FLYWHEEL INTENDED FOR ENERGY STORAGE

Abstract

Disclosed is a flywheel intended for energy storage, including a cylindrical mass body including a main material with compression resistance of at least 25 MPa, such as concrete, the body being surrounded on at least one portion of the outer surface thereof with fibers, the material that makes up the fibers having a tensile strength of at least 100 Mpa. The tension of winding the fibers around the body leads to the compression of the main material, and the tension applied to the fibers is such that the stress exerted on the material of the mass body is at least equal to half of the maximum acceptable stress, the maximum stress being lower than the compression yield strength of the material that makes up the mass body, the material of the latter thus being pre-stressed.

Claims

1-12. (canceled)

13. A flywheel intended for energy storage, comprising a cylindrical mass body comprising a main material that has a compression resistance of at least 25 MPa, said body being surrounded on at least one portion of the outer surface thereof with fibers the material that makes up the fibers having a tensile strength of at least 100 MPa, wherein the tension of winding the fibers around the body leads to the compression of said main material, and wherein the tension applied to the fibers is such that the stress exerted on the material of the mass body is at least equal to half of the maximum acceptable stress, said maximum stress being lower than the compression yield strength of the material that makes up the mass body, the material of the latter thus being pre-stressed.

14. The flywheel according to claim 13, wherein the fibers are made from a material that has a Young's modulus less than 100 GPa.

15. The flywheel according to claim 13, wherein the fibers are glass fibers.

16. The flywheel as claimed in claim 13, wherein the fibers are wound on the cylindrical surface of the body according to a direction that is tangent to the body, by creating an angle with respect to the longitudinal axis of the body.

17. The flywheel according to claim 13, wherein the fibers are combined with a polymeric material, with the polymeric material having the form of a coating for the fibers and impregnating the fibers, with the fibers and the polymeric material forming an enclosure under tension covering at least one portion of the outer cylindrical surface of the body.

18. The flywheel as claimed in claim 13, wherein the body is hollow and comprises an additional material that covers the inner wall of the cylindrical body, in particular this material is made of steel.

19. The flywheel according to claim 13, wherein the body of the flywheel is solid, and comprises a central shaft integrated to said body.

20. The flywheel according to claim 13, wherein the body of the flywheel is solid and homogeneous, and comprises a means of fastening on each distal end base.

21. An energy storage device comprising a protective enclosure, a motor which can be reversed as a generator and is housed in the enclosure, comprising a flywheel as claimed in claim 13, with the flywheel being associated with the motor, driven by the motor and guided in rotation by journal bearings.

22. A method for manufacturing a flywheel according to claim 13, comprising a step of manufacturing by molding, of the body made of a main material that has a compression resistance of at least 25 MPa, then, after hardening of said main material, a step of filament winding under tension of pre-stressed fibers which may be impregnated with a polymeric resin.

23. A method for manufacturing a flywheel according to claim 13, comprising the following steps: the fibers are wound on a mandrel with a minimum tension of winding, in order to prevent them from sliding and/or providing for their maintaining on the outer surface of the mandrel; the fibers are assembled by a resin in order to form a cylindrical enclosure; the mandrel is removed; the tube delimited by the fibers assembled by the resin is placed in a mold; concrete is injected into the inside space of the tube at a high pressure, at a pressure value that is able to create a tension in the fibers, with the pressure being maintained for the time required for the concrete to set.

24. A device for fastening a flywheel according to claim 13, said flywheel comprising a central shaft of which at least one end protrudes from the surface of the flywheel, comprising at least one annular flange, centered on the end portion of the shaft and pressed against the face of the body of the flywheel, and further comprising at least one means for tightening the flange on the flywheel able to cooperate with the end of the shaft.

25. The flywheel of claim 13, wherein the main material is concrete.

26. The flywheel of claim 14, wherein the material having a Young's modulus less than 100 GPa also has a density less than 4.

27. The flywheel of claim 26, wherein the material is glass.

28. The flywheel of claim 16, wherein the angle with respect to the longitudinal axis of the body is between 10 and 90°.

29. The flywheel of claim 28, wherein the angle with respect to the longitudinal axis of the body is an angle of or close to 90° with respect to the longitudinal axis of the body

30. the flywheel of claim 17, wherein the polymeric material is polyester of epoxy.

