A METHOD OF FORMING AN AIRFOIL FOR A WIND TURBINE

Field of the Invention

The present invention broadly relates to a method of forming a structure for causing generation of a force by an airflow, such as an airfoil, for a wind turbine.

Background of the Invention

Some wind turbines have airfoils or "blades" which have regular geometric shapes and are relatively easy to fabricate. However, increasingly the blades of wind turbines have much more complex shapes and consequently are more difficult to manufacture.

In addition, as the size of the wind turbine increases, so the physical loads on the blades increases. Thus, it is necessary for the turbine blades to be fabricated in a manner that will satisfactorily resist those loads. For example, a 15OkW wind turbine may have a vertical extent of approximately 13 metres and a diameter of approximately 5-6 metres.

Summary of the Invention

The present invention provides in a first aspect a method of forming a side portion of an airfoil for a wind turbine, the airfoil having a twist along an axis of the airfoil, the method comprising the steps of: forming a plurality of first support elements, each first support element having an edge, a portion of each edge having a shape that approximates that of a cross- sectional surface portion of the side portion of the airfoil; positioning the formed first support elements spaced apart from one another to form a series of the first

support elements that are rotated about an axis relative to each other in a manner such that the portions of the edges substantially align with a desired surface shape of the side portion; positioning at least one sheet of a material over the edge portions of the first support elements, the at least one sheet having a first and an opposite second surface, the at least one sheet being positioned so that the first surface faces the edge portions of the first support element and the second surface of the sheet, or of a plurality of the sheets, approximates the surface shape of the first side portion of the twisted airfoil; and applying a liquid material to the second side of the sheet, or the plurality of the sheets, the liquid material being arranged for hardening.

The method then typically comprises removing the hardened material from the sheet, or the plurality of sheets, to provide the side portion of the airfoil.

The side portion may be a first side portion and the airfoil may comprise a second side portion that is opposite the first side portion, the method may further comprise the steps of: forming a plurality of second support elements, each second support element having an edge, a portion of each edge having a shape that approximates that of a cross - sectional surface portion of the second side portion of the airfoil; positioning the formed second support elements spaced apart from one another to form a series of the second support elements that are rotated about an axis relative to each other in a manner such that the portions of the edges substantially align with a desired surface shape of the second side portion of the twisted airfoil; positioning at least one sheet of a material over the edge portions of the second support elements, the at least

one sheet having a first and an opposite second surface, the sheet being positioned so that the first surface faces the edge portions of the second support element and the second surface of the sheet, or of a plurality of the sheets, approximates the surface shape of the first side portion of the twisted airfoil; and applying a liquid material to the second side sheet, the liquid material being arranged for hardening.

The method then typically comprises removing the hardened material from the sheet, or the plurality of sheets, to provide the second side portion of the airfoil.

The present invention provides in a second aspect a method of forming an airfoil for a wind turbine, the airfoil having a first side portion and an opposite second side portion and having a twist along an axis of the airfoil, the method comprising the steps of: providing first and second side portions of the airfoil; positioning the first and second side portions relative to each other so that an interior space is defined; and introducing a liquid material into the interior space, the liquid material being arranged for hardening.

The method typically also comprises positioning the first and second side portions and a substantially rigid elongate truss in an interior space defined by the first and second mould elements.

The step of providing the first and second side portions of the airfoil typically comprises fabricating at least one of the first and second side portions. in accordance with the first aspect of the present invention.

In one embodiment the method comprises fabricating the

first and second side portions in accordance with the first aspect of the present invention.

It is to be appreciated that in the method in accordance with the first or second aspect of the present invention any number of layers of materials or any other suitable material or coating may be positioned between the at least one sheet and the liquid material or material may be removed .

The present invention provides in a third aspect a method of forming an airfoil for a wind turbine, the airfoil having a first side portion and an opposite second side portion and having a twist along an axis of the airfoil, the method comprising the steps of : forming a first side portion in accordance with the first aspect of the present invention; forming a plurality of second support elements, each second support element having an edge, a portion of each edge having a shape that approximates that of a cross- sectional surface portion of the second side portion; positioning the formed second support elements spaced apart from one another to form a series of the second support elements that are rotated about an axis relative to each other in a manner such that the portions of the edges substantially align with a desired surface shape of the first side portion of the twisted airfoil; positioning at least one sheet of a material over the edge portions of the second support elements, the at least one sheet having a first and an opposite second surface, the sheet being positioned so that the first surface faces the edge portions of the second support element and the second surface of the sheet, or of a plurality of the sheets, approximates the surface shape of the first side portion of the twisted airfoil; positioning the first side portion of the airfoil and the sheets, or the plurality of the sheets, relative to

each other so that a mould cavity is formed for moulding a twisted airfoil having the first and second side portions; and applying a liquid material into the cavity, the liquid material being arranged for hardening; and, after hardening of the liquid material; removing the formed airfoil from the sheet or the plurality of sheets .

The method in accordance with the second and third aspects of the present invention may also comprise embedding a substantially rigid elongate truss in the liquid material so that the substantially rigid elongate truss provides rigidity for the airfoil in the hardened material. The airfoil, typically the truss, may comprise fittings for attaching the airfoil to components of a rotary structure.

The liquid material in accordance with the first, second and third aspects of the present invention may be a polymeric material. Further, the method may comprise positioning of fibres in a manner such that the hardened material is fibre reinforced.

The present invention provides in a fourth aspect an airfoil for a wind turbine fabricated by the method in accordance with the second or third aspect of the present invention.

The present invention provides in a fifth aspect an airfoil for a vertical axis wind turbine, the airfoil comprising: a substantially rigid elongate truss having a twist along at least a portion of the elongate truss,- and a hardened polymeric material being formed in a mould, at least a portion of the elongate truss being embedded in the hardened polymeric material in a manner such that the substantially rigid elongate truss supports

the hardened polymeric material .

The formed airfoil for the vertical axis wind turbine typically has a horizontal cross-sectional shape that is the same for the full length of the airfoil and typically has a vertical warp and a constant linear pitch about a common vertical axis.

The airfoil may be a part of a vertical axis wind turbine assembly and an axis of the airfoil or "vertical wing" may be common with a vertical axis of a wind turbine assembly, or may alternatively be parallel and spaced apart form the axis of the turbine assembly.

The present invention provides in a sixth aspect a mould for moulding a side portion of an airfoil, the mould comprising: a plurality of first support elements, each first support element having an edge, a portion of each edge having a shape that approximates that of a cross-sectional surface portion of the first side portion of the airfoil, the first support elements being spaced apart from one another and form a series of the first support elements that are rotated about an axis relative to each other in a manner such that the portions of the edges substantially align with a desired surface shape of the first side portion of the twisted airfoil; at least one sheet of a material positioned over the edge portions of the first support elements, the at least one sheet being positioned so that a surface of the sheet or a plurality of the sheets approximates the surface shape of the first side portion of the twisted airfoil.

In embodiments of any aspect of the present invention the axis about which the airfoil is twisted is outside the airfoil, typically offset relative to an inner face of the airfoil. A wind turbine assembly may comprise a plurality

of such airfoils and each axis about which a respective airfoil is twisted may coincide with a vertical axis of a wind turbine assembly, or may alternatively be parallel and spaced apart form the vertical axis of the wind turbine assembly. Positioning of the axis of each airfoil relative to the vertical axis of the wind turbine assembly allows modification of performance properties of the wind turbine assembly. For example, by shifting or rotating the airfoils about the axis of the wind turbine assembly by a small amount, an outside diameter of the wind turbine assembly may be varied so that the wind turbine assembly is "opened" or "closed" to airflow.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings

Fig. 1 is a side elevation of a three blade vertical axis wind turbine,

Fig. 2 is a side elevation of the turbine blades, Fig. 3 is a plan view of the turbine blades of Fig. 2,

Fig. 4 is a side elevation of a metal truss to be embedded in each turbine blade,

Fig. 5 is a plan view of the top plate or upper disc of the turbine of Fig. 1 showing the top rung of the truss of Fig. 4,

Fig. 6 is a plan view of the base plate or lower disc of the turbine of Fig.l showing the bottom rung of the truss of Fig. 4,

Fig. 7 is a side elevation of a timber template to form the leading edge of each turbine blade,

Fig. 8 is an end elevation of a stack of the templates of Fig. 7, each adjacent template being rotated

by a like amount relative to a common axis passing through each template,

Fig. 9 is a side elevation of a timber template to form the trailing edge of each turbine blade, and Fig. 10 is a transverse cross-sectional view through the finished moulded blade showing the position of the embedded truss.

Detailed Description of Specific Embodiments

As seen in Fig. 1, a vertical axis wind turbine 1 has a base tower 2 which rotatably supports a mast 3 on which are mounted a plurality, and typically three, airfoils or "blades" or "sails" 4. As seen in Figs 2 and 3, each of the airfoils 4 is longitudinally twisted such as through 98 degrees or any other suitable angle. An axis about which the airfoil 4 is twisted is outside the airfoil 4.

The blades 4 have in this embodiment a constant cross- sectional shape, however, as one progress along the longitudinal axis of the blades 4 (which is substantially aligned with the mast 3) that constant cross-sectional shape is continuously rotated in order to provide a pitch for the blade 4. It will be apparent from Figs 2 and 3 that the individual blades 4 extend between an upper disc 6 and a lower disc 7.

In order to permit the blades 4 to withstand the necessary forces and loads applied to them, a steel truss 10 as illustrated in Fig. 4 is provided and the truss 10 also has the same pitch as the blades and extends between the upper disc 6 and lower disc 7. The truss 10 has a plurality of rungs which extend between a top rung 11 and a bottom rung 12. As seen in Figs. 5 and 6, the top rung 11 is connected to the upper disc 6 and the bottom rung 12 is connected to the lower disc 7.

In Figs. 5 and 6 the position of one of the rungs connected to the disc is shown in solid lines and the position of the opposite end rung is illustrated in broken lines in order to illustrate the twisted nature of the truss 10. It will also be apparent to those skilled in the art that the number of trusses typically equals the number of blades. Thus in this particular embodiment since there are three blades 4, there are also three trusses 10, only one of which is illustrated. Furthermore, each of the trusses 10 is embedded within the corresponding blade 4 which is moulded from a flowable hardenable material such as fibre reinforced resin (or fibreglass) .

To provide additional rigidity to the entire structure, it is desirable to interconnect the three trusses 10, at say, 3 or 4 vertically spaced apart locations with horizontally extending structural members. In this way the interconnected trusses 10 form a hollow rotatable structural mast. The lightweight mast 3 is typically left inside this structural mast to provide easy access for maintenance, cleaning, etc.

In order to fabricate the blades 4 , a mould is provided into which the truss 10 can be positioned and then the fibreglass is laid up into the mould. As seen in Figs. 2 and 3 , each of the blades 4 has a leading surface 14 and a trailing surface 15. For a conventional Savonius-type wind turbine, the leading convex surfaces of the turbine blades do not create any lift, instead the drag created by the wind blowing into the trailing concave surface of the semi-cylindrical blade is greater than the drag created by the semi-cylindrical leading surface moving into the wind. Consequently a net torque is provided, being the difference between the two drag forces, and the turbine rotates. However, for the type of turbines to which embodiments of the present invention are particularly

applicable, the leading surface 14 when the wind blows onto it, creates lift thereby turning the blade 4 into the wind. A blade on the opposite side of the turbine has its trailing surface 15 facing the wind and accordingly that trailing surface creates drag which the moves that blade in the direction of the wind. Thus two rotating forces or moments are created and the net torque on the wind turbine is the sum of those forces or moments, and not the difference as in the conventional "Savonius" -type turbine.

In this embodiment the leading surface 14 is carefully shaped in order to provide the correct aerodynamic surface to generate the desired lift, which complicates the fabrication of the turbine blades 4. In addition, the leading and trailing surfaces 14, 15 should be correctly positioned relative to the axis of rotation of the turbine 1 which is provided by the rotatable mast 3.

