"HEAT ACCUMULATOR INTEGRATED WITH THERMODYNAMIC CYCLE"

Technical field of the invention

The present invention is applied in the energy field, in particular by means of integrating thermodynamic systems with power cycles at the solar array .

Background art

Concentrating solar power systems (CSP) allow the solar energy to be converted into thermal energy by using mirrors which concentrate such an energy on an accumulator .

Carrier fluids, which are commonly selected from heat-transmitting oil, molten salt or water, are used for integrating with a power system.

In the case of heat-transmitting oil and molten salt, the carrier fluids transfer the heat of the solar array to a closed water circuit to produce pressurized vapor (working fluid) to be sent to a vapor power cycle for producing electricity.

With reference to the storage of thermal energy, the main CSP technology systems are:

• Parabolic Trough with heat-transmitting oil carrier fluid: indirect molten salt tanks at 380°C,

• Parabolic Trough with molten salt carrier fluid: direct molten salt tanks at 550°C,

• Solar Tower: direct molten salt tanks at 550°C,

• Linear Fresnel : pressurized vapor (about <10 min),

• Dish Stirling: no storage technology

A recent development of the solar tower has shown with the introduction of graphite storage technology developed by SOLASTOR, which uses graphite as the direct heat storage means, a module on the top of the tower which comprises the system consisting of: receiver, storage system also serving as a heat exchanger.

The carrier fluid (which is also the working fluid) is represented by pressurized water, which vaporizes inside the graphite modules, and is sent from a vapor tube network to the power unit where it expands in a vapor turbine; a vapor cycle is used as power cycle also for this technology.

The radiant energy from the solar array in such a system is concentrated on an opening provided in the graphite module, which serves as a black body. Each module has a heat capacity of about 3.2 MWht and receives the radiant energy collected from a solar array of about 100 mirrors with toroidal geometry. Inside the module is a tube coil for circulating a pressurized water flow (about 25 bar a and higher) , which vaporizes and is brought up to about 400°C and higher, operating as OTSG (once through steam generator) ; each module has a pressure control valve.

The vapor produced by the solar array (that is the group of solar arrays corresponding to each heat accumulation module) formed by several heat accumulations with related solar array (e.g. 160) is sent to the regenerated Rankine vapor power cycle, and the turbine inlet temperature is controlled through mixers ( tempering valve) which spray pressurized water which vaporizes when in contact with the vapor. The vapor is collected by a collector 18" (inches) brought to the power unit, where it expands in the turbine, thus generating electricity.

The diagram in Figure 1A refers to a solar array with 160 modules (45 gross hectors overall), where the vapor power cycle is carried out by a commercial power block (for example, manufactured by MAN) with 6 regenerative bleed-offs (not depicted) having power equal to 12.5 MWe and efficiency of about 33.7%.

All commercially available systems implement a Rankine water/vapor cycle for producing electricity.

For all the CSP technologies, the main technical problems are:

a heat-transmitting oil/molten salt tube system is required for transporting the heat from the solar array to the heat accumulation tanks and heat losses occur despite the insulation;

a need for heat-transmitting oil/molten salt tubes for transporting the heat from the solar array/accumulation tanks to the vapor power cycle;

losses of energy following the heat exchange between the heat-transmitting oil and heat accumulation tanks ;

the molten salt in the storage tanks (and even in the tubes when the carrier fluid is molten salt) is to be kept at higher temperatures with respect to the solidification temperature (about 200°C) to avoid their packing;

limited efficiency of the vapor power cycle; considering the efficiency chain, there is a limited overall efficiency of about 18.5%;

cumbersome volumes for the tubes and accumulation tanks;

there is an impact from an environmental viewpoint due to the leaks of heat-transmitting oil/molten salt in the ground;

the accumulation is quite limited for the

Linear Fresnel technology, indeed the use of water for the heat accumulation does not allow obtaining reasonable heat storage, if not with saturated vapor, the enthalpic contents of which highly limits the production efficiency of electricity due to the lower temperatures compared with the overheated vapor. The vapor stored in the form of saturate also requires challenging volumes which therefore limit the energy storage itself, with little predictivity of the electricity produced;

there is little predictivity of the electricity produced also for the Dish Stirling technology .

