FR2997445A1 - METHOD AND SYSTEM FOR CONVERTING THERMAL ENERGY INTO MECHANICAL ENERGY, IN PARTICULAR FOR CONVERTING THE THERMAL ENERGY OF THE SEAS - Google Patents

Abstract

The invention relates to a method and a system for converting thermal energy (SF, SC) into mechanical energy by means of a thermodynamic cycle. For this, the invention uses a thermodynamic cycle for which a working fluid composed of two miscible fluids and having different vaporization temperatures circulates in a closed circuit. In this circuit, all the steam flows successively in two turbines (3, 5), the steam being heated in a heat exchanger (17) before passing into the second turbine (5).

Description

The present invention relates to the field of the conversion of thermal energy into mechanical energy, in particular for the conversion of the thermal energy of the sea (ETM). One application of the present invention is in the field of Sea Thermal Energy (ETM or OTEC for Ocean Thermal Energy Conversion) which relates to the use of energy obtained by taking advantage of the temperature difference existing in tropical and subtropical regions between surface and deep sea waters, in particular of the order of 1000 m. Surface water is used for hot spring and deep water for the cold source of a thermodynamic 10 cycle engine. Since the temperature difference between the hot source and the cold source is relatively small, the expected energy yields are also low. Conventional ETM plants typically operate on a Rankine cycle. The patent application WO 81/002229 A1 describes the use of the Rankine cycle in the case of the ETM. Moreover, there is known a variant of this cycle with overheating (Hirn cycle). The Hirn cycle consists in sufficiently heating the working fluid so that, after the expansion, it is always gaseous. But these plants do not offer maximum optimization in terms of efficiency. Other thermodynamic cycles have been developed in order to recover this thermal energy. An example of a Uehara cycle is described in FIG. 1. For this cycle, a working fluid composed of ammonia (NH 3) and water (H 2 O) is used. The working fluid is partially vaporized in the evaporator (1) by means of the hot seawater source (SC). The flow of almost pure NH3 vapor is separated from the liquid in a separation tank (2) and is sent to a first turbine (3) which converts thermal energy into mechanical energy. At the outlet of the turbine (3) only a part of this stream is sent to a second turbine (5) by means of an extractor (4). The liquid at the outlet of the separation tank (2) is used to heat the liquid mixture coming out of a "rich" pump (14) by means of a heat exchanger (15), and after being relaxed (16), is mixed (6) with the flow of almost pure NH3 vapor at the outlet of the second turbine (5). At the inlet of a condenser (9), the liquid part of the flow is extracted in order to send the condenser (9) only the gaseous part (7). The condenser (9) allows a heat exchange between the gaseous portion of the flow and a source of cold seawater (SF). These two streams are mixed (10) at the outlet of the condenser (9). The flow extracted from the working fluid serves to preheat (12) the liquid at the outlet of the "lean" pump (11) before being mixed (13) with this liquid at the inlet of the "rich" pump (14). . The patent application EP 0 649 985 also describes a Uehara cycle.

The Uehara cycle provides slightly better thermal efficiency than the Rankine cycle but requires a higher calorie intake. The thermal efficiency is the ratio between the net energy generated on the calories supplied to the process by the condenser (9) and the evaporator (1). To calculate the net generated energy is added the mechanical energy at the output of the turbines. At this sum we subtract the energy consumed by the pumps included in the process. The invention relates to a method and a system for converting thermal energy into mechanical energy by means of a thermodynamic cycle which has a high thermal efficiency and which does not require a contribution of calories or too much frigories.

For this, the invention uses a thermodynamic cycle for which a working fluid composed of two miscible fluids and having different vaporization temperatures circulates in a closed circuit. In this circuit, all the steam circulates successively in two turbines, the steam being heated before passing into the second turbine.

