The present invention relates to wind turbines, and in particular to a support mast for a vertical axis wind turbines (VAWTs). Wind energy is rapidly emerging as a cost-effective form of renewable energy. VAWTs generally comprise a mill system which is formed of a plurality of vertical blades mounted for rotation about a vertical drive shaft. The drive shaft is connected to a generator, either directly or via a gear box, for generating electrical power.

The mill system is generally supported off the ground by a mast structure which houses the rotating drive shaft. Commonly, masts for VAWT are constructed from slim section steel tube members. The mast is generally able to house only the vertical drive shaft in order to connect the high level mill system to the low level gearbox/generator arrangement, or the direct drive generator (such as a permanent magnet motor).

Thus other equipment associated with the turbine is housed externally of the mast structure, either mounted to the outside of the mast at the top thereof, or adjacent the base of the mast in a separate housing. An important element of a VAWT is a safety brake system; this must be capable of stopping the mill in any foreseeable condition. As the brake is a safety critical item it must be connected directly to the main mill drive shaft; as close as practical to the mill system, and be fitted directly to the shaft without any shaft connections. As already stated tubular masts cannot readily house such a braking system. This means it can only be sited outside directly below the lower mill spreader connections on the main shaft. This places the brake system directly within the airflow of the mill system; this is detrimental to the performance of the mill due to air turbulence created by the brake system. A further problem with externally mounting the brake system is that it is exposed to the elements. To protect the brake system from the elements it must be enclosed, and or the components made from a weather resistant material such as stainless steel. Additionally maintenance is only possible using an access platform or by fitting a fixed external ladder and fall arrest system and a working platform.

All the other plant must be mounted in a separate housing at ground level. This design limitation leads to a connective drive shaft from the mill system to ground level commonly being very long; typically over 30m. Whilst such drive shafts are achievable they involve substantial cost and significant issues arise with design. For example, servicing the shaft couplings requires access hatches at several points on the mast. Additionally long drive shafts result in a shaft with a large mass. This reduces start-up performance and produces heavier induction braking loads.

As VAWTs generate a significant amount of the drive from blades positioned behind the mast centreline it is important that the upper section of the mast within the mill area is as slender as possible so as to minimise disruption to the wind flow behind the mast.

At least one or all of the mast sections may be designed with a taper which exceeds about 2 degrees. This minimises radar returns. In addition, the transition section may surround the upper mast section, so as to provide a lesser degree of air turbulence within the mill area than would be the case with an upper mast section comprising a straight tubular section.

Commonly obtaining a suitable external protective/decorative finish on masts can be problematic; hot dip galvanising large masts can prove very difficult and costly. Further, galvanised finishes may then need to be painted to achieve a suitable colour for planning permission requirements.

To reduce the overturning moment created by wind loads it is important that the foundation at the base of the mast covers a large enough area or 'footprint'. This usually requires a large concrete foundation, to which the mast is normally anchored. This is environmentally costly and it is also a complex specialist operation to erect the VAWT, with consequent cost implications. Large "lift-type" VAWT's of a "giromill", or "H-bar" rotor design, with fixed pitch vertical aerofoils or blades, usually require an uneven number of blades to ensure the rotor is better balanced in the vertical axis of the main rotor shaft and

tower. During operation the airflow over each blade generates both radial and tangential forces ("lift" and "drag"). When combined, these forces both act laterally on the vertical axis and produce the torque required to rotate of the rotor around the vertical axis of the main rotor drive shaft and be converted in to electrical energy via the generator. However, the forces generated by each blade at any given time are constantly changing depending on their orientation towards the wind, moving through various phases of operation (lift, drag, stall and so on) generating dynamic forces as they rotate around the vertical axis, thus generating cyclical lateral and torque loads on the tower and drive train. Each blade has two peak forces per revolution, an upstream primary peak of "lift" and a secondary downstream peak of "drag" force, for example a 3 bladed turbine will experience 3 major peaks per revolution. An uneven number of blades ensures the two peak forces acting upon opposing blades are not synchronised, minimising the combined radial forces acting on the tower whilst maintaining tangential forces acting on the rotor.

