Wind Energy

What makes an area good for windmills?

Ever wondered why there aren't windmills at your street corners, or even in the cities or towns you live? If you go on a road trip and drive on the highway, you'll see that wind mills are usually spinning on top of smooth hills.

We'll show you why.

Try this!

  1. Take out any small, light object lying around you (a napkin, paperclip etc.) and lay it out in front of you.
  2. Try blowing it with your mouth wide open and then try blowing it with your lips in a small “O” shape.

Which way moves the object more?
Blowing through the small “O” shape makes the object move more.
This is because you’re blowing out the same amount of air, but the “O” shape creates a smaller area. The same amount of air needs to move faster in order to get out of the smaller area.
Outdoor wind works the same way.

Over a hill

Wind flows within a boundary layer. When that wind encounters a hill, it is forced to flow faster through a smaller area. That's why wind is usually faster at the top of a hill.
What's a boundary layer?

Between hills

A similar thing happens when wind blows between two mountains. This is called a "tunnel" effect. The mountains force the wind to blow through a small opening between them, causing it to blow faster.
More examples.

Behind buildings or trees: Turbulence

The video to the right shows turbulent flow - rough and disorderly flow.

Obstacles, such as trees or buildings, act like the sphere as wind flows past them.

They cause the flow to break up and become turbulent.

Windmills do not usually work well in turbulent wind. Since the turbulent wind direction constantly changes, the windmill cannot spin steadily.

Where in the Bay Area has the best winds?

In order for a wind turbine to operate the wind must be faster than 8 meters per second and it must have low levels of turbulence.
We used a computer model to count the number of hours during a simulated day that the winds are both fast and smooth enough for a wind turbine to generate electricity.

Can you tell which locations have the best winds?

The best winds occur at the tops of hills, or where two hills form a funnel such as in the Golden Gate or at Altamont Pass (see video below). High quality winds also occur far off shore where the smooth ocean surface causes much less resistance to wind flow than land does.

Wind energy in Altamont Pass

Possible wind resources in the Bay Area

This website is part of a design project from CEE 105, Applied Fluid Mechanics by Charles Chang, Eddie Divita, Ching-Ching Liu, Milton Mi. For more information visit this page.