DESCRIPTION

Application field

The present invention refers to an energy storage, and particularly to a flywheel energy storage.

The above-mentioned storage is usefully employed in the energy sector and particularly for the energy supply in remote or difficult area installations, or anyway being not served by electrical supply networks, and in certain areas, it can provide a yearlong energetic self-sufficiency. . Known art

The possibility to store energy, in order to be able to use it accordingly to the needs, independently by the instantaneous availability of the energy source, is one of the criticality of the energy industry since the diffusion of the first electricity production plants. Recently, the diffusion of energy production plants exploiting renewable sources, for which a planning of the supplied power is not possible, since it varies depending on the energy resources being instantaneously available, further pushed the companies of that industry to develop devices being able , to accumulate energy reservoirs. Therefore, several types of energy storage have been spreading on the market, mainly being employed together with production plants, substantially providing to decouple the energy production by consumption.

The most common energy storages are the traditional chemical batteries, which however have a series of disadvantages such as a quick performance decay (with poor conversion efficiency as a consequence) and the risk of leaking polluting liquids.

Moreover, their market price is excessive if compared to the rather short useful life of the product. Instead, particularly interesting systems for storing/ supplying electrical energy are the so-called flywheel energy storages that use one or more vertical axis flywheels, which are inserted inside an environment being brought to a lower pressure than the external one and kept suspended by magnetic bearings being able to balance both axial and radial thrusts.

Those devices are particularly efficient because the energy dissipation due to mechanical and aerodynamic frictions is minimized thanks to the lack of contact between rotating flywheel and fixed casing and the lack of air in the inner volume. Moreover, the useful life of the flywheel energy storages is by far greater than the conventional chemical batteries.

Also from vibrations and noise level point of view, those devices show good performances.

However, the flywheel energy storages that currently can be found on the market are devices difficult to realize, because they require the mechanical system be integrated with a control and adjustment electrical circuit, suitable to stabilize the flywheel position.

Actually, the force field being generated by the magnetic bearings has an intensity that changes through the space, exerting on the flywheel a force that changes as a function of the position thereof.

Therefore, those devices are provided with sensors detecting the instantaneous flywheel position and transmitting a signal to a control unit, so that the above-mentioned unit can adjust the magnetic field generated by the bearings. Specifically, that adjustment can be carried out because the magnetic bearings are electromagnets, whose magnetic field changes as a function of the electrical current flowing into the coils of the above-mentioned electromagnets.

Clearly, such an assembly complexity level translates in a quite high cost that led to the diffusion of flywheel energy storages having a medium-big size only, which can be matched only to energy production plants justifying such an investment. Moreover, those storages are characterized by n on- negligible overall dimensions.

In other words, those flywheel energy storages are not suitable to be used together with small size plants or as simple energy reservoirs, being decoupled from the electrical supply networks.

Actually, the device production and servicing costs would not be suitable for a limited size storage. Furthermore, the magnetic bearings electrical consumption too will be excessive for storages able to store an energy quantity of few kWh. The above-mentioned facts clearly hinder the diffusion of those flywheel energy storages as energy reservoirs in areas where the supply through the common distribution network is not possible, for example in case of plants like phone repeaters that are often located in remote, difficult or however not served by electrical supply networks areas. Consequently, in those situations where it is not possible to access the common energy supply sources, it is mandatory to use less efficient solutions, generally being characterized by high noise levels: for example, generators provided with self- sufficiency tank.

Therefore, the technical issue underlying the present invention is to devise a flywheel energy storage preserving the advantages of the most complex flywheel energy storages of the known art, such as high efficiency and low noise level, but being easy to assemble, in order to be provided at a lower cost than the current devices, and to be used to supply energy in areas being not connected to the electrical distribution network. Invention Summary

The above-mentioned technical issue is solved by a flywheel energy storage comprising: a box casing, rigidly fastenable to an external' support, internally defining a vacuum chamber; a flywheel assembly, being housed inside said vacuum chamber, and comprising a rotatable flywheel being integral with a supporting shaft; a plurality of mechanical bearings, preferably provided with ceramic balls and preferably without lubrication, which rotatably fasten said supporting shaft to said casing along a vertical rotational axis; at least one magnetic element and at least one magnetically attractable element, being respectively integral to said casing and to said flywheel assembly, or vice versa, said magnetic element and said magnetically attractable element exerting such a reciprocal attracting force to balance a weight force acting on said flywheel assembly at least partially, so that to keep an axial load on said mechanical bearings under a threshold value. The phrase "to balance the weight force at least partially" means that at least one component of the attracting force is opposite to the weight force, comprising the three possible cases where said attracting force module is smaller than, equal to or greater than the weight force, with a downward, null, or upward resulting force as a consequence.

