WO2024259494A1

WO2024259494A1 - A vertical axis turbine

Info

Publication number: WO2024259494A1
Application number: PCT/AU2024/050655
Authority: WO
Inventors: Peter Kittel
Original assignee: KITTEL Corp Pty Ltd
Current assignee: KITTEL Corp Pty Ltd
Priority date: 2023-06-21
Filing date: 2024-06-21
Publication date: 2024-12-26
Anticipated expiration: 2025-12-21

Abstract

A vertical axis turbine includes at least one hub assembly. At least two struts extend radially from the hub assembly. The turbine includes at least two blades, each blade mounted on at least one respective strut in an operatively substantially vertical orientation. Each blade includes a leading edge, a trailing edge, and inner and outer surfaces interposed between the leading and trailing edges, wherein the inner and outer surfaces are in fluid communication with each other between the leading and trailing edges.

Description

A VERTICAL AXIS TURBINE

FIELD OF THE INVENTION

This invention relates to a vertical axis turbine.

BACKGROUND TO THE INVENTION

Several configurations of vertical axis turbines have been developed in the past. Examples include the Gorlov turbine (U.S. patent 6,036,443 A), the Darrieus turbine (U.S. patent 1,835,018) and the Savonius turbine (U.S. patents 1,697,574 and 1,766,765).

Vertical axis turbines can generally be divided into two different categories: lift-based turbines (e.g., Darrieus and Gorlov) and drag-based turbines (e.g., Savonius). The former uses the lift force generated by the incoming wind traveling across the airfoilshaped blades of the turbine to spin, which in turn rotates a generator and creates electricity. In contrast, the latter type uses curved or scooped blades that are forced to spin by the drag force exerted by the incoming wind. The lift-based turbines are capable of spinning faster than the wind speed. Well-designed lift-based turbines are more efficient than drag-based turbines. They can extract three to four times more energy contained in the wind for the same wind condition and swept area.

Straight-bladed Darrieus turbines use blades the span of which is parallel to the main axis of rotation. In contrast, Gorlov turbines use helical blades the span of which wraps around the main axis of rotation. It is generally accepted that straight-bladed Darrieus turbines can reach a higher coefficient of efficiency than Gorlov turbines. However, Darrieus turbines tend to suffer from self-starting issues as well as larger torque fluctuations due to a spacing between the blades, which is addressed by the helical blades of the Gorlov turbines. On the other hand, Darrieus turbines benefit from a much lower cost of construction as the blades are significantly less expensive to manufacture.

In the built environment, small-scale vertical axis wind turbines could have a major role to play alongside solar energy. Current building design trends emphasise energy efficiency and on-site energy generation. Horizontal axis wind turbines are not suited for this type of environment because they are too sensitive to the type of air flow in such environments and so do not perform well in turbulent conditions which are typical of such an environment. Obstacles and uneven structures that are often scattered around buildings disturb the flow of air, which becomes turbulent. This contrasts with laminar flows that can be found in open fields and areas away from manmade structures where horizontal axis turbines are usually installed. In the built environment, the layout of buildings can create channels in which the wind speed is increased and with the potential for significant energy extraction.

In addition to the vertical axis wind turbines being more suitable to the type of flow encountered in the built environment, they also tend to rotate at lower speeds and generate significantly less noise which are desirable attributes to possess in the built environment. Due to their lower cost of fabrication, straight-bladed vertical axis wind turbines can directly compete with solar energy whereas wind turbines using helical blades such as the Gorlov turbine tend to have a much higher cost per kW.

Blades for vertical axis wind turbines generally have an Airfoil profile, such as a NACA Airfoil profile. As such, they have a leading edge, a trailing edge, an outer or low-pressure surface, and an inner or high-pressure surface.

In figure 1, there is shown a schematic layout of four positions of a blade (B) of a vertical axis assembly. An angle describing a position of a wind turbine blade in a rotational cylinder about the axis of rotation is called the azimuthal angle (0). At an azimuthal angle of 0 degrees, the chord of the blade is parallel to the incoming wind direction (W) and the leading edge is facing into the wind. At an azimuthal angle of between 0 degrees and 180 degrees, the blade is in an upwind region, and is in a downwind region when the azimuthal angle is between 180 degrees and 360 degrees. As the blade travels from 0 degrees to 360 degrees, its alignment with the incoming wind direction changes and it experiences different angles of attack (a), which is the angle between the chord line (C) and a vector representing the relative motion between the blade and the fluid through which it is moving. Thus, a factor in the angle of attack is the speed of the blade. The faster the blade moves, the smaller the angle of attack. At 0 degrees azimuthal angle, the angle of attack is zero. In the upwind region, the angle of attack is positive, while in the downwind region, the angle of attack is negative.

A blade with an Airfoil profile experiences different values of lift and drag as the angle of attack changes. The reason for this is that a pressure differential across the blade varies with the amount of curvature of the flow. Airfoils tend to experience low lift and low drag for small angles of attack. As the angle of attack increases, the lift increases faster up to a point of maximum lift. At some point, the blade stalls because airflow begins to separate from the upper surface of the blade, causing a sharp drop in lift and a large increase in drag. In figure 2, there is shown an NACA0015 blade 2 with a 10-degree angle of attack. A boundary layer 4 can be seen separating from the upper surface 6 of the blade with a 10-degree angle of attack. In figure 3, there is shown the blade 2 experiencing even further separation of the boundary layer 4 from the upper surface 6 of the blade 2 with a 20-degree angle of attack, resulting in significant vortex shedding.

