GB2172752A - An energy storage unit - Google Patents

Abstract

An energy storage unit comprises a fixed armature 50; a housing 51 which includes a magnet and which surrounds and is rotatably mounted about the armature, and a flywheel 6 connected to the housing; there being provision to increase the kinetic energy of the combination of housing and flywheel to store energy, and provision to convert, when necessary, the stored kinetic energy into electrical energy. <IMAGE>

Description

SPECIFICATION An energy storage unit This invention relates to an energy storage unit and, more particularly, to a flywheel energy storage unit.

The concept of storing energy in a rotating flywheel is well known and systems exist whereby electrical energy can be converted into rotational energy of a flywheel for energy storage; the stored kinetic energy of the flywheel may then be converted, when necessary, into electrical energy. Such a system provides a means of coping with varying power demand.

Flywheel energy storage systems of the prior art are based upon rotating armature arrangements in which the flywheel is linked to the armature. In practice, this arrangement is not suitable for high speed operation in view of the necessary brush commutation which is apt to malfunction at high rotational velocities of the armature. Attempts to overcome this problem include the provision of a gear box between armature and flywheel to maximise the rotational velocity of the flywheel whilst maintaining the rotational velocity of the armature at operable levels, but this approach, by necessity, introduces complexity into the system.

According to the present invention, there is provided an energy storage unit comprising: a fixed armature; surrounding the armature and rotatably mounted about the armature, a housing which includes a magnet; and a flywheel connected to the housing; there being provision to increase the kinetic energy of the combination of housing and flywheel to store energy, and provision to convert, when necessary, the stored kinetic energy into electrical energy.

Such an arrangement avoids the necessity for a gearbox, although it is to be appreciated that a gearbox may still be included between the housing and flywheel. In an embodiment of the invention, the combination of housing and flywheel has been found to be rotatable at angular velocities of up to 6,000 rpm although higher velocities than this could be achieved with the simple arrangement of the present invention.

The flywheel may, in certain embodiments, be integral with the housing; in any case, the housing and magnet effectively constitute part of the flywheel and contribute to the energy storage capability of the unit.

Preferably, the armature and housing (drive/generatorl consist of a brushless, permanent magnet d.c. motor, the armature of which is preferably held stationary on a central axle which provides a support for the device of the invention.

The stationary, fixed armature thus permits brushless electronic commutation which may, if desired, be outside the housing of the flywheel.

In preferred embodiments of the present invention, the combination of armature, housing and flywheel are in vacuo or in an atmosphere which is reduced in pressure or replaced by an inert gas such as helium. This reduces atmospheric frictional losses.

The replacement of the mechanical brushes of prior art flywheel energy storage systems with an electronic commutation arrangement permits the motor to readily operate regeneratively, and at high speeds.

The energy storage system of the present invention could be applied in hybrid storage electrical vehicles where it could function in parallel with batteries, so as to permit full regenerative braking thereby improving the efficiency of such vehicles. In particular, the system could have wide applications in the area of electrified urban transport.

A further application envisaged is the utilisation of alternative energy sources such as wind and waves. The energy of wind and waves would be used to increase the kinetic energy of the combination of housing and flywheel to store energy. This stored energy could, for example, sustain output power to a local grid during short periods of reduced power. Such a system would reduce the requirement for resort to an auxiliary back-up power system such as a diesel generator.

Reference will now be made, by way of example only, to the accompanying drawings in which: Figure 1 shows a cross-sectional view of a flywheel energy storage unit according to the present invention; Figure 2 illustrates a typical flywheel drive arrangement; Figure 3 shows a thyristor converter which may be employed in combination with the energy storage system of the present invention; and Figure 4 illustrates a rotor position feedback system for synchronising energy transmission and rotation of the flywheel.

Referring to Fig. 1, a flywheel energy storage unit according to the present invention is shown in cross-section. The unit comprises a fixed shaft 50 upon which an armature (not shown) is supported. A cylindrical housing 51 is rotatably mounted on bearings 11, 12 about the combination of shaft 50 and armature 51. On the inner surface of the cylindrical housing 51 are disposed one or more permanent magnet (not shown). The housing 51, the permanent magnet, the armature and the shaft 50 may be in the form of a permanent magnet d.c. motor such as that produced by Small Electrical Motors Limited of London, United Kingdom in which the magnet comprises two permanent magnets inserted in the housing, which housing, together with the armature, completes the magnetic circuit.

In the embodiment illustrated in Fig. 1, a cylindrical flywheel 6 is supported on the housing 51 at one end of the housing 51. It is to be appreciated, however, that the flywheel 6 could extend along, and even beyond, the housing 51. The shaft 50, armature, housing 51, magnet and flywheel 6 are supported in an enclosed chamber 1 which may be evacuated, partially reduced in pressure or filled with an inert gas such as helium.

Electronic connections 24 to the windings of the armature are provided. An electronic commutator (not shown) replaces the conventional brush commutator utilised in flywheel energy storage units of the prior art. The embodiment shown has an overall height of about 20z inches (521 mm) and width of 194 inches (489 mm).

