"Plant and process for energy production"

DESCRIPTION

Field of the finding

The object of the present invention is a Brayton cycle plant and process for energy production. In particular, the present invention is situated in the field of plants for energy production which use gas turbines for carrying out Brayton cycles or cycles derived therefrom, open or closed. Preferably but not exclusively, the present invention is situated in the field of industrial plants for energy production. The present invention is applicable to many different types of cycles, in particular supercritical and transcritical cycles, such as: cycles with high external temperature source (cycles with external combustion, nuclear cycles, CSP- concentrating solar plants, etc.), cycles with residual heat recovery, oxy- combustion cycles.

Background of the finding

Brayton cycles of gas turbines for energy production have been known for some time.

The following technical documents each illustrate a summary of known Brayton cycles with CO2 (with or without condensation, with or without re-compression, with or without partial cooling, with or without partial expansion, with double expander with low LP and high LP pressure for recovery cycles etc.) and relative plant schemes.

The document "Grant Kimzey, "Development of a Brayton Bottoming Cycle Using Supercritical Carbon Dioxide as the Working Fluid, Electric Power Research Institute Report, Palo Alto (CA), 2012' illustrates different Brayton cycles with CO2 with the relative plant schemes in order to demonstrate their possible use as bottoming cycles.

The document "Supercritical carbon dioxide cycles thermodynamic analysis and comparison - Ing. Martin kulhanek, Ing. Vaclav dostal ph.d." contains a thermodynamic analysis of different supercritical CO2 Brayton cycles.

The document "Performance comparison of supercritical CO2 versus steam bottoming cycles for gas turbine combined cycle applications - Pierre Huck, Sebastian Freund, Matthew Lehar, Maxwell Peter - GE Global Research -The 5th International Symposium - Supercritical CO2 Power Cycles, March 28-31, 2016, San Antonio, Texas" illustrates a comparison in terms of performances between CO2 supercritical cycles and vapor bottoming cycles.

The document "Evaluation of Gas Turbine Exhaust Heat Recovery Utilizing Composite Supercritical CO2 Cycle - Leonid Moroz, Maksym Burlakal, Oleksii Rudenko, Clement Joly - SoftlnWay Inc., 15 New England Executive Park, Burlington, MA 01803, USA - Proceedings of International Gas Turbine Congress 2015 Tokyo November 15-20, 2015, Tokyo, Japan" contains a study made on a plurality of supercritical CO2 bottoming cycles usable as an alternative to the vapor cycles for the recovery of residual heat.

Gas turbine plants are also known in which the compressor is mechanically separated from the main turbine which generates power.

For example, the patent GB622053 (of 1949) illustrates a gas turbine plant used for the propulsion of a vehicle, in which the turbine which moves the compressor is separated from the turbine connected to the drive shaft of the vehicle, so that the temperature of the gas of the turbine that moves the compressor varies independently with respect to the load and to the speed of the turbine connected to the drive shaft.

The patent US5669216 illustrates a process and a plant for generating mechanical energy by means of a gas turbine. The plant comprises a compression unit with two compressors mechanically coupled to a drive turbine. The plant also comprises a main turbine which can be mechanically separated from the drive turbine or even be arranged on the same shaft.

The patent US2095991 illustrates a gas turbine system of combustion type comprising a low pressure turbine connected to a generator, a low pressure that moves a low pressure compressor, a high pressure turbine that moves a high pressure compressor. The three turbines are arranged in series with respect to the flow of the working fluid.

The patent GB614780 illustrates a gas turbine plant for marine installations comprising a high pressure turbine that moves a double-stage compressor. The discharge gas of the high pressure turbine is directed into a discharge turbine that moves a respective compressor.

The document US 2016/0298500 illustrates a closed thermodynamic cycle for generating power that comprises a high pressure expander, a first and a second low pressure expander. Downstream of the high pressure expander, a duct is configured for dividing the flow into a first and a second flow respectively directed towards the first and the second low pressure expander.

The document US 5 771 678 illustrates a gas turbine motor comprising a combustion chamber that receives air from a compressor and supplies two sets of turbines.

Summary

In such context, the Applicant has observed that the cycles and the plants described above can be improved from several standpoints, in particular with reference to the optimization of the machines constituting such plants, with consequent benefits regarding the overall efficiency of the plants themselves.

