Flywheel

Abstract

The present invention relates to a flywheel (100) comprising a plurality of discs (102) arranged in a stack (104), including at least first and second end discs at either end of the stack, each of the plurality of discs including a plurality of disc apertures therethrough, first and second plate members (106) disposed at opposing ends of the stack, one or both of the first and second plate members including a plurality of plate apertures therethrough for alignment with a corresponding series of disc apertures through the stack, and connection means for clamping the first and second plate members together about the stack of discs, in which one, some or all of the plurality of disc apertures includes circular disc apertures (120) for accommodating each of the connection means therethrough, and non-circular disc apertures (122a) spaced from a periphery (130) of each disc and slots (122b) extending from the periphery (130) of each disc for reducing stress at or around the circular apertures during flywheel rotation, the circular disc apertures being disposed between a pair of non-circular disc apertures and / or slots.

Description

[0001] FLYWHEEL

[0002] The present invention relates to a flywheel, particularly but not exclusively for use in energy storage and / or deployment, more particularly but still not exclusively for use in energy infrastructure; and a flywheel disc for a flywheel.

[0003] BACKGROUND TO THE INVENTION

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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 Ar2will be less than that of the adjacent main disc A . This will cause substantial shear stresses in the bonding which must therefore be strong enough to resist this.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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. The use of specially-shaped non-circular apertures is indicated to greatly improved the performance of laminated flywheels based on clamping means passing through the circular laminates. However, this requires the use of axially-oriented spacers inserted inside the apertures, the spacer having round holes into which bolted clamping means can be inserted.

[0024] It is an object of the present invention to reduce or substantially obviate at least some of the above problems, particularly by (but not limited to) providing a flywheel which has reduced containment requirements and a simpler construction with fewer parts which can preferably be manufactured at relatively low cost.

[0025] STATEMENT OF INVENTION

[0026] According to a first aspect of the present invention, there is provided a flywheel comprising a plurality of discs arranged in a stack, including at least first and second end discs at either end of the stack, each of the plurality of discs including a plurality of disc apertures therethrough, first and second plate members disposed at opposing ends of the stack, one or both of the first and second plate members including a plurality of plate apertures therethrough for alignment with a corresponding series of disc apertures through the stack, and connection means for clamping the first and second plate members together about the stack of discs, characterised in that one, some or each of plurality of disc apertures includes: a first set of circular disc apertures for accommodating the connection means therethrough, and a second set of apertures for reducing stress at or around the circular apertures during flywheel rotation, the second set of apertures including one or both of (i) non-circular disc apertures and / or (ii) slots extending from a periphery of each disc, each of the non-circular disc apertures and / or slots being arranged (or interspersed) between a given pair of the circular disc apertures.

[0027] According to a related aspect of the present invention, there is a flywheel as set out in claim 1 . Optional features are set out in the dependent claims.

[0028] This allows the flywheel to be operated at fast speeds with a reduced risk of catastrophic failure. A flywheel according to the invention does not suffer as significantly from the high stresses usually associated with circular disc apertures, while maintaining a simple design with fewer parts (i.e. spacers may not be required).

[0029] The circular disc apertures are provided to receive the connection means. The or each connection means may have a circular cross section. The connection means or each may be received through the circular apertures without requiring additional spacers. The additional spacers may otherwise occupy space within non-circular apertures to accommodate the difference in shape between the non-circular apertures and the connection means.

[0030] Additional non-circular apertures and / or slots may be provided on either side of the circular apertures for easing stresses on the circular apertures. The term ‘disc aperture’ is intended to mean an aperture in the disc.

[0031] The term slot should be taken to mean a groove or vacancy in an otherwise circular cross section of each disc. Each slot may extend substantially all of the way through each disc. The slots may provide vacancies in the periphery of the disc to accommodate expansion of the periphery during flywheel rotation. The plate members may be considered end plates.

[0032] The non-circular apertures and / or slots being ‘arranged between’ given pairs of circular apertures may be interpreted as meaning that, when considering the apertures incrementally or stepwise around the periphery, then a pair of circular apertures lie respectively angularly ahead and angularly behind a given non-circular aperture and / or slot.

