WO2017198725A1

WO2017198725A1 - Hybrid multistage gas compression/expansion systems and methods

Info

Publication number: WO2017198725A1
Application number: PCT/EP2017/061862
Authority: WO
Inventors: Sylvain Lemofouet
Original assignee: Enairys Powertech Sa
Priority date: 2016-05-17
Filing date: 2017-05-17
Publication date: 2017-11-23
Also published as: EP3807539A1

Abstract

Hybrid multistage systems and methods for converting the potential energy of a pressurised gas, particularly air, into mechanical work of two rotating shafts (L.16 and H.10) when operating in expansion mode, and for producing compressed gas from the mechanical work of the two shafts when operating in compression mode, by performing successive expansion/compression of the said gas. The system comprises: a low pressure multistage compression/expansion unit (L) made of several stages of positive displacement rotary compressor/expander (L1a, L1b) mounted on a common shaft (L.16), a high pressure multistage hydraulic compression/expansion unit (H) made of several stages of hydraulic compression/expansion enclosures (H.2a, H.2b) combined with variable-displacement, bidirectional flow motor/pump stages (H.1a, H. 1b), a buffer gas storage tank (T.1), for the temporary storage of the medium pressure gas as an interface of the two compression/expansion units, and a liquid conditioning unit (W), for supplying, cleaning and maintaining active liquid at ambient temperature.

Description

HYBRID MULTISTAGE GAS COMPRESSION/EXPANSION SYSTEMS AND

METHODS

FIELD OF THE INVENTION This invention concerns methods for efficiently producing high pressure gas, and for converting the potential energy of pressurized gas into mechanical work and vice-versa; as well as reversible hybrid systems, which directly convert the pressure energy of a compressed gas, particularly air, into mechanical work and vice-versa, by performing successive nearly isothermal compression/expansion.

BACKGROUND OF THE INVENTION

A list of references pertaining to this invention is given at the end of the Description.

This invention is related to the production of high pressure gas, such as compressed air for breathing purposes or as power source for various compressed air-powered tools and industrial processes. It is also related to the use of compressed air as energy storage media like in the case of compressed air-powered cars, or to circumvent the intermittency of some renewable energy sources such as solar and 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 liquid (oil, water) as active fluid. Isothermal compression/expansion of gas will yield the highest energy efficiency if the gas is cooled/heated before or during the compression/expansion process. Several patents and published patent applications propose means for cooling/heating the air during the compression/expansion process by spaying a liquid inside the compression/expansion chamber using a liquid piston (e. g. patent US1929350A by N.C. Christensen) or using a reciprocating mechanical piston (e. g. patent US2010329903 Al by D. Fong & AL). Other patents and published patent applications propose means for achieving nearly isothermal compression/expansion process by using several stage of gas-charged hydraulic accumulators and pressure intensifiers with separating membrane or piston between the two fluids (e. g. US2013240068 Al by S. Rubio-Dean & AL; US2011296823 Al by McBride Troy & Al. and patent DE102012003288 B3 by I. Cyphelly & AL). Some other patent publications (e. g. EP2273119 Al by AGO AG EN & ANLAGEN. and WO2011069015 A2 by UNIV Colorado Regents) propose special liquid/liquid or liquid/gas heat exchangers that can be used in air compression/expansion systems to improve the heat exchange performance.

The main limitations of existing conversion solutions were presented by Lemofouet & Al. in [5] US Patent 8,567,183 B2, where innovative, Multistage Hydraulic Gas Compression/Expansion systems and Methods, particularly the so-called "Second Inventive System (System 2)" are proposed to overcome these limitations. However the proposed multistage hydraulic compression/expansion solutions present themselves some limitations that reduce the power range and therefore the field of application of such solutions, for instance: - The low pressure stages require very high liquid flow rates to generate substantial power; therefore very big hydraulic machines are necessary in these stages to generate only a limited amount of mechanical power and vice-versa.

The standard fixed-displacement hydraulic motor/pump used in the "Second Inventive System (System 2)" requires a "liquid directional control unit" for controlling the flow direction of the active liquid and converting the unidirectional flow in the motor/pump into a bidirectional flow in the compression/expansion modules. This unit which is made of several directional valves increases the complexity of the system and its operation generates a lot of pressure harmer and losses. In addition, if the motor/pump are operated at constant speed as usual, the mechanical power will fluctuate drastically in relation with the gas pressure which varies all the time, thus the system provides low power flexibility and quality.

The efficiency of the proposed hydraulic gas compression/expansion modules depends on the amount of heat exchange surface provided by the heat exchanger integrated in the compression/expansion chambers. In the proposed designs of heat exchangers, this surface is limited to the external surface of the liquid channels which occupied at least half of the volume of the chambers. The useful volume left to the gas is therefore limited which reduces the power density of the compression/expansion modules and therefore that of the system. It would be desirable to provide a reversible conversion system with high power density, flexibility and quality to efficiently convert the potential energy of a high pressure gas, in particular air, into mechanical power and vice-versa. The present invention proposes original solutions to achieve these objectives and overcome the limitations of the solutions proposed in by Lemofouet & Al. in US Patent 8,567,183 B2.

SUMMARY OF THE INVENTION

This invention provides a hybrid multistage system for converting the potential energy of a pressurized gas, particularly air, into mechanical work of two rotating shafts when operating the system in expansion or discharge mode, and for producing high pressure gas (preferably above 40bar) from the mechanical work of these rotating shafts, when operating the system in compression or charge mode, by performing successive nearly isothermal expansion/compression of the gas. The inventive system comprises:

A low pressure multistage compression/expansion unit made of several stages of positive displacement rotary compressor/expander, such as a scroll or screw compressor/expander, mounted on one common driving/driven rotatable shaft and specially designed to allow the injection of a substantial amount of liquid (particularly water) inside the working chambers; which compress a low pressure gas (atmospheric air or gas below lObar) up to a medium pressure (preferably in the range 10 to 40bar) when rotating the shaft in one direction; or which convert pressure energy of a medium pressure gas (preferably in the range 10 to 40bar) into mechanical power when rotating the shaft in the opposite direction, by performing an essentially isothermal compression/expansion of the said gas thanks to the injection of liquid inside the working chambers.

A high pressure multistage hydraulic compression/expansion unit comprising mainly: · A multistage of hydraulic compression/expansion module having an even number of enclosures of different volume, each one integrating a special gas/liquid heat exchanger, designed to convert the high pressure (preferably above 40bar) gas power into hydraulic power and vice-versa, by performing an essentially isothermal compression/ expansion of the gas. · A multistage, variable-displacement, bidirectional flow hydraulic motor/pump on another common driving/driven rotatable shaft, for converting hydraulic power into mechanical power and vice-versa. The hydraulic motor/pump has several circuits or stages in correspondence with the number of enclosures of the compression/expansion module, each stage being of different displacement range and being directly connected to the compression/expansion enclosure of corresponding displacement or volume.

