WO2015063761A1

WO2015063761A1 - Impeller structure for a wind turbine

Info

Publication number: WO2015063761A1
Application number: PCT/IL2014/050931
Authority: WO
Inventors: Vladimir Kliatzkin
Original assignee: Winflex Ltd
Priority date: 2013-10-31
Filing date: 2014-10-28
Publication date: 2015-05-07

Abstract

A horizontal axis impeller for a wind turbine comprises a rigid hub rigidly attached to a gear box input shaft; a hollow annular shroud spaced outwardly from the hub and rotatable by the gear box; a plurality of blades made of a flexible stretched material extending within an annular space between the shroud and hub, each of the blades being attached to an outer region of the hub and to an inner region of the shroud; a plurality of shroud-connected cables extending between the shroud and the hub, for reducing impeller deformation and transferring forces to a yaw control unit; and a joint at a hub to shroud- connected cable connection having at least two degrees of freedom. The shroud- connected cables contribute to an increase in overall impeller rigidity as a result of a decrease in conic deformation and impeller twist deformation.

Description

IMPELLER STRUCTURE FOR A WIND TURBINE

Field of the Invention

The present invention relates to the field of wind turbines. More particularly, the invention relates to the structure of a horizontal axis wind turbine impeller having flexible blades and a rotating shroud of a sufficiently large diameter to generate power.

Background of the Invention

The main problem of increasing wind derived power generated by a single turbine is shown in Fig. 1, which is a graph illustrating the estimated costs of rotors manufactured by General Electric and Kaman, respectively. Another problem involved in increasing the generated wind derived power is related to the mechanical characteristics of the rotor. As the diameter of the rotor increases, the turbine power consequently increases by a power of 2. At the same time, the chord of the blade and its camber increase linearly, resulting in a bending moment and an impeller torque and blade weight that increase by a power of 3 at the same tip speed ratio (TSR), i.e. ratio between tangential speed of blades tips and wind speed. In other words, since the stress in the blade root resulting from overall impeller weight increases linearly with impeller diameter, the cost of the blades increases more than linearly, especially due to the mold cost of a 2O70m span single blade.

The fiberglass composite used for high speed turbines has a specific problem- inadequate strength for tension and compression at bending. In tension, strength and rigidity are determined by the parameters of the strength part of the fiber glass material, but in compression, the overall fiberglass parameters are determined by the binding part of the composite, i.e. ten times less. In other words, at the blade, 90% of load withstands only half of its materials- from the upwind side. Furthermore, the blades rotate at dangerously close to the resonant frequency of the tower and wind speed frequenc}^, approximately lHz. Accordingly, this kind of impeller is subject to a risk of resonance crush during which the impeller destroys itself and the supporting tower crashes, resulting in the complete destruction of the apparatus. For these reasons, a special turbine configuration is necessary.

Another problem is related to yaw control methods for a large turbine. It is absolutely forbidden to use a passive downwind system for yaw control, necessitating the use of upwind active elements, since such a system is very sensitive, very complicated, and, as a result, very dangerous. The same situation applies to the tower configuration, including the type of structure and impeller shaft. Another drawback of the rigid construction is the increase in weight of the large impeller, limiting the wind turbine capacity on one hand while requiring a need for very strong support structures, and on the other hand, resulting in a high specific cost per kW installed.

The following patent search was conducted for an impeller with a rotating shroud, which is adapted to overcome the aforementioned deficiencies.

US Patent No. 4,140,433 issued to Eckel discloses the use of a shrouded turbine with two wind speed accelerator the exterior being a shroud and the internal being a nose. It transforms wind speed energy like a jet turbine design. This construction is typical for a shrouded turbine requiring an additional architecture assembly. The shroud cannot withstand a propeller type rotor increase. Using an aerodynamic transformer in the form of gas turbine or, for farms, a multi blade turbine requiring a very low Tip Speed Ratio, less than 1, and, as a result, an increased system torque.

US Patent No. 4,547,124 issued to Kliatzkin discloses the tensing of flexible blades using an inflated torus. This patent does not solve the very important problem of the connection between the torus and the hub during the predicted large deformation of the rotor especially in the twist direction. It also does not solve very significant problems involving the blade profile at rotor distortion.

US Patent No. 4,832,571 issued to. Carrol discloses the tensing of blades using the centrifugal forces of the blades and wheel torus. This configuration is not adequate for large scale diameters of more than lm for two reasons. Firstly, the required radial (centrifugal) forces must be more than the radial projection of aerodynamic forces. Also, rotor twist rigidity in reaction to torque is not provided. Due to these drawbacks, this configuration therefore cannot be implemented as an optimal rotor design rotor for large diameters.

US Patent No. 5,823,749 issued to Green discloses a sail blade turbine with vertical axes and quadrate form of shroud having a structural role only. This turbine may be classified as a Savoniouse kind of turbine with similar results. The efficiency of this kind of turbine cannot exceed half of the Betz coefficient, excluding any aerodynamic and mechanical losses. With those losses included, full chain efficiency cannot exceed 10-12% of wind energy.

