(From:
http://news.bbc.co.uk/hi/english/sci/tech/newsid_1248000/1248068.stm
Reproduced without permission, for educational purposes.)

It looks a bit like a cross between a balsa-wood helicopter and a kite,
but Professor Bryan Roberts hopes this odd-looking craft will help meet
future energy needs. 
 
His gyromill, as he calls it, is actually a flying wind turbine.  It
uses its rotors to climb into the sky and then lies back in the wind
as those same rotors generate electricity. 

The plan is to send clusters of these vehicles 4.5 kilometres (14,700
feet) up into the jet stream to create a sort of flying power station. 

Professor Roberts, from the University of Western Sydney, has spent 20
years proving the concept and is now ready to put it into practice.  He
wants to build the first station near Woomera in South Australia 

The professor believes gyromills will prove to be a cheaper and more
flexible method of electricity generation than traditional wind turbines. 
 
By operating in the jet stream - the near-constant "river" of fast-moving
air high above the Earth - the gyromills can escape the more turbulent
winds found at ground level and which require a robust and expensive
design for traditional turbines. 

Each of the Professor's gyromills would be tethered to the ground. 

"The cable connecting it to the ground can draw energy from the ground
and use that energy to power the machine as a helicopter," Dr Roberts
told the BBC TV science programme Tomorrow's World.  "Then, when it gets
to altitude, the gyromill's motor can be switched to a generator and
energy is pushed back down the cable to the ground." 
 
Professor Roberts has been working on the gyromill idea since 1979.  The
design has gone through many phases, and prototypes have been flown
successfully in wind tunnels and in the sky.  The professor says it is
now time to scale up. 

"There would be a cluster of these things in the sky - like a range of
kites at an altitude of approximately four kilometres.  The power station
would cover an area about 20 kilometres (12 miles) in diameter. 

"To bring them down, you'd simply winch them in - or you could fly them
down.  In the best winds in Australia, the gyromills can stay up six days
out of seven." 

But Professor Roberts will have to convince the aviation authorities that
his project is a safe one.  Aircraft would have to be kept well clear of the
gyromills and their trailing cables.

13 responses total.

The BBC article is short on details, such as the amount of power
the power station is expected to generate.  It also appears to
confuse general upper-altitude winds with the jet stream.  On the
other hand, I've spent the last week watching the wind data for
the 500 millibar level over North America; it's obvious to me that
there is a whale of a lot of power there for the taking.  As a
for-instance, today (a windy day) winds at Detroit have been running
around 30 knots; that's a total wind power of about 2400 W/m^2.  At
the 500 millibar level, winds have been about 85 knots, and the
total wind power roughly 27 KW/m^2.  (Available power is around
half that.)

This idea deserves expedited consideration, IMHO.

I wonder about the possibility of powering an aircraft from the differential
between winds at different altitudes. Would, say, 1000 ft of altitude
difference between craft and rotor be sufficient? Perhaps two rotors, one
flying 1000 ft above the craft, and one 1000 ft below, for better leeway
control?

What sort of wire/cable is proposed for the gyrokite power generator in #0?

Re #2: It'd have to be light, but a good conductor.  Aluminum might be a 
good choice, maybe with a core of kevlar or some other 
high-tensile-strength material for strength.

Realistically, you can only put this up somewhere uninhabited, so if one 
of the contraptions sheds a blade it doesn't fall on someone's head.  
Over the ocean would work.  Australia's a handy place to test it in that 
it has lots of uninhabited land.  (Woomera was once an atomic bomb test 
site, IIRC.)

And oceans also have the advantage that you don't need to go up as high to
get uninterrupted fast winds.  I guess flat desert would do that as well. 

Re #2:  I did a bit of number crunching based on what seemed
like reasonable assumptions and got some answers I liked.

According to sources I found, Kevlar-49 has a density of 1.44
and an ultimate tensile strength of 3.6 GPa.  Assuming that it
can be used at a working strength of 1/3 its breaking strength,
and that a 2.2 megawatt generator system could have 100,000 lbs
(445 kN) of drag and 200,000 lbs (890 kN) of total force
(sqrt(lift^2 + drag^2)), I got a required cross-sectional area of
7.42 cm^2 of Kevlar.  This would have a mass of about 1.07 kg/m
of cable.  (Kevlar might not be a good idea due to its lack of
water and UV resistance, but better fibers exist or it could be
jacketed for protection.  I'm just using Kevlar as an example.)

Conductors are the other half.  According to my pocket reference
by Glover, #3 copper wire has a resistance of 232 micro-ohms per
foot, or 761 microohm/meter.  (This is equivalent to 8 12-gauge
strands, which could be woven as part of a cable.)  The area is
0.2664 cm^2, and the weight is 0.159 lb/ft or 0.236 kg/m.  If the
wire were divided into half the conductors going up and the other
half down, the circuit resistance would be 3.04 milliohms per meter.

