Wind Matters: Page 3 of 4

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Wind Matters
Wind Matters
Increasing swept area
Increasing swept area increases the captured wind energy proportionally.
The power in the wind is a function of the cube of its velocity.
The power in the wind is a function of the cube of its velocity.
Wind speed increases with height
Note how wind speed increases with the height above the ground.
An obstruction can create turbulence zones
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.
30-Foot Rule
The bottom of turbine’s swept area should be at least 30 ft. higher than any obstruction within 500 ft.
Trees have grown tall enough to render the wind turbine useless.
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.
Wind turbine siting
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.
Wind rose
The wind rose for this site shows that the strongest winds come predominately from a southwest and south-southwest direction.
Wind Matters
Increasing swept area
The power in the wind is a function of the cube of its velocity.
Wind speed increases with height
An obstruction can create turbulence zones
30-Foot Rule
Trees have grown tall enough to render the wind turbine useless.
Wind turbine siting
Wind rose

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.

Comments (2)

Tommy Taylor's picture

Mick: I have an installation that I'm considering that you didn't address. I live on a bluff that's rises 160' in elevation at a 2/1 slope on three sides. The tower could be located right out on the end of the ridge. What's the design critera for this installation. My site is higher than anything within 1/2 mile...

Mick Sagrillo's picture

Tommy, great question, and the answer is specific to your site. It sounds like your site may have real potential. The rules for siting on a bluff can get rather involved, which is why this was not included in the article. First of all, you never want to get closer to the edge of the bluff than 25% of the height of the tower. Wind turbines don't do very well in updrafts, and that's a concern when you get too close to the edge of a bluff. This means that if you decide to install a 100' tower, you need to be back at least 25' from the edge of the bluff. Next, you need to consider the ground cover below the bluff ,as the surface friction will influence the airflow up and over the bluff. If the ground cover is dense trees--in other words, a very high alpha--25% of the height of the tower will do. For a 100' tower this means 25'. However, if the ground cover is very smooth--open water having a very low alpha--then you need to site the tower back 2.5x the height. For a 100' tower, this means 250'. This has to do with the amount of turbulence generated at the top of the bluff as the wind rolls up and over the edge of the bluff. The next obvious question is how tall of a tower do you need. If you're following the principles of "taller will always generate more electricity", then put up the tallest tower that the manufacturer offers, typically at least 100'. But if money is a consideration--and when is it not--and you wish to install a shorter tower, then you need to so some "experimenting". This is going to involve a kite and maybe the neighborhood kids, as it's fun. Stand at the tower site as determined above, and get the kite flying as best you can. As the kite gets off the ground, back up towards the direction of the wind at the edge of the bluff so as to keep the kite above the tower location as best you can. Kites don't like turbulence, and they zig and zag around a lot to show their displeasure. But once they break above the zone of turbulence and get into the laminar flow of air--where you want the wind turbine to be--they get boring because they just fly. That's the minimum height--where they stop zigging and just fly--that you need for your tower height. This experiment is going to vary with the wind speed, but you can't continuously readjust your tower height. So you need to pick a wind speed that occurs most of the time at your site to optimize your energy production. Oh, and the kids? They the ones that will chase the kite around and pick it up so you can get it flying again. Let us know how you "site assessment" turns out.--Mick

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