US Patent No. 6,887,031 issued to Techer discloses decreasing the size of the rotor for the predetermined swept area through use of a single or multiple flow accelerator. The problem is that the accelerator is heavier than the turbine that has an equivalent outer span of the shroud relative to the wing span turbine diameter. A good estimate is that the length of the shroud (single or total of the multiple) is equal to the outer sweep diameter, such as the shroud surface 3.14/4* (Shroud diameter)*2, while the normal horizontal axis turbine surface of turbine blades is a few percent of the swept area. These parameters are the base of the weight and, as result, the cost of the overall turbine configuration. US Patent No. 6,951,443 issued to Blakemore discloses a horizontal axis turbine with a shroud that must reduce loads in the blade root sections. Cleariy, this simply increases the hub diameter to acceptable dimensions. Use of a teeth ring drive of a small electrical generator is very problematic due to the possible deformation of the systems in these places that may be hundreds of times more than what is expected for the structure.

It is an object of the present invention to provide a rotating shroud wind turbine of sufficient rigidity and structural strength to permit significant rotor diameter and weight and to thereby achieve increased aerodynamic efficiency at higher wind speeds.

It is an additional object of the present invention to provide a rotating shroud wind turbine of high rotor elasticity that permits use of a control system having a higher frequency than the change in wind speed, thereby increasing the ability to exploit wind energy.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

The present invention provides a horizontal axis impeller for a wind turbine, comprising a rigid hub rigidly attached to a gear box input shaft; a hollow annular shroud spaced outwardly from said hub and rotatable by means of said gear box! a plurality of blades made of a flexible stretched material extending within an annular space between said shroud and hub, each of said blades being attached to an outer region of said hub by a root thereof and to an inner region of said shroud by a tip thereof, a plurality of shroud-connected cables extending between said shroud and said hub, for reducing impeller deformation and transferring forces to a yaw control unit! and a joint at a hub to shroud-connected cable connection having at least two degrees of freedom.

The following are some of the advantages of this impeller configuration:

1. The configuration of a hollow shroud allows the weight and the amount of material needed in a production phase of the shroud to be reduced, resulting in a corresponding reduction in cost. Due to its low weight, the impeller is configured to withstand only tensile stress.

2. The annular shroud achieves a maximum ratio of moment of inertia to second moment of inertia, the former being indicative of the ability to withstand deformation and the latter being indicative of the ability to withstand stress.

3. This configuration permits the use of materials for the shroud structure with a very high strength to weight ratio, for example polyester fibers in the form of special oriented woven fabrics having a yield strength in tension (strength to weight ratio) of 15,000 N/cm², including the weight of UV protection.

4. As the flexible blades are subjected primarily to tensile forces, a composite matrix, e.g. consisting of epoxy and polyester as 20% of the total blade weight may be used. Fiberglass used for prior art blades is weak when subjected to tensile forces.

5. Several independent systems are associated with the rotating shroud, including an aerodynamic accelerator and an automatic brake system to prevent shroud overspeeding. The brake system is adapted to maintain the TSR at a value ranging from 2.5-3.5, while the TSR of a prior art impeller ranges from 5-6.

In one aspect, the hollow shroud is inflated. A pressure control unit may be provided for maintaining a desired internal pressure within the shroud. The pressure control unit may comprise a compressor, a differential atmospheric pressure sensor, and a control valve, said compressor being activated upon detection of a predetermined low pressure within the shroud by said differential sensor and said control valve being activated during periods of overpressure within the shroud.

Another means of maintaining the shape of the shroud is by filling it with a light, porous material, such as polyurethane. It can be understood that the cross section of the shroud in this case does not have to be circular, but may be elliptical or in the shape of an aerodynamic profile.

A blade tip, which may be curved, is connectable to the shroud, for example by a sleeve circumferentially fitted on the shroud and connection means by which the blade tip is connected to said sleeve.

In order to provide the required radial blade tension, the length of the blades may be somewhat shorter than the intended final distance between the hub and shroud since the shroud has a certain amount of elasticity. When the shroud is inflatable, the blades are preferably attached to the shroud prior to the inflation of the shroud. The shroud, under full pressure, will then expand and thus stretch and pre-tense the blades. When the shroud is solid and not inflatable, the radial tension may be achieved by calculating a difference in shroud rigidity.

Wind acting on the impeller will make the entire impeller rotate, including the shroud. Aerodynamic forces applied to the blades may be projected in two directions^ the rotating direction and the wind direction. Due to these aerodynamic forces, the shape of each blade changes along its length from the tip to the root, having an increase in the camber height, i.e. the maximum distance between camber line and chord, and a decrease in chord length. By virtue of the shroud-connected cables that provide additional support points, the overall impeller rigidity is able to increase as a result of a decrease in conic deformation and impeller twist deformation. For a wind speed of up to 12 m/s, the conic deformation is 5 degrees and less and the impeller twist deformation ranges from 5"15 degrees.

At least one additional cable may extend between the leading edge of one blade and the trailing edge of its adjacent blade, to prevent or limit blade twist deformation. A plurality of cross members may be fixed along the length of each blade in order to decrease camber deformation. For a wind speed of up to 12 m/s, the blade twist deformation ranges from 0.5- 1 degree and the camber deformation is 1 degree and less.

Brief Description of the Drawings

In the drawings^

- Fig. 1 is a graph of the estimated cost of a wind turbine rotor as a function of its diameter!

- Fig. 2a is a perspective view from the side of a diametrical section of a prior art impeller, showing the action of wind forces on the impeller to cause conic deformation,

- Fig. 2b is a front view of a prior art impeller, showing the action of wind forces on the impeller to cause twist deformation!

