Patent Description:
In conventional power systems and grids, supply must meet demand on a second-by-second basis. To avoid service interruption, unbalanced systems, and an increase in the cost of energy and unnecessary curtailment of renewable energy-based systems, EESS (electrical energy storage systems) will play an essential role in the management of energy in the future.

Power generators need to accomplish two essential functions. The first is to provide the base-load: the minimum energy requirement on a half-hourly basis. Forecasting algorithms and historic data can be used to anticipate the energy demand. The second is frequency regulation via services such as FFR (firm frequency response) procured by the National Grid. This service is of high value to the grid as it balances the second-by-second over/under supply of power. For this service, the priority is power over energy as a high power reactive load for short to medium durations is required. Other services are then procured based on a duration/energy requirement.

In today's market, the baseload is provided by nuclear and coal generation, due to its slow response time and inability to react quickly to changes in demand. Gas-fired generation are more flexible, and along with pumped hydro storage and renewable technologies, make up for the instantaneous load regulation. A main requirement of such systems is of course a permanent and constant availability.

The main concern with renewable energy sources such as wind and solar is that they cannot guarantee permanent availability due to their dependence on weather conditions. This variable and unpredictable nature can lead to a severe curtailment of renewable energy-based systems. In <NUM>, over <NUM> TWh of wind generation was curtailed due to an inability to store and manage the energy generation efficiently. In this context, EESSs are essential if renewable energy sources are ever to find their role as a critical player in the world of robust grids.

A particular type of energy storage system is a flywheel energy storage system (FESS), in which energy is stored as kinetic energy by a rotating flywheel. Challenges of designing current state of the art grid-connected FESS schemes relate to the presence of a separate motor/generator (M/G) and an outer containment system, needed to handle the high destructive forces arising in case of a fault. The two-bodied scheme can result in a quite bulky and heavy arrangement, due to the presence of numerous components. Furthermore, since flywheels are ideally vacuum-operated, an added problem lies in the presence of a rotating vacuum-sealed joint between the M/G and the main body, which represents one of the main sources of inefficiency.

<CIT> discloses a mobile mining machine including a plurality of traction elements, a plurality of motors, a power source in electrical communication with the plurality of motors, and an energy storage system in electrical communication with the plurality of motors and the power source. Each of the motors is coupled to an associated one of the plurality of traction elements. Each of the motors is driven by the associated traction element in a first mode, and drives the associated traction element in a second mode. The energy storage system includes a shaft, a rotor secured to the shaft, a stator extending around the rotor, and a flywheel coupled to the shaft for rotation therewith. In the first mode, rotation of the motors causes rotation of the flywheel to store kinetic energy. In the second mode, rotation of the rotor and the flywheel discharges kinetic energy to drive the motors.

<CIT> discloses a kinetic energy storage system which utilises a flywheel with a motor generator to store energy. A flywheel rotor is located in an elongate housing which forms at least part of a rigid framework. In use on a vehicle the framework provides a chassis for the vehicle and the vehicle may be powered from the flywheel. The flywheel rotates at high speed in a vacuum and the motor generator could be an inverted switched reluctance motor generator optionally providing a magnetic bearing function.

<CIT> discloses an energy storage system according to the preamble of claim <NUM>, forming part of a hybrid power train for an aircraft.

Aspects and embodiments of the present invention are set out in the appended claims.

These and other aspects and embodiments of the invention are also described herein.

According to an aspect of the invention, there is provided an energy storage system, comprising: a switched reluctance motor comprising a rotor and a stator; wherein the rotor is arranged to be capable of acting as a flywheel for storing energy; a casing enclosing the rotor and stator; wherein the rotor is supported for rotation on the casing; and a base arranged to support the casing; wherein the energy storage system is arranged such that, in use, the axis of the rotor is generally horizontal.

By providing a horizontal axis for the rotor/flywheel, and thereby a vertically-orientated flywheel, the system may take up less space on the ground (i.e. the system has a smaller 'footprint' than it would have if mounted with a horizontal axis), meaning the usage of available space can be improved. For example, a plurality of flywheel systems may be provided in an array, wherein the small form factor may be beneficial in allowing a larger number of such systems to be provided in a given area (thereby increasing the amount of energy that can be stored by the array). Furthermore, by providing the system on a horizontal axis, it may be easier to re-lubricate the bearings in-situ (since there is no need to reach above and below the rotor).

In addition, providing a horizontal axis for the flywheel rather than a vertical axis can improve stability if used within a vehicle. Vertically-orientated flywheels may cause undesirable gyroscopic effects, which need to be controlled. A typical way to mitigate this is to provide two oppositely-rotating flywheels to balance out external torque. However, this requires the positioning and the angular velocity of the flywheels to be matched. The present system may allow only a single flywheel to be used, without generating gyroscopic effects of a magnitude and direction to affect vehicle handling.

Preferably, the rotor is arranged within the stator. That is, the stator may be arranged around the rotor. The stator may be in the form of a hollow cylinder, while the rotor may be a cylindrical shape, optionally conforming to the inner surface of the stator. The rotor may be arranged to concentrically align with the stator. The rotor may be configured to be rotatable within the stator.

