ES2374891B2 - ENERGY STORAGE DEVICE THROUGH COMPRESSED FLUIDS AND ITS OPERATION PROCEDURE. - Google Patents

Abstract

Energy storage device by means of compressed fluids and its operating procedure # Energy storage system by containing fluids in a plurality of high pressure vessels, which are loaded one by one, sequentially, when there is availability of energy to be stored , using said excess energy in compressing the working fluid to the design pressure; said energy being recovered in an expanding machine that is traversed by the fluid that is discharged from the storage, which is done by joining all the previously loaded containers in parallel to the design pressure, in order to keep the remaining pressure as high as possible. the storage.

Description

Energy storage device using compressed fluids and its operating procedure.

Technical sector

The invention falls within the field of energy storage, mainly in the sector of power generation plants, or more generically of the electrical system as a whole, where situations occur in which the supply of energy exceeds demand , and therefore it should be stored; and on the other hand demand spikes occur, in which the available supply can hardly satisfy the demanded power; which is facilitated using energy storage means, such as hydraulic pumping; which is the most mature and widespread of the storage media, to which other methods can be added in the near future, among which the storage of compressed fluids at very high pressure, of several hundreds of bars (several tens of MPa), stands out .

Technical problem to be solved and Background of the invention

The generic problem to be solved is to find a storage configuration and a technical management procedure for it, which maximizes the benefits obtained from the storage system.

Within this objective, what is specifically to be solved is that the storage of mechanical energy by compression stores the compressed fluid at the design pressure (also called nominal or setpoint) and that it is discharged, through the expander machine corresponding, keeping the level of pressure in the discharges as high as possible, so that the aforementioned machine operates as close as possible to the nominal regime, which corresponds to an inlet pressure equal to the nominal value.

In existing inventions and applications, the use of air as a working fluid predominates, due to its high compressibility, but the invention presented here is applicable to any fluid. On the other hand, when P · V products (pressure by volume of the storage vessel) are considerably larger, there is a widespread technical conviction that sealed underground caverns, such as those containing natural gas, are necessary to have an application of High power and energy. Such is the scope of the invention presented in US 2011236134 (A1). However, it is not usual to have such caverns, and the most immediate solution is to use pressure vessels to store the compressed fluid, of which there are many inventions, and in particular those contained in WO 2011063071 A1, WO 2011056296 A2, US 2011219760 A1, and WO 2011054169 A1, the latter being dedicated to the storage of fluids above the critical point, which is also accessible in the invention proposed herein, although its configuration and mode of technical storage management has nothing to do with none of the inventions indicated.

Nor are the ideas set forth in various documents dealing with the same problem precedent of the invention, namely: a compressing machine, which serves to fill an artificial or natural container, but usually artificial, from which the compressed fluid is discharged by passing it by an expanding machine, often called a turbine (in this case the morphology is closer to a gas turbine) and performing this task with guidelines designed to take advantage of the available equipment, such as adiabatic compressed air, which is in the invention US 2011 100583 A1, WO 2011053410 A1 and in WO 2011056283 A2. Other inventions focusing on the same type of installation are US 2011061385 A1, US 2011219760 A1, US 2011196542 A1, US 2011167814 A1, CA 2749975 A1, WO 2011076926 A2, WO 2011071609 A1, EP 2378100 A2 and JP 2011106333 A, but none of they provide constructive or work procedure elements remotely similar to those proposed in this invention. Nor is US 2011167813 A1 considered precedent, although it contains two pressure vessels, but not a plurality of them, and not used with the procedure proposed in this invention.

Within the specific field that has been delimited, of an installation that has a compressed fluid storage, with a booster or compressor to fill it, and with an expander to perform a job when the compressed fl uid is released from the container, the invention makes reference to the storage itself, to the structuring of its components, and to the technical management of the fl ow fluxes, both in loading and unloading.

It should be added that in this sector of the technique, the compressor machine is the one that absorbs a flow of a fluid through its intake mouth, which drives a higher pressure through its discharge mouth, which executes thanks to the external power applies to him; an expanding machine being one that produces a mechanical power, generally applied to an axis (which in particular can be connected to an electric generator), thanks to the fact that it absorbs a flow of a fluid through its intake mouth, which ejects at lower pressure by Your discharge mouth.

