WO2008053282A1

WO2008053282A1 - Windturbine

Info

Publication number: WO2008053282A1
Application number: PCT/IB2007/000079
Authority: WO
Inventors: Laurence Robert Lemmon-Warde; Jan Phillipus Hiscock; Hendrik Jacobus Van Niekerk
Original assignee: Charmoon Close Corporation
Priority date: 2006-10-30
Filing date: 2007-01-11
Publication date: 2008-05-08

Abstract

A vertical axis wind turbine (10) for electricity generation includes a base structure (12) including a rotatable hub (14), three radial arms (16.1, 16.2 and 16.3) three aerofoils (18.1, 18.2 and 18.3) and a pitch control mechanism (20). The hub (14) is mounted to one side of the structure (12) by arm (15) and the radial arms (16) are spaced 120° apart and extend radially outwardly from the hub. The lower end of each aerofoil (18) is pivotally connected to the distal end of a different one of the radial arms. The pitch control mechanism (20) includes three tie rods (30, 31 and 32) which are pivotally connected between upper ends of the aerofoils (18) by means of V-shaped mounting formations (34) which are fixed to upper ends of the aerofoils. The pitch control mechanism (20) is operable to pivot the aerofoils so as to maintain an optimum angle of attack relative to the wind direction as the hub (14) rotates, thereby to optimise resultant lift forces acting on the aerofoils.

Description

WINDTURBINE

FIELD OF INVENTION

This invention relates to a turbine.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided a turbine including:

a base member including a rotatable hub;

at least one radial arm extending radially outwardly from the rotatable hub;

an elongate foil defining a longitudinal pivot axis and having a distal end and a proximal end, the foil being pivotally mounted at its proximal end to the radial arm near a distal end of the radial arm for driving the hub when the foil is acted upon by a working fluid; and pitch control means for pivoting the foil so as to maintain an optimum angle of attack relative to the flow direction of the working fluid as the hub rotates, thereby to optimise lift forces acting on the foil, in use.

Each foil may be in the form of a wing section having a longitudinal pivot axis and defining a leading edge and a trailing edge.

Each foil may be resiliently deformable to permit the foil to bow outwardly along its length due to centrifugal forces acting on the foil as the turbine rotates, in use.

The turbine may be configured so that, in operation, the longitudinal pivot axis of the foil may be disposed vertically.

The turbine may include at least two radial arms which are equi-spaced from one another and a corresponding number of foils which are each pivotally mounted to a different one of the radial arms.

The pitch control means may be in the form of at least one pitch control arm which extends between and which is pivotally connected at opposite ends of the pitch control arm to a different one of the foils. More particularly, the pitch control arm may be pivotally connected to the distal end of the foil. Each pitch control arm may be in the form of a tie rod which is pivotally connected at opposite ends thereof to the foils. The turbine may be a wind turbine wherein the foils are acted upon by ambient wind, in use.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention are described hereinafter by way of a non-limiting example of the invention, with reference to and as illustrated in the accompanying diagrammatic drawings. In the drawings:

Figure 1 shows a schematic perspective view of a turbine in accordance with the invention, mounted to a side of a support structure;

Figure 2 shows a schematic perspective view of the turbine of Figure 1 , mounted on top of a support structure;

Figure 3 shows a schematic side elevation of the turbine of Figure 1 ;

Figure 4 shows a schematic fragmentary top plan view of the distal end of a foil of the turbine of Figure 1 ;

Figure 5 shows a fragmentary schematic perspective view of the distal end of a foil of the turbine of Figure 1 ;

Figure 6 shows a fragmentary schematic perspective view of the hub of the turbine of Figure 1 ; Figure 7 shows a schematic perspective view of the turbine of Figure 1 , showing the manner in which the foils bow outwardly as the turbine rotates due to centrifugal forces acting on the foils;

Figure 8 shows a schematic top plan view of one of the foils of the turbine of Figure 1 ;

Figure 9 shows a schematic plan view of the turbine of Figure 1 at a particular azimuth at a relatively low rotational speed;