- Fig. 2c is a graph of blade staggering of a prior art impeller as a function of blade length;

- Fig. 3 is a perspective view of an impeller comprising a rotatable shroud, flexible blades, a rigid hub, and shroud-connected cables, according to one embodiment of the present invention!

- Fig. 4 is a top view of a blade according to one embodiment, when separated from the shroud and flattened out; ^■ Fig. 5 is an enlarged, perspective view of a portion of an impeller, according to another embodiment of the invention, interconnecting cables!

- Fig. 6 is a perspective view from the side of a hub and gearbox shaft, according to another embodiment of the invention;

^• Fig. 7 is a perspective view from the side of a portion of an intermediate connecting member, showing the connection thereto of the hub of Fig. 6 and of shroud-connected cables;

^■ Fig. 8 is a perspective view from the front of a root cross member serving as the intermediate connecting member;

- Fig. 9 is a front view from the interior of a flexible blade, showing the connection thereof to a shroud;

^■ Fig. 10 is a perspective view from the interior of a shroud, showing the connection of a cable thereto;

- Fig. 11 is a graph of conic deformation as a function of wind load, in comparison between a prior art impeller and the impeller of the present invention;

- Fig. 12 is a graph of twist deformation as a function of wind load, in comparison between a prior art impeller and the impeller of the present invention;

- Fig. 13 is a graph of camber deformation as a function of wind speed and relative distance from a cross member;

- Fig. 14 is a graph of stagger deformation as a function of relative blade length;

- Fig. 15a is an exploded view of a pressure control unit;

- Fig. 15b is a cross section view of the pressure control unit of Fig. 15a, when assembled and mounted on the shroud;

- Fig. 16 is a schematic perspective view of a ducted pressure charger^',

- Fig. 17 is a schematic perspective view of a braking air bag;

- Fig. 18 is a graph that correlates the distribution of wind speed, impeller rotational speed, and TSR over time; and ^■ Fig. 19 is a graph of the required speed to achieve a desired impeller rotational speed.

Detailed Description of Preferred Embodiments

The present invention is a novel horizontal axis wind turbine having a rotating shroud. Although prior art, horizontal axis wind turbines with a rotating shroud and flexible blades connecting the central hub and outer shroud, which are described in US 4,547,124, significantly decreases the impeller weight relative to impellers made of a fiberglass material, the prior art rotating shroud wind turbines nevertheless suffer from relatively high levels of deformation that compromises their structural integrity and reduces their reliability.

As shown in Fig. 2a, one type of deformation to which the prior art impeller is subjected is conic deformation. Due to axially directed wind forces C acting in a direction parallel to the axis 5 of impeller 10 and the rigidity of hub 7, the wind forces C are transmitted to the flexible blades 11. Since each blade tip 13 remains connected to the shroud 20 and each blade root 17 remains connected to hub 7 despite changes in wind pressure that lead to varying axially directed wind forces C, the angle of blade 11 changes with respect to nominal unloaded, i.e. when the wind forces are non-existent, axially projected blade disposition 15. As a result of this angular blade change, a plane passing through the diameter of shroud 20 will no longer coincide with hub 7, as was noticeable for the nominal unloaded blade disposition 15, but rather will be axially spaced from the hub. The realignment of the blades relative to the unloaded disposition 15 assumes a frustoconical shape, and this type of deformation will be therefore referred to as "conic deformation". Studies have revealed that the conic deformation for prior art rotating shroud impellers, on either side of the nominal unloaded blade disposition 15, depending on the direction of wind forces C, can be as much as 21 degrees. As shown in Fig. 2b, another type of deformation to which the prior art impeller is subjected is impeller twist deformation. When the flexible blades are stretched when being attached to the rigid hub and to the relatively flexible shroud, the resulting tensile force being applied to the shroud causes the shroud to be radially deformed and exhibiting a flower-like shape, such that the radial distance from the center of the shroud to a shroud surface, whether an outer surface or outer surface, radially coinciding with the point of connection with the blade tip either increases or decreases with a circumferentially adjacent shroud portion, e.g. by an amount of 100-300 mm for a shroud having a diameter of 20 m. Due to this radial deformation, each blade-shroud joint is subject to an increased amount of play.

As a result of circumferential aerodjoiamic forces applied to the rotating impeller and the increased amount of play, the entire impeller 10 is caused to be circumferentially shifted bout the impeller axis. The circumferential shifting of a blade 11 from a nominal unloaded and nonshifted blade position 22 to a circumferentially shifted blade position 23 is discontinuous during rotation of impeller 10, leading to rotation in one direction and an abrupt change to the other rotational direction, which will be referred to "twist deformation". Studies have revealed that the twist deformation for prior art rotating shroud impellers can be as much as 49 degrees.

A third type of deformation to which the prior art impeller is subjected is camber deformation. As a result of the change in wind pressure along the plane of the impeller, and in a radial direction from the tip to the root of a blade, the camber height, or the greatest distance between the pressure side and suction side of a flexible blade, becomes deformed. The wind derived forces determine the level of camber deformation, resulting in an increase in the camber height and a simultaneous decrease in the chord, or the distance from the leading edge to the trailing edge of the blade, while the impeller is rotating. This type of deformation is influenced by normal aerodynamic pressure, which has an asymmetrical parabolic form, with maximum pressure acting approximately on a middle blade section, spaced from the leading edge by a distance corresponding to 25-40% of the chord, and decreasing to zero pressure values at the blade root and tip section being connected to rigid holders. The blades are liable to buckle when the chord is decreased, due to reduced rigidity and structural strength.