Description of the invention

The device described in the present invention comprises a plurality of airtight containers of any size and shape, although particularly marked as canonical, within the strength of materials, the elongated cylindrical shape, topped at its ends by two hemispherical or torispheric caps, and in any case counting each container with a loading and a discharge pipe, each one of said pipes with at least one all / nothing or opening / closing type valve, which are called loading valves when they are in a pipeline load, and discharge valves if they are in a discharge pipe, all valves having the possibility of flow passage in one direction, all the filling or loading pipes of the containers, which are at least one per container, connected to the duct or discharge pipe of a compressor machine that takes the fl uid from a natural reservoir

or artificial, at low pressure; and all the emptying or unloading pipes of the containers, which are at least one per container, being connected to the inlet pipe or inlet of an expanding machine that discharges the fluid into said reservoir, natural or artificial, at low pressure; a plurality of circuits being configured in parallel through which the working fluid can flow from the compressor machine drive to the admission of the expander machine, each circuit consisting of the branch or load pipe to each container, followed by the container in yes, followed by the branch or discharge pipe of each container; and having in the drive duct of the compressor machine a general valve of that duct, and also having, in the intake duct of the expander machine, another general valve; and said circuits acting, partially or totally, as real flow passages according to the opening / closing state of their valves.

The open / closed position of each valve is determined either by a control action programmed in an opening / closing control system through the corresponding relay, or by a decision of the plant operator from its command post, depending on pressure gauge readings; said control system and said command post existing as a set of sensors and relays communicated by cable or by wireless electromagnetic signals, through which the opening or closing of each valve is activated, according to the operating procedure set forth below as part of the invention.

Eventually, a single machine can operate reversibly, as it turns in one direction or the other, as a compressor or expander, however the drive duct when it works as a compressor machine, and the intake duct when it functions as an expander machine, remain uniquely defined.

A variant is that a natural fluid be used as a working fluid, natural fluids being understood as those that are abundantly found in nature, such as air and water, in which case, instead of using a low pressure reservoir to close the circuit, the atmosphere is used directly, in the case of air, or a voluminous mass or stream of water, in the case of water, thus acting in an open circuit on the low pressure side.

The invention is completed by an operation procedure for loading and unloading the storage device, applied either automatically, or manually, which states that all valves must be closed, except those set forth below, which must be open, depending on the way in which it is being operated, for charging the energy storage, or for downloading it, prescribing that:

-

in the loading mode, in which the compressor machine operates, the inlet valve is open from the low pressure tank, or possibly the natural fluid supply valve, if this is used as a working fluid, and the general valve of the machine's discharge duct is open, and a container loading valve, selected for being the one with the highest pressure, below the setpoint, at the time of starting that storage loading stage, which ends when the pressure inside said container is equal to the nominal or setpoint, closing said valve that was open at that time, and opening the loading valve of the container that at that time has the highest pressure, below the setpoint, thus initiating a new charging stage;

-

in the storage discharge mode, in which the expander machine operates, the discharge valve is open to the low pressure reservoir, or possibly the discharge valve of a natural fluid, if this is the one used as a working fluid , and the general valve of the inlet duct of the machine is open, and all the discharge valves of all the containers that had the set pressure at the time of that phase of unloading of the storage are also open, which ends when the pressure inside these containers is insufficient to operate the expander at its technical minimum, or it ends when the specific demand for energy that was being met by storage ends.

The advantage provided by this invention, with respect to those that conventionally use a large, natural, underground cavern type or artificial deposit, is that in these, in the loading process, the pressure increases very slowly, to be diluted, in the large volume from the reservoir, the mass inflated by the compressor machine, being therefore difficult to reach the maximum or set pressure; However, it is the one that works best for the expander in the unloading of storage.

With the proposed invention, the containers are loaded one by one, therefore, it is common that at a time when storage energy is needed, a part of the containers is loaded to the maximum pressure, which is what Ideal for obtaining good performance of the expander machine. But in the discharge they are not emptied one by one, but all those that are loaded, at the same time, with which it is achieved that the pressure drop is slower, and therefore the expander machine will maintain a higher performance for longer .