The chamber within which the unit is disposed comprises a bottom cover 3 at the periphery of which is a flange 2. The flange supports the casing 1 and, at the top of the casing is connected another flange 2 at the periphery of a top cover 4. The top cover 4 is completed by an upper shaft bearing support 8 which supports the upper shaft bearing 34 which carries the drive/generator. Access plugs 25 to the inside of the chamber are provided and there is also a connection 37 for an air pump to reduce the pressure within the chamber.

A lower shaft support 7 is provided to support the lower end of the shaft 50.

Small balance weights 27 are provided on the flywheel and the upper part of the housing 51 to ensure stable rotation of the housing 51/flywheel 6.

The following data given below are derived from a flywheel energy storage unit as described above and are given merely by way of example.

At 3,000 RPM At 6,000 RPM Current rating 5 A 5 A Peak voltage 200 V 400 V Max. power 1 kW 2kW Run up time 30 sec. 60 sec.

Energy kJ 15 kJ 60 kJ Energy Wh 5 Wh 16 Wh The embodiment shown in Fig. 1 does not show all of the commutation segments which are present; in fact there are 42 commutation segments which means that the energy storage unit can regenerate a sinusoidal volage waveform at each of there 42 commutation segments, thus being operable in 3, 6, 21 or 42 phase delta connection. The frequency and voltage of the wave form generated depend upon the speed of rotation of the flywheel. By the use of electronic conversion, the variable voltage source may be converted into d.c. of any level, to single or three phase a.c. 50 Hertz or to variable frequency 3 phase a.c. for traction purposes.

When the unit is being used in its storage mode, an electronic commutation arrangement is necessary and a typical flywheel drive arrangement is shown in Fig. 2. In this embodiment, +400 V d.c. is fed into an inverter 60 which provides the electronic commutation, and a flywheel drive 41 is connected for 3 phase operation by selecting three commutation points each 1200 spaced in phase. The flywheel 6 is initially charged using forced commutated inversion of the applied voltage VAB utilising a thyristor converter as shown in Fig. 3.

In Fig. 3, a bridge converter 12 connected to the flywheel unit 63 is synchronised with the rotation of the flywheel by the scheme, shown in Fig. 4, utilising a dual optical transducer 64.

One of the optical transducers is pulsed once per revolution of the flywheel 6 and the other is pulsed six times per revolution of the flywheel 6 at the required thyristor firing times. These pulses are translated into gate switching pulses by a small dedicated microcomputer 65 which is connected to the six gate drive circuits 66.

Once the flywheel is above the base speed (i.e. the speed at which the regenerated voltage exceeds the input voltage VAB), naturally commutated inversion can be employed using broadly the same arrangement.

This converter may be used to regenerate power from the flywheel to the d.c. link by advancing the phase of the thyristor firing, thus reversing the polarity of VAB. The converter then functions in a controlled rectifier mode.

Alternatively, power may be extracted by an independent circuit connected to any of the flywheel unit output terminals.

An array of power field effect transistor switches connected to each of the output terminals may be used to synthesise a.c. By controlled switching of these transistors at a frequency of (f1-f2) Hz (where f1 is the frequency of flywheel machine EMF and f2 is the required output of frequency) then, by modulation theory, the output at the circuit terminals is f2 as required, which may be, for example, 50 Hz. Such a scheme could be used to synthesise a 50 Hz mains compatible voltage wave form for use in uninterruptable power supply applications, or to synthesise variable frequency current wave forms for traction motor drives in electric vehicles applications.

Claims (12)

1. An energy storage unit comprising: a fixed armature; surrounding the armature and rotatably mounted about the armature, a housing which includes a magnet; and a flywheel connected to the housing; there being provision to increase the kinetic energy of the combination of housing and flywheel to store energy, and provision to convert, when necessary, the stored kinetic energy into electrical energy.

2. An energy storage unit according to Claim 1, wherein the flywheel is integral with the housing.

3. An energy storage unit according to Claim 1 or 2, wherein the armature and housing consist of a brushless, permanent magnet d.c. motor.

4. An energy storage unit according to Claim 3, wherein commutation of the motor is brushless, electronic commutation.

5. An energy storage unit according to Claim 4, wherein the brushless, electronic commutation is carried out outside the housing of the unit.

6. An energy storage unit according to any preceding claim, wherein the combination of armature, housing and flywheel are disposed in an evacuated chamber.

7. An energy storage unit according to any one of Claims 1 to 5, wherein the combination of armature, housing and flywheel are in disposed in a chamber having an atmosphere of reduced pressure.

8. An energy storage unit according to any one of Claims 1 to 5, wherein the combination of armature, housing and flywheel are in disposed in a chamber having an atmosphere of an inert gas.

9. An energy storage unit according to Claim 8, wherein the inert gas is helium.

10. An energy generator comprising means for generating electricity and including an energy storage unit according to any one of Claims 1 to 9.

11. An electric vehicle including an energy storage unit according to any one of Claims 1 to 9.

12. An energy storage unit substantially as hereinbefore described, with reference to Fig. 1; Fig. 1 and 2, 3 or 4.