A fourth type of deformation to which the prior art impeller is subjected is blade twisting. Due to an asymmetric distribution of aerodynamic pressure as well as a variation in blade tension between the leading and trailing edges of a blade, the orientation of the chord of each blade with respect to the shroud tends to individually change in a direction normal to the impeller axis. The angle of the chord relative to a plane perpendicular to the impeller axis is often referred to as the "stagger". As a result of aerodynamic forces focused at an intermediate point on the blade (hereinafter the "aerodynamically focused point"), each blade twists about the aerodynamically focused point for a deformation of up to 8 degrees.

Fig. 2c illustrates the blade twisting deformation to which a prior art impeller having 7nrlong blades is subjected during periods of aerodynamic load.

The impeller of the present invention significantly reduces the first three aforementioned types of deformation by employing a plurality of shroud- connected cables that extend in the annular space between the central hub and the outer rotating shroud, thereby rigidizing the shroud by increasing the number of support points that are associated with the latter.

Fig. 3 is an isometric view of a horizontal axis wind turbine impeller generally designated by 200, according to one embodiment of the present invention. Impeller 200 comprises an outer shroud 202 in the shape of a torus, a rigid hub 201 positioned at the center of shroud 202 and rigidly attached to a gearbox spaced rearwardly from shroud 202, preferably directly to its input shaft, shroud- connected cables 206, a sleeve 212 at a connection region with each corresponding cable 206, and flexible blades 203.

Hub 201 may be star-shaped, having a central annular portion surrounding and engaged with the gearbox shaft, and circumferentially spaced pairs of arms that radially extend from the central portion. In each pair, a first arm is axially spaced from a second arm. The first and second arms have mutually aligned annular terminal elements through which a corresponding axially extending rod 221 for attachment to a shroud-connected cable 206 is insertable and engageable.

It will be appreciated that any other configuration of a hub is also in the scope of the invention.

Shroud 202 is hollow, to decrease the weight of impeller 200, and may be made of relatively rigid material, or alternatively of flexible material, e.g. cloth, which is covered with PVC, polyurethane or polyester and applied with an anti-UV coating, in which case the shroud is inflated in order to retain the flexible blades in their stretched condition. The blades 203 may be also be made of the same flexible material

In contrast to prior art impellers which are positionable to face against the instantaneous wind direction by means of a complicated and damage prone } aw control unit, in order provide a maximum power output when wind speed is under the rated speed, the impeller of the present invention employs a passive downwind system for yaw control since it is assured of not being subjected to resonance by virtue of its relatively low weight. As referred to herein, the "leading" and "trailing" edges of the blades are defined by the relative blade position when the impeller faces downwind to the wind direction.

Blades 203, which radially extend from hub 201 to outer shroud 202, are configured to withstand variable aerodynamic forces and reactions, particularly forces and torque, by virtue of increased camber height and decreased chord length along the span of each blade. The blades preferably have a concave curvature with respect to the wind direction, as shown in Fig. 5, in order to maximize wind derived rotor torque. The pitch, or angle with respect to the wind direction, of each blade 203 may range from 40-50 degrees.

Each shroud-connected cable 206 radially extends between a pair of adjacent blades 203, from a rod 221 located radially inwardly to the tip of a first blade to a conical anchoring element 207 protruding radially inwardly from an inner surface of shroud 202 between the first and second blades of a pair. Each shroud- connected cable 206, which is connectable to a corresponding rod 221, is sloped due to the axial spacing between anchoring element 207 and rod 221. The presence of the shroud-connected cables 206 increases the number of support points for shroud 202, i.e. at a blade-shroud connection and at a cable-shroud connection, so that the stress concentration at each support point will be decreased and that each blade-shroud joint will be more rigid, thereby reducing the level of conic and twist deformations.

Cables 206 provide stability to the impeller and also transmit forces to the yaw control unit hy means of hub 201 and the intervening shaft during changes of wind direction. The shroud-connected cables also ensure that the orientation of the impeller axis will be substantially constant despite an increase in wind speed at increased height due to inertia and Coriolis forces. The number and orientation of shroud-connected cables 206 that are employed in impeller 200 are selected so as to minimize shroud related deformation as well as the bending moment acting on the shroud.

For example, the outer radial deformation (ORD) noticeable on the shroud surface is given by the following relation^

ORD - Pr³/2EI * [(l/s²) * ((Θ/2) + (sc/2)) - (1/Θ)], (Equation l) where P is the tensile force applied by the blade, r is the shroud radius, E is Young's Modulus for the shroud, I is the shroud's moment of inertia, Θ is half the angular distance between two adjacent support points, s = sin θ , and c = cos Θ.

The bending moment M acting on the shroud is given by the following relation^

M = ½(Pr) * ((u/s) - (1/Θ)), (Equation 2) where u = cosa and a is one-quarter the angular distance between two adjacent support points.

Thus the radial deformation and bending moment can be reduced by increasing the number of support points, resulting in a reduced value of Θ.

One or more shroud-connected cables 206 may be positioned between a pair of blades 203. Each shroud-connected cable 206 may be radially extending, or, alternatively, slightly angularly spaced from the radial direction. Figs. 4 and 5 illustrates one embodiment of a flexible blade 203 on which variable wind forces act to induce impeller rotation and to thereb}^ generate electricity.