Brief description of the fi gures

Figure 1 shows the scheme of an energy storage facility with the structuring of components given by the invention.

Figure 2 shows the same scheme, but in the case of using the same machine, reversible, as a compressor and expander.

To facilitate the understanding of the preferred embodiments of the invention, the relevant elements thereof, which appear in the figures, are listed below:

one.

High pressure fluid storage container. The remaining containers are numbered, by analogy, 1a, 1b, 1c, 1d and 1e.

2.

Compressing machine

3.

Duct or pipe connected to the drive duct 33 of the compressor machine (2), each of the ducts (3, 3a, ... 3e) serving for loading the storage containers (1, 1a, ... 1e ).

Four.

Low pressure reservoir of working fluid.

5.

Intake duct, from the tank (4), to the compressor machine.

6.

All / nothing and non-return valve in the duct (5).

7.

Adding a natural fluid to the compressor machine.

8.

All / nothing and non-return valve in the duct (7).

9.

All-nothing and non-return valve in the general flow duct of the working fluid from the compressor machine.

10.

All / nothing and non-return valve in the mouth of the container (1). By numbering analogy, valves 10a, 10b, 10c, 10d and 10e have the same function for the plurality of containers.

eleven.

Motor (electric, hydraulic, aeromechanical, or any other nature) that activates the compressor machine (2).

12.

All / nothing and non-return valve in the discharge mouth of the container (1). By analogy of numbering, valves 12a, 12b, 12c, 12d and 12e have the same function for the plurality of containers.

13.

Pipe or discharge duct of the storage container 1. By analogy of numbering, the pipes 13a, 13b, 13c, 13d and 13e have the same function for the plurality of containers.

14.

All-nothing and non-return valve in the general inlet duct of the working fluid to the expander machine.

fifteen.

Expander machine.

16.

Electric generator powered by the expander machine.

17.

Discharge pipe of a natural fluid from the expander machine.

18.

All-nothing and non-return valve in the duct or pipe (17) for discharging the natural fluid from the expander.

19.

Discharge pipe for working fluid from the expander to the low pressure tank.

twenty.

All-nothing and non-return valve in the duct or pipe (19) for discharging the working fluid from the expander.

twenty-one.

Reversible machine, which can work as a compressor, turning in one direction, or as an expander, turning in the opposite direction.

22

Electric machine, which functions as a motor, rotating in one direction; or as a generator, turning in the opposite direction, acting in the first case on the storage loads, and on the discharges, in the second.

2. 3.

Duct or discharge pipe of the reversible machine (21) when it works as a compressor, so it is a duct analogous to (33).

24.

Intake duct, from the low pressure tank (4), to the reversible machine (21) when it works as a compressor.

25.

All / nothing and non-return valve in the duct (24).

26.

Adding a natural fluid to the reversible machine when it works as a compressor.

27.

All-nothing and non-return valve in the duct (26).

28.

Pipe or inlet duct of the reversible machine (21) when it works as an expander.

29.

Discharge pipe of the working fluid from the reversible machine, when it works as an expander, to the low pressure tank (4).

30

All / nothing and non-return valve in the duct (29).

31.

Discharge pipe of a natural fluid from the reversible machine when it works as an expander.

32

All / nothing and non-return valve in the duct (31).

33.

Supply pipe from the compressor 2, and from which the branches 3, loading the containers (1).

3. 4.

Inlet pipe or duct of the expander machine (15) and into which the storage containers (1, 1a, ... 1e) are discharged through the pipes (13, 13a, ... 13e).

Description of an embodiment of the invention

The invention is embodied on the basis of an electric power generation plant equipped with a compressor machine for storage working fluid, and with an expanding machine, or with a single reversible machine that performs both fluid-mechanical functions as moved by an external motor, or the machine exerts the power generating electricity, plus a low pressure tank where the working fluid accumulates when it is discharged from storage.

