GB2500643A

GB2500643A - Turbine braking system

Info

Publication number: GB2500643A
Application number: GB1205452.4A
Authority: GB
Inventors: Paul Vigars
Original assignee: Tidal Generation Ltd
Current assignee: Tidal Generation Ltd
Priority date: 2012-03-28
Filing date: 2012-03-28
Publication date: 2013-10-02
Anticipated expiration: 2032-03-28
Also published as: GB201205452D0; GB2500643B

Abstract

Braking elements 13, 38 for retarding rotation of a turbine 2, eg for generating current from wind underwater flow, are magnetically disengaged to allow free rotation of the turbine. The turbine 2 may comprise a rotor 3 having a rotor body 4 supporting at least one blade 6, the rotor 3 comprising a first braking element 38, eg an annular friction pad. The turbine 2 further comprises a hub 8 having a second braking element 13. The rotor 3 is rotatably mounted on the hub 8 such that the first and second braking elements 38, 13 engage one another when not magnetically displaced, eg to allow maintenance or repair work. The turbine further comprises an electromagnetic element 40, eg an inside out solenoid, operable to magnetically displace the first or second braking element, disengaging the first braking element 38 from the second braking element 13. The turbine may rotate about a horizontal axis. At least one of the braking elements may be movably or pivotably mounted on the rotor body of the hub. The braking system may be independent of the turbine.

Description

TURBINE AND BRAKING SYSTEM THEREFOR

The present invention relates to a turbine and to a turbine braking system. The invention is particularly applicable to tuibines used for power generation and most particulaily to turbines used for generating power from an underwater current flow.

Background

There is increasing interest in the use of underwater power generating equipment that makes use of the energy of water flows, such as tidal flows. Such equipment is secured to the bed of a body of watei, such as a sea, estuary or river, and employs a rotary generatol to generate electricity. The generator is diiven by a turbine having a rotor and a numbei of rotor blades placed in the water flow. Conventional underwater turbines used to drive electrical generators are typically mounted on a horizontal rotational axis and include yaw capability, allowing them to be directed to the most effective angle with respect to the direction of current flow.

In oidei to allow for the undertaking of maintenance and repail work, in the event of a failure in connection to the electrical supply network or for other reasons, it may be necessary to stop rotation of the turbine, and a braking system is thus required. Known braking systems have suffered in the past from problems of reliability, expense and/or complexity. The present invention seeks to addiess some or all of these issues.

Summary of the Invention

In the following specification, the phiases "magnetic displacement", or "magnetically displace' are employed to describe a displacement effected by an electromagnetic force.

These phrases are thus similar in meaning to the more widely recognised "magnetic levitation" but are not limited to any specific direction of displacement.

According to the present invention, theie is provided a turbine complising: a rotor having a rotor body, at least one blade supported on the rotor body, and a first braking element; and a hub comprising a second biaking element; wherein the rotol is iotatably mounted on the hub such that the first and second braking elements releasably engage one another; the turbine further comprising an electromagnetic element operable to magnetically displace the first or second braking element, thereby to disengage the first braking element from the second braking element.

The turbine rotor of the present invention is thus prevented from rotation by the braking elements until the electromagnetic element is energised. Once energised, the electromagnetic element causes the braking elements to be disengaged, allowing rotation of the turbine rotor. The turbine of the present invention thus embodies a failsafe design, ensuring that in the event of power loss, the braking elements are engaged as power to energise the electromagnetic element is no longer supplied.

The electromagnetic element may be operable to magnetically displace the rotor with respect to the hub. According to this embodiment, the braking elements may be mounted in fixed relation to the rotor and hub, movement of the rotor thus disengaging the first braking element from the second braking element.

According to a preferred embodiment, the turbine may further comprise an actuator mechanically coupled to one of the first or second braking elements and moveable under an electromagnetic force generated by the electromagnetic element.

The actuator may be mounted on one of the first or second braking elements. Alternatively, the actuator may be mounted on or adjacent the rotor body.

The actuator may for example comprise a permanent magnet, or may comprise a ferromagnetic material.

The actuator may form, together with the electromagnetic element, a solenoid, which may be of conventional arrangement or may be an "inside out" arrangement, with an internal element being surrounded by an outer actuator. The actuator of the solenoid may mechanically engage either one of the braking elements or the rotor itself in order to effect the necessary displacement to disengage the braking elements and allow rotation of the turbine rotor.

