External Conditions for one GW (DOWN) VInD Farms. Risø-R-Report

External Conditions for one GW (DOWN) VInD Farms Risø-R-Report Morten Nielsen Søren E. Larsen Sten Frandsen Risø-I-337(EN) January 21

Authors: Morten Nielsen, Søren E. Larsen and Sten Frandsen Title: External conditions for one GW DOWN VInD Farms Division: Wind Energy Risø-I-337(EN) January 21 Abstract (max. 2 char.): This report summarizes atmospheric external conditions and their implications for aspects of the layout of 1 GW wind farms at the two DOWNVInD offshore sites, the Södra Midsjöbanken site in the Baltic Sea and the Beatrice site in the North Sea. The work in the report is carried out in connection with the FP6 Downwind project and constitutes deliverable no. RIS 2.2 (for the Beatrice Site) and RIS 2.4 (for the Store Midsjöbanken site) work package 4.2. The environmental data and the one-turbine external conditions for Södra Midsjöbanken are summarized in Ris-I-, the report Deliverable RIS2.3 within the DOWNViND project. The environmental data and the one turbine external conditions for the Beatrice site is found in the Risø-I-234 report for the Beatrice site, DOWNWIND deliverable RIS 2.1 and RIS 2.1. Additional data are obtained from the WAsP Engineering home page. In the report a generic MW wind turbine is used as basic turbine. For each wind farm four alternative wind farms lay-outs scrutinized with respect to production, extreme winds( year recurrence period), and basic as well as effective turbulence intensity. Overall a wind turbine Class I are called for at Beatrice, and a Class II turbine at Södra Midsjöbanken. ISSN 16-284 ISBN xxxx-xxxx (can be obtained from Solvejg Bennov, BIG, 47) Contract no.: Group's own reg. no.: (Føniks PSP-element) Sponsorship: Cover : The size of both farms in all versions are about x km. The capacity factors for the two farms are 43% and 4 % respectively, corresponding to about 18.8 GWh for Beatrice and 17, GWh for Södra Midsjöbanken. Only minor variations are found between the different farm lay-outs, least for Beatrice, because of the broader wind direction distribution there. Pages: Tables: References: Information Service Department Risø National Laboratory for Sustainable Energy Technical University of Denmark P.O.Box 49 DK-4 Roskilde Denmark Telephone +4 46774 bibl@risoe.dtu.dk Fax +4 4677413 www.risoe.dtu.dk

Contents Preface 4 1 Introduction 2 Data and assumptions 2.1 Mean wind climate 2.2 Regional extreme wind climate 6 2.3 Digital maps 8 2.4 Turbine data 9 2. Windfarm layouts 9 3 Analysis 11 3.1 Wind resource estimates with WAsP 11 3.2 Extreme wind estimates with WAsP Engineering 13 3.3 Turbulence estimates with WAsP Engineering 3.4 Effective Turbulence Intensity 17 3. Check of wind speed distribution 19 Conclusions 2 References 21 Risø-I-337(EN) 3

Preface Abstract (max. 2 char.): This report summarizes atmospheric external conditions and their implications for aspects of the layout of 1 GW wind farms at the two DOWNVInD offshore sites, the Södra Midsjöbanken site in the Baltic Sea and the Beatrice site in the North Sea. The work in the report is carried out in connection with the FP6 Downwind project and constitutes deliverable no. RIS 2.2 (for the Beatrice Site) and RIS 2.4 (for the Store Midsjöbanken site) work package 4.2. The environmental data and the one-turbine external conditions for Södra Midsjöbanken are summarized in Ris-I-, the report Deliverable RIS2.3 within the DOWNViND project. The environmental data and the one turbine external conditions for the Beatrice site is found in the Risø-I-234 report for the Beatrice site, DOWNWIND deliverable RIS 2.1 and RIS 2.1. Additional data are obtained from the WAsP Engineering home page. In the report a generic MW wind turbine is used as basic turbine. For each wind farm four alternative wind farms lay-outs scrutinized with respect to production, extreme winds( year recurrence period), and basic as well as effective turbulence intensity. Overall a wind turbine Class I was called for at Beatrice, and a Class II turbine at Södra Midsjöbanken. The size of both farms in all versions are about x km. The capacity factors for the two farms are 43% and 4 % respectively, corresponding to about 18.8 GWh for Beatrice and 17, GWh for Södra Midsjöbanken. Only minor variations are found between the different farm lay-outs, least for Beatrice, because of the broader wind direction distribution there. 4 Risø-I-337(EN)

