DESCRIPTION

Field of the invention

The present invention relates to a process, a plant and a cascade thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources. A process/plant/cycle of this kind exploits variable temperature heat sources to heat one or more working fluids to which the source transfers heat and which produce powers transiting and expanding in one or more expanders. By way of non-limiting example, these variable temperature heat sources can be: exhaust fumes of gas turbines, exhaust fumes and heat of endothermic engines, CSP applications with solar collectors with oil collectors or salts (liquid carrier fluid, not vapour), inter-refrigeration heat of the compressors, intercooler cooling heat, oil and jackets of the endothermic engine, industrial waste heat (steel mills, cement factors, glassworks, petrochemical process heat, etc.), combustion fumes of biomasses and waste, etc.. Background of the invention

The public document US7942001 illustrates a system for generating power from the waste heat of internal combustion engines, such as microturbines or reciprocating engines, which uses cascaded organic Rankine cycles. A pair of ORC systems are combined and their respective organic fluid are chosen in such a way that the organic fluid (toluene) of the first organic Rankine cycle is condensed at a condensation temperature that is sharply above the boiling point of the organic fluid (R245fa) of the second organic Rankine cycle. A single heat exchanger is used as a condenser of the first Rankine organic cycle and as an evaporator of the second organic Rankine cycle.

The public document US8869531 illustrates a cascade system and method for recovering energy from waste heat. The system comprises an exchanger coupled with a waste heat source to heat a first flow of a working fluid (CO2, propane, ammonia), a first expansion device that receives the first flow from the exchanger to make it expand and to make a shaft turn, a first recuperator coupled to the first expansion device to receive the first flow therefrom and to transfer heat from the first flow to a second flow of the working fluid, a second expansion device that receives the second flow from the first recuperator and a second recuperator fluidly coupled to the second expansion device, to receive the second flow therefrom and transfer heat from the second flow to a combined flow of the first and second flows.

The public document US6857268 a cascading closed loop cycle for recovering power from a thermal energy source whose temperature is sufficiently high to vaporize a light hydrocarbon such as propane or propylene. The light hydrocarbon is vaporized in multiple indirect exchangers, expanded in multiple cascaded turbines and condensed in a cooling system. The light hydrocarbon is then pressurized with a pump and returned to the indirect heat exchangers.

The public document EP2607635 illustrates a cascaded organic Rankine cycle provided with a topping cycle and a bottoming cycle in thermal communication with the topping cycle through a condenser/evaporator in which a working fluid (R245fa) of the bottoming cycle is then evaporated and then heated and a working fluid (Siloxane MM) of the topping cycle is first cooled and then condensed. In this way, a percentage of the total heat transferred from the working fluid of the topping cycle during condensation is equal to or lower than a percentage of the total heat transferred to the bottoming cycle during evaporation.

Definitions

The term“at high temperature” means a maximum temperature of the variable temperature heat source between 300 and 600°C.

The term“stable and non-flammable” referred to the working fluid means that the fluid is stable, i.e. it does have reactions with significant kinetics, and not flammable in operating conditions at the working temperature, i.e. that its self- ignition temperature is higher than the maximum temperature of the variable temperature heat source from which it receives heat.

The term “reduced temperature” of a fluid means the ratio between the temperature of the fluid in degrees Kelvin and the critical temperature of said fluid in degrees Kelvin.

The term“reduced pressure” means the ratio between the pressure of the fluid and the critical pressure of said fluid. Summary

In this context, the Applicant has perceived the need to improve the efficiency of the processes/plants/cycles for the production of mechanical and/or electrical power from variable temperature heat sources so as, for example, to produce more power for the same available source.

The Applicant has in particular perceived the need to propose an even more efficient cycle able better to exploit the waste heat contained in the variable temperature heat sources, for example like the aforementioned ones.

In particular, the Applicant defined the following objectives:

^■ to devise a more efficient cascade process, a plant and a cycle to exploit the variable temperature resources preferably at high temperature;

^■ to devise a cascade process, a plant and a cycle that allow the extraction of a greater quantity of heat from said resources, cooling them to a lower temperature compared to the prior art.

^■ to devise a process, a plant and a cycle characterised by lower irreversibility and hence better conversion peformances;

^■ to devise a process, a plant and a cycle that are intrinsically safe, in particular with reference to the risk of fire associated with the working fluids used hitherto;

^■ to devise a process, a plant and a cycle that are relatively economical, in particular but not exclusively with reference to the technology used for the expanders.

The Applicant has found that the objectives indicated above and others besides can be achieved implementing multiple cascaded cycles, wherein the topping cycle of said cascaded cycles, i.e. the one that exchanges heat, preferably directly, with the high temperature source, is an organic supercritical Rankine cycle whose organic working fluid is selected from perfluorinated compounds.

