FIELD OF THE INVENTION This invention relates to a homopolar flywheel energy storage system. Background

Flywheel energy storage systems are used for a variety of applications for power averaging. During a low demand period energy is stored in a flywheel and during peak demand period energy is extracted from the flywheel. Possible applications for a highly efficient flywheel energy storage system include:

• an electric train that extracts energy from the flywheel during acceleration and returns the braking energy to the flywheel during deceleration;

• rolling mills that extract energy from the flywheel as a billet passes through rollers; · a battery for storing solar energy during the day and extracting it during night;

• wind turbine generators for storing excess power during peak wind periods and withdrawing it during lean wind periods;

• local electric power grids for storing power at cheaper rates during night and using it during peak rates during the day; and · a battery for powering an inner city/suburban bus transport system - unlike a conventional battery, it would be possible to recharge the flywheel within a few seconds.

Conventional flywheel energy storage systems have a rotor connected to one or more flywheels, and a motor/generator for transferring energy into and out of the rotor. The rotor comprises one or more permanent magnets and current is transferred to and from the system via copper wires.

Conventional flywheel systems have several drawbacks which make them uneconomical and/or unpractical.

Flywheel systems for the applications described above need to be able to cope with large currents as energy is introduced into the system. This requires a large amount of copper conductor, increasing the bulk of the system. In addition, energy is lost through heat generated in the copper. The high strength permanent magnets used in the rotors of conventional systems are relatively brittle. This limits the speed at which the rotor can rotate without the magnets failing. Brittle failure of the magnets is a safety concern .

Conventional induction motors/generators also suffer from losses due to alternating magnetic flux experienced in the rotor causing heating of the rotor body and its copper coils or copper squirrel cage ba rs.

Efficient flywheel energy storage systems have been proposed that employ

superconducting components. Superconducting components have the potential to be highly efficient because losses in the supercond ucting components a re almost nil . The only significant loss component is associated with power consumed by the refrigerator system for keeping the superconducting components at their intended operating temperatures.

The power, P, of an electrical machine is proportional to both its rotor excitation field, B, and rotational speed ω. Hence high power density machines must rotate large magnetic fields at very high speeds (ω > 15,000 rpm) . Copper windings cannot feasibly produce magnetic fields above ~ 1.5 T. Additionally, speeds above 15,000 rpm are not possible with active coils attached to the rotor due to excessive centrifugal stresses, as well as cha llenges of handling power and cooling system interfaces. The recent emergence of commercially produced high temperature superconductor (HTS) wire has radically changed both the cost-metric and technical viability of superconducting machines. HTS wires are superconducting at temperatures up to 93 K, enabling mechanical refrigerators to be used in conjunction with gas-exchange cooling. This eliminates ma ny of the problems associated with rotating liquid cryogens, whilst the elevated operating temperature provides 'thermal head-room', further enhancing system stability.

It is an object of at least preferred embodiments of the present invention to provide a flywheel energy storage system that provides a high electrical efficiency. It is an additional or alternative object of at least preferred embodiments of the present invention is to at least provide the public with a useful choice.

SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention, there is provided a flywheel energy storage system comprising :

a flywheel ; and a homopolar machine comprising :

a rotor that is operatively connected to the flywheel and arranged to rotate around a longitudinal axis, the rotor comprising a body having at least one first lobe at or toward a first end of the body and at least one second lobe at or toward an opposite second end of the body, wherein the first lobe(s) is/are rotationally offset from the second lobe(s);

an annular armature coil arrangement that surrounds the body of the rotor, the armature coil arrangement comprising a plurality of armature windings;

a stator that surrounds at least a section of the armature coil arrangement that surrounds the body of the rotor, wherein the stator comprises a first end section that is associated with the first lobe(s) of the rotor, a second end section that is associated with the second lobe(s) of the rotor, and a bridge section that extends between the first end section and the second end section of the stator, wherein the first and second end sections comprise a plurality of laminations that are arranged to enable flux to travel radially and axially, and wherein the bridge section comprises a plurality of laminations that are arranged to enable flux to travel in an arcuate path around the bridge section between the first end section and the second end section;

a superconductor field coil positioned between the stator and the rotor, wherein the superconductor field coil comprises high temperature superconducting (HTS) material; and

an electromagnetic shield positioned between the superconductor field coil and the armature coil arrangement, the electromagnetic shield configured to restrict the passage of alternating field from the armature windings to the superconductor field coil.

In an embodiment, a plane extending through the first lobes is non-coplanar with a plane extending through the second lobes.

In an embodiment, the rotor comprises at least two first lobes and at least two second lobes. In an embodiment, the rotor comprises three, four, or more first lobes and three, four, or more second lobes.

In an embodiment, the first end section of the stator is positioned radially outwardly of the first lobes of the rotor, and the second end section of the stator is positioned radially outwardly of the second lobes of the rotor. In an embodiment, the electromagnetic shield comprises a generally annular body that is positioned radially inwardly of the superconductor field coil and that is positioned radially outwardly of a portion of the armature coil arrangement adjacent the electromagnetic shield. In an embodiment, the electromagnetic shield further comprises a first end wall positioned at or adjacent a first end of the annular body and a second end wall positioned at or adjacent an opposite second end of the annular body, the first and second end walls extending radially outwardly from the annular body to extend over at least a portion of ends of the superconductor field coil. In an embodiment, the first and second end walls of the electromagnetic shield extend over the entire ends of the superconductor field coil.

In an embodiment, the superconductor field coil is located in a cryostat chamber, and insulating material is provided between the cryostat chamber and the electromagnetic shield. In an embodiment, the electromagnetic shield comprises copper or other low electrical resistivity material.

In an embodiment, the stator comprises a recess, and the superconductor field coil and electromagnetic shield are positioned in the recess. In an embodiment, the recess is located radially inwardly of the bridge section. In an embodiment, the recess is defined by the bridge section and the end sections of the stator.

In an embodiment, radially inner edges of the laminations of the first and second end sections of the stator define an aperture for receipt of the armature coil arrangement. In an embodiment, parts of the armature windings are provided in contact with or in close proximity to the radially inner edges of the laminations of the first and second end sections of the stator.

In an embodiment, the first and second end sections of the stator each comprise a plurality of substantially planar laminations that are oriented longitudinally and radially. In an embodiment, the end sections each comprise a plurality of laminated generally wedge-shaped or trapezoid-shaped members. In an embodiment, the end sections each comprise a plurality of laminated blocks, with at least some of the laminated members alternating with at least some of the laminated blocks. In an embodiment, at least some of the laminated members have a relatively long radial dimension and at least some of the laminated blocks have a relatively short radial dimension, wherein the radial dimensions provide a plurality of teeth and slots at a radially inward edge of the first and second end sections, and wherein parts of the armature windings are housed in the slots. In an embodiment, all of the laminated members have a relatively long radial dimension and at least some of the laminated blocks have a relatively short radial dimension.

In an embodiment, all of the laminated members alternate with the laminated blocks.

In an embodiment, the laminations in the bridge section are arranged to enable flux to travel in a helical path through the bridge section from one end of the stator to the other end of the stator. In an embodiment, the laminations in the bridge section are arranged to enable flux to travel about 90 degrees around the stator as well as along the length of the stator, to provide a flux path from one of the first lobes of the rotor to one of the second lobes of the rotor.

In an alternative embodiment, the armature windings may be helical. In an embodiment, the laminations in the bridge section are concentric annular laminations.

In an embodiment, the laminations in the bridge section are spiral laminations.

In an embodiment, the laminations in the end sections and the bridge section comprise iron material.

In an embodiment, the armature coil arrangement comprises a single layer armature winding.

In an embodiment, the armature coil arrangement comprises a double layer armature winding. In an embodiment, the flywheel energy storage system comprises two spaced-apart flywheels that are operatively connected to the rotor.

In an embodiment, the flywheel(s) comprise carbon fibre reinforced polymer material. Flywheel could be made of other materials too in order to achieve objectives of an intended application.

In an embodiment, the rotor and flywheel(s) are rotatably supported by superconducting bearings. Non-superconducting bearings could also be employed in some embodiments.

In an embodiment, the flywheel energy storage system further comprises current leads to selectively excite the superconductor field coil. In an alternative embodiment, the flywheel energy storage system is provided in combination with a flux pump to selectively excite the superconductor field coil. The term 'comprising' as used in this specification and claims means 'consisting at least in part of. When interpreting statements in this specification and claims which include the term 'comprising', other features besides the features prefaced by this term in each statement can also be present. Related terms such as 'comprise' and 'comprised' are to be interpreted in a similar manner.