Figure 10 shows a schematic top plan view of the turbine of Figure 1 at the same azimuth as shown in Figure 9, at an intermediate rotational speed;

Figure 11 shows a schematic top plan view of the turbine of Figure 1 at the same azimuth as shown in Figure 9, at a relatively high rotational speed;

Figure 12 shows a schematic top plan view of the turbine of Figure 1 , illustrating the manner in which the pitching angle of the aerofoils, vary at relatively low rotational speeds of the turbine for azimuths of the turbine from 0° to 345°, at 15° intervals;

Figure 13 shows a schematic top plan view of the turbine of Figure 1 , illustrating the manner in which the pitching angle of the aerofoils, vary at intermediate rotational speeds of the turbine for azimuths of the turbine from 0° to 345°, at 15° intervals; Figure 14 shows a schematic top plan view of the turbine of Figure 1 , illustrating the manner in which the pitching angle of the aerofoils, vary at relatively high rotational speeds of the turbine for azimuths of the turbine from 0° to 345°, at 15° intervals;

Figures 15 A - C show schematic top plan views of the turbine of Figure 1 at relatively low, intermediate and relatively high rotational speeds, respectively, illustrating the variation of the pitching angles of the foils and configuration of the pitch control arms at the various rotational speeds over a 360° rotation of the turbine;

Figure 16 shows a diagram illustrating trace paths of the leading edges of the foils of the turbine of Figure 1 at the relatively low, intermediate and relatively high rotational speeds of the turbine as illustrated in Figure 15A;

Figure 16 A shows detail A of Figure 16;

Figure 17 shows a diagram illustrating trace paths of the trailing edges of the foils of the turbine of Figure 1 at the relatively low, intermediate and relatively high rotational speeds of the turbine as illustrated in Figures 15 A - C;

Figure 17 A shows detail B of Figure 17; Figure 18 shows a diagram illustrating trace paths of the centroids of the aerofoils of the turbine of Figure 1 at the relatively low, intermediate and relatively high rotational speeds as illustrated in Figures 15 A - C;

Figure 18A shows detail C of Figure 18;

Figure 19 shows a schematic top plan view of the turbine of Figurei at a particular azimuth operating at a relatively low rotational speed, illustrating the resultant angular and ambient wind velocities acting on the foils;

Figures 19 A - C show details D, E and F, respectively, of Figure 19;

Figure 20 shows a schematic top plan view of the turbine of Figure 1 at the same azimuth as in Figure 19, operating at a relatively high rotational speed, illustrating the resultant angular and ambient wind velocities acting on the foils;

Figures 20 A - C show details G, H and I, respectively, of Figure 20;

Figure 21 shows a schematic top plan view of the turbine of Figure 1 , at the same azimuth as in Figure 19, operating at an intermediate rotational speed, illustrating the resultant angular and ambient wind velocities acting on the foils;

Figures 21 A - C show details L, M and N, respectively, of Figure 21; Figure 22 shows a schematic top plan view of the turbine of Figure 1 , at the same azimuth as in Figure 19, operating at a relatively low rotational speed, illustrating the lift and drag forces acting on the foils;

Figures 22 A - C show details J, K and L, respectively, of Figure 22;

Figure 23 shows a schematic top plan view of the turbine of Figure 1 , at the same azimuth as in Figure 19, operating at a relatively high rotational speed, illustrating the lift and drag forces acting on the foils;

Figures 23 A - C show details M, N and O, respectively, of Figure 23;

Figure 24 shows a schematic top plan view of the turbine of Figure 1 , at the same azimuth as in Figure 19, operating at an intermediate rotational speed, illustrating the lift and drag forces acting on the foils; and

Figures 24 A - C show details P, Q and R, respectively, of Figure 24.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, a turbine in accordance with the invention, is designated generally by the reference 10. The turbine 10 is a vertical axis wind turbine for use in generating electricity and includes a base structure 12 (12.1 in the case of the turbine shown in Figure 2) including a rotatable hub 14, three radial arms 16.1, 16.2 and 16.3, three aerofoils 18.1, 18.2 and 18.3 in the form of wing sections and a pitch control mechanism designated generally by the reference numeral 20.

