WO2024074873A1

WO2024074873A1 - Improvements in and relating to wind turbines

Info

Publication number: WO2024074873A1
Application number: PCT/IB2022/059562
Authority: WO
Inventors: Michael Ian PLUMMER
Original assignee: Mlc Wind Turbine Ltd
Priority date: 2022-10-06
Filing date: 2022-10-06
Publication date: 2024-04-11

Abstract

A wind turbine (10) comprises a bladed rotor (11) mounted from a pylon (12) for rotation about a substantially vertical axis with its blades (18) extending substantially horizontally from the pylon (12). The bladed rotor (11) comprises a series of hubs (13) each having three blades (18) mutually separated by 120°. The bladed hubs (13) are rotationally locked together and are mounted for concurrent rotation about the pylon (12). The six (twelve) hubs (13) are split into two (four) groups of six hubs (13). The hubs (13) forming each group are mutually staggered by 40° so that their nine blades are staggered by 40° to smooth the torque applied to the group by the wind. The pylon (12) is stabilised by a bridge member (16) supported by a tower (19). A platform (23) is supported for vertical movement along a tower (21) carried between the bridge member (16) and a carriage (22) mounted on a rail (20). This carriage (22) also permits horizontal movement of the platform (23) thereby providing access to each blade for inspection, servicing or replacement.

Description

IMPROVEMENTS IN AND RELATING TO WIND TURBINES

Field of the Invention

This invention relates to wind turbines for generating power from an ambient air flow and also to turbine blades therefor.

Background of the Invention

Many wind turbines have been constructed by mounting a bladed rotor at the top of a pylon secured in the ground or seabed. Such bladed rotor wind turbines typically comprise two or three aerofoil blades secured to a hub mounted for rotation about a substantially horizontal axis from a housing or nacelle usually supported at the top of the pylon.

The housing contains a generator driven by the rotor through a gear train and is rotatably mounted from the pylon so that the rotational axis of the rotor can be orientated into the direction of the wind. In this manner, the wind impinges constantly on all the blades and, within operational limits, the rotor speed is typically maintained constant by adjusting the electrical load taken from the generator.

As well as the torque generated by the blades, they apply a very significant horizontal thrust to the top of the pylon which consequently has to be provided with deep foundations.

Other blade configurations have been proposed. For instance, a series of vertically extending blades supported by a rotor having upper and lower hubs mounted for rotation about the vertical axis of a pylon. Such designs are always orientated into the wind and therefore do not require the provision of any rotatable housing for adjusting the attitude of the rotor relative to the wind direction. However, this advantage is offset by the need to provide a second hub, and by the mechanical demands for supporting the blades from the ends of their hubs.

Prior Art

UK patent application number GB 2 386 161 (Atkinson Design Associates limited) discloses a rotor element which comprises a circular hub portion and a pair of blade segments which are attached to and extend from diametrically opposite positions of the hub portion. The blade segments are helical in cross-section and are curved longitudinally. Several rotor elements may be stacked on a drive shaft in order to form a vertical axis wind turbine.

UK patent application number GB 2 412 948 (Learmonth) discloses a generator comprising a plurality of vanes, which may be hemi-spherical, and arranged so as to form a double helix. Twelve vanes are angularly spaced at thirty degrees, and a generator is housed in a single structure containing a plurality of generators.

US patent US 7 008 171 (Circle Wind Corp Inc) discloses a type of Savonius rotor used as a wind turbine which has an exhaust channel in each vane. The vane of the modified Savonius rotor is formed into an "S" shape. The air that enters a given end of the vane exits that end through an exhaust channel into the free air stream. A plurality of modified Savonius rotors may be stacked one on top of another for self-starting and greater power output.

US patent US 4 245 958 (Ewers) discloses improvements in vertical axis wind turbines. The improved apparatus has a vertically rising exoskeletal frame which permits modular addition of supplemental turbine components to the apparatus which is self-supporting.

An axially disposed drive shaft and associated turbines have upward, and radially inwardly extending arms, which distribute the weight of the operating structure to the exoskeletal frame. Guide bearings permit the drive shaft to move vertically to accommodate longitudinal expansions of the frame.

US patent application US 2004/0047723 (Horjus) discloses a horizontal windmill coupled to an electrical generator. The windmill includes a vertical drive shaft mounted for rotation in a base, with a plurality of wind drive units being mounted in wind catching positions at spaced axial locations along the drive shaft. The drive units comprise oppositely facing wind catching elements. The horizontal windmill rotates the electrical generator within its desired speed range without any governor or speed control.

