Hi All,
This
is a preliminary proposal for a new kind of buoyant kite that functions
as an advanced type of hot-air, or steam, balloon with superior
insulation, and solar heating, plus some heat storage to keep it aloft
at night or during dark days. It’s a crazy idea that probably can’t
work with today’s materials, and I can’t adequately analyze it, but it
at least offers a new approach. I don’t know enough to design the
complete balloon. These are just my first thoughts on this new type of
buoyant kite.
In my
opinion, small-scale energy kites cannot be competitive if they require
re-launching after lulls because the equipment will be too complicated
and expensive. I would be delighted to be proven wrong. But in the
meantime, I am looking for how to keep energy kites aloft continuously
for months at a time.
The
methods for doing that seem to include motorizing the kite so that it
can create lift from forward flight during lulls, or by using buoyancy.
I want to talk about buoyancy here because it seems to have more room
for improvement.
Buoyancy
can be accomplished by using helium or hydrogen (both of which are
problematic), using methane or ammonia gas (which are also
problematic), somehow creating a vacuum within a large and light
container, or using some form of hot-air balloon or steam balloon.
I
can't yet figure out how to make a vacuum container that can be very
light while resisting atmospheric pressure, which is enormous. Some
sort of multiple bicycle wheel construction around the balloon might
work, using a great many cord-spokes to the balloon. But I think the
most practical option might be an advanced hot-air balloon or a steam
balloon.
Most hot-air
balloons so far have relied on a flame heater below an open-bottom
balloon. That consumes a lot of energy and would not be energy
efficient. There are toy, solar-heated balloons consisting of a large,
thin-skinned, black, plastic bag. But the buoyancy is not large, and
the balloon would lose it buoyancy at night. So what else might work?
The
main problem to solve for a hot-air balloon is heat insulation. If the
container were large and the internal air, or steam, or other gas were
kept at reasonably high temperature, the balloon could provide a useful
amount of buoyancy.
The
limitation on this type of large balloon would seem to be its amount of
buoyancy as compared to its amount of wind drag. Without enough
buoyancy, wind pressure on the kite could force it all the way to the
ground. So the balloon would need additional lift. If it had additional
lift, then it would need to be just buoyant enough to keep it aloft
during lulls. That is my goal to explore here.
Consequently,
the two problems to solve are: How to achieve excellent thermal
insulation for very little weight, and how to provide additional lift
for very little weight.
My
guess is that adequate thermal insulation could be provided by using
alternating layers of reflective Mylar (used for emergency blankets)
and some of the new types of insulation used for cold weather clothing,
some sort of flexible aero-gel, or some sort of ultralight solid foam.
The result would be a very light, flexible skin with very good
insulation for internal temperatures up to maybe 100 degrees C, or
about the boiling temperature of water. The balloon would be sort of a
soft thermos bottle.
Assuming
that to be possible, what would be the weight of air or steam per cubic
meter at that temperature? From Wikipedia: At standard temperature (20
degrees C) and pressure, air weighs 1.2041 kg/m³. At 99 degrees
C, air weighs 0.9486 kg/m³. So at 99 degrees C internal temperature, the buoyancy provided by the hot air in the balloon would be .2555 kg/m3. Therefore, 1,000 m3 gives a buoyancy of 255.55 kg. 1,000 cubic meters would
be a cylindrical balloon 40 meters long and 8 meters in diameter. The
aspect ratio would be 5. If the surrounding air is colder than 20
degrees C, the balloon will have an increased buoyancy since colder air
is heavier. And if the surrounding air is warmer than 20 degrees C, the
balloon will have less buoyancy. Air at high altitudes is usually
cooler, and air at night is usually cooler. These cooler temperatures
work in a hot-air kite’s favor. However, normal balloon materials tend
to degrade quickly at a temperature of 99 degrees C, so a
heat-resistant inner layer material would be required, perhaps Nomex.
And at higher elevations the air density diminishes by about 3% for
each 1,000 meters of altitude.
Steam can provide more than twice as much lift as hot air at 100 degrees C. At 100 degrees C, steam weighs 0.587 kg/m3. So steam at 100 degrees C would provide 0.6171 kg/m3 of buoyancy. Here is a website that discusses a new type of Festo steam balloon with superior insulation: https://www.festo.com/net/SupportPortal/Files/42088/HeiDAS_UH_en.pdf
Where
would the energy come from to heat the air or steam? It could be
provided by a heating coil using electricity sent from the ground or
from a flexible solar panel on board. It could be provided by
constructing the entire balloon to function like a solar-heating
hot-air-panel. (I favor that option.) If the balloon or part of the
balloon rotated using wind energy, part of its rotation energy could be
used to create fluid-friction heating. I guess the balloon could also
be heated internally by aiming a microwave beam, or a laser beam at it
from the ground, but that strikes me as too expensive to be a practical
option, at least for now. Similarly, an intense sound beam could be
used to heat the balloon internally, but that too seems too expensive
for now. In any case, if the insulation had a high thermal insulation
value (R), very little energy would need to be provided to maintain the
buoyancy of the balloon. Consider that some houses are so well
insulated that a family's body heat can maintain a comfortable room
temperature in cold climates.