In figure 4, there is shown a graph generated using an NACA 0021 Airfoil. The axis (L) shows lift and drag coefficient values. The axis (A) shows angle of attack in degrees. The axis (LR) shows lift-to-drag ratios. As can be seen, the lift (line Cl) reaches a maximum value at around +/- 17 degrees attack angle. Beyond this value, the blade stalls, and the lift decreases as the drag (line Cd) increases. The maximum lift to drag ratio (line Cl/Cd) is achieved at around 8 degrees attack angle. In this example, due to the blade symmetry, the drag values are mirrored, but the lift is negatively reversed for angles of attack below zero.

Such data is usually determined for airplane wings which are never supposed to exceed the stall angle. Thus, such data is often incomplete for angles of attack above 20 degrees. In the case of a wind turbine, the angle of attack can vary between a much wider range, such as -120 degrees to +120 degrees, especially at start-up. As the turbine spins faster and faster, the range narrows down to values in the region of -30 degrees to +30 degrees

In a wind turbine, the angle of attack experienced by the blade tends to reach a maximum (positive or negative) when the azimuthal angle 0 nears 90 degrees or 270 degrees. Usually, the angle of attack at which the maximum lift to drag ratio is achieved is when the azimuthal angle is between 45 and 90 degrees or between 235 and 270 degrees. At larger angles of attack, the flow starts to detach from the blade surface which creates eddies and vortices. The lift value drops significantly while the drag increases as the blade stalls. Because it is inevitable that the angle of attack in a wind turbine will experience relatively large variations, a way to maximise the lift a blade experiences (and therefore the power a wind turbine produces) is to increase the lift-to-drag ratio in the upwind section.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a vertical axis turbine that comprises: at least one hub assembly; at least two struts that extend radially from the hub assembly; and at least two blades, each blade mounted on at least one respective strut in an operatively substantially vertical orientation, each blade including: a leading edge; a trailing edge; and inner and outer surfaces interposed between the leading and trailing edges, wherein the inner and outer surfaces are in fluid communication with each other between the leading and trailing edges.

Each blade may define at least one transverse slot, extending along at least part of an operatively vertical length of the blade, between the inner and outer surfaces.

Each blade may have an NACA Airfoil profile.

The, or each, slot may be angled towards the trailing edge from as it extends from the inner surface to the outer surface.

The, or each, slot may be angled towards the trailing edge at an angle of between about 20 degrees and 70 degrees relative to a chord of the blade.

The, or each, slot may have a transverse width that is between 0.005C and 0.1C, where C is a chord length of the blade.

A transverse width of the, or each, slot may narrow from the inner surface to the outer surface.

Each blade may include at least two blade segments that are separated from each other to define the, or each, transverse slot.

At least one blade segment may be substantially solid.

At least one blade segment may be hollow and may include an outer wall defining an internal volume.

The, or each, hollow segment may include internal ribs connected to the outer wall to span the internal volume.

The blade segments may include a leading segment that defines the leading edge, and a trailing segment that defines the trailing edge to define a single slot between the leading and trailing segments. The blade segments may include a leading segment that defines the leading edge, a trailing segment that defines the trailing edge, and at least one intermediate segment interposed between the leading and trailing segments, to define at least two slots between the leading and trailing segments.

The blade segments may include a leading segment that defines the leading edge, a trailing segment that defines the trailing edge, a leading intermediate segment, and a trailing intermediate segment interposed between the leading and trailing segments, to define three slots between the leading and trailing segments.

The vertical axis turbine may include an operatively vertical shaft and two of the hub assemblies. Each hub assembly may be mounted on a respective end of the shaft.

The turbine may include at least two pairs of struts, with the struts of each pair fastened to respective hub assemblies in substantial alignment with each other and each blade fixed to the struts of a respective pair of struts. The turbine may include three such pairs of struts arranged at 120 degrees relative to each other.

Each hub assembly may include two mounting plates that are fastened to the shaft to define a gap between the two mounting plates. Inner end portions of each strut may be received between the mounting plates and fastened thereto.

Each hub assembly may include at least two adjustment mechanisms that are secured to the shaft. Each adjustment mechanism may be engaged with the inner portion of a respective strut so that operation of the adjustment mechanism can adjust a position of the strut relative to the shaft prior to fastening the strut to the hub assembly.

A blade fixing assembly may be arranged on a radially outer end of each strut, the blade fixing assembly being fastened to the strut and to an associated blade to fasten the blade to the strut.

Each blade fixing assembly may include a fixing plate that is fixed to an end of each blade, and a tab that extends from the fixing plate and is fastened to the strut.

At least one locating member may be interposed between the two blade fixing member assemblies, the, or each, locating member including at least two openings to accommodate respective blade segments.

According to a second aspect of the invention, there is provided a blade for a vertical axis wind turbine, the blade comprising: a leading edge; a trailing edge; and inner and outer surfaces interposed between the leading and trailing edges, wherein the inner and outer surfaces are in fluid communication with each other between the leading and trailing edges.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic diagram illustrating four positions of a conventional blade of a vertical axis wind turbine, in operation.

Figure 2 shows a diagram illustrating separation of a boundary layer from an upper surface of a conventional blade for a vertical axis wind turbine.

Figure 3 shows another diagram illustrating separation of a boundary layer from the blade of figure 2.

Figure 4 shows a graph illustrating the performance of a conventional blade for a vertical axis wind turbine, for a range of angles of attack.

Figure 5 shows a three-dimensional view of one embodiment of a vertical axis wind turbine, in accordance with the invention.

Figure 6 shows a side view of the vertical axis wind turbine of figure 5.

Figure 7 shows a sectioned view of the vertical axis wind turbine of figure 5, taken through A-A in figure 4.