In such context, the Applicant has perceived the need to:

^■ make a plant provided with more efficient machines;

^■ make a plant in which the constituent machines are configured for working as close as possible to their highest-efficiency conditions;

^■ make a plant that facilitates the operations off design, e.g. during the starting of the cycle and/or the management of the transients (such as that which follows the irregular stoppage of a turbine);

^■ make a plant of compact size, relatively simple from the structural standpoint and hence having limited cost.

The Applicant has found that the above-indicated objectives and still others can be attained by mechanically decoupling the compressor or the compressors of the plant from the main turbine of the cycle and driving it/them by means of respective dedicated auxiliary turbine(s) through a power-balanced shaft, in which only a fraction of the total enthalpy change that moves the main turbine is exploited on said dedicated turbine(s), taking from an extraction of the main turbine and/or returning to an intermediate intake of the main turbine.

In other words, the innovation consists of the use of one or more turbochargers that are mechanically independent from the main turbine, in which the turbine of the turbocharger is moved by a fraction of the total enthalpy change that moves the main turbine and in which the compressor of the turbocharger can be the main compressor of the Brayton cycle or a further compressor used, for example, for carrying out a re-compression. In particular, the object of the present invention is a Brayton cycle plant for energy production, comprising:

a heater;

a main turbine connected or connectable to an external user, preferably to a generator;

a cooling device;

a turbocharger comprising an auxiliary turbine and a compressor, in which said auxiliary turbine and compressor are mechanically connected to each other in order to rotate together, preferably at the same number of revolutions, wherein the auxiliary turbine is mechanically decoupled from the main turbine;

conduits containing a working fluid and connecting together the heater, the main turbine, the cooling device, the auxiliary turbine and the compressor, to carry out a Brayton cycle;

wherein the conduits are configured to connect the auxiliary turbine with the main turbine so that said auxiliary turbine exploits a fraction of a total enthalpy change exploited by the main turbine.

In one independent aspect, the object of the invention is also a Brayton cycle process for energy production, comprising:

heating a working fluid;

- expanding the heated working fluid in a main turbine in order to rotate said main turbine and produce energy;

cooling the working fluid after expansion;

compressing the cooled working fluid and then heating it up again;

wherein the process also comprises: expanding a fraction of the working fluid in an auxiliary turbine driving a compressor, wherein the auxiliary turbine rotates at the same number of revolutions as the compressor, wherein the auxiliary turbine is mechanically decoupled from the main turbine;

wherein the auxiliary turbine exploits a fraction of a total enthalpy change exploited by the main turbine.

In the present application, by Brayton cycle it is intended the actual Brayton cycle and the cycles derived therefrom.

The Applicant has verified that the selection of the fraction of the total enthalpy change used in the auxiliary turbine allows optimizing the operation of the compressor of the turbocharger (which rotates at the same speed as the auxiliary turbine) independent of the number of revolutions at which the main turbine works. The Applicant has verified that the invention allows attaining the turbomachines that operate the compression and expansion transformations present in the cycle in an optimized manner, i.e. dividing the enthalpy changes and the flows into volumes so as to obtain machine configurations of industrially significant size which allow obtaining high efficiencies and simplifying and making more reliable the transients of starting, stopping and power variation of the cycle.

The Applicant has verified that the invention allows suitably configuring the cycle also in the presence of density variations of the fluid in proximity to the critical point, of the specific heat variation upon varying the thermodynamic conditions, of the intrinsic fluid characteristics such as the actual positioning of the limit curve in the thermodynamic diagrams.

The Applicant has also verified that each auxiliary turbine-compressor (turbocharger) assembly is small, simple, compact and relatively low-cost and this allows reducing the bulk and the costs of the plant in its entirety.

Preferred and non-limiting aspects of the invention are listed hereinbelow.

In one aspect, said fraction of the enthalpy change is comprised between 15% and

80%, more preferably between 20% and 60%, of the total enthalpy change that moves the main turbine.

In one aspect, the working fluid is selected from the group comprising: carbon dioxide (CO2), dinitrogen oxide (N2O), a mixture of N2O and CO2 and possible other gases.

The thermodynamic cycles with CO2 or similar gases allow exploiting the effects of real gas in an advantageous manner with respect to cycles, such as those with air, which use fluids in which these effects are nearly absent or much more limited. From the efficiency standpoint, such fluids allow greatly reducing the required compression work, approximating it to that of pumping required by a liquid. This translates into a considerably advantage in terms of thermodynamic efficiency. The fluids in question also allow reaching high maximum cycle temperatures with consequent advantage in the obtainable efficiencies.