Each blade 203 is attached at two connection lines- (a) from leading edge tip 204T to trailing edge tip 205T at the shroud periphery, and (b) from leading edge root 204R to trailing edge root 205R at the hub. As a result of these large-length connection lines and the provision of the shroud-connected cables 206, overall impeller rigidity will be advantageously increased.

Despite being flexible and being subjected to both aerodynamic pressure and mechanical reactions, blades 203 are afforded sufficiently rigidity by means of rigid cables that are embedded within the leading edge 204 and trailing edge 205 of each blade. The embedded cables may be made of e.g. alumina, oriented polyethylene, rope or wire, whether solid or woven. This blade frame structure supplies tension in two directions in addition to the blade tension in the radial direction.

The tension in each leading or trailing edge embedded cable is at least 2 times greater than the internal tension of the blade fabric in order to be able to adequately transmit the torque developed by the shroud to the hub.

Blade 203 may be configured with a trapezoidal shape when flattened out, such that the leading edge 204 defines an angle ranging from 5-25 degrees with respect to the trailing edge 205, while the chord length decreases from root to tip direction. Trailing edge 206 is substantially perpendicular to the chord of the blade, which is an imaginary line, measured in the direction of the normal airflow, from leading edge 204 to its trailing edge 205.

For additional rigiditj^ at the blade surface, there may be provided at least one cross member 219 e.g. 3-5 cross members. The cross members are spaced between the root and tip, and extend between, and are connected to, embedded cables 226 and 227. Each cross member 219 may be configured by a metallic pipe to maintain desired spacing between the leading and trailing edges, to therefore prevent blade deformation which is liable to result in buckling. Each cross member 403, which may have a profile similar to the camber of blade 203, may be manufactured from an aluminum alloy or from a flexible composite elastic material. For example, the cross member is produced from a bended rod having an annular cross section for high rigidity in the normal to chord direction, and the leading and trailing edges are made from wire or of from high strength polyethylene.

Also, one or more interconnecting cables 231 may be connected between the leading edge 204 of one blade 203 to the trailing edge 205 of an adjacent blade, to minimize stagger deformation, or change in orientation of a blade in the tangential direction with respect to shroud 202. The interconnecting cables 231 also help to reduce blade twisting, to a deformation of no more than 1 degree, as well as to prevent the blades from stalling by preventing an increase of angle of attack (AO A).

Also, by virtue of the trapezoidal shape of blade 203, each interconnecting cable 231 will be assured of remaining tense, therebj^ reducing impeller twist and conic deformations.

It will be appreciated that in other embodiments of the invention, an impeller may comprise a plurality of interconnecting cables without any shroud-connected cables.

Fig. 6 illustrates another embodiment of a hub. Star-shaped hub 270 comprises central portion 274 surrounding and engaged with axially extending gearbox shaft 277, and a plurality of circumferentially spaced projection units 271 that each radially extends from central portion 274. Each projection unit 271 comprises a first arm 272, a second arm 273 that is angularly spaced from first arm 272, and a planar terminal element 276 that extends between arms 272 and 273 at the outer end of projection unit 271, for connection with a shroud- connected cable.

Fig. 7 illustrates the connection of shroud-connected cables 206a^_c to the hub. An arcuate, intermediate connecting member 225 for providing increased surface area for connection to the cables 206 and to a flexible blade is attached, such as by welding or threaded attachment, to a corresponding hub projection unit 271. Each of the shroud-connected cables 206a^_c is connected to an intermediate connecting member 225 via a corresponding joint 211, e.g. a ring shaped joint, at an inner portion thereof, allowing cable movement in two degrees of freedom, sufficient for the twist direction of the impeller, ranging 5-10 degrees relative to hub 201 and the corresponding anchoring element 207. The sum of moments about the shroud axis resulting from the tensile force applied by cables 206a-c and blades 203 onto the shroud must be equal to zero.

Alternative^, as shown in Fig. 8, the intermediate connecting member 225 may be embodied by a root cross member of blade 203, which is attachable to a corresponding hub projection unit. A ring shaped joint 211 is attached to a central portion of intermediate connecting member 225 for connection to shroud- connected cable 206. Eyelet 243 attached to embedded cable 246 at the trailing edge of blade 203 is attached to one end of intermediate connecting member 225, e.g. by means of a strap. The embedded cable at the leading edge is attached in similar fashion.

Fig. 9 illustrates the connection of a blade 203 to shroud 202. A sleeve 212 for distributing the tensile force applied by blade 203 onto shroud 203 is circumferentially fitted on the latter. Blade 203 is attached to sleeve 212 by any suitable connection means 214 selected rom in a non-limiting way, or in combination with, stitching, straps, adhesion, and hook and loop material. This arrangement provides two types of impeller rigidity by enabling sufficient internal shroud pressure when the shroud is of the inflated type and also by compensating for movement of the impeller axis at increased height dependent wind speeds. Frictional forces between sleeve 212 and the shroud surface, particularly when the shroud is overinflated, prevent movement of the sleeve.

Fig. 10 illustrates the connection of a shroud-connected cable 206 to shroud 202. A sleeve 217 for distributing the tensile force applied by cable 206 onto shroud 202 is circumferentially fitted on the latter. Sleeve 217 is constructed from a high strength fabric having a UV protection coating. A reinforcement plate 231 is attached at each radial end of sleeve 217, and then the two plates are coupled together by a face to face relation. A shackle 236 or any other suitable fastening element connected to one end of cable 206 is received in aligned apertures of the two coupled plates 231. Due to the tensile force applied by shroud-connected cable 206, the coupled plates 231 are pulled inwardly from sleeve 217 so as protrude towards the shroud interior.

