Wind Matters: Page 2 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

Wind Quality Matters

You don’t necessarily need strong winds to get useful energy production from your wind-electric system. You might only need marginally stronger winds. So, how can you access those stronger winds? Time for another analogy.

Imagine floating down a river in a canoe, but rather than sitting back and enjoying yourself, you pay very close attention to what’s happening on the water. The first thing you notice when you put your canoe in is that there is slow flow near the bank. As you paddle out into the river, however, flow picks up somewhat. By the time you get to the center of the river, the farthest you can be from either bank, you notice that the flow is fastest.

Near the bank of the river, the flowing water is slowed by its interaction with the riverbank’s friction. Like water, air is a fluid. And just like water, its flow is inhibited by its contact with what’s on the ground: vegetation, land forms, and buildings. The farther from these sources of friction, the faster the wind.

The illustration above shows a wind profile, the length of the arrows representing wind speed—longer arrows mean higher speeds. As you move away from the surface of the earth and its ground cover, wind speed increases. To access stronger winds, you need to reduce friction by getting your wind turbine rotor higher up in the wind profile.

Different locations will have different wind profiles depending on the amount of friction presented to the air mass (see graph in sidebar). The friction that ground cover poses is known as ground drag. Different ground covers are akin to different grades of sandpaper. Smooth ground cover—such as a hay field—doesn’t present much drag to a moving air mass, whereas densely scattered trees and buildings present a lot of drag. The rougher the ground cover, the greater the drag and the more the air mass is slowed.

Turbulence is Trouble

Wind turbine blades are airfoils, similar to airplane wings. Both operate on the same principle of lift— the force that allows planes to fly, and wind turbine blades to rotate to extract energy out of the wind. Airfoils need laminar airflow­—constant and smooth flowing winds—over them to maximize the lift they can generate—which will maximize the kinetic energy they are able to extract from the wind.

Turbulence, which is caused when the wind tumbles over obstacles (trees and buildings, for example), is chaotic airflow. The greater the ground drag due to taller or more obstacles, the more turbulence that is created. Wind breaks, farmyard wind barriers, and snow fencing are often used to create turbulence to disrupt strong winds.

Turbulence changes laminar airflow into a chaotic, tumbling, churning mess. This wreaks havoc on lift devices that depend on laminar winds. Remember the last time you flew in an aircraft that hit a pocket of turbulence? The plane was tossed about and lost altitude because of decreased lift on the airfoils (wings). The same thing happens with a wind turbine: It is buffeted by turbulence as is evidenced by the way it changes direction, trying to follow the chaotic wind, and spins erratically without generating much electricity, since there is little usable energy in turbulent winds. Unfortunately, the “bubble” of turbulence around a house or on a farm can be of considerable distance, height, and width.

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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