A Multistage Hydro-Pneumatic Motor-Compressor

FIELD OF THE INVENTION

This invention concerns methods of converting the potential energy of pressurized gas into mechanical work and vice-versa, as well as a reversible, self-controlled hydro- pneumatic engine, which directly converts the pressure energy of compressed gas, particularly air, into mechanical work and vice-versa, by performing successive high efficiency compression/expansion.

BACKGROUND OF THE INVENTION

A list of the references quoted in this section is given at the end of the Description. This invention is related to the use of compressed air for power transmission like in the case of compressed-air-powered cars, or energy storage for example to circumvent the intermittency of some renewable energy sources such as solar or wind sources.

The potential energy of compressed air is generally exploited by firstly converting it into mechanical work. Two main categories of energy conversion systems have been proposed for that purpose: pure pneumatic conversion systems where the only active fluid is air and hydro -pneumatic conversion systems that use at least one additional liquid (oil, water) as active fluid.

Pneumatic conversion systems

Pneumatic conversion was the first (and still is the only commercially available) conversion solution used to exploit compressed air for the purpose of energy transmission. It consists, for low and medium power ranges or high compression ratio, in using mainly positive displacement (or volumetric) air machines to produce compressed air and later withdraw energy from it. In these machines, the variations of the working fluid's volume in a work-chamber produce equivalent displacements of the mechanical member, transmitting thus the energy and vice versa. The dynamic effect of the fluid is therefore of minor importance, unlike in kinetic (or turbo) machines where the kinetic energy of the working fluid is transformed into mechanical motion and vice versa. There are two main families of volumetric machines:

Rotary machines like lobe, vane, and screw machines.

Reciprocating machines like diaphragm and piston machines. In most power and pressure ratio ranges, the piston type is commonly used because of its higher efficiency and pressure ratio. The principle of such a conversion system is shown in Figure 1. Since it is difficult to realise an isothermal process in these work-chambers, the compression/expansion process is subdivided into several stages and heat exchangers are inserted in between as can be seen on Figure 1. Thus the complete cycle is more or less close to an isothermal cycle depending on the performances of the heat exchangers. This principle is as old as the first application of compressed air in propulsion in the 1800s and it is gaining nowadays more interest and improvements with the new developments in compressed-air-powered cars [I]. However, given the difficulties to implement a good heat exchange in the compression/expansion chambers, and the important leakage and friction related to the gaseous nature of air, the pressure ratings and conversion efficiency of this conversion system remain low and make it inefficient for most energy applications. Hydro-pneumatic conversion

The use of hydraulic machines to circumvent the drawbacks of pure air machines has been investigated, as they suffer less from the above problems and therefore exhibit very high conversion efficiencies. One of the main challenges in using hydraulic systems to compress/expand air is the liquid-to-air interface. A first solution has been proposed by Cyphelly & al. whose principle is depicted in Figure 2 and described in [2], [3] under the acronym "BOP: Batteries with Oil-hydraulics and Pneumatics". In this system air is compressed/expanded in alternating Liquid-Piston Work-chambers where a "Thin Plate Heat Exchanger" is integrated. During compression, the thin plates transfer the heat from gas top part to the liquid bottom part and the other way round during expansion. However, good heat exchange will require a high density of plate, which is not easy to realise and may also cause, depending on the properties of the used liquid, some viscosity problems.

Recently, another solution has been patented by Rufer & al. with as main original proposition work-chambers where the compression is performed by injecting the liquid in the form of a "shower" in the chamber, allowing a fast and effective abortion of the

compression heat [4]. This solution however requires an external liquid circulating pump to reheat the air during expansion.

In both cases, there is a concern about diffusion of the air into the liquid due to the direct contact between the two fluids. In addition, these hydro -pneumatic systems are somewhat bulky as they are assemblages of several distinct components and machines. Moreover, these split topologies require many ancillary devices for the command and control of the system's operation.

It would be desirable to provide a hydro -pneumatic conversion system with a simple and efficient integrated heat exchanger that can effectively operate both during compression and expansion. It would be also desirable to have a more compact, flexible and scalable solution that can be easily adapted to stationary as well as mobile applications. The present invention proposes original solutions to achieve these objectives.

