Flywheel

Abstract

A flywheel 100 comprises a plurality of discs 102 arranged in a stack 104, the discs including first and second end discs at either end of the stack and one or more discs between the first and second

Description

The present invention relates to a flywheel, particularly but not exclusively for use in energy storage and / or deployment. BACKGROUND TO THE INVENTION A flywheel is a device including a rotor assembly and an inertia element which can be used as a means for storing kinetic energy. The majority of the stored energy is stored in the inertia element which has a high moment of inertia relative to other elements of the rotor assembly, such as an electrical machine and mountings for low friction bearings. An electrical machine is commonly used for accelerating the inertia element in order to store energy, or decelerating the inertia element to release energy. The inertia element stores energy by virtue of its angular momentum; the faster it rotates, the more energy is stored, with energy storage being proportional the square of angular speed. In order to store a high level of energy (i.e. have high capacity for energy storage), the inertia element must operate at very high peripheral speeds, typically above the speed of sound (as measured in air under standard atmospheric conditions as opposed to conditions within the casing of the flywheel). The structure of the inertia element in an energy storage flywheel is therefore noticeably distinct from that of an inertia element in a flywheel designed for other applications. For example, a markedly different application is to provide a flywheel mounted on the crankshaft of an internal combustion engine for smoothing out intermittent torque and preventing the engine from stalling. A flywheel used for such smoothing typically has a rotor peripheral speed in the order of tens of metres per second (comparatively low energy capacity), whereas energy storage flywheels have rotor peripheral speeds in the order of hundreds of metres per second (comparatively higher energy capacity). The result is that the flywheel for such smoothing application is not subjected to the very high stresses that the energy storage flywheel type must resist. A further distinction is that the energy storage flywheel is able to retain energy for periods of many seconds, minutes or even hours and then delivery it when needed. In the case of a flywheel used for torque smoothing, small amounts of energy are transferred in and out of the flywheel in a passive manner in synchronicity with the pulsations of the internal combustion engine. The energy stored per unit mass of rotor material in an energy storage flywheel is thus around 10-100 times greater than that of a flywheel used for engine torque smoothing, and the energy storage flywheel needs to be volumetrically compact and light to offer performance which is competitive with other energy storage technologies such as electric batteries. A major design challenge for inertia elements used in the energy storage class of flywheels is how to withstand the high centrifugal stresses induced by the high peripheral speeds, whilst at the same time also maximising safety during operation of the energy storage flywheel. One approach is to ensure that the risk of rotor structural failure is made negligible, i.e. that the flywheel inertia element should never undergo material failure. This may be done by using materials of ultra-high quality, using non-destructive testing means to ensure material quality and carefully monitoring use of the rotor, in particular its number of operational cycles completed. Such techniques have been developed by the aerospace industry but the materials required and monitoring undertaken are both costly and time-intensive. Another approach is to accept that the inertia element may fail during operation and, even though this would be a rare occurrence, expelled fragments from the inertia element (having high linear kinetic energy) must then be contained by means of a casing to avoid damage to property, or, in more serious cases, human casualties. This may be achieved at a comparatively lower cost than the first approach above, because lower cost materials can be used for mass manufacture, and less rigorous monitoring is required so maintenance costs can be lower. However, to ensure safety, the mass of the containment element needs to be substantially greater than that of the inertia element to be contained, with experts recommending a figure of around ten times the mass of the inertia element. The size and expense involved in implementing such safety means that the main practical implementation is to put the energy storage flywheel (typically having monolithic steel inertia element) in an underground bunker. However, the installation cost is still high and there is reduced flexibility in how the stored energy can be installed. A third approach is to make the inertia element from fibrous composite materials which can result in materials substantially stronger than those in the above approaches. It was once thought that composite materials would always fail in a relatively benign manner, given that the inertia element would in theory break up into many small particles which could be contained more easily, e.g. using a lighter, cheaper casing than for the second approach. However, some failure modes of the inertia element can still be extremely violent due at higher energy storage levels, and can generate high pressure inside the casing. A flywheel energy storage system using composite material in the inertia element is therefore also normally placed in a very thick and heavy containment vessel or in an underground bunker for safety reasons. A fourth approach is to use metallic materials for the inertia element. The metallic material is typically steel but, instead of using a monolithic cylindrical design, the inertia element can be assembled from a stack of thin discs or laminates. Since the highest stresses in a rotating disc are tangential and radial, using a series of thin discs means that the rotor can operate at as high a peripheral speed as (or an even higher speed than) a monolithic cylinder. Indeed, the stresses can actually be lower using such an approach since axial stresses are reduced. In the case of a structural failure in the inertia element of such construction, only a fractional part of the whole inertia element will end up being ejected. Therefore, this approach substantially reduces level of containment required for safe operation of the flywheel, and a much lighter casing can be used. That is, neither a thick heavy casing nor bunker installation is required, substantially reducing cost and facilitating flywheel installation above ground and in compact formations. An important consideration for this fourth approach is to ensure that failure in one disc does not propagate to adjacent discs (and beyond), otherwise a cascade-type failure may occur and release fragments from more than one disc. In addition, the discs / laminates need to be firmly affixed to one another in the stack, and the stack must be connected to shafts such that the rotor can be located in bearings. It should be noted from a commercial perspective that the ability to manufacture the flywheel rotor at low cost is critical. Otherwise, it remains cheaper to use alternative energy storage means, such as electrochemical batteries or ultracapacitors, even if those other means provide less practical solutions for energy storage applications where the ideal energy storage solution is a flywheel-based system. US7267028 (Gabrys) discusses a laminate-type flywheel where it is recognised that a central hole in the discs would lead to high stresses, reduced peripheral speed and lower performance. In addition, favourable high strength properties could be obtained for thin steel as compared to a thick monolithic cylinder. Gabrys discusses two ways of joining the discs and connection means to the shafts for the bearings. The first is described with respect to Figure 12 of US7267028, which is substantially reproduced in Figure 1 (prior art) of the present application. This first way relies on bonding the surfaces of the discs (1) by means of adhesive, soldering or braze (2). The connection means and the shafts (3) are also bonded. The problem with this approach is that the joints are placed under very high stresses, requiring a very strong bond material to prevent failure of the bonds. However, if the bonds are strong, a crack which starts in one laminate is able to propagate to the other laminates, leading to undesirable cascade failure. The result is that the benign case of having only one laminate fail is not realised. The reasons for the high stresses on the bonds are described below, with reference to Figure 1 (prior art) of the present application, which has been annotated relative to Figure 12 of Gabrys to facilitate the explanation. When a disc rotates at high speed, the maximum radial and tangential stresses occur in the region around the centre of the disc. This causes the axial thickness of the disc to reduce due to the Poisson’s ratio effect by an amount shown as At. The sectional shape of each disc during rotation is shown in dotted lines, which the deformation exaggerated for the purpose of illustration. Since the bonding is relatively thin, it is difficult for this to absorb the effect of the discs pulling apart at the bore unless the bonding is very strong. There is an additional problem concerning the connection means of the shafts in which the small discs each have a lower diameter than the main discs. The radial growth of the upper small disc illustrated by A / 2 will be less than that of the adjacent main disc An. This will cause substantial shear stresses in the bonding which must therefore be strong enough to resist this. However, with strong joint bonding, a cascade failure will not be avoided if a crack develops in one of the discs for the reasons described earlier. In this case, a crack is most likely to form around the centre of one of the discs. As it grows radially outwards and across the disc, stress in the two adjacent discs increases as the load is transferred to these discs via the strong bonding. Given these adjacent discs are already operating under higher stresses, this increase in local stress will likely cause cracks to develop in the adjacent discs. This sequence will repeat until several, if not all, of the discs are cracked, leading to a multiple disc failure mode which is highly