Energy storage system with stand-by operation mode and method for operating the system

Abstract

An energy storage system (100) comprising a compressor (10), a fluid storage (40), an expander (20) and a generator (15) which is mechanically coupled to the expander (20) and is electrically coupled to an electric grid in order to supply power electricity to the electric grid. The compressor (1) is fluidly coupled to the fluid storage (40) and is configured to compress a first fluid flow and supply it to the fluid storage (40) during a charge mode of the energy storage system; the fluid storage (40) is configured to store the first fluid at least for a predetermined time. During a discharge mode of the energy storage system, the expander (20) is fluidly coupled to the fluid storage (40) so that the fluid storage (40) can supply a second fluid flow at a first temperature at an expander inlet (21) and the expander (20) can expand the second fluid flow according to a first pressure ratio so to discharge the second fluid flow at a second temperature at an expander outlet (29) and to drive the generator (15), so that it is synchronized with power electricity of the electric grid. The energy storage system further comprises a stand-by unit (50) which is fluidly coupled to the expander (20) and is configured to supply a third fluid flow at a third temperature at the expander inlet (21) during a stand-by mode of the energy storage system. During a stand-by mode of the energy storage system, the expander (20) is further configured to expand the third fluid flow according to a second pressure ratio so to discharge the third fluid flow at a fourth temperature (T4) at the expander outlet (29) and to drive the generator (15), so that it is synchronized with power electricity of the electric grid. The first temperature and the second temperature of the second fluid flow are substantially equal to the third temperature and the fourth temperature of the third fluid flow.

Description

TITLEEnergy storage system with stand-by operation mode and method for operating the systemDESCRIPTIONTECHNICAL FIELD

[0001] The subject-matter disclosed herein relates to an innovative energy storage system and a method for operating the innovative energy storage system.BACKGROUND ART

[0002] In order to improve reliability and profitability of energy systems and to meet the requirements between the supply and demand of electric energy, energy storage systems are studied and developed. Nowadays, battery energy storage systems (i.e. the use a group of batteries to store electrical energy) are the fastest responding dispatchable energy storage technology. However, battery energy storage systems have drawbacks such as for example the degradation of the storage capacity over the life of the system and the use of critical raw materials from the point of view of supply, environmental compatibility and safety (for example the use of lithium).

[0003] Promising energy storage solutions are also in mechanical form, such as for example storage in compressed air (Compressed Air Energy Storage - “CAES”) and storage in liquid air (Liquid Air Energy Storage - “LAES”). Mechanical energy storage solutions are promising especially in case of large size and long duration storage needs. Typically, these plants comprise a compressor driven by a motor, a tank for fluid storage and an expansion turbine driving a generator for power production. In general, ambient air or another gas is compressed and / or liquified through the compressor and stored under pressure in a suitable tank; when electricity is required, the pressurized fluid is heated and expanded through the expander which drives a generator forsupplying energy to a grid. Advantageously, the heat generated during compression can be stored and used to heat pressurized fluid before expansion.

[0004] However, turbomachinery used in mechanical energy storage systems have long response time (i.e. time required to reach full load) due to the fact that the run-up of the expander (to increase the rotating speed) and the synchronization of the expander to the grid require technical times. Moreover, it is also to be noted that the turbomachinery used in mechanical energy storage systems have long response time also due to the fact that there is the need to keep thermal stresses within a certain limit, especially for turbomachinery working at high temperature, for example temperatures higher than 150°C.

[0005] It is known that the run-up can be speeded up for example by supplying hot air to the expander during the standstill operation, i.e. when the expander is not working, for example during a charge mode of the energy storage system or during a switched-off mode of the energy storage system. However, even if by adopting this measure the run-up can be speeded up (since thermal stresses into the turbomachine are lowered if compared to a design with no heating during standstill), thermal stresses during run-up cannot be null as the temperature profile of the metal parts of the expander would be different from the nominal - or design - one (i.e. the temperature profile when the expander is synchronized to the grid); in fact, supplying an hot air flow into the expander would lead to an almost constant temperature profile inside the turbomachine (from the inlet to the outlet of the turbomachine) which is very different from the temperature profile that the turbomachine would have when the expander is working (i.e. during a discharge mode of the energy storage system) since the temperature profile would not be constant from the inlet to the outlet of the turbomachine. Therefore, even by supplying hot air to the expander during the standstill operation, the thermal stresses still impact the useful life of the expander.

