Energy storage plant and process

Abstract

An energy storage plant includes a casing for the storage of a working fluid other than atmospheric air, in a gaseous phase and in equilibrium of pressure with the atmosphere; a tank for the storage of said working fluid in a liquid or supercritical phase with a temperature close to the critical temperature; wherein said critical temperature is close to the ambient temperature. The plant is configured to carry out a closed thermodynamic cyclic transformation, first in one direction in a charge configuration and then in the opposite direction in a discharge configuration, between said casing and said tank; wherein in the charge configuration the plant stores heat and pressure and in the discharge configuration generates energy.

Claims

1. An energy storage plant, comprising: a casing configured to store a working fluid other than atmospheric air in a gaseous phase; a tank configured to store said working fluid in a liquid phase; a compressor; a turbine; and heat exchangers configured to store thermal energy from the working fluid or to release thermal energy to the working fluid, wherein the casing is selectively in fluid communication with an inlet of the compressor or with an outlet of the turbine; wherein the heat exchangers are selectively in fluid communication with an outlet of the compressor or with an inlet of the turbine; wherein the tank is in fluid communication with the heat exchangers; wherein the plant is configured to perform a closed cyclic thermodynamic transformation, first in one direction in a charge configuration and then in an opposite direction in a discharge configuration, between said casing and said tank; wherein in the charge configuration the plant stores heat and pressure and in the discharge configuration generates energy; wherein the heat exchangers are configured to operate a sub-critical transformation of the working fluid; wherein the plant is configured to make the tank work at a constant pressure or substantial constant pressure and to make the casing work at a constant pressure or substantial constant pressure; wherein in the charge configuration the working fluid is accumulated in the tank in the liquid phase with a temperature close to a critical temperature and said critical temperature is close to the ambient temperature; wherein in the discharge configuration the working fluid is evaporated, expanded, and stored in the casing; wherein the casing is a pressure-balloon or has the structure of a gasometer so that the working fluid in said casing is in equilibrium of pressure with the atmosphere, with low or no overpressure, in any operating condition, wherein the heat exchangers comprise a primary heat exchanger and a secondary heat exchanger, the secondary heat exchanger being located between the primary heat exchanger and the tank or being integrated into the tank; and wherein, in the discharge configuration, the working fluid is heated through the heat stored by the primary heat exchanger and secondary heat exchanger.

2. The plant of claim 1, wherein the secondary heat exchanger is configured to regulate the pressure in the tank in the charge configuration and in the discharge configuration through a direct or indirect heat exchange with the atmosphere.

3. The plant of claim 1, wherein the primary heat exchanger is a thermal energy storage or is operatively coupled to a thermal energy storage; and wherein the thermal energy storage is configured to store thermal energy released by the working fluid in the charge configuration and to provide thermal energy to the working fluid in the discharge configuration.

4. The plant of claim 1, wherein the secondary heat exchanger comprises a secondary circuit crossed by a secondary fluid; wherein the secondary circuit includes a heat exchange portion lapped by the working fluid, a secondary hot storage chamber, for the secondary fluid accumulated after removing heat from the working fluid in the charge configuration, and a secondary cold storage chamber, for the secondary fluid accumulated after releasing heat to the working fluid in the discharge configuration; and wherein the secondary hot storage chamber and the secondary cold storage chamber are connected to each other through a radiator equipped with fans and with recirculation ducts that cool the secondary fluid during night and heats it during day.

5. The plant of claim 1, wherein the secondary heat exchanger comprises a secondary circuit crossed by a secondary fluid; wherein the secondary circuit includes a heat exchange portion lapped by the working fluid and an additional heat exchange portion; and wherein the additional heat exchange portion is configured to exchange heat with air or sea water.

6. The plant of claim 1, wherein the secondary heat exchanger comprises a secondary circuit crossed by a secondary fluid; and wherein the secondary circuit includes a heat exchange portion lapped by the working fluid and a secondary tank with a two-phase system operationally connected to an auxiliary chiller.