An essential issue is to have the appropriate containers, which, having to withstand high pressures and according to which fluids, high temperatures, must be made of resistant material and with the most appropriate shape. A fundamental issue is the pressure at which the work fl uid in storage works, P1, which will be significantly higher than atmospheric, so that the storage has energy value. The mechanical stresses suffered by the walls of the vessel will depend on said pressure; and to characterize its value in a very representative manner, ring tension can be used, although this predisposes to the use of very elongated cylindrical vessels, which is the most conventional model for generic storage of compressed gases. For a thick cylindrical tank with a pressure P1, the ring tension σ is

σ = P1 · ((r2 + 1) / (r2-1))

where r is the ratio between outside and inside diameters of the tank. For high temperature values, the admissible σ is less than usual at room temperature, since the elastic period is shortened and the risk of fl uence is approaching. If P1 is relatively high, close to 10 MPa, for example, the allowable σ / P1 ratio is around 10, if the stored gas does not have a very high temperature (below 250ºC) and in that case the ratio between the diameters of container comes out 1.1; which already implies very thick thicknesses. Even worse, it is the case that the stored fluid is noticeably hot, as a result of the shortening of the elastic field. In this case, the allowable σ / P1 ratio drops to around 6. In this case, the ratio between diameters rises to 1.2; which implies much thicker walls. In the event that relatively high pressures and temperatures were combined, the allowable ratio would fall further, to around 4, which would give a ratio between diameters of approximately 1.3. This demonstrates the difficulties of occupying artificially large volumes spherically. On the contrary, it is more practical to go to very elongated cylindrical containers in order to provide an appreciable volume of storage, which is also buildable with relatively mature techniques. Keep in mind that there are hydraulic jumps of almost 1,000 meters high, which is equivalent to approximately 10 MPa, and that is the pressure at the foot of the forced conduction through which the water descends. A more important challenge is the compressed air, because in compression, if done very quickly, a practically adiabatic transformation is followed, and the heat of all irreversibilities remains practically in the air, which reaches very high temperature . The opposite option is to compress the air along an isothermal transformation, but that requires a very slow compression rate, which is not always acceptable, since the availability of excess energy that is either stored or lost is given often for relatively short times. Despite these precautions against compressed air, it seems the ideal candidate for this type of storage, for its compressibility, much greater than that of water.

In the case of storing gases with procedures that heat them noticeably, the containers must be thermally insulated; not only to prevent the temperature from falling, because with it the pressure would fall, but to prevent the temperature differences between the hot face of the vessel wall and the outer, cold face, producing a supplementary ring tension, compression in the hot face, and traction on the cold face, which would be a function of

ΔT = temperature difference between hot and cold faces,

α = coefficient of linear expansion,

E = Young's module,

the ring tension σt of thermal origin being the value

σt = α-ΔT-E / 2

The addition of tensions would imply greater thicknesses in the container, and to avoid this extra cost it is necessary to resort to a strong thermal insulation of the containers. In any case, the specifics of the invention is to configure the connection network of the compressor machine to the containers, and of the containers to the expander machine, so that the containers can be isolated, so that their filling is carried out sequentially, one by one; but its emptying through the expander is done in conjunction with all the containers that were loaded.

To this end, the pair of figures that explain the invention are clearly illustrative, which differ between them only because in the second there is only one machine, reversible, that operates as a compressor in the load, activated by an electric motor; which in turn becomes a generator when it rotates in the opposite direction and is moved by the axis of the expander machine, which in turn is moved by the expansion of the fluid that is released from its compression in storage. In three-phase electric machines, the change of direction of rotation, and therefore of electrical function, is carried out by changing the order of the phases, going from RST in a case (which is considered reference) to RTS. In simple, single-phase machines, this is done by reversing the polarity in the stator winding.

As an example, in Figure 1 a typical storage loading stage is seen, in which the compressor machine (2) whose inlet valves (6, 8) are open is open, as is the one of its drive (9). At that time, only the container 1c is being loaded, since only the valve 10c is open in connection with the supply line (33). The containers 1, 1a, and 1b are already loaded, and ready to be unloaded, whereby the valves 12, 12a and 12b are open (somewhat abruptly to illustrate the case); although the inlet valve (14) of the expander machine (15) is still closed, since the system is not in storage discharge; Although the storage is partially charged, and ready to start the production of energy, if required by the electrical system as a whole. Finally, it should be noted that containers 1d and 1e are unloaded, waiting for their loading shift.