Any of the above aspects of the invention may include any one or more of the features and/or functionality outlined above or herein in relation to any of the other aspects of the invention. Additionally, any of the above aspects may be provided in suitable

combination(s), such as those outlined in relation to other aspects, to provide desired functionality.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features. To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. Where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

As used herein the term '(s)' following a noun means the plural and/or singular form of that noun. As used herein the term 'and/or' means 'and' or 'or', or where the context allows both. The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only and with reference to the accompanying drawings in which :

Figure 1 shows a perspective cut-away view of a first embodiment of a flywheel energy storage system;

Figure 2 shows an end view and a side sectional view of the flywheel energy storage system;

Figure 3 shows a perspective view of a rotor of the flywheel energy storage system with the ends of the shaft omitted for clarity;

Figure 4 shows a perspective cut-away view of the rotor, stator, superconductor field coil and armature coil arrangement of the flywheel energy storage system;

Figure 5 shows a detailed view of the superconductor field coil of the flywheel energy storage system;

Figure 6A shows a perspective view of the stator of the flywheel energy storage system; Figure 6B shows a perspective view of the bridging section of the stator;

Figure 7A shows a perspective view of the armature coil arrangement of the flywheel energy storage system;

Figure 7B shows a perspective view of an alternative armature coil arrangement of the flywheel energy storage system;

Figure 8A shows a schematic illustration of the winding arrangement of the armature coil arrangement of figure 7A;

Figure 8B shows an exemplary arrangement of two armature windings connected in series within one pole-pitch;

Figure 8C shows an exemplary arrangement of phase-A armature windings connected in a wave fashion for four pole-pitches;

Figure 9A shows the 2D flux path through the rotor and stator for the armature coil arrangement of figure 7A;

Figure 9B shows the 2D flux path through the rotor and stator for the armature coil arrangement of figure 7B;

Figure 10A shows a perspective view of the flux path in relation to the rotor and armature windings for the armature coil arrangement of figure 7A; Figure 10B shows a perspective view of the flux path in relation to the rotor for the armature coil arrangement of figure 7A;

Figure 11A shows a schematic of a flux pump;

Figure 11B shows a cross-section of part of the flux pump;

Figure 12A shows the response of an exemplary electric train system that uses the flywheel energy storage system as a train leaves a station; and

Figure 12B shows the response of an exemplary electric train system that uses the flywheel energy storage system as a train arrives at a station . DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following describes exemplary embodiments of the present invention .

In some embodiments, a flywheel energy storage system comprises:

a flywheel ; and

a homopolar machine comprising :

a rotor that is operatively connected to the flywheel and arranged to rotate around a longitudinal axis, the rotor comprising a body having at least one first lobe at or toward a first end of the body and at least one second lobe at or toward an opposite second end of the body, wherein the first lobe(s) is/are rotationally offset from the second lobe(s) ;

an annular armature coil arrangement that surrounds the body of the rotor, the armature coil arrangement comprising a plurality of armature windings;

a stator that surrounds at least a section of the armature coil arrangement that surrounds the body of the rotor, wherein the stator comprises a first end section that is associated with the first lobe(s) of the rotor, a second end section that is associated with the second lobe(s) of the rotor, and a bridge section that extends between the first end section and the second end section of the stator, wherein the first and second end sections comprise a plurality of laminations that are arranged to enable flux to travel radially and axially, and wherein the bridge section comprises a plurality of laminations that are arranged to enable flux to travel in an arcuate path around the bridge section between the first end section and the second end section;

a superconductor field coil positioned between the stator and the rotor, wherein the superconductor field coil comprises high temperature superconducting (HTS) materia l; and an electromagnetic shield positioned between the superconductor field coil and the armature coil arrangement, the electromagnetic shield configured to restrict the passage of alternating field from the armature windings to the superconductor field coil.

General description

Figure 1 and figure 2 show a flywheel energy storage system 101 that comprises two flywheels 201, 203 and a homopolar machine that can operate as a homopolar motor/generator. The energy storage system is suitable for high power, fast charge and discharge applications.

In an exemplary embodiment, up to 6.7 kWh of energy is stored in a frictionlessly rotating mass of 325 kg that spins at up to 25,000 RPM.

In an exemplary embodiment, up to 500 kW is supplied to the energy storage system. At maximum load it takes 48 seconds to fully charge the system.

In an exemplary embodiment, 866 V three phase is supplied at up to 360 A to supply 535 kW of power at power factor of about 0.99.

Operating modes

The flywheel energy storage system 101 stores electrical energy as kinetic energy in rotating flywheels 201, 203. The machine has three operating modes:

1. motor, where electrical energy is added to the system causing the flywheels 201, 203 to accelerate;

2. storage, where the electrical circuit is broken and the flywheels 201, 203 spin at a constant rate; and

3. generator, where electrical energy is removed from the system, causing the flywheels 201, 203 to decelerate.

Flywheels overview

The flywheels 201, 203 store the majority of the system's stored energy. The dual flywheel arrangement reduces the required size of the shaft 205 as only half the total energy input to the system is transmitted to each flywheel 201, 203. Bearings overview / chamber overview

All of the rotating components of the system are housed in a common evacuated vacuum chamber 301, and are rotatably supported by frictionless, contactless, levitating, superconducting bearings 303, 305. The superconducting bearings 303, 305 utilise the flux pinning effect of type II superconductors. The only energy losses due to friction in this system are windage losses due to the evacuated chamber not being a perfect vacuum.

Superconductor field coil overview

In the centre of the energy storage system is the homopolar motor/generator, comprising a superconductor field coil 401, a rotor 501, a stator 601 and an armature coil arrangement 701. Figure 4 shows a sectioned perspective view of the homopolar motor/generator.

The superconductor field coil 401 provides a permanent (constant) DC magnetic field which magnetises the rotor 501. When operating as a generator, the rotor 501 rotates so a rotating magnetic field is experienced by the armature coil arrangement 701 making the armature windings 703 generate voltage within the armature windings 703. When operating as a motor, the armature windings create a rotating magnetic field which creates torque at the rotor 501 as it moves to align its magnetic orientation to that of the rotating field. As used herein, the phrase 'permanent magnetic field' refers to the orientation and intensity of the magnetic field while the superconducting field coil is energised. A magnetic field that does not change direction or intensity during machine operation is considered 'permanent'.

Rotor overview

A rotor 501 is operatively connected to the flywheels 201, 203 and is arranged to rotate around a longitudinal axis L-A. The rotor 501 is made from solid iron or other suitable ferromagnetic material and is concentric with the superconductor field coil 401. As the rotor 501 rotates, it is exposed to a constant magnetic field from the superconductor field coil 401. This causes the rotor 501 to behave as if it were a permanent magnet with north poles on one end and south poles on the other end.

Stator overview

The rotor 501 rotates within a stator 601. The stator 601 is positioned radially outwardly of the rotor 501. The stator 601 is made from laminated iron and provides a low reluctance flux path to complete the magnetic circuit from one end of the rotor 501 to the other, from one set of lobes to the other. Because the rotor lobes 503, 505 are rotationally offset, the flux paths through the stator 601 are helical. The stator has a lamination arrangement to reduce reluctance and minimise eddy currents, and therefore heating and losses. Armature coil arrangement overview

An annular armature coil arrangement 701 surrounds the body of the rotor 501. The armature coil arrangement 701 comprises a plurality of armature windings 703. The armature coil arrangement 701 comprises armature windings 703 made from Litz wire. The armature windings 703 are connected together in series in three circuits to receive and produce three phase power.

When operating as a generator, the rotation of the rotor 501 causes each armature winding to be passed over by the rotor lobes 503, 505, causing a voltage generation. Alternating voltage is generated in each armature winding 703 as alternating north lobe(s) 503 and south lobe(s) 505 pass over each armature winding 703. Voltage generated in one half of an armature winding 703 side is of opposite polarity to that of the other half of the armature winding 703. Since the rotor lobes are offset by 90° (in the example described here), the voltage generated under a rotor lobe is much higher than that generated where there is no lobe. The net voltage generated in each side of an armature winding 703 is the difference between these two voltages. When operating as a motor, alternating current is supplied to the armature windings 703 to produce a rotating magnetic field. The rotor 501 remains a permanent magnet and torque is produced as the rotor 501 rotates to align its magnetic field with the armature windings' 703 rotating field.