7. The plant of claim 1, wherein the secondary heat exchanger comprises a secondary circuit crossed by a secondary fluid; wherein the secondary circuit includes a heat exchange portion lapped by the working fluid; wherein the secondary circuit is located in a basin full of water consisting of several chambers; and wherein the secondary fluid in the secondary circuit is cooled or heated by the water in the basin.

8. The plant of claim 1, wherein the secondary heat exchanger comprises a secondary circuit crossed by a secondary fluid; wherein the secondary circuit includes a heat exchange portion lapped by the working fluid and a secondary storage chamber, for the secondary fluid accumulated after removing heat from the working fluid in the charge configuration and for the secondary fluid accumulated after releasing heat to the working fluid in the discharge configuration; and wherein the secondary storage chamber is combined with a radiator equipped with one or more fans placed on a recirculation duct which cools the secondary fluid during night and heats it during day.

Description

DESCRIPTION OF DRAWINGS

(1) This description will be set out below with reference to the attached drawings, which are provided for indicative and non-limiting purposes, in which:

(2) FIG. 1 schematically shows an embodiment of an energy storage plant according to the present invention;

(3) FIG. 2 shows a variant of the plant of FIG. 1;

(4) FIG. 3 is a T-S diagram showing a process according to the present invention implemented in the plants of FIG. 1 or 2;

(5) FIG. 4 shows a further embodiment of an energy storage plant according to the present invention;

(6) FIG. 5 shows a variant of the plant of FIG. 4;

(7) FIG. 6 is a T-S diagram showing a process according to the present invention implemented in the plants of FIG. 4 or 5;

(8) FIG. 7 is a T-Q diagram showing a part of the process according to the present invention implemented in the plants of FIG. 4 or 5;

(9) FIGS. 8, 9 and 10 show respective variants of a portion of the plant of FIG. 2;

(10) FIGS. 11 and 12 show respective variants of a different portion of the plants in FIGS. 1, 2, 4 and 5;

(11) FIG. 13 shows a further embodiment of an energy storage plant according to the present invention.

DETAILED DESCRIPTION

(12) With reference to the attached figures, with the reference number 1, a plant for the storage of energy (energy storage) according to the present invention has been indicated overall.

(13) The plant 1, for example, operates with a working fluid other than atmospheric air. For example, plant 1 operates with a working fluid chosen from the group including: carbon dioxide CO.sub.2, sulphur hexafluoride SF.sub.6, nitrous oxide N.sub.2O. In the following description, the working fluid used in combination with described plant 1 is carbon dioxide CO.sub.2.

(14) Plant 1 is configured to perform a closed cyclic thermodynamic transformation (TTC), first in one direction into a charge configuration/phase and then in the opposite direction into a discharge configuration/phase, in which plant 1 stores heat and pressure in the charge configuration and generates electrical energy in the discharge configuration.

(15) With reference to FIG. 1, plant 1 comprises a turbine 2 and a compressor 3 mechanically connected to a shaft of a single motor-generator 4. The motor-generator 4, the compressor 3 and the turbine 2 are arranged on a same axis. A shaft of the turbine 2 is coupled to one end of the shaft of the motor-generator 4 by means of connection devices, e.g. of the clutch type, which make it possible to connect and disconnect, on command, the turbine 2 to and from the motor-generator 4. Similarly, a shaft of the compressor 3 is coupled to an opposite end of the shaft of the motor-generator 4 by means of connection devices, e.g. of the clutch type, which allow the compressor 3 to be connected to and disconnected, on command, from the motor-generator 4. In other embodiments not shown here, the motor is firmly connected to the compressor 3 and the generator is firmly connected to the turbine 2. In such a case, the motor is permanently connected to compressor 3 and the generator is permanently connected to turbine 2.

(16) Plant 1 comprises a casing 5 preferably defined by a pressure-balloon made of flexible material, e.g. PVC coated polyester fabric. The pressure-balloon is placed on the earth's surface and is externally in contact with atmospheric air. The pressure-balloon delimits inside a volume configured to contain the working fluid at atmospheric pressure or substantially atmospheric pressure, i.e. in equilibrium of pressure with the atmosphere. The casing 5 may also be designed as a gasometer or any other gas storage system with low or no overpressure.

