High altitudes with low air density will lead to reduced energy yield below nominal performance. Depending on the aerodynamic design of the blades, Vortex Generators can reduce the loss in energy yield.
Vortex Generators have to be individually calculated for the blade and the site. Generic solutions or wrongly placed Vortex Generators can lead to a significant loss in performance.
Most reasonable approach: Tuft experiment (see picture) and CFD/empirical layout of VG pattern and size.