[0033] Where slots are provided and at least some of them extend inwards from the periphery, then the periphery of each disc may be considered to be an interrupted periphery. The periphery may have a similar overall shape in plan view, e.g. substantially circular, but the periphery may have a plurality of arcs or arc sections which lie between pairs of the slots.

[0034] The non-circular apertures may be elliptical, oval, elliptical-like, or oval-like. Throughout this specification, instances of the term “elliptical” are intended to cover any of these four terms and in particular is not intended to mean a shape which is mathematically elliptical.

[0035] The term elliptical is used to describe a shape that is roughly based on a circular shape that has been stretched in at least one axis.

[0036] Each non-circular aperture may have a first width. The first width may extend along a first axis passing through a centre of the non-circular aperture. The first width may be considered to be a maximum width or diameter.

[0037] Each non-circular aperture may have a second width. The second width may extend along a second axis passing through a centre of the non-circular aperture, perpendicular to the first axis. The second width may be considered to be a minimum width or diameter. The first width may be greater than the second width. The non-circular (or elliptical) apertures may be disposed near the periphery of the disc. That is, at least closer to the periphery than to the disc centre, and preferably within about the outermost 25% of the disc radius.

[0038] Elliptical disc apertures tend towards a circular shape under flywheel rotation. The elliptical disc apertures minimise stress on the circular disc apertures during flywheel rotation.

[0039] The flywheel may be any suitable size. That is, the exemplary embodiments described in this application are not strictly limited to any particular scale and may be increased and decreased in size as required for a particular use.

[0040] The first width may be greater than a diameter of each circular disc aperture. The second width may be similar to a diameter of each circular disc aperture.

[0041] The first width may be about double the length of the second width.

[0042] The second width may be about the same length as the neighbouring slot (where both the non-circular holes and the slots are provided).

[0043] The first width, the second width and (if provided) the slot may have a length ratio of 2:1 :1 (or just 2:1 if a neighbouring or adjoining slot isn’t provided).

[0044] Each slot of the non-circular second series of apertures may have a width in a direction tangential to the disc (or perpendicular to the radius of the disc). The width of each slot may be less than half of the diameter of a given circular disc aperture.

[0045] At least one of: the circular disc apertures, the non-circular disc apertures and the slots, may be disposed on a pitch circle. That is to say that the disc apertures and / or the slots may be arranged in (or centred on) an imaginary circle which smaller than and concentric with the periphery of each disc.

[0046] The circular and non-circular disc apertures may alternate around the periphery of the disc. An arrangement of the disc apertures may form a repeating sequence around at least a portion of the periphery of the plurality of discs. The number of circular disc apertures may be equal to the number of non-circular disc apertures and / or slots. In some flywheel discs, the ratio of circular to non-circular disc apertures may be many to one, for example 2:1 , or another ratio of (more than 1 ):1 . Two circular disc apertures may be provided between a pair of non-circular disc apertures. The slots and noncircular disc apertures may be provided in a rotationally symmetric arrangement.

[0047] In some flywheel discs, the ratio of circular to non-circular disc apertures may be one to many, for example 1 :2, or another ratio of 1 :(more than 1 ). Two non-circular disc apertures may be provided between a pair of circular disc apertures. The slots and non-circular disc apertures may be provided in a rotationally symmetric arrangement.

[0048] The circular disc apertures may be equidistantly spaced around the periphery of each of the plurality of discs. The non-circular disc apertures and / or slots may each be equidistantly spaced around the periphery of each of the plurality of discs. The circular and non-circular disc apertures may each be equidistantly spaced around the periphery of each of the plurality of discs.

[0049] The arrangements of the disc apertures and / or slots are intended to aid in minimising the maximum stresses expected to occur, because the maximum stress may limit the speed of flywheel rotation.

[0050] The non-circular disc apertures may each be spaced from the periphery of the host disc by a distance similar to a second width of each circular disc aperture.

[0051] The circular disc apertures may not meet / intersect the periphery of the host disc.

[0052] At least one of the non-circular disc apertures may be joined or intersected by at least one slot. The slot(s) may join the non-circular disc aperture(s) at a substantially central position.

[0053] If joined or in appropriate relative positions, then each non-circular aperture and slot may be considered to be substantially T-shaped or provide a substantially T-shaped stress relieving apertured portion. That is, shaped like a capital T. The ‘bottom’ of the T stem may meet the periphery of the disc.