undesirable. Lastly, the addition of stepped features (see Figure 13 of Gabrys) may strengthen the joints, but it will lead to high stresses in the discs at the corners of the stepped features in the female side of the fitting. Manufacturing the stepped features is also expensive. US10138980 (Sanders et al.) attempts to address some of the problems discussed above by the use of spigots on each of the discs, and the use of a collar between each of the joints. However, these joints require precision manufacture. This may be viable for a limited manufacture of very large flywheels of the weights described by Sanders, but would be very cost-prohibitive for mass production of flywheels. This structure may also be somewhat unstable during use given that the joints have a low diameter and the discs are not mechanically locked together. EP2759043 (Pullen) describes a laminated flywheel construction in which bolts can be inserted through the discs via specially-shaped apertures, for reduction of stresses at the apertures. Figure 2 (prior art) of the present application depicts Figure 14 of EP2759043, in which the disc stack (12) is clamped between two end plates (20) using an array of bolts (42, 44). The end plates (20) allow the disc stack to be connected to shafts for rotation. The bolts pass through specially shaped apertures in the discs (12) and are stabilised in position within the holes by inserts. The disc with openings in EP2759043 is intended to operate at the same speed as one without openings. However, to ensure flywheel stability, the structure requires precision fits between its parts, which is expensive. It is an object of the present invention to reduce or substantially obviate the above problems, particularly by (but not limited to) providing a flywheel which has reduced containment requirements and can preferably be manufactured at relatively low cost. STATEMENT OF INVENTION According to the present invention, there is provided a flywheel comprising a plurality of discs (or laminae / laminates) arranged in a stack, the plurality of discs including at least first and second end discs at either end of the stack and one or more discs between the first and second end discs, each disc including first and second opposing faces, each of the first and second opposing faces including a central region and a peripheral region disposed around (or adjacent to I near) the central region, the peripheral region being adjacent to a peripheral edge of that disc, and (primary) bonding means for bonding together adjacent peripheral regions of neighbouring discs for flywheel rotation, in which one, some or all of the one or more discs includes: a first set of bonding areas (which may be considered as pads) in the peripheral region of the first face, and one or more spacer areas in that peripheral region for spacing apart the bonding areas on the first face; a second set of bonding areas (which may be considered as pads) in the peripheral region of the second face, and one or more spacer areas in that peripheral region for spacing apart the bonding areas on the second face; and at least one of: one or more of the bonding areas at the first face is disposed opposite a spacer area on the second face of that disc; one or more of the bonding areas at the second face is disposed opposite a spacer area on the first face of that disc. The present invention addresses the problem of catastrophic flywheel failure by substantially preventing crack propagation through the stack of discs. In particular, the manner in which the discs are bonded to each other substantially prevents a crack which develops in one disc from passing or crossing to multiple other discs. This is because structural bonding (that is, bonding sufficient to maintain flywheel integrity at operational speeds) between the discs is limited to the bonding areas, and so a crack would need to grow into one of the bonding areas in order to propagate to another disc. Whilst a crack could potentially pass or cross to a single neighbouring disc, any crack is unlikely to propagate fully from one element to a second in practice due to the construction of the inertia element in the flywheel. Since a crack is substantially unable to propagate through the series of discs, the flywheel containment requirements (in the event of catastrophic disc failure) are substantially lower. In particular, it may only be necessary to account for a maximum of two discs bursting, instead of the entire flywheel, because any mass ejected as a result of the failure should have a comparatively lower mass or kinetic energy. The flywheel is therefore safer to operate and can be operated for a longer time at the high rotational speeds (preferably supersonic speeds, relative to the speed of sound in air at atmospheric pressure) required for substantial energy storage. The flywheel of the present invention also has a comparatively simpler construction than the prior art devices referenced above. In particular, it avoids the need for apertures in circular laminates and the associated disc stress as in EP2759043B1, whilst bonding is provided at a position of lower stress for the disc. It also substantially mitigates the potential of crack propagation when compared to the device in US7267028 and addresses the drawbacks described with respect to LIS10138980. The number and arrangement of the bonding areas and the spacer areas are selected to provide suitable structural integrity to the flywheel for rotation at flywheel speeds. During flywheel rotation, the discs can to some extent thin out and separate from each other, which minimises contact area between opposing central regions of adjacent discs. By having bonding means in the bonding areas within the peripheral region, disc separation during use has a lower or negligible impact on the interface between an adjacent pair of discs I rotor elements. It will be appreciated that the nature of disc-to-disc bonding in this flywheel requires only some of neighbouring peripheral regions to be bonded together, whilst leaving other portions of the neighbouring peripheral regions as spacer areas which are either not bonded to the neighbouring disc or not bonded in a manner that would facilitate crack propagation between the discs. The spacer areas between the primary (structural) bonding areas may be non-bonded areas. The spacer areas between the primary (structural) bonding areas may constitute a second type of disc-to-disc bonded area, which provides non-structural bonding. That is, bonding designed to resist flywheel rotation forces during normal operation but which fails when a crack tries to propagate through it. The spacer areas may be partially or entirely occupied by a secondary bonding means / agent, which is different to the primary bonding means / agent. A spacer area therefore may not need to comprise any ‘empty space’. The term spacer area is intended to signify that the primary bonding areas, for flywheel structural integrity, are discrete or separated from each other. That is, there is a discontinuity. The flywheel structure relies only on bonding of the discs to themselves and the plate members by the primary bonding means in order to maintain structural integrity. The flywheel may be capable of achieving a peripheral speed of at least 350 metres per second (ms'1) during use. The flywheel may have a kinetic energy storage capacity of at least 100 kilojoules (kJ) during use. Preferably, the flywheel may have a kinetic energy storage capacity of at least 200 kJ, at least 250kJ or at least 300kJ. The flywheel (in particular, the inertia element thereof) may have a minimum stored energy of 25 kJ per kilogram during use. Each disc may be considered as a laminated element or an inertia element (or laminated inertia element) of the flywheel. Each disc is configured or structured for the stack to store a majority of the kinetic energy in the flywheel during rotation. Any of the spacer areas may be disposed at substantially the same radial distance (or may cover the same radial extent) from a centre of the disc as the bonding areas it is disposed between. A spacer area on one face of a given disc may overlap a spacer area on the opposing face of that disc, in addition to the overlapping the opposing bonding area. The peripheral region may be considered to lie beyond a distance of at least half of a radius of the disc. The peripheral region may be considered to lie beyond a distance of at least two thirds of a radius of the disc. The peripheral region may be considered to lie beyond a distance of at least three quarters of a radius of the disc. The peripheral region may be considered to lie beyond a distance of at least 80% of a radius of the disc. The radius may be considered as a straight-line distance from a centre of the disc (which in use sits on the flywheel rotation axis) radially outwards to the perimeter of the disc. A castellated or crenellated (or pseudo-castellated or pseudo-crenellated) bonding arrangement may be provided. That is, arrangement of bonding areas may be considered as a series of castellations (particularly horizontal portions thereof) when viewed from a lateral side of a given disc. Put another way, bonding areas may alternate between the opposing faces of the disc in a direction around the perimeter of the disc. This may be achieved by having both of: each bonding area at the first face disposed opposite a corresponding spacer area on the second face of that disc; and each bonding area at the second face disposed opposite a corresponding spacer area on the first face of that disc. The castellated nature of the disc-to-disc bonding or joints ensures that no more than two discs or rotor elements will fail catastrophically. This greatly limits the requirements of whatever containment means may need to be provided around the flywheel. This correspondingly reduces both cost and weight of the containment. Individual flywheels according to the invention can be placed in close proximity to one another (preferably whilst contained) as part of a flywheel installation. This is because flywheel failure in a first flywheel is unlikely to substantially impinge on or impede continued operation of neighbouring flywheels in the vicinity of that failed flywheel. A plane taken perpendicular to the stack of discs, passing centrally through a spacer area and arranged on a radial path from the flywheel rotation axis, may not pass through a bonding area of primary bonding means. This may be the case for a single disc, for a pair of