Description

[0051] This invention is now described using examples solely for the purposes of illustration and in no way limiting the scope of the invention, and using the attached illustrations, wherein:

[0052] FIG. 1 shows a perspective view of an embodiment of the flywheel according to the invention, with the flywheel being hollow;

[0053] FIG. 2 shows a longitudinal cross-section view of an energy storage device comprising the flywheel of FIG. 1;

[0054] FIG. 3 is a partial cross-section view of an alternative embodiment of the flywheel of FIG. 1;

[0055] FIG. 4 is a partial longitudinal cross-section view of another alternative embodiment of the flywheel of FIG. 1;

[0056] FIG. 5 is a perspective view of the flywheel of FIG. 4;

[0057] FIG. 6 is a longitudinal cross-section view of another embodiment of the flywheel, with the flywheel being solid;

[0058] FIG. 7 is a partial longitudinal cross-section view of an alternative embodiment of the flywheel for which the flywheel is also solid;

[0059] FIG. 8 is a perspective view of the flywheel of FIG. 7;

[0060] FIG. 9 is a curve of the variation of the stress in the body according to the peripheral speed of the flywheel;

[0061] FIG. 10 is a partial diagrammatical cross-section view of the flywheel and of a means for fastening the central shaft of the latter.

[0062] The flywheel 1 of the invention shown in FIG. 1 has a body with a cylindrical shape of longitudinal axis X.

[0063] With regards to FIG. 2, the flywheel 1 is intended to be used in an energy storage device 2.

[0064] The energy storage device 2 comprises in a closed enclosure 3, the flywheel 1, an electric/generator motor 4 which is formed of a stator 40 and of a rotor 41, with the rotor 41 being mounted on the flywheel 1 and the stator 40 on a fixed shaft 5.

[0065] The shaft 52 is hollow in order to allow for the passage of the power cables 5A of the motor.

[0066] The shaft 52 carries at each one of its ends a ball bearing, respectively denoted 50 and 51.

[0067] In the example embodiment shown in the FIGS. 1 and 2, as in the alternatives of FIGS. 3 and 4, the flywheel 1 is hollow. The shaft 5 passes centrally through and according to its length, the longitudinal body of the flywheel.

[0068] The flywheel 1 is linked by its two opposite distal ends 10 and 11 to the shaft 5, and more exactly to the ball bearings 50 and 51, via connecting and fastening members 6. These connecting members 6 are for example two hubs associated respectively to the ends 10 and 11 of the flywheel, and cooperating with the two respective bearings 50 and 51.

[0069] As shall be seen hereinbelow, the two hubs 6 are integrated to the body of the flywheel 1, more particularly to the two end bases 10 and 11 of the cylindrical body of the flywheel.

[0070] Finally, a magnet 7 can be mounted on the shaft 5 creating on the lower hub 6 (in vertical mounted position of the flywheel), a force of attraction equal to the weight of the flywheel, in such a way as to cancel the axial force exerted on each bearing. This arrangement makes it possible to use bearings of small dimension that withstand high rotation speeds.

[0071] According to the invention, with regards to FIGS. 1 and 2, the body of the flywheel 1 comprises a mass 12 made of a material that makes it up, referred to as main material for the flywheel, for example made of concrete, and an enclosure 13 made of fibres 14 wound under tension and inducing compression forces on the mass 12.

[0072] The mass 12 made of concrete is manufactured by moulding. The enclosure 13 is obtained by winding under tension the fibres around the mass 12 in order to generate a compressive stress on said mass 12 when the latter is at rest, i.e. in the absence of rotation of the flywheel.

[0073] The material of the mass 12 is as such pre-stressed.

[0074] The tension applied to the fibres 14 during the winding is such that the stress exerted on the material of the mass 12 is at least equal to half of the maximum acceptable stress, with this maximum stress being less than the compression yield strength of the material that makes up the mass 12.

[0075] This yield stress of course depends on the material. For concrete, it will be appreciated that a concrete that has a high compression yield strength is used, and consequently that a concrete that is sufficiently loaded with cement is used.

[0076] The thickness (in the radial direction) of the concrete 12 will be much greater than the thickness of the enclosure 13 of fibres, with the latter thickness being sufficient to provide the appropriate stress.

[0077] In particular, the ratio e/D between the thickness “e” of the enclosure 13, and “D”, the diameter of the cylindrical body including the thicknesses of the material of the mass 12 and of said enclosure 13, is greater than 1/100.

[0078] Preferably, the ratio e/D is less than 1/10.

[0079] When the flywheel is hollow, the mass 12 is annular. Advantageously, when the mass is made of concrete, it is suitable that the annular concrete thickness be at least half of the radius of the cylindrical body of the flywheel, in order to supply with this inexpensive material a maximum of mass for the purposes of maximised energy storage.

