EP4669841A1

EP4669841A1 - PLANT AND METHOD FOR STORING ELECTRICAL AND/OR MECHANICAL ENERGY AND OPTIONALLY THERMAL ENERGY

Info

Publication number: EP4669841A1
Application number: EP24719275.0A
Authority: EP
Inventors: Stefano BRIOLA
Original assignee: Individual
Current assignee: Individual
Priority date: 2023-02-24
Filing date: 2024-02-20
Publication date: 2025-12-31
Also published as: WO2024176100A1

Abstract

The present invention relates to the field of plants for absorption/accumulation of electric energy and/or mechanical energy and possibly also thermal energy provided from external sources, subsequent storage of said energy for a certain time interval, and finally for the conversion of said energy into electric energy and/or mechanical energy supplied to the end-user. In particular, the present invention concerns a new plant, and related method, for energy storage in which energy is stored in the form of potential pressure energy and thermal energy of a suitable working fluid.

Description

TITLE

PLANT AND METHOD FOR THE STORAGE OF ELECTRICAL AND/OR MECHANICAL ENERGY, AND OPTIONALLY THERMAL ENERGY

DESCRIPTION

Technical field of the invention

This invention pertains to the field of plants for the absorption/accumulation of electrical and/or mechanical energy, and optionally also thermal energy supplied from external sources, subsequent storage of said energy for a certain period, and finally for the conversion of said energy into electrical and/or mechanical energy supplied to the end-users. Specifically, the present invention relates to a new energy storage plant and associated method, where energy is stored in the form of potential pressure energy and thermal energy of a suitable working fluid.

Background of the invention

Electric storage technologies can be classified into four categories: i) mechanical systems, specifically hydroelectric systems, compressed or liquid air systems, flywheels, and heat pump systems; ii) chemical systems, specifically hydrogen and synthetic natural gas; iii) electrochemical systems, particularly batteries; iv) electrical systems, notably capacitors and magnetic superconductors [1],

The mechanical systems category, to which this invention belongs, includes (beyond flywheels that exclusively store small amounts of energy) the systems described below.

Hydroelectric and compressed air energy storage systems allow for the storage of electrical energy in the form of potential gravitational energy of water or potential air pressure energy. In such systems, the working fluid (water or air) used for energy storage is stored at a site to later generate electricity. The hydroelectric and compressed air energy storage systems operate cyclically through charging and discharging processes: (i) during charging process, electricity powers pumps group (in hydroelectric systems) or compressors group (in compressed air systems) to store a predetermined mass of water or pressurized air in the storage site; (ii) during following discharging process, the mass of water or air, previously stored in the storage site, drives hydraulic turbines group (in hydroelectric systems) or gas turbines group (in compressed air systems) to produce electricity. Hydroelectric and compressed air energy storage systems are feasible only in sites with specific geomorphological conditions, such as water reservoirs (which imply a significant environmental impact) and underground cavities (hardly available), respectively. As an alternative, compressed air energy storage systems may use above-ground tanks for storing large volumes of pressurized air, which are prohibitively expensive. Moreover, the compressed air energy storage systems suffer from reduced isentropic efficiency of the turbomachinery (used in these systems) and the overall system efficiency of the same systems - termed "Round Trip Efficiency" (RTE), defined as the ratio of the electrical energy produced at the end of the discharging process to the electrical energy stored at the end of the charging process - due to pressure variations in the air storage site caused by changes in the stored air mass [2],

Liquid air energy storage systems operate similarly to compressed air energy storage systems but are not constrained by sites with specific geomorphological conditions, thanks to the storage of the working fluid in liquid phase, which has a significant energy density leading to reduced storage volume. However, storing air in liquid phase requires reaching cryogenic temperatures, introducing technological challenges [1],

Heat pump systems move the working fluid between two storage tanks at different temperatures: (i) during the charging process, electrical energy is used to operate the compressor to increase the temperature of the working fluid from the low-temperature storage tank to the high-temperature storage tank; (ii) in the following discharging process, the working fluid flows from the high-temperature storage tank to the low- temperature storage tank through the turbine, to generate electricity. Heat pump systems allow obtaining significant RTE values only with extremely high temperatures in the high-temperature storage tank and cryogenic temperatures in the low-temperature storage tank, leading to technological challenges [3],

Summary of the invention

This invention is based on the realization that an energy storage plant for electrical and/or mechanical energy can be remarkably effective by utilizing adiabatic fluid machines such as two-phase fluid expanders and two-phase fluid compressors. These unconventional adiabatic expansion and compression machines operate with a working fluid that is partly in saturated liquid phase and partly in dry saturated vapor phase. Although these machines are known or proposed, to the applicant's knowledge, they have never been employed and combined in an energy storage system before. The goal is to provide a plant and method for storing electrical and/or mechanical energy, and optionally also thermal energy, operating with said two- phase machines, with high Round Trip Efficiency (RTE), without the need for specific geomorphological site conditions, and featuring simplicity, cost-effectiveness, long lifespan, and the ability to quickly become operational.

Conceptually, the plant's first configuration involves the use of: (i) three storage tanks of the working fluid, including a supply tank, an intermediate tank, and a final tank;

(ii) a plurality of circuital means configured to move the mass of the working fluid between these storage tanks. Specifically, during the charging process, the working fluid circulates through said circuital means from said supply tank to said intermediate tank and also to said final tank to absorb/accumulate electrical and/or mechanical energy, and optionally also thermal energy from external sources. During the discharging process, the circulation of the working fluid occurs by said circuital means from said final tank and also from said intermediate tank to said supply tank to convert the previously stored energy into electrical and/or mechanical energy, and optionally also thermal energy, supplied to the end-users.

Said circuital means include a plurality of two-phase fluid adiabatic machines, specifically:

(a) two reversible two-phase fluid adiabatic machines, in particular the first reversible adiabatic two-phase fluid machine is in fluid communication with said supply tank and the second reversible adiabatic two- phase fluid machine is in fluid communication with said final tank. Each of these two reversible adiabatic machines is capable of: a) during the charging process, performing the adiabatic compression process of the two-phase working fluid, i.e., converting the electrical and/or mechanical energy supplied from an external source into an increase in the potential pressure energy and thermal energy of said fluid; b) during the discharging process, performing the adiabatic expansion process of the two-phase working fluid, i.e., converting the potential pressure energy and thermal energy of said fluid into electrical and/or mechanical energy supplied to the end-users; or

(b) Four non-reversible adiabatic two-phase fluid machines, specifically each first and fourth non- reversible adiabatic two-phase fluid machine is in fluid communication with said supply tank, each second and third non-reversible adiabatic two-phase fluid machine is in fluid communication with said final tank. Specifically, each first and second non-reversible adiabatic two-phase fluid machine is designed to perform, during the charging process, said adiabatic compression process of the two- phase working fluid. Moreover, each third and fourth non-reversible adiabatic two-phase fluid machine is designed to perform, during the discharging process, the adiabatic expansion process of the two- phase working fluid.

To summarize, in the first configuration of the plant (which will be described in detail later), the charging process consists exclusively of adiabatic compression processes of the two-phase working fluid, and the discharging process consists exclusively of adiabatic expansion processes of the two-phase working fluid. As an alternative, the plant proposes a second configuration (which will be detailed later), in which each charging and discharging process consists of both adiabatic compression processes of the two-phase working fluid and adiabatic expansion processes of the two-phase working fluid. In said second configuration of the plant, said circuital means include a plurality of adiabatic two-phase fluid machines:

(a) Two adiabatic reversible two-phase fluid machines, specifically the first reversible adiabatic two-phase fluid machine is in fluid communication with said supply tank and is capable of performing, during the charging process, said adiabatic expansion process of the two-phase working fluid, and, during the discharging process, said adiabatic compression process of the two-phase working fluid. The second reversible adiabatic two-phase fluid machine is in fluid communication with the final tank and is capable of performing, during the charging process, said adiabatic compression process of the two- phase working fluid, and, during the discharging process, said adiabatic expansion process of the two- phase working fluid; or

(b) Four non-reversible adiabatic two-phase fluid machines, specifically the first non-reversible adiabatic two-phase fluid machine is in fluid communication with said supply tank and is capable of performing, during the charging process, said adiabatic expansion process of the two-phase working fluid. The second non-reversible adiabatic two-phase fluid machine is in fluid communication with said final tank and is capable of performing, during the charging process, said adiabatic compression process of the two-phase working fluid. The third non-reversible adiabatic two-phase fluid machine is in fluid communication with said final tank and is capable of performing, during the discharging process, said adiabatic expansion process of the two-phase working fluid. The fourth non-reversible adiabatic two- phase fluid machine is in fluid communication with said supply tank and is capable of performing, during the discharging process, said adiabatic compression process of the two-phase working fluid.

