PLANT AND PROCESS FOR ENERGY STORAGE

Abstract

A plant for energy storage, comprises: a basin (2) for a work fluid having a critical temperature (T.sub.c) lower than 0°; a tank (3) configured to store the work fluid in at least partly liquid or super-critical phase with a storage temperature (T.sub.s) close to the critical temperature (T.sub.c); an expander (4); a compressor (5); an operating/drive machine (6) operatively connected to the expander (4) and to the compressor (5); a thermal store (8) operatively interposed between the compressor (5) and the tank (3) and between the tank (3) and the expander (4). The plant (1) is configured for actuating a Cyclic Thermodynamic Transformation (TTC) with the work fluid, first in a storage configuration and then in a discharge configuration. The thermal store (8), in the storage configuration, is configured for absorbing sensible heat and subsequently latent heat from the work fluid and, in the discharge configuration, it is configured for transferring latent heat and subsequently sensible heat to the work fluid.

Claims

1-20. (canceled)

21. A process for energy storage comprising: actuating a Cyclic Thermodynamic Transformation, first in one sense in a storage configuration/step and then in an opposite sense in a discharge configuration/step, between a basin for storing a work fluid having a critical temperature lower than 0° C. and a tank for storing said work fluid in at least partly liquid or super-critical phase with a storage temperature close to the critical temperature, wherein in the storage step the process stores heat and potential energy in the form of pressure and generates energy in the discharge step; in the storage step, first sensible heat and subsequently latent heat are removed from the work fluid by means of at least one heat carrier, in order to store — in the tank — said work fluid in the at least partly liquid or super-critical phase and with said storage temperature; and in the discharge step, first latent heat and subsequently sensible heat are transferred to the work fluid by means of said at least one heat carrier.

22. The process according to claim 21, wherein the storage step comprises compressing the work fluid before removing the sensible heat and the latent heat from said work fluid and subsequently storing the work fluid in the tank at a storage pressure substantially equal to or close to an end compression pressure.

23. The process according to claim 21, wherein the discharge step comprise: expanding the work fluid after having transferred the latent heat and the sensible heat to said work fluid and without any increase of pressure before the expansion.

24. The process according to claim 21, wherein in the storage step, the condensation of the work fluid occurs at variable pressure, and in the discharge step, the evaporation of the work fluid occurs at variable pressure.

25. The process according to claim 21, wherein in the storage step, the sensible heat is removed by means of a first heat carrier and the latent heat is removed by means of a second heat carrier, and in the discharge step, the latent heat is transferred by means of the second heat carrier and the sensible heat is transferred by means of the first heat carrier.

26. The process according to claim 21, wherein the work fluid is single-component or a mixture.

27. The process according to claim 21, wherein the basin is at a substantially constant pressure.

28. The process according to claim 21, wherein the critical temperature of the work fluid is lower than -70° C.

29. A plant for energy storage, comprising: a work fluid having a critical temperature lower than 0° C.; a basin for the work fluid; at least one tank configured to store said work fluid in at least partly liquid or super-critical phase with a storage temperature close to the critical temperature; pipes operatively interposed between the basin and the tank and connecting, directly and/or indirectly, the basin with the tank, the pipes delimiting: at least one storage path extended from the basin to the tank, and at least one discharge path extended from the tank to the basin; at least one expander arranged along the pipes and configured for expanding the work fluid; at least one compressor arranged along the pipes and configured for compressing the work fluid; at least one operating/drive machine operatively connected to the expander and to the compressor; and at least one thermal store arranged along the pipes and operatively interposed between the compressor and the tank and between the tank and the expander, wherein the plant is configured for actuating a Cyclic Thermodynamic Transformation with the work fluid, first in one sense in a storage configuration and then in an opposite sense in a discharge configuration, between said basin and said tank; said at least one thermal store, in the storage configuration, is configured for absorbing first sensible heat and subsequently latent heat from the work fluid, so as to store said work fluid in the at least partly liquid or super-critical phase at said storage temperature; and said at least one thermal store, in the discharge configuration, is configured for transferring latent heat and subsequently sensible heat to the work fluid.

30. The plant according to claim 29, wherein an outlet of the compressor is directly connected to said at least one thermal store, without any throttle/expansion member interposed between the compressor and the thermal store; and said at least one thermal store is directly connected to an inlet of the expander, without any pump interposed between the thermal store and the expander.

31. The plant according to claim 29, wherein said at least one thermal store comprises: a first section configured for transferring sensible heat to the work fluid or for absorbing sensible heat from the work fluid; and a second section configured for transferring latent heat to the work fluid or for absorbing latent heat from the work fluid.

32. The plant according to claim 29, wherein said at least one thermal store comprises: at least one first thermal store arranged along the pipes and operatively interposed between the compressor and the tank and between the tank and the expander, the first thermal store being configured for transferring sensible heat to the work fluid or for absorbing sensible heat from the work fluid; and at least one second thermal store arranged along the pipes and operatively interposed between the first thermal store and the tank or at least partially integrated in the tank, the second thermal store being configured for transferring latent heat to the work fluid or for absorbing latent heat from the work fluid.

