"Joe Strout" wrote in message
...
Hi Pete,

Thanks for an interesting series of posts. I believe I understand how
a
free-flying wing as you describe might be applied to aircraft, but I
don't see how it applies to wind turbines. OK, you could use a kite
structure instead of a tower to hold your turbine aloft (and hope the
wind never dies for even a few seconds!), but you still need rotors to
actually crank the generator, right? How do the principles you're
describing impact rotor design at all?


Yes, this method has been suggested in the past, so have similar systems
to what I am suggesting, I am just taking it one step further. Consider
a controllable kite which is continually looped within the centre of the
wind, an electric motor/generator and propeller are attached to extract
and supply energy to the system. The energy used to keep the kite aloft
is a small proportion of that which the kite generates. This much has
been suggested before. There are very important reasons why such
systems should exploit the apparent wind of the kite, which is many
times that of the true wind speed. By analogy with a standard wind
turbine, this is equivalent to driving the generator via a very small
high speed propeller mounted at the wind turbine blade tip.

Envisage an airplane with electric motor driven propellers, similar in
scale to a turboprop. These electric motors are powered from the ground
via an electric cable within the line faring. It just so happens that
there is sufficient thrust available for VTOL, this simplifies launching
and landing, which traditionally have been problematic. After launch
this airplane circles in the sky, like a wind turbine blade tip, (fast,
say 100m/s or more), loading up against the tether and the wind like a
kite. At these speeds the propellers are very effective wind turbines
and the electric motors become generators that feed electricity back
down the cable to the tether point. The area that the airplane sweeps
as it circles is equivalent to the swept area of a standard wind
turbine, though obviously it can be much greater.

The trick is that the propeller is operating at the speed of the
airplane, which is many times greater than that of the true wind speed.
At an overall lift to drag ratio of ten the air plane speed is ten times
that of the true wind, with power proportional to wind speed cubed, the
propeller can have a thousandth the swept area of a comparable wind
turbine for the same power. This makes for a very compact and effective
unit, it is important to exploit this apparent wind directly as it
allows for much higher specific speed of the propeller and generating
unit, (no gearing).

When the wind is light the propeller can revert to being driven,
powering the airplane along, if this continues for a period of time, the
airplane can be landed. Note also that the speed and kinetic energy of
the airplane is sufficient to coast through short lulls in the wind.
Such airplanes can fly and generate useful electricity in true winds
significantly lower than that required of standard wind turbines.

Pete.


Thanks,
- Joe

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