In particular, the objectives indicated and others besides are substantially achieved by a process, by a plant and by a cycle of the type claimed in the accompanying claims and/or described in the following aspects.

In an independent aspect, the present invention relates to a cascade process for the production of power from variable temperature heat sources. Preferably, the process comprises: circulating a main working fluid selected from the perfluorinated compounds in a main circuit according to a main organic supercritical Rankine cycle, wherein said main working fluid is heated and vaporized, expanded into a main expander, preferably enslaved to an electric generator or to a mechanical user, cooled, condensed and heated and vaporized again. The term“supercritical” cycle in the present description and in the claims means that supercritical are the conditions of the working fluid flowing into the turbine (while condensation is subcritical).

Preferably, the process comprises: operationally coupling in a boiler a variable temperature heat source, preferably fumes, to the main working fluid of the main circuit to perform said heating and vaporization of the main working fluid.

Preferably, the variable temperature heat source is directly coupled to the main working fluid, i.e. without the interposition of intermediate fluids.

Preferably, the process comprises: circulating an auxiliary working fluid in an auxiliary circuit according to an auxiliary Rankine cycle, wherein said auxiliary working fluid is heated and vaporized, expanded into an auxiliary expander, preferably enslaved to an auxiliary electric generator or to an auxiliary mechanical user, cooled, condensed and heated and vaporized again.

Preferably, the process comprises: thermally coupling the expanded main working fluid of the main Rankine cycle to the auxiliary working fluid of the auxiliary Rankine cycle, in order to cool the main working fluid and to heat and vaporize said auxiliary working fluid by heat transfer from said main Rankine cycle to said auxiliary Rankine cycle before the expansion of the auxiliary working fluid into the auxiliary expander.

In one aspect, the main working fluid is preheated in a main recuperator or first recuperator.

In an independent aspect, the present invention relates to a cascade plant for the production of power from variable temperature heat sources.

Preferably, the plant comprises: a main circuit comprising: a boiler operatively coupled to a variable temperature heat source, preferably fumes; a main expander; a main recuperator or first recuperator; a main condenser; a main pump; main pipes connecting each other the boiler, the main expander, the main recuperator, the main condenser and the main pump; a main working fluid selected from the perfluorinated compounds and flowing in the main circuit so as to implement a supercritical organic Rankine cycle.

Preferably, the plant comprises: at least one auxiliary circuit thermally coupled to the main circuit and comprising an auxiliary expander; wherein an auxiliary working fluid enters the auxiliary expander after exchanging heat with the main working fluid exiting the main expander.

In an additional independent aspect, the present invention relates to the combination of the cascade plant for the production of power from variable temperature heat sources claimed and described and of the main working fluid chosen from perfluorinated compounds.

In an additional independent aspect, the present invention relates to a cascade thermodynamic cycle for the production of power from variable temperature heat sources.

Preferably, the cascade cycle comprises: a supercritical main Rankine cycle with an organic main working fluid selected from the perfluorinated compounds; wherein the main Rankine cycle receives heat from a variable temperature heat source, preferably fumes.

Preferably, the cascade cycle comprises: an auxiliary Rankine cycle with an auxiliary working fluid; wherein the auxiliary cycle is thermally coupled to the main cycle to receive heat from said main cycle after an expansion of the main working fluid and before an expansion of the auxiliary working fluid.

Preferably, in the main cycle: a reduced temperature of the main working fluid immediately before the main expansion is between 1.1 and 1.7.

Preferably, in the main cycle: a reduced pressure of the main working fluid immediately before the main expansion is between 1 and 2.5.

Preferably, in the main cycle: a reduced condensation temperature of the main working fluid is between 0.6 and 0.9.

Preferably, in the main cycle: a reduced condensation pressure of the main working fluid is between 0.005 and 0.3.

The Applicant has verified that the present invention allows to exploit fully and efficiently the high temperature waste heat sources with a relatively economical and intrinsically stable and safe plant.

In particular, the Applicant has verified that the cascade structure of the cycles and of the circuits allows to exploit in series the heat source, to cool the fumes sufficiently and to obtain lower irreversibility and hence better conversion performances.

The Applicant has verified that the working fluid that reaches the highest temperatures, i.e. the one that directly exchanges heat with the high and variable temperature source, is the one chosen from the perfluorinated compounds and it is thermally stable and not flammable at high operating temperatures.

The Applicant has verified that the cascade structure of the cycles and of the circuits and the use of the working fluid chosen from the perfluorinated compounds allows to use the same technology, in particular of the expanders, on all cycles and circuits, also of the main expander that is directly coupled with the high and variable temperature heat source.