SUMMARY OF THE INVENTION

The invention concerns a method for converting the potential energy of pressurized gas, particularly air, into mechanical work by performing a set of transformations as set out in claim 1 ; a method for efficiently compressing a gas, particularly air, from the mechanical work of a rotating shaft by performing a set of transformations as set out in claim 4; and a multistage hydro -pneumatic system for converting the potential energy of a pressurized gas, particularly air, into mechanical work when rotating a shaft in one direction, and for producing compressed gas from the mechanical work of the rotating shaft, when rotating the shaft in the reverse direction, by performing successive expansion/compression of the said gas, as set out in claim 10. Further aspects of the invention are set out in the dependent claims.

In one embodiment, the engine according to the invention is made of several hydro- pneumatic stages mounted, on the bottom side, on a common crankshaft and connected, on the top side, in series in an air circuit to perform a highly efficient multistage compression or expansion process. Each stage consists of three main parts:

A special liquid-piston unit that converts the pressure energy into hydraulic power by performing isothermal compression-expansion processes thanks to an integrated, "Tubular Heat Exchanger", which allows the in and out-flowing active liquid to

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absorb/provide the compression/expansion heat. The active liquid thus plays the roles of power transmission and heat transport.

A forced-air external heat exchanger that maintains the active liquid at ambient temperature by exchanging its heat with the surroundings. The fan of this heat exchanger is directly driven by the crankshaft through a belt or chain.

A single cylinder, valve-free hydraulic motor-pump (SCMP) that converts the hydraulic power into mechanical work and vice- versa.

Several air and liquid valves operated by a common camshaft are used to control the compression/expansion and transfer of the two fluids from one enclosure to the other. Several pressure-controlled air valves are also used to operate the system in a variable stage configuration. The different parts are arranged in such a way to form a single compact embodiment and to ease the automatic control of the valve and the drive of the external heat exchanger's fan.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described by way of example with reference to the accompanying drawings which also show prior arrangements. In the drawings:

Figure 1 illustrates a known pneumatic storage with a multistage conversion system consisting of a piston air machine.

Figure 2 illustrates the principle of BOP-B, a known Hydro -pneumatic conversion system with "Thin Plates Heat Exchanger" integrated in the Liquid-Piston Work-chambers (Courtesy of Cyphelly & AL).

Figure 3 illustrates the principle of a Hydro -pneumatic conversion system with a "Shower Heat Exchanger integrated in the Compression/Expansion" Work-chambers. (Courtesy of Rufer & AL). Figure 4a (left) and Figure 4b (right) illustrate the principle of an inventive multistage hydro -pneumatic motor/compressor with direct "Ambient Air/ Active Liquid" External Heat Exchange.

Figure 5 a (left) and Figure 5b (right) illustrate the principle of a variation of the inventive multistage hydro -pneumatic motor/compressor with indirect "Ambient Air/ Active Liquid" External Heat Exchange.

Figure 6 illustrates a possible layout of the Compression/Expansion Unit. Figure 7 illustrates in cut-away perspective view the layout of the Tubular Heat Exchanger; for clarity, only part of the total number of tubes is represented.

Figure 8 illustrates in cut-away perspective and plan views the design of the heat exchanger tube's holding plate.

Figure 9 illustrates in cut-away perspective view the design of mobile fluid separation plate.

Figure 10 shows in two perspective views a "Single-in-line" vertical topology of the Hydro -pneumatic Motor-Compressor according to the invention.

Figure 11 sketches a "V" topology of the Hydro -pneumatic Motor-Compressor according to the invention. DETAILED DESCRIPTION OF THE INVENTION

Constitution

The machine according to the invention is made of several hydro -pneumatic stages (HPS) mounted, on one side, on a common crankshaft (1) and connected, on the other side, in series in the air circuit. A simplified diagram is presented in Figure 4a and Figure 4b for the case of a 4-stage machine where the hydro -pneumatic stages are designated HPS-I to HPS-4. Each stage consists of three main parts: A single cylinder hydraulic motor-pump (SCMP), an external heat exchanger (EHE) and a special liquid-piston compression- expansion unit (CEU).

The Compression-Expansion Unit The compression-expansion unit (CEU) is designed to perform fast and almost isothermal processes. Its simplified diagram is represented in Figure 6. It is mainly made of a vertical compression/expansion enclosure (12) that integrates a special tubular heat exchanger (THE). Its inner cavity is accessible through four valve-controlled ports: two air-ports (17x1) and (17x2) and two liquid-ports (18x1) and (18x2). Each valve plays either the role of intake valve or that of exhaust valve depending on the operation mode (i.e.

compression or expansion). The command of these valves is performed by the common camshaft (14), which is driven by the crankshaft (1) through a transmission belt or chain (7) and wheels (6) and (13).

The Tubular Heat Exchanger (THE) is the key element to achieve the isothermal process. Its possible layout is illustrated in the perspective drawing of Figure 7. It is made of a head distribution plate (19) where many heat exchange tubes (20) are fixed on their top end. These very thin tubes are uniformly distributed all over the plate surface and held together at their bottom end by a thin porous holding plate (21) that allows the flow of fluid between tubes. Such a holding plate is illustrated in the perspective and plan views of Figure 8.

The heat exchanger is mounted so as to provide a liquid distribution chamber (22) under the enclosure's top cap. The liquid ports are disposed and configured so as to ease a uniform distribution of liquid inside this chamber. The head distribution plate (19) provides an isolating central channel through which air manifolds run into the compression-expansion chamber.

In the ideal case, there is a direct contact between the air and the liquid in the compression-expansion chamber as illustrated in Figure 6a. In that case, it is possible to exploit almost the entire volume of the compression-expansion chamber including the tubes' inner volume during the intake stroke. This could be achieved, for instance, by making the central tubes as long as possible to absorb all the liquid at the bottom of the chamber and thus allowing air to flow inside the tubes.

Depending on the nature of the liquid and the pressure level, important diffusion of air into liquid could occur, which could cause an improper operation or premature failure of all or part of the system. To avoid such a situation, the two fluids can be separated with a mobile separating thin plate (23) as illustrated in Figure 6b. This plate (23) is almost a duplication of the head distribution plate (19) except for the central channel as can be seen on the perspective illustration of Figure 9. It must be precisely manufactured and well guided to avoid important friction with the tubes and the inner face of the compression- expansion chamber. With the fluid separation plate, the useful volume is limited to the space between the tubes; but in that case the CEU unit can be reverse upside down so as

to have the valves on the bottom side, which will shorten the hydraulic connections with the external heat exchanger.

The single cylinder hydraulic motor/pump

This device is a simplified hydraulic machine that transforms the alternating in/out flow of the active liquid into the rotational motion of the crankshaft and vice- versa. It is made of a liquid cylinder (4a - 4d) inside which a piston (3a - 3d) translates. The translational motion of the piston is transformed into a rotational motion thanks to a classical Rod (2a - 2d)-Crankshaft (1) association. Other motion conversion techniques such as axial piston or bent axis systems can be however envisaged. In any case, one important mechanical requirement is the parallelism of the different rotating axes of the engine, which will ease the motion transmission from the main shaft (1), for example by means of a simple driving belt (7). The access to the cylinder is made through two uncontrolled ports (5al - 5dl) and (5a2 - 5d2), each of which is connected to a heat exchange circuit. When the liquid flows in through one port, it flows out through the other port thanks to the control of the liquid valves (16al - 16dl) and (16a2 - 16d2). Thus, the liquid always flows in closed-circuit.

The external heat exchanger

The external heat exchanger (EHE) is a forced-air radiator that assures a fast and intensive heat exchange between the active liquid and the ambient air. This heat exchange can be performed either directly as illustrated in Figures 4a and 4b or indirectly as illustrated in Figures 5a and 5b. In the direct case, the EHE is made of several high-pressure hydraulic circuits realised inside a kind of "honeycomb" channel (11) where a fan (9) mounted at one extremity blows the ambient air. The fan (9) is driven by the crankshaft (1) through the belt or chain (7) and wheel (8). In the air channel (11), there is one pair of hydraulic circuits (lOal - IOdl) and (10a2 - 10d2) per hydro-pneumatic stage (HPS). Each pair of circuits is connected on its bottom side to uncontrolled ports (5al,5a2 - 5dl,5d2) of the corresponding liquid cylinder (4a - 4d), and on its top side to the liquid-ports of the corresponding compression-expansion unit (CEU). The volume of each hydraulic circuit (10al,10a2 - 10dl,10d2) of the heat exchanger is at least equal to that of the corresponding liquid cylinder (4a - 4d).