There are technical problems and limits for the SOLASTOR technology:

a vapor system having tubes with significant diameter (up to 18" for the example herein indicated) for transporting the heat from the heat accumulation modules to the vapor power cycle is required and there are heat losses, even if there is insulation;

cumbersome volumes for the vapor tubes;

limited efficiency of the vapor power cycle of about 33.7%;

considering the efficiency chain, there is a limited overall efficiency of about 22.7%;

the heat flow exiting the receiving cavity hits the vapor tubes in a non-uniform manner because it follows a non-linear path;

from the construction viewpoint, a series of graphite plates with a suitable groove are to be provided for running the vapor tubes;

there is a need to ensure an adequate argon flow in the grooves in which the pipes are accommodated in order to avoid corrosion, decarburization phenomena and metallurgical alterations of the steel forming the vapor tubes.

The patent document EP 2.326.886 describes a solar collector comprising heat regulator means defining a cavity therein and having an opening communicating with such a cavity to allow the solar energy incident to the opening to enter the cavity, and a device for collecting the solar energy arranged in the cavity and in thermal contact with the heat regulator means to collect the energy entering the cavity.

Summary of the invention

The authors of the present invention have surprisingly found that it is possible to exploit a power cycle for producing electricity using a fluid heated by a heat accumulator.

Object of the invention

In a first object it is described a process for producing electricity in a heat accumulation system integrated with a power cycle.

In a second object it is described a heat accumulation system integrated with a power cycle.

In another object it is described a heat accumulation, which is used for the purposes of the present invention.

Brief description of the figures

Figure 1 is a diagram of an integrated heat accumulation system with a Rankine vapor power cycle according to the prior art;

Figure 2A shows a heat accumulation system in solar applications with the working fluid circuit at one heat level ;

Figure 2B shows a heat accumulation system in solar applications, noting the angle between the outer opening of the receiver and the base thereof, which allows receiving the heat from the solar array to be optimized;

Figure 2C shows a particular configuration of the heat accumulation system at one heat level;

Figure 3 shows a heat accumulation system in solar applications with working fluid circuit at two heat levels ;

Figure 4 shows an embodiment of the system of the invention at a heat level which allows recovering and taking advantage of a flare gas or a tail gas;

Figure 5 shows an embodiment of the system of the invention at two heat levels for recovering and taking advantage of a flare gas or a tail gas;

Figure 6A shows the re-compressed thermodynamic cycle integrated with the accumulator of the invention;

Figure 6B shows the thermodynamic cycle with a mixing step which is integrated with the accumulator of the system, and Figure 6C shows a particular embodiment thereof;

Figure 7A shows the thermodynamic cycle at two heat levels integrated with the accumulator of the system, and Figure 7B shows a particular embodiment thereof;

Figure 8A shows an alternative embodiment of the system of the invention which uses a carrier fluid;

Figure 8B shows an alternative embodiment of the system of the invention which uses a direct exchange with the process combustion or heat fumes;

Figure 9A shows the re-compressed thermodynamic cycle integrated with the accumulator of the system by means of a carrier fluid and, in the version of Figure 9B, with a post-combustion step;

Figure 10A shows the re-compressed thermodynamic cycle at two heat levels integrated with the accumulator of the system by means of a carrier fluid and, in the version of Figure 10B, with a post-combustion step.

Detailed description of the invention

The process for producing electricity object of the present invention is carried out in a heat accumulation system AT 100 integrated with a power cycle 200.

For the purposes of the present invention, the heat accumulation AT 100, which represents per se one of the objects of the present invention, comprises three modules 110, 120, 130, each of which is characterized by its own function and in one embodiment of the invention, is not shared by one or more of the other modules:

the receiving module MR 110, which has the function of receiving the thermal energy and conveying it by conduction to the heat accumulation module;

the heat accumulation module MAT 120, which has the function of accumulating the thermal energy;

the heat exchange module MST 130, which has the function of exchanging heat with the working fluid (or with an intermediate carrier fluid) .

The receiving module MR 110 in turn comprises a black body, that is an opening 111 which collects heat.

In a preferred embodiment of the invention, such a heat comes from renewable energy sources.

In a preferred embodiment of the invention, the heat may be represented by solar energy which for example, may be collected by the mirrors in a solar array .

According to alternative embodiments of the present invention, such a heat may be directly or indirectly introduced .

If it is directly introduced, this is possible by the combustion in a burner of a combustible gas or by the flame in a furnace burning a gas selected from the group comprising: flare gas, syngas, tail gas after the separation from CO2 (biomethane), hydrogen, etc., or from the waste heat (thermal waste) from an industrial plant; or other liquid or solid fuels; or from the combustion of solid, liquid and gaseous waste.

If it is indirectly introduced, this is possible by means of a heat carrier fluid (FV) heated with the flame of a furnace burning a gas selected from the group comprising: flare gas, syngas, tail gas after the separation from CO2 (biomethane), hydrogen, etc.

In this case, the carrier fluid, heated for example also up to 550°C, transfers the heat to the heat receiving module MR 110, cooling down, for example down to 450°C and again 350°C.

Whether it originates directly or indirectly, the heat may also be represented by the exhaust fumes of a gas turbine or by the combustion gases of an existing furnace, possibly providing the addition of a further post-combustion step.

According to a further embodiment again, the thermal power (heat) may be provided by the products of a high temperature chemical reaction such as for example, the steam methane reforming reaction (SMR) , or from gasification.

The receiving module MR 110 serves the function of receiving the thermal energy from the solar array 500 (or from another natural and/or artificial source) and of conveying it by conduction to the heat accumulation module MAT 120.

In one embodiment, such a receiving module MR 110 has a truncated-conical shape and is made of materials with high thermal conductivity.

For example, such materials may be selected from the group comprising: graphite, solid sodium chloride, cast iron, steel, cement; metals selected from the group comprising: copper, aluminum, etc.; or from a refractory material selected from the group comprising: silicon carbide, vanadium carbide and zirconium oxide.

Graphite in particular may be characterized by the following values:

The use of other materials with respect to graphite has the advantage of limiting the graphite oxidation problems at the high temperatures with benefits on the oxidation containment and protection system of the storage material (through an inert-argon atmosphere) .

In one embodiment of the present invention, the receiving module 100 could be at least partially made of one or more materials with a conductivity greater than 25 W/mK, optionally greater than 35 W/mK, for example between 28 and 460 W/mK or between 28 and 200 W/mK.

As indicated above, an opening 111, which is normally cylindrical or conical in shape, having circular or elliptical section, which serves as a "receiving black body", is provided in the receiving module MR 110.

The receiving module MR 110 is shaped to conveniently guide the heat flow towards the heat accumulation module MAT 120, for example it may have a truncated-conical shape with a rectangular or circular, elliptical section.

The contact with the heat accumulation module MAT 20 occurs at the largest base (112), thereby transferring the heat by conduction.

In a preferred embodiment, the side portions 113,

heat accumulation module 120 through insulating materials selected from the group comprising: rock wool, glassy materials, mica, pumice and/or combinations thereof .

Opening 111 (black body) faces the external environment at the shortest base thereof 114 and in the absence of an incoming heat flow, may also be thermally insulated through a door or plug type device made of the above-described insulating materials (not shown in the figures) .

In one embodiment for solar concentration applications, the outer surface of such an opening 111 may have an angle a with respect to the horizontal (Fig. 2B) , which allows the luminous efficiency to be optimized: a merely geometrical factor given by the ratio between the radiant energy intercepted by the outer surface of the receiver opening and the total energy radiated by the solar array.

Since the technology is modular, solar arrays and related receiving modules 110 with different external opening may be provided for a given solar array consisting of several solar arrays 500 in order to optimize the luminous efficiency of each solar array also in the presence of a non-uniform orography of the site, such as for example a hilly ground.

The heat accumulation module 120, which only serves to accumulate the thermal energy, is made of materials which have a high conductivity and heat capacity.

These are preferably selected from the group comprising: copper, aluminum, graphite.

Moreover, in a preferred embodiment, it has a tetragonal shape, cylindrical having circular or elliptical section.

With regard to the external side surface, this is thermally insulated with insulating materials selected for example, from rock wool, glassy materials, mica, pumice and/or combinations thereof.

A base 112' is in contact with the receiving module 110 and acquires the heat collected by conduction, while the other base 115 is in contact with the heat exchange module MST 130 and transmits the heat by conduction thereto .

In a particularly preferred embodiment, the material which forms the heat accumulation module 120 is arranged, oriented, positioned, provided or poured so that a heat flow according to preferential directions is created therein, in the direction of the receiving module MR 110 or of the heat exchange module MST 130, rather than towards the side walls, which instead promote heat losses.

