WO2018104839A1

WO2018104839A1 - Thermodynamic cycle process and plant for the production of power from variable temperature heat sources

Info

Publication number: WO2018104839A1
Application number: PCT/IB2017/057600
Authority: WO
Inventors: Claudio SPADACINI; Luca Giancarlo Xodo
Original assignee: Exergy SpA
Current assignee: Exergy SpA
Priority date: 2016-12-05
Filing date: 2017-12-04
Publication date: 2018-06-14
Anticipated expiration: 2019-06-05
Also published as: IT201600123131A1

Abstract

The present invention relates to a thermodynamic cycle process for the production of mechanical and/or electrical power from variable temperature heat sources, comprising: circulating a gas (G) in a first closed circuit (2) according to a topping gas cycle, wherein the gas (G) is heated, expanded in a first expander (4), associated with a first electric generator (5) or a first mechanical user, cooled, compressed and again heated; operatively coupling in a boiler (100) a variable temperature heat source to the gas of the first closed circuit so as to perform a heating of the gas (G); circulating a working fluid (WF) in a second closed circuit (3) according to a bottoming Rankine cycle (RC), wherein the working fluid (WF) is heated and vaporized, expanded in a second expander (13), associated with a second generator (14) or a second mechanical user, condensed and again heated and vaporized; operatively coupling, in an exchanger (12), the expanded gas (G) of the topping cycle to the condensed and high-pressure working fluid (WF) of the bottoming Rankine cycle (RC), in order to cool the gas (G) and heat and vaporize the working fluid (WF) by transfer of heat from said topping cycle to the bottoming Rankine cycle (RC). The gas comprises, for example, carbon dioxide or sulfur hexafluoride or a mixture of sulfur hexafluoride and carbon dioxide. After the transfer of heat from the topping cycle to the Rankine cycle (RC) in the exchanger (12), the gas (G) of the topping cycle is further cooled in at least one cooling device (7', 7", 7"') to bring it close to its critical point.

Description

Title

"Thermodynamic cycle process and plant for the production of power from variable temperature heat sources"

DESCRIPTION

Field of the Invention

The present invention relates to a thermodynamic cycle process and plant for the production of mechanical and/or electrical power from variable temperature heat sources. A process/plant of this type exploits variable temperature heat sources in order to heat one or more working fluids to which the source transfers heat and which produce power by passing through and expanding in one or more expanders. By way of non-limiting example, such variable temperature heat sources can be: exhaust fumes of gas turbines, exhaust fumes and waste heat of internal combustion engines, CSP applications with solar collectors with oil or salts (liquid carrier fluid, not vapor), compressor intercooling heat, intercooler cooling heat, oil and jackets of internal combustion engines, industrial waste heat (steelworks, cement plants, glassworks, petrochemical process heat, etc.), biomass and waste combustion fumes.

Background of the invention

It is well known that the ideal cycle for exploiting variable temperature heat sources is the Lorenz or Triangular Cycle (see Fig.1 ). Said cycle consists of heating at a constant specific heat (from I to II), an isentropic expansion (from II to III) and an isothermal compression (from III to I).

Cycles for the exploitation of such sources are known, studied and/or used, for example: the Rankine steam cycle; ORC cycles, that is, organic Rankine cycles; binary cascade cycles (with a number of fluids); Kalina cycles, i.e. water - ammonia; trilateral cycles with expansion of saturated liquid (Trilateral Flash Cycle, TFC); open or closed Brayton gas cycles; gas cycles with a compression point around the critical point (e.g. C02, etc.).

Depending on the thermal level of the source and the environment, each of the above-mentioned cycles is optimized in order to approximate a triangular cycle as closely as possible, but in actual fact none of the above-mentioned cycles accurately reproduces said triangular cycle. In practice there are cycles that achieve efficiencies ranging from 40% to about 60% of the theoretical maximum obtainable with the triangular cycle.

In particular, water-based Rankine cycles are well suited to the cold source, as they condense at a constant temperature on the cold side. On the hot side, in order to come as close as possible to the variable temperature heat release curve on the T- Q or T-S plane (approximately linear, given the approximately constant specific heat of the source), the Rankine steam cycle takes on a configuration with multiple pressure levels. In cases in which the heat source is at high temperatures, even between 450°C and 580°C, since the critical temperature of the water is equal to 374°C, the maximum evaporation pressure of the cycle is normally limited to around a pressure corresponding to a temperature of 300°C. Therefore, a large heat exchange irreversibility is generated, partially and not effectively compensated for by carrying out superheating and re-superheating. Furthermore, the steam cycle shows additional limits in applications on reduced power levels and in the case of installation in areas with limited water availability and in very cold environments, due to the difficulty of managing the water cycle at extreme temperatures.

ORC cycles, like steam ones, are well suited to the cold source, whereas they must rely on multiple pressure levels in order to approximate the triangular cycle. However, especially when it comes to high temperatures, this is hardly compatible with the unavailability of fluids with a critical temperature higher than the temperature of the source combined with a condensation pressure that is not excessively low. Furthermore, for reasons tied to the thermal stability of organic fluids at high temperatures, in applications with ORC cycles an intermediate exchange circuit with diathermal oil or superheated water is provided, which has the effect of penalising performance due to its irreversibilities.