The energy storage can also comprise a wheel, fastened to said supporting shaft and comprising at least one peripheral portion developing at an upper wall of the box casing; said upper wall and said peripheral portion of the wheel including respectively said magnetic element and said magnetically attractable element, or vice versa.

Adjustment means, for example a central ring nut of the wheel engaged on a threaded portion of the supporting shaft, can be provided in order to modify the axial position of the wheel along the supporting shaft in order to change the distance between magnetic element and magnetically attractable element and adjust , the reciprocal attracting force therebetween.

Preferably, said central ring nut is joined with said peripheral portion by means of a shell or a plurality of spokes defining a convex upper section of the wheel.

The peripheral portion of the wheel comprises a ferromagnetic material, the magnetically attractable element being realized in said peripheral portion. Particularly, the whole wheel can be made of ferromagnetic material. Alternatively, the upper wall of the box body or an insert thereof can be made of ferromagnetic material. Said peripheral portion can be ring shaped and define an annular surface on the upper part being opposite to a reference surface of the upper wall.

The at least one magnetic element is realized by at least one magnet fastened to said upper wail or to said peripheral portion of the wheel as an alternative.

Particularly, those magnets can be a plurality, for example six, arranged equally spaced apart along a circular path, having an average diameter preferably between 20 cm and 40cm, opposite to and facing the peripheral portion of the wheel. The at least one magnet can be a permanent magnet being housed in a respective cavity obtained on the lower side of said upper wall, and being hold in position by a supporting cup.

Particularly, the flywheel assembly total mass can be between 250 Kg and 500Kg, said axial load on the mechanical bearings being operatively kept below the threshold value of 100 N.

It should be noted how the magnetic field creates a virtual cushion capable to stabilize and eliminate any vibration on the magnetically attractable element, that is on the wheel being integral to the supporting shaft radially during rotation. In an axial direction, the wheel is adjustable with respect to the supporting shaft, therefore it is possible to select the axial position based on the assembly choice selecting the residual load on the mechanical bearings placed on the bottom part, or reverse the weight force (that is selecting an attracting force higher than the weight force), letting the residual load on the mechanical bearings placed on the upper part.

An innovative aspect of the above-proposed construction is suspending in the void and eliminating the weight force vector on the mechanical bearings, consequently the weight force is distributed on the external casing, therefore improving the ground fastening stabilization. The dependent claims describe preferred and particularly advantageous embodiments, according to the present invention.

Further characteristics and advantages will best appear from the detailed description made here in the following of a preferred but not exclusive embodiment of the present findings, with reference to the attached figures given by way of a non-limiting example.

Brief description of drawings Figure 1 depicts a front view of a flywheel energy storage according to the present invention;

Figure 2 depicts a lateral view of the flywheel energy storage cut-away along the plane A- A of figure 1 ;

Figure 3 depicts an enlarged view of a first particular of the flywheel energy storage of figure 2;

Figure 4 depicts an enlarged view of a second particular of the flywheel energy storage of figure 2;

Figure 5 depicts an enlarged view of a third particular of the flywheel energy storage of figure 2; Figure 6 depicts a bottom view of the flywheel energy storage cut-away along the plane B-B of figure 1.

Detailed description

Referring to the attached figures 1 to 6, a flywheel energy storage suitable to store energy in the form of kinetic energy and to supply energy preferably in the form of electrical energy instead is identified generically with 1.

A cross-section view of the flywheel energy storage 1 is shown in figure 1 where the device is arranged vertically, according to its operative configuration; in the following of the present description, relative and absolute positions and directions of the different elements forming the device, defined by means of terms such as upper and lower, over and under, horizontal and vertical or other equivalent terms, must always be interpreted with reference to that configuration.

The flywheel energy storage 1 can be used individually in order to constitute an energy reservoir for machines, plants or other loads located in isolated areas, such as phone repeaters being not reached by the electrical distribution network.

The same flywheel energy storage 1 can be also connected to energy production plants and/ or be inserted inside a distribution network. Basically, the flywheel energy storage 1 comprises a box casing 2, which internally isolates and delimits a vacuum chamber 20, and a flywheel assembly 30 housed inside said vacuum chamber 20 and comprising a rotatable flywheel 3 and a supporting shaft 4, integral to each other.

