TECHNICAL FIELD OF THE INVENTION

The present invention relates to a vertical axis wind turbine having a rotor driven by wind for rotation about a vertically oriented axis of rotation, inwardly of stator vanes arranged about the rotor and operative for directing airflow to the rotor. The vertical axis wind turbine is intended and arranged to operate as an electric power generator.

BACKGROUND AND PRIOR ART Wind turbines having rotors rotating about a vertical axis of rotation and stator vanes for directing air in to the rotor are previously known. For example, US Application No. 2006/02222483 discloses a vertical axis wind turbine generator comprising a pair of rotor blades which are diametrically spaced in parallel disposition about an axis of rotation, and a plurality of stator vanes disposed in an annular array around the rotor. In a sectional view, each rotor blade comprises a first semicircular rotor vane segment connecting tangentially to a second arcuate rotor vane segment which is curved in the reversed direction. This known turbine contains no correlation between the designs of rotor and stator, and the shape of the rotor blades seems not able to take full advantage of the stator and of an undisturbed air flow through the open centre of the rotor.

Wind turbines operating as electric power generators are likewise previously known, such as the power generator disclosed in US Patent No. 7, 1 16,006, e.g. This document discloses rotor magnets which are arranged on a ring-shaped carrier member that is supported in the periphery of a vertically oriented rotor, to which an axial airflow is supplied via a curved inlet pipe. Stator windings are supported from the inner wall of a shroud enveloping the rotor.

Wind turbines and wind power generators in general are facing the problem of delivering useful output power at shifting wind velocities. In this connection, most wind turbines are designed to produce optimally within a limited range of wind velocities. Thus, wind velocities outside the suitable range, whether below or above the range, usually result in loss of power production.

SUMMARY OF THE INVENTION

The present invention addresses this problem, and aims at providing a vertical axis wind turbine generator of the type mentioned initially, and which by design is less affected by shifting wind velocities and thus performs well within a widened range of wind velocities.

This object is met in a vertical axis wind turbine generator comprising a rotor having at least two rotor vanes reaching from an upper rotor disc to a lower rotor disc defining a rotor periphery, the rotor vanes extending in parallel disposition spaced from the rotor centre for rotation about their common axis of rotation. Each rotor vane comprises a leading rotor vane segment connecting at an angle to a trailing rotor vane segment, the connection forming a knee which reaches from the upper rotor disc to the lower rotor disc at a radial distance from the axis of rotation. The vertical axis wind turbine generator of the present invention further has a stator comprising at least four stator vanes equidistantly spaced about the periphery of the rotor and oriented substantially in tangential direction to the rotor periphery. The invention further provides that the leading rotor vane segment and the trailing rotor vane segment are each formed with a cross-sectional profile which is curved forward in the direction of rotation from the knee connection to a peripheral end at the rotor periphery, the peripheral ends connecting to the rotor periphery at a circumferential distance between the peripheral ends substantially corresponding to the circumferential length of the rotor periphery between the stator vanes.

In other words, the rotor is formed with at least two wind-capturing pockets which are open towards the periphery of the rotor, and arranged in parallel with an intermediate free passage between them for air to pass through the unobstructed centre of the rotor. A wind-powered machine is this way provided, in which the rotor is pushed forward in the direction of rotation each time a rotor "pocket" passes the entrance into the rotor between two adjacent stator vanes. In a preferred embodiment of the invention the rotor comprises two rotor vanes, each of which cooperates with a pair of stator vanes. In a 3 -vane rotor configuration, six stator vanes are preferred, and a 4-vane rotor configuration correspondingly incorporates eight stator vanes, etc. However, a perfect correlation between the circumferential span between the peripheral ends of the rotor vanes and the intermediate distance between adjacent stator vanes is deemed not to be required in order to take partial advantage of the invention, and thus other and different numbers such as five or seven stator vanes may be arranged about the rotor in alternative, however less preferred, embodiments of the invention.