(17) First pipelines 6 develop between the casing 5 and an inlet 3a of the compressor 3 and between the casing 5 and an outlet 2b of the turbine 2 to connect the internal volume of the casing 5 with said compressor 3 and turbine 2. A valve or a valve system, not illustrated, may be operationally placed on the first pipelines 6 to put in fluid communication alternately the casing 5 with the inlet 3a of the compressor 3 or the outlet 2b of the turbine 2 with the casing 5.

(18) The plant 1 comprises a primary heat exchanger 7 which can be selectively put in fluid communication with an outlet 3b of compressor 3 or with an inlet 2a of turbine 2. For this purpose, second pipelines 8 develop between the inlet 2a of the turbine 2 and primary heat exchanger 7 and between the outlet 3b of the compressor 3 and the primary heat exchanger 7. A valve, or a valve system, not illustrated, is operationally located on the second pipelines 8 to connect the primary heat exchanger 7 with the inlet 2a of turbine 2 or the outlet 3b of compressor 3 with the primary heat exchanger 7. In a preferred embodiment, there is only one valve or valve system located on the second pipelines 8.

(19) A tank 9 is in fluid communication with the primary heat exchanger 7 and is configured to store the working fluid in liquid or supercritical phase.

(20) The tank 9 is preferably made of metal with a spherical outer wall.

(21) A secondary heat exchanger 10 is operationally active between the primary heat exchanger 7 and the tank 9, or in said tank 9, and is configured to operate on the working fluid accumulated or in accumulation phase in the tank 9. According to what is shown in the embodiment of FIG. 1, the secondary heat exchanger 10 is integrated in tank 9 in the sense that it has its own heat exchange portion 11 housed inside the tank 9 and configured to be touched by the working fluid contained in said tank 9. Third pipes 12 develop between the primary heat exchanger 7 and the tank 9 to put in fluid communication said primary heat exchanger 7 with said tank 9 and with said secondary heat exchanger 10.

(22) In the schematic representation of FIG. 1, the plant 1 may also comprise an additional heat exchanger 13 operationally placed between the casing 5 and the compressor 2 and between the casing 5 and the turbine 2 and possibly a cooler 13a positioned on a branch of the first pipelines 6 connected to the outlet 2b of turbine 2.

(23) The plant 1 also comprises a control unit, not shown, operationally connected to the different elements of the same plant 1 and configured/programmed to manage its operation.

(24) The plant 1 is configured to operate in a charge configuration or in a discharge configuration or to perform a process comprising a phase of energy charge and a phase of discharge and energy generation.

(25) In the charge configuration, the plant 1 starts from a first state in which the working fluid (CO.sub.2) in gaseous form is all contained in the casing 5 at atmospheric pressure or substantially atmospheric pressure and at a temperature substantially equal to the ambient temperature (point A of the T-S diagram in FIG. 3). Casing 5, through the valve system, is connected to the inlet 3a of the compressor 3 while communication with the outlet 2b of the turbine 2 is blocked. In addition, by means of the valve system, the primary heat exchanger 7 is in fluid communication with the outlet 3b of the compressor 3 and communication with the inlet 2a of the turbine 2 is blocked. The motor-generator 4 is coupled to the compressor 3 only and is decoupled from the turbine 2 (which is at rest) and works as motor to drive the compressor 3 such as to compress the working fluid coming from the casing 5.

(26) Before entering the compressor 3, the working fluid passes through the additional heat exchanger 13 which acts as a heater to pre-heat the working fluid (point B of the T-S diagram in FIG. 3). The working fluid is then compressed in the compressor 3 and heats up (point C of the T-S diagram in FIG. 3). The working fluid then flows through the primary heat exchanger 7 which works as a cooler to remove heat from the compressed working fluid, cool it (point D of the T-S diagram in FIG. 3) and store the thermal energy removed from the working fluid. At point D the working fluid is at a temperature lower than the critical temperature of the working fluid and at a point on the left side of the Andrews bell or slightly outside the bell in conditions of slight overheating. This compression may be adiabatic, inter-cooled or isothermal.