Flywheels and shaft - detail

The embodiment of the flywheel energy storage system 101 shown in figure 1 will now be described in more detail.

Two spaced-apart flywheels 201, 203 are operatively connected to the rotor 501. The flywheels 201, 203 are rigidly attached to the shaft 205 and kinetic energy is primarily stored by the flywheels 201, 203, and to a lesser extent by the shaft 205 and other rotating components. The symmetrical layout of the flywheels 201, 203 with one either side of the homopolar motor/generator reduces the required shaft 205 size as only half the energy transmitted from or to the motor/generator is transmitted to each flywheel 201, 203. The asymmetric features of the shaft 205 are for assembly purposes. In an exemplary embodiment the shaft 205 is made from steel. The shaft 205 may be made from any other material having suitable strength, such as carbon fibre reinforced polymer (CFRP) . In an exemplary embodiment, the flywheels 201, 203 comprise carbon fibre reinforced polymer (CFRP) material. CFRP is suitable for withstanding the centripetal force at high rotational speeds of the rotor and shaft such as 25,000 RPM. The flywheels 201, 203 may be made from any other suitable material that can withstand the required force, for example, high yield strength materials such as Constantan (Copper-Nickel alloy).

In an alternative embodiment, a single flywheel may be used. Bearings - detail

The superconducting bearings 303, 305 are passive and do not require a control system or energy input to create the magnetic fields as is the case in conventional magnetic contactless bearings.

The bearings 303, 305 have two main parts: a rotating component which is rigidly attached to the shaft 205 and constructed of one or more permanent magnets, and a static component which is rigidly attached to the vacuum chamber 301 wall and constructed from one or more superconducting elements, made from type II

superconductors. The permanent magnet(s) may be suitably strong magnets, for example, neodymium magnets. Additionally, there is a retractable support pin which supports the shaft 205 during commissioning (not illustrated), and a cryocooler which maintains the temperature of the superconducting elements.

Bearing operation relies on the flux pinning effect present in type II superconductors. This effect causes currents to be generated in the superconductor that oppose changes in the magnetic field which surrounds and penetrates the superconductor. The rotating component of each bearing 303, 305 has an axially symmetric magnetic field. This means the superconducting elements of the bearing do not oppose the axial rotation of the shaft 205 (as this rotation does not change the magnetic field strength or direction at the superconductor), but will oppose any other movements.

In an exemplary embodiment, the static component of the bearing has multiple blocks of superconductor which surround the rotating component in a rotationally symmetric arrangement. The flux pinning effect acts against both increases and decreases of the magnetic field strength, as well as changes in orientation. The arrangement of superconducting blocks means that each block supports the shaft 205. The bearings 303, 305 act as both thrust and radial bearings (they both lift and centre the shaft 205). The flywheel energy storage system 101 may be mounted with the shaft 205 in a vertical orientation as illustrated in Figure 1. In the vertical orientation both bearings 303, 305 are expected to act as thrust bearings and to support half the weight of the rotating components. Alternatively, the flywheel energy storage system 101 may be mounted in a different orientation, for example, a horizontal orientation.

The rotating component of the bearing is radially separated from the static component on all sides, creating a frictionless bearing. To commission the bearing the shaft 205 is initially held in the centre of the bearing, supported by the retractable pin. The superconducting elements are then cooled to their operating temperature while exposed to the magnetic field of the magnets, in a process known as Yield cooling'. The pin is then removed, and the flux pinning effect enables to shaft 205 to levitate in place and rotate frictionlessly about its axis.

The field cooling position sets the operating position of the shaft 205. The bearing maintains the shaft 205 position as the field cooled position. If the shaft 205 is not correctly aligned in the centre of the bearing during field cooling, the shaft 205 will not be correctly aligned in use. This means the position tolerance of the shaft 205 position is reliant on the position of the retractable pin and not on the bearing housing. If the shaft 205 is positioned closer to one side of the static bearing component the weight of the shaft 205 will not be carried equally by all the superconducting elements; the elements closer to the centre of the shaft 205 will be exposed to a higher field during the field cooling and will carry more of the shaft 205 weight. This ability for the shaft 205 operating position to be modified during commissioning and without modifying the operating hardware also applies to non-axial rotation of the shaft 205, and axial translation of the shaft 205. With a large enough force it is possible to overcome the flux pinning effect of the cooled superconducting elements and reposition the shaft 205 after cooling, at which point the bearing will continue to operate as if the bearing was field cooled with the shaft 205 in the new position, however, this is not the intended commissioning method.

The superconducting elements of the bearings 303, 305 are cryogenically conduction cooled via a copper thermal link to which they are rigidly attached. The bearings 303, 305 have higher conductive heating than the superconductor field coil 401 as the bearings 303, 305 must transmit the weight of the rotating components to the wall of the vacuum chamber 301, resulting in a large thermal path. The bearings 303, 305 are cooled separately to the superconductor field coil 401 as they are capable of operating at relatively high temperatures. Any other suitable frictionless bearings may be used to rotationally support the shaft 205. Friction bearings could also be employed to suit a given application.

Superconductor field coil - detail

The superconductor field coil 401 is positioned between the stator 601 and the rotor 501, and in the form shown between the stator 601 and the armature coil arrangement 701, and is shown in more detail in figure 5. The superconductor field coil 401 is axially located at a mid-point of the armature coil arrangement 701.

In an alternative embodiment, the superconductor field coil 401 is positioned between the rotor 501 and the armature coil arrangement 701. In this embodiment, the superconductor field coil 401 could be positioned substantially as shown in the figures, and the armature coil arrangement 701 could be shaped to pass radially outwardly of the superconductor field coil 401. Alternatively, the superconductor field coil 401 could be positioned between the lobes 503, 505 of the rotor 501 and the armature coil arrangement 701 could be shaped substantially as shown in the figures. The superconductor field coil 401 is made from a high temperature superconducting

(HTS) material and can be switched on and off. When the superconductor field coil 401 is switched on, it creates a permanent DC magnetic field. As the superconductor field coil 401 is a superconducting circuit, maintaining a large permanent magnetic field requires very little introduced energy as the coil has minimal resistance or other losses and is practically passive once energised.

The superconductor field coil 401 is constructed, in the exemplary embodiment, from two pancake wound annular shaped superconducting coils 403 separated by a conductive cooling ring 405. The cooling ring 405 is made from a suitably conductive material such as copper. Any number of superconducting coils 403 separated by conductive cooling rings 405 may be used. The coils 403 are connected in series to create an additive field circuit. As the circuit is superconducting, the DC current will flow continuously without degradation indefinitely as long as the coils are kept below the critical temperature to maintain superconductivity of the material. The continuously flowing current creates a constant and permanent magnetic field without additional energy being added to the system, as is the case with conventional coil magnets. The superconducting field coil produces a significantly larger magnetic field than is possible with the same volume or weight of conventional coil magnets, and can be switched off or attenuated by extracting current from the coil or heating the superconductor.

The superconductor field coil 401 comprises a suitable HTS material. An exemplary superconductor field coil 401 comprises ReBCO, and has 24 turns. It carries 317 A when the machine is operating at full-load, to provide a total of 7,600 amp-turns. Alternatively, the coil 401 could comprise an alternative HTS material such as DI-BSCCO for example. The coil could also comprise a different number of turns and carry a different current at full-load to provide the required total amp-turns. The ratio of operating current/critical-current for the superconductor is less than or equal to about 50%. A low ratio of operating current/critical-current helps to reduce AC loss and minimises the chance of a sudden quench .

In an embodiment, the coil 401 comprises second generation (2G) coated conductor HTS material (for example, ReBCO). The material is provided in ribbon or tape form, and comprises a thin film of superconductor on a tape substrate. Exemplary 2G HTS materials comprise a 2 micron HTS layer on 60 micron Hastelloy® that is either 4, 5, 10 or 12 mm wide. This allows a copper packaged 2 ^nd generation tape of aspect ratio 12 mm x 0.1 mm.

2G HTS superconductor material provides a wide but very thin coil material that is capable of handling very high magnetic field densities. High field densities provide high output power and high power to weight. As described in more detail below, the system comprises an electromagnetic shield 411/412. The electromagnetic shield attenuates AC field from the armature coil arrangement 701 that could otherwise cause AC losses in the superconductor field coil 401. The AC losses could degrade performance of the superconductor field coil 401 and increase cooling power requirement from the cryocooler. The system also includes laminations in the stator 601 that are arranged to provide efficiency and easy operation of the system.