Preferably, the rotor and the stator have the same depth. The depth is defined as the dimension parallel to the central axis of the cylindrical shape (i.e. parallel to the rotor axis). The depth is perpendicular to the diameter. Preferably, the depth of the rotor is less than the diameter of the rotor. Preferably, the depth of the stator is less than the diameter of the stator. For example, the depth of the rotor and/or the stator may be less than half the diameter of the rotor and/or the stator, respectively. Preferably, the casing has a depth less than a diameter of the casing. For example, the depth of the casing may be less than half the diameter of the casing. This arrangement can help provide a small footprint as described herein. Preferably, the base has a depth less than a width of the base, and more preferably less than the width of the casing.

In addition, the use of a switched reluctance motor (SRM) design may allow for a system to be provided without magnets or rare earth metals. In turn, this may allow the total number of components required can be reduced. Furthermore, the lack of a separate flywheel component significantly reduces the capital cost and component count of the system by obviating the need for many of the non-metallic components of known systems.

Preferably, the casing is generally cylindrical thereby to conform to the shape of the rotor and stator. On a circular face of the casing, a chord defined by the intersections of the casing and the base may be greater than the radius of the circular face and less than the diameter of the circular face.

The casing may be partially recessed into the base. A width of the base may be greater than a width of the part of the casing that is recessed into the base, the width of the base being perpendicular to the axis of the rotor. The base may be shaped as part of a triangular prism being cut-off by the casing. Optionally, a depth of the base does not extend beyond a depth of the casing, the depth of the casing being parallel to the axis of the rotor, preferably wherein a depth of the casing is defined by the length of an axle of the rotor.

The casing may comprise a plurality of fins on an outer surface for assisting heat dissipation. The casing may be configured to contain parts of the rotor within the casing in the event of a failure in use. The casing may be evacuated for use.

Preferably, the base is arranged to be mounted on a foundation in use. The casing may have a diameter of between <NUM> and <NUM>,<NUM>, preferably between <NUM> and <NUM>,<NUM>, more preferably between <NUM> and <NUM>,<NUM>.

The rotor may be supported for rotation on an axle which rotates on rolling element bearings provided on the casing. The bearings may be deep groove radial hybrid bearings. A compressible member may be provided between the bearings and the axle.

The stator may comprise a plurality of electrical windings positioned around the rotor. The rotor may comprise more than <NUM> stator poles, preferably at least <NUM> rotor poles, more preferably at least <NUM>, still more preferably at least <NUM>, most preferably at least <NUM>. Preferably, the switched reluctance motor has a <NUM>/<NUM> configuration.

The rotor and stator may be each formed from a plurality of laminations, preferably wherein each of the plurality of laminations has a width of <NUM> or less. At least an outer region of the rotor may be made from a ferromagnetic material.

According to an aspect of the invention, there is provided energy storage installation, comprising a plurality of energy storage systems as described herein. The energy storage installation may further comprise one or more power electronics units, each configured to control the operation of at least two energy storage systems. The energy storage installation may further comprise means for converting alternating current (AC) into direct current (DC) thereby to allow import of AC from an electrical grid to the installation. The energy storage installation may further comprise means for converting direct current (AC) into alternating current (DC) thereby to allow export of AC from the installation to an electrical grid. The energy storage installation may further comprise a master controller for controlling the operation of the energy storage systems.

According to another aspect of the invention, there is provided an energy storage system, comprising: a switched reluctance motor comprising a rotor and a stator; wherein the rotor is arranged to be capable of acting as a flywheel for storing energy; a casing enclosing the rotor; wherein the casing includes bearings for supporting the rotor; and a base arranged to support the casing from underneath; wherein the energy storage system is arranged such that, in use, the axis of the rotor is generally horizontal.

According to another aspect of the invention, there is provided an energy storage system, comprising: a switched reluctance motor comprising a rotor and a stator; wherein the rotor is arranged to be capable of acting as a flywheel for storing energy; a casing enclosing the rotor and stator; wherein the rotor is supported for rotation on the casing; and a base arranged to support the casing; wherein the rotor comprises more than <NUM> stator poles, preferably at least <NUM> rotor poles, more preferably at least <NUM>, still more preferably at least <NUM>, most preferably at least <NUM>. Preferably, the switched reluctance motor has a <NUM>/<NUM> configuration.

In general, the present invention may involve integrating a flywheel into a switchreluctance motor (SRM) to allow for more efficient flywheel energy storage. The rotor component may act as the flywheel that stores the energy. This is intended to balance supply and demand on the electrical grid, for example at a utility level. In one example, an application may be to balance the frequency of the grid - <NUM> in the UK and most worldwide territories, and <NUM> in the US. The use of the system may vary with the demands of the electrical grid. As energy is generated from intermittent generators (e.g. renewable generators) excess energy can be imported by the flywheels. If the generation output suddenly drops (e.g. from a drop in wind generation) the energy stored by the flywheels can be exported and used to balance the grid for short to medium durations.