International patent application WO 2011/106932 (Hsueh-Pen LV) discloses a shaped fan blade combination for a windmill. A blade combination of upper and lower layers is arranged at intervals of 45 degrees which stabilises the rotation speed and a bearing seat has an automatic lubricant oil circulation system to protect the axles.

International patent application WO 2012/008862 (Telbit Phu) comprises a vertical axis rotor in which working blades are fastened to top and lower elements of a rotor. In a central part of the rotor there is a means for controlling airflow which improves the aerodynamic properties of the rotor.

International patent application WO 2014/181585 (Eco Technology Co ltd) describes a hybrid wind power generator that can increase the amount of power generated by a solar panel.

International patent application WO 2016/019466 (Church) discloses a fluidredirecting structure which includes a rigid body having an upstream end, a downstream end, and an axis of rotation. The rigid body incorporates a plurality of troughs each spiralling from a tip at the upstream end to the downstream end about the axis of rotation. Troughs are splayed with respect to the axis of rotation. A drawback that is associated with some of the aforementioned wind turbine systems is that they require routine maintenance which often entailed travelling to remote and inhospitable locations in order to repair and replace damaged or worn-out wind turbine blades.

The present invention arose in order to overcome the aforementioned problem.

Summary of the Invention

According to first aspect of the present invention there is provided a wind turbine comprising: a plurality of bladed rotors supported on hub rotationally mounted on a pylon, the hub rotates about a substantially vertical axis, the blades extending substantially horizontally from the hub, an upper end of the pylon is stabilised by a bridge member which extends over space swept by the blades and the bridge member is supported by a tower and the tower includes a storage bay for at least one replacement blade.

As an upper end of the pylon is stabilised by the bridge member extending across the space swept by the blades and supported by the tower the tower acts to stabilise the pylon. In some embodiments a rail may be arranged between the bases of the pylon and the bridge, and a service platform with an inspection cabin may be supported between the bridge member on a carriage mounted on the service rail.

In some embodiments of the invention an inspection carriage is mounted from the rail.

As the blades rotate in a horizontal plane, no rotatable housing is required for adjusting the attitude of the pylon relative to the wind direction, as the blades rotate around the pylon. In some embodiments of the invention the blades have a scooped profile. Ideally at least two blades are connected to a hub. Optionally at least two hubs are provided on a pylon, one above another.

In some embodiments of the invention tips of the blades are hooked.

In some embodiments of the invention at least two hubs are coaxial and adapted to lock with respect to an adjacent hub.

Blades ideally have an interconnect at their root which enables removable connection of a blade to a hub.

Preferably each blade has a channel section defining a gutter that is directed laterally to catch the wind and an opposed aerodynamically dimpled surface to reduce drag.

In some embodiments of the invention the tip end of each blade is laterally curved away from the direction of rotation.

Preferably each blade is at least 40 metres from root to tip, more preferably at least 50 metres from root to tip and most preferably at least 60 metres from root to tip.

In some embodiments of the invention sets of counter rotating blades may be mounted on the same pylon.

In some embodiments of the invention the tower includes storage bays for a replacement blade. Ideally the storage bays are mounted in a carrousel which is adapted to rotate. The wind turbine and tower may be located on land or a semi-submersible platform. The semi-submersible platform is anchored to the seabed by way of cables or piers or tethers.

In some embodiments of the invention the hubs are supported on a tubular former which is capable of rotation in a race of ball bearings coated in lubricant.

Preferably at least two drive trains are provided: the first is preferably located below a top set of blades. The second drive train is preferably located at or towards a lower region of the pylon. A reason for this is that should the first drive train , or any of its power generating equipment, gearing or drive system, fail the second drive train continues to operate in order to provide a power output.

Preferably, the bladed rotor comprises a series of bladed hubs that are rotationally locked together and coaxially mounted from the pylon with their outer ends staggered to smooth the torque applied by the wind to the rotor.

Each blade is preferably of a channel section defining a gutter that is directed laterally to catch the wind and an opposed aerodynamically dimpled surface to reduce drag.

Preferably, the tip end of each blade is laterally curved away from the direction of rotation.