The present invention, in at least one of the aforesaid aspects, can have one or more of the additional preferred aspects that are described below.

In one aspect, a plurality of auxiliary working fluids is circulated in respective auxiliary circuits arranged in cascade according to respective auxiliary Rankine cycles.

In one aspect, the plant comprises: a plurality of auxiliary circuits arranged in cascade and thermally coupled with each other.

Below the main cycle/circuit can be present multiple cycle/circuits arranged in series, or in cascade one after the other, that exchange heat between them. Each cycle/circuit receives heat from the topping cycle/circuit and transfers heat to the bottoming cycle/circuit.

In one aspect, the process comprises: circulating an additional auxiliary working fluid in an additional auxiliary circuit according to an additional auxiliary Rankine cycle, wherein said additional auxiliary working fluid is heated and vaporized, expanded in an additional auxiliary expander, preferably enslaved to an additional auxiliary electric generator or to an auxiliary mechanical user, cooled, condensed and heated and vaporized again; thermally coupling the expanded auxiliary working fluid of the auxiliary Rankine cycle to the additional auxiliary working fluid of the additional auxiliary Rankine cycle, in order to cool the auxiliary working fluid and to heat and vaporize said additional auxiliary working fluid by heat transfer from said auxiliary Rankine cycle to said additional auxiliary Rankine cycle before the expansion of the additional auxiliary working fluid into the additional auxiliary expander. In one aspect, the plant comprises: at least one additional auxiliary circuit thermally coupled to the auxiliary circuit and comprising an additional auxiliary expander; wherein an additional auxiliary working fluid enters the additional auxiliary expander after exchanging heat with the auxiliary working fluid exiting the auxiliary expander.

In one aspect, the cascade cycle comprises: an additional auxiliary Rankine cycle with an additional auxiliary working fluid; wherein the additional auxiliary cycle is thermally coupled to the auxiliary cycle to receive heat from said auxiliary cycle after an expansion of the auxiliary working fluid and before an expansion of the additional auxiliary working fluid.

In one aspect, the main working fluid selected from the perfluorinated compounds is selected from the group comprising: Perfluoro-2-methylpentane / Perfluoro-i- hexane (Flutec ^tm PP1 ), Perfluoro-methylcyclohexane (PP2), Perfluoro-1 ,3- dimethylcyclohexane (PP3), Hexafluorobenzene.

In one aspect, the main working fluid and the auxiliary working fluid are the same fluid.

In one aspect, pressures of said same fluid entering the main expander and the auxiliary expander are substantially the same.

In one aspect, the auxiliary working fluid is different from the main working fluid. In one aspect, the auxiliary working fluid is an organic fluid.

In one aspect, the auxiliary working fluid is selected from the group comprising: cyclopentane, isopentane, isohexane, hexane, pentane, R245fa, R1234yf.

In one aspect, a maximum temperature of the variable temperature heat source is between 900 and 300°C.

In one aspect, a reduced temperature of the auxiliary working fluid immediately before the auxiliary expansion is between 0.8 and 1.2.

In one aspect, a reduced pressure of the auxiliary working fluid immediately before the auxiliary expansion is between 0.3 and 1.2.

In one aspect, a reduced condensation temperature of the auxiliary working fluid is between 0.5 and 0.75.

In one aspect, a reduced condensation pressure of the auxiliary working fluid is between 0.001 and 0.1.

In one aspect, a temperature of the main working fluid immediately before the main expansion is between 250°C and 400°C. In one aspect, a pressure of the main working fluid immediately before the main expansion is between 25 bar and 50 bar.

In one aspect, a condensation temperature of the main working fluid is between 10°C and 50°C.

In one aspect, a condensation pressure of the main working fluid is between 0.1 bar and 1 bar.

In one aspect, a temperature of the auxiliary working fluid immediately before the auxiliary expansion is between 150°C and 300 °C.

In one aspect, a pressure of the auxiliary working fluid immediately before the auxiliary expansion is between 20 bar and 50 bar.

In one aspect, a condensation temperature of the auxiliary working fluid is between 10°C and 50°C.

In one aspect, a condensation pressure of the auxiliary working fluid is between 0.1 bar and 2 bar.

The condensation pressure at the condensation temperature is such that it is possible to use air condensers, because said condensation pressure is not too far from atmospheric pressure.

In one aspect, the condensation temperature of the main working fluid is substantially equal to the condensation temperature of the auxiliary working fluid. In one aspect, the auxiliary circuit is a branch of the main circuit.

