GB2451670A

GB2451670A - A fluid driven rotor

Info

Publication number: GB2451670A
Application number: GB0715509A
Authority: GB
Inventors: Joseph Emans
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-08-09
Filing date: 2007-08-09
Publication date: 2009-02-11
Also published as: GB0715509D0

Abstract

A fluid driven rotor 10 comprises an axis transverse 12 to the direction of fluid flow, the rotor comprising one or more blades 11, 19 arranged round the axis of rotation. Each blade has a profile 20 such that the chord 23 of the profile is at an angle d between 0 and 90 degrees (ie excluding 0 and 90 degrees) from a plane 25 perpendicular to the rotational axis. Ideally, each blade defines a helical path in the axial direction. The helix angle r of the blade is ideally between 0 and 90 degrees from a plane 25 perpendicular to the axis, with r also being tangential to the rotational path of the blades. The blades may comprise tubercles / ribs separated by concave sections, which enhance lift by delaying stall and/or reducing stagnation on the leading edge. The blades may also comprise vortex generators. Preferably the fluid driven rotor is a vertical axis wind motor / turbine.

Description

* 2451670 Rotor The present invention relates to fluid-driven rotors, particularly lift-force based vertical axis wind turbines (VAWT).

Improving the efficiency of fluid-driven rotors such as wind turbines has become increasingly important as part of the drive to provide alternative energy sources to carbon-fossil fuels.

Wind turbine technology can generally be divided into two categories: horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). HAWTS are typified by the propeller type which is also currently the most widespread for both large commercial wind farms and smaller domestic units. VAWTs are less well known and used, but have potential advantages over HAWTs.

An important advantage for smaller domestic size turbines is that VAWTs are in theory suited to receiving wind from any direction, whereas HAWTs have to vane into the wind and have difficulty responding to winds which are blustery and rapidly changing (which is typical in urban environments).

Two most popular types of VAWT are termed the Savonius-type turbine or rotor (US1,697,574) and the Darrieus-type turbine or rotor (051,835,018). The Savonius-type rotor utilises differences in drag forces on opposite sides of the rotor to create motion. Rotors of this type are characterised by a large starting torque but the technology currently suffers from low efficiency at high rotational speeds due to large drag forces on the rotor. The Darrieus-type rotor, in Contrast, uses lift forces to produce rotation, and these forces are dependent on the effective wind direction experienced by the rotor blades.

Fig. 1 (prior art) illustrates how a Darrieus-type rotor works.

The rotor in the example shown in Fig. 1 has three airfoil-profiled blades (1) equally spread in a plane around an axis of rotation. When rotating, the blades experience both the velocity of the wind Vi,, and the velocity of an apparent wind V8 due to their rotation, the combination of which lead to the blade experiencing an equivalent wind V. The angle which this equivalent wind makes with the chord of the airfoil profile is termed the angle of attack, and this angle is important to the aerodynamics of the VAWT. Typically the lift force F1 experienced by the blade increases approximately linearly with the angle of attack until a critical angle or stall angle is reached, at which point the lift force drops rapidly. Stall angles are typically about 15° and as such, at low tip speed ratios, the Darrieus-type rotor blades experience stall over a significant portion of the rotor's revolution. Consequently, Darrieus-type rotors have low starting torque and after beginning to rotate slowly, usually (given the low Reynolds number "Re" typical of most Darrieus-type rotors and wind speeds experienced near ground level) experience a "dead band" of tip speeds for which there is a net negative torque on the rotors. It is therefore commonly necessary to rotate the Darrieus-type rotor through this dead band by external means. Once past this dead band, the Darrieus-type rotor can produce power efficiently due to high lift/drag force ratios.

The inability of the typical Darrieus-type rotor to self-start has been long known and several academics and inventors have sought to improve on the shortcomings of the original Darrieus design.

A key work in this area was a 1998 PhD thesis by Brian Kinloch Kirke entitled "Evaluation of self-starting vertical axis wind turbines for stand alone-applications" (Griffith University, Australia). The thesis reviewed then known attempts to solve the Darrieus-type rotor self-starting problem for both fixed pitch and variable pitch blades. These attempts included: using an auxiliary Savonius-type rotor; high solidity (solidity being defined as "nc/r" where "n" is the number of blades, "c" is chord length of the individual blades and "r" is their distance from the centre of rotation); flexible (sailwing) airfoils; and use of cambered airfoils as used to improve lift in model aircraft (which also typically experience low Re).