In said second configuration of the plant, in the charging process, said adiabatic expansion process can be carried out through an isenthalpic valve instead of said first reversible adiabatic two-phase fluid machine or said first non-reversible adiabatic two-phase fluid machine.

Furthermore, in each said first and second configuration of the plant, said circuital means may optionally include a device in fluid communication with both said intermediate tank and said two-phase fluid machines, wherein such device is capable of performing:

(a) during the charging process, the separation between a first part of the working fluid flow in the wet saturated vapor phase and a second part of the working fluid flow in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase having a quality (defined as the ratio between the mass flow rate of the dry saturated vapor phase and the mass flow rate of the two-phase fluid) lower than the quality associated with said first part of the working fluid flow. Said first part subsequently circulates in said two-phase fluid machine in fluid communication with said final tank and is capable of performing said compression process. Said second part subsequently circulates in said intermediate tank;

(b) during the discharging process, the mixing between said first part of the working fluid flow in the wet saturated vapor phase exiting from said two-phase fluid machine in fluid communication with said final tank and capable of performing said adiabatic expansion process and said second part of the working fluid flow in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase exiting from said intermediate tank.

Furthermore, in each said first and second configuration of the plant, said circuital means may optionally include a plurality of heat exchangers means, in particular:

(1) the first heat exchanger is configured for the heat exchange between two parts of the same working fluid (process called "thermal regeneration"), being in fluid communication (during the charging and discharging processes) with said two-phase fluid machines and said intermediate tank. In particular, in the heat exchange between the two parts of the working fluid, in said first heat exchanger:

(a) The side of said first heat exchanger, which is in fluid communication with said two-phase fluid machines, acts as an evaporator for absorbing thermal energy (cold side) during the charging process, and as a condenser for releasing thermal energy (hot side) during the discharging process. Additionally, the side of said first heat exchanger, which is in fluid communication with said intermediate tank, acts as a condenser for releasing thermal energy (hot side) during the charging process, and as an evaporator for absorbing thermal energy (cold side) during the discharging process; or

(b) The side of said first heat exchanger, which acts as an evaporator for absorbing thermal energy (cold side), is in fluid communication with said two-phase fluid machines via a system consisting of valves/piping bypass (known to an expert in the field and therefore not detailed in this description) during the charging process and is in fluid communication with said intermediate tank during the discharging process via said system consisting of valves/piping bypass. Moreover, the side of said first heat exchanger, which acts as a condenser for releasing thermal energy (hot side), is in fluid communication with said intermediate tank during the charging process via said system consisting of valves/piping bypass and is in fluid communication with said two-phase fluid machines during the discharging process via said system consisting of valves/piping bypass

Moreover, said first heat exchanger is further configured to allow for possible absorption of thermal energy supplied from an external heat source during the charging process. Said possible thermal energy can be supplied from said external heat source both in the presence of said thermal regeneration and in the absence of said thermal regeneration;

(2) The second heat exchanger is configured for the heat exchange between the working fluid and the external environment (including any possible heating power end-user) during the discharging process, being in fluid communication with said feed tank and said two-phase fluid machine (capable of performing said adiabatic expansion process of the two-phase working fluid in the first configuration of the invention or said adiabatic compression process of the two-phase working fluid in the second configuration of the invention).

The plant of the present invention operates cyclically according to the two processes of charging and discharging. In particular, during the charging process, the plant allows for the storage of electrical and/or mechanical energy (transferred from external sources to the working fluid through the operation of said two- phase fluid machines acting as two-phase compressors) and optionally also the storage of thermal energy (transferred from external sources to the working fluid through said first heat exchanger). During the subsequent discharging process, the plant enables the supply of electrical and/or mechanical energy to the end-user (produced via said two-phase fluid machines acting as two-phase expanders) and optionally also thermal energy (transferred via said second heat exchanger).

The machines under consideration, proposed within the scope of energy production systems, have the main characteristics mentioned below. A two-phase fluid expander converts the potential pressure energy and the thermal energy of a fluid in the wet saturated vapor phase (partially consisting of the dry saturated vapor phase and partially of the saturated liquid phase) into electrical (or mechanical) energy through simultaneous adiabatic expansion of the two said phases. Two categories of two-phase expanders are distinguished [4]: a) dynamic machines (radial action-reaction, radial reaction, axial or tangential action); b) volumetric machines (scroll, alternating piston, rotating piston, rotary vane, twin screw). The isentropic efficiencies of the radial reaction two-phase expanders marketed by Ebara International Corp. [5] and the axial impulse marketed by Energent Corp. [6] are both approximately 0.80.

A two-phase fluid compressor increases the pressure of a fluid in the wet saturated vapor phase by using electrical (or mechanical) energy supplied from external sources. Two categories of two-phase compressors are distinguished [7]: a) dynamic machines (axial and radial); b) volumetric machines (scroll, alternating piston, rotating piston, rotary vane, twin screw). The calculated isentropic efficiency of a twin- screw two-phase compressor operating with ammonia in the absence of lubricating oil is approximately 0.89 [8].

For a definition of the essential and preferred aspects of the invention, reference is also made to the independent claims and the dependent claims of the attached set of claims.

Brief description of the drawings

The features and advantages of the electrical and/or mechanical energy storage system, and possibly also thermal energy, according to the present invention will become more clearly from the following description of an exemplary but non-limiting embodiment thereof, with reference to the accompanying drawings in which:

Figure 1 is a general circuital scheme of a plant according to the invention;

Figure 2 is a T-s diagram of a charging process of the thermodynamic cycle associated with the plant in a first embodiment C1 ;

Figure 3 is a circuital scheme of the plant in said first embodiment C1 , highlighting the parts of the same plant where the transformations of the charging process (depicted in Figure 2) take place;

Figure 4 is a T-s diagram of a discharging process of the thermodynamic cycle associated with the plant in said first embodiment C1 ;

Figure 5 is a circuital scheme of the plant in said first embodiment C1 , highlighting the parts of the same plant where the transformations of the discharging process (depicted in Figure 4) take place;

Figure 6 is a T-s diagram of a charging process of the thermodynamic cycle associated with the plant in a first variant of said first embodiment C1 ;

Figure 7 is a circuital scheme of the plant in said first variant of said first embodiment C1 , highlighting the parts of the same plant where the transformations of the charging process (depicted in Figure 6) take place;

Figure 8 is a T-s diagram of a discharging process of the thermodynamic cycle associated with the plant in said first variant of said first embodiment C1 ;

Figure 9 is a circuital scheme of the plant in said first variant of said first embodiment C1 , highlighting the parts of the same plant where the transformations of the discharging process (depicted in Figure 8) take place; Figure 10 and Figure 11 are the counterparts of Figure 2 and Figure 3, respectively, associated with the plant in a second embodiment C2;

Figure 12 and Figure 13 are counterparts of Figure 4 and Figure 5, respectively, associated with the plant in said second embodiment C2;

Figure 14 and Figure 15 are counterparts of Figure 6 and Figure 7, respectively, associated with the plant in a first variant of said second embodiment C2;

Figure 16 and Figure 17 are counterparts of Figure 8 and Figure 9, respectively, associated with the plant in said first variant of said second embodiment C2.

Detailed description of the invention

1. Components and configurations of the plant

With reference to said Figures, and particularly for the moment to Figure 1 , in an exemplary configuration, the plant may comprise the following components, although not all necessarily present and/or used:

1) Three storage tanks of the working fluid, namely the feed tank FeT, intermediate tank IT, and final tank FiT. In particular, the mass of the working fluid is moved during the charging process from FeT to IT and FiT, and vice versa during the discharging process from FiT and IT to FeT;

2) Adiabatic two-phase fluid machines, specifically two reversible machines Ri and R2, which by convention will be referred to in order as the first and second reversible machine, respectively, or four non-reversible machines NR1, NR2, NR3, and NR4, which by convention will be referred to in order as the first, second, third, and fourth non-reversible machine, respectively. In particular, during the charging process, NR1 is capable of performing, in the first configuration, said adiabatic compression process of the two-phase working fluid, and in the second configuration, said adiabatic expansion process of the two-phase working fluid. Additionally, during the charging process, NR2 is capable of performing said adiabatic compression process of the two-phase working fluid in both configurations. Furthermore, during the discharging process, NR3 is capable of performing said adiabatic expansion process of the two-phase working fluid in both configurations. Lastly, during the discharging process, NR4 is capable of performing, in the first configuration, said adiabatic expansion process of the two- phase working fluid, and in the second configuration, said adiabatic compression process of the two- phase working fluid; 3) Separation or mixing device SM;

4) Two heat exchangers HEi and HE2.

