Wind turbines have become an accepted means of converting the energy of the winds to electricity and as such they provide an important contribution to the harnessing of renewable energy. However, present designs of these devices do suffer from notable problems, among them being the internal mechanisms used for converting the harnessed wind energy to electrical power. The present invention relates to improved energy conversion means or an energy converter for use within such turbines.

Energy conversion means as typically used within existing wind turbines comprises a gearbox, driven by the main axle to which the turbine blades are attached, which is used to step up the relatively slow but high torque rotational motion of the turbine blades to a sufficiently high speed to drive a conventional rotary generator. For example, the blades may rotate at just fifteen to twenty revolutions per minute, whereas the generator may require to be driven at a speed of three thousand revolutions per minute to generate effectively both at the required frequency, voltage and power output.

Normally, both the gearbox and the rotary generator are each housed in a nacelle mounted at the top of the turbine supporting tower. This presents a number of disadvantages. In the first instance, noise emanating from the rotary gearbox and generator may be offensive to nearby inhabitants. Secondly, it is not unknown for such gearboxes to fail or wear out. Where turbines have been located in difficult terrains, repair of these items may present insuperable difficulties, as well as great expense. In addition, it will be appreciated that the electrical power generated must be passed through commutators to reach ground level, in order to accommodate the rotational movement of the nacelle necessary to ensure the turbine blades are facing the prevailing direction of the winds acting thereupon.

According to the invention, energy conversion means for wind turbines comprises one or more rotary to linear motion conversion mechanisms, driven by the rotary motion of the main axle to which the turbine blades are attached, the linear motion obtained therefrom being so mechanically coupled to one or more linear generators as to cause relative reciprocal motion between the translator(s) and stator(s) thereof.

Electricity is thus generated as caused by the relative motion of the stator and translator parts of the linear generators.

It will be appreciated that the generators exercise a back electromotive force as a result of generating electricity, and thus the rotary to linear motion conversion mechanisms are made sufficiently robust to cope with this mechanical loading.

A facet arising from the use of any type of linear generator is that the resistance back electromotive force arising as a result of electrical generation is proportional to the speed of reciprocal motion of the translator relative to the stator. There may therefore be large variations in the force, from nothing at the extremes of travel, to a maximum at the centre of travel.

In a preferred embodiment, a plurality of generators are driven by the one or more rotary to linear motion conversion means, but out of phase with one another such that a substantially constant resistance force is presented to the rotary to linear motion conversion system. Depending on cost of implementation, one option is to use a single stator stack, with a plurality of translators moving along it, but driven out of phase to one another by the rotary to linear motion conversion mechanism(s).

The rotary to linear motion conversion mechanism may incorporate some degree of step up gearing, such as to effect a greater frequency of reciprocal motion between the translator and stator of the linear generators as compared to the original natural rate of rotation of the main axle driven by the turbine blades. A consideration affecting the stability of wind turbines is the significant reaction force exercised against the turbine blades as the prevailing winds press upon them. For substantial turbines, these forces can amount to several tonnes of pressure, so resulting in a couple tending to topple over the entire structure, namely the support tower, the nacelle and its contents and the turbine blades supported thereby. Very firm foundations are therefore necessary, and these can include ballast weight at the base of the support towers.

According to a feature of the invention, the stators of the linear generators are mounted at the base of the towers. Their considerable weight is thus available to stabilise them,

The rotary to linear conversion mechanisms may also be favourably located again at the base of the towers local to the generators in order to further increase the ballast weight. Drive from the main wind turbine main axle may be communicated by means of bevel gears mounted within the nacelle to drive a central shaft depending down (i.e. extending) from the nacelle to the rotary to linear motion conversion mechanism.

Linear guides may be provided to guide each of the translators of the linear generators along their respective stators.

In order to avoid imbalanced lateral forces as the armature of the linear generator is caused to traverse relative to its stator, the use of a design of generator in which the armature travels coaxially along its stator is preferable. By this means, no adverse lateral pull is transferred to the rotary to linear motion conversion mechanism.