In use, the base member 12 is typically located on a ground surface in a high wind area. In this example, the base structures 12 and 12.1 are in the form of tower structures. In the embodiment shown in Figure 2, the hub 14 is mounted on top of the tower structure 12, whereas in the embodiment shown in Figure 1 , the hub 14 is mounted to one side of the tower structure by means of a support arm 15. The turbine 10 includes an output shaft (not shown) that is coupled to the rotatable hub 14 and that is connectable to an electricity generator (not shown).

The radial arms 16 are fixedly connected to and extend horizontally outwardly from the rotatable hub 14. The hub 14 is rotatable about a hub axis HA. The radial arms 16 are spaced 120° apart when viewed in plan view and have an aerodynamic design, thereby to decrease drag when the rotatable hub 14 rotates.

Each aerofoil is in the form of a straight blade wing section defining a leading edge 22 and a trailing edge 24. Each aerofoil 18 has a distal (i.e. upper) end 26 and a proximal (i.e. lower) end 28 which is pivotally connected to a distal end of the corresponding radial arm. Each aerofoil 18 has a symmetrical wing design when viewed in cross section and has a uniform configuration along its length. The uniform configuration of the aerofoils renders them suitable for manufacture by extrusion. Each aerofoil 18 defines a longitudinal pivot axis AR which coincides with the neutral lift axis and centre of mass (centroids) of the aerofoil and a chord line CL extending between the leading edge and trailing edge. Each radial arm defines a longitudinal radial arm axis RAA extending along its length, the pivot axes of the aerofoils extending perpendicularly with respect to the radial arm axes of the corresponding arms to which the aerofoils are connected.

With reference to Figure 7, each aerofoil 18 is resiliently deformable thereby permitting the aerofoil to bow outwardly along its length when subjected to centrifugal forces when the turbine rotates, in use.

The pitch control mechanism 20 includes three pitch control arms in the form of tie rods 30, 31 and 32. The tie rod 30 extends between and is pivotally connected at opposite ends thereof to the aerofoils 18.1 and 18.2. Similarly, the tie rod 31 extends between and is pivotally connected at opposite ends thereof to the aerofoils 18.2 and 18.3. In similar fashion, the tie rod 32 extends between and is pivotally connected at opposite ends thereof to the aerofoils 18.1 and 18.3. The tie rods 30, 31 and 32 are pivotally connected to the distal ends of the aerofoils by means of a V-shaped mounting formation 34 comprising two legs 34.1 and 34.2, which is fixedly connected to the distal end of each of the aerofoils 18. The tie rods 30, 31 and 32 are pivotally connected to the legs of the mounting formation 34 by means pivot connectors 36. Each aerofoil thus has two tie rods connected thereto.

In use, the pitch control mechanism 20 is operable to pivot the aerofoils so as to maintain an optimum angle of attack relative to the wind direction (the wind direction being shown in the drawings by arrow indicator W) as the rotatable hub 14 rotates (the direction of rotation of the rotatable hub being indicated by arrow indicator T). In this manner, lift forces acting on the aerofoils are optimised. In Figures 19 to 24 of the drawings, the following reference numerals are used:

Fd = Drag force

Fl Lift force

V Angular velocity of aerofoil

U Ambient wind velocity

R Resultant angular and ambient velocity

"a" = angle of attack of aerofoil (i.e. angle between chord line CL and R)

FR = Resultant force acting on aerofoil d1 ,d2 = distances between FR and hub axis HA

As air flows over the aerofoils, lift forces act on the aerofoils thereby driving the rotating hub 14 via the radial arms 16. Both sides of the aerofoils are used to generate lift during a single revolution of the aerofoils about the rotatable hub 14. By way of illustration, in a single revolution, one side of each aerofoil 18 faces the oncoming airstream for almost 180° of revolution of the hub 14 and thereafter the other side of the aerofoil faces the airstream for the remaining approximately 180°.