Suitable tension of cable 206 in the axial and rotational directions is carried out by pre-tensing the impeller, for example by no more than 20% of desired tension, in order to resist conic and twist deformation.

The following are possible solutions for decreasing this distortion^

1. By changing impeller geometric characteristics with respect to orientation of the blades and shroud-connected cables and to the pre-tensing of the cables in order to increase the impeller rigidity, the shroud stress and internal pressure thereof will be decreased.

2. Impeller preliminary tensing opposite to the torque direction, for example in the trailing edge, leading edge and fabric part of camber. 3. Increasing the radius of the blade root at the hub connection if the active impeller swept area, i.e. the ratio Dhub/Dshroud, is less than 0.15, while increasing the tip/hub ratio from 0.05 to 0.15 by an optimization process.

4. Tensioning the shroud-connected cables such that the twist deformation to conic deformation ratio is 70-90%.

5. Increasing support points of the shroud by increasing the thickness of shroud- connected cables 206 to withstand bending of the shroud.

The description concerning impeller twist is also applicable for impeller deformation in the downwind direction.

By virtue of the apparatus of the present invention, the degree of each of the four aforementioned types of deformation decreased relative to that of prior art rotating shroud wind turbines, for example as described in US 4,547,124.

The deformation was determined by analytical computation, finite element methods, and experimental studies, both at a test bench and also when mounted on the turbine. The resulting impeller rigidity was found to increase by a factor of 3 with respect to a prior art impeller.

Fig. 11 is a graph of conic deformation as a function of wind load, in comparison between a prior art impeller having 6 support points and the impeller of the present invention having 12 support points. A dramatic reduction in conic distortion is noticeable! for example, at a load of 100% the conic distortion decreased from 23° to 6°.

Fig. 12 is a graph of impeller twist deformation as a function of wind load, in comparison between a prior art impeller having 6 support points and the impeller of the present invention having 12 support points. A dramatic reduction in twist distortion is noticeable! for example, at a load of 100% the twist distortion decreased from 82° to 20°.

This improved impeller rigidity allows the impeller to withstand wind speeds of more than 12 m/s, in comparison to a maximum value of 7.5 m/s for the prior art impeller.

Fig. 13 illustrates relative camber distortion with respect to a blade mounted on the impeller of the present invention employing at least three cross members, wherein the distance between them ranged from 0.7 m close to the tip to 2 m close to the root, for maintaining the needed blade rigidity, as a function of wind speed and relative distance from a cross member, to permit the needed rigidity at predetermined regions of the blade. For example, at a wind speed of 12 m/sec, a reduction in camber distortion of 0.1% was found. Such a reduction is significant as the configuration of the camber accounts for approximately 20% of the lift coefficient CL.

Fig. 14 is a graph of the change in stagger distribution along the span on the blade, as a function of relative blade length. A reduction in blade twist deformation along the span of the blade was found when interconnecting cables were employed, from a range of 59° to 55° for a prior art blade provided without any interconnecting cables to a range of 59° to 58° for a blade provided with interconnecting cables, these values being measures at a blade length ranging from 25-95%. Such a reduction is significant as each degree of blade twist deformation accounts for approximately 10% of the lift coefficient CL. 10% of the lift coefficient CL accounts for 20% of the torque and rotor efficiency.

Another advantage of the present invention is in terms of its control flexibility and speed. Fig. 18 illustrates typical wind speed fluctuations over time. The wind velocity sometimes changes as much as 30% within a second, resulting in a change in TSR of as much as 79%. Each fluctuation was measured during a standard measurement single sample period of 10 seconds. The wind speed fluctuations required 300-1000 averaged samples for each measurement point, in order to achieve a sufficient level of measurement accuracy.

The variable output power (OP) of a wind turbine is given by the following relation^

OP = ½ pAV³C(PA, TSR), (Equation 3) where p is the density of air, A is the swept area of the impeller, V is the wind speed, and C is the power coefficient, which is a function of the pitch angle PA and the tip speed ratio TSR.

Since utilization of the wind energy will be optimized when the power coefficient of the turbine is highest, and since the power coefficient is dependent on the TSR, it therefore follows that the angular speed of the impeller should change in response to the change of the wind speed in order to capture the maximum power.

The control system comprises a controller, a converter for rectifying the variable- magnitude and variable-frequency voltage to DC voltage, and an inverter coupled to the generator for inverting the DC voltage to AC voltage with the same magnitude and frequency as the grid. With this arrangement, the speed of the control system can be as high as 20 Hz, enabling the impeller efficiency in terms of utilizing wind energy to be as high as 55%, as opposed to an efficiency value of 48% in the prior art. By being of relatively low weight and being able to rapidly follow changes in wind speed by means of a dedicated algorithm, the impeller of the present invention is capable of changing its speed at a rate of up to 30% of its original speed per second, and its average rate is to 10% of its original speed per second. Also, it is capable of changing its power output at a rate of up to 60% of its original power output per second, and its average rate is 30% of its original power output per second.

Fig. 19 illustrates the control efficiency at a constant TSR frequency of 1.5 Hz, showing the synchronization between wind speed fluctuation and control speed of the turbine at work with AC converter in a regime of constant TSR. The ability of predicting changes in wind speed and to thereby increase power output is accordingly able to be achieved.