Accordingly, the present invention re-organizes the entire structure of a typical FESS by integrating both the M/G and inertia functions into the rotor body and incorporating the stator in the outer containment, thus realising a simple, one-bodied solution. This concept may have the added benefit of simplifying the mechanical structure and removing all the low-inertia rotating parts (e.g. transmission joints), thus reducing the overall component count.

The invention extends to methods, systems and apparatus substantially as herein described and/or as illustrated with reference to the accompanying figures.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory.

Furthermore, features implanted in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

As used herein, the terms 'rotor' and 'flywheel' should be understood to be synonymous when used in the context of descriptions of the present invention (but not in the context of descriptions of any known systems, where these are generally separate components).

As used herein, the term 'generally horizontal' (when used in the context of an axis) preferably connotes an angle of between -<NUM>° and <NUM>° with reference to a surface on which a system is mounted; more preferably between -<NUM>° and <NUM>°; yet more preferably between -<NUM>° and <NUM>°; still more preferably -<NUM>° and <NUM>°; and most preferably -<NUM>° to <NUM>°.

As used herein, the term 'approximately' preferably connotes an error margin of less than <NUM>%; more preferably less than <NUM>%; yet more preferably less than <NUM>%.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

The invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:.

<FIG> shows an energy storage system <NUM> comprising a switched reluctance motor (SRM) comprising a rotor <NUM> and a stator <NUM> (including windings, not shown), wherein the rotor <NUM> is arranged to be capable of acting as a flywheel for storing energy. The motor is arranged such that, in use, the axis of the rotor is generally horizontal. The system comprises an axle <NUM> (also referred to as a 'stub axle' or 'shaft') that defines the axis of the rotor. The axis of the rotor is perpendicular to the plane of the rotor such that the rotor <NUM> rotates about the axis of the rotor.

As is common with motor designs, the rotor <NUM> may be generally cylindrical in shape, with a generally circular cross-section, and the stator <NUM> may be a hollow cylinder configured to receive the rotor <NUM> therein. The rotor <NUM> may be disposed within the inner cylindrical surface of the stationary stator <NUM>, and configured to rotate about the axis of the rotor. For good magnetic coupling, the radial periphery of the rotor is in close proximity to the windings of the stator. The system <NUM> further comprises power electronics and control systems for controlling the operation of the system <NUM>.

In a switched reluctance motor, when a current is supplied to stator windings and they become energised, the windings form a stator pole. When stator windings are energised, the corresponding rotatable rotor poles become aligned with the energised stationary stator poles. This can be used to control the rotation of the rotor. In use, if an electric current is supplied to a pair of stator windings which are not aligned with poles on the rotor, the rotor experiences a torque. By controlling the electric currents supplied to the stator windings relative to the rotor angular position, the motor can be driven in either direction of rotation, or can be arranged to generate electrical power. By controlling the operating speed of the rotor, the relevant energy imported to or exported from the system can be controlled.

The rotor <NUM> comprise a plurality of protruding regions <NUM> extending radially outwards. The protruding regions <NUM> form the poles of the rotor <NUM>. At least the protruding regions <NUM> of the rotor <NUM> are made from a ferromagnetic material such as steel. In some examples, the entirety of the rotor <NUM> is made from a ferromagnetic material. In other examples, only the protruding regions <NUM> or an outer region of the rotor <NUM> is made from a ferromagnetic material. The protruding regions <NUM> are equally spaced around the circumference of the rotor <NUM>. As shown in <FIG>, the rotor <NUM> comprises <NUM> rotor poles. The stator <NUM> surrounds the rotor <NUM>. The stator <NUM> comprises a plurality of protrusions <NUM> around its inner cylindrical surface. In use, each protrusion <NUM> includes an electrical winding, forming the poles of the stator <NUM>. The windings are connected directly to a power converter for the system <NUM>. As shown in <FIG>, the stator <NUM> comprises <NUM> windings forming <NUM> stator poles. This forms a switched reluctance motor with a <NUM>/<NUM> configuration.

The SRM acts as a motor when power is transferred to it, and acts as a generator when power is transferred away from it. As is known, a SRM is a stepper motor with an unequal number of salient rotor and stator poles. There are no windings or permanent magnets on the rotor. The SRM power electronics further comprises a plurality of gate drivers to power the SRM. The gate drivers provide a high current drive to switches such as IGBTs (Insulated Gate Bipolar Transistors). In other examples, field effect transistors such as MOSFETs or thyristors can be used instead of IGBTs.

An SRM has a number of specific strengths that make it well suited for the application at hand. In particular, the simple and robust rotor structure permits its use as a source of inertia (i.e. as a flywheel) without the need for external sources and complications (i.e. PMs, rotor bars or field coils). The use of an SRM may also allow for the system to be manufactured at a cheap capital cost, as PMs or rotor bars mean added cost from both a material and from a manufacturing perspective.

The design of the SRM of the system is influenced by the need to use the rotor as a flywheel for storing energy. The kinetic energy stored (EK) in a flywheel is proportional to the polar moment of inertia (J) and the difference between the maximum and minimum square of the rotational speed (ωmax and ωmin), as shown in (<NUM>): <MAT>.