1 Introduction This report summarizes atmospheric external conditions and their implications for aspects of the layout of 1 GW wind farms at the two DOWNVInD offshore sites, the Södra Midsjöbanken site in the Baltic Sea and the Beatrice site in the North Sea. The work in the report is carried out in connection with the FP6 Downwind project and constitutes deliverable no. RIS 2.2 (for the Beatrice Site) and RIS 2.4 (for the Store Midsjöbanken site) work package 4.2. The environmental data and the one-turbine external conditions for Södra Midsjöbanken are summarized in Ris-I-, the report Deliverable RIS2.3 within the DOWNViND project. The environmental data and the one turbine external conditions for the Beatrice site is found in the Risø-I-234 report for the Beatrice site, DOWNWIND deliverable RIS 2.1 and RIS 2.1. Additional data are obtained from the WAsP Engineering home page. In the report a generic MW wind turbine is used as basic turbine. For each wind farm four alternative wind farms lay-outs scrutinized with respect to production, extreme winds( year recurrence period), and basic as well as effective turbulence intensity. Overall a wind turbine Class I was called for at Beatrice, and a Class II turbine at Södra Midsjöbanken. The size of both farms in all versions are about x km. The capacity factors for the two farms are 43% and 4 % respectively, corresponding to about 18.8 GWh for Beatrice and 17, GWh for Södra Midsjöbanken. Only minor variations are found between the different farm lay-outs, least for Beatrice, because of the broader wind direction distribution there. 2 Data and assumptions 2.1 Mean wind climate Risø-I- used measurements from Ölands Södra Grund, 1981-1991 to estimate the wind distribution at 1m height at Södra Midtsjöbanken. This resulted in the distribution shown in Table 1. Risø-I-34 found the wind distribution at 1m height at Beatrice Alpha by local measurements. This is shown in Table 2. The wind direction distributions are shown in Figure 1. Table 1 Mean wind climate at Södra Midtsjöbanken Sector Direction Frequency Weibull A Weibull k Mean speed Power # [ ] [%] [m/s] [-] [m/s] [W/m²] 1 6. 8.2 2.4 7.27 442 2 3 8.2 8. 2.11 7.3 474 3 6 6. 6. 1.96.77 229 4 9 4. 6.7 2.8.94 236 12.1 7. 2.2 6.22 231 6 4.9 6.9 2.31 6.11 23 7 18 7.2 7. 2.2 6.21 277 8 21 11.9 9. 2.21 7.97 4 9 24 2. 9.6 2.48 8.2 61 1 27 14.9 8.8 2.72 7.83 437 11 3 6.3 6.8 2.16 6.2 237 12 33 4.9 6.8 1.69 6.7 316 All 7.23 413 Risø-I-337(EN)

