WO2016041987A2

WO2016041987A2 - Flywheel for energy storage systems and energy storage systems comprising the same

Info

Publication number: WO2016041987A2
Application number: PCT/EP2015/071133
Authority: WO
Inventors: Jamshid Zamany; Martin SPEIERMANN
Original assignee: Wattsup Power A/S
Priority date: 2014-09-15
Filing date: 2015-09-15
Publication date: 2016-03-24
Also published as: WO2016041987A3

Abstract

Flywheel for energy storage systems and energy storage systems comprising the same. The flywheel according to the present invention is characterised by a 5 vertical structure and a compact layout, optimized to reach maximum efficiency and to be lighter and less cumbersome when compared to state of the art systems.

Description

FLYWHEEL FOR ENERGY STORAGE SYSTEMS AND ENERGY STORAGE SYSTEMS COMPRISING THE SAME

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Technical field of the invention

The invention relates to the technical field of energy storing and in particular to the technical field of energy storing apparatuses and systems based on flywheel energy storage devices.

State of the art

Energy Storage is a technical area that has developed rapidly over the past years, becoming a key to improve the management of energy production and delivery. Among energy storage systems, flywheels have emerged as one of the most interesting technologies, in particular for such electrical applications as uninterruptible power supplies, utility load leveling and shaving systems, alternative energy generation, and fast charge stations for electric and hybrid vehicles just to name a few.

As energy storage means, flywheels show several benefits with respect to electrochemical batteries, widely employed today in the above mentioned fields. Electrochemical batteries have many drawbacks: their life is short and unpredictable, they require periodic maintenance and inspection, they are subject to thermal degradation and can fail unpredictably when required. Most of the batteries are also not environmentally friendly and their disposal always represents an issue. Flywheels have potential to eliminate the disadvantages of batteries with an average life duration of twenty years and more with very little or no maintenance, temperature insensitivity, very high reliability and a high degree of environmental friendliness.

Most common Energy Storage systems based on flywheel operate in a way to convert energy between electrical energy and the rotational, or angular, kinetic energy of a spinning flywheel, and vice-versa.

A typical flywheel energy storage system includes a flywheel, an electric motor and generator and a bearing system, all preferably enclosed in a vacuum container to minimize the losses due to friction. During operation, the rotating flywheel stores the angular kinetic energy, the electrical motor and generator converts between electrical and mechanical energy while the bearing system physically supports the rotating flywheel minimizing friction losses. Flywheels, especially those designed for high rotational speed, are preferably contained in a vacuum or low-pressure container in order to minimize also the aerodynamic losses that would occur from operation in air at atmospheric pressure.

Energy storage apparatuses based on flywheel further comprise means to provide constant DC voltage adapted to power an electrical load as the flywheel speed slows during the discharging phase. An electrical power conversion unit is therefore electrically connected for operation and conversion of power to the flywheel.

The main object of present invention is therefore to disclose a new energy storage system based on flywheel, characterized by outstanding overall efficiency and performance.

Another object of the invention is to disclose a new flywheel optimized to achieve high energy efficiency with a simple, compact and cost competitive structure.

A further object of the invention is to disclose a new flywheel provided with a single phase DC motor controlled by a H-bridge driver adapted to minimize size and part count and optimize efficiency.

Brief description of the drawings

Further objects and features of the present invention will be understood from the following detailed description of preferred, but non-exclusive, embodiments of the energy storage system based on flywheel according to the invention, when taken in conjunction with the accompanying drawings in which like reference numerals designate like parts and wherein:

Fig. 1 shows a block diagram of a first example of energy storage system based on flywheel.

Fig. 1_bis shows a block diagram of a second example of energy storage system based on flywheel.

Fig. 2 shows a cross-sectional view of a first preferred embodiment of the new flywheel according to the present invention. Fig. 3 and 3_bis show cross-sectional views of the coil section of a first preferred embodiment of the new flywheel according to the present invention.

Fig. 4 and 4_bis show cross-sectional views of the coil section of a second preferred embodiment of the new flywheel according to the present invention.

Fig. 5 and 5_bis show a first embodiment of magnetic levitation means associated to the shaft of the flywheel according to the present invention.

Fig. 6 shows a second embodiment of magnetic levitation means associated to the shaft of the flywheel according to the present invention.

Fig. 7 shows a third embodiment of magnetic levitation means associated to the shaft of the flywheel according to the present invention.

