By James R. Stalker

The atmosphere made analogous to a spinning bucket

Visualize a bucket filled with water half the way to facilitate mixing, covered with a lid. Now, imagine this bucket spin out of control, jump, wobble, momentarily stop, and randomly resume any of these actions. Begin drawing a drop of water from a location within this bucket and measure its properties such as its speed. Put the water drop back and wait for ten minutes and draw another drop from the same exact location as before and measure its speed. Keep repeating this measurement effort for a while. Now, use these measurements to characterize the chaotic behavior of the entire fluid (water) in the bucket.

Wind measurement limitations explained

How successful will you be in reaching your objective? What is wrong with this approach? Can we use properties of water drops at a location to characterize the entire fluid? You don’t have to be an accomplished expert on fluid dynamics to clearly understand the limitations of such measurements.
 

Let’s apply this analogy to a real world situation. The water bucket here is the atmosphere and the bucket has hundreds of kilometers in diameter and ten’s of kilometers deep. The fluid here is the air, along with some water that changes into gas, liquid, and solid forms. Similar to measurements of water drops, wind energy developers, using conventional measurement platforms, are using wind measurements at a single location or two, to characterize the entire fluid behavior. It is the author’s hope that even non-technical folks (e.g., most financiers that bear the brunt of the risk resulting from such measurement limitations) can fathom this conceptual simplicity of very complex fluid behavior.

Limited wind measurements for secondary benefits (e.g., model validation) at best

Let’s dissect this analogy a bit further. Let’s assume a measurement platform that measures wind data with 100% accuracy at that location. One hundred per cent accuracy is a generous assumption indeed. Since the underlying causes (“wind forces”–read Dr. Stalker’s article for further details) are unknown, extrapolation methods to characterize wind behavior at other locations and heights (i.e., “within the spinning bucket”) can produce much larger errors as there are no a priori wind measurements at these other locations. On the other hand, if robust fluid models that are capable of capturing the effects of the above mentioned chaotic moves on the water drop’s speed and providing quantitative information with 90+% accuracy at hundreds of locations and heights, such models would deliver a much more accurate picture of the fluid behavior. This complex comparison of error statistics, between measurements and modeled wind speed, is illustrated in the following Table:

Note: Percentages shown in the above Table are accuracy rates of wind speed measurements and modeled wind speed values. 

So, financial risk takers, if you can get only one type of measurements and one type only, they ought to come from robust fluid models. If you can afford a second type of measurements, then ask for site wind measurements–as many as you can afford from the standpoints of budget and time. From the perspective of the complex fluid behavior of the atmosphere, the current approaches of single location wind measurements can only seem analogous to “groping” in the dark since such approaches represent only a secondary step at best and clearly can not represent a primary step in wind energy assessment efforts as concenptually shown by the spinning bucket analogy.

A critical take away from this article should be to “not put the cart in front of the horse.”

In the interest of full disclosure, and for ensuring technical soundness of the topic covered in this article, Dr. James Stalker would like to state that he owns Wind Forces, a Divsion of RESPR, that provides wind energy assessment services globally.  Dr. Stalker had over a decade of atmospheric computational fluid dynamics (CFD) research experience, without any commercial interest in wind energy, prior to founding RESPR/Wind Forces.