Attempts at improving Darrieus-type rotors include active self-starting mechanism, with actuator controlled rotor blades (described in W091/09225) and passive self-starting mechanisms, for example with blades hinged at the leading edge and using a spring/mass system (described in A1J-B-79418/91 and W086/05846).

Kirke in the above-mentioned PhD thesis notes that it appears that Darrieus turbines smaller than about 4m diameter are unlikely to achieve acceptable performance because of the need for low solidity and a chord length not much less than O.2m to avoid very low Re (Section 9.1.2, page 321).

The suggested and attempted improvements in Darrieus-type rotors and other VAWTs have had limited success, particularly pertaining to smaller domestic units. The present invention provides novel improvements in rotors, particularly applicable to VAWTs such as Darrieus-type rotors.

According to a first aspect of the present invention, there is provided fluid-driven rotor with an axis of rotation transverse to the direction of fluid flow, in which the rotor comprises one or more blades arranged around the axis of rotation and in which the or each blade has an airfoil profile such that the chord of the airfoil profile of the or each blade is positioned or tilted at an angle D from a plane perpendicular to the axis of rotation, where is between (but not including) �O and 900.

Thus may be greater than 00 but less than 90°.

The angle may be less than 900 and greater than or equal to about: 0.5°, 10, 2°, 30, 4, 5, 10°, 15°, 200, 25°, 30°, 350, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80° or 85°.

The one or more blades having an airfoil profile are also referred to herein as "airfoils".

According to the invention, therefore, the airfoil profiles are not corrected to lie flat on the plane perpendicular to the axis of rotation but remained tilted to it. Fluid flowing over a blade does not remain in its original plane but is affected by the presence of the rotor to take the path of least resistance through the rotor. The fluid may therefore preferentially flow in a plane other than that which is perpendicular to the axis of rotation, and tilting the profiles as described herein to lie on an expected preferential plane will ensure that the fluid experiences the desired airfoil profile.

Through improvement of the general aerodynamics of the rotors as described herein, the present invention results in an extension of the stall angle which leads to performance improvements. The invention also reduces or eliminates the dead band (as described above, and associated in particular with Darrieus-type rotors).

As used herein, the term "chord" refers to the line joining the leading and trailing edges of the airfoil profile of the blade.

The term "span" refers to the length of the blade perpendicular to the airfoil profile.

At least a part (or all) of the or each blade may be oriented at an angle e with respect to the plane perpendicular to the axis of rotation, with e tangential to the circumference of the rotational path of the or each blade (such that the axis of rotation itself would be at 8 = 900, which is the value for straight-bladed, i.e. non-helical, Darrieus blades), where 8 is less then 900 and more than 00.

The angle may have a value of between 0.5 x (8 -900) and 0.75 x (6 -900) at that part. This applies, for example, where the rotor has one or more blades which are helical around the axis of rotation.

According to the invention, the or each blade of the rotor may be straight along its span or may curve along its span around the axis of rotation (for example, helically). The or each blade may stay equidistant from the axis of rotation or vary in distance from the axis of rotation. Many airfoils have been conceived over the years and these are typically compiled and made available and classified by their key characteristics. The airfoil profile here may be of any such typical form, cambered or straight, but may in particular be selected to usefully produce high torque at low Re and be particularly suited to operating at high angles of attack.

According to the invention, the or each blade may be oriented so that fluid flows over the or each blade substantially in a plane containing the aerodynamically preferred airfoil profile of the or each blade. This aerodynamically preferred airfoil profile is a profile which enhances lift and/or delays stall and/or reduces drag of or on the or each blade.

In other words, preferred airfoil profiles may be on the plane in which it is anticipated or calculated that the fluid will flow due to the presence of the rotor. This fluid flow can be experimentally determined by computer or real modelling.

In another aspect of the invention, all or a part of the or each blade may be directed at an angle of less than 900 (but more than 00) towards the axis of rotation. The blade may, for example, be directed at an angle of less than 9�0 and greater than or equal to about: 0.5°, 10, 20, 30, 40, 50, ]Q0, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 500, 55°, 60°, 65°, 700, 750, 80° or 85°.

In a further aspect of the invention, the or each blade of the invention may comprise a plurality of leading edge tubercles, for example along the span of the or each blade. Each tubercie (except terminal tubercies located at each end of the or each blade) may be conjoined to an adjacent tubercle by a concave part. This concave part of the tuberc].es may serve to delay stall by at least partially or temporarily isolating the stall.