The faster the flywheel is able to rotate, the more energy it can store. The present system generally aims to store a large amount of energy. For example, the system may store above <NUM> kWh between <NUM> and approximately <NUM>,<NUM> rpm. Such power storage properties allow the system to act as an effective energy store, in particular for use in load balancing.

A high speed ratio (i.e. the ratio of the maximum and minimum rotational speed) of X = <NUM> is desired in order to obtain the desired power storage properties. Surface Mounted Permanent Magnet (SPM) machines offer a very poor speed ratio (X = <NUM>), Interior Permanent Magnet (IPM) machines and Induction Motors (IM) offer a speed ratio of about <NUM>, while Synchronous Reluctance (SynRel) machines can achieve speed ratios of up to <NUM>. Most SRMs achieve speed ratios of up to <NUM>. SRMs achieving speed ratios as high as required are not a common occurrence, and so a new SRM design is required.

A typical design for switched reluctance motors is a <NUM>/<NUM> configuration, with <NUM> rotor poles and <NUM> stator poles, used with three-phase power. In many applications, the use of more than a <NUM>/<NUM> configuration is unnecessary and adds additional cost and complexity in terms of control electronics and power electronics. In particular, by increasing the number of poles, the amount of switching required is increased which is generally undesirable in traditional SRMs, at least because electrical noise is significantly increased.

However, in the present system, where the rotor itself is used for flywheel energy storage, providing a higher number of poles is generally desirable to allow the system to achieve a high speed ratio. In particular, providing a high number of phases (corresponding to a high number of poles) reduces the torque that each phase needs to provide, resulting in a smaller flux and a smaller pseudo-back e. (electromotive force) at high speeds, which may improve the performance of the system. Reducing torque is particularly important in light of the large size and weight of the flywheel. Accordingly, in the present system a <NUM>/<NUM> configuration is used (i.e. a configuration having <NUM> rotor poles and <NUM> stator poles). In this case, <NUM>-phase power is used. The high-power input and output of the flywheel is dependent on the number of phases and the IGBTs which can share the load. For example, for a required torque of <NUM>, if a <NUM>/<NUM> configuration is used, each phase must be designed for the full <NUM> torque. However, in the <NUM>/<NUM> configuration, four phases are capable of conducting at the same instant, such that each phase can be designed for <NUM> (one quarter of the overall torque).

The physical size of the flywheel in the present system (which is relatively large thereby to store a large amount of energy) may also promote the use of a configuration having a relatively high pole count for the purposes of control. If the common <NUM>/<NUM> configuration was used, the system would be less controllable and might experience higher vibration (which would be detrimental to the operational life of the machine) as a result of the large torque that each phase needs to provide in order for the rotor to rotate. The use of more poles allows for more refined control of the rotation of the rotor.

A <NUM>/<NUM> configuration may greatly aid the design compared to other configurations such as a <NUM>/<NUM> motor. The geometry of the design such as the particular angles and pole lengths of the present system may maximise the efficiency of the system for large-scale energy storage purposes. Furthermore, torque ripple is decreased with an increase in pole count, and hence the system performance is increased.

The system <NUM> further comprises a casing <NUM> (or 'chassis'), which encloses the rotor <NUM> and stator <NUM>. In some cases, the stator <NUM> may be coupled to the casing <NUM>. The casing <NUM> is configured such that a width of the casing <NUM> is parallel to the axis of the rotor. The width of the casing <NUM> is configured such that the width of the rotor <NUM> and the stator <NUM> is enclosed within the casing <NUM>. For example, the casing <NUM> may be generally cylindrical, or comprise a cylindrical element around the outer surface of the stator <NUM>, such that it conforms to the shape of the stator <NUM> (and the rotor <NUM> within the stator <NUM>). The casing <NUM> comprises a cover <NUM> for each side of the casing, the cover being generally circular and extending over the face of the rotor <NUM>. The cover <NUM> is be configured to engage a face of the casing <NUM> thereby to close the generally cylindrical shape formed by the casing <NUM>, such that the casing <NUM> is sealed in use.

During assembly of the casing <NUM>, a vacuum pump and vacuum seals are used to seal the interior from the exterior. Therefore, the sealed casing <NUM> is evacuated in use such that the rotor <NUM> is operable under a vacuum. As such, the space around the rotor <NUM> is evacuated such that the rotor <NUM> is free to rotate under reduced friction. Once the system is installed on site, vacuum losses may occur over its lifetime, and as such a backup vacuum pump may be present "on site" (i.e. the location where the system is used) and connected to the system in order to compensate for any vacuum losses or deterioration in vacuum quality. The vacuum minimises frictional forces acting on the rotor <NUM>, and reduces viscous drag between the rotor <NUM> and the stator <NUM>.

Windage losses during rotation will be proportional to the rotational speed of the rotor cubed - hence, a suitable vacuum may be used to maintain efficiency and store a viable quantity of energy.