Table 2 Mean wind climate at Beatrice Alpha Sector Direction Frequency Weibull A Weibull k Mean speed Power # [ ] [%] [m/s] [-] [m/s] [W/m²] 1. 8.1 1.97 7.18 44 2 3 4.1 7.3 1.83 6.49 33 3 6 3.8 6.9 1.73 6.16 322 4 9 4.2 7. 1.61 6.27 371 12 8.7 8.4 1.83 7.46 3 6 1. 8.7 1.89 7.71 71 7 18 11.4 8.8 1.98 7.8 6 8 21 11.6 9. 2. 7.96 92 9 24 11.6 9.2 2.4 8. 62 1 27 11.4 9.7 2.17 8.9 686 11 3 9.3 9.2 2.14 8. 94 12 33 7.8 8.8 2.1 7.79 28 All 7.73 2 Beatrice Alpha Södra Midtsjöbanken Figure 1 Wind direction distributions at the two sites. Yamartino (1984) provides formulae for mean and standard deviations of the wind direction signals. μ = arctan 2 s, c DIR ( a a) ( ) b 3 σdir = arcsin ε 1 + ε ( ) = = 2 2 2 with ε 1 sa ca and b 2 3 1 The sums are calculated by the frequency of occurrence in wind direction sectors N 1 N 1 s = f sin 2 π i N and c = f cos 2π i a i a i i= i= The results are 222.6±84.2 for Södra Midsjöbanken and 247.±86.2 for Beatrice Alpha. 2.2 Regional extreme wind climate The extreme wind climates are based on NCEP/NCAR reanalysis data using the method of Larsén and Mann (26). These estimates are based on surface pressure gradients with correction for the 6-hour gabs in the available time series. These extreme wind climate files are downloaded from the WAsP Engineering homepage 1. Note that the reduced extreme wind distributions are referred to a standard 1 meter height and a standard roughness length. N 1 www.wasp.dk/products/weng/reanalysisewc.htm 6 Risø-I-337(EN)

Table 3 Reduced extreme-wind distribution (h=1m, z=.m) at Beatrice Alpha Sector Direction yr Gumbel Uncertainty Periods' data Periods' data (26) speed alpha (26) maximum # [ ] [m/s] [m/s] [m/s] [m/s] maximum in periods 1 19.7 1.8 1. 18.2 2 3 17.4 1.7 1.4.4 3 6 17.7 1.8 1..9 4 9 18.4 1.6 1.3 17. 12 21. 1.9 1.6 2. 18 6 24.3 1.9 1.6 23.7 ; 6; 23; 24 7 18 27. 2.3 1.9.7 4; 12; 14; 17; 19; 2; 8 21 23. 1.4 1.2 21.4 1; 1; 26 9 24 28. 2.6 2.2 27.1 8; 11; 13; ; 21 1 27.9 2.2 1.8.1 2; 7; 16; 22 11 3.6 2.6 2.3.2 3 12 33 21.8 1.9 1.7 19.8 9 All 27.8 1.8 1.6 27.1 Table 4 Reduced extreme-wind distribution (h=1m, z=.m) at Södra Midtsjöbanken Sector Direction yr Gumbel Uncertainty Periods' data Periods' data (26) speed alpha (26) maximum # [ ] [m/s] [m/s] [m/s] [m/s] maximum in periods 1 2.7 2.2 1.8 21. 1; 17; ; 26 2 3 18.3 2. 1.7.8 3 6 16. 1.6 1.4 16. 4 9 16.7 1.7 1.4 14.7 12.8 1.3 1.1.7 6 17.8 1.4 1.2.7 7 18 2.8 1.8 1. 2.7 24 8 21 21.8 1.6 1.3 21.4 2; 3; 6; 14; 21; 22 9 24 2.7 1.2 1.1 21. 7; 8; 11; 12; 13; 18; 2 1 27 21.7 1.7 1.4 19.7 1; 4; ; 9; ; 16; 19; 23 11 3 21. 1.9 1.6 19. 12 33 18.1 1.4 1.2 17.2 All - 23. 1. 1.2 21. Figure 2 Beatrice Alpha, all directions Risø-I-337(EN) 7

