System for Power Conversion and Energy Storage

Abstract

A system for power conversion and energy storage is described. In an embodiment the system comprise a DC rotating electrical machine (1) comprising a DC rotor; an AC rotating electrical machine (2) comprising an AC rotor; and a (thermo-) mechanical energy storage system (known as a TMESS) (3). The TMESS comprises a central shaft, said central shaft charged and discharged with shaft power and selectively mechanically coupled to the DC rotor and to the AC rotor via clutch (4) to form a shaft train. Some source of DC generation such as photovoltaic cells (5) feeds electrical power into the DC electrical machine (1). There may also be local DC loads (8) supported by the system. The AC electrical machine (2) may deliver power to local AC loads (6), or draw power from the AC electrical grid (7).

Claims

1. A system for power conversion and energy storage comprising: a DC rotating electrical machine comprising a DC rotor an AC rotating electrical machine comprising an AC rotor; and a (thermo-)mechanical energy storage system (TMESS) comprising a central shaft, said central shaft operable to charge and discharge the TMESS with shaft power and selectively mechanically coupled to the DC rotor and to the AC rotor to form a shaft train.

2. A system for power conversion and energy storage as described in claim 1 wherein the DC rotor and the AC rotor are selectively mechanically coupled to the central shaft by direct couplings such that rotor speed ratios between the DC rotor, AC rotor and the central shaft are unity.

3. A system for power conversion and energy storage as described in claim 1 wherein the DC rotor and the AC rotor are mechanical coupled to the central shaft at all times.

4. A system for power conversion and energy storage as described in claim 1 further comprising a clutch arrangement to mechanically decouple the DC rotor and the AC rotor from the central shaft so that the DC rotating machine and the AC rotating machine form a composite electrical machine, said composite electrical machine configured to convert power in one or both directions between AC and DC with no power being exchanged with the TMESS.

5. A system for power conversion and energy storage as described in claim 1 wherein the energy storage device further comprises a mechanical power transmission, and wherein shaft power used for charging the TMESS passes along one branch of the mechanical power transmission and shaft power emerging from discharging of the TMESS is routed along a different branch of mechanical power transmission.

6. A system for power conversion and energy storage as described in claim 1 wherein the TMESS comprises an energy store, said energy store storing and/or delivering shaft power to the central shaft in the form of pressurised air

7. A system for power conversion and energy storage as described in claim 1 where the TMESS stores energy by pumping heat from a cold store to a hot store, and wherein it releases energy by employing a heat engine to extract work from a reverse flow of heat.

8. A system for power conversion and energy storage as described in claim 1 where the TMESS stores its exergy in the form of liquefied air.

9. A system for power conversion and energy storage as described in of claim 1 where the TMESS comprises an energy store, said energy store storing energy obtained from the central shaft in the form of one or more of pressurised air, pumped heat, liquefied air and pumped water.

10. A system for conversion and energy storage as described in claim 1, wherein the central shaft is directly mechanically coupled to the AC rotor, and wherein the DC rotor is directly mechanically coupled to the AC rotor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiment of the invention shall now be described in detail by way of example and with reference to the accompanying drawings in which:

[0037] FIG. 1 shows a schematic representation of system for power conversion and energy storage comprising a DC rotating machine, an AC rotating machine and an energy storage device according to an embodiment of the present invention;

[0038] FIG. 2 shows a schematic representation of the energy storage device of FIG. 1.

[0039] It should be noted that the Figure is diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of the Figure have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

[0040] FIG. 1 shows a schematic diagram of the simplest representation of this system. This figure shows a DC electrical machine (1), such as a rotating DC motor/generator, comprising a DC rotor (along with other typical DC electrical machine components such as a stator and the like) directly-coupled to an AC electrical machine (2). The AC electrical machine 2 is also typically a rotating AC motor/generator and comprises an AC rotor (and other AC electrical machine components such as a stator and the like). The DC rotor and the AC rotor are mechanically coupled and form a composite electrical machine.

COMPONENTS OF THIS INVENTION

[0041] The essential components of the system of this invention are: [0042] (1) A DC electrical machine [0043] (2) An AC electrical machine coupled directly to the above DC machine [0044] (3) A (thermo-)mechanical energy storage system (TMESS) [0045] (4) A clutch capable of coupling/de-coupling the pair of electrical machines from the central shaft of the (thermo-)mechanical energy storage system.