Fig. 8 and shows a forth embodiment of radial magnetic bearing means associated to the shaft of the flywheel according to the present invention.

Fig. 9 and shows a fifth embodiment of radial magnetic bearing means associated to the shaft of the flywheel according to the present invention.

Detailed description of the invention

With reference to enclosed figures 1 and 1_bis, a first preferred embodiment of the apparatus according to the present invention comprises a DC/DC converter 10 associated to an input DC power line and to an output DC power line, a bidirectional DC/AC converter 1 1 associated to said input DC power line, a flywheel 12 associated to said DC/AC converter 1 1 and a controller 13 adapted to provide the control and driving signals to said DC/DC unit 10 and to said bidirectional DC/AC converter 1 1 .

During normal operation, the DC voltage of said input DC power line is converted to the output DC voltage of said output DC power line by said DC/DC converter 10. Furthermore, said bidirectional DC/AC converter 1 1 supplies power to said flywheel 12 converting the DC current of said input DC power line to synchronous alternating current that provides power to accelerate and maintain said flywheel 12 to normal operating speed.

During an interruption in the input DC power, energy from the rotating flywheel supplies AC power to said bidirectional DC/AC converter 1 1 . Said bidirectional DC/AC converter 1 1 , in turn, converts automatically the input AC power coming from the flywheel to DC power that is delivered on said input DC power line and finally converted, by said DC/DC converter 10, to the requested DC voltage on said output DC power line.

While supplying power to the DC power line, the flywheel speed slows and the voltage to the DC power line drops accordingly. Said DC/DC converter 10 maintains a constant DC output voltage on said output DC power line during discharging of the flywheel.

Both the input power and the output power can be AC power, simply by adding an input AC/DC converter and replacing said output DC/DC converter 10 with an output DC/AC converter.

In a preferred embodiment of the present invention, said bidirectional DC/AC converter 1 1 comprises a H-bridge comprising semiconductor switches provided with antiparallel diodes. Due to said antiparallel diodes, the power delivered back by the flywheel 12 is automatically rectified to said input DC power line whenever the voltage provided by the flywheel is greater than the voltage of said input DC power line.

Considering a flywheel with a single phase motor/generator, the stator winding comprises coils connected anti-pole in series. When connect to a DC voltage source said coils will generate opposite magnetic polarity due to their anti-pole connection. Placing a magnetic rotor provided of opposite permanent magnet poles in between said coils will make the rotor turn to the position where the south pole generated by one coil pulls the north pole of the rotor magnet and the north pole generated by the other coil will pull the south pole of the rotor magnet until the rotor stops at the polar aligned position (PAP).

Switching off the DC power supplying the coils just before the rotor reaches the polar aligned position will make the rotor keep on rotating and passing by the polar aligned position due to inertial energy of the rotor itself. Energizing back the coils with DC voltage of opposite polarity right after the rotor passes by the polar aligned position will push the rotor away from said polar aligned position and make it rotate towards the next polar aligned position. This way the rotor will keep on rotating and acquiring rotational kinetic energy. Thus the DC power supply of the coils needs to be switch on and off and polarity reversed twice every 360 degree of rotation. For this purpose the bidirectional DC/AC converter 1 1 according to the present invention comprises a H-bridge driven to change the polarity of the output voltage every half cycle, the driving signals of the H-bridge switches being provided by said controller 13 according to the control signals coming from a set of position detecting Hall sensors further included in said controller 13.

With reference to enclosed figure 2, the flywheel 20 according to a first preferred embodiment of the present invention comprise a vertical structure wherein a plurality of aligned high strength permanent magnets 21 configured in repelling mode and placed at the bottom of the flywheel structure, are adapted to bear the compound flywheel weight, making it levitate thus reducing rotational friction losses at the shaft to a minimum.

The flywheel according to said first preferred embodiment of the present invention comprises a container 22, in turn comprising a top cover 23, four side walls and a bottom cover 24. Said top and bottom cover 23, 24 comprise means to house one end of a vertical shaft 25 coupled with related bearings 26, 27 adapted to reduce rotational friction of said vertical shaft 25 to a minimum while guaranteeing mechanical strength to the junction between said vertical shaft 25 and said top and bottom cover 23, 24. Allowing the shaft 25 ends engaging with said top and bottom cover 23, 24 makes the disassembly of the flywheel 20 very straightforward and gives to the internal structure of the flywheel 20 a very simple access in order to keep maintenance operations very easy.

