WO2005068833A2

WO2005068833A2 - Wind turbine with variable pitch blades

Info

Publication number: WO2005068833A2
Application number: PCT/BR2005/000003
Authority: WO
Inventors: Luiz Cesar Sampaio Pereira
Original assignee: Luiz Cesar Sampaio Pereira
Priority date: 2004-01-14
Filing date: 2005-01-12
Publication date: 2005-07-28
Also published as: BRPI0400041B1; BRPI0400041A; WO2005068833A3; WO2005068833B1

Abstract

A wind turbine (1) is provided with a system for varying the pitch of the propeller blades (7) in accordance with wind velocity, using a combination of the dynamic wind force against the blades and the centrifugal force applied to the rotating blades due to their own weight. The system controls the speed of rotation of the propeller, thus preventing security limits from being surpassed during extreme wind speeds or gusts. Each blade is so hingedly (11) mounted to the propeller hub (8) so that its longitudinal axis can describe a segment of a conical surface of which the extreme radii are accompanied by the aerodynamic profile of the blade, from a first blade position representing greater torque production efficiency to a second blade position representing a lesser torque production efficiency. The combination of the dynamic wind force against each blade and the centrifugal force acting on the blade during rotation of the propeller tends to rotate the blade towards the second blade position, overcoming, at higher speeds, a force applied by biasing means (18) associated with the blades that tends to rotate the blades back towards the first blade position.

Description

Title: "WIND TURBINE"

Field of the invention The present invention relates to wind turbines and more particularly to a system for controlling and limiting the rotational blade speed of wind turbines. Background of the invention An important consideration in the preservation and improvement of environmental conditions is a progressive reduction of pollution created as a byproduct of numerous techniques utilized in the production of bulk electricity power. Although the hydroelectric alternative meets these requirements, it is also fraught with drawbacks, which include high costs and the damage caused by the flooding of large areas. It is also known that wind currents can be successfully harnessed for conversion to electrical power. The rudimentary use of wind currents dates back to sailing ships. Windmills for grinding grains or pumping water came much later and represented a great advance in the direction of the modern wind turbines. The use of generators having a plurality of magnetic poles and the adoption of structures allowing less expensive, lighter constructions, unlike more conventional electric machines, has increased the competitiveness of wind power production. In order to contribute to the improvement of the environment, the evolution of wind turbines depends largely on efforts to find alternatives which enable the construction of smaller, lighter and cheaper machines by making aerodynamic improvements, simplifying the assembly on higher towers and making the equipment commercially viable. A major obstacle in the use of wind turbines for the generation of electric power lies in the extreme variation of wind velocities. Considering that the wind energy capture of a windmill system varies with the cube of the velocity of the wind, a machine designed to operate under certain conditions may, within seconds, undergo stresses much greater than those for which it was designed. The alternative of manufacturing machines robust enough to withstand extreme conditions makes wind turbines costly and may still not provide the desired strength. It has thus become essential to include damping features in the machine which can change the configuration of the energy capture system, avoiding the transformation of wind energy into mechanical energy once the absorption limit is reached. There are two widely used strategies which limit the absorption of energy in a small wind turbine. One of them is flow deviation, where the collecting disc plane (circumference formed by the rotation of the blades) is mechanically deviated from its orthogonal position in relation to the wind direction. The other strategy involves a system of pitch control in which one or more blades are rotated about their longitudinal axis, which affects the wind vector angle on the profile of the blades, limiting the energy captured. Being simple and relatively cheap, systems based on flow deviation are the most widely used in small wind turbines. Unfortunately, these systems are not very effective in the event of non-uniform up flow or down flow wind currents or when there are sudden gusts. Such conditions may reduce the action of the flow deviation, causing damage to the equipment. An example of a pitch control system may be found in US Patent N°