According to another aspect of the invention a vertical axis wind turbine has at least two sets of counter rotating blades that are connected to separate hubs, each of the blades having a root that is to be secured to a rotor for rotation about a substantially vertical axis, each of the blades being in the form of a channel section defining a gutter that is directed laterally to catch wind and an opposed aerodynamically dimpled surface to reduce drag. Optionally the tip of the blade is laterally curved away from the intended direction of rotation and has a scooped profile.

In some embodiments the tips of the blades are hooked.

In some embodiments the at least two hubs are coaxial and adapted to lock with respect to an adjacent hub.

In some embodiments the, or each, blade is at least 40 metres, preferably at least 60 metres, from root to tip.

In some embodiments the blade has a root with a securement means to be secured to a rotor for rotation about a substantially vertical axis, the blade being in the form of a channel section defining a gutter that is directed laterally to catch incident wind and has an opposed aerodynamically dimpled surface to reduce drag.

Preferably the tip of the blade is laterally curved away from a direction of rotation.

Ideally the tip of the blade is laterally curved away from the intended direction of rotation.

According to a yet further aspect of the invention a wind turbine blade has a root to be secured to a rotor for rotation about a substantially vertical axis, the blade being in the form of a channel section defining a gutter that is directed laterally to catch wind and an opposed aerodynamically dimpled surface to reduce drag.

Preferably the tip of the blade is laterally curved away from an intended direction of rotation. Ideally blades are detachable individually. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Brief Description of the Drawings

Figure 1 is an overall view of one form of wind turbine in accordance with this invention;

Figure 2 is a plan view of one of the bladed hubs depicted in Figure 1 ;

Figure 3 is a transverse section taken through one of the blades close to its root as seen along the line 3 - 3 in Figure 2;

Figure 4 is a transverse section taken through one of the blades near its tip as seen along the line 4 - 4 in Figure 2;

Figure 5 is a view of a wind turbine with carousel which houses at least one spare blade;

Figure 6a is a plan view of three sets of bladed rotor depicted in Figure 1 ;

Figure 6b is a plan view the three sets of rotors stacked and shows the staggering of the blade tips of the bladed hubs;

Figures 7a and 7b show, in exploded diagrammatical form an example of a blade engagement mechanism;

Figure 7c shows, in diagrammatical form, successive views of how the blade engagement mechanism interconnects with a support ring; Figures 7d and 7e show operation of the spring mechanism as part of the blade locking system;

Figures 8, 9 and 10 show various views of a multi blade wind turbine connected by a bridge member to a storage pylon;

Figure 11 is a diagrammatical sectional view showing a ring of enclosed ball bearings in a support structure;

Figure 12 is a cross section through line A-A of Figure 11 ;

Figure 13 is a cross section of a blade holder which is used to remove, extract a damaged blade and fit a new blade;

Figures 14 and 15 are overall views of an example of a floating semisubmersible platform/structure which may be attached to the seabed with tethers; and

Figures 16A, 16B and 16C show sectional views through examples of a wind turbine blade.

Detailed Description of the Illustrated embodiment

With reference to Figure 1 , there is shown a wind turbine 10 comprising a bladed rotor 1 1 mounted from a pylon 12 of which the unshown lower end is carried by a deep foundation set into the ground or seabed. The bladed rotor 11 comprises a vertical series of three bladed hubs 13 that are rotationally locked together and are supported for rotation about the substantially vertical axis of the pylon 12 by thrust and journal bearings (not shown). There are six of the bladed hubs 13 arranged in two groups of three stacked, and offset, one group on another. Optionally groups of blades may be separated by spacers 14, as shown in Figure 1 .

Alternatively groups of blades (as shown in Figure 8) may be stacked directly one on another. Ideally groups of blades are staggered by an equal angular offset.

The upper end of the pylon 12 terminates in a conical casing 15 which is stabilised by a bridge member 16 extending over a space 17 swept by the blades 18 of the three bladed hubs 13. The other end of the bridge member 16 is supported by a tower 19 having its lower end carried by an appropriate foundation also set in the ground or seabed.

A rail 20 extends (as shown in Figure 1 ) between the bases of the pylon and is positioned approximately !4 way of the height of the pylon 12 and tower 19. A service tower 21 has its lower end carried by a carriage 22, the upper end of the service tower 21 is slidable along the bridge member 16. In this manner, a vertically moveable platform 23 can be moved vertically up or down the inspection/service tower 21 to the level of any of the three bladed hubs 13.

