Wind turbine tower

Abstract

A self-supporting wind turbine tower with walls comprising an upper portion (12) and a lower portion (14). Substantially all of the upper portion (12) is formed from a composite plastic. Substantially all of the lower portion (14) is formed from mild steel.

Claims

1. A self-supporting wind turbine tower with walls comprising: an upper portion formed from a composite plastic, the upper portion being subdivided into a plurality of segments arranged in a hoop direction of the tower; and a separate, lower portion mounted on a foundation, the upper portion mounted atop the lower portion so as to form the tower, the lower portion formed from a mild steel, wherein the self-supporting tower comprises a reduced weight and an increased natural frequency as compared to a tower of an equivalent size constructed entirely of steel.

2. A self-supporting wind turbine tower according to claim 1, wherein the upper portion comprises from 2.0% to 80% of the length of the tower.

3. A self-supporting wind turbine tower according to claim 1, further comprising a plurality of segments arranged in an axial direction of the tower.

4. A self-supporting wind turbine tower according to claim 1, further comprising a gasket positioned between the upper and lower portions.

5. A self-supporting wind turbine tower according to claim 1, wherein the upper portion is hollow.

6. A self-supporting wind turbine tower according to claim 1, wherein the lower portion is hollow.

7. A self-supporting wind turbine tower according to claim 1, wherein the composite plastic comprises a fibre reinforced plastic selected from a group including standard modulus carbon fibre, intermediate modulus carbon fibre, high modulus carbon fibre, and basalt.

8. A self-supporting wind turbine tower according to claim 7, wherein between 50% and 100% of the fibres are arranged at 0 degrees to the axial direction, up to 50% of the fibres are arranged at +/45 degrees to the axial direction and up to 30% of the fibres are arranged at 90 degrees to the axial direction.

9. A self-supporting wind turbine tower according to claim 1, wherein the composite plastic comprises a viscoelastic material.

10. A self-supporting wind turbine tower according to claim 9, wherein the viscoelastic material is provided as a viscoelastic core.

11. A self-supporting wind turbine tower according to claim 9, wherein the composite plastic comprises a fibre reinforced plastic having a viscoelastic polymer matrix.

12. A self-supporting wind turbine tower according to claim 1, wherein an outer surface of the tower comprises any of undulations, cavities, or protrusions arranged to reduce drag and/or vorticity downwind of the tower.

13. A self-supporting wind turbine tower according to claim 5, wherein the upper portion has a wall thickness which varies along the length of the upper portion.

14. A self-supporting wind turbine tower according to claim 6, wherein the lower portion has a wall thickness which varies along the length of the lower portion.

15. A self-supporting wind turbine tower according to claim 1, wherein the upper portion has a specific stiffness of at least 60 Gpa/(g/cm.sup.3).

16. A self-supporting wind turbine tower according to claim 1, wherein the lower portion has a specific stiffness of less than 30 Gpa/(g/cm.sup.3).

17. A wind turbine comprising a self-supporting wind turbine tower according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An example of the present invention will now be described with reference to the following drawings in which:

(2) FIG. 1 is a schematic section view of a wind turbine tower in accordance with the present invention;

(3) FIG. 2 is a partial side view of the wind turbine tower of FIG. 1, showing the connection between upper and lower portions;

(4) FIG. 3 is a schematic side view of the upper portion of the wind turbine tower of FIG. 1, which illustrates the fibre orientation;

(5) FIG. 4 is a partial section view of a connection between sections of the upper portion of the tower of FIG. 1;

(6) FIG. 5 is a partial section view of the tower of FIG. 1;

(7) FIG. 6 is a partial section view of the wind turbine tower of FIG. 1, showing a first alternative connection between upper and lower portions;

(8) FIG. 7 is a partial section view of the wind turbine tower of FIG. 1, showing a second alternative connection between upper and lower portions;

(9) FIG. 8 is a partial section view of the wind turbine tower of FIG. 1, showing a third alternative connection between upper and lower portions;

(10) FIG. 9 is a partial section view of a first alternative connection between sections of the upper portion of the tower of FIG. 1;

(11) FIG. 10 is a perspective view of a longitudinally divided section which may be used to form the tower of FIG. 1, showing Blade Dynamics' patent inserts; and

(12) FIG. 11 is a perspective view of the longitudinally divided section of FIG. 10 having protrusions on the outer surface thereof.

DETAILED DESCRIPTION

(13) As shown in FIG. 1, the tower 10 comprises an upper portion 12 made from a composite plastic and a lower portion 14 made from a mild steel. The upper portion 12 and the lower portion 14 are connected together to form the tower 10, which is mounted on a foundation 16 in a manner known in the art.

(14) To connect the upper portion 12 and the lower portion 14, each have at one end an outwardly extending peripheral flange 18, as shown in FIG. 2. The upper and lower portions 12, 14 are positioned such that they are coaxial along the longitudinal axis 20 of the tower 10 and the flanges 18 are connected together using bolts 22.

(15) The composite plastic, from which the upper portion 12 is made, has a high specific stiffness, i.e. has a specific stiffness of at least 60 GPa/(g/cm.sup.3).

(16) Suitable composite plastics for the upper portion 12 include, but are not limited to, plastic reinforced with any of standard modulus carbon fibre (HSC), intermediate modulus carbon fibre (IMC), high modulus carbon fibre (HMC), basalt, or a combination thereof. The composite plastic can be built using wet lamination, infusion, RTM or prepreg, among other conventional methods. The construction can be monolithic, sandwiched, or stiffened (e.g. orthogrid, stringers and rings, etc.), depending on the structural requirements of the upper portion 12. The material placement can be achieved by hand, filament winding, automated tape placement or by any other suitable method.

(17) Ideally the composite plastic is a laminate with between 50% to 100% of fibres at 0 degrees, 0% to 50% of fibres at +/45 degrees, and 0% to 30% of fibres at 90 degrees. As shown in FIG. 3, 0 degrees indicates that the fibres are parallel to the longitudinal axis 20 of the tower 10 and 90 degrees indicates that the fibres are perpendicular the axis 20, i.e. running along the hoop direction. The 0 degree material can be laid up uniformly distributed or can be added as pre-cured or pre-consolidated stacks.

(18) Other fibre orientations between +/20 degrees and +/70 degrees are also possible. Different materials can be combined, for example the 0 degree fibres can be made of HSC or Basalt and the off-axis plies can be made of fibre glass. Likewise, the 0 degree fibres can be made of IMC or HMC and the off-axis plies can be made of HSC.

(19) In this example, the upper portion 12 comprises Standard Modulus Carbon Fibre embedded in epoxy resin, with a Fibre Volume Fraction (FVF) of 56% and with 80% of the fibres at 0 degrees, 15% of the fibres at +/45 degrees and 5% of the fibres at 90 degrees. With this arrangement, the upper portion 12 has a specific stiffness of approximately 76 GPa/(g/cm.sup.3) and the lower portion 14 has a specific stiffness of approximately 27 GPa/(g/cm.sup.3).

(20) As shown in FIG. 4, the upper portion 12 is formed from a plurality of tubular sections 24. Each section is between 2 and 6 meters in diameter and between 5.8 to 45 meters in length. In this example, consecutive tubular sections 24 are connected together using root insert connections 26, as described in our earlier application International Patent Publication No. WO 2010/041008. The only difference is that the root insert connections 26 are provided on both the sections 24 being joined and studs 28 with right-handed and left-handed threads are used to join the sections 24 together. In WO 2010/041008, root insert connections are provided on one piece and conventional bolts are used to fix that piece to an adjacent structure. A gasket 30 is disposed between tubular sections 24 to create an even pressure distribution from the pretension.

(21) In this example, the upper portion 12 is 40 meters long, has an external diameter of 3.5 to 4 meters and a thickness of between 20 mm and 30 mm, and the lower portion 14 is 40 meters long, has an external diameter of 4 meters and a thickness of between 14 mm and 18 mm. The two are connected to form the tower 10, which is 80 meters tall.

(22) With this arrangement, the tower 10 has a natural frequency of 1.55 Hz, whereas an equivalent tower constructed entirely of mild steel would have a natural frequency of 0.97 Hz. This represents a 59% increase in natural frequency.

(23) Further, the total mass of the tower is reduced by approximately 24% in comparison to an equivalent tower constructed of mild steel. As the total mass of the tower 10 is reduced and its natural frequency increased, the static and fatigue loads at the foundation are reduced. Reducing the self-weight of the tower also further increases the natural frequency, due to diminished compressive load.

(24) Moreover, the use of composite materials yields an increased safety factor for a given component mass. The specific strength, which is defined as the material strength divided by its density, of mild steel is 32 MPa/(g/cm.sup.3), while for uni-directional HSC-epoxy 56% FVF along the fibre direction it is 767 MPa/(g/cm.sup.3).

(25) Although the wind turbine tower 10 is described as being formed from an upper portion 12 made from a composite plastic with a first stiffness and a lower portion 14 made from a mild steel with a second stiffness, the tower 10 could be formed from a number of sections each having different stiffnesses.

(26) The tower 10 may have any suitable cross-sectional shape, such as circular cross-section, or an elongated cross-section with a streamlined aerofoil shape, as shown in FIG. 5. Such an elongated cross-section can be used to minimise drag on the tower and the vorticity downwind from the tower if it is aligned in the direction of the predominant winds.

(27) Rather than having an outwardly extending flange 18, as shown in FIG. 2, an end of each of the upper portion 12 and lower portion 14 may have a flange 118 which extends inwardly by which the two portions 12, 14 may be connected, as shown in FIG. 6. Alternatively, connection may be effected using an outwardly extending flange 18 on one portion 12, 14 in combination with a root insert connection 26 on the outer surface of the other portion 14, 12 (see FIG. 7), or an inwardly extending flange 118 on one portion 12, 14 in combination with a root insert connection 126 on the inner surface of the other portion 14, 12 (see FIG. 8). Root insert connection 126 is essentially the same as root insert connection 26 but extends inwardly from the upper portion 12, rather than extending outwardly from it.

(28) Consecutive tubular sections 24 may be connected using any suitable fixing means. For example, the tubular sections 24 may be connected using root insert connections 126 extending internally from each section 24, as shown in FIG. 9.

(29) The upper portion 12, or tubular sections 24, may be divided along the direction of the longitudinal axis 20 of the tower 10 into longitudinally divided parts 32. The longitudinal connection of such divided parts 32 can be achieved by mechanical fastening, bonding, or a combination of both, using, for example, longitudinal flanges 34 (as shown in FIG. 10), lap joints or doublers. Alternatively, the upper portion 12 may be formed from a unitary component, i.e. one which is not subdivided either longitudinally or along the hoop direction.

(30) The outer surface of the tower 10 may include waves or protrusions 36, as shown in FIG. 11, to reduce drag and vorticity downwind from the tower.