[0006] Therefore, it would be desirable to have an energy storage system, in particular a mechanical energy storage system, which may provide energy toan electric grid (i.e. may be switched in a discharge mode) with a fast response, in particular minimizing the time necessary to reach 100% load during the discharge mode, hence limiting the time needed to synchronize the expander with the power electricity of the electric grid. Moreover, it would be desirable to have an energy storage system which has low thermal stresses, possibly null, on the expander (even with steep load ramp), in particular when the system is switched to the discharge mode, in particular to increase the expander life.SUMMARY

[0007] According to an aspect, the subject-matter disclosed herein relates to an energy storage system comprising a compressor, a fluid storage, an expander and a generator which is mechanically coupled to the expander and is electrically coupled to an electric grid in order to supply power electricity to the electric grid. The compressor is fluidly coupled to the fluid storage and is configured to compress a first fluid flow and supply it to the fluid storage during a charge mode of the energy storage system; the fluid storage is configured to store the first fluid at least for a predetermined time. During a discharge mode of the energy storage system, the expander is fluidly coupled to the fluid storage so that the fluid storage can supply a second fluid flow at a first temperature at an expander inlet and the expander can expand the second fluid flow according to a first pressure ratio so to discharge the second fluid flow at a second temperature at an expander outlet and to drive the generator, so that the power electricity supplied by the generator has substantially the same voltage frequency of the power electricity of the electric grid. The energy storage system further comprises a stand-by unit which is fluidly coupled to the expander and is configured to supply a third fluid flow at a third temperature at the expander inlet during a stand-by mode of the energy storage system. During a stand-by mode of the energy storage system, the expander is configured to expand the third fluid flow according to a second pressure ratio so to discharge the third fluid flow at a fourth temperature at the expander outlet and to drive the generator so the power electricity supplied by the generator has substantially the same voltage frequency of the power electricityof the electric grid. The first temperature and the second temperature of the second fluid flow are substantially equal to the third temperature and the fourth temperature of the third fluid flow.

[0008] According to another aspect, the subject-matter disclosed herein relates to a method for operating an energy storage system comprising the following steps:A. During a discharge mode of the energy storage system, supplying a second fluid flow to an expander, the expander being configured to expand the second fluid flow and to drive a generator electrically coupled to an electric grid, so that the voltage frequency of the power electricity of the generator is synchronized with the voltage frequency of power electricity of the electric grid in order to supply power electricity to the electric grid;B. During a stand-by mode of the energy storage system, maintaining the voltage frequency of the power electricity of the generator synchronized with voltage frequency of the power electricity of the electric grid by supplying through a stand-by unit a third fluid flow to the expander, the third fluid flow having inlet temperature and, outlet temperature and pressure ratio across the expander substantially equal to inlet temperature and, outlet temperature and pressure ratio across the expander of the second fluid flow during the discharge mode of the energy storage system.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:Fig. 1 shows a general schematic diagram of a first embodiment of aninnovative energy storage system implementing a closed-loop stand-by cycle,Fig. 2 shows a detailed schematic diagram of a second embodiment of an innovative energy storage system implementing a closed-loop stand-by cycle,Fig. 3 shows a detailed schematic diagram of a third embodiment of an innovative energy storage system implementing an open-loop stand-by cycle, andFig. 4 shows on a T-s plot an example of the expansion of air during a discharge mode of an innovative energy storage system and an example of expansions of air during a stand-by mode of an innovative energy storage system implementing a closed-loop stand-by cycle and an open-loop stand-by cycle.DETAILED DESCRIPTION OF EMBODIMENTS

