The invention relates to a device for converting wind energy to at least mechanical energy, comprising:

a rotor drivable rotatably about a rotation axis by wind and a duct disposed therearound with a wind inlet opening and a wind outlet opening, wherein a central axis of the duct substantially coincides with the rotation axis of the rotor; and

at least one valve which is arranged on the wind outlet opening of the duct and which is adjustable between a first open state, in which the at least one valve leaves the wind outlet opening substantially clear, and a second closing state in which the at least one valve substantially closes at least a part of the wind outlet opening.

Such a converting device is known from US 7 230 348 B2. This document describes a wind turbine with a rotor of the "Savonius" type rotatable about a vertical shaft, wherein the wind inlet opening is oriented horizontally, while the wind outlet opening is oriented vertically. The inflowing air is deflected downward through 90° by the rotor. The wind outlet opening is formed around the vertical shaft and is partially closable by means of movable valves which are controlled on the basis of an anemometer. The efficiency of the generator connected to the rotor is optimized by opening the wind outlet opening to greater or lesser extent.

A conventional device for converting wind energy to at least mechanical energy is generally referred to as a wind turbine or windmill. The invention can relate particularly to a relatively small wind turbine, also referred to as a microturbine or urban wind turbine, which wind turbine can be set up in an urban environment, and in particular optionally on a building. The invention can relate more particularly to a so-called horizontal wind turbine, wherein in use of the wind turbine the rotation axis of the rotor and the central axis of the duct are disposed substantially horizontally.

It is an object of the invention to improve the per se known device of the type stated in the preamble. A particular object of the invention can be to increase the efficiency of the per se known device.

This object is achieved with a device of the type stated in the preamble which is characterized according to the invention in that the at least one valve is a non-return valve such that the at least one valve adjusts automatically from its closing state to its open state when the wind flows from the wind inlet opening to the wind outlet opening, and adjusts automatically from its open state to its closing state when the wind threatens to flow into the duct through the wind outlet opening. This prevents air flowing back into the wind turbine when the wind direction and/or speed changes.

In the open state the wind can leave the duct substantially unobstructed via the wind outlet opening. In the closing state at least a part, in particular the peripheral zone, of the wind outlet opening is closed, whereby the wind is at least substantially prevented from flowing into the duct via the wind outlet opening, so in an upward back-flow direction.

In the case of varying wind speeds and/or a change in the wind direction it may be that a very low pressure occurs briefly at the outlet opening relative to an air pressure around the turbine and/or at the inlet opening, which can cause a back flow of wind and whereby wind can flow via the wind outlet opening into the duct. This has an adverse effect on the efficiency of the turbine because the flow can approach the rotor from a wrong side, which can thereby come to a standstill. In order to prevent this it is advantageous for the device according to the invention to be provided with valves which can temporarily substantially close the wind outlet opening at least partially. It is particularly advantageous that the at least one valve opens automatically when the air flows in downstream direction out of the duct via the wind outlet opening and closes automatically when the air threatens to flow in an opposite upstream direction into the duct via the wind outlet opening. This can for instance be detected by an anemometer or wind angle meter which can generate a signal to a drive connected to the valve.

It is noted that the duct can comprise any suitable cross-sectional form. The duct here preferably has a circular cross-sectional form at least in the area of the rotor so that the part of the duct where the rotor is disposed is substantially cylindrical. The wind inlet opening and/or the wind outlet opening of the duct can also have a substantially circular cross-section. In that case the duct preferably has a circular cross-sectional form at any random location along its length. The wind inlet opening and/or the wind outlet opening can alternatively have any other suitable cross- sectional form, such as for instance oval. In the case of such a non-circular cross-sectional form of the wind inlet opening and/or the wind outlet opening the duct preferably transposes gradually to the circular cross-sectional form in the area of the rotor.

In an embodiment of the device according to the invention the at least one valve is manufactured from a flexible material.

Such a valve of a flexible material can be automatically blown open to its open state when the wind flows in downstream direction and can be automatically blown shut to its closing state when the wind flows in upstream direction. A control is therefore then not necessary.

The flexible material can for instance be a fabric.

