ES2821746A1 - CLOSED CYCLE THERMODYNAMIC SYSTEM TO TRANSFORM THERMAL ENERGY INTO MECHANICAL ENERGY (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Closed-loop thermodynamic system to transform thermal energy into mechanical energy. Thermodynamic system with a closed working fluid circuit (15) comprising a hot-focus exchanger (2) that heats the fluid (15), a set of turbines (6, 7; 8, 9; 10, 11) where it expands the fluid (15), a cold focus exchanger (14) that cools working fluid (15), a set of compressors (16, 17; 20, 21) that successively compress the fluid (15) and a branch (27b) connected to an outer casing (5) of the turbine assembly; so that a regeneration portion of the working fluid (15) leaves the branch (27b), cools the set of turbines and returns to the first exchanger (2). (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

CLOSED CYCLE THERMODYNAMIC SYSTEM TO TRANSFORM

THERMAL ENERGY IN MECHANICAL ENERGY

TECHNICAL SECTOR

The present invention belongs to the technical field of energy and can be used, for example, in the energy industry sector.

More particularly, the present invention relates to thermodynamic systems for transforming thermal energy into kinetic energy.

TECHNICAL PROBLEM TO BE SOLVED AND BACKGROUND OF THE INVENTION

In the state of the art, various systems are known that use thermal machines, both compression and expansion, to generate useful mechanical energy in the form of a rotation of the shaft of one or more turbines or expansion machines.

For this, an external fluid flow is used as the starting energy source, for example, from combustion, which has a temperature well above that of the environment, and which transfers a large part of its thermal energy to a working fluid. , which undergoes a thermodynamic cycle, by means of which a part of its thermal energy is transformed into mechanical energy. Said working fluid is kept in the gas state throughout the cycle. In this sense, it can be classified as a Brayton-Joule cycle (it is not a Rankine cycle, so there is neither a boiling kettle nor a condenser).

Some of said thermodynamic systems are disclosed, for example, in Spanish patent application No. ES 2,776,024 A1, which shows a thermodynamic system provided with a closed circuit through which a working fluid circulates.

Figure 1 of the present specification corresponds precisely to a thermodynamic system according to document ES 2,776,024 A1. In it, the fluid of At high pressure, work accesses a first heat exchanger (called a hot-focus exchanger) where it is heated by receiving thermal energy from a fluid external to the circuit that has a very high temperature. As a consequence, the working fluid increases its specific enthalpy considerably, at constant pressure.

The working fluid then passes through three different turbines arranged in series with each other. By having a lot of specific enthalpy, the working fluid expands inside each turbine, moving the blades and inducing a rotation of the axis of each one of them. This implies that a part of the thermal energy acquired by the working fluid in the hot spot is converted into mechanical energy.

Subsequently, the working fluid passes through a second heat exchanger (called a cold-focus exchanger) where it is cooled by transferring thermal energy to a fluid external to the circuit that has a temperature close to that of the environment. As a result, the working fluid undergoes constant pressure cooling.

Finally, the working fluid completes a complete turn to the closed circuit after passing through three compressors arranged in series, by means of which its pressure increases successively. In order to improve compression performance, the working fluid is intercooled between two consecutive compression stages; this intermediate cooling being similar, in the temperature scale, to the cooling provided by the cold focus.

Likewise, in the thermodynamic systems disclosed by Spanish patent application no. ES 2,776,024 A1, it is also contemplated to inject an external heat-repairing fluid into the primary of the heat exchanger.

Despite the fact that the use of a heat-repairing fluid proposed by said document represents an advance with respect to other systems of the state of the art, in the sector there is still a need to devise systems and methods that use thermal compression and expansion machines, to generate mechanical energy, in which irreversibility losses are minimized.

Improving the performance of the system by minimizing energy losses within the circuit is precisely the main objective of the present invention, whose fundamental difference with what is disclosed in the application ES 2,776,024 A1 is that, in said document, the heat-repairing fluid It can be any that is compatible with the heat supply fluid to the hot spot, and it is provided at the spot, in its hot circuit. On the contrary, in the present invention, the heat repair fluid is the working fluid itself, and its flow is created as a bypass extracted at the exit of the compression phase; and after producing regenerative cooling in the expansion phase, in which this fraction is heated, said fraction is injected into the main stream of the working fluid, before the expansion stage.