Figs. 15a and 15b illustrate a pressure control unit 635 for maintaining a desired internal pressure within an inflatable shroud 202. Each unit comprises a compressor 615 shown in exploded view, a differential atmospheric pressure sensor 614, redundant control valves 612 and 613 for decreasing the internal pressure within the shroud or alternatively functioning as an automatic pressure relief control valve during periods of overpressure, and an automatic one-way valve 1011. Schematically illustrated computer 690 is in data communication with control valves 612 and 613, sensor 614, compressor 615, and one-way valve 1011. The pressure control unit may be disposed with a hermetic housing within the annulus of the shroud. Shroud 202 may be provided with one or more pressure control units.

Typical internal pressures within the shroud are as follows^ 7000 N/m² for a 6m shroud producing 10 kW, 1600 N/m² for a 15m shroud producing 200 kW, and 500 N/m² for a 40.5m shroud producing 1 MW. In other words, at significant diameters, variations in atmospheric pressure and temperature are enough to cause serious internal overpressure of the shroud, and therefore pressure control unit 635 is quite beneficial.

The internal pressure is maintained by the following algorithm. When the differential atmospheric pressure sensor detects a predetermined low pressure, one of the compressors 615 is activated. If other compressors are provided, the activated compressor is in communication with computer 690 and receives a signal to control and start the next compressor. One or more compressors may be activated simultaneously. If wind speed exceeds a predetermined internal pressure, the pressure relief valve is activated. The one-way valve 1011 compensates for ambient changes and emergency situations at a non-switched position from compressors 615.

A compressor subsystem 615 is embodied by a centrifugal compressor which is powered by electric motor 1004. Impeller 1002 and motor 1004 are inserted into rigid housing 1001. Housing 1001 is also provided with a valve housing 1005 into which is installed one-way valves 1011 for transferring air from compressor impeller 1002 to the rim shell. The compressor housing 1001 with inner flange 1006, and outer flange 1007 is connected with seal 1008 to the shroud surface. These flanges are connected to the surface with bolts 1009. Impeller cover 1003 distributes the air flow from the impeller 1002 to the housing 1001 via one-way valves 1011 to the rim. The pressure of the subsystem is limited by means of pressure valve 612 or 613, which is adjusted for the predetermined pressure difference between the internal rim pressure and the outer static pressure.

Fig. 16 schematicalhy illustrates a ducted pressure charger 700, for boosting the dynamic pressure on the rotating impeller, during approximately 50^_70% of the turbine working time. The charger may be located within, or adjacent to, compressor housing 1001. The internal volume of the shroud may be pressurized using the dynamic pressure from the outer flow around the rotating shroud. Pressure charger 700 comprises duct 701, which functions as the compressor inlet. Duct 701 may be oriented at various angles depending on the wind vector and the shroud rotating speed. A maximum vacuum level is achieved on the perpendicular open side of the duct, depending on the vector sum of the rotating shroud and the wind speed. Any intermediate open side position corresponds to an intermediate vacuum pressure. Duct 701 may be driven by a servo drive or by an air flow stabilizer, which relates to an open side leading edge of duct 701. In the internal duct volume are installed compressor 615 and pressure relief valve 613. Differential pressure sensor 710 measures the differential pressure between the internal pressure within shroud 202 and the internal static pressure on the shroud surface.

Fig. 17 schematically illustrates a braking air bag with centrifugal control. It uses aerodynamic pressure as a dynamic pressure brake on the rotating impeller, approximately up to 100% of turbine power at the predetermined rpm. This type of impeller brake prevents rotor, turbine transmission, and generator systems from over-speeding, for example during an external, electrical related malfunction in the grid. The system consists of special flexible fabric bag 801 which functions as a centrifugal weight. Force retractable spring 804, e.g. made of rubber, connects the leading edge of the bag to shroud surface 805. Small holes in the rear part of the bag serve to distribute, stabilize, and balance the pressure. The centrifugal force generated during over-speeding initially causes the leading edge of bag 801 to open by being separated from shroud surface 805. Further activity leading to full opening of the bag takes place as a result of dynamic pressure. At decreasing speed to a predetermined value, retractable spring 804 returns the leading edge of the bag to the shroud, causing the static pressure from both sides of bag 801 via the holes to equalize. Minimal air resistance is encountered when the bag returns to its initial position. Example 1

A prior art, rotating shroud turbine was used to generate 10 kW of electricity at an average wind speed of 15 m/s. The impeller had a diameter of 6m and six flexible blades having a 7% camber surface, providing a shroud with six support points. The blades were connected to a hub and to the surrounding shroud by universal joints that were resistant to bending moments and had two degrees of freedom to 20 degrees and a safety coefficient of more than 5.

The shroud was inflated to an internal constant pressure of 700 kg/m² by using an external compressor that pressurized air when the shroud was stationary.

The maximum impeller efficiency was 42%. A DC control system at a low speed sampling rate of 0.1 Hz was employed. The turbine utilized an rpm counter and a torque meter based on twist measurement of a calibrated Bowden shaft, which is flexible. The shaft was equipped with pickup sensors and with a frequency to voltage converter to measure torque. Wind speed measurement was carried out by a 3-cup anemometer.