Increasing the number of blades ensures the rotor has more blades in the primary "lift" force phase at any one time, thus helping the turbine to "self-start" more easily as it generates more torque in lower wind speeds. This is also beneficial for generating power in areas with lower average wind speeds. It also results in greater solidity (Solidity = Blade Area / Rotor Area) of the rotor. Greater solidity is preferable as it reduces the amplitude of the sinusoidal ("pulsing") torque curve and inherent cyclical stresses commonly referred to as "torque ripple", acting on both the tower and drive train, thus producing a smoother torque curve. If not adequately

accounted for in the tower design and drive train specification, the "torque ripple" can reduce power harvesting potential and overall reliability. Importantly, however, an increased blade number does not inherently deliver the most economical power-to- cost ratio. By contrast, towers and drive trains on conventional HAWT "propeller" rotor designs do not intrinsically suffer from the same detrimental "torque ripple" effects as

VAVVT's. As the rotor is perpendicular to the wind direction (either upwind or downwind), the forces acting on the rotors hub are relatively uniform across all the blades. Therefore the operational effect is negligible as the forces generated by the blades are balanced in uniform. Furthermore, any radial or tangential forces acting on the rotor hub and drive shaft, are perpendicular to the axis of the tower. As such, any lateral forces acting on the tower are more constant as they are induced solely by the force of the wind, and not the "torque ripple" experienced by most

VAVVT's. Overall, this means the HAWT tower design and drive train specification can be much simpler in terms of the structural tower strength and rigidity, and there is a lesser need for over specifying gearboxes and drive shafts to cope with the sinusoidal torque curve.

In particular, almost all VAWTs have resonant modes where, at a particular rotational speed, the frequency of the rotor's 'torque ripple' is the same as the natural frequency of the wind turbine's components; most critically the rotor components (blades, radial support arms and fixings), tower or drive train, which if poorly designed can cause them to fail eventually. For this reason, most VAWTs have mechanical brakes, or other speed control devices to keep the rotor from rotating at these detrimental speeds for any significant period of time.

Therefore, it is very important to ensure the natural frequency of the rotor components, drive train and tower, are designed so that any resonant frequencies occur when the operational forces and resultant mechanical frequency are either minimised or occur well beyond the expected operational speed of the rotor. The design of VAWTs that may experience significant resonant modes must be designed so the components can withstand the operational resonance for the expected lifetime of the turbine. A key challenge to designing VAWT's of this type include the relatively low rotational speed with the consequential higher torque and hence higher cost of the drive train, the 360 degree rotation of the aerofoil within the wind flow during each rotor rotation and inherent dynamic loading on the blades and rotor and the "torque ripple" generated by the rotor design on the tower and drive train. Increasing the amount of blades reduces the effects of the "torque ripple" on the tower but increases the cost of rotor; likewise reducing the amount of blades reduces the cost of the rotor but increases the effects of the "torque ripple" on the tower. Therefore altering the amount of blades can lead to inefficiencies in power harvesting and accelerated fatigue of the structure and foundations. It is therefore important to consider all the above factors to find the best overall cost to power ratio.

This invention aims to address these issues. According to the present invention, there is provided a support structure for a VAWT, the structure comprising a lower mast section comprising a frame and an outer skin, and an upper mast section having a smaller diameter than the lower mast section and being arranged to receive a rotating drive shaft, the upper mast section comprising a transition mast section for mounting the upper mast section to the lower mast section.

Thus the structure comprises two sections. Firstly, a large diameter tapered lower mast section which may comprise an internal frame and an outer skin with integral base and upper connective rings. Secondly a straight or tapered upper mast section which may have a relatively small diameter section constructed for example from heavy gauge steel tube and internal structural steel stiffening members; this also houses the main rotating drive shaft and bearings. A transition section connects the lower and upper mast sections. The transition section may simply comprise a base plate or similar structure connected at or adjacent the top of the lower section to which the upper section is connected. Alternatively the transition section may be tapering in shape and may substantially surround the upper mast section, with an upper cross sectional shape or diameter being arranged to correspond with the diameter of the upper mast section where they are connected together, and expanding to have a lower cross sectional diameter which substantially matches that of the lower mast section where they are connected together.

