This invention relates to energy storage and conversion apparatus, and in particular to a rotor for such an apparatus.

Flywheel energy storage and conversion apparatus are known which comprise a fixed stator having coils mounted on a core and a rotating rotor accelerated by the stator to store energy in the rotor. When energy is to be withdrawn from the apparatus, the stator and rotor act as a dynamo to deliver energy, thereby resulting in deceleration of the rotor.

As will be appreciated, to store any significant amount of energy, the rotor must rotate at high speeds, in the region of 100,000 rp in the case of the applicant's apparatus described in co-pending U.K. patent application number 9313926.9. Hence, significant stresses are produced in the rotor which can result in failure of the rotor and subsequent destruction of the apparatus. It is therefore desirable to incorporate in the apparatus some form of detection system which will provide information on the status of the rotor without affecting the normal running of the apparatus.

According to the present invention, there is provided a rotor for an energy storage and conversion apparatus, the energy storage and conversion apparatus comprising a stator for accelerating the rotor to store energy and for withdrawing energy from the rotor, wherein an optical guide

(which may be a single optical fibre, a multiplicity of optical fibres or some other appropriate optical device) is carried by the rotor such that a light beam can be passed along the optical guide during rotation of the rotor to deliver information indicative of a characteristic of the rotor. In one embodiment, the rotor may be substantially cylindrical with the optical guide mounted on the inside surface of the rotor. Alternatively, the optical guide may be embedded in the rotor.

The optical guide may be substantially straight and extend from one end of the rotor to the other.

Alternatively, the optical guide may be wound helically around the rotor progressing from one end of the rotor to the other.

The rotor is preferably formed of fibre reinforced plastics material. If such a material is used, the reinforcing fibres may be one or more of carbon, glass, boron, polyamide, polyaramid, polyolefin or kevlar (Registered Trade Mark) fibres.

The present invention further provides an energy storage and conversion apparatus comprising a stator, a rotor according to the present invention, a light source and a detector for the information delivered by the optical guide. The stator is preferably positioned within the rotor and both the stator and the rotor are within an outer housing supporting the light source and the detector. The light source and/or the detector may be either inside or outside the outer housing. If the light source or the detector is mounted outside the housing, the light beam may communicate with the rotor via a window in the housing or a fibre through the housing.

A vacuum is ideally formed within the housing, thereby allowing the rotor to rotate more freely. The light source is preferably a semiconductor laser diode. Any other appropriate light source may, however, alternatively be used. Furthermore, light of any appropriate wavelength can be used.

In a particular embodiment, two or more light sources may be provided having different wavelengths. In such an embodiment, the light sources are preferably arranged to strike the end of the optical fibre at different angles.

The detector may be a fibredyne interferometer sensitive to a speckle effect produced by the optical fibre. In an alternative embodiment, the detector may be a light meter sensitive to changes in light intensity received from the optical guide. In such an embodiment, the optical guide may include a modulator between its two ends.

Preferably the modulator is photoelastic, but a ruby crystal (or any other suitable crystal) could alternatively be used as the modulator.

In another embodiment, a second fibre may be wound around the optical guide to cause microbending in the optical guide as a result of changes occurring in the characteristics of the rotor. When microbending occurs in the optical guide, light passing along the optical guide will be dissipated, thereby producing a detectable effect indicative of a characteristic of the rotor.

Depending upon the information being detected by the detector, the detector output may be indicative of the temperature of the rotor, stresses in the rotor or vibrations in the rotor. Other characteristics of the rotor may possibly be detected, provided that they have some effect upon a light beam passing along an optical guide.

Specific embodiments of the present invention are now described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic cross-sectional side elevation of a flywheel energy storage and conversion apparatus incorporating a rotor according to the present invention;

Figure 2 is a schematic cross-sectional side elevation of a rotor according to the present invention with an optical fibre mounted on the inside surface of the cylindrical rotor;

Figure 3 is a schematic side elevation of a second embodiment of rotor incorporating an optical fibre according to the present invention; and

Figure 4 is a schematic representation of a fibredyne interferometer which could be used in an apparatus according to the present invention.

With reference to Figure 1 of the drawings, a flywheel energy storage and conversion apparatus l comprises a housing 3, a central stator 5 and a cylindrical rotor 7 arranged to spin around the stator 5. The stator 5 is

mounted on a central support 9 and includes a core and coil arrangement (not shown) which produces magnetic flux for driving the rotor 7 when power is applied by a control unit 13 to the stator 5. As a result, energy can be stored in the rotor 7 as kinetic energy of the rotor 7. Energy can also be withdrawn from the rotor by causing the stator 5 and rotor 7 to act as a dynamo.

The rotor 7 is suspended about the stator 5 by means of electromagnetic bearings 15 positioned at the top and bottom of the stator 5. A vacuum is also provided in the housing 3 such that the rotor 7 can spin substantially free from friction.

The rotor 7 is manufactured from carbon fibre composite material and includes an optical fibre 17 carried thereby. In this regard, the optical fibre 17 can be carried by the rotor 7 in a number of different ways, such as shown in Figures 2 and 3 for example. More particularly, the optical fibre 17 may be carried on the inside surface of the cylindrical rotor 7 (cf. Figure 2) , or embedded within the cylindrical rotor 7. Further, the optical fibre 17 may extend directly from one end of the rotor 7 to the other (cf. Figure 2) or, alternatively, may be helically or hoop wound along the rotor 7 (cf. Figure 3) . Furthermore, if appropriate, more than one optical fibre could be included in a single rotor.

