While I evoked some AWES which would include some inflatable blades or sails or other elements to lighten the AWES as it scales up, some information (in French) seems to emphasize the limitations of inflatable structures, while highlighting their advantages.
Pour les ballons souples et semi-rigides (qu’on devrait en fait appeler semisouples), la paroi fait l’étanchéité et transmet les efforts. Elle sert de structure. La forme du ballon est déterminée par la pression qui règne à l’intérieur. Plus le ballon est gros, plus il faut élever la pression pour tenir la forme, et comme le dimensionnement d’une paroi gonflée est directement fonction du rayon de courbure, un gros ballon souple devient rapidement trop lourd. C’est ce qui a justifié la création des ballons rigides, à partir d’une certaine taille.
Translation:
For soft and semi-rigid balloons (which should actually be called semi-soft), the wall provides sealing and transmits forces. It acts as a structure. The shape of the balloon is determined by the pressure inside. The larger the balloon, the higher the pressure needs to be maintained in order to hold its shape, and since the sizing of an inflated wall is directly related to the radius of curvature, a large soft balloon quickly becomes too heavy. This is what justified the creation of rigid balloons, starting from a certain size.
The video also seems to indicate that rigid boards are lighter than soft boards.
It can be deduced that it seems illusory for an inflatable wing or blade to achieve considerable dimensions while remaining lightweight, especially since thin and efficient profiles could only be obtained at relatively high pressure, regardless of the methods used (Drop Stitch…).
On the other hand, regarding an aerostat, could a spherical shape facilitate its shape stability even for a single giant and light flexible envelope? And for an open-bottom hot air balloon, what would be the limit of the dimensions?
Exploring Paddle Board Varieties: Inflatable vs Rigid
Disadvantages of Hard Shell Paddle Boards
On the flip side, hard shell paddle boards do have some drawbacks to consider:
Portability: These boards can be cumbersome compared to their inflatable counterparts, and transporting them can be a hassle for those who lack sufficient vehicle space or tools.
Storage: Hard shell boards require more physical space for storage compared to inflatable models, which can be more compact when deflated.
Weight: Often heavier than inflatables, these boards can be harder to manage for some paddlers, especially those looking to carry them significant distances or loading them alone.
Stitching: The drop-stitch fibers are sewn together within a pattern that will ensure a flat surface once inflated.
shapewave® could take on wing profiles.
“Often heavier than inflatables”, but not always. Although the video on the initial topic rather stated the opposite, one might think that the weight of the two types of boards is comparable, which does not bode well for the scalability for AWES, knowing the pressure inside the inflatable drop-stitch or shapewave® profile could increase with the dimensions.
The whole thread have been reflected in the preprint (PDF) Vertical axis wind turbine(s) connected to Flettner or Sharp balloons, also including:
As indicated below, rigid blades could perhaps advantageously accommodate to the cambered profile. Thus, as shown in the Fig. 11 [29], the medium solidity turbine, with chord length blades c of 0.5 m (in red) would replace inflatable high solidity turbine with chord length blades c of 1 m (in green) which are required for the structural outfit. Note that in the red curve (for c = 0.5 m), the best Cp (close to 0.5) corresponds to the slowest TSR (far below 1), which is ideal for low consumption of aligned balloons. And rigid 0.5 m chord length blades can be lighter that inflatable 1 m chord length blades. These numerical indications apply to the reference turbine represented in table 2 [29], with a radius of 3.333 m. The chosen radius here being 10 m, the proportions of the blades are accordingly adjusted.
However, since individual use is planned, it may be possible to reconsider the dimensions based on the aforementioned table 2 [29], which would result in a swept area of approximately 44.435 m² with a blade height substantially equal to the diameter of the turbine, which is 6.666 m.
Why Blimp Technology Doesn’t Scale Up Well
Blimps face a special scaling problem that arises from the way that hoop stress affects the strength of materials required for the envelopes. “Hoop stress” is a force acting on the wall of a vessel containing a fluid. The origin of the term comes from the wooden stave barrels that were held together by metal hoops. As larger barrels and vats were constructed with larger diameters, the strength of the hoops had to be increased in proportion to the diameter.