According to certain embodiments, the first and second braking elements may comprise surfaces formed on the rotor and hub. At least one of the first and second braking elements may comprise a friction pad. The braking elements may thus frictionally engage one another to retard rotation of the turbine.

The first and second braking elements may comprise friction pads mounted on annular contact surfaces of the rotor and the hub.

In one example, the hub may define the axis of rotation of the turbine, and the rotor may be mounted for rotation about the hub. The hub may extend into an internal aperture of the rotor.

In use, the axis of rotation of the turbine may be substantially vertical. According to this example, the rotor may be mounted on the hub in such a manner that gravity holds the rotor in position on the hub, and hence maintains the first and second braking elements in engagement. The electromagnetic element may thus magnetically levitate the rotor and/or first braking element to disengage the braking elements from one another and allow the rotor to rotate.

Alternatively, the axis of rotation of the turbine in use may be substantially horizontal.

According to this example, the rotor may be held against the hub by a substantially horizontal thrust force generated by passing fluid. The electromagnetic element may magnetically displace the rotor and/or first braking element against the horizontal thrust to disengage the braking elements from one another and allow the rotor to rotate.

According to another aspect of the present invention, there is provided a power generating apparatus comprising a turbine according to the first aspect of the present invention and a generator. The turbine rotor may provide the rotor of the generator and the turbine hub may provide the stator of the generator. Such an arrangement allows for a simplified and highly robust construction requiring minimal intervention during its operating life.

The apparatus may provide a water current power generating apparatus.

According to another aspect of the present invention, there is provided a braking system for a turbine which comprises a rotor having a rotor body and at least one blade supported thereon, and a hub on which the rotor is rotatably mounted, the braking system comprising: a first braking element mechanically coupled to the rotor; a second braking element mechanically coupled to the hub so as to engage the first braking element when the rotor is mounted on the hub; and an electromagnetic element operable to magnetically displace the first or second braking element, thereby to disengage the first braking element from the second braking element.

According to a further aspect of the present invention, there is provided a method of providing a braking system for a turbine comprising a rotor engaged against a hub by an axial force, the method comprising magnetically displacing the rotor out of engagement with the hub during operation, and allowing the rotor to return to engagement with the hub under the axial force when not in operation. The axial force may be provided by the weight of the rotor, the thrust provided by the water flow interacting with the rotor blade, or by buoyancy of the assembly.

Brief Description of the Drawings

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which:-Figure 1 illustrates a turbine hub and supporting structure; Figure 2 illustrates a turbine; Figure 3 illustrates a turbine rotor body and blades; and Figure 4 illustrates detail of a braking system for the turbine of Figure 2.

Detailed Description of Embodiments

The present invention provides a turbine and braking system therefor that operate in a failsafe manner. The turbine comprises a turbine rotor that is mounted for rotation on a supporting element or hub. The rotor comprises a rotor body and at least one turbine blade mounted on the rotor body. Each of the rotor and hub comprise a braking element, a first braking element being provided on the rotor and a second braking element being provided on the hub.

According to different embodiments of the present invention, the braking elements may comprise integral parts of the rotor and hub, or may be fixedly mounted on the rotor and hub, or may be moveably or pivotally mounted in relation to the rotor and hub. In one example, the braking elements may comprise abutting surfaces of the rotor body and hub element, which may for example be provided with friction pads to increase the frictional resistance between them. With the turbine rotor mounted in position on the hub, the first and second braking elements engage one another to retard rotation of the rotor. Disengagement of the first and second braking elements is effected via the activation of an electromagnetic element.

According to one example, an actuator in the torm of a magnetic or ferromagnetic element is provided to cooperate with the electromagnetic element and produce the required motion.

The actuator may be positioned to act directly on one or other of the braking elements, for example by being mounted on or immediately adjacent the first or second braking element.

Alternatively, the actuator may produce motion of one or other of the braking elements via another component. For example, the actuator may be fixedly mounted on the rotor, causing displacement of the rotor and associated first braking element. Alternatively, the actuator may be mounted immediately adjacent the rotor, able to mechanically engage the rotor to effect the required motion.

Movement of the actuator is controlled by the electromagnetic element, which, when energised. exerts an electromagnetic force on the actuator, causing the required motion.

The electromagnetic element may be realised in various forms, may surround or be surrounded by the actuator, or may be displaced with relation to the actuator. For example, in one embodiment, the electromagnetic element and actuator may comprise a solenoid, the actuator mechanically engaging the rotor andlor first braking element to cause the required displacement.