It is preferred that each stator vane has a length at least amounting to the radius of the rotor. In a 2-vane rotor configuration, the arrangement of at least four stator vanes substantially tangentially to the rotor minimizes the rotor's exposure to contrary wind, i.e. the rotor vanes move shielded behind a stator vane until the peripheral end of the leading rotor vane segment passes the end of a stator vane, whereupon the rotor is subjected to wind pressure acting in the direction of rotation.

It is further preferred that in a sectional view the rotor vanes are V-shaped or L- shaped. In a most preferred embodiment, the leading rotor vane segment is the shorter leg and the trailing vane segment is the longer leg of an L-shaped section of the rotor vane.

In the most preferred 2-vane rotor configuration, the knee connections of the L- shaped rotor vanes are located at diametrically spaced positions on opposite sides of the rotor centre. The knee connections may be located on or near a rotor diameter from the peripheral end of the leading vane segment through the centre of the rotor, thus locating the leading vane segment substantially on or in parallel with the diameter. In this embodiment, the intermediate angle between the leading and trailing rotor vane segments, as measured at the knee connection, is typically more than 90° and may amount to about 120°. It is however possible, in purpose of enlarging the operative area of the rotor vanes, that the knee connections are displaced forwardly of the diameter as seen in the direction of rotation. In such embodiments, the intermediate angle between the leading and trailing rotor vane segments at the knee connection may be less than 90°, such as down to about 75°, e.g. It is however preferred that the length extension of the trailing rotor vane segment by forward displacement of the knee connection is limited to a position wherein the free air passage through the rotor, between adjacent rotor vanes, does not go below an opening angle of about 45°, as measured at the rotor centre. Preferably, in a 2 -vane rotor embodiment, the opening angle for free air passage between the rotor vanes as measured at the rotor centre is in the range of 45°-75°, and most preferred within the range of 60°-75°. In consequence, the leading and trailing rotor vane segments thus connect under substantially right angles at the knee connection

Each of the above recited measures contributes individually to advantageous embodiments of the present invention.

In a further advantageous embodiment of the invention it is foreseen that the leading and trailing rotor vane segments each has the sectional profile of an aerofoil that is shaped to produce aerodynamic lift force in the rotational direction of the rotor. The rotor vane segments may be designed with an asymmetric wing profile, the rear surface of which has a negative or concave camber facing against the rotational direction of the rotor. The leading and trailing rotor vane segments may each have a sectional width at the knee connection which is successively reduced towards the peripheral end at the rotor periphery.

Through these measures the rotor vanes achieve aerodynamic properties that accelerate the rotor and increase its operability especially towards the lower range limit of useful wind velocities. Each of the recited measures are believed to contribute individually to increased pressure difference between the air mass passing on the suction side of the rotor vanes and the air mass acting on the pressure side thereof.

The stator vanes, which are stationary arranged in a supporting frame, are equidistantly spaced about the rotor and cover the vertical distance between the upper and lower rotor discs. In the preferred 2 -vane rotor embodiment four stator vanes are preferably arranged at intermediate angular distances of 90° as measured at the rotor axis in a horizontal plane. Basically, the four stator vanes are oriented substantially tangentially to the rotor periphery. In the preferred 2 -vane rotor configuration including four stator vanes tangentially oriented to the rotor, each stator vane thus points substantially at 90° angle towards the next stator vane, as viewed in the direction of the rotor's rotation.

Similar to the rotor vanes, the stator vanes may have a sectional profile which is curved forward in the rotary direction of the rotor. Each stator vane may further be designed to have an asymmetric wing profile having a maximum thickness which is displaced towards the downwind end adjacent to the rotor periphery. The stator vanes can be designed to have a typical asymmetric wing section, such as the Clark-Y profile known from aviation, e.g. However, and in contrast to aviation, the stator vanes are preferably arranged as illustrated with the wing's normally leading end pointing downstream in the wind direction, i.e. towards the rotor, and the wing's normally trailing end now instead pointing towards the wind, i.e. upstream in the wind direction and away from the rotor.

In a further development the stator vanes may be arranged adjustable with respect to their orientation relative to the rotor, in aspect of radial distance from the rotor centre and optionally in aspect of the stator vane's angular deviation from a tangent to the rotor periphery. Adjustability may be achieved through a pivot attachment of the stator vanes to a frame structure in which the rotor is journalled for rotation. Through these measures, the stator vanes may be controlled into an optimized angle of attack with respect to the wind direction.

Through these measures, the incoming air is accelerated in a substantially laminar flow over the curved stator vane surface. At the downwind end, i.e. upon entrance into the rotor, the laminar airflow near the stator vane is destroyed into a turbulent flow that creates a low pressure region downstream of the stator vane. This low pressure region initially reduces resistance as the rotor vane segments in succession pass the downwind end of the stator vane, to be successively caught and effected upon by the airflow passing outside the stator vane.

The vertical axis wind turbine generator as disclosed above is intended for operation in a system for producing electrical energy. To this purpose an electric power generator may be conventionally driven by a rotor shaft reaching axially outside the rotor assembly. However, in a most advantageous embodiment the rotor has no central through shaft, and is journalled for rotation via its upper and lower rotor discs being pivotally supported in a frame structure. This embodiment provides unhindered air flow through the rotor centre.

In the vertical axis wind turbine generator having no through shaft, power generation means are supported by the peripheral ends of the leading and trailing rotor vane segments and arranged for cooperation with power generation means supported by the stator vanes. To this purpose, permanent magnets may be arranged at the rotor periphery and effective to generate voltage through electromagnetic induction in stator windings that are supported on the stator vanes. Especially, rotor magnets may be arranged and distributed along the rotor periphery on a ring-shaped carrier member that is attached to the peripheral ends of the rotor vanes, and stator windings may be supported in the radially inner and downwind ends of the stator vanes, or supported on quarter-circular carriers arranged between the stator vanes. The rotor magnets and the stator windings may further be arranged at several power production levels between the upper and lower rotor discs, providing this way a number of power production cells which are arranged electrically separated and controllable between operative and non- operative states. Through this arrangement, the rotor speed will be adaptable to the current wind velocity by adjustment of the power output.

SHORT DESCRIPTION OF THE DRAWINGS The vertical axis wind turbine generator of the present invention is more closely explained below with reference made to the drawings, schematically illustrating a preferred embodiment of the invention. In the drawings,

Fig. 1 is a perspective view of the vertical axis wind turbine generator;

Fig. 2 is a horizontal sectional view of the same, and

Fig. 3 is a view corresponding to fig. 2, schematically illustrating an arrangement of power generating means in the vertical axis wind turbine generator according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The drawings illustrate a preferred embodiment of a vertical axis wind turbine generator according to the invention, comprising two rotor vanes and four stator vanes. As discussed above, alternative embodiments may include other and different numbers of rotor and stator vanes, as long as the relation between the interspacing of stator vanes and circumferential span of rotor vanes is substantially met.

With reference to fig. 1 , a vertical axis wind turbine generator comprises a rotor 1 arranged for rotation about a vertically oriented axis of rotation 2. The rotor 1 comprises rotor vanes 3, 4 running in spaced parallel disposition radially outside a rotor centre between an upper rotor disc 5 and a lower rotor disc 6. Each rotor disc 5, 6 can be formed as a one-piece element from a sheet material as illustrated, or be formed as a framework, or may simply comprise spoke elements extended from a central hub which is journalled for rotation, e.g.

The rotor 1 comprising rotor vanes 3, 4 and rotor discs 5, 6 is journalled for rotation in bearings (not disclosed) that are supported in a framework 7. The framework 7 also supports a number of stationary vanes or stator vanes 8 which are arranged radially outside the rotor to extend in parallel with the rotor between the upper and lower rotor discs. The framework 7 comprises a base 9 by which the wind turbine can be vertically erected on a ground surface, a mountain top or even on the roof of a building, if appropriate. The framework 7 may additionally comprise a top 10 by which a second wind turbine can be erected on top of the first wind turbine, in a tower structure including two or even more wind turbines, if desired and appropriate. The stator vanes 8 may alternatively form an integrated part of the framework.

The structure and function of the rotor and stator vanes will now be more closely explained with reference made to the sectional views of figs. 2 and 3, wherein the rotational direction of the rotor is illustrated by arrow R, and a wind direction to the rotor is illustrated by arrow W. Starting with the rotor vanes, the description of rotor vane 3 is applicable also to the rotor vane 4 which is identical to the rotor vane 3. The rotor vanes 3 and 4 are symmetrically arranged in the rotor, in the 2 -vane rotor embodiment on diametrically opposite sides of the axis of rotation.

In a cross-sectional view, the rotor vane 3 is basically of L-shape. The rotor vane 3 comprises a first rotor vane segment 3a and a second rotor vane segment 3b. The first rotor vane segment 3a is the leading rotor vane segment and the second rotor vane segment 3b is the trailing rotor vane segment, as viewed in the direction of rotation R. The rotor vane segments 3a, 3b are interconnected at a knee connection 3c running vertically, at a radial distance from the rotor's centre C, from the upper rotor disc to the lower rotor disc. The knee connection 3c thus runs separated from the rotor centre, in parallel with the axis of rotation 2, forming an air passage between the respective knees of the two rotor vanes through which air can flow freely through the rotor centre.

The leading rotor vane segment 3a reaches from the knee connection 3c to a peripheral end 3d connecting to the rotor periphery PE. The trailing rotor vane segment 3b reaches correspondingly from the knee connection 3c to a peripheral end 3e connecting to the rotor periphery PE. In the illustrated 2-vane rotor embodiment, the peripheral ends 3d-3e span substantially over one quarter of the circumferential length of the rotor periphery. The leading rotor vane segment 3 a may be arranged substantially on a diameter dl from the peripheral end 3d through the rotor centre, thus forming the shorter leg of the L-section, whereas the trailing rotor vane segment 3b forms the longer leg connecting to the rotor periphery on a diameter d2 perpendicular to the diameter d 1. In order to increase the effective rotor vane area which is exposed to wind pressure, the knee connection 3c may be displaced forward of the diameter d as seen in the direction of rotation. However, in order to maintain unrestricted airflow through the open centre of the rotor, a displacement of the knee connection forward of the diameter should preferably be limited to the extent that the free air passage towards the rotor periphery, between the peripheral end of a leading rotor vane and the knee connection of a trailing rotor vane, does not go below an opening angle β of about 45°, as measured at the rotor centre. Preferably, the opening angle for free air passage between the rotor vanes as measured at the rotor centre is in the range of 45°-90°, and most preferred within the range of 60°-75° in a 2-vane rotor embodiment.

In consequence of the above, a relative angle α between the leading 3a and trailing 3b rotor vane segments, as measured at the knee connection 3c, may be varied within a range of about 75- 120°, but is preferably about 90°, such as between 80° and 100°.

The rotor vane segments 3a, 3b may be formed as aerofoils shaped to produce aerodynamic lift in the direction of rotation R, having a curved front surface facing in the direction of rotation and a curved rear surface facing against the direction of rotation. The front surface of each rotor vane segment 3a, 3b is formed with a positive camber or convexity pointing in the rotational direction R. In this respect, each rotor vane segment 3a, 3b preferably has the sectional profile of an asymmetric wing, wherein the rear surface of the rotor vane segment has a negative camber or concavity facing against the rotational direction of the rotor. In particular, each rotor vane segment is preferably formed with a sectional width w at the knee connection 3c which gradually reduces towards the peripheral end 3d, 3e located on the rotor periphery.

In the illustrated 2-vane rotor embodiment, four stator vanes 8 ¹ S ⁴ are equidistantly spaced about the rotor 1. Basically, the stator vanes 8 are oriented tangentially to the rotor periphery. Each stator vane has a length which is at least equal to the radius of the rotor, the length preferably amounting to 1.5 - 2 times the length of the radius of the rotor.