In one aspect, the Brayton cycle is of trans-critical type (if the compressions are attained astride the critical pressure of the fluid) or super-critical (if even the minimal cycle pressure if higher than the critical pressure). In one aspect, the Brayton cycle is of condensation type. In this case, the compressor works as a pump (working fluid in liquid phase) or as a machine operating in part with the working fluid in liquid phase and in part in gaseous phase.

In one aspect, the Brayton cycle is provided with re-compression. The recompression allows reducing the irreversibility of heat exchange in the low temperature recuperator. In one aspect, the Brayton cycle is provided with intermediate cooling (inter-cooling between the compressions). In one aspect, the Brayton cycle is regenerative/recuperative.

In one aspect, the Brayton cycle is provided with re-heating, i.e. after a first expansion in the main turbine, the working fluid returns into the heater in order to newly increase its temperature before returning into the turbine and completing the expansion.

In one aspect, the auxiliary turbine and the compressor of the turbocharger are integral with each other in order to rotate together, preferably the auxiliary turbine and the compressor of the turbocharger are mounted on a common shaft or on two shafts that are aligned and joined together in order to rotate together. In one aspect, reducers are not present between the auxiliary turbine and the compressor of the turbocharger. In one aspect, the shaft of the turbocharger is separated from the main turbine. In one aspect, the turbocharger is mechanically separated from the other rotating machines of the plant.

In one aspect, the plant comprises a plurality of turbochargers of the described and/or claimed type.

This structural solution is simple and allows attaining turbochargers that are compact and relatively inexpensive, but perfectly suitable for allowing an optimized regulation of the plant.

In one aspect, the main turbine is multi-stage, preferably single-body or multi- body. In one aspect, the main turbine is axial, radial, radial/axial or axial/radial. In one aspect, the main turbine is connected to the external user by means of revolution multipliers/reducers, preferably defined by a gear train.

The compressor is singe-phase or multi-phase. With compression phase it is indicated a compressor section which comprises the same mass flow rate, regardless of the number of stages and their configuration, without the flow itself being entirely extracted from the compressor. In one aspect, the compressor is multi-stage.

In one aspect, the conduits are configured to connect an intermediate outlet (intermediate extraction) of the main turbine with an inlet of the auxiliary turbine and/or in which the conduits are configured to connect an outlet of the auxiliary turbine with an intermediate intake of the main turbine.

In one aspect, provision is made for: extracting, from an intermediate outlet from the main turbine, a fraction of the working fluid and sending it to the auxiliary turbine and/or re-introducing, in an intermediate intake of the main turbine, a fraction of the working fluid leaving the auxiliary turbine.

In one aspect, preferably according to one embodiment, the conduits are configured to connect an intermediate outlet (extraction) of the main turbine with an inlet of the auxiliary turbine and to connect an outlet of the auxiliary turbine with a discharge of the main turbine. In other words, the fraction of the total enthalpy change is obtained by extracting a fraction of the working fluid from the main turbine at an intermediate pressure (between that of inlet and that of outlet of the main turbine) and re-introducing it, after it has transited in the auxiliary turbine, downstream of the main turbine.

In one aspect, the process then comprises: extracting, from an intermediate outlet from the main turbine, a fraction of the working fluid and sending it to the auxiliary turbine and re-introducing, downstream of the main turbine, said fraction of the working fluid leaving the auxiliary turbine.

In one aspect, preferably according to one embodiment, the conduits are configured to connect an inlet of the auxiliary turbine with an inlet of the main turbine and to connect an outlet of the auxiliary turbine with an intermediate intake of the main turbine.

In other words, the fraction of the total enthalpy change is obtained by extracting a fraction of the working fluid upstream of the main turbine and re-introducing it, after it has transited in the auxiliary turbine, in an intermediate point of the main turbine. In one aspect, the process then comprises: drawing a fraction of the working fluid upstream of the main turbine and re-introducing, in an intermediate intake of the main turbine, the fraction of the working fluid leaving the auxiliary turbine.