DESCRIPTION OF THE INVENTION

An object of the invention refers to a closed cycle thermodynamic system for transforming thermal energy into mechanical energy, said system being provided with a closed circuit through which a working fluid circulates and comprising:

• at least one first heat exchanger (or hot-focus exchanger), provided with a primary duct and a secondary duct, the primary duct being fed by an external fluid that supplies heat to a flow of working fluid that circulates through the duct secondary to an inlet nozzle of a turbine;

• an expansion turbine, or alternatively a set of expansion turbines arranged in series with each other, and inside which the flow of the working fluid that leaves the last turbine expands through an outlet conduit towards a second heat exchanger , the turbine or set of turbines being covered by an external casing that constitutes a cooling channel around the turbines;

• at least one second heat exchanger, (or cold focus exchanger), provided with a primary conduit and a secondary conduit, the secondary conduit being fed by an external fluid at room temperature, which cools the working fluid flow, circulating through the primary conduit to a compressor;

• a compressor, or alternatively a set of compressors, arranged in series with each other and configured to successively compress the flow of the working fluid, which circulates inside the compressors and exits through an outlet conduit;

the thermodynamic system being characterized by:

the outlet duct of the compressor or set of compressors is provided with two different branches, a first branch connected to the secondary duct of the first heat exchanger and a second branch connected to the cooling channel of the external casing of the turbine or set of turbines; such that a first portion (or main flow) of the fluid flow returns directly to the first heat exchanger, from the hot spot, through the first branch and a second portion (or regeneration / heat repair flow) of the fluid flow through a second branch, accesses the cooling channel formed inside the outer casing of the turbine or set of turbines, and circulates inside and along said casing cooling the turbines, after which it is injected into the secondary duct of the first heat exchanger through a return duct, so that the total working flow, which is the sum of the main flow plus the heat-repairing flow, enters the nozzle of the first turbine.

The set of turbines of the closed-loop thermodynamic system according to the present invention is preferably provided with three turbines arranged in series with each other.

On the other hand, the set of compressors of the closed-cycle thermodynamic system according to the present invention is preferably provided with three compressors arranged in series with each other.

In a preferred embodiment of the invention, the compressor assembly is provided with at least one cooler through which fluid circulates at room temperature that cools the working fluid flow.

The closed cycle thermodynamic system according to the present invention is configured to carry out a closed thermodynamic cycle with four differentiated phases, which are sequentially of heating, expansion, cooling and compression, existing superimposed on these conventional elements, other components, which they channel a fraction of the working fluid, which regeneratively cools the expansion phase, acting countercurrently, thus reducing the effect of the entropy generated by irreversibilities in the expansion machines; and the system being composed of the components mentioned here, in which the thermodynamic evolutions that are indicated take place:

• there being at least one first heat exchanger, with a hot focus, where each heat exchanger comprises a primary conduit, fed by an external hot fluid that contributes heat to the main flow of the working fluid, which circulates through the secondary conduit of the exchanger; and at the end of the secondary circuit, a fraction of the total flow of the working fluid enters, through the lateral branch of regeneration, which corresponds to the regeneration flow, and the total flow being the sum of the main flow and the regeneration flow;

• the end of said secondary conduit coinciding with the beginning of the first expansion nozzle, with strong acceleration of the working fluid, and entry into the impeller of the first turbine, said nozzle and said impeller being cooled, to extract the heat generated by thermodynamic irreversibilities, the refrigerant flow being a fraction of the working fluid, called the regeneration fraction, traveling countercurrently inside its corresponding casing, and outside the nozzle and impeller;

• the fluid emerging from said refrigeration through a conduit that supplies it to the main flow of the working fluid, at the end of the secondary circuit of the exchanger of the hot spot, already mentioned;

• and the sum of the main flow and the regeneration fraction of the working fluid, which is the total flow, performs the expansion phase, the end of which connects with the cold source;

• the cold focus being constituted by a heat exchanger through whose primary the working fluid circulates, and through the secondary the ambient fluid that cools it;

• after which the working fluid enters the first compressor which is cooled countercurrently, by an ambient fluid, to extract the heat generated by thermodynamic irreversibilities;

• the total flow exiting from the diffuser of the last compression stage, dividing into two streams: a main flow, which enters the secondary circuit of the hot bulb exchanger, and a remaining fraction of working fluid, which is injected into the casing that surrounds the turbine or turbines in series of the expansion phase, to effect its regenerative cooling;

thus being configured the aforementioned elements, which establish a closed thermodynamic cycle with the following phases:

- heating of the main flow of the working fluid, which takes place in the heat exchanger, where the working fluid reaches its maximum temperature;

- expansion of the total flow of the working fluid, in the turbine or turbines concatenated in series, where the countercurrent cooling flow extracts the heat generated by the irreversibilities of the expansion phase, connecting said flow to the final part of the secondary pipe of the exchanger heat from the hot bulb;

- and from the last turbine, the working fluid enters the cold source, prior to the first compressor;

- The cooling and compression phases then occur in an integrated manner, since the cooling element is alternately interleaved with the compressor, with repetition of this cooling-compression sequence, a cold focus being configured as the set of refrigerations prior to the compressors.

