Just like airplanes use flaps to improve lift at low speeds, they are now experimenting with something similar on turbine blades.
[The article itself](https://www.omroepflevoland.nl/nieuws/365762/speciale-albatros-wieken-op-windmolen-gemonteerd) is in Dutch but the video shows some pictures.
In the lab they measured a 30% improvement (carefully controlled, I assume), in practice they’re hoping for 10-15%.
*No info on the min-max meter/second it works at, or if that 10-15% is ‘total’ or ‘partial’.*
Source: mrCloggy