The joins between the adjacent superconducting coils 403 are low resistance, but not superconducting. Because of this there will be a small degradation in current over time as the magnet operates and the current will need to be topped up from an external supply.

The superconductor field coil 401 is located in a cryostat chamber 407 and is

cryogenically conduction cooled via a heat transferring member 408 that extends from a cryocooler/vacuum chamber interface through openings in the stator 601. Conductive cooling of the coils simplifies construction of the coils within the stator 601 and housing the coils in a vacuum reduces the insulation requirements to radiation only. The cryostat chamber has a bore radius 406.

Cooling the superconductor field coil 401 to lower temperatures allows more current to flow unhindered in the superconductor, but requires exponentially more energy from the cryocooler. To maximise cooling efficiency, the superconductor field coil 401, which carries a large current, is cooled with a separate cooling system to the bearings 303, 305, which are capable at operating at relatively high temperatures. An exemplary operating temperature for the superconductor field coil 401 is 50 K.

An exemplary material for the cryostat chamber 407 is stainless steel. Stainless steel has low thermal conductivity at cryogenic temperatures, and its high reflectance protects against radiation heating.

The cryostat chamber 407 is mounted to the vacuum chamber 301 via insulated support rods 427 (shown in figure 2) that protrude through ports in the stator 601. This reduces heat transfer compared with mounting the cryostat to the stator 601.

The cryostat chamber 407 is surrounded by insulating material 409. As the

superconductor field coil 401 and cryostat chamber 407 are suspended inside the vacuum chamber 301, the insulating material 409 predominantly insulates the superconductor field coil 401 from radiation heating. An exemplary insulating material 409 is multi-layer insulation (MLI), for example composed of layers of polyimide film coated with aluminium on one or both sides. An exemplary MLI material is

CAPLINQ Linqstat™ PITIN-Alum Series.

Alternating current flowing in the armature windings 703 creates alternating magnetic fields. Exposure of the superconductor field coil 401 to alternating magnetic fields would cause hysteresis and eddy current heating and a reduction of current carrying capacity, reducing the size of the permanent magnetic field produced, and increasing thermal demand on the cryocooler.

To address this, an electromagnetic shield 411/412 is positioned between the

superconductor field coil 401 and the armature coil arrangement 701. The

electromagnetic shield attenuates the alternating field from the armature coil arrangement 701. The electromagnetic shield 411/412 is configured to restrict the passage of alternating field from the armature windings 703 to the superconductor field coil 401. The electromagnetic shield 411/412 absorbs alternating magnetic fields produced by rotation of the rotor 501 and/or current travelling in the armature windings 703 so that the superconducting field coil 401 is exposed to minimal AC field. The electromagnetic shield 411/412 absorbs the alternating magnetic field by allowing eddy currents to form.

In the embodiment shown, the electromagnetic shield 411/412 is a generally annular body of conductive material. The generally annular electromagnetic shield body could have any suitable cross-sectional shape when viewed in an axial direction of the system, For example, the cross-sectional shape could be circular. Alternatively, the cross- sectional shape could be a faceted polygon, for example hexagonal or octagonal. A U-shaped channel 412 is employed for housing the coils 403 and the insulating material 409. The insulating material 409 is provided between the cryostat chamber 407 and the U-shaped channel 412. In an embodiment, the primary purpose of the U-shaped channel is to house the coils 403, and the bottom part 413 of the U-shaped channel 412 is lined with an electromagnetic shield 411 having a rectangular cross section, shown in broken lines in Figures 4 and 5. In this embodiment, the U-shaped channel 412 may be made from stainless steel. The electromagnetic shield 411 is made from copper material. In alternative embodiments, the electromagnetic shield may be made from other low resistance materials, for example, gold or silver. In an alternative embodiment, the U-shaped channel 412 may be formed from a suitable low resistance material so that the U-shaped channel 412 serves the dual purposes of housing the coils 403 and insulating material 409, and acting as the electromagnetic shield. Again, the U-shaped channel 412 may be generally annular, and could have the cross-sectional shapes outlined above for the electromagnetic shield 411. In the embodiment shown in the figures, the U-shaped channel 412 is the electromagnetic shield and a separate electromagnetic shield 411 is not employed .

The U-shaped channel 412 shown in figure 5 has three parts 413, 415, 417. An annular body 413 is positioned radially inwardly of the superconductor field coil 401 and radially outwardly of a portion of the armature coil arrangement 701 adjacent the annular body 413, which predominantly protects from magnetic fields produced by current in the armature windings 703.

A first annular end wall 415 is positioned at or adjacent a first end of the annular body 413 and a second annular end wall 417 is positioned at or adjacent an opposite second end of the annular body 413. The first and second end walls 415, 417 extend radially outwardly from the annular body 413 to extend over at least a portion of the ends of the superconductor field coil 401. In the embodiment shown, the first and second end walls 415, 417 of the U-shaped channel 412 extend over the entire ends of the superconductor field coil 401. The first and second end walls 415, 417 predominantly protect from magnetic fields in the stator 601. The first and second end walls 415, 417 are optional as the magnetic fields in the stator 601 are essentially DC and are lower than the AC magnetic fields produced by current in the armature windings.

In the alternative embodiment where the superconductor field coil 401 is positioned between the rotor 501 and the armature coil 601, the annular body 413 is positioned radially outwardly of the superconductor field coil 401 and radially inwardly of a portion of the armature coil arrangement 701. In the embodiment where the superconductor field coil 401 is positioned substantially as shown in the figures, and the armature coil arrangement 701 is shaped to pass radially outwardly of the superconductor field coil 401, the electromagnetic shield would need to substantially surround the parts of the superconductor field coil 401 adjacent the armature coil arrangement 701. A suitable electromagnetic shield 412 comprises an annular body 413 and first and second end walls 415, 417 in a U-shaped configuration.

In the embodiment where the superconductor field coil 401 is positioned between the lobes 503, 505 of the rotor 501 and the armature coil arrangement 701 is shaped substantially as shown in the figures, an electromagnetic shield having a shape similar to shield 411 may be suitable.

The superconductor field coil 401 will be energised during commissioning of the machine, and may require either a small continuous current during operation, or to be occasionally topped up to full current. This is due to losses from the imperfection of the join between the adjacent coils 403 and electromagnetic shield 411/412. In an embodiment shown in figure 2, the commissioning and maintenance currents are supplied by current leads 421 that selectively excite the superconductor field coil 401. Exemplary current leads comprise copper current supply cables. However, copper cables create large thermal path from the cryogenic superconductor to the ambient power supply. In an alternative embodiment, the commissioning and maintenance currents are supplied by a

superconducting flux pump. The flux pump wirelessly transfers power from ambient to cryogenic temperatures, and transfers power across the vacuum chamber 301 wall without requiring a feedthrough, reducing thermal load on the superconductor field coil's cryocooler.

Flux pump

A superconducting flux pump of the type disclosed in WO 2016/024214 may be used to energise the superconductor field coil. The contents of that specification are incorporated herein in their entirety by way of reference.

Superconducting flux pumps use electromagnetic induction to generate a current within a superconducting circuit without physical connection to the circuit. The term 'flux pump' includes a wide range of devices which induce either persistent bulk magnetisation within a bulk superconducting material, or produce a net current to flow around a

superconducting circuit.

Figure 11A shows a schematic of an exemplary superconducting flux pump, disclosed in WO 2016/024214. The superconducting flux pump is used to energize the

superconductor field coil 401 that forms part of a superconducting circuit 920 that is enclosed within the vacuum chamber 301. The superconducting circuit 920 comprises the superconducting coils 403 and one or more superconducting elements 901 which are disposed about or partially within the flux pump stator yoke 915. The superconducting circuit is enclosed in a cryostat (not shown) and cooled to superconducting temperatures.

The flux pump comprises at least a flux pump rotor 911 and a flux pump stator 921, which are separated by a gap 906 through which extends a wall 930 of the vacuum chamber 301. The flux pump rotor 911 is provided externally of the vacuum chamber 301 and the flux pump stator 921 is provided internally of the vacuum chamber 301. The flux pump rotor 911 comprises at least in part a ferromagnetic yoke 916 and the flux pump stator 921 comprises at least in part a ferromagnetic yoke 915. The size of the gap 906 is at a minimum, the distance between the magnetic field generating elements 912a, 912b of the flux pump rotor 911, and the superconducting element 901 of the flux pump stator 921.