The casing <NUM> comprises bearings for supporting the rotor <NUM> (not shown in <FIG>). The bearings are mounted in the centre of each of the covers <NUM>, such that the ends of the axle <NUM> of the rotor <NUM> locate with and engage with the bearings when each of the covers <NUM> is mounted to the rest of the casing <NUM>. The bearings are provided in order to minimise friction during rotation. The rotor <NUM> is only supported by bearings (i.e. noncontact supports) and as such can rotate with very low frictional losses, especially in combination with an evacuated interior region. A compressible member (not shown) such as an O-ring or compressible gasket is provided between the cover <NUM> and the axle <NUM>. Once the system is sealed under vacuum, the cover <NUM> pushes against and compresses the O-ring. The cover <NUM> is secured to the casing <NUM> by fastening means such as screws. A plurality of screws is used to secure the cover <NUM> by screwing on the leads. The bearings are rotating element bearings, more specifically deep groove radial bearings.

The cover <NUM> may be removed from time to time for essential repair or maintenance. In this case, the O-ring is replaced, and the vacuum is re-introduced before operation starts again (for example by use of a vacuum back-up pump).

As the flywheel is supported in a vertical plane, the axis of the rotor is generally horizontal in use. This may reduce the stress on the bearings over time and may thereby increase their lifetime. In certain known systems, flywheels are conversely designed having vertical axes and are supported for rotation on angular contact bearings, or in some cases magnetic bearings. Magnetic bearings may be prohibitively expensive for many applications, and hence angular contact bearings are often used. However, in the present system, the flywheel may be arranged having a horizontal shaft, allowing the use of deep groove radial bearings.

<FIG> shows cross-sectional views of an example deep groove radial bearing <NUM> and an example angular contact bearing <NUM>. The angular contact bearing <NUM> comprises one or more rolling elements <NUM> which are arranged by contact surfaces <NUM> to rotate at an oblique angle relative to the axis of any shaft supported by the bearing, thereby to provided axial support to the shaft, allowing use of these bearings in the vertical case. Deep groove radial bearings <NUM> cannot be used in the vertical case as they do not provide axial support, since the rolling elements <NUM> and contact surfaces <NUM> are arranged to support radial loads only.

As the skilled person would appreciate, deep groove radial bearings <NUM> have a lower coefficient of friction (approximately <NUM>% less) than angular contact bearings <NUM> that are used in horizontal arrangements, due to increased slip of the rolling elements <NUM> as a result of the shape of the contact surface <NUM>. The design of the cage of the bearing <NUM> and the fact that the angular contact bearing <NUM> distributes load onto all rolling elements <NUM> (rather than approximately one third of the roiling elements <NUM> present in the bearing, as in deep groove radial bearings <NUM>) may be further reasons for the lower coefficient of friction of deep groove radial bearings <NUM> relative to angular contact bearings <NUM>.

The use of deep groove radial bearings <NUM> in the system may thereby improve the longevity of the bearings, thereby reducing the regularity at which bearings need to be maintained and/or replaced. This may reduce the downtime of the system - as will be appreciated, the system generally must be completely spun down for maintenance, requiring that energy is stored elsewhere. Accordingly, minimising downtime of the system is an important design consideration.

<FIG> shows bearings assembled in a system <NUM> with a cover <NUM> in place. In particular, the cover <NUM> shown in <FIG> is assembled with axle <NUM>, also shown in <FIG>. The deep groove bearings <NUM> (shown in <FIG>) are provided between the axle <NUM> (specifically, a thin end portion of the axle) and a flange <NUM> extending from the cover <NUM>. In other examples, the bearings <NUM> may be the same as angular contact bearings <NUM> shown in <FIG>, or they may be magnetic bearings.

The axle <NUM> is held in place in the cover <NUM> by pin <NUM> in order to hold the rotor <NUM> relative to the casing <NUM> and the rest of the system <NUM>. As described above, the bearings <NUM> are provided to facilitate rotation of the axle <NUM> and hence the rotor <NUM> relative to the cover <NUM>.

Referring again to <FIG>, the casing <NUM> further comprises heat dissipation means <NUM>. For example, heat dissipation means <NUM> may comprise fins, as shown in <FIG>, configured to increase the effective surface area of the casing <NUM> to assist in the dissipation of heat from the motor. In some examples, other heat dissipation <NUM> means such as heat sinks or heat pipes may be provided.

The system <NUM> comprises a base arranged underneath the casing for support. The casing is entirely supported from underneath by the base, which may assist in minimising the 'footprint' of the system. The base <NUM> is shaped generally as (part of) a triangular prism, into which the cylindrical casing is partially recessed (thereby to cut off the top part of the triangular prism shape of the base). The partial recession of the casing into the base lowers the centre of gravity of the system. The base <NUM> does not extend beyond the depth of the casing <NUM> (i.e. the dimension parallel to the shaft <NUM>), thereby to minimise the 'footprint' of the system. The underside of the base <NUM> is however wider than the portion that intersects with the casing <NUM>, such that a triangular surface extends between the edge of the underside of the base <NUM> and the edge of the portion of the base <NUM> meeting the casing <NUM>. Only a portion of the casing <NUM> sits on the base <NUM> - the portion sitting on the base <NUM> may be defined by a chord of the cover <NUM> (extending between points on the edge of the casing <NUM> which meet the base <NUM>). This chord is generally longer than the radius of the cover <NUM>, but shorter than the diameter of the cover <NUM>. The dimensions of the base <NUM> are generally selected to minimise the footprint of the system <NUM> and material usage while ensuring that an effective base <NUM> capable of supporting the motor in use (and in particular mitigating against toppling) is provided.