The box casing 2 is fixed and rigidly fastened to an external support 100 or frame that is fixed too; the supporting shaft 4 and the rotatable flywheel 3 can rotate with respect the casing, around a vertical rotational axis X.

The supporting shaft 4 is assembled on a plurality of mechanical bearings 5, mainly suitable to absorb the radial stresses, which are described in details in the following. In the present text, for mechanical bearings it is meant bearings providing for a direct or mediated mechanical contact between parts reciprocally rotating, in contrast to magnetic bearings applied in the known art.

On an upper portion of the supporting shaft 4 a wheel 7 made of ferromagnetic material is also assembled and arranged to be attracted by a plurality of magnets 6 housed in an upper wall 21 above the box casing 2. The magnetic attraction between the two elements is suitable to substantially balance the weight force acting on the whole flywheel assembly 30, in order to axially discharge the mechanical bearings 5. Wheel 7 and magnets 6 are described in details in the following. The rotation of the rotatable flywheel 3, provided with a considerable moment of inertia, allows storing inside the device a certain quantity of kinetic potential energy.

Conversion means are also arranged in order to allow transferring the kinetic energy to the flywheel and retrieving said kinetic energy when needed. Particularly, those conversion means comprise an alternator 9 connected to a lower portion of the supporting shaft 9.

As previously said, the box casing 2 defines a vacuum chamber 20, that is its internal volume is sealed and arranged to be at a lower pressure than the external environment, particularly at a pressure of about 0.2 bar.

In the preferred embodiment described herein, the box casing 2 has a cylindrical shape and comprises an upper wall or case back 21 , a lower wall or case back 22 and a lateral shell 23. Both the two upper 21 and lower 22 case backs have a circular protruding molding 24, in both cases being directed inwards the vacuum chamber 20, having central through openings 21a, 22a in their center whose purpose will be more evident in the following of the description. The lower surface of molding 24 of upper case back 21 , in the following being identified as reference surface 21a, presents a plurality of cavities 62 housing said magnets 6; in the particular case, six cylindrical cavities 62 containing just as many permanent magnets with a similar shape.

Particularly referring to fig. 6, it should be noted how the magnets 6 are arranged equally spaced apart along a circular path 60, whose center is crossed by the vertical rotation axis X. The average diameter of that circular path 60, in the embodiment described herein, is roughly equal to 30 cm.

It should be noted that each magnet 6 is blocked in the respective cavity 62 by means of a specific supporting cup 61 , preferably made of metallic material such as bronze. Those supporting cups 61 have a lower flange helping their exact placement inside the intended cavities 62.

The external support 100 comprises a plurality of supporting feet 101 , hinged to a floor. The above-mentioned box casing 2 is fastened by means of lateral flanges 102 connected to the individual supporting feet 101 through threaded rods 103 that allow adjusting the device configuration.

As already mentioned, the whole flywheel assembly 30 comprising the supporting shaft 4, as well as the rotatable flywheel 3 and the wheel 7 associated thereto is housed inside the box casing 2 in a vertical configuration.

On the supporting shaft 4, there is a central portion 42, which supports the rotatable flywheel 3 and wheel 7, and two upper 43a and lower 43b end portions, being supported by the mechanical bearings 5 and characterized by a diameter narrowing.

The rotatable flywheel 3, traditionally configured with a T cross- section in order to maximize the moment of inertia, in the present embodiment is made in a single piece with the supporting shaft. Two plates provided with a central hole, associated to the distal flange of the flywheel, enclose it on the upper and bottom part.

Over the rotatable flywheel 3, a threaded portion 41 is provided, extending from a lower shoulder up to the narrowing of the upper end portion 43a. The wheel 7 is engaged on that threaded portion 41 by means of a central ring nut 71 that advantageously allows its axial adjustment.

The above-mentioned wheel 7, which is made of ferromagnetic material, as previously mentioned, in order to promote the above magnets 6 attraction, substantially has a cup shape. That shape is defined by a frustoconical shell 72 developing upwards starting from said central ring nut 71, and ending in an annular peripheral portion 70. The peripheral portion has a horizontal annular surface 70a on the upper part, facing and being opposite to the reference surface 21a and particularly to the circular path 60 along which the magnets are assembled.

It should be noted how substantially the cup shaped wheel 7 advantageously allows its extension up close to the upper wall, near the magnets 6.

In this way, the reciprocal attracting force being exerted between the wheel 7 and the magnets 6 supports the flywheel assembly 30 inside the box casing 2, axially discharging the mechanical bearings 5.