A light source 19 is mounted on the inside of the housing 3 at one end of the rotor 7. At the other end of the rotor 7, a detector 21 is mounted on the inside of the housing 3. The light source 19 may be a laser, such as a semiconductor laser diode, or any other appropriate light source. Lenses may be provided between the light source 19 and the adjacent exposed end of the optical fibre 17 to focus the light beam from the light source 19 onto the end of the optical fibre 17 as it passes the light source during rotation of the rotor 7 about the stator 5. As a result, a burst of light enters the optical fibre 17 each time the rotor 7 passes the light source 19. Hence, the time period between bursts of light passing down the optical fibre 17 is

indicative of the speed of rotation of the rotor 7. Further, if the speed of rotation of the rotor 7 is known, a change in the time of travel of the light pulse along the optical fibre 17 may give an indication as to whether the optical fibre 17 has expanded or contracted due to temperature rises, for example. Other information may also be obtained from the light pulse passing down the optical fibre 17, as discussed below.

Fibredyne interferometry involves phase modulation within an optical fibre. The sensitivity of an interferometer can be increased simply by increasing the length of the optical fibre being used. A "fibredyne" interferometer (fibreoptic self-homodyne) may be constructed from multimode fibre as shown in Figure 4. Fibredyne interferometry utilises the modal noise or "speckle" produced by multimode fibre. Its attraction is its simplicity which, in most practical sensing situations, offsets its less refined performance, when compared with more complex systems. In fibredyne interferometry, the interference pattern 23 produced by impurities in the fibre consists of a random array of speckles both in the near and far field. When the optical fibre 17 is disturbed by a mechanical signal or vibration, the distribution of speckles is altered. This change in speckle distribution can be measured by the detector 21 to give an indication as to the vibrations or stresses occurring in the optical fibre 17, and hence in the rotor 7 itself. Indeed, vibration displacements of the order of 10 ^"12 meters can be detected. Thus, the "health" of the rotor 7 can be evaluated by detecting the speckle pattern produced by the optical fibre 17.

The integrated intensity change in the speckle pattern in the far field is negligible. Spatial filtering 25 is therefore required to facilitate examination of a small proportion of the far field pattern in which intensity variations will take place. This spatial filtering can be achieved relatively simply, because the output from the optical fibre 17 moves past the detector 21 as the rotor 7

rotates. Hence, the output from the optical fibre 17 reaching the detector 21 varies as the output passes the detector 21, thereby effectively being spatially filtered automatically. Phase modulation of signals in an optical fibre can be induced by the effects of mechanical pressure such as an acoustic wave, marginally changing the refractive index of the core according to

<5n = n ² p _e P/2E where tSn = change in refractive index p _e = effective photoelastic constant

P = applied pressure E = Young ^■ s Modulus

This refractive index change will alter the velocity of light within the medium and hence the velocity of the various propagation modes supported in the fibre. This produces a phase change given by the relationship

where

1 = interaction length λ = optical wavelength •50 = phase change <Sn = change in refractive index

In a multimode fibre, each mode is phase modulated to a different degree, so individual modes act as the references for the other modes. Essentially the fibre

"self-interferes", hence the terminology fibreoptic self homodyne.

Fibredyne interferometry is discussed in IEE PROCEEDINGS - A, Vol. 140, No. 5, September, 1993.

As will be appreciated from the foregoing, by using fibredyne interferometry, significant information regarding the characteristics of the rotor can be ascertained.

Further information may also be obtained by applying more than one light beam to the input end of the optical fibre 17. In this regard, if light beams of different wavelengths are made instant upon the end of the optical fibre 17 at different angles, interference occurs between the light beams along the fibre producing output patterns indicative of the status of the rotor 7. Hence, by interpreting these output patterns, additional information can be obtained about the status of the optical fibre 17 and hence the rotor 7. Thus, in the event that the rotor 7 were to begin to disintegrate, an early warning may be given by the detector 21 to shut down automatically the flywheel energy storage and conversion apparatus. Hence, significant damage which could be caused by such an apparatus failing can be avoided. Although fibredyne interferometry is designed to use a single strand optical fibre 17, the rotor 7 may include an optical fibre 17 with a second, smaller fibre wound around the optical fibre 17 along its length. This second fibre may be used to produce microbending in the optical fibre 17 due to stresses and vibrations in the rotor 7. As a result of this microbending, the light transmitted by the optical fibre 17 will vary due to losses from the optical fibre, which variations can be interpreted to give information on the rotor 7 itself. In another embodiment, a modulator, such as a photoelastic material or a ruby crystal, may be included between the ends of the optical fibre 17. As a result, the output from the optical fibre 17 (and hence the detector 21) will be dependent upon the light transmitted by the photoelastic material or ruby crystal due to temperature changes, stresses or vibrations within the rotor 7. Once again, when the output from the optical fibre 17 has been initially understood, subsequent outputs can be readily interpreted to give an indication as to the status of the rotor 7.

With regard to outputs from the detector 21, if a steady change in output is recorded, this may be due to a gentle temperature change in the rotor 7; appropriate action could

then be taken. Alternatively, if a significant change in the output from the detector 21 is recorded, this may indicate that the rotor 7 is deteriorating drastically and may result in failure of the complete flywheel energy storage and conversion system. When this occurs, the apparatus should perhaps be shut down completely. Hence, the rate of change of an output from the detector 21 should be considered in detail.

As will be appreciated, the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the invention.