(27) The working fluid enters the tank 9 where the secondary heat exchanger 10, which in this configuration works as a cooler, removes further heat from the working fluid and accumulates further thermal energy. The working fluid passes through the saturated vapor zone until it reaches the liquid phase (point E of the T-S diagram in FIG. 3). The tank 9 therefore accumulates the working fluid in the liquid phase at a temperature lower than its own critical temperature Tc. In this second state, the working fluid (CO.sub.2, Tc=31° C.) in liquid form, for example at 20° C., is all contained in the tank 9. The secondary heat exchanger 10 and the primary heat exchanger 9 are therefore configured to perform a sub-critical transformation of the working fluid so that the working fluid is accumulated in the tank 9 in liquid phase.

(28) In the discharge configuration, the plant 1 starts from the second state (point F of the T-S diagram in FIG. 3). The casing 5, through the valve system, is put in communication with the outlet 2b of turbine 2 while communication with the inlet 3a of the compressor 3 is blocked. In addition, by means of the valve system, the primary heat exchanger 7 is in fluid communication with the inlet 2a of the turbine 2 and the communication with the outlet 3b of the compressor 3 is blocked. The motor-generator 4 is coupled to turbine 2 only and is decoupled from compressor 3 (which is at rest) and works as a generator driven in rotation by the turbine 2 driven by the expanding working fluid.

(29) The secondary heat exchanger 10 works as a heater and transfers some of the heat previously accumulated in the charge configuration to the working fluid in the tank 9. The working fluid passes through the saturated steam zone until it reaches the steam phase (point G of the T-S diagram in FIG. 3). The working fluid passes through the primary heat exchanger 7 which now works as a heater and releases additional heat, previously accumulated in the charge configuration, to the working fluid and heats it (point H of the T-S diagram in FIG. 3).

(30) The heated working fluid enters the turbine 2, expands and cools (point I of the T-S diagram in FIG. 3) and causes the rotation of the turbine 2. The turbine 2, rotated by the heated working fluid, drives the motor-generator 4, which works as a generator and generates electrical energy. The working fluid expansion in the turbine may be adiabatic, inter-heated or isothermal.

(31) The working fluid coming from turbine 2 is cooled in the additional heat exchanger 13 (point J of diagram T-S in FIG. 3) and returns into the casing 5 at atmospheric or substantially atmospheric pressure. The additional heat exchanger 13 in this phase stores additional thermal energy in a respective additional thermal energy storage device, which will be used in the next charge phase to pre-heat the working fluid.

(32) In the transformation illustrated in FIG. 3, the secondary circuit 20 is configured to remove heat from the working fluid, in the charge configuration, or to transfer heat to the working fluid, in the discharge configuration, at a temperature close to the ambient temperature, for example, of about 20° C.

(33) Both in the charge and in the discharge configuration/phase, since the secondary heat exchanger 10 operates in conditions close to the ambient temperature, due to the fact that the fluid has a critical temperature close to the ambient temperature, it is possible that the heat removal phase and/or the heat supply phase by the secondary heat exchanger is/are assisted by a phase of direct or indirect exchange with the atmosphere.

(34) For example, a working fluid temperature (CO.sub.2) accumulated in the tank 9 is 24° C. and a working fluid pressure accumulated in the tank 9 is 65 bar. The density of CO.sub.2 at 25° C. and atmospheric pressure is about 1.8 kg/m.sup.3. The density of CO.sub.2 in the tank 9 is about 730 kg/m.sup.3. The ratio between the density of the working fluid when contained in the tank 9 under the indicated conditions and the density of the same working fluid when contained in the casing 5 under atmospheric conditions is therefore about 400. It should be noted in this regard that if instead of CO.sub.2 the atmospheric air stored at 65 bar and 24° C. in the tank 9 were used, its density would be only 78 kg/m.sup.3 and the volume of the tank 9 theoretically required would be about ten times greater.

