July 20, 2009
The following documents [see below*] are a pretty good summary of
where we got to in 2004. The main inventions were all made by our CTO,
Jon Howes, who was previously Head of Loads and Dynamics at the UK CAA
- consequently he knows quite a bit about aero structures, control, and
fatigue. The document that compares VAWT [Variable Area Wind Turbine]
vs a conventional turbine was the result of our most detailed
theoretical model and shows the key points. The main conclusion was that
the optimum cable length was about 9 times the glider width and you could
generate about 3 times the power of a fixed turbine blade - beyond this
cable length the cable drag overcame the benefits of more swept area. We
were assuming solid gliders (no flaps etc..) and steel cables as we had
concerns about the life of the more exotic materials like DyneemaŽ.
The key features we liked about this proposal was that the control of the
gliders flight path is totally automatic as any deviation away from the
main circular path is self correcting (see patent description for more
detail). This means no clever electronics or control systems to go wrong.
The power is generate by the models flying ahead of the arms on the
generator and effectively leading them by a certain angle - so fairly
simple all round. You control the length of the cable with torque, this
automatically meant that if the wind drops the gliders wind in and this
keeps generating lift so they fly in, likewise they automatically launch
when the wind speed is high enough to support them. The The gliders are
flying at about 1/3 speed of sound to avoid compressibility issues and
keep the torque right down. The loads are still pretty ferocious and we
were looking at 2inch steel cables for bigger versions.
The bits we did not like--and still do not like--is that the glider and
cable will see a pretty horrible load cycle (much worse than a
conventional aircraft) as each revolution you will go from full load to
part load and back again. This is equivalent to continual take-off/landing
events but every couple of seconds. We think the fatigue issues are
substantial. We had concerns about complexity compared to a conventional
wind turbine. You ideally want something that can be left in place and not
worried about - it seemed that with cables you will always have a higher
complexity and hence maintenance requirement. Lastly we were worried about
cable wear at the base and around the rollers. None of this are
insurmountable, but we could not convince ourselves that it gave a
sufficient advantage over conventional turbines to pursue - especially
when no one else believed us or saw the need for wind turbine structures
that could be based offshore.
As usual with anything new we were rejected by the UK DTI - I have
attached their rejection letter as an advert for what happens when you
think up something really different. The cable-wear issue is a fair point,
but all of the aerodynamic stuff is wrong. The irony is that the UK
government (via the Energy Technologies Institute) is now spending a small
fortune trying to develop offshore floating turbines.
On the physical side we only did a small amount of small scale model work,
which showed that you need some torque control on the line for controlled
launching and recovery and the launch gear needs to be reasonably slippery
(low profile). We did have some designs for a multi-stacking one, where
each wing launches from the top of the one below (too complex) and for one
with more than two wings (too much risk of interference between them).
We probably should have done some more testing, but for the reasons
mentioned above and our small company size, we let this drop.
Anyway I thought you might like to pass it on to anyone you think might be
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