Here
is something that might prove to be critically important for insulating
the balloon: I saw a very interesting hot-fluid, solar heater with a
unique form of insulation. As I recall, it consisted of a glass pipe
with a dark fluid that flowed through the glass section of the pipe. A
parabolic mirror focused light on the glass tube. What was special was
that the insulation used around the glass section of the pipe was a
large-diameter, slowly-rotating cylinder made of clear plastic. The
cylinder was mounted on bearings so that it could rotate, and a tiny
motor was used to rotate it. The ends of the cylinder used normal
insulation materials. The parabolic reflector shone light through the
large diameter cylinder onto the glass pipe. When it rotated, the
cylinder dragged the air inside it along and subjected that air to
centrifugal force. The centrifugal force created an artificial gravity.
Hot air rises. Cool air falls. So inside the slowly spinning cylinder,
the cooler (heavier) air "fell" to the perimeter of the cylinder, and
the hot (lighter) air "rose" to the rotational axis of the cylinder
where the clear pipe was located. As I recall, that insulation was
quite effective. The fluid flowing through the glass pipe got quite hot
-- even when wind was blowing on the cylinder and cooling it due to the
wind chill factor. Here is a more recent version of the same concept:
http://solarenergyengineering.asmedigitalcollection.asme.org/article.aspx?articleid=1458179 It prevents heat loss via normal convection, which is a major cause of heat dissipation.
What
that means is that by rotating, a hot-air balloon could increase its
thermal insulation rating substantially. The internal air in contact
with the skin of the cylinder would be significantly cooler than the
average temperature of the air inside of the cylinder, and much cooler
than the very hot air along the central axis of the cylinder. So there
would be reduced heat loss, and that should more than compensate for
the cooling effect of the wind. And the lower temperature air in
contact with the insulation should increase the insulation’s working
life. That suggests that a good candidate for a hot-air balloon, or
steam balloon, might be a Flettner rotor or a Sharp Rotor since they
rotate, have a large internal volume, and can be constructed to be
light. They could be slowly rotated during lulls to maintain their
thermal insulation, by using a small motor and a small battery. A Sharp
Rotor could be slowly rotated by a small motor powered by flexible
solar panels.
The
hot-air balloon would need to adjust its internal pressure to that it
did not burst from high pressure or collapse from low pressure. That
should be easy to do by using two, small, spring-loaded, one-way air
valves. If the pressure got too high, some air would be allowed to
escape. If the pressure got too low, some air would be admitted before
the balloon started to collapse. The balloon would need to have just
enough skin stiffness to withstand an internal pressure that was just
slightly below atmospheric pressure. That stiffness might be provided
by using inflatable rings. A lot of work has been done on using
inflatable parts to create relatively rigid structures. Some kites
already using inflatable spars or inflatable rings. Or, the stiffness
could be molded in to the outer skin of the balloon.
A
Sharp Rotor might be constructed with a thin, clear, polycarbonate skin
with chord-wise ribs molded into the skin to give it rigidity. Sunlight
would shine through the clear skin and heat the surface of a black
colored cylinder that was thermally insulated. The rotation of the
balloon would keep the warmest air away from the clear skin of the
rotor. The spinning kite would add insulation to the cylinder holding
the heated air. A small fan would circulate the heated air from the
outside of the cylinder to the inside of the cylinder at close to the
long axis of the cylinder where a pipe holding eutectic salts would lie
along the long axis of the cylinder where the air was hottest. During
the day, the salts would absorb part of the solar heat. At night, the
salts would give off heat to keep the balloon aloft.
An
alternative would be to store heat in high temperature, pressurized
tank of water located along the spin axis if the kite. It would be used
to supply steam. A small water pump would return condensed steam from
the periphery of the cylinder to the outside of the central tank where
it would be boiled to produce steam. The water in the tank could be
heated to well above 100 degrees C because it would be in contact with
the hottest air inside of the balloon that would not come in contact
with the wall of the cylinder containing the buoyant steam.
The
Sharp Rotor could be used as a pilot kite to support various wind
energy devices, or it could function as a long-pull kite or as a
stretch kite working with other kites.
Can
you think of ways to improve this basic concept or to replace it with
something better? Does this concept seem feasible? Can you think of
additional problems to solve?
Further reference: http://www.heidas.de/docs/HeiDAS_AIAA2003.pdf
“Alternative Buoyancy Concepts: First Numerical and Experimental
Results for a Hot Steam Balloon (HeiDAS)”, by Bormann, et. al.,
2003, About the best materials for steam balloons. The best
material, PEEK, is still too expensive.
Note:
Normal passenger balloons could use steam and could be slowly spun
using a large-diameter bottom bearing so that the basket did not spin.
Spinning would make them much more efficient. Blimps and dirigibles
could be designed to slowly spin around their long axis so as to
provide insulation for steam. 10% to 15% hydrogen can be added to steam
without making an explosive mixture.