[0054] It will be appreciated that the shape may be approximately T-shaped. Where the non- circular aperture is elliptical or similar, then the T branch may appear to be ‘bloated’ (in plan view) but the overall T-shape is still generally evident.

[0055] In some cases, some or all of the slots may not join to the non-circular apertures, but may be provided elsewhere on the disc (preferably still on the periphery). Adhesive may be provided between the plurality of discs to prevent relative movement of the flywheel discs.

[0056] At least one connection means may be in contact with a side wall of at least one circular disc aperture (or the sidewalls of the series of aligned disc apertures it fills). A diameter of the circular disc apertures may be substantially matched with a diameter of a cross section of the connection means. In other words, there may be no vacant space provided between the side wall of the circular disc apertures and the connection means. A spacer may not be required providing a simpler construction with fewer parts.

[0057] The connections means may include a rod or plurality of rods. For example, the connection means may include one or more bolts or threaded studs.

[0058] Each rod may be substantially solid or may be tubular (that is, hollow inside).

[0059] The or each connection means may comprise one or more necked portions for reducing stress on the connection means and plate members.

[0060] The connection means may be necked down in diameter. The necked portions may be located in a corresponding one of the plate apertures of the first plate member.

[0061] Each necked portion may be fully received in a plate member. The necked portion(s) may be disposed adjacent to but inset from distal end(s) of the rod. The distal ends of the rod may be disposed in the plate members.

[0062] The necked portion may reduce the mass of the connection means and / or the stiffness in bending. The or each necked portion may reduce stresses in any one or more of: the connection means itself, the plate members and / or the laminates adjacent or close to the plate members.

[0063] The connection means may comprise one or more hollow portions. The or each hollow portion may be disposed in a head or distal end of the or each rod / threaded stud.

[0064] The hollow portion may reduce the mass of the connection means and the load induced during flywheel rotation. The hollow portion and / or necked portion may alleviate pressure on the threaded stud during flywheel rotation.

[0065] The arrangement of disc apertures on a given disc may have rotational symmetry about the centre of that disc, which may be at least 2-fold or may be multi-fold at the number of circular apertures or non-circular apertures or slots provided on that disc. The width of one, some or all of the circular apertures may be at least about 3% of the diameter of the disc.

[0066] The width of one, some or all of the circular apertures may be up to about 10% of the diameter of the disc.

[0067] The width of one, some or all of the circular apertures may around 5% of the diameter of the disc.

[0068] The flywheel may be an energy storage flywheel. The flywheel may be an energy infrastructure flywheel. The flywheel may be designed for installation in (or provided as part of) a flywheel energy storage system for energy infrastructure.

[0069] The type of flywheel described in this application may in particular not be the type of flywheel which is provided for an internal combustion engine. For example, not the type that may be used in a car or as part of a starter mechanism or motor.

[0070] 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. The flywheel may be capable of achieving a peripheral speed of at least 350 metres per second (ms'1) during use.

[0071] The flywheel (in particular, the inertia element thereof) may have a minimum stored energy of 25 kJ per kilogram during use.

[0072] Each disc may be considered as an inertia element (or laminated 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.

[0073] One, some or all of the discs may be substantially Laval shaped or rimmed-Laval shaped, or may have a substantially Laval-shaped portion (which may be a central portion of the disc). In other words, any one, some or all of the discs may have a thin (or thinner) portion or annular region relative to the rest of the disc.

[0074] The thin portion may have an asymmetric cross-section when considering the crosssection in a direction radially outwards away from a central longitudinal axis of the disc (i.e. the intended axis of rotation) towards the periphery of the disc. The thin portion may extend around the disc. The thin portion may be adjacent to the peripheral region of the disc. The thin portion may be considered to be a radially-inward portion of the peripheral region of the disc, or may be considered to be a radially-outward portion of the central region of the disc. The terms radially-inward and radially-outward are intended to be construed relative to the central longitudinal axis of the disc (which is the intended axis of rotation of the disc when it is in a flywheel).

[0075] The flywheel may be mounted on or to a drive assembly for facilitating rotation of the flywheel for storing energy in or deploying energy from at least one of the flywheels.

[0076] According to a second aspect of the invention, there is provided a flywheel disc for a flywheel, the flywheel disc comprising a plurality of disc apertures which include: a first set of circular disc apertures for accommodating connection means therethrough; and a second set of apertures for reducing stress at or around the circular apertures during flywheel rotation, the second set of apertures including one or both of (i) non- circular disc apertures and / or (ii) slots extending from (or from near) a periphery of each disc, each of the non-circular disc apertures and / or slots being arranged between a given pair of the circular disc apertures.

[0077] According to a related aspect of the present invention, there is a flywheel disc as set out in claim 20.

[0078] The advantages are similar to those set out for the first aspect of the invention. Any feature or independent combination of features presented with respect to the first aspect of the invention may be provided in the second aspect or related aspect of the invention.

[0079] According to a third aspect of the present invention, there is provided flywheel comprising a plurality of discs arranged in a stack, including at least first and second end discs at either end of the stack, each of the plurality of discs including a plurality of disc apertures therethrough, first and second plate members disposed at opposing ends of the stack, one or both of the first and second plate members including a plurality of plate apertures therethrough for alignment with a corresponding series of disc apertures through the stack, and a plurality of rods for clamping the first and second plate members together about the stack of discs, a length of each rod being disposed through a corresponding series of disc apertures, and one, some or all of the rods (each) having a first necked portion which is located in a corresponding one of the plate apertures of the first plate member.

[0080] The necked portions of the rods may substantially reduce stresses in any one or more of: the connection means itself, the plate members and / or the laminates adjacent or close to the plate members. This mitigates the risk of catastrophic flywheel failure, providing a more reliable flywheel at relatively lower cost.

[0081] The rod may be (or be part of) a threaded bolt or stud.

[0082] Some or all of the rods may each have a second necked portion. The second necked portion may be located in a corresponding one of the plate apertures of the second plate member. The second necked portion may be disposed through the stack of discs. The second necked portion may oppose the first necked portion about the rod, stack or plate members.

[0083] The necked portion may be considered as a tapered neck or portion of narrower diameter / cross-section. The necked portion can reduce the mass of the rod and the stiffness in bending. The necked portion can reduce stress on the rod and plate members. The necked portion may reduce stress on the discs disposed adjacent to or near the plate members.

[0084] Each rod may include at least one hollow portion. The at least one hollow portion may be disposed at distal end(s) of one, some or all of the rods. The rod may include a hollow portion at opposing ends of the rod. The hollow portions may be located in at least one of the plate members.

[0085] The hollow portion and / or necked portion(s) may alleviate pressure on rod during flywheel rotation.

[0086] According to a fourth aspect of the present invention, there is provided a flywheel assembly comprising one or more flywheels of the first or third aspect of the present invention, or one or more discs of the second aspect, 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.

[0087] Any aspect of the invention may independently include any feature or features presented with respect to any other aspect(s) of the invention.

[0088] BRIEF DESCRIPTION OF THE DRAWINGS

[0089] 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:

[0090] Figure 1 shows a side view of a first prior art flywheel device from US7267028;

[0091] Figure 2 shows a cross-sectional side view of a second prior art flywheel device from EP2759043;

[0092] Figure 3 shows a cross-sectional side view of a first embodiment of a flywheel according to the invention;

[0093] Figure 4 shows a top view of a first embodiment of a disc of the flywheel of Figure 3;

[0094] Figure 5 shows an enlarged partial cross-sectional side view of the flywheel of Figure 3;

[0095] Figure 6 shows a partial cross-sectional side view of a second embodiment of a flywheel according to the invention;

[0096] Figure 7 shows a top view of a second embodiment of a disc for a flywheel according to the invention; and

[0097] Figure 8 shows a top view of a third embodiment of a disc for a flywheel according to the invention.

[0098] DESCRIPTION OF PREFERRED EMBODIMENTS

[0099] Figures 1 and 2 relate to prior art devices, which are described in the background section.

[0100] Referring firstly to Figure 3, a flywheel according to the present invention is generally shown 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 shaft spigot (or any other suitable type of drive means) plus any suitable bearings required, to provide a means of storing electrical energy.

[0101] 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.

[0102] 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, unless otherwise specified.

[0103] The flywheel 100 includes a plurality of discs 102 (also referred to as laminates). In this embodiment fourteen 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.

[0104] 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 a stack 104 of discs. The stack 104 is substantially cylindrical in this embodiment.