The electrical and/or mechanical energy storage plant, and optionally also thermal energy, can operate according to the following two configurations depending on the ratio between the pressures of the feed tank FeT and the intermediate tank IT:

1) First configuration C1 where the pressure of the feed tank FeT is lower than the pressure of the intermediate tank IT;

2) Second configuration C2 where the pressure of the feed tank FeT is greater than the pressure of the intermediate tank IT. Additionally, an isenthalpic valve IV can be used in the charging process instead of a first reversible adiabatic two-phase fluid machine R1 or instead of a first non-reversible adiabatic two-phase fluid machine NR1. In both said cases, during the discharging process, both a first reversible adiabatic two-phase fluid machine R1 and a fourth non-reversible adiabatic two-phase fluid machine NR4 are capable of performing said adiabatic compression process of the two-phase working fluid.

In both said configurations C1 and C2, the pressure of the final tank FiT is greater than the pressures of the intermediate tank IT and the feed tank FeT.

2. First configuration C1 of the plant

Figures 2 and 3 represent the T-s diagram and the schematic layout of the charging circuit for the first configuration C1 , respectively:

• The overall flow rate of the working fluid (a+b) extracted from FeT in the wet saturated vapor phase (point 1) circulates through a first reversible adiabatic two-phase fluid machine R1 or a first non- reversible adiabatic two-phase fluid machine NR1, each performing said adiabatic compression process of the two-phase working fluid (transformation 1-2*);

• The first part of the working fluid flow (a) extracted in the wet saturated vapor phase at an intermediate section of the first reversible adiabatic two-phase machine R1 or the first non-reversible adiabatic two- phase machine NR1 at pressure and temperature (point 2*) higher than the homologous quantities associated with the inlet section of the first reversible adiabatic two-phase machine R1 or the first non- reversible adiabatic two-phase machine NR1 (point 1) circulates in the cold side of HE1.

The second part of the working fluid flow (b) in the wet saturated vapor phase at the outlet section of the first reversible adiabatic two-phase machine R1 or the first non-reversible adiabatic two-phase machine NRi at pressure and temperature (point 2) higher than the homologous quantities associated with the first part of the working fluid (point 2*) circulates in the hot side of a first heat exchanger HEi.

The first part of the working fluid flow (a) circulating in the cold side of the first heat exchanger HEi absorbs thermal power at constant pressure (transformation 2*-4) released by the second part of the working fluid flow (b) circulating in the hot side of the first heat exchanger HEi at constant pressure (transformation 2-3).

The first part of the working fluid flow (a) circulating in the cold side of the first heat exchanger HEi may optionally absorb thermal energy supplied from an external heat source;

• The second part of the working fluid flow (b) exiting the first heat exchanger HEi in the form of subcooled liquid or saturated liquid or wet saturated vapor (point 3) with a lower quality than the quality of the same second part of the working fluid flow (b) entering the first heat exchanger HEi (point 2) is stored in the intermediate tank IT.

The first part of the working fluid flow (a) exiting the first heat exchanger HEi in the form of wet saturated vapor (point 4) with a higher quality than the quality of the same first part of the flow entering HEi (point 2*) circulates into the inlet section of a second reversible adiabatic two-phase fluid machine R2 or a second non-reversible adiabatic two-phase fluid machine NR2.

• The first part of the working fluid flow (a) circulates into the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2, performing the adiabatic compression process of the two-phase working fluid (transformation 4-5), being in the outlet section of the second reversible two-phase fluid machine R2 or the second non-reversible two-phase fluid machine NR2 in the subcooled liquid phase, saturated liquid phase, wet saturated vapor phase, or dry saturated vapor phase (point 5), and finally stored in FiT.

To summarize, during the charging process, electrical and/or mechanical energy supplied from the external source to the working fluid through the operation of R1 and R2 (first and second reversible two- phase fluid machine, respectively) or NR1 and NR2 (first and second non-reversible two-phase fluid machine, respectively), performing said adiabatic compression process of the two-phase working fluid, is stored in the form of potential pressure energy and thermal energy of the first (a) and second (b) parts of the working fluid flow (stored in the final tank FiT and the intermediate tank IT, respectively).

Figures 4 and 5 depict the T-s diagram and the schematic layout of the discharging circuit for the first configuration C1 , respectively: The first part of the working fluid flow (a) extracted from the final tank FiT in subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase, or dry saturated vapor phase (point 5) circulates into the second reversible adiabatic two-phase fluid machine R2 or a third non-reversibie adiabatic two-phase fluid machine NR3, performing said adiabatic expansion process of the two-phase working fluid (transformation 5-6);

The first part of the working fluid flow (a) in the wet saturated vapor phase at the outlet section of the second reversible adiabatic two-phase fluid machine R2 or the third non-reversible adiabatic two- phase fluid machine NR3 at pressure and temperature (point 6) lower than the homologous quantities associated with the inlet section of the second reversible adiabatic two-phase fluid machine R2 or the third non-reversible adiabatic two-phase fluid machine NR3 (point 5) circulates in the hot side of the first heat exchanger HE1.

The second part of the working fluid flow (b) in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase extracted from the intermediate tank IT (point 3) circulates in the cold side of the first heat exchanger HE1.

The first part of the working fluid flow (a) circulating in the hot side of the first heat exchanger HE1 transfers thermal power at constant pressure (transformation 6-6*) to the second part of the working fluid flow (b) circulating in the cold side of the first heat exchanger HE1 at constant pressure (transformation 3-3*);

The first part of the working fluid flow (a) exiting the first heat exchanger HE1 in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase (point 6*) with a lower quality than the quality of the same first part of the flow (a) entering the first heat exchanger HE1 (point 6) circulates in the inlet section of the first reversible adiabatic two-phase fluid machine R1 or a fourth non-reversible adiabatic two-phase fluid machine NR4.

The second part of the working fluid flow (b) exiting the first heat exchanger HE1 in the wet saturated vapor phase (point 3*) with a higher quality than the quality of the same second part of the flow (b) entering the first heat exchanger HE1 (point 3) circulates in an intermediate section of the first reversible adiabatic two-phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4.

Due to the mixing of the first (a) and second (b) parts of the working fluid flow, the overall mass flow rate of the working fluid is located in the said intermediate section of the first reversible adiabatic two- phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4 at an intermediate quality (point 8) between the qualities of the first part (a) and the second part (b) of the working fluid flow

• The first (a) and second (b) parts of the working fluid flow circulate in the first reversible adiabatic two- phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NR4, performing the said process of adiabatic expansion of the two-phase working fluid (transformation 6*-1*);

• The overall flow rate of the working fluid (a+b) exiting the first reversible adiabatic two-phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4 in the wet saturated vapor phase (point 1*) circulates in a second heat exchanger HE2 where it releases thermal power to the external environment (e.g., thermal power end-user or atmospheric air or water) at constant pressure (transformation 1*-1) such that the thermodynamic state of the overall flow rate of the working fluid (a+b) exiting the second heat exchanger HE2 coincides with the thermodynamic state of the same overall flow rate of the working fluid (a+b) extracted from the feed tank FeT during the charging process (point 1).

To summarize, during the discharging process, the electrical and/or mechanical energy (generated by R1 and R2, the first and second reversible adiabatic two-phase fluid machine, respectively, or NR3 and NR4, the third and fourth non-reversible adiabatic two-phase fluid machine, respectively, operating the said adiabatic expansion process of the two-phase working fluid) is provided to the end-user along with the possible thermal energy supplied through the second heat exchanger HE2.

Within said first configuration C1 under consideration, a first variant involves deactivating the first heat exchanger HE1 and simultaneously activating the separation or mixing device SM in both the charging and discharging circuits.

