Background of the Invention Conventional horizontal axis wind turbines have one or more rotor blades which rotate about an axis substantially parallel to the wind direction. The turbine is typically connected to a mast via a yaw bearing and the turbine is rotatable about the mast so that it may be oriented correctly with respect to the wind direction. Typically, a fin or like structure is provided downwind to ensure that the rotors are upwind of the mast.

The force exerted by a wind stream upon a wind turbine increases with the square of the wind speed. A challenge in wind turbine design is limiting the thrust and torque of the rotor blades towards the higher end of the design wind speed range. Fatigue considerations are an important factor when sizing horizontal axis wind turbines. Good designs aim to limit the maximum bending and torsional cyclic stresses.

A primary design goal in designing a wind turbine is to maximise economic performance in the conversion of wind energy to extracted power.

A common horizontal axis wind turbine design utilises a pair of opposed rotor blades which maintain a fixed relationship to the axis of rotation.

Whilst this design is relatively simple to implement the design is subject to changes in yaw axis rotational inertia as the spinning rotor blades move about the yaw axis. Horizontal axis wind turbines with three or more blades behave better when yawing, as the yaw axis rotational inertia does not change significantly as the spinning rotor moves around the yaw axis.

However, multiple blade wind turbines limit the range of rotational speeds that may be utilised by the designer. Such designs are also subject to high wind loadings in strong winds. Rotors with fewer blades give the designer

the freedom to increase the rotational speed or increase the blade chord to obtain the optimum axial interface factor or blocking ratio. Increased rotational speed provides a better match between rotor rotational speed and the desired rotational speed of the generator. Increasing the blade chord benefits the airfoil Reynolds Number.

Wind turbines having a single rotor blade and a single opposing counter weight have been used. In such designs the rotor blade is disposed at a substantially fixed angle with respect to the drive shaft. This design suffers from changes in yaw axis rotational inertia as the spinning rotor moves around the yaw axis. The rotor blade is also exposed to high ladings in high winds and it can be difficult to dynamically balance generator load with turbine output.

A number of techniques have been employed to balance turbine output to a generator load. Rotor blades may be feathered to match the aero- dynamic lift generated by the rotors to the generator load. It can be difficult to dynamically adjust feathering, especially during wind gusts, and the support structure may still be subject to considerable wind loading during gusts. The load applied to the generator may also be adjusted.

However, as the blades are fixed this can place considerable strain upon the wind turbine in high winds.

US4,517, 467 discloses a single rotor blade wind turbine utilising a single counter weight. Blade pitch control is utilised to feather the rotor blade to match rotor output with generator'requirements. The rotor blade is swingable over a narrow angular range so that a blade can be kept substantially perpendicular to wind directions deviating slightly from the horizontal. Blade feathering is the technique used for load balancing.

It is an object of the present invention to provide a wind turbine which overcomes the prior art disadvantage or which at least provides the public with a useful choice.

According to a first aspect of the invention there is provided a wind turbine including :.

a generator; a rotor coupled to the generator ; and a plurality of weights distributed about the axis of rotation of the rotor to balance centripetal forces generated by the rotor in use.

Preferably two weights are provided disposed about 120 degrees apart from each other and the rotor. The rotor is preferably tiltable between 0 to about 90 degrees. Control means are preferably provided to control the load applied to the generator to balance centripetal forces and lift and drag generated by the rotor.

According to a further aspect of the invention there is provided a wind turbine including: a generator; a rotor coupled to the generator via a drive coupling wherein the rotor is tiltable with respect to the drive coupling over a sufficient range to match the power supplied by the rotor to the torque required by the generator.

Each rotor is preferably tiltable by in excess of 30 degrees and preferably between 0 to about 90 degrees. Preferably a single rotor is employed having multiple opposed counterweights disposed at about 120 degrees apart from each other and the rotor. Control means are preferably provided to control the load applied to the generator to balance centripetal forces and lift and drag generated by the rotor. Adjustable control surfaces, such as ailerons, blade spoilers, flaps, pitch changing blades or dampers, may be employed to maintain a desired degree of teeter of the blade.