In another embodiment of the device according to the invention the at least one valve extends over at least a part of the periphery of the outlet opening and is connected with one peripheral end zone thereof to the duct, wherein the other peripheral end zone is freely suspended.

The freely suspended peripheral end zone can be easily moved here between the open state and the closing state of the valve.

The valve can alternatively take a stiff form. It is in that case advantageous for the at least one valve to extend over at least a part of the periphery of the outlet opening and to be connected pivotally with one peripheral end zone thereof to the duct such that the at least one valve is pivotable between its open state and its closing state.

In another embodiment of the device according to the invention the device comprises at least one annular element disposed concentrically with the central axis close to the outlet opening and having a smaller cross-sectional dimension than the wind outlet opening.

The valve extending over the outlet opening can rest in its closing state against the at least one annular element and thus close the space between the outlet opening and the annular element, whereby at least the peripheral zone of the outlet opening is closed in the closing state.

It is noted that the substantially annular element can have any suitable cross-sectional form, such as circular, but also non-circular, for instance oval. The substantially annular element can particularly have a form adapted to the local cross-sectional form of the duct. The local cross- sectional form is understood here to mean the cross-sectional form of the part of the duct situated at the same position in radial direction.

In another embodiment of the device according to the invention the at least one annular element is provided with a said valve, wherein the valve extends over at least a part of the periphery of the at least one annular element and is connected, optionally pivotally, with one peripheral end zone thereof to the at least one annular element.

Said valve can in its closing state at least partially close a surface area of the outlet opening of the duct enclosed by the at least one annular element.

In another embodiment of the device according to the invention the device comprises at least one subsequent annular element disposed concentrically with the central axis close to the outlet opening and having a smaller cross-sectional dimension than the at least one annular element, which at least one subsequent annular element is optionally provided with a said valve, wherein the optional valve extends over at least a part of the periphery of the at least one subsequent annular element and is connected, optionally pivotally, with one peripheral end zone thereof to the at least one subsequent annular element.

The valve connected to the at least one annular element can rest in its closing state against the at least one subsequent annular element and thus close the space between the at least one annular element outlet and the at least one subsequent annular element in the closing state of the valve.

When the at least one subsequent annular element is also provided with a valve, this valve can in its closing state at least partially close a surface of the outlet opening of the duct enclosed by this subsequent annular element.

When a plurality of subsequent annular elements are provided, they then each have a differing cross-sectional dimension, and in particular a cross-sectional dimension smaller than the at least one subsequent annular element.

In the case of a plurality of subsequent annular elements this valve connected to the at least one subsequent annular element can rest against another subsequent annular element.

All subsequent annular elements can optionally be provided with a valve so that substantially the whole outlet opening can be closed by the valves. A central part of the outlet opening can however also not be closable by a valve because the risk of a back-flow wind stream which threatens to flow via the outlet opening into the duct is relatively small.

The annular elements can be disposed successively of each other as seen in downstream direction, wherein the annular elements are disposed in a sequence of decreasing cross-sectional dimension.

In yet another embodiment of the device according to the invention the or each annular element comprises a peripheral surface which widens in downstream direction.

An advantage of the peripheral surface of the substantially annular element which widens in downstream direction, i.e. the cross-sectional dimension of the substantially annular element increases in downstream direction, is that the air is guided outward to some extent. This creates an underpressure close to the wind outlet opening, particularly in a central part thereof, relative to the wind inlet opening, whereby the speed of the airflow in the duct can increase and the efficiency of the device can be enhanced.

The widening annular element can widen in any suitable manner. In an embodiment of the device according to the invention the peripheral surface extends obliquely outward in downstream direction at an angle to the central axis. The substantially annular element in this way widens in a substantially tapering manner.

Said angle can for instance be greater than 0° and smaller than 60°.

In yet another embodiment of the device according to the invention the device comprises bounding means for bounding a movement of the valve in its closing state such that the valve cannot move from its closing state in upstream direction to the wind inlet opening.

The valve is hereby prevented in efficient manner from being blown open in upstream direction from its closing state.

In practical manner the bounding means can comprise at least one elongate bounding element extending in substantially radial direction.