The base <NUM> is configured to support the casing <NUM> and hence the rotor <NUM> and stator <NUM> of the motor in a vertical orientation. Thereby the base <NUM> supports the axis of the rotor at a generally horizontal axis when the base is horizontal, for example when it rests on a horizontal surface. The system <NUM> may be arranged to be free-standing for use, but is preferably 'dug in' to a further support to increase the stability of the system. For example, the system may be mounted on a concrete platform and/or may be bolted to a foundation. In a particular example the system <NUM> is bolt mounted on a reinforced steel concrete floor with concrete bolts to provide stability and to mitigate any toppling problems.

In some examples the base <NUM> may be coupled to the casing <NUM>, for example it may be made from the same extrusion as the casing <NUM>. In other example, the base is a separate component, onto which the casing is mounted.

The system <NUM> also comprises an encoder (not shown) for monitoring the angular position of the axle <NUM>, where the encoder is provided in communication with the control circuitry. Where a <NUM>/<NUM> configuration (or another configuration having a high pole count) is used, the angular position of the shaft which retains the flywheel must be known to a reasonably high accuracy for the purposes of control (since the windings are located relatively close together). This precludes the use of low cost optical rotary encoders. Instead, custom-sized encoders, such as magnetic incremental encoders must be used. For example, custom magnetic incremental encoders (such as those manufactured by Pepperl+Fuchs) may be used to achieve an angular resolution of less than <NUM>°. The encoder is directly attached to the axle <NUM> and is configured to transmit measurements of angular position to a controller (such as a digital signal processor (DSP) controller), which performs signal processing on data received from the encoder. The controller then informs a main or master controller, such as a field-programmable gate array (FPGA) controller, of the position of the rotor <NUM>.

<FIG> shows the assembled energy storage system <NUM>. The assembled system has a total height of approximately <NUM> (including the fins <NUM>), a total length of approximately <NUM>, and a total width of approximately <NUM>. The casing <NUM> has a total diameter of approximately <NUM>. As will be appreciated, the dimensions of the system <NUM> are primarily dictated by the size of the rotor <NUM>, which size is selected as a balance between energy density and ability to withstand the mechanical stress at high speeds (along with the ability to be controlled as part of an SRM). Generally, the mechanical and thermal aspects of the system dictate the dimensions of the system, rather than magnetic limits. The system is generally arranged such that its size (notwithstanding the size of the rotor) is minimised, thereby to provide a system having a small form factor and footprint to improve its usability as part of an array of systems <NUM>.

In use, the rotor can be rotated and act as a flywheel. The evacuation of the casing during manufacture (or after maintenance) leads to very little friction opposing the flywheel rotation. If electrical current is supplied to the pairs of windings in succession, as the rotor rotates, this applies a torque to the rotor causing it to rotate faster. It will be understood that the phasing of the currents supplied to the windings must be synchronised with the rotation of the rotor, so that each pole is attracted to the winding as it approaches it. The input electrical power is hence converted to kinetic energy of the rotating rotor.

<FIG> shows an example of a rotor <NUM> of the system <NUM>. The rotor <NUM> is a solid body which comprises a plurality of regions <NUM> protruding from a generally cylindrical body <NUM>, where the protruding regions <NUM> form the rotor poles. For example, <FIG> shows <NUM> rotor poles (suitable for use in an SRM having a <NUM>/<NUM> configuration). Each of the protruding regions corresponds to around <NUM>° of the rotor. The rotor <NUM> also comprises a plurality of recesses <NUM> between the protruding regions <NUM>. The rotor <NUM> also comprises a hole <NUM> in the centre, configured to receive the axle <NUM>. The rotor has a radius (including the protruding regions <NUM>) of approximately <NUM>, and a width of approximately <NUM>. The hole <NUM> has a diameter of approximately <NUM>.

The rotor <NUM> is formed from a plurality of laminations of type <NUM>-<NUM> precipitation hardened steel (chosen for its high strength and hardness, allowing high speed operation). The layers of laminations may serve as a barrier to eddy currents, meaning that eddy currents are restricted to small loops within the thickness of each lamination.

This may reduce the magnitude of any eddy currents substantially, and hence may reduce power loss.

A single lamination <NUM> is shown in <FIG>. The lamination has a thickness of less than <NUM>, and preferably is approximately <NUM> thick. The use of an SRM for large scale flywheel storage would not be feasible using laminations above this thickness as the electromagnetic properties would not be desirable (i.e. eddy currents would become non-negligible).

<FIG> shows an example of a stator <NUM> of the system. The stator <NUM> comprises a plurality of protrusions <NUM>, each protrusion having windings around them. When the windings are energised, these form the stator poles. The stator <NUM> is configured to surround the rotor to allow the rotation of the rotor, while ensuring the poles of the stator are close to the poles of the rotor, allowing for maximum magnetic interaction and hence torque. The stator <NUM> has an outer radius of approximately <NUM>, and an inner radius (i.e. a radius to the inner surface of the stator <NUM>, including the protrusions <NUM>) of approximately <NUM>. Each protrusion <NUM> is approximately <NUM> long. The stator <NUM> has a width of approximately <NUM>, which is the same as the width of the rotor <NUM>.