In a nonrigid airship, or blimp, the gas wants to form a bubble. Consequently, the most hoop stress forms on the sides of the airship. The formula for hoop stress (approximately), in a “thin-walled” vessel is:
σ = (piDi – peDe)/2t
Where:
σ = hoop stress
pi = internal pressure of the vessel
pe = external pressure of the vessel
Di = internal diameter of the vessel
De = external diameter of the vessel
t = thickness of the vessel’s wall
In a blimp, as in a car tire, but not in a rigid airship, there must be a significant difference between pi, the internal pressure on the envelope of the airship, and pe, the external pressure on the airship; again, this pressure difference gives the airship (or the tire) its shape and stiffness. As the diameter of the airship gets larger D, the hoop stress rises proportionally, and the envelope of the airship needs to be made thicker/stronger to resist the forces trying to pull it apart.
Expanding the diameter of the blimp is problematic because the extra thickness of the envelope increases its weight and reduces the benefit of the non-rigid structure. Assuming that strength is a function of the wall thickness, the weight of the envelope scales with roughly the cube of the dimension (the area with the square and the thickness with the dimension; the product of these, which is the weight, scales with the cube). Gross lift also scales with the cube of the dimension, and does not particularly outrun weight. Consequently, blimps lack the tendency to become dramatically more efficient as they become larger. In the words of one engineer, the scaling problem of the blimp becomes like “a dog chasing its tail.”
Heavier envelopes are also harder to fabricate (stiffer materials) and more difficult to transport for assembly of the blimp. In order to reduce the strength of the materials required for a larger diameter blimp, catamaran designs have been created that have two or three lobes. These blimps are usually referred to by the builders (Lockheed-Martin and HAV) as “hybrids” because they are designed to use both aerostatic and aerodynamic lift to fly. The smaller diameters of the two lobes reduces the hoop stress and allows these blimps to lift more than an equivalent single cigar-shaped envelope.
Economies of size also impact the competitive distance of operations. Without being able to gain efficiency by scaling up, blimps will have difficulty becoming a profitable mode of transport for intercontinental shipping.
See also Inflation of a Spherical Rubber Balloon (equations and curves) and
This illustration shows a high-altitude balloon ascending into the upper atmosphere. When fully inflated, these balloons are 400 feet (150 meters) wide, or about the size of a football stadium, and reach an altitude of 130,000 feet (24.6 miles or 40 kilometers). Credits: NASA’s Goddard Space Flight Center Conceptual Image Lab/Michael Lentz
Unusual dimensions for a blimp (?), but at such an altitude, does the lower pressure make this kind of thing possible? Of course, this question is situated within the context of the ‘hoop stress’ issue that has just been mentioned.
About weather balloons (in French):
Would there be usable as lifters for AWES?
Let’s compare the smallest with the largest:
Filling quantity at maximum payload in liters: 500 l
Weight: 0.2 kg
Diameter in the space: 3 m.
Filling quantity at maximum payload in liters: 8300 l
Weight: 3 kg
Diameter in the space: 14 m.
There is no scaling issue. On the contrary: the larger the balloon, the more voluminous it is,
particularly its volume in space (scale advantage) but also its volume during filling (slightly a scale advantage), the less it is penalized by mass.
As a result numerous weather balloons could lift a large wind turbine with mass benefit (only concerning the balloons) instead of mass penalty. When a balloon pops, there should be enough left. Sure, it wouldn’t last long, but why not for demonstrations or tests before expensive balloon use?
There are specifications including “Balloon Material Thickness Thou/Inch”, volume (m³), length (ft), width (ft).
For a balloon of 1 m³, length of 5.5 ft and width of 4 ft, thickness is 1 thou/inch.
For a balloon of 34 m³, length of 22 ft and width of 15 ft, thickness is 6 thou/inch.
Some very rough calculation based on usual statements: the surface area increases by the square, and the volume increases by the cube. The volume is counted only as a basis for estimation, without considering the mass of the helium contained.
The surface area increase would be between squared 22/5.5 (= 4²) and 15/4 (= 3.75²), let’s say squared square root of 15 = about 3.87². The thickness increases 6 times. Total of skin mass increase: 90.
The volume increase is 34. We will note that the cube root of 34 is 3.24, and not 1 m³ which is the volume of the smaller balloon between the two ones.This is likely due to the fact that the two blimps contain surfaces with very little volume (kite and keel) that can be clearly seen in the photo, said surfaces being taking account for the calculation of the length.
Thus, the volume of the balloon should increase to 3.87³ = 58. But 58 is still not 90. Even considering a real cubic volume scale for the balloon, we find that the increase in the mass of the skin remains higher, which gives 3.87 raised to the power of 3.325 = around 90.
We understand “why Blimp Technology Doesn’t Scale Up Well”, leaving the room to rigid airships.