[0080] According to the invention, the fibres 14 are for example made of glass.

[0081] The winding is carried out at least on the cylindrical surface of the body of the flywheel.

[0082] The embodiment of FIG. 1 is a flywheel with a hollow body. The fibres are arranged over the entire cylindrical surface, except on the end bases 10 and 11.

[0083] The mass 12 is made of a single material, such as concrete. However, this material could be loaded with fibres.

[0084] In the alternative embodiment of FIG. 3, the hollow flywheel 1 similar to that of FIG. 1 further comprises, an additional material 15 that forms the inner wall of the cylindrical body. Advantageously, this material offers a resistance to the winding of the mass 12 by the fibres 14, which has for effect to further increase the compressive stress with regards to the main material 12 (concrete) that makes up the flywheel.

[0085] The material 15 is for example steel. In particular, the inner wall made of steel is formed by a duct 16 which was made integral with the concrete during the moulding of the concrete.

[0086] The alternative embodiment shown in FIGS. 4 and 5 corresponds to a hollow flywheel for which the end bases 10 and 11 (with only the base 10 visible) are covered by a filament winding 17 of fibres 14. The filament winding 17 forms on the cylindrical surface of the body a second enclosure that covers the first enclosure 13.

[0087] The bases 10 and 11 are also surrounded in order to make integral the flywheel with the connecting and fastening members 6. FIGS. 4 and 5 show one of the bases 10 only, as the other base is identical. A part 6, here forming a hub, is maintained on the hollow central portion of the end base 10 of the cylindrical body thanks to the filament winding 17.

[0088] The part 6 has a base 60 which is integrated to the filament winding 17. Only the orifice 61 of the hub is not covered with fibres (FIG. 5) in order to render it visible and accessible for the purposes of a later mounting of the flywheel on the shaft 5 and/or on the ball bearing 50 or 51.

[0089] In another embodiment for the mass 12 of the flywheel, the latter is not hollow but solid.

[0090] As such, FIG. 6 shows a solid flywheel comprising the concrete 12 and the enclosure 13 of fibres. In order to mount the flywheel in the storage device 1A, the flywheel 1 integrates a shaft 52 that extends beyond the end bases 10 and 11. The shaft 52 has the same function as the shaft 5 of FIG. 2. The shaft 52 is made integral with the body of the flywheel during the manufacturing by moulding of the mass 12, with the shaft 52 having been placed in the mould wherein the concrete was cast.

[0091] The alternative of FIGS. 7 and 8 also shows a solid body. This alternative will be preferred to that of FIG. 6 for which the risk exists over the course of time, of detaching the shaft 5 from the concrete.

[0092] In this alternative, the body 12 of the flywheel is homogeneous, i.e. it does not contain any material other than the main material that makes it up. In this way, the stress in the body remains substantially constant, while it increases near a heterogeneity such as a void or a rigid material. A solid and homogeneous flywheel can therefore be made to rotate faster than a hollow flywheel, or faster than a solid and heterogeneous flywheel, and therefore finally to store more energy therein.

[0093] In the example of the FIGS. 7 and 8, the mass body 12 on the end bases 10 and 11 (with only the base 10 visible) comprises a central boss 18 coming from moulding. This boss 18 receives via mutual cooperation a means of fastening 6 such as a shaft, with the base 60 of said means being added by mutual engagement around the boss. The means of fastening 6 can possibly be assembled by gluing. Said means 6 is made integral with the body of the flywheel thanks to the filament winding 17 that covers the end base 10.

[0094] In place of the boss 18, a central blind orifice (during moulding) can be arranged in the base of the cylinder in order to arrange the means of fastening. However, it is preferable to avoid any cavity in the concrete is order not to generate any additional stress.

[0095] As an example, a flywheel of the invention has the following characteristics:

[0096] The flywheel 1 is solid and cylindrical according to the configuration of FIGS. 7 and 8. [0097] The main material is made of concrete, with the concrete having a compression yield strength of 100 MPa. [0098] The diameter of the cylinder is 0.6 m. [0099] It has a length (height) of 2 m. [0100] Its mass is 1.4 t. [0101] The thickness of the enclosure 13 made of glass fibres is 12 mm. [0102] The mass of glass fibres is 0.11 t, which is much less than the mass of concrete. [0103] The glass fibres were wound according to an angle of 90° with respect to the longitudinal axis of the cylinder, and under a tension that leads to a stress of 1500 MPa. [0104] The pre-stress in the concrete (compression) is −50 MPa. [0105] The flywheel can rotate up to 7,700 rpm, a speed at which the pre-stress in the concrete becomes zero. The energy stored is then 23 MJ or 6.4 kWh.