The failsafe nature of the braking system embodied in the turbine arises from the fact that the resting mounted position of the rotor on the hub is constrained by the braking elements.

Thus, with no energy connection applied to the turbine, rotation of the turbine rotor is prevented by the braking elements. Only when an electrical connection allows energising of the electromagnetic element are the braking elements disengaged, allowing the turbine rotor to rotate. Thus in the event of a failure in electrical connection, the rotor is maintained in a safe position, allowing for intervention if necessary.

The turbine may be employed to drive a generator as part of a power generation apparatus.

In one example, the generator rotor may be provided by the turbine rotor, and the generator stator may be provided by the turbine hub.

An example of a turbine and braking system according to the present invention will now be described with reference to the drawings. The example is described with reference to an underwater application, but it will be appreciated that other applications, including a wind power installation, may also be considered. It will also be appreciated that, in addition to where explicitly stated or illustrated, elements described or shown in relation to one aspect, embodiment or example of the present invention may be incorporated within another aspect, embodiment or example of the invention without departing from the scope of the claims.

Referring initially to Figure 2, a turbine 2 comprising a rotor 3 and hub 8 is illustrated. The rotor 3 comprises a rotor body 4 and blades 6, and is mounted for rotation about the hub 8.

The hub 8 is mounted on a foundation 9 comprising at least one pier 10 and surrounding support structure 12. The hub 8 defines the axis of rotation of the turbine 2 and is substantially vertically oriented. The turbine 2 is arranged to drive a generator 14 comprising a rotor 16 and a stator 18.

With particular reference to Figure 1, the stator 18 of the generator 14 comprises a stator winding 20 housed within the hub 8, which performs the function of a stator housing. The stator winding 20 may be canned and/or potted to protect the winding 20 from ingress of surrounding water. The hub 8 is sufficiently robust to support the turbine rotor 3 and comprises an annular supporting surface 11 on which a braking element in the form of an annular friction pad 13 is mounted. The annular friction pad 13 may comprise a continuous annular friction pad or may be formed from a series of discrete friction pads arranged around the annular support surface 11. In other embodiments, the braking element may comprise a surface treatment or other surface application to increase the surface roughness of the support surface 11 and hence increase the coefficient of friction between the support surface 11 and an abutting surface of the rotor body 4. The hub 8 may be releasably mounted on the foundation 9 via a clamp or may be affixed to the foundation in a more permanent manner, for example by the use of a welded joint. The stator winding 20 housed within the hub 8 is connected via cables 22 to an electricity supply network (not shown) which may be a local, national or international network.

Referring now to Figure 3. the rotor 3 comprises a substantially cylindrical annular rotor body 4 on which are supported at least two turbine blades 6. The rotor body 4 is dimensioned to be received about the hub 8, with a first end 24 of the rotor body 4 being closed by an end stop 26. At least an inner surface of the end stop 26 may be formed by a bearing 28, which may be of any suitable shape for bearing axial loads and may for example be a disc bearing.

The bearing 28 may additionally provide lateral support to the turbine rotor body 4, or additional bearings (not shown) may be provided to supply lateral support to the turbine rotor body 4, ensuring the rotor body 4 remains centred upon the hub 8.

In the example shown, support arms 30 extend substantially radially from the rotor body 4 to support the turbine blades 6. Each turbine blade 6 may be supported by at least two arms 30. In another example, the turbine blades 6 may be supported by direct connection of the blade with the rotor body 4, or may have any appropriate number of suitable support members.

Encased within the annular wall of the rotor body 4 is the generator rotor 16. In the example shown, the generator rotor 16 comprises a winding 32 which is entirely housed within the rotor body 4 of the turbine 2. However the generator rotor 16 may take any form appropriate the type of generator employed, which itself may be of any suitable construction or type including induction, permanent magnet, brushless excited etc. The generator winding 32 is preferably canned and/or potted to protect the winding 32 from ingress of surrounding water.

At a second end of the rotor body 4, an annular end surface 34 is defined. Mounted on the annular end surface 34 is a braking element in the form of an annular friction pad 38. The friction pad 38 may be continuous or may be formed by a series of discrete friction pads arranged around the annular end surface 34. The friction pad 38 abuts the cooperating annular friction pad 13 mounted on the annular support surface 11 of the hub 8. In other embodiments, the braking element on the annular end surface 34 may comprise a surface treatment or other surface application to increase the surface roughness of the annular end surface 34 and hence increase the coefficient of friction between the annular end surface 34 and the abutting support surface 11 of the hub 8.

