On Sep 4, 2007, at 9:08 AM, Jed Rothwell wrote:
http://tinyurl.com/2lqyyr
The authors estimate that it would take 43 groups of 600 of these
"FEGs" to power the U.S. 1 FEG produces 20 MW, so that's 12 GW per
group and 516 GW total. That's about right. I do not think 12 GW
groups would be cost effective for many rural locations. For
example, the whole of North Dakota has only 4.8 GW of peak electric
generator capacity. See:
http://www.eia.doe.gov/cneaf/electricity/st_profiles/north_dakota.html
I expect you could put all 600 groups in North Dakota with room to
spare, and they would operate at peak efficiency, but they do not
need 516 GW up there. Like most super-large scale wind power grids,
this would work best with HTSC power transmission.
It probably isn't necessary to locate in North Dakota. Also the
article implies an altitude of 15,000 ft is necessary: "But how do we
get a working turbine up to the necessary height -- at least 15,000
ft (4600 meters) above the earth's surface? That's where helicopter
technology comes in." It doesn't seem likely that altitude is
necessary either. There is a diminishing return for higher altitudes.
http://en.wikipedia.org/wiki/Wind_power states
“The wind blows faster at higher altitudes because of the reduced
influence of drag of the surface (sea or land) and the reduced
viscosity of the air. The variation in velocity with altitude, called
wind shear, is most dramatic near the surface. Typically, the
variation follows the 1/7th power law, which predicts that wind speed
rises proportionally to the seventh root of altitude. Doubling the
altitude of a turbine, then, increases the expected wind speeds by
10% and the expected power by 34%.”
The power from wind is proportional to the cube of the velocity, so
the power increases with the 3/7 power of altitude. At 15,000 ft the
power is only 60 percent more than at 5000 ft. The majority of that
altitude benefit can be obtained by building wind walls on high
rugged mountain tops, which concentrate wind over their ridges. The
power cable, a major weight problem, is more than 3 times heavier at
15,000 ft than 5,000 ft. A major weight problem is associated with
protecting the power cable from lightning strikes, which would be
extremely frequent to say the least.
A non-economic wind power class 2 location at an altitude of 50 m has
average wind speed of 5.6 m/s and power density of 200 W/m^2.
Applying the 1/7th power law, a 1 km tower in that location would
experience an average wind speed of (1000m/50m)^(1/7) *(5.6 m/s) =
1.53*(5.6 m/s) = 8.54m/s. This turns a useless wind class 2
location, like the coast of Georgia, into a wind class 6 location,
with 600 W/m^2 wind power density.
One problem is the fundamental fact that a drag proportional to the
square of the wind velocity is necessary to achieve the power
proportional to the cube of velocity. In any event, for a given
aerodynamic configuration, drag is roughly proportional to the square
of the velocity. At high altitudes fast feathering and getting out
of the sky fast to avoid tether breaking in high wind becomes an issue.
Horace Heffner
http://www.mtaonline.net/~hheffner/
