VINDKRAFTNET MEETING ON TURBULENCE On-going Work on Wake Turbulence in DONG Energy 28/05/2015 Cameron Brown Load Engineer Lucas Marion R&D graduate
Who are we? Cameron Brown Load Engineer from Loads Aerodynamics and Control Department - Calculation of loads for design of offshore Wind turbine sub structures (tower + monopile foundations) - Collection of inputs needed for load calculations (e.g. turbulence intensities) cambr@dongenergy.dk Lucas Marion In the DONG Energy graduate program from Wind Power Research & Development Department (R&D) - Responsible for managing R&D projects across DONG Energy - One major R&D topic is offshore meteorology (incl. research on turbulence intensity) Loaned out to LAC for 8 months lucma@dongenergy.dk 2
Turbulence from a Loads Perspective Turbulence intensity affects loads - Turbulence intensity is one of the main inputs for load calculations We need to ensure the WTG structure can withstand the turbulence level - We need to compare site specific turbulence to RNA type certificate turbulence - We design foundations to withstand site specific turbulence The turbulence in wake is higher than free turbulence - We want to understand the turbulence in wake to be able to accurately predict loads 3
What methods do we have to estimate turbulence in wake? IEC 61400-1 specifies the recommended methods Currently, only Frandsen [1] is recommended - Fairly simple method - Wake is simply modelled by increasing turbulence levels In next version of IEC standard, the Dynamic Wake Meandering model (DWM) [2] is acceptable - More complicated method - More aspects of the wake physics are modelled - Wake is a wind speed deficit, increased wake turbulence, and wake movement (meandering) estimated Should we switch to DWM? - We are investigating the simple Frandsen method to find out 4 [1] S. Frandsen (2007) Turbulence and Turbulence generated loading in wind turbine clusters, Risø report T-1188 [2] G.C. Larsen, H. Aa. Madsen, K. Thomsen, and T.J. Larsen (2008) Wake meandering a pragmatic approach. Wind Energy, 11, 2008, pp. 377-395
How does the Frandsen method work? The wakes are simply modelled by increasing the turbulence intensity The wakes from the 8 closest wind turbines (in direct sight) are modelled with the center wake equation σ T = 2 V hub 1.5 + 0.8 d i C T 2 + σ c 2 The gaps between the center wakes are filled in with either: - 5 turbines from "edge": Ambient turbulence (90th percentile) - σ c = σ + 1.28 σ σ - >5turbines from "edge": Large Park Turbulence - σ c = 1 2 σ w 2 + σ 2 + σ + 1.28 σ σ - σ w = 0.36 V hub 1+0.2 d r d f C T 5
Questions to be answered 1. Are the added turbulence equations still valid? - E.g. the center wake fits were made for small wind turbines in small arrays Wind Farm Wind Turbine Rated Power Rotor Diameter Total Capacity Andros 7x V27-225 225kW 27m 1.58MW Taff Ely 20x Nordtank 450kW 37m 9.00MW Alsvik 3x Danwin 23/180 180kW 23m 0.54MW Vindeby 11x Bonus B35/450 450kW 35m 4.95MW 2. How is the turbulence spectrum affected in wake? - Is it valid to simply scale up the free turbulence spectrum with the center wake turbulence intensity? 3. What about atmospheric stability? - What kind of impact does atmospheric stability have on the wake turbulence intensity? 6
Data sources Horns rev 2, M8 Close to some of the turbines Cup anemometers (10 minute statistics only) Nysted Upstream and downstream masts Sonics with high-frequency data Relatively far downstream 7
Questions to be answered 1. Are the added turbulence equations still valid? - E.g. the center wake fits were made for small wind turbines in small arrays 2. How is the turbulence spectrum affected in wake? - Is it valid to simply scale up the free turbulence spectrum with the center wake turbulence intensity? 3. What about atmospheric stability? - What kind of impact does atmospheric stability have on the wake turbulence intensity? 8
HR2 site and M8 HR2 uses SWT 2.3MW-93 - Pitch controlled, variable speed 91 positions for total of 209MW Wakes from ~2.5 to up to 32D -Closest 8 shown in figure to right Cup anemometer at hub height 9
Directional Turbulence intensity Magnitude of the Center wake peaks appears good except K06 wake (~2.5D) is not well represented - Not bell shaped but more M shaped 10
Center wake turbulence intensity The center wake turbulence intensity seems to predict reasonable turbulence levels except At <4D, the center wake equation seems quite conservative 11