The pitch control mechanism 20 provides the aerofoils 18 with an enhanced angle of attack over a wide range of rotational speeds. The enhanced angle of attack of the aerofoils ensures that lift forces acting on the aerofoils are optimised (i.e. kept as high as possible), while at the same time maintaining drag forces at a minimum. The pitch control mechanism further provides the turbine with improved self- starting capabilities without affecting the omni-directional characteristics of the turbine and induces a diminishing pitch response amplitude at higher rotational speeds.

At relatively low rotational speeds of the turbine 10, centrifugal forces acting on the aerofoils are almost negligible and the predominant moment about each aerofoil is that exerted by the lift forces acting on the aerofoil. The tie rods of the pitch control mechanism exert reaction forces on the aerofoils in reaction to the centrifugal forces acting on the aerofoils. At such low rotational speeds, the tie rods remain slightly bent with respect to the legs of the mounting formation 34 due to the relatively low centrifugal forces acting on the aerofoils. In Figures 9, 12, 15A and 22, the turbine is illustrated operating at relatively low rotational speed. Having regard to Figures 9, 19 and 22, the leading edge of the aerofoil 18.1 (shown at an azimuth of 0°) faces inwardly and cannot produce useful lift in the direction of rotation T as drag forces Fd acting on the aerofoil result in a negative torque being applied to the turbine. The ideal angle of attack "a" for the aerofoil at this position will be 0° as this will produce the least amount of drag. The pitch control mechanism 20 allows for an almost 0° angle of attack at this point, with the lift and drag forces acting through the pitch control mechanism from all three aerofoils, being in equilibrium.

The aerofoil 18.2 is 120° ahead of the aerofoil 18.1. It can be seen from Figure 22, the resultant force FR acting on the aerofoil 18.2 produces useful torque as it is offset by distance d1 from the hub axis HA. The angle of attack "a" of the aerofoil 18.2 depends on several variables, including the rotational speed of the turbine, ambient wind velocity U and the mass and lengths of the tie rods of the pitch control mechanism. It will be appreciated that a momentum force generated by the rotation of the turbine acts in a radial direction relative to the hub axis HA and thus produces no torque in the direction of rotation. In this instance, it will be appreciated that the resultant force FR acting on the aerofoil 18.2 forces the leading edge of the aerofoil to move towards the hub thus decreasing the effective arc of rotation and "loosening" the tie rods of the pitch control mechanism to allow for deflection of the aerofoils. In this case, the momentum force acting on the aerofoil is less than the resultant force FR due to the relatively low rotational speed of the turbine.

Aerofoil 18.3 is 120° ahead of aerofoil 18.2. It can be seen from Figure 22 that the resultant force acting on the aerofoil produces useful torque as it is offset distance d2 from the hub axis HA.

The turbine 10 is illustrated operating at a relatively high rotational speed in Figures 11 , 14, 15C, 20 and 23. At relatively high rotational speeds of the turbine 10, the leading edges of the aerofoils are forced outwardly with the result that the tie rods are straightened relative to the legs 34.1 and 34.2 of the mounting formation 34, due to the relatively high centrifugal forces acting on the aerofoils. The aerofoils act as cantilevers about a torque arm in the radial direction. Equal and opposite reaction forces are provided by the tension in the tie rods in reaction to the centrifugal forces acting on the aerofoils. The mechanism starts to act as a stable, simply supported beam in suspension at the wing tips of the aerofoils. As both lift and centrifugal forces act on the aerofoils, a righting moment is induced and the angle of incidence is kept low for an increased speed / power yield. The Applicant envisages that the centrifugal forces acting on the aerofoils may be adjusted by, for example, the addition of counter weights. It will be appreciated that an equal centrifugal reaction force will be provided by the tension in the tie rods. At high rotational speeds, the aerofoils tend to flex outwards as is illustrated in Figure 7 of the drawings, thereby reducing the pitch amplitude to the point where the centrifugal forces acting on the aerofoils are far greater than the pitching forces. As a result, each aerofoil will be virtually locked in position with the chord line CL thereof extending perpendicularly to the corresponding radial arm.