The stator vanes 8 are curved in the direction of rotation of the rotor, and may have the sectional profile of an aerofoil having a cambered or convex surface facing out from the rotor. Advantageously, the wing profile of the stator vane 8 can have an inner surface, i.e. a surface facing towards the rotor, which includes another camber than the outer surface, giving the stator vane a thickness. The inner surface may be flat, or formed with a slightly negative or positive camber, in other words the stator vane has a flat or a slightly convex or a slightly concave inner surface. However, the wing profile is oriented such that the maximum thickness w ^MAX is displaced towards the downwind end of the stator vane, which is the end nearest to the rotor. In other words, the outer surface camber is such that the camber radius at the downwind end is smaller than the camber radius at the upwind end, which is the end that is pointing away from the rotor. Thus, in contrast to aviation, the stator vane's wing profile is reversely oriented with the trailing end instead pointing towards the wind direction.

The operation and aerodynamic forces acting on the vertical axis wind turbine generator, as presently understood, will now be explained with reference made to fig. 2. In a case of wind direction W in alignment with a stator vane 8 ² , as illustrated through the dashed lines, a portion of the air mass is captured between stator vanes 8 ¹ , 8 ² , 8 ³ to be directed towards the rotor. On each side of the air mass of interest, the air either passes the wind turbine or is deflected away from the wind turbine along the upwind surfaces of the stator vanes 8 ¹ and 8 ³ . Adjacent the curved outer surface of stator vane 8 ² the flow is substantially laminar and accelerated before entrance into the rotor. Upon passage of the downwind or inner end of stator vane 8 ² , the laminar flow is destroyed and a low pressure region of turbulent air, in fig. 2 illustrated by a vortex, is created in front of the approaching leading rotor vane segment 4a of rotor vane 4. On the opposite or inner side of stator vane 8 ² the air mass is captured between the leading and trailing rotor vane segments 4a and 4b, together forming a wind-capturing "pocket" which is open towards the rotor periphery between the adjacent stator vanes 8 ¹ and 8 ² . The pressure difference between the air volumes in front of and behind the leading rotor vane segment 4a is at its peak when the peripheral ends of the rotor vane 4 correlate with the corresponding inner ends of the adjacent stator vanes. In this sequence of revolution the opposite rotor vane 3 moves on the lea side of the stator vane 8 ¹ which significantly reduces air resistance for rotor vane 3. When in continued rotation the rotor vanes 3 and 4 successively pass the surrounding stator vanes, air flow through the rotor centre creates lift on the forward side of the curved or aerofoil-shaped leading rotor vane segments, and pressure on the backside of the trailing rotor vane segments.

Obviously, in most geographic places the wind direction is not constant over time. It will be realized that in a vertical axis wind turbine generator comprising four equidistantly spaced stator vanes as illustrated, identical conditions are re- established at every 90° change of wind direction. In order to ensure best possible operation in most wind directions, the stator vanes may be arranged adjustable in the framework 7. To this purpose, the stator vanes 8 may be arranged movable about a pivot 1 1 which is journalled in the downwind end of the stator vane, and controlled by an actuator 12 to assume the most effective angle of attack towards the incoming wind. The range of angular adjustability of the stator vanes may amount to 0-45°, measured from the tangent T to the rotor periphery. Alternatively or in addition, the framework 7 may itself be arranged pivotable about the vertical axis.

In fig. 3 the wind turbine is schematically equipped as a power plant for generation of electric energy. Permanent magnets 13 are supported on a ring-shaped carrier member 14 which is attached to the peripheral ends of the rotor vanes 3, 4. Stator windings 15 and associated magnet cores are arranged and supported at the inner ends of the stator vanes 8. The detailed circuitry and control electronics are omitted from the drawing, and considered to be within reach of a person of ordinary skill in the art of electric power generation. It should be mentioned though, in order to control the magnetic field strength, that an air gap between the rotor magnets and the stator windings may be adjustable by arranging the stator vanes, and /or the stator windings, movable in the radial direction of the rotor. To this purpose, an actuator 16 may be arranged to adjust the radial position of the pivot 1 1 , e.g.

The vertical axis wind turbine generator as disclosed provides a compact structure and ability to operate within a widened range of wind velocities. The skilled person will realize that modification of the detailed structure is possible without departing from the invention. The features that provide the enhanced operation are listed in the set of claims, wherein advantageous alternatives and preferred embodiments of the vertical axis wind turbine generator are defined in the subordinated claims.