• A multistage gas directional control module. - An intermediate buffer storage tank, for the temporary storage of the medium pressure gas (preferably in the range 10 to 40bar) so as to interface the continuous mass flow of the low pressure compression-expansion unit with the intermittent mass flow of the high pressure compression-expansion unit.

A low pressure (below lObar) liquid conditioning unit, for supplying, cleaning and maintaining the active liquid at ambient temperature thanks to heat exchange with external heat sink or/and heat source.

Further features of the inventive system are set out in the claims. Other aspects of the invention are a method for converting the potential energy of pressurized gas, particularly air, into mechanical work of two rotating shafts by performing a sequence of transformations, and a complementary method for producing high pressure gas, particularly air, from the mechanical work of two rotating shafts by performing a sequence of transformations, as set out in further details in the claims.

In these methods, advantageously the sequence of transformations, involving several stages of low pressure positive displacement rotary compression/expansion units, several stages of high pressure hydraulic gas expansion/compression enclosures with the corresponding stages of a multistage variable displacement bidirectional flow hydraulic motor/pump, external buffer gas storage tank and active liquid conditioning unit, is repeated several times to perform a multistage expansion/compression process.

Specifically, a method for producing a high pressure gas, particularly air, from the mechanical work of two rotating shafts by performing a sequence of transformations, in particular in a system in accordance with any preceding claim, comprises the steps of:

Firstly, pre-compressing low pressure gas, in particular below lObar, in particular atmospheric air, in a dedicated multistage low pressure gas compression unit made of several stages of a positive displacement rotary compressor, using the mechanical power of a first rotating shaft to force said gas to flow inside a continuously decreasing working chamber of one stage of the said unit, and then into that of the next smaller stage, while continuously cooling said gas by means of injecting a liquid in particular water in said working chambers, thus compressing said gas in a nearly isothermal manner up to a medium pressure, preferably in the range 10- 40bar.

Secondly, temporarily storing the resulting medium pressure gas in an intermediate buffer storage tank.

Thirdly, converting the mechanical power of a second rotating shaft into hydraulic power of an active liquid, particularly water, by means of a multistage variable displacement bidirectional flow hydraulic motor/pump; one stage of said motor/pump being used to pump active liquid into a corresponding chamber of one stage of a multistage hydraulic gas compression unit containing said medium pressure gas, to force said gas into a smaller chamber of a next stage initially full of liquid, while using another stage of smaller displacement of the said motor/pump to pump the active liquid out of said smaller chamber, in such a way that the liquid being driven out of the smaller chamber and the liquid being pumped into the bigger chamber indirectly cool the compressing gas by means of a heat exchanger, so as to perform an essentially isothermal compression.

Finally cooling the liquid flowing out of the working or compression chambers and which has been heated by the gas being compressed, by means of external heat exchangers in a dedicated liquid conditioning unit, in order to maintain said liquid at ambient temperature.

Correspondingly, a method for converting the potential energy of a high pressure gas, preferably above 40bar, particularly air, into mechanical work of two rotating shafts by performing a sequence of transformations, in particular in a system in accordance with any preceding claim, comprises the steps of:

Firstly, expanding high pressure gas, preferably above 40bar, in a dedicated multistage hydraulic gas expansion unit down to a medium pressure preferably in the range 40 to lObar by transferring said gas from a chamber of one stage of said unit into a bigger chamber of a next stage initially full of a liquid, in such a way as to drive out said liquid, while both the liquid being driven out of a bigger chamber and the liquid being pumped into a smaller chamber indirectly heat the expanding gas by means of a heat exchanger integrated in each chamber, so as to perform an essentially isothermal expansion; then converting hydraulic power produced by pressurised liquid flowing out of said bigger chamber of the expansion unit into mechanical power by means of a stage of a multistage variable displacement bidirectional flow hydraulic motor/pump, while another stage of smaller displacement range of said motor/pump uses part of the produced power to pump the liquid into said smaller chamber.

Thirdly, further post-expanding said medium pressure gas in a dedicated multistage gas expansion unit made of several stages of a positive displacement rotary expander, operating from medium to low pressure preferably from 40bar down to atmospheric pressure, by forcing said gas to flow inside a continuously increasing working chamber of one stage of the said unit and then into a bigger continuously increasing working chamber of a next stage, while continuously heating said gas by means of liquid injection in said working chambers, thus decreasing the pressure of said gas in a nearly isothermal manner down to atmospheric pressure.

Finally reheating the liquid flowing out of the working or expansion chambers and which has been cooled by the expanding gas, by means of external heat exchangers in a dedicated liquid conditioning unit, in order to maintain said liquid at ambient temperature. These and further features of the invention will be apparent from the following specific description.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described by way of example with reference to the accompanying drawings, in which:

Figures la and lb present one embodiment of the inventive hybrid multistage gas compression/expansion system, using a pressurised liquid reservoir as active liquid source, scroll compression/expansion stages in the low pressure conversion, and hydraulic compression/expansion modules with hydraulic ports on the top cover. Figures 2a and 2b illustrate general embodiments of the active liquid conditioning unit, with different possibilities for liquid supply and heat exchanges with external heat sinks/sources.

Figure 3a presents the flow characteristic of one stage of the multistage variable- displacement bidirectional flow hydraulic motor/pump used in the inventive hybrid system, and Figure3b illustrates an example of pump technology which allows achieving the flow characteristics.

Figure 4 depicts the layout of one compression/expansion enclosure of the inventive hybrid multistage gas compression/expansion system, using an innovative design of the integrated heat exchanger.

Figure 5 a, 5b and 5 c present respectively an isometric view, a cut-away isometric view and a bottom view of a new design of the heat exchanger integrated in the hydraulic gas compression/expansion enclosure presented in Figure 4.

Figures 6a and 6b present another embodiment of the inventive hybrid multistage gas compression/expansion system, using an unpressurised boosted liquid reservoir as active liquid supply, screw compression/expansion stages in low pressure conversion, and hydraulic compression/expansion modules with hydraulic ports on the bottom cover.

Figure 7 illustrates the layout of another compression/expansion enclosure using another new design of the integrated heat exchanger. - Figures 8a, 8b and 8c present respectively an isometric view, a cut-away isometric view and a bottom view of another new design of the heat exchanger integrated in the hydraulic gas compression/expansion enclosure presented in Figure 7.

Figures 9a and 9b present respectively a top view and an isometric view of another shape of a base honeycomb structure with square shape, that can be used to build the integrated heat exchangers presented in Figures 5 and 8.