adjacent discs, for a portion of the stack comprising three of more discs, or preferably for the entire stack of discs. For a given disc, the first set of bonding areas on the first side may be rotationally offset from (or staggered relative to) the second set of bonding areas on the second side about a flywheel rotation axis. The first and second sets of bonding areas may be in antiphase about the flywheel rotation axis. That is, a given bonding area may be disposed directly opposite a spacer area and equidistantly spaced (in a direction around the disc perimeter) from the nearest opposing bonding areas on the other side of that disc. This mitigates crack propagation between adjacent discs in a longitudinal direction along the stack. Each spacer area may be larger in area or have a greater angular extent than the bonding area it opposes for spacing apart bonding areas on the opposing faces of the disc. This reduces the likelihood of crack propagation between adjacent discs by increasing the relative through-disc distance between adjacent bonding areas on opposing sides of the disc. The bonding areas and the spacer areas may be regularly or equidistantly spaced around the peripheral region. Each bonding area may be substantially identical to each other bonding area on a given side of a given disc. Each spacer area may be substantially identical to each other spacer area on a given side of a given disc. Each bonding area on a given side of at least one disc may be the same. Each spacer area on a given side of at least one disc may be the same. The bonding and spacer areas on that disc side may accord to the equation 2N(p+g) = 360°, where N is the number of bonding areas on the given side, p is the angular extent in degrees of a given one of the bonding areas, and g is the angular extent of in degrees of a given one of the spacer areas. Having a relatively uniform bonding pattern contributes to more even distribution of force or stress in the flywheel during operation. This can reduce the likelihood of disc failure in the flywheel. Any suitable number of discrete bonding areas may be provided on each disc in the flywheel. The number of the bonding areas may differ between different discs in the flywheel. Preferably, at least eight discrete bonding areas may be provided in the peripheral region. In some embodiments, there may be any integer number of discrete bonding areas in the range 8 to 32 inclusive, on each side of the disc. This includes any even or odd number of bonding areas such as nine, ten, eleven, twelve, thirteen, fourteen, fifteen or sixteen (or more) discrete bonding areas in the peripheral region. Even higher numbers of bonding areas may be provided, if desired. A corresponding number of spacer areas should be provided in the peripheral region. In some embodiments, it may be preferred to provide a series of substantially U-shaped bonding areas. If U-shaped areas are provided, then the straight sides of the U-shape may be parallel to each other or may be disposed in a substantially radial direction on the surface of the disc. So, the U-shape may not have exactly parallel sides. It will be appreciated, however, that any suitable size and / or shape of bonding area and / or spacer area may be provided for each disc. The bonding areas may meet or extend from / to the peripheral edge of the disc. This maximises the distance of the bonded area from the centre of the flywheel, to a position where disc thinning during flywheel rotation is minimised. One, some or all of the bonding areas may each comprise a recess formed in the peripheral region. The recess(es) may be formed by chemical etching or mechanical stamping of the disc(s). The depth of the recess(es) is small relative to the thickness of a disc, typically on the order of tens of micrometres. The or each recess can provide a region into which bonding means or material is accommodated between the discs. The bonding material can in some instances flow into or be drawn into the recess(es) during the process of bonding the discs together into a stack. This can mitigate the need to use shims or shim plates. At least one shim may be provided in the stack. A plurality of shims may be provided in the stack. The or each shim may be disposed between a given pair of neighbouring discs. The shim(s) may not be substantially bonded to the discs but may be held in place between the discs primarily by virtue of the disc-to-disc bonding. The shim(s) may be made of metal or metal alloy, or any other suitable material. If the shim(s) are made of metal or metal alloy, then this may be or include any of steel, aluminium or titanium, although other suitable metals / alloys are also contemplated and this list should not be construed as exhaustive. Any of the shim(s) may include a plurality of peripheral spaces or cut-outs at its perimeter. The peripheral spaces may correspond to or define the bonding areas at respective faces of the neighbouring discs. One, some or all of the peripheral spaces may extend inwards towards a centre of the shim from the perimeter of the shim. The use of one or more shims between one or more respective pairs of adjacent discs facilitates more accurate control of the shape of the bonding areas, compared to depositing bonding agent on the disc surface and then clamping the stack (for the duration required to bond the discs together). The peripheral gaps in the shim(s) can provide small and accurately sized axial gaps on the order of tens of micrometres. The use of shims can be particularly advantageous in respect of brazed or soldered bonds between discs, because the bonding agent can flow into the spaces or pockets provided by positioning of the shim(s) between the pair(s) of discs. Any suitable size, shape and / or arrangement of the peripheral spaces may be provided in a shim. Any of the shim(s) may include a central aperture sized to correspond to a central region of the adjacent discs. The central aperture may have a diameter greater than half of a diameter of the shim. This reduces the weight of the shim. Where a shim is provided between a set of pairs or each pair of neighbouring discs in the stack, the peripheral spaces of neighbouring shims (to either side of a disc) are rotationally offset from (or staggered relative to) each other or in antiphase about the flywheel rotation axis. This mitigates crack propagation between adjacent discs in a longitudinal direction along the stack. At least one carrier sheet for carrying the bonding means may be provided. A plurality of carrier sheets may be provided. The or each carrier sheet may be provided between a pair of neighbouring discs in the stack. Any of the carrier sheet(s) may be porous or fibrous. The carrier sheet may be considered to be a mat, such as a porous or fibrous mat. This provides another means for bonding adjacent pair(s) of discs together. Bonding agent or material can be provided in the desired positions on each side of the carrier sheet for bonding neighbouring discs. This facilitates accurate placement of the bonding means and makes it easier to ensure the required bonding pad pattern is provided during the process of bonding the stack of discs together, including where required clamping the stack for the time required to bond the discs together. Where disc surface recesses, shims and / or carrier sheets, or any combination thereof, are used to facilitate disc-to-disc bonding in a stack, suitable consideration may be given to the relative orientation of bonding areas provided or facilitated by those features a first pair of discs (discs 1 and 2) and a second pair of discs (discs 2 and 3) in order to preferably rotationally offset or stagger the bonding areas thereof. That is, to provide the bonding areas out of phase. The shim(s) and / or carrier sheet(s) may be thin compared to the thickness of a given disc. For example, each may have a thickness which is substantially 10% or less of the thickness of a disc. The bonding means may bond only part of each disc to each neighbouring disc in the stack for mitigating crack propagation between adjacent discs in a longitudinal direction along the stack. The bonding means may include a first bonding agent and a second bonding agent. The first bonding agent may be on the first face and / or second face of a given disc. The second bonding agent may be on the first face and / or second face of a given disc. The second bonding agent may be different to the first bonding agent. The first and second bonding agents may be provided interleaved or ‘out of phase’ on the first and second sides of the disc. That is, first bonding agent on the first side may be out of phase with first bonding agent on the second side. Second bonding agent on the first side may be out of phase with second bonding agent on the second side. Secondary bonding means may be provided for conducting heat between at least one pair of neighbouring discs. The secondary bonding means may be more thermally conductive than the primary bonding means. This can facilitate heat flow axially, i.e. in a longitudinal direction along the stack, from one disc (or rotor element) to another by means of conduction. Heat can be generated within the rotor assembly during use, for example by the electrical machine used to drive it, and it is important to dissipate this heat effectively. Although the bonding means through the pads gives one pathway for heat conduction, heat conduction may be enhanced by having an additional thermal conducting means, which can be achieved by providing secondary bonding means in positions between the bonding areas. The secondary bonding means may provide weaker disc-to-disc bonding than the primary bonding means for mitigating crack propagation via the secondary bonding means. In that case, axial overlap of two areas of secondary bonding means on either side of a given disc may be provided for maximising heat conduction between discs. The secondary bonding means should be strong enough to resist the high centrifugal field (relative to the rotor) in order that it stays in place during flywheel rotation. However, it should also be weak enough that it does not facilitate crack propagation through the stack. For example, a comparatively weaker type of solder may be used as thermal conducting means which will fail before allowing a crack to pass through it from one disc to another. Thus, if a crack grows into the region bonded by the secondary bonding means, then the secondary bonding means fails before the neighbouring disc fails, i.e. before