According to a preferred realisation of the invention, the linear generators may therefore be of the coaxial type invented by Hugh-Peter Granville Kelly as disclosed in granted EP 1839384 and in granted patent, GB 2079068 and EPC/international equivalents, but in the latter case, used as a linear generator, both of which are hereby incorporated by reference in their entirety.

In the case of the preferred use of a plurality of linear generators, in order to combine their respective electrical outputs, the frequency and amplitude varying voltage outputs provided thereby may be fed to dedicated ac to dc inverters for combining on a dc bus for subsequent processing into grid ready ac.

The invention will now be described with reference to the accompanying drawings in which:

Fig 1 shows a wind turbine fitted with the energy conversion mechanism of the invention,

Fig 2 shows a rotary to linear motion conversion mechanism driving out of phase, three linear generators,

Fig 3 a shows the generators mounted at the base of a conventional wind turbine tower and Fig 3b shows the generators mounted at the base of a horizontal type wind turbine.

Fig 4 shows electronics means for combining the outputs of the respective generators for feeding further processing electronics.

Referring to Fig 1 , energy conversion means for a wind turbine is constructed as follows. Blades 10 of the turbine cause rotation of a main axle 1 1. This is in the form of a crank as shown at 12. Resulting from the rotation of the crank, as caused by the reaction of the winds upon the blades 10, a con rod 13 causes relative motion of the armature 15 of a linear generator relative to its stator 16, as shown by the arrow at 17. The con rod 13 is swivably attached to a yoke 14. By this means, the mechanical forces exerted on the turbine blades 10 are converted into electricity, but without the need for a high revolution speed and potentially noisy conventional gearbox driving a rotary generator.

The armature 15 may be guided along the length of the stator 16 by linear guide means 18 mounted parallel to the stator 16.

It will be appreciated from consideration of Fig 1 that the force exercised upon the crank will be non-uniform, in terms of driven movement of the armature along the stator. The non-uniformity resulting from the effects of gravity is further exacerbated in practice by the fact that a back electromotive force is exercised by the armature as electricity is being generated. The greater the speed of motion, the greater this force.

The arrangement shown in Fig 2 illustrates a method of ameliorating this effect. The crank shaft is shown driving three eccentric discs, 20, 21 and 22. Three linear generators 23, 24 and 25 are mounted below each disc. Conrods 26, 27 and 28 depend down from (e.g. extend between) each disc to their respectively driven armatures 29, 30 and 31 , preferably using the same type of yoke arrangement as shown in Fig 1. However, the swivel points on the discs are arranged to be at 120° out of phase to one another. The effect is to reduce the variation in torque force upon the drive axle 19. Careful regulation of the power take offs from the armatures using a controller can further reduce the non-linearity, and indeed, almost eliminate it (such regulation can be used with any number of generators including one). This ensures the turbine blades can precess around their described disc of motion without transient lateral torque variations. Any number of generators may be used. Using more than one generator, as described above, may have the advantage that they may be positioned out of phase with one another such as to reduce the variation in resistance force presented by the rotary to linear motion conversion mechanisms on the main axle on which the turbine blades are mounted which is present with only one generator. In one embodiment the resistance force is substantially constant.

Referring to Fig 3a, an arrangement is shown in which the linear generators are shown at the base of the turbine. A tower 35 mounted on a firm base 36, is shown supporting a nacelle 37. Bearings 38 and 39 at each end of the nacelle 37 support a main drive shaft 40.