The method and the system according to the invention The invention relates to a method of converting a thermal energy into mechanical energy, in which a working fluid composed of a first and a second fluid is circulated in a closed circuit. miscible with different vaporization temperatures. For this process, the following steps are carried out: a) heating said working fluid by heat exchange with a first heat source at a temperature above the vaporization temperature of said first fluid; b) separating said heated working fluid into a first vapor-shaped portion essentially comprising said first fluid and a second portion in liquid form comprising at least said second fluid; C) converting a portion of the thermal energy contained in said first portion into mechanical energy by means of a first turbine; d) heating said first portion at the outlet of said first turbine by a heat exchange with said second portion; e) transforming a portion of the heat energy contained in said first heated portion into mechanical energy by means of a second turbine; and f) reforming said working fluid by condensing the vapor contained in at least said first portion by means of a second heat source at a temperature below the vaporization temperature of said first fluid, and mixing said two portions.

According to the invention, said working fluid comprises ammonia and water. Preferably, said working fluid comprises between 90 and 98 mol% of ammonia, preferably substantially 95 mol% of ammonia. Advantageously, said heat sources consist of seawater taken at different depths. Advantageously, the second source of heat consists of seawater taken from a depth greater than or substantially equal to 1000m. According to an alternative embodiment of the invention, said working fluid is heated before the separation step.

According to another embodiment of the invention, said first portion is heated before at least one of the steps of transforming the thermal energy into mechanical energy. According to one embodiment of the invention, after step d), said second portion is cooled with said reformed working fluid.

According to one embodiment of the invention, said first portion is relaxed after said second step of transforming the thermal energy into mechanical energy and after said cooling, said second portion is expanded, said second portion is separated into a vapor phase of said second portion. first fluid and a liquid phase of said second fluid and said vapor phase is mixed with the first portion, only the first fluid being condensed in step f). According to one embodiment of the invention, after said cooling, said second portion is expanded, said two portions are mixed and the mixture is separated into two phases: a vapor phase of said first fluid and a liquid phase of said second fluid, only the first fluid being condensed in step f).

According to one embodiment of the invention, said first portion is expanded before said mixture of said two portions. According to one embodiment of the invention, after said cooling, said second portion is expanded and said two portions are mixed, said two fluids being condensed in step f).

Alternatively, said first portion before the implementation of the condensation is cooled by said first portion after the implementation of the condensation. The invention also relates to a system for converting thermal energy into mechanical energy comprising a closed circuit in which circulates a working fluid composed of a first and a second miscible fluid having different vaporization temperatures. Said closed circuit comprises consecutively: a first heat exchanger for heating said working fluid by means of a first heat source at a temperature higher than the vaporization temperature of said first fluid, a separation flask, in which said working fluid is separated into a first portion essentially comprising said first fluid in vapor form and into a second liquid portion comprising at least said second fluid, a first turbine for converting the thermal energy contained in said first portion into mechanical energy, a second heat exchanger for heating said first portion by means of said second portion, a second turbine for converting the thermal energy contained in said first portion into mechanical energy, means for reforming said working fluid comprising at least a third heat exchanger to condense at least said first po by heat exchange with a second heat source at a temperature below the vaporization temperature of said first fluid and at least one mixer for mixing said two portions. According to the invention, said working fluid comprises ammonia and water.

Preferably, said fluid comprises between 90 and 98 mol% of ammonia, preferably substantially 95 mol% of ammonia. Advantageously, said heat sources consist of seawater taken at different depths. According to an alternative embodiment of the invention, said circuit further comprises at least one heating means before said separation tank and / or before the first turbine and / or before said second turbine. Advantageously, said heating means is a heat pump or a solar water heater. According to one embodiment of the invention, said circuit further comprises a fourth heat exchanger disposed after the second heat exchanger, for cooling said second portion by means of said reformed working fluid. According to one embodiment of the invention, said circuit further comprises means for expanding said second portion disposed after said fourth heat exchanger, a second separation flask for separating said second portion into a vapor phase consisting essentially of said first fluid and a liquid phase comprising the second fluid and a mixer for mixing said vapor phase with said first portion.