Having regard to Figures 11 , 20 and 23 of the drawings, at relatively high rotational speed, the aerofoil 18.1 faces directly into the oncoming wind and cannot produce useful lift in the direction of rotation and as such, drag forces acting on the aerofoil will result in a negative torque produced by the resultant force FR which extends along the chord line CL of the aerofoil. When the turbine operates at a relatively high rotational speed, the momentum force acting on the aerofoil 18.2 is greater than the resultant force FR, causing the resultant force FR to act on the aerofoil 18.2 in a direction which is slightly offset a distance d1 from the hub axis HA₁ thereby producing a small useful torque and effectively forcing the leading edge of the aerofoil to move towards the hub. The momentum forces and resultant forces FR acting on the aerofoils effectively stiffen the pitch control mechanism by straightening the tie rods and allow for a reduced pitch amplitude which is required at higher tip speed ratio's (TSR's). TSR is commonly used to evaluate wind turbine performance and is the ratio of the actual aerofoil angular velocity V vs the ambient wind speed U. For example, when the turbine is rotating at V=U, the TSR=L At relatively high TSR's, the pitching of the aerofoils is reduced as the leading edges of the aerofoils are forced gradually outwards by the momentum forces to a point where the tie rods are in equilibrium with virtually equal tension forces acting on the tie rods as a result of the predominant centrifugal force acting radially outwards which restricts the pitch amplitude to a point where pitching is virtually zero.

Having regard to Figure 23, it can be seen that the resultant force FR acting on the aerofoil 18.3 produces a relatively large useful torque as it is offset distance d2 from the hub axis HA.

Figures 10, 13, 15B and 24 show the turbine operating at intermediate rotational speeds (i.e. rotational speeds between the relatively low and relatively higher rotational speeds described above). At such intermediate rotational speeds, the tie rods begin to straighten as the tension in the tie rods increases with increasing centrifugal force. The lift moment about the longitudinal pivot axis AR of each aerofoil and the tension forces in the tie rods provide a resultant lift force FR which is transmitted to the radial arms.

With reference to Figures 10, 21 and 24 of the drawings, the leading edge of the aerofoil 18.1 faces slightly inwardly and as a result, cannot produce useful lift in the direction of rotation T as the drag forces acting on the aerofoil result in a negative torque being applied to the turbine 10. At intermediate rotational speeds, the momentum forces and centrifugal forces acting on the aerofoil increase and the pitch control mechanism 20 stiffens as the tie rods straighten. The pitch amplitude of the aerofoil also begins to reduce at intermediate rotational speeds. Having regard to Figure 24, the resultant force FR acting on the aerofoil 18.2 produces a useful torque as it is offset a distance d1 from the hub axis HA. Similarly, the aerofoil 18.3 also produces useful torque, with the resultant force FR acting on the aerofoil being offset a distance d2 from the hub axis.

With regard to Figures 19, 20 and 21 , it can be seen that for relatively low rotational speeds wherein the angular velocity V of the aerofoils is substantially less than the ambient wind velocity U, the angles of attack "a" of the aerofoils will be relatively large. This is also clearly illustrated in Figures 15 A - C which show the turbine operating at relatively low intermediate and relatively high rotational speeds. For relatively high rotational speed values, the angular velocity V of the aerofoils will thus be much greater than the ambient wind velocity U and hence a smaller angle of attack "a" will be obtained as the wind velocity U will tend to flow in the direction of the aerofoil chord line CL. For low rotational speeds, the relative angle of attack "a" varies considerably as the wing is rotated through a full revolution as the ambient wind velocity U is far greater than the angular velocity V of the turbine, leading to angles of attack that produce stall. For relatively high rotational speeds, the ambient wind velocity U becomes negligibly small with flow virtually parallel to the aerofoil chord line CL, requiring a very small or no pitch amplitude in order for the aerofoils to avoid stall. At such speeds, the angle of attack "a" is virtually constant due to the higher relative wind speeds and the only pitching required would be for an optimal angle of attack, but not to reduce stall as is the case with lower rotational speeds. At intermediate rotational speeds, a stage exists during which equal pitching and centrifugal forces act on the aerofoils. Figures 12, 13 and 14 illustrate, in sequence, for comparison purposes, the manner in which the pitching angle of the aerofoils vary as the turbine rotates at relatively low, intermediate and high rotational speeds, respectively.