In one aspect, preferably according to one embodiment, the conduits are configured to connect an intermediate outlet (extraction) of the main turbine with an inlet of the auxiliary turbine and to connect an outlet of the auxiliary turbine with an intermediate intake of the main turbine. In other words, the fraction of the total enthalpy change is obtained by extracting a fraction of the working fluid from the main turbine at an intermediate pressure (between that of inlet and that of outlet of the main turbine) and re-introducing it, after it has transited in the auxiliary turbine, in an intermediate point of the main turbine. In one aspect, the process then comprises: extracting, from an intermediate outlet from the main turbine, a fraction of the working fluid and sending it to the auxiliary turbine and re-introducing, in an intermediate intake of the main turbine, the fraction of the working fluid leaving the auxiliary turbine.

In order to operate the compression and expansion transformations in an optimized manner, it is possible to select the position of the intermediate outlet from the main turbine (i.e. select the extraction point after a specific number of stages of the main turbine) and/or select the position of the intermediate intake in the main turbine (after a specific number of stages) as a function of the number of revolutions at which the compressor must work.

In one aspect, preferably according to one embodiment, the compressor of the turbocharger is the only compressor of the plant. The main turbine therefore is not mechanically connected to any compressor and the only compressor is driven by the auxiliary turbine.

In one aspect, preferably according to other embodiments, the plant comprises a main compressor and at least one auxiliary compressor or a plurality of auxiliary compressors.

The power absorbed by the auxiliary compressor on the shaft of the turbocharger is considerably lower than that required by the main compressor, due to the limited enthalpy change operated on the fluid and the limited volume flow rate at its suction. The solution of the present invention allows optimizing the fluid-dynamic performances of the auxiliary compressor, by attaining a very small and fast machine, mechanically independent of the main turbine.

In one aspect, preferably according to one embodiment, the main compressor is mechanically connected to the main turbine, preferably by means of a transmission, preferably with gears.

In one aspect, preferably according to one embodiment, the main compressor is part of a respective turbocharger of the described and/or claimed type. In one aspect, said at least one auxiliary compressor is part of a respective turbocharger of the described and/or claimed type.

In one aspect, a motor and/or auxiliary electric generator is connected to the auxiliary turbocharger. The object of such motor and/or auxiliary electric generator is to make the cycle transients easier to manage and/or to start the circulation of the fluid in the circuit in the plant starting and/or to maintain a minimum flow transiting in the hot exchangers during the stop steps or following block situations of the main turbine.

In one aspect, the plant comprises a plurality of auxiliary compressors and auxiliary turbines defining a plurality of auxiliary turbochargers. Preferably, said auxiliary turbochargers are arranged in parallel from the standpoint of the fluid connection. Preferably, said auxiliary turbochargers are mechanically independent. The invention allows dividing the compression attained by the turbocharger over multiple turbochargers, all with the same characteristics and, preferably but not exclusively, identical to each other. The flow can be divided over multiple compressors in parallel, so to be able to activate the necessary number thereof in order to allow keeping each single turbocharger operating in proximity to its design point, to the advantage of the overall cycle efficiency and avoiding limit conditions of the compressor operating fields (with particular reference to pumping) which could negatively affect the reliability and the overall availability of the plant.

In one aspect, the process comprises: compressing the working fluid in a main compressor and/or in an auxiliary compressor, in which the main compressor and/or the auxiliary compressor are driven by the auxiliary turbine.

In one aspect, the conduits are configured to connect an outlet of the main turbine with an inlet of the main compressor.

In one aspect, preferably according to one embodiment (inter-cooled cycle), the conduits are configured to connect an intermediate outlet of the main compressor (multi-step compressor) with an inlet of the auxiliary compressor. The auxiliary compressor works at a pressure higher than the main compressor. The auxiliary compressor carries out a re-compression of a fraction of the working fluid.

In one aspect, an auxiliary cooling device is operatively arranged between the inlet of the auxiliary compressor and the intermediate outlet of the main compressor, in order to execute a partially inter-cooled cycle. In one aspect, the process comprises: extracting, from an intermediate outlet of the main compressor, a fraction of the working fluid and introducing it into an inlet of the auxiliary compressor. In one aspect, the process comprises: cooling the fraction of the working fluid extracted from the main compressor before introducing it into the inlet of the auxiliary compressor. In the auxiliary cooling device, the working fluid transfers thermal power to a thermal source at low or ambient temperature.