33. The plant according to claim 29, wherein the basin is the atmosphere and the work fluid is air.

34. The plant according to claim 29, wherein said at least one expander is a compressed air user machine.

35. The plant according to claim 33, comprising a device for the carbon dioxide capture present in the work fluid.

36. The plant according to claim 35, wherein the device for the carbon dioxide capture is operatively coupled to the pipes through which the work fluid flows or is operatively coupled to the or integrated in said at least one thermal store.

37. The plant according to claim 36, wherein the device for the carbon dioxide capture comprises: a tank for the work fluid, elements configured for cooling the mixture and solidifying the carbon dioxide, and a system for extracting from the tank the carbon dioxide solidified in said tank.

38. The plant according to claim 29, comprising an additional heat exchanger operatively placed on the discharge path and placed at least between said at least one thermal store and said at least one expander, wherein the additional heat exchanger is operatively coupled to a source of external heat.

39. The plant according to claim 29, comprising at least one combustion chamber operatively placed on the discharge path and placed at least between said at least one thermal store and said at least one expander, wherein the work fluid flows through the combustion chamber and receives heat from the combustion generated in said combustion chamber.

40. The plant according to claim 29, wherein a bottoming system is operatively coupled to the discharge path of the plant in order to recover residual heat from expander discharge.

Description

DESCRIPTION OF THE DRAWINGS

[0173] Such description will be set forth hereinbelow with reference to the enclosed drawings, provided only as a non-limiting example, in which:

[0174] FIG. 1 illustrates a diagram of the plants according to the Prior Art;

[0175] FIG. 2 illustrates a general diagram of the plant for energy storage according to the present invention;

[0176] FIG. 3 is a T-S diagram illustrating a transformation according to the process for energy storage of the present invention;

[0177] FIG. 4 is an enlargement of a part of the diagram of FIG. 3;

[0178] FIG. 5 illustrates the enlargement of FIG. 4 according to a variant of the transformation;

[0179] FIG. 6 is a T-Q diagram illustrating the thermal exchange in a first thermal store of the plant of FIG. 2;

[0180] FIG. 7 is a T-Q diagram illustrating the thermal exchange in a second thermal store of the plant of FIG. 2;

[0181] FIGS. 8, 10, 12, 14, 16, 18 each schematically illustrate an embodiment of the plant for energy storage according to the present invention;

[0182] FIGS. 9, 11, 13, 15, 17, 19 each illustrate a T-S diagram relative to one of the plants of FIGS. 8, 10, 12, 14, 16, 18;

[0183] FIGS. 20 and 21 illustrate examples of a portion of the plant according to the invention;

[0184] FIGS. 22, 23 and 24 illustrate respective embodiments of a device of the plant according to the invention.

DETAILED DESCRIPTION

[0185] With reference to the enclosed Figures starting from FIG. 2, reference number 1 overall indicates a plant for energy storage according to the present invention. The plant 1 comprises a basin 2 in which a work fluid at a constant or substantially constant pressure is present or can be stored. The basin 2, represented schematically as a rectangle in FIG. 2, can be a closed environment or it can be the terrestrial atmosphere. The work fluid is part of the plant 1 and can be the atmospheric air or a different fluid, single-component, such as nitrogen or oxygen or methane, or a mixture, such as natural gas. A particular characteristic of the abovementioned work fluid is that it has a critical temperature T.sub.c, lower than 0° C. For example, if the work fluid is air, the critical temperature T.sub.c is about -140° C. (140° C. below 0°), if the work fluid is methane, the critical temperature T.sub.c is about -83° C. (83° C. below 0°), if the work fluid is argon, the critical temperature T.sub.c is about -122° C. (122° C. below 0°).

[0186] The plant 1 comprises a tank 3 configured to store the work fluid in an at least partly liquid or super-critical phase with a storage temperature close to its critical temperature T.sub.c.

[0187] Pipes, defined for example by tubes and/or conduits made in another manner, are operatively interposed between the basin 2 and the tank 3 and connect, directly and/or indirectly, the basin 2 with the tank 3. Such pipes delimit a storage path which is extended from the basin 2 to the tank 3 and a discharge path which is extended from the tank 3 to the basin 2.

[0188] The plant 1 comprises an expander 4, a compressor 5 and an operating/drive machine 6, which in FIG. 2 is represented as an electric motor-generator, operatively connected to the expander 4 and to the compressor 5. The expander is configured for expanding the work fluid; the compressor 5 is configured for compressing the work fluid. In FIG. 2, the compressor 5 is represented as a turbo-compressor, the expander 4 is a turbine. In embodiment variants, the expander 4 can be any one compressed air user machine, for example part of an iron and steel plant or of an air separation unit (ASU). In embodiment variants, the operating machine and the drive machine can be separate machines. The drive machine is mechanically connected to the compressor 5 and actuates the compressor 5. The operating machine is mechanically connected to the expander 4 and is actuated by the expander 5, producing mechanical and/or electrical energy.

[0189] The basin 2 is in fluid communication, by means of the pipes, with an inlet 5a of the compressor 5. The basin 2 is also in fluid communication, by means of the pipes, with an outlet 4b of the expander 4. The electric motor-generator 6 is mechanically connected/connectable to rotation shafts of the compressor 5 and of the expander 4 by means of respective clutches 7a, 7b.