(35) For example, for a plant 1 according to the invention able to store 100 MWh of energy, the volume of the pressure-balloon is about 400000 m.sup.3 while the volume of the tank is about 1000 m.sup.3.

(36) The variant of FIG. 2 shows a type of primary heat exchanger 7, i.e. a fixed bed heat regenerator comprising a thermal mass 14 consisting, for example, of metal balls. In the charge configuration/phase, the thermal mass 14 is lapped by the hot, compressed working fluid, which transfers heat to the metal balls that store thermal energy. In the discharge configuration/phase, the thermal mass 14 is lapped by the cold working fluid, which absorbs heat from the metal balls and heats up. In a variant not shown, the heat regenerator may also be of the moving bed type. The primary heat exchanger 7 is therefore a thermal storage (Thermal Energy Storage TES).

(37) Instead of the fixed bed heat regenerator shown in FIG. 2, other types of heat regenerator may be used.

(38) For example, a possible primary heat exchanger 7 is shown in FIG. 11. As shown in FIG. 11, the primary heat exchanger 7 comprises a primary circuit 15 crossed by a primary fluid, such as water, oil or salts. The primary circuit 15 comprises a heat exchange portion 16 configured to exchange heat with the working fluid. For example, in the schematic embodiment illustrated above, a section of the second 8 pipelines through which the working fluid flows passes through the heat exchange section 16, so that the primary fluid touches said section. The primary circuit 15 comprises a hot primary storage chamber 17, for the hot primary fluid accumulated after removing heat from the working fluid in the charge configuration/phase of the apparatus/process, and a cold primary storage chamber 18, for the cold primary fluid accumulated after transferring heat to the working fluid in the discharge configuration/phase of the apparatus/process. The heat exchange portion 16 is placed between the hot primary storage chamber 17 and the primary cold storage chamber 18. In the charge configuration/phase of the apparatus/process, the primary fluid flows from the cold primary storage chamber 18 to the hot primary storage chamber 17, removing heat from the working fluid. In the discharge configuration/phase of the apparatus/process, the primary fluid flows from the hot primary storage chamber 17 to the cold primary storage chamber 18 releasing heat from the working fluid.

(39) A different possible primary heat exchanger 7 is shown in FIG. 12. According to what is illustrated in FIG. 12, the primary circuit 15 of the primary heat exchanger 7 comprises a heat exchange portion 16 defined by a section of primary circuit 15 that is lapped by the working fluid that passes through the second pipelines 8. The primary circuit 15 also comprises a fixed bed heat regenerator 19, preferably operating at atmospheric pressure and preferably similar to that described above, which is lapped by the primary fluid.

(40) The variant of FIG. 2 is not equipped with the additional heat exchanger 13 so that the corresponding T-S diagram, not illustrated, does not show, with respect to the diagram of FIG. 3, points B and J.

(41) The variant in FIG. 2 also has a special structure of the secondary heat exchanger 10. The secondary heat exchanger 10 shown includes a secondary circuit 20 crossed by a secondary fluid, such as air or water. The secondary circuit 20, in addition to the heat exchange portion 11 housed inside the tank 9, comprises a secondary hot storage chamber 21, for the secondary hot fluid accumulated after removing heat from the working fluid in the charge configuration/phase of the apparatus/process, and a secondary cold storage chamber 22, for the secondary cold fluid accumulated after releasing heat to the working fluid in the discharge configuration/phase of the apparatus/process. The above mentioned chambers 21, 22 are also connected to each other, in addition to the above mentioned heat exchange portion 11, through a radiator 23 equipped with fans 24 and with recirculation ducts that cools the secondary fluid during the night and heats it during the day.

(42) FIGS. 8, 9 and 10 show other variants of the secondary heat exchanger 10 associated with tank 9.

(43) In FIG. 8, the secondary circuit 20, in addition to the heat exchange portion 11, is equipped with an additional heat exchange portion 25 through which it exchanges heat with e.g. air or sea water.