Figure 8 shows a detailed view of a hub assembly of the vertical axis wind turbine of figure 5.

Figure 9 shows a sectioned view of the vertical axis wind turbine taken through B-B in figure 6.

Figure 10 shows a detailed view of a blade and strut of the vertical axis wind turbine taken through B-B in figure 6.

Figure 11 shows a detailed view of part of a blade fixed to a strut of the vertical axis wind turbine.

Figure 12 shows a further detailed view of the hub assembly of the vertical axis wind turbine taken through B-B in figure 6. Figure 13 shows part of a blade of the turbine, with a locator for blade segments.

Figure 14 shows a sectioned view of blades and a shaft of the vertical axis wind turbine of figure 5, taken through C-C in figure 6.

Figure 15 shows a cross-sectional detail D in figure 14 of one embodiment of a blade, in accordance with the invention, for the vertical axis wind turbine.

Figure 16 shows a cross-sectional detail of an embodiment of a blade, in accordance with the invention, for the vertical axis wind turbine.

Figure 17 shows a cross-sectional detail of an embodiment of a blade, in accordance with the invention, for the vertical axis wind turbine.

Figure 18 shows a cross-sectional detail of an embodiment of a blade, in accordance with the invention, for the vertical axis wind turbine.

Figure 19 shows a cross-sectional detail of an embodiment of a blade, in accordance with the invention, for the vertical axis wind turbine.

Figure 20 shows a cross-sectional detail of an embodiment of a blade, in accordance with the invention, for a vertical axis wind turbine.

Figure 21 shows the blade of figure 15, with streamlines illustrating air flow across a surface of the blade, for a 10-degree angle of attack.

Figure 22 shows the blade of figure 15, with streamlines illustrating air flow across a surface of the blade for a 20-degree angle of attack.

Figure 23 shows a graph comparing lift to drag ratio as a function of angle of attack with a conventional blade and the blade of figure 15.

DETAILED DESCRIPTION

In figures 5, 6, 7, 9, reference numeral 10 generally indicates an embodiment of a vertical axis wind turbine, in accordance with the invention.

The turbine 10 includes two, opposed hub assemblies 12. A set of three, equally circumferentially spaced struts 14 extends radially from each hub assembly 12. The struts 14 of each set are aligned with the struts 14 of the other set. Thus, there are three pairs of struts 14, with the struts 14 of each pair in alignment with each other. The turbine 10 includes three blades 16. Each blade 16 is fixed to respective aligned struts 14 of respective pairs. It will be appreciated that the turbine 10 can have two, four, or more sets of struts 14 extending radially from each hub assembly 12.

A shaft 17 interconnects the hub assemblies 12 and is rotatable with respect to an energy sink (not shown), such as an electricity generator.

The turbine 10 is a lift-based turbine. Thus, each blade 16 has an Airfoil profile. Each blade 16 can have various Airfoil profiles. In this example, each blade 16 has an NACA Airfoil profile. It follows that each blade 16 has a leading edge 18, a trailing edge 20, an overall high pressure or inner surface 22, and an overall low pressure or outer surface 24.

In this embodiment, the blades 16 can have any symmetrical 4-digit NACA Airfoil profile. An example of a suitable Airfoil profile is NACA0015. In tests, it was found that chord lengths of 266 mm and 346 mm were suitable in this profile. Other chord lengths may also be suitable because the principle of air flow through the slots described below to reduce a propensity for boundary layer separation will still hold.

The inner and outer surfaces 22, 24 are in fluid communication with each other between the leading and trailing edges 18, 20.

As shown in figure 15, each blade 16 includes three blade segments, in the form of a leading segment 26, an intermediate segment 28, and a trailing segment 30. The segments 26, 28, 30 together define the Airfoil profile.

The segments 26, 28, 30 are held together with locating plates 98, which have openings 100, 102, 104 that correspond with the segments 26, 28, 30, as described in further detail below.

The segments 26, 28, 30 are located with respect to each other to define leading and trailing transverse slots 31, 33 that provide fluid communication between the surfaces 22, 24.

The slots 31 , 33 are angled towards the trailing edge 20 at an angle of between about 20 degrees and 70 degrees relative to the chord of the blade 16, as they extend from the inner surface 22 to the outer surface 24. For example, the slot 31 can be angled at between about 30 degrees and 60 degrees and the slot 33 can be angled at between about 20 degrees and 45 degrees. The slots 31 , 33 have a transverse width that is between 0.005C and 0.1C, where C is a chord length of the blade. The transverse width of each slot 31, 33 narrows from the inner surface 22 to the outer surface 24. For example, for a 266 mm chord length, the slot 31 can have a transverse width that is about 14 mm at the inner surface 22 and about 4 mm at the outer surface 24. An entrance of the slot 31 is flared. The slot 33 can have a transverse width that is about 9.5 mm at the inner surface 22 and about 4.7 mm at the outer surface 24. These dimensions can vary, for example, by 10 percent. Furthermore, these dimensions can vary in proportion to chord length.

The leading segment 26 includes the leading edge 18. The leading edge 18 is interposed between an inner surface 32 and an outer surface 34 of the leading segment 26. The leading segment 26 includes a trailing surface 36 that interconnects the lower and upper surfaces 32, 34. The leading segment 26 is shaped so that a junction of the inner surface 32 and the trailing surface 36 leads a junction of the outer surface 34 and the trailing surface 36.