In one aspect, preferably according to one embodiment (cycle only re- compressed, not inter-cooled), the conduits are configured to connect an outlet of the main turbine with an inlet of the auxiliary compressor. Downstream of the main turbine, the working fluid is divided between the main compressor and the auxiliary compressor. In one aspect, the process then comprises: drawing a fraction of the working fluid downstream of the outlet of the main turbine and introducing it into an inlet of the auxiliary compressor. The auxiliary compressor substantially works at the same pressure as the main compressor.

In one aspect, the conduits comprise a first stretch extended between the delivery of the auxiliary compressor and the heater. In one aspect, the conduits comprise a second stretch extended between the outlet of the main turbine and the inlet of the main compressor.

In one aspect, preferably according to one embodiment, a recuperator assembly is operatively interposed between the outlet of the main turbine and an inlet of the main compressor and between a delivery of the auxiliary compressor and the heater, in order to recover heat from the working fluid leaving the main turbine and transfer it to the working fluid entering the heater.

In one aspect, an outlet of the auxiliary turbine is connected to a point of the second stretch placed upstream of the recuperator assembly and downstream of the main turbine.

The main turbine and the auxiliary turbine discharge, excluding the load losses, at the same pressure. From this, and from the efficiencies of the two turbines that are different from each other, it follows that the actual inlet conditions in the high temperature recuperator are determined by the mixing between the flows discharged by the main turbine and the auxiliary turbine.

In one aspect, the recuperator assembly is operatively active on the first stretch and on the second stretch in order to exchange heat between said two stretches. In other words, the first stretch and the second stretch are operatively coupled at the recuperator assembly in order to exchange heat between said two stretches. In one aspect, the recuperator assembly comprises a high temperature recuperator and a low temperature recuperator.

In one aspect, the high temperature recuperator and the low temperature recuperator are arranged in series along the first stretch. In one aspect, the high temperature recuperator and the low temperature recuperator are arranged in series along the second stretch.

In one aspect, a delivery of the main compressor is connected with an intermediate point of said first stretch placed between the high temperature recuperator and the low temperature recuperator.

The high temperature recuperator pre-heats the working fluid flows compressed by the auxiliary compressor and by the main compressor (after their mixing), using the heat transferred by the flows of the working fluid discharged by the turbines up to the inlet of the heater (boiler or heat exchanger) which serves to provide thermal power entering at high temperature.

After the discharge of the high temperature recuperator, the low temperature recuperator preheats the fluid compressed by the auxiliary compressor, up to the mixing point with the delivery of the main compressor, by means of the heat transferred by the flows discharged by the turbines that have already transited through the high temperature recuperator.

In one aspect, preferably according to one embodiment, the cooling device is operatively arranged on the second stretch and downstream (with respect to a direction of the working fluid in the second stretch) of the recuperator assembly, in particular downstream of the low temperature recuperator. After the discharge of the low temperature recuperator, the cooling device (or cold exchanger) is traversed by the entire flow discharged from the main and auxiliary turbines, which transfers thermal power to a cold source (or environment).

In one aspect, preferably according to one embodiment, a branch from the second stretch connects said second stretch to the auxiliary compressor. In one aspect, the branch connects a point of the second stretch located downstream of the recuperator assembly, in particular downstream of the low temperature recuperator, with the inlet of the auxiliary compressor. In one aspect, the cooling device is operatively arranged on said branch. In one aspect, the process comprises: cooling the fraction of the working fluid extracted downstream of the outlet of the main turbine before introducing it into the inlet of the auxiliary compressor.

After the discharge of the low temperature recuperator, the cooling device (or cold exchanger) is traversed by the single flow sent to the auxiliary compressor.

In accordance with further aspects and embodiments, the exchangers which operate the regeneration, high and low temperature recuperators, are not present in non-regenerative cycles still according to the invention, and the high and low temperature recuperators can alternatively not be present in partially regenerative cycles.

Further characteristics and advantages will be clearer from the detailed description of embodiments of a plant and of a process according to the present invention.

