Energy and Human Ambitions on a Finite Planet, 2021a

12 Wind <strong>Energy</strong> 186
patterns over the course of 24 hours: a full day of the driving solar input
around the globe. The associated power works out to 700 TW. Notice
that the value for wind in Table 10.2 (p. 168) is pretty-darned close to this,
at 900 TW. 6 As the margin note indicates, we should be pleased to get
within a factor of two for so little work <strong>and</strong> very off-the-cuff assumptions
about global average air speed (see Box 12.1 for related thoughts). Figure
12.2 shows the annual average wind velocity at a height of 80 m (typical
wind turbine height) for the U.S. Note that the 5 m/s we used above
falls comfortably within the 4–8 m/s range seen in the map.
See if you can confirm; own it
yourself!
6: In the first draft of this textbook, a different
data source was used for Table 10.2
that had wind at 370 TW. Even so, the
700 TW estimate corroborated the orderof-magnitude
scale <strong>and</strong> was deemed a satisfactory
check: within a factor of two.
United States - Annual Average Wind Speed at 80 m
Wind Speed
m/s
>10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
< 4.0
Source: Wind resource estimates developed by AWS Truepower,
LLC for windNavigator . Web: http://www.windnavigator.com |
http://www.awstruepower.com. Spatial resolution of wind resource
data: 2.5 km. Projection: Albers Equal Area WGS84.
01-APR-2011 2.1.1
Figure 12.2: Average wind velocity at a
height of 80 m across the U.S. [69]. Boundaries
between colored boxes are every
0.5 m/s from 4.0 m/s to 10.0 m/s. Nothing
on this map exceeds 9 m/s, <strong>and</strong> the deepest
green is below 4 m/s. The plains states are
the hot ticket. Note that Alaska is not to
scale. From NREL.
Box 12.1: The Value of Estimation/Checking
Calculations like the one above offer a way to see if something at
least checks out <strong>and</strong> seems plausible. If we had found that the whole
atmosphere would have to be moving at 50 m/s to get the 900 TW
figure in Table 10.2 (p. 168), we would suspect a problem, <strong>and</strong> either
distrust the 900 TW number—seeking another source to confirm—or
re-evaluate our own underst<strong>and</strong>ing. If we could get to 900 TW by
only having wind speeds of 0.1 m/s, 7 we would also have cause for
skepticism. When crude estimates of this type l<strong>and</strong> in the general
vicinity 8 of a number we see in a table, we can at least be assured that
the number is not outl<strong>and</strong>ish, <strong>and</strong> that our basic underst<strong>and</strong>ing of
the phenomenon is okay. Checking underst<strong>and</strong>ing against presented
data is excellent practice!
7: ...orrequiring weeks rather than a day
to re-establish, once sapped
8: ...say,withinafactoroften
But we can’t capture the entire atmospheric wind, because doing so
would require wind turbines throughout the volume, up to 10 km high!
In fact, some estimates [70] of practical global wind installations come in
as low as 1 TW—well below our 18 TW dem<strong>and</strong>. Wind alone is unlikely
[70]: Castro et al. (2011), “Global Wind Power
Potential: Physical <strong>and</strong> Technological Limits”
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.