The intermediate segment 28 includes opposed leading and trailing surfaces 38, 40, and opposed inner and outer surfaces 42, 44. The intermediate segment 28 is shaped so that a junction of the leading surface 38 and the inner surface 42 leads a junction of the leading surface 38 and the outer surface 44. Likewise, the intermediate segment 28 is shaped so that a junction of the inner surface 42 and the trailing surface 40 leads a junction of the trailing surface 40 and the outer surface 44. A transition from the inner surface 42 to the trailing surface 40 is contoured to minimise turbulence as air enters and flows through the slot 33.

The trailing segment 30 includes opposed inner and outer surfaces 46, 48 that converge to define the trailing edge 20. The trailing segment 30 includes a leading surface 50. The trailing segment 30 is shaped so that a junction of the inner surface 46 and the leading surface 50 leads a junction of the outer surface 48 and the leading surface 50. The junction of the inner surface 46 and the leading surface 50 is angled to facilitate the direction of air into the slot 33. A transition from the leading surface 50 to the outer surface 48 is contoured to minimise air turbulence as air flows through and exits the slot 33.

The leading segment 26 is solid. Both the intermediate segment 28 and the trailing segment 30 are hollow and include an outer wall 52, 54, respectively. The intermediate segment 28 and the trailing segment 30 include internal ribs 56, 58, respectively, connected to the outer wall 52, 54, respectively. The ribs 56, 58 span internal volumes 60, 62 of the segments 28, 30, respectively. Thus, the internal ribs 56, 58 serve to provide structural integrity to the segments 28, 30 while optimising a mass of the segments 28, 30.

As shown in figure 15, the trailing surface 36 overlaps the leading surface 38. Likewise, the trailing surface 40 overlaps the leading surface 50.

Detail of one of the hub assemblies 12 can be seen in figures 8 and 12. Each hub assembly 12 includes a pair of mounting members or mounting plates 64.

Each mounting plate 64 defines a central opening 66, with the mounting plates 64 positioned so that the central openings 66 are aligned. End portions 68 of the shaft 17 extend through the openings 66 in the mounting plates 64 of respective pairs. The mounting plates 64 can be secured to the shaft 17 in various ways, for example, by being keyed or otherwise fastened to the shaft 17, to define a gap 70 between the mounting plates 64 of each pair.

Each strut 14 has an appropriate aerodynamic transverse profile. For example, each strut 14 can be flattened with tapered leading and trailing edges 72, 74 (figure 7). Inner end portions 76 of each set of three struts 14 are received between the mounting plates 64 of respective pairs.

As can be seen in figure 12, each hub assembly 12 includes three adjustment mechanisms 78 that are secured to the shaft 17. Each adjustment mechanism 78 is engaged with an inner end portion 76 of a respective strut 14 so that operation of the adjustment mechanism 78 can adjust a position of the strut 14 relative to the shaft 17 prior to fastening the strut 14 to the mounting plates 64.

Each strut 14 includes a generally flat intermediate portion 80 interposed between the leading and trailing edges 72, 74 (figure 9). The mounting plates 64 are secured to the struts 14 with fasteners 82 received through the mounting plates 64 and the intermediate portions 80.

The turbine 10 includes blade fixing assemblies 84 (figure 5, 11) arranged on radially outer ends 95 of respective struts 14 for mounting ends 86 of the blades 16 to the respective struts 14. Each blade fixing assembly 84 includes a fixing plate 88 with a profile that corresponds with, and is slightly larger than, that of the blades 16. Each fixing plate 88 is secured to a respective end 86 of each blade 16 with suitable fasteners 90 that can be screwed into socket formations 91. Each strut 14 includes a channel 92 (figure 10), between the leading and trailing edges 72, 74, with the intermediate portion 80 defining a floor of the channel 92. A tab 94 extends from each fixing plate 88 into the channel 92 of a respective strut 14. The tab 94 is secured to the intermediate portion 80 with suitable fasteners 96.

The turbine 10 includes a plurality of locating members in the form of locating plates 98 (figure 11 ,13) that are arranged in a spaced manner on each blade 16. Each plate 98 has an external profile that corresponds with that of the blade 16 and is slightly larger. Each plate 98 has a leading opening 100 that accommodates the leading segment 26 in a snug fit, an intermediate opening 102 that accommodates the intermediate segment 28 in a snug fit, and a trailing opening 104 that accommodates the trailing segment 30 in a snug fit. Thus, the plates 98 serve to locate and retain the segments 26, 28, 30 in their relative positions to reduce deflection and maintain a profile of the blade 16. It will be appreciated that the plates 98 can be replaced with other plates having openings that correspond with the segments of the other blades described below.

In use, the slots 31 , 33 provide airflow from the inner surface 22 to the outer surface 24 to mitigate separation of the boundary layer from the upper surface 24. The additional flow through the slots 31 , 33 serves to re-energise or maintain the boundary layer and prevent or delay such separation thereby increasing the lift to drag ratio of the blade 14 at an angle of attack in which separation would occur without the slots 31 , 33.

The struts 14 can be dimensioned to provide the turbine 10 with a diameter that suits a particular environment. For example, with the blade 16, having a chord length of 266 mm, the turbine can have a diameter of 1.73 m.

In figure 16, reference numeral 110 generally indicates a sectioned plan view of an embodiment of a blade suitable for the vertical axis turbine 10. The material and wall thickness of the blade 110 is the same as that of the blade 16. The dimensions, material and profile of the blade 110 is the same as that of the blade 16, unless otherwise set out below.

