EP3510257B1 - Mechanical system for generating mechanical energy from liquid nitrogen, and corresponding method - Google Patents

Description

1. Field of the invention

The invention relates to a system and method for producing mechanical energy from liquid nitrogen and/or producing liquid nitrogen or other liquefied gas.

In variations, the invention relates to a system for storing energy in the form of liquid nitrogen or other liquid gases such as air.

2. Prior art

• un mode générateur d'azote liquide lors de la mise en oeuvre duquel de l'azote liquide est produit et stocké, et
• un mode moteur lors de la mise en oeuvre duquel l'azote liquide préalablement produit et stocké est consommé pour produire de l'énergie mécanique, pouvant par exemple être exploitée pour entraîner un alternateur et produire du courant électrique ou pour mouvoir un véhicule.

The international patent application bearing number WO-A1-2014/154715 , describes a reversible mechanical system that can operate in two modes, namely:

• a liquid nitrogen generator mode during the implementation of which liquid nitrogen is produced and stored, and
• a motor mode during the implementation of which the liquid nitrogen previously produced and stored is consumed to produce mechanical energy, which can for example be used to drive an alternator and produce electric current or to move a vehicle.

In generator mode, the system includes a piston compressor into which nitrogen gas is admitted in order to be compressed. The compressed nitrogen is introduced into an exchanger in which it is cooled before being admitted into a piston expander in which it is expanded and partly liquefied. The liquid nitrogen produced is stored. The unliquefied nitrogen inside the expander is admitted to the heat exchanger to cool the compressed nitrogen gas from the compressor to be admitted back into the compressor.

In engine mode, the liquid nitrogen, pumped under high pressure, is vaporized in an exchanger then admitted into a first piston expander (which acts as a expander where the liquid nitrogen is formed in generator mode) then into a second piston expander. piston (which acts as a low pressure compressor in generator mode). The pistons are connected to the same crankshaft which is rotated due to the expansion, inside the regulators, of the vaporized nitrogen.

The implementation of such generator and motor modes makes it possible to store energy in large quantities by producing liquid nitrogen in a simple and efficient manner and by restoring mechanical energy from said liquid nitrogen by example to drive an alternator and produce electric current or to move a vehicle.

The document WO 2015/138817 shows a system for producing energy from nitrogen evaporation according to the state of the art.

However, it remains possible to further improve the efficiency of the generator mode in order to produce a greater quantity of liquid nitrogen at each expansion, and the efficiency of the motor mode in order to produce a greater quantity of mechanical energy at each expansion.

3. Objectives of the invention

The invention aims in particular to provide an effective solution to at least some of these different problems.

In particular, according to at least one embodiment, an objective of the invention is to increase the efficiency in engine mode of a mechanical system for producing mechanical energy from liquid nitrogen or other liquefied gas.

Another objective of the invention is, according to at least one embodiment, to increase the efficiency in generator mode of a mechanical system for producing liquid nitrogen or other liquefied gas.

In particular, the invention aims, according to at least one embodiment, to provide such a system which is simple and/or efficient and/or robust and/or inexpensive.

Another objective of the invention is, in at least one embodiment, to increase the overall efficiency and reduce the cost of the energy storage system both in terms of storage and in terms of restitution of energy by the combination of the three objectives mentioned above.

4. Presentation of the invention

• un compresseur ;
• un détendeur ;
• un échangeur de chaleur ;

ledit système possédant un mode de fonctionnement moteur dans lequel ledit système comprend en outre :

• des moyens d'admission d'azote liquide sous pression dans une entrée d'admission d'azote liquide dudit échangeur, des moyens d'admission d'air ou d'azote gazeux dans une entrée d'admission d'air ou d'azote gazeux dudit échangeur, des moyens d'échappement d'azote vaporisé à une sortie d'azote vaporisé dudit échangeur et des moyens d'échappement d'air ou d'azote refroidi à une autre sortie d'air ou d'azote gazeux refroidi dudit échangeur ;
• des moyens d'admission à l'intérieur dudit détendeur dudit azote vaporisé pour l'y détendre ;
• des moyens d'admission dans ledit compresseur dudit air ou azote gazeux refroidi pour y produire de l'air ou azote gazeux comprimé ;
• des moyens de détente dudit air ou azote gazeux comprimé ;
• des moyens de réchauffage de l'air ou azote gazeux comprimé avant admission dans lesdits moyens de détente ou à l'intérieur desdits moyens de détente ;
• des moyens de récupération d'énergie provenant de la détente dudit azote vaporisé et de la détente dudit air ou azote gazeux comprimé.

For this, the invention is described in claims 1, 11 and 13 and proposes in particular a mechanical energy production system comprising at least:

• a compressor;
• a regulator;
• a heat exchanger;

said system having a motor operating mode in which said system further comprises:

• means for admitting liquid nitrogen under pressure into a liquid nitrogen intake inlet of said exchanger, means for admitting air or gaseous nitrogen into an air or nitrogen intake inlet gas from said exchanger, means for exhausting vaporized nitrogen to a vaporized nitrogen outlet from said exchanger and means for exhausting air or cooled nitrogen to another air or cooled nitrogen gas outlet from said interchange;
• means for admitting said vaporized nitrogen inside said regulator to relax it there;
• means for admitting said cooled air or gaseous nitrogen into said compressor to produce compressed air or gaseous nitrogen;
• means for expanding said compressed air or gaseous nitrogen;
• means for heating the air or compressed gaseous nitrogen before admission into said expansion means or inside said expansion means;
• means for recovering energy from the expansion of said vaporized nitrogen and the expansion of said compressed air or gaseous nitrogen.

For the purposes of the invention, the term gaseous or liquid nitrogen means essentially composed of nitrogen but may include a small proportion of other elements and a lower but sufficient oxygen content if combustion is desired. The nitrogen content of the fluid considered will preferably be between 90 and 98%.

However, it is possible to use the invention with air. In this case, the invention can be used as a simple and economical air liquefier and for use other than energy storage.

Thus, according to this aspect of the invention, the cooling of air or gaseous nitrogen in the exchanger followed by compression, preferably adiabatic, then expansion thereof, with a supply of thermal energy before and/or during expansion, with a significantly larger volume, for example of the order of four times, produces additional mechanical energy.

This makes it possible to recover the mechanical energy resulting from the phase change and the heating of the liquid nitrogen and thus to increase the efficiency in engine mode compared to a simple expansion of the liquid nitrogen after pressurization and vaporization.

5. List of figures

• la figure 1 illustre un schéma d'un système de production d'énergie mécanique à partir d'azote liquide selon une variante simplifiée de l'invention ;
• la figure 2 illustre un schéma d'un système de production d'énergie mécanique à partir d'azote liquide selon une variante évoluée de l'invention;
• les figures 3 et 4 illustrent des moyens de réchauffage ou de refroidissement en amont et dans un détendeur ou un compresseur respectivement simple ou étagé ;
• la figure 5 illustre un schéma d'un système de production d'azote liquide selon la variante évoluée de l'invention ;
• la figure 6 illustre un logigramme d'un procédé de production d'énergie mécanique selon l'invention
• la figure 7 illustre un logigramme de la phase de mise en route d'un procédé de production d'azote liquide selon l'invention ;
• les figures 8 et 8 bis illustrent la phase de fonctionnement stabilisé d'un procédé de production d'azote liquide selon l'invention ;
• la figure 9 illustre une variante d'un système selon la variante évoluée de l'invention comprenant plusieurs compresseurs ou détendeurs ;
• la figure 10 illustre une variante d'un système selon la variante simplifié de l'invention comprenant un compresseur/détendeur étagé avec deux chambres de détente basse pression.

Other characteristics and advantages of the invention will appear on reading of the following description of particular embodiments, given by way of simple illustrative and non-limiting example, and of the appended drawings among which:

• there figure 1 illustrates a diagram of a system for producing mechanical energy from liquid nitrogen according to a simplified variant of the invention;
• there figure 2 illustrates a diagram of a system for producing mechanical energy from liquid nitrogen according to an advanced variant of the invention;
• THE figures 3 and 4 illustrate means of heating or cooling upstream and in an expander or compressor, respectively simple or staged;
• there figure 5 illustrates a diagram of a liquid nitrogen production system according to the advanced variant of the invention;
• there Figure 6 illustrates a flowchart of a process for producing mechanical energy according to the invention
• there Figure 7 illustrates a flowchart of the start-up phase of a process for producing liquid nitrogen according to the invention;
• THE figures 8 and 8 bis illustrate the stabilized operating phase of a process for producing liquid nitrogen according to the invention;
• there Figure 9 illustrates a variant of a system according to the evolved variant of the invention comprising several compressors or expanders;
• there Figure 10 illustrates a variant of a system according to the simplified variant of the invention comprising a stepped compressor/expander with two low pressure expansion chambers.

6. Description of particular embodiments 6.1. Mechanical energy production 6.1.1. Mechanical system for producing mechanical energy from liquid nitrogen

The invention relates to a mechanical system for producing mechanical energy from liquid nitrogen.

i. Simplified version

In reference to the figure 1 , we present such a system in a simplified version.

Such a system comprises a pipe 300 of liquid nitrogen under pressure which opens into a liquid nitrogen inlet 301 of a heat exchanger 302.

The heat exchanger 302 comprises an outlet of vaporized and heated nitrogen 303 which is connected by a pipe 304 to an inlet of vaporized and heated nitrogen 305 of a compressor/expander 306. The heat exchanger 302 comprises an inlet of ambient air 311.