The flux pump comprises one or more magnetic field generating elements 912a, 912b carried by the flux pump rotor 911 to provide magnetic flux across the gap 906 to penetrate the superconducting elements 901 associated with the flux pump stator 921. The pair of ferromagnetic yokes 915, 916 of the flux pump rotor 911 and flux pump stator 921 jointly form a magnetic circuit of low reluctance which provides a pathway for the magnetic flux generated by the magnetic field generating elements 912a, 912b to cross the gap 906 and penetrate the superconducting elements 901, and then return to the flux pump rotor yoke 916 without penetrating the superconducting circuit 920 for a second time.

In an embodiment, the stator yoke 915 comprises flux-concentrating ferromagnetic protrusions 922a, 922b which are located opposite to the magnetic field generating elements 912a, 912b. The superconducting element 901 passes through the gap 906 between the flux pump rotor 911 and the flux pump stator 921 where it is exposed to focused magnetic flux lines 940 provided by the magnetic field generating elements 912a, 912b. In the embodiment shown, the superconducting element 901 first enters the gap 906 from a periphery of the flux pump stator 921, and then leaves the gap 906 via an exit opening 923 formed in the flux pump stator yoke 915. Thus the superconducting element 901 passes between the field generating elements 912a, 912b of the flux pump rotor 911 and the ferromagnetic protrusions 922a, 922b of the flux pump stator 921 in one direction but not another direction. The exit opening or aperture 923 comprises a higher reluctance region which enables the superconducting element 901 to exit the flux pump stator 921 under low or no opposite magnetic field. It then re-enters the gap 906 after it passes another opening 923 and leaves the gap from an opposite end of the flux pump stator 921.

The flux pump rotor 911 is driven by an electric motor 913. When a magnetic field generating element 912a, 912b moves past a ferromagnetic protrusion 922a, 922b of the flux pump stator 921, the magnetic flux imposed at the surface of the superconducting element 901 is greater than B _pe n which is the minimum imposed magnetic field required for flux penetration of the superconductor. When the magnetic field generating elements 912a, 912b move relative to the superconducting element 901, magnetic flux vortices enter one side of the superconducting element 901 and subsequently exit from the opposite side. This results in a net flow of magnetic flux lines 940 across the

superconductor element 901 which causes a net electrical current to be pumped around the superconducting circuit 920 thereby energizing the superconductor field coil 401. Figure 11B shows a cross-section of part of an embodiment of a superconducting flux pump. Flux pump rotor 911 is driven by motor 913 to rotate about axis 914 and sweep the magnetic field generating elements 912a, 912b past the flux pump stator 921 to drive lines of magnetic flux 940 across the superconducting element 901 thus driving an electrical current to flow around the superconducting circuit 920. Rotor - detail

Figure 3 shows the rotor 501. The rotor 501 comprises a body 502 having at least one first lobe 503 at or toward a first end of the body and at least one second lobe 505 at or toward an opposite second end of the body. Preferably, the rotor has two or more first lobes and two or more second lobes to provide rotational symmetry. The first lobes 503 are rotationally offset from the second lobes 505 so that a plane extending through the first lobes 503 is non-coplanar with a plane extending through the second lobes 505. In the embodiment shown, each end of the rotor 501 comprises two lobes spaced 180° from each other. The lobes of the first end are rotationally offset from the lobes of the second end by 90°. In alternative embodiments, there could be one, three, four, or more lobes at or toward each end.

The lobes are electrically offset by substantially 90° (in phase time) to provide the required flux path through the armature windings 703. In rotor embodiments comprising three, four, or more lobes, the lobes would be electrically offset by substantially 90° to provide the required flux path through the armature windings 703. For alternative armature coil arrangements, a different degree of electrical offset may be required.

The rotor 501 is fixedly attached to the shaft 205 and is concentric with the

superconductor field coil 401. In an exemplary embodiment, the rotor 501 material comprises iron or other suitable ferromagnetic material. In alternative embodiments, other materials with high magnetic permeability and high saturation field may be used, for example, iron cobalt alloys. Other ferromagnetic materials could also be used, but may not provide optimal results. As the rotor 501 rotates, the magnetic flux in the rotor 501 caused by the

superconductor field coil 401 is constant in intensity and orientation due to the axially symmetric magnetic field of the superconductor field coil 401. This causes the rotor 501 to behave as if it were a permanent magnet. Constant flux in the rotor 501 means that there are no electric currents induced in the rotor 501, and therefore no heating that would cause energy losses. Therefore, the rotor 501 does not need to be laminated. The rotor lobes 503, 505 at either end cause the magnetic field outside the rotor 501 to rotate with the rotor 501.

One end of the rotor 501 will effectively become the magnetic north pole, and this will be aligned with the north pole of the superconducting permanent magnet. The other end will be the magnetic south pole.

When combined with the stator 601, the lowest reluctance path for the magnetic circuit is along the rotor 501 (parallel to the shaft 205), then radially out through the north lobes 503, across the small flux gap 602, and into the stator 601 via either stator teeth 615 or the armature windings 703; whichever is directly alongside the rotor lobes 503, 505. Inside the stator 601, the magnetic circuit extends around the outside of the

superconducting field coil 401 then radially inward through the stator teeth 615 or armature windings 703, across the flux gap 602 and back into the rotor 501 at the south lobes 505.

The rotor lobes 503, 505 are arranged so the magnetic circuit only crosses each parallel intermediate portion of the armature coil arrangement 701 at one position at any one time. An intermediate portion of the armature coil arrangement 701 is never at the same time being passed by both a north 503 and south 505 rotor lobe.

In the embodiment shown there are two north lobes 503 and two south lobes 505, the south lobes are offset from the north lobes by 90°, so the arms of the armature windings 703 are alternately passed over by the north and south lobes, at different axial positions along the windings.

Stator - detail

The stator 601 surrounds at least an intermediate section of the armature coil arrangement 701 that corresponds to the rotor body 502 and lobes 503, 505. With reference to figures 6A and 6B, the stator 601 comprises a first end section 603 that is associated with, and in particular is positioned radially outwardly of, the first lobes 503 of the rotor, a second end section 605 that is associated with, and in particular is positioned radially outwardly of, the second lobes 505 of the rotor, and a bridge section 607 that extends between the first end section 603 and the second end section 605 of the stator 601.

The first and second end sections 603, 605 comprise a plurality of laminations 609 that are arranged to enable flux to travel radially and axially (arrows RA in figures 10A and 10B). The bridge section 607 comprises a plurality of laminations 621 that are arranged to enable flux to travel in an arcuate path (arrows AP in figures 10A and 10B) around the bridge section 607 between the first end section 603 and the second end section 605.

The end sections 603, 605 and the bridge section 607 have the same exterior radius. The interior radius of the bridge section is larger than the interior radius of the first and second end sections 603, 605. This provides a recess 631 located radially inwardly of the bridge section and defined by the bridge section and the end sections of the stator 601 (see figure 4). In the embodiment shown, the superconductor field coil 401 and electromagnetic shield 412 are positioned in that recess 631.

Radially inner edges of the laminations of the first and second end sections 603, 605 define an aperture for receipt of the armature coil arrangement 701. Parts of the armature windings 703 are provided in contact with or in close proximity to the radially inner edges of the laminations of the first and second end sections 603, 605, to provide high efficiency transfer of flux between the rotor 501 and the stator 601.

In the embodiment shown, the radially inner edges of the laminations of the first and second end sections 603, 605 define teeth 615 which extend between the armature windings 703. In alternative embodiments, the radially inner edges of the laminations of the first and second end sections 603, 605 define other geometries suitable for receiving an armature coil arrangement, for example a circular aperture.

Positioning the armature windings 703 of the armature coil arrangement 701 between teeth defined by an aperture in the stator end sections 603, 605 provides a very small flux gap between the rotor lobes 503, 505 and the first and second end sections 603, 605 of the stator. This enables very high efficiency transfer of flux between the rotor 501 and the stator 601.

With reference to figures 6A, 6B, 10A, and 10B, the flux path FP through the stator 601 is helical due to the rotor lobes 503, 505. The flux path FP enters the stator 601 at a position near a north rotor lobe 503, then travels around the stator 601 to exit the stator 601 near a south rotor lobe 505. The laminations 609, 621 in the stator 601 reduce eddy currents and reluctance of the flux path FP. In the first and second end sections 603, 605, the flux path FP readily travels in a radial direction from the interior to the exterior of the stator 601, but is restricted in a circumferential direction. In the bridge section 607, the flux path readily travels in a circumferential direction and moves around the stator 601 in a helix, but is restricted in a radial direction. The flux path travels axially in all sections 603, 605, 607 of the stator 601.

