Patent Application
20110138704
This invention relates generally to towers. In particular, but not limited thereto, the present invention relates to wind turbine towers having tensioning cables.
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Several technical installations require a tower or a mast to which the installation is mounted. Non-limiting examples of such installations are wind turbines, antenna towers used in broadcasting or mobile telecommunication, pylons used in bridge work, or power poles. Typically, the tower is made of steel and must be connected to a foundation made of reinforced concrete. In these cases, the typical technical solution is to provide a large, solid reinforced concrete foundation at the bottom of the tower. In typical applications the tower foundation extends about 12 meters below the ground level, and can be about 18 meters or more in diameter.
In larger utility grade wind turbines (e.g., 2.5 MW or more) it is often desired to have towers with heights of 80 meters or more. The higher hub heights provided by larger towers enable the wind turbine's rotor to exist in higher mean wind speed areas, and this results in increased energy production. Increases in tower height invariably have lead to corresponding increases in the mass, length and diameter of the tower. However, it becomes difficult to construct and transport large wind turbine towers as the local transportation infrastructure (e.g., roads, bridges, vehicles, etc.) often impose limits on the length, weight and diameter of tower components.
According to one aspect of the present invention, a tower is provided. The tower includes at least one concrete tower section having a plurality of tensioning cables. The tensioning cables are configured to induce a compressive force on the concrete tower section. The tensioning cables are spaced from an exterior surface of the concrete tower section by a substantially uniform distance.
According to another aspect of the present invention, a wind turbine tower is provided. The tower includes at least one concrete tower section having a plurality of tensioning cables. The tensioning cables are configured to induce a compressive force on the concrete tower section. The tensioning cables are spaced from an exterior surface of the concrete tower section by a substantially uniform distance.
Reference will now be made in detail to the various aspects of the invention, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation of the invention, and is not meant as a limitation of the invention. For example, features illustrated or described as part of one aspect can be used on or in conjunction with other aspects to yield yet a further aspect. It is intended that the present invention includes such modifications and variations.
The wind turbine 100 shown in
It would be advantageous to increase tower height in order to capture more energy due to higher mean wind speeds. An aspect of the present invention provides a tower or tower section fabricated from concrete. A concrete base section can be used to elevate a conventional rolled-steel tower, or the entire tower can be formed of concrete. Concrete is defined as a mixture of aggregates and binder or any suitable masonry support. As one non-limiting example only, the aggregates may be sand and gravel or crushed stone, and the binder may be water and cement.
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. In conventional reinforced concrete, the high tensile strength of steel is combined with concrete's great compressive strength to form a structural material that is strong in both compression and tension. The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.
Compressive stresses can be induced in prestressed concrete either by pretensioning or post-tensioning the steel reinforcement. In pretensioning, the steel is stretched before the concrete is placed. High-strength steel tendons or cables are placed between two abutments and stretched to a portion of their ultimate strength. Concrete is poured into molds around the tendons/cables and allowed to cure. Once the concrete reaches the required strength, the stretching forces are released. As the steel reacts to regain its original length, the tensile stresses are translated into a compressive stress in the concrete.
In post-tensioning, the steel or cable is stretched after the concrete hardens. Concrete is cast in the desired shape first. Once the concrete has hardened to the required strength, the steel tendons or cables are attached and stretched against the ends of the unit and anchored off externally, placing the concrete into compression. According to an aspect of the present invention, post-tensioned concrete is used for wind turbine towers or wind turbine tower sections.
The tensioning cables 280 are located circumferentially around and external to the concrete walls 260, and are positioned close to and at a substantially uniform distance from an outer or exterior surface of concrete walls 260. The term “substantially uniform” can be defined as having approximately the same, or having a slightly varying distance (e.g., a slight taper). In other words, the tensioning cables 280 can be parallel to or nearly parallel to the outer surface of concrete walls 260. As one non-limiting example only, the tensioning cables 280 may be spaced from an exterior surface of a top portion of concrete wall 260 by about two inches, whereas the cables 280 may be spaced from an exterior surface of a bottom portion of concrete wall 260 by about six inches. The cables 280 can be of the post-tensioned type, and they apply a compressive force to concrete walls 260. The use of external cables results in a larger moment arm and lower cable forces, and eventually, smaller cables would be required when compared to using the cables internal to the concrete segments. In other aspects of the invention, the tensioning cables 280 are positioned close to an exterior surface of concrete walls 260, but may be configured to have a slightly increasing or slightly decreasing distance from the exterior surface of concrete walls 260.
During operation of the wind turbine 200, wind flows in the direction indicated by arrow 202. The force of the wind creates a load on the wind turbine and tower. The up-wind side of the tower (i.e., the left side of the tower as shown in
One advantage provided by the present invention is the reduction of the effective moment-arm on tower section 215. By positioning the tensioning cables 280 close to and external to the exterior surface of concrete walls 280 the tower 210, 215 reduces its effective moment-arm to provide resistance to wind loads. This invention moves the cables outside, but in close proximity to the tower walls. For example, a very small diameter tower having internal cables would need thicker walls and thicker cables to counteract the forces applied by the wind, when compared to a larger diameter tower having external cables. The larger diameter tower could be made with thinner concrete walls and have smaller diameter cables when compared to the very small diameter tower.
In another aspect of the present invention,
The grooves 690 in concrete wall 660 provide several advantages, a few of which are, (1) protecting the external tensioning cables 680 (even more so with cover 810), (2) keeps the cables 680 away from view (i.e., reduces visual impact), (3) allows for easy maintenance of the cables 680 by facilitating external access, and (4) centers the compressive load on the concrete (due to the post-tensioned cables 680) in the body of the concrete wall 660.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