For said first variant of said first configuration C1 , Figures 6 and 7 represent the T-s diagram and the plant layout of the charging circuit, respectively, while Figures 8 and 9 are the corresponding counterparts associated with the discharging circuit:

• Charging process: the overall flow of working fluid (a+b) extracted from the feed tank FeT (point 1) in the wet saturated vapor phase circulates into the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 performing said adiabatic compression process of the two-phase working fluid (transformation 1-2) and is divided at constant pressure (via the separation device SM) at the outlet section of the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 (point 2) into two parts. The first part of the working fluid flow (a) is in the wet saturated vapor phase (point 4), and the second part of the working fluid flow (b) is in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase (point 3) with a lower quality than the quality of said first fraction of the working fluid flow (a*). The latter circulates into the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 performing said adiabatic compression process of the two-phase fluid (transformation 4-5) and is then stored in the final storage tank FiT (point 5) in the wet saturated vapor phase, or saturated liquid phase, or subcooled liquid phase, or dry saturated vapor phase. The second part of the working fluid flow (b) is stored in the intermediate tank IT;

• Discharging process: the first part of the working fluid flow (a) extracted from the final storage tank FiT (point 5) circulates into the second reversible adiabatic two-phase fluid machine R2 or the third non- reversible adiabatic two-phase fluid machine NR3 performing the adiabatic expansion process of the two-phase fluid (transformation 5-6) and is then mixed at constant pressure (via the mixing device SM) with the second part of the working fluid flow (b) extracted from the intermediate tank IT (point 3). The overall flow of working fluid (a*+b*) obtained after mixing (point 7) circulates first into the first reversible adiabatic two-phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4 performing said two-phase adiabatic expansion process (transformation 7-1*) and finally into the second heat exchanger HE2 (transformation 1*-1) for the transfer of thermal power to the external environment (e.g., thermal power end-user or atmospheric air or water).

Within the scope of the first configuration C1 under consideration, two additional variants are distinguished: a) The second variant involves activating the first heat exchanger HE1 in the charging circuit (as described in Figures 2 and 3), deactivating the first heat exchanger HE1, and simultaneously activating the mixing device SM in the discharging circuit (as described in Figures 8 and 9); b) The third variant entails deactivating the first heat exchanger HE1 and simultaneously activating the separation device SM in the charging circuit (as described in Figures 6 and 7), and activating the first heat exchanger HE1 in the discharging circuit (as described in Figures 4 and 5).

3. Second configuration C2 of the plant

Figures 10 and 11 represent the T-s diagram and the plant layout of the charging circuit for the second configuration C2. In this configuration (similarly to said first configuration C1), a first and second reversible adiabatic two-phase fluid machine, R1 and R2, respectively, are used, or alternatively four non-reversible adiabatic two-phase fluid machines, NRi, NFb, NR3, and NR4, corresponding to the first, second, third, and fourth non-reversible adiabatic two-phase fluid machine, respectively. The setup will be as follows.

• The overall flow rate of the working fluid (a+b) extracted from the feed tank FeT in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase (point 1) circulates in the first reversible adiabatic two-phase fluid machine R1 or in the first non-reversible adiabatic two-phase fluid machine NR1 operating said adiabatic expansion process of the two-phase fluid (transformation 1-2*);

• The first part of the working fluid flow (a) in the wet saturated vapor phase at the outlet section of the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 at a pressure and temperature (point 2) lower than the homologous quantites associated with the inlet section of the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 (point 1) circulates in the cold side of the first heat exchanger HE1.

The second part of the working fluid flow (b) extracted in the wet saturated vapor phase at an intermediate section of the same first reversible adiabatic two-phase fluid machine R1 or the first non- reversible adiabatic two-phase fluid machine NR1 at a pressure and temperature (point 2*) higher than the homologous quantities associated with the first part (a) of the working fluid flow (point 2) circulates in the hot side of the first heat exchanger HE1.

The first part of the working fluid flow (a) circulating in the cold side of the first heat exchanger HEI absorbs thermal power at constant pressure (transformation 2-4) supplied by the second part of the working fluid flow (b) circulating in the hot side of the first heat exchanger HE1 at constant pressure (transformation 2*- 3).

The first part of the working fluid flow (a) circulating in the cold side of the first heat exchanger HE1 can optionally absorb thermal power supplied by an external heat source;

• The second part of the working fluid flow (b) exiting the first heat exchanger HEi in the subcooled liquid phase or saturated liquid phase or wet saturated vapor phase (point 3), with a lower quality than the quality of the same second part of the working fluid flow (b) entering the first heat exchanger HE1 (point 2*), is stored in the intermediate tank IT.

The first part of the working fluid flow (a) exiting the first heat exchanger HE1 in the wet saturated vapor phase (point 4), with a higher quality than the quality of the same first part of the flow (a) entering the first heat exchanger HE1 (point 2), is directed to the inlet section of the second reversible adiabatic two-phase fluid machine R₂ or the second non-reversible adiabatic two-phase fluid machine

NR₂;

• The first part of the working fluid flow (a) circulates in the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 operating said adiabatic compression process of the two-phase fluid (transformation 4-5), being in the outlet section of the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 in the subcooled liquid phase or saturated liquid phase or wet saturated vapor phase or dry saturated vapor phase (point 5), and finally stored in the final tank FiT.

To summarize, during the charging process, electrical and/or mechanical energy (supplied from an external source to the working fluid through the operation of the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 performing said adiabatic compression process of the two-phase fluid and reduced by the electrical and/or mechanical energy generated by the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 performing said adiabatic expansion process of the two-phase fluid) are stored in the form of potential pressure energy and thermal energy of the first (a) and the second (b) part of the working fluid flow (stored in the final tank FiT and the intermediate tank IT, respectively).

The Figures 12 and 13 depict the T-s diagram and the schematic layout of the discharging circuit for the second configuration C2, respectively:

• The first part of the working fluid flow (a) extracted from the final tank FiT in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase, or dry saturated vapor (point 5) circulates through the second reversible adiabatic two-phase fluid machine R2 or the third non- reversible adiabatic two-phase fluid machine NR3 operating the said two-phase adiabatic expansion process (transformation 5-6);

• The first part of the working fluid flow (a), in the wet saturated vapor phase, exiting from the outlet section of the second reversible adiabatic two-phase fluid machine R₂ or the third non-reversible adiabatic two-phase fluid machine NR3 at a pressure and temperature (point 6) lower than the homologous quantities at the inlet section of the same machines (point 5), circulates in the hot side of the first heat exchanger HE1.

The second part of the working fluid flow (b), in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase, extracted from the intermediate tank IT (point 3), circulates in the cold side of the first heat exchanger HE1. The first part of the working fluid flow (a) circulating in the hot side of the first heat exchanger HEi transfers thermal power at constant pressure (transformation 6-6*) to the second part of the working fluid flow (b) circulating in the cold side of the first heat exchanger HEi at constant pressure (transformation 3-3*).

The first part of the working fluid flow (a) exiting the first heat exchanger HEi in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase (point 6*), with a lower quality than the quality of the same first part of the working fluid flow (a) entering the first heat exchanger HE i (point 6), circulates in an intermediate section of the first reversible adiabatic two-phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NF

The second part of the working fluid flow (b) exiting the first heat exchanger HEi in the wet saturated vapor phase (point 3*), with a higher quality than the quality of the same second part of the working fluid flow (b) entering the first heat exchanger HEi (point 3), circulates in the inlet section of the first reversible adiabatic two-phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NR4.

Due to the mixing of the first (a) and second (b) parts of the working fluid flow, the overall mass flow rate of the working fluid (a+b) in the said intermediate section of the first reversible adiabatic two- phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NR4 is at an intermediate quality (point 8) between the qualities of the first part (a) and the second part (b) of the working fluid flow;

The first (a) and second (b) parts of the working fluid flow circulate in the first reversible adiabatic two- phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NR4, operating said adiabatic compression process of the two-phase working fluid (transformation 8-1*);

The overall flow rate of the working fluid (a+b) exiting the first reversible adiabatic two-phase fluid machine Ri or the fourth non-reversible adiabatic two-phase fluid machine NR4 in the saturated wet vapor phase (point 1*) circulates in the second heat exchanger HE2 (transformation 1*-1), where it transfers thermal power to the external environment (e.g., thermal power end-user, or atmospheric air, or water) at constant pressure, ensuring that the thermodynamic state of the overall flow rate of the working fluid (a+b) exiting the second heat exchanger HE2 coincides with the thermodynamic state of the same overall flow rate of the working fluid (a+b) extracted from the feed tank FeT during the charging process (point 1). To summarize, during the discharging process, the electrical and/or mechanical energy (generated by the second reversible adiabatic two-phase fluid machine R2 or the third non-reversible adiabatic two-phase fluid machine NR3 operating said adiabatic expansion process of the two-phase fluid, and reduced by the electrical and/or mechanical energy used for the operation of the first reversible adiabatic two-phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4 operating said adiabatic compression process of the two-phase fluid) is supplied to the end-user, along with the possible thermal energy provided via the second heat exchanger HE2.

It is noted that, in the charging circuit, the adiabatic expansion process (1-2) can be performed using an isenthalpic valve IV instead of the first reversible adiabatic two-phase fluid machine R1 or the first non- reversible adiabatic two-phase fluid machine NR1.