When the bladed rotor 11 is locked against rotation, the moveable platform 23 can be slid horizontally towards the pylon 12 to allow inspection, servicing or replacement of a blade 18.

Replacement of blades is described in greater detail below with reference to Figures 7 and storage carousel is supported in the service tower. This is shown for example in Figure 5 which indicates the location of the carousel 24 which houses at least one spare blade.

Figures 2, 3 and 4 show the configuration of one of the three bladed hubs 13.

Three identical blades 18 have their respective roots 30 secured in any convenient manner to a common hub 31 so that the angle between adjacent blades is 120°. If a four bladed hub were to be used, this angle would be 90°. Adjacent hubs may be offset.

Each of the blades 18 is formed as a channel section, as seen in Figures 3 and 4, which defines a gutter 32 that is directed laterally to catch the wind. Each gutter 32 consequently faces horizontally whilst extending laterally along its blade 18. Each channel section also defines an aerodynamically dimpled surface 33 that is opposite to its gutter 32. This aerodynamic dimpled surface 33 reduces drag, particularly when the blade 18 rotates progressively into wind.

From Figures 3 and 4 it will be noted that each blade 18 tapers towards its tip and, from Figure 2, that each blade 18 has an integral tip 34 laterally curved away from the direction of rotation R. These curved tips 34 inhibit wind from “spilling” radially from the end of the gutter 32 as the blades 18 turn past a position normal to the air flow and increase the force generated by the wind on the blades 18.

Figure 5 shows the relative rotational positions of three bladed hubs 13a, 13b and 13c which are rotationally locked together to form part of the bladed rotor 11. These hubs are mutually staggered 40° out of phase to smooth the torque applied by the wind to the bladed rotor 11 .

As already described with reference to Figure 1 , the bladed rotor 11 comprises six bladed hubs 13 divided into 2 groups of three sets of blades each arranged at 120° one from another. Additional torque smoothing can be achieved by mutually staggering each group of blades from one another, for example by 90°. Figure 6 shows the relative blade positions of the three blade hubs 13a, 13b and 13c shown in Figure 5. It will be noted that the nine blades are mutually staggered by 40°.

In the embodiment of the wind turbine shown in Figures 8 and 9 there are three blades on each rotor and ideally three rotors arranged in a set. Advantageously four sets of blades are arranged on each wind turbine. Each set is offset from an adjacent set by 10°, with the result that a complete generator, as depicted for example in Figures 9 and 10, comprises 36 blades staggered in a helical configuration.

A practical advantage of the embodiments shown in Figures 8 to 10, is that torque smoothing occurs as at almost any instant one of the blades is presented at an ideal or optimum angle to the impinging wind. This feature helps to avoid vibrations and provides for more efficient energy conversion.

It is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the description or illustrated in the drawings. For example, smaller versions of the invention may be fitted to boats, caravans or houses.

Referring briefly to Figures 7a, 7b and 7c there is shown in diagrammatical form, rotor hub 82 a hollow right circular cylinder and is intended to represent a portion of the pylon 12. A cylindrical portion is housed within the hollow right circular cylindrical bush and has slots 85 formed therein. The slots 85 are adapted to receive keys 83 and 86 (Figures 7a and 7b) formed integrally with the turbine blades 18.

In use therefore a damaged or faulty turbine blade 18 is able to be removed by lifting and translating or by lifting and twisting or otherwise displacing the blade with respect to the support so as to release a turbine blade from the pylon. The five views in Figures 7a to 7c show how a keyed blade 18 (blade not shown) is introduced into the rotor hub 82 (view 1), located therein in an axial direction (view 2), abuts an end stop (view 3), turns through 45° (view 4) and then through a further 45° (view 5) so as to reach an end lock point and to be locked therein. Blade is then held in place by large spring 81 at base of rotor hub Figures 7d to 7e.

Personnel (not shown) carrying out this process are aided by way of the moveable platform 23 which is able to raise them to the desirable height in order to gain access to a specific blade 18.

It is apparent that an advantage of this locking mechanism, when applied to the particular stacked arrangement of horizontally disposed blades, is that it no longer is necessary to remove all blades above or below a faulty or damaged blade for repair or replacement. Instead, only a specific blade has to be removed, which is easier and therefore cheaper to do and results in less down time of the wind turbine.

It will also be appreciated that the invention is capable of other embodiments and of being practised and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

Solar panels may be attached to or formed integrally with the surface of the support pylon or tower, or any other part of the structure which is able to receive sunlight. The outer surfaces of the entire assembly may be coated with photo voltaic cell material for additional energy production.