[0190] The plant 1 comprises a thermal store 8 arranged along the pipes and operatively interposed between the compressor 5 and the tank 3 and between the tank 3 and the expander 4. According to that illustrated in FIG. 2, the thermal store 8 comprises a first thermal store 9 and a second thermal store 10 arranged in series. The first thermal store 9 is in fluid communication, by means of the pipes, with an outlet 5b of the compressor 5 and with an inlet 4a of the expander 4. The second thermal store 10 is in fluid communication, by means of the pipes, with the first thermal store 9 and is placed downstream of the latter.

[0191] The second thermal store 10 is physically placed upstream of the tank 3 or it is at least partly integrated in said tank 3.

[0192] The first thermal store 9 is configured for exchanging sensible heat with the work fluid, in particular for absorbing sensible heat from the work fluid, in order to store said absorbed heat and in order to newly transfer said sensible heat to the work fluid, as a function of the operating configuration of the plant 1.

[0193] The second thermal store 10 is configured for exchanging latent heat with the work fluid, in particular for absorbing latent heat from the work fluid, in order to store said absorbed heat and in order to newly transfer said latent heat to the work fluid, as a function of the operating configuration of the plant 1.

[0194] In possible embodiment variants, there is only one thermal store but it has distinct sections: a first section configured for transferring sensible heat to the work fluid or for absorbing sensible heat from the work fluid and a second section configured for transferring latent heat to the work fluid or for absorbing latent heat from the work fluid.

[0195] The plant 1, in accordance with the process for energy storage according to the invention, is configured for actuating a Cyclic Thermodynamic Transformation TTC with the work fluid, first in one sense, in a storage configuration, from the basin 3 to the tank 2, and then in an opposite sense, in a discharge configuration, from the tank 3 to the basin 2.

[0196] In the storage configuration, the plant 1 is configured for absorbing first the sensible heat and subsequently the latent heat from the work fluid, so as to store, in the tank 3, said work fluid in at least partly liquid or super-critical phase at a storage temperature close to the critical temperature (lower than 0°) of said work fluid. In the discharge configuration, the plant 1 is configured for transferring the latent heat and subsequently the sensible heat to the work fluid. In the storage step, the process/plant 1 stores heat and potential energy in the form of pressure and generates energy in the discharge step.

[0197] More in detail and with reference to FIGS. 2, 3 and 4, in the storage configuration/step, the work fluid present in basin 2 (point I), e.g. atmospheric air, is suctioned by the compressor 5, compressed (from I to II) and sent to the first thermal store 9.

[0198] At the inlet 5a of the compressor 5, a system of filters and an air purification system can be present, not illustrated in the enclosed Figures.

[0199] An end compression pressure, i.e. at a pressure of the work fluid at the outlet of the compressor 5, is comprised for example between 30% and 80% of the critical pressure. The mechanical energy which moves the compressor 5 is supplied by the motor-generator 6 which functions as electric motor and absorbs electrical energy. The compression can be adiabatic or intercooled.

[0200] At the first thermal store 9, the compressed work fluid exchanges heat with a first heat carrier belonging to the first thermal store 9 or operatively coupled to the first thermal store 9. For example, the first thermal store 9 comprises a heat exchanger operatively coupled to the pipes or to the work fluid and to the first heat carrier. The first heat carrier can be directly or indirectly coupled to the work fluid. In other words, the removal of the sensible heat from the work fluid can be actuated by means of direct or indirect exchange with the first heat carrier. The first heat carrier can be of liquid type, solid type, with phase change, chemical type, etc..

[0201] The work fluid is then cooled by means of the removal of the sensible heat from said work fluid (from II to III) and such heat is stored in the first thermal store 9, for example it is stored in the same first heat carrier. By means of the removal of the sensible heat from the work fluid, the work fluid is cooled to a temperature close to a saturation temperature at the corresponding pressure.

[0202] The cooled work fluid flows towards the second thermal store 10 and towards the tank 3. At the second thermal store 10, the work fluid transfers — to a second heat carrier — latent heat or transfers in part sensible heat and in part latent heat (from III to IV) and is stored in the tank 3. The heat removed from the work fluid is stored in the second thermal store 10.

[0203] The second thermal store 10 can be of the type with direct or indirect thermal exchange, so that the removal of the latent heat from the work fluid is actuated by means of direct or indirect exchange with said second heat carrier. For example, the second thermal store 9 comprises at least one heat exchanger operatively coupled to the pipes or to the work fluid and to the second heat carrier.

[0204] The second thermal store 10 can be at least partly integrated in the tank 3. In one embodiment, the second thermal store 10 for example comprises a heat exchanger inserted in the tank 3. The heat exchanger is operatively coupled to the second heat carrier and to the work fluid contained in the tank 3. In a different embodiment, the second heat carrier is directly housed in the tank 3. The second heat carrier can be of liquid type, solid type, with phase change, chemical type, etc..

[0205] The removed latent heat is stored in the second thermal store 10, for example it is stored in the same second heat carrier.

[0206] In the storage configuration, a single flow without recirculations of the work fluid flows from the environment 2 to the tank 3.

[0207] As a function of the operative parameters of the specific process and/or of the properties of the work fluid, the latter can be stored in the tank 3 in totally liquid phase, partially liquid phase (i.e. the work fluid is at least partly condensed) or slightly super-critical phase. In case of storage in slightly super-critical phase, there is a transfer of heat with temperature variation but with very high C.sub.p, since it is close to the critical point.

