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Publication Title | Tower Economics 102

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Tower

Economics 102

Mick Sagrillo

©1993 Mick Sagrillo Turbulence and ground drag are

the enemies of any wind

generator, robbing it of its precious fuel, the wind. As we move away from the surface of the earth, the effect of turbulence and ground drag are reduced. The higher up we go, the stronger the wind speed. The lesson is that tall towers are a must for wind generators. But how high is high enough? What is cost effective?

Recap

In the last issue of Home Power (HP#37), we examined why turbulence and ground drag are detrimental to the production of wind generated electricity. We looked at the relationship of height above ground and wind speed, and how the power available to a wind generator increases with wind speed. Finally, we analyzed the economics of a simple wind installation at two different tower heights.

The conclusion we reached is that a taller tower is always cost effective. Now, being essentially terrestrial animals, most folks don’t like to hear this. After all, if we were meant to service wind generators on 100 foot towers, we would have been given 95 foot legs!

Try to put your fear of heights aside for a little while as we examine the economics of increasing tower height cost versus additional power output.

Shear Factor

How much wind speed increases with height is fairly well documented. The major variable of wind speed versus height is the terrain over which the wind is blowing. Land forms, vegetation, and buildings all impact the speed of the wind in their own way.

The increase in wind speed with height over various ground obstacles is known as “wind shear”. Wind shear is less pronounced over relatively flat open ground and

Percent Increase of Wind Speed with Height

160

140

120

100

80 60 40 20

Over Open Water

Over Smooth Ground

Over Row Crops & Hedges

Over Trees & Buildings

Wind

0

0 10 20 30 40 50 60

Percent Increase (over baseline of 30 feet)

Chart 1

considerably greater over hilly terrain with many buildings and trees. Wind shear can be represented as a percentage of increased wind speed as height increases over the ground or over vegetation and buildings. The above chart depicts wind shear for us.

Thirty feet is the baseline used to determine wind shear at higher levels. It is a typical adjusted height for measuring wind speed at weather bureaus and airports.

Percent Increase of Wind Power with Height

160

140

120

100

80 60 40 20

0

0 50 100 150 200 250 300

Percent Increase (over baseline of 30 feet)

Chart 2

Over Open Water

Over Smooth Ground

Over Row Crops & Hedges

Over Trees & Buildings

Home Power #38 • December 1993 / January 1994 27

Height (feet)

Height (feet)

Image | Tower Economics 102



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