The tubercies may enhance lift by reducing or minimising stagnation on the leading edge of the or each blade. I0

Hydrodynamic properties of tubercles present on the front edge of humpback whale flippers have been described (see Fish & Battle, 1995, J. Morphology 225: 51-60; Miklosovic et al., 2004, Physic of Fluids 16: L39-L42). Such tubercies (also known as a "scalloped leading edge") have been applied on standard rotor blades (W02006/042401) and aeroplane wings (US2006/0060721), but not previously suggested for fluid-driven rotors with an axis of rotation transverse to the direction of fluid flow, such as for example Darrieus-type rotors.

The or each blade of the rotor of the present invention may additionally or alternatively have one or more undulations along the span thereof which do not alter the chord length. These one or more undulations may form concave sections which serve to at least partially or temporarily isolate stall.

The or each blade may additionally or alternatively comprise vortex generators which enhance flow adjacent to the or each blade surface to improve lift and delay stall. Such vortex generators may comprise projections, optionally knife-shaped, which extend into the fluid flow and mix fast-flowing fluid away from the or each blade with fluid being slowed by friction adjacent to the or each blade.

The or each blade may comprise stall delaying features, as described herein, such that the airfoil blades produce positive torque throughout a larger portion of their cycle about the axis of rotation. These features may operate by effectively segmenting the flow around the airfoil, focusing stall at sections of the or each blade and temporarily delaying stall from spreading along the span. This can additionally or alternatively be done through the use of undulations in the thickness along the span of the or each blade which serve to temporarily isolate stall in the concave sections. These undulations may be produced by increasing and decreasing the chord length of the airfoil profile whilst maintaining a similar profile, which may produce protuberances (or tubercles") at the forward or rear end of the blades. Forward tubercies have the added advantage of minimising the areas of stagnation.

Alternatively, undulations along the or each blade may occur whilst maintaining chord length, thereby altering the airfoil profile along the or each blade. A similar segmentation effect could be obtained with other span-wise changes, e.g. more abrupt step-like undulations or ribs. Alternatively or additionally, stall could be delayed through the use of vortex generators placed along the span of the or each blade, which re-energise the boundary layer of the or each blade, improving fluid flow around the airfoil and delaying separation and stall.

In aspects of the invention where the or each blade of the rotor comprises tubercies, undulations, vortex generators and/or other stall delaying features as described herein, optionally in combination with other recited features, the chord of the airfoil profile of the or each blade may positioned or tilted at an angle from a plane perpendicular to the axis of rotation, where is between or including 00 to 90°. Here, the angle may be less than or equal to 90° and greater than or equal to about: 0°, 0.5°, 1°, 2°, 30, 40, 5, 100, 15°, 200, 250, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80° or 85°.

The rotor of present invention comprises one or more airfoil blades positioned around an axis of rotation such that fluid flow over the or each blade causes a torque about the axis of rotation, as discussed previously with reference to Fig. 1. The torque the or each blade produces may be transmitted to a rotating object, the details of which would depend on the application (for example, a pump or electric generator). This could be done, for example, via a rotating central shaft to which the or each blade is connected, for example via one or more struts. Alternatively, the or each blade could be connected top and bottom to each other by a connector such as a plate, and either of these structures subsequently attached to a rotating object, again typically via a shaft. The shaft for such a rotor will be mounted transverse to the direction of the wind.

The or each blade may be connected with a rotatable central shaft onto which torque generated by the blades is transmittable. The or each blade may be connected with the central shaft via support struts.

The central shaft or a drive couple connected therewith may be coupled with a magneto-electric generator or a pump.

In one aspect, a rotor according to the invention comprises three helical blades each of which covers a turn of 120°. The proximal end of one blade and the distal end of an adjacent blade may be in a plane parallel to the axis of rotation of the rotor.

A rotor according to the invention may comprise a single blade, for example a single helical blade.

The fluid may be air, but the invention is applicable to other fluids including liquids.

The rotation of axis of the rotor may be substantially vertical, as found in VAWTs. The rotor may in particular be a lift-based VAWT, such as a modified Darrieus-type rotor.

The or each blade of the present invention may be substantially rigid.

Blade position (or pitch) may be fixed. A simple fixed-pitched rotor can be cost-effective and self-starting, which would be particularly suited to domestic installations for example.

Also provided according to the invention is a rotor blade per se comprising blade features as described herein.