A (substantial) bevel gear 41 is mounted upon the drive shaft 40, causing the rotation of an orthogonally located bevel gear 42. As is clear from the comparative diameters of the gears, they serve to step up the rate of rotation of a central drive shaft 43, but could equally well step down or not alter the rate of rotation. This drive shaft thus transfers the torque exercised by the wind turbine blades down towards the base of the tower 35. Preferably the torque is transferred at a higher number of revolutions per minute. At this lower end, a further set of bevel gears 45 and 46 serves to effect rotation of a further drive axle 47. The drive axle 47 may be in the form of a three portioned crank, as shown, but wherein each portion of the crank is at 120° to the other. Con rods 48, 49 and 50 serve to drive armatures 51 , 52 and 53 relative to their stators 54, 55 and 56. The variations described above with reference to Figure 2 may also be applied to the Figure 3a embodiment.

In will be apparent that in this arrangement, the weights of the stators effectively provide valuable ballast weight at the bottom of the towers, and so provide improved stability to the whole structure. The gear ratio step up effected by the bevel gears 41 and 42 serves to increase the rate of movement of the armatures relative to their stators, and thus a favourable increase in their respective power outputs. This increase may be judiciously selected (e.g. be preselected) to suit the dynamic performance of the turbine blades and the optimum translation rate of the linear generators. Fig 3b shows an alternative type of wind turbine, the horizontal type, again driving linear generators mounted in the base of the unit. As is self-evident, the principle of operation is the same as that shown for Fig 3 a, except that, on account of the fact that the main drive shaft from the turbine blades is already vertical, the bevel gears 41 and 42 of Fig 3 a are not required. Gearing up the rate of oscillation of the armatures relative to their stators may instead be achieved by means of the lower bevel gears, 45 and 46.

The one or more linear generator 23, 24, 25 may be of any sort. The one or more linear generator 23, 24, 25 may be comprised of a stator of a stack of permanent magnets and a translator of one or more coils or vice versa. The one or more linear generators 23, 24, 25 may be of the coaxial type.

In an example, the translator and stator of the linear generator may be each adapted to provide a sequence of north and south poles along the length of the apparatus, at least one of the translator and stator having at least two coils to produce the poles thereof, the pole pitches of the stator and the translator being different from one another such that on relative motion of the stator and translator by said means the difference in pole pitch of the translator and stator can result, in use, in electricity generation at any location within a range of relative longitudinal positions of the translator and stator, as disclosed in GB 2,079,068 A.

The linear generator may have an annular stator containing coils and a translator containing permanent magnets or vice versa. The translator may be located concentrically through the stator. The stator and translator are movable relative to one another along a longitudinal axis. A magnetically permeable sleeve may be affixed to and circumferentially surround the stator. The permeability of one or both ends of the sleeve may be contoured around its circumference such that the variation of the longitudinal cogging force on the sleeve in its travel with the stator relative to the translator, is reduced. The contouring of the permeability may be achieved by a variation in the amount of material around the circumference of the sleeve end at one or both ends of the sleeve. The variation may be in the form of varying absence of sleeve material at locations around the circumference of the armature. The sleeve may include or is constructed from a multiplicity of individual ferromagnetic elements for drawing out lines of flux from the translator, but each being of a shape and individually insulated such as to eliminate substantially the circulation of eddy currents around and/or along the circumference sleeve, as disclosed in EP 1 ,839,384.

It is commonly known for the rate of rotation of wind turbine blades to be carefully controlled to ensure synchronisation of the frequency of the alternating currents generated thereby with a predefined value.

In the case of the linear generators, clearly the armatures are moving each at differing velocities at any one point during their respective cycles up and down their stators. A power adaption strategy to cope with the consequent varying velocities is shown with respect to Fig 4.

In this, three phase power as generated by the linear generators of Fig 3, shown here notionally at 57, 58 and 59, is converted to dc by three ac to dc inverter rectifiers, 60, 61 and 62.

The outputs of the three inverter rectifiers are combined onto a common dc bus, 63. Control inputs 64, 65 and 66 are used to regulate the inverters to ensure the desired output dc voltage is obtained from each inverter.

The power available from the dc bus is controlled by a further inverter, 67, this time dc to ac, which provides regular three phase ac 68 at the desired frequency and transmission voltage for feeding onto an ac grid.

Numerous variations will be apparent to those skilled in the art.