The heat exchange module MST 130 serves the function of transferring the heat by conduction (and partially by radiation) from the heat accumulation module MAT 120 to the working fluid (or intermediate carrier fluid) .

In particular, the heat exchange module 130 comprises means for transferring heat to the working fluid or intermediate carrier fluid and is made of a material having high thermal conductivity, such as for example: copper, aluminum, graphite for the receiving body and tubes 116 made of steel, copper, aluminum accommodated therein in the cavities provided in the receiving body.

It is worth noting that a part of the heat may also be transmitted by radiation (not solar) between the heat accumulation module MAT 120 and the receiving module MR 110 and between the receiving module MR 110 and the same tubes 116; with respect to a conventional exchanger where there are two fluids which exchange heat with each other, there are one or more fluids in the heat exchange module (MST) which enter and exit and which receive heat, also at various heat levels, from the heat exchange module.

The heat exchange module MST 130 may have one or more portions in contact with the heat accumulation module MAT 120 and the remaining outer surfaces are preferably thermally insulated.

The heat flow may be interrupted at the portions in contact with the heat accumulation module MAT 120, creating a gap in which a gas with a low heat exchange coefficient may be caused to flow; such a fluid may be for example, selected from the group comprising: air, argon, krypton, etc., argon in a preferred embodiment.

In an alternative configuration, the heat exchange module MST 130 may consist of a plate-type exchanger, which is conveniently worked to be in contact with the heat accumulation module MAT 120.

In another alternative configuration, the tubes 116 may be absent and may be replaced by the same cavities in the heat exchange module MST 130 when for example, the heat exchange module 130 is obtained through 3D printing techniques (additive manufacturing) .

Due to the 3D printing technique, such cavities may have circular, but also rectangular, hexagonal, elliptical section and other geometrical shapes in order to optimize the heat exchange; moreover, the axis of the cavity may also be helical, curvilinear in general (in addition to rectilinear) .

In a further alternative configuration, the heat exchange module MST 130 may be an exchanger obtained with techniques which are typical of PCHEs (printed circuit heat exchanger) , in which instead of having two fluids which exchange heat with each other, there are several fluids entering and exiting, which receive heat at various heat levels.

For the purposes of the present invention, the three modules: receiving module MR 110, heat accumulation module MAT 120 and heat exchange module MST 130 are structurally integrated in a single insulating container and form a heat accumulation AT 110.

In a preferred embodiment of the invention not shown in the figures, a gap may be provided at the side walls, in which a gas with a low heat exchange coefficient is contained, for example represented by argon .

The heat accumulation AT 110 may be installed on a dedicated structure on each heat accumulation or in a single structure housing more than one thereof.

Such structure (s) may also be pre-existing in a plant, a disused platform or a floor of an industrial plant or of a renewable energy park.

According to a particularly preferred embodiment of the present invention, the heat accumulation module MAT 120 does not comprise means for heat exchange to the working fluid.

The consequence thereof is the fact that the mass of material forming the heat accumulation module MAT 120 only performs the function of heat accumulation and not that of heat exchange as well.

Indeed, in one embodiment of the invention, the heat accumulation module MAT 120 is represented by a uniform mass which does not contain means for the heat exchange towards the working fluid (or carrier fluid) , rather it acquires the heat from the receiving module MR 110 and transfers it to the heat exchange module MST 130.

For example, it is not provided that the tubes of the tube bundle actuating the heat exchange are immersed in the mass of high conductivity and high heat capacity material .

For the purposes of the present description, "high heat capacity" means a material characterized, in the range of temperatures above 200°C, by a heat capacity equal to or greater than 1.40-kJ/kgK, for example for a heat capacity equal to 1.84 kJ/kg°C.

Preferably, the heat accumulator module MAT 120 is conveniently externally insulated so as to avoid external heat loss.

Such an insulation may be obtained for example, by means of an insulating material or with a gap inside of which a low heat exchange coefficient fluid circulates; such a fluid may be for example, selected from the group comprising: air, argon, krypton, etc., argon in a preferred embodiment.

According to a particular embodiment of the present invention, the possibility of inserting an insulating module (not depicted in the figures) between the receiving module MR 110 and the heat accumulation module MAT 120 may be provided.

