Synergistic effects of turbine wakes and atmospheric stability on power production at an onshore wind farm

Tall wind turbines, with hub heights at 80 m or above, can extract large amounts of energy from the atmosphere because they are likely to encounter higher wind speeds, but they face challenges given the complex nature of wind flow in the boundary layer. At large wind farms, power production is dependent both on atmospheric stability and the presence of turbine wakes. Depending on whether the boundary layer is stable, convective or neutral, mean wind speed and turbulence may vary greatly across a tall turbine swept area (40 m to 120 m). In addition, turbine wakes increase ambient levels of atmospheric turbulence while the mean wind speed is slowed. Our study examines the influence that atmospheric mixing or stability has on turbine wakes and power production at a West Coast North American wind farm. The site has an extensive observational network including a SODAR remote sensing platform, two meteorological towers, 10s of 80 m tall turbines with nacelle wind speed and power data, and a nearby research flux station.

We first discuss the applicability of instrument-derived stability parameters, including the wind shear-exponent, α, turbulence intensity, I_U, and turbulence kinetic energy, TKE, against the more physically-based Obukhov length, L, to accurately describe the wind speed and turbulence profiles in the rotor area. Next, we divide the measurement period into five stability classes (strongly stable, stable, neutral, convective, and strongly convective) to discern stability-effects on power output at both upwind and downwind turbines to assess the role of atmospheric stability on turbine wakes and power production. Stability effects at upwind turbines are significant: when I_U is considered, the power generated for a given wind speed is twenty percent higher during strongly stable conditions than during strongly convective conditions. Power produced by downwind turbines is more complex due to the interactions between atmospheric stability and turbine wakes. During convective conditions, wake effects on downwind power production are expected to be small because turbulence dissipates turbine wakes. Under stable conditions, in contrast, turbine wakes will not be largely dissipated by turbulence and power production is expected to be attenuated at downwind turbines as compared to upwind turbines. Therefore, although leading-edge turbines at this farm generate more power during stable conditions than during convective conditions, the total farm power collection is more complicated.