[0208] In the storage configuration/step, the condensation can occur at constant or variable pressure.

[0209] The storage of the thermal energy by the second thermal store 10 thus occurs via removal of “latent” heat also in the supercritical case, where — even if it is desired to work at constant pressure in the storage tank 3 — inevitably up to the completion of the loading of the tank, the initial pressure will be slightly subcritical and therefore the removal of the heat is in any case mainly of latent type. In other words, one starts from a subcritical condition in order to terminate in a supercritical condition by exploiting a higher temperature jump. This allows reducing the dimensions of the second thermal store 10.

[0210] If the storage pressure is lower than the critical pressure, the condensation of the work liquid occurs only via removal of latent heat. If the final storage pressure is higher than the critical pressure, and optionally lower than about 1.3 times said critical pressure, the condensation occurs by transfer of latent heat up to at least 70%-90% of the fill of the tank 3, instead exchanging with a small temperature difference only in the final charge transient step.

[0211] The outlet 5b of the compressor 5 is directly connected to the first thermal store 9 and between the compressor 5 and the tank 3 there is no throttle valve, so that the storage pressure in the tank 3 is substantially equal to or close to the end compression pressure.

[0212] In the discharge configuration/step (FIGS. 2, 3 and 4), the pressure of the work fluid present in the tank 3 is reduced from IV to V. The work fluid absorbs the heat, latent or mainly latent, stored in the second heat carrier of the second thermal store 10. The work fluid evaporates and/or is overheated to a pressure lower than the storage pressure (point VI). In the discharge configuration/step, the evaporation of the work fluid can occur at constant or variable pressure. If the work fluid is a mixture, such as air, said work fluid can also have a variable composition over time during the discharge step, since the lighter component of the mixture in terms of molar mass will tend to evaporate “first” or with a higher percentage.

[0213] The work fluid passes through the first thermal store 9, where it absorbs sensible heat from the first heat carrier and is heated up to point VII, and then it transits through the expander 4 which exploits the enthalpy jump of the work fluid from point VII to point VIII. The expander 4 actuates the motor-generator 6, which works as electric generator and converts the mechanical energy into electrical energy.

[0214] In the discharge configuration/step, a single flow without recirculations of the work fluid flows from the tank 3 to the environment 2.

[0215] The inlet 4a of the expander 4 is directly connected to the first thermal store 9 without any pump interposed therebetween, so that there is no increase of pressure between the tank 3 and the expander 4 in the discharge step.

[0216] In FIG. 4, the line A-B is observed which represents the second heat carrier which starts from an initial condition (point A) of start storage step and reaches a final condition (point B) at end storage; the opposite takes place during the discharge step.

[0217] It is observed that: [0218] a pressure of the work fluid in the storage step and before compression is equal or substantially equal to a pressure of the work fluid in the discharge step and at the end of expansion; [0219] a storage pressure in the tank 3 in the storage step is higher than a storage pressure in the tank 3 in the discharge step; for example, a difference between the storage pressure in the storage step and the storage pressure in the discharge step is comprised, for example, between 0.1 bar and 30 bar, optionally between 0.5 bar and 3 bar; [0220] in the discharge step, from the tank to the inlet of the expander, pressures of the work fluid are lower than pressures of the work fluid in the storage step, from the outlet of the compressor to the tank; it is indicated that the pressures in the discharge step are lower than the pressures in the storage step but as close as possible to each other in order to optimize the Round Trip Efficiency (RTE) of the plant/process; for example, a difference between the pressures in the storage step and the pressures in the discharge step is comprised, for example, between 0.5 bar and 20 bar, optionally between 0.5 bar and 5 bar. [0221] evaporation pressures of the work fluid, during the evaporation, are comprised between 99% and 20%, optionally between 90% and 70%, of the condensation pressure; [0222] a temperature of the work fluid in the storage step and just before the removal of the latent heat from the work fluid (point III) is higher than or equal to a temperature of said work fluid in the discharge step and at the end of the transfer of the latent heat to said work fluid (point VI). [0223] a temperature of the work fluid in the tank at the end of the storage step (point IV) is higher than a temperature of the work fluid in the tank at the start of the discharge step (point V); [0224] the latent heat and/or sensible heat absorbed by the work fluid in the discharge step is/are higher than or equal to the latent heat and/or sensible heat transferred from the work fluid in the storage step; it derives that the Cyclic Thermodynamic Transformation TTC behaves as a cooling cycle.

[0225] As can be observed, the lines of condensation and evaporation are not horizontal (i.e. at constant temperature) since the work fluid is air, which is a mixture of fluids that are different from each other. If the work fluid was “pure”, the lines of condensation and evaporation would be horizontal and the condensation/evaporation would occur at constant temperature.

[0226] FIG. 5 illustrates a variant of the process in which points IV and V, respectively at end storage step and start discharge step, are not in total liquefaction. This variant can be useful for the purpose of optimizing the ratio (given the same stored energy) between the volume of the tank 3 and the quantity of the material necessary (liquid, solid, with phase change, chemical, etc.) for the storage of the latent heat (i.e. the second heat carrier in the second thermal store 10). Indeed, by completing the charge with a two-phase situation in the tank 3, the volume required for the storage of the work fluid under pressure increases, but a reduction of the latent heat to be removed is obtained.