The exchanger 302 is crossed by the pipe 302' which leaves from the inlet 301 of the exchanger to the outlet 303 of the exchanger. The pipe 302' serves as a heat exchange surface in the exchanger with the fluid which passes through it internally, liquid nitrogen, and the fluid which passes through it externally, air or nitrogen. It can consist of a set of plates stacked on top of each other and which form conduits or even, consist of multiple pipes connected to the inlet 301 and the outlet 303 of the exchanger.

The compressor/expander 306 includes an exhaust outlet 307 of expanded air or nitrogen gas. It includes a cooled air inlet 308 which is connected by a pipe 309 to a cooled air outlet 310 of the exchanger 302.

The compressor/expander 306 can for example be a system comprising at least one piston 314 movable in a chamber 315 and connected to a crankshaft 316 by a connecting rod 317. This crankshaft can for example be connected to an alternator 318 to produce electric current, be used to set a vehicle or other vehicle in motion.

Rather than being connected to a crank-rod type system, the compressor/expander piston can be connected to a linear electric motor or alternator.

The system includes means for reheating the vaporized nitrogen before admission into the regulator 306 or for reheating inside the regulator 306. They may include a heater 500 placed on the pipe 304. They may alternatively or in addition include means fluid injection 313 in the regulator to ensure heating.

In a variant (shown in dotted lines on the figures 1 And 10 ), the inlet 311 can be connected directly to another pressurized nitrogen outlet (residual expansion pressure) 312 of the compressor/expander 306 by means of a pipe 311'. As will become clearer later in relation to the description of the operation of the system, this makes it possible to reduce the size of the system (in particular the exchanger 302, the compressor/expander 306).

In a variant, which can be combined with the previous variant, the compressor/expander 306 ( figure 1 ) can be staged to compress or expand several times at different pressures, according to the principle shown in figure 4 . Several expanders/compressors (two or more) can thus be implemented with means to admit into a compressor/expander the fluid coming from another expander/compressor with a view to expanding or compressing it at a different pressure than in the previous one. .

For two stages of compressor/expander, the high pressure compressor/expander will include in addition to an additional orifice with the pipeline which connects it to an additional orifice of the low pressure expander, the inlet ports 308, and 305, the compressor expander/low pressure will include the exhaust ports 312 and 307, (each of the ports 308, 305, 312, 307 being connected to their respective pipe according to the figure 1 ).

In another variant according to the Figure 10 and compared to the previous variant where the compressor/expander is staged, the second low pressure stage can include two regulators 306' and 306" each connected to the high pressure stage by the pipes 400 and 400' themselves connected to the orifices 600, 601, 602, 603. One of the low pressure regulators may include an exhaust to the open air via outlet 307 and the other low pressure regulator, an exhaust via outlet 312 which can be connected to the inlet of the exchanger 311 via the pipe 311' as indicated in a previous variant.

In this variant, the means for heating the vaporized nitrogen before admission into a regulator or for heating inside a regulator can be arranged in each of the compressor regulators and alternatively or in addition on the intake pipe of each two. So on the Figure 10 , the external heaters 500, 501 and 502 are placed on the pipes 304, 400 and 400', the internal heaters 313, 313' and 313 "are placed inside the compressor/expander 306, 306' and 306".

These heating means can for example be provided by the direct injection of an energetic fluid such as gasoline with combustion or by a fluid having a significant specific heat such as water. In the variants in which the reheating takes place outside the expander, it can be obtained by means of a heat exchanger heated by a heat transfer fluid itself heated by a heat source: solar concentration, gas or gasoline combustion.

This reheating increases the volume of the gas to be expanded and reduces the consumption of liquid nitrogen for the same quantity of mechanical energy produced.

A pressure probe P is optionally placed on pipe 311' and a temperature probe T° is optionally placed on pipe 304.

ii. Advanced version

In reference to the figure 2 , we present such a system in a more advanced version.

Such a system includes a liquid nitrogen tank 10.

This tank 10 includes a liquid nitrogen outlet 11 which is connected to a pipe 12 between two portions of which a pump 13 is placed. The pump 13 can be placed in the tank 10. It is optional the pressure which can be obtained by the heating of the tank for example.

Pipeline 12 opens into a valve 14.

The valve 14 comprises an outlet which is connected by a pipe 15 to the liquid nitrogen inlet 160 of a heat exchanger 16.

Valve 14 is optional and pipes 12 and 15 can constitute a single pipe when valve 14 is not used.

The heat exchanger 16 includes an outlet 161 of heated vaporized nitrogen.

The exchanger 16 is crossed by the pipe 16' which starts from the inlet 160 of the exchanger to the outlet 161 of the exchanger. Pipe 16' serves as a heat exchange surface in the exchanger with the fluid which passes through it internally, liquid nitrogen, and the fluid which passes through it externally, air or nitrogen. It can consist of a set of plates stacked on top of each other and which form conduits or even, consist of multiple pipes connected to the inlet 160 and the outlet 161 of the exchanger.

The heated vaporized nitrogen outlet 161 is connected to a pipe 17 which opens to the vaporized nitrogen inlet 180 of a regulator 18 and optionally carries a T° temperature probe.

The heat exchanger 16 includes an air intake inlet 162 preferably at ambient temperature or lower. This inlet 162 can optionally and alternatively be connected by a pipe 19 to an outlet optional nitrogen gas under pressure (residual pressure from the expansion) 181 of the regulator 18.

The heat exchanger 16 includes an outlet 163 for cooled air or nitrogen. This outlet is connected to a pipe 20 which opens into a valve 21.

Pipe 20 optionally includes a temperature probe T.

The valve 21 is connected by a pipe 22 to an air or cooled nitrogen intake inlet 230 of an adiabatic compressor 23.

Valve 21 is optional and pipes 20 and 22 can constitute a single pipe when valve 21 is not used.

The adiabatic compressor 23 includes an outlet 232 for compressed air or nitrogen.

The compressed air or nitrogen outlet 232 is connected by a pipe 24 to an air or compressed nitrogen inlet 182 of the regulator 18.

The regulator 18 includes a nitrogen gas exhaust outlet 183.

The valve 14 is optionally connected to an optional pipe 25 which is connected to an optional orifice 231 of the adiabatic compressor 23. The valve 21 is optionally connected to an optional pipe 26, connected to an optional orifice 11', of the liquid nitrogen tank 10 optional. These optional elements are not useful for the production of mechanical energy. As will be described later, they are used for the production of liquid nitrogen.

The expander 18 and the adiabatic compressor 23 each include a drive shaft 184, 233.

The system includes an output shaft 27. This can for example be connected to an alternator 28 to produce electric current, be used to set a vehicle in motion or the like.

the drive shaft 184 of the regulator 18 constitutes or is connected to the output shaft 27 of the system directly or by a transmission.

The drive shaft 184 of the expander 18 is preferably connected to that of the adiabatic compressor 23 directly or by a transmission so that the adiabatic compressor 23 is driven by the expander 18. Otherwise, auxiliary motor means must be placed used to drive the drive shaft of the compressor 23.

The drive shaft 233 of the compressor 23, the drive shaft 184 of the expander 18 and the output shaft 27 of the system can constitute a single and same tree, as shown on the figure 2 . In this case, the expander causes both the adiabatic compressor and the output shaft to move.

The expander 18 and the adiabatic compressor 23 each comprise one or more pistons 185, 235 mounted movable in translation in one or more chambers 186, 236 and connected by means of connecting rods 187, 237 to a crankshaft 188, 238. The crankshaft of the expander constitutes the expansion valve drive shaft and the compressor crankshaft constitute the compressor drive shaft.

The compressor and the expander preferably share the same crankshaft which constitutes or is connected to the output shaft of the system.

Alternatively, the expander and the compressor could each consist of a turbine comprising a stator housing a rotor comprising respectively the drive shaft of the expander and the drive shaft of the compressor. The compressor rotor shaft, the holder rotor shaft and the output shaft may constitute a single shaft.

Rather than being connected to a connecting rod crank type system, the pistons of the compressor and/or expander can be connected to a linear electric motor or alternator.

The compressor and the regulator can make it possible to carry out staged expansions and/or compressions. In this case, the compressor and the expander will be staged to compress or expand several times at different pressures, according to the principle represented in figure 4 . Several expanders/compressors (two or more) can thus be implemented with means for admitting into a compressor/expander the fluid coming from another compressor expander with a view to expanding or compressing it at a different pressure than in the previous one.

Inside the expander 18 and the compressor 23, a plurality of cycles are successively produced which will be described in more detail subsequently in relation to the description of the mechanical energy production process.

The system according to the invention obviously comprises control means for controlling the opening and closing of the different orifices (inlets, outlets) of the regulator and the compressor in order to synchronize these cycles and their different phases (admission, expansion, compression, exhaust). Such means are known per se and are not described in detail.

The system includes means for heating the vaporized nitrogen and/or compressed air or nitrogen gas before admission into the regulator or reheating inside the regulator.

In reference to the Figure 3 , such heating means include an external heating system 40 placed on the pipes 17 and/or 24. They can alternatively or in addition comprise an internal heating system 41 allowing the injection of fluid into the regulator which ensures heating.

In the different variants, the heating means increase the temperature of the gas, for example by the direct injection of a hot fluid such as water without combustion or of a fluid with combustion such as gasoline. In the variants in which the reheating takes place outside the regulator, it can be obtained by means of a heat exchanger heated by a heat transfer fluid itself heated by a heat source: solar concentration, gas or gasoline combustion . It could also be a system for heating the walls of the regulator. This is applicable in the context of the simplified version.

There figure 4 illustrates a variant according to which the regulator is stepped, that is to say it comprises several regulators 18, 18' placed in series, the partially expanded vaporized nitrogen exhaust 181 from one (18) being connected to the inlet of partially expanded vaporized nitrogen 180' from the other (18') via a pipe 42. The inlets/outlets 181' (optionally connected to the orifice 162 of the exchanger 16 via pipe 19) and 183 are arranged in the low pressure regulator 18' and the inlets/outlets 180 and 182 are arranged in the high pressure regulator 18.