The rotor 3 is mounted on the hub 8 in a manner illustrated in Figure 2. The rotor body 4 is received over the hub 8, the two components defining an annular clearing 36 therebetween.

In use, the annular clearing 36 is flooded by water from the body of water within which the apparatus is installed. The rotor body 4 is supported on the hub 8 via the end stop 26 and bearing 28, together with the cooperating friction pads 13, 38 formed on the mating annular surfaces of the rotor body 4 and hub 8. The rotor body 4 is mounted on the hub 8 in a direction along the axis of rotation of the turbine 2. With the hub 8 positioned so that the axis is substantially vertical, the rotor body 4 is lowered into place on the hub 8, such that the hub 8 extends inside the internal aperture of the rotor body 4. The rotor body 4 is then maintained in position on the hub by virtue of the effect of gravity. That is, the weight of the rotor 3 keeps it in place on the hub 8. The weight of the lotor 3 also ensures the engagement of the two annular braking elements (friction pads 13, 38) which act to retard rotation of the rotor, providing a braking effect.

With particular reference to Figure 4, the turbine further comprises an electromagnetic element 40, for example a coil, and an actuator 42, which, together with the braking elements mounted on the rotor body 4 and hub 8, form the braking system for the turbine 2.

In the illustrated example, the electromagnetic element 40 is provided in an upper region 44 of the hub 8, substantially adjacent the end stop 26 of the rotor body 4. In other embodiments, the electromagnetic element 40 may be provided in other regions of the hub 8, and may be vertically or horizontally aligned, as explained in further detail below. The electromagnetic element 40 is preferably powered via the cables 22 that connect the generator stator 18 to the electrical supply network.

According to one embodiment, the actuator 42 comprises a permanent magnet, and is mounted within the end stop 26 of the rotor body 4. However, according to other embodiments, the actuator 42 may be formed from a ferromagnetic material and may be mounted on another region of the rotor body or adjacent the rotor body, as discussed in further detail below. In the illustrated embodiment! the position of the actuator 42. in the end stop 26 of the rotor body 4, ensures that it is immediately adjacent the electromagnetic element 40 when the rotor body 4 is mounted in position on the hub 8.

When no current is flowing into the electromagnetic element 40, the element is not energised and the rotor body 4 rests on the hub 8 as described above, with the cooperating friction pads 13.38 acting as a brake to rotation of the rotor. When it is desired to release the brake and allow rotation of the rotor, electrical current is provided by the supply network to energise the electromagnetic element 40. The energised electromagnetic element 40 generates an electromotive force acting to repel the permanent magnet actuator 42 located in the rotor body 4. The repulsive force is sufficiently strong to overcome the effects of gravity and magnetically levitate the rotor body 4 a small distance away from the hub 8. This levitation separates the friction pads of the adjacent annular surfaces, and with these friction braking elements no longer engaged, the rotor body 4 is free to rotate about the hub 8. In this released operating condition, water flowing past the turbine 2 exerts a force on the turbine blades 6 causing rotation of the rotor 3. Owing to the location of the generator rotor 16 within the rotor body 4, rotation of the rotor body 4 directly causes rotation of the generator rotor 16, which cooperates with the generator stator 18 to generate electricity.

Should connection to the electrical network be lost for any reason, the power flowing to the element 40 is lost, and the turbine rotor body 4 settles back under gravity to rest on the hub 8, thereby re-engaging the friction pads 13, 38 on the annular surfaces of the rotor body 4 and hub 8 and thus retarding rotation of the rotor 3. The buoyancy of the turbine rotor body 4 and blades 6 may be designed to optimise the levitation and braking effects of the brake system.

In another embodiment (not shown) the electromagnetic element and actuator may foim a solenoid. The solenoid may be of conventional arrangement, with the actuator being at least partially received within the element and acting as a "plungei". Alternatively, the solenoid may be an "inside out' type solenoid, with the element being at least partially received within an outer actuator. In these arrangements, the actuator is not formed within or fixedly attached to the rotor body but iather is mounted immediately adjacent the rotor body.

Activation of the electromagnetic element displaces the actuator which in turn mechanically engages against the rotor body to magnetically displace the rotor body a sufficient distance axially away from the hub to disengage the annular friction pads and enable rotation of the turbine. In still further embodiments, the location of the electromagnetic element may dictate that it is an attractive electiomagnetic force that is generated to effect the requiied motion of the rotor body.

The turbine and associated braking system can be managed via a control system which, according to various embodiments, may be based on live flow speed measurement, power measurement or may be timetabled according to flow patterns in the area of installation.

According to another example of the present invention (not shown), a turbine substantially similar to that described above but orientated for rotation about a horizontal axis may be provided. In this case, it is the thrust of the passing fluid that creates the axial force holding the turbine rotor against the hub, and forcing the braking surfaces of the rotor body and hub into engagement with one anothei. When energised, the electromagnetic element and actuator force the rotor to move along its axis of rotation away from the hub, disengaging the braking surfaces and allowing rotation of the rotor. It will be appreciated that further arrangements of bearing elements may be required to allow for the action of gravity on the rotor and maintain the rotoi in a centred position for rotation about the hub. A yaw capability is also required to enable the hub and rotor to be directed to the most effective angle for rotation with respect to the surrounding current flow at any particular time. However, the principle elements of the above desciibed vertical turbine and braking system may be incorporated.

According to a still further example of the present invention, at least one of the braking elements may be moveably 01 pivotally connected to the rotor body 4 oi hub 8. In this example, the actuator 42 may be mounted on or located adjacent to the moveable braking element, such that energising of the element 40 causes the actuator to effect movement of the associated braking element, bringing the associated braking element out of engagement with the other biaking element and thus allowing rotation of the turbine iotoi body 4. In this example, it is not necessary to magnetically displace the entire rotor body and associated turbine blades 6, but merely one of the braking elements.

It will be appreciated that, while the above disclosure concerns a turbine including a braking system, the braking system of the present invention may be provided independently of the turbine. Additionally, the braking concept of electromagnetic energising to disengage braking elements may be embodied in forms other than those discussed above. The present disclosure thus provides preferred ways in which the invention may be realised but is not in any way exhaustive.

Claims

Claims 1. A turbine comprising: a rotor having a rotor body, at least one blade supported on the rotor body, and a first braking element; and a hub having a second braking element; wherein the rotor is mounted on the hub such that the first and second braking elements releasably engage one another; the turbine further comprising an electromagnetic element operable to magnetically displace the first or second braking element, thereby to disengage the first braking element from the second braking element.
2. A turbine as claimed in claim 1, wherein the electromagnetic element is operable to magnetically displace the rotor with respect to the hub.
3. A turbine as claimed in claim 1 or 2, further comprising an actuator mechanically coupled to one of the first or second braking elements and moveable under an electromagnetic force generated by the electromagnetic element.
4. A turbine as claimed in claim 3, wherein the actuator is mounted on one of the first or second braking elements.
5. A turbine as claimed in claim 3, wherein the actuator is mounted on the rotor body.
6. A turbine as claimed in claim 3, wherein the actuator is mounted adjacent the rotor body.
7. A turbine as claimed in any one of claims 3 to 6, wherein the actuator comprises a permanent magnet.
8. A turbine as claimed in any one of claims 3 to 6, wherein the actuator comprises a ferromagnetic material.
9. A turbine as claimed in any one of claims 3 to 8, wherein the actuator forms with the electromagnetic element a solenoid.
10. A turbine as claimed in claim 9, wherein the solenoid has an inside out arrangement.
11. A turbine as claimed in any one of the preceding claims, wherein the first and second braking elements comprise surfaces formed on the rotor body and hub.
12. A turbine as claimed in any one of the preceding claims, wherein at least one of the first and second braking elements comprises a friction pad.
13. A turbine as claimed in claim 12, wherein the first and second braking elements comprise friction pads mounted on annular contact surfaces of the rotor and the hub.
14. A turbine as claimed in any one of the preceding claims, wherein the hub defines the axis of rotation of the turbine, and wherein the rotor is mounted for rotation about the hub.
15. A turbine as claimed in claim 13, wherein the hub extends into an internal aperture of the rotor.
16.A turbine as claimed in any one of the preceding claims, wherein, in use, the axis of rotation of the turbine is substantially vertical.
17. A turbine as claimed in any one of claims 1 to 15, wherein, in use, the axis of rotation of the turbine is substantially horizontal.
18. A power generating apparatus comprising a turbine as claimed in any one of the preceding claims and a generator.
19. Apparatus as claimed in claim 18, wherein the turbine rotor provides the rotor of the generator and the turbine hub provides the stator of the generator.
20. Apparatus as claimed in claim 18 or 19, wherein the apparatus provides a water current power generating apparatus.
21. A braking system for a turbine which comprises a rotor having a rotor body, at least one blade supported thereon, and a hub on which the rotor is mounted, the braking system comprising: a first braking element mechanically coupled to the rotor; a second braking element mechanically coupled to the hub so as to engage the first braking element when the rotor is mounted on the hub; and an electromagnetic element operable to magnetically displace the first or second braking element, thereby to disengage the first braking element from the second braking element.
22. A method of providing a braking system for a turbine comprising a rotor engaged against a hub by an axial force, the method comprising magnetically displacing the rotor out of engagement with the hub during operation, and allowing the rotor to return to engagement with the hub under the axial force when not in operation.
23. A method as claimed in claim 22, wherein the axial force is provided by weight of the rotor, thrust exerted on the rotor by a water flow, or by buoyancy of the rotor.
24. A turbine and braking system for a turbine substantially as described herein, with reference to and as shown in the accompanying drawings.Amendments to the claims have been filed as follows Claims 1. A turbine comprising: a rotor having a rotor body, at least one blade supported on the rotor body, and a first braking element; and a hub having a second braking element; wherein the rotor is mounted on the hub such that the first and second braking elements releasably engage one another; the turbine further comprising an electromagnetic element operable to magnetically displace the first or second braking element, thereby to disengage the first braking element from the second braking element, wherein the electromagnetic element is operable to magnetically displace the rotor with respect to the hub.2. A turbine as claimed in claim 1 or 2, further comprising an actuator mechanically coupled to one of the first or second braking elements and moveable under an electromagnetic force generated by the electromagnetic element. C')3. A turbine as claimed in claim 3, wherein the actuator is mounted on one of the first or Li") second braking elements.N-4. A turbine as claimed in claim 3, wherein the actuator is mounted on the rotor body.5. A turbine as claimed in claim 3, wherein the actuator is mounted adjacent the rotor body.6. A turbine as claimed in any one of claims 3 to 6, wherein the actuator comprises a permanent magnet.7. A turbine as claimed in any one of claims 3 to 6, wherein the actuator comprises a ferromagnetic material.8. A turbine as claimed in any one of claims 3 to 8, wherein the actuator forms with the electromagnetic element a solenoid.9. A turbine as claimed in claim 9, wherein the solenoid has an inside out arrangement.10. A turbine as claimed in any one of the preceding claims, wherein the first and second braking elements comprise surfaces formed on the rotor body and hub.11. A turbine as claimed in any one of the preceding claims, wherein at least one of the first and second braking elements comprises a friction pad.12. A turbine as claimed in claim 12, wherein the first and second braking elements complise friction pads mounted on annular contact surfaces of the lotol and the hub.13. A turbine as claimed in any one of the preceding claims, wherein the hub defines the axis of rotation of the turbine, and wherein the rotor is mounted for rotation about the hub.14. A tuibine as claimed in claim 13, wheiein the hub extends into an internal apeiture of the rotor.15. A turbine as claimed in any one of the preceding claims, wherein, in use, the axis of rotation of the turbine is substantially vertical.C') 16. A tuibine as claimed in any one of claims 1 to 15, wherein, in use, the axis of rotation of the turbine is substantially hoiizontal.1.1)20 17. A power generating apparatus comprising a turbine as claimed in any one of the preceding claims and a generator.N0 18. Apparatus as claimed in claim 18, wherein the turbine rotor provides the rotor of the generator and the tuibine hub piovides the statol of the generatol.19.Appaiatus as claimed in claim 18 or 19, wherein the apparatus provides a watel current power generating apparatus.20. A braking system for a turbine which comprises a rotor having a rotor body, at least one blade suppoited thereon, and a hub on which the rotol is mounted, the braking system complising: a fiist braking element mechanically coupled to the rotor; a second braking element mechanically coupled to the hub so as to engage the first braking element when the rotor is mounted on the hub; and an electromagnetic element operable to magnetically displace the first oi second braking element, thereby to disengage the first braking element from the second braking element.21.A method of providing a braking system for a turbine comprising a rotor engaged against a hub by an axial force, the method comprising magnetically displacing the rotor out of engagement with the hub during operation, and allowing the rotor to return to engagement with the hub under the axial force when not in operation.22. A method as claimed in claim 22, wherein the axial force is provided by weight of the rotor, thrust exerted on the rotor by a water flow, or by buoyancy of the rotor.23. A turbine and braking system for a turbine substantially as described herein, with reference to and as shown in the accompanying drawings. C')LUN