The at least one bounding element can for instance comprise a rod or tensioned wire.

The at least one bounding element can for instance extend between the outlet opening of the duct and the at least one annular element and/or, when applicable, between adjacent annular elements.

The invention will be further elucidated with reference to the figures shown in a drawing, in which - figures 1 A- 1 D show schematically the wind turbine according to a first embodiment of the invention, wherein figure 1 A is a perspective view from a wind inlet side, figure IB is a side view, figure 1C is a perspective view from a wind outlet side, and figure 1 D is a longitudinal vertical cross-section;

- figure 2 shows schematically a perspective view of rotor and guide blades disposed in a duct of the wind turbine of figure 1;

- figures 3A and 3B show schematically in detail the valves on the wind outlet opening, wherein figure 3A shows the valves in an open state and figure 3B shows the valves in a closing state;

- figures 4A-4C show schematically a nanostructure which can be arranged on a number of surfaces of the wind turbine, wherein figure 4 A is a top view of the nanostructure, figure 4B shows a detail of figure 4A and figure 4C shows a cross-section through the naiioslruclure.

- figures 5 A-5EG show schematically a rotor of the wind turbine of figure 1 , wherein figure 5A is a perspective front view, figure 5B is a front view, figure 5C is a section in the longitudinal direction of the rotor of figure 5B; figure 5D shows a pressure side of a rotor blade and figure 5E is a rear view of the rotor blade; and

- figures 6A and 6B show schematically the wind turbine according to a second embodiment of the invention, wherein figure 6A is a perspective view from a wind inlet side and figure 6B is a front view.

The various aspects of the invention will be elucidated with reference to the figures. The same elements will be designated here with the same reference numerals. The different aspects of the invention can be applied individually or in any random combination.

Figures 1 A- ID show a wind turbine 1 according to a first embodiment of the invention. Wind turbine 1 comprises a duct 2 with a central axis 3. A rotor 4 is disposed in duct 2, wherein the central axis 3 of duct 2 substantially coincides with a rotation axis of rotor 4. Duct 2 has a wind inlet opening 5 and a wind outlet opening 6. In this first embodiment wind inlet opening 5 and wind outlet opening 6 are circular.

According to an aspect of the invention, duct 2 is provided on its outer side close to wind inlet opening 5 with a number of wind capture elements 7, in this example three, extending radially outward. Each wind capture element 7 is provided with a channel 8 extending to the inner side of duct 2. The three wind capture elements 7 are arranged distributed at an equal mutual angular distance over the outer surface of duct 2. Each channel 8 extends over substantially its full length in helical form in flow direction round the central axis through duct 2, and debouches with an outlet opening 9 on the inner surface of duct 2. Wind capture elements 7 capture wind flowing on the outer side of duct 2 and feed this wind in helical form to the inner surface of duct 2 via outlet openings 9. According to another aspect of the invention, see also figure 2, wind turbine 2 comprises a number of stator blades 10, in this example six, which are disposed upstream of rotor 4 in duct 2 and which extend radially outward from the central axis 3. Stator blades 10 have a main plane which extends radially from central axis 3 and which is disposed at an oblique angle relative to central axis 3. Because of the oblique angle of the main plane of stator blades 10 the wind flow flowing in duct 2 is guided in an oblique direction relative to central axis 3 so that the wind flow is guided in a substantially helical movement round the central axis 3. Each stator blade 10, in particular the main plane thereof, is provided with a number of upright ribs 11 , in this example three. The upright ribs 11 extend from the pressure side of each stator blade 10 from an upstream wind entry side of blade 10 to a downstream wind exit side of stator blade 10. Ribs 11 extend obliquely outward as seen in radial direction over the wind guiding surface so that on the wind exit side each rib 11 is located at a greater radial distance from the central axis than on the wind entry side. The ribs support the change in the flow direction of the airflow to said helical movement round central axis 3. The desired angle of the helical movement of the wind round central axis 3 is preferably adjustable. Stator blades 10 are connected for this purpose to a connecting shaft 12 extending radially from central axis 3, which connecting shafts 12 are each connected at their radial outer end to duct 2. Stator blade 10 is pivotable about or with connecting shaft 12 for the purpose of adjusting the oblique angle of stator blade 10 relative to central axis 3. Each stator blade 10 is provided with a number of openings 13, in this example three. On the wind exit side each stator blade 10 is provided with a substantially sine-shaped end edge 14, the second derivative of which changes sign more than once.