The blade 110 has the leading segment 26, a leading intermediate segment 112, a trailing intermediate segment 114, and a trailing segment 116. The leading intermediate segment 112 is similar to the intermediate segment 28. The trailing intermediate segment 114 and the trailing segment 116 together define a profile that is overall the same as that defined by the trailing segment 30 of the blade 16. The trailing intermediate segment 114 is spaced from the leading intermediate segment 112 to define the slot 33. The trailing segment 116 is spaced from the trailing intermediate segment 114 to define a trailing slot 132. The trailing slot 132 is angled towards the trailing edge 20 at an angle of between about 20 degrees and 45 degrees, as it extends from the inner surface 22 to the outer surface 24. The trailing slot 132 has a transverse width that is between 0.001 C and 0.1C, for example between 0.005C and 0.04C, where C is the chord length The transverse width of each slot 31 , 33, 132 narrows from the inner surface 22 to the outer surface 24. For example, for a 266 mm chord length, the slot 132 can have a transverse width that is about 14 mm at the inner surface 22 and about 4 mm at the outer surface 24. An entrance of the slot 31 , 33, 132 is flared. The slot 132 can have a transverse width that is about 6.5 mm at the inner surface 22 and about 2.5 mm at the outer surface 24. These dimensions can vary, for example, by 10 percent. Furthermore, these dimensions can vary in proportion to chord length.

The trailing intermediate segment 114 has a leading surface 118, a trailing surface 120, an inner surface 122, and an outer surface 124. The trailing intermediate segment 114 is shaped so that a junction between the leading surface 118 and the inner surface 122 leads a junction between the leading surface 118 and the outer surface 124. Likewise, a junction between the trailing surface 120 and the inner surface 122 leads a junction between the outer surface 124 and the trailing surface 120. A junction of the leading surface 118 and the inner surface 122 is angled to facilitate the flow of air into and through the slot 33. A junction of the leading surface 118 and the outer surface 114 is contoured to minimise air turbulence as air flows through and exits the slot 33. A transition from the inner surface 122 and the trailing surface 120 is contoured to minimise turbulence as air enters and flows through the slot 132.

The trailing segment 116 has inner and outer surfaces 126, 128 that converge to define the trailing edge 20. The trailing segment 116 has a leading surface 130. A junction of the inner surface 126 and the leading surface 130 leads a junction of the outer surface 128 and the leading surface 130.

A junction between the leading surface 130 and the inner surface 126 is angled to facilitate the flow of air into and through the slot 132. A transition from the leading surface 130 to the outer surface 128 is contoured to minimise air turbulence as air flows through and exits the slot 132. The trailing surface 40 overlaps the leading surface 118 and the trailing surface 120 overlaps the leading surface 130.

The trailing intermediate segment 114 and the trailing segment 116 are hollow and include an outer wall 134, 136. The trailing intermediate segment 114 and the trailing segment 116 include internal ribs 138, 140 connected to the outer wall 134, 136 to span internal volumes 142, 144. Thus, the internal ribs 138, 140 serve to provide structural integrity to the segments 114, 116 while optimising a mass of the segments 114, 115.

In figure 17, reference numeral 150 generally indicates a sectioned plan view of an embodiment of a blade suitable for the vertical axis turbine 10. The material and wall thickness of the blade 150 is the same as that of the blade 16. The dimensions, material and profile of the blade 150 is the same as that of the blade 16, unless otherwise set out below.

The blade 150 has a leading segment 152 and a trailing segment 154.

The leading segment 152 has an inner surface 156 and an outer surface 158 that converge to define the leading edge 18. The leading segment 152 has a trailing surface 160. A junction between the lower surface 156 and the trailing surface 160 leads a junction between the upper surface 158 and the trailing surface 160.

The leading segment 152 and the trailing segment 154 are spaced to define a slot 168 that is similar to the slot 33.

The trailing segment 154 has an inner surface 162 and an outer surface 164 that converge to define the trailing edge 20. The trailing segment 154 has a leading surface 166. A junction between the leading surface 166 and the lower surface 162 leads a junction between the leading surface 166 and the upper surface 164.

The leading segment 152 and the trailing segment 154 are hollow and include an outer wall 153, 155. The leading segment 152 and the trailing segment 154 include internal ribs 157, 159 connected to the outer walls 153, 155 to span internal volumes 161 , 163. Thus, the internal ribs 157, 159 serve to provide structural integrity to the segments 152, 154 while optimising a mass of the segments 152, 154.

The trailing edge 160 overlaps the leading edge 166.

A transition from the inner surface 156 of the leading segment to the trailing surface

160 is contoured to minimise air turbulence as air flows into and through the slot 168. A transition from the leading surface 166 of the trailing segment 154 and the outer surface 164 is contoured to minimise turbulence as air flows through and exits the slot 168. A junction of the leading surface 166 and the inner surface 162 is angled to facilitate the direction of air into the slot 168.

In figure 18, reference numeral 170 generally indicates a profile of an embodiment of a blade suitable for the vertical axis turbine 10.

The blade 170 is the same as the blade 110, with the exception that it has a leading segment 172, a leading intermediate segment 174, a trailing intermediate segment 176, and a trailing segment 178, all of which are solid.

In figure 19, reference numeral 180 generally indicates a profile of an embodiment of a blade suitable for the vertical axis turbine 10.

The blade 180 is the same as the blade 16, with the exception that it has a leading segment 182, an intermediate segment 184 and a trailing segment 186, all of which are solid.

In figure 20, reference numeral 190 generally indicates a profile of an embodiment of a blade suitable for the vertical axis turbine 10.

The blade 190 is the same as the blade 150, with the exception that it has a leading segment 192 and a trailing segment 194, both of which are solid.

