Part 1: Average Wind Speed
Mick Sagrillo

You’ve decided that it’s time to consider a wind electric system. It sure seems windy in your area,
at least part of the time anyway. And you could really use the power! But how can you tell if a wind generator will really do well at your site? How do you know if you have enough wind, and where the wind is? And if it turns out that you do have a good resource, where’s the best place to put the tower?

Tools
The common response to the above questions is that you must monitor the wind speed at your site for a year or two to find out if a wind generator will work for you. For a residential-sized system, this is nonsense! PV installers don’t record available sunlight forever, so why do it for wind? Actually, PV installers have a couple of tools at hand to help them site PV arrays. We can do
the same for wind generators. In order to correctly site and size a wind electric system, it is helpful to have the following information about your location:
• average annual wind speed
• prevailing wind directions
• vegetation and buildings at the site
• surface roughness
• the type of energy storage
What we need to do is quantify your resource in general terms first. Once that is done, we can fine tune your resource for your specific site. So, let’s analyze each of these tools and see how to use them.
Options
Because it is so easy to address, let’s consider the type of energy storage first. If utility electricity is available to you, by all means, consider a utility tie-in system. By using the grid, you eliminate the need for a very expensive and time consuming component: the batteries. In a grid intertie system, the utility stores your electricity in the form of a credit whenever the wind is blowing. When the wind is still, you reclaim that credited power. In this case, your primary concern is producing enough electricity over extended periods of time to offset your consumption of utility-generated
power. Daily or weekly production is irrelevant. If yours is a remote system utilizing batteries rather
than the utility grid, then you can actually do with less wind than the minimum cost-effective grid intertie system. Any renewable source of energy is more cost effective than running, maintaining, and fueling a gas or diesel generator for several decades.
In the case of a utility intertie system, a ten mph
average annual wind speed is usually considered the cut-off. Below ten mph, the wind generator cannot be justified on a purely economic basis compared to purchased utility power. With a stand-alone system, wind generators are certainly cost effective in the nine
mph and even the eight mph average wind speed ranges. These numbers refer to wind speeds at the
height of the blades, referred to as hub height. The big question is, how do you get these numbers for yoursite?
You What???
Determining your exact average annual wind speed is difficult, expensive, and time-consuming. But, Mick, you say, you just told us that monitoring the wind for a year or so was nonsense. What gives? While it is necessary to have an idea of your average annual wind speed, you do not need an exact
measurement. We’re not prospecting for a wind farm here. Therefore, monitoring wind speed at hub height for several years is unnecessary. So how do we get this elusive number called average
wind speed. We guess. Don’t laugh! It’s done all of the time, and fairly accurately, I might add. What we need to do is get a ballpark idea of the average annual wind speed for your area. I’ll explain how it was done at my site, and we can work backwards from there.
Conventional Wisdom
When we moved to our homestead in Wisconsin, we investigated the use of a wind generator because it always “seemed” so windy. Conventional wisdom said to check the airport and weather bureau data for our area. We found that the average annual wind speed at the Green Bay airport, 35 miles away, was barely nine mph, hardly worth considering due to the cost of electricity we were buying from the utility. After replanting the same peas in our first garden fourtimes (they kept blowing out of the ground), we asked the neighbors what they thought

Let’s be daring and assume that the recording station knows the whereabouts of their anemometer, and that it is on the standard 30 foot tower it’s supposed to be on. You visit their location and find that the anemometer is in a very well-exposed site, far from the influence of any trees or buildings. The recording station tells you that the average wind speed at the site for a 35 year period of time is 9.0 mph. What does this translate to at your site? The first thing you do is estimate the similarity of
your site to theirs and assign your site a surfacefriction coefficient from Table 1.
Assuming that their site is wide open and your siteis like ours, a farm with the usual buildings and trees plus a nearby wooded area, you might choose a surface friction coefficient of 0.24. We
have a 100 foot tower that we would like to use to mount the wind generator. At 100 feet, we will easily pass the “30 feet above anything within 500 feet” rule
What kind of average wind speed can we“flagging”. Flagging is usually more pronounced for
single, isolated trees with some height. The upper part of the tree is swept back and away from the solar shower next to it. On the upwind side of the tree, the branches are noticeably stunted. On the downwind side, they’re long and horizontal. The flagging was caused by persistent winds from, more or less, one direction. Ecologists have been using flagging in trees for decades to ballpark the average wind speed for an area. (Vegetables don’t lie!) Pick up any ecology book and you will find a diagram similar to the one bottom right on the page that quantifies an area’s wind speed as a function of tree deformity.
Note that you can get an idea of the average wind speed by the amount of flagging. Comparing the
picture to the diagrams, we could categorize my pine tree as Class III, an 11 to 13 mph average wind speed. The monitoring done at my site certainly bears this out.If you do not have any conifers in your area, look around at the deciduous trees. They may also show evidence of flagging. Look around especially for single trees, or trees on the outskirts of a grove. Unless they have grown considerably above the common tree line, trees in a forest will not show flagging because the collective body of trees tends to reduce the wind speed over the area. While the presence of flagging positively indicates a wind resource, you should not conclude that the absence of flagging in your area precludes any suitable average wind speeds. Other factors that you are not aware of
may be affecting the interaction of the wind with the trees.
Tools Revisited
So far, the tools that we have available to us for approximating our wind resource are:
• The various experiences of your area’s neighbors, especially the old timers
• Any flagging that might be observed in the surrounding areas
• Local wind generated problems (snow drifting, severe soil erosion, tumbleweeds)
• The average wind speed established by the nearest airport or weather bureau, which can
be used as our baseline
• The average wind speed for your site as determined by the shear factor
• Your gut feelings based on all of the above

Remember from “Tower Economics 101” that even small increases in wind speed can yield substantial increases in power to the wind generator because velocity is cubed in the power equation. This is especially true at lower average annual wind speeds, as depicted in the graph above.
The Bottom Line
So what is the bottom line number for an acceptable average wind speed? Because there seems to be such a disparity between monitoring stations (airports and weather bureaus) and actual wind generator installations, we can actually fudge a little here. For a utility intertie system with a good exposure comparedto the monitoring station site, an average wind speed of eight mph at the recording station or at ground level at your site is acceptable. Remember, our economic cutoff point (if you’re doing this to make money) is ten mph at hub height when we’re competing with cheap utility power.
For a battery charging wind system in which the monitoring station has nearly identical topographical exposure to your site, an annual average wind speed as low as seven mph at the recording station or at ground level at your site is acceptable. If the monitoring site is very sheltered compared to your site, this number could drop to about six mph. However, I wouldn’t consider any site with average wind speeds less than these.
Next whitepaper we’ll take a look at maximizing the
usefulness of your site.