Once the invention is clearly described, it is noted that the particular embodiments described above are susceptible to modifications of detail provided that they do not alter the fundamental principle and essence of the invention.

Claims (6)

1. Energy storage device by means of compressed fl uids, located in an electric power generation plant equipped with a compressor machine (2) for the working fl uid of storage, and with an expander machine (15), or with a single reversible machine (21) that alternately exert both fluid-mechanical functions, plus a natural or artificial reservoir of low pressure (4) where the working fluid accumulates when it is discharged from storage, characterized in that the storage comprises: a plurality of airtight containers (1, 1a, 1b, 1c, 1d, 1e) counting each container with a load pipe (3, 3a, 3b, 3c, 3d, 3e) and another discharge pipe (13, 13a, 13b , 13c, 13d, 13e), equipped with each of said pipes (3, 3a, ... 3e, 13, 13a, ..., 13e) with at least one valve (10, 10a, ..., 10e, 12, 12a, ..., 12e) of the all / nothing or opening / closing type, which are called loading valves when they are in a load pipe, and discharge valves if they are in a discharge pipe, having all the valves possibility of fluid flow in only one direction; being all the filling or loading pipes (3, 3a, 3b, 3c, 3d, 3e) of the containers (1, 1a, ..., 1e), which are at least one per container, connected to the conduit or pipe of drive (33, 23) of a compressor machine (2, 21) that takes the fl uid of a reservoir, natural or artificial, at low pressure (4); and all the drainage or discharge pipes (13, 13a, 13b, 13c, 13d, 13e) being of the containers (1, 1a, ..., 1e), which are at least one per container, connected to the conduit or pipe of admission (34, 28) of an expanding machine (15, 21) that discharges the fluid in said reservoir, natural or artificial, at low pressure (4); a plurality of circuits being configured in parallel through which the working fluid can flow from the drive (33, 23) of the compressor machine (2, 21) to the intake (34, 28) of the expander machine (15, 21 ), each circuit consisting of the branch or load pipe (3, 3a, ..., 3e) to each container (1, 1a, ... 1e), followed by the container itself, followed by the branch or discharge pipe (13, 13a, ... 13e) of each vessel, and said circuits acting, partially or totally, as actual flow passageways according to the opening / closing state of their valves; having in the discharge duct (33, 23) of the compressor machine (2, 21) a general valve (9) of that duct, and also having, in the intake duct (34, 28) of the expander machine (15 , 21), another general valve (14).

2.

Energy storage device by means of compressed fluids, according to claim one, characterized in that a single machine (21) can function reversibly, as it turns in one direction or the other, as a compressor or expander, the discharge duct (23) being uniquely defined. ) when it works as a compressor machine, and the intake duct (28) when it works as an expander machine.

3.

Energy storage device by means of compressed fl uids, according to the first claim, characterized in that a working variant is to use a natural fl uid as a working fl uid, natural fl uids being understood as those which are found abundantly in nature, such as air and water, in which case, instead of using a low pressure reservoir to close the circuit, the atmosphere is used directly, in the case of air, or a bulky mass or stream of water, in the case of water, thus acting in open circuit (7, 17) on the low pressure side.

Four.

Energy storage method by means of compressed fluids, which employs a storage device according to any one of the preceding claims, characterized in that the storage operation procedure, in loading and unloading, applied either automatically, or manually, establishes that all the valves they must be closed, except those listed below, which must be open, depending on the way in which they are operating, loading the energy storage, or unloading it, prescribing that

-

in the loading mode, in which the compressor machine (2, 21) operates, the valve (6) of its intake is open from the low pressure tank (4), or possibly the supply valve (8) of a natural fluid, if this is the one used as a working fluid, and the general valve (9) of the drive duct (33) of the machine (2, 21), and a loading valve (10c) of a container ( 1c), selected for being the one with the highest pressure, below the setpoint, at the time that storage loading stage begins, which ends when the pressure inside said container (1c) equals the nominal or of setpoint, closing said valve (10c) that was open at that time, and opening the loading valve of the container that at that time has the highest pressure, below the setpoint, thus initiating a new loading stage;