The blades 16, 110, 150, 170, 180, 190 can be formed to provide a consistent profile across the blades. However, it is envisaged that it may be possible to vary the shape of the segments across a span of the blades. For example, the segments could increase or reduce in size, a camber along their chord lengths could change and so could the shape and radius of the leading and trailing edges. Furthermore, other embodiments could include more segments to define four or more slots. Such an arrangement may be useful for blades having long chord lengths.

Figure 21 illustrates the way a boundary layer 200 remains attached using the blade 16 at an angle of attack of 10 degrees. In figure 21 , there is shown the blade 16 with its profile exposed to a given air velocity relative to the blade 16. As can be seen, the flow of air through the slots 31, 33 serves to maintain the boundary layer 200 along the entire upper surface 24. This contrasts with the separation of a boundary layer from the upper surface of a conventional NACA0015 blade, with the given relative air velocity, as shown in figure 2 and as described in the Background. Figure 22 illustrates the way the boundary layer 200 remains attached using the blade 16 at an angle of attack of 20 degrees. In figure 21 , there is shown the blade 16 with its profile exposed to a given air velocity relative to the blade 16. As can be seen, the flow of air through the slots 31 , 33 serves to maintain the boundary layer 200 along a significant portion of the upper surface 24. This contrasts with the separation of a boundary layer from the upper surface of an NACA0015 blade, with the given relative air velocity, as shown in figure 2 and as described in the Background.

Figure 23 shows a graph of lift to drag ratio versus angle of attack of the blade 16 with an external contour that matches the NACA0015 profile and a conventional blade with an NACA0015 profile without the slots 31 , 33. The line 210 represents the lift to drag ratio as a function of angle of attack of the blade 16, while the line 212 represents the lift to drag ratio as a function of angle of attack of the blade without the slots 31 , 33. As can be seen, the lift to drag ratio is higher for the blade 16 for angles of attack that are greater than about 5 degrees, positive or negative. It follows that the power generated by the turbine 10 will be greater than that generated by a turbine using conventional blades with the same external profile, all other factors being equal.

The blade 16 is asymmetrical, so a drop in performance is observed in the downwind section (see the Background for explanation of downwind and upwind sections), which is a region where only a small portion of the power is produced due to the blade 16 experiencing an unsteady airflow, with vortices caused by a blade in an upwind section partially obstructing the airflow.

It was found that an improvement of the lift-to-drag ratio of the blade in the upwind section results in a larger power production, compared with the blade without slots, even with a drop of performance in the downwind section. The achieved power or performance coefficient (C_p) is significantly greater than what is achieved with conventional straight-bladed vertical axis wind turbines.

Blades in accordance with the invention were tested using computational fluid dynamics (CFD). Four blades were tested. These were:

A. A reference blade with a profile of the blade 2 described in the above background and having a chord length of 266 mm.

B. An NACA0015 blade with a chord length of 266 mm and a single profiled slot. This is the blade 150, 190 described above. C. An NACA0015 blade with a chord length of 266 mm and three profiled slots. This is the blade 110, 170 described above.

D. An NACA0015 blade with a chord length of 266 mm and two profiled slots. This is the blade 16, 180 described above.

A two-dimensional model of the vertical axis turbine 10 using the blades was tested at three different tip speed ratios (TSR). The C_p for the blades with a chord length of 266 mm and 346 mm was extracted for an ambient wind velocity of 8m/s. The model of the turbine 10 had a diameter of 1 .73 m. A two-dimensional model of the turbine using these blades was tested in a 60 m/s ambient wind condition with the turbine 10 locked against rotation. This was intended to determine the likely loads on the blades under very high wind conditions.

An analysis space was divided into two sections: a rotating domain and a stationary domain. The blade 16 was modelled within the rotating domain. The rotating domain was rotated such that the angle of attack of the blades could be changed relative to the incoming wind or the blades could be rotated at a specific TSR such that the effect of the rotation of the turbine 10 within the ambient condition could be captured.

The turbine 10 of the model comprised three blades orientated at 120° and the shaft 17 had a radius of 20mm. The blades had a 1° pitch.

A boundary of the model was positioned far enough away from the turbine to ensure boundary effects would not affect the analysis and the airflow around the turbine. The overall extent of the model was a 75m by 50m rectangular grid with the turbine located 25m from the inbound boundary.

A combination of triangular and quadrilateral mesh was used for the model. Resolving a velocity profile at or near the inner and outer surfaces was critical for calculating the correct drag and lift forces on the blades.

Up to five prism cells were used at the walls (inner and outer surfaces, leading edge, and trailing edge) with a cell growth rate of 1.2, giving a minimum cell size of approximately 0.5 mm. The aspect ratio of the mesh on the surface of the blades was kept to a maximum aspect ratio of four (i.e., the maximum length of the mesh on the surface of the blades was four times the thickness of the mesh).

The mesh sizing was selected to reduce wall y+ values. The y+ value is a dimensionless value that characterizes the accuracy in modelling the turbulent and laminar flow at the wall based on the turbulence model chosen. For the model used in the analysis, a y+ value of ~10 was found to provide reasonably accurate results.

The mesh was graded to provide a higher mesh density near critical areas, e.g., the slots.

A velocity inlet with a constant velocity, in-line with an x-axis, was set on an inlet side of the model. Top and bottom sides of the model were set as symmetry boundaries. An outlet side of the model was set at a constant fixed gauge pressure of 0 Pa. This ensured the turbine’s response was not affected by boundary effects and provided a constant ambient wind condition.

A constant angular velocity was applied to the rotating domain to model the rotation of the turbine.