In one aspect, the thermal coupling of the main working fluid to the auxiliary working fluid is performed in a second recuperator located downstream of the main expander and upstream of the auxiliary expander. The second recuperator is a high temperature recuperator and the main recuperator is a low temperature recuperator.

In one aspect, it is provided for separating the auxiliary working fluid from the main working fluid upstream of the main expander and for re-joining the auxiliary working fluid and the main working fluid downstream of the main expander and the auxiliary expander.

In one aspect, the auxiliary working fluid re-joins the main working fluid in the main recuperator.

In one aspect, the auxiliary working fluid is the main working fluid.

In one aspect, the plant further comprises a second recuperator located on the main circuit and downstream of the main expander and placed on the auxiliary circuit and upstream of the auxiliary expander; the heat between the working fluid of the main circuit and the working fluid of the auxiliary circuit being exchanged in the second recuperator.

In one aspect, the auxiliary circuit branches from the main circuit in a point positioned between the main recuperator and the boiler and re-joins the main circuit in the main recuperator.

In one aspect, the plant with a single working fluid comprises a single pump, i.e. the main pump.

In one aspect, the plant with a single working fluid comprises a single low temperature recuperator, i.e. the main recuperator.

In one aspect, the plant with a single working fluid comprises a single condenser.

In one aspect, the plant with a single working fluid comprises a single set: condenser, recuperator, pump.

In one aspect, the main working fluid and the auxiliary working fluid are fluidly separated; wherein the plant comprises a heat exchanger; wherein the main working fluid and the auxiliary working fluid are thermally coupled in the heat exchanger.

In one aspect, the auxiliary circuit is fluidly separated from the main circuit; wherein the plant comprises a heat exchanger; wherein the main circuit and the auxiliary circuit are thermally coupled in the heat exchanger.

In one aspect, the auxiliary circuit comprises: an auxiliary recuperator; an auxiliary condenser; an auxiliary pump; auxiliary pipes connecting each other the auxiliary expander, the auxiliary recuperator, the auxiliary condenser and the auxiliary pump.

In one aspect, the heat exchanger is located on the main circuit between the main expander and the main recuperator and it is located on the auxiliary circuit between the auxiliary recuperator and the auxiliary expander.

In one aspect, the main expander has an expansion ratio between 30 and 200.

In one aspect, the main expander and/or the auxiliary expander and/or the additional auxiliary expander is/are selected from: axial turbines, radial outflow/inflow turbines, radial/axial turbines. Preferably, the main expander and/or the auxiliary expander and/or the additional auxiliary expander is/are a radial outflow turbine. Preferably, the radial outflow turbine comprises: a casing, a rotor disk provided with a front face and rotatably housed in the casing, annular arrays of rotor blades positioned concentrically on the front face, annular arrays of stator blades mounted on the casing and interposed between the annular arrays of rotor blades to define a first radial path for a working fluid.

Preferably, the casing has an inlet in fluid communication with said first radial path and preferably positioned in proximity to a centre of the rotor disk.

Preferably, the casing has an outlet positioned in proximity to a radially peripheral portion of the rotor disk.

Preferably, the rotor disk is provided with a rear face, opposite to the front face, wherein annular arrays of rotor blades are positioned concentrically on the rear face, wherein annular arrays of stator are blades mounted on the casing and interposed between the annular arrays of rotor blades of the rear face to define a second radial path for a working fluid.

Preferably, the casing has an additional inlet positioned in front of the rear face and in fluid communication with said second radial path.

In one aspect, said first radial path is a part of the main circuit and said second radial path is a part of the auxiliary circuit.

The radial outflow turbine with two bladed faces defines both the main expander (through the first radial path) and the auxiliary expander (through the second radial path) when the main working fluid and the auxiliary working fluid are the same fluid. In one aspect, the boiler comprises a container and a plurality of ducts housed in the container, wherein the main working fluid flows in said ducts.

In one aspect, the container has an inlet and an outlet for fumes defining the variable temperature heat source.

In one aspect, the fumes directly lap the ducts to transfer heat to the main working fluid.

In one aspect, the ducts are bent pipes, preferably without junctions/welds.

In one aspect, the ducts comprise a plurality of coiled tubes.

