Tubular steel towers have come to be preferred over lattice-type towers for wind energy applications, even though they typically use approx. twice as much steel for a given rating. Tubular steel towers are inefficient in their use of material due to their tendency to buckle on the compressive or downwind side while the tensile stress in the upwind side is still a relatively small percentage of the yield strength of the steel. Buckling starts with an elastic instability of the relatively-thin steel shell, and can progress rapidly to catastrophic failure.
ISO-e technology will reduce the amount of steel used in the construction of tubular towers by at least 50% while achieving a large increase in the tower's survival wind speed. We regard ISO-e as a critical, “must have” technology for the wind energy industry, or any other industry that uses large, tubular towers. It is based on several well-established principles, the first being that a pressurized cylinder can withstand much higher bending loads than an un-pressurized cylinder. The reason for this is that the pressure stabilizes the relatively thin steel shell by maintaining its shape, and opposing the inward elastic instability that precedes buckling failure. Secondly, it uses the well-known advantage of prestressing concrete in providing for optimal use of the concrete composite (residual compressive strain) and steel (residual tensile strain) components, and preventing the concrete composite component from experiencing any tensile stress, even under worst-case loading. Thirdly, by having the much-thicker reinforced and prestressed concrete composite jacket on the outside, the area moment-of-inertia of the outer jacket will be over 10 times larger than that of the inner shell, meaning that the outer jacket will carry nearly all of the bending stress. The vertical loads of the tower and wind turbine will be distributed nearly-equally between the inner shell and outer jacket.
Each tower section would start out as a tapered steel tube that would be very similar in appearance to current towers, however, the steel would be approx. 1/3rd as thick. Oversized end flanges would be then be welded on. These would use the conventional means of internally bolting the tower sections, and would also extend outward beyond the outside diameter of the steel tube. A suitable assembly of reinforcing steel would then be welded in place, and this would likely be done by robotic welders. Once all of the welding had been inspected, the tower section would be sealed with suitable endcaps and gaskets, and each end fixed to a set of motorized rollers. The proprietary concrete – polymer layer would then be robotically-applied using the “Shotcrete” process. The mix would be optimized for this application and would be significantly lighter than conventional concrete. When the outer jacket has cured, the pressure is slowly released and its stored strain energy becomes optimally distributed between the shell and the jacket. The name “ISO-e” alludes to this equal but opposite shared strain energy. This acts to increase the tower's resistance to buckling, since it provides the same benefit as internally-applied pressure. while ensuring that the concrete composite jacket never experiences any tensile stress. It is expected that the finished tower will weigh about the same as a similar, all-steel tower, and the key to achieving this is in the density of the concrete – polymer composite. Manufactured in standard-length sections, the completed tower will demonstrate a much higher damping ratio than conventional all-steel construction which will reduce tower sway and provide for large reductions in radiated noise levels. In addition, a thin, chloride-resistant outer layer may be applied to towers destined for offshore or near-shore installations, and the outside may be colored to match the environment, providing a more pleasing appearance.
Buckling failure of a wind turbine tower resulting from high winds.
