Bidirectional wakes over complex terrain using SCADA data and wake models

We investigate bidirectional wake effects between two wind turbines of the same type in a hill region in northern Japan using supervisory control and data acquisition (SCADA) data and validate the simulated wakes using 12 wake models. The extent to which complex terrain affects wake behavior has not yet been fully studied, and further understanding of the fundamental characteristics of bidirectional wakes over complex terrain is required. The two wind turbines are located 3.17 times the rotor diameter apart, with a different elevation of 0.44 times the rotor diameter. First, we identify the wake effects in terms of wind speed ratio, which is defined as a ratio of wind speed at the downstream wind turbine to that at the upstream wind turbine. By comparing the conditions according to the operating state of the upstream wind turbine, the wakes are clearly detected as minimum wind speed ratios for northeasterly and southwesterly winds. The wind speed ratio with a maximum wake effect occurs below the rated wind speed. Increases in turbulence intensity and decreases in power output are greater for southwesterly wind than for northeasterly wind. This difference arises from the combined effects of the turbine-induced wake and the terrain-induced variation in wind speed. Then, we examine the winds and the wakes over complex terrain by using Wind Atlas Analysis and Application Program Computational Fluid Dynamics (WAsP CFD) in combination with PyWake. The wind speed ratios derived from the wake models show strong dependence on inflow wind speed, reflecting the thrust coefficient curve used in the wake models. The wake models commonly overestimate the reduction in wind speed ratio for northeasterly wind and underestimate it for southwesterly wind. This comparative study contributes to understanding the additional effects of topography on wake effects in onshore wind power plants and offshore wind power plants near the coast.