Additional features and advantages will become more readily apparent from the detailed description of preferred, but not exclusive, embodiments, of a cascade thermodynamic process/plant/cycle for the production of power from variable temperature heat sources according to the present invention. Description of the drawings

Such description will be made herein below with reference to the accompanying drawings, provided for indicative purposes only and therefore not limiting, in which:

^■ Figure 1 schematically shows a cascade plant for the production of power from variable temperature heat sources according to the present invention;

^■ Figure 2 shows a T-S diagram of a cascade thermodynamic cycle implemented by the plant of Figure 1 ;

^■ Figure 3 shows a T-Q diagram relating to the heat exchange between the two circuits of the plant of Figure 1 in subcritical conditions;

■ Figure 4 shows a T-Q diagram relating to the heat exchange between the two circuits of the plant of Figure 1 in supercritical conditions;

^■ Figure 5 shows a different embodiment of the plant according to the present invention;

^■ Figure 6 shows a T-S diagram of a cascade thermodynamic cycle implemented by the plant of Figure 5;

^■ Figure 7 shows a variant of the plant of Figure 5;

^■ Figure 8 shows a T-S diagram of a cascade thermodynamic cycle implemented by the plant of Figure 7;

^■ Figure 9 shows a variant of the cascade plant of Figure 1 ;

■ Figure 10 schematically shows a turbine usable in the plants of Figures 5 or

7.

Detailed description

With reference to the aforementioned figures, the reference numeral 1 globally designates a cascade plant for the production of (mechanical and/or electrical) power from variable temperature heat sources according to the present invention. With particular reference to Figure 1 , the plant 1 comprises a closed main circuit 2 and a closed auxiliary circuit 3 thermally coupled to each other but fluidly separated from each other.

The main circuit 2 comprises a boiler 4, a main expander 5, a main recuperator 6, a main condenser 7, a main pump 8. Main pipes connect each other the boiler 4, the main expander 5, the main recuperator 6, the main condenser 7 and the main pump 8, so as to allow the implementation of a recuperative supercritical organic Rankine cycle, by means of a working fluid that circulates through the main pipes and the aforementioned elements.

In particular, with respect to a direction of the flow of the working fluid, the main expander 5 is positioned immediately downstream of the boiler 4, the main condenser 7 is positioned downstream of the main expander 5, the main pump 8 is positioned immediately downstream of the main condenser 7 and the boiler 4 is positioned downstream of the pump 8.

The main recuperator 6 is operatively positioned on a first segment of the main pipes that extends from the pump 8 towards the boiler 4 and on a second segment of the same pipes that extends from the main expander 5 towards the main condenser 7. The main recuperator 6 has the function of pre-heating the main working fluid before the entry into the boiler 4 by means of heat transferred by the same main working fluid exiting from the main expander 5.

The main working fluid is selected from the perfluorinated compounds and it preferably is Perfluoro-2-methylpentane/Perfluoro-i-hexane (for example known by the commercial name: Flutec ^tm PP1 ).

The boiler 4 is operatively coupled to a variable temperature heat source, for example fumes 9 of a turbogas or of an industrial process, with high temperature, for example with a maximum temperature of 600°C.

As is schematically shown in Figure 1 , the boiler 4 comprises a container 10 provided with an inlet and an outlet for the fumes 9 and a plurality of ducts housed in the container 10. These ducts are a part of the aforementioned main pipes and are defined by bent/curved pipes arranged in a coil. The fumes 9 directly lap the ducts to transfer heat to the main working fluid. The bent/curved pipes are preferably made of stainless steel and are made without junctions/welds.

The main expander 5 is enslaved to an electric generator 11 that generates energy by means of the rotation imparted to the main expander 5 by the working fluid expanding in said main expander 5. The main expander 5 is preferably but not exclusively a radial outflow turbine, known in itself, which comprises: a casing, a rotor disk provided with a front face and rotatably housed in the casing, annular arrays of rotor blades positioned concentrically on the front face, annular arrays of stator blades mounted on the casing and interposed between the annular arrays of rotor blades to define a first radial path for a working fluid, wherein the casing has an inlet positioned in proximity to a centre of the rotor disk and an outlet positioned in proximity to a radially peripheral portion of the rotor disk. For example, the radial outflow turbine is like the one described in the patent EP2699767 in the name of the same Applicant. In other embodiment variants, the main expander can be an axial turbine, a radial inflow turbine, a radial/axial turbine, for example like the one described in the patent EP2743463 in the name of the same Applicant.

The auxiliary circuit comprises: an auxiliary expander 12 enslaved to a respective auxiliary electric generator 13, an auxiliary recuperator 14, an auxiliary condenser 15, an auxiliary pump 16 and auxiliary pipes connecting each other the auxiliary expander 12, the auxiliary recuperator 14, the auxiliary condenser 15 and the auxiliary pump 16 so as to allow the implementation of a recuperative organic Rankine cycle, by means of an auxiliary working fluid that circulates through the auxiliary pipes and the aforementioned elements.

The auxiliary expander 12 can be an axial turbine, a radial outflow/inflow turbine, a radial/axial turbine.

The auxiliary working fluid is an organic fluid, for example cyclopentane.

The plant 1 comprises a heat exchanger 17 that thermally couples the main circuit 2 and the auxiliary circuit 3.

The heat exchanger 17 is placed on the main circuit 2 between the main expander 5 and the main recuperator 6 and is placed on the auxiliary circuit 3 between the auxiliary recuperator 14 and the auxiliary expander 12.

Through said heat exchanger 17, the main working fluid of the main circuit 2 transfers heat to the auxiliary working fluid of the auxiliary circuit 3.

In particular, with respect to a direction of the flow of the auxiliary working fluid, the main expander 12 is positioned immediately downstream of the heat exchanger 17, the auxiliary condenser 15 is positioned downstream of the auxiliary expander

12, the auxiliary pump 16 is positioned immediately downstream of the auxiliary condenser 15 and the heat exchanger 17 is positioned downstream of the auxiliary pump 16.

The auxiliary recuperator 14 is operatively positioned on a first segment of the auxiliary pipes that extends from the auxiliary pump 16 towards the heat exchanger 17 and on a second segment of the same pipes that extends from the auxiliary expander 12 towards the auxiliary condenser 15. The auxiliary recuperator 14 has the function of pre-heating the auxiliary working fluid before the entry into the heat exchanger 17 by means of heat transferred by the same auxiliary working fluid exiting from the auxiliary expander 12.

In use and in accordance with the cascade process and thermodynamic cycle according to the invention and with reference to the T-S diagram of Figure 2, the main working fluid (Flutec ^tm PP1 ) circulating in the main circuit 2 is pumped by the main pump 8 and slightly increases its own temperature passing from point A to point B. The main working fluid passes through the main recuperator 6 heating to point C and then transits into the boiler 4 where it absorbs heat from the fumes 9 (which for example have a maximum temperature of 600°C) heating and vaporizing to point D (at approximately 400°C). The vaporized main working fluid then expands in the main expander 5 cooling to point E and causing the rotation of the expander 5 and of the respective electric generator 11.

The main working fluid (Flutec ^tm PP1 ) immediately before the main expansion (point D) has the following parameters:

Temperature: 400°C

Reduced temperature: 1.5

Pressure: 40 bar

Reduced pressure: 2.1

The main working fluid is cooled further in the heat exchanger 17, transferring heat to the auxiliary working fluid of the auxiliary circuit 3 and then in the main recuperator 6 (Points F and G). The subsequent passage in the main condenser 7 determines the condensation of the main working fluid that then returns to point A, ready to start a new main Rankine cycle.

The main working fluid (Flutec ^tm PP1 ) has the following parameters:

Condensation temperature: 25°C

Reduced condensation temperature: 0.65

Condensation temperature: 0.25 bar

Reduced condensation pressure: 0.01

The auxiliary working fluid (cyclopentane) circulating in the auxiliary circuit 3 is pumped by the auxiliary pump 16 and slightly increases its own temperature passing from point FI to point I. The auxiliary working fluid passes through the auxiliary recuperator 14 heating to point L and then transits into the heat exchanger 17 where it absorbs heat from the main working fluid, heating and vaporizing to point M (at approximately 250°C). The vaporized auxiliary working fluid then expands in the auxiliary expander 12 cooling to point N and causing the rotation of the expander 12 and of the respective electric generator 13.

The auxiliary working fluid (Cyclopentane) immediately before the auxiliary expansion (point M) has the following parameters:

Temperature: 250°C

Reduced temperature: 1.02

Pressure: 30 bar

Reduced pressure: 0.7

The auxiliary working fluid cools further in the auxiliary recuperator 14 (point 0) and then in the auxiliary condenser 15. The passage in the auxiliary condenser 15 determines the condensation of the auxiliary working fluid that then returns to point H ready to start a new auxiliary Rankine cycle.

The auxiliary working fluid (Cyclopentane) has the following parameters:

Condensation temperature: 25°C

Reduced condensation temperature: 0.6

Condensation temperature: 0.32 bar

Reduced condensation pressure: 0.007

Figure 3 shows the T-Q diagram relating to the heat exchange that takes place between the two circuits of the plant of Figure 1 at the heat exchanger 17 (points E-F and L-M) in subcritical conditions. Figure 4 shows a T-Q diagram relating to the same heat exchange in supercritical conditions.

Figure 5 shows a different embodiment of the plant 1 according to the present invention wherein the auxiliary circuit 3 is a branch of the main circuit 2 and therefore the working fluid is the same in the two circuits 2, 3.