the crack can pass or cross to the neighbouring disc. This is why overlap of these areas is acceptable, whereas it is critical not to have overlap of the structurally bonded areas (provided by the primary bonding means). One, some or all of the areas of secondary bonding means may be larger than the bonding areas of the primary bonding means. This can improve the rate of heat conduction between the discs. The secondary bonding means may be provided in one or more portions of the spacer areas. The secondary bonding means may be provided spaced apart from the primary bonding means. This may be preferred where the spacer areas are larger than the corresponding bonding areas. The secondary bonding means (e.g. relatively weaker solder, or weaker bonding agent / means than the primary bonding pads) may entirely fill the spacer areas. That is, there may be no empty space or gap between the primary bonding means and the secondary bonding means. Sections of the spacer areas may accommodate the secondary bonding means to provide areas of suitably-sized heat-conducting (but non-structural) disc-to-disc bonding, in order to better dissipate heat between the discs of the stack. The bonding means may be selected to comprise any of the following: welding, braze, solder, diffusion bonding, adhesive such as strong engineering or structural adhesive (for example, epoxy resin). It will be appreciated that any suitably strong bonding means for tolerating flywheel rotation may be used. Braze and solder are given as examples of a soft metal or alloy with a low melting point which could be used, and it will be appreciated that other soft metals or alloys (preferably having a melting point below around 600°C) could be used instead. This is because temperatures above 600°C can adversely affect the structure / strength of the discs where they are made of steel, i.e. its heat treatment. First and second plate members (or cheek plates) may be disposed at opposing ends of the stack. The first and second plate members may be bonded to opposing ends of the stack. The stack of discs may not include a clamping means for clamping the discs in the stack together. That is, there may be no clamping elements which clamp the stack together. The first and second plate members may lack apertures for clamping the stack together. The or each plate member should ideally be structured such that during use it is subject to levels of stress comparable or similar to the stresses that the discs are subjected to. Each disc may be of similar thickness. Either or both of the first and second plate members may have a thickness substantially the same as the disc thickness. The first and second end plates may be thicker than a disc. This can improve end plate stiffness for connecting the shafts to the rest of the stack. At the periphery of either or both end plates, the end plate(s) may be as thin as any or a majority of the rotor discs. Either or both end plates may have a shallow conical shape (in terms of a side view of the stack) for increased stiffness. That is, to provide a stiffer element or plate onto which the shaft can be connected. By having plate members which are axially much thinner than conventional heavy clamping plate members (that is, thinner in a direction parallel to the flywheel rotation axis), the containment means can also be thinner and lighter than compared to flywheel devices having clamping means for the inertia element of the flywheel. For similar reasons, the mass of the plate member(s) may be selected such that if the plate member(s) fail catastrophically. The containment requirements for an installation around the flywheel to safely contain outbound plate fragments should therefore not significantly exceed the containment requirements for the scenario where part of the disc stack fails catastrophically. It will be appreciated that, for bonding the plate members to the stack, either or both plate members may have bonding areas and spacers areas corresponding to the adjacent disc. The above disclosure in respect of disc-to-disc bonding can therefore also apply in respect of disc-to-plate bonding. Each disc may be made of a metal or may be a metallic laminate. Each disc in the stack may lack through apertures for receiving clamping means. One or more additional stack elements may be disposed at or bonded to one or both ends of the stack. The additional stack elements may be provided instead of plate members. Each additional stack element may include a laminate having any of the following: a circular perimeter of lesser diameter than the discs in the stack; a noncircular perimeter; a central aperture. It will be appreciated that any of the discs may also comprise any of these features. The stack elements (or additional rotor elements) may be formed using a relatively low-cost method such as stamping, waterjet cutting or laser cutting, for example. One or more of the rotor elements may be formed by joining or bonding together any of: circular laminates of decreased outer diameter, laminates of a non-circular outer shape or laminates with a central aperture. The bonding can be achieved by having castellated or offset bonded areas as disclosed above, or by continuous bonding in an annular ring. The latter is acceptable in the case where the diameter of the rotor elements is less than that of the discs making up the main body of the stack. This is because if more than two of the rotor elements making up the end plates failed, the energy released should not be substantially greater than that of two discs or laminates failing. Note that if the end plates are somewhat thicker than the disc thickness (or an average or mean disc thickness), then the energy released during catastrophic flywheel failure may be relatively higher. At a maximum, the end plates may have about twice the mass of a majority of the discs. That said, the energy involved (in terms of containment requirements) should not be double. This is because much of the mass should be closer to the centre of the flywheel. This is where the additional elements may (when viewed from the side of the stack) be considered to form a shallow cone. This cone type shape can be created by bonding shapes of smaller size and shapes, or could be made from a machined disc made from material thicker than a sheet used for forming a majority (or any) of the discs. A plurality of the additional stack elements may be provided at one or both ends of the stack. The width or diameter of successive additional stack elements decreases in a direction towards the end of the stack. Put another way, in a direction away from the end of the stack, the first stack element adjacent to the stack may have a width or diameter less than that of the stack and each subsequent stack element may have a width or diameter less than that of the preceding stack element. This can help to provide cheek plates which have relatively lower weight yet remain stiff enough to resist bending forces applied to the flywheel shaft. Such forces are generally small but if the end plate stiffness is too low then the whole shaft may resonate, which can lead to failure. One option is to use end plate(s) having a relatively thicker solid shallow cone (or a cone with sculptured profile), or for similar reasons to stack planar discs of decreasing diameter. A flywheel assembly may be provided which comprises one or more flywheels according to the invention. The or each flywheel may be mounted on or to a drive assembly for facilitating rotation of the or each flywheel, for storing energy in or deploying energy from at least one of the flywheels. The drive assembly may be mechanical, electrical, or any other suitable form of drive means. 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 only to the accompanying drawings, in which: Figure 1 shows a side view of a first prior art flywheel device from US7267028; Figure 2 shows a cross-sectional side view of a second prior art flywheel device from EP2759043; Figure 3 shows a side view of a first embodiment of a flywheel of the invention; Figure 3A shows a top view of a first embodiment of disc for the flywheel of Figure 3; Figure 3B shows an underside view of the disc of Figure 3A; Figure 4 shows a top view of the disc of Figure 3A, depicting bonding areas on the top and bottom surfaces of the disc; Figure 5 shows a top view of the disc of Figure 4, depicting exemplary pathways of cracks that could form in the disc; Figure 6 shows a perspective view of a second embodiment of disc for the flywheel of Figure 3, including bonding areas which are welded areas; Figure 7 shows an enlarged partial perspective view of a portion of the flywheel of Figure 3; Figure 8A shows a top perspective view of a third embodiment of disc for the flywheel of Figure 3, depicting bonding areas of deposited bonding material on the top surface of the disc; Figure 8B shows a bottom perspective view of the disc of Figure 8A, depicting bonding areas of deposited bonding material on the bottom surface of the disc; Figure 9A shows a top view of a first embodiment of a shim for a flywheel; Figure 9B shows a top view of a second embodiment of a shim for a flywheel; Figure 90 shows an enlarged partial perspective view of the shim of Figure 9A or 9B on a disc for a flywheel; Figure 10A shows an exploded partial side view of a second embodiment of a flywheel, including the shims of Figures 9A or 9B; Figure 10B shows a partial side view of the flywheel of Figure 10A; Figure 11 shows a partial perspective view of a third embodiment of a disc for a flywheel; Figure 12A shows a top view of a first embodiment of a carrier sheet for a flywheel; Figure 12B shows a top view of a second embodiment of a carrier sheet for a flywheel; Figure 13A shows a top view of a fourth embodiment of disc for a flywheel; Figure 13B shows a bottom view of the disc of Figure 13A; Figure 14 shows a side view of a third embodiment of a flywheel; Figure 14A shows a top view of a first disc for a flywheel end plate of Figure 14; Figure 14B shows a top view of a second disc for a flywheel end plate of Figure 14; and Figure 14C shows a top view of a third disc for a flywheel end plate of Figure 14. DESCRIPTION OF PREFERRED EMBODIMENTS Figures 1 and 2 relate to prior art devices, which are described in the background section. Figure 3 relates to a first embodiment of a flywheel, indicated generally at 100. The flywheel 100 is designed for use in kinetic energy storage which can be used, in conjunction with a suitable electrical motor-generator or