In this case, the heating means comprise an external heating system 40 placed on the pipes 17 and/or 24. They can alternatively or in addition comprise an internal heating system 41 allowing the injection of fluid into the regulator (hot fluid such as water without combustion inside the regulator or fluid with combustion inside the regulator) which ensures heating. They may also include an external heating system 43 placed on the pipe(s) 42 and/or an internal heating system 44 (of the type of system 41) placed in the regulator(s) 18'. It could also be a system for heating the walls of the regulator(s).

A system such as that described in relation to the figure 4 can also make it possible to create a staged compressor by placing several compressors 18' and 18 in series. In this case, the internal and/or external heating means are in made means of cooling.

In a variant, in which the pressure in the pipes 17 and 24 is substantially equal, these two pipes can be connected and open into the same and single inlet 180 or 182.

In another variant in which the pressures inside the pipes 17 and 24 will be different, two regulators can be used, one into which the pipe 17 opens and the other into which the pipe 24 opens.

In another variant, the expansion of the cooled air or nitrogen can take place after compression in the compressor 23 which will then act as an expander after the compression phase.

The mechanical energy due to the expansion inside the compressor 23 will then be recovered at its drive shaft. Compressor 23, which will then be a compressor/expander, will include an additional orifice 234 for the exhaust of cold air or nitrogen compressed therein and means of heating in the expander/compressor and/or on line 24 .

The tank, the pipes and the valve connected to it are optional. What is important is that the system includes a liquid nitrogen inlet intended to be connected to a pressurized liquid nitrogen supply device.

6.1.2. Mechanical energy production process

A process for producing mechanical energy from liquid nitrogen will now be described in relation to the Figure 6 .

i. Simplified version

The method described in this paragraph corresponds to the implementation of the simplified version of the system described in relation to the figure 1 in a variant in which the expander-compressor thereof comprises a liner-piston assembly whose piston is connected to a crankshaft.

When starting the system, it is necessary to fill with gaseous nitrogen (i.e. vaporized) under pressure (for example 300 bar) the pipe 302' of the exchanger and the pipe 304 which is connected to the regulator, before sending the liquid nitrogen under pressure into these pipes, in order to avoid vaporizing too large a quantity of liquid nitrogen due to the fact that these pipes can be at ambient temperature and which would have the effect of increase the pressure in the circuit more than necessary.

For this, it is possible to plan for example to implement a step of admitting liquid nitrogen into the pipe 302' (and which constitute a buffer reserve with the pipe 304), by opening the orifice 301 in order to introduce into the pipe 302' a small quantity of liquid nitrogen which, once vaporized and heated, will establish the desired pressure level.

The quantity of liquid nitrogen to be introduced into the pipes 302' and 304 for this start-up phase is related to their volume and the desired pressure (approximately 200/300 bar).

The start-up phase is followed by an engine launch phase then a stabilized operating phase during which the pressure in the pipes 302', 304 is regulated so as to maintain it at a pressure level determined by playing on the quantity of liquid nitrogen admitted through the orifice 301 of the exchanger and in relation to the quantity of air or gaseous nitrogen admitted into the exchanger.

When the pipes 304 and 302' are pressurized by nitrogen gas, the engine launch phase is implemented.

The piston is initially at top dead center, inlet 305 is open, ports 308, 307 and where applicable 312 are closed. Ports 301 and 303 are also open throughout the launch phase and the stabilized operating phase.

Liquid nitrogen is admitted (step 501) under pressure (approximately 200/300 bars) and at approximately -195°C via pipe 300 into the intake inlet 301 of the exchanger 302 then pipe 302'.

The liquid nitrogen close to -195°C is heated inside the exchanger 302 by the air circulating in the exchanger and is thus vaporized there and reheated until it reaches a temperature close to ambient temperature (step 502). This has the effect of cooling the ambient air in the exchanger to a temperature close to the temperature of the liquid nitrogen which has been introduced (-195°C) (step 53). It takes approximately 1.7 kg of ambient air to bring 1 kg of liquid nitrogen to ambient temperature.

A step 504 of admission, into the regulator 306, of vaporized nitrogen and heated to a temperature close to ambient temperature is implemented.

For this, the nitrogen vaporized and heated to ambient temperature escaping from the heat exchanger 302 via the vaporized nitrogen outlet 303 is routed into the regulator 306 via the pipe 304 and the open inlet 305.

Of course, steps 501 to 504 are simultaneous.

The vaporized nitrogen introduced into the regulator 306 undergoes a step 505 of expansion inducing the descent of the piston towards its bottom dead center and the setting in motion of the crankshaft: this setting in motion constitutes a step 507 of mechanical energy recovery.

A nitrogen reheating step 602 will be implemented before admission (approximately 300/600°C) and/or during expansion (approximately 20 to 140°C if fluid injection without combustion) so as to preferably have an exhaust temperature equal to or greater than the ambient temperature. For this, the heating means 500 and/or 313 will be used. If the reheating takes place before admission, the expansion will be preferentially adiabatic, if the reheating takes place during the expansion, the expansion will be preferentially isothermal.

The inlet 305 closes and the outlet 307 opens to allow the escape of the expanded nitrogen from the regulator 306 (step 506) during the rise of the piston towards its top dead center.

Exhaust outlet 307 closes. The engine launch phase thus ends while the stabilized operating phase begins.

The stabilized operating phase begins with the choice of operating in expansion mode or operating in compression mode.

To properly vaporize the liquid nitrogen in the exchanger, a certain quantity of air must be introduced. Otherwise, the temperature of the vaporized nitrogen leaving the exchanger is not high enough.

To ensure that the temperature of the vaporized nitrogen is sufficient, a step 65 of choosing an expansion mode or a compression mode is implemented.

According to a first variant, this choice step 65 consists of measuring the nitrogen temperature inside the pipe 300 at the outlet 303 of the exchanger or the pipe 304.

When the measured temperature reaches a predetermined threshold signaling that the sufficient temperature has been reached, the process will continue by implementing the relaxation mode.

When the measured temperature does not reach this predetermined threshold, that is to say the nitrogen is insufficiently heated, the process will continue by implementing the compression mode.

The expansion mode includes a step 50 of admitting liquid nitrogen under pressure (approximately 200/300 bars) and at approximately -195°c via pipe 300 into the admission inlet 301 of the exchanger 302 then the pipeline 302'.

The liquid nitrogen close to -195°C is heated inside the exchanger 302 by the air circulating in the exchanger and is thus vaporized there and reheated until it reaches a temperature close to ambient temperature (step 52). This has the effect of cooling the ambient air in the exchanger to a temperature close to the temperature of the liquid nitrogen which has been introduced (-195°C) (step 53). It takes approximately 1.7 kg of ambient air to bring 1 kg of liquid nitrogen to ambient temperature.

A step 54 of admitting, into the regulator 306, vaporized nitrogen and heated to a temperature close to ambient temperature is implemented.

For this, the nitrogen vaporized and heated to ambient temperature escaping from the heat exchanger 302 via the vaporized nitrogen outlet 303 is routed into the regulator 306 via the pipe 304 and the open inlet 305.

Of course, steps 51 to 54 are simultaneous.

The vaporized nitrogen introduced into the regulator 306 undergoes an expansion step 55 inducing the descent of the piston towards its bottom dead center and the setting in motion of the crankshaft: this setting in motion constitutes a step 57 of mechanical energy recovery.

A nitrogen reheating step 62 will be implemented before admission (approximately 300/600°C) and/or during expansion (approximately 20 to 140°C if fluid injection without combustion) so as to have a exhaust temperature preferably equal to or greater than ambient temperature. For this, the heating means 500 and/or 313 will be used. If the reheating takes place before admission, the expansion will be preferentially adiabatic, if the reheating takes place during the expansion, the expansion will be preferentially isothermal.

The inlet 305 closes and the outlet 307 opens to allow the escape of the expanded nitrogen from the regulator 306 (step 56) during the rise of the piston towards its top dead center.

Exhaust outlet 307 closes.

• étape 51 d'admission dans l'échangeur d'air ambiant;
• étape 53 de refroidissement de l'air dans l'échangeur ;
• étape 53' d'admission dans le compresseur.

Compression mode includes simultaneous implementation of the following steps:

• step 51 of admission into the ambient air exchanger;
• step 53 of cooling the air in the exchanger;
• stage 53 'of admission into the compressor.

To carry out steps 51, 53 and 53', the piston moves again towards its bottom dead center. The cooled ambient air leaving the exchanger 302 via the outlet 310 undergoes an admission step 53' into the compressor 306, flowing into the pipe 309 and passing through the inlet 308 of the compressor.

Inlet 308 closes and the piston rises towards its top dead center. The cold air (approximately -195°) then undergoes a step 58 of adiabatic compression (with a pressure ratio of approximately 50) while the piston rises towards its top dead center in the compressor 302. This adiabatic compression has the effect to increase the air temperature to room temperature.

The compressed air is kept in the chamber and under the effect of the pressure inside the compressor, the piston moves back to its bottom dead center, while the compressed air expands there (step 59) . An air heating step 63 will be implemented during the expansion so that it is preferably isothermal. For this, the heating means 313 will be used.

This relaxation induces the movement of the piston and the rotation of the crankshaft and thus the recovery of mechanical energy (step 61). When the piston arrives at bottom dead center, port 307 opens then the piston rises towards its top dead center. The air which still has a certain pressure then escapes from the compressor (step 60) into the ambient air.

Exhaust outlet 307 closes.