The main circuit 2 comprises: the boiler 4, the main expander 5 (enslaved to the electric generator 11 ), the main recuperator 6, the main condenser 7, the main pump 8. In particular, with respect to a direction of the flow of the working fluid, the main expander 5 is positioned immediately downstream of the boiler 4, the main condenser 7 is positioned downstream of the main expander 5, the main pump 8 is positioned immediately downstream of the main condenser 7 and the boiler 4 is positioned downstream of the pump 8. The main recuperator 6 is operatively positioned on a first segment of the main pipes that extends from the pump 8 towards the boiler 4 and on a second segment of the same pipes that extends from the main expander 5 towards the main condenser 7. The auxiliary circuit 3 further comprises the auxiliary expander 12 enslaved in this specific embodiment to the same electric generator 11 of the main expander 5. The auxiliary circuit 3 branches from the main circuit 2 at a branching point 18 located between the main recuperator 6 and the boiler 4 and re-joins the main circuit 2 in the main recuperator 6. The main expander 5 and the auxiliary expander 12 therefore lie fluidly in parallel and the working fluid entering the two expanders 5 and 12 has substantially the same pressure. The working fluid of the main circuit and the working fluid of the auxiliary circuit exchange heat in a second recuperator 19. The second recuperator 19 is placed on the main circuit down stream of the main expander 5 and is placed on the auxiliary circuit upstream of the auxiliary expander 12. The second recuperator 19 is a high temperature recuperator. The main recuperator 6 is a low temperature recuperator.

The single working fluid is selected from the perfluorinated compounds and it preferably is Perfluoro-2-methylpentane/Perfluoro-i-hexane (for example known by the commercial name: Flutec ^tm PP1 ).

In use and in accordance with the cascade process and with the thermodynamic cycle according to the invention and with reference to the T-S diagram of Figure 6, the main working fluid (Flutec ^tm PP1 ) is pumped by the main pump 8 and slightly increases its own temperature passing from point A to point B.

The working fluid passes through the main recuperator 6 heating to point C and then divides into a main flow, which transits into the boiler 4 where it absorbs heat from the fumes heating and vaporizing to point E (at approximately 350°C), and into an auxiliary flow that passes into the second recuperator 19 heating to point D.

The vaporized main flow then expands in the main expander 5 cooling to point F and causing the rotation of the expander 5 and of the respective electric generator 11 and then passes into the second recuperator 19 where it transfers heat to the auxiliary flow (which heats to point D) and cools to point G.

The auxiliary flow then expands in the auxiliary expander 12 cooling to point G and causing the rotation of the expander 12 and of the electric generator 11. The main and auxiliary flow join in the main recuperator 6 and there they transfer heat, cooling to point H. The single flow of the working fluid at this point traverses the condenser 7, condenses (at approximately 30°C) and returns to the pump 8, i.e. to point A. Figure 7 shows a variant of the plant 1 of Figure 5. With respect to the plant of Figure 5, the variant of Figure 7 comprises an additional auxiliary circuit 20 which is thermally coupled with the auxiliary circuit 3. The additional auxiliary circuit 20 comprises an additional auxiliary expander 21. The additional auxiliary circuit 20 branches from the branching point 18 described above or from an additional branching point 22 of the auxiliary circuit 3 positioned between the branching point 18 and the second recuperator 19. The working fluid of the auxiliary circuit and the working fluid of the additional auxiliary circuit exchange heat in a third recuperator 23.

The third recuperator 23 is placed on the auxiliary circuit downstream of the auxiliary expander 12 and is placed on the additional auxiliary circuit upstream of the additional auxiliary expander 21. The third recuperator 21 is a high temperature recuperator. The second recuperator 19 is a medium temperature recuperator and the main recuperator 6 is a low temperature recuperator.

The additional auxiliary circuit 20, the auxiliary circuit 3 and the main circuit 2 join in the main recuperator 6. The main expander 5, the auxiliary expander 12 and the additional auxiliary expander 21 therefore lie fluidly in parallel and the working fluid entering the three expanders 5, 12 and 21 has substantially the same pressure.

In use and in accordance with the cascade process and thermodynamic cycle according to the invention and with reference to the T-S diagram of Figure 7, the working fluid is pumped by the main pump 8 and slightly increases its own temperature passing from point A to point B. The working fluid passes through the main recuperator 6 heating to point C and then divides into a main flow, which transits into the boiler 4 where it absorbs heat from the fumes, heating and vaporizing to point F (at approximately 400°C), into an auxiliary flow that passes into the second recuperator 19, heating to point E, and into an additional auxiliary flow, which passes into the third recuperator 23, heating to point D.

The main flow then expands in the main expander 5 cooling to point G and causing the rotation of the expander 5 and of the respective electric generator 11 and then passes into the second recuperator 19 where it transfers heat to the auxiliary flow (which heats to point E) and cools to point I.