[0105] 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.

[0106] The discs 102 may each be made of steel in this embodiment, although another suitable metal, alloy or composite material may be used in other embodiments.

[0107] Each disc 102 has a plurality of circular disc apertures 120 in this embodiment. Each disc 102 in the stack is substantially the same in this embodiment, preferably to a tolerance on the order of microns or tens of microns.

[0108] A pair of plate members 106 (also referred to as cheek plates) are disposed at either end of the stack 104. The plate members 106 are secured or clamped together about the stack 104. The plate members 106 in this embodiment each have a diameter which is less than a diameter of the disc stack 104. The cheek plates 106 allow the laminate stack 104 to be connected to bearing landings 150. 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 mechanical drive part 160. This allows for use of the flywheel in energy harvesting and / or deployment.

[0109] Connection means secures the plate members 106 together about the stack of discs. In this embodiment, a plurality of connection means is provided. The connection means in this embodiment includes a plurality of threaded studs 114, which are described in more detail with reference to Figure 6.

[0110] The circular disc apertures 120 are shown in the exemplary disc of Figure 4. In this embodiment, each disc 102 has twelve circular disc apertures 120. The circular disc apertures 120 are disposed around (and inset from) a periphery 130 of the disc 102. The circular disc apertures 120 are disposed on a pitch circle 170 in this embodiment, although it will be appreciated that other arrangements or distributions of circular apertures may be provided in other embodiments. The circular disc apertures 120 are equidistantly spaced apart in this embodiment.

[0111] In this embodiment, the minimum distance between each circular disc aperture 120 and the periphery 130 is approximately equal to a diameter of the circular disc aperture 120. No particular limitation on the absolute size or dimensions of the apertures should be inferred as long as the apertures are suitably sized to accept connection means and do not significantly affect the structural integrity of the disc. It will of course be appreciated that the distance of the circular apertures from the periphery may be varied as needed, although preferably centres of the circular holes are disposed closer to the periphery than to the centre of the disc.

[0112] The threaded studs 114 have a substantially circular cross-section. The threaded studs 1 14 are each received in respective ones of the circular disc apertures 120. A crosssection of the threaded studs 1 14 is substantially matched with a cross-section of the circular apertures 120. That is, there is a snug fit between pairs of the threaded studs 1 14 and circular apertures 120. An outer surface of each threaded stud 144 is in contact with an inner surface of each circular disc aperture 120. This arrangement is advantageous because spacers are not required between the connection means and circular disc apertures 120. The claimed invention provides a simpler design with a reduced number of parts required. In some embodiments, adhesive may be provided between the plurality of flywheel discs 102 for preventing relative motion between them and / or improving thermal conductivity, for example. The adhesive should not be too strong, in order to mitigate crack propagation between discs.

[0113] In some embodiments, the threaded studs 114 could be replaced by other fixing means, for example bolts with nuts or rivets.

[0114] Non-circular disc apertures 122a are disposed through the flywheel disc 102. A given pair of two non-circular disc apertures 122a are disposed to either side of a given circular disc aperture 120. In this embodiment, the number of non-circular disc apertures 122a is equal to the number of circular disc apertures 120 (i.e. twelve non- circular disc apertures 122a are provided).

[0115] The non-circular disc apertures 122a are equidistantly spaced around the disc. Both the circular disc apertures 120 and non-circular disc apertures 122b are equidistantly spaced around the disc.

[0116] In this embodiment, the non-circular disc apertures 122a are elliptical. The elliptical shape is generally preferred because it tends towards circular under rotation of the flywheel disc 102. It will be appreciated that, in other embodiments, other shapes which tend towards circular under flywheel rotation may be used. For example, the non- circular disc apertures 122a may be formed from two or more intersecting circular apertures.

[0117] The minimum distance between each non-circular disc aperture 122a and the periphery 130 is substantially similar to the minimum distance between the circular disc aperture 120 and the periphery 130. The non-circular apertures 122a may be on the same pitch circle as the circular apertures.

[0118] It will of course be appreciated that the distance of the non-circular apertures from the periphery may be varied as needed (subject to reducing stress in the circular apertures), although preferably centres of the non-circular holes 122a are disposed closer to the periphery than to the centre of the disc.