Figure 3 Södra Midsjöbanken, all directions Figure 4 Digital maps of the Beatrice site (left) and the Södra Midsjöbanken (right). 2.3 Digital maps Coastline data were downloaded from the US National Geophysical Data Centre 2 and edited by the WAsP Map editor 3. All land surface roughness is set to.1m. The roughness map was for the Beatrice Alpha site was combined with a digital elevation map based on data from the NASA Shuttle Radar Topography mission, which was converted to WAsP format by in-house software. It was not found necessary to add elevation data to the map for Södra Midsjöbanken. 2 http://rimmer.ngdc.noaa.gov/coast/ 3 http://www.wasp.dk/products/wasp/terrainmaps.html 8 Risø-I-337(EN)

Table Turbine data for WAsP calculations Speed [m/s] Power [MW] Thrust coefficient 4..167.818..38.86 6..7.84 7. 1..8 8. 1.74.86 9. 2.49.78 1. 3.33.737 11. 4.3.649 12. 4.66.71 13. 4.89.41 14. 4.97.314. 4.993.249 16. 4.998.22 17...167 18...14 19...118 2...11 21...88 22...76 23...67 24...9...2 2.4 Turbine data The calculations are made for a fictive MW turbine which we call D126 (D for Downwind). It is a scaled Vestas V8 2MW turbine and it has hub height 16m and rotor diameter 126m. The power and trust-coefficient curves are shown in Table. 2. Windfarm layouts We examine four layouts for each location. Turbine rows are perpendicular to the local mean wind direction with variable spacing. Beatrice (1x2, 1DxD) [1 MW] Beatrice (2x1, Dx1D) [1 MW] Risø-I-337(EN) 9

Beatrice (x14, 7Dx7D) [ MW] Beatrice (x14, 7Dx7D, stag) [1 MW] Figure Alternative wind-farm layouts for Beatrice Alpha Midsjö (1x2, 1DxD) [1 MW] Midsjö (2x1, Dx1D) [1 MW] Midsjö (x14, 7Dx7D) [ MW] Midsjö (x14, 7Dx7D, stag) [1 MW] 1 Risø-I-337(EN)

Figure 6 Alternative layouts for Södra Midtsjöbanken 3 Analysis 3.1 Wind resource estimates with WAsP Calculations are made with WAsP 9.1 4 using standard parameters, except for the wake decay constant which was set to k=.4. The results are shown in Table 6 and Table 7. The production estimates are generally higher at Beatrice Alpha and it is less sensitive to the layout type than at Södra Midtsjöbanken, see Figure 7. The lower sensitivity to wind-farm layout isa consequence of the more uniform directional distribution at Beatrice Alpha, see Figure 1. Table 6 WAsP production estimates for Beatrice Alpha Beatrice (1x2, 1DxD) [1MW] Beatrice (2x1, Dx1D) [1 MW] Variable Total Mean Min Max Total Mean Min Max Total gross AEP [GWh] 4312.72 21.64 21.13 21.96 4312.693 21.63 21.9 21.97 Total net AEP [GWh] 3797.74 18.98 18.321 2.81 3787.642 18.938 18.277 2.6 Proportional wake loss [%] 11.96-3.34.4 12.17-4.17.27 Mean speed [m/s] - 9.29 9.27 9.3-9.29 9.27 9.3 Power density [W/m2] - 991 986 994-991 986 994 Beatrice (x14, 7Dx7D) [ MW] Beatrice (x14, 7Dx7D, stag) [1MW] Variable Total Mean Min Max Total Mean Min Max Total gross AEP [GWh] 427.42 21.6 21.497 21.96 4376.723 21.6 21.496 21.97 Total net AEP [GWh] 3984.29 18.972 18.298 2.741 3787.773 18.69 17.872 2.739 Proportional wake loss [%] 12. - 3.. 13.46-3.94 17.11 Mean speed [m/s] - 9.28 9.26 9.3-9.28 9.26 9.3 Power density [W/m2] - 99 98 994-991 984 994 Table 7 WAsP production estimates for Södra Midsjöbanken Midsjö (1x2, 1DxD) [1 MW] Midsjö (2x1, Dx1D) [1 MW] Variable Total Mean Min Max Total Mean Min Max Total gross AEP [GWh] 471.412 2.37 2.37 2.37 471.412 2.37 2.37 2.37 Total net AEP [GWh] 3438.632 17.193 16.411 19.77 33.613 17.768 17.36 19.62 Proportional wake loss [%].4-3.83 19.38 12.72-3.71 16.31 Mean speed [m/s] - 8.72 8.72 8.72-8.72 8.72 8.72 Power density [W/m2] - 7 7 7-7 7 7 Midsjö (x14, 7Dx7D) [ MW] Midsjö (x14, 7Dx7D, stag) [1 MW] Variable Total Mean Min Max Total Mean Min Max Total gross AEP [GWh] 4274.982 2.37 2.37 2.37 4132.483 2.37 2.37 2.37 Total net AEP [GWh] 3682.96 17.38 16.767 19.6 3492.42 17.24 16.326 19.621 Proportional wake loss [%] 13.8-3.69 17.64.49-3.62 19.8 Mean speed [m/s] - 8.72 8.72 8.72-8.72 8.72 8.72 Power density [W/m2] - 7 7 7-7 7 7 4 www.wasp.dk Risø-I-337(EN) 11

