The Roughness Rose
If we have measured the wind speed exactly at hub height over a long period at the exact spot where a wind turbine will be standing we can make very exact predictions of energy production. Usually, however, we have to recalculate wind measurements made somewhere else in the area. In practice, that can be done with great accuracy, except in cases with very complex terrain (i.e. very hilly, uneven terrain).
Just like we use a wind rose to map the amount of wind energy coming from different directions, we use a roughness rose to describe the roughness of the terrain in different directions from a prospective wind turbine site.
Normally, the compass is divided into 12 sectors of 30 degrees each, like in the picture to the left, but other divisions are possible. In any case, they should match our wind rose, of course.
For each sector we make an estimate of the roughness of the terrain, using the definitions from the Reference Manual section. In principle, we could then use the wind speed calculator on the previous page to estimate for each sector how the average wind speed is changed by the different roughness of the terrain.
Averaging Roughness in Each Sector
In most cases, however, the roughness will not fall neatly into any of the roughness classes, so we'll have to do a bit of averaging. We have to be very concerned with the roughness in the prevailing wind directions. In those directions we look at a map to measure how far away we have unchanged roughness.
Photograph Soren Krohn, © 1999 DWIA
Accounting for Roughness Changes Within Each Sector
Let us imagine that we have a sea or lake surface in the western sector (i.e. roughness class 0) some 400 m from the turbine site, and 2 kilometres away we have a forested island. If west is an important wind direction, we will definitely have to account for the change in roughness class from 1 to 0 to 3.
This requires more advanced models and software than what we have shown on this web site. It is also useful to be able to use the software to manage all our wind and turbine data, so at a future update of this site we'll explain how professional wind calculation software works.
Meanwhile, you may look at the Links page to find the link to Risoe's WAsP model and Energy & Environmental Data's WindPro Windows-based software.
Accounting for Wind Obstacles
It is extremely important to account for local wind obstacles in the prevailing wind direction near the turbine (closer than 700 m or so), if one wants to make accurate predictions about energy output. We return to that subject after a couple of pages.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 8 June 2003