An alternative configuration of the EHE is proposed in Figures 5a and 5b, which avoids the higher difficulty to realise a high pressure compact air heat exchanger. In this configuration, the heat is first transferred from the active liquid to low pressure water thanks to an embedded Liquid/Liquid heat exchanger (10)-(11). The low pressure water is thereafter driven by small pump (32) into a separate classical radiator (33), where heat exchange with the ambient air is takes place. The pump (32) and the radiator's fan are driven like in the previous case, by the crankshaft (1) through the belt or chain (7) and wheels (8) and (31). This split configuration of the External Heat Exchanger (EHE), which is similar to that of a car's Engine, eases the mono-block design and construction of the motor-compressor while allowing to increase the surface of the separated forced-air radiator and thus to improve its efficacy. Principle of operation General Principle The presented machine is a 2-stroke engine: An intake stroke and a compression (or expansion during motor operation) stroke which is combined with the transfer or exhaust process. In a multistage operation, two consecutive stages always operate in opposite stroke. For example, in the case of a 4-stage machine as illustrated in Figures 4a and 4b, the stages A and C perform an intake stroke when stages B and D perform a compression (or expansion) stroke and vice-versa. Consequently, some stage is always active, which yields a more regular mechanical torque.

The number of stages depends on the desired pressure level. The higher this number, the lower the stage's compression ratio will be and the higher the thermodynamic efficiency will be also. An even number of stages will however ensure a more constant mechanical torque over one complete turn of the crankshaft, as the number of active stages will be equal during the two strokes.

The hydraulic liquid in each stage plays two important roles:

It is the Power Transmission Link between the compression-expansion chamber and the single cylinder hydraulic machine.

It is also the Heat Carrier Medium between the same compression-expansion chamber and the ambient air flowing through the external heat exchanger's channel (11).

Because of the multistage compression process, the required liquid volume is not equal for all the stages. Attention must be paid that this difference does not create unbalanced mechanical constraints on the crankshaft. For that purpose, the forces applied on all the pistons, which are the product of each chamber's pressure and the corresponding piston's surface, must be equal for all the stages. In case of equal compression ratios (CR) for all the stages, the following pressure relation can be written:

If isothermal compression processes are assumed as targeted, then relation (I) can be written:

V V V V V Cλ = -^ = -^ = ^■ ?-*- = -^ = .... = -^ (H)

where V _x is the volume of the liquid cylinder (4x) ((4a - 4d) as labelled in Figure 4). For normal multi-cylinder hydraulic machine, all the pistons would have the same displacement; therefore the volume ratios in (II) can be replaced by the following radius ratios:

The diameter of the liquid cylinder (x) is thus related to that of the consecutive lower pressure cylinder (4x-l) as follows:

^{U ~ •} (IV)

CR

It should be noted that this relation is not mandatory for the compression-expansion units (CEU) as they can be made in different lengths and even shapes. However, the useful volume of their compression-expansion chamber should fulfil relation (II).

The mechanical force Fb applied to piston (3b) is given by:

^F b = Pb ^S _b (V)

where Sb denotes the surface of piston (3b). Similarly, the mechanical force Fd applied to the synchronous piston (3d) is given by:

^F _d = P _d -S _d = = p _b .S _b = F _{b (VI)}

Where Sd denotes the surface of piston (3d). This relation shows that in case of equal compression ratio and piston displacement for all the stages, the crankshaft (1) is subject to balanced mechanical efforts.

Compressor operation mode

The compressor operation is described on the basis of the schematic diagram of Figures 4a and 4b. The 3-way-2-position distribution valves (24xx) (i.e. 24al;24bl,24b2; 24cl,24c2;24dl,24d2) are kept in the position illustrated on those Figures. The compression process consists in a series of intake - compression/transfer cycles over the sequential stages, from the atmospheric pressure (p ₀ ) to the maximum admissible pressure ipd) in the allowed in the storage tank (28).

Each cycle lasts one turn of the crankshaft (1). As stated above, two consecutive stages always operate in opposite phases. In addition, the compression and transfer processes are performed simultaneously to reduce the negative effects of dead volumes; thus the air- exhaust and liquid- intake valves of a given stage are operated in phase with the air- intake and liquid-exhaust valves of the following, higher pressure level, stage.

The initial point is defined as that where the piston (3a) of the first stage (HPSl) is at the top dead centre (TDC) and is starting an intake stroke by moving downward. The air- intake valve (15al) and the liquid-exhaust valve (16al) are opened simultaneously by the cams of the camshaft (14). At the same time, pistons (3b) and (3d) of stages (HPS2) and (HPS4) are at the bottom dead centre, ready to start a compression stroke. The air-exhaust valves (15b2) and (15d2) and the liquid- intake valves (16b2) and (16d2) are therefore opened. Stage (HPS3) is in the same state as stage (HPSl); the air-intake valve (15cl) and the liquid-exhaust valve (16cl) are opened and the compression-expansion chamber of the enclosure (12c) is communicating with that of enclosure/cylinder (12b).

