Existing flywheels for energy storage are sometimes constructed such that the rotating mass of the flywheel rotates inside a chamber containing a vacuum. Operating the rotating mass inside a vacuum is advantageous since it reduces energy losses due to air resistance also known as windage. However, in order to transfer energy into and out of the rotating flywheel mass, a coupling means is required. Some existing flywheels use a rotating shaft passing through a rotating seal in the vacuum chamber to couple torque from an energy source to the flywheel energy storage means. Rotating seals are never perfect, however, since they inevitably leak and therefore require an environmental management system to be coupled to the vacuum chamber in order to maintain the vacuum despite leakage. Furthermore, the seals become more “leaky” with age and as rotational speed increases, and also wear more quickly at higher speeds. The mass, volume and cost of such an environmental management system is undesirable. The use of rotating seals is therefore undesirable.
Magnetic couplings can be used with flywheels to transfer torque through a vacuum chamber wall, thereby obviating the need for rotating seals. For example, a magnetic gear used to couple force between movable members, for example drive shafts, is described in International Patent Application PCT/GB2010/000590, filed by Ricardo UK Limited, the entire contents of which are incorporated herein by reference. A rotational magnetic gear 100 as described in PCT/GB2010/000590 is shown in FIG. 1a herein. The device has first and second movable members 110, 120, each having a circumferentially distributed array of alternating magnetic poles 115, 116, 125, 126. Magnetic flux is coupled between the pole arrays by coupling elements 130. The coupling elements 130 minimise the air gap 150 between the moveable members, especially when a membrane 140 is present in the air gap. FIG. 1b shows the lines of magnetic flux 160, 170 in a portion of the arrangement of FIG. 1a. The membrane 140 allows the two movable members 110, 120 to be operated in different respective atmospheric conditions. For example, one member may be operated in a vacuum. As one member rotates in a clockwise direction, the other member counter rotates in an anticlockwise direction as the lines of magnetic flux 170 pass from one array of poles to the other array of poles through the coupling elements 130. No physical connection is required therefore the use of rotating seals can be eliminated which is advantageous in that it allows expensive environmental management systems to also be eliminated. The membrane 140 of course needs to be strong enough structurally to withstand the forces exerted by air pressure.
Although not limited to flywheel applications, the arrangement shown in FIG. 1a can be advantageously used to couple a high speed flywheel operating inside a vacuum enclosure to a lower speed drive shaft under atmospheric pressure since, if the number of poles of the first member is dissimilar to the number of poles on the second member, a gearing effect results which allows the driveshaft in atmospheric pressure to operate at a lower speed than the flywheel, thereby reducing windage losses. However, in order to achieve a high gearing ratio, the dimensions of the magnetic poles on one of the members must be made as small as possible in order to fit as many as possible in. This, coupled with the need to make the whole assembly as compact as possible, dictates that the coupling elements 130 should also be relatively small. Further, in order to maximise the transfer of flux and thereby maximise the torque capacity of the magnetic gear coupling, the device may be extended along its axial length so that it is generally elongate cylindrical. This can mean that the coupling elements 130 have a relatively long length dimension and a relatively narrow cross sectional area. The coupling elements are therefore prone to suffering from a lack of rigidity and can bend, move, or vibrate. This can lead to non-optimal functioning of the device and/or eventual degradation and/or failure. It is also difficult to manufacture such a device since careful alignment is necessary and many production steps are needed to individually assemble the coupling elements into the correct position and hold them there.
The angular offset between the input and output shafts of a magnetic gear such as the one shown in FIG. 1a varies according to the torque applied and to the torque coupling capacity of the magnetic gear at a given meshing position. Such variation of the torque coupling capacity with meshing position will result in a torsional vibration in the shafts. This can reduce the life of the associated mechanical components, and/or can result in failure and/or disengagement. This is an especially serious problem if the rotational speed is such that the frequency of the torsional vibration coincides with a resonance of the mechanical system. Therefore it would be advantageous if the variation in torque coupling capacity of a magnetic gear could be reduced or eliminated. This would allow smaller, cheaper, magnet arrays to be used, since the minimum torque coupling capability would then be much closer to the mean torque coupling capability. Torsional vibration of the shafts would also be reduced, allowing cheaper, lighter and smaller components to be used. A flywheel energy storage system employing such smaller, cheaper and lighter components would have a higher energy storage density.
Existing magnetic couplings suffer from further disadvantages. For example, in existing systems where magnetic coupling is used to transfer energy into and out of a rotating flywheel situated in a vacuum to and from means outside that vacuum, a cooling arrangement is required in the vacuum. Such a cooling arrangement acts to reduce heat caused by operation of the flywheel and magnetic coupling, including heat generated due to variation in the magnetic field of the magnets on the flywheel side of the coupling. Such cooling arrangements can be complex, adding to the overall complexity, bulk and expense of the system. Existing magnetic coupling arrangements also suffer from general fatigue over time, and in particular there is a tendency for the rotating magnets to shift out of place over time due to the effects of rotation. Similarly, any stationary electromagnetic poles provided between the two respective rotating magnetic members can encounter slip and fatigue over time.
There is no known coupler using rotatable magnets which can be used to transfer energy into an out of a flywheel situated in a vacuum in an efficient, cost effective and compact manner.
An invention is set out in the claims.
According to an aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel and wherein the magnetic strength of a first magnet arranged on the first movable member exceeds the magnetic strength of a second magnet arranged on the second movable member. The first magnet may be larger than the second magnet. The first magnet may be a sintered magnet and/or the second magnet may be a bonded magnet. As a result, the magnetic field across the coupling apparatus is skewed so that losses due to variation in magnetic field occur mainly in the vicinity of the second moveable member, away from the flywheel.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The coupling apparatus further comprises a retainer for physically retaining the one or more magnets arranged on the first movable member. The retainer may take the form of a winding which can be wound around an outer surface of the magnets on the first movable member. Alternatively it may take the form of a sleeve which can be fitted over an outer surface of the magnets on the first movable member. The retainer may be formed from a retaining material and an adhesive material, wherein the adhesive material may include fragments of other materials embedded therein for the provision of electronic stress relief.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The coupling apparatus further comprises a membrane intermediate the first and second movable members, said membrane comprising a groove or recess for locating one or more magnetic poles. The membrane may be formed from Polyether ether ketone (PEEK).
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The coupling apparatus further comprises a liner intermediate the second movable member and the one or more magnets arranged thereon. The liner may comprise one or more discontinuous sections and/or may be formed from a suitable material such as Somaloy.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The first and second movable members are rotatable about a common rotation axis, with the second movable member being provided radially outward of the first movable member. The second movable member is formed from a material with low electrical conductivity and low permeability. For example it is formed of Peek or Glass fibre. The second movable member may comprise first and second sections formed from first and second respective materials.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The apparatus is provided inside a housing or casing, which can also house the flywheel to which the first movable member is coupled. A formation on an inner surface of the casing is arranged to provide magnetic flux shielding during operation of the coupling apparatus.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The first and second movable members are rotatable about a common rotation axis, with the second movable member being provided radially outward of the first movable member and connected to a shaft at a first end. An end plate is provided at a second end of the second movable member, substantially axially opposite the shaft.
According to another aspect, a coupling apparatus for transferring energy to or from a flywheel is provided. The coupling apparatus comprises first and second movable members, each having one or magnets arranged thereon, wherein the first movable member is arranged to be coupled to a flywheel. The apparatus further comprises a stator, between the first and second movable members. Means is provided on the stator, and/or on at least one of the movable members, for enhancing air flow around the apparatus. The means may comprise a scrolling or groove. The means may comprise a projection such as a fin or a blade. A plurality of such projections may be provided. The means may comprise an opening or channel through the stator and/or through the second movable member.
According to another aspect, a method for constructing any coupling apparatus as described herein is provided.