According to one embodiment of the invention, the circuit further comprises means for expanding said second portion disposed after said fourth heat exchanger, a second mixer for mixing said two portions and a second separation flask for separating the mixture into a single phase. vapor essentially consisting of said first fluid and a liquid phase comprising at least said second fluid. According to one embodiment of the invention, the circuit further comprises means for expanding the first portion before said second mixer. According to one embodiment of the invention, said system further comprises means for expanding said second portion disposed after said fourth heat exchanger and a second mixer for mixing said two portions. According to one embodiment of the invention, said system further comprises a fifth heat exchanger between said first input portion of said third heat exchanger and said first output portion of said third heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics and advantages of the method according to the invention will appear on reading the following description of nonlimiting examples of embodiments, with reference to the appended figures and described below.

FIG. 1, already described, illustrates a thermodynamic cycle according to the prior art. Figure 2 illustrates the thermodynamic cycle according to the invention. FIG. 3 is a graph showing the evolution of the energy generated and the efficiency as a function of the composition of the working fluid. FIG. 4 is a graph showing the evolution of the energy generated and the efficiency as a function of the inlet pressure of the condenser. FIG. 5 is a graph showing the evolution of the energy generated and the efficiency as a function of the pressure drops. Figure 6 illustrates an alternative embodiment of the invention. Figures 7 to 12 illustrate a portion of the cycle according to the invention for different embodiments of the invention. DETAILED DESCRIPTION OF THE INVENTION The invention relates to a method and a system for converting thermal energy from heat sources into mechanical energy. The heat sources may for example consist of seawater taken from different depths: the hot source (for example at 28 ° C) can be taken from the sea surface, whereas the cold source (for example 4 ° C) may be taken at depths close to or greater than 1000 m. The method and the system are based on the use of a thermodynamic cycle employing a working fluid composed of two miscible fluids in a closed circuit. These two fluids also have the characteristic of having separate vaporization temperatures. For example, the two fluids used can be ammonia (NH 3) and water (H 2 O), these two fluids being miscible and the saturated vapor pressure at 26 ° C. of the ammonia is 1013 kPa whereas for water it is of the order of 23 kPa. The closed circuit used by the method and the system according to the invention is illustrated in FIG. 2. In this FIG. 2, elements similar to the elements used for FIG. 1 have the same reference signs.

The working fluid (for example composed of NH 3 and H 2 O) is partially vaporized in a first heat exchanger, said evaporator (1) by means of heat exchange with the hot source (SC). The hot source is at a temperature greater than the vaporization temperature of the first fluid comprising the working fluid but less than the vaporization temperature of the second fluid comprising the working fluid.

Thus, in the evaporator (1), it vaporizes almost only the first fluid. A source of hot water (SC) at a temperature of about 28 ° C is suitable in the case where the first fluid is ammonia and the second water. The first portion (for example essentially comprising NH3) in almost pure vapor form is separated from the liquid (second portion) in the separation flask (2), called the "Flash HT" flask. This flow is called the first portion of the working fluid. Then, this first portion is sent to a first turbine (3), called "HT turbine" (HT: high temperature). The first turbine (3) allows the transformation of a portion of the thermal energy contained in the first portion into mechanical energy. This mechanical energy may possibly be converted into electrical energy by means of a generator. At the outlet of this first turbine (3), the first portion is heated in a second heat exchanger, said "superheater" (17) which allows a heat exchange with the liquid flow leaving the separation tank (2), this flow is called the second portion of the working fluid. The first almost pure portion is then sent to a second turbine (5), called "BT turbine" (BT: low temperature). The second turbine (5) allows the transformation of a portion of the thermal energy contained in the first portion into mechanical energy. This mechanical energy may possibly be converted into electrical energy by means of a generator. The "superheater" (17) thus makes it possible to heat up the vapor phase, and thus to bring it energy that will then be partially recovered in the "BT turbine" (5) and with the certainty that at the outlet of this turbine the first portion remains essentially in the vapor phase. Thus, it is possible to completely reuse the flow of steam passing in the first turbine (3) in the second turbine (5). This allows a better efficiency of the cycle according to the invention. At the outlet of the second turbine (5), the first portion essentially in vapor form is sent into a third heat exchanger, called "condenser" (9) to be liquefied. The condenser (9) allows a heat exchange of the first portion with the cold source (SF). This fluid is at a temperature below the vaporization temperatures of the two fluids comprising the working fluid. A cold source (SF) at a temperature of about 4 ° C is suitable in the case where the first fluid is ammonia and the second fluid is water. The first condensed fluid is then repressurized in a pump (18), called "NH3 pump", if appropriate. At the outlet of the separation tank (2), the second portion containing a mixture of the two fluids comprising substantially all of the second fluid is sent to the "superheater" (17) to heat the first portion leaving the first turbine (3). The second portion then passes through a pump, called "rich pump" (20). In the diagram, a valve (19) has been shown to symbolize the pressure drops for this part of the process. The two streams are then mixed in a mixer (21) to reform the working fluid. The resulting stream is then sent to the evaporator (1). Comparison with the cycles of the state of the art In order to show the advantages of the method and of the system according to the invention, various tests have been carried out. To carry out these tests, we have assumed that:> the heat sources (SF, SC) consist of seawater, so that: o hot water temperature (SC): 28 ° C, o cold water temperature (SF): 4 ° C,> condenser outlet working fluid temperature (1): +2 ° C compared to seawater temperature, D working fluid temperature evaporator outlet (9): -2 ° C compared to that of seawater, D the working fluid is composed of 94% ammonia and 6% water,> the pressure losses are represented by the valve (19) and the pump (20) counteract this pressure drop. Table 1 gives the main values at various points of the process: Table I - Working fluid status during the process Unit Inlet Inlet Outlet Inlet Outlet Evaporator outlet 1st 1st 2nd 2nd condenser (1) turbine turbine turbine turbine (9) (3) (3) (5) (5) Pressure bar 9.00 9.00 6.80 6.80 5.40 5.40 Temperature ° C 12.84 26.00 13.41 25.00 10.44 6.00 Flow rate kg / s 100.00 48.66 48.66 48.66 48.66 48.66 Fraction molar% 94 99.99 99.99 99.99 99.99 99.99 Ammonia Molar fraction% 6, 00 0.01 0.01 0.01 0.01 0.01 Water Steam Phase% 0 100 98.96 100 99.94 100 Liquid Phase% 100 0 1.04 0 0.06 0 Unit Output Inlet Outlet Liquid Outlet Pump (20) pump (20) pump balloon (2) (18) Pressure bar 9.00 9.00 9.00 9.00 Temperature ° C 26.00 18.22 18.23 6.08 Flow rate kg / s 51, 34 51.34 51.34 48.66 Fraction molar% 88.29 88.29 88.29 99.99 ammonia Fraction molar% 11.71 11.71 11.71 0.01 water Vapor phase% 0 0 0 0 Phase liquid% 100 100 100 100 For this example, the energy balance is given in Table 2: Table 2- Energy balance of the process according to the invention Unit Energy generated Turbine HT (3) MW 1,61 Turbine BT (5) MW 1,35 Total MW 2.96 Energy consumed Pump NH3 (18) MW 0.029 Pump (20) MW 0.002 Total MW 0.05 Balance MW 2.93 The heat exchanges at the condenser (9) and at the evaporator (1) are respectively 62.57 and 65.50 MW. As a result, the thermal efficiency of this process calculated for this operating point is 4.48%. The thermal efficiency is the ratio between the net energy generated on the calories supplied to the process by the condenser (9) and the evaporator (1). To calculate the net generated energy is added the mechanical energy at the output of the turbines. At this sum we subtract the energy consumed by the pumps included in the process. Next, the cycle according to the invention is compared with different cycles used in the prior art: it is the Rankine cycle, the Uehara cycle (see FIG. 1) and the Guohai cycle, cycle derived from the cycle. Uehara. The comparison is made keeping the same composition of the working fluid for the three cycles using a mixture: 94 mol% of NH 3 and 6 mol% of H 2 O. The hot and cold water temperatures are respectively 28 ° C and 4 ° C. Finally, for the exchangers with water, a difference of 2 ° C is set with the water temperature, on the evaporator side and on the condenser side. Table 3 summarizes the different simulations. For each process, except for the Rankine process, for which this does not make sense, the points of optimized operation were searched for either generated energy or efficiency.

Table 3- Energy comparison of the cycle according to the invention to the cycles of the prior art Cycles Energy generated Efficiency Energy Energy (MW) (%) exchanged to exchanged generated the evaporator at par (MW) condenser frigories (MW) provided ( MW / MW) Rankine 1.71 3.50% 48.81 47.10 0.036 Uehara / Optimized Efficiency 2.60 3.93% 66.10 63.50 0.041 Guohai / Optimized Efficiency 2.05 4.43% 46.30 44.25 0.046 Invention / Efficiency-optimized 2.41 4.77% 50.62 48.21 0.050 Uehara / Energy-optimized 2.78 3.57% 77.75 74.97 0.037 Guohai / Optimized in energy 3.00 3.29% 91.41 88.40 0.034 Invention / Optimized in energy 3.39 4.09% 82.98 86.38 0.039 It is noted that the method according to the invention allows: D to reach an efficiency maximum thermal gain of 4.77% while generating more energy than the Rankine cycle with the consequence of exchanges of thermal energy at the evaporator and the condenser very close to those of the Rankine cycle.

D to generate the maximum energy (3.4 MW) while keeping a good thermal efficiency (4.09%). For the operating point of the energy-optimized method according to the invention, twice as much energy is produced as the Rankine cycle. > to obtain a better energy ratio generated by frigories made compared to the methods of the prior art. For the thermal energy of the seas (ETM), this ratio is important because the better it is, the smaller the size of the condenser and the pipe (riser) that brings the cold water and therefore the lower the costs of investments . Optimization of the Process According to the Invention The process according to the invention has numerous optimization parameters including: the composition of the working fluid, the pressure downstream of the pumps, and the pressure drop in the turbines.

For the composition of the working fluid, we studied the influence of the percentage of water in constant total flow ammonia. For these tests, the values kept constant are the total flow rate upstream of the evaporator (1): 100 kg / s, the pressure downstream of the pumps (18, 20): 9 bar and the pressure drops at the first turbine (3): 2.5 bar and at the second turbine (5): 1.1 bar.

Figure 3 shows the evolution of energy (Ene) and efficiency (Eff) as a function of the ammonia composition of the working fluid. This graph shows that the lower the percentage of water, the more energy is generated. Beyond 98 mol%, the superheater (17) upstream of the second turbine (5) can no longer function because it no longer brings energy, the liquid flow leaving the separation tank (2) becoming almost no. It is noted that the maximum efficiency is obtained for 95 mol% of NH3. To study the influence of the inlet pressure of the evaporator (1), we studied the influence of the outlet pressure of the pumps (18, 20). For these tests, the values kept constant are the total flow rate upstream of the evaporator (1): 100 kg / s, the composition of the working fluid: 96 mol% of NH 3 and 4 mol% of water, and pressure drops at the first turbine (3): 2.5 bar and at the second turbine (5): 1.1 bar.

Figure 4 shows the evolution of energy (Ene) and efficiency (Eff) as a function of the pressure of the pumps. There is an energy optimum generated at 8.4 bar and thermal efficiency at 9.2 bar. An operating point having both a high thermal efficiency without unduly degrading the energy could be chosen around 8.8 bar.

For the pressure drop in turbines, the study focuses on pressure drops in turbines (3, 5). In fact, these pressure drops are related because their sum must be equal to the pressure gain of the pump (18). Also it is necessary only to study the effect of the relaxation in the "Turbine HT" (3).

For these tests, the values kept constant are the total flow rate upstream of the evaporator (1): 100 kg / s, the composition of the working fluid 96 mol% of NH 3 and 4 mol% of water, pressure in pump outlet: 9 bar and outlet pressure of the turbine (5): 5.4 bar. Figure 5 shows the evolution of energy (Ene) and efficiency (Eff) as a function of the pressure drop. The optimum for the energy generated and for the efficiency are reached at the same value of the expansion of the turbine (3): 2.2 bar. Embodiments of the invention According to a first variant embodiment, it is possible to provide calories at certain points of the cycle in order to improve the energy produced or the thermal efficiency. The thermal energy can be supplied by at least one heating means (22; 23; 24) disposed in at least one of the following three points: upstream of the separation tank (2), upstream of the first turbine (3) and / or upstream of the second turbine (5). This embodiment variant with the three heating means (22; 23; 24) is illustrated in FIG. 6. The heating means (22; 23; 24) can consist, for example, of a heat pump or a heater. - solar water. The heating means (23) upstream of the first turbine (3) are the most efficient. However, in the case considered, beyond 2 MW brought the temperature at the outlet of the first turbine (5) becomes greater than the temperature of the fluid leaving the separation tank (2). In addition, the heating means (22) upstream of the separation tank (2) are slightly more efficient than the heating means (24) upstream of the second turbine (5). Figures 7 to 12 illustrate a part of the cycle according to the invention (downstream of the second turbine (5)) for different embodiments of the invention. These various embodiments make it possible to increase the energy generated and / or the efficiency of the method according to the invention.

All these embodiments comprise, in addition to the embodiment of FIG. 2, a regenerator (25), which is a fourth heat exchanger between the fluid leaving the superheater (17) and the reformed working fluid at the outlet of the mixer. (21).

The first embodiment of FIG. 7 comprises the following modifications with respect to the embodiment of FIG. 2: D The gas leaving the second turbine (5) is expanded in a gas diffuser (27), which produces an expansion some gas. The second outlet portion of the superheater (17) exchanges some of its heat in the regenerator (25) with the reformed working fluid. Its pressure is then lowered into a liquid diffuser (26) which provides a relaxation of the second portion. A separating balloon (29), called a "flash BT" balloon, separates the gas produced during the expansion, this gas being then mixed by means of a mixer (28) with that leaving the gas diffuser (27). The mixed gas is then introduced into the condenser (9). > After repressurization by the pump (19) and the pump (18), the two streams are mixed to be sent to the regenerator (25) and then to the evaporator (1). The second embodiment of FIG. 8 comprises the following modifications with respect to the embodiment of FIG. 2: D The second outlet portion of the superheater (17) exchanges some of its heat in the regenerator (25) with the fluid reformed. Its pressure is then lowered in a liquid diffuser (26) which performs a relaxation of the liquid. It is then mixed in the mixer (30) to the gas stream from the second turbine (5). > A separating balloon (31), called "Flash BT" separates the liquid from the gas. Only the gas is sent to the condenser (9). > After repressurization by the pump (19) and the pump (18) the two streams are mixed by means of a mixer (21) to be sent to the regenerator (25) and then to the evaporator (1). The third embodiment of FIG. 9 comprises the following modifications with respect to the embodiment of FIG. 2: the second portion leaving the superheater (17) exchanges some of its heat in the regenerator (25) with the fluid reformed. Its pressure is then lowered into a liquid diffuser (26). It is then mixed by means of a mixer (30) to the gas stream (first portion) from the second turbine (5).

D After mixing, the entire fluid passes through the condenser (9). D After repressurization by the pump (18) the flow is sent to the regenerator (25) and then to the evaporator (1).