DETAILED DESCRIPTION OF THE INVENTION

Constitution of the inventive system The machine according to this invention is made of four main parts or units as illustrated in Figure 1 and Figure 6: A low pressure active liquid conditioning unit (W);

A low pressure multistage gas compression/expansion unit (L);

A buffer gas storage tank (T); and

A high pressure multistage hydraulic gas compression/expansion unit (H).

Each unit may have one or several different possible configurations as explained below. The low pressure active liquid conditioning unit (W)

The present inventive system exploits the incompressibility and heat capacity of an active liquid, in particular water or water emulsion, to compress and/or maintain a gas at almost constant temperature during the compression/expansion processes. The role of the active liquid conditioning unit (W) is to supply, clean and maintain the active liquid at constant temperature. Several configurations of this unit are possible depending on the application context, however two embodiments are presented in Figure 2a and 2b:

Figure 2a presents a possible layout of the active liquid conditioning unit, which uses a pressurised reservoir (W.l) serving as boosted liquid source for the conversion units and in the supply line a heat exchanger (W.9) connected to an external heat source (W.10) allowing, particularly during the expansion mode where the liquid is cooled down by the expanding gas, to reheat the active liquid before supplying it again to the conversion units. A check valve (W.12) and 3-way/2-positon valve (W. l l) allow connecting the heat exchanger (W.9) to the supply line or bypassing it when needed. The heat source (W.10) could be any available waste heat from another application or a specific heat source in the form of a solar thermal system for instance.

The return line of this layout includes a liquid filter (W.2) which extracts the impurities from the liquid flowing back from the conversion units and, another heat exchanger (W.5') connected to a heat sink/source (W.6) allowing to regulate the temperature of the active liquid flowing back from the conversion units before feeding it back into the reservoir (W. l). A check valve (W.8) and 3-way/2-position valve (W.7) allow connecting the heat exchanger (W.5') to the return line or bypassing it when needed. The heat sink/source (W.6) could be any heat application providing an important heat capacity or the ambient environment as illustrated in the systems of Figure 1 and Figure 6. In that case, the heat exchanger (W.5') is simply an ambient air/liquid radiator (W.5) used to regulate the liquid temperature.

The active liquid conditioning unit (W) provides one connection port D on the supply line for the liquid injection pumps of the low pressure conversion unit, one connection port C on the return line for liquid/gas separators of the whole system and, one connection port (A, or B) for each stage of the high pressure conversion unit (H). In the case of a pressurised liquid reservoir as illustrated in Figure 1, a set of check valves (W.3a and W.3b, or W.4a and W.4b) is used to automatically convert each bidirectional liquid flow from high pressure conversion unit (H) into the unidirectional liquid flow of the active liquid conditioning unit (W).

Another possible layout of the active liquid conditioning unit (W) is presented in Figure 2b. The main difference with the previous layout is the use of an unpressurised liquid source (W.1 '), which could be a reservoir at atmospheric pressure or a natural water source like a river or a lake. In this case a variable boost pump (W.13) driven by the electric motor (W.14) could be necessary to supply the active liquid to the high pressure conversion unit (H). In this case, a quick exhaust valve (W.3'a or W.3'b) is used to automatically convert each bidirectional liquid flow from high pressure conversion unit (H) into the unidirectional liquid flow in the active liquid conditioning unit (W).

The low pressure multistage gas compression/expansion unit (L)

The low pressure multistage compression/expansion unit (L) is intended to efficiently compress a low pressure gas (preferably atmospheric air or gas below lObar) up to a medium pressure (preferably in the range 10 to 40bar), and to efficiently convert back the pressure energy of a medium pressure gas into mechanical power by performing an essentially isothermal compression/expansion of the said gas. As the unit operates at low pressure range, it must be able to generate a high gas mass flow to deliver substantial mechanical power. Positive displacement rotary compressor technologies are able to achieve this objective and some of them like scroll or screw technologies are reversible, e. g. they have working chambers with continuously decreasing (or increasing when rotating in the opposite direction) displacement and can therefore operate as compressor when rotating the shaft in one direction and as expander when rotating the shaft in the opposite direction. The low pressure compression/expansion unit (L) of the current inventive system is therefore made of several stages of positive displacement rotary compressor/expander, such as a scroll (orbiting or co-rotating) compressor/expander like in Figure l a (L. la and L.lb) or a screw compressor/expander like in Figure 6a (L.l 'a and L.l 'b), mounted on a common driving/driven shaft (L.16) and specially designed to allow the injection of substantial amount of liquid (particularly water) inside the working chambers, in order to cool the gas during compression and heat it during expansion, thus performing nearly isothermal compression/expansion processes and therefore highly efficient power conversion. Each stage has a liquid injection circuit composed of an injection variable pump (L.5a, L.5b), a 3-way/2-positon directional valve (L.7a, L.7b) which allows directing the liquid either on the intake spray nozzle (L.6a, L.6b) during the gas compression mode or on the exhaust spray nozzle (L.8a, L.8b) during the gas expansion mode. Depending on the technology and construction of the compressor/expander stage, other liquid injection nozzles can be mounted on the casing of the unit to directly access intermediate working chambers. Intake (L.3a), exhaust (L.3b) and inter-stage (L.4) gas/liquid separators are provided to extract the liquid from the gas at the end of each compression/expansion stage. Each pressurised separator has a liquid discharge circuit including a discharge valve (L.10; L.12) and a pressure relief valve (L.l l; L.13) allowing to return the liquid back into the liquid conditioning unit (W) without causing a gas pressure drop. An exhaust 2-way/2-positon valve with integrated check (L.14) is used to control the reverse flow of the gas and thus avoid and uncontrolled operation of the unit in discharge (expansion) mode.

The buffer gas tank (T) The buffer gas tank is a storage unit connected at the junction of the two multistage gas compression/expansion units for the temporary storage of the medium pressure gas flowing from one unit to the other. To generate a constant power, the average gas mass flow needs to be constant all over the compression/expansion chain. The low pressure compression/expansion unit produces/absorbs an almost continuous gas mass flow because of its rotary operation, whereas the high pressure compression/expansion unit produces/absorbs an intermittent gas mass flow because of its reciprocating operation. The buffer gas tank therefore acts like a damper that regulates the average gas mass flow and allows an appropriate interconnection of the two units. The high pressure multistage hydraulic gas compression/expansion unit (H)

This unit is designed to perform nearly isothermal high pressure gas compression/expansion processes. Figure lb presents a possible configuration of this unit, which is made of an even number (at least 2) of compression/expansion stages. The multistage hydraulic gas compression/expansion unit (H) according to this inventive system comprises three key parts: a) A multistage bidirectional flow, variable displacement hydraulic motor/pump b) A multistage hydraulic gas compression/expansion module

c) A multistage gas directional control module a) The multistage, variable displacement, bidirectional flow hydraulic motor/pump

