US20050069415A1

US20050069415A1 - Wind turbine diffuser

Info

Publication number: US20050069415A1
Application number: US10/495,502
Authority: US
Inventors: Pascal Ferracani
Original assignee: CITA
Current assignee: CITA
Priority date: 2001-11-19
Filing date: 2002-11-19
Publication date: 2005-03-31
Also published as: WO2003044361A1; CA2466897A1; AU2002360179A1; FR2832465A1; FR2832465B1; EP1446577A1

Abstract

The invention concerns a diffuser for a whine turbine, in particular a large-dimension wind turbine mounted on a mast (2) and comprising a wind-driven propeller (3) equipped with blades (4) as well as an alternator converting wind kinetic energy into electric, power. The invention is characterized in that it consists of a circular element (7) enclosing the ends (9) of the blades (5), and consisting of a skin (14) made of a stretched textile membrane associated with a rigid internal and/or external framework (15) supporting the loads and enabling to stretch the membrane and forming it.

Description

The invention relates to a diffuser for a wind turbine, in particular a wind turbine of large dimensions mounted on a mast and having a wind-driven propeller as well as an alternator co-operating with the propeller to supply electrical energy.
Specialists have been trying to recover wind energy for a long time, its advantages being that it is clean, i.e. does not create thermal or chemical pollution, and inexhaustible.
However, these advantages are compensated by a series of disadvantages, linked in particular to the dispersed and intermittent nature of wind; it also well known that wind turbine “parks” occupy a large amount of space and cannot operate without creating a noise nuisance.
Furthermore, the wind turbines currently in use are often fitted with propellers having radial blades and a horizontal shaft similar to those used to propel aircraft but generally much bigger as a rule. Such propellers conventionally co-operate with industrial dynamos or alternators with geared drives, which makes them heavy, expensive and low in output.
As a result of these disadvantages, the market for wind turbines has not taken off as might have been expected in recent years and the potential for development in this field today is very high.
In order to overcome these disadvantages, a more robust wind turbine has been proposed more recently, disclosed in document FR-2 793 528, which is more compact but produces the same output and is less noisy than previously proposed wind turbines.
A wind turbine of this type is mounted on a vertical mast and has a wind-driven propeller fitted with helical blades which are inclined towards the upstream end, supported on a hub with a horizontal axis, as well as an alternator co-operating with the propeller to supply electrical energy, and which is also equipped with a magnetic rotor secured to the hub and a stator with a magnetic coil adjacent to the rotor and secured to a fixed underframe.
This wind turbine is also equipped with a static diffuser, which consists, on the one hand, of a fixed circular element of relatively short length mounted concentrically with the hub on arms integral with the underframe and co-operating, with a slight clearance, with the ends of the blades, at a slight clearance therefrom, and on the other hand, a rounded leading edge followed by a thick body and a thin divergent trailing edge.
It has been proven from numerous tests conducted in wind tunnels that this wind turbine has aerodynamic characteristics which are very much superior to earlier conventional wind turbines.
This result is due to the optimised profile of the rotor blades and the presence of the divergent diffuser.
A diffuser of this type conventionally has a skin, which is what imparts its contour, and an internal structure which is designed to absorb load and accommodate the ends of support arms enabling the diffuser to be centred around the wind turbine blades.
In the case of small wind turbines (up to a few meters in diameter), it is possible to use solutions based on boiler technology or conventional manufacturing technologies, using either thin composite skins in conjunction with an internal box with a combined foam/epoxy glass structure or thick rigid skins, in particular of plastics, steel or Duralumin® which also fulfil a structural function.
However, these conventional technologies are out of the question when it comes to wind turbines of bigger diameters (which may be up to several tens of metres): in practice, steel skins with a thickness of 1 to 2 mm would create much too heavy a mass and would incur too high a cost, whilst still requiring the presence of an appropriate rigid internal structure.
For economic reasons, a diffuser made from a composite material would not be feasible either, because the cost of machining and raw materials would be far too high.
Consequently, the known solutions cannot be transposed to large-dimension wind turbines of the type described above because they would not be viable on an industrial scale.
To date, no technologies have ever been proposed that would reduce the weight and cost of such diffusers.
The objective of the present invention is to fill that gap.
To this end, it relates to a diffuser for a wind turbine, in particular a wind turbine of large dimensions mounted on a mast and having a wind-driven propeller fitted with blades, as well as an alternator, for converting kinetic wind energy into electricity.
For the purposes of the invention, a diffuser of this type is characterised in that it comprises a circular element enclosing the ends of the blades and has a skin in the form of a stretched textile membrane in conjunction with a rigid internal and/or external framework to support loads and enable the membrane to be kept taut and to impart the requisite shape to it.
It should be pointed out that the invention may advantageously be used for a wind turbine of the type described above but could also be adapted for use with any wind turbine system with a turbine casing.
The textile membrane proposed by the invention preferably has a density in the order of 1 kg/dm³.
The use of such a textile membrane has proved to offer various advantages: in effect, there are currently membranes of this type on the market which have high mechanical properties, high resistance to UV radiation and good resistance to bad weather conditions; in addition, manufacturers offer maintenance facilities in the event of tearing.
It should also be pointed out that the tension of this textile membrane, produced by means of the internal framework, can be controlled over time in order to compensate for any creep which might occur in the materials.
By virtue of another characteristic of the invention, the textile membrane has a lagging, thereby enabling it to be reinforced, especially in the region disposed in proximity to the blades.
Large depressions can be generated locally across the entire contour of the diffuser, i.e. its internal and external surfaces, depending on the main direction of the wind relative to the rotation axis of the blades, which can cause the textile membrane to become detached from the framework.
It is vital that this phenomenon does not occur in the region located in proximity to the blades in order to avoid any risk of tearing.
For this reason, it is crucial to provide either a reinforcing lagging or means for securing the textile membrane to the internal framework at this level.
As a result of another feature of the invention, the rigid framework consists of a series of identical radiating ribs, which are intrinsically closed with a planar curve forming a wing-shaped contour matching the contour of the diffuser and constituting its internal and external surfaces.
These radiating ribs may advantageously be made from a metal such as steel or aluminium, or alternatively from timber or a composite material.
By virtue of another characteristic of the invention, the rigid framework also has a series of peripheral members linking the radiating ribs and, if necessary, a series of stiffening elements enabling the thickness of the diffuser contour to be kept constant.
These members and these stiffening elements may be made from the same materials as those used for the radiating ribs. In a first variant of the invention, the radiating ribs as well as the peripheral members are made from steel tubes.
In this variant, adjacent peripheral members may advantageously be linked by stiffening elements provided in the form of steel sections disposed in the plane of the radiating ribs along the internal and external faces of the diffuser and/or transversely thereto.
Connecting steel rods, also intended to fulfil a stiffening function, may optionally be incorporated parallel with the peripheral members in order to link the radiating ribs.
In a second variant of the invention, the radiating ribs are provided in the form of aluminium sections, the cross-section of which is specifically of a Ω shape, whilst the peripheral members are provided in the form of aluminium tubes.
In this variant, the various aluminium tubes forming the peripheral members may advantageously be inter-linked by other aluminium tubes serving as stiffening elements.
It should be pointed out that the framework used in this second variant is essentially similar to that used for the first variant of the invention but has the advantage of being lighter in weight due to the nature of the materials used.
In a third variant of the invention, the radiating ribs are provided in the form of timber sections, in particular T-shaped, and the peripheral members are provided in the form of carbon beams, specifically with a rectangular cross section, linked in pairs by cores of plywood in particular, on either side of the radiating ribs so as to define box-type spars.
These box-type spars preferably have cut-outs at their median part in order to reduce the weight of the rigid framework as far as possible and, as a general rule, are linked in pairs by stainless steel cables.
In a fourth variant of the invention, the radiating ribs are provided in the form of elements with a rectangular cross-section, in particular, made from a laminated-bonded composite material, and the peripheral members are provided in the form of beams, in particular with a rectangular cross-section made from a laminated-bonded composite material, linked in pairs on either side of the radiating ribs by a series of rings rigidly secured to one another and also made from a laminated-bonded composite material.
In this variant of the invention, the internal and external faces of each of the radiating ribs are preferably linked by a series of rings, rigidly secured to one another and made from a laminated-bonded composite material, serving as stiffening elements.
It is also of advantage to provide connecting timber strips linking the radiating ribs, incorporated parallel with the peripheral members.
The characteristics of the wind turbine diffuser proposed by the invention are described in more detail below with reference to examples illustrated in the appended drawings, although these should not be construed as restrictive in any way:
FIG. 1 shows a perspective view of a wind turbine fitted with a static diffuser as proposed by the invention,
FIG. 2 is a view showing a partial axial section of this wind turbine,
FIG. 3 is a partial perspective view illustrating the mounting of the static diffuser,
FIG. 4 is an axionometric view depicting a detail of the framework of a diffuser used for the first variant of the invention,
FIG. 5 is an axionometric view similar to that shown in FIG. 4 but depicting the framework of a diffuser used for the second variant of the invention,
FIG. 6 is an axionometric view similar to that shown in FIG. 4 but depicting the framework of a diffuser used for the third variant of the invention,
FIG. 7 is an axionometric view similar to that shown in FIG. 4 but depicting the framework of a diffuser used for the fourth variant of the invention.
The wind turbine 1 illustrated in FIGS. 1 and 2 is mounted on a vertical mast 2 and has a wind-driven propeller 3 fitted with helical blades 5 inclined towards the upstream side.
The blades 5 are supported by a hub 4 with a horizontal axis.
The wind turbine 1 also has an alternator, not illustrated in the drawings, which co-operates with the propeller 3 to generate electrical energy.
This alternator is fitted with a magnetic rotor, fixed to the hub 4, and a stator with magnetic coils adjacent to the rotor and secured to a fixed underframe 6.
The rotor and the stator are not illustrated in the drawings.
The wind turbine also has a static diffuser 7 provided in the form of a fixed circular element of relatively short length.
This diffuser 7 is mounted concentrically with the hub 4 on arms 8 integral with the underframe 6 and co-operates with the ends 9 of the blades 5 at a slight clearance therefrom.
As illustrated in FIG. 2, the static diffuser 7 has an aerodynamic profile with a rounded leading edge 10 followed by a thick body 11 and a thin divergent trailing edge 12.
As illustrated in FIG. 1, the divergent trailing edge 12 is supported by stays 13 distributed in radial planes.
As may be seen from FIG. 2, this configuration enables a divergence to be created in the air flow downstream of the diffuser as indicated by arrows F and F′.
The design of the static diffuser proposed by the invention will now be described with reference to FIGS. 3 to 6.
It should be pointed out that in these drawings, the blades 6 of the propeller 3 are not illustrated and are merely schematically indicated by their axis XX′.
In FIG. 3, the diffuser 7 consists of a skin 14 made from a textile material with a very low density. This skin 14 is stretched around a rigid framework 15, which imparts its shape and enables it to be centred around the blades 6.
The framework 15 is essentially made up, on the one hand, of a series of radiating ribs 16, all of which are identical, and on the other hand, a series of peripheral members 17, the purpose of which is to link the radiating ribs 16 and receive the ends of the arms 8 on which the diffuser 7 is mounted.
In FIG. 3, the radiating ribs 16 are provided in the form of intrinsically closed elements with a planar curve forming a wing-shaped contour corresponding to the contour of the diffuser 7 as illustrated in FIG. 2.
These radiating ribs 16 therefore constitute the internal surface 18 and external surface 19 of the diffuser 7.
As illustrated in FIG. 4, the radiating ribs 16 ₁as well as the peripheral members 17 ₁are provided in the form of steel tubes.
The tubes forming the peripheral members 17 ₁are linked to one another by various stiffening elements in order to maintain a constant thickness of the contour of the diffuser 7.
In FIG. 4, these elements are steel sections 20 disposed in the planes of the radiating ribs 16 ₁and stainless steel cables 21 disposed transversely to these planes.
These various stiffening elements 20, 21 also co-operate with connecting steel rods 22, linking the radiating ribs 16 ₁, disposed parallel with the peripheral members 17 ₁.
In FIG. 5, the radiating ribs 16 ₂are aluminium sections having a cross-section with a Ω shape, whereas the peripheral members 17 ₂are aluminium tubes.
The various adjacent tubes 17 ₂on the internal surface 18 or on the external surface 19 of the diffuser 7 or on either side of the radiating ribs 16 ₂are linked by aluminium tubes 23 disposed in the plane of the radiating ribs 16 ₂or transversely thereto in a pyramid design to impart rigidity to the internal framework 15 of the diffuser 7.
In FIG. 6, the radiating ribs 16 ₃are timber T-shaped sections whilst the peripheral members 17 ₃are carbon beams with a rectangular cross-section.
With the exception of the front and downstream ends of the radiating ribs 16 ₃, the beams 17 ₃are linked in pairs on either side of these ribs 16 ₃by plywood cores 24 with cut-outs 25 in their median part.
The beams 17 ₃and the plywood cores 24 thus define box-type spars enabling the thickness of the contour of the diffuser 7 to be maintained.
In FIG. 6, adjacent box-type spars are linked to one another by rigidity-imparting cables 26 disposed obliquely relative to the plane of the radiating ribs 16 ₃.
In FIG. 7, the radiating ribs 16 ₄are elements with a rectangular cross-section with a timber base made from a laminated-bonded composite material, whereas the peripheral members 17 ₄are beams with a rectangular cross-section made from a similar material.
The beams 17 ₄are linked in pairs on either side of the radiating ribs 16 ₄by a series of rings 27 rigidly secured to one another and also made from a laminated-bonded composite material.
In FIG. 7, the internal face 18 and external face 19 of the radiating ribs 16 ₄are also linked to one another by a series of rings 28 rigidly secured to one another and also made from a laminated-bonded composite material.
These rings 28 also co-operate with the timber strips 29 linking the radiating ribs 16 ₄, disposed parallel with the peripheral members 17 ₄.
It should be pointed out that the fixing arms 8 in this configuration are not secured directly to the peripheral members 17 ₄at the level of their outer ends 8 ₁but are mounted at this level on mounting plates 30 which are in turn secured to the rings 27, 28.