Figure 7 Average annual energy production of a turbine for different turbine layouts. Figure 8 Wind field 16m above sea level at Beatrice Alpha with wind from 33 12 Risø-I-337(EN)

Figure 9 Transect of selected properties at Beatrice Alpha with wind from 33. From the top: terrain elevation, surface roughness, friction velocity, plus wind speed and nondimensional shear at hub height. The dashed rectangle indicates the wind farm. 3.2 Extreme wind estimates with WAsP Engineering WAsP Engineering was used to estimate extreme wind speeds at 16m hub height for all turbine positions. Although the distance from the shore is relatively large, see Figure 8, the wind field is still developing as it passes through the wind farm, see Figure 9. Table 8 lists extreme winds and statistical uncertainties for the corners of one of the layouts at each location. A class I turbine (max m/s) should be safe for Beatrice Alpha while a class II turbine (max 42. m/s) should be safe for Södra Midtsjöbanken. The uncertainties shown are estimated by the statistical fit. Additional uncertainty may arise from using reanalysis data. Real project of this scale should be based on measured data. Table 8 Extreme wind and uncertainty estimates for the corners or the wind farms. Beatrice Alpha Södra Midtsjöbanken Position -year wind Position -year wind Site (W) 43.6±2.4 m/s Site (NW) 38.4±2.2 m/s Site 9 (S) 43.±2.4 m/s Site 9 (SW) 38.4±2.1 m/s Site 19 (N) 44.2±2. m/s Site 19 (NE) 38.1±2.1 m/s Site 199 (E) 42.9±2. m/s Site 199 (SE) 38.1±2. m/s www.wasp.dk/products/weng.html Risø-I-337(EN) 13

Figure 1 Predicted extreme wind climate for the North corner of Beatrice (1x2, 1DxD layout) 14 Risø-I-337(EN)

Figure 11 Predicted extreme wind climate for the SW corner of Södra Midtsjöbanken (1x2, 1DxD layout) 3.3 Turbulence estimates with WAsP Engineering WAsP Engineering calculates turbulence by the Mann model (2), which both accounts for effects of orography and variable surface roughness. The equilibrium surface roughness of water is modeled by a fetch-dependent Charnock s relation. The model assumes neutral atmospheric stability. In reality the stability effect will both affect the equilibrium turbulence level and the width of the transition zone. 1.% 9.% 8.% 7.% 6.%.% 4.% 3.% 2.% 1.%.% Site (W) Speed (m/s) Site 9 (S) Speed (m/s) Site 19 (N) 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 Speed (m/s) Site 199 (E) Speed (m/s) 3 6 9 12 18 21 24 27 3 33 Figure 12 Turbulence intensity for the corners of the Beatric Alpha wind farm. Figure 12 shows turbulence intensity as a function of wind speed for sites at the corners of the Beatrice (1x2, 1DxD) layout and for different wind directions. As expected for an offshore site the turbulence intensity generally increases with wind speed due to the increasing surface roughness. It also depends on wind direction and it is noted that the wind direction with the highest turbulence is not the same for all Risø-I-337(EN)