Conveniently, for easier transportation and assembly, the transition section may be formed in at least two parts arranged to be secured together. Thus the transition section may be placed around the upper mast section, and secured in place to connect the upper section to the lower larger diameter section. The transition section may also comprise integral flanges to accept the mounting of components such as the brake system, gearbox and generator. The upper mast section including the transition section may be clad in a lightweight, non-structural galvanised and plastic coated steel skin. This eliminates the need to galvanise or paint the internal structural steel core elements. The skin also incorporates a ventilation zone which allows warm air generated by the internal equipment to vent freely using the stack effect. The zone preferably includes measures such as soffit vents to prevent birds and insects to access the interior of the mast. Air flowing into the mast can be controlled for example via thermostatically controlled louvers sited in the main access door at ground level. Forced ventilation may optionally be used in hot climates. Importantly the taper of the mast sections is designed to provide improved airflow characteristics especially within the mill area where turbulence needs to be minimised and also provides radar mitigation by reflecting radar signals away from the source radar. The large diameter tapered lower mast section importantly allows components such as a disc brake system, gearbox, generator, inverter controls and all other plant to be sited within the mast. Access to all equipment may be from inside the mast, which may be electrically lit, from permanent access ladders and working platforms.

Drive may be taken directly from the mill system, through the upper mast and transition sections via a single drive shaft. The gearbox is preferably mounted directly onto the main shaft such that a flexible coupling is not required. Such a gearbox may be controlled by use of a torque arm to prevent the gearbox from rotating, but allowing flexibility of movement. Alternatively the gearbox can be separately mounted and coupled via a flexible coupling.

The use of a single piece drive shaft from the mill system connected directly to the gearbox without a flexible coupling has important advantages. This arrangement requires only one shaft coupling, between the generator or direct drive generator, which minimises losses and reduces cost and complexity.

The design of the driveshaft may also be aimed at minimising mass to improve efficiency. This may be achieved by the upper and lower sections of the drive shafts comprising solid steel. The central connective section may be constructed from tube, which is permanently fixed to the shaft end section, by welding or a similar method. This connective tube section can be constructed from steel, fibreglass, carbon fibre or other light weight composite, the aim being to minimise mass.

Lightning protection for the mill system and upper mast may be provided by connective copper strips fully connected to the main transmission shaft. Current is then transferred to the rotating shaft which flows down the main shaft. Current is then transferred to the main fixed earth grounding system by means of a spring loaded graphite or similar conductor running in contact with a track on the disc brake or similar disc attached to the main shaft. The support structure is suitable for a 3 or 5 blade rotor configuration that will operate with acceptable levels of lateral movement at the tower head accounting for the effects of "torque ripple", ensuring the resonance of the components are not detrimental within the turbine's operational life. A critical part of the tower design requires that the upper section of the tower, which also houses the main drive shaft and bearings, is as slender as possible to minimise air turbulence within the rotor area which would reduce the performance of the wings during their "lift" phase. The upper tower may then be formed from a tubular section with the main drive shaft supported within. The lower part may be formed by an upwards facing cone with internal support gussets. All these elements may be attached to a main circular upper tower base plate. Where this design could alone prove unstable and allow too much lateral movement which would amplify the effects of "torque ripple", additional structural support may be provided.

Preferably, the upper most section is supported on the lower section using a plurality of stays. This is essentially a similar principal to a sailing boat mast and rigging, supporting a lightweight slender mast utilising tensioned cordage (e.g. rope, cable or rod) to keep the main mast securely in position and adequately braced to handle loads induced by sails.

The stays or cordage may be connected between a base member on the lower most section and an attachment position part way up the upper section. The lower mast section is also preferably supported using a plurality of internal stays, which may be attached between a base member and an attachment position part way up the lower section. With the lower section of the mast, the rigging principals are inverted whereby instead of utilising the cordage externally to support a slender lightweight central mast, the cordage is placed internally to support the external lightweight weatherproof shell to house the internal componentry. This allows the use of fewer internal peripheral and intermediate floors and steel sections which might be otherwise required to support the lightweight outer shell. This design further allows a central maintenance and access shaft/column to enable the equipment (e.g. gearbox, motor etc.) to be readily lowered to ground level.