Description of the drawings

Such description will be set forth hereinbelow with reference to the enclosed drawings, provided only as a non-limiting example, in which:

^■ figure 1 schematically illustrates a Brayton cycle plant for energy production according to the present invention;

^■ figure 2 is a T (temperature) - S (entropy) diagram that illustrates the Brayton cycle carried out by the plant of figure 1 ;

^■ figure 3 is a T-S diagram that illustrates a variant of the Brayton cycle carried out by the plant of figure 1 ;

^■ figure 4 illustrates an embodiment variant of the plant of figure 1 ;

^■ figure 5 is a T-S diagram that illustrates the Brayton cycle carried out by the plant of figure 3;

^■ figure 6 is a T-S diagram that illustrates a variant of the Brayton cycle carried out by the plant of figure 3;

^■ figure 7 illustrates a simplified embodiment of the plant of figure 1 ;

^■ figure 8 illustrates a variant of the plant of figure 7.

Detailed description

With reference to the abovementioned figures, reference number 1 overall indicates Brayton cycle plant for energy production in accordance with the present invention. The plant 1 comprises a main turbine 2 which receives in a respective inlet 2a, by means of conduits, a working fluid (e.g. CO2) previously heated and brought to vapor state in a heater 3 (e.g. constituted by a boiler or by a hot exchanger which serves to provide thermal power entering at high temperature) placed upstream of said main turbine 2 (with respect to a direction of the flow of the working fluid). The main turbine 2 is, for example, an axial multistage turbine and is mechanically connected to a generator 4 by means of a gear train 5. In embodiment variants, the turbine 1 can be a radial turbine, radial/axial or axial/radial of single-body or multi-body type.

The plant 1 comprises a main compressor 6 of two-phase and two-stage type, which is mechanically connected to the abovementioned gear train 5. In the illustrated embodiment, the main compressor 6 comprises two bodies mounted on the same shaft connected to the gear train 5.

The plant 1 comprises a turbocharger 7 comprising an auxiliary turbine 8 and an auxiliary compressor 9 connected by a common shaft (or on two aligned shafts, joined together in order to rotate together), in a manner such that the respective rotors rotate together at the same number of revolutions. The turbocharger 7 is mechanically decoupled and independent of the main turbine 2.

A first stretch of the conduits of the plant 1 connects a delivery 9a of the auxiliary compressor 9 with the heater 3 and subsequently with the inlet 2a of the main turbine 2. A second stretch of the conduits connects a discharge 2b of the main turbine 2 with an inlet 6a of the main compressor 6.

The first stretch and the second stretch are operatively coupled together in order to exchange heat. In particular, the plant 1 comprises a recuperator assembly comprising a high temperature recuperator 10 and a low temperature recuperator 1 1 which are both arranged in series along the first stretch and along the second stretch and are operatively active on the first stretch and on the second stretch in order to exchange heat between said two stretches, i.e. in order to recover heat from the working fluid leaving the main turbine 2 and transfer it to the working fluid entering the heater 3. With respect to a direction of the working fluid in the second stretch, the high temperature recuperator 10 is placed upstream of the low temperature recuperator 1 1 . With respect to a direction of the working fluid in the first stretch, the high temperature recuperator 10 is located downstream of the low temperature recuperator 1 1. A cooling device 12 is operatively arranged on the second stretch and downstream (with respect to the direction of the working fluid in the second stretch) of the low temperature recuperator 1 1 .

A third stretch connects a delivery 6b of the main compressor 6 with an intermediate point of said first stretch placed between the high temperature recuperator 10 and the low temperature recuperator 1 1 .

A fourth stretch connects an outlet 8a of the auxiliary turbine 8 with a point of the second stretch placed between the main turbine 2 and the high temperature recuperator 10, i.e. with a discharge 2b of the main turbine 2.

The main turbine 2 has an intermediate outlet 2c placed between subsequent stages and configured for extracting working fluid (extraction) at an intermediate pressure between an inlet pressure and a discharge pressure of said main turbine 2. A fifth stretch of the conduits connects the intermediate outlet 2c of the main turbine 2 with an inlet 8b of the auxiliary turbine 8.

The connection between the auxiliary turbine 8 and the main turbine 2 is such that said auxiliary turbine 8 exploits a fraction "ΔΗ1 " of a total enthalpy change "ΔΗ" exploited by the main turbine 2.

The main compressor 6 has an intermediate outlet 6c placed between an outlet 6d from the first of the two bodies mounted on the same shaft and an inlet 6e into a second of said two bodies, which is connected, by means of a sixth stretch of the conduits, to an inlet 9b of the auxiliary compressor 9.

On the sixth stretch and interposed between the main compressor 6 and the auxiliary compressor 9, an auxiliary cooling device 13 is arranged. In the auxiliary cooling device 13, the working fluid transfers thermal power to a thermal source at low or ambient temperature.