Example 2

A rotating shroud turbine was used to generate 132 kW of electricity at an average wind speed of 11.5 m/s. The impeller had a diameter of 19.5 m, six flexible blades having a 7% camber surface, and six shroud-connected cables, providing a shroud with twelve support points. The blades were connected to a hub and to the surrounding shroud by universal joints that were resistant to bending moments and had two degrees of freedom to 20 degrees and by high strength cables, and were provided with the cross members illustrated in Figs. 4 and 5 and with embedded cables. As a result, the conic and twist rigidity increased by a factor of 4 with respect to that of a prior art impeller. The rigidity is dependent upon the number of support points, the distance between adjacent support points, and the change in wind speed. The impeller was inflated to an internal pressure of 1700 kg/m² controlled by an optimizing program using a built-in, computer driven compressor. The maximum impeller efficiency was 51%. An electronic control system characterized by high speed sampling of up to 5 Hz was employed.

While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without exceeding the scope of the claims.

Claims

1. A horizontal axis impeller for a wind turbine, comprising^'- a) a rigid hub rigidly attached to a gear box input shaft;

b) a hollow annular shroud spaced outwardly from said hub and rotatable by means of said gear box!

c) a plurality of blades made of a flexible stretched material extending within an annular space between said shroud and hub, each of said blades being attached to an outer region of said hub by a root thereof and to an inner region of said shroud by a tip thereof;

d) a plurality of shroud-connected cables extending between said shroud and said hub, for reducing impeller deformation and transferring forces to a yaw control unit; and

e) a joint at a hub to shroud-connected cable connection having at least two degrees of freedom.

2. The impeller according to claim 1, further comprising one or more interconnecting cables which are connected between a leading edge of a first blade to a trailing edge of a second blade adjacent to said first blade.

3. The impeller according to claim 1, wherein the sum of moments about the shroud axis resulting from tensile force applied by each of the plurality of shroud- connected cables and by each of the plurality of blades onto the shroud is equal to zero.

4. The impeller according to claim 1, wherein each blade is attached to the shroud from its leading edge tip to its trailing edge tip and to the hub from its leading edge root to its trailing edge root.

5. The impeller according to claim 1, wherein the shroud is inflatable.

6. The impeller according to claim 2, wherein each of the blades has a trapezoidal shape when flattened out such that its leading edge defines an angle ranging from 5 to 25 degrees with respect to its trailing edge, to ensure that each of the one or more interconnecting cables remains tensed during rotation of the impeller.

7. The impeller according to claim 4, wherein each of the blades comprises an embedded cable that is embedded within the leading edge and trailing edge thereof, for ensuring sufficient blade rigidity.

8. The impeller according to claim 7, wherein tension in each leading edge embedded cable and trailing edge embedded cable is at least twice as great as internal tension of the blade material.

9. The impeller according to claim 1, wherein each of the blades has a concave curvature with respect to the wind direction, in order to maximize wind derived rotor torque.

10. The impeller according to claim 9, wherein an angle of each of the blades with respect to the wind direction ranges from 40 to 50 degrees.

11. The impeller according to claim 7, wherein each of the blades further comprises at least one cross member extending between, and connected to, the leading edge embedded cable and the trailing edge embedded cable.

12. The impeller according to claim 1, further comprising a sleeve circumferentially fitted on the shroud which is connected to a corresponding blade, for distributing a tensile force applied by the blade onto the shroud.

13. The impeller according to claim 1, further comprising a sleeve circumferential^ fitted on the shroud for distributing a tensile force applied by one of the shroud-connected cables onto the shroud, and a reinforcement plate attached at each of two radial ends of said sleeve, wherein said two plates are coupled together by a face to face relation and a fastening element connected to one end of said shroud-connected cable is received in aligned apertures of said two coupled plates, causing said coupled plates to be pulled inwardly from said sleeve towards a shroud interior due to a tensile force applied by said shroud- connected cable.

14. The impeller according to claim 1, which has a conic deformation of 5 degrees or less and an impeller twist deformation ranging from 5 to 15 degrees, for a wind speed of up to 12 m/s.

15. The impeller according to claim 2, wherein each of the blades has a camber deformation of 1 degree or less and a blade twist deformation ranging from 0.5 to 1 degree, for a wind speed of up to 12 m/s.

16. The impeller according to claim 1, wherein the shroud is made of polyester fibers having a yield strength in tension of at least 15,000 N/cm².

17. The impeller according to claim 1, further comprising a control system for changing the impeller rotational speed by a rate of up to 30% of its original speed per second.

18. The impeller according to claim 17, wherein the control system is operable to change the impeller power output at a rate of up to 60% of its original power output per second.

19. The impeller according to claim 17, wherein the control system is operable to produce an impeller efficiency of up to 55%.

20. The impeller according to claim 1, wherein the material of each of the blades is pretensed in two orthogonal directions.

21. The impeller according to claim 1, wherein one or more of the shroud- connected cables is connected between each pair of adjacent blades.

22. The impeller according to claim 1, wherein each of the blades has an angle of attack ranging from 4 to 6.

23. The impeller according to claim 5, further comprising a pressure control unit for maintaining a desired internal pressure within the shroud.

24. The impeller according to claim 23, wherein the pressure control unit comprises a compressor, a differential atmospheric pressure sensor, and a control valve, said compressor being activated upon detection of a predetermined low pressure within the shroud by said differential sensor and said control valve being activated during periods of overpressure within the shroud.

25. The impeller according to claim 1, further comprising a dynamic pressure brake for preventing conditions of overspeed.

26. The impeller according to claim 25, which has a tip speed ratio ranging from 2.5 to 3.5.