According to a further aspect of the invention there is provided a wind turbine including: A generator; one or more rotor coupled to the generator via a drive coupling wherein each rotor is tiltable with respect to the drive coupling; and control means for varying, in use, the load applied to the generator so as to adjust the balance between centripetal force and lift and drag for

each rotor to adjust the angle of tilt of each rotor.

The control means preferably controls the load applied to the generator to match the power output of the one or more rotor to the power requirements of the generator. The control means preferably maintain the angle of tilt of the rotor at a normal optimum value up to the maximum allowed generator shaft speed.

The control means preferably controls the load applied to the generator to maintain substantially constant generator shaft speed when maximum generator shaft speed has been attained.

Each rotor is preferably tiltable in excess of 30 degrees, more preferably in excess of 60° and more preferably between 0 to about 90 degrees. A single rotor having multiple counterweights disposed at about 120 degrees spacing is preferably employed. The control means preferably controls the load applied to the generator to balance centripetal forces and lift and drag generated by the rotor.

According to a further aspect of the invention there is provided a rotor assembly comprising ; a rotor; and a plurality of weights connected to the rotor and distributed about an axis of rotation of the rotor assembly to balance centripetal forces of the rotor and the weights.

In each aspect the rotor is preferably provided downwind of the generator.

The generator may be a pump, electrical generator or the like. Where an electrical generator is employed, a permanent magnet generator is preferably utilised and directly driven by the rotor.

Brief Description of the Drawings The invention will now be described by way of reference to the accompanying drawings in which:

Figure 1 shows a side view of a wind turbine according to one embodiment of the present invention.

Figure 2 shows a cross sectional view (along the line B-B) of the rotor and counterweights of the wind turbine shown in Figure 1.

Figure 3 shows a rear view of the wind turbine of Figures 1 and 2.

Figure 4 shows a cross sectional view through line C-C of the wind turbine shown in Figure 1.

Figure 5 shows a front view of the wind turbine shown in Figures 1-4.

Figure 6 is a side cross sectional view along line A-A shown in Figure 5.

Figure 7 shows the range of tilt of the blade of the wind turbine of figures 1 to 6.

Figure 8 is a block diagram of an electrical generator and its associated control means.

Figure 9 shows a blade having a spoiler mounted upon it.

Figure 10 shows a blade having an aileron.

Figure 11 shows a blade having a damper.

Detailed Description of Best Mode for Carrying out the Invention An exemplary wind turbine will now be described with reference to Figures 1-7. Wind turbine 1 is rotatably mounted about mast 2. The turbine housing 3 consists of an airfoil section 4 and a generally conical section 5.

A drive coupling in the form of drive shaft 7 is directly connected from generator 6 to rotor blade 8. Although in this example the generator is a permanent magnet generator, it will appreciated that other of types of

generators could be employed in conjunction with suitable gearing.

Alternatively, the generator could be a fluid pump or other like device for utilising the torque generated by the rotor. A direct drive permanent magnet generator is particularly preferred as it may be designed to operate over a desired range of rotational shaft speeds without requiring gearing.

Rotor blade 8 has a pair of counterweights 9 and 10 attached thereto via shafts 11 and 12. The centre line of rotor blade 8 and the centre lines of shafts 11 and 12 are preferably displaced from each other by about 120 degrees in the plane of rotation of the rotor blade. Counterweights 9 and 10 serve to balance rotational inertia as the wind turbine yaws about mast 2. The assembly of rotor blade 8, shafts 11 and 12 and counterweights 9 and 10 is tiltable with respect to shaft 7 about a teeter bearing 13.

It will be appreciated that more than two counterweights may be employed.

Two counterweights are preferred as this provides the desired weight distribution with minimum complexity. Other configurations providing balanced operation may be employed.