(44) In FIG. 9, the secondary circuit 20 is equipped with a secondary tank 26 with water/ice or another two-phase system operationally connected to an auxiliary chiller 27.

(45) In FIG. 10, the secondary circuit 20 is located in a basin full of water consisting of several chambers 28a, 28b, 28c. The embodiment illustrated in FIG. 10 shows a chamber 28a for hot water storage, a chamber 28b for cold water storage and a chamber 28c in fluid communication with the others and housing part of the secondary circuit 20. The secondary fluid in the secondary circuit 20 is cooled or heated by the water in the basin. The working fluid is condensed in the charge phase and evaporated in the discharge phase by the suitably circulated water, preferably through immersion pumps and through the secondary fluid. The chambers 28 of said basin can be covered or uncovered and in communication or not with the environment so that the chamber from which the water is circulated for condensation during charging is always cooled by the surrounding environment, by appropriate panels 29, while the one from which the water is circulated for evaporation during discharging is always heated by the surrounding environment and possibly kept warm by a cover. The above can be further supported by special exchange systems that absorb heat or release heat, both through convection and radiation, with the environment, all in order to improve the RTE of the system.

(46) The embodiments of FIGS. 4 and 5 differ structurally from what has already been described because the secondary heat exchanger 10 is placed between the primary heat exchanger 7 and the tank 9, i.e. it is not integrated in the tank 9. The secondary heat exchanger 10 is in line on the third pipelines 12. FIG. 4 schematically illustrates a generic secondary heat exchanger 10. FIG. 5 shows a schematic design example of the secondary heat exchanger 10.

(47) The secondary heat exchanger 10 shown in FIG. 5 comprises a secondary circuit crossed by a secondary fluid, e.g. water. The secondary circuit 20 has a heat exchange portion 11 that is lapped by the working fluid that passes through the third pipelines 12 and is configured to exchange heat with the working fluid.

(48) The secondary circuit 20 of FIG. 5 comprises a secondary hot storage chamber 21, for the secondary hot fluid accumulated after removing heat from the working fluid in the charge configuration/phase of the apparatus/process, and a secondary cold storage chamber 22, for the secondary cold fluid accumulated after releasing heat to the working fluid in the discharge configuration/phase of the apparatus/process.

(49) The heat exchange portion 11 is located between the secondary hot storage chamber 21 and the secondary cold storage chamber 22. In the charge configuration/phase of the apparatus/process, the secondary fluid flows from the secondary cold storage chamber 22 to the secondary hot storage chamber 21, removing heat from the working fluid. In the discharge configuration/phase of the apparatus/process, the secondary fluid flows from the secondary hot storage chamber 21 to the secondary cold storage chamber 21, releasing heat from the working fluid. The secondary circuit 20 also comprises one or more intermediate secondary storage chambers 30 to adjust/vary the flow rate of the secondary fluid in the heat exchange portion 11 and the temperature variation of the working fluid exchanging heat with this secondary fluid. FIG. 5 shows two intermediate secondary storage chambers 30.

(50) The embodiments of FIGS. 4 and 5 differ structurally from what has already been described also because the tank 9 comprises a separation membrane 31 configured to internally separate the tank 9 in a first chamber with variable volume 32 for the working fluid in super-critical phase and in a second chamber with variable volume 33 in fluid communication with a compensation circuit 34 containing water. The compensation circuit 34 is configured to maintain a substantially constant pressure in the super-critical working fluid coming from the secondary heat exchanger 20 and contained in the first variable volume chamber 32 of the tank 9.

(51) The compensation circuit 34 comprises an auxiliary tank 35 for water at atmospheric pressure, which is in fluid communication, through appropriate pipelines, with a lower portion of the tank 9 and with the second variable volume chamber 33. An auxiliary turbine 36 has an inlet in communication with the second variable volume chamber 33 and an outlet connected to the auxiliary tank 35. The auxiliary turbine 36 is connected to an auxiliary generator 37 and is configured to be rotated by the water coming from the second variable volume chamber 33 in the charge configuration/phase of the apparatus/process. A pump 38 has an inlet in communication with the auxiliary tank 35 and an outlet connected to the second variable volume chamber 33. The pump 38 is connected to an auxiliary motor 39 and is configured to pump water from the auxiliary tank 35 into the second variable volume chamber 33 in the discharge configuration/phase of the apparatus/process.