With the upper section of the mast which houses the main drive shaft and bearings, it is important that the outer profile is as narrow as possible to maximise the efficiency of the wind, therefore the stays or cordage is utilised in a more conventional manner (externally). Importantly this provides rigidity to the upper section with as little disruption to the airflow as possible. This use of a rigging method is both enabled and enhanced by the stability and rigidity of the large diameter lower section utilising internal cordage.

In another aspect, the invention provides an anchoring arrangement for a support structure for a VAVVT, the anchoring arrangement comprising a plurality of substantially radially extending legs, the legs arranged to be attached to a base of the support structure and to extend beyond the cross sectional area of the support structure, each leg being provided at or adjacent the distal end thereof with a downwardly extending foot for fixing to the ground.

Such an arrangement may reduce the amount of concrete required compared to conventional screwpile foundations. The anchoring arrangement may be used with the support structure as defined above.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a side view of a vertical axis wind turbine having a support tower according to one embodiment of the invention;

Figure 2 is a perspective view of the support tower of Figure 1 , with the outer skin removed for ease of reference;

Figures 2a and 2b are enlarged views of the transition section and base section respectively of the support tower of Figure 2;

Figure 3 is a side view of the support tower of Figure 1 , partly broken away;

Figure 4 is a cross sectional side view of the tower of Figure 3 taken along line C-C, and Figure 4a is an underneath view of the tower;

Figure 5 is a further cross-sectional side view of the tower of Figure 3 taken along line A-A, and Figure 5a is an underneath view of the tower;

Figure 6 is an enlarged view showing the transition section of the tower of Figure 4; Figure 7 shows an alternative enlarged view, with a shaft mounted planetary gearbox and motor arrangement;

Figure 8 is an enlarged cross sectional side view of the transition section of the tower according to another alternative configuration;

Figure 9 is an exploded view of the upper and transition sections of the tower;

Figure 10 is a perspective view of the base ring of the tower;

Figures 1 1 and 1 1 a are a perspective and exploded perspective view of part of the base ring of Figure 10, respectively;

Figures 12 and 12a are a side view of showing the tower frame and an exploded view of part of the lower section of the frame of the tower respectively;

Figures 13, 13a and 13b are exploded and schematic views of skin panels of the tower respectively;

Figures 14a to 14d depict a procedure for assembling the tower;

Figures 15a to 15d depict further steps in the procedure for assembling the tower; Figure 16 depicts a further step of the assembly procedure;

Figures 17 and 17a to 17e are a cross side view of a mill assembly and perspective views of parts of the upper and transitional sections of a tower;

Figures 18 and 18a to 18d are side and perspective views of a middle and base section of the tower; and

Figures 19 and 19a to 19c are cross-sectional views of a VAVVT having a support tower according to an alternative embodiment.

Referring to Figures 1 to 3, a VAVVT comprises a mill system 1 and a support structure in the form of a tower 2, which comprises a first lower section 4, and a second upper section 6, linked by a transition section 8. The first section is a monocoque structure, formed of a frame 10 covered by a skin 12 (see Figure 3).

In order to provide a light weight, low carbon tapered mast, the internal structural frame 10 is preferably formed from high tensile galvanised steel which is pressed into 5 structural members to support the lightweight skin or shell 12. The skin comprises high tensile steel, and may be multifaceted or round in cross section. As best shown in Figure 2, the frame 10 comprises a plurality of upright members 14, which may for example be U, I or T sections, which are radially arranged and inclined slightly inwardly. A plurality of equally spaced stiffening rings 16 are formed for example from pressed U sections and serve to brace the upright members 14 in position. The complete frame therefore creates a substantially conical shape for the shell to be attached.

