taken undisturbed from the mast for all wind directions. At 63 meters, only one anemometer was mounted on a boom, pointing towards south. The top anemometer at a 65-meter height was mounted on a pole on top of the tower. The wind direction was measured at 23 meter and 61 meter heights using Vector wind vanes. At 8 meters above sea level the temperature and humidity was measured using Vaisala temperature/humidity probe in a self-ventilated radiation shield. (Humidity was not stored in the database). The temperature difference between 61 meters and 8 meters was also measured, together with air pressure at a 5- meter height. All data has been sampled as 10 min averages, together with standard deviations and extreme values during each averaging period, on a Campbell logger (CR23X). Lillgrund X Figure 1: Map of southern Öresund showing the location of the Lillgrund tower. 5(25)

Figure 2: Drawing of the meteorological tower at Lillgrund. 6(25)

3 RESULTS Time series of wind speed, wind direction, temperature, and air pressure at the Lillgrund site during the period 1 st September 2003 to 28 th February 2006 are shown in Figure 3. Only a few occasions with wind speed above 20 m/s have occurred with the highest wind speed during the Gudrun-storm in January 2005. The maximum 10-minute-average wind speed at 65 m height was 32.4 m/s and the highest gust wind speed was 41.9 m/s, both observed on January 8, 2005 at 1550 hours. The temperature has varied between -8 and +26 C with the average 8.2 C at 8 m height. The average air pressure at 5 m height was 1013.3 hpa, and the pressure has varied between 962 and 1046 hpa. Both temperature and pressure are close to what could be expected on the average. The data availability on a monthly basis is plotted in Figure 4, and shows that for the whole period data was available during 100 % of the time, expect for some minor interruptions in October 2003 and in July 2004. Some errors could be accounted by the too small standard deviations of the wind speeds or un-realistically small or large numbers found in the data during shorter periods. In January and February 2006 the number of erroneous data increased. The anemometer at 63 m stopped working on January 20 th and also the southern anemometer at 25 m showed erroneous values from time to time. Following this the data availability for January-February 2006 was about 90 %, while for the whole measurement period the average data availability was 99 %. The effects of disturbances from the tower are illustrated in Figure 5. The error typically amounts to a maximum of some 15-25 % for winds through the tower. But also for other directions small systematic deviations (1-3%) between the two anemometers mounted at the same heights may be seen. All results presented below were arrived at using the least disturbed anemometers. 7(25)

Figure 3: Time series (10 min averages) of, from top to bottom: Wind speed, wind direction, temperature, and air pressure. Measurements were taken at Lillgrund 1 st September 2003 to 28 th February 2006. 8(25)

Figure 4: Data availability on a monthly basis for the measurement system at Lillgrund 1 st September 2003 to 28 th February 2006. The left hand bars (blue) represent amount of data before checking for errors. The right hand bars (brown) show data availability after removing erroneous data. 9(25)

Figure 5: Ratios between wind speeds measured with the different anemometers at the 65 m, 40 m, and 25 m levels, plotted versus wind direction, showing the tower disturbances for winds through the tower. 10(25)

The mean wind speed distribution at 65 m height is shown in Figure 6a. The peak is found at about 8 m/s, and the observed mean wind speed for the observation period, 1 st September 2003 to 28 th February 2006, is 8.36 m/s. The Weibull distribution, ( c 1) c U A c U f ( U ) = e, (1) A A which has been adapted to the observations, is given by the full line in Figure 6a. The corresponding scale parameter, A, was estimated to 9.42 m/s and the shape parameter, c, to 2.41. The observed and corresponding Weibull wind speed distributions are given in Table 1. Both give the probability 88 % for wind speed being above 4 m/s. Table 1: Observed wind speed distribution at 65 m height together with the corresponding Weibull distribution. Wind speed (m/s) Weibull 65 m (%) Observed 65 m (%) 0 1.00 0.41 0.53 1.01 2.00 1.90 2.25 2.01 3.00 3.79 4.06 3.01 4.00 5.78 5.46 4.01 5.00 7.63 7.52 5.01 6.00 9.11 8.54 6.01 7.00 10.07 9.46 7.01 8.00 10.41 10.06 8.01 9.00 10.13 10.37 9.01 10.00 9.32 9.67 10.01 11.00 8.13 8.47 11.01 12.00 6.72 6.97 12.01 13.00 5.28 5.43 13.01 14.00 3.94 4.07 14.01 15.00 2.78 2.93 15.01 16.00 1.87 1.90 16.01 17.00 1.19 1.04 17.01 18.00 0.72 0.57 18.01 19.00 0.41 0.32 19.01 20.00 0.22 0.16 20.01 21.00 0.11 0.09 21.01 22.00 0.06 0.05 22.01 23.00 0.03 0.02 23.01 24.00 0.01 0.03 24.01 25.00 0.00 0.01 The wind direction distribution is given in Figure 6b. The peak in the distribution is found for winds from around west-southwest but a smaller secondary peak for winds from southeast is also seen. The dual peak type of wind direction distribution is common for southwestern Sweden, but typically the southwesterly winds are the most common. The observed wind direction distribution is also given in Table 2. 11(25)

