Jed Rothwell wrote:
OrionWorks wrote:
CNN.COM Article on future flying wind farms.
http://www.cnn.com/2007/TECH/science/08/31/sky.turbines/index.html
http://tinyurl.com/2lqyyr
This describes the SkyWindPower device that we have discussed here
before. I think this is more practical and promising than the LadderMill
approach for the following reasons:
* The device stays at a high altitude, rather than continuously climbing
and descending.
* The power is transmitted to the ground electrically rather than
mechanically.
* The tether can be lighter -- I think. Most of the mechanical energy of
the wind converts to electricity.
We can put a lower bound on tether strength. Amusingly, the force
needed is actually smaller in a higher wind: the maximum power generated
is the pull on the cable TIMES the wind speed. That product is the rate
at which the kite would "do work" on the air if its mechanical coupling
to the air were perfect.
If 1 HP is 555 ft-pounds/sec and 1 hp is 745.2 watts, then a watt is
0.738 foot-pounds/sec. A million watts, then, are 738,000 ft-lb/sec.
If the wind at altitude is blowing at 50 MPH, or 73 feet/sec, then to
generate 738,000 ft-lb/sec we need a pull of about 10,000 pounds.
To generate 20 MW we need 20 times that, or a pull of about 2 million
pounds.
I don't know -- that sounds like a pretty fierce level of tension to me.
And that's just a lower bound -- in practice, coupling to the air is
less than perfect, alternator mechanical efficiency is less than
perfect, and the pull isn't horizontal so we also need to divide by
cos(theta) where theta is the "downward tilt" of the cable where it
connects to the kite. (But on the other side, maybe they're looking at
faster (stronger) winds than 50 MPH -- again, faster winds mean you can
generate the same power with less tension on the cable.)
According to Wikipedia the yield strength of Kevlar is 3620 MPa, which,
if I understand how to convert this into pounds, means a 2 million pound
test Kevlar cable would have a cross sectional area of about 4 square
inches. That's a pretty heavy cable; for a high kite it means the
catenary is going to be pretty steep at each end, which means the angle
of pull is going to be pretty steep, so that factor of 1/cos(theta) due
to the "tilt" of the cable may be substantial.
* The device can power itself up to altitude, and adjust its position as
needed, by reversing the flow of electricity to the turbine propellers
instead of from them. So even if there are no low-altitude winds on the
day you launch it, it can reach high altitude.
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.
- Jed
