The invention relates to a flywheel energy storage device, comprising a housing supported by the environment, a central stator supported therein, and a rotor which is supported rotatably by said stator by means of two ball- bearings, is coaxial with said stator, and encloses it.

Such a flywheel energy storage device is described in Applicants' own publication: New Results on Flywheel

System EMAFER by F. Thoolen, 1ECEC 1988, Paper No. 889228.

In order to achieve both a high efficiency and a great energy storage capacity, it is essential for the rotor to rotate at very high speeds, of the order of magnitude of 18,000 to 20,000 revolutions per minute. Of course, vibration-free running must be ensured all the time while it is passing through the full speed range from standstill up to this value, and this makes particularly great demands on the rotor bearings. In fact, such demands cannot be met with the known bearings and bearing supports.

The object of the invention is to provide a solution to this problem. According to the invention, this solution is achieved by said ball-bearings having a first radial stiffness with value SI and each being accommodated between the rotor and a top and bottom auxiliary carrier respectively, which auxiliary carriers are supported on the stator with a second radial stiffness S2 _b and S2 ₀ respectively, the value of which lies between 1/50 SI and

1/2 SI, while the stator is connected at the top and bottom side to the housing by means of a support whose radial stiffness has a value S3 lying between 1/50 and 1/5 of the mean value of S2 _b and S2 ₀ :

( S2 _b + S2 _Q )

2 and the following also applies: 1E8 < SI < 5E8 N/m The housing support preferably has a radial stiffness with a total value S4 lying between 1/500 SI and

1 / 50 SI .

A preferred embodiment is characterized by the following values of the respective radial stiffnesses: SI = 2 E8 N/m S2(b) = 0.7 E7 N/m S2(0) = 6 E7 N/m

53 = 2 E6 N/m

54 = 2 E6 N/m

Further preferred embodiments of the flywheel energy storage device according to the invention are described in claims 4 to 6.

The invention is explained with reference to the drawing, in which:

Figure 1 is a diagrammatic longitudinal section through a flywheel energy storage device in which the measures according to the invention are used;

Figure 2 is a diagrammatic illustration of the equivalent components of a coupling element used in said flywheel energy storage device; Figures 3 to 6 are diagrammatic half longitudinal sections of specific embodiments of such a coupling element.

The flywheel system for energy storage indicated in its entirety by reference numeral 1 in the figure comprises a housing 2, which is evacuated during operation and which is by means of a top coupling element 4a and a bottom coupling element 4b, supported on the solid environment, diagrammatically indicated by 6a and 6b. The specific design of said coupling elements 4a, 4b will be explained further below.

Said housing 2 bears, by means of a set of second coupling elements, i.e. a top element indicated by 8a and a bottom element indicated by 8b, a top auxiliary carrier 10a, which is stationary during operation, and a bottom auxiliary carrier 10b, which is also stationary during operation. Each of these auxiliary carriers 10a, 10b is directly coupled to the top end 12a and the bottom end 12b respectively of the central, cylindrical stator 14, which bears electrical windings (not shown) .

The auxiliary carriers 10a, 10b are directly connected by meahs of two third coupling elements, i.e. the top coupling element 16a and the bottom coupling element 16b respectively, to the stationary outer race of a top and bottom ball-bearing 18a, 18b respectively, the inner race of which is connected by means of an S-shaped connecting piece (which is the subject of Dutch Patent Application 9201585 in the name of Applicants, which is not a prior publication) 20a, 20b respectively to the rotor 22. The latter bears on its inner surface the permanent magnets (not shown) and on its outer surface the disc-shaped flywheels 24. The radial stiffness of the connection between the flywheels 24 and the rotor 22, which can be designed, for example, in the way described in Dutch Patent Application 9201584 (in the name of Applicants) , is very great, for example of the order of magnitude of 1E9 N/m.

The invention is based on the insight that good vibration behaviour of this whole unit, also while passing through the full operating speed range (which lies between zero and twenty thousand revolutions per minute) is obtained if the radial stiffnesses, expressed in N/m, of the respective supports mentioned above comply with or lie between certain values. The following terms are used below in the definition thereof: Radial stiffness of the ball-bearings 18a, 18b = SI

(per bearing) .

