The present disclosure relates to wind turbines for electric power generation and more particularly to a vertical axis wind turbine having improved efficiency.
The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Modern wind turbines generally exist in two configurations: horizontal axis and vertical axis, the designations referring to the axis about which the turbine blade disk rotates. Horizontal axis wind turbine (HAWT) configurations are inherently more efficient than vertical axis wind turbines (VAWT) because the full blade disk faces the ambient wind, whereas the blades in a vertical axis turbine alternately advance into and retreat from the wind. Nonetheless, both configurations have certain recognized benefits and drawbacks. Commercial HAWTs are generally immense devices, mounted high in the air where they are exposed to higher wind velocities but where they contribute to visual pollution and interference with migratory and local bird flight and produce strobe-like effects during periods of low incident sunlight, i.e., dawn and dusk. On the other hand, VAWTs are generally installed closer to the ground. While this siting yields lower effective wind speeds, it effectively overcomes the visual pollution, bird and strobe effect problems of HAWTs. Being nearer the ground also allows VAWT's to be more readily repaired and maintained. Moreover, VAWTs can be placed in close proximity to each other, an attribute that is beneficial, especially for wind farm installations.
In both wind turbine types the ultimate objective is the maximization of the resultant power (the product of the shaft torque and the angular rate) delivered to an electrical generator for every ambient wind direction and velocity condition. As the foregoing summary highlights, it would be advantageous to develop wind turbines that have the inherent advantages of VAWTs that are also more efficient and thus competitive with HAWTs. The present invention is so directed.
The present invention provides a vertical axis wind turbine that provides greatly improved efficiency over prior art vertical axis turbine configurations because of the mechanical elements that lead to the maximum torque being applied to the electrical generator from the aerodynamic blades. A maximally efficient vertical axis wind turbine (MEVAWT) according to the present invention includes a rotatable circular frame having upper and lower concentric flat rings or disks which support a plurality of, typically three, four, five or six, pivotable cascades, each including a plurality of fixed, configurable airfoils. The airfoils preferably include single, pivotable trailing flaps and may include lateral extensions. The center and periphery of the lower ring are supported in suitable bearings to facilitate free rotation of the frame. Wind direction and velocity sensors provide data utilized to control drive mechanisms which orient each cascade and the flap of each airfoil to maximize the resultant power produced by the turbine. An integral electrical generator includes permanent magnets on the periphery of the lower ring of the frame which cooperate with adjacent stator windings.
Thus it is an aspect of the present invention to provide a vertical axis wind turbine having improved efficiency,
It is a further aspect of the present invention to provide a vertical axis wind turbine having a rotatable frame including a bearing supported lower ring.
It is a still further aspect of the present invention to provide a vertical axis wind turbine having a wind direction sensor and a wind velocity sensor.
It is a still further aspect of the present invention to provide a vertical axis wind turbine having a plurality of cascades each having a plurality of airfoils.
It is a still further aspect of the present invention to provide a vertical axis wind turbine having a plurality of pivotable cascades each having a plurality of airfoils.
It is a still further aspect of the present invention to provide a vertical axis wind turbine having a plurality of pivotable cascades each having a plurality of fixed, configurable airfoils.
It is a still further aspect of the present invention to provide a vertical axis wind turbine having a plurality of pivotable cascades having a plurality of airfoils and drive assemblies for pivoting the cascades and configuring the airfoils.
Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
With reference to
Below the lower circular plate or disk 24 or integrally assembled therewith are a plurality of equally circumferentially spaced radial support arms 28. The circular frame 20, including the upper and lower circular disks 22 and 24 and the radial arms 28 are freely, rotatably supported on the central support 14 by a center anti-friction bearing 30 disposed between the central support 14 and the radial arms 28. For a commercially viable VAWT, the circular frame 20 will typically be quite large and have a diameter between ten and twenty meters or more or less. The circular base 16 locates and supports a concentric circular track 32 having an upper terminal portion 34 defining a generally circular cross section. In contact with, stabilized by and riding on the circular track 32 are a plurality of main support bearings 36A, a plurality of outside, anti-tipping bearings 36B and a plurality of inside, guide bearings 36C which are connected to and support the radial arms 28 and the frame 20.