The total mass of the cable would be about 1.31 kg/meter.

If the "gyromill" were flying at an altitude of 5000 meters with a
total cable run of 7000 meters one way, the total mass of the the
cable would be about 9200 kg and it weight under 90 kN.  This is
far less than the lift force of 385 kN.  The circuit resistance
would be 14000 m * 3.04 mohm/meter = 42.6 ohms.  If you transmitted
2.2 MW from the top at 50,000 volts at 44 amps, you'd have less than
1900 volts drop across the cable, or less than 4%.  The heat loading
of the cable would be about 2.9 watts per meter per conductor (5.9
W/m for both directions).  This appears unlikely to cause undue heating.

I think the fundamentals are very solid, all this requires is a bunch
of engineering (and financing, and politicking) to take care of the details.

I'd use aluminum, not copper.  It's nearly as good a conductor, and it's 
both cheaper and much lighter.  It's used pretty extensively in 
long-distance power lines, usually with a steel core for better tensile 
strength.  (This is similar in concept to 'copperweld' antenna wire.)

Kevlar was just a thought; there are probably better materials.  
Regardless you would jacket it with the aluminum conductor.  You want 
the conducting material on the outside anyway, due to skin effect.

Re #6:  I suggested copper because it works and it's relatively
corrosion-resistant.  Aluminum builds up a very highly insulating
oxide layer, which can present problems.

By my numbers, the load-bearing part of the cable would have to be
rather thick (over an inch across).  Using metal as an outer jacket
would cause it lots of stretching and compression when it was reeled
in and wound around a takeup drum.  It seems likely that a solid
outer jacket wouldn't be solid after a few cycles.

The obvious response is to go to a stranded conductor.  Using stranded
wire eliminates much of the flexing problem.  It would also allow a
cable to be made by adding some spools of wire to the machine which
twists yarns of the load-bearing fiber (in other words, cheap).  The
issue of skin effect is largely eliminated if the strands do not
interconnect much and weave in and out of the body of the cable.
This is the principle behind Litz wire.  (Skin effect is not an issue
if the power transmission is DC - though I doubt that would be done
because it would require power converters on the flying platform.)

An insulating coating could present a problem.  If one of the strands
breaks, load will be re-distributed when the wires touch above and
below the break.  The better the insulating properties of the oxide,
the more heat would be generated at these points.  Too much heating
could result in additional conductor failures or burning of the
load-bearing fibers, culminating in breakage of the rope.  I don't
like this idea, and after the problems associated with aluminum wire
in housing I wouldn't want to spec it for a first-cut design.

Right now I'm trying to find my drawings of a motor housing to get
an idea of the power/volume ratio of small motors.  No luck so far.

I made an error above; I already considered the entire loop resistance
per unit length as 3.04 milliohm/meter, but doubled it for the cable
run.  The figure for a run of 7000 meters should be 21.3 ohms, and the
voltage drop at 44 amps would be less than 950 volts.  If the transmission
voltage were 50,000 volts this would be less than 2% losses.

I wrote Roberts and received a reply.  He pointed me to some conference
papers which I intend to look up if I have time, and mentioned that his
group has a 50-KW machine in the making.  He didn't mention the rotor
size, but he did say that they have dropped the single-blade rotor design
in favor of a more conventional two-blade rotor.

#2 reminds me of a technique for model sailplanes called "dynamic slope
soaring"  Rather than fly in the updraft on the windward side of a rigdge,
the sailplane loops in the fast moving air behind and above the ridge, and
the slow moving air behind and _below_ the top of the ridge.  A simple
explaination is available at
http://ourworld.compuserve.com/homepages/dlstone/dsoar.htm .  No relation
to power generation, but I think you might find it interesting!  -Alex

Re #9:  That sounds an awful lot like the technique used by the
wandering albatross to dynamically soar over the ocean.  If so,
I guess nature has prior art.

Boy, and I thought the Altamont Pass windmills were ugly...  Now this!

There's an interesting technique I saw presented at a Mars Society convention
a couple of years ago for constructing tethers - have several vertical strands
connected by a lattice of smaller diagonal strands, around in a circular
arrangement, somewhat like this:

           |  |  |  |
           |\ | /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|/ |
           |\ | /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|/ |
           |  |  |  |
 
If one, or several, of the strands breaks, the load is taken up by the
lattice:

           |  |  |  |
           |\ | /|\ |
           | \|/   \|
           | /|\   /|
           |/   \|/ |
           |\   /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|  |
           |  |  |  |
 
This type of assembly, constructed with Spectra fiber (ultimate tensile
strength of 3.25gpa), was the basis of experiments in the use of tethers for
orbital maneuvering and space power generation.  (Spectra is resistant to
chemicals, water, and UV.)

new item  (??) #83 ... about real life wind powered electricity generation.

russ; why are you a homophobe?

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