The stator 601 is conduction cooled. As it is in a vacuum, convection is not possible so cooling is a combination of conduction cooling to the vacuum chamber wall via stator mounts 623 (figure 1), and conduction cooling from the armature windings 703 which are liquid cooled. In an embodiment, the stator 601 is liquid cooled.

The first 603 and second 605 end sections of the stator each comprise a plurality of substantially planar laminations 609 that are oriented axially and radially. The end sections 603, 605 are formed from a plurality of laminated generally wedge-shaped or trapezoid-shaped members 611 and a plurality of laminated blocks 613. The laminated members 611 alternate with the laminated blocks 613. Any suitable number of members 611 and blocks 613 could be used.

In an alternative embodiment, some (but not all) of the laminated members 611 alternate with some (but not all) of the laminated blocks 613.

At least some of the laminated members 611 have a relatively long radial dimension and at least some of the laminated blocks 613 have a relatively short radial dimension. In an embodiment, all of the laminated members 611 have a relatively long radial dimension and all of the laminated blocks 613 have a relatively short radial dimension. The radial dimensions provide a plurality of teeth 615 and slots 617 at a radially inward edge of the first 603 and second end sections 605. Windings of the armature coil arrangement 701 are housed in the slots so they are in close proximity to the field coil 401 and so that the rotor lobes 503, 505 are in close proximity to the field coil 401.

The laminations 621 in the bridge section 607 are concentric annular laminations arranged to enable flux to travel in a helical path through the bridge section 607 from one end 603 of the stator to the other end 605 of the stator. The laminations in the bridge section are arranged enable flux to travel about 90 degrees around the stator 601 as well as along the length of the stator 601, to provide a flux path FP from one of the first lobes 503 of the rotor to one of the second lobes 505 of the rotor. The stator sections 603, 605, 607 are made from laminated layers of low reluctance material.

Exemplary materials comprise iron. The laminated members 611 and laminated blocks 613 may be machined from commercially available epoxy bonded lamination blocks, such as Japanese steel 10JNEX900 adhesively bonded blocks for example. Alternatively, the laminated members 611 and laminated blocks 613 may be formed by bonding the necessary number of laminations 609 of a suitable material. The bridge section 607 is constructed by wrapping lamination sheet like a scroll with adhesive between the layers until the desired radial thickness is obtained. These spiral laminations are an efficient use of space and are easier to manufacture than concentric rings. Alternatively, the bridge section 607 may be formed from concentrically laminated rings.

An exemplary material for the laminated members 611, laminated blocks 613 and laminations 621 in the bridge section 607 is 0.1 mm thick JFE Steel Super Core™

10JNEX900. This material has a core loss of 10 W/kg in a 1 T field and 1 kHz frequency, and a saturation magnetic field of 1.8 T. This material has very low power loss and high efficiency. In alternative embodiments, steel sheet could be used. This stator design is relatively simple to construct and reduces both material usage and losses in the stator.

The laminated members 611, laminated blocks 613, and bridge section 607 are adhesively bonded and banded using one or more bands that extend around the sections 603, 605, 607 (if necessary) to form the stator 601.

This arrangement of stator laminations 609, 621 allows the flux path FP to travel helically through the bridge section, while ensuring flux lines at the flux gap between the rotor and the stator 601 are perpendicular to the armature windings 703. It is important for the flux lines to be perpendicular to the windings of the armature coil arrangement 701 for maximum efficiency of the homopolar motor/generator. The helical path is required as the rotor poles are offset by 90°, so a path from a north to a south rotor pole must travel 90° degrees around the stator 601, as well as along its length.

Armature coil arrangement

Figures 7A and 8A show an exemplary double layer armature coil arrangement 701. The armature coil arrangement 701 comprises armature windings 703 made from Litz wire with an embedded liquid cooling tube and fully transposed copper strands. The armature coil arrangement 701 has a cylindrical intermediate section 705 that is located between two enlarged opposite end sections 707, 709. The intermediate section 705 is located in the central recess of the stator 601. The enlarged end sections 707, 709 are positioned externally of the central recess of the stator 601.

The armature windings 703, in the intermediate section 705 of the armature coil arrangement 701, are parallel with the magnetic field created by the superconductor field coil 401 so this field combined with the current flowing in the armature windings 703 does not cause a force on the wire. When operating as a generator, the rotation of the rotor 501 causes each armature winding 703 to be passed over by the rotor lobes 503, 505. As the lobes 503, 505 are rotationally offset, at any one time each armature winding 703 is only being passed over by either a magnetically north lobe 503 (causing voltage in the winding) or a

magnetically south lobe 505 (causing voltage in the winding in the opposite direction). Alternating voltage is generated in each armature winding 703 as alternating north and south lobes are passed over each armature winding 703.

The armature windings 703 may be liquid cooled as the stator 601 sits in a vacuum and conduction cooling via the stator 601 may be insufficient. Referring to figure 2, the armature windings 703 are connected to six power feedthroughs 423 which enable current to pass from the inside of the vacuum chamber 301 to the outside of the vacuum chamber 301. Three pairs of power feedthroughs 423 correspond to three phases of AC power. The power feedthroughs connect to the armature coil arrangement 701 at ends of the armature windings 711. The liquid cooling lines pass through a separate feedthrough 425 to a pump outside the vacuum chamber 301.

Vacuum is applied to the vacuum chamber 301 via a port 302 (Figure 2).

As shown in figure 8A, the armature coil arrangement 701 may be a double layer construction using diamond shaped coils. Each coil is identical, and multiple coils are connected in series for creating a single phase armature winding 703. In a 3-phase winding, 1/3 of the total number of coils are connected in series to form each single phase. Since there are two slots 706A, 706B per pole per phase in this embodiment, two coils of a given phase having a pole-pitch 704 could be connected in series as shown in figure 8B. N and S show the relative locations of the north and south poles of the rotor respectively at a point in time. A, B and C represent the three electrical phases. The phases A, B, C in the top slot 706A and the bottom slot 706B at each of the 24 slot locations 708 are shown above the respective coils.

The armature windings 703 are connected in series with windings under other poles to form a phase - all windings being connected with the correct polarity. Alternatively, the armature windings 703 may be wave wound in series, with all windings of a single phase constructed from a single length of cable, as shown in Figure 8C. A single cooling path for each phase is possible provided the liquid pumping pressure and temperature rise are acceptable. Otherwise, the cooling path could be split into two or more circuits. As and AF are the start and finish of the length of cable, where leads would be connected. The armature windings 703 are arranged so that the passing of the different rotor lobes 503, 505 across different armature windings 703 in the same circuit produces a voltage from one end of the armature winding 703 to the other. The portions of the armature windings 703 in the intermediate section 705 of the armature coil arrangement 701 extend parallel to each other in a longitudinal or axial direction of the apparatus. Within each armature winding circuit, the portions of the armature windings 703 that go from north to south are connected to the portions of the armature windings 703 that go from south to north 90° further around the stator so all the voltages generated are in phase and in the same direction along the armature winding wire. As shown in figure 8A, adjacent armature windings 703 are arranged so that a portion of one winding overlaps with a portion of an adjacent winding, in a radial direction. The two overlapping portions are each positioned in one of the slots 617 in the stator 601.

There are three armature winding circuits, which are identical but rotationally offset 120° (electrically) from each other around the armature coil arrangement 701 to receive and produce three phase power.

Alternative designs with different numbers of lobes and/or teeth and winding formats are also possible. For example, figure 7B shows a lightweight single layer armature coil arrangement layout 701A. In this configuration, the stator 601 may not be provided with teeth 615 and recesses 617. Instead, the portions of the armature windings 703 in the intermediate section 705 of the armature coil arrangement layout 701A will be positioned in close proximity to the wall of the central recess of the stator 601.

In another example of an alternative design, the armature windings may be helical. The angle through which flux travels in an arcuate path around the bridge section 607 of the stator 601 to travel between offset rotor lobes 503, 505 may vary from that of the armature coil arrangements 701, 701A discussed above. Helical armature windings would be more complicated to manufacture than the armature windings shown in the figures.

Figures 9A and 9B show axial models of the respective 2D flux paths through the rotor 501 and stator 601 for the armature coil arrangements of figures 7A and 7B respectively. The axial models assume a vacuum space of 15 mm on all sides of the superconductor field coil 401. Figures 9A and 9B also show some exemplary dimensions.

