Device and method for converting and storing electrical energy in the form of compressed air

Abstract

A device and method for converting electric energy into pneumatic energy and vice versa, which involves: pumping a liquid forming a liquid piston into a conversion chamber in which a quantity of air is trapped until the air reaches a pressure of a compressed air storage vessel; or churning a liquid by expanding the compressed air in a conversion chamber which is filled with a quantity of liquid, in which device or method the pumping or the churning of the water takes place in the same conversion chamber and consecutively in at least two pumping or churning stages, respectively, provided so as to operate in different pressure ranges. The present device and method can be used in particular in the field of converting and storing electric energy.

Claims

1. An energy conversion device for converting electrical energy into aeraulic energy and vice versa, and for storing this in the form of compressed air, the device comprising: dynamo-electric machines which have an electrical power link to a grid; dynamo-hydraulic machines mechanically linked to the dynamo-electric machines; at least one conversion chamber capable on the one hand of containing liquid pumped by the dynamo-hydraulic machines operating as pumps or receiving liquid intended to supply power to the dynamo-hydraulic machines operating as expanders, and on the other hand of containing air, such that the liquid present in the chamber forms a liquid piston for compression or expansion of air; a reservoir for storing compressed air at a storage pressure; sealable means for bidirectional aeraulic communication between the conversion chambers and the storage reservoir; and each dynamo-hydraulic machine is provided in order to operate within a respective pressure range at its high-pressure opening in order to carry out the pumping or the hydraulic expansion in stages inside each conversion chamber successively with several said dynamo-hydraulic machines up to or, respectively, from the desired storage pressure, the pressure range being narrower than the difference between the low pressure and the storage pressure, and in that distribution means are provided to connect each conversion chamber successively to at least two dynamo-hydraulic machines provided in order to operate within different pressure ranges.

2. The device according to claim 1, characterized in that at least one dynamo-hydraulic machine is provided in order to operate within a narrow pressure range substantially corresponding to the pressure of the storage reservoir.

3. The device according to claim 1, characterized in that the dynamo-hydraulic machines are mounted hydraulically in parallel with each other between a low-pressure liquid source and the at least one conversion chamber.

4. The device according to claim 1, characterized in that the storage reservoir is underwater and open in the lower part to receive water from the aquatic environment, enclosing a pocket of air at a pressure defined by the submersion depth of the reservoir.

5. The device according to claim 1, characterized in that, to carry out the conversion of the electrical energy into aeraulic energy and vice versa, several cycles of pumping or turbining respectively are provided, each cycle passing through the successive pressure ranges.

6. The device according to claim 1, characterized in that it comprises at least two conversion chambers in order to continuously maintain the energy flow in the dynamo-hydraulic machines.

7. The device according to claim 6, characterized in that the cycles of varying the level of liquid in the conversion chambers are phase shifted between conversion chambers, each dynamo-hydraulic machine being connected successively to several conversion chambers which have a time offset within the pressure range corresponding to this dynamo-hydraulic machine.

8. The device according to claim 1, characterized in that a pause is provided at the time when the at least one conversion chamber passes from one dynamo-hydraulic machine to another.

9. The device according to claim 1, characterized in that it comprises hydraulic readjustment means for readjusting the level of liquid to its initial state in order to carry out the pumping or the turbining in the at least one conversion chamber.

10. The device according to claim 1, characterized in that it comprises more conversion chambers than dynamo-hydraulic machines.

11. The device according to claim 1, characterized in that the dynamo-hydraulic machines are of the pump-turbine type capable of operating as a pump or, conversely, as a turbine.

12. The device according to claim 11, characterized in that the dynamo-hydraulic machines are pump-turbines of the Kaplan or Deriaz type.

13. The device according to claim 1, characterized in that the dynamo-electric machines are reversible motor-generators.

14. The device according to claim 1, characterized in that the bidirectional communication means are closed, except for during a final phase of compression and during an initial phase of expansion.

15. A method for converting electrical energy into aeraulic energy and vice versa, in which: a liquid is pumped, forming a liquid piston in a conversion chamber in which a quantity of air is trapped until this air reaches a pressure of a compressed air storage reservoir, then the compressed air is transferred from the conversion chamber to the storage reservoir; and/or a liquid is turbined by allowing compressed air to enter a conversion chamber containing a quantity of liquid such that the liquid is pushed through a turbine; the pumping or the turbining of the liquid is carried out successively at least in two stages of pumping or, respectively, turbining provided to take place within different pressure ranges.