Referring now to
Each of the cascades 40 includes an upper plate, base or end member 44, a lower plate, base or end member 46, a hollow support tube 48 that receives the vertical support or shaft 42 and a plurality of configurable airfoils 60 that are fixedly secured to the upper and lower bases or end members 44 and 46. A suitable antifriction thrust bearing 50 preferably resides between each of the vertical supports or shafts 42 and the hollow support tube 48 to facilitate free, pivoting motion of the cascade 40 about the axis of the vertical support or shaft 42. A drive mechanism 52 that is capable of constantly and independently rotating and re-orienting each of the cascades 40 relative to the circular frame 20 is associated with each of the cascades 40. The drive mechanisms 52 may be electrically, pneumatically or hydraulically operated, are disposed on the lower circular disk 24 and drive, i.e. rotate, each cascade 40 through, for example, a gear train, chain or timing belt 54. The drive mechanisms 52 receive signals from a microprocessor 56 having data inputs and outputs, storage, algorithms incorporating the equations set forth more completely below and other conventional electronic modules. The microprocessor 56, in turn, receives data from a wind speed and direction sensor 58 that is located near the vertical axis wind turbine 10 so that its measurements accurately reflect the wind direction and speed to which the turbine 10 is exposed but not so near as to be affected by the presence of the turbine 10.
The number of airfoils 60 on each of the cascades 40 is equal and will be three, four, five, six or more or fewer depending upon various operational parameters. The illustration of three airfoils 60 on each of the cascades 40 in
Referring now to
In
Referring now to
Referring now to
Referring now to
The airfoil configurations are shown in
Zone I: π/4≲φc≦3π/4, bluff body, maximum torque, deployment of the optional width extenders 74 or the flaps or tails 66 to increase the aerodynamic drag;
Zone II: 3π/4≦φc≲5π/4, the single flaps or tails 66 pivoted clockwise for maximum lift;
Zone III: 5π/4≲φc≲7π/4, the single flaps or tails 66 straight and the optional width extenders 74 withdrawn for minimum drag; and
Zone IV: 7π/4≲φc≲π/4, the single flaps or tails 66 pivoted counterclockwise for maximum lift.
Note that the downwind: φc=π/2→3π/2, region will be influenced by the upwind cascades 40. The zone boundaries must be corrected for these effects.
An additional degree-of-freedom is provided by the orientation of the cascade: θc=θc(φc), with respect to the radial support arm 28; see
A subtle, but important aspect of the maximally efficient claim, involves the power to establish the airfoil conditions as a function of φc. Namely,
b) The relative chord length=C/D=0.327 is representative of that for the planned prototype. c) The radial support arms 28 (below the indicated disk 24) are not shown in this figure. d) The four cascades 40 and the three airfoils 60 per cascade are merely representative.
The derived power is maximized by causing each airfoil 60 in a cascade 40 to maximize the component of the net aerodynamic force: {right arrow over (F)}L+{right arrow over (F)}D that is perpendicular to the support arm 28 for that cascade 40. The incoming wind direction will be monitored for a suitable period by the sensor 58 (to gain its locally averaged value) and the control system will continuously position the cascade 40 during the 0→2π revolution of φc in keeping with that inflow direction and velocity magnitude. Implicit in this description is the condition that Ω(=dφ/dt) will be controlled to permit the required position adjustments to be made during the period of the revolution. The angular speed (Ω) will be controlled by the extracted power from the generator 90 as is described below.
Limiting Ω to account for the positioning requirements has the negative attribute that the extracted power is also limited. In this regard, it is a positive attribute that:
These are the factors that will establish the optimal Ω value. For the present purpose of assigning numerical values, 5 seconds will be allowed for the φc=13π/8→15π/8 transition or Ω=1.5 rpm.
The approach flow for a given airfoil 60 can be described as (see
{right arrow over (V)}a/A={right arrow over (V)}a/g+{right arrow over (V)}g/A
where a=air, g=ground and A=airfoil. {right arrow over (V)}g/A will be perpendicular to the radial support arm 28. That is, {right arrow over (V)}g/A is opposite to that of the optimal sum of the aerodynamic forces.
Using {right arrow over (V)}a/g=10 mph=4.4 m/sec as the start-up speed and dφc/dt=Ω=1.5 rpm as an angular speed that will allow the θc and γ positions to be established for a twenty meter diameter vertical axis wind turbine 10, it is seen that {right arrow over (V)}g/A must be accounted for in the θc (φc, {right arrow over (V)}a/g) control system; see
Arranging the split-flap airfoils 60 in the cascade 40 does more than simply multiply the aerodynamic forces of one airfoil by N blades, it makes the airfoils exhibit larger lift coefficients than the CL(α) values of an isolated airfoil since the adjacent airfoils provide an attached flow condition (on the suction side) for a higher angle of attack than that for an isolated airfoil. The cascade of airfoils will also be responsible—to some extent—for a flow blockage effect that will cause the flow to divert around the cascade.
The operational strategy can be described in summary form as:
The large diameter of the vertical axis wind turbine 10 makes it an ideal generator of electric power.
The vertical axis wind turbine 10 offers an ideal combination of aerodynamic effectiveness and electrical power generation. Specifically, the large diameter of the frame 20 will permit the revolving permanent magnets 92 and the slightly larger diameter current carrying stator coils 94 to represent an electrical generator that may readily be 20 meters in diameter. Its relationship to the standard generator design ensures its functionality. Specifically, the electric machine in this design is functionally equivalent to surface permanent magnet machines, in which the permanent magnets are mounted on the surface of the rotor. In the present case, the electric machine rotor is part of the wind turbine rotor. In consideration of the electric machine design, the number of magnetic poles can be determined such that the desired output electric frequency (in the order of tens Hz) is matched with the maximum operating speed of the turbine.
The basic operating principle of the electric machine equipped with permanent magnets is that the alternating currents in the stator winding will produce a rotating magnetic field, which interacts with the magnetic field created by permanent magnets to produce torque. By regulation of the stator currents, both the magnitude and the orientation of the magnetic field excited by the stator currents can be controlled. Hence, the torque of the machine can be controlled and the speed of the electric machine-turbine rotor can be regulated to track the speed command. The turbine speed command will typically come from an optimal power point tracking control block that maximizes the captured power given a measured wind speed.
The exceptionally large circumferential distance (20π meters=62.8 meters) means that there can be an exceptionally high fundamental frequency, which is ideal for electrical efficiency since it eliminates the need for a speed increasing gearbox.
The inherent energy storage capability of the large mass of the rotating frame 20 will ensure stable operation against short-term intermittency of wind speed variations. This provides stable output power with limited requirements for further power electronics controls—a desirable condition from the point of view of power system control.
The required control of the angular speed (Ω), that is essential for the maximum efficiency (electrical power output/wind power input) to be provided by the vertical axis wind turbine 10, is quite simply enabled by standard power electronics components. Technically, the function of speed control of the machine/turbine is accomplished with a power converter that is connected between the terminals of the machine's stator winding and electric power grids. The power converter can effectively and efficiently synthesize the appropriate voltage by controlling internal semiconductor switches. At a very simplified level, the electric machine can be modeled with a set of winding inductances and induced voltages (or electromotive forces) that result from the rotating permanent magnets. Thus, dynamic control of the stator currents can be realized with a set of dynamically controlled voltages synthesized by the power converter. Accordingly torque and speed control of the wind turbine is achieved.
The cascade 40 will be rotated about the axis of its support shaft 42 (see
The angles of attack of the airfoils 60 with respect to the local oncoming wind can be reliably estimated (a'priori) for the forward half of the φc rotations: φc=3π/2→π/2. The approach flow angles in the leeward region (φc=π/2→3π/2) will be influenced by the upwind cascades. For the present analysis, the required θc angles will be determined as if there is no upwind effect.
The vector triangles:
{right arrow over (V)}a/A={right arrow over (V)}a/g+{right arrow over (V)}g/A (A.1)
for each φc value identify the approach flow of the air (a) ({right arrow over (V)}a/A) with respect to the airfoil (A). The negative of the velocity of the airfoil 60 with respect to the ground: {right arrow over (V)}A/g={right arrow over (Ω)}×{right arrow over (R)}c, provides the {right arrow over (V)}g/A velocity. An isolated flapped airfoil gains its maximum lift at ca 8 degrees angle of attack. (This value is dependent on the cascade configuration and may change.) Hence, with representative values for Rc(=10 m) and Ω(=2πrad/40 sec), the θc(φc) calculations can be established for a given {right arrow over (V)}a/g magnitude. (By definition, {right arrow over (V)}a/g is aligned with φc=0). A Cartesian system is then useful as
{right arrow over (V)}a/g=îu (A.2)
and
{right arrow over (V)}g/A={right arrow over (R)}c×{right arrow over (Ω)}=−î(RΩ)sin φ+j(RΩ)cos φ (A.3)
which yields
{right arrow over (V)}a/A=î[u−RΩ sin φ]+ĵRΩ cos φ (A.4)
Introducing the angle β as the orientation of {right arrow over (V)}a/A yields
The tip-to-tail orientation of the airfoil 60 for maximum lift can then be designated as (β+8°). The base of the cascade 40 can be described by the vector (îBx+ĵBy) with the understood orientation that {right arrow over (B)} points to the half-plane: φ>π→2π.
The orientation of the cascade base is obtained by subtracting π/2 from β. That is,
Given that the centerline of the airfoil 60 is perpendicular to the cascade base and given that θ=0 is the condition wherein the base is aligned with the radial arm 28 (that is, when the base is aligned with φ) the θc=θc(φc) relationship is obtained by rotating θ to the position
θc=π/2−φc+β
The function: θc=θc(φc) is shown in
The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/680,596, filed Aug. 7, 2012, which is hereby incorporated in its entirety herein by reference.
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