mechanical drive (or any other suitable type of drive means) to provide a means of storing electrical energy. Many applications are possible for the flywheel 100, including but not limited to any one or more of: local grid boosting for fast charging electric vehicles; uninterruptible power supplies; trackside rail; demand side management; and electrical grid services as some examples. The flywheel 100 could be provided in a vehicle with an electric propulsion system, such as a car, truck, bus, train, plane or boat. The flywheel 100 could also be used to provide kinetic energy storage where the stored energy can be transmitted mechanically to assist a vehicle accelerating and to retrieve otherwise lost kinetic energy. The structure of the flywheel 100 is primarily described with respect to the flywheel at rest (i.e. not moving / rotating), unless otherwise specified. The flywheel 100 includes a plurality of discs (also referred to as laminates or rotor elements), some of which are indicated at 102. Each disc 102 is substantially the same size and shape in this embodiment. Each disc 102 is bonded to each of its neighbouring discs to form a stack 104. None of the discs 102 requires any disc apertures because the stack 104 does not need to accommodate any connecting or clamping elements therethrough. In this embodiment fifteen discs 102 are provided, but it will be appreciated that any suitable number of discs may be provided in other embodiments subject to there at least being first and second end discs, and at least one disc between those discs. The discs 102 are substantially circular in profile in this embodiment. Centres of the discs 102 are aligned along a common longitudinal axis A-A to provide the stack 104 of discs. The stack 104 is substantially cylindrical in this embodiment. The stack 104 may be considered to provide an inertial element of the flywheel 100. The stack 104 may be considered as a laminate stack. Each of the discs 102 can be bonded to its neighbouring discs by means of a primary bonding means or agent selected from any of: welding, brazing, soldering, diffusion bonding or strong engineering adhesive. The bonding is provided in a castellated pattern or arrangement, described further below with respect to Figures 3A and 3B. The discs 102 may each be made of a metallic laminate or steel in this embodiment, although another suitable metal, alloy or composite material may be used in other embodiments. Each disc 102 lacks apertures for receiving a bolt or other clamping means. Each disc 102 in the stack is preferably substantially the same in this embodiment, optionally to a tolerance on the order of microns or tens of microns. Plate members or cheek plates (which may also be referred to as rotor elements) 106 are provided at either end of the stack 104. The plate members 106 are bonded to the discs at the respective ends of the stack 104 by any of the means described above with respect to bonding the discs together. In this embodiment, the plate members 106 are substantially the same diameter as the discs 102. Each plate member 106 is about the same thickness as a given one of the discs 102. An outer surface region of each plate member 106 lies approximately parallel to the plane of the neighbouring disc, moving from a radial edge of the plate member 106 towards the axis A-A to a position approximately at a third of the plate member radius. The plate member 106 thickness then gradually increases moving further towards the axis A-A. The plate members 106 facilitate connection of the laminate stack 104 to bearing landings 108. The cheek plates 106 also allow the stack 104 to be connected or fitted (or more generally provided in operative engagement) together with an electrical machine rotor part or motor generator rotor part 110. It will be appreciated that the precise form of the plate members is not considered to be essential to the present invention. Figures 3A and 3B depict one of the discs 102 in the stack from above and below respectively. The top view corresponds to the plane X-X of that disc 102 in Figure 3. The bottom view corresponds to the plane Y-Y at the opposite side of the same disc in Figure 3. The disc faces shown in Figures 3A and 3B show the disc 102 in the same relative rotational position about the flywheel axis of rotation A-A, but from opposing sides. Figure 3A shows a first series of twelve bonding areas or pads (one of which is indicated at 112) on the first side of the disc 102. The pads 112 are regularly spaced around the perimeter P of the disc. The positions of the pads in this view may be considered to correspond to the positions of the numerals 1-12 on a clock face. It will of course be appreciated that this is not essential and any suitable number of bonding areas and positions / sizes of bonding areas can be provided in other embodiments. Figure 3B shows a second series of twelve bonding areas or pads (one of which is indicated at 114) on the second side of the disc 102. The pads 114 are again regularly spaced around the periphery of the disc. The positions of the pads in this view may be considered to correspond to positions midway between those of the numerals 1-12 on a clock face. Put another way, the bonding pattern on this side is the same as that of Figure 3A except that it is incremented around the disc by half the pitch of one bonding pad to another. It will of course again be appreciated that this arrangement is not essential and any suitable number of bonding areas and positions / sizes of bonding areas can be provided in other embodiments, subject to not overlapping with the pads 112 on the other side. The resultant bonding arrangement may be considered to provide a castellated or crenellated pattern of bonding, particularly within the peripheral region of the disc. That is, when considered from a lateral side of a disc in a direction moving around the disc, a bonding pad 112 on the top face is succeeded by a bonding pad 114 on the bottom face, which is in turn succeeded by a bonding pad 112 on the top face, which is succeeded by a bonding pad 114 on the bottom face, and so on. These pads can thus be considered as the horizontal portions of castellations I crenellations. Overlapping portions of the spacer areas 112a, 114a to either side of each bonding pad can be considered as the ‘vertical’ portions of the castellations / crenellations. The pads 112, 114 are substantially U-shaped in this embodiment. Each U-shape is substantially the same size and shape as the other U-shapes in this embodiment. The U-shape of each pad is arranged such that the U-shape is at the perimeter of the disc and the curved end of the U-shape is closest to the centre of the disc. It must be emphasised that the pads 112, 114 in the respective peripheral regions of the disc can be any shape and any size and any position, which may be independently selected for each pad, subject to not overlapping with the opposing pads 114, 112 of primary bonding means on the other side of the disc. It should be note that the pads 112, 114 do not extend too far towards the centre of the disc. This is to prevent the bonds being placed under high tensile stress during flywheel rotation, which can occur as a result of reducing thickness of the laminates due the Poisson’s ratio effect when operating at high speed. In this particular embodiment, the pads 112, 114 extend inwards by a distance which is approximately 15% of the disc radius. Spacer areas (or gaps) 112a are disposed between each pair of neighbouring pads 112. Spacer areas (or gaps) 114a are disposed between each pair of neighbouring pads 114. The spacer areas 112a, 114a are regions of the surface of each side of the disc that do not have the primary bonding means (or any disc-to-disc bonding means in some embodiments). The spacer areas may be considered to complement the shapes of the pads. Each spacer area 112a, 114a is larger in area than its two neighbouring bonding areas 112, 114 on the same side of the disc 102. In this embodiment, the angular extent of each spacer area around the periphery of the disc is approximately double (or slightly over double) the angular extent of each bonding area around the periphery of the disc. A similar bonding pattern may be used for both disc-to-disc bond and disc-to-end plate bonding. Figure 4 depicts the pads 112, 114 of both Figures 3A and 3B, treating the disc 102 as transparent solely for the purposes of illustration. The pads 114 of Figure 3B at the bottom surface are shown in ‘light’ shading to distinguish them from those at the top surface. By having a series of identical, regularly-spaced pads and a series of identical, regularly spaced apart spacer areas between pairs of those pads on the same side of the disc, i.e. an alternating arrangement, the equation 2N(p+g) = 360° can be used to describe the relationship between the number of pads N, the angular extent p of a given pad at the perimeter, and the angular extent g of a given spacer area next to that pad. This assumes the entire perimeter of the disc is occupied by a combination of bonding areas and spacer areas. The bonding pattern is next considered as part of the stack 104, in the case where the discs all have the same number of bonding pads and spacer areas on each of their sides. In the case of an alternating set of twelve pads and twelve spacer areas on a first disc, the pitch is equivalent to 30 degrees (i.e. one twelfth of 360°). The set of bonding pads on a second disc, adjacent to the first disc, is incremented by 15 degrees. The set of bonding pads on a third disc, adjacent to the second disc on the far side to the first disc, is again incremented by 15 degrees and thus gives rise to substantially the same bonding pattern (including orientation about the flywheel axis A-A) as the first disc. It will be appreciated that this gives rise to a bonding pattern which alternates between pattern 1 (that of odd numbered discs in the stack) and pattern 2 (that of even numbered discs in the stack). Of course, other patterns may manifest in other embodiments of stacks having identical discs and bonding pads but this is not essential and in some cases there may be no discernible repeating pattern. It does not matter whether the stack has an even or odd number of discs, and does not matter whether the two discs at the ends of the stack share or do not share the same bonding pattern or bonding patterns in the same relative orientation around the stack. It will also be appreciated that this arrangement means that each