Like the rotor <NUM>, the stator <NUM> is formed of a plurality of laminations so as to reduce eddy currents. A single lamination <NUM> is shown in <FIG>. Each lamination is under <NUM> thick, and more specifically is approximately <NUM> thick. The laminations are formed from maraging steel, specifically M270-35A with an electrical steel coating, such as C3, C5, or C6 ASTM International standard coatings.

The stator <NUM> also comprises at least one notch <NUM> on the outer surface (i.e. the outward-facing circumference). The notches are provided to allow the lock the stator to the casing <NUM> shown in <FIG>. In <FIG>, there are four notches in the stator <NUM>.

<FIG> shows simulation results of a section of a rotor <NUM> which has failed. A solid steel fragment <NUM> of the rotor <NUM> which has failed and collided with the stator <NUM> at an initial angular velocity of <NUM>,<NUM> RPM is shown, causing damage and deformation to the stator <NUM> and casing <NUM>. For example, a region <NUM> is shown to be deformed from the plastic strain caused by the rotor collision. This may cause the rotor <NUM> to contact the stator <NUM> at other points along its circumference. The stator <NUM> has also separated from the casing <NUM>, for example at point <NUM>. This damage makes the system highly unsafe, for example large pieces of material may be ejected at high speed.

<FIG> shows simulation results where a rotor <NUM> is made from a single lamination of thickness <NUM>, which has failed at an initial angular velocity of <NUM>,<NUM> RPM. The widespread buckling through the rotor <NUM> and subsequent rupturing appears to have caused little effect on the integrity of the stator <NUM> or chassis <NUM>. Therefore, the failure of a section of lamination appears not to compromise the safety of the unit - however, the failure of a lamination may unbalance the system.

Accordingly, as well as reducing electrical losses, the use of laminations can improve safety via improved plastic deformation under component failure and a reduction in the likelihood that large components of material are ejected simultaneously. The ability for laminations to deform and lose energy through plastic deformation mechanisms means that the kinetic energy in the system is rapidly reduced to safer levels in the event of failure. Furthermore, the integrity of the rolled steel chassis is less likely to be compromised by a rotor rupture event when laminations are used. The simulation in <FIG> shows the safety improvements afforded by a laminated construction - a one-piece rotor would deform plastically to a lesser degree, and so would be more likely to rupture the casing <NUM>.

<FIG> shows an example installation of an array (or "installation") <NUM> of energy storage systems <NUM>. Each of the energy storage systems <NUM> may be the energy storage system <NUM> of <FIG>. The energy storage systems <NUM> are configured so as to form one unit of a modular array.

Each of the energy storage systems <NUM> are arranged such that the axis of the motor is aligned horizontally. The small form factor of the systems <NUM> allows for more efficient arrangement of an array <NUM> of energy storage systems <NUM>. The energy storage systems <NUM> are separated by a certain distance for safety reasons. The array <NUM> consists of <NUM> systems. It will be appreciated, of course, that various different sizes of arrays may be provided for different purposes.

The installation <NUM> comprises four power electronics units <NUM>. The function of the power electronics units <NUM> will be described in more detail with reference to <FIG>.

<FIG> shows an exemplary system diagram of an array <NUM> of multiple flywheel systems <NUM> for a <NUM> MW arrangement (i.e. an array being capable of storing <NUM> MW). Each flywheel system <NUM> may be the same as the system <NUM> shown in <FIG>, and the systems <NUM> in <FIG>. The array <NUM> comprises <NUM> flywheel systems <NUM>, which are split into two sub-arrays <NUM> comprising <NUM> flywheel systems <NUM> each. Each system <NUM> operates at <NUM> kW, providing a total power of <NUM> MW for the array <NUM>. The maximum energy stored per system <NUM> is <NUM> kWh - accordingly, the array <NUM> stores a maximum of <NUM> kWh.

Each sub-array <NUM> is connected to a power electronics unit <NUM>. Each power electronics unit <NUM> comprises power electronics for each system <NUM> within the particular sub-array <NUM>. The power electronics unit <NUM> comprise eight racks, where each rack respectively provides the power electronics for each flywheel system <NUM> in the sub-array <NUM>.

The flywheel systems <NUM> are connected to the power electronics unit <NUM> via a common DC bus <NUM>. The bus voltage is generally between <NUM> and <NUM> V for a <NUM> MW array. Each power electronics unit <NUM> is connected to a power convertor <NUM>, specifically an active front end (AFE), via the bus <NUM>. The power convertor <NUM> is capable as acting as an inverter and as a rectifier, so as to transform DC current from the flywheel storage systems <NUM> to AC current for use in e.g. the electrical grid, and vice versa.

The power convertor <NUM> is connected directly to a transformer <NUM> which converts current being exported from the system for onward transmission, and converts energy from the grid into an (AC) form suitable for use with the flywheel systems <NUM>.

Each power electronics unit <NUM> comprises a controller <NUM> (such as a PCS "PLC on a Chip" (RTM) controller) which receives communicates data between the flywheel systems <NUM> in the sub-array <NUM> and a master controller <NUM>, which receives data from all power electronics units <NUM> in the array <NUM> (and hence from all flywheel systems <NUM> in the array <NUM>). For example, the controller <NUM> may communicate encoder data back to the master controller <NUM> or may transmit control commands from the master controller <NUM> to individual flywheel systems <NUM>. The master controller <NUM> communicates with a process control system <NUM>, which may use a supervisory control and data acquisition (SCADA) architecture. The process control system <NUM> receives operator input via a command interface <NUM>. A power plant operator or an electrical grid operator may be able to provide such an input. The described control system allows for the operation of the various flywheel systems to be controlled, for example to change mode between energy input and export, to change particular set points, and to start up or shut down systems. The use of a SCADA architecture allows for multiple arrays <NUM> to be controlled at once.

Although the present system has been principally described for load balancing applications, it will be appreciated that the described system may also be provided in various other configurations. For example, the system may be used in voltage control, load time-shifting, peak avoidance, high power electric vehicle charging support (to allow deferment of network infrastructure upgrades), energy arbitrage, electro-chemical storage support (to prolong the lifetime of such systems), and micro-grid energy management and balancing. In certain applications, the relatively small size and portability of the system may be used to provide flexible load balancing and/or energy storage - for example, a truck-mounted system may be set-up to provide temporary energy storage at any location accessible by truck.

Optionally, the system is set up so as to minimise the effects of the rotation of the Earth on the system. Such rotation causes forces to be applied to the flywheel. To conserve angular momentum, the flywheel will try to resist these forces, which may cause increased load and consequently wear on the bearings. This effect may be mitigated by orientating the system such that the axis of the flywheel is parallel to the rotation of the Earth (i.e. from west to east).

In an alternative, a plurality of systems may be combined (for example, in a 'stack') so as to form a further system comprising a plurality of flywheels. In such a case, certain components (such as power electronics) may be shared across the plurality of systems. Optionally, such a plurality of systems may be integrated within a single outer casing. In one example, an array of systems, for example four, are packaged together into a unit. Each of the flywheel systems within the unit are positioned on the ground at very close proximity. For example, one configuration involves each system arranged one behind the other, such that the axles are substantially aligned. The unit may further comprise a sliding mechanism to allow the systems to project out e.g. for maintenance. This example may be particularly useful for scenarios with very tight spatial constraints such as within buildings.

In an alternative, the casing may comprise at least one coolant channel defining a fluid flow path for a coolant. The coolant may for example be a liquid, being recirculated through a pipe system and cooled down through a heat exchanger. In other examples, the coolant is passed through a radiator to exchange heat with the ambient air; or the coolant may be air; or it may be a recirculated refrigerant fluid that evaporates during passage through the coolant channels.

Optionally, the casing may be coated in a high emissivity paint to increase heat dissipation from the system. Cooling is a substantial problem with this vacuum arrangement, and painting the surface with high emissivity paint can help mitigate this issue. This will help to reduce the temperature of the system through radiation in use, and increase safety by helping prevent overheating and component failure. By cooling through radiation, an active cooling system may also not be required.

It will be further be appreciated that the dimensions and materials described as being used in the system are exemplary, and a variety of other materials and dimensions may be used to achieve at least some of the benefits of the present system. Similarly, it will be appreciated that a suitably adapted version of the system may be configured to operate with a flywheel on a vertical axis, to use bearings other than those described (in particular, magnetic bearings) and/or to use a different SRM configuration (in particular, a <NUM>/<NUM> configuration (using <NUM>-phase) may be used in an alternative). Typically, the number of rotor poles is n, while the number of stator poles is m, where m = n + <NUM>. Preferably, n > <NUM>, more preferably n > <NUM>, still more preferably n ><NUM>, yet more preferably still n > <NUM>.

Although the array <NUM> has generally been described with the power electronics and control componentry being shared between a plurality of flywheel systems <NUM>, it will be appreciated that in an alternative such components may be provided in respect of individual systems, or shared between a greater or lesser number of flywheel systems <NUM> than described.

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

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claim 1:
An energy storage system (<NUM>), comprising:
a switched reluctance motor comprising a rotor (<NUM>) and a stator (<NUM>); wherein the rotor (<NUM>) is arranged to be capable of acting as a flywheel for storing energy, and wherein the rotor (<NUM>) is configured to be rotatable within the stator (<NUM>);
a casing (<NUM>) enclosing the rotor (<NUM>) and stator (<NUM>); wherein the rotor (<NUM>) is supported for rotation on the casing (<NUM>); and
a base (<NUM>) arranged to support the casing (<NUM>);
wherein the energy storage system (<NUM>) is arranged such that, in use, the axis of the rotor (<NUM>) is generally horizontal;
characterised in that
the energy storage system (<NUM>) is configured to import electrical energy and convert the electrical energy into kinetic energy stored in the flywheel, and to convert kinetic energy stored in the flywheel to electrical energy and export the electrical energy.