The described plant 1 allows carrying out, in accordance with the process according to the invention, a super-critical Brayton cycle, partially re-compressed, partially inter-cooled and regenerative, as illustrated in figure 2.

The working fluid, heated and brought to the vapor state in the heater 3 (point A of the T-S diagram of figure 2), is expanded in the main turbine 2, is cooled (point B) and causes the rotation of the generator 4 and the electrical energy production. Subsequently, the working fluid passes through the high temperature recuperator 10 where it transfers heat, point C in the T-S diagram, and then traverses the low temperature recuperator 1 1 where it transfers further heat, point D of the T-S diagram. Then, the working fluid is further cooled through the cooling device 12 and reaches the point E close to the Andrews bell curve. At this point, the working fluid enters into the two-stage main compressor 6. The entire flow of the working fluid is compressed by the first stage and reaches the point F, then a fraction which transits through the second stage is further compressed up to point G while a fraction that exits from the intermediate outlet 6c passes through the auxiliary cooling device 13 and reaches the point H. The fraction at G is introduced in the intermediate point of the first stretch placed between the high temperature recuperator 10 and the low temperature recuperator 1 1. The fraction at H is compressed in the auxiliary compressor 9 up to the point I and then heated in the low temperature recuperator 1 1 , it too reaching point G. The total flow of the working fluid is heated, moving through the high temperature recuperator 10 up to point L and then once again enters into the heater 3. The auxiliary compressor 9 is rotated by the auxiliary turbine 8 which in turn exploits the partially expanded fraction of the working fluid extracted from the intermediate outlet 2c of the main turbine 2 and then re-introduced into the second stretch and upstream of the high temperature recuperator 10 (and the fraction "ΔΗ1 " of the total enthalpy change "ΔΗ"). For example, the fraction of the enthalpy change is comprised between 15% and 80%, more preferably between 20% and 60%, of the total enthalpy change that moves the main turbine 2. The auxiliary compressor 9 carries out a recompression of a fraction of the working fluid and works at a pressure higher than the main compressor 6.

Figure 3 illustrates a variant of the Brayton cycle of figure 2, in which the Brayton cycle is of condensation type. In this case, the auxiliary compressor 9 works as a pump (working fluid in liquid phase) or as a machine operating in part with the working fluid in liquid phase and in part in gaseous phase.

Figure 4 illustrates a variant of the plant of figure 1 . The reference numbers are the same for the same components. The difference with respect to the plant 1 of figure 1 lies in the configuration of the connection of the rest of the plant 1 with the inlet 9b of the auxiliary compressor 9. Indeed, instead of receiving a fraction of the working fluid coming from the intermediate outlet 6c of the main compressor 6 (as in figure 1 ), the inlet 9b of the auxiliary compressor 9 is connected, by means of a branch, to the second stretch. The branch connects a point of the second stretch located downstream of the low temperature recuperator 1 1 with the inlet 9b of the auxiliary compressor 9. In addition, the cooling device 12 is operatively arranged on said branch. The main compressor 6 is single-phase and does not have any intermediate outlet 6c.

The described plant 1 allows carrying out, in accordance with the process according to the invention, a super-critical Brayton cycle, partially re-compressed, not inter-cooled and regenerative, as illustrated in figure 5.

In this variant, the working fluid that flows into the second stretch and exits from the low temperature recuperator 1 1 (point D) in part flows towards the main compressor 6 where it is compressed, reaching point G, and in part passes through the cooling device 12, being cooled up to point H and then it is compressed in the auxiliary compressor 9, reaching the point I.

Figure 6 illustrates a variant of the Brayton cycle of figure 5, in which the Brayton cycle is of condensation type.

The embodiments of figures 7 and 8 illustrate respective further configurations of the plant 1 , simpler than those of figures 1 and 4, in which the main compressor 6 of the turbocharger 7 is the only compressor of the plant 1 . The main turbine 2 is therefore not mechanically connected to any compressor and the only compressor present, the main compressor 6, is driven by the auxiliary turbine 8.

The plant of figure 7, like that of figures 1 and 4, draws from the intermediate outlet 2c of the main turbine 2 the partially expanded fraction of the working fluid and directs it towards the inlet 8b of the auxiliary turbine 8. Such fraction, after expansion in the auxiliary turbine 8, is introduced downstream of the outlet 2b of the main turbine 2, before the cooling device 12.