Within the scope of the second configuration C2 under consideration, a first variant involves deactivating the first heat exchanger HE1 and simultaneously activating the mixing and separation device SM in both the charging and discharging circuits.

For the first variant of the second configuration C2, Figures 14 and 15 represent the T-s diagram and the plant layout of the charging circuit, respectively, while Figures 16 and 17 depict the counterparts associated with the discharging circuit:

• Charging process: the overall flow of working fluid (a+b) extracted from the feed tank FeT (point 1) in the wet saturated vapor phase, or saturated liquid phase, or subcooled liquid phase, circulates through the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1 operating said adiabatic expansion process of the two-phase fluid (transformation 1-2), and is divided at constant pressure in the outlet section of the same machines (point 2) into two parts. The first part of the working fluid flow (a) is in the wet saturated vapor phase (point 4), and the second part of the working fluid flow (b) is in the subcooled liquid phase, or saturated liquid phase, or wet saturated vapor phase (point 3) with a lower quality than the quality of said first part. The latter circulates in the second reversible adiabatic two-phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 operating said adiabatic compression process of the two- phase fluid (transformation 4-5), and is then stored in the final tank FiT (point 5) in the wet saturated vapor phase, or saturated liquid phase, or subcooled liquid phase, or dry saturated vapor phase. The second part of the working fluid flow (b) is stored in the intermediate tank IT (point 3) in the saturated liquid phase, or subcooled liquid phase, or wet saturated vapor phase with a lower quality than the quality in the outlet section of the first reversible adiabatic two-phase fluid machine Ri or the first non- reversible adiabatic two-phase fluid machine NRi (point 2).

• Discharging process: the first part of the working fluid flow (a) extracted from the final tank FiT (point 5) circulates through the second adiabatic reversible two-phase fluid machine R2 or the third non- reversible adiabatic two-phase fluid machine NR3 operating said adiabatic expansion process of the two-phase fluid (transformation 5-6) and is then mixed with the second part of the working fluid flow (b) extracted from the intermediate tank IT (point 3). The overall flow of working fluid (a*+b*) obtained after mixing (point 7) circulates first in the first reversible adiabatic two-phase fluid machine R1 or the fourth non-reversible adiabatic two-phase fluid machine NR4 operating said adiabatic compression process of the two-phase fluid (transformation 7-1*) and finally in the second heat exchanger HE2 (transformation 1*-1) to transfer thermal power to the external environment (e.g., thermal power enduser or atmospheric air or water).

Still within the scope of the second configuration C2 under consideration, two additional variants are distinguished: a) The second variant consists of activating the first heat exchanger HE1 in the charging circuit (as described in Figures 10 and 11), deactivating the first heat exchanger HE1, and simultaneously activating the mixing device SM in the discharging circuit (as described in Figures 16 and 17); b) The third variant consists of deactivating the first heat exchanger HE1 and simultaneously activating the separation device SM in the charging circuit (as described in Figures 14 and 15) and activating the first heat exchanger HE1 in the discharging circuit (as described in Figures 12 and 13).

It is noted that, in each of the three variants associated with the second configuration C2, in the charging circuit, the adiabatic expansion process (1-2) can be carried out using an isenthalpic valve instead of the first reversible adiabatic two-phase fluid machine R1 or the first non-reversible adiabatic two-phase fluid machine NR1.

4. Further two options in said first configuration C1 and second configuration C2 of the plant

In said first configuration C1 , in said second configuration C2, and in each of said respective three variants of the same configurations C1 and C2, the following two additional options are distinguished (possibly usable simultaneously):

1) The first option involves activating, during the discharging process, an adiabatic compression means located downstream of said intermediate tank IT. Specifically, said adiabatic compression means is driven by a (negligible) part of the electrical or mechanical energy generated during the discharging process to increase the pressure of the second part of the working fluid exiting said intermediate tank IT. Each additional transformation in the discharging process remains unchanged; ) The second option involves activating, during the charging and discharging processes, a thermal storage medium of sensible heat type, or latent heat type, or thermochemical type located in each first configuration C1 and second configuration C2: in the charging circuit between the second reversible adiabatic two-phase fluid machine R2 (or the second non-reversible adiabatic two-phase fluid machine NR2) and the final tank FiT, and in the discharging circuit between the second reversible adiabatic two- phase fluid machine R2 (or the third non-reversible adiabatic two-phase fluid machine NR3) and the final tank FiT.

In particular, during the charging process, the working fluid exiting the second reversible adiabatic two- phase fluid machine R2 or the second non-reversible adiabatic two-phase fluid machine NR2 (point 5 of Figures 2, 3, 6, 7, 10, 1 1 , 14, and 15) first flows through said thermal storage medium, transferring thermal energy at constant pressure, and is finally stored in said final tank in the subcooled liquid phase or saturated liquid phase. Therefore, the electrical and/or mechanical energy (supplied from external sources to the working fluid through the operation of the two-phase fluid machines performing the adiabatic compression process of the two-phase working fluid) is stored in the form of potential pressure energy in the first and second parts of the working fluid (stored in the final tank FiT and the intermediate tank IT, respectively) and in the form of thermal energy stored in said thermal storage medium. Each additional transformation remains unchanged during the charging process. During the subsequent discharging process, the working fluid (exiting said final tank FiT in the subcooled liquid phase or saturated liquid phase) first flows through said thermal storage medium, absorbing thermal energy at constant pressure, and then flows through the second reversible adiabatic two-phase fluid machine R2 or the third non-reversible adiabatic two-phase fluid machine NR3 (transformation 5-6 of Figures 4, 5, 8, 9, 12, 13, 16, and 17). Each additional transformation remains unchanged during the discharge process. Said types of thermal storage are extensively described in the literature, as known, for example, from [9], and therefore are not detailed in this description. . Configuration of the components of the plant The configuration of the components of the plant described below is associated with said first configuration C1 , said second configuration C2, each of said respective three variants of the same configurations C1 and C2, and each of said two additional options.

(1) Each feed tank FeT, intermediate tank IT, and final tank FiT is configured to maintain the thermodynamic state (pressure, temperature, quality) of the working fluid constant over time (both during the charging and discharging processes and also during the time interval between said charging and discharging processes). For example, each of said tanks is delimited by rigid walls, possibly thermally insulated, and internally equipped with: (i) Deformable casing (to allow volume variation) containing the working fluid and possibly thermally insulated; (ii) Mass of a suitable fluid in liquid phase or in vapor phase in instantaneous pressure equilibrium with said deformable casing, where said mass of the suitable fluid varies over time through a regulation/control system to allow said volume variation of the deformable casing.

(2) The adiabatic two-phase fluid machines can be:

(a) Reversible (first machine Ri and second machine R2), meaning each capable of performing both the adiabatic expansion process of the two-phase working fluid over a certain time interval and the adiabatic compression process of the two-phase working fluid over a different time interval. In order to carry out the adiabatic expansion of the working fluid for converting its potential pressure energy and thermal energy into electrical and/or mechanical energy, and reversibly, the adiabatic compression of the same working fluid for converting the electrical and/or mechanical energy supplied from the outside into an increase in the potential pressure energy and thermal energy of the working fluid itself, the two-phase machines can be designed using solutions already extensively tested on single-phase machines, as known, for example, from [10] and [11], or,

(b) Non-reversible (first machine NR1, second machine NR2, third machine NR3, and fourth machine NR4), namely each capable of performing exclusively either said adiabatic expansion process of the two-phase working fluid or said adiabatic compression process of the two-phase working fluid. In particular, NR1 during the charging process is capable of performing in the first configuration said adiabatic compression process and in the second configuration said adiabatic expansion process. Additionally, NR2 during the charging process is capable of performing in both configurations said adiabatic compression process. Furthermore, NR3 during the discharging process is capable of performing in both configurations said expansion adiabatic process. Lastly, NR4 during the discharging process is capable of performing in the first configuration said adiabatic expansion process and in the second configuration said adiabatic compression process.

(3) The first exchanger HE1, configured for the heat exchange between two parts of the same working fluid, can be:

(a) reversible, namely each of the two sides of the first heat exchanger HE1 is capable of functioning both as an evaporator for absorbing thermal power over a certain period and as a condenser for transferring thermal power over a different period. By way of example, to function as condensers or evaporators, heat exchangers can be realized using solutions that have already been extensively experimented with, as known, for instance, from [12] and [13], or

(b) non-reversible, namely each of the two sides of the first heat exchanger HE1 is capable of operating exclusively as either an evaporator for absorbing thermal power or as a condenser for transferring thermal power. For this purpose, each of the two sides of the first heat exchanger HE, is in fluid communication with said adiabatic two-phase fluid machines or with said intermediate tank IT through a system consisting of valves/pipelines bypass.

Furthermore, said first heat exchanger HE1 is further configured to allow for the possible absorption of thermal energy provided by an external heat source during the charging process.

6. Advantages of the invention

The present invention pursues the following advantages compared to the energy storage systems present in the prior art:

1) High values of RTE, particularly up to about 0.90 in the absence of thermal storage medium and higher than about 0.90 in the presence of thermal storage medium. In fact, in the present invention, RTE is mainly determined by the isentropic efficiency of the adiabatic two-phase machines and the very modest thermal dissipation to the external environment in the three tanks (due to imperfect thermal insulation).

High RTE values are achievable at temperatures not excessively low of the working fluid stored in the feed tank FeT (as opposed to the liquid air energy storage systems where the necessary achievement of cryogenic temperatures implies technological challenges) and at not excessively high temperatures of the working fluid stored in the final tank FiT (as opposed to heat pump storage systems), due to the appropriate choice of the working fluid type and operating conditions.

In addition, high RTE values are practically constant during operation thanks to the adoption of the systems (previously mentioned) for maintaining constant values over time of the thermodynamic state of the working fluid in each feed tank FeT, intermediate tank IT, and final tank FiT.

2) Located in sites that do not require particular geomorphological conditions (unlike hydroelectric and compressed air energy storage systems, which require artificial reservoirs and underground cavities, respectively) thanks to moderate storage volumes. Indeed, these volumes are determined by the significant energy density of the working fluid obtained due to the appropriate choice of the same fluid type and operating conditions;

3) Simplicity with consequent reduction in purchase-installation costs and operating costs;

4) High lifespan (over thirty years) due to the negligible degradation of the system over time, unlike different types of energy storage (e.g., batteries); 5) Ability to start up quickly.

The present invention has been described herein with reference to its preferred embodiments. It is to be understood that other embodiments may exist that relate to the same inventive concept, all falling within the scope of protection of the claims set forth below.

Bibliographical and web references

[1] S. Briola, R. Gabbrielli, A. Delgado. Energy and economic performance assessment of the novel integration of an advanced configuration of liquid air energy storage plant with an existing large-scale natural gas combined cycle, Energy Conversion and Management 205 (2020) 112434.

[2] S. Briola, P. Di Marco, R. Gabbrielli, J. Riccardi. A novel mathematical model for the performance assessment of diabatic compressed air energy storage systems including the turbomachinery characteristic curves, Applied Energy 178 (2016) 758-772.

[3] B. Eppinger, D. Steger, C. Regensburger, J. Karl, E. Schlucker, S. Will. Carnot battery: Simulation and design of a reversible heat pump-organic Rankine cycle pilot plant, Applied Energy 288 (2021) 116650.

[4] M. Francesconi, S. Briola, M. Antonelli. A review on two-phase volumetric expanders and their applications, Applied Sciences 12 (20), 10328.

[5] P. Welch, P. Boyle, M. Giron, M. Sells. Construction and startup of low temperature geothermal power plants, GRC Conference, San Diego, 26 October 2011 .

[6] C.D. Finley. Continuously transient operation of two-phase LNG expanders, Conf. Proc, of AIChE Spring National Meeting, New Orleans, 2004

[7] S. Briola. Analisi delle prestazioni di cicli termodinamici di co-trigenerazione operanti con espansori e compressori a fluido bifase, PhD thesis, University of Pisa, 2015.

[8] C.A. Infante Ferreira, D. Zaytsev, C. Zamfirescu. Wet compression of pure refrigerants, Int. Compressor Engineering Conference, Purdue University, 2006.

[9] L.F. Cabeza, I. Martorell, L. Miro, A.I. Fernandez, C. Barreneche. Advances in Thermal Energy Storage systems - Cap. 1 Introduction to thermal energy storage (TES) systems, Elsevier, 2015.

[10] M. Rossi, M. Righetti, M. Renzi. Pump-as-turbine for energy recovery applications: The case study of an aqueduct, Energy Procedia 101 (2016) 1207-1214.

[11] D. Steger, M. Feist, E. Schlucker. Using a screw-type machine as reversible compressor-expander in a Carnot Battery: An analytical study towards efficiency, Applied Energy 316 (2022) 118950.

[12] https://www.kelvion.com/products/product/td-series/

[13] https://www.swep. net/refrigerant-handbook/7.-condensers/asd1/

Claims

1. A storage plant for at least electrical energy and/or mechanical energy and possibly also thermal energy, comprising:

(i) three storage tanks of a working fluid, comprising a feed tank (FeT), an intermediate tank (IT), and a final tank (FiT);

(ii) a plurality of circuital means configured to move the mass of said working fluid among said storage tanks (FeT, IT, FiT) and comprising at least a plurality of adiabatic machines operating with said two-phase working fluid, i.e., consisting of the vapor and liquid phases (Ri, F , NRi, NR2, NR3, NR4) whereby during a charging process, the circulation of said working fluid occurs through said circuital means from said feed tank (FeT) towards said intermediate tank (IT) and also towards said final tank (FiT) to perform an absorption/accumulation of electrical energy and/or mechanical energy and possibly also thermal energy supplied from external sources, and during a discharging process, the circulation of said working fluid occurs through said circuital means from said final tank (FiT) and also from said intermediate tank (IT) towards said feed tank (FeT) to carry out the conversion of said previously stored energy into electric energy and/or mechanical energy supplied to an end-user and possibly also thermal energy supplied to an end-user.

2. The plant according to claim 1 , wherein at least one of said three storage tanks of said working fluid (FeT, IT, FiT) is configured to maintain constant over time (in said charging and discharging processes and also during a time interval between them) the thermodynamic state of said working fluid.

3. The plant according to each of claims 1 and 2, wherein said adiabatic two-phase fluid machines (R1, R2, NR1, NR2, NR3, NR4) comprise a combination selected from the following:

(i) two reversible adiabatic two-phase fluid machines, in particular a first reversible adiabatic two-phase fluid machine (R1) in fluid communication with said feed tank (FeT) and a second reversible adiabatic two- phase fluid machine (R2) in fluid communication with said final tank (FiT), each of said two machines (R1, R2) being configured to perform: - in said charging process, an adiabatic compression of said two-phase working fluid, i.e., the conversion of the electrical energy and/or mechanical energy supplied from an external source into the increase in potential pressure energy and thermal energy of said fluid; - in said discharging process, an adiabatic expansion of said two-phase working fluid, i.e., the conversion of said potential pressure energy and said thermal energy of said fluid into electrical energy and/or mechanical energy supplied to the enduser;

(ii) four non-reversible adiabatic two-phase fluid machines including a first non-reversible adiabatic two- phase fluid machine (NR1), a second non-reversible adiabatic two-phase fluid machine (NR2), a third non- reversible adiabatic two-phase fluid machine (NR3), and a fourth non-reversible adiabatic two-phase fluid machine (NR4), wherein said first machine (NR1) and second machine (NR2) in fluid communication with said feed tank (FeT) and said final tank (FiT), respectively, both configured to exclusively perform an adiabatic compression of said two-phase working fluid in said charging process, said third machine (NR3) and said fourth machine (NR4) in fluid communication with said final tank (FiT) and said feed tank (FeT), respectively, both configured to exclusively perform an adiabatic expansion of said two-phase working fluid in said discharging process.

4. The plant according to each of claims 1 and 2, wherein said adiabatic two-phase fluid machines (R1, R2, NR1, NR2, NR3, NR4) comprise a combination selected from the following:

(i) two reversible adiabatic two-phase fluid machines, in particular a first reversible adiabatic two-phase fluid machine (R1) in fluid communication with said feed tank (FeT) and configured to perform an adiabatic expansion of said two-phase working fluid in said charging process and an adiabatic compression of said two-phase working fluid in said discharging process, and a second reversible adiabatic two-phase fluid machine (R2) in fluid communication with said final tank (FiT) and configured to perform an adiabatic compression of said two-phase working fluid in said charging process and an adiabatic expansion of said two-phase working fluid in said discharging process, and wherein said circuital means further optionally comprise an isenthalpic valve (IV) in fluid communication with said feed tank (FeT) and configured to perform said adiabatic expansion of said two-phase working fluid in said charging process instead of said first reversible adiabatic two-phase fluid machine (R1), which is consequently configured to exclusively perform said adiabatic compression of said two-phase working fluid in said discharging process;

(ii) four non-reversible adiabatic two-phase fluid machines, including a first non-reversible adiabatic two- phase fluid machine (NR1), a second non-reversible adiabatic two-phase fluid machine (NR2), a third non- reversible adiabatic two-phase fluid machine (NR3), and a fourth non-reversible adiabatic two-phase fluid machine (NR4), wherein: said first non-reversible adiabatic two-phase fluid machine (NR1) in fluid communication with said feed tank (FeT) and said second non-reversible adiabatic two-phase fluid machine (NR2) in fluid communication with said final tank (FiT) configured to exclusively perform an adiabatic expansion of said two-phase working fluid and exclusively an adiabatic compression of the two-phase working fluid in said charging process, respectively; said third non-reversible adiabatic two-phase fluid machine (NR3) in fluid communication with said final tank (FiT) and said fourth non-reversible adiabatic two- phase fluid machine (NR4) in fluid communication with said feed tank (FeT), configured to exclusively perform an adiabatic expansion of said two-phase working fluid and exclusively an adiabatic compression of said two-phase working fluid in said discharging process, respectively; and wherein said circuital means further optionally comprise an isenthalpic valve (IV) in fluid communication with said feed tank (FeT) and configured to perform said adiabatic expansion of said two-phase working fluid in said charging process instead of said first non-reversible adiabatic two-phase fluid machine (NRi).