In use the wind turbine is oriented so that rotor blade 8 is downwind of body 3. Teeter bearing 13 permits rotor blade 8 to tilt with respect to shaft 7 over a range of angles (preferably from about 0 degrees to about 90 degrees) with respect to vertical as illustrated in figure 7. When the rotor blade 8 is tilted by 90° it is horizontal and is exposed to minimum wind loading.

Cowling 14 is provided with a slot 15 to accommodate the teetering movement of the rotor blade 8 and slots 16 to accommodate the teetering moving of shafts 11 and 12.

In use wind turbine 1 is rotatable about mast 2 and so rotor blade 8 automatically orientates to a downwind position. If a low load is applied to generator 6 rotor blade 8 rotates relatively freely. The rotation of rotor blade 8 gives rise to centripetal forces tending to force rotor blade 8 outwardly. The aerodynamic force generated by rotor blade 8 in a wind

stream tends to force rotor blade 8 away from mast 2.

Rotor blade 8 and weights 9 and 10 lie in a common plane and so as blade 8 is forced away from mast 2 rotor blade 8 adopts a conical pattern of rotation. As the path of rotation of blade 8 becomes increasingly coned, less wind force from the wind stream is captured by rotor blade 8.

Accordingly, by controlling the balance between centripetal force and aerodynamic lift and drag generated by blade 8 a desired degree of rotor blade coning may be achieved. In use, this balance between centripetal force and aerodynamic lift and drag may be achieved by adjusting the load applied to generator 6. When a low load is applied, rotor blade 8 will describe a relatively flat conical path. As the load applied by generator 6 increases the path described by rotor blade 8 will become increasingly coned. The increased load decreases the speed of rotation of rotor blade 8 and thus the centripetal force. This changes the balance between aerodynamic lift and drag and centripetal force to increase rotor blade coning.

Control of the rate of change of the blade coning angle is preferably maintained by the aerodynamic damping of the blade associated with the rate of change of the cone angle. Additional damping may be introduced to maintain smooth running of the blade at the desired cone angle by any of the following devices, singly or in combination: blade damper, aileron, flap, spoiler, blade pitch change mechanism. Figure 9 shows a blade 20 having a spoiler 21 mounted on its surface. Figure 10 shows a blade 22 having an aileron 23 which may be adjusted via suitable control means.

Figure 11 shows a blade 24 having a damper 25.

Generator 6 is preferably a permanent magnet generator. This enables the number of poles to be matched to the operating shaft rotational speeds of the wind turbine to achieve a desired match. Directly driving generator 6 avoids losses incurred in gearing arrangements.

Generator 6 will typically be utilised to charge storage batteries (or other power sink: AC power system, resistance heater, hydrogen generator,

flywheel energy storage device etc. ). The control means may adjust the load applied to generator 6 so that it generates the optimum power for the given wind conditions. As the incident wind stream increases, the variable load applied to the generator may be matched to the turbine output until the rated maximum shaft rotational speed is reached. From this point, the rotor blade may be flown by controlling the external load to stall and cone the rotor blade to limit the torque delivered. When a maximum desired wind speed is reached, the machine may be shut down and the rotor blade may be parked near its maximum coned position to minimise the static aerodynamic force on the wind turbine and mast. Parking the rotor blade may be achieved by shorting the windings of the generator then braking the rotor at a desired position. The blade may then simply pivot about the teeter bearing. The parked blade may be locked at the maximum cone angle in high winds.

Figure 8 shows a schematic diagram of a possible control means for connection between the generator and a power system. Controller 30 is connected to an interface 33 which receives user input and displays status information. The user may set operational parameters and control strategies etc via interface 33. Controller 30 may interface to external apparatus to provide information will receive control commands via external communications unit 34.

Controller 30 receives information from angle sensor 31 as to the degree of tilt of blade 8 and information from frequency shaping unit 32 as the angular velocity of the rotor. Controller 30 also receives information from current sensor 38 as to the output power from generator 6.