16. The conversion method according to claim 15, characterized in that, during the turbining and after having allowed a quantity of compressed air to enter the conversion chamber still containing water, the inlet for compressed air originating from the storage reservoir is closed, and the compressed air present in the conversion chamber is expanded, while the remaining liquid is pushed back in order to be turbined.

17. The conversion method according to claim 15, characterized in that: the liquid is pumped by dynamo-hydraulic machines driven by at least one electric motor operating with electrical energy originating from an electricity grid; and the liquid is turbined by dynamo-hydraulic machines which drive an electric generator in order to generate electrical energy returned to the grid.

18. The conversion method according to claim 15, characterized in that: during the pumping and after having transferred the compressed air into the storage reservoir, the liquid contained in the conversion chamber is drained; and during the turbining and after having expanded the air contained in the conversion chamber, said chamber is filled with liquid again.

19. The conversion method according to claim 15, characterized in that the compression of the air and/or the expansion of the air in the at least one conversion chamber is quasi-isothermal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics of the invention will become apparent on examination of the detailed description of embodiments which are in no way limitative, and the attached drawings, in which:

(2) FIG. 1 is a diagrammatic view of the device for converting electrical energy into aeraulic energy;

(3) FIG. 2 is a connection diagram of the different elements of the energy conversion device according to a preferred embodiment of the invention;

(4) FIG. 3 is a schematic diagram of the operation of the device in the case of a conversion of electrical energy into aeraulic energy;

(5) FIG. 4 is a schematic diagram of the operation of the device in the case of a conversion of aeraulic energy into electrical energy;

(6) FIG. 5 is a table showing the temporal organization of the connection of the conversion chambers to the dynamo-hydraulic machines and to readjustment means, according to a preferred embodiment;

(7) FIG. 6 is a table showing the temporal organization of the connection of the conversion chambers to the dynamo-hydraulic machines and to the readjustment means, according to a second preferred embodiment; and

(8) FIG. 7 is a timing diagram for the hydraulic powers corresponding to the table of FIG. 6.

DETAILED DESCRIPTION.

(9) As these embodiments are in no way limitative, variants of the invention can be considered comprising only a selection of the characteristics described hereinafter, in isolation from the other characteristics described (even if this selection is isolated within a phrase containing other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the art. This selection comprises at least one, preferably functional, characteristic without structural details, and/or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

(10) First of all, with reference to FIGS. 1 and 2, a device for converting electrical energy into aeraulic energy and vice versa will be described. One of the preferred embodiments is a device for converting and storing electrical energy in the sea. It has the objective of absorbing the electrical surpluses in order to convert and store them in the form of aeraulic energy, then to convert the aeraulic energy back into electrical energy so as to return it to the grid when the grid demands it. Between these two active phases, the energy is stored in the form of compressed air in at least one underwater, typically subsea, storage reservoir 20 at a storage pressure.

(11) According to a preferred embodiment and with reference to FIG. 1, the subsea storage reservoir 20 is open in the lower part in order to communicate with the aquatic environment and, in the upper part, above the water level in the reservoir, encloses a pocket of compressed air at a desired pressure defined by the submersion depth of the reservoir. Preferably, the reservoir is constituted by several contiguous cells, firmly fixed together. Typically, the reservoir is placed on the bottom of the stretch of water, typically the seabed. A typical submersion depth is comprised between 70 and 200 m, preferably in the order of 100 m.

(12) With reference to FIG. 1, the energy conversion device is placed on a floating platform 10. The floating platform groups together the electromechanical, hydromechanical and hydropneumatic conversion systems as well as the associated electrical and electronic systems in order to allow the conversion of electrical energy into pneumatic (also called aeraulic) energy and vice versa. A summary of the main parts of these items of equipment is shown in FIG. 2. The platform is linked to the electricity grid by high-voltage undersea power cable 18. The position of the platform on the surface above the tanks 20 for storing the compressed air is maintained using a set of permanent anchorage lines 19.