At the end of the expansion mode as well as that of the compression mode, a new step 65 of choice of the expansion mode or the compressor mode is implemented, then a new cycle is initiated.

According to this first variant, step 65 of choice between compression mode and expansion mode is optimized by controlling the temperature of the vaporized nitrogen at the outlet of the exchanger. According to a complementary or alternative variant, this step 65 can be replaced by programming the sequencing of the expansion and compression modes according to the need for heating of the liquid nitrogen and based on the quantity of air necessary for the heating and vaporization of liquid nitrogen. Indeed, we know that to vaporize and heat up to ambient temperature 1 kg of liquid nitrogen requires approximately 1.7 kg of air at ambient temperature, it will therefore be necessary to alternately introduce into the expansion compressor approximately 1 kg of vaporized liquid nitrogen heated to 300 bar for 1.7 kg of air cooled to 1 bar, or even up to 6 bar if the air is recovered in the exhaust as we will see in a variant a little further on.

These different stages follow one another (in a very short time) to cause the crankshaft to move.

Several identical stages of compression or expansion can follow one another, the aim being that the liquid nitrogen is vaporized and reheated to a temperature close to ambient temperature before being reheated by the heater or being admitted into the expander and that the The air which passes through the exchanger before being compressed is cooled to a temperature close to that of the liquid nitrogen which supplies the engine.

Cooling the air in the exchanger then its adiabatic compression makes it possible to subsequently expand it in the expander with a significantly larger volume, for example of the order of four times, to produce additional mechanical energy. This makes it possible to recover the mechanical energy resulting from the phase change and the heating of the liquid nitrogen and thus to increase the efficiency in terms of mechanical energy production.

Setting the crankshaft of the expander-compressor 306 in motion can, for example, make it possible to rotate an alternator to produce electric current or to move a vehicle.

In the variant according to which the inlet 311 is connected to the outlet 312 of the expander/compressor via the pipe 311', the gas admitted into the exchanger to vaporize the liquid nitrogen is no longer ambient air but comes from there recovery of exhaust gases from the system in engine mode. In this case, the exhaust step 56 (506) consists of causing the expanded nitrogen to escape no longer through the outlet 307, but through the outlet 312, (the exhaust through the outlet 307 occurs alternately with the outlet 312 when the pressure in the pipe 311', which constitutes a buffer tank, has reached a determined pressure threshold, if the pressure is sufficient in the pipe 311', the exhaust takes place into the open air through the orifice 307 , if the pressure in the pipe 311' is insufficient, the exhaust takes place via the outlet 312 in the pipe 311'). These exhaust gases, still under pressure (approximately 6 bars), are introduced into the exchanger (step 51) and are used to vaporize the liquid nitrogen during step 52. This facilitates heat exchange inside. of the exchanger and reduce its size. In addition, more gas is compressed at each revolution of the engine during the compression step 58. The implementation of this variant requires a start-up step during which the liquid nitrogen, vaporized, heated and expanded in the regulator, escapes into the pipe 311' through the orifice 308. This step is repeated until the pressure in the pipe 311' reaches a predetermined pressure threshold, for example between 1 to 6 bar. Then, during stabilized operation, the pressure in the pipe is maintained by adjusting the quantity of nitrogen admitted through the orifice 312, when the pressure in the pipe 311' is sufficient the exhaust takes place alternately with air free through port 307.

The other advantage of this variant is that if we recover the exhaust gases at around 6 bars and apply the pressure ratio of 50 for the adiabatic compression of cold nitrogen, we end up with a large quantity of nitrogen at 300 bars at ambient temperature in the compression chamber (when the piston reaches top dead center). We can therefore let this gas escape at high pressure through the orifice 305 to store it momentarily in the pipe 304, where the vaporized liquid nitrogen is located which is at the same pressure of 300 bars and at the same temperature and continue subsequently by several expansions of gas coming from pipeline 304 before carrying out a new compression. The fact of reintroducing the newly compressed gases into the pipe 304 possibly makes it possible to implement the external reheating step 62. After the compression step 58, the inlet 305 opens so that all or part of the compressed air or nitrogen flows into pipe 304 where it joins the nitrogen vaporized under pressure (exhaust stage 400). If all of the air or compressed nitrogen flows into line 304, the cycle continues with step 54. If only part of the air or compressed nitrogen flows into the pipe 304, the cycle continues with step 59. The compression of the cold gas coming from the exchanger can generate the production of too large a volume of compressed gas in the compressor. This volume is so large that it is not possible to completely relax it afterwards except by admitting a smaller quantity of cold gas, or by releasing gases still under pressure into the ambient air, which reduces the effectiveness of the system.

In another variant where the compressor/expander is staged with a high pressure chamber and a low pressure chamber and which can be combined with the variant according to which the inlet 311 of the exchanger is connected to the outlet 312 of the regulator/ compressor through pipe 311', the compression of the cold air coming from the exchanger (step 58) can be done only in the high pressure chamber, following which part of the compressed cold air escapes of the compressor to join the heated vaporized nitrogen in line 304 (step 400) while the other part is directly expanded in the high pressure chamber before joining the low pressure chamber to be completely expanded (step 59).

Steps 51, 53, 53', 58 of admission and compression of cold air in the exchanger and the expander, intervene as previously when the liquid nitrogen in line 304 is insufficiently heated.

The high pressure compressor/expander will include ports 308 and 305, the low pressure compressor/expander will include ports 312 and 307. The two compressors will each include an additional port connected by a pipe acting as a buffer tank and which may include a heater.

In another variant and according to the Figure 10 , the low pressure stage may include two regulators, one of which may include an exhaust to the open air via outlet 307 and the other an exhaust via outlet 312 connected to the inlet of the exchanger 311 via the pipeline 311'. In this last variant, the exhaust will therefore take place simultaneously in the open air and in pipe 311'. This last variant which has 3 cylinders perhaps transformed into a liquid nitrogen generator according to the general indications expressed in the liquid nitrogen generator section depending on the variant where the compressor can be staged. For this we will use the low pressure compressor/expander 306' and the high pressure expander 306 as an isothermal staged compressor while the low pressure expander 313" will be used as an expander.

ii. Advanced version

The process described in this paragraph corresponds to the implementation of the evolved version of the system described in relation to the figure 2 in a variant in which the expanders and compressor each comprise a liner-piston assembly whose piston is connected to a crankshaft.

When starting the system, it is necessary to fill the pipe 16' of the exchanger and the pipe 17 (connected to the regulator 18), with nitrogen gas under pressure (for example 300 bar) before sending the liquid nitrogen under pressure in these pipes, this is to avoid vaporizing too large a quantity of liquid nitrogen due to the fact that these pipes are at ambient temperature and which could have the effect of increasing the pressure in the circuit more than necessary .

For this, it is possible to plan for example to implement a step of admitting liquid nitrogen into the pipe 16' (and which constitute a buffer reserve with the pipe 17), by opening the orifice 160 in order to introduce into pipe 16' a small quantity of liquid nitrogen which, once vaporized and heated, will establish the desired pressure level.

The quantity of liquid nitrogen to be introduced into the pipes 16' and 17 for this start-up phase is related to their volume and the desired pressure (for example 300 bar).

The start-up phase also requires placing pipe 24 under a pressure of approximately 50 bar (300 bar if the exhaust gas recovery option is at 6 bar) during the first rounds and to the extent that the volume of the pipe represents a certain volume in relation to the displacement of the compressor. The start-up and therefore the pressurization of this pipeline makes it possible to reach the compression rate necessary for heating the cold gas, during stabilized operation, and which comes from the exchanger, by the adiabatic compression in the compressor. For this it is possible to provide for example that the cooled air which comes from the exchanger 16 or the vaporized nitrogen expanded in the regulator 18 and which arrives in the adiabatic compressor 23 via the pipe 22 via the orifice 230 is admitted in the compressor 23 during the descent of the piston 235 then compressed when the piston rises to escape into the pipe 24 and the cycle is started again until the pressure in the pipe 24 reaches a predetermined threshold value and while continuing to maintain the pressure in pipes 16' and 17.

The start-up phase is followed by a stabilized operating phase during which the pressure in the pipe 16', 17 is regulated so as to maintain it at a pressure level determined by adjusting the quantity of liquid nitrogen admitted by the orifice 160 of the exchanger and in relation to the quantity of air or nitrogen gas admitted into the exchanger.

The pressure in the pipe is 24 is also regulated for example by measuring the pressure inside it and by varying the quantity of gas which enters through the orifice 232 of the compressor and which exits through the orifice 282 of the regulator.

When the pipes 16', 17 and 24 are pressurized by nitrogen gas, stabilized operation can begin.

In stabilized operation, ports 160 and 161 are open.

The regulator piston is initially at top dead center, port 180 is open. Ports 183, 181 and 182 are closed.

The process comprises a step of vaporizing liquid nitrogen, in the heat exchanger 16 in which ambient air or nitrogen passes substantially at ambient temperature and still under pressure coming from the expander 18 and which is cooled during of its passage through the exchanger 16 via pipe 16'. The gaseous nitrogen obtained during vaporization which, given the pressure, is in a critical phase (vapour/liquid), is heated before being expanded.

For this, liquid nitrogen at approximately -195°C is taken from the tank 10 by means of the pump 13 so that it passes through the outlet 11 of the tank and flows into the pipes 12 and 15 until 'to be admitted into the intake inlet 160 of the exchanger 16, then 16' (intake stage 50) at a pressure close to approximately 300 bar. Ambient air is admitted via inlet 162 in the exchanger 16 (intake step 51).

The liquid nitrogen in the pipe 16' is heated inside the exchanger 16 by the air circulating in the exchanger and is thus vaporized there (vaporization step 52) and reheated to a temperature close to ambient temperature , while the air circulating in the exchanger is cooled (step 53) to a temperature close to the temperature at which the liquid nitrogen is introduced into the exchanger (approximately -195°C).