Further aspects of the present invention will now be described by way of example only and with reference to the accompanying illustrative drawings in which: Fig. 1 is a schematic illustration in plan view of a rotating three-bladed vertical axis wind turbine (VAWT; prior art); Fig. 2 is a perspective view of a vertical axis wind turbine according to the present invention; Fig. 3 is a schematic illustration of airfoil-blades optimally tilted into the plane that contains a unique point for the axis of rotation; Fig. 4 is a schematic illustration of airfoil blades not optimally titled; Fig. 5 is a schematic illustration showing an airfoil blade according to a further embodiment of the invention with leading edge tubercies; and Fig. 6 is a schematic illustration showing an airfoil blade according to another embodiment of the invention with span-wise undulations.

With reference to Fig. 2, a rotor (10) according to a first embodiment of the invention consists of three helical elongate blades (11) spaced equally around a vertical axis of rotation (dotted line 12). In this embodiment, the rotor diameter is approximately equal to its height (for example, about 1.8 m) to minimise visual impact when placed on building roofs. Together the three blades cover a full turn of the helix, 120° each, such that the base (13) of one blade is vertically aligned with the top (14) of the next blade, thus spreading the torque on the rotor most continuously about its cycle of rotation, and minimisirig fatigue of the blades and other components of the rotor. The rotor has a central shaft (15) which travels along the axis of rotation. Each of the three helical blades is attached to the shaft by means of an upper strut (16) near the top of the blade and a lower strut (17) near the base of the blade.

The blades are constructed of a composite material or for mass production may be injection moulded using a suitable engineering plastic. The upper and lower struts are as stiff, strong and light as possible so as to minimise rotor inertia and parasitic drag. Composite materials such as carbon fibre reinforced plastic would be suitable. The central shaft should be stiff and strong, but its proximity to the axis of rotation means weight is of less importance provided its diameter is not excessive. Stainless steel tubing may therefore be suitable for the central shaft.

The rotor produces a torque about its axis of rotation due to forces on the blades. These forces are generated by fluid flowing over the blades which have an airfoil profile. With reference to Fig. 3, the airfoil (19) has a leading edge (21) and a trailing edge (22) and a chord (dash line 23) defined between them. The profile of the airfoil is symmetrical about the chord. The aerodynamically preferred blade airfoil profile (20) is in a plane such that the chord makes an angle 4) to the plane which is perpendicular to the axis of rotation of the rotor shown in Fig. 2. This angle is such that the airfoil lies somewhere between flat on the plane perpendicular to the axis of rotation, to lying on the plane where the airfoil profile is perpendicular to the leading edge of the helical blade. If the angle that the helix makes to the plane that is perpendicular to the axis of rotation is e then 4) is preferably somewhere between 1/2(0-90°) and 3/4(8-90°).

Having the blades angled in this manner has distinct advantages for helical blades in particular. The fluid flowing over the blades seeks to take the path that minimises pressure losses and as such the flow may be affected by the presence of the rotor to flow in a plane other than that which is perpendicular to the axis of rotation. This is particularly applicable if the flow is highly three dimensional, as is the case with helical blades.

By choosing the correct plane in which to fixedly orient the airfoil profiles (e.g. the plane into which the flow is directed by the presence of the rotor), the flow over the correct profile is maximised and that over less favourable aerodynamic profiles is minimiseci. Thus the air flowing over the blade experiences the preferred profile, rather than an elongated profile (25) as shown in Fig. 3, which would be experienced if the rotor did not affect the flow in this manner.

To further illustrate the above point, Fig. 4 shows a blade with the chosen profile lying horizontally (which is typical of prior art blades). If this configuration is adopted, the air flowing over the blade will experience a profile (30) which is clearly different to the aerodynamically preferred profile as shown in Fig. 3.

An additional advantage to inclining the airfoil profile as described herein relates to domestic use of such a rotor. A significant portion of housing makes use of pitched roofs, and the flows over these roofs naturally have a significant vertical component. A forward leaning helical bladed rotor with an inclined airfoil would naturally allow the flow coining up the roof to pass over its favourable airfoil profile with much less deviation as compared to that without such an incline.

Fig. 5 shows an embodiment of the invention in which the chord of the airfoil changes in length along the span of the blade (40) in a sinusoidal manner affecting the leading edge. This change is brought about by tubercies (41) on the leading edge.

The airfoil profile is maintaining the same relative dimensions however, so that both the width of the blade and length of the chord are expanding and contracting in a cyclical manner along the span. This expansion and contraction represents an approximately 5% change in the length of the chord and one cycle completes in a distance along the span that is approximately equivalent to the chord length. The trailing edge (42) of the airfoil blade is linear, meaning that it is only the leading edge of the blade which is expanding and contracting to form the tubercies or protuberances.

In another embodiment, the chord length changes do not occur in a sinusoidal manner as shown in Fig. 5 but are modulated to produce smooth step-like changes.

