The present invention relates to wind turbine towers. In particular, the present invention relates to self- supporting wind turbine towers.

Conventional towers for large wind turbines are generally either tubular steel towers, lattice towers, or concrete towers. Most are tubular steel towers which are formed from several 20-30 metre sections bolted together in situ. The tower is then fixed either to a foundation made of concrete using anchor bolts (onshore), to a mono pile, gravity or tripod foundation (off-shore, shallow), or to a floating foundation (off-shore, deep water) .

When designing a wind turbine tower, it is necessary to consider its natural frequency in relation to the passing frequencies of the rotor. These passing frequencies are defined as the frequency of one complete revolution of the rotor and the frequency of any blade passing the tower, i.e. the rotating frequency of the complete rotor, divided by the number of blades. If the natural frequency of the tower is in the region of either of the passing frequencies of the rotor, resonance may occur causing the magnitude of the vibrations felt by the turbine to increase.

The natural frequency of a tower decreases in proportion to its length squared. Thus, the natural frequencies of longer towers are typically lower than those of shorter towers. As tower lengths have increased in order to accommodate larger blades, the natural frequency of a conventional tower may lie close to the passing frequencies of the rotor. Consequently, longer towers can be susceptible to resonance which may result in damage to the turbine components or tower foundations. This problem is compounded by the fact that turbines are usually designed to work within a certain range of rotational speeds. Thus, the passing frequencies to be avoided are generally frequency ranges, rather than narrow bands or fixed amounts.

US 2009/0266004 discloses a wind turbine tower formed from a carbon fibre composite. The tower is fabricated by preparing a flexible textile preform offsite, transporting the preform to the assembly location, placing the preform over a mandrel and laminating the preform with a resin to form the composite shell. As the natural frequency of a tower is proportional to the square root of the specific stiffness of the material from which the tower is made (the specific stiffness being an inherent property of the material and which is defined as E/p, where E is the Young's modulus and p is the density) , the use of a stiff carbon fibre composite results in a tower with an increased natural frequency. Consequently, the tower is less prone to excitation modes under varying load conditions. However, carbon fibre composite towers are significantly more expensive to manufacture than equivalent steel towers.

US 2011/0138707 discloses a wind turbine tower having a concrete lower portion and a steel upper portion and teaches that this arrangement allows the height of a conventional steel tower to be increased without a corresponding increase in the difficulty of tower construction and transport. However, when constructed, such a tower will have a lower natural frequency in comparison to a smaller, conventional steel tower. Consequently, the natural frequently may lie close to the passing frequencies of the rotor, increasing the risk of resonance and the associated damage to the turbine components or tower foundations.

According to the present invention, there is provided a self-supporting wind turbine tower with walls comprising an upper portion and a lower portion, wherein the upper and lower portions are connected together to form the tower, wherein substantially all of the upper portion is formed from a composite plastic, and wherein substantially all of the lower portion is formed from mild steel.

With this arrangement, favourable frequency characteristics can be achieved for a long tower, relative to one formed entirely from steel, and without the expense of forming the tower entirely from a composite plastic. This is due to the fact that the specific stiffness of the lower portion of the tower has been found to have far less of an effect on the overall natural frequency of the tower in comparison to the stiffness of the upper portion of the tower .

Further, by forming the upper portion from a composite plastic, a lower mass of material is required to meet the particular stiffness requirements for a given installation. Thus, the total mass of the tower can be reduced. This, combined with the increased natural frequency of the tower, can result in a noticeable reduction in static and fatigue loads at the foundation. Preferably, the upper portion comprises 20 to 80% of the length of the tower. The upper portion may be a unitary component.

Alternatively, the upper portion may be subdivided into a plurality of segments. In addition to simplifying the transport of the upper portion, subdividing the upper portion also reduces the cost of tooling and of any ovens used during production and makes the upper portion easier to manufacture and inspect. It also allows the mechanical properties of the upper portion to be closely controlled by using factory controlled pre-cured segments. This is not possible with the textile preform arrangement of US 2009/0266004. The plurality of segments may be arranged in an axial direction of the tower and/or in a hoop direction of the tower.

The upper and lower portions may be connected directly. Alternatively, a gasket may be positioned between the upper and lower portions to create an even pressure distribution between the two parts. The upper portion and/or the lower portion may be hollow. The composite plastic may be a fibre reinforced plastic selected from a group including standard modulus carbon fibre, intermediate modulus carbon fibre, high modulus carbon fibre, and basalt. In a preferred example, between 50% and 100% of the fibres are arranged at 0 degrees to the axial direction, up axial direction and up to 30% of the fibres are arranged at 90 degrees to the axial direction.

Optionally, the composite plastic comprises a viscoelastic material to increase the hysteric damping characteristics of the tower. The viscoelastic material may be provided as a viscoelastic core. Alternatively, the composite plastic may comprise a fibre reinforced plastic having a viscoelastic polymer matrix.

Preferably, an outer surface of the tower comprises any of undulations, cavities, or protrusions arranged to reduce drag and/or vorticity downwind of the tower. Preferably, the upper portion and/or the lower portion has a wall thickness which varies along the length of that portion. In this manner, material can be removed from where it is not needed in order to improve the dynamic behaviour of the tower.

The specific stiffness of the upper portion may be at least 60 GPa/ (g/cm ³ ) .

In a preferred example, the specific stiffness of the lower portion is less than 30 GPa/ (g/cm ³ ) .

An example of the present invention will now be described with reference to the following drawings in which: Figure 1 is a schematic section view of a wind turbine tower in accordance with the present invention; Figure 2 is a partial side view of the wind turbine tower of Figure 1, showing the connection between upper and lower portions;

Figure 3 is a schematic side view of the upper portion of the wind turbine tower of Figure 1, which illustrates the fibre orientation;

Figure 4 is a partial section view of a connection between sections of the upper portion of the tower of Figure i;

Figure 5 is a partial section view of the tower of

Figure 1;

Figure 6 is a partial section view of the wind turbine tower of Figure 1, showing a first alternative connection between upper and lower portions;

Figure 7 is a partial section view of the wind turbine tower of Figure 1, showing a second alternative connection between upper and lower portions;

Figure 8 is a partial section view of the wind turbine tower of Figure 1, showing a third alternative connection between upper and lower portions;

Figure 9 is a partial section view of a first alternative connection between sections of the upper portion of the tower of Figure 1;

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

Figure 11 is a perspective view of the longitudinally divided section of Figure 10 having protrusions on the outer surface thereof.

As shown in Fig.l, 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 .

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.

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 ³ ) .

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. 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 .

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.

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 ³ ) and the lower portion 14 has a specific stiffness of approximately 27 GPa/ (g/cm ³ ) . 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 metres in diameter and between 5.8 to 45 metres 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.

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

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.

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. 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 ³ ) , while for uni-directional HSC-epoxy 56% FVF along the fibre direction it is 767 MPa/ (g/cm ³ ) . 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.

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 . 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.

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. 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.

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.