Hi DaveS,
Intuitively,
it would seem right, as you suggest, to add ballast, perhaps water, to
a Bird Windmill blade, flying in either horizontal or vertical circles
(Cyclo-Kite). The ballast, it makes sense to assume, would increase
energy accumulation, and the flywheel effect, and might also, one might
assume, increase the speed of the blade. So your guesses are good ones.
But the physics of the Bird blade are quite complex, even though it is
dirt simple to make and costs almost nothing. So there is a problem
with adding water ballast to a Bird blade. I’ll explain, but please
forgive me if I get to detailed.
Per
orbit, the blade can extract only a certain amount of energy from the
wind. The centrifugal force of the blade then pulls on cords to
transmit that energy to a driven device such as a pump. If the blade is
made heavier, it can transmit a stronger pulling force. So it would
seem sensible to make the blade heavier. But a heavier blade can act to
transmit more energy than the blade can capture during an orbit. That
energy has to come from somewhere, and so it is taken from the momentum
of the blade. The result is that the blade slows down. But if the blade
slows down, it captures less energy per orbit.
So,
sadly, above a blade weight of roughly 0.5 pounds per square foot
(roughly 5 pounds per square meter), the blade speed seems to reduce,
meaning that the tip speed ratio of the blade becomes smaller. The
bottom line is that above a certain weight per blade area, a heavier
blade cannot capture more energy. But future research might find a way
around that limit.
So
if the goal is to get a Bird blade to accumulate as much energy as
possible per orbit, the best way to do that is to reduce the diameter
of the blade’s orbit. If the orbit diameter is reduced to one half, the
centrifugal force will double, and the rpm will double, so the energy
capture will increase 4 times.
That
is different from conventional wind turbines which are designed to
operate at close to their best tip speed ratio. The Bird rotor, due to
its tip speed ratio of only 2, has a solidity ratio that is way too low
to begin with for high efficiency. So the solidity ratio can be
increased a lot without reducing the tip speed ratio. An efficient VAWT
with a tip speed ratio of 3 would have a solidity ratio of about 0.15
to 0.20. So a Bird blade with its tip speed ratio of only 2 will almost
never reach its optimum solidity ratio of something like 0.3 to 0.5.
Consequently, the orbit diameter can be made as small as practical. The
limit is set by whatever frequency (rpm) will cause destructive shaking
of the whole windmill. The Bird Windmill is deliberately unbalanced.
That imbalance is used to great benefit. But there is a practical
safety limit on the rpm as a result.
Well,
these details are probably boring, but I wanted to include enough to
explain why your good idea runs into a problem in this particular case
due to the very odd characteristics of the Bird Windmill.
However,
let me mention an experiment I did with the Bird Windmill that is
relevant to your good suggestion. Water ballast could be used in a
particular way to enhance the versatility of a Bird Windmill. The
experiment I did was very simple, but the resulting physics was quite
complex. So a lot more experimenting is required to work out the basic
proportions and dynamics.
I
typically test a Bird blade by using two horizontal poles. I hold one
pole and put the other one on the ground and step on it. The blade’s
elastic cords attach to the two poles. The blade orbits and causes the
poles to flex back and forth.
For
the experiment, I eliminated the bottom pole. I tied the bottom cord
from the blade to a short length of dowel. Since the dowel hung down,
gravity acting on the dowel created a steady pull -- sort of like an
elastic cord would normally do. The blade started normally and began to
orbit. The hanging stick began to orbit too. But it didn’t orbit with
the blade. It orbited opposite the blade and served as a counterbalance
to the blade. The stick was ballast that served as a counterbalance.
So
instead of a stick, a container of water might be used instead. It
might need to be a tube filled with water, or just a plastic bag filled
with water might work. I would need to do more experiments.
What
that means is that a Bird blade could be suspended from a kite, and the
bottom cord of the Bird blade would not have to extend all the way down
to the ground. The ballast might even weigh less than a cord to the
ground. The bottom cord could extend down a relatively short distance
to a water ballast container that would enable the blade to orbit
normally even though the bottom cord was not anchored to a stationary
object. So the Bird blade could be lifted to a great height and still
function normally. And it would function as a dynamically balanced
windmill to some extent. Here is a sketch of the basic idea. In this
case, I was thinking about using the concept for an advertising sign
instead of hanging it from a kite. As an advertising sign, I think it
would sell well. So here is a kite that could be used for advertising
along city streets.
Incidentally,
the Bird blade is actually partially airborne. That is because its
interaction with the wind increases its elevation above its starting
point.
A
Bird blade suspended from a kite might, for example, have a
ram-air-turbine (RAT) mounted on it. In one of my videos, I show a Bird
blade with a RAT mounted on it. It worked. The LED is fully lit, but
it’s hard to see in the sunlight. The blade is way too small for the
RAT, and that slowed the blade, but it worked anyway.
https://www.youtube.com/watch?v=It2tx110ON8
But
a RAT on the blade is not such a great idea because the blade normally
only moves at a TSR of 2, and a RAT needs a high apparent wind speed,
preferably a tip speed ratio of 3 or more.
Another
option would be to use the Bird blade to twist the cords of a
twist-cord-accumulator/transmission (TCAT). I’ve used a Bird blade to
do that, and it works easily.
https://www.youtube.com/watch?v=W85Yr1BwIsw
So
instead of using a water ballast below the blade, a generator could be
used as the ballast below the blade. A tail vane would prevent the body
of the generator from spinning with the TCAT spun the shaft of the
generator.
A
better option might be to use a RAT as the ballast below the blade.
Maybe it could be designed so that it would orbit with a much larger
diameter than the blade. That would increase its tip speed ratio of the
RAT to 3 or 4, and that would enable a small, light RAT to produce a
lot of power for its size. The electricity would go from the RAT up the
cords, past the Bird blade, and to the main tether of the supporting
kite, and then down to the ground.
Neither
of the blade cords needs to be elastic because gravity substitutes for
elasticity in this case. The blade orbit can expand normally, and so
can the orbit of the ballast. As their orbits expand, they rise upward.
The
end result would be similar to a Makani energy kite system, but
potentially far simpler and far cheaper. So your suggestion of using
water ballast for a Bird blade leads to an unexpected, and different,
application where the water ballast concept functions as an
intermediate step in the invention process. Much thanks for your
suggestion.
-------------------------------------
Here
is a related idea based on your suggestion. A Bird blade could be
constructed using kite materials. The counterweight is, in effect, a
form of ballast. So the counterweight could be a streamlined, closed
container of water. It would then be easy to balance the blade
correctly by just adding or subtracting some water. In an emergency,
the water could be dumped, thus unbalancing and feathering the blade.
-----------------
Careful
readers may have noted that perhaps the ballast weight below a Bird
blade could be replaced by another Bird blade, and so on, thus creating
a very tall column of Bird blades counterbalancing each other. That
would be a most interesting looking device. If that worked, then the
next step would be to determine the best way to use the column to
produce energy.
------------------
On
the general subject of using water ballast to accumulate energy, here
is a crazy way to do that with a Sharp Cycloturbine. It’s admittedly a
wild idea, but it might actually work. I’m including it here just for
fun, even though it’s not airborne. Sometimes seemingly unrelated
trains of thought can lead to desirable, but unexpected, destinations.
Maybe this one will.
PeterS
PS:
I would like to think about your suggestion of classifying “looping
foils” as a group, but I can’t yet find the technical details on the
other foils you mentioned. Can you or someone else help me? I’m not
actually logged onto the AWES site because Yahoo thinks I’m somebody
else and won’t let me tell them any different.
From: AirborneWindEnergy@yahoogroups.com [mailto:AirborneWindEnergy@yahoogroups.com]
Sent: Saturday, September 17, 2016 8:11 PM
To: yahoogroups <airbornewindenergy@yahoogroups.com