For assembly, delivery, installation and operational purposes, intermediate floors 20 may be provided instead of selected ones of the horizontal stiffening rings 16. The intermediate floors 20 (for example three) comprise a peripheral walkway to create a central access column 22 for at least one access ladder 24 (see Figure 3) which may be affixed to the intermediate floors 20, and to allow service components to be installed or removed via a portable hoist system (not shown). The access ladder 24 may have an integral cable tray (not shown) for housing both power and communication cables. A suitable fall arrest system is preferably installed, and the intermediate floors may provide rest platforms for engineers, or be positioned to provide convenient access to components. An access door 9 may be provided at the base of the mast. Referring also to Figures 12 and 13, the shell 12 is formed by connecting elongate sheets 18 between adjacent upright members 14 and adjacent rings 16 so that the sheets 18 overlap both horizontally and vertically. The rings 16 may be formed of ring sections 17 extending between adjacent upright members 14, and connected to the upright members with brackets 19.

The sheets may be pressed into facets or may be rolled for either a faceted or round mast cross section. The sheets 18 are secured to the frame for example using either two piece rivets or nuts and bolts. As shown in Figures 13 and 13a, the lower edge 21 of each panel 18a, 18c overlaps the upper edge 23 of an adjacent panel 18b, 18d and covers the join. Similarly, one edge 25 of a back panel 18b butts up to the underside of the join with the adjacent edge 27 of the next panel 18d.The vertical overlap at the upright members may be weather proofed for example using a double strip of self-adhesive sealant, either side of the rivet line, to provide a long life weather shielding to protect internal componentry. The horizontal overlap should not require sealant if the overlap is sufficient to prevent water ingress. The external shell 12 is preferably manufactured from high tensile pre galvanised steel, which may have an additional plastic colour coating added as part of the manufacturing process at the steel mill. The use of this material provides both better weather protection for lightweight steel and a durable paint finish. A leathergrain embossed finish may also be used to create a matt effect on the surface which in various light conditions may reduce the reflectiveness of the surface. This can offer benefits to the local environment by reducing glare in high sunlight and making the structure less reflective in low light so as to reduce the attraction of insects and hence animals that feed on flying insects such as birds and bats. As the pre-coated steel is available in a range of colours the mast can be provided in either a solid colour or a mixture. This may be preferential regarding planning restrictions, as the need for structures to blend in with their surroundings can be of high importance in certain areas of natural beauty.

Referring to Figures 3 to 6, the upper mast section 6 comprises a tubular member 26 housing the drive shaft 28 for the turbine and housings for the two main shaft bearings. An upper bearing 1 1 accepts the main radial loads imposed by the mill system 1 and the lower bearing 42 a lower radial load transferred by wind loads from the mill and all the axial loads from the weight of the mill and drive shaft 28. Once fully assembled this arrangement is referred to as a 'hub'. The hub has upper and lower flanges 45, 47 which house the bearings (see also Figure 9). The lower 47 doubles as a means to connect the hub to the transition section 8. An intermediate flange 48 provides a connection to an upper lip on the transition section 8 as described below. The tapered semi-monocoque lower mast section 4 and the straight upper tubular section 6 are connected via the transition section 8 to accommodate the transfer of the associated wind loading forces from the upper mast through to the lower mast sections.

Referring in particular to Figure 6, the transition section 8 is substantially frustoconical in shape so as to match the diameters of the first and second sections 4, 6 at each end thereof. Thus at a first end 30, the transition section has a circular lip 32 which passes closely over the upper end of the lower mast section. At the lower end 30, the transition section 8 is provided with a an annular plate 38 extending inwardly for attachment to the topmost upright member 40 of the first section 4. Thus the tubular member 26 is retained in place in the centre of the mast structure.

The transition section 8 is provided with a gearbox mount for a gearbox 53 and a single piece low speed drive shaft. This arrangement of the gearbox and driveshaft couples the mill system directly to the gearbox, minimising moving parts and power transmission distance. A torque arm 64 and disc brake 66 are also mounted adjacent the gearbox. The gearbox is 53 is connected to a motor/generator 65 by a secondary drive shaft 44. Alternatively, as shown in Figure 7, a planetary gearbox 51 may be used.