-

and in the storage discharge mode, in which the expander machine (15, 21) operates, the valve (20) of its discharge is opened towards the low pressure tank (4), or possibly the discharge valve (18 ) of a natural fluid, if this is used as a working fluid, and the general valve (14) of the intake duct (34, 28) of the machine (15, 21) is open, and all valves are also open of discharge (12, 12a, 12b) of all the containers (1, 1a, 1b) that had the set pressure at the time of starting that phase of discharge of the storage, which ends when the pressure inside these containers is insufficient to operate the expander machine (15, 21) at its technical minimum, or it ends when the punctual demand for energy that was being met by storage ends.

SPANISH OFFICE OF THE PATENTS AND BRAND Application no .: 201101340 SPAIN Date of submission of the application: 20.12.2011 Priority Date: REPORT ON THE STATE OF THE TECHNIQUE 51 Int. Cl.: See Additional Sheet RELEVANT DOCUMENTS

Category

56 Documents cited Claims Affected

TO

US 4237692 A (AHRENS FREDERICK W et al.) 09.12.1980, 1,3,4

column 3, lines 12-41; Figure 1.

TO

WO 2011104556 A2 (ISENTROPIC LTD et al.) 01.09.2011, 1-4

page 19, line 31 - page 20, line 2; page 20, line 28 - page 25, line 19; Figures 1-3.

TO

US 5845479 A (NAKHAMKIN MICHAEL et al.) 08.12.1998, 1,3,4

column 5, line 22 - column 7, line 34; figures.

TO

US 3818698 A (BECKMANN G et al.) 25.06.1974, 1.4

column 2, line 56 - column 3, line 31; column 4, line 20 - column 5, line 12; figures.

TO

US 2010205960 A1 (MCBRIDE TROY O et al.) 08.08.2010, 1.4

paragraphs [41], [49-50]; Figures 3.8.

Category of the documents cited X: of particular relevance Y: of particular relevance combined with other / s of the same category A: reflects the state of the art O: refers to unwritten disclosure P: published between the priority date and the date of priority submission of the application E: previous document, but published after the date of submission of the application

This report has been prepared • for all claims • for claims no:

Date of realization of the report 07.02.2012

Examiner E. García Lozano Page 1/5

REPORT OF THE STATE OF THE TECHNIQUE CLASSIFICATION OBJECT OF THE APPLICATION F02C6 / 16 (2006.01) F01K3 / 12 (2006.01) F15B1 / 02 (2006.01) Minimum documentation sought (classification system followed by classification symbols) F02C, F01K, F15B Electronic databases consulted during the search (name of the database and, if possible, terms of search used) INVENTIONS, EPODOC  WRITTEN OPINION Date of Written Opinion: 07.02.2012 Statement

Novelty (Art. 6.1 LP 11/1986)

Claims Claims 1-4 IF NOT

Inventive activity (Art. 8.1 LP11 / 1986)

Claims Claims 1-4 IF NOT

The application is considered to comply with the industrial application requirement. This requirement was evaluated during the formal and technical examination phase of the application (Article 31.2 Law 11/1986).  Opinion Base.- This opinion has been made on the basis of the patent application as published.  WRITTEN OPINION 1. Documents considered.- The documents belonging to the state of the art taken into consideration for the realization of this opinion are listed below.

