I mentioned yesterday that while the data I've been collecting so far is at best imprecise. What I didn't mention is that climatologists and weather industry folks have put together a formula to estimate wind speeds at higher altitudes based on measurements as lower altitudes. It's not perfect, of course, but it's an interesting guide.
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I'd read about it in a recent issue of Home Power magazine (which I highly recommend for anybody who has to live off-grid), so I dug up the issue and ran some numbers. The results are somewhat interesting (at least to those with a geekier bent).
The formula itself is fairly straightforward:
V2/V1 = (H2/H1)a
where V1 is your measured speed at height H1, height H2 is the height your prospective turbine will be located at, and a is a wind shear factor based on the terrain surrounding the turbine. It runs from 0.1 (for perfectly smooth surroundings like calm water) to 0.6 (surrounded by tall trees or buildings). I decided to choose 0.3 since it's in the middle.
With a 10.1 mph measured average taken at 6 feet, and a desired tower of 60 feet, we can crunch the numbers and get roughly 20 mph. If I kick the height up to 80 feet, the calculated speed bumps up to 22 mph; at 100 feet it reaches 24 mph.
In other words, height may not help much during the "summer doldrums". I'll know more after I'm able to compare the measured value at 30 feet (where the anemometer will eventually be) against the calculated value (16.3 mph). I'll also be very interested to see what the numbers are once we get outside the summer doldrums.
Ain't this kind of stuff fun?