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

a rotor with a number of rotor blades drivable rotatably about a rotation axis by wind and a duct disposed round the rotor, wherein a central axis of the duct substantially coincides with the rotation axis of the rotor; and

guide means disposed downstream of the rotor for guiding the wind away in a substantially downstream direction.

Such a converting device is known from DE 20 2010 004 320 Ul. Described herein is a ducted wind turbine with a rotor which is rotatable about a horizontal shaft. Located downstream of the rotor is a disc-like deflector which, together with the duct, defines an annular gap through which the wind flows to the outside. This deflector is located a relatively great distance from the rotor in the plane of the outlet opening of the duct.

Devices for converting wind energy to at least mechanical energy are generally known and are also referred to as wind turbines or windmills. 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 guide means as seen in flow direction are disposed directly behind the rotor blades and/or have a form adapted to the rotor blades.

A vacuum can occur on the underpressure side of the rotor blades. Because of this vacuum the airflow round the rotor blades coming from the rotor tends to turn in the direction of the vacuum created there, whereby air vortices occur. These air vortices decrease the efficiency of the rotor because the pressure difference over the front and rear side of the rotor blades decreases as seen in rotation direction. Using the guide means disposed downstream of the rotor the airflow or wind is guided away in a substantially downstream direction. This prevents or at least reduces the possibility of the airflow round the rotor blades turning and vortices occurring behind the rotor blades.

Another function of the guide means can be that the airflow from the rotor is distributed by the guide means over an outflow surface area at an outlet opening of the duct. The velocity of the airflow flowing out of the duct through the outlet opening is hereby substantially the same over the periphery of the duct. The velocity of the airflow flowing out of the duct is preferably lower, or at most equal to, the velocity of the ambient airflow flowing outside the duct. The air pressure at the outlet opening of the duct is hereby substantially smaller, or at most equal to, the air pressure at an inlet opening of the duct, whereby the efficiency of the device can increase.

The guide means preferably connect substantially to the rotor blades as seen in flow direction of the wind. Because the guide means as seen in flow direction are disposed directly behind the rotor blades and/or have a form adapted to the rotor blades, the rotor blades and the guide means have a substantially continuous form in a rotation position of the rotor where the rotor blades are disposed in register with the guide means. To the extent a gap remains between the rotor blades and the guide means, it is only a narrow one and has a substantially constant value over the periphery of the rotor blades. The width of the gap can amount to 0-40% of the diameter of the rotor, preferably 0-20%, and more preferably 0-10%. For optimum guiding of the outgoing flow the width of the gap amounts to 0-5% of the diameter of the rotor, preferably 0-2%, and most preferably 0-1%.

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 guide means comprise a number of rear stator blades disposed in the duct, which rear stator blades extend radially outward from the central axis.

The intended effect of guiding the wind away in a substantially downstream direction can take place effectively by selecting a suitable geometry and/or disposition of the rear stator blades.

In another embodiment of the device according to the invention the height of the rear stator blades increases in downstream direction along the length of the rear stator blades.

The airflow from the rotor is hereby guided radially outward to some extent in the direction of the inner surface of the duct in order to thus distribute the airflow uniformly over the duct surface.

The height and/or the increase in the height along the length of the rear stator blades can be the same for each rear stator blade, this being advantageous when the duct has a substantially circular cross-sectional form. In the case of a non-circular cross-sectional form of the duct the height and/or the increase in the height along the length of the rear stator blades can be adapted to the cross-sectional form of the duct. The duct can thus comprise a substantially oval form, wherein rear stator blades disposed close to the long half-axis of the oval can have a greater height than rear stator blades disposed close to the short half-axis of the oval.

In an embodiment of the device according to the invention at least the inner side of the duct widens in downstream direction from the rotor. The height of the rear stator blades preferably increases here such that an upper edge thereof has substantially the form of the local inner side of the duct.

The widening duct can substantially take the form of a Venturi, whereby the height of each rear stator blade increases substantially in accordance with the form of a Venturi.

In another embodiment of the device according to the invention a tangent of the upper edge of each rear stator blade makes an angle with the central axis which lies between 0-90°.

The angle of the tangent to the central axis can vary along the length of the upper edge of the rear stator blade, and can increase particularly in downstream direction.

A maximum cross-sectional dimension of the wind outlet opening of the duct, in particular an outer diameter in the case of a circular wind outlet opening, can be greater than a maximum cross-sectional dimension of the wind inlet opening of the duct, in particular an outer diameter in the case of a circular wind inlet opening.

In another embodiment of the device according to the invention the rear stator blades each comprise two blade parts extending at an angle relative to each other, wherein a first blade part has a first main plane extending radially from the central axis and the second blade part has a second main plane extending radially from the central axis.

