Flagging as an Indication of Wind Speed - Selecting a site for your Wind Turbine

by: Sara on 06/05/2017

Today's post is an excerpt from Power from the Wind: A Practical Guide to Small-scale Energy Production - Revised 2nd Edition by Dan Chiras. In it he describes flagging and the Griggs-Putnam Index of Deformity and there use in wind site assessment. 

Mike Bergey states that a stand-alone (off-grid) wind energy system makes sense if you’re in an area with an annual average wind speed of eight miles per hour (3.6 meters per second) or higher at hub height. It should, he contends, be economically feasible. Mick Sagrillo agrees, and says this is especially true for those whose off-grid systems incorporate PV. This makes a lot of sites in the United States candidates for off-grid homes powered by wind systems.

Remember, though, off-grid homes tend to use very little electrical energy — one-tenth to one-twelfth as much energy as a grid-tied home. Off-gridders dry clothes on clotheslines, cook on propane or natural gas, install only super-energy- efficient appliances and lighting, and work hard to save energy. They are not your typical US or Canadian energy guzzler.

4.7 Flagging

FIGURE 4.7. Flagging. This photo shows a windy site in the foothills of the Rockies in Colorado. The pine tree in the distance exhibits moderate flagging, indicating an average annual wind speed of 11 to 13 miles per hour. The pine tree in the middle exhibits complete flagging, indicating 13 to 16 mile-per- hour average annual windspeed. Both trees are on the same property, illustrating how wind can vary within a short distance. Credit: Dan Chiras.

While an 8-mile- per-hour average wind speed may suffice for an off-grid system,according to Bergey, grid-connected systems typically require a higher average annual wind speed of about 10 miles per hour (4.5 meters per second) to be economical.

Again, this speed is at the hub height of the turbine. I advise caution when viewing general recommendations such as these. With the higher cost of wind turbines these days, higher wind speeds might be necessary to make a wind system economical — perhaps 12 miles-per- hour (5.4 meters per second). All in all, it is best determine the wind speed at your site; determine which turbine you need to install to meet your needs; determine the cost of the system, including the tower; and run calculations to determine the economics of this proposed installation. I’ll show you some ways to do this at the end of this chapter.

Even if a state wind map or the NASA data suggests the presence of a good resource on a site, it is important — no, essential — that you or your site assessor study other factors such as the topography, vegetative cover, the proximity of the wind turbine to trees, hills, cliffs, and buildings, all of which could decrease — or, in some cases, increase — wind speeds.

You or your site assessor should also take into account local wind patterns like mountain-valley winds and offshore and onshore winds that might boost your wind system’s output. 

In some areas, vegetation provides an important clue as to how windy a site is. Figure 4.7 shows a tree in the windswept foothills of the Rockies. This tree exhibits a phenomenon known as flagging. What is flagging?

As readers know, a flag blowing in the wind blows to one side of the pole. Strong winds produce a similar but permanent visual effect in trees, especially coniferous trees; this is called flagging. When flagging occurs, the branches of a tree appear to stream downwind. Persistent strong winds do not cause the branches of a tree to bend around and “fly” on one side of the trunk. Rather, they damage or stunt the growth of upwind branches, so it appears as if the downwind branches are streaming downwind like a flag. In really strong winds, the upwind branches may be stripped away entirely.

Flagging can be used to approximate the average wind speed at some sites, as speed at some sites. This scale is known as the Griggs-Putnam Index.

According to this index, slight deformity, known as brushing and slight flagging, indicates a probable mean annual wind speed in

4.9

FIGURE 4.9. Blowing in the Wind. Strong, persistent winds can cause trees to bend over like this one at a roadside stop in western Kansas. Credit: Dan Chiras.

the range of seven to nine miles per hour. Slightly more flagging, cleverly referred to as slight flagging, indicates an approximate average annual wind speed of 9 to 11 miles per hour. Moderate flagging indicates an average annual wind speed of 11 to 13 miles per hour, and so on. On the extreme upper end, the trees are bent to the ground, a phenomenon called carpeting.

Flagging is typically observed in single trees — that is, a tree growing alone in a field isolated from other trees and obstacles. Trees in forests or groves are not prone to flagging except at their tops, because they shelter one another.

Flagging can be used to verify data from NASA or wind maps. If the wind map or NASA data indicates that a site has an average wind speed of 15 miles per hour and the trees exhibit complete flagging, you can feel confident that the data is correct.

Although flagging helps verify other data, it is important to note that the absence of flagging does not necessarily indicate a lack of wind. Some trees aren’t very susceptible to flagging. Most deciduous trees, for example, are less prone to flagging than coniferous trees. That said, if you observe carefully, you can find signs of heavy winds in the way deciduous trees grow. Take a look at Figure 4.9. It shows a tree along I-70 in western Kansas that is bent away from the wind thanks to the persistent southwesterly winds that sweep across the Plains.

Site assessors and homeowners can also look for other telltale signs of strong winds on a site. If neighbors or local governments use snow fences to prevent snow from drifting over driveways and roads, it’s a good indication of strong winter winds (Figure 4.10). Clouds of dust and dirty snow are signs of soil erosion by wind, a good sign for wind site assessors (a bad sign for farmers). The use of windbreaks, trees planted upwind from homes and farmsteads to protect them from winds, is another good sign that there’s wind to be reckoned with.

Tattered flags flying on flagpoles tell the same story. Telephone and electric poles that tilt at an angle may convey a similar message.

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