(13) The energy conversion device comprises dynamo-electric machines MG1, MG2, MG3 provided in order to operate as a motor taking the electrical energy to be converted originating from the grid facility and to transform it into mechanical energy and/or to operate as a generator using mechanical energy produced from the aeraulic energy stored in the reservoir 20 in order to transform this mechanical energy into stored electrical energy to be returned to the grid. According to a preferred embodiment, the dynamo-electric machines MG1, MG2, MG3 are reversible motor-generators.

(14) The diagram of FIG. 2 is broken down into two parts: a part enclosed by dotted lines which corresponds to a purely hydraulic part and a part enclosed by dashed lines which corresponds to a purely aeraulic part.

(15) With reference to FIG. 2, the device comprises dynamo-hydraulic machines PT1, PT2 and PT3 for respectively pumping or turbining water. In the non-limitative but preferred example, the dynamo-hydraulic machines PT1, PT2 and PT3 are reversible machines capable of operating either as pumps, in particular turbopumps, or as expanders, in particular turbines. However, the invention can be applied using specific machines for the pumping and for the expansion respectively.

(16) The shaft of each dynamo-hydraulic machine PT1, PT2, PT3 is coupled to the shaft of the respective one of the dynamo-electric machines MG1, MG2, MG3, as shown by the reference numbers 21. The dynamo-hydraulic machines operating in pumping mode make it possible to convert the mechanical energy from the dynamo-electric machines operating as a motor into hydraulic energy by pumping a liquid drawn from a source such as the water of the surrounding aquatic environment and to push this liquid into a discharge opening 16 at an increased pressure by pumping. The dynamo-hydraulic machines operating as a hydraulic expander, in particular for turbining, make it possible to convert the hydraulic energy into mechanical energy provided at the shaft of the dynamo-electric machine operating as a generator, by turbining the liquid arriving under a certain pressure at the high pressure opening 16 and exiting the dynamo-hydraulic machine through its low-pressure opening 14 to return to a reservoir, in particular the surrounding aquatic environment.

(17) Preferably, the dynamo-hydraulic machines can be pump-turbines of the Kaplan or Deriaz type. These pump-turbines make it possible to vary their throughput at a constant speed, which makes it possible in particular to tend towards a stabilization of the power despite the variation in the pressure during the compression of the air, and thus to limit the variation in electrical power experienced by the electric machines.

(18) The device comprises conversion chambers CH1, CH2, CH3, CH4, CH5, CH6, each having a lower opening capable of being connected to the high-pressure opening 16 of the dynamo-hydraulic machines PT1, PT2, PT3 via a system of distribution gate valves 17, an upper opening capable of being connected via a gate valve 11 to the pipe 13 for bidirectional communication with the reservoir 20, and an upper opening capable of being connected to the open air via a gate valve 12 and a filling/draining pipe 22. In general, each conversion chamber contains air in the upper part and working liquid, typically water from the aquatic environment, in the lower part. The water present in the lower part of the chamber forms a liquid piston for compressing or expanding the air. The conversion chamber makes it possible to convert hydraulic energy into pneumatic energy and vice versa. The compressions and expansions of the air are carried out inside the conversion chambers. Preferably, enough conversion chambers are provided to continuously maintain the energy flow even while at least one conversion chamber is in the phase of filling with or draining its water. In particular, the device comprises more conversion chambers than dynamo-hydraulic machines PT1, PT2, PT3 capable of operating as a pump and more conversion chambers than dynamo-hydraulic machines PT1, PT2, PT3 capable of operating as hydraulic expanders. This feature makes it possible to maintain the activity of all the dynamo-hydraulic machines operating, depending on the case, as pumps or as turbines even during the filling or the draining of at least one conversion chamber. In the example there are twice as many conversion chambers as dynamo-hydraulic machines, thus more particularly six conversion chambers CH1-CH6 for three reversible dynamo-hydraulic machines PT1, PT2, PT3.

(19) The device also comprises hydraulic readjustment means P1, P2, P3 for readjusting the level of liquid to its initial state in order to carry out the pumping or the turbining in the conversion chambers. They take the liquid from the same source as the dynamo-hydraulic machines PT1, PT2, PT3, in the example the surrounding aquatic environment, and return the liquid to said source. Preferably, the readjustment means are pumps (P1, P2 and P3) which operate at a small pressure difference, just enough to counterbalance the head losses and any hydrostatic pressure differential resulting from the water level in the conversion chambers in relation to the level of the source. Preferably, the readjustment means are bidirectional pumps also capable of draining or accelerating the draining of the conversion chambers when they have to be filled with air prior to a compression cycle. The distribution means 17 are designed to also ensure the selective connection of each conversion chamber CH1-CH6 to a readjustment pump P1, P2 or P3.

(20) According to the invention, the dynamo-hydraulic means for pumping and for hydraulic expansion comprise machines PT1, PT2, PT3, which differ from each other by their respective pressure range measured in operation at their high-pressure opening 16, and which also differ by their maximum flow rate.

(21) There are at least two dynamo-hydraulic machines, one for moderate pressures and high flow rates at the start of pumping and at the end of turbining, the other for higher pressures and lower flow rates in the more advanced phase of pumping or in the earlier phase of turbining.

(22) In the example shown there are three different machines, namely: a machine PT1 for the start of the rise in pressure during the pumping and the end of the drop in pressure during the turbining, operating within a moderate pressure range and a high flow rate range; a machine PT2 for the end of the rise in pressure during the pumping and the start of the drop in pressure during the turbining, operating within a high pressure range and a moderate flow rate range; and a machine PT3 for pushing the compressed air into the reservoir 20 at the end of the storage cycle and for allowing the compressed air 20 to enter the conversion chamber at the start of the release cycle, operating within a narrow pressure range close to the pressure of the reservoir 20 and a corresponding narrow flow rate range.

(23) It is also provided to shift the phase of the respective cycles of the conversion chambers CH1-CH6 so that the dynamo-hydraulic machines and the associated dynamo-electric machines are continuously in active conversion operation with one or other of the conversion chambers being at that time within the corresponding pressure range.

(24) The operation of the device will now be explained and, at the same time, the description of the device and of the method will be completed.

(25) Storage Phase

(26) With reference to FIG. 3, the operation of the device during the conversion of the electrical energy into aeraulic energy in the conversion chamber CH1 will be described. FIG. 3 comprises three windows 3a, 3b, 3c respectively showing the characteristic stages of the storage phase for a conversion chamber. The windows 3a and 3b correspond to the productive stages and the window 3c corresponds to a non-productive stage, which could be called the resetting stage.

(27) At the start (window 3a), the conversion chamber CH1 is full of air at atmospheric pressure, the water is at a minimum level. The bidirectional communication gate valve 11 and the venting gate valve 12 are closed, such that the upper part of the conversion chamber, occupied by the air, is hermetically sealed. The electrical energy to be stored supplies power to the dynamo-electric machine MG1 coupled to the dynamo-hydraulic machine PT1 which pumps the water into the conversion chamber CH1 under a moderate pressure.

(28) At a certain intermediate stage of filling the conversion chamber CH1 with water, substantially corresponding to the maximum pressure for which the dynamo-hydraulic machine PT1 is provided, the distribution means 17 interrupt the link of the conversion chamber CH1 to the dynamo-hydraulic machine PT1 and establish the connection of the conversion chamber to the high-pressure opening 16 of the dynamo-hydraulic machine PT2 coupled to the dynamo-electric machine MG2. The electrical energy to be stored supplies power to the dynamo-electric machine MG2 coupled to the dynamo-hydraulic machine PT2 which pumps the water into the conversion chamber CH1 under an increased pressure.

(29) When another intermediate stage of filling the conversion chamber CH1 with water is then reached, substantially corresponding to the maximum pressure for which the dynamo-hydraulic machine PT2 is provided, the distribution means 17 interrupt the link of the conversion chamber CH1 to the dynamo-hydraulic machine PT2 and establish the connection of the conversion chamber to the high-pressure opening 16 of the dynamo-hydraulic machine PT3 coupled to the dynamo-electric machine MG3. At the same time the gate valve 11 opens (window 3b of FIG. 3). The electrical energy to be stored supplies power to the dynamo-electric machine MG3 coupled to the dynamo-hydraulic machine PT3 which pumps the water into the conversion chamber CH1 under the pressure of the storage reservoir 20 until substantially all of the compressed air present in the conversion chamber CH1 has been pushed into the reservoir 20.

(30) Owing to the dynamo-hydraulic means, the liquid, typically the water from the aquatic environment, is pumped in order to form a liquid piston in the conversion chamber in which a quantity of air is trapped. With reference to the windows 3a and 3b, the water/air interface moves from the bottom to the top of the conversion chamber, forming a piston compressing the air trapped in the conversion chamber until this air reaches the pressure prevailing in the storage reservoir.