[0227] FIG. 6 illustrates a T-Q diagram relative to the exchanges of heat operated by means of the first thermal store 9 during the storage step and the discharge step.

[0228] In the storage step, the work fluid transfers the sensible heat, being cooled from point II to point III, while the first heat carrier used in order to store the sensible heat is heated, entirely or in part, from point C to point D.

[0229] In the discharge step, the work fluid is heated, absorbing sensible heat, passing from point VI to point VII, while the first heat carrier is cooled, passing from point D to point C. For the process to be attainable, the temperature at VI must be lower than or at most equal to the temperature at III and hence the pressure in the discharge step lower than or equal to the pressure in the storage step.

[0230] As already specified above, the thermal exchange in the first thermal store 9 can be made in different modes, including: indirect thermal exchange or direct thermal exchange.

[0231] An indirect system of thermal exchange can for example comprise a heat exchanger interposed between the work fluid and a fluid used as first heat carrier and/or as storage. The first heat carrier is stored from a condition that corresponds with point C (at the start of the storage step) to a condition that corresponds with point D (at the end of the storage step). The indirect system of thermal exchange comprises one (in this case it functions as a thermocline) or more tanks for the storage of the first heat carrier and/or alternatively a direct exchange storage system, i.e. a thermocline at whose interior there is a solid material with which the first heat carrier in turn transfers/absorbs the heat, with the objective of minimizing the fluid of the first heat carrier and reducing the dimensions of the tanks, increasing the thermal energy storage capacity.

[0232] A system of direct thermal exchange can be attained through a container/pressure vessel, a connection system connected to the process/interface with the rest of the system; fill material that constitutes the first heat carrier inserted within the pressure vessel. The fill material serves to absorb and transfer heat respectively during the storage step and discharge step.

[0233] FIG. 7 illustrates a T-Q diagram relative to the heat exchanges operated by means of the second thermal store 10 during the storage step and the discharge step.

[0234] In the storage step, the work fluid transfers the latent heat, being cooled from point III to point IV, while the second heat carrier in order to store the latent heat is heated from point A to point B. In the discharge step, the work fluid is heated by absorbing latent heat, passing from point V to point VI, while the second heat carrier is cooled, passing from point B to a point close to point A. By suitably adjusting the discharge pressure, i.e. evaporation pressure, it is possible to ensure that the latent heat absorbed for evaporating the work fluid during the discharge step is greater than or equal to the latent heat transferred from the work fluid to the second heat carrier during the storage step.

[0235] As already specified above, also the thermal exchange in the second thermal store 10 can be attained in different modes, including: indirect thermal exchange or direct thermal exchange.

[0236] If the latent heat is removed from the second heat carrier in an indirect manner, a system can for example be present that comprises heat exchanger interposed between the work fluid and a fluid used as second heat carrier and/or as storage. Such second heat carrier is stored from a condition that corresponds with point A (at the start of the storage step) to a condition that corresponds with point B (at the end of the storage step). The indirect exchange system comprises one (in this case it functions as thermocline) or more tanks for the storage of the second heat carrier and/or alternatively a direct exchange storage system (“modified” thermocline), i.e. a thermocline at whose interior there is a solid material with which the second heat carrier in turn transfers/absorbs the heat with the objective of minimizing the fluid of the second heat carrier and reducing the dimensions of the tanks, increasing the thermal energy storage capacity.

[0237] If the latent heat is removed from the second heat carrier in a direct manner, a system can be present comprising a container/pressure vessel, a connection system connected to the process/interface with the rest of the system; inert fill material (which constitutes the second exchange/thermal storage carrier) inserted within the pressure vessel. The tank 3 for the storage of the work fluid in liquid phase can be the same that contains the fill material. In such case, the work fluid occupies by gravity the interstices delimited by the fill material and the volume not occupied by the solid material, or it occupies a volume left empty for such purpose within the tank 3. Alternatively, the tank 3 and the container/pressure vessel can be separate elements.

[0238] In order to limit the quantity (and the relative costs) of the second heat carrier, two routes can be followed.

[0239] According to a first route, it is possible to increase the enthalpy difference between point A and point B by maintaining constant the pressures during the step of condensation and evaporation and further reducing the evaporation pressure. Indeed, by increasing the pressure difference between condensation and evaporation (reducing their ratio), an increase of temperature difference is obtained.

[0240] In accordance with a second route, by allowing a variable pressure during the step of condensation (and evaporation), in particular a pressure that increases with the increase of the mass of the work fluid to be stored, it is possible to reduce the necessary mass of the second heat carrier (or for storage of the latent energy) and thus the size of the second thermal store. Indeed, by reducing the mass but increasing the pressure during the storage step, it is possible to maintain an evaporation pressure, in discharge step, close to that of condensation without negatively affecting the efficiency. The storage step starts for example from 30% of the critical pressure and finishes at 80% of the critical pressure, while the discharge step starts from 75% of the critical pressure and finishes at 28% of the critical pressure, therefore the discharge step passes by points with pressure lower than the relative storage points, with a ratio of about 0.9 - 0.95. In addition, due to the reduction of specific latent heat for higher pressures, also the enthalpy difference of the work fluid between point III and point IV is reduced, contributing to the reduction of mass necessary for the condensation medium.

[0241] The plant 1, if it uses air as work fluid, can also be comprised or be coupled to a device 11 for the capture of the atmospheric carbon dioxide CO.sub.2. Such device 11 is operatively coupled to the pipes in the sense that they are in fluid communication with the storage path and/or discharge path. Such device 11 can be operatively coupled to the thermal store 8.

[0242] If atmospheric air is used, there can also be a device for capturing and removing moisture (H.sub.2O) and carbon dioxide CO.sub.2 which otherwise - reaching the freezing point during the storage step, would create problems for the storage process.

[0243] For example, molecular sieves can be employed in which the water, the carbon dioxide and most of the other residue impurities are eliminated. There are two molecular sieves and they work alternately; when one is operating for purifying the air, typically at the intercooled outlet of a first compressor at low pressure, the other is regenerated. The reversal of the molecular sieves as well as their regeneration can be completely automated.

[0244] The device 11 for the carbon dioxide capture CO.sub.2 can also be configured for reusing the recuperated carbon dioxide CO.sub.2, e.g. in solid form, in advanced energy systems.

[0245] In one embodiment, such device 11 for the carbon dioxide CO.sub.2 capture is integrated in the first thermal store 9 which stores the sensible heat, as schematically represented in FIG. 2. The first thermal store 9 works at temperatures lower than 0° C. and exploits the passage through the freezing point of CO.sub.2 at the work pressure. Indeed, holding true the rule of the partial pressures, since the content of CO.sub.2 in the air is equal to about 0.04%, within the first heat store 9 there is a passage of the carbon dioxide CO.sub.2 from the gaseous state to the solid state and the carbon dioxide CO.sub.2 is then separated from the rest of the mixture, which remains in the gaseous state, and extracted. Such device is thus configured for: solidifying the CO.sub.2, separating the CO.sub.2 from the mixture of air, extracting the solidified CO.sub.2 (dry ice) from the device.

[0246] FIG. 8 illustrates an embodiment of the plant 1 which comprises a plurality of turbocompressors 5 fluidically connected in series and a plurality of turbines 4 fluidically connected in series.

[0247] The first thermal store 9 comprises a first portion 9a provided with heat exchangers 12 interposed between two successive turbocompressors 5 and configured for absorbing sensible heat from the work fluid between successive compressions in the storage configuration/step. The turbocompressors and the relative compressions are therefore intercooled. The first portion 9a also comprises heat exchangers 13 interposed between two successive turbines 4 for transferring heat to the work fluid between one expansion and the next in the discharge configuration/step. The turbines 4 and the relative expansions are therefore interheated. The first thermal store 9 also comprises storage tanks 14, 15 of the first heat carrier (e.g. liquid) thereof in fluid connection, by means of suitable circuits, with the heat exchangers 12, 13. The first portion 9a of the first thermal store 9, in addition to storing the sensible heat, operates the inter-cooling and the inter-heating operations.

[0248] The first thermal store 9 comprises a second portion 9b which is placed downstream of the first portion 9a in the storage configuration/step and is placed upstream of the first portion 9a in the discharge configuration/step.

[0249] FIG. 9 illustrates the transformation of the plant of FIG. 8 in a T-S diagram, in which the intercooled compressions (from point I to II) are observed as well as the interheated expansions (from point VII to point VIII).

[0250] The embodiment of the plant 1 of FIG. 10 differs from that of FIG. 8 since it comprises an additional heat exchanger 16 operatively placed on the discharge path and placed between the first thermal store 9 and a single turbine 4 which is not interheated. The additional heat exchanger 16 is operatively coupled to a source of external heat 17 which, in the discharge step, transfers additional heat to the work fluid before its expansion, operated in the turbine 4. In addition, unlike the embodiment of FIG. 8, the heat exchangers 12 interposed between two successive turbocompressors 5 are coupled to a system 18 for removing the heat which is different from the first thermal store 9.

[0251] In a non-illustrated variant, one of the heat exchangers 12 interposed between two successive turbocompressors 5 is operatively coupled to the first thermal store 9 and the other heat exchanger 12 is operatively coupled to the system 18 for removing the heat. The system 18 for removing heat, in such case, can serve for removing the residual heat stored in the first thermal store 9, so as to ensure a constant temperature at the inlet to the compressor placed more downstream.

[0252] FIG. 11 illustrates the transformation of the plant of FIG. 10 in a T-S diagram, which shows the introduction of the additional heat Q from outside (from point VII to point VII′).

[0253] The embodiment of the plant 1 of FIG. 12 differs from that of FIG. 10 since it comprises a further additional heat exchanger 16′ operatively interposed between two successive turbines 4 (interheated in discharge step) and the source of external heat 17 is operatively coupled also to said further additional heat exchanger 16′. In addition, a recuperator 19 is present which operatively couples a first portion of the discharge path placed between the first thermal store 9 and a first turbine 4 with a second portion of the discharge path placed between a second turbine 4 and the basin 2, not illustrated in FIG. 12, in order to pre-heat the work fluid before the expansion.

[0254] FIG. 13 illustrates the transformation of the plant of FIG. 12 in T-S diagram, in which the recovery of heat (VII - VII″ and VIII′ - VIII) is observed along with the double introduction of the additional heat Q′, Q″ from outside (from point VII″ to point VII′ and from point VII‴ to point VII‴′).