The transition section 8 may be provided in two halves 55, 57 which may be secured together around the upper section 6. The transition section 8 may include apertures 59 for providing a ventilation zone and cooling of the components mounted within the mast.

Lightning conducting carbon pads 61 run in contact with a disc brake 63, and are spring loaded. Copper conductors (not shown) may be used to transmit current from the carbon pads 61 to the main earth conductor system to ground. The VAVVT may be designed as a low maintenance wind turbine, such that it is envisaged the gearbox and motor will only require annual or biannual service inspections and may only need to be replaced once in its expected life cycle. On units fitted with a direct drive motor 65 (see Figure 8) maintenance is further reduced. On this basis mounting these elements at high level is the optimum position considering the performance and costs associated with mounting it at the base of the turbine. For practical reasons the inverter/control equipment are likely to be mounted at ground level as this may require more rapid service access for example in the event of any software or electronic failures. The direct drive motor 65 may be mounted to an access floor 20 directly below the transition section, and connected to the main drive shaft 28 by a sliding Cardan shaft 67. The mast is mounted by means of a mounting frame 58 attached to the base of the lower section 4. The mounting frame comprises several radially extending legs 50,52, for example comprising box sections, joined together at the centre of the frame. For ease of construction, there may be two leg sections 50 extending across the diameter of the frame in a cross shape with one being cut and joined to the other at the centre, and four bracing sections 54 extending between the legs in a square shape to hold them in position and provide a base for mounting the remaining radial legs 52; in this case two legs 52 on each bracing section 54. The mounting frame 58 also comprises a substantially circular fixing bracket 73 arranged to correspond to the base ring 75 of the mast for attachment thereto.

Referring to Figures 10 and 1 1 , the construction of the fixing bracket 73 and base ring 75 is shown. The base ring comprises a plurality of side panel segments 77 joined by welding flanges 79 which connect the panels segments 77 to a corresponding plurality of base ring segments 80. These sit on top of further fixing bracket side segments 82 separated by sideplates 84 and joined to bracket base segments 86.

Each radial leg 50, 52 terminates in a foot 56, for example comprising a further short box section. The feet 56 are attached to footing pieces 58 arranged to be attached to a concrete foundation. In order to reduce the size of the concrete foundation, the feet 56 may extend radially outwardly from the mast base, providing a larger footprint which therefore requires a shallower foundation. The feet may be mounted perpendicular to the legs, or may be angled outwardly at an angle a, as shown in Figure 3.

Thus depending upon the dimensions of the mast and the mill assembly, the required 10 diameter of footing may be calculated, and a mounting frame may be supplied to the required diameter. This is readily achieved simply by manufacturing the radial legs with a greater length.

The first section of the mast may be formed of several modules so that the first section of the tower can be constructed in modular form, by placing the required number of modules one on top of another depending upon the required height of the mast. For example, the first section may be 20m high, and formed of ten such modules. Thus the internal framework 10 and external shell 12 may be assembled in to position using a central lifting point and specialist lifting jig working from the top to the bottom of the mast section.

As shown in Figures 14 to 16, to form the lower part of the lower section of the mast, a first subsection 90 is lifted onto a second section 92 using a jig 96. Then this is lifted onto a third subsection 98. The complete lower part 100 is then mounted onto a base ring 102 (see figures 14a to 14d). Subsequently, in a similar manner the top part of the lower mast section is constructed by lifting a top subsection 104 sequentially onto the next two subsections 106, 108, and these are then placed on top of the lower part 100 to complete the monocoque section (see Figures 15a to 15d).

As shown in Figure 16, the upper and transition sections 6, 8 of the mast may then be mounted on top of the monocoque section 4.

In an alternative embodiment shown in figure 17, the transition section 1 10 comprises a base plate 1 12 for connection to the lower mast section as before, but is elongated so as to extend further up the upper mast section 6, for connection to the upper flange 45 where the mast section meets the mounting plate 1 14 of mill system 1 by means of an upper connection lip 1 16. The transition section thus enclosed the upper mast section 6, and locates and supports the upper section 6 by means of bracing members 1 18.