The method then comprises a step 54 of admitting, into the regulator 18, vaporized nitrogen coming from the exchanger 16 and which is at a pressure close to approximately 300 bar and at a temperature for example close to ambient temperature. .

For this, the vaporized nitrogen escaping from the heat exchanger 16 via the vaporized nitrogen outlet 161 is routed into the regulator 18 via the pipe 17 and the vaporized nitrogen inlet 180 which opens while the regulator piston is at top dead center.

Of course, steps 50, 52 and 54 take place simultaneously.

The vaporized nitrogen introduced into the regulator 18 undergoes an expansion step 55 inducing a movement of the piston, towards its bottom dead center, and the setting in motion of the drive shaft 184 of the regulator, that is to say say the crankshaft. This corresponds to a mechanical energy recovery step 57.

A reheating step 62 (between approximately 300°C to 600°C) of the nitrogen will be implemented before admission and/or during expansion (10°C to 140°C if fluid injection without combustion). For this, the heating means 40 and/or 41 will be used.

After the piston reaches bottom dead center, outlet 183 opens and the piston rises toward top dead center. The vaporized and then expanded nitrogen then undergoes an exhaust stage 56 via outlet 183.

In a variant in which the outlet 181 is connected to the inlet 162 of the exchanger, the outlet 181 opens alternately with the free air exhaust outlet 183, in such a way as to obtain a constant pressure (approximately 1 to 6 bars) in the network between orifice 181 and orifice 230 and to ensure the exhaust stage 56.

In this case, the nitrogen coming from outlet 181 of the regulator is admitted under pressure (approximately 1 to 6 bar) via pipe 19 into inlet 162 in exchanger 16 during admission step 51, rather than ambient air. When the pressure in pipe 19 is sufficient, the exhaust takes place through port 183 into the open air.

The implementation of this variant requires a start-up step during which the liquid nitrogen, vaporized, heated and expanded in the regulator, escapes into pipe 19 through orifice 181 (step 56) without being admitted into the compressor. This step is repeated until the pressure in the network between orifice 181 and orifice 230 reaches a predetermined pressure threshold, for example 1 to 6 bar. In stabilized operation, the exhaust takes place alternately through port 183 or 181 in such a way as to maintain the desired pressure in the network between port 181 and 230.

According to this variant, during the vaporization step 52, the liquid nitrogen is then heated inside the exchanger 16 by the nitrogen coming from the expander circulating in the exchanger, the nitrogen coming from the expander being cooled there during step 53.

The admission into the exchanger of nitrogen under pressure from the regulator rather than ambient air makes it possible to increase the efficiency of the exchanger and therefore to reduce the size of the system.

The cooled air or nitrogen leaving the exchanger 16 via the outlet 163 undergoes an admission stage 53' into the compressor 23, flowing through the pipes 20, 22 and passing through the inlet 230 of the compressor. . To do this, inlet 230 opens while the compressor piston goes from top dead center to bottom dead center and the regulator piston goes from bottom dead center to top dead center during the exhaust step 56.

Of course, steps 51, 53 and 53' take place simultaneously.

Inlet 230 closes and the piston rises towards top dead center. The cooled air or nitrogen undergoes a compression step 58 in the compressor 23. This compression is preferably adiabatic and has the effect of heating the gas which is at a temperature close to -195°c for example, up to a temperature close to ambient temperature due to compression. The outlet 232 opens and the compressed air or nitrogen then escapes from the compressor (with a temperature close to ambient temperature for example and which is due to compression) then flows into line 24 acting as a buffer tank during an exhaust stage 58'. Port 232 closes and the compressor piston returns to its bottom dead center while port 182 opens so that this compressed air or nitrogen undergoes a step 54' of admission into the expander through the inlet 182. It undergoes an expansion stage 59 inducing the descent of the piston of the regulator to its bottom dead center and the setting in motion of the drive shaft 184 of the regulator (step 61), then exhaust (step 60) when the piston rises to its top dead center while port 183 opens.

A nitrogen reheating step 62 will be implemented before admission and/or during expansion. For this, the heating means 40 and/or 41 will be used.

The cooling of air or gaseous nitrogen in the exchanger (passed through by the liquid nitrogen which comes from the pump and the tank and is vaporized and heated there) followed by a compression, preferably adiabatic, and a expansion with a thermal energy input into the expander which generates a significantly larger volume of gas, for example of the order of four times, produces an additional mechanical energy.

This makes it possible to recover the mechanical energy resulting from the phase change and the heating of the liquid nitrogen and thus to increase the yield compared to a simple expansion of the liquid nitrogen after pressurization and vaporization.

When the pressures in pipes 17 and 24 are substantially equal (approximately 300 bars), the admission into the regulator of vaporized nitrogen and the admission of air or compressed nitrogen can be simultaneous. These pipes can also be connected together to achieve admission in a single step and at a single entry point. When the pressures in these two pipes are different, the expansion of vaporized nitrogen and the expansion of air or compressed nitrogen will be offset in time, the first expansion taking place with the higher pressure fluid circulating in them. and the next with the fluid at lower pressure circulating in them.

When the pressures in these two pipes are different, two different regulators can be used, one to expand the vaporized nitrogen, the other to expand the air or compressed nitrogen. The two relaxations can then take place simultaneously.

If the pressure in line 19 is 6 bars, automatically the pressure at the outlet of the compressor with a pressure ratio of 50 (to obtain the necessary heating) will be 300 bars while the pressure of the vaporized nitrogen will be 300 bars .

The setting in motion of the crankshaft or more generally of the drive shaft 184 of the regulator 18 due to the expansion of the vaporized nitrogen coming from the exchanger and the compressed air or nitrogen coming from the compressor constitutes a stage of recovery (or production) of mechanical energy.

These different stages follow one another to cause the crankshaft to move.

Setting the drive shaft 184 of the regulator 18 in motion can for example make it possible to rotate an alternator 28 to produce electric current or to move a vehicle. When the drive shaft of the expander and that of the compressor are connected or constitute the same shaft, the mechanical energy generated by the expander makes it possible to drive the compressor. Otherwise, means of driving the compressor must be used, such as for example an electric motor or the like.

In another variant in which the compressor 23 is a compressor/expander, after the compression step 58, the orifice 232 opens so that all or part of the compressed air or nitrogen flows into the pipe 24 which acts as a buffer pipe.

If all of the air or compressed nitrogen flows into line 24, the cycle continues with a new admission into the regulator 23 of the air or compressed nitrogen coming from line 24 via the orifice 232 then expansion and exhaust through orifice 234.

The compressed air or nitrogen present in pipe 24 could be partly expanded in regulator 23 and partly in regulator 18.

If part of the compressed air or nitrogen remains in the compressor/expander without flowing into the buffer pipe 24, the cycle includes an expansion in the regulator 23 of this compressed air or nitrogen then and an exhaust through the orifice 234 .

The advantage of expanding part of the gas in the regulator 23 is that if the machine is reversible into a liquid nitrogen generator, the compressor 23 which is then reversible, must represent a larger displacement than if it is used as a compressor alone.

All of the pipes and the heat exchanger preferably constitute buffer reserves so as to have the different fluids necessary at each stage. This will make it easy to synchronize the different stages of the process.

6.2. Liquid nitrogen production 6.2.1. Mechanical liquid nitrogen production system

The invention relates to a mechanical system for producing liquid nitrogen.

Components of this system structurally identical to components of the mechanical energy production system previously described in relation to the figure 2 bear the same numbers although their functions in one system or the other may be different.

• une entrée d'air ou d'azote gazeux 183 ;
• une première sortie d'air ou d'azote comprimé 180 ;
• une deuxième sortie d'air ou d'azote comprimé 182 ;
• une entrée d'azote non liquéfié 181.

In reference to the figure 5 , such a system includes an isothermal compressor 18. This compressor 18 includes:

• an air or nitrogen gas inlet 183;
• a first compressed air or nitrogen outlet 180;
• a second compressed air or nitrogen outlet 182;
• an inlet of non-liquefied nitrogen 181.

The first compressed air or nitrogen outlet 180 is connected by a pipe 17 to the compressed air or nitrogen intake inlet 161 of a heat exchanger 16.

The heat exchanger 16 includes an outlet of compressed and cooled air or nitrogen 160. This outlet 160 is connected to a pipe 15 connected to a valve 14.

The exchanger 16 is crossed by the pipe 16' which starts from the inlet 161 of the exchanger to the outlet 160 of the exchanger. The pipe 16' serves as a heat exchange surface in the exchanger with the fluid which passes through it internally, the compressed nitrogen and the fluid which passes through it externally the cold nitrogen which comes from the regulator 23. The pipe 16' can be made up of a set of plates stacked on top of each other and which form conduits or even, be made up of multiple pipes connected, therefore, to the inlet 161 and the outlet 160 of the exchanger.

The valve 14 is connected by a pipe 25 to an inlet 231 of compressed and cooled air or nitrogen of a regulator 23.

The valve 14 is connected to a pipe 12 on which a pump 13 is mounted and which is connected to a liquid nitrogen outlet 11 of the tank 10. The valve 14 as well as the pipe 12, the pump 13 and the outlet 11 are optional and are not useful for producing liquid nitrogen.

The regulator 23 comprises an inlet 232 of compressed air or nitrogen which is connected by a pipe 24 to the second outlet of compressed air or nitrogen 182 of the compressor 18.

The regulator 23 includes an outlet 230 of a mixture of liquid nitrogen and non-liquefied nitrogen. This outlet 230 is connected to a pipe 22 which opens into a valve 21.

The valve 21 integrates means for separating a liquid phase and a gas phase.

The valve 21 comprises an outlet connected by a pipe 26 to the liquid nitrogen inlet 11' of the liquid nitrogen tank 10.

The valve 21 has an outlet connected to a pipe 20 which opens to the non-liquefied nitrogen inlet 163 of the heat exchanger 16.

The heat exchanger 16 comprises a heated non-liquefied nitrogen outlet 162 which is connected by a pipe 19 to the non-liquefied nitrogen inlet 181 of the compressor 18.

The expander 23 and the compressor 18 each include a drive shaft 233, 184.

Exit 234 is not necessary.

The system includes an output shaft 27.

The system comprises means for driving the movement of the drive shafts, such as for example an electric or wind motor 28.

the drive shaft of the expander is preferably connected to that of the compressor so that the drive means are common to the compressor and the expander.

The compressor drive shaft, the expander drive shaft and the system output shaft may constitute one and the same shaft. The drive means are then connected to the output shaft and can for example include an electric motor 28 or a wind turbine.

The compressor and/or the expander can be staged to compress or expand several times at different pressures, according to the principle shown in figure 4 . Several expanders/compressors (two or more) can thus be implemented with means for admitting into a compressor/expander the fluid coming from another compressor expander with a view to expanding or compressing it at a different pressure than in the previous one.

The expander 18 and the compressor 23 each comprise one or more pistons 185, 235 mounted movable in translation in one or more chambers 186, 236 and connected by means of connecting rods 187, 237 to a crankshaft 188, 238. The crankshaft of the expander constitutes the The expansion valve drive shaft and the compressor crankshaft constitute the compressor drive shaft.

Rather than being connected to a connecting rod crank type system, the pistons of the compressor and/or expander can be connected to a linear electric motor or alternator.

The compressor and the expander preferably share the same crankshaft which constitutes or is connected to the output shaft of the system.

Alternatively, the expander and the compressor could each consist of a turbine comprising a stator housing a rotor comprising respectively the drive shaft of the expander and the drive shaft of the compressor. The compressor rotor shaft, the holder rotor shaft and the output shaft may constitute a single shaft.

The system includes means for cooling the compressed nitrogen in the compressor either inside the compressor and/or at the exhaust when the compressor is staged. The cooling means make it possible to evacuate the heat generated by compression and to reduce the compression force in such a way that the volume of the gas does not increase.

These cooling means are preferably identical to the means for heating the system in engine mode which are then reversible. They can in any case be placed in approximately the same places. In the case of staged compression, they allow cooling between each compression.

In reference to the Figure 3 , such cooling means include an external cooling system 40' placed on the pipes 17 and/or 24. They can alternatively or in addition include a cooling system. internal cooling 41' allowing the injection of fluid into the regulator (cold fluid like water) which ensures cooling. It can also be a compressor wall cooling system.

In reference to the figure 4 , they include internal cooling means 44', 41' and/or external cooling means 43', 40'.

In a variant, two regulators could be used. One of these regulators will include an inlet 231 connected to pipe 25 and an outlet 230 connected to pipe 22. The other will include an inlet 232 connected to pipe 24 and an outlet 230' connected to pipe 25.

In a variant, two compressors can be used. One of the compressors will include an inlet 181 connected to pipe 19, an outlet 180 connected to pipe 17 and, where appropriate, an inlet 183. The other will include an inlet 181' connected to pipe 19, an outlet 182 connected to pipe 24 and, where applicable, an inlet 183.

In a variant illustrated in Figure 9 , if two expanders and two compressors are used, according to the previous variants, there can be two separate return and return circuits.

The first circuit is composed of the original circuit with the isothermal compressor 18 and the adiabatic expander 23, the exchanger 16, the two pipe networks between the orifices 230,181 and 231,180, the separator 21 with the liquid nitrogen outlet 26 and the 'entry 183 of nitrogen gas. Line 24 is found in the second circuit.

The second circuit is composed of the second compressor 18' and which then includes the outlet 182 connected to the pipe 24 itself connected to the second regulator 23' via the inlet 232 while a new network of pipes 1, 2, 3 connects the outlet 230' of the expander 23' by crossing the exchanger 16 via the inlet 163' and the outlet 162' to the inlet 181' of the second isothermal compressor 18'. The exchanger 16 is therefore crossed by two pipes 2 and 16'.

The two circuits composed mainly of two networks each are independent of each other and can then operate at different pressures, the second circuit which is completely closed and which can include a gas other than nitrogen ensures the cooling of the exchanger 16 while the first circuit generates liquid nitrogen by expanding the nitrogen cooled in the exchanger and compressed in the compressor. The cooling circuit compensates for the liquid nitrogen produced and which does not participate in the cooling of the compressed nitrogen admitted into the exchanger. The regulation of the cooling circuit can be ensured by a temperature probe placed on pipe 25 which supplies the regulator where the liquid nitrogen is generated. The compressor 18 and the expander 23 can be driven by an electric motor connected by the same drive shaft. While the compressor 18' and the regulator 23' can also be connected by another drive shaft and driven by another electric motor and rotating at a different speed, so as to be able to finely regulate the temperature of the compressed nitrogen which enters the regulator 23, this temperature being preferentially must be close to the liquefaction point.

As will be described later in relation to the process for producing liquid nitrogen, the system is designed so that, during its implementation, a succession of cycles occurs in the compressor and in the expander.

The system according to the invention obviously comprises control means for controlling the opening and closing of the different orifices (inlets, outlets) of the regulator and the compressor in order to synchronize these cycles and their different phases (admission, expansion, compression, exhaust). Such means are known per se and are not described in detail.

In a variant and in relation to figures 5 And 9 , inlet 183 is not necessary, the gaseous nitrogen being injected, already under pressure, for example in line 19.

6.2.2. Liquid nitrogen production process

A process for producing liquid nitrogen will now be described in relation to the Figure 7 .

In operation, the system includes two circuits each connected to the compressor and the regulator so that the nitrogen circulates there in a closed circuit.

The first circuit, operating at high pressure (for example between 5 and 100 bars), connects the orifices 180 to 231. The second circuit, operating at low pressure (for example between 1 and 10 bars), connects the orifices 230 to 181. The nitrogen gas contained in the first circuit under pressure by means of the compressor is expanded in the second circuit by means of the regulator.

The pressures in the two circuits will be regulated so as to maintain at a predetermined pressure level by adjusting the quantities of fluids admitted into the regulator on the one hand and the nitrogen brought into the system by the compressor on the other hand.

When the system is started, an air or nitrogen admission step 70 via the inlet 183 and a compression step 72 in the compressor 18 are implemented so as to supply the first circuit (step d exhaust 73) via port 180 to put it under pressure. To do this, the compressor 18 is operated until the pressure in the first circuit reaches a predetermined threshold value (step 74).

Once the first high pressure circuit has been pressurized, the second circuit is filled via the regulator while maintaining the pressure in the first circuit via the compressor.

To do this, we continue to admit air or nitrogen gas into the compressor (step 70) and compress it isothermally in the compressor (step 72) while implementing an admission step 76 in the expander. of compressed nitrogen gas in order to relax it there (step 79) then to extract it (exhaust step 790) in order to introduce it into the second circuit via the orifice 230. These steps are implemented in loop until the pressure inside the first circuit and the pressure inside the second circuit reach their respective predetermined threshold value (steps 74 and 75). As soon as these pressures are reached, which means that the system is stabilized, the step 70 of admitting external nitrogen gas stops.

This is an example of a getting started phase. Another start-up mode could be implemented.

During this start-up phase, the nitrogen, expanding adiabatically, cools. Its circulation in the second circuit therefore induces a start of cooling of the compressed nitrogen gas in the first circuit due to the fact that these two circuits pass through the heat exchanger.

The first and second circuits constitute buffer reserves.

The start-up phase is followed by a stabilized operating phase, the implementation of which makes it possible to produce liquid nitrogen.

During the stabilized operation phase, the ambient nitrogen admission step 70 is stopped. The system then operates in a closed circuit.

• une étape d'admission 80 dans le compresseur d'azote non liquéfié réchauffé en provenance de l'échangeur 16 via la canalisation 19 et l'entrée 181 (la production de cet azote non liquéfié apparaîtra plus clairement par la suite ;
• une étape 81 de compression isotherme dans le compresseur de l'azote non liquéfié ;
• une étape 82 d'échappement du compresseur d'azote gazeux non liquéfié comprimé ;
• une étape 83 d'admission de l'azote gazeux non liquéfié comprimé s'échappant du compresseur par la sortie 180 à l'entrée 161 de l'échangeur de chaleur 16 ;
• une étape 84 de refroidissement à l'intérieur de l'échangeur de l'azote gazeux non liquéfié comprimé (elle sera plus clairement décrite ultérieurement) ;
• une étape 85 d'admission, dans le détendeur 23, d'azote gazeux comprimé puis refroidi en provenance de l'échangeur de chaleur 16 : pour cela, l'azote gazeux comprimé puis refroidi s'échappe de l'échangeur de chaleur 16 par la sortie 160 puis est admis dans le détendeur 23 via l'entrée 231 ;
• une étape de détente adiabatique 86 dans le détendeur de l'azote gazeux comprimé puis refroidi : cette détente induit la production d'azote liquide, la liquéfaction de l'azote dans le détendeur n'étant pas totale ;
• une étape d'échappement 87 d'un mélange d'azote liquide et d'azote non liquéfié par la sortie 230 du détendeur 23 ;
• une étape 88 de séparation de phases permettant de séparer l'azote liquéfié de l'azote gazeux non liquéfié : le mélange d'azote liquide et d'azote non liquéfié est acheminé via la canalisation 22 dans la vanne 21 intégrant des moyens de séparation d'une phase liquide et d'une phase gazeuse ;
• une étape 89 de récupération d'azote liquide et éventuellement une étape d'acheminement de l'azote liquéfié dans le réservoir 10 via la canalisation 26 et l'entrée 11' ;
• une étape 90 de récupération d'azote non liquéfié ;
• une étape 91 d'admission dans l'échangeur de chaleur via l'entrée 163 de l'azote gazeux non liquéfié : la circulation de l'azote gazeux non liquéfié, qui est froid, dans l'échangeur permet de refroidir l'azote gazeux comprimé à l'étape 84 et donc de réchauffer l'azote gazeux non liquéfié (étape 92).

In this closed circuit operation, are successively put into operation performs the following steps in a loop:

• an admission step 80 into the heated non-liquefied nitrogen compressor coming from the exchanger 16 via pipe 19 and inlet 181 (the production of this non-liquefied nitrogen will appear more clearly later;
• a step 81 of isothermal compression in the compressor of non-liquefied nitrogen;
• a step 82 for exhausting the compressed non-liquefied nitrogen gas compressor;
• a step 83 of admitting the compressed non-liquefied gaseous nitrogen escaping from the compressor via the outlet 180 to the inlet 161 of the heat exchanger 16;
• a cooling step 84 inside the compressed non-liquefied gaseous nitrogen exchanger (it will be more clearly described later);
• a step 85 of admitting, into the regulator 23, gaseous nitrogen compressed then cooled from the heat exchanger 16: for this, the nitrogen gas compressed then cooled escapes from the heat exchanger 16 through the outlet 160 then is admitted into the regulator 23 via the inlet 231;
• an adiabatic expansion step 86 in the compressed and then cooled gaseous nitrogen expander: this expansion induces the production of liquid nitrogen, the liquefaction of the nitrogen in the expander not being complete;
• a step 87 of exhausting a mixture of liquid nitrogen and non-liquefied nitrogen through the outlet 230 of the regulator 23;
• a phase separation step 88 making it possible to separate the liquefied nitrogen from the non-liquefied gaseous nitrogen: the mixture of liquid nitrogen and non-liquefied nitrogen is conveyed via pipe 22 into the valve 21 integrating separation means a liquid phase and a gas phase;
• a step 89 of recovering liquid nitrogen and possibly a step of conveying the liquefied nitrogen into the tank 10 via the pipe 26 and the inlet 11';
• a step 90 for recovering non-liquefied nitrogen;
• a step 91 of admission into the heat exchanger via the inlet 163 of the non-liquefied gaseous nitrogen: the circulation of the non-liquefied gaseous nitrogen, which is cold, in the exchanger makes it possible to cool the gaseous nitrogen compressed in step 84 and therefore heat the non-liquefied nitrogen gas (step 92).

During this closed circuit operation, the production of liquid nitrogen induces a drop in pressure in the circuit consisting of the first and second circuits. To compensate for this drop in pressure, nitrogen gas from outside the circuit must be introduced. For this purpose, rather than admitting non-liquefied nitrogen from the exchanger into the compressor (step 80), ambient gaseous nitrogen is admitted via orifice 183 during an intake step. 93 similar to that implemented during the system start-up phase.

To determine the instant at which the admission step 93 replaces the admission step 80 during the stabilized operating phase, a step 94 of determining at least one piece of information representative of the quantity of nitrogen liquid produced is implemented. This information is compared to a first predetermined threshold value (step 95). The admission step 93 then replaces the admission step 80 as soon as the information representative of the quantity of liquid nitrogen produced reaches this first predetermined threshold value.

One way of determining the moment to switch from an admission 80 to an admission 93, consists of implementing a step of measuring the pressure in the first or in the second circuit, then a step of comparing the pressure value measured at a predetermined low threshold value, the switch from admission step 71 to step 70 taking place upon reaching this threshold value.

Another way of determining the moment to switch from an admission 80 to an admission 93, consists of implementing a step of measuring the quantity by mass or volume of liquid nitrogen produced, then a step of comparing the measured value to a predetermined threshold value, the switching from the admission step 80 to step 93 taking place upon reaching this threshold value.

Once the intake switch has been made, the system operates to produce liquid nitrogen, but no longer in a closed circuit momentarily, the reintroduced gaseous nitrogen being compressed isothermally inside the compressor 18 (step 98).

One operating cycle in external admission mode is enough to put the circuit back under pressure for a few subsequent cycles in internal admission mode.

In the variant according to which the inlet 183 is not necessary, the nitrogen gas will be injected, already under pressure, for example into the pipe 19. The admission step 93 and the compression step 98 are not required.

Part of the cold nitrogen coming from the regulator escapes from the circuit in liquid form. The quantity of hot nitrogen coming from the compressor and introduced into the exchanger is therefore greater than the quantity of cold non-liquefied nitrogen coming from the expander and introduced into the exchanger. When the use of the heat exchanger no longer makes it possible to sufficiently cool the compressed nitrogen gas coming from the compressor, a direct admission cycle into the compressed nitrogen expander is implemented. The quantity of cold nitrogen introduced into the exchanger is thus increased to improve the cooling of the compressed nitrogen.

In stabilized operation, the process thus comprises a step 99 of direct admission into the regulator 23 of another part of the nitrogen gas compressed in the compressor 18. The admission step 99 temporarily replaces step 85 of admission of compressed nitrogen gas.

For this, this part of compressed nitrogen gas escapes from the compressor 18 via the outlet 182 and is routed into the regulator 23 via the pipe 24 and the inlet 232.

The process also includes a step 100 of expanding the compressed gaseous nitrogen admitted directly into the regulator 23. This expansion induces the production of cold non-liquefied nitrogen which, after having escaped from the regulator (step 101), is then admitted into the exchanger via line 20 and inlet 163 to improve the efficiency of the exchanger.

The direct admission 99 into the regulator 23 of gaseous nitrogen coming from the compressor 18 via the pipe 24 is implemented according to the need for cooling in the exchanger 18. A temperature probe T° can for example be placed at the outlet 160 of cooled compressed nitrogen from the exchanger to implement a step 800 of measuring the temperature of the cooled nitrogen leaving the exchanger then a step 801 of comparing this temperature with a high temperature threshold predetermined, to control the implementation of direct admission step 99 when this high threshold is reached. The cooling of the compressed nitrogen inside the exchanger is thus maximized, which makes it possible to increase the subsequent production of liquid nitrogen in the expander. The efficiency of the system is thus improved in terms of liquid nitrogen production.

To stop the implementation of direct admission step 99, a step 802 of comparing the measured temperature with a predetermined low temperature threshold is implemented. Direct admission step 99 is stopped when this low threshold is reached. Compressed nitrogen gas admission step 85 once again replaces direct admission step 99.

An initialization phase must be implemented prior to the direct admission step 99 in order to pressurize the pipe 24. To do this, the orifice 182 is opened instead of the orifice 180 of the compressor in operation until until the pressure in pipe 24 reaches a predetermined threshold value. When this threshold value is reached, we move into a phase of stabilized operation.

During the stabilized operating phase, line 24 is maintained under pressure by opening outlet 182 of the compressor, the nitrogen being able to come either from inlet 183 (when external nitrogen must be reintroduced into the system) either from entry 181.

Two different regulators could be used, one to expand compressed and then cooled nitrogen circulating in line 15, the other to expand compressed nitrogen circulating in line 24. The two expansions could then take place simultaneously. One of this regulator will include an inlet 231 and an outlet 230. The other will include an inlet 232 and an outlet 230.

Two compressors can be used. One of these compressors will include an inlet 181, an outlet 180 and where appropriate an inlet 183. The other will include an inlet 181, an outlet 182 and where appropriate an inlet 183. The two compressions in these compressors can take place simultaneously.

According to Figure 9 , if two expanders and two compressors are used, there can be two separate return and return circuits.

The first circuit is composed of the original circuit with the isothermal compressor 18 and the adiabatic expander 23, the exchanger 16, the two pipe networks between the orifices 230,181 and 231,180, the separator 21 with the liquid nitrogen outlet 26 and the 'entry 183 of nitrogen gas. Line 24 is found in the second circuit.

The second circuit is composed of the second compressor 18' and which then includes the outlet 182 connected to the pipe 24 itself connected to the second regulator 23' via the inlet 232 while a new network of pipes 1, 2, 3 connects the outlet 230' of the expander 23' by crossing the exchanger 16 via the inlet 163' and the outlet 162' to the inlet 181' of the second isothermal compressor 18'. The exchanger 16 is therefore crossed by two pipes 2 and 16'.

The two circuits composed mainly of two networks each are independent of each other and can then operate at different pressures, the second circuit which is completely closed and which can include a gas other than nitrogen ensures the cooling of the exchanger 16 while the first circuit generates liquid nitrogen by expanding the nitrogen cooled in the exchanger and compressed in the compressor. The cooling circuit compensates for the liquid nitrogen produced and which does not participate in the cooling of the compressed nitrogen admitted into the exchanger. The regulation of the cooling circuit is ensured by a temperature probe placed on pipe 25 which supplies the regulator where the liquid nitrogen is generated. The compressor 18 and the expander 23 can be driven by an electric motor, all connected by the same drive shaft. While the compressor 18' and the regulator 23' can also be connected by another drive shaft and driven by another electric motor and rotating at a different speed, so as to be able to finely regulate the temperature of the compressed nitrogen which enters the regulator 23, this temperature can be close to the point of liquefaction. This variant consists of creating an external cold production circuit which passes through the heat exchanger used during the production of liquid nitrogen to cool the gaseous nitrogen compressed in the exchanger.

Inside the system, a plurality of cycles are thus produced successively making it possible to produce liquid nitrogen which is for example stored in the tank 10.