2,464,234, in which the blades are rotated about their longitudinal axes. This rotary movement occurs solely in the turning plane of blades and is due exclusively to the action of centrifugal force. Another example in which pitch change is effected using centrifugal force is found in US Patent N° 4,678,923. On the other hand, US Patent N° 4,310,284 disclosed a system for varying the pitch of the blades without using centrifugal force, but rather the dynamic force of the wind itself. Other examples of prior art patents that disclose blade pitch variation for wind turbines are US Patents Nos. 6,514,043, 5,028,209, 5,584,655 and 4,534,705 but they all depend on external action to produce the rotation of the blades about their axes. Object of the invention It is the primary object of the present invention to provide a system to control and limit the rotational speed of the blades of wind turbines based on pitch control which is both simple and cheap, enabling its use in small and mid-sized pieces of equipment. Summary of the invention According to the present invention a wind turbine comprises a propeller having a plurality of aerodynamic shaped blades extending outwardly from a central hub, the pitches of the blades being variable, a propeller shaft driven by the propeller; and a generator driven by the propeller shaft. Each blade is so hingedly mounted to the hub that its longitudinal axis can describe a segment of a conical surface of which the extreme radii are accompanied by the aerodynamic profile of the blade, from a first blade position representing greater torque production efficiency to a second blade position representing a lesser torque production efficiency, said mounting being such that both the dynamic wind force against each blade and the centrifugal force acting on the blade due to its own weight during rotation of the propeller, tend to rotate the blade towards said second blade position. Biasing means associated with the blades apply a bias force thereto tending to rotate the blades back towards the first blade position. In the preferred embodiment, each blade has an inner end mounted to the propeller hub so as to be able to rotate partially about an axis that is inclined with respect to the longitudinal axis of the blade. Moreover, the biasing means may comprise pressure means, preferably a spring, mounted on the propeller shaft, the inner end of each propeller blade having a lever extension against which such means apply pressure to bias the blade towards the first blade position. In one alternative the pressure means may include a hydraulic system connected to counter the pressure applied by the pressure means, in accordance with the rotational speed of the propeller. It is also preferred that each propeller blade is provided with a lateral projection generally parallel to the propeller axis, the projection carrying a counterweight at its free end for increasing the centrifugal force during rotation of the propeller. The system of the present invention is based on the principles of aerodynamics which show that a profile (airfoil) presents a determined incidence angle allowing maximum lift. Greater or smaller angles cause the reduction in the lift force associated with an increase in the drag force. The invention, as will be seen from the above, adopts a geometrical arrangement which uses the centrifugal force acting upon the blades to produce a rotational movement of the profile, causing a loss of lift. In this case, the increase in rotational speed of the blades causes an opposing aerodynamic reaction and, up to a point, control of the angular speed. In addition, the structure is such that the rotation of the blades about their own axes is not only caused by the centrifugal force, but also by the aerodynamic force of the wind, the hinged connections with the propeller hub permitting the wind the force the tips of the individual blades backwards towards the above mentioned second blade position. A further aspect of the present invention, when applied in its presently preferred form, is that the mechanism that permits the turning of the blades about their longitudinal axes, is also the means of fixing the blades to their central disc or hub.