As piston (3a) moves downward, fresh air is admitted into the compression-expansion chamber of cylinder (12a) through silencer (25) and filter (26). At the same time, the

liquid which was heated by the previous compression stroke is taken out of the enclosure (12a) and transferred into the heat exchange circuit (lOal), in place of the liquid which had been cooling there during the previous stroke and which is now transferred into the liquid cylinder (4a). Simultaneously to the descent of piston (3a), piston (3b) rises and expels its liquid content into the heat exchange circuit (10b2) through port (5b2), where it replaces an equal amount of cooled liquid which is in turn injected into the enclosure (12b) to compress the enclosed air while transferring it into the higher pressure stage's enclosure (12c). The pressurized air transferred into enclosure (12c) causes its liquid contents to flow out through the liquid exhaust valve (16cl). This out-flowing liquid enters the heat exchange circuit (lOcl) and transfers its contents into the liquid cylinder (4c) where piston (3 c) is being driven downward. As stated before, stage (HPS4) operates in phase with stage (HPS2); piston (3d) moves upward and injects the liquid contents of the heat exchange circuit (10d2) into the compression chamber of cylinder (12d), to compress the contained air. As the air exhaust valve (15d2) is open, when the pressure inside cylinder (12d) is slightly higher than that inside the tank (28), a check valve (27) opens and the compressed air is transferred into the tank (28).

During the compression stroke, the cool compressing liquid that flows downward into the tubular heat exchanger and enters the compression chamber is firstly in contact (through the very thin tubes) with the air which is being heated by compression and pushed upward. These opposite flows of the two fluids allow the liquid to quickly absorb the compression heat and thus maintain the air at almost constant temperature. The liquid is further cooled down in the external heat exchanger. This is the key for high compression efficiency.

The intake stroke of stages (HPSl) and (HPS3) is ended when their respective pistons reach the bottom dead centre, as shown in Figures 4a and 4b. At the same time pistons

(3b) and (3d) reach the top dead centre to end the compression/transfer stroke of stages

(HPS2) and (HPS4). The phases are now inverted, i.e. stages (HPSl) and (HPS3) start a compression stroke while stages (HPS2) and (HPS4) will start an intake stroke. All the valves previously opened are closed and those previously closed are opened. As pistons (3a) and (3c) translate upward, the air contents of cylinder (12a) and (12c) are compressed and transferred into cylinders (12b) and (12d) respectively, while pistons (3b) and (3d)

descend. When pistons (3 a) and (3 c) reach their top dead centres, the strokes are ended, the system is back to the initial position and a new cycle can restart.

Motor operation mode

The engine operates in motor mode in a similar way to the compressor mode, but with opposite air-flow and rotation directions. The motor operation process consists in a series of intake - expansion/transfer cycles over the sequential stages, from the tank pressure ipd) to the atmospheric pressure (p ₀ ). In this mode, the check valve (27) is opened through the pilot control port to allow the air outflow from the tank.

Each cycle lasts one turn of the crankshaft (1) and two consecutive stages always operate in oppose phases as well. The expansion and transfer processes are performed simultaneously like in the compressor mode; thus the air-exhaust and liquid-intake valves of a given stage are operated in phase with the air-intake and liquid-exhaust valves of the following, lower pressure level, stage.

During the expansion stroke, the heat exchange process is performed in a similar way as for compression; the hot out flowing liquid reheats the inflowing and expanding air through the heat exchange tubes, so as to maintain it at almost constant temperature. The liquid will be further reheated in the external heat exchanger. This is the key for high expansion efficiency.

Variable stage configuration The multistage architecture allows reaching high pressure levels with high efficiency; however the operation is optimal only if all the stages are used at their optimal compression ratio, but this would hardly be the case because the tank pressure will vary both during compression and expansion. In the case of an equal compression ratio (CR) for all the stages, the sequential use of "«" stages, during compression as well as expansion, will be efficient only if the tank pressure "/? _< /' is greater than "(n-l)p ₀ ". Otherwise, the "n ^th " stage will simply serve as a transfer stage and air will expand when entering the tank.

It can be therefore interesting to adapt the number of sequential stages to the tank pressure. This is the role of the distribution air valve (24xx), i.e. (24al;24bl,24b2;24cl,24c2;24dl,24d2) represented in Figures 4a and 4b. By

appropriately controlling these valves with regards to the tank pressure, it is possible to configure the engine as follow:

-> 4-channel 1-stage when the tank pressure is less than 4bar. All the stages are in parallel on the air circuit, which results in a higher air flow rate. -^ 2-channel 2-stage when the tank pressure is between 4 and lobar.