Said countercurrent cooling actions make up what can be called a heat-repairing cycle, applied both to compressors, particularly their outlet diffusers, and to turbines, particularly their inlet nozzles; the compressors being externally cooled by an ambient fluid, and the turbines being cooled by a fraction of the working fluid, this cooling having a regenerative thermodynamic purpose, since the heat extracted from the cooling of the turbines is recovered in a high fraction, as high pressure heat from the working fluid, as it is injected into the main flow of the working fluid, in the final section of the secondary conduit of the hot spot exchanger, from which the expansion phase begins.

BRIEF EXPLANATION OF THE FIGURES

Figure 1 shows a schematic representation of a closed cycle thermodynamic system according to the document of the state of the art ES 2776024 A1, with three compressors and three turbines, the heat-repairing fluid being supplied from outside, and injected at the end in the primary of the exchanger of the hot bulb.

Figure 2 shows a scheme similar to the previous one, but representing the present invention, where it can be seen that the heat-repairing fluid, which goes through the cooling channel provided inside the shell housing the elements of the expander phase (ie, the set of turbines), is a fraction of the working fluid, which ends up joined to its main flow in the final section of the secondary circuit of the hot spot heat exchanger.

Figure 3 presents a thermodynamic diagram in which three types of cycles are drawn: an ideal one; another realistic one, with irreversibilities, corresponding to isentropic yields of 0.85 in both compression and expansion, with a cascade of 4 pressure steps, each with a pressure ratio of 2. In compression, cooling occurs intermediate between successive steps. The diagram is given with the temperature on the abscissa, on a linear scale, and with the pressure on the ordinate, on a logarithmic scale.

To improve understanding of the explanation of the figures, the elements that make up the invention are listed below:

1. Fluid flow that provides heat from outside the circuit, and enters the first conduit of the hot bulb exchanger.

2. First heat exchanger (hot spot exchanger). 2nd. Primary duct of the first heat exchanger.

2b. Secondary duct of the first heat exchanger.

. Injection, in the supply of the primary circuit of the exchanger (2) of the heat-repairing fluid flow that leaves the regeneration casing (5). It is part of a prior art system. . Heat-repairing fluid conduit, from the casing (5) to the injection (3). It is part of a prior art system.

. External casing of the turbine assembly.

to. Cooling channel, to drive the regeneration portion of the working fluid flow around the turbines.

. Inlet nozzle on the highest pressure turbine.

. Impeller of the highest pressure turbine.

. Inlet nozzle in the intermediate pressure turbine.

. Impeller of the intermediate pressure turbine.

0. Lowest pressure turbine inlet nozzle.

1. Lowest pressure turbine impeller.

2. Entry into the casing (5) of the external heat-repairing fluid. It is part of a prior art system.

3. Environmental cold fluid flow.

4. Second heat exchanger (cold bulb exchanger).

4th. Primary duct of the second heat exchanger.

4b. Secondary duct of the second heat exchanger.

5. Work flow.

6. Lowest pressure compressor blade crown.

7. Lower pressure compressor diffuser.

8. Environmental cold fluid flow.

9. First cooler in the compressor set.

0. Crown of blades of the intermediate pressure compressor.

1. Intermediate pressure compressor diffuser.

2. Environmental cold fluid flow.

3. Second refrigerator of the compressor set.

4. Higher pressure compressor blade crown.

5. Higher pressure compressor diffuser.

6. Valve for regulating the flow rate of the working fluid, which is used for thermo-repair.

7. Final outlet duct of the compression phase.

7a.Main branch of the outlet conduit (27) through which the main flow of the working fluid circulates.

27b. Branch of the outlet conduit (27) through which the heat repair flow is derived.