A solely partial and insufficient correction of the defects of the ORC cycle is given by binary cascade cycles (multi-fluid), which by combining, for example, two fluids, seek to decouple the maximum and minimum operating pressures so as to obtain two coupled topping + bottoming cycles that better approximate the triangular cycle. The main problem with this type of solution is the complicated management of two extremely different organic fluids and in particular of metal vapours, which represent the best performing topping cycle. The use of supercritical ORC cycles in the attempt to minimize the losses due to the irreversibility of exchange does not, in any case, make it possible to get much closer to the theoretical efficiencies of the triangular cycle, due to the scarcity of fluids with sufficiently high critical temperatures and acceptable condensation pressures. In addition to the above-mentioned problems of supercritical cycles in relation to the lack of fluids with sufficiently high critical temperatures and acceptable condensation pressures, trilateral flash cycles (TFCs) suffer problems tied to the low performance of two-phase fluid expanders, which impact the overall efficiency of the cycle.

Kalina cycles moderate some irreversibilities of Rankine cycles, but are likewise unable to approximate the triangular cycle.

Brayton cycles naturally closely approximate the hot part of the triangular cycle, being characterised by a variable temperature heat input curve and Cp typical of perfect gases, i.e. substantially constant, but in order to approximate the cold part of the triangular cycle they must adopt a compression with multiple intercooling. These cycles, though they approximate the shape of a triangular cycle better than others, suffer greatly in terms of turbomachinery efficiency. Practically speaking, the expansion and compression work depends on specific volume. The specific volumes at the end of expansion and start of compression are very similar and even with very high compressor and expander efficiencies (even greater than 80% or 90%) the efficiency of the cycle is highly penalized due to the same and in fact these cycles are not used for the applications in question.

The only variant of the Brayton cycle that offers quite good efficiencies in the applications in question is the Brayton cycle with a compression point close to the critical point. These cycles are normally proposed in the CO2 configuration. However, in the structure without a recuperator they still have high irreversibilities (in the part transferring heat to the cold source, since part of the heat is discharged to the cold source at very high temperatures), whereas with a recuperator they have much lower irreversibilities, but do not exploit all the potentially exploitable heat from a variable temperature source (the temperature at the end of heat acquisition is limited by the temperature at the end of the internal heat exchange in the recuperator, which tends to be only slightly lower than the turbine discharge temperature, which is very high).

Also known in this field is public document WO 201 1/059563, which illustrates a residual heat (waste heat) recovery system that employs a Brayton cycle system using carbon dioxide (C02) combined with a Rankine cycle system of the steam or organic fluid type. The source that supplies the residual heat to be recovered can comprise: exhaust gases of an internal combustion engine or gas turbine or geothermal, solar, industrial, residential heat sources, etc. The recovery system comprises: the Brayton cycle system, which is provided with a heater configured to circulate carbon dioxide vapor in a heat exchange relationship with a hot fluid, so as to heat the aforesaid vapors, a turbine and a compressor; the Rankine cycle system, which is coupled to the Brayton cycle system and configured to circulate a working fluid in a heat exchange relationship with the carbon dioxide vapors in a number of heat exchangers in order to heat said working fluid.

Summary

In this context, the Applicant has perceived a need to further improve the efficiency of the processes/plants for the production of mechanical and/or electrical power from variable temperature heat sources in such a way, for example, as to produce more power, the available source being equal.

The Applicant has in particular perceived the need to propose an even more efficient cycle, i.e. one that even better approximates the Lorenz triangular cycle.

In particular, the Applicant has set itself the following objectives:

■ to devise a more efficient process and plant for exploiting variable temperature resources;

■ to devise a process and plant characterized by lower irreversibilities, better approximation of the triangular cycle and hence better conversion efficiencies;

■ to devise a process and a plant which also enable the effective use of a secondary heat source at a relatively low temperature, typically between about 120-80°C and about 70-50°C. A secondary source of this type is typically present in applications such as gas turbines used for gas compression (pipeline compression stations), internal combustion engines and gas turbines with intercooled compression (such as, for example, the well-known GE LMS100).

The Applicant has found that the above-mentioned objectives and still others can be reached by a combined triangular cycle made up of: ^■ a gas topping cycle with multistage intercooled compression or compression near the critical point with or without condensation, wherein said gas topping cycle receives all the heat input by the variable temperature source(s).

^■ a Rankine type bottoming cycle, for example a subcritical or supercritical steam or organic Rankine cycle, which receives part of the heat discharged from the gas topping cycle.

The gas topping cycle is characterized by a variable temperature heat input curve and specific heat Cp that is substantially constant and isoentropic expansion. The gas topping cycle is thus a Brayton type cycle or a modified Brayton type cycle with condensation, as will be clearer below.

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

In an independent aspect, the present invention relates to a thermodynamic cycle process for the production of mechanical and/or electrical power from variable temperature heat sources, comprising:

circulating a gas in a first closed circuit according to a topping gas cycle, wherein said gas is heated, expanded in a first expander, preferably associated with a first electric generator or with a first mechanical user, cooled (and possibly condensed), compressed and heated again;

operatively coupling in a boiler (or a gas-liquid or gas-gas heat exchanger) a variable temperature heat source to the gas of the first closed circuit to perform said heating of the gas;

circulating a working fluid in a second closed circuit according to a bottoming Rankine cycle, wherein said working fluid is heated and vaporized, expanded in a second expander, preferably associated with a second generator or with a second mechanical user, condensed and again heated and vaporized;

operatively coupling, in an exchanger, the expanded gas of the topping cycle to the high pressure condensed working fluid of the bottoming Rankine cycle, in order to cool the gas and heat and vaporize said working fluid by transfer of heat from said topping cycle to said bottoming Rankine cycle.

After the transfer of heat from the topping cycle to the bottoming cycle in the exchanger, the gas of the topping cycle is further cooled in at least one cooling device to bring it close to its critical point. In an independent aspect, the present invention relates to a plant for the production of mechanical and/or electrical power from variable temperature heat sources, comprising:

a first closed circuit for a topping gas cycle comprising: a first expander; a first electric generator or a first mechanical user coupled to the first expander; a boiler; first piping configured to connect the first expander and the boiler according to the first closed circuit in which a gas circulates;

a second closed circuit for a bottoming Rankine cycle comprising: an exchanger; a second expander; a second electric generator or a second mechanical user operatively connected to the second expander; a condenser; a pump; second piping configured to connect the exchanger, the second expander, the condenser and the pump according to the second closed circuit in which a working fluid circulates; wherein the boiler of the first closed circuit is operatively coupled to a variable temperature heat source; wherein a portion of the first closed circuit placed downstream of the first expander is operatively coupled to the exchanger of the second closed circuit.

The first closed circuit comprises at least one cooling device placed between the exchanger and the boiler, wherein said at least one cooling device is configured to further cool the gas to bring it close to its critical point.

In a further independent aspect, the present invention relates to a thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources, comprising: a topping closed gas cycle coupled to a variable temperature heat source so as to receive the incoming heat from said variable temperature source; a bottoming closed Rankine cycle operatively coupled to the topping cycle so as to receive at least part of the heat discharged from said topping cycle. After the transfer of heat from the topping cycle to the bottoming Rankine cycle in the exchanger, the gas of the topping cycle is further cooled to bring it close to its critical point.

The terminology "close to the critical point" means that the T-S diagram of the gas, after the transfer of heat from the topping cycle to the bottoming Rankine cycle, has at least one portion tangent to the Andrews curve or intersecting the Andrews curve, i.e. at a minimum temperature that is equal to or less than the temperature of the critical point (peak of the Andrews curve). Preferably, the topping gas cycle is a Brayton type cycle or a modified Brayton cycle with condensation.

Preferably, the gas comprises sulfur hexafluoride (SF6) or a mixture of sulfur hexafluoride and carbon dioxide (C02), trifluoromethane or R23 (CHF3), difluoromethane or R32 (CH2F2), xenon (Xe) or nitrogen monoxide (N20).

Preferably, the gas is carbon dioxide (C02).

The Applicant has verified that:

the triangular combined cycle makes it possible to have a gas cycle (with a low expansion ratio) with expansion temperatures that are much hotter than the compression temperatures and thus good efficiency, not excessively influenced by the efficiencies of the machines;

the topping cycle is very simple;

the boiler typically in contact with fumes (variable temperature heat source), is very simplified if fumes or a gaseous stream are involved, since on the topping cycle side has a single-phase fluid (a gas) and not a phase-change fluid;

the topping cycle replaces the typical heat transfer circuit, for example with diathermic oil or pressurized water, in the case of a heat recovery application with an ORC cycle;

the bottoming cycle can be very well optimized thanks to the lower inlet temperature of the heat source and thus a greater availability of fluids with an adequate critical T.

The Applicant has further verified that:

the isothermal compression typical of the triangular cycle occurs in zones with modest specific volumes and thus with an overall efficiency that is not greatly penalized by the efficiency of the compressors/pumps;

the topping cycle also carries out the intermediate cycle, which enables the Rankine cycle (if ORC) to be decoupled from the high temperature source; the need for diathermic oil (and consequently a fire safety system, etc.) is thus eliminated; the gas circuit, if used in systems requiring storage (such as, for example, CSP) enables easy storage with systems with inert solids;

the reduction in the Rankine cycle incoming heat level (which enables the identification of many fluids with sufficiently high critical temperatures but acceptable condensation pressures, i.e. no lower than 5-10 hundredths of a bar) makes it possible to adopt a Rankine cycle with multiple (or supercritical) pressure levels that closely approximate the Lorenz cycle;

the topping cycle typically has a low compression ratio and the expansion starting point coincides with the maximum temperature and the compression starting point coincides with the minimum temperature of the cycle.

The present invention, in at least one of the aforesaid aspects, may have one or more of the further preferred aspects described below.

The Applicant has also verified that the present invention makes it possible to exploit variable temperature heat sources with temperatures lower than those exploitable by the prior art. The invention enables lower temperatures and more horizontal recovery curves to be obtained (because of lower thermal levels and much lower apparent specific heats Cp) and the heat produced by sources with lower temperatures can thus be exploited.

In one aspect, after the transfer of heat from the topping gas cycle to the Rankine cycle in the exchanger, through said further cooling, condensation of the gas is provided for. In one aspect, said at least one cooling device is configured to condense the gas. In one aspect, a pump is placed downstream of said cooling device and it is configured to pump the condensed gas in liquid phase or supercritical fluid towards the boiler.

In one aspect, after the transfer of heat from the topping gas cycle to the Rankine cycle in the exchanger, it is envisaged to perform an intercooled compression of said gas through a series of compressors alternating with cooling devices. In one aspect, the first closed circuit comprises a series of compressors alternating with cooling devices placed between the exchanger and the boiler, wherein it is envisaged to perform an intercooled compression of said gas through said series of compressors alternating with cooling devices.

In one aspect, after the transfer of heat from the topping cycle to the Rankine cycle in the exchanger, it is envisaged to perform first an intercooled compression of said gas, preferably through at least one compressor and a cooling device, more preferably by means of a series of compressors alternating with cooling devices, and then a condensation, preferably through a further cooling device.

In one aspect, after the cooling, possibly with condensation, and the compression and/or pumping, and before the heating of the gas in the boiler, it is envisaged to operatively couple, in an auxiliary boiler (or heat exchanger), an auxiliary heat source to the gas of the first closed circuit (Brayton or modified Brayton with condensation) so as to perform an auxiliary heating of the gas at an auxiliary variable temperature lower than a main variable temperature of the variable temperature heat source. In one aspect, the first closed circuit comprises an auxiliary boiler placed downstream of the pump or series of compressors alternating with cooling devices and the boiler, so as to perform an auxiliary heating of the gas at an auxiliary variable temperature lower than a main variable temperature of the variable temperature heat source.

In one aspect, the thermodynamic cycle provides for a condensation of the gas and subsequently an auxiliary heating to an auxiliary variable temperature lower than a main variable temperature of the variable temperature heat source. In one aspect, the thermodynamic cycle provides for a cooled compression of the gas and subsequently an auxiliary heating to an auxiliary variable temperature lower than a main variable temperature of the variable temperature heat source.

The Applicant has verified that the preferred solutions described above make it possible to exploit, in addition to the variable temperature heat source which we define as the main one, one or more auxiliary sources at a lower temperature(s). In fact, as already pointed out previously, the preferred solutions described above (with condensation or cooled compression of the gas around the critical point) enable lower temperatures and more horizontal recovery curves to be obtained (because of lower thermal levels and much lower apparent specific heats Cp) and the heat produced by auxiliary sources with lower temperatures can thus also be exploited. For example, it is possible to exploit the exhaust fumes of internal combustion engines as a main variable temperature heat source and use the heat from cooling jackets or the intercooler or oil as an auxiliary source. Or else, in gas compression trains, the main source can be obtained from the gas turbine and the secondary heat from the intercooling of the compressor of the pipeline or similar entrained heat. In short, the Applicant has verified that the preferred solutions described above (with condensation or cooled compression of the gas) enable substantially all of the usable heat dispersed by the variable temperature sources to be exploited.

In contrast, the solution proposed in the prior document WO 201 1/059563 envisages that the carbon dioxide leaving the compressor is about 210°C, so that it cannot be exploited to receive heat from an auxiliary source. In order to lower this temperature, in fact, WO 201 1/059563 adopts a further exchanger, which complicates the structure and, furthermore, even after the passage through the further heat exchanger, it is not able to lower the temperature to below 120°.

In one aspect, the working fluid of the bottoming cycle is selected from the group comprising: water (H20) or an organic fluid, preferably a hydrocarbon (cyclepentane, normal pentane, isopentane, normal butane, isobutane), a siloxane, a perfluoride or a commercial refrigerant fluid (R245fa, R134a, R1233zde, R1234ze).

In one aspect, after the condensation or compression and before the heating of the gas in the auxiliary boiler, a minimum temperature of the gas is comprised between about 30°C and about 90°C, preferably between about 40°C and about 80°C.

In one aspect, the auxiliary variable temperature is comprised between about 50°C and about 120°C.

In one aspect, the main variable temperature is comprised between about 100°C and about 600°C.

In a preferred embodiment, the gas of the topping cycle is close to the critical point in the final heat transfer phase and in the compression phase and the first closed circuit comprises at least one cooling device placed between the exchanger and the boiler, wherein it is envisaged to condense the gas by means of said cooling device. Preferably, after the condensation and before the heating of the gas in the auxiliary boiler, a minimum temperature of the gas is comprised between about 30°C and about 40°C.

In a different preferred embodiment, the gas of the topping cycle is close to the critical point and the first topping cycle closed circuit comprises a series of compressors alternating with cooling devices placed between the exchanger and the boiler, wherein it is envisaged to perform an intercooled compression of said gas through said series of compressors alternating with cooling devices. Preferably, after the intercooled compression and before the heating of the gas in the auxiliary boiler, a minimum temperature of the gas is comprised between about 30°C and about 40°C.

In one aspect, it is envisaged to vary the percentages of the components of the mixture, preferably of SF6 and C02, in order to optimize the cycle and machines both at the design stage and during the process. Such variations are preferably used to maintain the volumetric flow rates of the gas substantially constant irrespective of variations, for example, in the temperature of the heat sources and in the temperature of the cold source.

The mixture may be varied in order to be better adapted to the installation conditions (optimization of the efficiency of the machines and cycle during design) and furthermore it may be varied in order to optimize efficiency both throughout the day and in the context of processes characterized by seasonal factors (for example, gas turbines work under different conditions with hot and cold temperatures, so the underlying cycles will also see a difference in both the hot and cold parts). Furthermore, an adjustment of the mixture can be used in the adjustment of the minimum pressure.

In one aspect, the first closed circuit comprises a compression-expansion device comprising at least one compressor and a third expander mechanically connected to each other; wherein the compressor is configured to perform the intercooled compression; wherein the third expander receives the gas from the second expander and/or directly from the boiler and moves said at least one compressor. In one aspect, said at least one compressor and the third expander are connected by a system of gears.

In one aspect, the compression-expansion device is connected to the first generator or to an additional generator.

In one aspect, the expanders are of the centripetal and/or centrifugal radial type. In one aspect, the expanders are radial turbines. In one aspect, the second expander is a fixed-speed expander.

In one aspect, an intake valve or a device for varying the turbine IGV (inlet guide vane) geometry is placed at the inlet of the third expander in order to adjust the speed and pressure of the compression-expansion device in such a way as to optimize the efficiency of the cycle.

In one aspect, the compressors are of a centrifugal and/or positive displacement type.

In one aspect, the expanders are of the centrifugal and/or centripetal and/or axial type.

In one aspect, the plant comprises an off-line storage reservoir operatively connectable, preferably by means of an intake valve, more preferably a turbine with a variable IGV geometry, to the first circuit between the third expander and the exchanger, wherein said storage reservoir is configured to contain the gas or at least one component of the gas. The gas in the storage reservoir serves to regulate the minimum/medium pressure of the topping cycle so as to optimize the efficiency of the cycle and regulate the volumetric flow rate during compression and expansion as the load varies.

In one aspect, the variable temperature heat source comprises: exhaust fumes of gas turbines or exhaust fumes and waste heat of internal combustion engines or CSP applications with solar collectors with oil or salts or compressor intercooling heat or intercooler cooling heat, oil and jackets of internal combustion engines or industrial waste heat or biomass and waste combustion fumes.

Additional features and advantages will be more apparent from the detailed description of preferred, but non-exclusive embodiments, of a process/plant and of a thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources in accordance with the present invention. Description of the drawings

This description is provided herein below with reference to the attached drawings, which are provided solely for the purpose of providing approximate and thus non- limiting examples, and of which:

^■ figure 1 illustrates an ideal Lorenz thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources;

^■ figure 2 illustrates a thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources in accordance with the present invention;

^■ figure 3 illustrates a plant configured to carry out the cycle of figure 2;

■ figure 4 illustrates a variant of the cycle of figure 2;

^■ figure 5 illustrates a variant of a part of the plant of figure 3 configured to carry out the cycle of figure 4;

^■ figure 6 illustrates a further variant of a part of the plant of figure 3;

^■ figure 7 schematically illustrates a variable temperature heat source couplable to the plants of figures 3, 4 and 5;

^■ figure 8 illustrates a different variable temperature heat source couplable to the plants of figures 3, 4 and 5.

Detailed description With reference to the aforementioned figures, the reference number 1 denotes in its entirety a plant for the production of mechanical and/or electrical power from variable temperature heat sources in accordance with the present invention.

With particular reference to figure 3, the plant 1 comprises a first Brayton cycle BC closed circuit 2 and a second Rankine cycle RC closed circuit 3.

The first closed circuit 2 comprises: a first expander 4, a first electric generator 5 coupled to the first expander 4, a boiler 100, a series of compressors 6 alternating with cooling devices 7', 7", 7"' and actuated by a motor 8, and first piping configured to connect the first expander 4, the boiler 100 and the series of compressors 6 alternating with cooling devices 7', 7", 7"' according to the first closed circuit 2. A gas G consisting of a mixture of sulfur hexafluoride (SF6) and carbon dioxide (C02) circulates in the first closed circuit 2.

The boiler 100 is interposed between the compressors 6 and the first expander 4. Downstream of the compressors 6, a line 9 of the first piping forks off into a main line 10 and an auxiliary line 1 1. The main line 10 runs through the boiler 100, where, for example, the exhaust fumes of an internal combustion engine, schematically illustrated in figure 7, pass over it. The exhaust fumes constitute a variable temperature heat source.

The auxiliary line 1 1 runs through an auxiliary boiler 101 where it receives, for example, the heat coming from the cooling jackets of the same internal combustion engine. The auxiliary line 1 1 reconverges with the main line 10 in a point inside the boiler 100, so that the auxiliary boiler 101 is in part parallel to the boiler 100. In other words, the boiler 100 comprises a first portion 100' into which a first fraction of the gas coming from the compressors 6 flows and a second portion 100" into which the entire gas stream coming from the compressors 6 flows. The first portion 100' is disposed parallel to the auxiliary boiler 101 , through which a second fraction of the gas coming from the compressors 6 flows.

The second closed circuit 3 comprises: an exchanger 12, a second expander 13, a second generator 14 operatively connected to the second expander 13, a condenser 15 and a pump 16. According to one cycle, second piping connects the exchanger 12, the second expander 13, the condenser 15 and the pump 16 according to the second closed circuit, in which an organic working fluid WF circulates, for example a hydrocarbon such as cyclepentane. A portion of the first closed circuit placed downstream of the first expander 4 and upstream of the series of compressors 6 is operatively coupled to the exchanger 12 of the second closed circuit.

The gas of the first closed circuit 2 is heated in the boiler 100 and in the auxiliary boiler 101 , flows through the first expander 4, where it expands, bringing about the movement of the first expander 4 and the first electric generator 5, which thus generates electrical energy. The expanded gas subsequently enters the exchanger 12, where it transfers heat to the organic fluid of the Rankine cycle and cools.

Subsequently, it is envisaged to perform an intercooled compression of the gas through the compressors 6 alternating with cooling devices 7', 7", 7"' before the gas returns into the boiler 100 and the auxiliary boiler 101 .

The organic fluid of the second circuit 3 is heated and vaporized in the exchanger 12 by virtue of the heat transferred by the gas. The organic fluid in a vapor state leaving the exchanger 12 enters the second expander 13, where it expands, bringing about the movement of the second expander 13 and the second electric generator 14, which thus generates electrical energy. The expanded organic fluid subsequently enters the condenser 15, where it is brought back to the liquid phase and from here again pumped by the pump 16 into the exchanger 12.

With reference to the T-S diagram in figure 2, the thermodynamic cycle carried out by the above-described plant 1 for the production of mechanical and/or electrical power from variable temperature heat sources comprises a closed topping gas G Brayton cycle BC coupled to a variable temperature heat source to receive the incoming heat from said variable temperature source; and a closed bottoming Rankine cycle RC operatively coupled to the closed topping Brayton cycle to receive at least part of the heat discharged from said closed topping Brayton cycle.

The letters A,B,C,D,E,G,X,Y,T,V indicate the correspondence between the points of the cycle of figure 2 and the points of the plant of figure 3.

As may be noted, the gas mixture in point A of figure 2 is at the outlet of the last compressor 6 of figure 3 with a temperature of about 48°C, just above the Andrews curve. The mixture is heated as it absorbs heat between points A and B both from an auxiliary heat source (in the auxiliary boiler 101 ) and the main heat source (in the first portion 100' of the main boiler 100). The auxiliary heat source is at about 70 °C and the main heat source is at about 560°C. The mixture continues to be heated between points B and C, absorbing heat only from the main heat source in the second portion 100" of the main boiler 100 until reaching about 530°C (in point C). The mixture expands and cools (to about 420°C) between points C and D, through the first expander 4, and then transfers heat, between points D and E, to the Rankine cycle working fluid, which is heated and vaporized (points V and X). In point E the mixture is at about 55°C. The Rankine cycle working fluid is heated from about 66°C in point V to about 250°C in point X. The mixture continues to transfer heat in the first 7' of the cooling devices 7', 7", 7"' it meets (section E - G), is then compressed and cooled twice (to a minimum temperature of about 35°C) and then arrives again in point A by virtue of a further compression. The straight line representing the last cooling before the final compression is substantially tangent to the Andrews curve.

The vaporized working fluid of the Rankine cycle expands between X and Y (where it reaches 200°C) through the second expander 13 and then transfers heat (up to about 35°C) and is brought back into the liquid phase between Y and T through the condenser 15. Between V and T the pump 16 brings it back to point V.

Figure 5 illustrates a variant of the plant 1 wherein the first circuit 2 shows differences compared to figure 3 (the second circuit 3 has been represented only schematically because it can be identical to the one in figure 3) and figure 4 is the T-S diagram achieved by the plant 1 of figure 5.

As may be noted, the second circuit 2 has only two compressors 6 and a pump 17 placed downstream of the last cooling device 7"'.

A different composition of the mixture (SF6 and CO2) and the configuration of the cooling devices 7', 7", 7"' and compressors 6 are such that, as illustrated in the T- S diagram in figure 4, the mixture continues to transfer heat in the first 7' of the cooling devices 7', 7", 7"' it meets (E - G section), is then compressed and cooled twice (G - G') and then condensed between G' and G" (horizontal section inside the Andrews curve) in the last cooling device 7"' before again reaching point A by virtue of its passage in the pump 17.

The condensed liquid mixture is at about 35°C in point G' and still at about 48°C in point A. In point C the mixture is at about 530°C and after expansion (in D) it is at about 405°C.

The plant 1 according to this variant is also capable of absorbing heat from an auxiliary heat source at about 70 °C and from a main heat source at about 560°C. Figure 6 illustrates a further embodiment of the first circuit 2 configured to implement the cycle of figure 2. As may be noted, all the elements are substantially the same as in the first circuit 2 of figure 3 and have been identified with the same reference numbers.

Unlike the first circuit 2 of figure 3, the series of compressors 6 is part of a compression-expansion device 18 which, in addition to the first, second and third compressors 6', 6", 6"', comprises a third expander 19, preferably a radial centrifugal or centripetal one. In particular, the compression-expansion device 18 comprises the first compressor 6', which has a shaft in common with the third expander 19 and/or said shaft is axially aligned with that of the third expander 19. The compression-expansion device 18 comprises the second compressor 6", which has a shaft in common with the third compressor 6"' and/or said shaft is axially aligned with the third compressor 6"'. The third compressor 6"' is preferably a high- pressure centrifugal compressor (HP compressor). For example, the third compressor 6"' illustrated in figure 6 is multi-stage. The pair formed by the first compressor 6' and the third expander 19 is disposed parallel to the pair formed by the second and third compressors 6", 6"' and the two pairs are mechanically connected by a system of gears 20, schematically illustrated in figure 6, so that the three compressors 6', 6", 6"' receive motion from the third expander 19.

The fluid outlet of the first expander 4 is connected to the inlet of the third expander 19 and the mixture leaving the third expander 19 enters the exchanger 12 before passing through the three compressors 6', 6", 6"'. The expansion of the mixture in the third expander thus generates the mechanical energy that moves the compressors 6', 6", 6"'. An intake valve 21 or a device for varying the turbine IGV (inlet guide vane) geometry serves to adjust the speed and pressure of the compression-expansion device.

The first circuit 2 of figure 6 further comprises a storage reservoir 22 connected via a control valve 23 to a point of the first circuit 2 situated between the third expander 19 and the exchanger 12. The storage reservoir 22 can contain the mixture or one of the components thereof and serves to adjust the minimum/medium pressure of the cycle in order to optimise efficiency by maintaining higher volumetric flow rates even under partial loads as a result of the reduction in the minimum pressure (within the limits of the effects of closeness to the critical point). For example, the reservoir contains SF6 and is used to change the percentage of SF6 and C02 in the mixture circulating in the first circuit 2 so as to maintain the volumetric flow rates of the gas substantially constant with variations, for example, in temperature.