[0119] The non-circular disc apertures 122a have a first width or diameter in a direction tangential to the flywheel disc 102. The first width is approximately twice that of the diameter of the circular disc apertures 120. The non-circular disc aperture 122a has a second width or diameter in a direction radial to the flywheel disc 102. The second width is substantially similar to the diameter of the circular disc apertures 120. No particular limitation on the absolute size or dimensions of the apertures 122a should be inferred as long as the apertures are suitably sized to reduce disc stress during flywheel rotation and do not significantly affect the structural integrity of the disc.

[0120] Slots 122b are provided around the periphery 130. In this embodiment, the slots 122b are each arranged substantially radially on the disc.

[0121] The slots 122b are equidistantly spaced around the periphery 130. In this embodiment, the number of slots 122b is equal to the number of non-circular apertures 122a. The slots 122b extend from the periphery 130. The slots 122b extend in a direction towards the centre of the flywheel disc 102.

[0122] The slots 122b each have a width in a direction tangential to the flywheel disc 102. The width is less than the first width of the non-circular disc aperture 122a.

[0123] Note that whilst the slots and non-circular apertures are each substantially identical in this embodiment, this is not essential and variations are envisaged within the scope of the invention.

[0124] The slots 122b each intersect the respective non-circular disc apertures 122a. Each slot 122b meets a central region or point of each non-circular disc aperture 122a.

[0125] The slots 122b provide a vacancy in the periphery 130 of the flywheel disc 102. It will be appreciated that in other embodiments, other shaped vacancies may be provided. Various shapes of vacancies will be apparent to persons skilled in the art without departing from the scope of the present invention.

[0126] The approximately T-shaped non-circular apertures 122a and slots 122b described above have been found to be particularly advantageous in reducing the stresses at the circular apertures 120.

[0127] It will be appreciated that any given joined pair of non-circular hole 122a and slot 122b may be considered to together provide a non-circular hole. In that case, the larger non- circular hole may be considered to a slotted portion and a non-circular portion.

[0128] It will be appreciated that in other embodiments, the size of the disc apertures may vary in relation to the size of the flywheel disc 102. For example, if a different number of disc apertures 102 is provided, then the apertures may be larger (for a lesser total number) or comparatively smaller (for a greater total number) in order to fit into an arrangement that is comparable to the Figure 4 arrangement.

[0129] In various embodiments, the flywheel disc 102 may have any integer number of each type of disc aperture, where the integer number is selected to be between 2 to 50 (inclusive).

[0130] It will be appreciated that any suitable number of disc apertures may be provided in other embodiments. Further embodiments of flywheel disc are described below with respect to Figures 7 and 8.

[0131] Two of the threaded studs 114 are shown in more detail in Figure 5. Each threaded stud 1 14 may be considered to be a rod. Each threaded stud 1 14 extends through the corresponding circular apertures 120 in the stack of discs 104. Each threaded stud has a head 1 16 at distal ends of the threaded stud 114. Second heads (not shown) can similarly be provided at the other ends of the studs 1 14. Each head 1 16 is threaded for receiving a nut 126. The threaded studs 114 are used to clamp the plate members 106 about the stack 104 as described above.

[0132] Each threaded stud 1 14 has a necked portion 118. Each necked portion 1 18 may be considered as a tapered neck or section of reduced thickness.

[0133] In any embodiment, each necked portion may have a cross section including opposing inwardly curved arcs. The inwardly curved arcs may be separated by a minimum thickness of the stud 114 substantially at a mid-point of the necked portion.

[0134] Each necked portion 118 is disposed adjacent to each head 116. Each necked portion 1 18 is disposed adjacent to one of the plurality of discs 102.

[0135] Each pair of head 1 16 and necked portion 118 occupy an aperture in a plate member 106 in this embodiment, although it will be appreciated that the head may in some less preferred embodiments lie proud of the plate member. Each necked portion 1 18 is fully received in a plate member 106 (but preferably not the stack of discs).

[0136] The length of the head 1 16 is similar to the length of the necked portion 118. Each necked portion 118 reduces the mass of the stud 114 and the stiffness in bending. The necked portions 118 may reduce stress on the threaded studs 114, the plate members 106 and the discs 102 disposed adjacent (particularly immediately adjacent) to the plate members 106. Each head 1 16 in this embodiment has a hollow portion 124. The hollow portion 124 may be considered as a recess in the head 116 of the threaded bolt 114. The hollow portion 124 reduces the mass of the threaded stud 114 and the load induced during rotation of the flywheel.