(52) FIG. 6 shows the T-S diagram for the embodiments of FIGS. 4 and 5.

(53) FIG. 7 shows the T-Q diagram relating to a part of the thermodynamic transformation carried out by the embodiment of FIG. 5.

(54) The secondary heat exchanger 10 and the primary heat exchanger 7 of the embodiments of FIGS. 4 and 5 are configured to operate a super-critical transformation of the working fluid so that said working fluid is accumulated in the tank in super-critical phase. In fact, unlike what is shown in FIG. 3, the primary heat exchanger 7 removes heat from the working fluid up to bring it (point D of FIG. 6) to a temperature higher than the critical temperature and above the Andrews bell.

(55) Subsequently, the secondary heat exchanger 10 brings the working fluid into super-critical phase (point E) making it follow the right side of Andrews' bell. FIG. 7 shows the temperature decrease from point D to point E of the working fluid during the charge phase and the corresponding temperature increase of the secondary working fluid of the secondary heat exchanger 10 of FIG. 5 (points U, V, W, Z).

(56) The same FIG. 7 also illustrates the temperature increase from point F to point G of the working fluid during the discharge phase and the corresponding temperature decrease of the secondary working fluid of the secondary heat exchanger 10 of FIG. 5 (points Z, W, V, U).

(57) For instance, a working fluid temperature (CO.sub.2) accumulated in super-critical phase in the tank 9 is 25° C. and a working fluid pressure accumulated in super-critical phase in the tank 9 is 100 bar. The density of CO.sub.2 at 25° C. and atmospheric pressure is about 1.8 kg/m.sup.3. The density of CO.sub.2 in the tank 9 is about 815 kg/m.sup.3. The ratio between the density of the working fluid when contained in the tank 9 under the indicated conditions and the density of the same working fluid when contained in the casing 5 under atmospheric conditions is therefore about 450.

(58) It should be noted that the structure of the secondary heat exchanger of FIG. 10 may also be adopted in the embodiment of FIGS. 4 and 5.

(59) In addition, the secondary heat exchanger may be equipped with flow rate and/or temperature control systems for secondary fluid, typically water or air, capable of regulating the pressure in the storage tanks within certain limits, when the system operates in sub-critical conditions. Temperature control may, for example, be carried out by adding heat from the atmosphere or removing heat to atmosphere, also taking advantage of the normal fluctuations in the ambient temperature of air and water at different times of the day.

(60) In the illustrated embodiments using CO.sub.2 as the working fluid, a CO.sub.2 dehydration system, a de-humidifier, for example with zeolites, is also preferably present to avoid potential formation of carbonic acid in the circuit.

(61) FIG. 13 shows a further variant of plant 1. It shows the main elements common to FIG. 1, i.e. the turbine 2, the compressor 3, the motor generator 4, the casing 5, the primary heat exchanger 7 (TES thermal storage), the tank 9 and the secondary heat exchanger 10. Plant 1 shown here also comprises the additional heat exchanger 13. As in the embodiment shown in FIG. 4, the secondary heat exchanger 10 is located between the primary heat exchanger 7 and the tank 9, i.e. it is not integrated in tank 9. Similar to the plant shown in FIG. 2, the secondary heat exchanger 10 comprises a secondary circuit 20 crossed by a secondary fluid, e.g. water. The secondary circuit 20, in addition to the heat exchange portion 11 comprises a secondary storage chamber 200, for the secondary hot fluid accumulated after removing heat from the working fluid in the charge configuration/phase of the apparatus/process and for the secondary cold fluid accumulated after releasing heat to the working fluid in the discharge configuration/phase of the apparatus/process. The above mentioned secondary storage chamber 200 is also combined with a radiator 23 equipped with one or more fans 24 placed on a recirculation duct which, for example, cools the secondary fluid during the night and heats it during the day. The above-mentioned secondary storage chamber 200 is also connected to the additional heat exchanger 13 via a corresponding circuit 210.