corners. This is because the length of the open fetch differs for individual corners. The wind directions are color coded. Figure 13 Developing wind speed over a water surface. The effect of fetch in WAsP Engineering is studied by a test case with the wind perpendicular to a linear coast. The flow solver is cyclic so the terrain is constructed as 4 km of water followed by 4 km of land. In the figure the wind is coming from west and the shoreline is at x=2 km. It takes some distance before the wind speed responds at the selected m height and the increase is quite slow. The simulation is repeated for several wind speeds and the turbulence intensity is extracted for variable fetch. Figure 14 shows the dependence on local wind speed and fetch. The development to offshore conditions is gradual and fastest in the beginning. The difference between onshore and offshore conditions becomes less significant at higher wind speeds because of higher offshore surface roughness. Had the calculations been done for a fixed water surface roughness there would still have been fetch dependence but no wind-speed dependence. 16 Risø-I-337(EN)

18 Turbulence intensity for variable fetch 16 14 12 ] 1 T I [% 8 6 4 2 Shoreline 1 km 2 km 3 km 4 km 6 km 1 km 2 km 3 km 4 km 1 2 3 4 U [m/s] Figure 14 Turbulence intensity as a function of wind speed and fetch. 3.4 Effective Turbulence Intensity The effective turbulence intensity is a constant-level turbulence causing the same fatigue damage as variable turbulence intensity in winds coming from different directions (Frandsen 27). These variations include added wake turbulence from neighbor turbines. Different materials respond differently to dynamic loads and thus the effective turbulence intensity depends on a material constant called the Wöhler exponent, which here is set to m=1, a typical value for glass fiber. Figure shows directional turbulence distributions by the Windfarm Assesment Tool 6 (WAT). It shows the directional dependence for a wind speed of m/s at a turbine in the middle of the Beatrice wind farm with the four different layouts. The background turbulence is indicated by purple and added wake turbulence is indicated by gray color. The added turbulence is strongest for the closest turbines. IN the 1x2, 1DxD layout, see top-left sub figure, the turbines are deployed with the closed spacing in the SW-NE direction and this is alos the direction of the highest added turbulence. Also shown is a red circle indicating the effective turbulence intensity. The plots for Södra Midsjöbanken look very similar. According to the IEC 614-1 (Ed.3) turbine safety standard, the effective turbulence intensity must include natural variation between random 1-min periods, e.g. due to variable atmospheric stability. The safety standard accounts for this effect by prescribing use of the 9% percentile of the background turbulence rather than the mean level. Unfortunately, WAsP Engineering only predicts turbulence for neutral atmospheric stability, so WAT simply use the empirical model as the standard, except that it matches the WAsP Engineering prediction for high wind speeds. This method is probably too conservative. Even so, we have made a comparison with the IEC-614-1 design limits. 6 www.wasp.dk/products/wat Risø-I-337(EN) 17