The fourth embodiment of FIG. 10 corresponds to the second embodiment, in which an additional step of expansion of the gas flow (first portion) is carried out at the outlet of the BT pump (5) and before mixing in the mixer (30). ) by means of a gas diffuser (27).

The fifth embodiment of FIG. 11 comprises the following modifications with respect to the embodiment of FIG. 2:> The second portion leaving the superheater (17) exchanges some of its heat in the regenerator (25) with the fluid reformed. Its pressure is then lowered into a liquid diffuser (26). In this case, the diffuser (26) represents the head losses of this portion of the process. > The gas (first portion) at the outlet of the second pump (5) is sent to the condenser (9). D After repressurization by the pump (19) and the pump (18), the two streams are mixed by means of the mixer (21) to be sent to the regenerator (25) and then to the evaporator (1). The sixth embodiment of FIG. 12 comprises the following modifications with respect to the embodiment of FIG. 2: The gas leaving the second turbine passes into a fifth heat exchanger, called a pre-condenser (32) which carries out a heat exchange of the gas leaving the second turbine (5) with the first portion at the outlet of the condenser (9). The flow at the outlet of the condenser being previously repressurized by the pump (18). The second outlet portion of the superheater (17) exchanges some of its heat in the regenerator (25) with the reformed working fluid. Its pressure is then lowered into a liquid diffuser (26). > After repressurization by the pump (19) and the pump (18) the two streams are mixed by means of the mixer (21) to be sent to the regenerator (25) and then to the evaporator (1).

The comparison of the different embodiments for identical operating conditions gives the following results: TABLE 4 Comparison of Embodiments of the Invention Embodiment Frigory Exchange Power "NH3 Condenser Pump" (18) (MW) ( MW) Figure 2 35.75 0.022 Figure 7 36.19 0.067 Figure 8 36.10 0.066 Figure 9 37.24 0.065 Figure 10 36.19 0.067 Figure 11 35.75 0.022 Figure 12 35.58 0.022 The various embodiments require therefore a different exchange of frigories, which affects the efficiency of the process. In addition, the power required for the pump (18) also varies from one embodiment to another. Moreover, it is noted that the embodiment of Figure 12 allows to slightly lower the power to be exchanged with the condenser (9) relative to the embodiment of the figure and therefore can advantageously reduce the size of the condenser (9).

Claims (25)

CLAIMS1) A method for converting thermal energy into mechanical energy, in which a working fluid is circulated in a closed circuit composed of a first and a second miscible fluid having different vaporization temperatures, characterized in that the following steps are carried out: a) said working fluid is heated by a heat exchange with a first heat source (SC) at a temperature higher than the vaporization temperature of said first fluid; b) separating said heated working fluid into a first vapor-shaped portion essentially comprising said first fluid and a second portion in liquid form comprising at least said second fluid; c) transforming a portion of the thermal energy contained in said first portion into mechanical energy by means of a first turbine (3); d) heating said first portion at the outlet of said first turbine (3) by a heat exchange with said second portion; e) transforming a portion of the heat energy contained in said first heated portion into mechanical energy by means of a second turbine (5); and f) reforming said working fluid by condensing the vapor contained in at least said first portion by means of a second heat source (SF) at a temperature below the vaporization temperature of said first fluid, and mixing said two portions. 2) The method of claim 1, wherein said working fluid comprises ammonia and water. 3) Process according to claim 2, wherein said working fluid comprises between 90 and 98 mol% of ammonia, preferably substantially 95 mol% of ammonia. 4) Method according to one of the preceding claims, wherein said heat sources (SF; SC) consist of seawater taken at different depths. 5) The method of claim 4, wherein the second heat source (SE) is seawater taken from a depth greater than or substantially equal to 1000m. 6) Method according to one of the preceding claims, wherein said working fluid is heated before the separation step. 7) Method according to one of the preceding claims, wherein said first portion is heated before at least one of the steps of converting thermal energy into mechanical energy. 8) Method according to one of the preceding claims, wherein after step d) is cooled said second portion with said reformed working fluid. 9) Process according to claim 8, wherein said first portion is relaxed after said second step of transforming the thermal energy into mechanical energy and after said cooling, said second portion is expanded, said second portion is separated into a vapor phase of said second portion. first fluid and a liquid phase of said second fluid and said vapor phase is mixed with the first portion, only the first fluid being condensed in step f). 10) The method of claim 8, wherein after said cooling, said second portion is expanded, said two portions are mixed and the mixture is separated into two phases: a vapor phase of said first fluid and a liquid phase of said second fluid, only the first fluid being condensed in step f). 11) The method of claim 10, wherein said first portion is expanded before said mixture of said two portions. 12) The method of claim 8, wherein after said cooling, said second portion is expanded and said two portions are mixed, said two fluids being condensed in step f). 13) The method of claim 8, wherein said first portion before the implementation of the condensation is cooled by said first portion after the implementation of the condensation. 14) System for converting thermal energy into mechanical energy comprising a closed circuit in which circulates a working fluid composed of a first and a second miscible fluid having different vaporization temperatures, characterized in that said closed circuit consecutively comprises: a first heat exchanger (1) for heating said working fluid by means of a first heat source (SC) at a temperature above the vaporization temperature of said first fluid, a separation tank (2), wherein said working fluid is separated into a first portion substantially comprising said first fluid in vapor form and into a second liquid portion having at least said second fluid, a first turbine (3) for converting thermal energy contained in said first portion in mechanical energy, a second heat exchanger (17) for heating said first portion to means of said second portion, a second turbine (5) for converting the thermal energy contained in said first portion into mechanical energy, means (9; 21) for reforming said working fluid comprising at least a third heat exchanger (9) to condense at least said first portion by a heat exchange with a second heat source (SE) at a temperature below the vaporization temperature of said first fluid and at least one mixer (21) for mixing said two portions. The system of claim 14, wherein said working fluid comprises ammonia and water. 16) System according to claim 15, wherein said fluid comprises between 90 and 98 mol% of ammonia, preferably substantially 95 mol% of ammonia. 17) System according to one of claims 14 to 16, wherein said heat sources (SF, SC) consist of seawater taken at different depths. 18) System according to one of claims 14 to 17, wherein said circuit further comprises at least one heating means (22; 23; 24) before said separation tank (2) and / or before the first turbine (3). ) and / or before said second turbine (5). The system of claim 18, wherein said heating means (22; 23; 24) is a heat pump or a solar water heater. 20) System according to one of claims 14 to 19, wherein said circuit further comprises a fourth heat exchanger (25) disposed after the second heat exchanger (17), for cooling said second portion by means of said reformed working fluid . 21) System according to claim 20, wherein said circuit further comprises means (26) for expanding said second portion disposed after said fourth heat exchanger (25), a second separation balloon (29) for separating said second portion into a second portion. vapor phase consisting essentially of said first fluid and a liquid phase comprising the second fluid and a mixer (28) for mixing said vapor phase with said first portion. 22) System according to claim 20, wherein the circuit further comprises means for expansion (26) of said second portion disposed after said fourth exchanger (25), a second mixer (30) for mixing said two portions and a second balloon separation device (31) for separating the mixture into a vapor phase consisting essentially of said first fluid and a liquid phase comprising at least said second fluid. 23) System according to claim 22, wherein the circuit further comprises expansion means (27) of the first portion before said second mixer (30). 24) System according to claim 20, wherein said system further comprises expansion means (26) of said second portion disposed after said fourth exchanger (25) and a second mixer (30) for mixing said two portions. The system of claim 20, wherein said system further comprises a fifth heat exchanger (34) between said first input portion of said third heat exchanger (9) and said first output portion of said third heat exchanger (9). ).