[0255] The embodiment of the plant 1 of FIG. 14 differs from that of FIG. 8 since, in place of the additional heat exchanger 16 and of the source of external heat 17, a combustion chamber 20 is present, i.e. a source of internal heat. The combustion chamber 20 is operatively placed on the discharge path and is placed between the first thermal store 9 and the turbine 4. Fuel F is introduced into the combustion chamber in order to generate a combustion with a comburent present in the work fluid (e.g. the oxygen of the air) and produce heat. The work fluid flows through the combustion chamber 20 and receives heat from the combustion generated in the combustion chamber 20 itself.

[0256] FIG. 15 illustrates the transformation of the plant of FIG. 14 in a T-S diagram, in which the introduction of the additional heat Q by the combustion (from point VII to point VII′) is observed.

[0257] The embodiment of the plant 1 of FIG. 16 differs from that of FIG. 12 since, in place of the additional heat exchangers 16, 16′, it has a combustion chamber 20 and an additional combustion chamber 20′. In addition, a “bottoming” cycle or system 21 is present. The “bottoming” system 21 is operatively coupled to the plant 1 by means of an exchanger 22 placed on the discharge path and between the second turbine 4 and the basin 2, not illustrated in FIG. 16, in order to recover residual heat from the outlet/discharge 4b of the second turbine 4. For example, the “bottoming” system 21 can produce extra power or be a source for a heat user (for example: district heating, urban teleheating, industrial processes).

[0258] FIG. 17 illustrates the transformation of the plant of FIG. 16 in a T-S diagram which is analogous to that of FIG. 13 and additionally illustrates the “bottoming” cycle with generation of extra power Pw.

[0259] The embodiment of the plant 1 of FIG. 18 differs from that of FIG. 16 since there is no “bottoming” system 21 but it also comprises a pair of additional intercooled compressors 22 arranged on the discharge path, between the first thermal store 9 and the turbines 4, in particular between the first thermal store 9 and the recuperator 19. The intercooling is operated by an exchanger 23 placed between the two additional compressors 22 and operatively connected to the heat removal system 18.

[0260] FIG. 19 illustrates the transformation of the plant of FIG. 18 in a T-S diagram which is analogous to that of FIG. 17 and additionally illustrates the double compression intercooled in the discharge step (between VII and VII″).

[0261] FIG. 20 illustrates an embodiment of a thermal store 8 comprising the first 9 and the second 10 thermal store, in which the second thermal store 10 is integrated in the tank 3.

[0262] The first thermal store 9 is of “packed bed” type and comprises a cistern, for example made of stainless steel, which contains a thermal mass 24 defined, for example, by inert material such as gravel or metal spheres or ceramic spheres. The thermal mass 24 defines the first heat carrier. The work fluid which flows through the first thermal store 9 fills the interstices delimited in the thermal mass and hits the loose material, exchanging the sensible heat therewith.

[0263] Also the second thermal store 10 is of “packed bed” type and comprises a cistern, for example made of stainless steel, which contains a thermal mass defined, for example, by inert material such as gravel or metal spheres or ceramic spheres. The thermal mass defines the second heat carrier. The work fluid that comes into contact with the thermal mass condenses and is stored in the cistern that therefore also defines the tank 3.

[0264] FIG. 21 illustrates a different thermal store 8 embodiment, in which the first thermal store 9 is identical to that of the preceding FIG. 20 while the second thermal store 10 is separate from the tank 3. The second thermal store 10 comprises a heat exchanger 25 placed between the first thermal store 9 and the tank 3 and a cistern 26 containing a respective thermal mass 27 defined by inert material. A circuit provided with a pump 28 connects the cistern 26 with the exchanger 25 and contains a carrier fluid. The carrier fluid and the thermal mass 27 define the second heat carrier. The work fluid exchanges the latent heat with the carrier fluid in the exchanger 25 and the carrier fluid, together with the thermal mass with which it comes into contact, stores such heat. In the exchanger 25, the work fluid condenses and thus flows in the tank 3, where it is stored up to the next discharge step.

[0265] One example of the device 11 for the carbon dioxide capture CO.sub.2 is illustrated in FIG. 22. The device 11 comprises a tank 29 for containing pressurized air. The tank 29 has an inlet 30 for the air, an outlet 31 for the mixture of air with carbon dioxide removed therefrom and an outlet 32 for the carbon dioxide that is separate from the air. The first store 9 comprises elements 33 configured for cooling the mixture and solidifying the carbon dioxide and a system for extracting the solidified carbon dioxide from the tank 29. In the illustrated embodiment, the elements 33 for cooling the mixture and solidifying the CO.sub.2 comprise a plurality of fixed 34 and/or moving plates that were previously cooled (in the discharge step of the TTC cycle) to a temperature lower than the temperature necessary for the solidification of the CO.sub.2 itself. In order to separate the solidified CO.sub.2, it is first provided to remove from the plates 34 the CO.sub.2 that is solidified on the walls of the plates 34 themselves and then to separate from the mixture the solid CO.sub.2 already removed from the plates 34. For such purpose, a mechanical system is used which works via rubbing. In the illustrated embodiment, the rubbing system comprises rotary blades 35, moved by a respective motor 35a, which have a double function: the first, to separate the dry CO.sub.2 from the plates; the second, to move solid particles of CO.sub.2 towards the system for separating the CO.sub.2 from the mixture via gravity, so that the CO.sub.2 particles with greater density fall downward. The blades 35 can have a shape such to generate a movement of the solid particles that is centrifugal radial or centripetal radial, so as to transport said solid particles into the zone of separation via gravity. A system of mechanical extraction 36, e.g. a screw moved by a respective motor 36a, allows the extraction of the solid particles of CO.sub.2 through the outlet 32.

