Richard J.A.M. Stevens

Physics of Fluids

University of Twente

Publication 109 ยท Wind-farm flow

Why large wind farms lose power at two distinct scales

R.J.A.M. Stevens, Journal of Fluid Mechanics 958, F1 (2023).

The article and figure are open access under CC BY 4.0.

Main finding

The paper separates wind-farm power loss into a turbine-scale component from interactions within the array and a farm-scale component from the atmospheric boundary layer's response to aggregate drag. This conceptual split clarifies why layout optimization cannot remove the whole-farm momentum-supply limit.

Conceptual bars comparing turbine-scale and farm-scale losses with wake and farm-blockage losses using four power-coefficient reference levels
How to read the figure. Both bars begin at the farm-average power coefficient Cp and end at the Betz reference Cp,Betz. The upper bar separates the turbine-scale loss (TSL), from Cp to the two-scale-model reference Cp,Nishino, from the farm-scale loss (FSL). The lower bar instead separates traditional wake loss (WL), ending at the first-row coefficient Cp,1, from farm-blockage loss (FBL). This is a conceptual decomposition, not a measured dataset, and it does not encode the reported two-to-one farm-scale/turbine-scale loss ratio. Open the full-resolution figure. Figure 1, cropped. R.J.A.M. Stevens (2023), adapted from Kirby et al. (2022), CC BY 4.0.

Why this matters

The distinction separates losses that layout changes may reduce from losses imposed by the atmospheric response to the wind farm as a whole. Internal wake interactions create the layout-sensitive turbine-scale component. At farm scale, aggregate turbine drag slows the atmospheric boundary layer and limits the momentum available to the array; turbulent entrainment from above replenishes it. Layout optimization can therefore address only part of the total loss, while farm-scale strategies must target the momentum supply.

Research context

This Focus on Fluids article synthesizes the two-scale momentum analysis and large-eddy simulations of Kirby et al. (2022); it does not independently derive the model or generate the underlying simulation evidence. The reported result that farm-scale losses are typically about twice turbine-scale losses for large offshore wind farms is attributed to that earlier work and is not demonstrated numerically by Figure 1. The discussion concerns neutral, statistically stationary large wind farms. It does not test atmospheric stability, nonstationarity, heterogeneous terrain, gravity waves, wind-farm controls, or economics. The article argues that the farm-to-atmosphere interaction becomes increasingly important with farm size, while identifying momentum entrainment and its dependence on atmospheric conditions, layout, and size as open research questions.

Read the article Open-access version View in the complete publication list Related wind-farm research