The multistage, variable displacement bidirectional flow hydraulic motor/pump (H. la, H.2b) is intended to convert hydraulic power into mechanical power and vice-versa. The multistage hydraulic motor/pump has several stages mounted on a common driving/driven shaft (H.10), in correspondence with the number of stages of the multistage compression/expansion module, each circuit being of different displacement range and being directly connected, on the high pressure side to a compression/expansion enclosure of corresponding volume and, on the low pressure side to the liquid conditioning unit. Figure 3 a presents the flow characteristic of one stage of such a multistage variable- displacement bidirectional flow motor/pump, and Figure 3b depicts the operation of a variable displacement axial piston motor/pump with a swash plate rotating over the centre; as an example of pump technology which achieves such flow characteristics. By varying the angle of the swash plate over its rotation centre it is possible to vary the flow rate and moreover to reverse the flow direction without changing the rotational direction of the shaft. This feature of the motor/pump provides two key advantages to this inventive system compared to the standard fixed-displacement motor/pump used in the "Second Inventive System" of US Patent 8 567 183 B2:

It avoids using a "Liquid directional control unit" for controlling the flow direction of the active liquid. This unit is made of several hydraulic valves which increases the complexity of the system and its operation generates a lot of pressure harmers and losses. In addition, it allows switching the connections between high circuit and low pressure circuit, and in case of failure, high pressure could be accidentally applied on low pressure components and destroy them. With the current variable-displacement bidirectional flow motor/pump, the operation of the unit is simpler, smoother and safer.

The bidirectional variable flow rate allows regulating and varying the mechanical power in a wide range even at constant speed operation, which drastically increases the power flexibility and quality of this conversion unit. The continuous change in pressure during the compression and expansion processes is compensated by an equivalent opposite change in flow rate, thus a constant (over one stroke) and variable power can be generated even at constant speed operation.

At the end and the beginning of each compression/expansion stroke, the liquid flow direction has to be reversed and the sign of the flow rate will then change. Consequently, mechanical power sags are generated during the zero-point crossover of the flow rate. This is why the common driving/driven shaft (H.10) of the multistage hydraulic motor/pump (H. la, H.2b) is coupled to a flywheel (H.9) which smooths the resulting power generated/absorbed by an electrical motor/generator (H.8) through a small and short speed variation. b) The multistage hydraulic gas compression/expansion module

This multistage module comprises several hydraulic gas compression/expansion enclosures (H.2a, H.2b) of different volume, which contains a special gas/liquid heat exchanger (H.2.2a, H.2.2b). A simplified cutaway drawing of a hydraulic gas compression/expansion enclosure is represented in Figure 4. It is mainly made of a vertical compression/expansion cylinder (H.2.1) that integrates the special heat exchanger (H.2.2a or H.2.2b). Its inner cavity is accessible through a central gas port (H.2.3) on the top cover and, one or several hydraulic ports (H.2.4) on the top cover (Figure 4).

Figure 5a, 5b and 5c present different views of a new design of the heat exchanger integrated in the presented hydraulic gas compression/expansion enclosures, which is the key element to achieve nearly isothermal compression/expansion processes. It is made of a thin corrugated metal sheet (H.2.2.2) placed on another thin flat metal sheet (H.2.2.1), both wound to form a "spiral honeycomb" structure. A number of thin metal tubes (H.2.2.5) are threaded through holes of the honeycomb structure which they match almost perfectly the shape. These tubes form water channels and are uniformly distributed over the entire section of the structure to ensure good distribution of water flow and heat transfer. They are then fixed at their upper end in a distribution plate (H.2.2.3) so as to leave a space (H.2.2.4) between the upper edge of the honeycomb structure and the underside face of the distribution plate as can be seen on Figure 5b. The space (H.2.2.4) ensures a good distribution/collection of the gas during its entry/exit into/from the compression/expansion chamber. The whole assembly is then brazed or welded for instance to ensure good metallic contacts and therefore good mechanical strength and thermal conduction of the heat exchanger. The underside face of the distribution plate (H.2.2.3) may also have a conical shape to facilitate gas expulsion from inside the compression/expansion chamber.

Other shapes (triangular, square, rectangular, and polygonal) of the channels formed by the base honeycomb structure are foreseeable as illustrated in Figure 9a for a square shape. The section's shape of the metal tubes (H.2.2.5) should then be adapted accordingly. In addition, the metal sheets (H.2.2.1) and (H.2.2.2) can be perforated with small holes to facilitate a uniform distribution of pressure and fluid flow within the honeycomb base structure.

As shown in Figure 4, the heat exchanger is mounted so as to provide a liquid distribution chamber (H.2.5) under the top cover of the compression/expansion enclosure (H.2.1). The liquid ports are disposed and configured so as to ease a uniform distribution of liquid inside this chamber. The enclosure's top cover provides an isolating central channel (H.2.3) through which gas flow into/out of the compression-expansion chamber

A key difference between the hydraulic gas compression/expansion module of the current inventive system and that of US Patent 8 567 183 B2 is the design of the integrated heat exchanger. The new design described above presents at least two major advantages:

The metallic heat exchange surface provided to the gas is separated from the liquid channels whose total section is defined according to the liquid flow rate and not heat exchange surface. As result, the new heat exchanger occupied less than 20% of the total inner volume of the compression/expansion chamber, which is very low compared to at least 50% for the previous designs. This affects the power density of the compression/expansion unit and therefore the size of whole system.

Contrary to the previous heat exchangers where hundreds of metallic hollow channels had to be assembled together, the new heat exchangers need only a small amount of pieces and are mainly assembled by winding 2 metal sheets together. The manufacturing process is therefore highly simplified which drastically reduces the production cost.

A simplified cutaway drawing of another embodiment of hydraulic gas compression/expansion chamber, based on another design of the integrated heat exchanger is represented in Figure 7. In this embodiment, the hydraulic port (H.2.4') is placed on the bottom cover of the compression/expansion enclosure. The drawings of the corresponding integrated heat exchanger are presented in Figures 8a, 8b and 8c. This heat exchanger is limited to the "spiral honeycomb" structure formed by a thin corrugated metal sheet (H.2.2.2') placed on another thin flat metal sheet (H.2.2.1 ') and wound together.

As there is no liquid channel contributing to regulate the temperature of the honeycomb structure in this case, the heat exchange with the gas mainly relies on the high heat capacity and conductivity of this honeycomb metallic structure, which absorbs the heat generated by the gas during compression and transfers it to a liquid in which it is always immersed at least partially. This process works the other was round during expansion, e. g. the gas in indirectly heated by the liquid through the honeycomb metallic structure of the heat exchanger. c) The multistage gas directional control module The multistage gas-directional control module is made of several directional valves for controlling the gas flow direction. As can be seen on Figure lb or Figure 6b, each stage is made of a pair of 2-way/2-position valves (H.3a and H.4a; H.3b and H.4b, Figure lb) with integrated check valves that allow connecting the gas port (H.2.3) of the corresponding compression/expansion chamber to that of the next stage (on the left or in the right). These valves are connected in such a way that the integrated check valves are placed in series in the gas circuit, allowing an automatic control of the gas flow during the compression mode.

The intake valve (H.3a) of the lowest pressure stage is connected to the buffer storage tank (Tl) through a gas/liquid separator (H.5a), which removes the residual liquid from the gas mainly during discharge mode (downstream flow). Similarly, the exhaust valve (H.4b) of the highest pressure stage is connected to the high pressure exhaust port (S) through a gas/liquid separator (H.5b), which removes the residual liquid from the gas mainly during compression mode (upstream flow). Principle of operation

The compression process is realised in two phases by the inventive system:

A pre-compression phase performed by the low pressure compression/expansion unit, which aims at compressing the low pressure gas (atmospheric air or gas below lObar) up to a medium pressure (pb preferably in the range 10 to 40bar) and temporary storing it in the buffer tank (T.l).

A final compression phase performed by the high pressure compression/expansion unit, which aims at further compressing the medium pressure gas (pb) up to the final high pressure (p_f above 40bar) and storing it in a high pressure tank (not represented on the drawings) connected on the exhaust port (S).

Similarly, the expansion process is realised in two phases by the inventive system:

The first expansion phase is performed by the high pressure compression/expansion unit, which aims at expanding the high pressure gas down to a medium pressure (pb) and temporary storing it in the buffer tank (T .1 ) .

The post-expansion phase performed by the low pressure compression/expansion unit, which aims at further expanding the medium pressure gas down to the initial pressure (p_a).

Thus, the two compression/expansion (or conversion) units are connected in series on the gas circuit as can be seen on Figures la, lb and Figures 6a, 6b, and a buffer gas tank (T.l) is connected on their junction point to ensure a proper interface of the two units.

To realise the above compression/expansion processes with high efficiency, the conversion units use an active liquid which plays two important roles in the system:

In the high pressure conversion unit, it serves as "Power transmitter" (liquid piston) between the gas in each compression/expansion chamber and the corresponding stage of the multistage hydraulic motor/pump.

In both conversion units, it also serves as "Heat carrier" between the gas being compressed/expanded and the external heat sink/source (W.6 or W.10) through the external heat exchanger (W.5 or W.9). Active liquid conditioning

To perform the nearly isothermal gas compression/expansion processes that are required to achieve high efficiency, the inventive system needs that the temperature of the active liquid remains constant and close to ambient temperature all the time. This temperature regulation is performed by the active liquid conditioning unit (W) which is designed in such a way that the active liquid always flows in the same time direction, from the return line to the reservoir and then to the supply line. Thus, the heat exchanger (W.5) located on the return line is used to compensate the temperature gradient undergone by the active liquid in the conversion units:

During the compression (or storage) mode, the temperature of the liquid coming from the conversion units through the port A, B or C is slightly higher than that of the liquid in the reservoir (W. l). By performing an active heat transfer with the surrounding air or the heat sink/source (W.6), the heat exchanger (W.5) cools the active liquid down before feeding it back to the reservoir (W. l) through the filter (W.2) (Figure la).

During the expansion (or discharge) mode, the temperature of the liquid coming from the conversion units through the port A, B or C is slightly lower than that of the liquid in reservoir (W. l). By performing an active heat transfer with the surrounding air or the heat sink/source (W.6), the heat exchanger (W.5) heats the active liquid up before feeding it back to the reservoir (W.1) through the filter (W.2).

In case an external heat source (W.10) is available (Figure 2a, 2b), this heat can be exploited to improve the performances of the system during the expansion (or discharge) mode. The active liquid coming from the liquid tank (W. l) is heated up through the heat exchanger (W.9) prior to its supply to the conversion units. It then transfers this heat to the gas inside the compression/expansion during the expansion causing the gas temperature's gradient to be reduced and therefore the system performances to be improved. Pre-compression/post-expansion phase in the low pressure conversion unit

The low pressure conversion unit is made of several stages of reversible, positive displacement rotary technologies like scroll or screw compressor/expander stages mounted on a common shaft. The volume of the internal working chamber of these machines continuously decreases when rotating the shaft in one direction to achieve the gas compression function, and it continuously increases when rotating the shaft in the opposite direction to achieve the gas expansion function. In this case, the machine is specially designed to tolerate the presence of a substantial amount of active liquid (mainly water) that will keep the gas at constant temperature thanks to its higher heat capacity.

Pre-compression operating mode: (Figure la or 6a)

By driving the common shaft (L.16) through the electric motor (L.2) in the appropriate direction, the gas is sucked in the first stage (L. la) through the gas filter/silencer (L.15), is compressed and transferred in the second stage (L.lb) for further compression and finally discharged in the buffer storage tank (T.l). During this mode, the directional valves (L.7a and L.7b) are kept at the illustrated position, thus the injection pump (L.5a) and the spray nozzle (L.6a) are used to inject the liquid inside the first stage to cool the gas being compressed. At the same time the injection pump (L.5b) and the spray nozzle (L.6b) do the same thing for the second stage. The inter-stage gas/liquid separator (L.4) and the exhaust gas/liquid separator (L.3b) are used to separate the active liquid from the gas at the end of each compression stage. When these separators are full of liquid, their liquid discharge circuit made of the valves (L.10 and L.12) and the pressure relief (L. l 1 and L.13) respectively are activated to discharge the liquid without causing a pressure drop in the main line. The check valve integrated in the exhaust valve (L.14) always allow the discharge of the compressed gas inside the buffer tank (T. l).

Post-expansion operating mode:

When the exhaust valve (L.14) is switched to the "open" position, medium pressure gas is fed into the high pressure stage (L.lb). This pressure generates a starting torque that drives the common shaft (L.16) in the opposite direction than that of the pre-compression mode. The torque generation continues as the gas expands in the continuously increasing working chamber. It is further transferred to the lower pressure stage (L.la) where the expansion continuous until the initial pressure (atmospheric pressure p_a in case of air). During this mode, the directional valves (L.7a and L.7b) are switched to the second position, thus the injection pumps (L.5a and L.5b) and the spray nozzle (L.8a and L.8b) are used to inject liquid inside the working chambers to heat the gas being expanded. The inter-stage gas/liquid separator (L.4) and the intake gas/liquid separator (L.3a) are used to separate the active liquid from the gas at the end of each expansion stage. The liquid in separator (L.4) is discharged in the same way like in pre-compression mode, whereas the intake separator (L.3a) is continuously discharged through its drain port as it isn't pressurised.

Compression/expansion phase in the high pressure conversion unit The high pressure conversion unit performing this phase is composed of an even number of hydraulic gas compression/expansion stages, each stage operating as a reciprocating engine in two strokes:

An intake stroke when the gas is admitted in the compression/expansion enclosure (H.2a and H.2b). This stroke is passive during the compression operation mode and active during the expansion operation mode.

An exhaust (or transfer) stroke when the gas is exhausted from the compression/expansion enclosure (H.2a and H.2b). This stroke is active during compression operation mode and passive during expansion operation mode.

The compression/expansion process is always performed simultaneously with the gas transfer from one stage to the next one, except for the first and the last stages of the chain that can perform gas transfer with the external tanks (buffer tank or high pressure tank). In that case, the gas transfer from or into the chamber may last only part of the stroke time depending on the pressure level in the related tank. Due to this simultaneity of the compression/expansion and transfer operations, two consecutive stages always operate in opposite stroke, one performing an intake stroke and receiving the gas from the other (or from the external tank) which is performing an exhaust stroke and supplying the gas. Thus, in a 2-stage system as illustrated in Figure lb and 6b, stage A and stage B always operate in opposite stroke.

The number of stages depends on the desired maximum final pressure (p_f). For a given desired pressure, the higher this number is, the lower the inter-stage compression ratio "Cr" will be and the higher the thermodynamic efficiency will be also. For a smooth and synchronised operation, the ratio between the water capacities of two consecutive compression/expansion chambers is equal to the ratio of the maximum displacements of the related motor/pump stages and corresponds to the interstate compression ratio "Cr". An even number of stages will ensure a more constant mechanical torque all over the cycle, as the number of active stages will be the same during the two strokes (intake and exhaust). With an "N" stages conversion unit, a given mass of gas will flow all over the unit in (N+l) strokes.

Compression operation mode:

The compressor operation is described on the basis of the 2-stage conversion unit (H) illustrated in Figures lb and 6b. The 2-way/2-position valves (H.3a, H.4a, H.3b and H.4b) of the multistage gas directional control module are kept in the position illustrated on these Figures; therefore the gas flow is automatically controlled by their integrated check valves. The compression process consists in cycles of two intake/exhaust strokes over the two stages, from the buffer tank pressure (p_b) to the maximum admissible pressure (p_f) of the storage tank connected on port (S).

Assuming as starting point the situation where the low pressure chamber (H.2a) is full of liquid and the high pressure chamber (H.2b) is full of gas previously compressed from (H.2a). The displacement of the hydraulic motor/pump stage (H.la) is controlled to generate a negative flow (from the high pressure port "b" to the low pressure port "a") in order to withdraw the liquid from the chamber (H.2a). At the same time the displacement of the hydraulic motor/pump stage (H.lb) is controlled to generate a positive flow (from the low pressure port "a" to the high pressure port "b") in order to fill the chamber (H.2b) with liquid. As the liquid flows out of the chamber (H.2a), its inner pressure drops which causes the check valve integrated in (H.3a) to open and allow fresh gas from the buffer tank (T.l) to fill the chamber (H.2a). At the same time, liquid filling the chamber (H.2b) causes the gas pressure to increase and when this pressure is slightly higher than that of the final storage tank (not represented) connected to port (S), the check valve integrated in (H.4b) opens and allows the gas transfer inside the final storage tank through the exhaust gas/liquid separator (H.5b), which removes the residual liquid from the gas. During this stroke, as the HP stage B is the active one, the displacement of (H. lb) is controlled to reduce the liquid flow rate as the pressure increases in order to regulate the power and the displacement of the hydraulic motor/pump (H.la) is adapted to that of the hydraulic motor/pump (H.lb) in order to synchronise the operation of both stages. At the end of this stroke, when chamber (H.2a) is full of fresh gas at pressure (p_b) and chamber (H.2b) is full of liquid, the displacements of the two motor/pump stages are reversed so that the hydraulic motor/pump stage (H.la) generates a positive flow to fill the chamber (H.2a) with liquid and the hydraulic motor/pump stage (H. lb) generates a negative flow to withdraw the liquid from the chamber (H.2b). The resulting pressure variation causes the check valves integrated in (H.4a) and (H.3b) to open and allow a continuous transfer of the gas from chamber (H.2a) to chamber (H.2b). The difference in the water capacity of the two chambers causes the pressure in the two chambers to increase almost equally. As the HP Stage A is the active one during this stroke, the displacement of motor/pump (H. la) is controlled to reduce the liquid flow rate as the pressure increases in order to regulate the power and the displacement of motor/pump (H. lb) is adapted to that of motor/pump (H. la) in order to synchronise the operation of both stages. And the end of this stroke, the initial conditions where the low pressure chamber (H.2a) is full of liquid and the high pressure chamber (H.2b) is full of gas previously compressed from chamber (H.2a) are recreated and a new cycle can start again.

During the compression mode, the gas is cooled down and maintained at a nearly constant temperature thanks to the huge, wet metallic contact surface provided by the honeycomb structure of the integrated heat exchanger which stores the heat for a short time. In the case of the first heat exchange design illustrated in Figure 5a, 5b, 5c, this metallic surface is in turn cooled by the cool liquid flowing inside the liquid channels (H.2.2.4) and further immersing the lower part of this structure thanks to the very good thermal conductivity of the metal. In the case of the second heat exchange design illustrated in Figure 8a, 8b, 8c, this metallic surface is in turn cooled only by the liquid immersing the lower part of the honeycomb structure thanks to the very good thermal conductivity of the metal. This second design therefore needs a higher mass of metal compared to the first one as a higher thermal inertial (or heat capacity) is needed. The liquid will be further cooled down in the external active liquid conditioning unit. Expansion operation mode:

The system operates in expansion (or motor) mode in a similar way to the compression mode, but with a reverse gas flow. The motor operation process consists in a series of intake/expansion-exhaust cycles over the sequential stages, from the high pressure tank pressure (p_f) to the medium pressure (p_b). Assume as starting point the situation where the low pressure chamber (H.2a) is full of gas previously expanded from (H.2b) and the high pressure chamber (H.2b) is full of liquid. The valve (H.4b) is forced to the "open" position for a determined duration, allowing the right amount of gas from the high pressure tank (not represented) connected to port (S) to enter the high pressure chamber (H.2b) where it pressurises the active liquid. The displacement of the hydraulic motor/pump stage (H. lb) is controlled to generate a negative flow, thus the pressurised liquid drives this stage in motor mode which generates a driving torque on the common shaft (H.10) while the gas expands in chamber (H.2b). At the same time, the valve (H.3a) is forced to the "open" position during the whole stroke time and the displacement of the hydraulic motor/pump stage (H.la) is controlled to generate a positive flow in order to fill the chamber (H.2a) with liquid and therefore force the previously expanded gas to flow out towards the buffer tank (Tl). During this stroke, as the HP stage B is the active one, the displacement of motor/pump (H. lb) is controlled to increase the liquid flow rate as the pressure decreases in order to regulate the mechanical power and the displacement of motor/pump (H.la) is adapted to that of motor/pump (H. lb) in order to synchronise the operation of both stages.