[0010] According to an aspect, the subject-matter disclosed herein relates to an energy storage system having at least one expander which is coupled to an electric generator during a discharge mode of the system so to provide power electricity to an electric grid when it is required. The energy storage system has an additional loop (referred also as “stand-by unit”) which is coupled to the inlet and to the outlet of the expander during a stand-by mode of the system, i.e. when the system is turned on but is not in a discharging condition. The additional loop processes and recirculates the fluid from the outlet to the inlet so that the temperatures of the fluid at the expander inlet and at the expander outlet are the same of the temperatures of the fluid at the expander inlet and at the expander outlet when the system is in a discharge mode. By recirculating a small mass flow through the expander during the stand-by mode of the system, the pressure of the fluid at the expander inlet can be lower with respect to the pressure of the fluid at the expander inlet during discharge mode and therefore the energy required to implement the stand-by loop or to operate the stand-by unit during stand-by mode can be low. Thanks to the additional loop, the thermal stresses of the expander are limited, possibly eliminated, since the temperature profile of the expander when the system works in the stand-bymode is kept equal to the temperature profile of the expander when the system is in the discharge mode. Furthermore, the energy storage system has advantageously much shorter start-up time when switched in the discharge mode with respect to known mechanical energy storage technologies.

[0011] In general, known mechanical energy storage systems have two operating modes: a “charge mode” in which the compressor processes a fluid so to store pressurized fluid into a fluid storage, and a “discharge mode” in which the expander receives the fluid stored into the fluid storage and processes it, so to generate mechanical work to be converted into electric power by an electric generator. With the state-of-the art technology, expanders are maintained at standstill when the system is not discharging energy, being energetically unaffordable to maintain them ready at substantially full speed and no load.

[0012] As it will be better described in the following, the innovative energy storage system according to the present disclosure has further another operating mode, called in the following “stand-by mode”, provided by a standby unit implemented in the system in which the expander is maintained synchronized with the grid, with the aim to have a much faster response when the energy storage plant initiates the discharge phase.

[0013] Reference now will be made in detail to embodiments of the disclosure, examples of which are illustrated in the drawings. The examples and drawing figures are provided by way of explanation of the disclosure and should not be construed as a limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. In the following description, similar reference numerals are used for the illustration of figures of the embodiments to indicate elements performing the same or similar functions. Moreover, for clarity of illustration, some references may be not repeated in all the figures.

[0014] Fig. 1 shows a simplified diagram of a first embodiment of aninnovative energy storage system 100 implementing a closed-loop stand-by cycle, referred in the following as “system 100”. A second and a third embodiment of an innovative energy storage system 200 and 300 will be described in the following with the aid of Fig. 2 and Fig. 3 respectively.

[0015] With non-limiting reference to Fig. 1, the system 100 comprises a compressor 10, a fluid storage 40, an expander 20 and a generator 15. For the sake of clarity, it is to be noted that Fig. 1 shows only one compressor 10 and one expander 20. However, the compressor 10 and / or the expander 20 may have multiple stages which are divided into different turbomachinery; for example, the expander may have a high-pressure stage (e.g. having an inlet pressure about 55-50 bar), an intermediate-pressure stage (e.g. having an inlet pressure about 25-20 bar) and a low-pressure stage (e.g. having an inlet pressure about 8-7 bar).

[0016] The generator 15 is mechanically coupled to the expander 20; in particular, the generator 15 is configured to transform mechanical work generated by the expander 20 into electric energy. The generator 15 is further electrically coupled to an electric grid in order to supply power electricity to the electric grid (see the big black arrow in Fig. 1). It is to be noted that the electric grid may be a national electric grid or a stand-alone grid. In particular, as it will be better described in the following, during the discharge mode of the system 100, the expander 20 generates mechanical work by expanding a fluid and drives the generator 15 so that it is synchronized with the electric grid. It is to be noted that, for the purpose of the present invention, the synchronization between the generator and the electric grid means that the power electricity supplied by the generator has substantially the same voltage frequency of the power electricity of the electric grid (while voltage amplitude may be different; for example, if the electric grid is the Italian national grid, the voltage frequency of the power electricity is 50 Hz.