[0046] As shown in FIG. 1, the AC rotor is coupled to a central shaft 31 of a TMESS (3) via a clutch (4) mechanism. Some source of DC generation (5), such a PV cells or the like, feeds electrical power into the DC electrical machine (1), and the AC electrical machine (2) delivers power to local AC loads (6) or draws power from the AC electrical grid (7). In some implementations of the present invention, there could be local DC loads (8) also supported by the system.

[0047] The DC machine 1 is typically a wound-field DC machine 1 such that the (back-)EMF developed by the machine 1 can be controlled independently of the speed. In some embodiments a main magnetic field of the DC machine 1 can be supplied mainly by permanent magnets on a stator magnetic circuit, with relatively small field windings also provided so that the magnetic field of the stator magnetic circuit can be adjusted slightly.

[0048] Similarly, the AC machine 2 is typically a wound-field synchronous machine such that the (back-) EMF developed by the AC machine 1 can be controlled independently of the rotor speed. In some embodiments the main magnetic field of this AC machine 1 can be supplied mainly by permanent magnets on the AC rotor magnetic circuit with relatively small field windings also provided so that the AC rotor magnetic field can be adjusted slightly. The currents acting on the AC rotor to either provide or adjust the rotor magnetic field would typically be sourced from an exciter—following standard practice in synchronous generator design.

[0049] FIG. 2 shows a decomposition of the TMESS (3) into the central shaft (31) which may be linked to a charging machinery set (33) via a mechanical-transmission-for-charging (32) or to a discharging machinery set (35) via a mechanical-transmission-for-discharging (34). The charging action results in energy being placed into an energy store (36) (which may comprise two or more discrete energy stores of the same or different types). Similarly, the discharging action results in energy being drawn out of that energy store (36).

Operating Modes of the System of this Invention

[0050] The system disclosed in this invention has four main operating modes:

[0051] (A) Quiescent

[0052] (B) Charging

[0053] (C) Discharging

[0054] (D) Free-Spinning

[0055] When the system is quiescent (mode A), neither the electrical machines nor the central shaft of the (thermo-)mechanical energy storage system (TMESS) are turning. Both electrical machines would normally be isolated from their respective buses in this state so as to prevent losses from occurring in the electrical machines and to prevent torques from arising in them.

[0056] When the system is charging (mode B), net electrical power is flowing into the two electrical machines. In this mode, it may often be that substantial power is flowing into the DC machine whilst at least some AC power is being withdrawn from the AC machine, and the remainder of the power (apart from losses) is flowing into the central shaft of the TMESS.

[0057] When the system is discharging (mode C), net electrical power flows out from the two electrical machines. In mode C, it would most often be the case that negligible power emerges from the DC machine and that the only significant power is AC power being drawn from the AC machine. The net output electrical power is all drawn from the TMESS via its central shaft. Some DC loads (such as computer power rails and power for LED lights) may be present “behind the meter” and these loads could be drawn from the DC machine.

[0058] In the free-spinning mode (mode D), the central shaft of the TMESS is declutched from the combined DC+AC electrical machine set. The DC+AC electrical machine set may be considered to be a composite electrical machine. Most commonly in this mode, significant electrical power flows into the DC electrical machine and most of this power is fed into the AC electrical machine through the mechanical connection of the two rotors. Then AC electrical power emerges from the AC electrical machine—either for direct use to drive some load or for injection into the AC electricity grid.

[0059] The (thermo-)mechanical energy storage system in this case is charged by injecting net electrical power into the pair of electrical machines such that mechanical power flows into the central shaft of the (thermo-)mechanical energy storage system. The mechanical energy is converted into a storable form and held. The (thermo-)mechanical energy storage system is charged by converting the stored energy back into the form of mechanical energy at the central shaft. The mechanical power flowing from the central shaft then turns the two electrical machines simultaneously and enables both to generate—though normally most output power would emerge from the AC machine.

[0060] From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of energy storage or conversion, and which may be used instead of, or in addition to, features already described herein.

[0061] Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

[0062] Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

[0063] For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, and reference signs in the claims shall not be construed as limiting the scope of the claims.