Brief description of the drawings The invention will now be described in more detail, by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a view in elevation of a wind turbine in accordance with the preferred embodiment of the present invention; Figure 2 is a diagrammatic representation of the construction of the preferred mounting of the turbine blades that permits geometry the blade movements required by the present invention; Figure 3 is a detail showing an individual blade for mounting in the arrangement shown in Figure 2; Figure 4 is a side view of the same turbine mounting with the nose cone removed and showing a spring loaded system biasing the turbine blades towards a maximum torque configuration; Figure 5 is a view similar to that of Figure 1 but showing the extreme positions of the turbine blades; Figure 6 is a side view similar to Figure 4 but with an alternative arrangement for increasing the centrifugal forces that tend to rotate the turbine blades about their longitudinal axes; Figure 7 shows a second embodiment differing from that of Figures 1 to 5 in that a hydraulic biasing system is used; and Figure 8 is a rotational velocity vs. wind speed plot comparing a fixed blade turbine with that of the preferred embodiment of the present invention. Detailed description of the drawings Fig. 1 illustrates a wind turbine 1 rotatably mounted by means of a suitable bearing device 2 at the top of a pole or tower 3, bearing 2 permitting the alignment of the turbine so as to face the wind. In the illustrated preferred embodiment, such alignment is guaranteed by a rudder 4 at the end of a tail portion 5. Independently of how this alignment is actually obtained, the heart of the present invention is located in an assembly formed by a generator 6 and a propeller having turbine blades 7, a central blade fixing disc or hub 8 and elements contained within a propeller nose cone 9. Figures 2 and 3 show diagrammatically the form of construction that will produce the geometry embodying the invention. Each wind turbine blade 7 has a tubular foot 10 attached to a tubular segment 11 by means of which it is hingedly attached to the central disc or hub 8. Tubular segment 11 is inclined by a small angle of, for example, 20 to 30 degrees with respect to the longitudinal axis of foot 10, that is to say, of blade 7, defining a "Y" configuration, as seen in Figure 2. A sliding sleeve (not visible in the drawings) fitted within tubular element 11 receives a pin 12, the ends of which are journaled in a pair of lugs 13 fixed to central disc 8. This arrangement permits each blade 7 freely to execute a conical movement, as shown in Figure 2. Each tubular element 11 is provided centrally with a lever pin 14 that extends away from the blade 7, normal to axis of element 11. The synchronization of the movements of the blades will be ensured by the action of lever pin 14 against a synchronizing disc 15, as will be further explained below. The angle α through which each blade 7 is permitted to travel is the key to the functioning of the system. The forwardmost position of the blade (with the blade inclined forwardly of the plane of rotation) corresponds to the optimal angle of the wind incidence and consequently for the maximum production of torque. Such position is maintained until, as a combined result of the centrifugal force and of the aerodynamic force of the wind on the blade, the blade will start to execute a control movement towards the position closer to the rotational plane. Figure 5 illustrates a typical configuration of the two extreme situations. Figure 4 illustrates the mechanical arrangement that causes speed control in addition to the synchronized movement of the blades. The central blade- fixing disc or hub 8 rotates together with the shaft of generator 6 and an axial extension 16 thereof. Each blade 7, as mentioned above, is journaled by means of pin 12 to lugs 13. Due to this assembly each blade is free to turn about the pin 12 between the limits of the control angle α. The blades 7 are maintained in their in operational position and the synchronism of the movements is promoted by the action of the above mentioned synchronization disc 12 which is at the end of a shaft sleeve 17 which slides aligned with the shaft extension 16 under the bias of a spring 18 bearing against a flange 19 at the end of shaft sleeve 17. The initial speed adjustment is controlled by the tightening of a nut 20 acting against the seat 21 of the spring 18. Figure 6 illustrates an alternative in which a lever 22 having a counterweight 23 at its end is added to the arrangement of Figure 4, so as to increase the effect of the centrifugal force which causes the counterweight 23 to move towards end of the course established by angle α. Fig. 7 shows an alternative biasing arrangement in substitution of the simple spring bias system of Figure 4. This alternative uses a hydraulic system of control. In this case a closed hydraulic oil system relieves the pressure against the end of lever pins 14, using a hydraulic cylinder 24 having an internal return spring. The pressure of the oil is generated by a positive sliding pump 25 of which the housing is mounted on a flange fixed to the rotary shaft of an axial flange mounted generator 26. The generator 26 is attached to the structure 27 of the wind turbine in a way that enables the shaft 28 of pump 25 to be attached to the structure as well. In this assembly the pump shaft 28 is fixed and the housing of the pump spins with the shaft 6 of the generator. The oil sucked from a tank 29, which also rotates with generator shaft 16, is pressured against a needle valve 30 which controls the flow through a return line 31 and consequently the trapped oil pressure is transmitted through a transfer line 32, a duct that runs along shaft 16 and an external tube 33 to the right hand end of cylinder 24. This forces the piston of cylinder 24 against the internal bias spring corresponding to spring 18 of Figure 4. In use, when the angular velocity of blades 7 increases, so also does the pressure generated in cylinder 24, sliding collar 19 to the left which, in turn, causes a rotation of blades 7 about their axes in the direction of speed reduction. Needle valve 30 permits adjustment of the pressure and, consequently, of the controlled speed. The arrangement presented is one among a range of hydraulic arrangements which may be used for the control action which is the purpose of the present invention. Figure 7 is a typical experimental graph represented by the relation between the wind speed versus the rotational speed of the turbine blades where curve (A) shows a system operating with fixed blades and curve (B) illustrates the rotational velocity control action when using the system of the preferred embodiment of the present invention.

Claims

CLAIMS 1. Wind turbine comprising: a propeller having a plurality of aerodynamic shaped blades extending outwardly from a central hub, the pitches of the blades being variable; a propeller shaft driven by the propeller; and a generator driven by the propeller shaft; characterized in that each blade is so hingedly mounted to said hub that its longitudinal axis can describe a segment of a conical surface of which the extreme radii are accompanied by the aerodynamic profile of the blade, from a first blade position representing greater torque production efficiency to a second blade position representing a lesser torque production efficiency, said mounting being such that both the dynamic wind force against each blade and the centrifugal force acting on the blade due to its own weight during rotation of the propeller, tend to rotate the blade towards said second blade position, biasing means associated with the blades applying a bias force thereto that tends to rotate the blades towards said first blade position. 2. Wind turbine according to claim 1 , characterized in that each blade has an inner end mounted to said hub so as to be able to rotate partially about an axis that is inclined with respect to the longitudinal axis of the blade. 3. Wind turbine according to claim 2, characterized in that said biasing means comprises pressure means mounted on said propeller shaft, said inner end of each propeller blade being provided with a lever extension against which said means applies pressure to bias the blade towards said first blade position. 4. Wind turbine according to claim 3, characterized in that said pressure means comprises a spring. 5. Wind turbine according to claim 3 or 4, characterized in that said pressure means includes a hydraulic system connected to counter the pressure applied by said pressure means in accordance with the rotational speed of the propeller. 6. Wind turbine according to any one of claims 3 to 5, characterized in that each blade is provided with a lateral projection generally parallel to the propeller axis, Said projection carrying a counterweight at its free end for increasing said centrifugal force during rotation of the propeller.