According to another aspect of the invention, see figure ID, the inner side of duct 2 takes the form, from wind inlet opening 5 up to for instance the location where connecting shaft 12 is disposed, of a Venturi narrowing in flow direction. In a part of duct 2 where rotor 4 is disposed the inner side of duct 2 is substantially cylindrical. Particularly the combination of the Venturi form of the inner side of duct 2 and the stator blades 10 ensures that the wind flows in helical form with a radially outward component upstream of the stator blades 10, so that the diameter of the wind flow supplied to wind turbine 2 upstream of wind inlet opening 5 increases in upstream direction, see also figure 1A.

According to another aspect of the invention, see figure ID and figure 2, wind turbine 2 comprises a number of rear stator blades 20, in this example six, disposed in duct 2 downstream of rotor 4 and substantially connecting thereto for guiding the wind away from rotor 4 in a substantially downstream direction. Rear stator blades 20 extend radially outward from central axis 3. Each rear stator blade 20 is provided with a number of upright ribs 21, in this example three. Upright ribs 21 extend from the pressure side of each rear stator blade 20 from an upstream wind entry side of blade 20 to a downstream wind exit side of rear stator blade 20. Ribs 21 extend obliquely outward as seen in radial direction with a determined curvature over the wind guiding surface so that on the wind exit side each rib 21 is located at a greater radial distance from central axis 3 than on the wind entry side. Ribs 21 substantially convert a possible helical airflow coming from rotor 4 to a radially outward expanding airflow flowing substantially parallel to central axis 3. The angle of rear stator blades 20 to the central axis is preferably adjustable. Rear stator blades 20 are connected for this purpose to a connecting shaft 22 extending radially from central axis 3, which connecting shafts 22 are each connected at their radial outer end to duct 2. Rear stator blade 20 is pivotable about or with connecting shaft 22 for the purpose of adjusting the angle of rear stator blade 20 relative to central axis 3. On the wind exit side each rear stator blade 20 is provided with a substantially sine-shaped end edge 24, the second derivative of which changes sign more than once. Each rear stator blade 20 has substantially two blade parts 25, 26 disposed at an angle (x4 relative to each other, wherein blade part 25 substantially connects to rotor 4 and blade part 26 is disposed downstream of blade part 25. Depending on the adjusted angle of rear stator blade 20, blade part 25 can extend substantially at an angle to central axis 3 and blade part 26 can extend substantially parallel to central axis 3. The angle al between blade parts 25, 26 is in this example around 130°. Blade part 26 has an increasing height so that the wind is guided substantially radially outward, and thereby expands. The increasing height of blade part 26 is optionally adapted to the form of the inner side of that part of duct 2 where blade part 26 is disposed, as will be further elucidated below.

According to another aspect of the invention, see figure ID, a part of duct 2 extending from rotor 4 to wind outlet opening 6 widens in flow direction, particularly in the form of a Venturi. Duct 2 widens in Venturi form particularly on both its inner side and its outer side. Due to the Venturi form of the outer side of duct 2 the airflow flowing on the outer side of duct 2 is guided radially outward to some extent, whereby an underpressure is created in the area of outlet opening 6. An outlet angle al 1 of wind outlet opening 6 to central axis 3 is in this example about 60°.

As elucidated above with reference to rear stator blades 20 and as shown in figure ID and figure 2, the height of blade part 26 can be adapted here to the inner side of duct 2 widening in the form of a Venturi. A tangent of an upper edge 27 of each rear stator blade 20, and in particular of blade part 26 thereof, can make an angle a2 with central axis 3 which is adapted to the inner side of duct 2 widening in the form of a Venturi, and thereby increases in this example along its length in downstream direction from about 20° to about 80°.