Moreover, by rotating the central ring nut 71 on the threaded portion 41 of the shaft, it is possible to change the wheel 7 position on the supporting shaft 4 and therefore the distance d between the wheel 7 itself and the magnets 6. Doing so, the reciprocal attracting force between the wheel 7 and the magnets 6 is adjusted in order to balance the weight force acting on the flywheel assembly 30 and to keep the axial load on the mechanical bearings 5 under a threshold value, being roughly quantifiable in 100 N.

The air gap being measured between the lower surface of magnets 6 and the ferromagnetic annular surface 70a can be adjusted exactly; roughly, its value is between 1 and 4 mm, preferably about 2.5 mm. The previously said mechanical bearings 5, thanks to the magnetic attraction exerted on the flywheel assembly 30, can carry out a mainly radial supporting function of the supporting shaft 4, discharging the force on the box casing 2.

Given the limited axial thrust acting on them, it is possible to use ceramic mechanical bearings 5, which allow an outstanding internal friction reduction, without resorting to the quite expensive magnetic bearings technology.

Preferably, the mechanical bearings 5 are plain bearings and thus include an internal ring 51, being integral with the supporting shaft 4, and an external ring 52, being integral with the box casing 2, as it can be seen in figure 1 , having rotating ceramic balls suitable to work without lubrication.

Alternatively, the mechanical bearings 5 can be ball bearings.

It should be noted that the ceramic mechanical bearings 5 used in the present invention do not need lubrication, so that the above-mentioned mechanical bearings 5 can be housed inside the vacuum chamber 20, whose low internal pressure, on the contrary, would cause the lubricant spill.

In the preferred embodiment being currently described, the mechanical bearings 5 are five; two mechanical bearings 5 are arranged on the upper end 43a of the supporting shaft 4 and three mechanical bearings 5 are arranged on the lower end 43b of the supporting shaft 4.

The mechanical bearings 5 fastening on the supporting shaft 4 is by means of a lower bearings support 8b and an upper bearings support 8a, being shaped to house respectively three and two mechanical bearings 5.

The above-mentioned lower and upper bearings supports 8b, 8a are inserted inside the central openings 21a, 22a obtained on the upper and lower walls of the box casing 2, namely interposed between the walls of casing 2 and the mechanical bearings 5.

Substantially, the lower and upper bearings supports include a hollow cylindrical body 80, provided with internal grooves suitable to house the mechanical bearings 5, and a protruding flange 82 suitable to abut the upper or lower wall of the box casing.

Said upper 43a and lower 43b end portions of the supporting shaft 4 are completely housed inside the hollow body of the bearings supports 8a, 8b, with the shoulder due to the diameter narrowing abutting on the final edge thereof.

Between the protruding flanges 82 of the two bearings supports 8a, 8b and the walls on which they abut, respective channels are obtained, which house as many seals 81, suitable to guarantee the proper isolation of the vacuum chamber 20.

Under the lower bearings support 8b, a cup shaped body 90 is assembled defining a lower appendix of the vacuum chamber 20. Inside this cup shaped body 90 the alternator 9 is housed, whose rotor is rotatably fastened to the lower end portion of the supporting shaft 4, which passes through the lower bearing support 8b.

In a preferred embodiment, cited by way of example only, the flywheel assembly mass is equal to 337 kg, and it. rotates at a maximum speed of 20000 rpm storing a maximum energy of few tens kWh. The magnetic compensation allows reducing the axial load on the mechanical bearings 5 to about 70 N.

Those skilled in the art will appreciate how the combination of mechanical bearings and magnets, suprx>rting the flywheel weight, allows realizing a flywheel energy storage easy to assemble, which does not need any control and adjustment electrical circuit. Advantageously, the magnetic attraction action also promotes and facilitates the flywheel stabilization at high rotational speeds (about 20000 rpm) . Advantageously, the cost of such a flywheel energy storage is also reduced with respect to the flywheel energy storages being currently available on the market.

Moreover, the flywheel energy storage, according to the present invention, has reduced dimensions, which facilitates its use in domestic environments too.

Advantageously, the permanent magnets use allows reducing vibrations and satisfying the low noise requirement of the flywheel energy storage.

Moreover, the ceramic bearings, although being in contact with the rotating supporting shaft, are able to reduce mechanical friction losses, ensuring a high conversion efficiency and an enduring energy storage.

Clearly, to what above described, those skilled in the art, with the purpose of satisfying contingency and specific needs, can bring several changes and variations, however all included in the protection scope of the invention as defined by the following claims.