28. Connection of the branch (27b) with the casing (5) that channels the working fluid, which is used for heat repair.

29. Outlet (at high temperature) of the working fluid, which is used for heat repair.

30. Thermodynamic point where the working fluid acquires its minimum value of pressure and temperature in the cycle.

31. End point of the ideal compression phase. Point 31r is that of the end of the realistic compression phase. In the corresponding one, depending on the ideal or real cycle, the heating phase in a hot spot begins.

32. End point of heating of the ideal cycle. The 32r marks the end of realistic warm-up.

33. End point of the expansion of the ideal cycle. The 33rd marks the end of the realistic expansion; and 33c is the end of the cycle corrected with the invention, that is, with regenerative cooling.

34. Cooling line, in the cold focus.

35. Intermediate refrigeration line, (particularly after the second compressor stage, in figure 3). All similar lines are of the same type.

36. Isentropic ideal compression line, specifically in the last compression stage. It ends at point 31. Line 36r represents the last stage of realistic compression, which ends at point 31r.

37. Heating line (isobar) in the hot spot.

38. Isentropic ideal expansion line.

39. Realistic cycle expansion line.

40. Realistic cycle expansion line, corrected with the regenerative refrigeration of the invention.

MODE OF EMBODIMENT OF THE INVENTION

The invention is materialized by grouping in a circuit the successive components that have been prescribed in the invention, using suitable materials at the temperature and pressure levels that exist in each case. For example, for the heat exchangers of the cold source, which have temperatures moderately by On top of the environmental one, it is possible to use aluminum or copper, due to their much higher thermal conductivity than carbon steel, in turn superior to stainless steel. Another important aspect in the selection of the material is its resistance to corrosion, although using a noble gas, such as argon, as a working fluid, internally the corrosion would be inhibited (even with the intrusion of water vapor). However, if CO2 is used as the working fluid, the intrusion of water vapor would generate carbonic acid, which could attack non-electrochemically protected materials.

The most specific aspect of this invention is the determination of the fraction of the working fluid that is diverted to the heat repair circuit, which is essentially made up of a bypass pipe (branch 27b) provided with a flow regulation valve (26), said derivation reaching the interior of the casing (5) through the connection (28). There it will have to extract the excess heat, operating under the technical criteria of adjusting the temperature at each level of the expander machines, to the value of the pressure at said level if there had been an isentropic expansion, so that part of the heat from irreversibilities is regeneratively recover. For this, the heat-repairing fluid, which is a fraction of the working fluid, comes from the point of highest pressure in the entire circuit, which is where the branch (27b) leaves, so that it can overcome the loss of manometric head that It requires it to pass along the casing (5), and to be injected back into the main flow of the working fluid through the conduit (29) from which it will go towards the expansion phase.

The determination of this fraction goes in parallel with the calculation of the improvement that the base cycle can experience, when adding the regeneration of heat from irreversibilities. If that heat is not regenerated, it is lost, because when the working fluid reaches the minimum pressure level of the circuit, all the excess heat to close the cycle has to be evacuated through the cold source. In the case of the invention, this is not the case, since a fraction of the working fluid carries away said heat, said fraction being at such high pressure that it can be injected upstream of the expansion phase.

To make the pertinent calculations, the following magnitudes and parameters of the problem are defined, for which we will assume that the working fluid is an ideal gas, specifically argon, since it simplifies things the fact that its specific heat at constant pressure is in turn constant. This means that the enthalpy increments are always proportional to the temperature increments, and therefore the yields can be expressed as a function of these.

The following performances and parameters are relevant:

Isentropic startup of the expansion (it would be necessary to define a similar one for compression machines; but for reasons of exposition simplicity, it will be assumed that in the realistic cycle, the performance of both types of machines is the same)

£ = heat recovery performance from irreversibilities

(relative performance of the regenerated heat expansion, with respect to the main heat performance

AT = temperature increase in the hot spot (1250-412 = 838 K in the example)

AT '= temperature increase in regeneration (at most it can be equal to AT; a temperature performance can be defined, ^t as AT' / AT ratio).

In reality, the application of the invention can only be justified if the specific returns that it entails, £, cp, and ^t , are very close to 1 (and of course much greater than p).

The mechanical power of the expander part, W, is defined by

W ⁼ p ( mAT ( pm AT ⁾

and compression

And the energy balance of regeneration is

mAT

( mAT m AT ) (1 ^- p)

and

Where is it obtained from?

AT ^/?

m = m

A T 'l - p

being /? = e (l - p)

The expansion power can be expressed as

<pf3

W = pm AT ( l - ^)

It should be noted that when the heat recovery yields of regeneration, £, and that of turbinate, cp, are close to 1, the result of W, regardless of the value of p, is the maximum possible, that is, the one that is delivery in hot spot, i.e.

W = mAT = Q

Remember, for the purposes of rigor in units, that in these equations the isobaric specific heat of the fluid has been omitted, since the objective is to determine the yields of each type of cycle, and this can be done with temperatures (in the case of ideal gas ).

Figure 3 shows a thermodynamic diagram in which three types of cycles are drawn: an ideal one (without irreversibilities); another realistic one, with irreversibilities, corresponding to isentropic yields of 0.85 in both compression and expansion, with a cascade of 4 pressure steps, each with a pressure ratio of 2. In compression, cooling occurs intermediate between successive steps. The temperature values at the various representative points are also provided, with which the performances of each type of case can be determined.

Now, the inclusion of the invention causes:

-An increase in the mass flow rate in compression (line 36r) and in expansion (line 40) with respect to the mass flow rate in the hot spot. This last expense is called m, and that of the fraction of fluid for regeneration, m '. In both compression and expansion, the mass flow M will be the sum of both, that is, M = m + m '

-An increase in the work generated in the expander part, as there is a greater flow. (To be able to compare, the pressure and temperature maps do not change from the realistic case without invention, to the case with invention, except in the expansion line, which obviously changes);

- an increase in power consumed in the compression phase, also due to an increase in mass flow rate (since without the invention it would be m, and with the invention it goes to M).

To make an illustrative example, the following pressure and temperature values are taken at the most relevant points:

-minimum pressure, 0.1 MPa

-max pressure, 1.6 MPa

-minimum temperature, 300 K

Note that in the isentropic transformations of argon, the quotient between isobaric and isochoric specific heats is 5/3; Therefore, if the pressure ratio between the end and the beginning of an isentropic evolution is called r, the temperature ratio is

In the ideal cycle we have

-temperature at the end of compression, coinciding with the end of expansion, and the beginning of the hot spot = 396 K

- hot spot outlet temperature = 1200 K

Cycle performance is defined by

Where Q is the heat contributed to the hot spot. Since in this case the mass flow is the same for the 3 magnitudes, it can be ignored, and we obtain

(1200 ^- 396 ^{) -} 4 (396 ^- 300 ⁾

( ú = ⁼ 0.522

1200 - 396

In which it has been taken into account that there are 4 steps in compression For its part, the realistic cycle (without invention applied at the moment) with 85% performance in the machines, yields the following results

-temperature at the end of compression, coinciding with the beginning of the hot spot = 412 K

-heat bulb outlet temperature = 1250 K

-temperature of the end of expansion, 538 K

And you get

(1250 - 538) - 4 (412 - 300) 712 - 448

^to) = ________ _______ ^ttt : ------------ = ----- rrr: ---- = 0.315 _{1200 -412 838}

It can be seen that the effect of irreversibilities is very pronounced, even greater than the product of compression and expansion performances (which gives 0.7225 in this case, while the quotient between 0.315 and 0.522 is 0.603). Finally, the effect of the invention must be computed on the realistic case. When applying the invention, the heat in excess with respect to the isentropic (line 40) is continuously extracted from the expansion (line 39), and said heat is recycled; and to extract it, a flow (a fraction of the mass flow) is required, which means that for both expansion and compression, circulate more expense. This addition is

A T p

m = m

A T 'l - p

That for the hypothesis that the returns £, cp and ^t are almost equal to 1, it remains

, ¹ - p

m = m -------

P

And for the expected yields on the machines, it remains m-0.176m.

It is appreciated that it is fulfilled

^, m

M ⁼ m m ^{= -}

P

With which it is arrived at that the work carried out in the expansion happens to

W = p Q - = Q

And similarly, the compression work is multiplied by (1 / p), so the performance remains

Wr 448 ^/ 0.85

Cú = 1 ----- - = l ⁼ 0.37

Q 838

With the invention, a yield increase from 0.315 to 0.37 is obtained, which means 17.5% higher.

The added components will certainly not be perfect, so it is useful to redo the above calculations with returns below 1. For example, £ = 0.95; ^t = 0.97 and cp = 0.99, consistent with what they mean. The p-value drops slightly to 0.1425 (from 0.15). This modifies the increase in flow in compression and expansion, obtaining M = 1.1713m (which is slightly higher than in the previous case). This leads to the following values

Q = 838 (does not vary)

W = 830 (decreases from 838)

Wc = 448 1.1713 = 515

And the yield of the cycle remains (830-515) / 838 = 0.364, slightly less than that of the thermo-repair with all its yields equal to 1 (0.37).

As a summary of the invention, it can be said that it consists of taking advantage of two sources of heat: the exterior, which is the one that is really paid for, (line 37 in the diagram in figure 3) and the interior, which is not paid, since the heat is generated by irreversibilities (and comes from cooling the system so that line 39 passes to line 40, in the expansion of the total flow, sum of the main and the regeneration).

The external heat (1) is transferred in the hot spot (2) to the main flow, which once heated enters the expansion zone, made up of an alternate succession of nozzles (6, 8, 10) and impellers (7, 9 , eleven). These components are cooled by the regeneration flow, which is channeled inside a casing (5) that constitutes a second heat source, generated internally. Hence, it emerges through the conduit (29) to join the main flow and enter the expansion zone. After this, the total flow passes to the cold focus (13,14), which is followed by the compression phase, through the blade crowns (16, 20, 24) and the diffusers (17, 21, 25), with which are intercalated the refrigeration exchangers (19, 23). From the complete flow already compressed, the fraction of flow used to obtain the heat from the second hot spot, which is generated internally, is extracted (through conduit 27b, and according to the regulation of valve (26)). This focus puts in thermodynamic value what initially is only a loss, due to irreversibilities.

The present invention is by no means limited to the embodiments disclosed herein. Different possible embodiments of this invention will be apparent to the person skilled in the art in light of the present disclosure. Consequently, the scope of protection of the present invention is defined exclusively by the claims that follow.

Claims (4)

1.- Thermodynamic system to transform thermal energy into mechanical energy, provided with a closed circuit through which a working fluid circulates (15) and comprising: • at least one first heat exchanger (2) provided with a primary conduit (2a) and a secondary conduit (2b), the primary conduit being fed by an external fluid (1) that supplies heat to a main flow of working fluid circulating through the secondary conduit (2b) towards an inlet nozzle (6) of a turbine (6, 7); • a turbine, or alternatively a set of turbines (6, 7; 8, 9; 10, 11) arranged in series with each other and inside which the total flow of the working fluid that leaves the set of turbines (6, 7; 8, 9; 10, 11) through an outlet conduit to a second heat exchanger (14), the set of turbines being covered by an external casing (5) that provides a cooling channel (5a); • at least one second heat exchanger (14) provided with a primary conduit (14a) and a secondary conduit (14b), the secondary conduit (14b) being fed by an external fluid (13) at room temperature, which cools the flow total working fluid circulating through the primary conduit (14a) towards a first compressor (16, 17); • a compressor, or alternatively a set of compressors (16, 17; 20, 21; 24, 25), arranged in series with each other and configured to successively compress the total flow of the working fluid, leaving said compressor or set of compressors (16, 17; 20, 21; 24, 25) through a conduit output; the thermodynamic system being characterized by: - the outlet duct (27) of the compressor or set of compressors is provided with two different branches, a first branch (27a) connected to the secondary duct of the first heat exchanger, and a second branch (27b) connected through a duct (28) to the cooling channel (5a) of the outer casing (5) of the set of turbines; from such that a first portion of the fluid flow, called the main flow, goes directly to the first heat exchanger (2) through the first branch, and a second portion of the working fluid flow, called the heat-repair flow, accesses the outer casing (5) of the set of turbines through the second branch (27b), and circulates along said casing, through its inner channel (5a), cooling the set of turbines, and is incorporated at the end of the duct secondary (2b) of the first heat exchanger through a return duct (29), from which the total work flow, which is the sum of the main flow plus the heat-repair flow, enters the nozzle the first turbine (6). 2. Thermodynamic system according to claim 1, in which the set of turbines (6, 7; 8, 9; 10, 11) is provided with three turbines arranged in series with each other. 3. - Thermodynamic system according to any of the preceding claims, in which the set of compressors (16, 17; 20, 21; 24, 25) is provided with three compressors arranged in series with each other. 4. - Thermodynamic system according to any of the preceding claims, in which the set of compressors (16, 17; 20, 21; 24, 25) is provided with at least one refrigerator (19, 23) through which fluid circulates ( 18, 22) at room temperature and cooling the total flow of working fluid (15).