[0017] With non-limiting reference to Fig. 1, the compressor 10 is driven by a motor 14 and is configured to compress a first fluid flow. In particular, the compressor 10 receives the first fluid flow at a compressor inlet 11, processesit and discharge a compressed first fluid flow at a compressor outlet 19. During a charge mode of the energy storage system 100, the compressor 10, in particular the compressor outlet 19, is fluidly coupled to the fluid storage 40 and is configured to supply the first fluid flow to the fluid storage 40. In other words, during the charge mode of the system 100 the motor 14 drives the compressor 10 so to supply the first fluid flow, in particular the compressed first fluid flow, to the fluid storage 40; the fluid storage 40 is configured to store the first fluid at least for a predetermined time, e.g. for some hours.

[0018] During a discharge mode of the energy storage system 100, the expander 20, in particular the expander inlet 21, is fluidly coupled to the fluid storage 40 so that a second fluid flow at a first temperature Tin is supplied to the expander 20. The expander 20 is configured to expand the second fluid flow according to a first pressure ratio so to discharge the second fluid flow at a second temperature Tout at an expander outlet 29 and generate mechanical energy. The generator 15 is mechanically coupled to the expander 20 so that it can convert mechanical energy into electrical energy. In other words, the expander 20 drives the generator 15 so that the power electricity supplied by the generator 15 has substantially the same the voltage frequency of the power electricity of the electric grid.

[0019] The innovative system 100 further comprises a stand-by unit 50, in particular a stand-by unit 50 configured to implement a stand-by cycle (which can be a closed-loop stand-by cycle, see e.g. Fig. 1 and Fig. 2, or an open-loop stand-by cycle, see e.g. Fig. 3) during a stand-by mode of the system 100. As it will be apparent from the following, the stand-by mode of the system 100 can be implemented when the system 100 is not in the discharge mode, i.e. when the system 100 is in the charge mode or when the system is neither in the discharge mode or in the charge mode.

[0020] During the stand-by mode of the system 100, the stand-by unit 50 is fluidly coupled to the expander 20, in particular to the expander inlet 21, and is configured to supply at the expander inlet 21 a third fluid flow at a third temperature T3. It is to be noted that the first fluid flow and / or the second fluidflow and / or the third fluid flow may be the same fluid or different fluids. Advantageously, the first fluid flow and / or the second fluid flow and / or the third fluid flow have different mass flow; in particular, the mass flow of the third fluid flow is much lower than the mass flow of the second fluid flow; for example, it is 80% lower that the mass flow of the second fluid flow.

[0021] During the stand-by mode of the system 100, the expander 20 is configured to expand the third fluid flow according to a second pressure ratio so to discharge the third fluid flow at a fourth temperature T4 at the expander outlet 29 and to drive the generator 15 so that it is synchronized with power electricity of the electric grid. In order to minimize thermal stresses of the expander 20, the first temperature Tin and the second temperature Tout of the second fluid flow during the discharge mode of the system 100 are substantially equal to the third temperature T3 and the fourth temperature T4 of the third fluid flow during the stand-by mode of the system 100; advantageously, the first temperature Tin and the second temperature Tout are equal to the third temperature T3 and the fourth temperature T4. It is to be noted that if the fluid has an ideal gas behavior (i.e. the ideal gas law may be applied), also the first pressure ratio of the second fluid flow is substantially equal to the second pressure ratio of the third fluid flow.

[0022] According to a possibility, the first fluid flow and / or the second fluid flow and / or the third fluid flow is air, in particular ambient air, or CO2, in particular CO2 in supercritical conditions. For example, the innovative energy storage system 100 may be a Liquid Air Energy Storage (=LAES) system or a Compressed Air Energy Storage (=CAES) system.

[0023] With non-limiting reference to Fig. 1, the stand-by unit 50 comprises a stand-by compressor 60 and a stand-by motor mechanically coupled to the stand-by compressor 60 so to drive the stand-by compressor 60. The stand-by compressor 60 is fluidly coupled to the expander 20 so that the third fluid flow at the expander outlet 29 is supplied to a stand-by compressor inlet 61, it is pressurized by the stand-by compressor 60 and discharged at a stand-by compressor outlet 69. As already mentioned above, the stand-by unit 50 mayimplement a closed-loop stand-by cycle; hence, the stand-by compressor outlet 69 may be further fluidly coupled to the expander inlet 21 and the third fluid flow discharged at the stand-by compressor outlet 69 is recirculated to the expander 20 (see for example Fig. 1 and Fig. 2).