Such a module allows the residual heat to be used when the receiving module MR 110 does not receive heat, such as for example during the night.

According to a further embodiment of the present invention, the possibility of inserting an insulating module between the heat accumulation module MAT 120 and the heat exchange module MST 130 may be provided.

Such a module allows the heat in the heat accumulation module MAT 120 to be maintained when the system is not operating.

Said modules may consist of devices which realize when gaps are required in which a solid, liquid, gaseous insulating material is to be inserted, such as for example, air, argon, krypton, etc., which preferably is represented by argon.

According to a particular embodiment of the present invention, if the heat is provided directly by combustion fumes or by the products of a chemical reaction, the receiving module MR 110 and the heat exchange module MAT 130 have the same general structure (Figure 8A and 8B) .

As described above, the working fluid of the power cycle 200 integrated with the heat accumulator AT 100 of the invention may be directly or indirectly heated by means of the use of an intermediate carrier fluid.

Preferably, a carrier fluid is represented by a fluid capable of carrying out a transfer of heat from one heat source to another.

In particular, a carrier fluid may be selected from the group comprising: water, superheated water, saturated vapor, superheated vapor, heat-transmitting oil; CO2 may alternatively be involved, possibly at a pressure of about 1/3 or 1/2 of the pressure of the CO2 of the power cycle.

For the purposes of the present invention, the working fluid is represented by a gas or a gas mixture selected from the group comprising: hydrocarbons, nitrogen, CO2, organic fluids, refrigerant fluids, vapor .

In a particularly preferred embodiment of the present invention, the working fluid is represented by CO2, possibly in the supercritical or subcritical state (thereby configuring a supercritical or transcritical CO2 cycle) .

"Working fluid flow" refers to the working fluid inside the circuit of the power cycle 200.

For simplicity, reference is made to CO2 in the continuation of the description, when any working fluid may be equally used.

A "CO2 flow" therefore means the CO2 (or other working fluid) in the power cycle circuit.

For the purposes of the present invention, such a flow may have a value up to 100 kg/s, >100 kg/s, preferably > 120 kg/s, and even more preferably >150 kg/s or also up to about 500 kg/s and higher.

In particular, the flow may be about 150 kg/s in a system with two thermal levels (see below) .

In a single thermal level system, the flow instead may be about 200 kg/s .

"Supercritical state" means a CO2 flow in the supercritical temperature and pressure conditions.

In particular, such conditions correspond to higher temperature and pressure conditions than those of the critical state.

In more detail, the CO2 is in critical condition under the following conditions: pressure Temperature

73.83 bar a 31.06°C

Under the supercritical conditions, the CO2 has certain properties which are similar to liquids and other properties which are similar to gases.

For the purposes of the present invention, when reference is made to a transcritical CO2 cycle, reference is made to a supercritical CO2 cycle which takes place at a supercritical pressure and temperature, in which after an expanding step below the critical pressure (73.83 bar a), the CO2 itself is cooled to below the critical temperature (31.06°C) (subcritical state) .

Since the temperature decreases below the condensation point causing the condensation of the CO2, it is correct to indicate a transcritical CO2 cycle as a CO2 condensation cycle.

In particular, the CO2 may be at the pressure of about 200 bar a and at a temperature of about 90-650°C, preferably of about 300-650°C or even >650°C.

According to the present invention, the tubes 116 in the heat exchange module MST 130 distribute the CO2 to a system of exchangers which are external to the heat accumulation modules 120.

With regard to the power cycle 200 integrated in the heat accumulator 100 of the present invention, it comprises :

- a turbine 220,

a heat recovery section represented by a heat recovery unit 230,

- a cooling section 240,

- a compressor 250 or pump per condensation cycle, a heat exchange section represented by an exchanger 260.

In a particular embodiment, the power cycle 200 may further comprise a re-compressor 270 (as described later) .

According to a first object of the invention therefore, the process for producing electricity in a system 400 comprises a heat accumulator AT 100 (inserted in a solar array and/or industrial plant) and a power cycle 200 which uses a working fluid, comprising a step in which said working fluid is heated by the heat accumulated by said heat accumulator AT 100.

In a preferred embodiment of the present invention, the working fluid is heated by the heat accumulated by a plurality of heat accumulators.

One (or more for the cycle with two thermal levels, as described later) tube from the power cycle carries the working fluid to the solar array, or heat accumulator system, where smaller tubes feeding the heat accumulation itself branch off at each heat accumulator AT 100.

Once heated in the heat accumulators AT 100, the working fluid is collected by a system of tubes which merge into a single tube (or several tubes for the cycle with two heat levels) which carries the working fluid to the power cycle 200.

The process of the invention in particular comprises a first step a) , in which a CO2 flow expands in turbine 220 cooling with the production of electricity (through an electric generation system, for example represented by an electric generator) .

Such a flow in particular is the CO2 flow heated in the heat exchange module MST 130 of the accumulator ( s ) of the solar array 500.

In a subsequent step b) , the CO2 flow exiting turbine 220 is cooled in the heat recovery section 230.

In a preferred embodiment of the invention, such a step is carried out:

in a high temperature recovery unit RAT 230' (step bl ) , and

in a low temperature recovery unit RBT 230'

(step b2 ) .

Then, the CO2 flow is further cooled in a step c) in the cooler 240 to be sent then to a compressor 250 for a compression step d) .

Exiting compressor 250, the CO2 flow is sent back to the recovery unit 230 for a heat recovery step g) with the expanded CO2 flow exiting the turbine 220 (step b) ) .

In one embodiment of the invention, the CO2 flow exiting compressor 250 (step d) ) may be sent to a step:

gl) of recovery in a low temperature recovery unit RBT 230' ' ; and/or to

g2) of recovery in a high temperature recovery unit RAT 230' .

From step g) , the CO2 flow is sent to the exchanger 260 for a heat exchange step e) in which the CO2 flow is heated .

In one embodiment of the present invention, the heat exchanger 260 comprises:

a high temperature heat exchanger SAT 260' for a high temperature heat exchange step el), and

a low temperature heat exchanger SBT 260 for a low temperature heat exchange step e2) .

For the purposes of the present invention, each section of high temperature heat exchange 260' and low temperature heat exchange 260 may comprise one or more modules .

For simplicity, reference is made to a section, this meaning a single- or multiple-module section.

The CO2 flow is sent back from sent e) to the turbine 220 for step a) .

In a preferred embodiment of the present invention, a portion of the CO2 flow exiting the low temperature recovery unit 260 of step b2) may be re-compressed in a step f) in a re-compressor 270.

Then said flow exiting re-compressor 270 is sent to the high temperature recovery unit RAT 230' for an additional further heat exchange step g2') with the expanded CO2 flow in turbine 220 (step b) .

In an alternative embodiment of the present invention, upon exiting the exchanger 260 (step e) and before entering the turbine 220 (step a), the CO2 flow is subjected to a post-combustion step h) in a post combustor 280, as depicted in figures 9B and 10B.

In a preferred embodiment, said step h) is carried out in the configuration in which a heat exchange with the intermediate carrier fluid (FV) occurs in exchanger 260 of the power cycle.

According to particular embodiments of the present invention, mixing steps are carried out in order to control the temperature of the CO2 flow in certain steps of the process.

According to a first embodiment, depicted for example in Figure 6A, step a) is carried out after a mixing step m) in which a portion of the CO2 flow obtained from step e) is mixed with a portion of the CO2 flow obtained from step g2 ) , and possibly from step g2') as well.

According to another embodiment, represented for example in Figure 6B, step g) is carried out after a mixing step m' ) in which a portion of the CO2 flow obtained from step e) is mixed with a portion of the CO2 flow obtained from the compression step d) obtained after a mass split (SMI in Figure 6B) .

According to a particular embodiment represented in Figure 6C, a portion of the CO2 flow from the compression step d) (obtained after a mass split - SM2 in Figure 6c) instead is used in the high temperature heat recovery step g2 ) after a mixing step m' ' ) with a portion of the CO2 flow exiting the exchanger 260 (step e) ) , while the remaining CO2 flow from the compression step d) is used in the low temperature heat recovery step gl ) .

According to the embodiment represented in Figure 7a, the CO2 flow from the low temperature heat exchange step e2) is used in the high temperature heat recovery step g2 ) after a mixing step ' ' ' ) with a portion of the CO2 flow from the compression step d) . For the purposes of the present invention, without being limited to the representation in the figures, one or more of the above-described mixing operations may be carried out according to what those skilled in the art deem it suitable.