In a further embodiment shown in Fig. 6, the chord length of the blade (50) does not change but the airfoil profile width changes in a sinusoidal manner along the span of the blade creating span-wise undulations (51). These span-wise features have an effect in common, which is to delay the onset of stall by temporarily containing it within the concave sections of the span. Some stall is inevitable as the blades make a full rotation with respect to the wind direction, but delaying stall by even a small amount leads to a significant increase in net torque on the rotor particularly as lift force is highest immediately preceding stall.

In yet a further embodiment, vortex generators are augmented along the span of the blades. These vortex generators project into the flow, in a knife-like or other fashion, and mix fast flowing fluid away from the blade with the fluid being slowed by friction adjacent to the blade, thus enhancing the flow adjacent to the blade surface which improves lift and delays stall.

All documents cited herein are hereby incorporated by reference in their entirety.

Claims

Claims 1. A fluid-driven rotor with an axis of rotation transverse to the direction of fluid flow, in which the rotor comprises one or more blades arranged around the axis of rotation and in which the or each blade has an airfoil profile such that the chord of the airfoil profile of the or each blade is positioned at an angle from a plane perpendicular to the axis of rotation, where is between but not including 00 and 90°.
2. The rotor according to claim 1, in which at least a part of the or each blade is oriented at an angle ê with respect to a plane perpendicular to the axis of rotation, with e tangential to the circumference of the rotational path of the or each blade, where 0 is less then 900 and more than 0°.
3. The rotor according to claim 2, in which the or each blade is helical around the axis of rotation.
4. The rotor according to either claim 2 or claim 3, in which has a value of between 0.5 x (8 -90°) and 0.75 x (8 -90°) at that part.
5. The rotor according to any preceding claim, in which the or each blade is oriented so that fluid flows over the blade substantially in a plane containing the aerodynamically preferred airfoil profile of the or each blade.
6. The rotor according to claim 5, in which the aerodynamically preferred airfoil profile is a profile which enhances lift and/or delays stall and/or reduces drag of or on the or each blade.
7. The rotor according to any preceding claim, in which the or each blade comprises a plurality of leading edge tubercles.
8. The rotor according to claim 7, in which each tubercie except terminal tubercies is conjoined to an adjacent tubercie by a concave part.
9. The rotor according to claim 8, in which the concave part of the tubercles serve to delay stall by at least partially or temporarily isolating the stall.
10. The rotor according to any of claims 7-9, in which the tubercles enhance lift by reducing or minimising stagnation on the leading edge of the or each blade.
11. The rotor according to any preceding claim, in which the or each blade has one or more undulations along the span thereof.
12. The rotor according to claim 11, in which the one or more undulations form concave sections which serve to at least partially or temporarily isolate stall.
13. The rotor according to any preceding claim, in which the or each blade comprises one or more vortex generators which enhance flow adjacent to the blade surface to improve lift and delay stall.
14. The rotor according to claim 13, in which the one or more vortex generators comprise projections, optionally knife-shaped, which extend into the fluid flow and mix fast-flowing fluid away from the blade with fluid being slowed by friction adjacent the blade.
15. The rotor according to any preceding claim, in which the or each blade is connected with a rotatable central shaft onto which torque generated by the blades is transmittable.
16. The rotor according to claim 15, in which the central shaft or a drive couple connected therewith is coupled with a magneto-electric generator.
17. The rotor according to either of claim 15 or claim 16, in which the or each blade is connected with the central shaft via support struts.
18. The rotor according to any preceding claim, comprising three helical blades each of which covers a turn of 1200.
19. The rotor according to claim 18, in which the proximal end of one blade and the distal end of an adjacent blade are in a plane parallel to the axis of rotation of the rotor.
20. The rotor according to any preceding claim, in which the rotation of axis is substantially vertical.
21. The rotor according to any preceding claim, in which the rotor is a vertical axis wind turbine (VAWT).
22. The rotor according to claim 21, in which the rotor is a lift-based VAWT.
23. The rotor according to claim 22, in which the rotor is a modified Darrieus-type rotor.
24. The rotor according to any preceding claim, in which the fluid is air.
25. The rotor according to any preceding claim, in which the blades are substantially rigid.
26. The rotor according to any preceding claim, in which the blade position (or pitch) is fixed.
27. A rotor blade as defined in any of claims 1 to 26.
28. A fluid-driven rotor substantially as herein described and shown with reference to the accompanying figures.
29. A rotor blade substantially as herein described and shown with reference to the accompanying figures.