UTM32 Northing [m] Large Park Turbulence x 10 6 In some wind directions (between center wakes), we should see the large park turbulence Especially at wind directions of about 126 6.167 6.166 6.165 6.164 6.163 6.162 6.161 6.16 6.159 6.158 4.06 4.08 4.1 4.12 4.14 4.16 UTM32 Easting [m] x 10 5 12
Large Park Turbulence The large park turbulence equation greatly overestimates the turbulence in these directions Free wind at ~126 13
Large Park Turbulence as a function of Wind Speed The large park turbulence equation greatly overestimates the turbulence in these directions Wind direction 122 to 154 This is also true across a wide range of wind speeds 14
Questions to be answered 1. Are the added turbulence equations still valid? - E.g. the center wake fits were made for small wind turbines in small arrays 2. How is the turbulence spectrum affected in wake? - Is it valid to simply scale up the free turbulence spectrum with the center wake turbulence intensity? 3. What about atmospheric stability? - What kind of impact does atmospheric stability have on the wake turbulence intensity? 15
Nysted wind farm and method Sonic anemometers up and down-stream the wind farm (for Eastern and Western winds) Power spectra for the three wind components Atmospheric stability 16
F 11 (m 2.s -2 /Hz) Longitudinal power spectra 10 3 Eastern winds - Longitudinal part 10 2 10 1 10 0 10-1 10-2 10-4 10-3 10-2 10-1 10 0 f (Hz) 17
F 11 (m 2.s -2 /Hz) Longitudinal power spectra 10 3 Eastern winds - Longitudinal part 10 2 10 1 Ratio 6 5 10 0 Mean ratio ~2.5 4 Ratio 3 10-1 2 18 1 10-2 10-4 10-3 10-2 10-1 10 0 f (Hz) 0 0 0.1 0.2 0.3 0.4 0.5 f [Hz]
F 11 (m 2.s -2 /Hz) Longitudinal power spectra 10 3 Eastern winds - Longitudinal part 10 2 10 1 10 0 10-1 10-2 10-4 10-3 10-2 10-1 10 0 f (Hz) 19
F 11 (m 2.s -2 /Hz) Longitudinal power spectra 10 3 Eastern winds - Longitudinal part 10 2 10 1 10 0 10-1 NOT OK OK 10-2 10-4 10-3 10-2 10-1 10 0 f (Hz) 20
Questions to be answered 1. Are the added turbulence equations still valid? - E.g. the center wake fits were made for small wind turbines in small arrays 2. How is the turbulence spectrum affected in wake? - Is it valid to simply scale up the free turbulence spectrum with the center wake turbulence intensity? 3. What about atmospheric stability? - What kind of impact does atmospheric stability have on the wake turbulence intensity? 21
Atmospheric Stability Two mechanisms Wind Shear Thermal convection NEUTRAL Strong thermal convection Ts>Ta UNSTABLE Cooling from the down Ts<Ta STABLE 22
Impact of stability conditions on TI 23
Impact of stability conditions on TI U N S 24
Impact of stability conditions on TI = up-stream = down-stream Leveling effect of the wind farm on the turbulence intensity over the 3 classes 25
Impact of stability conditions on TI New classification 26
Impact of stability conditions on TI New classification cl 1 cl=2 cl=3 27
Impact of stability conditions on TI New classification = up-stream = down-stream 28
EASTERN WINDS WESTERN WINDS Impact of stability conditions on TI New classification = up-stream = down-stream 29
Results conclusions Nysted study No impact of the wind farm on very large turbulence scale. General leveling effect of the wind farm over the stability classes on the turbulence intensity level TI ambient Sta. + TI park Sta. TI ambient Unsta. + TI park Unsta. Seems difficult to use atmospheric stability as an input variable 30
Questions to be answered: Conclusions 1. Are the added turbulence equations still valid? - The center wake turbulence equation seems ok at distances between 4D and 32D - Wake at <4D may not be well represented in Frandsen method - Wake turbulence is not bell shaped at <4D - Large Park Turbulence equation seems to overestimate turbulence levels 2. How is the turbulence spectrum affected in wake? - Scaling undisturbed spectrum might misrepresent turbulence on time scales of >10s 3. What about atmospheric stability? - The wake turbulence seems to be dominated by the mechanical turbulence added by the presence of the turbine lessening the importance of atmospheric stability 31
Next steps Further investigations being carried out under Carbon Trust Offshore Wind Accelerator project - Doing similar analysis on Rødsand II and Greater Gabbard data Danish EUDP* project with DTU Wind Energy - Investigating the impact of atmospheric stability and wakes on loads - Load measurement data will be analysed 32 *ENERGY TECHNOLOGY DEVELOPMENT AND DEMONSTRATION PROGRAMME