Referring to figure 9A, which shows the axial model for the double layer armature coil arrangement 701 :

Length of rotor lobes 503, 505 and stator end sections 603, 605 : d l = 130 mm.

Length of stator bridge section 607: d2 = 70 mm.

Flux gap 602: d3 = 3mm. Referring to figure 9B, which shows the axial model for the single layer armature coil arrangement 701A:

Length of rotor lobes 503, 505 and stator end sections 603, 605 : d4 = 85 mm.

Length of stator bridge section 607: d5 = 70 mm.

Total length of the rotor 501 and stator 601 : d6 = 240 mm.

Gap between the intermediate section 705 of the armature coil arrangement 701A and the rotor 501 : d7 = 3 mm.

Flux gap 602: d8 = 20 mm.

Radial dimension of rotor lobes 503, 505 : d9 = 24 mm.

Radial dimension of the rotor body 502: dlO = 45 mm.

Radial dimension of the stator end sections 603, 605 : d l l = 67 mm.

Radius of the outer surface of the stator 601 : d l2 = 194 mm.

Radial dimension of the superconductor field coil 401 : d l3 = 5 mm.

Figures 10A and 10B show a 3D illustration of the flux path FP through the armature coil arrangement 701 of figure 7A.

Any other suitable armature coil arrangement and winding configuration may be used.

Adding alternating current to the system instead of extracting it causes the homopolar machine to act as a motor where the lobes 503, 505 are attracted and repelled by the rotating magnetic field created by the current in the armature windings 703. Use of the apparatus

Figures 12A and 12B diagrammatically show an example application of the flywheel energy storage system 101 used in an electric train system, such as a subway. Power is supplied to the electric train system via a supply line. It is advantageous for the supply line voltage to stay within a given operational range for keeping current in armature windings 703 within their design value. For example, to generate a given torque value, it would be necessary to carry larger current in the armature windings 703 when the voltage is low. Larger current also causes larger voltage drop in the supply lines. On the other hand, voltage much higher than the design value will stress the armature winding 703 insulation and will cause accelerated aging of the electrical insulation. The supply line voltage is monitored by a suitable control system. The control system is configured to detect when the supply line voltage drops below a predetermined undervoltage trigger value, and when the supply line voltage exceeds a predetermined overvoltage trigger value. The control system is also configured to control the flywheel energy storage system 101. Figure 12A shows the response of the electric train system as a train leaves a station. In the example shown, the train accelerates from 0 km/hr up to 80 km/hr over 20 seconds. As the train accelerates it draws power from the supply line, causing the voltage on the supply line to drop. When the supply line voltage drops below the undervoltage trigger (eg 11,000 V), a control system connects the flywheel energy storage system 101 to the supply line as a homopolar generator. Kinetic energy from the flywheels 201, 203 is converted to electrical energy by the homopolar generator and added to the supply line to prevent the supply line voltage dropping further. As energy is extracted from the flywheels 201, 203 the rotational speed of the flywheels 201, 203 decreases. Once the train is at full speed the power it draws from the supply line decreases and the supply line voltage increases to a level above the undervoltage trigger (eg 12,000 V). The control system detects this and disconnects the flywheel energy storage system 101. The flywheels 201, 203 remain spinning at the new slower speed until another train leaves the station (where the flywheel energy storage system 101 will again be used as a generator, and the rotational speed of the flywheels 201, 203 will further decrease), or another train arrives at the station (where the flywheel energy storage system 101 will be used as a regenerative braking motor, and the rotational speed of the flywheels 201, 203 will increase, as described in more detail below.)

Figure 12B shows the response of the electric train system as a train arrives at a station. In the example shown, the train decelerates from 80 km/hr to 0 km/hr over 20 seconds. As the train decelerates, energy from a regenerative braking system is added to the supply line, causing the voltage on the supply line to increase. When the supply line voltage increases above the overvoltage trigger (eg 13,000 V), the control system connects the flywheel energy storage system 101 to the supply line as a homopolar motor. Electrical energy from the supply line is converted to kinetic energy in the flywheels 201, 203 by the homopolar motor, taking energy away from the train, slowing the train and increasing the rotational speed of the flywheels 201, 203.

Once the train has come to rest it is no longer adding energy to the supply line and the supply line voltage drops to a level below the overvoltage trigger. The control system detects this and disconnects the flywheel energy storage system 101. The flywheels 201, 203 remain spinning at the new faster speed until another train arrives at the station (where the flywheel energy storage system 101 will again be used as a motor, and the rotational speed of the flywheels 201, 203 will further increase), or a new train leaves the station (where the flywheel energy storage system 101 will be used as a generator, and the rotational speed of the flywheels 201, 203 will decrease, as described above). Commissioning

To commission the system, vacuum is applied to the vacuum chamber 301. Then the superconducting bearings 303, 305, superconductor field coil 401 and flux pump (if used) are cryocooled to their respective operating temperatures. The superconductor field coil is energised using current provided by current leads 421, and/or a flux pump. The current leads 421 and/or flux pump can be used to provide top up current to the system as/when needed.

Exemplary machine parameters and performance

The following describes parameters and performance of a flywheel energy storage system in accordance with an exemplary embodiment of the present invention. The described exemplary parameters are not intended to be limiting. A flywheel energy storage application is selected for storing 9 MJ of kinetic energy. This electric

motor/generator machine is coupled to a flywheel and rated at 500 kW. Its requirements are summarized in Table 1.

Table 1: Requirements for a flywheel energy storage device employing AC homopolar motor/generator rotating machine

Parameter Value

Energy storage capability 9 MJ

Rotating machine rating 500 kW

Rotary speed 25,000 RPM

Line voltage 866 V

Rotor diameter <200 mm

Axial length of stator <500 mm

Axial length of rotor <420 mm

Overall armature coil current density <3.0 A/mm ²

Maximum flux density in stator laminations < 1.5 T

This machine is designed using the following assumptions:

a) The rotating machine is synchronous AC homopolar type with 4-poles b) The field coil is located within the stator, and wound from HTS (either ReBCO or DI-BSCCO)

c) The operating temperature for HTS windings in 50 K for ReBCO and 30 K for DI-BSCCO

d) The field coil is cooled using a suitable cryocooler available off-the-shelf e) The armature on the stator is a 3-phase winding employing suitable Litz copper wire cable f) The armature windings have current densities; 3 A/mm ² (overall) and 6 A/mm ² (in the copper)

g) The armature winding is liquid cooled

h) The rotor is made of high permeability magnetic iron

i) The stator laminations are 0.1 mm thick Japanese JNEX-Core (model

10JNEX900) intended to reduce iron losses

A preliminary comparison between machines employing single layer (SL) 701A and double layer (DL) 701 armature coil arrangements is summarised in Table 2. Both machines are designed to generate 500 kW at a 3-phase line voltage of 830-900 V. The SL machine is shorter in overall length and lighter than the DL machine. However, the DL machine is more efficient than the SL machine and uses much shorter length of ReBCO wire (185 m for SL versus 54 m for DL). This makes the DL machine less costly to build than the SL machine. Table 2 also lists preliminary component weights for the two machines; the DL machine weighs 27% more than the SL machine. If size and weight are not of concern then the DL design is preferable over the SL design.

Table 2: Preliminary Design of SL and DL Machines

Parameter Si ale Laver Double Laver

Power Rating, MVA 524 540

Output power at full-load, MW 519 535

Line voltage, V-rms 900 866

Phase current, A-rms 336 360

Overall axial length, m 0.32 0.46

Overall diameter, m 0.44 0.44

Mass of the machine alone, kg 345 467

Mass of cryo-cooling system, kg 30 49

Total mass, kg 375 516

Efficiency at full-load, % 98.9 99.2

Cryocooler load, kW 0.80 1.36 Other Parameters of Inter est

Rated speed, RPM 25000 25000

Number of pole 4 4

Frequency, Hz 833 833 FIELD COIL DETAILS

Number of turns 204 24

Field winding current, A 252 278

Field winding Io/Ic ratio 0.54 0.49 HTS wire width, mm 3 3

HTS wire length, m 185 22 ARMATURE COIL ARRANGEMENT

DETAILS

Active length under each pole,

85 130 mm

Number of armature turns/phase 20 8

Number of coils in armature coil

6 24 arrangement

Number of turns/ coil 1 1 Machine component weight

summary

- Shaft, kg 4 4

- Rotor yoke, kg 42 59

- Poles, kg 7 12

- Stator case, kg 144 178

- Cooling system, kg 30 49

Total machine mass, kg 345 467

Total system mass, kg 375 516

The Litz wire with cooling tube that is used for the armature windings 703 of the armature coil arrangement 701 has a substantially rectangular cross-section with a stainless steel liquid cooling tube extending through its centre. The overall cross-section of each turn is 10 mm x 12 mm and the stainless steel tube at the centre has a diameter of 5 mm. The total cross-section of the Litz cable is equivalent to that of a single AWG 1/0 conductor. More details are provided in Table 3.