The coil section of said first preferred embodiment of the flywheel according to the present invention is located about the top half of said shaft 25 and coupled with it. Said coil section comprises an inner rotating permanent magnet 29 coupled to said shaft 25, a field winding coil 30 wound on said inner rotating permanent magnet 29 and an outer rotating magnet 31 placed around said field winding coil 30 and integral to the body 32 of the flywheel.

The above described topology implements a closed magnetic circuit 33 comprising design the shaft and the inner body of the flywheel. Said closed magnetic circuit 33 is characterized by maximum field strength in the space between two mutually facing magnets 29, 31 , where the electrical coils 30 are placed, as illustrated in figures 3 and 3_bis. This way the currents induced by the magnetic field in the electrical coils 30 are maximized together with the efficiency of the flywheel. In the above described first preferred embodiment of the flywheel according to the present invention, the whole or a part of the flywheel body and the rotating shaft are made of ferro-magnetic material and they are integrated parts of the motor/generator assembly thus making said motor/generator assembly lighter and less cumbersome when compared to state of the art flywheel systems. Furthermore, the electrical coils of said motor/generator assembly are coreless meaning there is no ferro-magnetic material within said coils making them lighter and more compact and easy to manufacture and adapted to eliminate the iron hysteresis losses often affecting the state-of-the-art flywheel.

A further embodiment of the flywheel according to the present invention, illustrated in enclosed figures 4 and 4_bis comprise a coil section wherein the closed magnetic circuit 33 lies on a plane comprising the axis of the rotating shaft.

Said further embodiment comprises an upped disc 40 and a lower disc 41 made of ferro-magnetic material, said discs face each other and are assembled coaxial to the rotating shaft 25. A plurality of magnets 42 are integral to the inner surface of both discs and a plurality of coils 43 are placed within the gap between said discs 40, 41 and said magnets 42 protruding from the inner surface of said discs, in a sandwich-like structure. Again, the electrical coils 43 are coreless meaning there is no ferro-magnetic material within said coils making them lighter and more compact and easy to manufacture and adapted to eliminate the iron hysteresis losses often affecting the state-of-the-art flywheel.

This embodiment has the further advantage to be modular in that a plurality of coil sections can be assembled on the same shaft in order to extract more energy from the flywheel in a shorter time or to accelerate the flywheel to speed up in a shorter time.

Bearings are very critical in flywheels, like in any other apparatus comprising fast rotating shafts, therefore improving the bearings reliability and increasing their life is of crucial importance to the flywheel. Since the loading of the bearings has a significant influence on bearings life, the reduction of bearing loadings is the key to increase the bearing life and therefore to improve the flywheel reliability.

In the flywheel, the supporting bearings need to stand the heavy weight of the flywheel rotating body and rotating shaft. The flywheel according to the present invention comprises means for supporting the total vertical weight of the vertically rotating shaft and the rotating body based on a set of coupled repelling magnets that induce a closed magnetic circuit to provide magnetic vertical levitation.

Magnets in repelling mode exert a repelling force proportional to the strength of the magnets and inversely proportional to the distance between the magnets.

Thus, at least one end of said shaft 25 is coupled with a plurality of support magnets 21 configured in repelling mode and adapted to bear the weight of the core of the flywheel and of the shaft itself. The magnets are arranged in a way to repel each other and to repel the end of said shaft 25 they are coupled with, thus making the end of said shaft 25 to levitate on them, reducing the revolving friction of the shaft 25 down to a minimum.

In greater detail and with reference to figures 5 and 5_bis, in a preferred embodiment of the present invention the coupled magnets comprise each an inner magnetic ring 50 and an outer magnetic ring 51 with opposite polarity. Said inner and outer magnetic rings create a fully closed magnetic circuit 52, and hence the established magnetic fields are limited within the common surface areas on both sides of each coupled rings. This topology maximize the strength and the stability of the magnetic field

In another preferred embodiment of the present invention, adapted to limit the size of the ring magnets and to make their construction fully flexible, the magnetic rings are made of separated segmented magnets shaped, for example as wedges or cubes, as shown in enclosed figure 6. Using this combination of magnets allows designing magnetic bearing supports for any heavy load by increasing the number of segmented magnets in a circle of any arbitrary radius.

To optimize the magnetic bearing support for the magnetic circuitry the inner ring, or segment, magnet surface area can be chosen as large as the outer ring magnet surface area in order to induce equal magnetic circuit coupling in between the upper and the lower coupled ring magnets.

