The windmills were supposed to be a source of cheap clean energy to power the entire world. But they weren't enough.

the Sign Painter
[Blustery Day]

Windmills are a hazard in the video game World of Goo. They are seen only in Chapter 2, in the level Blustery Day. The spinning blades of the windmill function identically to regular blades and will destroy Goo Balls and pop Balloon Goo on contact. While they only appear once, the windmills are mentioned by the Sign Painter to have been a failed attempt at harnessing energy to power the world. The failure of the windmills led to the use of the Beauty Generator, the centerpiece of the plot of Chapter 2.