Generator 6, in this case an alternator, may deliver power to either dump load 39 or output 40. Dump load control 35 is controlled by controller 30 to connect the output of alternator 6 to dump load 39. Dump load 39 may be a hot water heater or other load. Voltage control/rectifier 36 is controlled by controller 30 to connect the output of alternator 6 to output 40. Output 40 may supply storage batteries, an electricity network etc.

Starting circuit 37 is controlled by controller 30 to supply a pulse width

modulated voltage waveform to alternator 6 to effect starting in low wind conditions. An external electrical supply 41 is utilised to effect starting.

The external supply 41 also supplies power to controller 30. External supply 41 may be storage batteries or an electrical network.

In standby mode, the rotor teeter angle sensor 30 is used to measure the rotor teeter angle, which can be used by controller 30 to infer the wind speed. The rotor blade 8 is provided with some stiffness in rotation about the teeter bearing to cause the blade to rest in a vertical plane at zero wind speed. In marginal conditions below the natural starting wind speed for the rotor, controller 30 may start the windmill from external supply 41 using starting circuit 37.

Once rotor 8 is rotating at an angular velocity where sufficient positive torque is generated, controller 30 may disconnect the starting circuit and allow the rotor to accelerate to the optimum rotational speed for the wind speed. This is again inferred from the teeter angle. There is a relationship between the lift and drag on the blade, the wind speed, the rotational speed and the centripetal reactions on the blade and counterweight masses that resolves the teeter angle for the optimum blade running condition. The controller may increase or reduce the electrical load on the alternator using the teeter angle as the controlled variable. Up to the rated output of the turbine, the rotational speed at which the optimum teeter angle is achieved can be used to determine the wind speed.

As the'wind speed and rotor angular velocity approach the values corresponding to the maximum rated power of the turbine, the controller will act to limit the increase of rotor angular velocity. The increasing aerodynamic lift and drag on the blade as the wind velocity increases and the airfoil angle of attack increases will cause the teeter angle to increase from the optimum which progressively reduces the projected swept area of the blade rotor. As this process develops with increased wind speed, the controller continues to set the rotor rotational speed to achieve a predetermined teeter angle (these may be derived from testing using a parameter mapping process) and keep the alternator within its rated power. The blade progressively stalls from the hub towards the tip with

increasing section angles of attack until stable running of the coned rotor is no longer possible, and the controller shuts the turbine down to await less extreme wind conditions. The controller will continue to monitor wind strength, and may restart the turbine if wind conditions abate.

Control strategies may also be employed which are dependent upon the stress to which the turbine is exposed. The controller may determine the wind velocity and degree of gustiness from the angular velocity and blade angle and the variations and rates of variation of these parameters. An estimate of the stress state in the blade may be calculated from these parameters at all times while running. The controller may apply different control strategies for the turbine for different sets of conditions. For example, in a situation where the primary energy store (e. g. a battery) is not fully charged the controller may adopt an energy capture maximisation approach, where it continues running at relatively high stress. If the battery is fully charged, and the machine is supplying power to a secondary store, such as the hot water dump load, it may be programmed to change to a life maximisation strategy where it shuts down and waits for less gusty or lower velocity conditions before restarting. A measure of damage for the machine may be obtained based on historical measurements of these parameters. This may be used for schduling inspection once a safe percentage of the design fatigue life is used.

The wind turbine of the invention provides a design that automatically adjusts the configuration of the rotor to match power demand of the generator. Excess wind may be spilled by automatic adjustment of the degree of coning of the blade.

The provision of a pair of counterweights equally spaced from the rotor results in only small changes in rotational inertia as the wind turbine yaws about its axis of rotation. As well as reducing the stress applied to the wind turbine, the construction minimises the force applied to the structure supporting the wind turbine also.

The use of the single blade enables higher rotational blade speeds to be employed, providing better matching between the generator and rotor

blade. Use of a single blade also allows the blade chord to be increased, benefiting the airfoil Reynolds number.

Where in the foregoing description reference has been made to integers or components having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention.