WO2016042335A1

WO2016042335A1 - Energy storage system and control method thereof

Info

Publication number: WO2016042335A1
Application number: PCT/GB2015/052696
Authority: WO
Inventors: George PRASSINOS; Theodoros Theodorou
Original assignee: Sdnc Ltd
Priority date: 2014-09-18
Filing date: 2015-09-18
Publication date: 2016-03-24
Also published as: GB201416534D0

Abstract

An energy storage system and a control method thereof are disclosed. The energy storage system comprises: a first section comprising a first rotor and a first stator configured to act as a switched reluctance machine, the first stator comprising a plurality of first electrical windings and the first rotor being configured to act as an energy-storage flywheel; a second section comprising a second rotor and a second stator, the second stator comprising a plurality of second electrical windings, at least part of the second rotor being formed of a permanent magnetic material, and the second rotor being arranged to co-rotate with the first rotor; and an electrical circuit arranged to transfer electrical power between the first electrical windings and the second electrical windings. The second electrical windings can supply excitation power to the first electrical windings to being extracting energy from the system. The electrical circuit can include switching means for electrically disconnecting the second electrical windings, to prevent energy being lost in eddy currents in the second electrical windings whilst the system is in a coasting mode.

Description

Energy Storage System and Control Method Thereof Technical Field

The present invention relates to energy storage systems. More particularly, the present invention relates to an energy storage system comprising a switched reluctance motor, and a control method thereof.

Background of the Invention

In recent years, flywheel energy storage devices based on switched-reluctance machines (SRMs) have become increasingly popular. In comparison to conventional energy storage solutions, SRM-based energy storage devices offer relatively long operating lifetimes, high energy density, and large maximum power output. An SRM-based energy storage device stores kinetic energy in a rotating flywheel formed of soft magnetic material. Energy can be stored in the system by passing an electrical current through stator coils disposed around the flywheel, to cause the rotor to accelerate. Similarly, electrical energy can be extracted from the stator coils while the rotor is rotating. However, an excitation energy must first be supplied to the stator coils to generate a magnetic field and magnetise the spinning rotor, before electrical power can be extracted from the stator coils.

The invention is made in this context. Summary of the Invention

According to the present invention, there is provided an energy storage system comprising: a first section comprising a first rotor and a first stator configured to act as a switched reluctance machine, the first stator comprising a plurality of first electrical windings and the first rotor being configured to act as an energy-storage flywheel; a second section comprising a second rotor and a second stator, the second stator comprising a plurality of second electrical windings, at least part of the second rotor being formed of a permanent magnetic material, and the second rotor being arranged to co-rotate with the first rotor; and an electrical circuit arranged to transfer electrical power between the first electrical windings and the second electrical windings.

The first and second rotors can be physically connected to rotate around an axis as a single body during operation of the energy storage system. The first and second rotors maybe spaced apart along the axis, and in some embodiments the first and second rotors are physically connected by a non-ferromagnetic material.

The first stator and the second stator can be spaced apart and separated by a non- ferromagnetic material.

The electrical circuit can comprise switching means for electrically disconnecting the second electrical windings. In some embodiments, the switching means comprises two Field Effect Transistors FETs connected in a bidirectional configuration.

The energy storage system can further comprise a controller configured to control the electrical circuit to transfer electrical power from the second electrical windings to the first electrical windings to generate a magnetic field across the first rotor, and to subsequently transfer electrical power from the first electrical windings to a load. The controller can also be configured to control the switching means to electrically disconnect the second electrical windings when the energy storage system is operating in a coasting mode.

In some embodiments the energy storage system is included in a vehicle. However, in other embodiments the energy storage system can be included in a stationary device.

According to the present invention, there is also provided a method of controlling the energy storage system of any one of the preceding claims, the method comprising: using the electrical circuit, transferring electrical power from the second electrical windings to the first electrical windings to generate a magnetic field across the first rotor; and using the electrical circuit, subsequently transferring electrical power from the first electrical windings to a load. The method can further comprise electrically disconnecting the second electrical windings when the energy storage system is operating in a coasting mode.

A computer-readable storage medium can be arranged to store computer program instructions which, when executed by one or more processors, perform the method.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates the cross-sectional shape of a rotor in an energy storage system, according to an embodiment of the present invention;

Figure 2 schematically illustrates the energy storage system when viewed from the side, according to an embodiment of the present invention;

Figure 3 schematically illustrates a cross-section through the first rotor section and first stator section, according to an embodiment of the present invention;

Figure 4 schematically illustrates a cross-section through the second rotor section and second stator section, according to an embodiment of the present invention;

Figure 5 schematically illustrates a possible configuration of the second rotor section, according to an embodiment of the present invention;

Figure 6 is a flowchart showing a method of controlling an energy storage system, according to an embodiment of the present invention;

Figure 7 schematically illustrates part of an electrical control circuit for transferring electrical power to or from the first electrical windings, according to an embodiment of the present invention; and

Figure 8 schematically illustrates part of an electrical control circuit for transferring electrical power to or from the second electrical windings, according to an embodiment of the present invention. Detailed Description

Figures 1 to 4 schematically illustrate an energy storage system according to an embodiment of the present invention. The energy storage system of the present embodiment comprises a multi-sectioned rotor, which hereinafter will be referred to as a 'hybrid rotor'. Embodiments of the present invention can find use in various energy- storage applications. In some embodiments, the energy storage system can be included in a vehicle and configured to store energy under braking and/or when the vehicle is coasting. For example, the vehicle may be one which is powered by a conventional internal combustion engine, a hybrid drive system, or an electric motor. The energy storage system can provide electrical power to the drive train in order to accelerate the vehicle, and/or can supply electrical power to auxiliary systems. In the context of a motor vehicle, an 'auxiliary system' is a system that performs a function other than generating power to drive the vehicle. Examples of auxiliary systems include air conditioning, lighting and hydraulic systems. Figure 1 schematically illustrates the shape of the hybrid rotor 100 in cross-section. As shown in Fig. 1, the hybrid rotor 100 is generally cylindrical in shape but includes a plurality of poles 102a, 102b, 102c, i02d, which may also be referred to as 'teeth'. The poles 102a, 102b, 102c, i02d protrude from the surface of the rotor 100, and extend in a direction parallel to the axis of rotation A. In the present embodiment the rotor 100 includes four poles, but in other embodiments any number of poles maybe provided.

The hybrid rotor includes a first rotor section 110 formed of a soft magnetic material and a second rotor section 120 formed of a permanent magnetic material. Figure 2 schematically illustrates the energy storage system when viewed from the side, showing the first rotor section 110 and the second rotor section 120. In the present embodiment the first and second rotor sections 110, 120 have different lengths in the direction of the rotational axis A, but in other embodiments the first and second rotor sections 110, 120 could be of the same size. The first and second rotor sections 110, 120 are arranged to co-rotate during operation of the energy storage system. That is, the first and second rotor sections 110, 120 are connected such that rotation of one of the rotor sections 110, 120 causes rotation of the other one of the rotor sections 110, 120.

In the present embodiment the first rotor section 110 and second rotor section 120 are spaced apart along the axis of rotation A, and are physically connected by a non- ferromagnetic material 130. By being physically connected, the first and second rotor sections 110, 120 are arranged to co-rotate as a single body during operation of the energy storage system. Although in the present embodiment the first rotor section 110 and second rotor section 120 of the rotor are physically connected in a fixed manner, to rotate as a single body, other arrangements are possible. For example, in some embodiments the first rotor section 110 and the second rotor section 120 can be connected by one or more gears. In such embodiments, the first and second rotor sections 110, 120 can be arranged to rotate at different angular velocities, and/or around different axes, during operation of the energy storage system.

To minimise energy losses due to friction, in some embodiments the first and second rotor sections and the first and second stator sections may be included in a common casing, which can be evacuated to avoid energy losses being incurred due to air resistance. The first rotor section and/ or the second rotor section can also be supported by magnetic bearings to further reduce mechanical losses. In addition to the first and second rotor sections 110, 120 the energy storage system further comprises a first stator section 210 and a second stator section 220. In the present embodiment, the first stator section 210 comprises a plurality of first electrical windings and the second stator section 220 comprises a plurality of second electrical windings. As shown in Fig. 2, the first and second electrical windings are spaced apart from one another in the vertical direction and are separated by an air gap 230. In the present embodiment, the first and second stator sections 210, 220 are integrally formed as a single body comprising separate protruding cores around which the first electrical windings and second electrical windings are formed. In other embodiments, the first and second stator sections 210, 220 can be formed as physically separate components separated by a non-ferromagnetic material. Figure 3 shows a cross-section through the first rotor section 110 and first stator section 210, and Fig. 4 shows a cross-section through the second rotor section 120 and second stator section 220. As shown in Figs. 3 and 4, the first and second stator sections 210, 220 have a similar shape in cross- section. However, in other embodiments the first and second stator sections 210, 220 can have different cross-sectional shapes.

The inner wall of the first stator section 210 comprises a plurality of equally-spaced recesses 212 separated by protrusions 214. The recesses 212 and protrusions 214 extend longitudinally, parallel to the axis of rotation. The first stator section 210 also comprises a plurality of first electrical windings 216, each one of the first electrical windings 216 comprising an electrical conductor wound around one of the protrusions 214. In the present embodiment a total of six electrical windings 216 are provided, but in other embodiments a different number of electrical windings may be used. Together, the first rotor section 110 and first stator section 210 are configured to act as a switched reluctance machine (SRM), and the first rotor section 110 is configured to act as an energy-storage flywheel. The section of the energy storage system comprising the first rotor section 110 and the first stator section 210 can be referred to as an SRM energy storage section. The SRM energy storage section stores energy in the form of kinetic energy in the first rotor section 110. As an example, the first rotor section 110 can be formed from a laminated soft magnetic material such as silicon steel. In some embodiments, the first rotor section 110 can have a composite structure in which different parts are formed from different materials. For example, an outer part which includes the rotor poles may be formed from a high- density ferromagnetic material, whilst an inner part may be formed from a lower- density high-strength material such as a carbon-fibre composite. Such a construction is particular suitable for devices that are intended to operate at high angular velocities. The operating principles of SRM-based energy storage devices are well-understood, and a detailed description will not be provided here to avoid obscuring the present inventive concept. Similar to the first stator section 210, the inner wall of the second stator section 220 comprises a plurality of equally-spaced recesses 222 separated by protrusions 224. The recesses 222 and protrusions 224 extend longitudinally, parallel to the axis of rotation. The second stator section 220 also comprises a plurality of second electrical windings 226, each one of the second electrical windings 226 comprising an electrical conductor wound around one of the protrusions 224. In the present embodiment a total of six electrical windings 226 are provided, but in other embodiments a different number of electrical windings may be used. As shown in Fig. 4, at least part of the second rotor section 220 is formed of a permanent magnetic material having fixed magnetic poles. Together, the second rotor section 120 and second stator section 220 are configured to act in a similar manner to a brushless direct-current (DC) motor. The operating principles of brushless motors are well-understood, and a detailed description will not be provided here. The section of the energy storage system comprising the second rotor section 120 and the second stator section 220 can be referred to as permanent magnetic section.

The second rotor section 120 and second stator section 220 can also act as a generator of electrical power when the second rotor section 120 is rotating, since the movement of the permanent magnetic poles past the second electrical windings 226 will induce a current in the second electrical windings 226. Since the second rotor section 220 is arranged to co-rotate with the first rotor section 210, electrical energy can be extracted from the second electrical windings 226 whenever the first rotor section 210 is rotating, that is, whenever there is stored energy in the SRM energy storage section.

As with conventional SRM-based energy storage devices, the SRM energy storage section in embodiments of the present invention is relatively cheap and easy to construct, and does not suffer from cogging effects. However, as explained above, a drawback of SRM-based energy storage devices is that a certain excitation energy is required to establish a magnetic field across the spinning rotor, before energy can be extracted from the system. This means that a conventional SRM-based energy storage device requires an external power supply of sufficiently high voltage in order to begin extracting power from the system. In embodiments of the present invention, the excitation energy can be provided by induced current in the stator windings of the permanent magnetic section. Depending on the amount of energy currently stored in the rotor, the permanent magnetic section may be able to provide the full excitation power required, or can be used to supplement an external power supply. In addition, electrical power can be drawn from the stator coils in the permanent magnetic section to supply power to auxiliary devices connected to the hybrid energy storage system.

In the present embodiment the first and second rotor sections no, 120 are configured to have a similar shape in cross-section, in other embodiments the first and second rotor sections 110, 120 may have different shapes. For example, instead of having a plurality of magnetic poles protruding from a central region as shown in Fig. 4, in some embodiments the second rotor section could have a cylindrical or toroidal shape, as shown in Fig. 5. In the example shown in Fig. 5, a second rotor section 520 comprises two permanent magnetic sections, each having a permanent south pole 522a, 524a and a permanent north pole 522b, 524b. The two sections are arranged so that the north pole of one section is disposed adjacent to the south pole of the other section. Although two permanent magnetic sections are shown in Fig. 5, in other embodiments a second rotor section could include any number of permanent magnetic sections. In the present embodiment the first and second stator sections 210, 220 surround the first and second rotor sections 110, 120, respectively, and can be referred to as external stators. Similarly, the first and second rotor sections 110, 120 of the present embodiment can be referred to as internal rotors. However, the present invention is not limited to an internal rotor design. In other embodiments, one or both of the first and second rotor sections can be formed as an external rotor arranged to rotate around an internal stator.

In addition to the multi-sectioned rotor and stator described above, the energy storage system further comprises an electrical control circuit arranged to transfer electrical power between the first electrical windings and the second electrical windings. Figure 6 illustrates a control method of the energy storage system, which can be implemented using the energy control circuit. First, in step S601 the electrical control circuit is switched to supply the excitation energy to the SRM energy storage section from the permanent magnetic section, by transferring electrical power from the second electrical windings 226 to the first electrical windings 216. This establishes a magnetic field across the first rotor section 110. Then, in step S602 the electrical control circuit is switched to begin transferring electrical power from the first electrical windings to a load, for example auxiliary systems or an electric motor in the drive train of a vehicle in which the energy storage system is included.

Figure 7 schematically illustrates part of an electrical control circuit for transferring electrical power to or from the first electrical windings 216. In the present

embodiment, the circuit is an asymmetric bridge converter circuit. However, in other embodiments a different circuit design may be used, such as an (n + 1) switch and diode configuration.

Figure 8 schematically illustrates part of an electrical control circuit for transferring electrical power to or from the second electrical windings 226, according to the present embodiment. In the present embodiment, the circuit is configured to transfer three- phase electrical power to or from the second electrical windings 226, which are connected in a Ύ configuration. However, in other embodiments the second electrical windings 226 may be connected differently, for example a delta configuration may be used.

Also, in the present embodiment the circuit includes switching means 810 for electrically disconnecting the second electrical windings 226, in the form of a pair of Field Effect Transistors (FETs) arranged in a bidirectional configuration. The switching means can be controlled to disconnect the second electrical windings 226 when the energy storage system is operating in a coasting mode, that is, when it is not required to transfer electrical power to or from the second electrical windings. The FETs are arranged to electrically isolate the second electrical windings 226 from the power line, to prevent eddy currents from flowing in the coils while the hybrid rotor is spinning. Electrically disconnecting the second electrical windings 226 in this way avoids energy being lost due to eddy currents in the permanent magnetic section, whilst the energy storage system is operating in the coasting mode. Although FETs are used to electrically disconnect the second electrical windings in the present embodiment, in other embodiments any suitable switching means may be used, for example other types of solid state switch or a mechanical switch such as a relay. Embodiments of the invention have been described in which an energy storage system includes a first section configured to act as a switched reluctance machine, and a second section in which the rotor includes a permanent magnetic material. In some embodiments, the rotor in the second section may itself comprise a plurality of sections, each having a different arrangement of permanent magnetic poles. The stator in the second section may also comprise a plurality of sections having different configurations of the second electrical windings, corresponding to the arrangement of the permanent magnetic poles in the different sections of the second rotor. In this way, the different sections of the permanent magnetic part of the energy storage system can be optimised for different applications, for example by positioning the magnetic poles in the different sections to generate power at different phases of the cycle.

As a few examples, the arrangement of magnetic poles and/ or electrical windings in one part of the permanent magnetic section can be optimised for generating excitation power for the SRM section when the hybrid rotor is already spinning, another part of the permanent magnetic section can be optimised for generating power to help with the motoring phase, and another part of the permanent magnetic section can be optimised for helping to start the SRM when the hybrid rotor is stationary. Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the

accompanying claims.

Claims

1. An energy storage system comprising:

a first section comprising a first rotor and a first stator configured to act as a switched reluctance machine, the first stator comprising a plurality of first electrical windings and the first rotor being configured to act as an energy-storage flywheel; a second section comprising a second rotor and a second stator, the second stator comprising a plurality of second electrical windings, at least part of the second rotor being formed of a permanent magnetic material, and the second rotor being arranged to co-rotate with the first rotor; and

an electrical circuit arranged to transfer electrical power between the first electrical windings and the second electrical windings.

2. The energy storage system of claim l, wherein the first and second rotors are physically connected to rotate around an axis as a single body during operation of the energy storage system.

3. The energy storage system of claim 2, wherein the first rotor and second rotor are spaced apart along the axis.

4. The energy storage system of claim 3, wherein the first rotor and second rotor are spaced apart and physically connected by a non-ferromagnetic material.

5. The energy storage system of any one of the preceding claims, wherein the first stator and the second stator are spaced apart and separated by a non-ferromagnetic material.

6. The energy storage system of any one of the preceding claims, wherein the electrical circuit comprises:

switching means for electrically disconnecting the second electrical windings.

7. The energy storage system of claim 6, wherein the switching means comprises two Field Effect Transistors FETs connected in a bidirectional configuration.

8. The energy storage system of any one of the preceding claims, further comprising: a controller configured to control the electrical circuit to transfer electrical power from the second electrical windings to the first electrical windings to generate a magnetic field across the first rotor, and to subsequently transfer electrical power from the first electrical windings to a load.

9. The energy storage system of claim 8, when dependent on claim 6 or 7, wherein the controller is configured to control the switching means to electrically disconnect the second electrical windings when the energy storage system is operating in a coasting mode.

10. The energy storage system of any one of the preceding claims, included in a vehicle.

11. A method of controlling the energy storage system of any one of the preceding claims, the method comprising:

using the electrical circuit, transferring electrical power from the second electrical windings to the first electrical windings to generate a magnetic field across the first rotor; and

using the electrical circuit, subsequently transferring electrical power from the first electrical windings to a load.

12. The method of claim 11, further comprising:

electrically disconnecting the second electrical windings when the energy storage system is operating in a coasting mode.

13. A computer-readable storage medium arranged to store computer program instructions which, when executed by one or more processors, perform a method according to claim 11 or 12.