Figure 6: a) Observed (bars) distribution of mean wind speed at the Lillgrund site, 65 m height, for the period 1 st September 2003 to 28 th February 2006. The curve shows the Weibull distribution adapted to the observations. b) Observed distribution of wind direction at Lillgrund, 61 m height. 12(25)

Table 2: Observed wind direction distribution at Lillgrund, 61 meters in height. Wind direction ( ) Frequency (%) 0.1-30.0 5.55 30.1-60.0 5.13 60.1-90.0 5.32 90.1-120.0 7.63 120.1-150.0 9.39 150.1-180.0 7.14 180.1-210.0 8.93 210.1-240.0 12.22 240.1-270.0 14.44 270.1-300.0 13.66 300.1-330.0 5.87 330.1-360.0 4.72 The daily variation of mean wind speed is shown in Figure 7. The daily amplitude is small, as a mean 0.3-0.4 m/s. The daily cycle is very similar at all heights, with a maximum in the afternoon, in contrary to what is observed over land where the daily amplitude is larger and the afternoon maximum is commonly reduced with increasing height. Above about 50 m the daily maximum over land is often found during the night. This is due to a combined effect of the transition to stable stratification and the development of a nocturnal jet, both increasing the wind gradient. Offshore, the daily cycle in thermal stability is typically small and the observed daily variations are also consequently small. Figure 7: Hourly average wind speed at Lillgrund 1 st September 2003 to 28 th February 2006. 13(25)

Figure 8 shows how the mean wind speed depends on wind direction. We can see that the highest average winds are found for wind directions from west-southwest to west, but almost as high average wind speed is also found for winds from east to east-southeast. The annual variation of average wind speed is shown in Figure 9. The highest monthly averages are found for December, while the lowest values are found for July. The length of the measurement period is however too short to get any details about the annual variation. The higher winds observed in June might thus not be climatologically representative, and January might have as high or even higher winds than December looking at a longer period. Figure 8: Mean wind speed versus wind direction (10 bins in wind direction) at Lillgrund 1 st September 2003 to 28 th February 2006. 14(25)

Figure 9: Monthly average wind speed at Lillgrund 1 st September 2003 to 28 th February 2006. The observed average wind speed profile at Lillgrund is shown in Figure 10. For the period 1 st September 2003 to 28 th February 2006 the average wind speed increased from 7.5 m/s at 25 m to 8.4 m/s at 65 m height. Assuming that the vertical wind profile could be described by an exponential profile given by U ( z U ( z 2 1 ) z = ) z 2 1 α (2) where U=wind speed, z=height, the value of the exponent α is determined to 0.108 using the average wind speeds at 25 m and 65 m heights. The exponential profile is also shown in Figure 10, and gives the average wind speed 8.7 m/s at 100 m height. Also included in Figure 10 is a long-time corrected profile arrived at by using the geostrophic winds (horizontal air pressure gradients) at the site during the measurements period. The geostrophic wind (U g ) was determined from the height to the 850 hpa pressure surface using the NCEP/NCAR re-analysis data [1]. According to this data the measurements period should have had an average wind about 2 % below the long time average, giving the long time corrected average wind speed 8.5 m/s at 65 m height. A long-time correction was also made using the Danish wind energy production index (www.vindstat.dk). The result of this correction was, assuming that the production is proportional to the square of wind speed, that the measuring period should have had an average wind about 4 % below the long-time average, thus giving the long-time corrected average wind speed is 8.7 m/s at 65 m height. The long-time corrected exponential average wind profiles following the geostrophic wind index and the Danish wind index are shown by the dash-dotted lines in Figure 10. 15(25)