Referring now to Figures 7a, 7b and 7c views depict the principle of connecting a turbine blade to a hub. Examples of the hub are shown in Figures 7b and the interconnect key Figure at 7a the principle of connecting and locking a hub onto a pylon is the pictured in Figures 7c in a series of stages a to e (Figure 7d and 7e) in which the keyed portions inter-connect and enable interlocking so as to prevent blades from inadvertent dislocation from the turbine.

Figure 8 is a view depicting the location of blades a lift arrangement with an inspection and a rail which connects the tower 19 to the pylon Figures 9 and 10 show alternative views illustrating the relationship between the inspection gantry, with an optional safety rail (not shown), and the pylons and how a platform is provided onto which pylon can be placed and then lowered into a storage facility shown in the tower 19 in Figure 5.

Figures 11 and 12 show views of the principle of a rotating turbine supported on a series of steel bearings encased in a grease covered ball race. Figure 12 is a section showing the engagement of a base support and the rotating turbine pylon. Figure 13 shows a view of the mould which supports a replacement blade 11 which is subsequently stored in a carousel as described below.

Figures 14 and 15 show examples of a support on which platform the pylon tower and bridge structure are mounted for offshore floated deployment which are fixed to the seabed, for example by tethers or cables (not shown).

The structure is ideally formed from large tubular steel welded components which are familiar to the offshore drilling and oil and gas industry.

The invention has been described by way of examples and it will be appreciated that the offshore floated systems may support the combined tower (with storage facility, bridge and pylon, for example of the type shown in Figure 5, 9 to 14. A light and/or radar warning beacon or radar reflector may be mounted at or near the apex of the structure so as to warn aircraft. An access door leads to a lift or staircase (not shown) that transports technicians to the inspection platform and upper drive train.

Support tower has a storage sleeve around it for storing at least one and preferably two or three spare blades and an empty space for faulty blade storage. The sleeve turns around in order to bring spare blade into position with the inspection and maintenance platform. Each blade is suspended in a vertical position with the tip of the blade at the top and the base at the bottom in order to optimise its efficiency.

Blades are typically suspended at an angle sloping inwards. When a new blade needs replacing the following procedure takes place:

1. The old blade is removed by positioning the platform blade mould into the blade and securing it into position then using a mechanical plunger that pushes the blade in then turns 90 degrees to release it. The blade is then withdrawn by reversing the platform backwards, it then swivels 180 degrees and moves towards the support tower.

2. The support tower sleeve is turned around until the empty space for storing faulty blades is in position.

3. The platform blade mould with blade is then raised at tip end and moved forward till faulty blade rests in blade mould within the support tower sleeve the support tower sleeve then moves back into the storage sleeve. Platform blade mould moves back.

4. Storage sleeve moves around till spare blade is in position by platform. Replacement blade moves out away from support sleeve. 5 Platform blade mould still in vertical position moves in to support replacement blade.

6 Base of storage sleeve slowly moves in till blade is vertical then continues till tip of blade is now tilted out, blade moves from resting in storage sleeve to resting in platform blade mould. The mould then slowly lowers back into a horizontal position as shown in Figure 13.

The mould with blade then swivels 180 degrees and moves towards tower using mechanical plunger the blade is pushed into the rotor hub turned 90 degrees and then releases blade which remains secured in position due to spring inside hub

7. Finally the platform reverses back into position.

Figures 1 , 5, 8, 9 and 10 show various views of a multi blade wind turbine connected by a bridge member to a storage pylon.

Figure 11 is a diagrammatical cross-sectional view showing a ring of enclosed ball bearing in a support structure.

Figure 12 is a cross section through line A-A of Figure 11. Figure 13 is a cross section view of a platform blade mould with replacement blade.

Figures 14 and 15 are overall diagrammatic views of an offshore semisubmersible support structure and a lozenge shaped support area on which the wind turbine sits. The offshore semi-submersible support structure 200 has a lozenge shaped support area 210 on which the wind turbine (not shown) is supported.

The offshore semi-submersible floating multi-level co-axial vertical axis wind turbine (VAWT). The multi-layered co-axial vertical axis wind turbine ensures maximum use of available wind by allowing a large surface area of blades to come into contact with the wind at all times.