[0137] The hollow portion and / or necked portion alleviate pressure on the threaded stud during flywheel 100 rotation.

[0138] Figure 6 shows a further embodiment of a flywheel 200 which is substantially the same as the flywheel 100 shown in Figure 5, with the differences as follows. In this embodiment, each disc is a Laval shaped disc or a rimmed Laval shape. That is to say, each disc has a thin portion (or thinner / thinned portion) 180 somewhat inset from the region having the circular and non-circular apertures.

[0139] In this case, the thin portion 180 has an asymmetrical cross section (e.g. akin to an elongated tear drop shape) when considering the cross section in a direction radially outwards away from a central longitudinal axis of the disc (i.e. the intended axis of rotation) towards the periphery of the disc, but it will be appreciated that other shapes of cross-section may be provided and may be symmetrical.

[0140] The thin portion 180 is provided in both faces of the disc, i.e. the top and bottom faces. The thin portion 180 is a central portion of the disc and extends annularly around the disc. The Laval shape reduces the stress at the centre of the disc, allowing for higher speeds at the expense of increased stresses around the circular apertures 120.

[0141] Figure 7 shows a further embodiment of a flywheel disc, generally indicated at 202. This embodiment is substantially similar to the discs described above, but has some differences as now described. In this embodiment, the ratio of circular disc apertures 220 to pairs of non-circular apertures 222a and slots 222b is 2:1. Two circular disc apertures 220 are provided between each non-circular aperture 222a and slot 222b. The arrangement may be considered to be an AAB type pattern, where A refers to a circular aperture 220 and B refers to a non-circular aperture 222.

[0142] Figure 8 shows a further embodiment of a flywheel disc, generally indicated at 302. This embodiment is substantially similar to the discs described above. However, in this embodiment, the ratio of pairs of non-circular apertures 322a and slots 322b to circular disc apertures 320 is 2:1 . Two pairs of non-circular apertures 322a and slots 322b are provided between each circular disc aperture 320. The arrangement may be considered to be an ABB type pattern, where A refers to a circular aperture 320 and B refers to a non-circular aperture 322.

[0143] In some embodiments of the flywheel disc, an (nA)(mB) type pattern may be provided where n and m are positive integers. For example, where n is 2 and m is 3, the (nA)(mB) type pattern would be AABBB. In some embodiments m and n have different values. In some embodiments m and n have the same value. In embodiments where non-circular apertures 322a are not provided, m is 0.

[0144] For both of the embodiments depicted in Figures 7-8, whilst the apertures 220, 222a and apertures 320, 322a are respectively arranged on common pitch circles, it will be appreciated that other embodiments of flywheel disc may have some of the apertures on other pitch circles, or in another arrangement which does not require the apertures to be on a pitch circle.

[0145] The embodiments described above are provided by way of example only. Various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1 . A flywheel comprising a plurality of discs arranged in a stack, including at least first and second end discs at either end of the stack, each of the plurality of discs including a plurality of disc apertures therethrough, first and second plate members disposed at opposing ends of the stack, one or both of the first and second plate members including a plurality of plate apertures therethrough for alignment with a corresponding series of disc apertures through the stack, and connection means for clamping the first and second plate members together about the stack of discs, characterised in that one, some or each of the plurality of disc apertures of the discs includes: a first set of circular disc apertures for accommodating the connection means therethrough, and a second set of apertures for reducing stress at or around the circular apertures during flywheel rotation, the second set of apertures including (i) non-circular disc apertures spaced from a periphery of each disc and (ii) slots extending from the periphery of each disc, each of the non-circular disc apertures and / or slots being arranged between a given pair of the circular disc apertures.

2. A flywheel as claimed in claim 1 , in which the non-circular disc apertures are substantially elliptical.

3. A flywheel as claimed in claim 1 or 2, in which the non-circular disc apertures each have a first width in a substantially tangential direction and a second width in a substantially radial direction relative to the disc, wherein the first width is greater than the second width.

4. A flywheel as claimed in claim 3, in which the first width of the non-circular disc apertures is greater than a diameter of a given one of the circular disc apertures.

5. A flywheel as claimed in claim 3 or 4, in which the second width of the non- circular disc apertures is similar to a diameter of a given one of the circular disc apertures.

6. A flywheel as claimed in any preceding claim, in which the slots have a width in a tangential direction less than half of a diameter of the circular disc apertures.

7. A flywheel as claimed in any preceding claim, in which the circular disc apertures and the non-circular disc apertures are disposed on a pitch circle.

8. A flywheel as claimed in any preceding claim, in which, on one, some or all of the discs, an arrangement of the circular and non-circular disc apertures forms a repeating or alternating sequence relative to at least a portion of the periphery of each disc.

9. A flywheel as claimed in any preceding claim, in which the number of circular apertures for receiving the connection means is equal to the number of non- circular apertures or slots.

10. A flywheel as claimed in any preceding claim, in which one or both of the following are provided: the circular disc apertures are each equidistantly spaced around the periphery of each disc; and the slots are each equidistantly spaced around the periphery of each disc.1 1. A flywheel as claimed in any preceding claim, when directly or indirectly dependent on claim 3, in which the non-circular disc apertures are spaced from the periphery of each disc by a distance similar to the second width of at least one of the non-circular disc apertures.

12. A flywheel as claimed in claim 7, or any of claims 8 to 1 1 when dependent on claim 7, in which the flywheel has a greater number of the circular disc apertures on the pitch circle than the number of non-circular disc apertures on the pitch circle, or in which the flywheel has a greater number of the non-circular disc apertures on the pitch circle than the number of circular disc apertures on the pitch circle.

13. A flywheel as claimed in any preceding claim in which at least one of the non- circular disc apertures is joined to or intersected by at least one of the slots.

14. A flywheel as claimed in any preceding claim, in which one or more pairs of non-circular disc apertures and slots together provide one or more substantially T-shaped apertures or apertured portions of one, some or all of the discs.

15. A flywheel as claimed in any preceding claim in which one, some or all of the connection means are in contact with a side wall of each corresponding circular disc aperture.

16. A flywheel as claimed in any preceding claim, in which a diameter of each circular disc aperture is substantially matched with a diameter of a cross section of the connection means.

17. A flywheel as claimed in any preceding claim in which the or each connection means comprises one or more hollow portions.

18. A flywheel as claimed in any preceding claim in which the connection means includes one or more bolts or threaded studs.

19. A flywheel as claimed in claim 18, in which the or each threaded stud comprises a necked portion.

20. A flywheel disc for a flywheel, the flywheel disc comprising a plurality of disc apertures which include: a first set of circular disc apertures for accommodating connection means therethrough; and a second set of apertures for reducing stress at or around the circular apertures during flywheel rotation, the second set of apertures including (i) noncircular disc apertures spaced from a periphery of each disc and (ii) slots extending from the periphery of each disc, each of the non-circular disc apertures and / or slots being arranged between a given pair of the circular disc apertures.21 . A flywheel comprising a plurality of discs arranged in a stack, including at least first and second end discs at either end of the stack, each of the plurality of discs including a plurality of disc apertures therethrough, first and second plate members disposed at opposing ends of the stack, one or both of the first and second plate members including a plurality of plateapertures therethrough for alignment with a corresponding series of disc apertures through the stack, and a plurality of rods for clamping the first and second plate members together about the stack of discs, a length of each rod being disposed through a corresponding series of disc apertures, and one, some or all of the rods each having a first necked portion which is located in a corresponding one of the plate apertures of the first plate member.

22. A flywheel as claimed in claim 21 , in which one, some or all of the rods each has a second necked portion which is located in a corresponding one of the plate apertures of the second plate member, the second necked portion opposing the corresponding first necked portion of the or each rod.

23. A flywheel as claimed in claim 21 or claim 22, in which one, some or all of the rods includes at least one hollow portion.

24. A flywheel as claimed in claim 23, in which the at least one hollow portion is disposed at one or both of first and second distal ends of each rod.

25. A flywheel assembly comprising one or more flywheels as claimed in any of claims 1 to 19 or 21 to 24, and / or one or more flywheel discs as claimed in claim 20, the or each flywheel or flywheel disc being mounted on or to a drive assembly for facilitating rotation thereof for storing energy in or deploying energy from at least one of the flywheels or discs.

Images