For static cases (single blade and turbine assembly), a steady state formulation was used for the analyses. The rotating domain was fixed for flow around the turbine to develop and for any artificial effects to stabilise. The results were assessed to ensure that the changes in the pressure and force distribution on the blades were no longer fluctuating.

For transient runs, several revolutions of the rotor were modelled until the torque generated for each rotation stabilised. The size of the time step, internal iteration per time step, and convergence criteria were also considered to provide an accurate representation of the turbine.

The CFD simulation of the turbine was carried out in the following steps:

1. A steady state of the ambient condition within the model was set up. The rotating domain was fixed for flow around the turbine to develop and for any artificial effect to stabilise. This step was used to provide static forces on the turbine as well.

2. A transient model of the turbine was then carried out. The rotating domain in the model was activated and rotated at a constant predefined angular velocity.

3. The torque produced on the turbine was recorded and monitored over each rotation until it stabilised. The transient analysis was run until the change in torque over one rotation was less than about 2%. To connect the rotating domain with the stationary domain, General Grid Interface (GGI) connections were used. GGI is a method for connecting the grid on either side of a boundary where the two connected surfaces do not match. The GGI connections were made in an implicit way even if the nodes on either side of the connection were not aligned.

The properties of the fluid (air) were as follows:

• Density: 1.225 kg/m3

• Molar Mass: 28.96 g/mol

• Reference Temperature: 25 degrees.

Turbulent flows are highly complex, inherently unsteady, and depend largely on the interaction with structures along the flow path. To model this occurrence, simplified equations were used to predict the formation and the effects of turbulence.

For these tests, the shear stress transport (SST) k — omega (k-co) turbulence model based on the Reynolds Average Navier-Stokes (RANS) equations was used.

The RANS equations are time-averaged equations of motion for fluid flow. These equations give the approximate time-averaged solutions to the Navier — Stokes equations to represent the effect of turbulence.

One of the more recent formulations of the RANS based equation is the SST k-co model. The SST k-co turbulence model (Menter, 1993) is a two-equation eddyviscosity model. The SST formulation uses the k-co formulation for the near wall portion of the boundary layer. It is used all the way down to the wall through the viscous sub-layer. The SST formulation switches to a k-e formulation for the free stream behaviour which reduces the sensitivity of the k-co equations to the inlet free stream turbulence. The SST k-co model provides a reasonable representation of the flow in situations where adverse pressure gradients and separating flow can occur.

To accurately model the near wall effect of the interaction between the ambient condition and the turbine (e.g., pressure differential, flow separation etc.), two major verification functions were used. The resultant values for these verification functions were kept constant across the analysis to ensure the results obtained were comparable. y+ is a non-dimensional distance used to describe how coarse or fine a mesh is for a particular flow pattern. In general, a lower y+ will result in a more accurate result as the near wall effects are better captured by the mesh. With a faster flow, the value of y+ will increase and thus the grid size near the wall will need to be reduced to maintain the y+ value. However, to achieve a low y+, the required mesh density and mesh size near the surface can become prohibitive. For the analysis, the y+ value was kept to approximately 10 across all models to ensure the results were comparable across all models.

The Courant number, Cr, provides a representation of the advection across a cell in one single time step. This signifies the speed at which flow will travel across a single mesh. Usually, the Courant number should be limited to a value of around 1 to obtain reasonable result in a transient analysis. Therefore, time step size will be dependent on the rotational speed. For this analysis, the time step used was equal to a 0.5- degree rotation of the turbine.

The cases analysed were as follows:

1. 22 cases were set up for the two-dimensional static simulation. This involved just the blade to determine lift and drag on the blade at different attack angles.

2. 6 cases were set up to determine the Cp for the rotating blades.

3. 4 cases were set up for the two-dimensional static simulation. The blade assembly was stationary to check the pressure on the blade at high velocity.

Table 1 : Case 1 - Boundary conditions

Table 2: Case 2 - Boundary conditions

Table 3: Case 3 - Boundary conditions

The software used was ANSYS CFX version 17.2.

The following assumptions were made:

• Ambient conditions remained constant.

• Rotational speed of the turbine was constant.

• No interaction with other objects other than the turbine or duct (floor not modelled)

• Constant gas density, temperature, and viscosity

The drag, lift and torque on the aerofoil at different attack angles are summarised in the tables below. The torque was extracted at a position one third of the distance along the chord of the aerofoil.

Table 4: Forces and torque on Blade A - Per metre length of blade.

Table 5: Forces and torque on Blade B - Per metre length of blade

Table 6: Forces and torque on blade C - Per metre length of blade

Table 7: Forces and torque on blade D - Per metre length of blade

The results showed that the slotted blades generated more lift and less relative drag as the attack angle increased (from about +/- 5 degrees - figure 23) compared to the conventional blade A. This would likely correspond to an increase in the potential efficiency in the power generation capability of the aerofoil when used in the turbine 10.

For example, at 30 degrees attack angle, the blade A had a lift of 71.8 N/m and a drag of 32.1 N/m. At that angle, the blade B had a lift of 84.1 N/m and a drag of 34.6 N/m, the blade C had a lift of 183.4 N/m and a drag of 21.7 N/m, the blade D had a lift of 171.3 N/m and a drag of 20.3 N/m.

The appended claims are to be considered as incorporated into the above description.

Throughout this specification, reference to any advantages, promises, objects or the like should not be regarded as cumulative, composite, and/or collective and should be regarded as preferable or desirable rather than stated as a warranty.

Throughout this specification, unless otherwise indicated, "comprise," "comprises," and "comprising," (and variants thereof) or related terms such as "includes" (and variants thereof)," are used inclusively rather than exclusively, so that a stated integer or group of integers may include one or more other non-stated integers or groups of integers.

