# Wind Matters: Page 3 of 4

Beginner

Wind Matters
Increasing swept area increases the captured wind energy proportionally.
The power in the wind is a function of the cube of its velocity.
Note how wind speed increases with the height above the ground.
An obstruction can create turbulence zones that are, above it, twice its height (2H); upwind, twice its height (2H) horizontally; and downwind, 20 times its height (20H) horizontally.
The bottom of turbine’s swept area should be at least 30 ft. higher than any obstruction within 500 ft.
This wind turbine was installed in 1982 using the 30-foot rule for fixed obstacles—tree growth was not accounted for. In the last three decades, the trees have grown tall enough to render the wind turbine useless.
The site for the first turbine (1) turned out to be highly compromised—downwind from almost all obstacles on the property relative to the prevailing wind direction. Subsequent turbines were placed at sites 2 and 3.
The wind rose for this site shows that the strongest winds come predominately from a southwest and south-southwest direction.

### Height Matters

The problem facing a prospective wind turbine site should now be obvious: You need to minimize turbulence and take advantage of the power of incremental wind speed by getting the turbine higher into the wind. The solution is pretty obvious: Mount your turbine on a tall tower. If ground-dwellers afraid of heights don’t like to hear that message, they shouldn’t consider a wind turbine for their electricity.

How tall does the tower need to be? The first rule used in the small wind industry for determining minimum tower height is that the entire rotor of the wind turbine must be at least 30 feet higher than any obstacles within 500 feet of the tower. This rule is based on several facts:

• Wind speed increases with height above the ground.
• Turbulent winds have little extractable energy.
• Increased vertical separation between ground clutter (trees and buildings) and the wind turbine rotor means that the rotor is moved out of chaotic turbulent winds and into clean laminar wind flow with more extractable energy.

Wind turbines are presumably installed for two to three decades of service, during which time the trees in the area will probably grow taller. As such, we also need to consider the mature tree height, not the current tree height, when determining tower height.

So, we need to modify the 30-foot rule to take into consideration tree growth over the life of the wind system. It now states: The entire rotor of the wind turbine must be at least 30 feet higher than any obstacles within 500 feet of the tower, or the mature tree height or tree line in the area, whichever is higher.

Keep in mind that the 30-foot rule with consideration for mature tree height dictates the minimum tower height for your site. Installing a taller tower reduces turbulence even further, while getting your turbine higher into the wind profile. All of this will result in more electricity production over the life of your wind system. Don’t cut corners by scrimping and installing a short tower—you’ll be sacrificing long-term performance.

### Location Matters

Most locations have winds that blow out of somewhat-specific directions over the bulk of the year. These are known as prevailing winds and are plotted graphically in a wind rose.

Using the wind rose for your site will help determine the best location for your tower. Over the course of the year, the wind blows from all directions, so any location on your property is going to be a compromise. However, you can minimize turbulence while optimizing your site’s wind profile by placing your tower as far upwind in the direction of the area’s prevailing winds as possible.

My Wisconsin homestead is typical—a house and a few buildings, all with electricity, fencerows for privacy and to block winter storm winds, and a few tall trees. At first glance, one might opt to site the wind turbine close to the house for a shorter wire run, which is exactly what we did with our first turbine. If you look at the wind rose for our site (at right), however, you’ll notice that this places our tower downwind of almost all obstacles on our property relative to the prevailing wind direction. While the turbine generates adequately, it would actually generate more if it had been sited somewhere else. The locations of our two other wind turbines are much less subject to turbulence most of the year.