In a preferred embodiment of the invention, the re compression step f) is not provided in the above- described embodiments in which there is a mixing between the CO2 flow (or a portion thereof) exiting the compressor (step d) ) and the CO2 flow (or a portion thereof) exiting the exchanger (step e) ) .

As described above, a portion of the CO2 flow exiting compressor 250 after a mass split (SM in Figures 7A and 7B) may be sent to the exchanger 260 rather than to the low temperature recovery unit RBT 230' ' , thereby configuring a power cycle with two thermal levels.

In a preferred embodiment, such a portion is between 0 and 100%, more preferably between about 10 and 50%, and even more preferably between about 15 and 50% of the total current exiting compressor 250.

According to a first embodiment of the invention, the heat exchange step e) carries out a direct heat exchange between the heat of the heat accumulator or of the heat accumulators, in particular exchanged by the heat exchange modules MST 130 thereof and the working fluid .

There, the exchanger 260 (or heat exchange section) of the power cycle 200 corresponds to the heat exchange module (s) MST 130 of accumulator ( s ) 100.

According to a further embodiment of the present invention (represented for example, in Figures 8A, 8B, 9A, 9B, 10A and 10B) , the heat exchange occurs between the heat accumulated in the heat accumulator AT 100, and more specifically with the heat exchange modules MST 130 thereof, and an intermediate carrier fluid (FV) is carried out in step e) , thus causing an indirect exchange between the heat accumulator AT 100 and the working fluid.

In particular, a heat exchange occurs in the heat exchange section 260 between the CO2 flow and the intermediate carrier fluid heated in the heat exchange module MST 130.

In particular, the intermediate carrier fluid may be introduced into the heat exchange module MST 130 of the accumulator at a temperature of about 100-150°C, for example, preferably of 150-200°C, for example, and be heated up to a temperature >300°C, for example, preferably of 450°C and higher.

In one embodiment of the invention, the intermediate carrier fluid (FV) may be subjected to a heat recovery step for a further cooling thereof prior to the entry into the exchange module of accumulator MST 130, thus taking advantage of fluids outside the power cycle (Figures 8A and 8B) .

Once cooled after the heat exchange, the intermediate carrier fluid (FV) is recirculated in accumulator 100.

According to a preferred embodiment of the invention, in step a) , turbine 220 is capable of producing a quantity of electricity of about 20 MWe .

Following the expansion in the turbine 220 of step a) , the temperature of the CO2 flow for example, decreases from about 600°C to about 480°C.

Simultaneously, the pressure following the expansion may be reduced from about 200 bar a to about 78 bar a.

Step bl) instead is capable of further lowering the temperature, for example down to 280°C, while step b2) for example down to about 80°C.

The possible re-compression step f) may increase the temperature of the portion of CO2 flow for example up to about 220°C.

Simultaneously, the pressure following the re compression increases from about 78 bar a to about 200 bar a .

With regard to step c) , according to an embodiment of the present invention, the cooler 240 operates with water .

This may in turn be cooled by a cooler, in a cooling tower; alternatively, it may be obtained from natural sources (sea, waterbed, river, etc.) .

Alternatively, a low temperature fluid may be used in place of the water, for example represented by LNG to be regasified or by another gas to be regasified (e.g. hydrogen, nitrogen, liquid air, etc.) or again, by a gaseous, liquid or solid cryogenic storage.

Otherwise according to embodiments not represented in the figures, the cold source may be obtained artificially by a refrigeration cycle.

If the circumstances are considered in which the ambient temperature is about 35°C, given that the critical temperature of the CO2 is close to the ambient one, in order to liquefy it by cooling it, for example down to 25°C, a refrigeration cycle would be carried out which operates with a COP (coefficient of performance) value of about 10-15, therefore very high; this would have the advantage of a reduced energy consumption required by the refrigeration cycle and pumping work of the CO2 (rather than increased compression work) , thus causing an increase in the efficiency of the CO2 cycle.

For the purposes of the present invention, the cooling circuit 300 operates on the power circuit 200, in particular downstream of the low temperature recovery unit RBT 230' ' .

If the (natural or artificial) cold source is available at a sufficiently low temperature as described above, the cooling may lead to the liquefaction of the C0 ₂ .