bonding area 112 at the first face is disposed opposite a spacer area 114a on the second face of the disc, and that each bonding area 114 at the second face is disposed opposite a spacer area 112a on the first face of the disc. Figure 5 shows variations of hypothetical crack propagation routes through a given disc having a castellated bonding arrangement (which in this case is the disc of Figure 4). A crack is most likely to develop at the most highly stressed region of the rotor element, which is at the centre of the rotor element, although the present invention is equally applicable for cracks originating at other positions. Analysis and testing of flywheels has shown that such cracks tend to grow in a direction that is predominantly radially outwards, perpendicular to the plane of maximum principal stress which is the hoop or tangential direction. It is possible for cracks to bifurcate into two but such two cracks will still tend to grow in a direction that is predominantly radial. Since the rotor element is thin, the crack will also remain in a predominantly radial axial plane. That is, the crack surface will be perpendicular to the disc surface of the rotor elements. The mechanism for crack growth is metal fatigue. Before a crack reaches the outer radius, it typically reaches a critical size after which it grows extremely quickly as a ‘fast fracture’. At this point, the forces within the rotor element become so large that the cracked rotor element will open up. This causes the whole rotor to become so out of balance that the machine will be shut down. Alternatively, the joints at the pads will fail in shear, resulting in the cracked rotor element rubbing on the containment casing and bringing the rotor to a halt. Should the crack follow a path shown by way of example as path X in Figure 5, which is disposed between the pads 112 and 114, the crack cannot transfer into a second rotor element. Should a crack follow a path shown by way of example as path Y in Figure 5, it may pass through one of the pads 112. There is a possibility that the crack could also transfer into a second rotor element 102 connected to the same pad 112 as the first rotor element 102 within which the crack started. Even should this be the case, a crack in the second rotor element is unlikely to cause fast fracture since the crack in the second element is not sufficiently long for this to happen. However, the castellated nature of the joints ensures that no more than two rotor elements will fail catastrophically. This greatly limits the requirements of the containment means. An example of a bifurcating crack is shown as path Z in Figure 5, which in this case has the bifurcations both passing through the same pad 112. The result of such a crack would also not lead to any failure of a major part or whole of the rotor for the reasons described for single cracks above. For two or more cracks to originate separately in different places and each grow to the same critical size to cause fast fracture simultaneously is statistically improbable. It is possible for two cracks to grow in the same rotor element in different places and merge to form a single crack, which may then grow to a critical size which causes fracture. However, this would also not lead to any failure of a major part or whole of the rotor for the reasons described for single cracks above. The disc-to-disc and disc-to-plate bonds should be made using a bonding material or agent (or other means) which is strong enough to maintain structural integrity of the flywheel 100 during high-speed rotation. As touched on above, bonding may be achieved in some cases by welding, soldering, brazing or other processing techniques to provide bonded interfaces between adjacent discs. Specific examples are discussed below. Figure 6 shows an example of a disc 102’ having welded (or diffusion bonded) areas, one of which is indicated at 116. One method of achieving this is first to clamp the rotor elements (discs 102 and / or plate member 106) together, either one by one or all together in a full stack 104’. The castellated pads 116 are then created by welding means. Electron beam welding is a preferred method because this avoids significantly altering the material properties of the rotor elements. However, other means of welding may also be selected, e.g. for cost effectiveness, if the properties of the rotor element material are not substantially adversely affected. Even then, given the stresses in the rotor elements are generally lower at the position of the pads, i.e. in the peripheral region of the disc, some reduction or weakening in material properties may be permissible in the region of the pads when compared to the material around the centre of the rotor elements. Figure 7 shows detail of the welding and how it results in a castellated pattern for a stack 104’ of the discs 102’. The bonding areas 116 are offset for each joint in the pattern in the manner previously described. The shape and depth of each weld 116 can be selected to be any shape and depth suitable for the disc-to-disc bonds to permit the flywheel 100 to operate properly and safely. Figures 8A and 8B show an example of a disc 102’ having bonded areas where bonding agent has been deposited directly onto the surface of the disc. One of bonding areas is indicated at 118. Using bonding agent is one alternative to welding, but it will be appreciated that these options are not considered to be mutually exclusive. The shapes of the pads 118 means may vary from one pad to another if the bonding means is deposited and squeezed down during the stack assembly process. More preferably the shape of each pad should be controlled to ensure a strong and uniform structure. The pad pattern depicted on the disc 102” is substantially similar to that of Figures 3A and 3B, albeit this time with sixteen pads 118 on each side, each of which has a random or irregular shaped outline. The shape can of course be any other shape. The bonding means for the pads 118 may be braze, solder or adhesive. In some embodiments, it may be advantageous to deposit two different types of bonding agent on adjacent rotor elements. One example of this includes having braze on one side of the rotor element 102” and flux on the other side. In another example, two different parts of an engineering adhesive system could be placed on neighbouring faces of adjacent rotor elements. Once the rotor elements have been assembled into an inertia element or stack, they are heated in order to form the bonds or set the bonding material. The exception is for engineering adhesive, which does not require heating for the joint to be cured. Note that targeted heating may be done using electron beams similar to the those generated by machines used for electron beam welding. This may be particularly suitable for brazed joints, whereby the braze is melted locally by means of an electron beam. The amount of bonding agent deposited should be carefully controlled. This is so that, the pads of bonding agent on the same disc side remain discrete or separate from each other when the rotor elements are pressed together. It is also to avoid causing bonding agents of pads 118 on opposing sides of the rotor element to overlap other as the material spreads under compression between the discs. Figure 9A shows a first embodiment of a shim 120 for positioning between two discs 102. The shim 120 is thin compared to a disc 102. The shim 120 is substantially circular and has a series of peripheral indents or cut-outs 120a. This provides spaces 120a which in this case are substantially U-shaped, extending inwards towards the middle of the shim 120. The U-shaped spaces may in some cases correspond to the U-shaped pads 112, 114 discussed earlier with respect to Figures 3A and 3B, but the flywheel of Figure 3 does not necessarily require shims to provide those pads. There may of course be other shapes of pad in other embodiments. Figure 9B shows a second embodiment of shim 122, which is similar to the first shim 120. The shim 122 also has U-shaped spaces 122a at its perimeter. However, in this case the shim 122 is substantially annular, having a central aperture or hole 122b through the middle. The central aperture 122b corresponds to the central region of a disc. Figure 9C shows either of the shims 120, 122 in position against a disc 102, providing a pocket indicated generally at 124 for receiving bonding agent. For creating a brazed or soldered joint, it is often advantageous to create a small and accurately sized axial gap, h, which may be between 10 to 100 micrometres or so in depth. This can be achieved at low cost in this case by using the shim 120, 122. This allows the shape of the pads to be controlled more readily than the method of depositing bonding means and clamping the rotor elements discussed with respect to Figures 8A and 8B. The cut-outs 120a, 122a facilitate the flow of solder or braze into the stack to form the pads 112, 114 precisely where they are needed. Solder or braze will not substantially flow into regions other than the pockets provided by the shim 120, 122 between a pair of discs 102. Variations of the above flywheel embodiment and component parts are contemplated within the scope of the present invention. Features of the following embodiments are the same as above except where described otherwise. Like reference numerals are used to refer to like features in the following embodiments, incrementing each numeral by a multiple of 100, e.g. feature 102 corresponds to features 202, 302 and so on in later embodiments. Figure 10A and 10B show a second embodiment of flywheel 200. The flywheel 200 includes a series of the shims 120 (and / or 122), described with respect to Figures 9A to 9C. An alternating arrangement of discs 202 and shims 120 is provided. Whilst not fully illustrated, it will be appreciated that similar bearings 208 and motor generator rotor part 210 are provided. The shims 120 are positioned in an alternating pattern relative to each other in a manner previously described with respect to the pads in Figures 3A and 3B. That is, in order to obtain the castellated pad pattern, a given pair of neighbouring shims 120 are rotationally offset to avoid any cut-outs 120a of one shim 120 from overlapping any cut-outs 120a of the immediately adjacent shims 120. In order to create the bonding pads, one of the above methods can be used, or another option is to assemble the whole rotor with shims 120 (or 122) in place and to use the dip braze or dip solder method. This