-^ 1 -channel 3-stage when the tank pressure is between 16 and 64bar: In this case the 4 ^th stage is deactivated by connecting the two air ports to the intake line. As mentioned before, such an odd number of stages might produce irregular torque on the crankshaft as there will be two active stages (HPSl and HPS3) during one half turn and only one stage (HPS2) during the other half turn. Using a 4-stage configuration within this pressure range won't solve the problem because the 4 ^th stage will simply serve as a transfer stage as explained before. The 2-stage configuration will be preferred up to the threshold pressure of 64bar.

-> 1-channel 4-stage when the tank pressure is greater than 64bar. This is the normal configuration of the machine, suited for high pressure operation.

The variable configuration allows an optimal utilization of the entire contents of the compressed air tank, or an optimal filling of an empty tank. But in practice, for a given application there is a minimum pressure under which the produced power becomes useless. Main advantages

The proposed machine provides many technological improvements compared to the state- of-the art Pneumatic-to-Mechanical energy conversion systems; a few of them are listed below:

• Simple and efficient heat exchanger integrated in the compression/expression chamber. This is achieved thanks to the use of the very thin heat exchange tubes that allow:

• A better isolation of the air and active liquid, which restricts the risk of diffusion only on the horizontal separation surface in case of a non- isolated interface.

• A single way for the inflow and outflow of the liquid inside the compression/expansion chamber, which avoids the use of an external recycling

pump during expansion as proposed by Rufer & al. in PCT/IB 2007/051736, and a permanent heat exchange between the two fluids.

• A higher heat capacity related to the metallic tubes that improves the quality of heat exchange. • Simple and compact power conversion topology embedded in a single embodiment. The simplest topology for the multistage engine is the vertical, in-line configuration as illustrated in perspective in Figure 10. Such a topology is simply scalable and the overall size of the engine will depend on its power range. A "V" configuration like in some Internal Combustion Engines (ICE) can be also envisaged as illustrated Figure 11. This configuration will improve the power density of the system and make it suited for mobile applications.

• Simple and automatic control of the valves. Part of the mechanical energy provided to or produced by the crankshaft is used to synchronously operate the air and liquid valves like in ICE. This possibility drastically reduces the number of the needed ancillary devices such as electro-valves, and provides the system with more autonomy.

Main Limitations

The efficiency of the machine according to the invention strongly depends on the quality of heat transfer between the air and the liquid during the compression/expansion process inside the compression/expansion chamber. Given the thermal time constant of various elements involved, a high quality heat transfer will require at least a certain minimum amount of time.

In the presented configuration, the compression/expansion process would last only for a half-turn of the crankshaft. For a rotational speed of 3000rpm for example, this process will last only for 10ms, which might be quite short depending on the design of the Tubular Heat Exchanger. As a consequence, the optimal speeds of the presented machine lie in the lower speed range, which might not fit with the optimal speed range of some electrical machines or applications. This limitation can however be circumvented by using a speed adaptor, such as a mechanical gear box with high speed ratio.

Main Fields of Application

This invention is mainly intended to the production and exploitation of the potential energy of compressed air, for the purpose of power transmission and energy storage.

A potential stationary application would be Pneumatic Energy Storage (or Fuel- free Compressed Air Energy Storage) for renewable energy sources support. In association with an electrical machine and power electronic converters it can be use to circumvent the intermittency of some renewable energy sources such as solar or wind sources.

This invention can be also easily used for mobile applications. With the proposed configurations, compact high efficiency hydro -pneumatic engines can be built and the available mechanical energy directly exploited for car propulsion for instance.

Finally, the proposed engine can be used like any classical compressor to condition any gas under high pressure, but with high efficiency. Depending on the application, a gas treatment (or purifying) device would be necessary. References

[1] Sylvain Lemofouet; Investigation and Optimisation of Hybrid Electricity Storage Systems Based on Compressed Air and Supercapacitors; PhD Thesis number 3628 available on: http://library.epfl.ch/theses/?nr=3628

[2] I. Cyphelly, A, Rufer, P. Bruckmann, W. Menhardt, A. Reller; Usage of Compressed Air Storage System" DIS project 240050, Swiss Federal Office of Energy, May 2004 www, electricity-research . ch/

[3] I. Cyphelly; Pneumo -Hydraulic Converter for Energy Storage; US Patent n° 6,145,311, Nov 2000.

[4] Rufer and Al; Hydro -Pneumatic Storage System; PCT/IB 2007/051736.