The ML VAWT works on a series of 3 levels of blade at the top of the wind turbine turning clockwise. Each level has three blades set at an angle of 120 degrees apart, with the middle level being offset to the bottom level by 40 degrees, and upper level being offset by 40 degree to the middle level and 80 degrees to the bottom level. This means that the series has a blade every 40 degrees.

This helps to ensure the immediate maximum utilisation of any available wind from any direction. The docking hub base that the blades fit into at the top of the tower is secured to the tower as shown in Figure 12 which shows the base of docking hub 95 standing on a large number of ball bearings 90 which are sitting in a bed of lubricant to minimise friction and ensure the hub 95 turns freely.

Bottom level blades are positioned at 0° 120° 240° Middle level blades are positioned at 40° 160° 280° Top level blades are positioned at 80° 200° 320°

Referring to Figures 16A, 16B and 16C, the front edge of the blades is shaped to ensure that they trap the wind and do not release it until it has fully utilised its energy creating potential. Whilst the reverse side of the blade is so shaped with dimples to minimise drag as it turns into the wind on its way round.

The top drive train houses the generator, gear systems and control equipment (not shown). The drive train is incorporated within the pylon below the blades.

Below the nacelle is located another set of 3 levels of blade this time turning anti-clockwise. The generator etc for this set of blades is situated within the base of the tower. Each blade is connected to the tower by pushing in the blade into its spring- loaded hub docking pod turning it 90 degrees and releasing so that it is partly pushed out and locked into position by the spring. The blades can be inspected by utilising an inspection platform which is connected to the VAWT.

The VAWT is located on a semi-submersible platform which is anchored to the seabed by a number of cables out at sea.

It is also understood that the above description is not intended to be limiting in that the optimum dimensional relationships for the parts of the invention, may include variations in size, materials, shape, form, function and manner of operation, assembly and use; and all are deemed readily apparent to one skilled in the art.

Likewise, all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention as defined in the claims.

Claims

1 . A wind turbine comprising: a plurality of bladed rotors supported on hub rotationally mounted on a pylon, the hub rotates about a substantially vertical axis, each blade extends substantially horizontally from the hub, an upper end of the pylon is stabilised by a bridge member which extends over space swept by the blades; and the bridge member is supported by a tower and the tower includes a storage bay for at least one replacement blade.

2. A wind turbine according to claim 1 , in which a rail is provided between the pylon and the tower, and a carriage is mounted from and moveable along the rail.

3. A wind turbine according to claim 1 or 2 wherein the tower includes at least one storage bay for storing at least one replacement blade.

4. A wind turbine according to claim 3 wherein the at least one storage bay is mounted in a carrousel which is operative to rotate.

5. A wind turbine according to any preceding claim wherein blades have a scooped profile.

6. A wind turbine according to any preceding claim, in which each blade has an opposed aerodynamically smoothed surface to reduce drag.

7. A wind turbine according to any preceding claim wherein at least two blades are connected to a hub.

8. A wind turbine according to any preceding claim wherein tips of the blades are hooked. A wind turbine according to any preceding claim wherein at least two hubs are provided on a pylon. A wind turbine according to claim 9 wherein the at least two hubs are coaxial and adapted to lock with respect to an adjacent hub. A wind turbine according to claim 8 wherein hubs are counter-rotating. A wind turbine according to any preceding claim, in which each blade is at least 40 metres from root to tip. A wind turbine according to any preceding claim wherein the wind turbine and tower are located on a semi-submersible platform. A wind turbine according to claim 13 wherein the semi-submersible platform is anchored to the seabed by a plurality of tethers or cables. A wind turbine according to any preceding claim wherein hubs are supported on a tubular former which rotates in a race of ball bearings in lubricant. A wind turbine according to any preceding claim wherein two drive trains are provided. A wind turbine blade wherein at least two sets of counter rotating blades are connected to separate hubs, each of the blades having a root that is secured to a rotor for rotation about a substantially vertical axis, each of the blades being in the form of a channel section defining a gutter that is directed laterally to catch wind and an opposed aerodynamically dimpled surface to reduce drag. A wind turbine blade according to claim 17, in which the tip of the blade is laterally curved away from the intended direction of rotation and has a scooped profile. A wind turbine according to any preceding claim wherein tips of the blades are hooked. A wind turbine according to any preceding claim wherein blades have an interconnect at their root which enables removable connection of a blade to a hub. A wind turbine according to any preceding claim, in which each blade is at least 20 metres from root to tip.