Patent Application
20100135821
The invention relates generally to wind turbine tower construction and more specifically to a wind turbine tower and its method of construction that permits rail transport of sections for large towers.
For many years it has been common practice to build steel wind tower sections separately in a workshop facility and then to move each complete section to the site, where the wind turbine tower installation was performed. The tower sections would typically have a cylindrical or slightly tapered shape, and each of the sections could in turn be divided along axial lines into an adequate number of shells.
Due to the ever-increasing demand for taller and larger capacity towers and consequently larger dimensions of all parts needed to build such towers, a physical limit has been imposed by the infrastructure, e.g. the clearance on a bridge or in a tunnel or underpass.
For a given Wind Turbine, the wind load increases as square of the wind speed. As the tower height is increased, the wind shear increases non-linearly due to wind shear (ground effect on wind speed). Consequently, the higher the turbine towers are, the stronger should the structure be dimensioned, which in turn means that either the wall thickness should be increased or the diameter extended. Increased thickness would mean higher material costs and a requirement for heavier transportation vehicles, whether trucks, trains, ships, or helicopters, while diameters need to be appropriately dimensioned in order to pass over bridges and through tunnels and underpasses. Also, thicker steel stock is more difficult and more costly to form and fabricate.
The present invention relates to an apparatus and method for allowing sections of large wind turbine towers to be transported to a windfarm site by rail transport by construction of the tower sections to fall within an allowable space envelope for rail transport.
Briefly in accordance with one aspect of the present invention, a tower is provided having a plurality of axial sections. The tower includes at least one lower axial section having a non-circular cross-section in at least a portion thereof. The lower axial section is located near a bottom of the tower. At least one upper axial section has a substantially circular cross-section in at least a portion thereof, and is located near a top of the tower. The tower has a cross-sectional profile that transitions from the non-circular cross-section to the substantially circular cross-section, and an outer diameter of the tower is less than a designated maximum diameter.
In accordance with another aspect of the present invention, a wind turbine tower is provided having at least one lower section having a non-circular cross-section, and at least one upper section having a substantially circular cross-section. The cross-sectional profile of the tower transitions from the non-circular cross-section in the lower section to the substantially circular cross-section, in the upper section.
In accordance with yet another aspect of the present invention, a wind turbine is provided having at least one lower tower section having a non-circular cross-section in at least a portion thereof. At least one upper tower section has a substantially circular cross-section in at least a portion thereof. The cross-sectional profile of the wind turbine tower transitions from the non-circular cross-section in the lower tower section to the substantially circular cross-section in the upper tower section.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The following embodiments of the present invention have many advantages, including permitting wind turbine tower sections that have previously required large diameters for structural integrity to incorporate reduced diameters that fall within allowable space envelopes for rail transport.
A material shell, of which the tower consists, is responsible for carrying the loads induced by the wind or other necessary application. A shell of this kind most cost efficiently resists these bending loads by having the maximum possible diameter about a neutral axis 116. By not using the maximum outer diameter, a cost inefficiency is manifested by having to increase tower shell thickness not only in the bottom tower shell, but potentially in all tower sections where the maximum potential outer diameter is not utilized. Each additional increase in thickness at a fixed outer diameter is less efficient than the previous due to ever decreasing average shell diameter and load bearing capability. Increasing tower shell diameter will allow for relative shell thicknesses to decrease roughly to a square power.
Accordingly, it may be advantageous to increase tower diameter rather than the thickness of the steel plate or other wall material. For a given wind load, the wall thickness needed to resist this load varies inversely as the square of the diameter. A ten percent reduction in diameter would require a wall thickness that is twenty one percent larger, to have a tower that structurally resists the same load as before. In principle, this means that, any tower design that has portions of it situated far outboard from the tower center would result in less wall thickness. Since the available railable or truckable window is somewhat rectangular, we can utilize the vacant corner spaces between a circular cross-section and a square one to maximize effectiveness of the structure.
The lowermost cross-section of the base of the tower sees the highest loads and stresses due to the wind loads. It is this section that needs the most tower material (wall thickness at a chosen diameter). The loads at higher sections will gradually diminish, thus requiring thinner walls. Hence a tower whose cross-section at the lower parts of the base is nearly square (e.g., rectangular with rounded corners, octagonal, etc.) could be lighter, and have the thinnest possible walls. Sections approaching the middle and upper parts of a tower can blend into the circular cross-section at the very top, near the yaw bearing (not shown). This transition to circular can occur anywhere in the tower, since at some intervening heights, the diameter of the tower would be easily railable, and the tower weight manageable.
The advantage of this design is in the improved strength to weight ratio and more optimal dimensions (from a transportation standpoint) of the tower. By using a polygonal base section having a non-circular cross-section, the outer circumference of the tower can be expanded to take advantage of the available shipping window.
In all the embodiments of the present invention, the transition from a non-circular cross-section to a substantially circular cross-section may occur in one tower section, multiple tower sections or within part of one tower section. In addition, the transition may occur between tower sections by using an adapter that is configured to join one tower section with a non-circular cross-section to another tower section having a substantially circular cross-section. An adapter could also be used in a single tower section as well.
The non-circular cross-section in any, or a portion, of the tower sections can have any suitable shape as desired by the specific application. By non limiting example, the non-circular cross-section could have a shape that is polygonal, polygonal with rounded corners, triangular, rectangular, rectangular with rounded corners, pentagonal, pentagonal with rounded corners, hexagonal, hexagonal with rounded corners, heptagonal, heptagonal with rounded corners, octagonal, octagonal with rounded corners, nonagonal, nonagonal with rounded corners, decagonal, decagonal with rounded corners, hendecagonal, hendecagonal with rounded corners, dodecagon, or dodecagon with rounded corners.
Typically, rail carriers permit items of a maximum weight, width, height and length. The tower, according to aspects of the present invention, can be sized to fit within these limitations. The weight of each tower section can be designed to be under about 140,000 lbs (˜64,000 kg), or under any weight limit imposed by typical rail carriers or trucking companies. The width and height of each tower section can be designed to be under about 13 feet 6 inches (˜4.1 meters), or under any height and/or width limit imposed by typical rail carriers. The length of each tower section can be designed to be under about 89 feet (˜27 meters), or under any length limit imposed by typical rail carriers. Rail transport outside the U.S. is also constrained by similar considerations of weight, width, height and length, but with sizes specific to the locale. Accordingly, an improved tower has been provided that can be shipped by rail, enabling less costly transportation for large towers. A single train can transport many wind turbine towers, whereas at least three trucks were required to transport a single tower.
The present invention was described in conjunction with a tower for a wind turbine; however, it is to be understood that the tower, according to aspects of the present invention, may be useful for any application needing elevated towers. For example, the present invention could be applied to electrical utility power transmission wire towers, communication towers, on or off-shore wind turbine towers, lighthouses, fire monitoring towers, agricultural silos, residential or commercial applications, and any other application requiring a tower.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