The difference between the two correction results is typical for the uncertainty in the long time corrections such as these. An uncertainty analysis using wind observations during 18 years at Näsudden on Gotland, [2], gives a standard deviation that typically is 0.15 m/s for a measuring period of 30 months. Thus we could expect with a 95 % confidence limit that the long-time average wind speed at 65 m height is within the interval 8.2 to 8.9 m/s (observed long-time corrected average ±two standard deviations) taking account of the difference between the two long time correction methods. A comparison between observed monthly average wind speed at 65 m height and the monthly average geotropic wind speed on one hand and the Danish wind index on the other hand is shown in Figure 11, again with the assumption that the production is proportional to the square of the wind speed. The results show similar agreement between both indexes and the observed wind, with correlation coefficients above 0.90. There is consequently no reason to believe that one of the indexes should be more reliable than the other. Also shown in Figure 10 is the annual average wind speed profile at Lillgrund. This profile was estimated from wind climate modelling results for the area according to the national wind resource mapping using the MIUU-model, [3]. The modelled average wind speed is at 65 m 8.2 m/s, which is slightly below the 8.36 m/s that was the observed average during the 30-month measurement period. After having long time corrected the observations, the difference between model results and observations increased to 0.4 m/s. The observed wind gradient is also smaller than the modelled gradient. A reason for this might be a somewhat too stable thermal stratification on the average in the model. This in turn might be due to somewhat too low surface water temperature. These are not calculated by the model but given as lower boundary values using climate averages. The modelled average wind speed at 72 m height over the southern Öresund area is shown in Figure 12. 16(25)

Figure 10: Average wind speed profiles at Lillgrund during period 1 st September 2003 to 28 th February 2006. The full line gives the corresponding exponential profile estimated from average wind speed at 25 m and 65 m levels. The dashed-dotted lines give the long-time corrected wind profiles, and the dashed line the estimated profile using the MIUU-model. 17(25)

Figure 11: Relation between observed monthly average wind speed at Lillgrund, 65 m height, and a) Monthly average geotropic wind speed, and b) Danish wind index. The data was taken during the period 1 st September 2003 to 28 th February 2006. The correlation coefficients R are given in the plots. 18(25)

Figure 12: Modelled annual average wind speed at 72 m height according the national wind resource mapping using the MIUU-model. 19(25)

Figure 13: Observed monthly average wind speed at 65 m height together with cumulative averages. In Figure 13, the monthly average wind speed at 65 m height is plotted for the period September 2003 to February 2006. The highest average, 11.4 m/s, is found for January 2005, the month with the storm Gudrun, and the lowest average, 6.6 m/s, is found for July 2005. Also included in Figure 13 is the cumulative average. By accumulating more and more data into the average, resulting in a cumulative average wind speed (thick read line), we could expect a more and more stable average, eventually levelling off at the climatological average. The cumulative average, after using data from 30 months, shows a value of about 8.4 m/s at 65 m height. The turbulence intensity, as in the ratio between standard deviation of wind speed and average wind speed, was calculated for the 25 m, 40 m, and 65 m levels for the period 1 st September 2003 to 28 th February 2006 based on 10 min average statistics. The resulting distributions for average wind speed larger than 4 m/s are plotted in Figure 14, and show values in the range 0.01 to 0.18. The peak in the distribution is located at about 0.07 at 25 m height and at about 0.06 at 65 m height. Figure 15 shows how the turbulence intensity varies with average wind speed. As expected, high values are found for low wind speed and a minimum is observed for wind speed between about 5-10 m/s with typical values of turbulence intensity at about 0.06-0.07. For higher winds the turbulence intensity increases with increasing wind speed but is at 65 m height still below 0.07 at 15 m/s and reaches 0.09 at 25 m/s. Figure 16 illustrates how the turbulence intensity varies with wind direction. The lowest values are found for wind directions between southeast and southwest, that is for winds entering Öresund from the Baltic Sea. As expected the largest turbulence intensities are observed when the wind comes from northeast (the Malmö area) or from northwest (the Copenhagen area). 20(25)