Claims

1. Diffuser for a wind turbine, in particular a wind turbine of large dimensions mounted on a mast (2) and having a wind-driven propeller (3) fitted with blades (5) and an alternator for converting kinetic wind energy into electricity, characterised in that it comprises a circular element (7) surrounding the ends (9) of the blades (5) and comprising a skin (14) in the form of a stretched textile membrane in conjunction with a rigid internal and/or external framework (15) supporting load and enabling the membrane to be kept taut and impart the requisite shape to it.

2. Diffuser as claimed in claim 1, characterised in that the textile membrane (14) has a density in the order of 1 kg/dm³.

3. Diffuser as claimed in claim 1, characterised in that the textile membrane (14) is provided with a lagging enabling it to be reinforced, in particular in its zone located in proximity to the blades (5).

4. Diffuser as claimed in claim 1, characterised in that the rigid framework (15) comprises a series of identical radiating ribs (16), intrinsically closed and having a planar curve with a wing-shaped contour corresponding to the contour of the diffuser (7) and constituting the internal surface (18) and external surface (19) thereof.

5. Diffuser as claimed in claim 4, characterised in that the rigid framework (15) comprises:

a series of peripheral members (17) enabling the radiating ribs (16) to be linked and, if necessary, a series of stiffening elements enabling the thickness of the contour of the diffuser (7) to be kept constant.

6. Diffuser as claimed in claim 5, characterised in that the radiating ribs (16 ₁) and the peripheral members (17 ₁) are steel tubes.

7. Diffuser as claimed in claim 5, characterised in that the radiating ribs (16 ₂) are provided in the form of aluminium sections with a cross-section with a a shape, whilst the peripheral members (17 ₂) are aluminium tubes.

8. Diffuser as claimed in claim 5, characterised in that the radiating ribs (16 ₃) are provided in the form of timber sections which are T-shaped in particular, and the peripheral members (17 ₃) are carbon beams, in particular with a rectangular cross-section, linked in pairs by cores (24) in particular of plywood, on either side of the radiating ribs (16 ₃) so as to define box-type spars.

9. Diffuser as claimed in claim 8, characterised in that the box-type spars have cut-outs (25) at their median part.

10. Diffuser as claimed in claim 5, characterised in that the radiating ribs (16 ₄) are provided in the form of elements, in particular with a rectangular cross-section, made from a laminated-bonded composite material, and the peripheral members (17 ₄) are beams, in particular with a rectangular cross-section made from a laminated-bonded composite material linked in pairs on either side of the radiating ribs (16 ₄) by a series of rings (27) rigidly secured to one another and also made from a laminated-bonded composite material.

11. Diffuser as claimed in claim 10, characterised in that the internal face (18) and external face (19) of each of the radiating ribs (16 ₄) is linked by a series of rings (28) rigidly secured to one another and made from a laminated-bonded composite material.