[0266] A different embodiment of the device 11 for the carbon dioxide CO.sub.2 capture is illustrated in FIG. 23. The elements in common with the device 11 of FIG. 22 have the same reference numbers. The device 11 of FIG. 23 comprises an external unit 37 for extracting heat from the mixture, such as a “chiller”, connected to a heat exchanger 38, for example, to a tubular exchanger, in which the carrier fluid of the “chiller” passes in the tubes. Outside the tubes, the CO.sub.2 will solidify. In a subsequent step, the CO.sub.2 can be mechanically removed and recovered, for example with a hammering of the tubes or by means of a reciprocating movement of the plates 39 which are moved parallel to the tubes and slide along the same by means of a respective motor 40. Such plates 39 have holes for the passage of the tubes and can also have the function of “baffle” of the heat exchanger 38. The separated CO.sub.2 particles are transported towards the separation and collection zone via gravity and by exploiting the flow of the mixture in the passage through the tank 29.

[0267] A further embodiment of the device 11 for the carbon dioxide CO.sub.2 capture is illustrated in FIG. 24. The elements in common with the devices 11 of FIGS. 22 and 23 have the same reference numbers. Such device 11 comprises two tanks 29, each provided with a respective “chiller” 37 and with a tubular heat exchanger 38. The two tanks 29 are connected in parallel with the inlet 30 and the outlet 32 for the mixture and each has a respective outlet for the CO.sub.2. First valves 40 on the inlet 30, second valves 41 on the outlet 31 and throttle valves 42 on the outlets 32 for the CO.sub.2 allow working – in an alternating manner - the two tanks 29 and the two “chillers” 37 and heat exchangers 38. While the mixture flows through one of the two tanks 29, exiting from the respective outlet 31 (first and second valves 40, 41 open and throttle valve 42 closed) and the solid CO.sub.2 is formed on the tubes of the respective heat exchanger 38, from the other tank 29 (first and second valves 40, 41 closed and throttle valve 42 open) the CO.sub.2— previously formed on the tubes -newly passes into the gaseous phase and exits from the tank 29 by means of the depressurization of the tank 29 in stand-by condition.

NUMERICAL EXAMPLE

[0268] In an embodiment example, the plant 1 can be schematized like that of FIG. 2. The work fluid is atmospheric air and the basin at nearly constant pressure is the environment.

[0269] The plant 1 comprises only one axial compressor 5 of turbogas type which works at nearly constant delivery pressure (with fixed revolutions), with compression ratio comprised between 12 and 24, connected to an electric motor for absorbing the electrical energy and converting it into thermal and potential energy (from point I to point II in FIG. 3). A single axial turbine 4 is connected to an electric generator in order to convert the potential and thermal energy into electrical energy, by exploiting the jump from point VII to point VIII in FIG. 3.

[0270] The plant 1 comprises a first thermal store 9 (TES) of “packed bed” type which absorbs sensible heat from the air, between the maximum delivery temperature of the compressor 5 (point II) and the proximal condensation temperature of the air at pressure equal to the delivery of the compressor 5, except for the load losses (point III) during the storage step, while during the discharge step, it transfers heat, heating the air from a temperature close to evaporation of the air from point VI to point VII.

[0271] The plant 1 comprises a second thermal store 10 (TES) for the latent heat of “packed bed” type which absorbs the latent heat between the maximum and the minimum temperature of condensation of the air at a pressure equal to the delivery of the compressor 5 except for the load losses, i.e. from point III to point IV during the storage step, in order to condense the air. During the discharge step, the second thermal store 10 operate at a pressure lower than the condensation pressure and evaporates the air by transferring latent heat, i.e. it works between the point V and the point VI. The second thermal store 10 integrates the tank 3, i.e. it is constituted by a single tank made of stainless steel, or by a battery of identical tanks, which have the object both of containing the inert material that defines the second heat carrier (useful for the purpose of the heat storage) and of containing the liquefied air.

[0272] Characteristic values of the abovementioned work points (from I to VIII) are summarized in the following Table.

TABLE-US-00001 Table Point I II III IV V VI VII VIII Pressure [bar] Nominal 1.01 18 17 17 15 15 14.5 1.01 Min Patm* 12 11.3 11.3 9 9 8.7 Patm* Max 24 22.7 22.7 20.8 20.8 19.7 Temperature [°C] Nominal 15 415 -155 -158 -160 -158.5 400 75 Min Tamb* 340 -162.5 -165 -168 -166 330 70 Max 470 -150 -152 -154 -152 460 80 N.B. Al point I, Patm and Tamb are parameters that depend on the place of installation. Pressures and temperatures in the subsequent points are calculated starting from the nominal values of point I.