With reference to Figures 16 and 16A, the trace paths of the leading edges of the aerofoils at relatively low, intermediate and relatively high rotational speeds, is shown. In Figures 16 and 16A, the reference letters LS, IS and HS are used to designate the trace paths of the leading edges at relatively low, intermediate and relatively high speeds, respectively. Similarly, in Figures 17 and 17A, the trace paths of the trailing edges of the aerofoils operating at relatively low, intermediate and relatively high rotational speeds are designated by the reference letters LS, IS and HS, respectively.

With reference to Figures 18 and 18A, the trace paths of the centroids (i.e. centres of mass) of the aerofoils operating at relatively low, intermediate, and relatively high rotational speeds are similarly designated by the reference letters LS, IS and HS, respectively.

The pitch control mechanism 20 thus enables the wind turbine 10 to passively accommodate for an improved angle of attack of the aerofoils over a wide variety of rotational speeds while at the same time exhibiting self-starting capabilities and inducing diminished pitch response amplitude at higher speeds. The pitch control mechanism 20 thus allows for larger pitch angles at lower TSR's thereby reducing stall and improving lift. It is well known that a straight bladed fixed pitch vertical axis wind turbine will not reliably achieve a TSR over unity due to the resultant lift and drag forces which impose a negative drag force on the turbine just after start up, caused mainly by the stalling of blades through a large portion of the azimuth of rotation. The pitch control mechanism 20 thus allows for sufficiently improved lift and drag force vectors to propel to turbine 10 to higher more efficient TSR's.

It will be appreciated that the exact configuration of the turbine in accordance with the invention may vary greatly while still incorporating the essential features as defined and described hereinabove. More particularly, the Applicant envisages that the turbine in accordance with the invention will be suitable for use in both gaseous or liquid working fluids.

Claims

CLAIMS:

1. A turbine including:

a base member including a rotatable hub;

at least one radial arm extending radially outwardly from the rotatable hub;

an elongate foil defining a longitudinal pivot axis and having a distal end and a proximal end, the foil being pivotally mounted at its proximal end to the radial arm near a distal end of the radial arm for driving the hub when the foil is acted upon by a working fluid; and

pitch control means for pivoting the foil so as to maintain an optimum angle of attack relative to the flow direction of the working fluid as the hub rotates, thereby to optimise lift forces acting on the foil, in use.

2. The turbine as claimed in claim 1, wherein each foil is in the form of a wing section having a longitudinal pivot axis and defining a leading edge and a trailing edge.

3. The turbine as claimed in claim 2, wherein each foil is resiliently deformable to permit the foil to bow outwardly relative along its length due to centrifugal forces acting on the foil as the turbine rotates, in use.

4. The turbine as claimed in claim 2 or claim 3, wherein the turbine is configured so that the longitudinal pivot axis of the foil is disposed vertically in operation.

5. The turbine as claimed in claim 4, wherein the foil is pivotally mounted at the proximal end thereof to the distal end of the radial arm.

6. The turbine as claimed in claim 5, wherein the radial arm defines a longitudinal radial arm axis, the pivot axis of the foil extending substantially perpendicularly with respect to the radial arm axis.

7. The turbine as claimed in any one of the preceding claims, which includes at least two radial arms which are equi-spaced from one another and a corresponding number of foils which are each pivotally mounted to a different one of the radial arms.

8. The turbine as claimed in claim 7, wherein the pitch control means is in the form of at least one pitch control arm which extends between and which is pivotally connected at opposite ends of the pitch control arm to a different one of the foils.

9. The turbine as claimed in claim 8, wherein the pitch control arm is pivotally connected to the distal end of the foil.

10. The turbine as claimed in claim 9, wherein each pitch control arm is in the form of a tie rod which is pivotally connected at opposite ends thereof to the foils.

11. The turbine as claimed in any one of the preceding claims, wherein the turbine is a wind turbine wherein the foils are acted upon by ambient wind, in use.