[0024] Advantageously, the expander 20 further comprises at least one dry gas seal, in particular if the expander 20 is an overhung expander, configured to minimize leakage of ambient air into the expander 20 during the stand-by mode of the energy storage system 100. Preferably, if the expander 20 is not an overhung expander, the expander 20 has two dry gas seals located respectively at the two axial ends of the expander 20. This is particularly advantageous since, during the stand-by mode of the system 100, the pressure at the expander outlet 29 is lower than ambient pressure. Advantageously, also the pressure of the third fluid flow at the expander inlet 21 may be lower than ambient pressure (please note that this condition is very useful to reduce the power required by the stand-by unit 50 but can be achieved only if the standby unit 50 implements a closed-loop stand-by cycle). Advantageously, the expander 20 further comprises at least one magnetic bearing, in particular a radial magnetic bearing, preferably two magnetic bearings located at the axial ends of the expander 20, the magnetic bearing(s) being configured to radially support the expander 20. The use of magnetic bearing(s) is particularly advantageous for having very low mechanical losses, thus decreasing the power necessary to run in stand-by mode.

[0025] With non-limiting reference to Fig. 1, the stand-by unit 50 further comprises a heat removal unit 81 located downstream the stand-by compressor 60 and fluidly coupled to the stand-by compressor outlet 69 so that the heat removal unit 81 may remove heat from the third fluid flow before being supplied to the expander inlet 21. In other words, the heat removal unit 81 is configured to remove heat from the third fluid flow in order to supply at the expander inlet 21 the third fluid flow at the third temperature T3.

[0026] A second embodiment of an innovative energy storage system 200 will be described in the following with the aid of Fig. 2. It is to be noted thatelements 250, 260, 261, 269, 270 and 281 in Fig. 2 may be identical or similar respectively to elements 50 (stan-by unit), 60 (stand-by compressor), 61 (stand-by compressor inlet), 69 (stand-by compressor outlet), 70 (stand-by motor) and 81 (heat removal unit) in Fig. 1 and perform the same or similar functions. It is also to be noted that also the energy storage system 200 of Fig. 2 is implementing an open-loop stand-by cycle similar to the one shown in Fig. 1.

[0027] The system 200 shown in Fig. 2 is similar to the system 100 shown in Fig. 1 and operates similarly to the system 100 shown in Fig. 1. With respect to the system 100 shown in Fig. 1, the system 200 further comprises a thermal storage 35 and two heat exchanger 31 and 32. In particular, the first heat exchanger 31 is located downstream the compressor 10 and is fluidly coupled to the compressor outlet 19 and the second exchanger 32 is located upstream the expander 20 and is fluidly coupled to the expander inlet 21. The first heat exchanger 31 is configured to remove heat from the first fluid flow discharged by the compressor outlet 19 before being supplied to the fluid storage 40. Advantageously, the first heat exchanger 31 is thermally coupled to the thermal storage 35 so that the heat removed from the first fluid flow is efficiently stored in the thermal storage 35; for example, the thermal storage 35 may be a hot water storage or molten salt storage. The second heat exchanger 32 is configured to provide heat to the second fluid flow supplied at the expander inlet 21, in particular before being supplied to the expander inlet 21. Advantageously, the second heat exchanger 32 is thermally coupled to the thermal storage 35 so that the heat provided to the second fluid flow comes from the thermal storage 35.

[0028] A third embodiment of an innovative energy storage system 300 will be described in the following with the aid of Fig. 3. It is to be noted that elements 350, 360, 361, 369, 370 and 381 in Fig. 3 may be identical or similar respectively to elements 50 (stan-by unit), 60 (stand-by compressor), 61 (stand-by compressor inlet), 69 (stand-by compressor outlet), 70 (stand-by motor) and 81 (heat removal unit) in Fig. 1 and perform the same or similarfunctions. It is also to be noted that also the energy storage system 300 of Fig. 3 is implementing an open-loop stand-by cycle, as it will be apparent from the following.

