Hi Gordon,
Thank you very much for your response. I finally found and read your
proposal (message #19899) and I think you did a great job. Very clear. You
clearly anticipated me. Thanks also for the additional guideline information
about how much lift the balloon kite needs to produce. (JoeF made it easy to
find with his search system.)
In answer to your question about how much lift a Sharp Rotor can produce, my
guess is that the rotor could be treated as a wing with a surface area
equivalent to one of the three surfaces. The coefficient of lift is probably
somewhere close to 2 when auto-rotating, and the L/D is 2 for small sizes.
If spun with a motor, then the Cl might go as high as 10. In large sizes,
the L/D should reach about 3, similar to a large Flettner rotor. Part of the
power of an array could be delivered to a supporting Sharp Rotor to spin it
and to produce extreme lift. For storage, it could probably be made to
collapse like an accordion bellows if it were constructed as a thin-skin
balloon with internal cords used to maintain the profiles of its three wing
surfaces. However, I'm unsure as to whether it can be used for a pilot kite
because I don't yet know how to reduce its drag and rpm in high winds.
My current opinion is that energy kites need to be able to remain aloft in
too-high wind speeds because it would be too costly and too dangerous to
lower them during high winds. I see that you disagree. But assuming I'm
right, then they need ways to reduce their drag while maintaining adequate
lift.
For that reason, I think that an array of HAWT rotors may need to be
arranged with their rotational axes horizontal rather than
inclined-vertical. When horizontal, the HAWT rotors are sideways to the wind
and have minimal drag and produce no power. That's how Doug protects his
Super Turbines in high winds, which is the conventional way to protect HAWT.
So that could be used as the default position, with a tail vane used to
orient the array to the wind for producing power. Some additional tweaks
will be required for placing the rotors at the best angle to the wind, such
as keeping the array in a vertical plane, and then turning it around a
vertical axis with a wind vane. In that case, the loop belt drive would need
to be at the top of the array rather than at the bottom. But I haven't
figured that all out yet.
My reason for using X-Joints instead of bevel gears is to save weight, and
to lower costs. Bevel gears need to be precisely aligned and they usually
require lubrication, which means their housing needs to be very stiff and
potentially heavy -- or relatively expensive is formed using carbon fiber.
X-Joints have low tolerances, are light, and can transmit high torque.
The array I showed would probably require a tail-vane or two side-vanes to
keep it reliably oriented to the wind. Your use of double belt loops would
keep your array oriented to the wind.
I wasn't sure why you wanted to control your kite because my assumption is
that the kite will remain at its optimum position simply by being directly
downwind. So I assume that you mentioned control as merely an option that is
available when two loop belts are used.
I like your idea of using inflatable, closed tubes to create a wing shape.
There would seem to be a wide range of possible configurations based on that
basic idea. I assume that people building inflatable kites have explored
some of them already. I like the idea of using multiple containers (despite
the added weight) for the lifting gas so that a leak in one won't bring down
the whole balloon.
A related idea is to use multiple, inflatable Sharp Rotors or Donaldson
rotors, with each rotor spinning independently. The rotors would be placed
one behind the other, but arranged to create a curved shape like a sail. Or,
the rotors could be arranged one behind the other, but they could have
different diameters. That would create a streamlined wing profile. The L/D
ratio could be relatively high for such a wing, and the Cl would probably be
more than 2 because the wing could probably operate at a high angle of
attack without stalling. If the rotors were also spun using a power source,
the effect would be like using a moving belt for the entire wing surfaces,
which can produce a Cl as high as 3.5, if I recall correctly.
PeterS

Hi Rod,
Thanks for your comment. Yes, that would work: Give the upper rotors a
larger diameter than the lower rotors so that all of the rotors have the
same tip speed ratio, and all of the rotors can operate efficiently.
But there is still the power to weight ratio to consider. The upper rotors
will probably have a much better power to weight ratio than the lower rotors
(due to the higher wind speed), so it could be lighter and more powerful to
keep all of the rotors at a high elevation.
There is also a question about whether the smaller rotors down toward the
generator could handle the much higher torque from the larger diameter
rotors above them -- assuming that the torque is transmitted using lines in
tension and various means to keep the lines from twisting and collapsing. I
don't know of any studies on how much torque different tension-cord
arrangements can handle before collapsing or snapping. And I haven't done
any experiments focused on that question. So you would have a much better
idea about that than me. For a tall stack reaching 300 to 500 meters, the
torque near the ground would be extremely high. That concerns me. A lot
depends upon the tension of the stack as a whole, but I don't know how to
estimate the ratio of torque to tension or what is practical, while keeping
everything as light as possible. The lower the tip speed ratio of the
rotors, the bigger the problem becomes because the torque will be higher for
rotors with a lower tip speed ratio, given that power equals the torque
times the rpm. Reducing the rpm increases the torque. So it seems best to
use rotors with a high tip speed ratio of 5 or 6, like most HAWT. But there
are lots of variables to consider, such as the diameter of the rotors as
compared to the diameter of the tension-cord "shaft", and the aerodynamic
drag caused by the rotating tension cords.
PeterS

Hi DaveS,
You make a very good point since objects generally increase their mass by
the cube as they become larger. Double the size and the mass increases eight
times. Yet the swept area of a HAWT rotor increases only by the square, or 4
times. So if the wind speed stays the same, the power to weight ratio of the
rotor that is twice as big is only half that of the smaller rotor.

But if the wind higher up acting on the larger rotor is 50% faster, then the
available power will increase by 1.5 cubed, or 3.38 times. Half of that is
1.69. So the power to weight ratio of the 2 times larger, higher rotor will
be 1.69 times that of the smaller, lower rotor. This assumes that both size
rotors are designed to handle the same wind speeds, as is typically the case
for HAWT rotors.

There is something else to consider in this case. HAWT rotors generally
increase their mass by an exponent of 2.4 rather than 3 as they become
larger. That is because they save weight due to internal bracing of the
blades combined with thin blade skins, which leaves relatively more empty
volume. So double the size of the rotor and the mass increases by 5.28 times
instead of 8 times. That will further increase the power to weight ratio of
the larger, higher rotors.

Another consideration is that larger rotors of the same type will have a
higher Reynolds number and so will be more efficient. That too will increase
the power to weight ratio of the larger, higher rotors.

So what you are saying may apply to most kites and kite-like HAWT rotors
that strictly obey the square/cube law of scaling, but it may not apply to
rigid wind turbine rotors used as parts of energy kites. Lots of variables
to consider.

However, my math is terrible, so please forgive me if I blundered. And there
may be additional factors I have not considered.
PeterS

DaveS,

 

Keep in mind that for his patent Rudy Harburg is a precursor of Doug Selsam which developed both the torque based and multirotor concepts to such a point that he was able to build prototypes based on his patents. Moreover his realizations are the basis of later realizations comprising Rod's, Christof's, mine. So the main consideration is due to Douglas Spriggs Selsam.

 

PierreB

 

 

 

 

 

Dear Dave,

Hi Dave,
Thank you for your appreciation of the design. I agree that the Sharp Rotor
Pumping Kite is not as simple as I would like it to be, as in "KISS". And I
worry that icing could interfere with any sort of sliding device. But I
think I could re-design it to solve that problem if necessary. What I do
like about it is that it achieves a complex function using only simple,
mechanical means, so no batteries or circuits are needed. (Although, I'm not
yet addressing the launching and retrieval problem.)

The basic idea of the Sharp Rotor Pumping Kite (to start with a balanced
device that becomes unbalanced as the orbit diameter increases) can be used
with a Donaldson rotor, or a sail-blade, or a rigid blade with pitch
control. I think that it has the potential to achieve an especially high
power to weight ratio, but that would probably require operating at the
highest possible tip speed ratio so as to produce maximum centrifugal force.


Thanks for mentioning the idea of using a shock cord between the pilot kite
and the pumping kite. I was wondering about that. The shock cord would to
some extent function as an energy accumulator, as you suggest. I had
considered that but concluded that the shock cord, by acting as a shock
absorber, would reduce the power delivered to the ground pump because it
would have a dampening effect. I want the ground pump to act as the shock
absorber so as absorb as much energy as possible. But maybe I'm wrong. Do
you have any measurement data or a vector analysis or some line of reasoning
that shows the use of a shock cord increases the power delivered to the
ground rather than reducing it? It does seem like a shock cord would provide
smoother operation though, which you confirmed. And a resonant condition
might serve to increase efficiency.

A related idea I thought about is to use a static Sharp Rotor kite as the
pilot kite. Here is why: When the pilot kite is pulled downward and windward
during the power stroke, that will increase the apparent wind speed acting
on the Sharp Rotor and increase its spin ratio. So when the Sharp Rotor
pilot kite starts to move upward and away from the wind, it will have a
significantly higher coefficient of lift because it will be spinning at a
spin ratio greater than one. That should compensate for moving away from the
wind, which would normally decrease the lift. So the static Sharp Rotor kite
would help to lift the orbiting Sharp Rotor during the upward part of its
orbit. So I'm thinking that the static Sharp Rotor kite would be a better
energy accumulator than using a shock cord.

A similar idea would be to use a kite with flapping wings, like a bird, for
the pilot kite. The tether would attach to the wings so as to force them to
flap when pulled down quickly, thus forcing the kite to fly forward so as to
increase its apparent wind speed and its lift. That type of static kite
might be better able to resist the down-pull of the pumping kite. But I
would have to think about this some more to be sure that there is any
advantage.

I very much like your idea of doing a comparison test between different
energy conversion devices by using the same pilot kite in each case. Can you
please recommend a specific pilot kite that I should consider using? It
should probably be relatively small and cheap so that the pumping models can
be small and cheap. That would make it possible to build and test a lot of
iterations of any design. Using a standard pilot kite might be a way for
various kite inventors, in different parts of the world, to compare their
devices to standardized efficiency and cost of energy measurements. The goal
would be to get the highest efficiency, or the lowest cost energy, or both.
The AWE group could maintain the records.

The driven device should also be standardized to simplify comparisons. There
might need to be two standardized driven-devices: one for electricity
production and one for water pumping or air compression. Would you recommend
using the air compressor foot pump that you used in one of your videos, or
might something else work even better? What might be universally available
around the world? Maybe a bicycle bottle dynamo would make a good standard
device, such as the Tung-Lin dynamo (12 volts). It's cheap and probably sold
almost everywhere. Maybe a DIY PVC water pump could be used for devices that
are made to pump water.

Or maybe better, a hand-cranked rotary water pump could be driven by both
kinds of kites (rotary, and short-pull). Some rotary pumps are reasonably
inexpensive and available on-line. I bought one to use with my windmills
when I build larger models. It's plastic, so not greatly durable, but fine
for testing. Rotary kites connected to the pump would produce a high flow
with a low head, and short-pull pumping devices would produce a low flow
with a high head. Then the volume of pumped water, times the head, could be
used as the measurement standard to compare the efficiency of different
kinds of energy kites. Then the efficiency divided by the cost could be used
to compare the cost of energy from different kinds of kites (assuming other
variables to be equal, such as durability, safety, etc.).
PeterS

Attachments :

HAWT Kite with Belt Loop.jpg

Good. We now have the image in two places. Here is the other place: 

Dear Dave,

One related glossary: