Thursday, July 15, 2010
I noticed we hadn't published any pictures in a while, and the blog was a lot of text.
Here's our most recent anemometry project: We placed a RainWise model anemometer (I call them "disposable anemometers" because they're so much cheaper than our expensive NRG ones, although not as capable) on a State o' Maine-owned communications tower in support of Maine wind mapping and to begin to satisfy the Town of Fayette's interest in knowing more about its wind resource.
The results, available in a year, after two more exciting trips up this big tower, will begin to test a hypothesis I have about Maine wind shear factors.
This is good college level physics and math. For the geeks among you, let me elaborate, at risk of becoming boring again:
The wind shear factor is the exponent in the Power Law equation. Power law models are essentially predictive analytical models used to predict a range of phenomena, among them how much stronger the wind blows as you get away from the ground. The variables involved in this are more or less obvious: how flat the terrain is, how tall and "rough" the ground cover vegetation is, how many buildings, etc. Any variable that might slow down the wind is subsumed in what my statistician professors a "kitchen sink" term called the "wind shear factor."
Kitchen sinks are combined variables that you put a lot of other, possibly hard to measure, variables in to. Here we subsume buildings, trees, flatness, and so on, in one number, generally a decimal fraction from 0.1 to 0.9. The wind shear factor is thus defined a priori as an exponential variable of the order of a fraction of the number 1, which is however still an exponent, "geometric" change, not "arithmetical", as Malthus would have said. Non-linear, we would say today. This makes sense because the further away you get from all those kitchen-sink wind-slowing variables, the faster and faster the wind is going to blow. Up to a point, and then it of course it won't make any difference.
I would like to mention here that much of my working life is about exponential change, so wind shear factors have much in common with population growth models, oil depletion models, climate tipping points and on and on.
I'm never happier except when I'm rummaging in a data set and I realize that there's an exponent, because exponents make life exciting.
Sometimes too much so.
All this sounds kind of assumption-ridden, but I can assure you many MS and PhD theses were devoted to testing the wind shear model over the decades that it's been known, and so the assumptions have been tested too.
The more interesting question is "what difference does this make for wind power in Maine?"
My answer to that is, "just about everything."
Our Maine wind map as currently configured uses a standard wind shear factor typically used for forested ground of 0.30. But in practice, because engineers and analysts do wind power planning, not scientists, this number is assumed, not experimentally derived. I never assume anything where numbers are concerned. That might be what makes me more of a scientist and less of an engineer or analyst.
When I and other science researchers measure the real wind shear factors in Maine and indeed all over New England, we often get much higher ones. The Massachusetts Renewable Energy Lab has reported a wind shear factor of 0.57 on one particular coastal hilltop. I've measured real wind shear factors as low as 0.42 and as high as 0.90. The latter is a bit of an outlier, to be sure -- a site where my anemometer was affected by buildings. But the first number, which my hypothesis posits is more like the average, was on a site that most anemometrists would naturally expect to have a wind shear number of about 0.30.
Since this term, the wind shear factor, is an exponent, it defines the behavior of the wind as exponential. The higher the number, the faster the wind speed climbs as you climb up the air column from ground level. The faster the wind speed increases, the more likely it is that you will have stronger wind for a turbine at turbine hub height, and the more likely that any turbine placed on the site makes loud noise at ground level.
This last is because high wind shear factors result in high wind at turbine hub height and low wind at ground level. Turbines appear noisier if there's no wind noise in trees and other vegetation to drown out mechanical noise. Of course, they're not actually noisier -- they just appear so.
So the higher the wind shear factor, the more potential for Maine wind power, but the more potential for noise. A two-edged sword, isn't it?
But you would think that the good people of Maine would want their wind power planners, commercial or government, to know what the number really was before planning turbines and wind farms, wouldn't you.
Even our anti-wind activists would want to know, you would think.
Watch this space and I'll let you know for sure, in a year or three.
There. That was interesting, I thought. But I'm a geek, so what would I know?