(31) The liquid piston has the advantage of limiting the energy losses due to friction compared with a traditional rigid piston compressor. In addition, the use of the liquid piston makes it possible to limit the heat losses, i.e. to limit the heating due to the compression, which is quasi-isothermal because of this. In order to strengthen this feature of being quasi-isothermal, the conversion chambers preferably contain heat conductors which thermally link the air and the liquid in the conversion chambers. These heat conductors are, for example, a bundle of vertical metal tubes, open at both ends, extending over substantially the whole height of each chamber. These conductors expel into the water the heat of the compression of the air in the compression cycle, which reduces the work needed for the compression, and reheat the air with heat originating from the water in the expansion cycle, which increases the work provided by the expansion of the air.

(32) For example, for storage of air in a storage reservoir placed at a depth of 100 meters, therefore subjected to an absolute hydrostatic pressure of about 1.1 MPa, the dynamo-hydraulic machine PT1 operates within the low pressure range (atmospheric pressure to 0.3 or 0.4 MPa), the dynamo-hydraulic machine PT2 operates within the intermediate pressure range (from 0.3 to 0.4 MPa up to 1.1 MPa) and the dynamo-hydraulic machine PT3 operates within a narrow range around 1.1 MPa.

(33) Then, with reference to the window 3c of FIG. 3, a non-productive time makes it possible to drain the water contained in the conversion chamber CH1 in order to be able to start a storage cycle in this chamber again. The bidirectional communication gate valve 11 is then closed and the venting gate valve 12 is opened. The hydraulic readjustment means are actuated in order to drain the conversion chamber until it is emptied of almost all of its water.

(34) In this way, the conversion device carries out several pumping cycles, each cycle passing through the successive pressure ranges. The duration of a cycle is provided between 30 seconds and 5 minutes.

(35) Release phase

(36) With reference to FIG. 4, the operation of the device during the conversion of the pneumatic energy into electrical energy will be described. FIG. 4 comprises three windows 4a, 4b, 4c respectively showing the characteristic stages of the release phase for a conversion chamber CH1. The windows 4a and 4b correspond to the productive stages and the window 4c corresponds to a non-productive or resetting stage.

(37) At the start (window 4a), the conversion chamber is full of water. The venting means 12 are closed. For the release, the bidirectional communication gate valve 11 is opened so that the compressed air present in the storage reservoir 20 is partially transferred to the conversion chamber CH1 and a part of the water contained in the conversion chamber is pushed through the high-pressure dynamo-hydraulic machine PT3 operating as a turbine driving the dynamo-electric machine MG3 operating as a generator.

(38) When the quantity of air present in the conversion chamber CH1 is such that this air is capable of occupying the whole volume of the conversion chamber if expanded to atmospheric pressure, the bidirectional communication gate valve 11 is closed (window 4b of FIG. 4). At the same time, the distribution means 17 interrupt the link of the conversion chamber CH1 with the dynamo-hydraulic machine PT3 and establish the link of the conversion chamber with the dynamo-hydraulic machine PT2. The air expands quasi-isothermally, continuing to push the water but now through the dynamo-hydraulic machine PT2, dedicated to medium pressures, then, after a new switchover operated by the distribution means 17, through the dynamo-hydraulic machine PT1 dedicated to moderate pressures.

(39) It is provided to act on the dynamo-hydraulic machines in order to allow dynamic variation in certain physical variables (such as the rotational speed of the machine, the angle of the blades, the position of the distributor, etc.). It is thus possible to regulate the flow rate of the dynamo-hydraulic machines in order that the associated dynamo-electric machine operates at constant power. This regulation has the advantage of improving the power stability of the device and therefore of limiting the variations in the electrical power exchanged between the device and the electricity distribution grid.

(40) Then, with reference to the window 4c of FIG. 4, a non-productive or resetting time makes it possible to fill the conversion chamber with water in order to start a release cycle again. The bidirectional communication gate valve 11 is then closed and the venting gate valve 12 is opened in order to allow the addition of water. The hydraulic readjustment means P1 are connected to the conversion chamber CH1 via the distribution means 17 and actuated in order to fill the conversion chamber with water.

(41) In this way, the conversion device carries out several turbining cycles, each cycle passing through the successive pressure ranges. The duration of a cycle is provided between 30 seconds and 5 minutes.

(42) Temporal Organization of the Connection of the Dynamo-Hydraulic Machines with the Conversion Chambers

(43) With reference to FIGS. 5 to 7, the pumping or turbining cycles of each conversion chamber are temporally offset such that at each given moment each dynamo-hydraulic machine is connected to a conversion chamber that is in the corresponding pumping or turbining phase. In other words, in the example with three pressure ranges, at all times there are three conversion chambers of which one is within the moderate pressure range, another is within the intermediate pressure range and the third is within the upper pressure range, this latter communicating with the storage reservoir 20.

(44) The table of FIG. 5 shows an example of the temporal organization of the connection of the dynamo-hydraulic machines with the conversion chambers over a cycle, in the example of three pumps and/or turbines and six conversion chambers. Each conversion chamber (CH1, CH2, CH3, CH4, CH5, CH6) is linked successively to the three dynamo-hydraulic machines PT1, PT2, PT3 (in the example of pumping) before being linked to one of the readjustment means P1, P2 or P3. A complete cycle (twice the duration of a compression or of an expansion for one chamber) is shown there, of which each column represents a duration of of the cycle. In this organization, each pump and/or turbine carries out its work synchronously on the chambers. The switching thereof from one chamber to another is also carried out at the same time. This organization contributes to limiting the variations in electrical power exchanged between the conversion device and the distribution grid. Moreover, this makes it possible to limit the energy losses of the thermal type.

(45) According to another embodiment and with reference to FIGS. 6 and 7, it is provided to introduce a shift of phase between the moments of passing from one pump-turbine to another in a chamber, such that the hydraulic switchovers, corresponding to power outages, are no longer simultaneous. The table of FIG. 6 shows an example of a 1/18 shift of phase of the duration of a cycle between each pump and/or turbine. The substantially continuous operation of the pumps and/or turbines is retained, but there is now a pause for each chamber and for each passage from one pump and/or turbine to the other in each conversion chamber. Having preserved the same number of conversion chambers, the time available for returning the chamber to its initial state (by P1, P2 or P3) is reduced by the duration of the pauses produced.

(46) In this way, the power fluctuations are even further limited. With reference to FIG. 7, an example of a timing diagram for the compression hydraulic powers envisaged for a device with three pumps, the last of which works only at constant pressure and power, is shown. There is a 1/12 shift of phase of the cycle between the pump 1 and the pumps 2 and 3. The pumps 1 and 2 make it possible to reach the storage pressure and the pump 3 makes it possible to push the compressed air to the reservoir. Shifting the phase of the switching of the pumps makes it possible to limit the fluctuations in the resulting total hydraulic power and therefore makes it possible to limit the fluctuations in the total electrical power.

(47) Of course, the invention is not limited to the examples which have just been described, and numerous adjustments can be made to these examples without exceeding the scope of the invention.

(48) Of course, the different features, forms, variants and embodiments of the invention can be combined together in various combinations unless they are incompatible with each other or mutually exclusive. In particular, all of the variants and embodiments described previously can be combined together.

(49) Embodiment examples have been shown with three dynamo-hydraulic machines PT1, PT2 and PT3, which can be three separate pumps and three separate turbines or can be three pump-turbines. The device can comprise a number of pumps and/or turbines other than three. According to other embodiments, it is provided that there are different numbers of pumps and expanders.

(50) One or more additional systems, for example electrical capacitors, can be added in order to smooth out the electrical power consumed (or provided for the release).

(51) Similarly, the number of conversion chambers can be different from that indicated in the example. However, it is desirable to have at least one more of them than the number of pumps or turbines. The number of chambers is not necessarily a multiple of the number of pumps and/or turbines.

(52) With respect to the readjustment means, the use of one or more other technologies, combined or not, can be envisaged to carry out the filling or the draining of the conversion chambers. There may be mentioned in particular the use of hydraulic ejectors, the natural use of gravity or the recovery of potential energy present in another chamber.

(53) The bidirectional communication means can comprise separate paths for the air going to the storage reservoir 20 and coming from the storage reservoir 20, optionally with each one having its gate valve instead of the common gate valve 11.