Patent Application
20130156596
The present disclosure relates generally to wind turbines and, more particularly, to an improved design for wind turbine towers that reduces or eliminates aerodynamic wake downwind of the tower.
A conventional wind turbine typically includes a set of two or three large blades mounted to a hub. The blades and the hub together are referred to as the rotor. Wind causes the rotor to rotate about a horizontal main shaft, which in turn is operatively connected via a speed increasing gear box to a generator or a set of generators that produce electric power. The main shaft, the gear box and the generator(s) are all situated within a nacelle, which is situated on top of a tower.
In recent years, engineers have designed wind turbines of all sizes and integrated them with electric power generation systems to create electricity to support the needs of both industrial and residential applications. As the wind power market has matured, there has been a large push to reduce the cost of energy (COE) of the electric power being produced by these wind turbine systems. The COE for a wind farm development is almost entirely driven by the manufacturing costs, assembly costs and maintenance costs of the wind farm itself, as there are no fuel costs associated with generation of electricity at the wind farm site. There are several large cost drivers associated with a wind farm site: (1) the tower, (2) the blades, (3) the speed increasing gear box, (4) the electric generator, (5) the power conditioning system and (6) the high power transmission lines that bring the wind farm electricity to the grid.
Existing tower designs typically have a predominantly circular cross section and are symmetric about their central vertical axis. This symmetry allows for the use of a fixed (non-rotating) tower, with a yaw bearing attaching the nacelle to the top of the tower. The yaw bearing allows the nacelle, and thus the blades, to rotate with respect to the tower so the blades can point into the wind regardless of wind direction. These symmetric, circular cross section towers have two distinct drawbacks.
First, circular cross section towers must be designed to react with the turbine's aerodynamic forces in any direction. Since the aerodynamic forces are predominantly along the wind direction, the symmetric design is non-ideal from a material usage standpoint. This use of extra material—typically steel—drives up the cost of the wind turbine.
Second, circular cross section towers create a pronounced “tower shadow effect.” The tower shadow effect refers to the aerodynamic wake that is present immediately downwind of the tower. In this aerodynamic wake zone, the wind velocity is dramatically reduced compared to the free stream (unencumbered) velocity. The tower shadow effect prohibits the use of a downwind turbine (one in which the blades are downwind of the tower), since the tower shadow effect momentarily unloads each blade as it passes through the aerodynamic wake zone. This momentary unloading of the aerodynamic force causes a loud audible noise to be generated. The noise is proportional to the magnitude of the tower shadow. In addition to the audible noise, the tower shadow effect also causes periodic force perturbations to impact the wind turbine system.
For these reasons, wind turbines having symmetrical, circular cross section towers operate pointing into the wind, with the blades upwind of the tower. Such wind turbines are referred to as “upwind wind turbines” or simply “upwind turbines.”
Another drawback to upwind turbines is that the blades must be rigid enough to ensure that they will not bend and strike the tower when subjected to high wind loads. Stiffer blades require more material to make, thereby increasing the cost of the wind turbine. Engineers have designed wind turbines in which the blades are set at a pitch (slight angle) off the vertical so that the distance between the blades and the tower is greatest when the blades are at their lowest point, but this solution has the disadvantage of reducing the wind driving force, and thus, electrical output.
Accordingly, it would be beneficial to provide a wind turbine tower that eliminates the tower shadow effect so that a downwind wind turbine could be utilized. The use of a downwind wind turbine would reduce the cost of the turbine system by allowing for lighter, more flexible blades, since the wind loads would tend to bend the blades away from the tower structure.
A novel wind turbine is disclosed. In a first embodiment of the invention, the wind turbine comprises a tower and a rotor having a hub and at least one blade. The rotor is rotatably mounted to a nacelle (along a horizontal axis of rotation), and the nacelle is mounted to the tower in stationary (fixed) relationship therewith. The tower has a top, a bottom, a height, a maximum thickness (width), and a yaw bearing at its base so that it is rotatable about a vertical axis.
In one embodiment of the invention referred to as the “single airfoil design”, the tower comprises a left side wall and a right side wall connected to each other along a rounded leading vertical edge and a relatively narrower trailing vertical edge to form a substantially hollow shell. The tower has a cross-sectional airfoil shape having a major chord extending from the leading vertical edge to the trailing vertical edge and a minor chord running perpendicular to the major chord and intersecting the major chord at the area of maximum tower thickness. The left and right side walls are curvilinear and symmetrical about the major chord. The length of the minor chord preferably is less than 50% that of the major chord, and may be less than about 33% that of the major chord. The maximum tower thickness may be smaller at the top of the tower than at the bottom of the tower.
In another embodiment of the invention referred to as the “twin airfoil design”, the tower comprises two separate, spaced apart airfoils. Each airfoil comprises a curved outer wall and a relatively flatter (less curved) inner wall connected along a rounded leading vertical edge and along a relatively narrower trailing edge. The inner walls are spaced apart and face each other. Each airfoil has a cross-sectional airfoil shape along all or a substantial portion of its height, with a major chord extending from the leading vertical edge to the trailing vertical edge and a minor chord running perpendicular to inner walls and intersecting the major chord at the area of maximum airfoil thickness. Each airfoil is asymmetrical about its major chord.
The twin airfoils are arranged symmetrically about a vertical plane and preferably are oriented at a slight angle of about seven degrees with respect to each other, with the leading edges farther apart than the trailing edges, so as to define a space there between through which a relatively high speed jet of air can travel to fill the aerodynamic wake zone behind the tower.
Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings.
For a more complete understanding of the disclosed apparatuses, reference should be made to the embodiments illustrated in greater detail on the accompanying drawings, wherein:
While the following detailed description has been given and will be provided with respect to certain specific embodiments, it is to be understood that the scope of the disclosure should not be limited to such embodiments, but that the same are provided simply for enablement and best mode purposes. The breadth and spirit of the present disclosure is broader than the embodiments specifically disclosed and encompassed within the claims eventually appended hereto.
In the description that follows the term “chord” means a straight line connecting two points on a closed two dimensional shape and the term “major chord” means a straight line connecting the two points farthest apart on a closed two dimensional shape.
Referring now to
In addition to the components of the wind turbine 10 described above, the tower 14 may include several auxiliary components, such as a yaw system on which the nacelle 18 may be positioned to pivot and orient the rotor in a direction facing the prevailing wind current (and thus upwind of the tower 14) or another preferred wind direction, a pitch control unit (PCU) (not shown) situated within the hub 16 for controlling the pitch (e.g., angle of the blades with respect to the wind direction) of the blades 12, a hydraulic power system (not shown) to provide hydraulic power to various components such as brakes of the wind turbine, a cooling system (not shown) and the like.
The tower 14 shown is generally shaped like a circular cylinder having a constant diameter from top to bottom. Alternatively, many conventional towers are generally shaped like a circular cone having a larger diameter at the bottom than at the top. In either case conventional towers have a predominately circular cross section and therefore are substantially symmetrical throughout their height (vertical axis). This symmetry allows for the use of a fixed (non-rotating) tower, with a yaw bearing attaching the nacelle 18 to the top of the tower 14. The yaw bearing allows the rotor and nacelle 18 to rotate with respect to the tower 14 so the turbine 10 can always point into the wind.
The present invention is designed to eliminate the tower shadow effect so that a downwind turbine can be utilized. The use of a downwind turbine reduces the cost of the turbine system by allowing the designers to create lighter, more flexible, turbine blades, since upwind turbines require that the blades 12 be sufficiently rigid to ensure that they will not strike the tower 14 when subjected to high wind loads. With a downwind turbine, the wind loads tend to bend the blades away from the tower 14.
In accordance with this goal, the present invention is a new wind turbine design having a rotatable tower that incorporates a yaw bearing at the tower base, rather than at the top of the tower. By moving the yaw bearing from the top of the tower to the bottom of the tower, the entire tower structure is allowed to rotate along with the turbine and the nacelle, thereby maintaining the alignment of the tower with the nacelle/ turbine. By maintaining the alignment of the tower with the nacelle and rotor, a tower can be constructed with a smaller cross section in the direction normal to prevailing wind direction than the cross section parallel to the prevailing wind direction.
The tower 24 cross section may be any suitable airfoil shape, but preferably is rounded in the area around the forward (wind) facing edge 36 and relatively narrower in the area around the trailing edge 38. The length of the minor chord B, i.e., the maximum thickness of the tower 24, is substantially less than the length of the major chord (A), i.e., the distance between the leading and trailing edges 36, 38, and preferably is less than 50% that of the major chord (A), and may be only about one third (⅓) the length of the major chord (A). In any case, the tower 24 is rotatable so that its smaller cross section (represented by minor chord (B)) is generally normal to the prevailing wind direction (W).
A second embodiment of the invention will now be described that also utilizes a full tower yawing system like the first embodiment. As shown in
Each airfoil 62, 62′ comprises an outer wall 66, 66′ and an inner wall 68, 68′ connected at a leading edge 70, 70′ and at a trailing edge 72, 72′ to form a substantially hollow shell. In contrast to the first embodiment, each airfoil 62, 62′ is asymmetrical about its major chord (C, C′), with one wall, the outer wall 62, 62′, being substantially more curved than the relatively flatter opposite (inner) wall 68, 68′.
A minor (shorter) chord (D, D′) is defined by a line running perpendicular to the inner walls 68, 68′ and intersecting the major chord (C, C′) at the area of maximum airfoil (and thus tower) thickness. While the major chord (C, C′) is by definition straight, the camber line, defined as the locus of points midway between the outer and inner walls, is curved. Each tower airfoil 62, 62′ may be any suitable airfoil shape, but preferably is rounded in the area around the forward (wind) facing edge 70, 70′ and relatively narrower in the area around the trailing edge 72, 72′.
As shown in
The airfoils 62, 62′ are arranged symmetrically about a vertical plane (P). In operation the vertical plane (P) generally will coincide with (be parallel to) the prevailing wind direction (W). The angle defined by the major chords (C, C′) may be about seven degrees. As in the first (single airfoil) embodiment, the tower 54 is rotatable so that its smaller overall cross section is generally normal to the prevailing wind direction (W).
The twin airfoil design of this second embodiment has two significant benefits. First, it eliminates or nearly eliminates the tower shadow effect, thereby reducing the acoustic signature of the wind turbine, and further reducing the structural perturbations being reacted by the turbine structures. Second, the twin airfoil tower 54 further reduces the mass of the tower, and therefore reduces the overall cost of the wind turbine 50. This may be accomplished by moving the two airfoil sections 62, 62′ away from one another, thereby increasing the effective width of the tower 54 without increasing its aerodynamic drag. As the effective width of the tower 54 is increased, the stresses on the tower 54 are proportionately lowered. Since the tower stresses are lower, the tower walls 66, 66′, 68, 68′ can be made with thinner materials, thereby lowering the overall tower mass and manufacturing costs.