As seen in Fig. 7, a template 20 is fabricated and may comprise relatively stiff timber sheets such as 5 ply or 7 ply plywood. Each sheet is provided with a flat bottom edge 21 ("reference edge") and a carefully shaped leading edge 24. In addition, the template 20 is provided with a circular aperture 30 which is coincident with the position of the mast 3.

Since for this particular embodiment the cross-sectional shape of the blades 4 is constant, a number of the templates 20, each having an identical shape are fabricated. In this embodiment, the shape the edges 24 corresponds to a cross-sectional shape of the leading surface of the airfoil. In the embodiment illustrated in Fig. 8, eight of the templates 20 are fabricated and these are placed on a pipe or circular rod 31 which forms a close fit with each of the apertures 30.

Templates 20 are spaced apart from each other by a

predetermined identical distance and successive templates 20 are rotated by an identical amount which corresponds the overall desired pitch of the blade 4 divided by the number of templates less one. Thus for eight templates and a 98 degree pitch, each adjacent template is rotatable to relative to its neighbour by approximately 14 degrees. Once the templates 20 are in this position they are maintained in place by any conventional fixing means such as noggins or spacing timbers which are nailed between each pair of adjacent templates 20.

With the templates 20 so arranged, a thin sheet or sheets (not illustrated) of substantially constant thickness, such as a single ply plywood, is then placed over the leading edge 24 and thus naturally conforms to the shape which the leading edges 24 defines. Thus the thin timber sheet (s) bridge (s) the adjacent templates 20 and can be easily temporarily secured thereto be means of glue and / or nails or other fasteners. Thereafter the upper surface of the sheet (s) is evenly covered with a flowable hardenable material such as gel coat or fibreglass and allowed to set.

An exposed surface of the hardended material forms the leading surface 14 of the airfoil 4. It is to be appreciated that in an alternative embodiment the templates 20 may be inversely curved. In this case a surface at the interface of the hardened material with the sheet forms the leading surface 14 of the airfoil. The formed side portion of the airfoil is then removed from the sheet (s) .

Turning now to Fig. 9, the procedure described above in relation to Figs. 7 and 8 and template 20 is now repeated in relation to the template 120 of Fig. 9 which again has a flat bottom or reference edge 121 and a circular aperture 130. The template 120 has a trailing edge 25

which corresponds to a surface shape of the trailing surface 15 of the blade 4. The procedures described in relation to Fig. 8 are then repeated with the template 120 of Fig. 9. In this case a surface at the interface of the hardened material with the sheet (s) forms the trailing surface 15 of the airfoil. The formed side portion of the airfoil is then removed from the sheet (s) .

The first and second side portions and an elongate rigid truss are the position so that a moulding cavity is formed between the first and second side portions and the rigid elongated truss is positioned in the mould cavity. A flowable hardenable material is the introduced into the cavity to form the airfoil.

Turning now to Fig. 10, the final moulded blade 4 is illustrated in transverse cross-section. The blade 4 has the leading surface 14 and trailing surface 15 having shapes as determined by the templates 20, 120. In addition, the blade 4 has embedded in it the truss 10 of Fig. 4.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the moulding arts, can be made thereto without departing from the scope of the present invention. For example, the female mould surfaces produced from the sheet which covers the spaced apart templates 20, 120 can be used either singularly to mould a single surface (as by building up individually laid layers using fibreglass techniques) , or used jointly so as to form a mould cavity in which the leading surface 14 and trailing surface 15 of the blade 4 are simultaneously fabricated.

Further, one side portion of the airfoil may be fabricated in he above-defined manner and then positioned over a mould for forming the other side portion so that forming

of the other side portion and filling the flowable hardenable material into the cavity between the first and second side portions is combined into one step.

Since the shape of the leading surface 14 is more critical than the shape of the tailing surface 15, the female mould surface created by use of the templates 20 can be polished smooth and laid horizontally on the floor. Then gel coat is applied together with a first layer of fibreglass mat reinforcing. Then the steel truss 10 is positioned and more resin and reinforcing applied to build up the blade. The template 120 is used to manually check the blade thickness until the correctly dimensioned blade has been formed.

Reference that is being made to prior publications does not constitute an admission that these prior publications form a part of the common general knowledge in Australia or any other country.

Although the invention has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms .