(Yes, I’m talking about the modern windmills being pitched for wind farms)
The classic way to capture wind is seen in the 1800s sailing ships - add more and more masts with more and more sails, really wringing the last puff of wind passing over the hull. More cloth meant more power, more speed, simple as that.
It’s a modern adaptation that reduces their vulnerability to lances.
Think about a sailplane wing rather than a ship’s sails. The goal of the wing is to create the most amount of lift with the least amount of drag. Thus, sailplane wings are long and narrow. It’s the same thing with a windmill blade. You want the most ‘lift’ with as little drag as possible. If the blades were wide, there would be more drag; thus making them less efficient.
They are long because they need to generate a lot of tourqe to turn their generators. Being long increases leverage. Now if they were long AND broad then it would be difficult to build towers and blades strong enough to stand-up under the load.
Yeh, it annoys me too: “Dudes you’re wasting all that wind energy!!”.
It’s a question of low solidity for high speed applications. We had a thread about windmill blades a while back dealing with the number of blades issue. Solidity is also the reason there are only three blades.
No, they are long and narrow for the reason Johnny LA gave: to increase aerodynamic efficiency.
A windmill’s power output is proportional to its tip speed ratio, which is the ratio of the speed of the tip of a blade to the windspeed. This is closely analogous to L/D (lift to drag, or glide ratio) in an airplane. It has been known for a long time that to achieve a high L/D you must use a wing with a high aspect ratio (i.e. a long and narrow wing). The result is a small amount of induced drag, which means that wingtip vortices consume a small amount of energy.
If broad blades offered an advantage, it would be no problem at all to build them and towers easily capable of supporting them (indeed, broad blades would be in some ways easier to build).
I asked a similar question last year.