5. The plant according to each of claims from 1 to 4, wherein said circuital means further comprise at least a first heat exchanger (HEi) configured for the heat exchange between two parts of the same working fluid (a) and (b), being in fluid communication in said charging and discharging processes with said reversible adiabatic two-phase fluid machines (Ri, R2) or said non-reversible adiabatic two-phase fluid machines (NR1, NR2, NR3, NR4) and said intermediate tank (IT).

6. The plant according to claim 5, wherein said circuital means further comprise at least one second heat exchanger (HE2) configured for the heat exchange between said working fluid and the external environment in said discharging process, being in fluid communication with said feed tank (FeT) and with at least one of said reversible adiabatic two-phase fluid machines (R1) or of said non-reversible adiabatic two-phase fluid machines (NR4) configured to perform the adiabatic expansion of said two-phase working fluid according to claim 3, or the adiabatic compression of said two-phase working fluid according to claim 4.

7. The plant according to claim 5 or 6, wherein the heat exchange between said two parts of the working fluid (a) and (b), in said first heat exchanger (HE1), comprises a combination selected from the following:

(i) a first side of said first heat exchanger (HE1), operating as an evaporator for the absorption of thermal energy in said charging process, and as a condenser for providing thermal energy in said discharging process, is in fluid communication with said reversible adiabatic two-phase fluid machines (R1, R2) and said non-reversible adiabatic two-phase fluid machines (NR1, NR2, NR3, NR4); and a second side of said first heat exchanger (HE1), operating as a condenser for providing thermal energy in said charging process, and an evaporator for the absorption of thermal energy in said discharging process, is in fluid communication with said intermediate tank (IT);

(ii) a first side of said first heat exchanger (HE1), operating as an evaporator for the absorption of thermal energy, is in fluid communication with said reversible adiabatic two-phase fluid machines (R1, R2) and said non-reversible adiabatic two-phase fluid machines (NR1, NR2) through a bypass system in said charging process and in fluid communication with said intermediate tank (IT) in said discharging process through the same bypass system; and a second side of said first heat exchanger (HE1), operating as a condenser for providing thermal energy, is in fluid communication with said intermediate tank (IT) in said charging process through said bypass system and in fluid communication with said reversible adiabatic two-phase fluid machines (Ri, F ) and said non-reversible adiabatic two-phase fluid machines (NR3, NR4) in said discharging process through said bypass system.

8. The plant according to each of claims from 1 to 7, wherein said first heat exchanger (HE1) is further configured to allow the possible absorption of thermal energy supplied from an external thermal source during said charging process.

9. The plant according to each of claims from 5 to 8, wherein said circuital means further comprise a fluid separation/mixing device (SM) in fluid communication both with said intermediate tank (IT) and with said reversible adiabatic two-phase fluid machines (R1, R2) and with said non-reversible adiabatic two-phase fluid machines (NR1, NR2, NR3, NR4), configured to perform, instead of said heat exchange between said two parts of the working fluid (a) and (b) carried out through said first heat exchanger (HE1) in said charging process, the separation of the overall flow rate of the working fluid (a*+b*) exiting said reversible adiabatic two-phase fluid machines (R1) or said non-reversible adiabatic two-phase fluid machines (NR1) in a first part of the working fluid flow rate (a*) in the wet saturated vapor phase and a second part of the working fluid flow rate (b*) in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase having a quality lower than the quality of said first part of the working fluid flow rate (a*), said first part (a*) circulating in at least one of said reversible adiabatic two-phase fluid machines (R2) or said non-reversible adiabatic two-phase fluid machines (NR2) in fluid communication with said final tank (FiT) and configured to perform said adiabatic compression of the two-phase working fluid, said second part (b*) circulating in said intermediate tank (IT).

10. The plant according to each of claims from 5 to 9, wherein said circuital means further comprise a fluid separation/mixing device (SM) in fluid communication both with said intermediate tank (IT) and with said reversible adiabatic two-phase fluid machines (R1, R2) and with said non-reversible adiabatic two-phase fluid machines (NR1, NR2, NR3, NR4), configured to perform, instead of said heat exchange between said two parts of the working fluid (a) and (b) carried out through said first heat exchanger (HE1) in said discharging process, the mixing of said first part of the working fluid flow rate (a*) in the wet saturated vapor phase at the outlet section of one of said reversible adiabatic two-phase fluid machines (R2) or of one of said non- reversible adiabatic two-phase fluid machines (NR3) in fluid communication with said final tank (FiT) and configured to perform said adiabatic expansion of the two-phase working fluid, and said second part of the working fluid flow rate (b*) in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase at the outlet section of said intermediate tank (IT).

11 . The plant according to any of claims from 5 to 10, comprising an adiabatic compression means placed downstream of said intermediate tank (IT), configured to be activated in said discharging process by a fraction of the electrical energy or mechanical energy generated during said discharging process, in order to determine the pressure increase of said second part of the working fluid (b or b*) at the outlet section of said intermediate tank (IT).

12. The plant according to any of claims from 5 to 11 , comprising a thermal energy storage medium of the sensible heat type, or latent heat or thermochemical types, configured for the heat exchange with said first part of the working fluid (a or a*) and being: placed in said charging process between one of said reversible adiabatic two-phase fluid machines (F ) or one of said non-reversible adiabatic two-phase fluid machines (NR2) and said final tank (FiT), so that said first part of the working fluid (a or a*) at the outlet of said reversible adiabatic two-phase fluid machines (R2) or of said non-reversible adiabatic two-phase fluid machines (NR2) first circulates in said thermal energy storage medium performing the supply of the thermal energy at constant pressure and finally is stored in said final tank (FiT) in subcooled liquid phase or saturated liquid phase; located in said discharging process between one of said reversible adiabatic two- phase fluid machines (R2) or one of said non-reversible adiabatic two-phase fluid machines (NR3) and said final tank (FiT), whereby said first part of the working fluid (a or a*) at the outlet section of said final tank (FiT) in the subcooled liquid phase or saturated liquid phase first circulates in said thermal energy storage medium carrying out the absorption of thermal energy at constant pressure and finally circulates in said reversible adiabatic two-phase fluid machine (R2) or in said non-reversible adiabatic two-phase fluid machines (NR3).

13. Storage method of at least electrical energy and/or mechanical energy and possibly also thermal energy, which provides for:

(i) storing a working fluid in three storage tanks, comprising a feed tank (FeT), an intermediate tank (IT), and a final tank (FiT);

(ii) moving the mass of said working fluid among said storage tanks (FeT, IT, FiT) by carrying out adiabatic transformations of said two-phase working fluid, i.e., consisting of the vapor and liquid phases so that during a charging process, the circulation of said working fluid occurs from said feed tank (FeT) towards said intermediate tank (IT) and also towards said final tank (FiT) in order to carry out an absorption/accumulation of electric energy and/or mechanical energy and possibly also thermal energy supplied from an external source, and during a discharging process, the circulation of said working fluid occurs from said final tank (FiT) and also from said intermediate tank (IT) towards said feed tank (FeT) in order to convert said previously stored energy into electrical energy and/or mechanical energy supplied to an end-user and possibly also thermal energy supplied to an end-user.

14. Method according to claim 13, wherein in the storage of said working fluid in at least one of said three storage tanks (FeT, IT, FiT), the thermodynamic state of said working fluid remains constant over time (in said charging and discharging processes and also during a time interval between them).

