Impact of Negative Geostrophic Wind Shear on Wind Farm Performance

Abstract

Baroclinicity, which leads to height-dependent driving pressure gradients, occurs in various situations such as the flow transition between land and sea, and sloping terrain. It has been shown that baroclinicity modifies the structure of the atmospheric boundary layer. For example, negative shear baroclinicity creates additional turbulence at higher elevations, which might influence the energy entrainment into large wind farms. Here, we use large-eddy simulations to study the effect of baroclinicity-induced negative shear on the wind farm power production and energy entrainment into a large wind farm. In agreement with the literature, our simulations show that negative geostrophic wind shear significantly modifies the mean wind velocity in the atmospheric boundary layer. Specifically, for the cases considered in the study, the negative geostrophic shear causes a change in the mean velocity up to 2.3 m/s at hub height, which greatly alters the wind farm power production. Additionally, we demonstrate with an energy budget analysis that a wind farm does not necessarily benefit from the additional turbulence created by the negative geostrophic wind shear. The reason for this is that the baroclinicity-induced negative shear alters the height and strength of the low-level jet and creates an upward flux above the jet, limiting the energy entrainment into the wind farm. Our results show that wind resources are altered in the boundary layer due to negative geostrophic wind shear and should be considered in wind farm modeling and power forecasts.