At the end of this stoke chamber (H.2a) is full of liquid and chamber (H.2b) is full of gas at the pressure equal to "Cr.pb". The valves (H.4a) and (H.3b) are forced to the "open" position and the pressures of the two chambers get equalized. The displacements of the two motor/pump stages are reversed so that the hydraulic motor/pump stage (H.la) generates a negative flow in order to withdraw the liquid from chamber (H.2a) and the hydraulic motor/pump stage (H. lb) generates a positive flow in order to fill the chamber (H.2b). The gas contained in chamber (H.2b) can further expand while being continuously transferred into chamber (H.2a) where it pressurises the out flowing liquid that drives in turn the hydraulic stage (H.la) in motor mode. The difference in the water capacity of the two chambers causes the pressure in both chambers to decrease almost equally. The HP stage A is the active one during this stroke. The displacement of motor/pump (H. la) is controlled to increase its flow rate as the pressure decreases in order to regulate the power and the displacement of motor/pump (H.lb) is adapted to that of motor/pump (H.la) in order to synchronise the operation of both stages. At the end of this stroke, the initial conditions where the low pressure chamber (H.2a) is full of gas previously expanded from chamber (H.2b) and the high pressure chamber (H.2b) is full of liquid are recreated and a new cycle can restart. The heat exchange process during the expansion mode is performed in a similar way as for compression mode; e. g. indirectly through the honeycomb structure of the integrated heat exchanger. The liquid reheats the inflowing and expanding gas indirectly through thin wall of the heat exchanger's honeycomb structure, so as to maintain its temperature almost constant. The liquid will be further reheated in the external active liquid conditioning unit.

Main advantages

The proposed inventive system and methods provide many technological improvements compared to the state-of-the art compressed gas energy conversion, particularly the system proposed in the US Patent 8 567 183 B2; the most relevant ones are the following:

- Hybrid topology combining two different technologies, each of which been optimized to achieve the highest conversion efficiency in the dedicated pressure range thanks to nearly isothermal compression/expansion processes. The overall power density, system performances are drastically improved. - Simplified topology of the hydraulic gas compression/expansion unit with smooth, valve- less hydraulic directional control that drastically improves the efficiency and reliability of the whole system

- Simple, efficient and cost effective designs of the heat exchanger integrated in the compression/expression chamber. This is achieved thanks to its special honeycomb structures that provide largest heat exchanger surface in a minimum volume, which contributes to increase the power density of the whole system.

- Comprehensive low pressure active liquid conditioning unit, with automatic hydraulic directional control, providing various heat exchange possibilities with external heat sink/source in order to regulate the temperature of the active liquid or exploit available waste heat.

Main Fields of Application

This invention is mainly intended to the production of high pressure gas, particularly air, and use of its potential energy, for the purpose of power transmission and energy storage. One potential application of this invention would be the production of compressed air for industrial applications or for medical and breathing purpose like diving, and fire-fighting. Another potential 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 used to circumvent the intermittency of some renewable energy sources such as solar or wind sources.

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.

[5] Lemofouet and Al; Multistage Hydraulic Gas Compression/Expansion systems and Methods; PCT/IB2008/053691 ; US Patent 8 567 183 B2

Claims

A hybrid multistage system for converting the potential energy of a pressurized gas, particularly air, into mechanical work of two rotating shafts when operating the system in an expansion or discharge mode, and for producing compressed gas from mechanical work of these rotating shafts, when operating the system in a compression or charge mode, by performing successive nearly isothermal expansion/compression of the gas, wherein the system comprises:

- A first multistage compression/expansion unit made of several stages of a positive displacement rotary compressor/expander such as a scroll compressor/expander or a screw compressor/expander comprising working chambers and mounted on a first common shaft, and designed to allow the injection of a substantial amount of an active liquid particularly water into the working chambers, which first multistage compression/expansion unit is arranged to compress a low pressure gas, preferably below lObar, in particular atmospheric air, up to a medium pressure, preferably in the range 10 to 40bar, from the mechanical power of said first common shaft, and/or which converts the pressure energy of a medium pressure gas, preferably in the range 10 to 40bar, into mechanical power of said first common shaft, by performing an essentially isothermal compression/expansion of the gas through the heat transfer capacity of the active liquid for example water injected inside the working chambers;

- A second multistage hydraulic gas compression/expansion unit made of a pair or several pairs of hydraulic compression/expansion stages, mounted on a second common shaft and driven by an active liquid for example water, which second multistage hydraulic gas compression/expansion unit is arranged to compress a medium pressure gas, preferably in the range 10 to 40bar, up to a high pressure, preferably above 40bar, from the mechanical power of said second common shaft indirectly through the hydraulic power of the active liquid, and/or which converts the pressure energy of a high pressure gas, preferably above 40bar, into mechanical power of said second common shaft indirectly through the hydraulic power of said active liquid, by performing an essentially isothermal compression/expansion of said gas through the heat transfer capacity of said active liquid working as liquid piston;

- A common gas circuit connecting the first and second compression/expansion units on either side of a junction; - A buffer gas storage tank connected at the junction of the first and second compression/expansion units in the said gas circuit, for the temporary storage of medium pressure gas, preferably in the range 10 to 40bar, adapted to store a continuous gas mass flow of the first compression/expansion unit and an intermittent gas mass flow of the second compression/expansion unit; and

- An active liquid conditioning unit, for supplying, cleaning and maintaining the active liquid of the first and second compression/expansion units at low pressure, preferably below lObar, and substantially constant temperature by heat exchanges with external heat sinks or sources.