The first blade part can be embodied here such that the first blade part substantially connects to the rotor blades. The second blade part, which is disposed behind the first blade part as seen in flow direction, can be embodied here such that the second blade part guides the wind away in the substantially downstream direction and optionally radially outward.

The angle between the first blade part and the second blade part can for instance lie between 90-180°.

Particularly at an angle greater than 0° it can be the case that the first blade part is disposed at an angle relative to the central axis, wherein the second blade part is disposed substantially parallel to the central axis. The wind coming from the rotor blades can hereby be properly captured by the first blade part and the wind can be guided away substantially parallel to the central axis.

The angle between a main plane of the rear stator blade or between the first and/or second main plane of the rear stator blade and the central axis is preferably selected subject to the velocity of the airflow in the duct and/or the rotation speed of the rotor. Since this velocity and/or the rotation speed can vary, it is advantageous for the rear stator blades to be adjustable for the purpose of adjusting the angle between the main plane or between the first and/or second main plane of the rear stator blade and the central axis.

In practical manner the rear stator blades are automatically adjustable subject to the velocity of the airflow in the duct and/or the rotation speed of the rotor. The device can be provided for this purpose with measuring means for measuring the velocity of the airflow in the duct and/or the rotation speed of the rotor, wherein the device is configured to adjust the angle between the main plane or between the first and/or second main plane of the rear stator blade and the central axis subject to the measured velocity and/or rotation speed.

In yet another embodiment of the device according to the invention each rear stator blade is connected to the duct via a connecting shaft extending radially relative to the central axis such that the rear stator blade is pivotable about or with the connecting shaft for the purpose of adjusting the angle between the main plane or between the first and/or second main plane of the rear stator blade and the central axis.

The device can alternatively comprise a central shaft coinciding with the central axis, wherein each rear stator blade is connected to the central shaft via a connecting shaft extending radially relative to the central shaft such that the rear stator blade is pivotable about or with the connecting shaft for the purpose of adjusting the angle between the main plane or between the first and/or second main plane of the rear stator blade and the central shaft.

The connecting shaft can optionally be connected to both the duct and the central shaft. In practical manner each rear stator blade is connected fixedly to a respective connecting shaft so that, by pivoting the connecting shaft, the angle between the main plane or between the first and/or second main plane of the rear stator blade and the central shaft (axis) can be adjusted.

In yet another embodiment of the device according to the invention each rear stator blade is provided with a number of upright ribs extending from a pressure side thereof, which ribs extend from a wind entry side of the blade to a wind exit side of the rear stator blade, wherein the ribs extend radially outward with a determined curvature over said side such that on the wind exit side each rib is located at a greater radial distance from the central axis than on the wind entry side.

The ribs provide the advantage that they guide the airflow from the rotor substantially radially outward.

The airflow from the rotor can optionally flow in substantially helical form round the central axis. In such a case the ribs can provide the advantage that they substantially convert the substantially helical airflow to a radially outward expanding airflow flowing substantially parallel to the central axis.

The ribs can here have a height lying between 0.1% and 25% of the maximum height of the stator blade.

In yet another embodiment of the device according to the invention each rear stator blade comprises a wind entry side and a wind exit side, wherein the rear stator blade is provided on its wind exit side with an end edge, the second derivative of which changes sign more than once.

Such an end edge of the rear stator blade of the device according to this embodiment of the invention provides the advantage that the formation of air vortices behind the outflow edge is limited, whereby the airflow over the rear stator blades can be improved. The efficiency of the device according to the invention can hereby be relatively high.

Applicant has found that a substantially sine-shaped end edge is particularly effective in limiting the formation of air vortices behind the outflow edge.

It is however also possible for the end edge to be substantially block tooth-shaped or sawtooth-shaped, these shapes likewise being able to at least partially provide the intended effect.

As further alternative the end edge can be provided with a number of elements which extend in a main plane of the rear stator blade and which each take the form of a parabola or a part of a circle. These shapes can also at least partially provide the intended effect.

In yet another alternative the end edge can be provided with a number of elements which extend in a main plane of the rear stator blade and which each take a substantially feather-like form. This shape can also at least partially provide the intended effect.

The end edge can optionally have a thickness varying along its length.

In yet another embodiment of the device according to the invention the rear stator blades are provided with a structure, which structure has a pattern of recesses for receiving substantially stationary air.

An advantage of the pattern of recesses according to the invention, which serve to receive substantially stationary air, is that the surface of the rear stator blades in contact with the airflow flowing in the duct consists partially of the stationary air present in the recesses. For the part where the airflow is in contact with the stationary air present in the recesses air-to-air friction will occur, which provides for a lower friction than the parts where the airflow is in contact with the rear stator blades. The efficiency of the device can increase as a result of the reduction in the air friction of the airflow.

According to the invention the structure is characterized by one of the following features or a random combination thereof:

- a depth of each recess is between 0.1 x - 2 x the length of each recess;

- a width of each recess is between 0.8 x - 3.5 x the length of each recess;

- the recesses have an oval shape, a longitudinal axis of which is disposed at an angle relative to the central axis, wherein the angle lies for instance between 0° and 45°;

- the peripheral wall of each recess extends at an angle to the inner surface of the duct, wherein the angle lies for instance between 90 and 100°;

- the peripheral wall of each recess is connected at a rounded angle to the bottom of each recess, wherein the rounded angle has for instance a radius of between 0 x - 1 x the length of each recess;

- the recesses are disposed adjacently of each other in a number of substantially straight lines, wherein the straight line extends at an angle relative to the central axis, wherein the angle lies for instance between 0° and 90°, wherein a centre-to-centre distance between two recesses disposed adjacently of each other in one line lies for instance between 1 x - 4 x the width of each recess, and wherein the recesses of two mutually adjacently disposed lines of recesses are for instance arranged offset relative to each other, wherein the offsetting is for instance greater than 0 x the length of each recess and a maximum of 2 x the length of each recess.

In another embodiment of the device according to the invention each rotor blade comprises a rotor blade wind entry side and a rotor blade wind exit side, wherein each rotor blade is provided on its rotor blade wind exit side with a rotor blade end edge, the second derivative of which changes sign more than once.

Such a rotor blade end edge of the device according to this embodiment of the invention provides the advantage that the formation of air vortices behind the outflow edge is limited, whereby the airflow over the rotor blades can be improved. The efficiency of the device according to the invention can hereby be relatively high.

Because the formation of air vortices behind the rotor blade is limited and the airflow is guided away in a substantially downstream direction by the guide means according to the invention disposed downstream of the rotor, turning of air vortices round the rotor blade to the underpressure side thereof is limited still further.

Applicant has found that a substantially sine-shaped rotor blade end edge is particularly effective in limiting the formation of air vortices behind the outflow edge.

It is however also possible for the rotor blade end edge to be substantially block tooth- shaped or sawtooth-shaped, these shapes likewise being able to at least partially provide the intended effect.

As further alternative the rotor blade end edge can be provided with a number of elements which extend in a main plane of the rotor blade and which each take the form of a parabola or a part of a circle. These shapes can also at least partially provide the intended effect.

In yet another alternative the rotor blade end edge can be provided with a number of elements which extend in a main plane of the rotor blade and which each take a substantially feather-like form. This shape can also at least partially provide the intended effect.

In an embodiment of the device according to the invention the rotor blade end edge comprises an inner end, which is disposed close to the rotation axis, and an outer end, wherein a main line of the rotor blade end edge between the inner end and the outer end takes a substantially curved form.

In another embodiment of the device according to the invention an angle of the main line close to the inner end of the end edge relative to a straight line between the inner end and the outer end is greater than -45° and smaller than 45°.

In another embodiment of the device according to the invention an angle of the main line close to the outer end of the end edge relative to a or the straight line between the inner end and the outer end is greater than -45° and smaller than 45°, preferably greater than -25° and smaller than 25°.

The angles of the main line close to the inner end and the outer end are preferably selected here in accordance with the guide means according to the invention disposed downstream of the rotor, so that the rotor blades substantially connect properly to the guide means disposed downstream thereof, in particular said rear stator blades.

The rotor blades can be disposed at an angle to the rotation axis, wherein the angle is greater than 35° and smaller than 75°, preferably greater than 40° and smaller than 65°.

The number of rotor blades which the rotor comprises is preferably equal to the number of rear staler blades, which number can for instance be between two and eight.

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 Λ 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 I 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 3 A 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 4A is a top view of the iianostmcture, figure 4B shows a detail of figure 4A and figure 4C shows a cross-section through the nanostructure.

- figures 5A-5B? 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 1A-1D 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 are disposed as seen in flow direction directly behind rotor blades 70. They further have a form adapted to the rotor blades, in the sense that the form of the front edge of each rear stator blade 20 is complementary to the form of the rear edge of each rotor blade 70. Rotor blades 70 and rear stator blades 20 thus have a substantially continuous form in a rotation position of the rotor where the rotor blades lie in register with rear stator blades 20. To the extent there is a gap between rotor blades 70 and rear stator blades 20, the width of this gap amounts to just a fraction of the diameter of rotor 4. 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 a4 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.