Table 3: Litz Wire Parameters

Parameter Value Units

Space available for Litz 100 mm ²

Equivalent AWG size 1/0 Cable

Strand diameter 0.32 mm

Number of strands 665

Copper cross-section 54 mm ²

Resistance of cable 0.00035 ohm/m Resistive loss per coil 51 W

Cooling tube diameter 5 mm

Water flow velocity 1 m/s

Pressure drop over two coils in series 6.7 kPa

Difference in cooling tube wall and water

0.34 K

temperature

Temperature rise of water through 2 coils in

1.24 K

series

Only 6.7 kPa pressure drop is experienced for a sustained water flow rate of 1 m/s in a tube length (2.3 m) equal to two coils connected in series (lap winding). The water temperature rise is about 1.2 K. Based on this data, it is feasible that all coils of a phase could be connected in series to form a single cooling circuit for each phase. The total resistive loss in all armature coils is 1.2 kW while carrying rated load.

Table 4 summarizes performance data for the machine. The performance has been calculated for a full-load current of 360 A at a power-factor of 0.99 lagging. The power- lagging factor is a conservative estimation. A more accurate value for power-factor could be determined based on the power electronics inverter design. The load angle with respect to the terminal voltage is 1.34°, which is quite small. However, in absence of a fixed terminal voltage, the value of load angle is of little interest. Induced voltage during the full-load is 1.14 pu : 14% higher than the nominal voltage at the machine terminals. This higher voltage is experienced at the machine terminals if it suddenly loses load while carrying full-load.

The nominal shaft torque for the machine is 208 Nm. But during short-circuit at the machine terminals, the torque will increase to 1487 Nm. Thus, the rotor shaft 205 and the armature coil arrangement 701 mechanical supports can be designed to bear this torque with an acceptable safety margin, e.g. a safety factor of at least 2. Attractive forces between rotor and stator are 9282 N under a solid-pole and 246 N under void- pole. Table 4: Calculated Performance

Performance

Rated line voltage 866 V

Rated current 360 A

Power rating 540 kVA

Field current - no-load 282 A

Power factor at full-load-lagging 0.99

Field current - full-load 317 A

Load angle at full-load 1.3 o

Induced voltage at full-load 1.14 pu

Power generated at full-load 1.01 pu

Torque on armature at full-load 208 Nm

Torque on armature during a short-

1487 Nm

circuit

Attractive force between rotor and stator

Under a salient pole 9282 N

Under non-salient pole 246 N Losses at full-load

Stator iron core loss 1.7 kW

Armature resistive loss 1.2 kW

Armature cooling system power 0.13 kW

Field winding cooler power 1.37 kW

Refrigerator Coefficient of Performance

21

(COP)

Total losses at full-load 4.45 kW

Efficiency at full-load 99.2 %

Total losses at no-load 3.23 kW

Table 4 also lists various loss components when the machine is carrying full-load. Stator core loss and armature winding resistive loss are two most significant components.

Cryogenic cooling system power is estimated to be 1.36 kW based on a refrigerator COP of 21 at 50 K. A cooler system selected by a manufacturer may have different COP. Furthermore, thermal load calculations for the HTS coil might be different as they are a function of HTS coil type, its construction and interface with the room-temperature systems. Base on the assumptions made here, total loss at full-load is 4.4 kW, which yields an efficiency of 99.2%. Superconducting machines typically experience significant losses under no-load operation. For example, if the field current is kept at its full-load operating value, the machine thermal load (cryo-cooler power) and core loss will remain unchanged and total loss would be 3.2 kW. However, if the field current is turned off during a prolonged no- load operation then the only loss component (cryo-cooler power loss) would be less than 1.36 kW.

The field current will be turned off when the flywheel is turning and storing energy for a long period of time. For example, if the flywheel is turning while the field current is on, core-loss experienced in the stator laminations will be supplied from the energy stored in the flywheel. This will slow the flywheel.

Design concepts for armature and field windings as discussed above will employ technologies familiar to individual manufacturer capabilities and practices. Because of this, the results reported here may be preliminary. However, the parameters reported here are useful for setting goals and for designing electronic, cooling and other subsystem interfaces. Only the double layer (DL) armature winding design is discussed in the following section. Table 2 summarizes the flywheel machine design, the 2nd column refers to the DL architecture. The machine is designed for generating 535 kW at line voltage of 866 V. The corresponding phase current is 360 A with a power factor of 0.99 lagging. The homopolar motor/generator has axial length and diameter of 455 mm and 438 mm, respectively. These dimensions are within the design guidance of Table 1. The mass of the machine alone is 467 kg. Total machine mass with cryo-cooling system is 516 kg. These mass numbers are likely to change as a function of selected machine hardware and cooling system. At the rated load, the efficiency of this machine is estimated to be 99.2%. The ReBCO field coil has 24 turns and carries 317 A when the machine is operating at full-load. A different superconductor could be used for generating the total amp-turns equal or greater than 7,600 (~ 24 x 317). Furthermore, the ratio of operating-current (Io)/critical-current (I _c ) for the superconductor is preferred to be less than or equal to -50%.

The armature coil arrangement on the stator employs double layer lap winding using single turn coils. There are 2 coil sides per pole per phase, resulting in 8 coils per phase (equal to 8 turns/phase). Machine component weight estimates are also included in Table 2. Table 5 includes machine parameters for calculating machine performance under different loads and operating conditions as well as for interfacing it with inverters.

Parameters are expressed in per-unit values; referenced to a base impedance of 1.39 ohm. Synchronous reactance of this machine is very low due to the small air-gap between rotor and stator. Small synchronous reactance is usually beneficial for stable operation of the machine on an electric grid. However, the small value also creates very large fault current during sudden short-circuit of the field coil. Attention needs to be paid to this aspect during inverter design. No-load and full-load field currents are 278 A and 317 A, respectively. The field coil should be designed to carry 317 A with an I _c /I ₀ of about 2. However, it must be noted that if the machine operating at full-load suddenly loses load, the armature voltage will rise to 1.14 pu (= 866*1.14 = 987 V-rms). Since it is difficult to change HTS field coil current rapidly, the voltage will be modulated

electronically in the inverter.

Table 5: Machine Parameters

Parameters

Rated line voltage 866 V

Rated current 360 A

Power rating 540 kVA

Base impedance 1.39 ohm

D-axis synchronous reactance 0.14 pu

Q-axis synchronous reactance 0.02 pu

Armature resistance 0.0023 pu

Induced voltage with pf= l load 1.14 pu

Field current - no-load 278 A

Field current - full-load 317 A

Field winding inductance 0.96 rmH

Maximum field winding discharge voltage 100 V

Resistance of field winding discharge

0.36 ohm

register

Field winding discharge time constant 2.7 ms

Mutual inductance between Field and

6.29xl0 ^"5 H

Armature

In event of a sudden quench of the field winding, it could be discharged using a resistor of 0.36 ohm. This will impose 100 V across the field coil terminals and provide a discharge time constant of 2.7 ms. Manufacturers could select a different discharge resistor value or different protection scheme that suits the HTS winding design. The preferred embodiments provide compact and lightweight devices of very high torque/weight and electrical efficiencies. One possible application is as machines which act as both motor and generator integrated with high-speed flywheels for energy storage. This system could be employed on moving assets such as locomotives and automobiles, both as a method of energy recovery and for fast energy delivery.

Alternately, the flywheel energy storage system could act as a battery for storing surplus solar and wind energy for later use during lean periods of energy generation. HTS field coils enable compact and light-weight machines to be realised. An AC homopolar synchronous machine topology enables high rotational speeds to be achieved by removing the requirement for active rotating coils and simplifies the cryogenic cooling system for enhanced system reliability, making it ideal for high power applications. Building such a machine using conventional copper rotor excitation coils would present significant cooling challenges, and would limit the size of such a machine to a power rating of only a few kilowatts. However, mega-watt rated machines become possible by replacing the DC field excitation coil with a suitable superconducting coil.

The above describes exemplary embodiments of the present invention, and modifications may be made thereto without departing from the scope of the present invention.