[0029] According to a possibility shown in Fig. 3 but not limited to the embodiment shown in Fig. 3, the compressor 10 and the expander 20 are mechanically coupled to a motor-generator 14 / 15 which is alternatively configured to drive the compressor 10 and to generate power electricity by converting mechanical power generated by the expander 20 into electrical power.

[0030] As already stated above, the energy storage system 300, in particular the stand-by unit 350, is implementing an open-loop stand-by cycle. With nonlimiting reference to Fig. 3, the expander inlet 21 is fluidly coupled to atmosphere (or to a first suitable tank storing the third fluid) in order to receive the third fluid flow; the expander 20 processes the third fluid (i.e. expands it) and discharge the third fluid flow at the expander outlet 29. The stand-by compressor 360, in particular the stand-by compressor inlet 361, is fluidly coupled to the expander outlet 29 and is configured to receive the third fluid flow; the stand-by compressor 360 processes the third fluid (i.e. compresses it) and discharge the third fluid flow at the stand-by compressor outlet 69. It is to be noted that the stand-by compressor outlet 69 is fluidly coupled to atmosphere (or to a second suitable tank storing the third fluid), so that the stand-by unit 350 implements an open-loop of the third fluid flow.

[0031] Advantageously, the stand-by unit 350 further comprises a heat supply unit 383 located upstream the expander 20 and fluidly coupled to the expander inlet 21, the heat supply unit 383 being configured to supply heat to the third fluid flow before being supplied to the expander inlet 21. Advantageously, the heat supply unit 383 is thermally coupled to the thermal storage 35 so that the thermal storage 35 can provide heat to the heat supply unit 383, in particular to the third fluid flow before being supplied to the expander inlet 21.

[0032] Advantageously, the stand-by unit 350 further comprises a heatremoval unit 382 located downstream the stand-by compressor 360 and fluidly coupled to the stand-by compressor outlet 369, the heat removal unit 382 being configured to remove heat from the third fluid flow discharged by the standby compressor outlet 369. Advantageously, the heat removal unit 382 is thermally coupled to the thermal storage 35 so that the heat removal unit 382 can discharge heat to the thermal storage 35, in particular heat removed from the third fluid flow discharged by the stand-by compressor outlet 369. In other words, the thermal storage 35 is configured to store heat discharged by the heat removal unit 382 and to supply heat to the heat supply unit 383 so that, during a stand-by mode of the system 300, the third fluid flow at the third temperature T3 may be supplied to the expander inlet 21 and the third fluid flow at the fourth temperature T4 may be discharged by the expander outlet 29.

[0033] With not-limiting reference to Fig. 4, it is shown on a temperatureentropy plot (T-s plot) an example of- the expansion of air during a discharge mode of the energy storage system 100 and 200 and 300 (see reference 400 in Fig. 4), in particular an expansion between the first temperature Tin of the air at the expander inlet 21 and the second temperature Tout of the air at the expander outlet 29;- the expansion of air during a stand-by mode of the energy storage system 100 and 200 implementing an open-loop stand-by cycle (see reference 500 in Fig. 4), in particular an expansion between the third temperature T3 of the air at the expander inlet 21 and the fourth temperature T4 of the air at the expander outlet 29; and- the expansion of air during a stand-by mode of the energy storage system 300 implementing a closed-loop stand-by cycle (see reference 600 in Fig. 4), in particular an expansion between the third temperature T3 of the air at the expander inlet 21 and the fourth temperature T4 of the air at the expander outlet 29.

[0034] According to another aspect, the subject-matter disclosed herein relates further to a method for operating an energy storage system comprisingthe steps of:A) During a discharge mode of the energy storage system, supplying a second fluid flow to an expander, the expander being configured to expand the second fluid flow and to drive a generator electrically coupled to an electric grid, so that the voltage frequency of the power electricity of the generator is synchronized with voltage frequency of the power electricity of the electric grid in order to supply power electricity to the electric grid;B) During a stand-by mode of the energy storage system, supplying through a stand-by unit a third fluid flow to the expander, the third fluid flow having inlet temperature and outlet temperature across the expander substantially equal to inlet temperature and outlet temperature across the expander of the second fluid flow during the discharge mode of the energy storage system.

[0035] It is to be noted that during step B the expander drives the generator so that it is maintained synchronized with power electricity of the electric grid. In other words, the method disclosed herein allows to have the expander of the energy storage system which, during the stand-by mode of the system, is maintained at substantially the same temperatures as during the discharge mode; moreover, the method disclosed herein allows the expander of the energy storage system to reach the 100% load very quickly when switched from the stand-by mode to the discharge mode as the expander drives the generator so that it is already synchronized with the electric grid.

[0036] According to a preferred embodiment the second fluid flow and / or the third fluid flow is air, in particular ambient air, or CO2, in particular CO2 in supercritical conditions.

[0037] According to a first possibility, the stand-by mode may implement a closed-loop stand-by cycle. In particular, according to this possibility, the third fluid flow is supplied to the expander, processed by the expander, dischargedby the expander and then recirculated back to the expander, in particular after being compressed and cooled down.

[0038] According to a second possibility, the stand-by mode may implement an open-loop stand-by cycle. In particular, according to this possibility, the third fluid flow is supplied to the expander from atmosphere or a first suitable thank, in particular after being heated up, is processed by the expander and then discharged by the expander to atmosphere or a suitable second tank, in particular after being compressed and cooled down. In other words, the third fluid flow, after being processed by the expander, is not recirculated back to the expander but is discharged to atmosphere or a suitable second tank and another third fluid flow is supplied to the expander from atmosphere or a first suitable thank.

Claims

CLAIMS1. An energy storage system (100, 200, 300) comprising a compressor (10), a fluid storage (40), an expander (20) and a generator (15) mechanically coupled to the expander (20), wherein the generator (15) is configured to be electrically coupled to an electric grid in order to supply power electricity to the electric grid, wherein, during a charge mode of the energy storage system (100, 200, 300), the compressor (10) is fluidly coupled to the fluid storage (40) and is configured to compress a first fluid flow and supply it to the fluid storage (40), wherein the fluid storage (40) is configured to store the first fluid at least for a predetermined time, wherein, during a discharge mode of the energy storage system (100, 200, 300), the expander (20) is fluidly coupled to the fluid storage (40), wherein the fluid storage (40) is configured to supply at an expander inlet (21) a second fluid flow at a first temperature (Tin), wherein the expander (20) is configured to expand the second fluid flow according to a first pressure ratio so to discharge the second fluid flow at a second temperature (Tout) at an expander outlet (29) and to drive the generator (15), so that the power electricity supplied by the generator (15) has substantially the same the voltage frequency of the power electricity of the electric grid, wherein the energy storage system (100, 200, 300) further comprises a stand-by unit (50), wherein, during a stand-by mode of the energy storage system (100, 200, 300), the stand-by unit (50) is fluidly coupled to the expander (20), wherein the stand-by unit (50) is configured to supply at the expander inlet (21) a third fluid flow at a third temperature (T3), wherein, during a stand-by mode of the energy storage system (100, 200, 300), the expander (20) is further configured to expand the third fluid flow according to a second pressure ratio so to discharge the third fluid flow at a fourth temperature (T4) at the expander outlet (29) andto drive the generator (15) so that the power electricity supplied by the generator (15) has substantially the same the voltage frequency of the power electricity of the electric grid, wherein the first temperature (Tin) and the second temperature (Tout) of the second fluid flow are substantially equal to the third temperature (T3) and the fourth temperature (T4) of the third fluid flow.

2. The energy storage system (100, 200, 300) of claim 1, wherein the first pressure ratio of the second fluid flow is substantially equal to the second pressure ratio of the third fluid flow.

3. The energy storage system (100, 200, 300) of claim 1, wherein the first fluid flow and / or the second fluid flow and / or the third fluid flow is air or CO2, in particular CO2 in supercritical conditions.

4. The energy storage system (100, 200, 300) of claim 1, wherein the mass flow of the third fluid flow is much lower than the mass flow of the second fluid flow.