Radial stiffness of the top coupling element 16a between stationary outer race of ball-bearing 18a and auxiliary carrier 10a = S2 _b . Radial stiffness of the bottom coupling element 16b between stationary outer race of ball-bearing 18b and bottom auxiliary carrier 10b = S2 ₀ .

Combined radial stiffness of the coupling elements 8a, 8b between auxiliary carriers 10a and 10b and housing 2 = S3.

Radial stiffness of the coupling element 4a, 4b respectively between the housing 2 and the environment 6a, 6b respectively = S4.

Then the following applies: 1E8 < SI < 5E8 1/50 SI < S2 _b < 1/2 SI 1/50 SI < S2 ₀ < 1/2 SI 1/50 (S2 _b + S2 ₀ ) < S3 < 1/5 (S2 _b + S2 ₀ )

2 2 and the following also applies: 1/500 SI < S4 < 1/50 SI The following values are preferably used: SI = 2 E8 N/m S2 _b = 0.7 E7 N/m S2 ₀ = 6 E7 N/m S3 = 2 E6 N/m S4 = 2 E6 N/m

Figure 2 shows in a diagrammatical longitudinal section an equivalent representation of such a coupling element which fulfils both a specific stiffness function and a specific damping function. Showing the coupling between two cylindrical concentric elements, i.e. the inner element 30 and the outer element 32, between which an annular gap 34 (shown exaggeratedly large) is present, but which cannot rotate relative to each other. The stiffness function in the radial direction of the mutual coupling is represented diagrammatically by the spring 36, while the damping function of any radial movements relative to each other is represented by the hydraulic piston-cylinder combination 38.

Figure 3 shows a half longitudinal section through a first practical implementation of such a coupling element, which is rotationally symmetrical about the axis 40. The cylindrical outer part is indicated by 42, and the cylindrical inner part by 44; an annular gap 46 is present between them, to which gap damping oil can be supplied through the oil supply opening 48. Above and below said oil supply opening an annular groove 50a, 50b respectively is formed in the outer surface of the part 44, in which grooves O-rings 52a, 52b respectively are accommodated. Said O-rings have a threefold function: they ensure the necessary stiffness of the coupling between the parts 42

and 44, they damp radial movements between said parts, and they also ensure a seal in order to prevent excessive leakage of damping oil supplied through the connection 48. The oil film in the annular gap 46 also provides additional damping of radial movements between the parts 42 and 44.

Figure 4 shows a diagrammatic longitudinal section, symmetrically about the axis 60, of a second embodiment of a coupling element. Here again the parts to be coupled, i.e. the outer part 62 and the inner part 64, are concentric and rotationally symmetrical about the axis 60; they enclose each other while leaving free a narrow annular gap 66. Damping oil can be supplied to said annular gap through oil supply channel 68, and leakage thereof is greatly reduced by the fact that the end surfaces lying opposite each other, at right angles to the axis 60, i.e. the surface 72a of the element 62 and the surface 74a of the element 64, on the one hand, and the surface 72b of the element 64 and the surface 74b of the element 62, on the other hand, fit closely together. In this embodiment the stiffness function is provided by a ring of axial bending bars 76, accommodated between the two cylindrical parts. In the exemplary embodiment shown they go out from the outer part 62 and are fixed by their ends in openings 63, formed in the inner part 64. They can deflect radially with a specific radial stiffness.

Of course, many constructional variants are possible as regards the design and fixing of said bending bars. The embodiment according to Figure 5 corresponds in broad outline to that of Figure 4; corresponding elements are indicated by identical reference numbers. The difference from the embodiment according to Figure 4 lies in the fact that the sealing function is performed by piston rings 82a, 82b above and below the damping oil supply 68 respectively.

The embodiment according to Figure 6 largely corresponds to that according to Figure 4 and differs therefrom through the fact that the sealing function is

performed in this case by the O-rings 70a', 70b'.

Embodiments without sealing constructions for the damping oil are also possible, but then a higher damping oil flow rate is needed.