(62) In this embodiment, plant 1 also comprises at least one additional heat exchanger 220 which receives heat from an additional heat source 230. The additional heat exchanger 220 is located on the second pipeline 8, between the inlet 2a of turbine 2 and the primary heat exchanger 7. The additional heat source 230 is, for example but not exclusively, a solar source (e.g. solar field), residual heat deriving from industrial recovery (Waste Heat Recovery), exhaust heat from gas turbines, etc.

(63) The additional heat source 230 provides additional heat during the discharge phase.

(64) The temperature at which the working fluid is brought during the discharge phase and just before it enters turbine 2, via the additional heat source 230 and the additional heat exchanger 220, is higher than the temperature of the working fluid that is obtained at the end of the compression during the charge phase. For example, the temperature at which the working fluid is brought by the additional heat source 230 and the additional heat exchanger 220 is about 100° C. but also 200° C. or 300° C. or 400° C. higher than the temperature of the working fluid at the end of compression.

(65) Plant 1 is also equipped with an auxiliary thermal storage 240 (Thermal Energy Storage TES) connected, through appropriate circuits, to the compressor 2 and the turbine 2 in order to achieve, in the compressor 3 (during the charge phase), an inter-cooled compression (with one or more inter-coolings) and to achieve, in the turbine 2 (during the discharge phase), an inter-heated expansion (with one or more inter-heatings). The heat accumulated in the auxiliary heat accumulator 240 during the inter-cooled compression is used in whole or in part to achieve the inter-heated expansion.

(66) In an embodiment of the process performed with the plant of FIG. 13, it is provided not to carry out inter-coolings in the charge phase and not to carry out inter-heatings in the discharge phase and to provide additional heat in the discharge phase through the additional heat source 230 and the additional heat exchanger 220.

(67) In variants of the process performed with the plant of FIG. 13, it is provided to make one or more inter-coolings in the charge phase and an equal number of inter-heatings in the discharge phase, in addition to providing additional heat in the discharge phase through the additional heat source 230 and the additional heat exchanger 220.

(68) In a further embodiment of the process performed with the plant of FIG. 13, it is provided to carry out a number of inter-coolings in the charge phase and to carry out a single inter-cooling in the discharge phase using the heat (accumulated in the auxiliary thermal accumulator 240) only of the last inter-cooling, in addition to heat with the additional heat through the additional heat source 230 and the additional heat exchanger 220. The heat stored in the auxiliary heat storage 240 and coming from the remaining intercoolings can be used for other purposes, e.g. for co-generation.

LIST OF ELEMENTS

(69) 1 energy storage plant 2 turbine 2a turbine inlet 2b turbine outlet 3 compressor 3a compressor inlet 3b compressor outlet 4 motor-generator 5 casing 6 first pipelines 7 primary heat exchanger 8 second pipelines 9 tank 10 secondary heat exchanger 11 heat exchange portion of secondary heat exchanger 12 third pipelines 13 additional heat exchanger 13a cooler 14 thermal mass 15 primary circuit 16 heat exchange portion of the primary circuit 17 primary hot storage chamber 18 primary cold storage chamber 19 fixed bed heat regenerator 20 secondary circuit 21 secondary hot storage chamber 22 secondary cold storage chamber 23 radiator 24 fans 25 further heat exchange portion 26 secondary tank 27 auxiliary chiller 28a, 28b, 28c water basin chambers 29 panels 30 intermediate secondary storage chambers 31 separation membrane 32 first variable volume chamber 33 second variable volume chamber 34 compensation circuit 35 auxiliary tank 36 auxiliary turbine 37 auxiliary generator 38 pump 39 auxiliary motor 200 secondary storage chamber 210 additional heat exchanger circuit 220 additional heat exchanger 230 additional heat source 240 auxiliary thermal storage