5. The energy storage system (100, 200, 300) of claim 1, wherein the stand-by unit (50) comprises a stand-by compressor (60) and a standby motor (70) mechanically coupled to the stand-by compressor (60) so to drive the stand-by compressor (60), wherein the stand-by compressor (60) has a stand-by compressor inlet (61) and a stand-by compressor outlet (69), wherein the stand-by compressor (60) is fluidly coupled to the expander (20) so that the third fluid flow at the expander outlet (29) is supplied to the stand-by compressor inlet (61), pressurized by the stand-by compressor (60) and discharged at the stand-by compressor outlet (69).

6. The energy storage system (100, 200, 300) of claim 1, wherein the expander (20) further comprises at least one dry gas seal, preferably two dry gas seals located at the axial ends of the expander (20), the dry gas seal being configured to minimize leakage of ambient air into the expander (20) during the stand-by mode of the energy storage system (100, 200, 300).

7. The energy storage system (100, 200, 300) of claim 1, wherein the expander (20) further comprises at least one magnetic bearing, in particular a radial magnetic bearing, preferably two magnetic bearings located at the axial ends of the expander (20), the at least one magnetic bearing being configured to radially support the expander (20).

8. The energy storage system (100, 200) of claim 5, wherein the stand-by compressor outlet (69, 269) is fluidly coupled to the expander inlet (21), so that the stand-by unit (50, 250) implements a closed-loop of the third fluid flow.

9. The energy storage system (100, 200) of claim 8, wherein the stand-by unit (50, 250) further comprises a heat removal unit (81, 281) located downstream the stand-by compressor (60,160) and fluidly coupled to the stand-by compressor outlet (69, 169), wherein the heat removal unit (81, 181) is configured to remove heat from the third fluid flow before being supplied to the expander inlet (21).

10. The energy storage system (100, 200) of claim 8, wherein the pressure of the third fluid flow at the expander inlet (21) is lower than ambient pressure.

11. The energy storage system (300) of claim 5, wherein the expander inlet (21) is fluidly coupled to atmosphere or to a first suitable tank in order to receive the third fluid flow, wherein the stand-by compressor outlet (69) is fluidly coupled to atmosphere or to a second suitable tank in order to discharge the third fluid flow, so that so that the stand-by unit (350) implements an open-loop of the third fluid flow.

12. The energy storage system (300) of claim 11, wherein the stand-by unit (350) further comprises a heat supply unit (383) located upstream the expander (20) and fluidly coupled to the expander inlet (21), wherein the heat supply unit (383) is configured to supply heat to the third fluid flow before being supplied to the expander inlet (21).

13. The energy storage system (300) of claim 11, wherein the stand-by unit (350) further comprises a heat removal unit (382) located downstream the stand-by compressor (360) and fluidly coupled to the stand-by compressor outlet (369), wherein the heat removal unit (382) is configured to remove heat from the third fluid flow discharged by the stand-by compressor outlet (369).

14. The energy storage system (300) of claim 12 and 13, wherein the heat removal unit (382) and the heat supply unit (383) are thermally coupled to the thermal storage (35), the thermal storage (35) being further configured to store heat discharged by the heat removal unit (382) and to supply heat to the heat supply unit (383).

15. Method for operating an energy storage system (100, 200, 300), the method comprising the following steps:A) During a discharge mode of the energy storage system (100, 200, 300), supplying a second fluid flow to an expander, the expander being configured to expand the second fluid flow and to drive a generator electrically coupled to an electric grid, so that the voltage frequency of the power electricity of the electric grid is synchronized with the voltage frequency of power electricity of the electric grid in order to supply power electricity to the electric grid;B) During a stand-by mode of the energy storage system (100, 200, 300), supplying through a stand-by unit a third fluid flow to the expander, the third fluid flow having inlet temperature and outlet temperature across the expander substantially equal to inlet temperature and outlet temperature across the expander of the second fluid flow during the discharge mode of the energy storage system, wherein during step B the expander drive the generator so that it is maintained synchronized with power electricity of the electric grid.

16. The method of claim 15, wherein the second fluid flow and / or the first fluid flow is air or CO2, in particular CO2 in supercritical conditions.

17. The method of claim 15, wherein the stand-by mode comprises implementing a closed-loop stand-by cycle.

18. The method of claim 15, wherein the stand-by mode comprises implementing an open-loop stand-by cycle.

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