In the variant of figure 8, the conduits connect the inlet 8b of the auxiliary turbine 8 with an inlet 2a of the main turbine 2 so that the fraction directed towards the inlet 8b of the auxiliary turbine 8 is extracted upstream of the main turbine 2. In addition, the conduits connect the outlet 8a of the auxiliary turbine 8 with an intermediate intake 2d of the main turbine 2.

In other non-illustrated embodiment variants, both of the plants of figures 1 and 4 and of those of figures 7 and 8, the extraction from the main turbine 2 can be present as well as the intermediate intake into the main turbine 2.

In other embodiment variants, not illustrated in detail, the working fluid can be dinitrogen oxide (N2O) or a mixture of N2O and CO2 and possible other gases, the Brayton cycle can be of transcritical type, and the plant can comprise a plurality of turbochargers 7. The turbochargers 7 can be arranged in parallel from the standpoint of the fluid connection and be mechanically independent. In other embodiment variants, not illustrated in detail, a motor and/or auxiliary electric generator can be connected to the auxiliary turbocharger 7.

Example

The following table 1 reports the values relative to a cycle like that of figure 2, relative to the plant of figure 1 only for the part relative to the turbocharger 7 and to the main turbine 2, in which the partially expanded fraction of the working fluid extracted from the intermediate outlet 2c of the main turbine 2 feeds the auxiliary turbine 8 and is then introduced downstream of the main turbine 2.

The following table 2 instead illustrates the case in which the auxiliary turbine 8 works in parallel with the main turbine 2, i.e. the working fluid introduced into the inlet 8b of the auxiliary turbine 8 is extracted upstream of the main turbine 2 and re-introduced downstream of the main turbine 2.

Dh isentropic enthalpy change

Pin pressure at the inlet [bar]

Pout pressure at the outlet [bar]

Vin volumetric flow rate at the inlet [m ^A 3/s]

Vout volumetric flow rate at the outlet [m ^A 3/s]

Rpm number of revolutions per minute

ns characteristic velocity

nsturbine=rotation velocity [rad/s]*(volume flow rate at the discharge [m ^A 3/s]) ^A .5/(isentropic change[kJ/kg]) ^A .75

nscompresso rotation velocity [rad/s]*(volume flow rate at the inlet[m ^A 3/s]) ^A .5/(isentropic change[kJ/kg]) ^A .75

eta efficiency Table 1

Dh Dh/Dh main Pin Pout Vin Vout rpm ns eta turbine

Main turbine 185.65 1 178.2 56.5 0.8 2.1 1 1 1000

Auxiliary 15.21 0.08 84.57 185.5 0.07 30000 0.6 compressor

Auxiliary turbine 41 .17 0.22 74.28 56.51 0.74 30000 0.94 0.87 with feed from

extraction Table 2

It is observed that in the plant of figure 1 (Table 1 with extraction), the enthalpy change of the auxiliary turbine 8 of the turbocharger is 22% of the enthalpy change of the main turbine 2 while in the case of Table 2 the enthalpy change is the same.

It is observed that in the plant of figure 1 (Table 1 with extraction) the characteristic velocity ns of the auxiliary turbine 8 of the turbocharger is 0.94 while in the case of Table 2 such characteristic velocity ns is 0.14.

Normally, the following characteristic velocity ns values are deemed acceptable. For the compressors like that of the turbocharger 7: 0.55 < ns < 0.75.

For the turbines like that of the turbocharger 7: 0.4 < ns < 1.2.

Hence, the plant of figure 1 (Table 1 with extraction) allows a considerable improvement, actually allowing the attainment of the compression in the turbocharger 7 independent of the main turbine 2.

List of elements

1 Plant

2 main turbine

2a inlet of the main turbine

2b discharge of the main turbine

2c intermediate outlet

2d intermediate intake heater

user/generator

gear train

main compressor

a inlet of the main compressor

b delivery of the main compressorc intermediate outlet of the main compressord outlet from the first body

e inlet into the second body

turbocharger

auxiliary turbine

a outlet of the auxiliary turbine

b inlet of the auxiliary turbine

auxiliary compressor

a delivery of the auxiliary compressorb inlet of the auxiliary compressor

0 high temperature recuperator

1 low temperature recuperator

2 cooling device

3 auxiliary cooling device