involves immersing the rotor element (i.e. the stack of discs) in liquid braze or liquid solder, whether immersing partially or fully. The braze or solder is thus able to wick into the pockets 124 and form bonding areas or pads 212, 214 once the flywheel is removed and the braze / solder has solidified. Figure 11 shows a portion of a disc which may be provided in either embodiment of the flywheel 100, 200, without the need for a shim 120, 122. Here, a small recess of depth h is provided in one or more regions of the disc. This physical depression or recess 224 in the surface of the disc 202 is on the order of tens of micrometres in depth. Any suitable number of recesses 224 can be provided on both sides of the disc 202 to facilitate bonding at similar regions to those discussed in the above examples. That is, no recess 224 on one side of the disc 202 should overlap any recess on the other side of the disc. Braze or solder can flow into the recess by any of the previous methods during flywheel assembly. The recesses 224 in the disc 202 are created directly in the surfaces of the disc by means of chemical etching or mechanical stamping. Figure 12A relates to another means of providing bonding agent between a pair of discs 202 in the stack 204. Instead of shims (or in place of some shims), a suitable number of flexible thin sheets 226 can be provided for each positioning between an adjacent pair of discs 202 in the flywheel 200. Bonding agent has been applied to the carrier sheet 226 in the desired pattern of pads 212, near or at the sheet edge in the peripheral region of the sheet 226. Any suitable sheet can be used, but preferably the sheet 226 is made of a porous material. The carrier sheet 226 may be a fibrous mat. The bonding agent is thus provided at the desired positions to achieve the desired bonding pattern in the stack of discs 202 as the stack is clamped and the bonding process carried out. As in the previous examples, care needs to be taken to avoid having overlapping areas 212 of bonding agent at immediately adjacent carrier sheets 226 in the stack 204. That is, for two carrier sheets disposed at either side of a given disc, areas of bonding agent in one carrier sheet should not overlap areas of bonding agent in the second carrier sheet. Figure 12B shows a variation of carrier sheet 228 which differs from the carrier sheet 226 in that there is a central aperture 228a to minimise the weight of the sheet. Figures 13A and 13B depict another embodiment of disc 302 from above and below respectively. The disc faces shown in Figures 13A and 13B show the disc 102 in the same relative rotational position about the flywheel axis of rotation A-A, but from opposing sides. A plurality of discs 302 can be provided in a stack that is suitable for use in the preceding flywheels, whether involving shims, carrier sheets, disc surface recesses, or any combination thereof. Figure 13A shows a first series of twelve bonding areas or pads 312 (darker shaded pattern) on the first side of the disc 302, and a second series of twelve pads 330 (lighter shaded pattern) on the same side of the disc 302. The first series of pads 312 are intended to receive primary bonding means that provides structural bonding for the stack of discs. The second series of pads 330 are intended to receive a different bonding agent that provides weaker disc-to-disc bonding but provides better heat conduction between discs. The first and second series of pads 312,330 are regularly spaced around the perimeter of the disc. The positions of the pads 312 in this view may be considered to correspond to the positions of the numerals 1-12 on a clock face, whilst the other series of pads 330 are positioned in between adjacent pairs of the first series of pads 112. In this particular example, the pads 312, 330 are all U-shaped in plan view. However, as before, any suitable number, position and size of pads / areas can be provided. Figure 13B shows a third series of twelve bonding areas or pads 314 (darker shaded pattern) on the second side of the disc 302, and a fourth series of twelve pads 332 (lighter shaded pattern) on the same side of the disc 302 as those pads 314. The third series of pads 312 are intended to receive primary bonding means that provides structural bonding for the stack of discs. The fourth series of pads 332 are intended to receive a different bonding agent that provides weaker disc-to-disc bonding but provides better heat conduction between discs. The third and fourth series of pads 314, 332 are again regularly spaced around the periphery of the disc. The positions of the pads 314 in this view may be considered to correspond to positions midway between those of the numerals 1-12 on a clock face, whilst the fourth series of pads 332 are positioned in between adjacent pairs of the third series of pads 112. Put another way, the bonding pattern on this side is the same as that of Figure 13A except that it is incremented around the disc by half the pitch of one bonding pad to another. In this particular example, the pads 314, 332 are all U-shaped in plan view. However, as before, any suitable number, position and size of pads / areas can be provided, subject to no pad 314 overlapping with any pad 312 on the other side. It is acceptable for any of the pads 330 in the third series to overlap any of the pads 332 in the fourth series. It will be appreciated that spacer areas 312a, 314a are provided between the pads 312, 314 on both sides of the disc 302 for the reasons given previously. The pads 330, 332 of secondary bonding agent may be considered to sit within the spacer areas. Note that other embodiments are contemplated in which the more thermally-conducting pads 330, 332 are larger than the other pads 312, 314. This can further improve heat transfer between the discs. It will also be appreciated that various embodiments of the invention may involve having the whole disc assembly electron beam welded, according to any suitable variation of the previously-described castellated pattern. If so, the whole rotor I stack of discs may be dipped in a solder bath and there may not be any gaps or spacer areas between the (primary) structural bonding agent and the (secondary) thermal bonding agent. The reason for providing the second and fourth series of pads 330, 332 is to dissipate heat in the flywheel during use. Heat is generated within the rotor assembly by, for example, the electrical machine. That is, in addition to ensuring the structural integrity of the flywheel via the primary bonding means, it is important to facilitate heat flow axially from one rotor element to another via thermal conduction. This can be achieved by providing additional areas where the discs 302 can be bonded together in a way that does not facilitate crack propagation, because the secondary bonding means selected should fail before allowing a crack to propagate through it to a neighbouring disc. This improves heat conduction relative to embodiments where only the primary bonding means at the pads 312, 314 provides a pathway for heat conduction. The secondary bonding means in this example may be a weak type of solder (e.g. weaker than any solder / braze used as the primary bonding agent), although it can be used in examples where welding or engineering adhesive are provided as the primary bonding means, for example. Figure 14 shows another embodiment of a flywheel 300. Instead of the plate members of preceding embodiments, a potentially lower cost route involves providing end plates 306 which are constructed from various shapes of laminate 334. Examples of the possible laminate shapes 334a, 334b, 334c are depicted at Figures 14A, 14B and 14C, although any other suitable shape may be provided. The shapes may be created by stamping, waterjet cutting or laser cutting. In the example shown, the end plate 306 at each end of the stack is provided by an assembly of any of the rotor elements 334 depicted in Figures 14A to 14C. That is, circular laminates of decreased outer diameter, and / or laminates of a non-circular outer shape, and / or laminates with a central aperture, can be sequentially joined together to provide the end plate 306. The shapes are bonded together either using castellated bonding in the manner described above or by continuous bonding in an annular ring. The latter is acceptable because the diameter of these rotor elements 334 is lower than the main discs 302. Thus, if more than two of the rotor elements 334 making up the end plates 306 were to fail, the energy released would not be any greater than that of two laminates 302 failing. It is emphasised that any of the discs 102, 202, 302 can be used in any of the flywheel embodiments, and any combination of shims, carrier sheets and disc recesses can be used with any suitable primary bonding means. The flywheel 100, 200, 300 can provided as part of an assembly with a suitable drive mechanism. The assembly may be considered to be part of an energy harvesting and deployment system. The assembly is provided in containment suitable for safe operation of the flywheel, taking account of the mass and energy storage capacity of the flywheel. During use, the flywheel 100, 200, 300 is rotated by the drive mechanism about the axis A-A to store kinetic energy in the flywheel. The flywheel 100, 200, 300 can deform radially outwards at speeds on the order of hundreds of metres per second, when used to store substantial amounts of kinetic energy. The drive mechanism can be used to deploy a portion of the stored kinetic energy in a rotating flywheel by reducing the rotational rate of the flywheel, harvesting the stored kinetic energy and converting it into another useful form, e.g. electrical energy. Further instances of energy gathering and harvesting may be carried out as needed, which may include thousands or millions of cycles of energy deployment on the order of hundreds of kilojoules, several megajoules, or greater e.g. if multiple flywheels are provided in an assembly and work together in parallel. Whilst different examples and embodiments of flywheel and flywheel components (such as discs and bonding means) have been presented above, it will be appreciated 5 that other embodiments of flywheel can be provided in which any combination of features from any two or more embodiments can provided as a new embodiment, within the scope of this disclosure. The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without 10 departing from the scope of the present invention as defined by the appended claims.