Beatrice (1x2, 1DxD) Beatrice (2x1, 1DxD) Beatrice (x14, 7Dx7D) Beatrice (x14, 7Dx7D, stag) Figure Turbulence conditions for a site in the middle of wind farm for four different layouts. Beatrice (1x2, 1DxD) Beatrice (2x1, 1DxD) IEC Ieff range Ief f A B C TI [%] Turbulence Intensity at Site 9 w ith Wöhler coefficient m = 1. 4 4 3 3.6 Vrated - Vcutout 2 1 1 2 u [m/s] Warning: Ieff exceeds IEC614-1(ed.3) class b design limit IEC Ieff range Ieff A B C TI [%] Turbulence Intensity at Site 11 w ith Wöhler coefficient m = 1. 4 4 3 3.6 Vrated - Vcutout 2 1 1 2 u [m/s] Warning: Ieff exceeds IEC614-1(ed.3) class b design limit Beatrice (x14, 7Dx7D) Beatrice (x14, 7Dx7D, stag) IEC Ieff range Ief f A B C TI [%] Turbulence Intensity at Site 112 w ith Wöhler coefficient m = 1. 4 4 3 3.6 Vrated - Vcutout 2 1 1 2 u [m/s] Conclusion: Ieff within IEC614-1(ed.3) class b design limit IEC Ieff range Ieff A B C TI [%] Turbulence Intensity at Site 19 w ith Wöhler coefficient m = 1. 4 4 3 3.6 Vrated - Vcutout 2 1 1 2 u [m/s] Conclusion: Ieff within IEC614-1(ed.3) class b design limit Figure 16 Effective turbulence intensity for a site in the middle of the Beatrice Alpha wind farm calculated for four alternative layouts. 18 Risø-I-337(EN)

Midsjö (1x2, 1DxD) Midsjö (2x1, 1DxD) IEC Ieff range Ieff A B C TI [%] 4 4 3 3 2 1 Turbulence Intensity at Site 14 w ith Wöhler coefficient m = 1..6 Vrated - Vcutout 1 u [m/s] Warning: Ieff exceeds IEC614-1(ed.3) class b design limit 2 IEC Ieff range Ief f A B C TI [%] 4 4 3 3 2 1 Turbulence Intensity at Site 19 w ith Wöhler coefficient m = 1..6 Vrated - Vcutout 1 2 u [m/s] Conclusion: Ieff within IEC614-1(ed.3) class b design limit Midsjö (x14, 7Dx7D) Midsjö (x14, 7Dx7D, stag) IEC Ieff range Ieff A B C TI [%] 4 4 3 3 2 1 Turbulence Intensity at Site 111 w ith Wöhler coefficient m = 1..6 Vrated - Vcutout 1 2 u [m/s] Conclusion: Ieff within IEC614-1(ed.3) class b design limit IEC Ieff range Ief f A B C TI [%] 4 4 3 3 2 1 Turbulence Intensity at Site 18 w ith Wöhler coefficient m = 1..6 Vrated - Vcutout 1 2 u [m/s] Conclusion: Ieff within IEC614-1(ed.3) class b design limit Figure 17 Effective turbulence intensity for a site in the middle of the Södra Midtsjöbanken wind farm calculated for four alternative layouts. Figure 16 shows the effective turbulence intensity as function of wind speeds and compared to the IEC 614-1 design curves. The rule is that the design limit must exceed the local values in a range from 6% of rated wind to the cut-out wind speed. The local curves seem to follow the design limit for turbulence category B and with minor differences between the four alternative layouts. Figure 17 shows a similar comparison for Södra Midtsjöbanken. Here we see a larger difference between the four layouts with the lowest effective turbulence intensity for the 2 1 turbine layout, or the one with the largest turbine separation in the predominant wind direction. The reason why a similar effect is not observed at the Beatrice site is the more uniform wind direction distribution, see Figure 1. 3. Check of wind speed distribution Site-assessment, according to IEC 614-1, both involves a check of the effective turbulence intensity, as above, and a check of the probability distribution. The reasoning is that if both quantities stay beyond the design limits used for the IEC turbine fatigue analysis, then the turbine safety will not be compromised. The design curves are determined by the turbine class. Figure 18 shows the comparison for the Beatrice site and the design curve may is slightly exceed near 1 m/s but not very much so. For Södra Midsjöbanken, see Figure 19, the local probability exceeds the design curve at more wind speeds. This means that this wind farm may need a class I turbine after all. On the other hand there seem to be a safety margin in the effective turbulence intensity so the conclusion is not clear perhaps calling for more detailed fatigue-load analysis. Risø-I-337(EN) 19