Document

Publication or Identification Number publication date

D01

US 4237692 A (AHRENS FREDERICK W et al.) 09.12.1980

D02

WO 2011104556 A2 (ISENTROPIC LTD et al.) 01.09.2011

D03

US 5845479 A (NAKHAMKIN MICHAEL et al.) 08.12.1998

2. Statement motivated according to articles 29.6 and 29.7 of the Regulations for the execution of Law 11/1986, of March 20, on Patents on novelty and inventive activity; quotes and explanations in support of this statement The present application refers to an energy storage device by means of compressed fluids and its operating procedure. The storage device object of the invention is located in an electric generation plant that includes a compressor machine (2), an expansion machine (15) and a low pressure tank where the working fluid accumulates, and comprises a plurality of hermetic elements ( 1) each of them with loading and unloading pipe and at least one valve in any of said pipes. All the load pipes are connected to the supply pipe of the compressor machine, in which there is a valve, and all the discharge pipes are connected to the intake duct of the expander machine, in which there is also a valve, of so that the flow is established in parallel by the tanks and their respective loading and unloading pipes from the drive of the compressor machine to the admission of the expander machine. The number of pass-through circuits / tanks will depend on the opening / closing state of their valves (Reiv.1). Optionally, there can be only one machine that works reversibly as a compressor or expander (Reiv. 2). Another variant is the use of a natural fluid such as air or a mass of bulky water instead of a low pressure reservoir to close the circuit (Reiv.3). Likewise, the system operation procedure is part of the object of the invention (Reiv.4). According to said procedure, the system can act manually or automatically on the valves of the system depending on the way in which it is operating, as follows:

-

In load mode, the compressor machine operates and the intake valve is open from the low pressure tank. The valve of the discharge duct and one of the load valves of a container, the one with the highest pressure below the nominal pressure at the time of loading, are also open. When the nominal pressure is equalized, said valve is closed and the corresponding valve is opened to the next vessel that meets the above requirements.

-

In the discharge mode, the expander machine operates, the discharge valve is open to the low pressure tank and also the general valve of the intake duct. In addition, all the discharge valves of the vessels that have the nominal pressure at the time the discharge begins are open.

Document D01 discloses an energy storage system by means of underground tanks that in a period of demand feed a turbine at constant pressure. The storage system has two tanks (18,19), each of which has two valves in its load (20,21) and in its discharge to the turbine (22,26), said tanks being communicated by a pipe which has another valve (25). In the discharge pipe of the second tank (19) downstream of the connection pipe with the first tank (18) is an ejector device (24) whose function is to unify the flow of air from both tanks at the inlet of the turbine. The differences between document D01 and the system of the invention are found in the use of a low pressure tank as a source of the working fluid and the use of isolation valves in the supply and intake ducts. The use of a reservoir or a natural fluid without the use of the reservoir are common alternatives in the state of the art known to one skilled in the art, and whose practical realization is achieved without the use of inventive activity. The use of the valves in the supply and intake pipes is not required in D01 because the mode of operation of the system is not the same as that of the application. As disclosed in D01, in the loading mode the two tanks (18 and 19) are filled at the same pressure and in the discharge mode the tank 18 is initially used feeding the turbine (valves 20, 21, 25 and 26 closed), and when the pressure drops below one point, valve 22 is closed and 25 and 26 are opened so that the air from the two tanks together feeds the turbine (column 3 lines 12 to 41). Unlike the application procedure, the deposit load is not selective, one is not completed and the next one is loaded, but both are loaded at the same time. The discharge, however, is selected one or both tanks depending on the pressure in them, so that the two tanks can be fed in parallel, as in the device claimed in the application.  WRITTEN OPINION As already indicated, D01 does not contemplate the possibility that the loading of the deposits is selective, which implies that a full load of them must be given for optimum system operation. The use of a differential load depending on the pressures gives the system of the invention greater flexibility and improved installation performance. This difference in operation means that the device is also different and the passage of D01 to the solution proposed in the application is not evident. Document D02 discloses a thermal storage system comprising a high pressure tank (13), two low pressure tanks (11.12) and a gas pressure storage (20). The system can operate continuously by first filling the reservoir 10 and the storage 20, changing the valve position towards the reservoirs 11 and then 12. Reservoirs 11 and 12 have common manifolds with valves 42 and 43 as valves of said manifolds, and in each of their sockets they have other independent valves that make them both working in parallel or independently. However, the other deposits and warehouses in the system (13, 20) have different functions that make said system substantially different from that proposed in the application. Document D03 discloses a storage system for use in electrical systems, comprising several tanks (37) working in parallel. Unlike the system proposed in the application, the deposits in D03 always work in the same conditions, not considering the loading or unloading of them. Therefore, it is considered that none of the documents cited in the Search Report, or any relevant combination thereof, reveals a system of the characteristics of the application system. The above documents only reflect the state of the art, and it is considered that the invention is new and implies inventive activity (Art. 6 and 8 Patent Law 11/1986).