ASK THE EXPERTS: Bluff Wind Sites

Intermediate
Area of Wind Turbulence
Area of Wind Turbulence

I found Mick Sagrillo’s recent article (“Wind Matters” in HP158) very useful. However, I have an installation I’m considering that the article didn’t address. I live on a bluff that rises 160 feet in elevation at a 2:1 slope on three sides. I was considering locating the tower near the edge of the ridge. What are the design criteria for this type of installation? My site is higher than anything within a half-mile.

Tommy Taylor • via email

The rules for siting on a bluff can be complicated. The first rule is to never get closer to the bluff edge than 25% of the tower height, because wind turbines don’t do very well in bluff-edge turbulence from updrafts. This means a 100-foot tower needs to be located at least 25 feet from the bluff edge.

The significant area of turbulence at the leading edge of the bluff top will disrupt your wind profile. The extent of this turbulent area depends on the ground cover below the bluff, which shapes the wind profile as it approaches. If the ground cover is dense trees, for example, then 25% of the height of the tower will do as a setback.

However, if the ground cover is very smooth—like open water—then site the base of the tower back from the bluff edge at a minimum of 2.5 times the tower height. For a 100-foot tower, that’s 250 feet from the edge.

The next question is how tall the tower should be. 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 feet. You might get by with a shorter tower, but you’ll need to “experiment.” Stand at the tower site, and get a kite flying as well as you can. As the kite gets off the ground, back up toward the direction of the wind at the bluff edge to keep the kite above the tower location.

In turbulence, kites will zig and zag. But once they break above the zone of turbulence, they will be in the laminar flow of air—where you want the wind turbine to be. That’s your minimum tower height, and you’ll need to estimate this height since there’s no easy way of measuring it. This experiment is going to vary with the wind speed. But since you can’t continuously readjust your tower height, you need to pick a wind speed that occurs most of the time at your site, and that will optimize your turbine’s energy production.

Mick Sagrillo • Forestville, Wisconsin

Comments (2)

Ruth Zweidinger's picture

Or your can calculate the kites height by using a ruler at arms length to sight the kite while holding your arm out full length while sighting the bottom of the ruler on the ground at the tower site. Measure the distance to the tower site and the length of your arm and you can calculate the tower height by similar trangles. Just like estimating tree heights.

The problem with using the string length is the string curves so your angle measurement is off.

Mathew Elder's picture

" But once they break above the zone of turbulence, they will be in the laminar flow of air—where you want the wind turbine to be. That’s your minimum tower height, and you’ll need to estimate this height since there’s no easy way of measuring it. "

One way to accomplish this measurement would be to mark the kite string. Use a black marker to colour a 1 inch section of line say every twenty feet. Count the number of markers as the line pays out and you know how much kite string is out. That could be your base tower height. For further accuracy measure the approximate angle of the string with an angle finder. Then it's back to high school geometry for an actual height. Just a thought.

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