The present invention relates to an inertial energy accumulating device. A device such as this makes it possible, for example, to absorb the fluctuations in the production and/or consumption of energy which are associated with a unit for producing energy, particularly by means of a wind turbine. A device of this type may also be used to recuperate and then restore or use in some other way a retarding and/or slowing power. The device may also be used to stabilize a rotational speed.

Energy storage proves necessary particularly in order to: absorb fluctuations in energy consumption; - store energy when it is at a lower cost, then advantageously restore it thereafter; absorb fluctuations in the production of energy, particularly in the case of wind turbine energy production that is dependent on an irregular primary energy source, namely the wind.

These days, this storage is sometimes performed using a battery. Such a storage means not only represents a relatively high cost of the order of five euro cents per kWh, but also comes with a whole series of disadvantages such as reduced life, need for frequent maintenance, and above all, presents problems of pollution.

Storage using pumping and turbines is also known, this involving pumping water to a height, for example from the bottom to the top of a dam, and recovering the potential energy at the desired moment by allowing the water to flow back down through a turbine. However, a method of this type is naturally not suited to all geographies, and once again involves relatively high costs.

Another type of storage is based on flywheels, that is to say on at least one mass set in rotation by in input of energy, which will continue to rotate, under inertia, after the energy input has ceased. The rotating mass is connected to a motor which constitutes a means of inputting energy during the energy-storage periods, or a generator during energy-restoring periods. The heavier the flywheel and the more able it is to rotate quickly with the lowest possible friction, the greater the amount of energy that can be stored. The problem with the flywheel bearings, or more generally with how it is pivot-mounted, is therefore of key importance. The company Beacon Power for example has developed a system for storing energy in flywheels with a capacity ranging up to 250 kWh. This system, known as "Smart Energy Matrix" consists of ten flywheels with a capacity of twenty-five kWh each, arranged in series each in a separate container. The system uses flywheels made of carbon fibre, a very expensive material. The bearings of each flywheel are partially relieved of the weight of the flywheels through the application of an electromagnetic force. The cost of such a system does, nonetheless, remain relatively high, of the order of 1.5 million euros per system.

It is an object of the present invention to provide an inertial energy accumulating device that employs one or several flywheels that is economical, effective, and less restrictive.

This objective is attained by means of an inertial energy accumulation device comprising a frame and at least one flywheel mounted so that it can rotate relative to the frame about an axis of rotation, characterized by means for exposing at least one face of the flywheel to a gas pressure which, by comparison with the pressure applied to a substantially opposite face of the flywheel, generates an upward differential pressure force that at least partially compensates for the weight of the flywheel.

Thus, not only are the flywheel bearings relieved at least partially of the weight of the flywheel, thus increasing their life, but the cost per kWh is also greatly reduced.

Preferably, the face of the flywheel that is exposed to the gas pressure is surrounded by gas flow slowing means. These flow slowing means make it possible to create a drop in pressure head in the leakage space. They are typically formed between the flywheel and a surface integral with the frame. They are also preferably secured to an internal wall itself integral with the frame and/or to the external peripheral wall of the flywheel. In a preferred embodiment, the gas flow slowing means are secured to the external peripheral wall of the flywheel.

Preferably, the flow slowing means work without there being any contact between the surfaces connected to the flywheel and those connected to the frame, so as to limit or eliminate energy losses through friction and wear. However, the space between the external surface of the flywheel and the flow slowing means and/or the said internal wall integral with the frame and the flow slowing means is also fairly small, in order to limit the leakage flow rate. The flow slowing means involve the proximity of the external peripheral wall of the flywheel to an internal wall of the cage. According to a first embodiment, the flow slowing means comprise at least one flexible lip seal. This may preferably be an inflatable ring, preferably made of elastomer, of the kind used in hovercraft type carriers. It is possible to obtain better control over the air leaks by filling the inflatable ring or rings with more or less gas. In a preferred embodiment, the flow slowing means consist of a series of flexible lip seals or a series of inflatable rings arranged one after the other and forming a succession of chambers at pressures that vary by successive bearings.

According to a much more advantageous embodiment, the flow slowing means comprise a labyrinth seal. In such a seal, the gas flow path comprises a succession of special features that generate drops in pressure head ("head drops"). For example, the cross section for the passage of the gas is alternately reduced and enlarged.

In an advantageous embodiment, the flywheel has a vertical axis, the face of the flywheel exposed to the pressure is an end face, and the gas flow slowing means are then preferably formed between the external peripheral face of the flywheel and the internal peripheral face of a cage integral with the frame. The seal is made up, for example, of a series of rings that are substantially concentric relative to the flywheel, secured to the internal wall of the cage and/or to the external surface of the flywheel. A labyrinth seal is preferably made of polyester, but may also be made of polyethylene or polystyrene.

In a preferred embodiment, the device according to the invention comprises a compressor having a suction side and a delivery side, one of which supplies the pressure that is applied to the exposed face of the flywheel and the other of which is connected to carry the gas that is present on the other side of the flow slowing means relative to the exposed face. The energy consumed by the compressor during a typical storage period represents only a very small percentage of the energy that can be stored in the flywheel. Thus it becomes possible, with practically no loss of energy, to recirculate the gas that has escaped via the flow slowing means and reinject it at the desired pressure into the device in order continuously to expose at least one face of the flywheel to a gas pressure that generates a force that at least partially compensates for the weight of the flywheel.

As a preference, in the device according to the invention, the gas present at the other side of the flow slowing means is in contact with an opposite face of the flywheel to the said exposed face so that the lift force applied to the flywheel is a function of the difference between the suction pressure and the delivery pressure. Because it is only the pressure difference that matters, it is possible, for example, to deliver a gas under pressure to a lower face of the flywheel which then defines the exposed face, and/or by suction to create a partial vacuum above an upper face of the flywheel, which then defines the exposed face, both methods still leading to a difference in pressure between the suction pressure and the delivery pressure.

It may be advantageous for the device according to the invention to comprise regulation of a flow rate of the compressor with a view to maintaining predetermined lift conditions on the flywheel, particularly a predetermined pressure difference between two opposite faces of the flywheel and/or a predetermined load on bearings such as rolling bearings or plain bearings of the flywheel. Thus, the load on the bearings is kept at the intended value without the risk either of overloading the flywheel or, conversely, of destabilizing it by lifting it.

The device advantageously comprises a radiator between the gas suction side and the gas delivery side. As a result the gas flowing against the rotating flywheel heats up as a result of the compression and/or of the friction between the gas and the flywheel when it is rotating. By positioning a radiator between the gas suction and delivery sides, it is possible to cool this gas when it is not in contact with the flywheel, before once again introducing it into the frame and bringing it into contact with the flywheel. In this way, the gas can be prevented from little by little reaching too high a temperature which could, for example, increase the mean temperature inside the frame and degrade certain elements located therein, such as the exterior faces of the flywheel or the flow slowing means.

The flywheel is preferably mounted in a cage secured to the frame, the cage and the exposed face of the flywheel together defining, for the gas, at least one chamber surrounded by the flow slowing means. The said cage is preferably made of concrete. This material is economical and is able to cushion the impact if the flywheel accidently explodes as a result, for example, of a sudden halt in the rotation of the flywheel or flywheel overspeed. The at least one chamber defined by the cage and the exposed face of the flywheel communicates with the gas suction side or the gas delivery side. The substantially opposite face of the flywheel and the cage preferably define a second chamber which communicates with the gas suction side or the gas delivery side.

According to the second embodiment described hereinabove, the internal wall of the cage and/or the external peripheral wall of the flywheel comprise a coating in the form of a labyrinth, particularly made of polyester.

According to a preferred embodiment, the gas is delivered at a relative (gauge) pressure of between 50 and 60 kPa (about 0.5 and 0.6 bar). Preferably, the pressure near the suction side is substantially zero, for example 10 kPa, so as to limit the friction of the flywheel with the surrounding gas. A pressure of between 50 and 60 kPa near the delivery side therefore constitutes a raised pressure.

The flywheel is advantageously secured to a shaft mounted in bearings. A pinion is coupled in terms of rotation to, but axially decoupled from, the flywheel. Thus, the flywheel shaft allows the flywheel to rotate about the common axis of the flywheel and of the shaft. The pinion provides connection to equipment external to the flywheel. The axial decoupling between the pinion and the flywheel prevents any high axial and/or radial stresses there might be from being transmitted to the gear sets. The pinion allows the rotation of the flywheel to be coupled to a source of motive power and/or to energy-consuming external equipment. The coupling may be indirect, particularly if several flywheels are coupled together.

In a preferred embodiment, the flywheel shaft is held in position particularly by the self- centring effect of the flywheel about its shaft through a gyroscopic effect and through the effect of the pressure of the gas.

The outermost mass of the flywheel is the mass that contributes most to its moment of inertia. Thus, it is preferable to use a flywheel in the form of a hollow cylinder, in order to increase the moment of inertia of the flywheel, for the same overall mass.

For preference, the rim of the flywheel is made of reinforced concrete, which is a material that is both dense in order to increase inertia of the rotor, and economical.

In a preferred embodiment and when the choice and location of flow slowing means so permit, the rim of the flywheel is surrounded by a smooth shell over at least part of its external surface. The smooth shell is, for example, made of steel and/or of carbon fibre. The smooth shell makes it possible to limit energy losses and wear in the event of friction. The use of steel is fairly advantageous, because steel and concrete have substantially the same expansion coefficient. This then reduces the risk of dangerous internal stresses within the flywheel in the event of temperature variations. When the flow slowing means are secured to the external peripheral wall of the flywheel, it is then the internal peripheral wall of the cage which is preferably covered over at least part of its surface with the said smooth coating.

The rim of the flywheel is preferably in the form of a hollow cylinder, connected to the flywheel shaft by crossed spokes. These crossed spokes are preferably made of carbon fibre. Typically, there are two conical layers of spokes, one widening towards the top and the other towards the bottom. The two layers cross one another. At the same time, within each layer, the spokes are arranged in pairs forming an X. The crossed spokes arrangement is very simple to implement, affords good precision in the positioning of the rim of the flywheel with respect to its shaft and ensures good balancing of the whole. In one embodiment, two plates, for example metal plates, preferably made of steel, are fixed one to the upper face and one to the lower face of the flywheel, for example using metal spikes that pass through the metal plate and are embedded in the concrete of the flywheel. These plates are fixed to the flywheel shaft and pierced at their centre to allow the flywheel shaft to pass from one end of the cylinder to the other. A proportion of the total number of spokes, substantially half the spokes, runs substantially from the exterior periphery of the lower plate to the interior periphery of the upper plate, and the other proportion of the spokes runs substantially from the interior periphery of the lower plate to the exterior periphery of the upper plate. This creates the aforementioned two conical layers. The spokes may also be fixed directly to the flywheel shaft and/or directly to the flywheel. The spokes furthermore preferably rest against a metal rim integral with one or both of the two plates, preferably made of metal.

The gas sucked in and/or delivered is preferably predominantly made up of air, hydrogen or helium, because of their low coefficients of friction and low viscosities. Generally helium is preferred on account of its stability and its low coefficient of friction.

According to a preferred embodiment, several flywheels can be joined together to form a matrix of flywheels, all connected directly or otherwise to the same motor and/or the same consumer. It is thus possible to multiply the amount of energy stored and that can be restored by the number of flywheels that the matrix comprises. Specifically, each flywheel is relatively limited in terms of the amount of energy it can store, because it rapidly reaches the speed of sound, a speed around which the material of the flywheel is unable to withstand the centrifugal force and is destroyed. It is therefore advantageous to use matrix configurations. Each flywheel has its own pinion, the flywheels therefore being connected to one another by gear sets, and by means of one or more transmission wheels. Each pinion has its own bearing.

The flywheels are then preferably assembled along the lines of concentric rings, with a central flywheel and several peripheral flywheels. Thus, the number of pinions mounted in series is reduced, thus limiting the phenomena whereby jerks are amplified.

According to a second embodiment, several pinions are connected together by two points of contact each. This configuration entails that at least some of the transmission wheels be superposed. Specifically, pairs of transmission wheels composed of two concentric wheels, one externally toothed and the other internally toothed and surrounding the first are preferably used. Each pair preferably drives three pinions by two points of contact each, one point of contact being with the inner wheel of the pair and the other with the outer wheel of the pair. A configuration of this type gives far greater stability in the transmission of energy between the flywheels. The mechanical stresses are lower and there is a better distribution of energy between the pinions.

The flywheels may also be used stacked on top of one another.

One or more control device(s) is/are preferably positioned outside the device according to the invention, notably with a view to controlling the rotational speed or speeds of the flywheels.

The device according to the invention is used particularly to absorb fluctuations in the production and/or consumption of energy which are associated with a unit for producing energy, particularly electrical energy, particularly using a wind turbine.

Other particulars and advantages of the invention will become apparent from reading the detailed description of an entirely nonlimiting exemplary embodiment, and from studying the attached drawings:

Figure 1 is a schematic elevation of the device according to the invention; Figure 2 is a perspective view of the device according to the invention, with cutaway; - Figure 3 is a perspective sectioned view of the flywheel alone;

Figure 4 is a perspective view of a matrix of flywheels according to the invention, not depicting the frame;

Figure 5 is a perspective view of a flywheel matrix device; Figure 6 is a view in section on IV-IV of the coupling between the shaft of a flywheel and its pinion;

Figure 7 is a top view of a means of coupling flywheels in a matrix configuration;

Figure 8 is a detailed sectional view of the gas flow slowing means.

The device according to the invention comprises at least one flywheel 1 mounted to rotate with respect to a frame 2 about an axis of rotation 3 by means of bearings 14. In the embodiment of Figure 1 , the axis of rotation 3 is vertical. In this same embodiment, the flywheel 1 is housed inside a cage 4 defined inside the frame 2 and secured thereto. The cage 4 is sealed closed by a cover 112, for example made of metal.

A compressor 5 establishes a circulation of gas through the cage 4. An exposed face 6 of the flywheel 1 and the cage 4 define, for a gas said to be a delivered gas and injected under the exposed face 6, at least one chamber 100 surrounded by gas flow slowing means 7. The substantially opposite face 11 and the cage 4 define, for the sucked-in gas, at least one chamber 101. The chambers 100 and 101 are separated by the gas flow slowing means 7. The suction side 9 of the compressor 5 communicates with the chamber 101 , and the delivery side 10 of the compressor 5 communicates with the chamber 100. The cage 4 is sealed, in particular by a seal 113 around the shaft 14 of the flywheel 1 , on the side on which this shaft emerges from the cage 4 through the cover 112. At the other end of the flywheel, the shaft 14 is mounted in a blind bore 115 formed in the bottom of the cage.

The gas is delivered for example at a relative (gauge) pressure of between 50 and 60 kPa into the cage 4, in the chamber 100, under the flywheel 1. This gas advantageously contains a high proportion of helium, or of hydrogen, or of air, the first two in particular having a very low coefficient of friction. Prferably, use is made of pure helium. In order further to limit the friction between the rotating flywheel and the surrounding gas, a substantially zero pressure may be created at the outset, inside the cage 4. In this particular embodiment, it is the lower face 6 of the flywheel 1 which is exposed to a gas pressure by injecting gas under this exposed face 6 and into the chamber 100. This generates an upwards force that at least partially compensates for the weight of the flywheel 1. In this embodiment, it may also be considered that the exposed face is the face 11 , exposed to a reduced pressure by suction of gas from the chamber 101. It is only the difference in pressure between the pressure in the chamber 100 and the pressure in the chamber 101 that matters. This pressure difference needs to be such that the resultant force is an upwards force. The upwards force corresponds to a large proportion of the weight of the flywheel. The remaining proportion of the weight of the flywheel, which is not balanced by the upwards force, is carried by at least one of the bearings 114, which is selected to be capable of carrying such a load.

The gas flow slowing means 7 are formed between the flywheel 1 and a surface integral with the frame 2. In the schematic embodiment depicted in Figure 1 , the gas flow slowing means 7 are secured to the cage 4 and therefore to the frame 2. However, the gas flow slowing means 7 are preferably secured to the flywheel 1. They may also comprise a part integral with the frame 2 and a part integral with the flywheel 1. In any event, the space between the gas flow slowing means 7 and the flywheel 1 and/or a surface integral with the frame 2 has to be small in order to limit the flow rate of gas along the gas flow slowing means 7, but has to be large enough to limit the risk of the flywheel 1 rubbing against the walls of the cage 4 or of the frame 2. In general, the gas flow slowing means 7 involve proximity of the external peripheral wall 13 of the flywheel 1 to an internal peripheral wall of the cage 4.

In the embodiment according to Figure 1 , the gas flow slowing means 7 are created in the form of a labyrinth seal. Such a seal forms a succession of changes in shape and/or in direction for the flow, here a succession of widenings and narrowings of the leakage path 8, thus creating a drop in pressure head at each variation in the cross section of the leakage path. The seal of the labyrinth seal type is created by a coating that has evenly spaced circumferential ribs. The coating is preferably made of polyester or of polyethylene, situated entirely on the external peripheral wall 13 of the flywheel 1. According to another embodiment, the said seal may also be situated partially or completely on the internal peripheral wall of the cage 4, facing the external peripheral surface 13 of the flywheel 1.

According to a preferred embodiment described in Figure 8, the labyrinth seal is secured to the external peripheral wall 13 of the flywheel 1. It is formed of a succession of peripheral grooves formed in the gas flow slowing means 7, which are, for example, made of polyester. The depth of the grooves is two to three times the width thereof. In a preferred embodiment, the grooves are spaced by approximately e=0.5 mm (or 0.2 mm, or 0.1 mm). Their depth p measures for example 6 mm, preferably 9 to 10 mm, and their height d preferably measures 3 mm. In a configuration such as this a gas flow rate through the gas flow slowing means 7 of about three to six litres/minute is obtained. Only a few per cent of the energy stored by a flywheel 1 is then used to carryrt the said flywheel during a typical storage period. More advantageously the greatest possible proportion of the height of the flywheel is covered by the gas flow slowing means.

It is advantageous to construct the flywheel 1 equipped with the gas flow slowing means 7 tailored to the inside diameter of the cage 4, and then by rotating the flywheel 1 inside its housing in the cage 4, to wear away or lap the gas flow slowing means 7 against the adjacent wall until such point that the diameter of the flow slowing means allows the flywheel 1 to rotate without any rubbing between the flow slowing means 7 and the internal peripheral wall of the cage 4.

The compressor 5 comprises a suction side 9 and a delivery side 10. One of them, in this instance the delivery side 10, supplies the pressure applied to the exposed face 6 of the flywheel 1 , and the other, in this instance the suction side 9, is connected to carry the gas that lies on the other side of the gas flow slowing means 7. Thus, the fact that the device operates in a closed loop avoids the ingress of undesirable particles or other polluting elements likely to impede the flow in the flow circuit. Closed-circuit operation also allows that a gas other than air can easily be used inside the cage 4.

The gas on the other side of the gas flow slowing means 7 is in contact with the opposite face 11 of the flywheel 1 opposite to the exposed face 6, so that the lift force applied to the flywheel 1 is dependent on the difference between the suction pressure and the delivery pressure.

This device according to the invention advantageously comprises control of the flow rate of the compressor 5 with a view to maintaining predetermined lift conditions on the flywheel 1 , particularly a predetermined pressure difference between two opposite faces 6 and 11 of the flywheel 1 and/or a predetermined vertical load on bearings of the flywheel. The control device for example comprises a pressure probe 102 in the upper chamber 101 , a pressure probe 103 in the lower chamber 100, a calculation unit 104 for calculating the pressure difference, comparing this difference to an instruction (stored value) and generating a speed or power command for the compressor 5 to increase this speed or power when the pressure difference is too low, or, on the other hand, to decrease this speed or power when the pressure difference is too high.

In the embodiment according to Figure 1, the device comprises a radiator 12 situated between the gas suction side and the gas delivery side, in this instance between the discharge of gas from the cage 4 or from the frame 2 and the injection of gas under 'the exposed face 6.

In a preferred embodiment illustrated in Figures 2 and 6, the flywheel 1 is secured to a shaft 14 rotationally connected to a pinion 15 which is axially decoupled from the shaft 14 and therefore from the flywheel 1. Figure 6 shows an example of a splined coupling between the flywheel 1 and a sleeve 16 secured to the pinion 15.

As shown by Figure 3 which is a sectioned view of a flywheel 1 , the rim 18 of the flywheel 1 is in the form of a hollow cylinder, made of reinforced concrete surrounded by a smooth skin or shell, for example made of steel or of carbon fibre. The reinforced concrete cylinder is known as an insert. In a preferred embodiment, a flywheel has the following characteristics:

The concrete insert therefore provides most of the total weight of the rotor. Thus, since concrete is an economical material, a high total rotor weight and therefore a high rotor inertia and therefore high storable energy values are obtained economically. The rotation speed indicated is a speed for normal operating conditions, that is to say without the risk of rapid deterioration of the rim 18 of the flywheel 1 , and allowing a large amount of energy to be stored.

According to a second embodiment, a flywheel 1 is produced on a small scale and has the following characteristics:

Figure 3 shows a sectioned perspective view of the flywheel 1. The flywheel 1 comprises the rim 18 connected to the shaft 14 of the flywheel 1 by crossed spokes 19. Two plates 20, for example made of steel, connect the rim 18 and the shaft 14, one of them defining the upper face of the flywheel 1 and the other the lower face of the flywheel 1. The plates 20 are connected to the rim 18 by metal spikes 30 that pass through the metal plate and are embedded in the concrete of the rim 18. The crossed spokes are, in part, fixed to the lower plate near its outer periphery and to the upper plate near its inner periphery, and in part fixed to the upper plate near its outer periphery and to the lower plate near its inner periphery. This then forms two conical layers of spokes 19 widening in opposite directions and which cross one another. Preferably, the plates are shaped as double cones in order to define at least one support, preferably a metal support 21 , for the spokes. The crossed spokes 19 are, for example, made of carbon fibre. They may also be fixed directly to the shaft 14 of the flywheel 1 rather than to the inner edges of the plates 20, and/or directly to the rim 18 rather than to the outer periphery of the plates 20. This type of crossed-spokes arrangement is inspired by the wheels of sports cars from the last century. The plates 20 are mounted in such a way as to prevent any passage of gas through the annular space between the rim 18 and the shaft 14.

It is possible to assemble several flywheels 1 to form a matrix 22 of flywheels 1. The various flywheels 1 are joined together by gear sets and by one or more transmission wheels 25. In the example depicted in Figure 4, the flywheels 1 are assembled along the lines of concentric rings with a central flywheel 22 and several peripheral flywheels 1 , the flywheels 1 being rotationally coupled to the central flywheel 26. This concentric rings configuration has the advantage of transmitting energy to a high number of flywheels 1 for a limited number of intermediate pinions 15 between the flywheel 26 which is the most directly connected to the motor and/or to the consumer, and the peripheral flywheel or flywheels 1 which are more indirectly connected to the motor and/or to the consumer, that is to say for which the number of pinions 15 through which the energy has to pass in order to be transmitted to these flywheels 1 is maximum. It is possible to provide at least one second ring of flywheels 1 around the first ring depicted.

It is advantageous to limit the maximum number of pinions 15 which transmit the energy in series between a flywheel 1 and the central flywheel 26 because transmission jerks are amplified on each transfer of movement to an adjacent pinion and may cause premature deterioration to the device according to the invention if they reach too great an intensity. It is therefore preferably limited to two intermediate pinions 15 between the pinion of the central flywheel 26 and the pinion or pinions furthest from the pinion of the central flywheel 26.

In the example depicted, each pinion 15 is protected by a sheath 23, for example made of concrete, which has at least one opening 24 to allow the pinion 15 to be coupled to an element external to the flywheel 1. In a second embodiment depicted in Figure 7, several pinions 15 of several flywheels 1 are each connected together by two points of contact. Thus, each transmission wheel 25 is in actual fact a pair comprising a central wheel 110 with external teeth and an internally toothed annulus 111, the pinions 15 rotating in the opposite direction to the central wheel 110. Each pinion 15 is positioned between a central wheel 110 and an annulus 111 of one and the same pair and meshes with both. Each transmission wheel 25 or, respectively, each pair, advantageously connects three pinions 15 together. Specifically, if the said three pinions 25 form an equilateral triangle, the assembly is then perfectly balanced. The three pairs according to Figure 7 are positioned in different planes in order not to interfere with one another.

The table which follows sets out the energies available for different sizes of matrix 22

The values given correspond to various sizes of matrix sets out in concentric circles. A module in fact corresponds to a flywheel 1 and its associated pinion 15. A seven-module matrix corresponds to a hexagonal matrix comprising one central module and six peripheral modules. Then, each additional concentric circle contains six modules more than the previous circle. Very high available energies are therefore obtained for fairly small container diameters, the container here being a concrete shell 27.

The matrices 22 are enclosed together in a concrete shell 27 on the outside of which are positioned the compressor 5 and the motor 121 and the system 122 for transmitting energy between the motor and the central pinion 26 or, more generally, the pinion which will then transmit all the energy received from the motor to the other pinions 15 of the matrix 22.

Of course, the invention is not restricted to the examples that have just been described, and numerous amendments may be made to these examples without departing from the scope of the invention. The flywheel could have a horizontal axis and the opposite faces exposed to different pressures would then be the respectively upper and lower parts of the lateral wall. The flow slowing means 7 would then, for example, comprise axial grooves on the cage on each side of the axis of rotation, and circumferential grooves on the cage 4 or the flywheel 1 along the periphery of the end faces of the flywheel.