Figure 14: Distribution of turbulence intensity at 25 m, 40 m, and 65 m heights using data from Lillgrund for the period 1 st September 2003 to 28 th February 2006. 21(25)

Figure 15: Turbulence intensity versus wind speed at 25 m, 40 m, and 65 m heights using data from Lillgrund for the period 1 st September 2003 to 28 th February 2006. Figure 16: Turbulence intensity versus wind direction at 25 m, 40 m, and 65 m heights using data from Lillgrund for the period 1 st September 2003 to 28 th February 2006. 22(25)

The temperature distribution during the measurement period is shown in Figure 17. The typical double maximum observed for temperatures is seen with one maximum between 0 C and 6 C, and a second maximum at 15 C. The temperature range is -8 C to +26 C, and the annual average was 8.7 C. As the air pressure was measured at 5 m height the air density was calculated from the gas law assuming dry air. (The humidity was not stored in the database). The resulting distribution is shown in Figure 18. The density has varied between 1.18 and 1.36 kg/m 3 with the average 1.255 kg/m 3. The density at 65 m is reduced to an average of 1.246 kg/m 3 assuming that the pressure decreases with the typical value of 1 hpa per 8 m. Figure 17: Temperature distribution at Lillgrund 1 st September 2003 to 28 th February 2006, 8 m height. 23(25)

Figure 18: Air density distribution at Lillgrund 1 st September 2003 to 28 th February 2006, 8 m height. 24(25)

4 COMMENTS AND CONCLUSIONS Wind measurements from Lillgrund during the period 1 st September 2003 to 28 th February 2006 have been analysed. The measurements were taken from a 65 m high mast. The observed mean wind speed was found to be 8.4 m/s at 65 m height. Correction of the measurements was made to make them represent a long time average using geotropic wind data estimated from 850 hpa pressure height data from the NCEP/NCAR reanalysis of the data set. The result gave the expected long-time average wind speed 8.5 m/s at 65 m height. A similar correction using the Danish wind index based upon wind energy production resulted in the long-time average 8.7 m/s. With a 95 % confidence limit the long time average is expected to be in the range of 8.2-8.9 m/s. The observed mean wind speed distribution at 65 m height during the measurement period corresponds to a Weibull distribution with the shape parameter 2.41, and the long-time corrected average wind speed then gives the corresponding scale parameter of 9.8 m/s. The wind direction distribution showed a maximum for the west-south-westerly sector. The daily variation shows a maximum wind speed value in the late afternoon and a minimum during the morning, with the difference on the average being about 0.4 m/s at 65 m height. To check how representative the observed wind direction distribution may be for the long time average, the geotropic wind direction distribution during the observation period was compared to its long time average distribution. Using 30 sectors the differences were for all sectors within ±1 between the two. This is not the true wind direction close to the surface, but should be representative of the long time wind direction distribution. The turbulence intensity showed an average of about 0.06 at 65 m in height for the most common wind speeds, but slightly increases with increasing wind speed. The average density at 65 m was 1.25 kg/m 3 during the measurement period. The observed and long time corrected average wind is somewhat higher than estimated by the MIUU-model at the site using data from the national wind resource mapping with 1 km 2 horizontal resolution. The difference was found to be 0.3-0.6 m/s depending on choice of long time correction. The modelled value 8.2 m/s was, however, within the 95 % confidence limit of the long time corrected observed value. 5 REFERENCES [1] Kalnay, E. et al.: 1996: The NCEP/NCAR 40-year reanalysis project, Bull. Amer. Meteor. Soc., 77, 437-470. [2] Nilsson, E. and Bergström, H., 2008: Från mätt vid till vindklimat normalårskorrigering. Elforsk rapport 09:03, 27 pp. Report available at www.elforsk.se. [3] Bergström, H., 2008: Wind resource mapping of Sweden using the MIUU-model. Wind Energy report WE2008:1, Department of Earth Sciences, Uppsala University, 34 pp. 25(25)