When any number or range is described herein, unless clearly stated otherwise, that number or range is approximate. Recitation of ranges of values herein are intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value and each separate subrange defined by such separate values is incorporated into the specification as if it were individually recited herein.

Words indicating direction or orientation, such as “front”, “rear”, “back”, etc, are used for convenience. The inventor(s) envisages that various embodiments can be used in a non-operative configuration, such as when presented for sale. Thus, such words are to be regarded as illustrative in nature, and not as restrictive.

The term “and/or”, e.g., “A and/or B” shall be understood to mean either “A and B” or “A or B” and shall be taken to provide explicit support for both meanings and for either meaning.

The use of common reference numerals in the description of different embodiments refers to like parts, unless otherwise indicated. This is for convenience and is not to be understood as meaning that the components labelled with such common reference numerals are identical.

Features which are described in the context of separate aspects and embodiments of the invention may be used together and/or be interchangeable. Similarly, features described in the context of a single embodiment may also be provided separately or in any suitable sub-combination.

It is to be understood that the terminology employed above is for the purpose of description and should not be regarded as limiting. The described embodiments are intended to be illustrative of the invention, without limiting the scope thereof. The invention is capable of being practised with various modifications and additions as will readily occur to those skilled in the art.

Claims

1. A vertical axis turbine that comprises: at least one hub assembly; at least two struts that extend radially from the hub assembly; and at least two blades, each blade mounted on at least one respective strut in an operatively substantially vertical orientation, each blade including: a leading edge; a trailing edge; and inner and outer surfaces interposed between the leading and trailing edges, wherein the inner and outer surfaces are in fluid communication with each other between the leading and trailing edges.

2. The vertical axis turbine as claimed in claim 1 , wherein each blade defines at least one transverse slot, extending along at least part of an operatively vertical length of the blade, between the inner and outer surfaces.

3. The vertical axis turbine as claimed in claim 1 , wherein each blade has an NACA Airfoil profile.

4. The vertical axis turbine as claimed in claim 2, wherein the, or each, slot is angled towards the trailing edge as it extends from the inner surface to the outer surface.

5. The vertical axis turbine as claimed in claim 4, wherein the, or each, slot is angled towards the trailing edge from the internal surface to the external surface at an angle of between 20 degrees and 70 degrees relative to a chord of the blade.

6. The vertical axis turbine as claimed in claim 2, wherein the, or each, slot has a transverse width that is between 0.005C and 0.1C, where C is a chord length of the blade.

7. The vertical axis turbine as claimed in claim 2, wherein a transverse width of the, or each, slot narrows from the inner surface to the outer surface.

8. The vertical axis turbine as claimed in claim 2, wherein each blade includes at least two blade segments that are separated from each other to define the, or each, transverse slot.

9. The vertical axis turbine as claimed in claim 8, wherein at least one blade segment is substantially solid.

10. The vertical axis turbine as claimed in claim 8, wherein at least one blade segment is hollow and includes an outer wall defining an internal volume.

11 . The vertical axis turbine as claimed in claim 10, wherein the, or each, hollow segment includes internal ribs connected to the outer wall to span the internal volume.

12. The vertical axis turbine as claimed in claim 8, wherein the blade segments include a leading segment that defines the leading edge, and a trailing segment that defines the trailing edge to define a single slot between the leading and trailing segments.

13. The vertical axis turbine as claimed in claim 8, wherein the blade segments include a leading segment that defines the leading edge, a trailing segment that defines the trailing edge, and at least one intermediate segment interposed between the leading and trailing segments, to define at least two slots between the leading and trailing segments.

14. The vertical axis turbine as claimed in claim 8, wherein the blade segments include a leading segment that defines the leading edge, a trailing segment that defines the trailing edge, a leading intermediate segment, and a trailing intermediate segment interposed between the leading and trailing segments, to define three slots between the leading and trailing segments.

15. The vertical axis turbine as claimed in claim 1 , which includes an operatively vertical shaft and two of the hub assemblies, each hub assembly mounted on a respective end of the shaft, and at least two pairs of struts, with the struts of each pair attached to respective hub assemblies in substantial alignment with each other and each blade fixed to the struts of a respective pair of struts.

16. The vertical axis turbine as claimed in claim 15, wherein each hub assembly includes two mounting plates that are fastened to the shaft to define a gap between the two mounting plates, inner end portions of each strut being received between the mounting plates and being fastened thereto.

17. The vertical axis turbine as claimed in claim 15, wherein each hub assembly includes at least two adjustment mechanisms that are secured to the shaft, each adjustment mechanism being engaged with the inner portion of a respective strut so that operation of the adjustment mechanism can adjust a position of the strut relative to the shaft prior to fastening the strut to the hub assembly.

18. The vertical axis turbine as claimed in claim 1 , wherein a blade fixing assembly is arranged on a radially outer end of each strut, the blade fixing assembly being fastened to the strut and to an associated blade to fasten the blade to the strut.

19. The vertical axis turbine as claimed in claim 18, wherein each blade fixing assembly includes a fixing plate that is fixed to an end of each blade, and a tab that extends from the fixing plate and is fastened to the strut.

20. The vertical axis turbine as claimed in claim 8, wherein at least one locating member is interposed between the two blade fixing assemblies, the, or each, locating member including at least two openings to accommodate respective blade segments.

21 . A blade for a vertical axis wind turbine, the blade comprising: a leading edge; a trailing edge; and inner and outer surfaces interposed between the leading and trailing edges, wherein the inner and outer surfaces are in fluid communication with each other between the leading and trailing edges.