The auxiliary flow expands in the auxiliary expander 12 cooling to point FI and causing the rotation of the expander 12 and of the electric generator 11 and then passes into the third recuperator 23 where it transfers heat to the additional auxiliary flow (which heats to point D) and cools to point I.

The additional auxiliary flow expands in the additional auxiliary expander 21 cooling to point I and causing the rotation of the expander 21 and of the electric generator 11.

The main, auxiliary and additional auxiliary flow join in the main recuperator 6 and there they transfer heat, cooling to point L. The single flow of the working fluid at this point traverses the condenser 7, condenses (at approximately 30°C) and returns to the pump 8, i.e. to point A.

Other variants of Figures 5 and 7, not shown, comprise further cascaded additional auxiliary circuits/cycles, all operating with the same working fluid. Other variants of the plant of Figure 1 comprise multiple cycle/circuits arranged in series, i.e. in cascade one after the other, with different fluids that exchange heat between them.

For example, Figure 9 shows an additional auxiliary circuit 20 arranged in cascade with respect to the auxiliary circuit 3 of Figure 1. The additional auxiliary circuit 20 is structurally similar to the auxiliary circuit 3. The additional auxiliary circuit 20 is operatively coupled with the auxiliary circuit 3 at an additional heat exchanger 24. The additional auxiliary circuit 20 comprises an additional auxiliary expander 21 with a respective additional auxiliary electric generator 25, an additional auxiliary recuperator 26, an additional auxiliary condenser 27 and an additional auxiliary pump 28.

In other embodiments, the main expander 5 and the auxiliary expander 12 of the plants of Figure 5 and/or of Figure 7 are incorporated in a single radial outflow turbine 100 of the type shown in Figure 10.

The radial outflow turbine 100 comprises a casing 101 , a single rotor disk 102 rotatably housed in the casing 101. The rotor disk 102 is provided with a front face 103 and with a rear face 104, opposite to the front face 103.

Annular arrays of rotor blades 105 are arranged concentrically on the front face 103 and also on the rear face 104. Annular arrays of stator blades 106 are mounted on an inner face of the casing 101 positioned in front of the front face 103 of the rotor disk 102 and lie interposed between the annular arrays of rotor blades 105 of the front face 103 to define a first radial path for the working fluid. Annular arrays of stator blades 106 are also mounted on an inner face of the casing 101 positioned in front of the rear face 104 of the rotor disk 102 and lie interposed between the annular arrays of rotor blades 105 of the rear face 104 to define a second radial path for the working fluid.

The casing 101 has an inlet 107 positioned in front of the front face 103, in proximity to a centre of the rotor disk 102 and in fluid communication with the first radial path, an additional inlet 108 positioned in front of the rear face 104 and in fluid communication with the second radial path, and an outlet 109 positioned in proximity to a radially peripheral portion of the rotor disk 102.

In the illustrated embodiment, the additional inlet 108 is defined by openings obtained through a wall of the casing 101 positioned around a support sleeve 110 of a shaft 111 of the turbine 100 mounted in said sleeve 110 on bearings. The shaft 111 bears and supports in overhang the rotor disk 102.

The first radial path defined on the front face 103 is a part of the main circuit of Figures 5 or 7 and the second radial path defined on the rear face 104 is a part of the auxiliary circuit of Figures 5 or 7.

The radial outflow turbine 100 with two bladed faces therefore defines both the main expander 5 (through the first radial path) that the auxiliary expander 12 (through the second radial path) of Figure 5 or 7. In other words, the inlet 107 corresponds to point E of Figure 5 or 7 and the additional inlet 108 corresponds to point D of Figures 5 or 7. These points E and D are substantially at the same pressure, so that the rotor disk 102 is intrinsically balanced.

List of elements

1 cascade plant

2 main circuit

3 auxiliary circuit

4 boiler

5 main expander

6 main recuperator

7 main condenser

8 main pump

9 fumes

10 container body

11 electric generator 12 auxiliary expander

13 auxiliary electric generator

14 auxiliary recuperator

15 auxiliary condenser

16 auxiliary pump

17 heat exchanger

18 branching point

19 second recuperator

20 additional auxiliary circuit

21 additional auxiliary expander

22 additional branching point

23 third recuperator

24 additional heat exchanger

25 additional auxiliary electric generator

26 additional auxiliary recuperator

27 additional auxiliary condenser

28 additional auxiliary pump

100 radial outflow turbine

101 casing

102 rotor disk

103 front face

104 rear face

105 rotor blades

106 stator blades

107 inlet

108 additional inlet

109 outlet

110 sleeve

111 shaft