To improve the confinement of the magnetic field within the magnets and minimize magnetic field distortion, a ferro-magnetic plate can be employed on the top and on the bottom of each coupled magnet. Alternatively the shaft 25 itself and its supporting flange 24 can do the job of said ferro-magnetic plates, resulting in a more compact and cost effective structure, depicted in enclosed figure 7.

Alternatively, a double flange 26 can be employed, one at each end of said shaft 25.

In another preferred embodiment of the present invention, a closed circuit passive magnetic bearing can be arranged to support radial loads. This arrangement is shown in figure 8. The radial magnetic bearing consist of an inner ring 61 and an outer ring 62, and any of them, depending on design requirements, can act as the rotor or stator. In this case, the eccentric forces in radial direction will be opposed by repelling forces in the gap 60 between the outer 62 and inner ring 61 arrangement.

In both rotor and stator rings the close magnetic circuity is arranged by two rows of segmented magnets on top of each other which are paired by opposite polarity (N, S). In this configuration magnet poles within outer and inner ring are arranged radially so there will be a strong repulsion force between the segmented magnetic rings in radial direction as show in the figure 8. Any relative eccentric displacement of the rings will induce an opposite radial force that will correct the displacement and locate the rings in the center again. The radial magnetic bearing arrangement can be further supported by an iron support, for example an iron ring 63 to further strengthen the magnetic flux within the magnetic circuit of the described arrangement, as shown in figure 9.

Claims

1 . Flywheel device comprising a container (22) provided with a top cover (23) and a bottom cover (24); a vertical shaft (25) made of ferro-magnetic material and provided with bearings (26, 27) at both ends, said top and bottom cover (23, 24) comprising means to house one end of said vertical shaft (25) and engage with said bearings (26, 27); a coil section coupled with said shaft (25) and comprising an enclosure made of ferro-magnetic material, said enclosure including at least one electrical coil (30, 43) placed between and in close proximity of at least a couple of permanent magnets (29, 31 , 42).

2. Flywheel device according to claim 1 characterised in that said enclosure comprises an inner rotating permanent magnet (29) coupled with said shaft (25), an electrical coil (30) wound on said inner rotating permanent magnet

(29) and an outer rotating magnet (31 ) placed around said electrical coil

(30) and integral to said coil section (32).

3. Flywheel device according to claim 1 characterised in that said enclosure comprises an upped disc (40) and a lower disc (41 ) made of ferro-magnetic material, said discs facing each other and being coaxial to the rotating shaft (25); a plurality of magnets (42) integral to the inner surface of both said discs (40, 41 ) and a plurality of electrical coils (43) placed within the gap between said discs (40, 41 ) and said magnets (42), protruding from the inner surface of said discs (40, 41 ).

4. Flywheel device according to claim 3 characterised in that it comprises a plurality of said enclosures placed coupled to said rotating shaft (25) on top of each other in a modular manner.

5. Flywheel device according to one or more of claims 1 to 4 characterised in that said electrical coils (30, 43) are coreless.

6. Flywheel device according to one or more of claims 1 to 5 characterised in that at least one end of said shaft (25) is coupled with a plurality of coupled support magnets (28) configured in repelling mode and arranged in a way to repel each other and to repel the end of said shaft (25) they are coupled with, thus making the end of said shaft (25) to levitate on them, reducing the revolving friction of the shaft 25 down to a minimum.

7. Flywheel device according to claim 6 characterised in that said coupled support magnets (28) comprise each an inner magnetic ring (50) and an outer magnetic ring (51 ) with opposite polarity.

8. Flywheel device according to claims 6 to 7 characterised in that said coupled support magnets (28) are made of separated segmented magnets shaped as wedges.

9. Energy storage systems comprising a DC/DC converter (10) associated to an input DC power line and to an output DC power line, a bidirectional DC/AC converter (1 1 ) associated to said input DC power line, a controller (13) adapted to provide the control and driving signals to said DC/DC unit (10) and to said bidirectional DC/AC converter (1 1 ) characterised in that it comprises a flywheel (12) according to one or more of claims from 1 to 8, associated to said DC/AC converter (1 1 ).

10. Energy storage systems according to claim 9 characterised in that said bidirectional DC/AC converter (1 1 ) comprises a H-bridge in turn comprising semiconductor switches provided with antiparallel diodes.