Session 16 Exportation for wind sites

TOTAL TIME: 2 Hours

OBJECTIVES: To learn how to visualize the flow of the wind

To locate the good windsites on or around the training area

To learn about wind movement characteristics

MATERIALS: Bubble materials (soap, wire hoop & water container)

Trainer Note

Bubble materials are the best way to see the small scale variations of the wind flow in an area. Make bubbles using flow of wind if velocities are sufficient or by moving wire hoop through air. Track the bubbles noting their path and movement.

PROCEDURES:

Step l: 2 Hours

This is an outdoor session. Walk around the area and investigate the wind using bubbles to show the wind flow in various areas.

Notice areas of stronger wind and quiet low velocity areas.

Notice areas of turbulence and areas of smooth flow.

Discuss the causes of the differences, noting that the wind has the same kinds of motion that water has (eddies, backflows, rough and smooth turbulent areas, etc.)

Discuss the use of vegetation as an aid to judging average wind speed, pointing out examples.

Explain the local peculiarities of wind flow-flow over ridges, flow in valleys, day and night winds. Explain solar heating and the resulting connection and downslope cooling connection.

Note some sites to avoid (side of the hill, just beyond the ridge line, near trees or buildings.

Discuss and select the best local site for a windmill.

Trainer Note

Distribute and discuss handout before going outdoors.

RESOURCES: Copies of Attachment 16-A

Attachment 16-A

ECOLOGICAL INDICATORS OF SITE SUITABILITY

Vegetation deformed by high average winds can be used both to estimate the average speed (thus power) and to compare candidate sires. This technique works best in three regions: (1) along coasts, (2) in river valleys and gorges exhibiting strong channeling of the wind, and (3) in mountainous terrain. Ecological indicators are especially useful in remote mountainous terrain not only because there are little wind data, but also because the winds are often highly variable over small areas and difficult to characterize. The most easily observed deformities of trees (illustrated in Figure 1) are listed and defined below:

* BRUSHING- Branches and twigs bend downwind like the hair of a pelt that has been brushed in one direction only., This deformity can be observed in deciduous trees after their leaves have fallen. It is the most sensitive indicator of light winds.
* FLAGGING- Branches stream downwind, and the upwind branches are short or have been stripped away.
* THROWING - A tree is wind thrown when the main trunk and the branches bend away from the prevailing wind.
* CARPETING- This deformity occurs because the winds are so strong that every twig reaching more than several inches above the ground is killed, allowing the carpet to extend far downward.

Figure 1 is one of the best guides to ranking tree deformities by wind speed. Both a top view and a side view of the tree are shown to demonstrate the brushing of individual twigs and branches and the shape of the tree trunk and crown. The figure uses the Griggs-Putnam classification of tree deformities described by indices from 0 to VII. When WECS sites are ranked by this scheme, only like species of trees should be compared, because different types of trees may not be deformed to the same degree.

Another good indicator of relative wind speeds is the deformation ratio (Hewson, Wade and Baker, 1977). It also measures how much the tree crown has been flagged and thrown. Figure 2 shows the tree angles, A, B. and C, that must be measured to compute the deformation ratio "D". To measure these angles, the trees can either be photographed or sketched to scale. (The user might sketch the tree on clear acetate while he looks at it through the acetate.)

He should draw or take the tree pictures while viewing the tree perpendicular to the prevailing wind direction so that he can see the full effects of nagging and throwing.

To compute D, the three angles shown in the figure (A on the downwind side, B on the upwind side, and C, the angle of deflection) should be measured in degrees using a protractor. The larger the value of D, the stronger the average wind speed.

Mean annual wind speed is correlated with the Griggs-Putnam Index (Figure 1) in Table 1, and with the deformation ratio (Figure 2) in Table 2. These reflect only preliminary research results based on studies of two species of conifers, the Douglas Fir and the Ponderosa Pine. Further studies are examining these and other tree species to improve predictions of mean annual winds with ecological indicators. However, these tables do agree well with similar research conducted by Griggs and Putnam on Balsam and Fir trees in the Northeast (Frost and Nowak, 1977).

Estimates of mean annual wind speed using vegetation can be improved if several trees in a siting area are sampled, using the Griggs-Putnam Index and the deformation ratio. The results of all the sampling should then be averaged. However,, ecological indicators should be used primarily to identify possible high wind areas, to locate candidate sites, and to establish roughly the annual average wind speed. Selection of a WECS should not be based solely on ecological indicators; WECS economics and performance analysis should include either a wind measurement program or available wind data in addition to ecological indicators.

Though the presence of one type of deformity (or a combination) may indicate an area of high average winds and the degree of deformity may give estimates of the relative strengths of the winds, there are still pitfalls to rating sites according to tree deformity. Because past or present growing conditions can greatly affect the size and shape of trees, only isolated trees appearing to have grown under similar conditions should be compared. For example, a tree in or near a dense stand of timber should not be compared to an isolated tree. In addition, trees being compared should be of nearly the same height (preferably 30 ft. or more). Another fact to be aware of is: limbs are stripped from trees not only by strong nagging. They can be damaged by man, disease, other trees that once grew nearby, or possibly ice storms. Misinterpreting such signs could lead to the wrong assumptions about the prevailing wind direction and the average speed. Common sense, however, should reveal whether or not all the deformities observed in an area fit together into a consistent pattern.

Attachment 16-A

TABLE 1

MEAN ANNUAL WIND SPEED VERSUS THE GRIGGS-PUTTNAM INDEX*

Griggs Putnam Index (as in Figure 1)	I	II	III	IV	V	VI
Probable Mean Annual Wind Speed Range (mph)	7-9	9-11	11-13	13-16	15-18	16-21

TABLE 2

MEAN ANNUAL WIND SPEED VERSUS THE DEFORMATION RATIO*

Deformation Ratio (as in Figure 2)	I	II	III	IV	V	VI
Probable Mean Annual Wind Speed Range (mph)	5-9	8-11	10-13	12-16	14-18	15-21

* These data were prepared by E. W. Hewson, J. E. Wade, and R. W. Baker of Oregon State University.

The following guidelines summarize this section and suggest how to use ecological indicators effectively:

1. direct ecological indicators of strong wind;

2. compare isolated trees of the same species and height within the strong wind areas to select candidate sites;

3. consider flow patterns over barriers, terrain features, and surface roughness in the final selection;

4. measure the wind in complex terrain to ensure that a suitable site is selected; and

5. base selection of a particular WECS and any detailed estimation of its power output on wind measurements, not on ecological indicators alone.

Figure 1: Wind Speed Rating Scale Based on the Shape of the Crown and Degree Twigs, Branches, and Trunk are Bent (Griggs-Putnam Index; Hewson, Wade and Baker, 1977)

Figure 2: Deformation Ratio Computed as a Measure of the Degree of Flagging and Throwing (Hewson, Wade, and Baker, 1977)