6.3. Production of liquid nitrogen or alternatively mechanical energy

As described above, the invention relates to a mechanical system for producing mechanical energy from liquid nitrogen and a mechanical system for producing liquid nitrogen.

These two systems can be implemented completely independently of each other.

Alternatively, the system may be reversible so that it can operate alternately in a motor mode as a mechanical power production system and in a generator mode as a liquid nitrogen production system.

A mechanical system for producing non-reversible mechanical energy does not include, for example, the valve 14, the valve 21, the pipe 25, the pipe 26 and the orifices 231 and 11'. It may or may not contain pipe 19 and port 181.

A mechanical system for producing non-reversible liquid nitrogen, for example, does not include valve 14, pipe 12, pump 13 and orifices 11 and 234.

A system for producing mechanical energy and producing liquid nitrogen, that is to say a reversible system having a motor mode and a generator mode, comprises all of the components necessary for operation in motor mode and in mode. generator, the valves 14 and 21 making it possible to make certain pipes non-functional in each of the operating modes.

A valve can be placed along pipe 19 to allow in engine mode the admission into the heat exchanger of ambient air and/or compressed gas coming from the regulator, and in generator mode the circulation between outlet 162 of the exchanger and inlet 181 of the compressor.

6.4. Variant

In engine mode, the system that compresses the gas that was used to vaporize the liquid nitrogen, makes it possible to produce a large quantity of compressed gas in a compact system. In fact, the gas is compressed after being cooled since it is used to vaporize liquid nitrogen.

According to a variant, this compressed gas can be injected into an existing regulator, such as for example into the cylinders of the engine of a vehicle in the intake phase. The system according to the invention then constitutes a compressed gas generator capable of performing the function of a turbocharger.

Such a compressed gas generation system could also, for example, power a compressed air engine or a compressed air energy storage system.

Claims (15)

1. System for producing mechanical energy comprising at least:

   - a compressor (306);

   - an expansion valve (306);

   - a heat exchanger (302);

   said system having a motor operating mode in which said system further comprises:

   - means for intake of pressurised liquid nitrogen into an inlet for intake of liquid nitrogen (301) of said exchanger (302), means for intake (311) of air or of gaseous nitrogen into an inlet for intake of air or of gaseous nitrogen of said exchanger (302), means for escape of vapourised nitrogen to a vapourised nitrogen outlet (303) of said exchanger (302) and means for escape of cooled air or nitrogen to another outlet (310) for cooled air or gaseous nitrogen of said exchanger (302);

   - means for intake (305) into said expansion valve (306) of said vapourised nitrogen to expand it therein;

   - means for intake (308) into said compressor (306) of said cooled air or gaseous nitrogen to produce therein compressed air or gaseous nitrogen;

   - means for expanding said compressed air or gaseous nitrogen;

   - means for recovering energy (318) coming from the expansion of said vapourised nitrogen and from the expansion of said compressed air or gaseous nitrogen;

   characterised in that it further comprises:

   - means for reheating (500, 313) the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means;

   - means for reheating (500, 313) said vapourised nitrogen before intake into said expansion valve or inside said expansion valve.
2. System according to claim 1, comprising means for intake of gaseous nitrogen still under pressure, coming from said expansion valve (306) and/or from said expansion means, into the inlet for intake (311) of air or of gaseous nitrogen of said exchanger (302).
3. System according to any one of claims 1 to 2, wherein said expansion valve, said expansion means and said compressor are reversible.
4. System according to any one of claims 1 to 3, wherein said expansion valve, said expansion means and said compressor form a single assembly ensuring at times the function of expansion valve, at times that of expansion means and at times that of compressor.
5. System according to any one of claims 1 to 4, wherein said expansion valve and said compressor are distinct from one another.
6. System according to any one of claims 1 to 5, comprising means for intake into said expansion valve of said compressed air or nitrogen, said expansion valve thus forming said means for expansion of said compressed air or gaseous nitrogen.
7. System according to any one of claims 1 to 6 having an operating mode for generating liquid nitrogen, wherein it comprises:

   - means for intake of compressed gaseous nitrogen into an inlet for intake (161 fig.5) of compressed gaseous nitrogen of said heat exchanger (16, 16' fig.5) to cool it therein;

   - means for intake (231 fig.5) of the compressed and cooled gaseous nitrogen, coming from said exchanger (16 fig.5), into expansion means (23, 235, 236 fig.5) with a view to producing liquid nitrogen;

   - means for separating (21 fig.5) the liquid nitrogen and the non-liquefied nitrogen at the outlet of said expansion means (23, 235, 236 fig.5);

   - means for intake, into an inlet (163 fig.5) for non-liquefied nitrogen of said exchanger (15 fig.5), of the non-liquefied nitrogen coming from said expansion means (23, 235, 236 fig.5);

   - means for intake of the non-liquefied and heated nitrogen coming from said exchanger into compression means with a view to compressing it therein;

   - means for intake, into said inlet (161 fig.5) for compressed gaseous nitrogen of said exchanger (16, 16' fig.5), of the non-liquefied then compressed nitrogen coming from said compression means (18, 185 fig.5);

   - means for direct intake (24 fig.5), into said expansion means (23, 235, 236 fig.5), of the non-liquefied and compressed nitrogen coming from said compression means (18, 182, 186 fig.5) to produce compressed then expanded non-liquefied nitrogen;

   - means for intake of said non-liquefied nitrogen, compressed then expanded, coming from said expansion means (23, 235, 236 fig.5), into said inlet (163 fig.5) for non-liquefied nitrogen of said exchanger (16 fig.5);

   - means for intake (183 fig.5) of gaseous nitrogen into said compression means (18, 185, 186 fig.5) to produce said compressed gaseous nitrogen.
8. System according to claim 7, comprising means for determining at least one piece of information representative of the temperature of the compressed and cooled gaseous nitrogen at the outlet of said exchanger, the direct intake means being implemented to the detriment of said means for intake into said exchanger of non-liquefied then compressed nitrogen coming from said compression means when said piece of information representative of said temperature reaches a predetermined upper threshold then not implemented until said piece of information representative of said temperature reaches a predetermined lower threshold.
9. System according to claim 7 or 8, wherein said means for expanding the non-liquefied then compressed nitrogen coming from said compression means are distinct from said means for expanding said compressed then cooled gaseous nitrogen coming from said exchanger.
10. System according to any one of claims 7 to 9, said system comprising means for going from one mode to the other in a reversible manner.
11. Method for producing mechanical energy from liquid nitrogen, said method comprising at least:

    - a step of vaporising liquid nitrogen in a heat exchanger (302) through which air or gaseous nitrogen substantially at ambient temperature cooled during its passage through said exchanger passes;

    - a step of intake of the vaporised nitrogen coming from the exchanger into an expansion valve (306);

    - a step of expansion in said expansion valve of said vaporised nitrogen generating mechanical energy;

    - a step of intake into a compressor (306) of the air gaseous nitrogen cooled in said exchanger;

    - a step of compression in said compressor (306) of said cooled air or cooled gaseous nitrogen;

    - a step of intake into expansion means of said cooled then compressed air or gaseous nitrogen;

    - a step of expanding said cooled then compressed air or gaseous nitrogen generating mechanical energy;

    - a step of recovering the mechanical energy resulting from said expansions;

    characterised in that it further comprises:

    - a step of reheating the compressed air or gaseous nitrogen before intake into said expansion means or inside said expansion means;

    - a step of reheating said vaporised nitrogen before intake into said expansion valve or inside said expansion valve.
12. Method according to claim 11, wherein the nitrogen taken into said exchanger is expanded gaseous nitrogen coming from said expansion valve and/or from said expansion means.
13. Method for producing liquid nitrogen comprising:

    - a step of cooling compressed air or gaseous nitrogen in a heat exchanger (16 fig.5);

    - a step of expanding in expansion means (23, 235, 236 fig.5) said compressed and cooled air or gaseous nitrogen inducing the production of a mixture of liquid nitrogen and of non-liquefied nitrogen;

    - a step of separating the liquid nitrogen and the non-liquefied nitrogen of said mixture;

    - a step of intake into said heat exchanger (16 fig.5) of the non-liquefied nitrogen to cool therein said compressed air or gaseous nitrogen;

    characterised in that it further comprises:

    - a step of compressing in compression means (18, 185, 186 fig.5) the cooled non-liquefied nitrogen coming from said exchanger;

    - a step of reheating, in said exchanger (16 fig.5), the non-liquefied then compressed nitrogen coming from said compression means;

    - a step of direct intake, into said expansion means (23, 235, 236 fig.5), of the non-liquefied and compressed nitrogen coming from said compression means (18, 185, 186 fig.5) to produce compressed then expanded non-liquefied nitrogen;

    - a step of compressing air or nitrogen to produce said compressed air or gaseous nitrogen;

    - a step of intake of said non-liquefied nitrogen, compressed then expanded, coming from said expansion means (23, 235, 236 fig.5), into said exchanger to reheat it therein.
14. Method according to claim 13, comprising a step of determining at least one piece of information representative of the temperature of the compressed and cooled gaseous nitrogen at the outlet of said exchanger (16 fig.5), said step of direct intake being implemented to the detriment of said means for intake into said exchanger of non-liquefied then compressed nitrogen coming from said compression means when said piece of information representative of said temperature reaches a predetermined upper threshold then not implemented until said piece of information representative of said temperature reaches a predetermined lower threshold.
15. Method according to claim 13 or 14, wherein said step of expanding the non-liquefied then compressed nitrogen coming from said compression means and said step of expanding said compressed then cooled air or gaseous nitrogen coming from said exchanger are carried out simultaneously in distinct expansion means.

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