[0106] According to an alternative implementation of the invention, the enclosure made of fibres is made before the moulding of the concrete (called to form the body of the flywheel). The pre-stress of the concrete is obtained during the moulding of the concrete inside the enclosure formed of fibres. For example, concrete in liquid state is subjected to a very high pressure (of about its compression yield strength), during the entire duration of the setting/curing of the concrete.

[0107] According to this alternative method: [0108] the fibres are wound on a mandrel with a minimum tension of winding, corresponding to a stress of about a few MPa, in order to prevent them from sliding or providing for their maintaining on the outer surface of the mandrel; [0109] the fibres are assembled with a resin (polymerisable or thermosetting or thermoplastic) in order to form a cylindrical enclosure; [0110] after hardening or polymerisation of the resin, the mandrel is removed; [0111] the tube delimited by the assembled fibres by the resin is placed in a mould; [0112] concrete is injected into the inside space of the tube at a high pressure, at a pressure that is able to create a tension in the fibres; the pressure is sufficiently high to take the natural shrinkage phenomenon of the concrete into account; this pressure of the concrete leads to a tension in the fibres, during moulding, which is greater than that required in service; the pressure is maintained for the time required for the concrete to set.

[0113] We can also use a concrete referred to as “expansive” which has the property of increasing in volume as it sets.

[0114] FIG. 9 describes a curve of the variation of the stress in the body according to the peripheral rotation speed of the flywheel.

The curve 1 refers to a flywheel made of conventional reinforced concrete, while the curve 2 refers to a flywheel made of sintered concrete according to the invention.
The horizontal dotted lines delimit the field of use of the concrete and correspondent respectively to:

[0115] a stress value σ.sub.T which is the maximum acceptable traction, and of about a few MPa;

[0116] a stress value σ.sub.C which is the maximum acceptable stress, and of about several tens of MPa.

Due to the centrifugal force, the stress “σ” increases as the square of the peripheral speed “V” of the flywheel: the curves σ=f(V) are therefore parabola.
For a flywheel body made of conventional concrete (curve 1), the initial stress (therefore when the peripheral speed is zero, i.e. V=0) is zero. The limit σT is reached for a low rotation speed (referenced as V1, of about a few dozen kilometres per hour). The quantity of energy stored in the concrete is therefore very low.
For a flywheel body made of concrete of the invention, combined with the pre-stressed fibres (curve 2), the compression applied by the winding of fibres makes for the initial stress to be close to the maximum acceptable stress. This has the consequence of reaching the limit σT for a high rotation speed (referenced as V2, of about a few hundred kilometres per hour). As such, the amount of energy stored in the concrete is very high.
FIG. 10 shows a detailed view of a mode for fastening the flywheel 10 of the FIG. 6 provides with a central shaft 5, on a structure. Only the upper end portion of the flywheel is shown, since the lower portion is identical.
The upper end 52 of the shaft 5 extends beyond the upper surface of the flywheel. An annular flange 20 is centred on the end portion 52, and placed against the upper face of the body 12 of the flywheel.
The lower portion of the flange, turned towards the flywheel, comprises a central annular portion 20A bearing against the body 12 and an outside annular portion 20B offset upwards (by moving away from the flywheel) by a shoulder 20C.
A nut 21 cooperates with the threaded end of the end portion 52 of the shaft, and bears against a central annular recess 22, provided on the upper face of the flange 20. This fastening alternative has several advantages. The tightening of the nuts 21 places the shaft 52, which is made of steel, in traction, and the body 12, which is made of concrete, in compression, which is the preferred working mode of each material. This induces a pre-stressing of the flywheel in the axial direction, which makes it very robust. In addition, the flange 20 facilitates the operations required for the operation of the flywheel: handling, balancing, magnetic suspension, fastening of bearings, axial fastening, etc.

[0117] Consequently, the invention, thanks to the main material of the mass of the flywheel, which is pre-stressed via the winding of the fibres under tension, makes it possible to provide a compression of said material such that it is possible to reach high rotation speeds before reaching the rupture of the material, which very advantageously makes it possible to store a large quantity of energy. The main material is in particular concrete, with a low cost price and is compression resistant.