Wind Resource Reconstruction In Complex Terrains Regarding Thermal Stratification Chengcheng Chen, Tingting Jiang, Genyong Wu State Key Laboratory of Wind Power System, China Zhejiang Windey Co., Ltd, China Chi-Yao Chang, Xiao Wang, Bernhard Stoevesandt Fraunhofer IWES, Germany

Introduction Methodology Results analysis Summary

Objectives Development of a methodology to calculate the wind resource in complex terrain wind farm, cooperating with Fraunhofer IWES: Topography capture by an advanced mesh strategy, key feature, less element… Derive distinct thermal stratification classes from Met. Mast data, and execute the CFD simulation with the extended simpleFoam. Obtain the wind data of the target position by a speed up ratio matrix of wind sectors and atmospheric stability.

Monin-Obukhov Similarity theory Mean velocity profile in homogeneous flat plane with heat transfer correct the log-law to (stable stratification) (neutral) (unstable stratification) Vertical heat flux induced buoyant force → changing the mean velocity profile Night time: air warmer then the ground → downward heat flux : stable condition Day time: ground warmer than the air → upward heat flux :unstable/ convective condition

Buoyant induced effects on inhomogeneous boundary layer Attached flow in stable stratified pattern Enlarged recirculation bubble in convective state Stable Neutral Unstable

Met mast data classification The u*-MoL diagram provides the wind analysts an overview of the stratification conditions on the site, for which the met mast data over several height levels are fit according to Monin-Obukhov similarity law.

Wind farm in complex terrain Hill land wind farm in the south east of China. Domain size 10km×10km×2.5km TerrainBlockMesher generated structural mesh, element 11×106 Inhomogeneous roughness distribution

+ Processing Stable/unstable/ neutral conditions Each simulation with 1-D precursor boundary generation Extended k-Epsilon model for buoyant force with correction +

Speed-up-correlation in Stratified ABL The general description of speed-up ratio behavior regarding two velocities based on one certain point reads:. 𝑈(𝑀𝑂𝐿,𝐷ir) 𝑎𝑖𝑚 = 𝐴(𝑀𝑂𝐿,𝐷ir) 𝑎𝑖𝑚 ∙ 𝑈(𝑀𝑂𝐿,𝐷ir) 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 Mol Dir unstable （Mol=-1000） （Mol=-500） （Mol=-100） neutral （Mol=+∞&-∞） stable （Mol=500） A(-1000, 0) A(-500, 0) A(-100, 0) A(0, 0) A(500, 0) 15 A(-1000, 15) A(-500, 15) A(-100, 15) A(0, 15) A(500, 15 ) 30 A(-1000, 30) A(-500, 30) A(-100, 30) A(0, 30) A(500, 30) 60 A(-1000, 60) A(-500, 60) A(-100, 60) A(0, 60) A(500, 60) 90 A(-1000, 90) A(-500, 90) A(-100, 90) A(0, 90) A(500, 90) 120 A(-1000, 120) A(-500, 120) A(-100, 120) A(0, 120) A(500, 120) 150 A(-1000, 150) A(-500, 150) A(-100, 150) A(0, 150) A(500, 150) 165 A(-1000, 165) A(-500, 165) A(-100, 165) A(0, 165) A(500, 165) 180 A(-1000, 180) A(-500, 180) A(-100, 180) A(0, 180) A(500, 180) 195 A(-1000, 195) A(-500, 195) A(-100, 195) A(0, 195) A(500, 195) 210 A(-1000, 210) A(-500, 210) A(-100, 210) A(0, 210) A(500, 210) 240 A(-1000, 240) A(-500, 240) A(-100, 240) A(0, 240) A(500, 240) 270 A(-1000, 270) A(-500, 270) A(-100, 270) A(0, 270) A(500, 270) 300 A(-1000, 300) A(-500, 300) A(-100, 300) A(0, 300) A(500, 200) 330 A(-1000, 330) A(-500, 330) A(-100, 330) A(0, 330) A(500, 330) 345 A(-1000, 345) A(-500, 345) A(-100, 345) A(0, 345) A(500, 345)

Commercial software

Flow pattern regarding thermal stratification Interaction with complex topology, stable and neutral show similar attached flow pattern. Flow runaway upward more serious in the downstream of hill in unstable condition, because of the convective in vertical direction. 3 4 6 8 9 U/m·s-1 Stable Neutral Unstable (Area 1) (Area 2) (Area 3)

Summary Monin-Obukhov similarity law was used to class the thermal stratification of wind farm. Flow over terrain could have significant different patterns due to thermal stratification class. Taking thermal stratification into account of wind resource reconstruction in complex terrain shows the closer result.

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