Hello everybody, I recently discovered this fascinating forum and thought I’d share my idea for a high altitude pneumatic wind turbine tower.
The boundary layer for wind traveling along the surface of the earth is up to 350 meters in height even on relatively smooth grass surfaces. This frictional resistance considerably reduces the available kinetic energy in the wind, constraining the potential power that existing wind turbines can extract. If a method existed to build wind turbines up to 350 meters tall, their power output would increase by 3.4x from a height of 50 meters (hub height for the Enercon E-44), without making any changes to the basic design and aerodynamics of the turbine, which has already reached a physical limit. Wind speed for a hypothetical location in Nebraska USA increases from 8.39 m/s at 50 m to 12.64 m/s at 350 meters as predicted by the power law using an exponent of 0.15-0.16 for grassy surfaces. Since the energy yield from wind is the cube of velocity, a 1.5 increase in wind speed translates into a much larger 3.4x increase in power. A near quadruplicating of the power density of a wind turbine is a very significant thing, allowing it to produce electricity for less than third of the current cost. Unfortunately, such an ultra-tall tower is simply not feasible with current engineering. Existing wind turbine towers make use of cantilevered masts, generating immense bending stress at their mounting point, requiring very thick steel construction, which results in high fabrication and material costs.
A better solution is desired so that the higher kinetic energy of higher altitude winds beyond the boundary layer can be tapped into economically.
Existing tall and slender structures are limited by the elastic deformation of the plastic material, primarily metal, used in their construction. Elastic deformation is a major constraint on the structural efficiency of metallic structures. An ideal structure would be constructed using only tension-loaded components, such as cables and pressure vessels, so that each structural element could be loaded just below the yield point without any structurally compromising deformation. To create a truly high-altitude structure, a means is sought whereupon the elastic deformation can occur only via the circumferential stretching of a cylinder which does not compromise the integrity of the tower. If the tower is made out of thin-wall metal but with a large diameter to evade Euler buckling, it will merely fail by crumpling or what’s called “flexural buckling”, so this is not a solution.
A classic way to increase the lateral stiffness of a tower is by fastening guy cables to transfer the lateral bending moment into vertical compression. Unfortunately, such a scheme does not increase the load capacity of a slender-column tower, since it simply discretizes the compression forces between the guy-cable mounting points. Guy cables therefore only impart an improvement in the stiffness of the tower, but they do not increase the load capacity, they are not in themselves a source of strength. A wind turbine is heavy and generates strong torsional loads, which cannot be born by a thin-wall tubular or lattice tower, regardless of whether it is laterally stabilized by guy cables. In order for an ultra-tall tower to work for a wind turbine, it must be very stiff and able to carry tens of tons of both static dead-weight and occasional gust loads which generate strong bending and subsequent compressive forces.
An elegant and perhaps genius solution is to employ the power of pneumatics to create a continuously pressurized gas cylinder acting as a tower to absorb the entirety of the compressive loads. Upon first examination, this idea appears obvious and seems to solve virtually every problem faced by the designer of a tall tower. Surprisingly, such an idea has never hitherto been proposed.
A the end of this cylinder a sliding piston would be placed to carry the loads acting on the tower. A method to seal the pressurized gas would be devised. In such a scheme, the lateral loads are transferred to compression along with any dead weight placed on the tower, which is then converted to hoop stress in the cylinder by pushing against the piston. But since the cylinder is filled with pressurized gas, the downward force is resisted by the force acting on the piston by the compressed air, which is at a steady-state pressure generating mild hoop stresses on the cylinder wall. A 750mm diameter constant diameter tube could generate 150 tons of force or equal load bearing capacity before displacement using only 4 MPa of gas pressure, requiring only 8mm in wall thickness to keep hoop stress under 160 MPa. This simply an unparalleled degree of structural efficiency, the ability of a light-weight thin-wall tube to carry 150 tons can only be realized using the power of pneumatics. The weight of this tube would only be 55 kg/m, or 19 tons for the entire tower! In this scheme, we have completely eliminated elastic deformation of the tower’s main structural member, and we are now able to generate a tower as tall as 350 meters using lightweight aluminum pipes filled with compressed air and stayed with cables connecting to the piston at the apex of the tower. The designer is no longer required to use thick gauge material to generate the required stiffness and strength to bear compressive loads since cables can produce all the needed stiffness by transferring their bending moment into the upwardly forced piston, and the walls of the cylinder can carry this compression force via gas pressure. This scheme is extraordinarily elegant since it optimizes the distribution of forces to minimize loading in ways that utilizes a plastic materials greatest asset: its strength in tension, while minimizing its greatest weakness: its susceptibility to deformation, itself a corollary of the very ductility we want for such a structure.
A series of cables can then be spanned vertically connecting intermediate guy cables to stabilize the thin-wall tube. A 0.75-1 meter diameter thin-wall aluminum cylinder/tube can be braced by cables every 25-40 meters, allowing an otherwise infinitely slender tubular column to be broken up into discrete rigid sections. Such a structure would be unique in the world of structures in that strictly speaking, it has no compression-loaded structural members other than its foundation. The guy cables experience only tension, and the thin-wall pressure-bearing column experiences circumferential and longitudinal tension. It should be noted that pressure vessels can be constructed out of materials with no intrinsic compressive strength or stiffness, such as aramid fiber (Kevlar) widely used to construct ultra-lightweight pressure vessels for space vehicles, so a pressure vessel is a solely tension-loaded structure. In order for such a structure to maintain long term structural integrity, redundant air compressors can be placed at the base of the tower to counter any leaks that could develop along the piston seal.