The upper mast section including the transition section is clad in a lightweight, nonstructural galvanised and plastic coated steel skin. This eliminates the need to galvanise or paint the internal structural steel core elements. The skin also incorporates a ventilation zone which allows warm air generated by the internal equipment to vent freely using the stack effect. The zone includes measures to prevent birds and insects to access the interior of the mast. Air flowing into the mast can be controlled for example via thermostatically controlled louvers sited in the main access door at ground level. Forced ventilation may optionally be used in hot climates.

Importantly the taper of the mast sections is designed to provide improved airflow characteristics especially within the mill area where turbulence needs to be minimised and also provides radar mitigation by reflecting radar signals away from the source radar.

Referring now to Figures 19 to 19c, in another embodiment, in order to provide further rigidity to the tubular or upper tower or mast section 120, a plurality of high tension rigging rods 122 are employed, typically 8 in number. These are attached to a circular base plate 124 of the upper tower section 120 and to gusset attachment points 126 typically two-thirds of the distance up the height of the tower.

The upper tower section 120, when supported by the tensioned rods 122, is in general terms independent of the lower mast or tower section 128. Its triangulated shape comprising the stressed tie rods 122 provides real structural stiffness whilst minimising the effect on airflow in the upper tower area. The upper tower section 120 is over-clad with lightweight sheet steel. The cladding 130 is in sections and is formed into at least one and preferably several circular or multi-faceted cones. Importantly these cones are tapered at 2 degrees from the vertical plane to minimise the turbine's radar signature. These cladding panels may be galvanised and coated with coloured PVC to suit the lower tower colour. This system also provides an economic method of finishing the tower with the main structural elements beneath only requiring to be painted in a low cost primer paint system.

To provide additional stiffness to the tower a central shaft comprises vertical structural ribs 132 which are continued to ground level to take the forces from the upper tower via several diaphragm decks 134 directly into the mass concrete base via the base fixing ring 136. The decks may for example comprise channel sections 133 extending between the ribs 132. When further additional strength is required the central shaft can be braced with horizontal struts 145 midway between the diaphragm deck and cross braces 146. A further enhancement is to add struts 147 and cross braces 148 to link the central shaft to the vertical legs of the outer shell. Adding these final elements essentially has the same effect of hugely increasing the size of the vertical legs of the outer shell.

The central shaft comprising vertical structural ribs performs four main functions:- a) assisting in transmitting forces from the upper tower 120 via the lower tower 128 to ground level,

b) providing vertical support to the intermediate diaphragm decks 134,

c) providing additional overall support the tower in general,

d) when the additional braces are added to the central shaft and the outer shell it has the same effect of hugely increasing the size of the vertical legs of the outer shell, e) spreads the vertical loads of the gearbox and motor 138 housed at the top of the lower section 128. To provide support for the lightweight generally single skin lower tower, a plurality of diaphragm decks are installed. The number and location of decks within the tower can be varied to suit the structural loads required for a specific tower. To perform their design function effectively the diaphragm decks should be restricted from deflecting vertically in either direction. To prevent such flexing, the central shaft comprising vertical structural ribs, typically 8 in number, are fixed at every deck level and to the upper tower and to the base ring/concrete base 136. The design of the diaphragm decks 134 provides:- a) radial support for the internal leg members 140 of the tower to which the outer skin 138 is attached to prevent any buckling of the outer support shell or members, b) a central opening 142 of sufficient dimensions to allow the passage of large plant such as the gearbox, motor or brake assembly from the top of the tower to ground level if this needs to be achieved during the lifetime of the tower and access ladder and fall arrest system, and

c) access decking where access is required for either construction, maintenance or support for equipment. The effect of the central shaft and vertical structural ribs allows both tension and compression loads to be reduced in the outer ribs and outer shell of the tower, mainly in the lower areas close to the base. If overstressed, inward buckling of the lightweight shell 138 may result. Therefore on specific tower designs with higher than normal loads this effect can be negated by introducing an internal double skin fixed to the internal vertical ribs.

In the upper part of the lower mast section 128, a machinery platform 144 may also be provided for supporting the motor and gearbox.