Claims

1. A flywheel comprisinga plurality of discs arranged in a stack, the plurality of discs including at least first and second end discs at either end of the stack and one or more discs between the first and second end discs, each disc including first and second opposing faces, each of the first and second opposing faces including a central region and a peripheral region disposed around the central region, the peripheral region being adjacent to a peripheral edge of that disc, andbonding means for bonding together adjacent peripheral regions of neighbouring discs for flywheel rotation, in which one, some or all of the one or more discs includes:a first set of bonding areas in the peripheral region of the first face, and one or more spacer areas in that peripheral region for spacing apart the bonding areas on the first face;a second set of bonding areas in the peripheral region of the second face, and one or more spacer areas in that peripheral region for spacing apart the bonding areas on the second face; andat least one of:one or more of the bonding areas at the first face is disposed opposite a spacer area on the second face of that disc;one or more of the bonding areas at the second face is disposed opposite a spacer area on the first face of that disc.

2. A flywheel as claimed in claim 1, in which a castellated or pseudo-castellated bonding arrangement is provided, when viewed from a lateral side of a given disc, by having both of:each bonding area at the first face disposed opposite a corresponding spacer area on the second face of that disc; andeach bonding area at the second face disposed opposite a corresponding spacer area on the first face of that disc.

3. A flywheel as claimed in claim 1 or claim 2, in which, for a given disc, the first set of bonding areas on the first side are rotationally offset from the second set of bonding areas on the second side about a flywheel rotation axis.

4. A flywheel as claimed in claim 3, in which the first and second sets of bonding areas are in antiphase about the flywheel rotation axis.

5. A flywheel as claimed in any preceding claim, in which each spacer area is larger in area or has a greater angular extent than the bonding area it opposes for spacing apart bonding areas on the opposing faces of the disc.

6. A flywheel as claimed in any preceding claim, in which the bonding areas and the spacer areas are regularly or equidistantly spaced around the peripheral region.

7. A flywheel as claimed in any preceding claim, in which for the first side and / or the second side of a given disc: each bonding area is substantially identical to each other bonding area on that side, and each spacer area is substantially identical to each other spacer area on that side.

8. A flywheel as claimed in claim 7, when dependent on claim 6, in which each bonding area on a given side of at least one disc is the same, each spacer area on the given side is the same, and the bonding and spacer areas on the given side accord to the equation:2N(p+g) = 360°where N is the number of bonding areas on the given side, p is the angular extent in degrees of a given one of the bonding areas, and g is the angular extent of in degrees of a given one of the spacer areas.

9. A flywheel as claimed in any preceding claim, in which at least eight discrete bonding areas are provided in the peripheral region.

10. A flywheel as claimed in any preceding claim, in which at least some of the bonding areas each comprise a recess formed in the peripheral region.

11. A flywheel as claimed in any preceding claim, in which at least one shim is provided in the stack, the or each shim being disposed between a given pair of neighbouring discs, the or each shim including a plurality of peripheral spaces at its perimeter which correspond to or define the bonding areas at respective faces of the neighbouring discs.

12. A flywheel as claimed in claim 11, in which the shim includes a central aperture sized to correspond to a central region of the adjacent discs, the central aperture having a diameter greater than half of a diameter of the shim.

13. A flywheel as claimed in claim 11 or claim 12, when dependent on claim 3, in which a shim is provided between each pair of neighbouring discs in the stack, and the shims in a given neighbouring pair of shims are rotationally offset from each other or in antiphase about the flywheel rotation axis.

14. A flywheel as claimed in any preceding claim, in which at least one porous or fibrous carrier sheet for carrying the bonding means is provided between at least one pair of neighbouring discs in the stack.

15. A flywheel as claimed in any preceding claim, in which the primary bonding means bonds only part of each disc to each neighbouring disc in the stack.

16. A flywheel as claimed in any preceding claim, in which, for at least one of the discs, the bonding means includes a first bonding agent on the first face and the second face of that disc, and a different second bonding agent on the first face and the second face of that disc.

17. A flywheel as claimed in any preceding claim, in which secondary bonding means is provided for conducting heat between at least one pair of neighbouring discs, the secondary bonding means being more thermally conductive than the primary bonding means but providing weaker disc-to-disc bonding than the primary bonding means for mitigating crack propagation via the secondary bonding means.

18. A flywheel as claimed in claim 17, in which the secondary bonding means is provided in one or more portions of the spacer areas and optionally spaced apart from the primary bonding means.

19. A flywheel as claimed in any preceding claim, in which the or each bonding means comprises one of the following: welding, braze, solder, diffusion bonding, strong engineering adhesive.

20. A flywheel as claimed in any preceding claim, in which first and second plate members are disposed at opposing ends of the stack, the first and second plate members lack apertures for clamping the stack together, and the first and second plate members are bonded to opposing ends of the stack.

21. A flywheel as claimed in claim 20, in which each disc has similar thickness, and either or both of the first and second plate members have a thickness substantially the same as the disc thickness.

22. A flywheel as claimed in any preceding claim, in which each disc in the stack lacks through apertures.

23. A flywheel as claimed in as claimed in any preceding claim, in which one or more additional stack elements are disposed at or bonded to one or both ends of the stack, each additional stack element including a laminate having one or more of the following:a circular perimeter of lesser diameter than the discs in the stack;a non-circular perimeter;a central aperture.

24. A flywheel as claimed in as claimed in claim 23, in which a plurality of the additional stack elements are provided at one or both ends of the stack, and the width or diameter of successive additional stack elements decreases in a direction towards the end of the stack.

25. A flywheel assembly comprising one or more flywheels as claimed in any preceding claim, the or each flywheel being mounted on or to a drive assembly for facilitating rotation of the or each flywheel for storing energy in or deploying energy from at least one of the flywheels.

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