According to another aspect of the invention, duct 2 has a thickness and/or form such that the flow distance of the wind through duct 2 is smaller than the flow distance round the outer side of duct 2, and that because of the form the flow direction round the outer side of duct 2 changes direction at the position of wind outlet opening 6. An underpressure is hereby created in the area of outlet opening 6. According to another aspect of the invention, the diameter of wind outlet opening 6 of the duct is greater than an outer diameter of wind inlet opening 5 of duct 2.

According to another aspect of the invention, the outer periphery of duct 2 is provided with a helical upright rib 30. This lengthens the flow distance of the wind on the outer side of duct 2 compared to the flow distance of the wind through the inner side of duct 2, and it changes the flow direction round the outer side of duct 2. An underpressure is hereby created in the area of outlet opening 6.

According to another aspect of the invention, see also figures 3 A, 3B, wind turbine 1 is provided in the area of wind outlet opening 6 of duct 2 with a number of annular elements 40, in this case two, disposed concentrically with outlet opening 6. Annular elements 40 each have a different diameter which are both smaller than the diameter of outlet opening 6. Annular elements

40 each comprise a cylindrical peripheral surface which extends obliquely outward in downstream direction at an angle to central axis 3. Annular elements 40 are therefore substantially conically widening annular elements. Due to the outward tapering form of annular elements 40 the wind flowing out of outlet opening 6 is guided radially outward. Arranged on duct 2 extending over the periphery of outlet opening 6 is a flexible valve 41 which is connected with one end zone to duct 2. Arranged on the outer annular element 40 is a flexible valve 41 which extends over the periphery thereof and which is connected with one end zone to annular element 40. In figure 3 A valves 41 are shown in their open state, in which they leave outlet opening 6 substantially clear. The wind flowing out of wind outlet opening 6 moves the valves automatically into this open state. When the wind turns and threatens to flow into duct 2 via outlet opening 6, the wind pushes valves 41 automatically to their closing state as shown in figure 3B. In the closing state the valve 41 connected to duct 2 lies with its free end zone against the outer annular element 40, and the valve connected to the outer annular element 40 lies against the inner annular element 40 so that valves

41 substantially close at least the peripheral zone of wind outlet opening 6. Particularly the valve 41 connected to outlet opening 6 substantially closes the space between outlet opening 6 and the outer annular element 40. Particularly the valve 41 connected to outer annular element 40 substantially closes the space between outer annular element 40 and inner annular element 40. Bounding elements in the form of rods 42 extend between the peripheral end zone of outlet opening 6 of duct 2 and outer annular element 40 and between outer annular element 40 and inner annular element 40. These rods 42 prevent the flexible valves 41 blowing the valves 41 further inward from their closing state by the wind threatening to flow into outlet opening 6. In this example the inner annular element 40 is not provided with a valve, so that a central part of outlet opening 6 cannot be closed. This inner annular element 40 can if desired also be provided with a valve so that the central part of outlet opening 6 can be closed and outlet opening 6 can be substantially completely closed. Wind turbine 1 according to the invention can particularly be a relatively small wind turbine, also referred to as a microturbine or urban wind turbine, which wind turbine can be set up in an urban environment, and in particular optionally on a building. Wind turbine 2 can for this purpose comprise a leg 50, using which the wind turbine can be set up. As shown in the figures, wind turbine 1 is particularly a so-called horizontal wind turbine, wherein the rotation axis of the rotor and the central axis 3 of duct 2 are disposed substantially horizontally during use of wind turbine 1.

An inner surface of the duct and/or rotor blades of the rotor is/are provided with a structure, which structure has a pattern of recesses for receiving substantially stationary air.

Figures 4A-4C show a nanostructure 60 which can for instance be arranged on the inner surface of duct 2 and/or on stator blades 10 and/or on rear stator blades 20. Nanostructure 60 has a pattern of recesses 61 for receiving substantially stationary air. The dimensions of recesses 61 lie in the order of magnitude of several μπι to several mm. In this example the dimensions are substantially oval, but can take any desired form. In this example the length 62 of each recess is about 4.2 mm. The width 63 of each recess in this example is about 2.3 mm. In this example the depth 64 of each recess is about 0.7 mm. The peripheral wall of each recess 61 extends in this example at an angle a3 to the inner surface of the duct and/or the surface of stator blade 10 and/or rear stator blade 20, wherein the angle a8 is in this example about 95°. The peripheral wall of each recess 61 is connected in this example at a rounded angle 65 to the bottom of each recess, wherein the rounded angle 65 in this example has a radius of about 0.6 mm. In this example the recesses 61 are disposed adjacently of each other in a number of substantially straight lines 69, wherein the straight line extends at an angle a4 relative to the central axis 3, wherein the angle a4 in this example is about 41°. In this example a centre-to-centre distance 66 between two recesses 61 disposed in one line adjacency of each other is about 3.8 mm. In this example recesses 61 of two mutually adjacent lines 69 of recesses 61 are disposed offset relative to each other, wherein the offsetting 67 in a direction perpendicularly of the longitudinal direction of duct 2 is in this example about 1.1 mm. A centre-to-centre distance 68 between two adjacent recesses 61 of adjacent lines 69 is in this example about 5.2 mm.

Figures 5A-5E show a rotor according to an aspect of the invention. The rotor comprises a number of rotor blades 70, in this example six, which are connected with a peripheral edge to a rotor body 71 of a generator, see also figure ID. Rotor 4 is driven rotatingly by a wind flow flowing in duct 2, whereby rotor body 71 co-rotates. A stator body 77 of the generator disposed in duct 2 is arranged round rotor body 71, see figure ID. As shown in figure 5C, rotor blades 70 are disposed at an angle a5 to rotation axis 3, this angle a5 being about 53° in this example. As shown in, among others, figures 5A, 5B and 5D, the rotor blades have a wind entry side with a front end edge 72 and a wind exit side with an end edge 73. End edge 73 is substantially sine-shaped over a curved main line 74. An angle a6 of main line 74 close to an inner end of end edge 73, which is disposed close to the rotation axis coinciding with central axis 3, relative to a straight line 75 between the inner end and the outer end of end edge 73, which is disposed close to rotor body 71, is in this example about 38°. An angle l of the main line 74 close to the outer end of end edge 73 relative to the straight line 75 between the inner end and the outer end is in this example about 17°. The front end edge 72 is substantially arcuate. An angle a8 of front end edge 72 close to an inner end of front end edge 72, which is disposed close to the rotation axis coinciding with central axis 3, relative to a straight line 76 between the inner end and the outer end of front end edge 72, which is disposed close to rotor body 71, is in this example about 28°. An angle al4 of front end edge 72 close to the outer end of front end edge 72 relative to the straight line 76 between the inner end and the outer end is in this example about 48°. As can be seen in, among others, figures 5C and 5E, rotor blades 70 are twisted in a direction between an inner end zone and the peripheral edge connected to generator body 71, in this example through an angle al5 of about 5°.

Figures 6A and 6B show a wind turbine 1 according to a second embodiment of the invention. Only the differences from the wind turbine of figures 1-5 will be elucidated here, and for a further specification of figures 6A and 6B reference is made to the figure description associated with figures 1-5.

Wind turbine 1 according to the second embodiment of the invention differs from the wind turbine according to the first embodiment in that inlet opening 5 and outlet opening 6 are substantially oval-shaped instead of circular. Duct 2 transposes gradually from its oval end zones or openings 5, 6 to a round cross-sectional form so that the part of duct 2 where rotor 4 is disposed is substantially cylindrical, just as in the wind turbine according to the first embodiment.

It is noted that the invention is not limited to the shown embodiments but also extends to variants within the scope of the appended claims.

The stated values for dimensions, angles and the like are thus given only by way of example. Applicant has found that said values are particularly suitable, but the invention is thus not limited thereto.

It will also be apparent that the form of the inlet opening and/or outlet opening is not limited to the shown circular shape or oval shape, but that it can have any suitable shape. The part where the rotor is disposed is however preferably of circular cross-section, and thereby cylindrical, wherein in the case of a non-circular inlet opening or non-circular outlet opening a gradual transition to this cylindrical part will take place.