2. A multistage system in accordance with claim 1 wherein each stage of the second compression/expansion unit comprises:

- A hydraulic compression/expansion enclosure of specific volume having an inner cavity that is accessible through a central gas port on a top cover and through one or several hydraulic ports on the top cover or on a bottom cover; said enclosure containing a gas/liquid heat exchanger designed to produce heat transfer between the gas and the active liquid, wherein the active liquid acts like a piston to compress the gas while cooling it during a compression mode and to recover expansion energy of the gas while heating it during an expansion mode; and

- A variable displacement, bidirectional flow, hydraulic motor/pump of specific displacement range, having a high pressure port connected to the compression/expansion enclosure, and a low pressure port connected to the liquid conditioning unit, said motor/pump having a shaft kinetically connected to the said second common driving/driven shaft for converting hydraulic power transmitted by said active liquid flowing out of said compression/expansion enclosure into mechanical power of the said second common shaft and vice-versa; wherein the multistage system further comprises:

- A gas-directional control module, comprising two directional valves with parallel check valves, connected in series over the gas circuit with said junction connected to the gas port of the corresponding compression/expansion enclosure, for controlling the gas flow direction between said stages and the external gas storage tank. A multistage system in accordance with claim 1 or 2, further comprising an integrated heat exchanger designed to achieve indirect heat transfer between the gas and the active liquid during the compression/expansion process, said heat exchanger being constructed principally as a honeycomb structure of various shapes including spiral, square and polygonal, made of brazed or welded metal sheets, possibly perforated to facilitate pressure equilibration and fluid flow within the structure; said structure being adapted to perform an indirect heat transfer between gas and active liquid during compression/expansion processes, due to the thermal capacity and conductivity of said metal sheets.

A multistage system in accordance with claim 3, wherein the integrated heat exchanger comprises a number of thin metal tubes uniformly distributed over the entire section of the said honeycomb structure and threaded through holes in the honeycomb structure, said tubes serving as water channels and being threaded at their upper end in a distribution plate so as to provide a space between the upper edge of the honeycomb structure and an underside face of the said plate, to ensure distribution/collection of the gas during its entry/exit into/from the compression/expansion enclosure, the assembly being assembled by brazing or welding for instance to ensure a metallic contact, mechanical strength and thermal conduction of said heat exchanger.

A multistage system in accordance with any preceding claim, comprising an active liquid conditioning unit operable at a low pressure, preferably below lObar, said conditioning unit comprising a return line including a liquid filter and a controllable heat exchanger for compensating temperature variation of liquid flowing back from the compression/expansion units by bidirectional heat exchange with an external heat sink/source; a supply line including another heat exchanger for exploiting an external heat source during the expansion operating mode; each return or supply line providing a connection to a liquid injection circuit of the first multistage low pressure compression/expansion unit, and being connected on one end to a pressurised liquid reservoir or to an unpressurised boosted liquid reservoir/source, and connected on the other end to all the low pressure ports of the hydraulic motor/pump stages of the second multistage high pressure compression/expansion unit, through a set of check valves or exhaust valves that automatically convert bidirectional flow in said second high pressure compression/expansion unit into a unidirectional flow within the said active liquid conditioning unit.

A multistage system in accordance with any preceding claim wherein each stage of the first multistage compression/expansion unit has a liquid injection circuit comprising a controllable injection pump having an intake port connected to a supply line of a liquid conditioning unit and exhaust port connected to a 3-way/2-position directional valve for directing liquid either to intake injection nozzles during compression, or to exhaust injection nozzles during expansion; said first multistage compression/expansion unit having a liquid extraction circuit comprising a main intake gas/liquid separator, a main exhaust gas/liquid separator and inter-stage gas/liquid separators, said separators being connected to a return line of the liquid conditioning unit through a discharge valve and pressure relief valve, for returning active liquid to the liquid conditioning unit.

A method for producing a high pressure gas, particularly air, from the mechanical work of two rotating shafts by performing a sequence of transformations, in particular in a system in accordance with any preceding claim, comprising the steps of:

- Firstly, pre-compressing low pressure gas, in particular below lObar, in particular atmospheric air, in a dedicated multistage low pressure gas compression unit made of several stages of a positive displacement rotary compressor, using the mechanical power of a first rotating shaft to force said gas to flow inside a continuously decreasing working chamber of one stage of the said unit, and then into that of the next smaller stage, while continuously cooling said gas by means of injecting a liquid in particular water in said working chambers, thus compressing said gas in a nearly isothermal manner up to a medium pressure, preferably in the range 10-40bar;

- Secondly, temporarily storing the resulting medium pressure gas in an intermediate buffer storage tank;

- Thirdly, converting the mechanical power of a second rotating shaft into hydraulic power of an active liquid, particularly water, by means of a multistage variable displacement bidirectional flow hydraulic motor/pump; one stage of said motor/pump being used to pump active liquid into a corresponding chamber of one stage of a multistage hydraulic gas compression unit containing said medium pressure gas, to force said gas into a smaller chamber of a next stage initially full of liquid, while using another stage of smaller displacement of the said motor/pump to pump the active liquid out of said smaller chamber, in such a way that the liquid being driven out of the smaller chamber and the liquid being pumped into the bigger chamber indirectly cool the compressing gas by means of a heat exchanger, so as to perform an essentially isothermal compression;

- Finally cooling the liquid flowing out of the working or compression chambers and which has been heated by the gas being compressed, by means of external heat exchangers in a dedicated liquid conditioning unit, in order to maintain said liquid at ambient temperature.

8. A method for converting the potential energy of a high pressure gas, preferably above 40bar, particularly air, into mechanical work of two rotating shafts by performing a sequence of transformations, in particular in a system in accordance with any preceding claim, comprising the steps of:

- Firstly, expanding high pressure gas, preferably above 40bar, in a dedicated multistage hydraulic gas expansion unit down to a medium pressure preferably in the range 40 to lObar by transferring said gas from a chamber of one stage of said unit into a bigger chamber of a next stage initially full of a liquid, in such a way as to drive out said liquid, while both the liquid being driven out of a bigger chamber and the liquid being pumped into a smaller chamber indirectly heat the expanding gas by means of a heat exchanger integrated in each chamber, so as to perform an essentially isothermal expansion; then converting hydraulic power produced by pressurised liquid flowing out of said bigger chamber of the expansion unit into mechanical power by means of a stage of a multistage variable displacement bidirectional flow hydraulic motor/pump, while another stage of smaller displacement range of said motor/pump uses part of the produced power to pump the liquid into said smaller chamber;

- Secondly, temporarily storing the resulting medium pressure gas in an intermediate buffer storage tank; - Thirdly, further post-expanding said medium pressure gas in a dedicated multistage gas expansion unit made of several stages of a positive displacement rotary expander, operating from medium to low pressure preferably from 40bar down to atmospheric pressure, by forcing said gas to flow inside a continuously increasing working chamber of one stage of the said unit and then into a bigger continuously increasing working chamber of a next stage, while continuously heating said gas by means of liquid injection in said working chambers, thus decreasing the pressure of said gas in a nearly isothermal manner down to atmospheric pressure; and

- Finally reheating the liquid flowing out of the working or expansion chambers and which has been cooled by the expanding gas, by means of external heat exchangers in a dedicated liquid conditioning unit, in order to maintain said liquid at ambient temperature.