In a further embodiment of the first circuit 2, not illustrated, the compressors of the compression-expansion device 18 are of the positive displacement piston type. In a further embodiment of the first circuit 2, not illustrated, the second expander 4 is not present, the third expander 19 also performs the function of the second expander 4 and the gear system 20 is further connected to the first generator 5. In a further embodiment of the first circuit 2, not illustrated, the third compressor 6"' is not part of the compression-expansion device 18, but is rather independent and provided with its own motor.

Figures 7 and 8 schematically illustrate the heat sources exploitable by means of the cycle, process and plant of the present invention. In figures 7 and 8, only the boiler 100 and auxiliary boiler 101 belonging to the plants of figures 3, 5 and 6 are represented.

Figure 7 represents an internal combustion engine 24, for example with pistons. The main heat source consists of the exhaust fumes 25 of the engine 24 which flow through the boiler 100. Furthermore, the auxiliary heat source comprises heat extracted, through specific exchangers, from the oil circuit 26, the water of the jackets 27 and the intercooler 28. These sources transfer heat, for example, to pressurized water that is pumped into the auxiliary boiler 101.

Figure 8 represents a gas turbine (turbine 29, combustion chamber 30 and compressor 31 ) connected to a driven compressor 32. The main heat source consists of the exhaust gases 33 of the turbine 29 of the gas turbine 28 that flow through the boiler 100. The auxiliary source consists of the compressed gases 34 leaving the driven compressor 32 which flow through the auxiliary boiler 101 .

In a further variant embodiment wherein the compressor 31 of the gas turbine 28 is intercooled and the auxiliary source can comprise the heat extracted from the intercoolers of the compressor of the gas turbine.

Shown below is a table relating to a further example of the invention compared with a cycle of the type described in document WO201 1/059563 (with a carbon dioxide cycle far from the critical point). Considering the same source, i.e. an internal combustion engine (such as the one in figure 7) with a temperature (T in) of the exhaust fumes 25 of 347°C, it may be noted first of all that the outlet temperature (Tout) of the fumes 25, after having transferred heat to the gas (G) between points A and C of the invention, is 100°C, whereas in the prior art the temperature is higher, i.e. equal to 120°C. Furthermore, the invention also exploits an auxiliary source (figure 7, intercooler 27, jacket 28, oil circuit 26), which the known system cannot exploit. It follows that the net total power (1 1200 kWte) generated by the first generator 5 connected to the first expander 4 according to the invention is greater than the power (9540 kWte) generated by the generator connected to the turbine of the system according to the prior art.

Table

List of elements

1 plant

2 first closed circuit

3 second closed circuit

4 first expander

5 first generator

6, 6', 6", 6'" compressors

7', 7", 7"' cooling devices

8 motor

9 line

10 main line

1 1 auxiliary line

12 exchanger 13 second expander

14 second generator

15 condenser

16 Rankine cycle pump

17 Brayton cycle pump

18 compression-expansion device

19 third expander

20 gear system

21 intake valve

22 storage reservoir

23 adjustment valve

24 internal combustion engine

25 engine exhaust fumes

26 engine oil circuit

27 engine intercooler

28 engine jackets

29 turbine

30 combustion chamber

31 compressor

32 driven compressor

33 exhaust gas of gas turbine

34 compressed gases

100 boiler

100' first part of the boiler

100" second part of the boiler

101 auxiliary boiler

Claims

1 . A thermodynamic cycle process for the production of mechanical and/or electrical power from variable temperature heat sources, comprising:

^■ circulating a gas (G) in a first closed circuit (2) according to a topping gas cycle, wherein said gas (G) is heated, expanded in a first expander (4), associated to a first electric generator (5) or to a first mechanical user, cooled, compressed and heated again;

^■ operatively coupling in a boiler (100) a variable temperature heat source to the gas of the first closed circuit (2) to perform said heating of the gas (G);

■ circulating a working fluid (WF) in a second closed circuit (3) according to a bottoming Rankine cycle, wherein said working fluid (WF) is heated and vaporized, expanded in a second expander (13), coupled to a second electric generator (14) or to a second mechanical user, condensed and again heated and vaporized;

^■ operatively coupling, in a heat exchanger (12), the expanded gas (G) of the topping cycle to the high pressure condensed working fluid (WF) of the bottoming

Rankine cycle, in order to cool the gas (G) and to heat and vaporize said working fluid (WF) by transfer of heat from said topping cycle to said bottoming Rankine cycle;

wherein, after the transfer of heat from the topping cycle to the bottoming Rankine cycle in the heat exchanger (12), the gas (G) of the topping cycle is further cooled in at least one cooling device (7', 7", 7"') to bring it close to its critical point.

2. The process of claim 1 , wherein the topping gas cycle is a Brayton type cycle.

3. The process of claim 1 or 2, wherein, through said further cooling, condensation of gas (G) is provided for.

4. The process of claim 1 or 2 or 3, wherein, after the transfer of heat from the topping cycle to the bottoming cycle in the heat exchanger (7'), the gas (G) of the topping cycle is located close to the critical point and it is envisaged to perform an inter-cooled compression of said gas (G) through a series of compressors (6, 6', 6", 6"') alternating with cooling devices (7', 7", 7"').

5. The process of claim 3 or 4, wherein, after condensation and/or compression and before heating the gas (G) in the boiler (100), it is envisaged to operatively couple in an auxiliary boiler (101 ) an auxiliary source of heat to the gas of the first closed circuit to perform an auxiliary heating of the gas (G) to an auxiliary variable temperature (Taux) lower than a main variable temperature (Tmain) of the variable temperature heat source.

6. The process of claim 3 or 4 or 5, wherein, just before the heating of the gas (G) in the auxiliary boiler (101 ), a gas (G) minimum temperature (Tmin) is between about 30°C and about 90°C.

7. The process of one of the preceding claims, wherein the main variable temperature (Tmain) is between about 100°C and about 600°C.

8. The process according to claim 5, wherein the auxiliary variable temperature (Taux) is between about 100°C and about 50°C.

9. The process according to one of the preceding claims, wherein the gas comprises sulfur hexafluoride (SF6) or a mixture of sulfur hexafluoride and carbon dioxide (C02), trifluoromethane or R23 (CHF3), difluoromethane or R32 (CH2F2), xenon (Xe) or nitrogen monoxide (N20) and wherein the working fluid (WF) is selected from the group comprising water or an organic fluid.

10. The process according to claim 5, wherein the gas is carbon dioxide (C02).

1 1 . A plant for the production of mechanical and/or electrical power from variable temperature heat sources, comprising:

a first closed circuit (2) for a topping gas cycle comprising:

a first expander (4);

a first electric generator (5) or a first mechanical user coupled to the first expander (4);

a boiler (100);

first piping configured to connect the first expander (4) and the boiler (100) according to the first closed circuit (2) in which a gas (G) circulates;

a second closed circuit (3) for a bottoming Rankine cycle comprising:

a heat exchanger (12);

a second expander (14);

a second generator (13) or a second mechanical user operatively connected to the second expander (13);

a condenser (15);

a pump (16);

second piping configured to connect the heat exchanger (12), the second expander (13), the condenser (15) and the pump (16) according to the second closed circuit (3) in which a working fluid (WF) circulates; wherein the boiler (100) of the first closed circuit (2) is operatively coupled to a variable temperature heat source;

wherein a portion of the first closed circuit (2) placed downstream of the first expander (4) is operatively coupled to the heat exchanger (12) of the second closed circuit (3);

wherein the first closed circuit (2) comprises at least one cooling device (7', 7", 7"') placed between the heat exchanger (12) and the boiler (100), wherein said at least one cooling device (7', 7", 7"') is configured to further cool the gas (G) to bring it close to its critical point.

12. The plant of claim 1 1 , wherein said at least one cooling device (7', 7", 7"') is configured to condense the gas (G).

13. The plant of the preceding claim, wherein a pump (17) is located downstream of said cooling device (7', 7", 7"') and it is configured to pump the condensed gas in liquid phase towards the boiler (100).

14. The plant of claim 1 1 or 12, wherein the first closed circuit (2) comprises a series of compressors (6, 6', 6", 6"') alternating with cooling devices (7', 7", 7"') placed between the heat exchanger (12) and the boiler (100), wherein it is envisaged to perform an inter-cooled compression of said gas (G) through said series of compressors (6, 6', 6" , 6"') alternating with cooling devices (7', 7", 7"').

15. The plant of claim 13 or 14, wherein the first closed circuit (2) comprises an auxiliary boiler (101 ) placed between the pump (17) or the series of compressors (6, 6', 6", 6"') alternating with cooling devices (7', 7", 7"') and the boiler (100), to perform an auxiliary heating of the gas (G) to an auxiliary variable temperature (Taux) lower than a main variable temperature (Tmain) of the variable temperature heat source.

16. A thermodynamic cycle for the production of mechanical and/or electrical power from variable temperature heat sources, comprising:

a topping closed gas cycle coupled to a variable temperature heat source to receive the incoming heat from said variable temperature source;

a bottoming closed Rankine cycle operatively coupled to the topping closed cycle for receiving at least part of the heat discharged from said topping closed cycle; wherein, after the transfer of heat from the topping cycle to the bottoming Rankine cycle, the gas (G) of the topping cycle is further cooled to bring it close to its critical point.

17. The thermodynamic cycle of claim 16, wherein the topping gas cycle is a Brayton type cycle.

18. The thermodynamic cycle of claim 16 or 17, wherein the topping closed gas (G) cycle, downstream of the transfer of heat to the bottoming Rankine cycle, provides for a condensation of the gas (G) and subsequently an auxiliary heating to an auxiliary variable temperature (Taux) lower than a main variable temperature (Tmain) of the variable temperature heat source.

19. The thermodynamic cycle of claim 16 or 17, wherein the topping closed cycle, downstream of the transfer of heat to the bottoming Rankine cycle, provides for a cooled compression of the gas (G) and subsequently an auxiliary heating to an auxiliary variable temperature (Taux) lower than a main variable temperature (Tmain) of the variable temperature heat source.

20. The thermodynamic cycle of one of claims 16 to 19, wherein the gas (G) comprises sulfur hexafluoride (SF6) or a mixture of sulfur hexafluoride and carbon dioxide (C02), trifluoromethane or R23 (CHF3), difluoromethane or R32 (CH2F2), xenon (Xe) or nitrogen monoxide (N20) and wherein the working fluid (WF) is water or an organic fluid.

21 . The thermodynamic cycle of claim 19, wherein the gas is carbon dioxide (C02).

22. The thermodynamic cycle of one of claims 16 to 21 , wherein the variable temperature heat source comprises: exhaust fumes of gas turbines or exhaust fumes and waste heat of internal combustion engines or CSP applications with solar collectors with oil or salts or compressor intercooling heat or cooling heat of intercooler, oil and jackets of internal combustion engines or industrial waste heat or biomass and waste material combustion fumes.