IEC pdf range pdf Site pdf IEC 4 Wind-speed probability density distribution at Site 9 4 3 pdf [%/(m/s)] 3 2.2 Vref -.4 Vref 1 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Warning: Actual PDF sometimes above design PDF in IEC range! Figure 18 Comparison of probability distribution in the middle of the Beatrice Alpha wind farm and the IEC 614-1 design distribution. IEC pdf range pdf Site pdf IEC 4 Wind-speed probability density distribution at Site 14 4 3 pdf [%/(m/s)] 3 2.2 Vref -.4 Vref 1 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Warning: Actual PDF sometimes above design PDF in IEC range! Figure 19 Comparison of probability distribution in the middle of the Södra Midsjöbanken wind farm and the IEC 614-1 design distribution. Conclusions Alternative wind-farm layouts with approximately 1GW installed effect at Beatrice Alpha and Södra Midtsjöbaken have been tested with WAsP, WAsP Engineering and related tools. The findings are that The annual energy production, corrected for wake effects, is about 18.8 GWh per turbine at Beatrice Alpha and 17. GWh at Södra Midsjöbanken. This corresponds to capacity factors of 43% and 4%, respectively. The extreme wind is about 44 m/s at Beatrice and 38 m/s at Södra Midsjöbanken; All wind farms layouts cover approximately km. Extreme and mean wind climates vary over the wind farm area, especially at Beatrice alpha which is closer to land than Södra Midsjöbanken; Annual energy production and effective turbulence intensity seem less sensitive to the chosen layout at Beatrice Alpha than at Södra Midsjöbanken. This is due to a more uniform wind direction distribution; 2 Risø-I-337(EN)

The ambient turbulence intensity calculated by WAsP Engineering is valid for neutral atmospheric stability only. The 9% level of the actual turbulence intensity including stability effects is estimated in a crude and conservative way. With the chosen wind-farm layouts the background turbulence is important for the effective turbulence intensity, so the effective turbulence estimate may be too conservative. The analysis indicates that the IEC 614-1 turbine class needs to be I B for the Beatrice Alpha wind farm and II B for the Södra Midsjöbanken wind farm. The conclusion is clear regarding the main wind class, whereas more analysis regarding turbulence category is recommended. References Frandsen, S. (27) Turbulence and turbulence generated fatigue loads in wind turbine clusters. Risø-R-1188(EN) Larsen, S., Tarp-Johansen, N.J., Frandsen, S., Jørgensen, E.R (27) Södra Midsjöbanken Environmental data. Risø-I-. Jørgensen, E.R., Larsen, S, Frandsen, S. Tarp-Johansen, N.J. and Trumars, J. () Beatrice Environmental Data, Risø-I-34 Larsén, X. G. and Mann, J. (26) The effects of disjunct sampling and averaging time on maximum mean wind speeds, J Wind Eng. Ind. Aerodyn. 94 81 62 Mann, J. (2) The spectral velocity tensor in moderately complex terrain, J. Wind Eng. Ind. Aerodyn. 88 3-169 Nielsen, M., Jørgensen, H. E. and Frandsen, S. E. (29) Wind and wake models for IEC 614-1 site assessment, EWEC 29. Yamartino, R. J. (1984) A comparison of several single-pass estimators of the standard deviation of wind direction, J. Clim. Appl. Meteorol. 23 1362-1366 Risø-I-337(EN) 21

Risø DTU is the National Laboratory for Sustainable Energy. Our research focuses on development of energy technologies and systems with minimal effect on climate, and contributes to innovation, education and policy. Risø has large experimental facilities and interdisciplinary research environments, and includes the national centre for nuclear technologies. Risø DTU National Laboratory for Sustainable Energy Technical University of Denmark Frederiksborgvej 399 PO Box 49 DK-4 Roskilde Denmark Phone +4 4677 4677 Fax +4 4677 688 www.risoe.dtu.dk