15. Method according to each of claims 13 and 14, wherein said adiabatic transformations of said two- phase working fluid comprise a combination selected from the following:

(i) an adiabatic compression transformation of said two-phase working fluid fed from said feed tank (FeT) and an adiabatic compression transformation of said two-phase working fluid fed towards said final tank (FiT) are performed during said charging process for the conversion of the electrical energy and/or mechanical energy supplied from an external source into an increase in the potential pressure energy and thermal energy of said fluid; an adiabatic expansion transformation of said two-phase working fluid fed from said final tank (FiT) and an adiabatic expansion transformation of said working fluid fed towards said feed tank (FeT) are performed during said discharging process for the conversion of said potential pressure energy and said thermal energy of said fluid into electrical energy and/or mechanical energy supplied to the end-user; wherein said adiabatic compression transformation of said two-phase fluid fed from said feed tank (FeT) and said adiabatic expansion transformation of said two-phase fluid fed towards said feed tank (FeT) are performed in the charging and discharging processes, respectively, both in a same antecedent reversible adiabatic two-phase fluid machine and wherein said two-phase adiabatic compression transformation of said two-phase fluid fed towards said final tank (FiT) and said two-phase adiabatic expansion transformation of said two-phase fluid fed from said final tank (FiT) are performed in the charging and discharging processes, respectively, both in the same subsequent reversible adiabatic two-phase fluid machine, the last one different from said antecedent reversible adiabatic two-phase fluid machine;

(ii) an adiabatic compression transformation of said two-phase working fluid fed from said feed tank (FeT) and an adiabatic compression transformation of said two-phase working fluid fed towards said final tank (FiT) are performed during said charging process in the respective antecedent non-reversible adiabatic two-phase fluid machines; an adiabatic expansion transformation of said two-phase working fluid fed from said final tank (FiT) and an adiabatic expansion transformation of said two-phase working fluid fed towards said feed tank (FeT) are performed during said discharging process in the respective successive non-reversible adiabatic two-phase fluid machines, the last ones different from said antecedent non-reversible adiabatic two-phase fluid machines.

16. Method according to each of claims 13 and 14, wherein said adiabatic transformations of said two- phase working fluid comprise a combination selected from the following:

(i) an adiabatic expansion transformation of said two-phase working fluid fed from said feed tank (FeT) and an adiabatic compression transformation of said two-phase working fluid fed towards said feed tank (FeT) are performed in said charging and discharging processes, respectively, both in a same antecedent adiabatic reversible two-phase fluid machine; an adiabatic compression transformation of said two-phase working fluid fed towards said final tank (FiT) and an adiabatic expansion transformation of said two-phase working fluid fed from said final tank (FiT) are performed during said charging and discharging processes, respectively, both in a same subsequent reversible adiabatic two-phase fluid machine, the last one different from said antecedent reversible adiabatic two-phase fluid machine; and optionally an isenthalpic expansion transformation of said two-phase working fluid performed in an isenthalpic valve instead of said adiabatic expansion transformation in said antecedent reversible adiabatic two-phase fluid machine;

(ii) an adiabatic expansion transformation of said two-phase working fluid fed from said feed tank (FeT) and an adiabatic compression transformation of said working fluid fed towards said final tank (FiT) are performed during said charging process in the respective antecedent non-reversible adiabatic two-phase fluid machines; an adiabatic compression transformation of said two-phase working fluid fed towards said feed tank (FeT) and an adiabatic expansion transformation of said two-phase working fluid fed from said final tank (FiT) are performed during said discharging process in the respective subsequent non-reversible adiabatic two-phase fluid machines, the last ones distinct from said antecedent non-reversible adiabatic two- phase fluid machines; and optionally an isenthalpic expansion transformation of said two-phase working fluid performed in an isenthalpic valve instead of said adiabatic expansion transformation in said antecedent non- reversible adiabatic two-phase fluid machine.

17. Method according to each of claims from 13 to 16, wherein a heat exchange is carried out between two parts of the same working fluid flow rate (a) and (b), through at least one first heat exchanger: - wherein the heat absorption process of said part of the working fluid flow rate (a) fed by said adiabatic two-phase fluid compression transformation and fed towards said adiabatic two-phase fluid compression transformation and the heat supply process of said part of the working fluid flow rate (b) fed towards said intermediate tank (IT) occur in said charging process, and where the heat absorption process of said part of the working fluid flow rate (b) fed by said intermediate tank (IT) and the heat supply process of said part of the working fluid flow rate (a) fed by said adiabatic two-phase fluid expansion transformation and fed towards said adiabatic two- phase fluid expansion transformation occur in said discharging process; - wherein the heat absorption process of said part of the working fluid flow rate (a) fed from said adiabatic two-phase fluid expansion transformation and fed towards said adiabatic two-phase fluid compression transformation and the heat supply process of said part of the working fluid flow rate (b) fed towards said intermediate tank (IT) occur in said charging process, and where the heat absorption process of said part of the working fluid flow rate (b) fed by said intermediate tank (IT) and the heat supply process of said part of the working fluid flow rate (a) fed from said adiabatic two-phase fluid expansion transformation and fed towards said adiabatic two-phase fluid compression transformation occur in said discharging process.

18. The method according to claim 17, wherein, instead of said heat exchange between said two parts of the working fluid flow rate (a) and (b) performed through said first heat exchanger in said charging process, the separation between a first part of the working fluid flow rate (a*) in the wet saturated vapor phase and a second part of the working fluid flow rate (b*) in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase having a quality lower than the quality of said first part of the working fluid flow rate (a*), through a fluid separation device where: - said overall working fluid flow rate (a* + b*) is fed from said adiabatic two-phase fluid compression transformation, said first part of the working fluid flow rate (a*) is fed towards said adiabatic two-phase fluid compression transformation and said second part of the working fluid flow rate (b*) is fed towards said intermediate tank (IT); - said overall working fluid flow rate (a* + b*) is fed from said adiabatic two-phase fluid expansion transformation, said first part of the working fluid flow rate (a*) is fed towards said adiabatic two-phase fluid compression transformation and said second part of the working fluid flow rate (b*) is fed towards said intermediate tank (IT).

19. The method according to any of claims 17 or 18, wherein, instead of said heat exchange between said two parts of the working fluid flow rate (a) and (b) performed through said first heat exchanger in said discharging process, the mixing between a first part of the working fluid flow rate (a*) in the wet saturated vapor phase and a second part of the working fluid flow rate (b*) in the saturated liquid phase or subcooled liquid phase or wet saturated vapor phase having a quality lower than the quality of said first part of the working fluid flow rate (a*), through a fluid mixing device where: - said first part of the working fluid flow rate (a*) is fed from said adiabatic two-phase fluid expansion transformation and said second part of the working fluid flow rate (b*) is fed from said intermediate tank (IT) and said overall working fluid flow rate (a* + b*) is fed towards said adiabatic two-phase fluid expansion transformation; - said first part of the working fluid flow rate (a*) is fed from said adiabatic two-phase fluid expansion transformation and said second part of the working fluid flow rate (b*) is fed from said intermediate tank (IT) and said overall working fluid flow rate (a* + b*) is fed towards said adiabatic two-phase fluid compression transformation.

20. Method according to any of claims from 17 to 19, comprising an adiabatic compression transformation fed from said intermediate tank (IT), and activated in said discharging process through a fraction of the electrical or mechanical energy generated during said discharging process, to determine the increase in pressure of said second part of the working fluid (b or b*) exiting said intermediate tank (IT).

21 . Method according to any of claims from 17 to 20, comprising a thermal energy exchange between said first part of the working fluid (a or a*) and a thermal storage medium of sensible heat type, or latent heat type, or thermochemical type, said exchange occurring: in said charging process between one of said reversible adiabatic two-phase fluid machines (F ) or one of said non-reversible adiabatic two-phase fluid machines (NR2) and said final tank (FiT), so that said first part of the working fluid (a or a*) at the outlet of said reversible adiabatic two-phase fluid machines (R2) or of said non-reversible adiabatic two-phase fluid machines (NR2) first circulates in said thermal energy storage medium performing the supply of the thermal energy at constant pressure and finally is stored in said final tank (FiT) in subcooled liquid phase or saturated liquid phase; and in said discharging process between one of said reversible adiabatic two-phase fluid machines (R2) or one of said non-reversible adiabatic two-phase fluid machines (NR3) and said final tank (FiT), whereby said first part of the working fluid (a or a*) at the outlet section of said final tank (FiT) in the subcooled liquid phase or saturated liquid phase first circulates in said thermal energy storage medium carrying out the absorption of thermal energy at constant pressure and finally circulates in said reversible adiabatic two-phase fluid machine (R2) or in said non-reversible adiabatic two-phase fluid machines (NR3).