For this purpose, the temperature of the cold source preferably needs to be about <25°C.

Therefore, a condensation cycle is achieved under such conditions.

As a consequence, the compressor 250 may be replaced by a pump in the power cycle.

The cooling of step c) may lower the temperature of the CO2 flow down to about 20-40°C.

The compression step d) instead may increase the temperature of the CO2 flow, for example up to about 58- 60°C if the mass split is implemented, or also up to about 100°C.

The heat recovery steps following compression may further increase the temperature of the CO2 flow, for example up to about 400°C.

With regard to the heat exchange step e) , this may cause an increase of the temperature of the CO2 flow, for example from about 392°C to about 610°C. In the embodiment of the invention providing two thermal levels, the low temperature heat exchange section SBT 260' ' may receive a CO2 flow at a temperature which is >50°C (for example of about 58°C) and heat it up to >300°C (for example up to about 306°C) ; the high temperature heat exchange section SAT 260' may receive a CO2 flow at a temperature which is >390°C (for example of about 392°C) and heat it up to >600°C (for example up to about 610°C) .

The mixing steps mentioned above instead serve the purpose of slightly modifying - in particular lowering - the temperature of the CO2 flow.

Such steps may be carried out in suitable mixers or may be obtained simply with specific communication valves between tubes which guide the flows to be mixed.

The purpose of the mixing step m) is to control the temperature of the CO2 flow; for example, it may allow the temperature to be decreased from about 610°C to about 600°C.

The purpose of the mixing step ' ' ' ) is to control the temperature of the CO2 flow; for example, it may allow the temperature to be decreased from about 306°C to about 300°C.

According to a second object of the invention, a system 400 is described for producing electricity comprising a heat accumulator AT 100 and a power cycle 200.

Said accumulator and said power cycle in particular are the above-described heat accumulator (AT) and the power cycle.

For the purposes of the present invention therefore, a system 400 is described for producing electricity, comprising a heat accumulator AT 100 integrated with a power cycle 200 which uses a working fluid, said heat accumulation AT 100 comprising a thermal energy receiving module MR 110, a thermal energy accumulation module MAT 120 and a heat exchange module MST 130 of said thermal energy, said power cycle 200 comprising a turbine 220 for generating electricity by expanding the working fluid, a heat recovery section 230, a cooler 240, a compressor 250, an exchanger 260, and possibly a re-compressor 270, in which said heat exchange module MST 130 of accumulator 100 is in thermal connection with said exchanger 260 of the power cycle.

This means that the working fluid in exchanger 260 of the power cycle is directly or indirectly heated, by means of an intermediate carrier fluid, by the heat accumulated by the heat accumulator 100.

In a preferred embodiment of the invention:

the receiving module MR 110 is made of materials with high thermal conductivity;

the heat accumulation module MAT 120 is made of materials with high conductivity and heat capacity and is externally thermally insulated; and

said heat exchange module MST 130 comprises means for transferring heat to the working fluid or carrier fluid (FV) and is made of a material having high thermal conductivity.

Alternatively, said accumulator is the above- described heat accumulator AT 100 and the power cycle 200 is a cycle selected from the group comprising: Rankine vapor cycle (overheated, regenerated, overheated and regenerated), Rankine organic cycle.

As described above, the thermal power (heat) may be provided by the products of a high temperature chemical reaction, for example steam methane reforming reaction (SMR) and gasification.

In a preferred embodiment of the invention for example shown in Figure 7B, a CO2 flow of about 150 kg/s at the pressure of about 200 bar a and temperature of about 600°C (1) expands in turbine 220 up to the pressure of about 78 bar a and a temperature of about 480°C (2) and carrying out a work of about 20 MW; the

CO2 current in the high temperature recovery unit RAT 230' cools down to about 280°C (3) and further cools to about 80°C in the low temperature recovery unit RBT 230' and cools again in cooler 240 down to about 32°C (5) ; it is brought to about 200 bar a in the compressor 250, with a compression work of about 4 MW, and to the temperature of about 60 °C and divided in SMI into a fraction of 20 kg/s (6b) and a fraction of 130 kg/s (6a); the fraction of 20 kg/s is sent to the solar array (or heat accumulation module system of an industrial plant) where it reaches about 58 °C (6b) due to the heat losses, and it heats up to about 306°C (7c) by crossing the heat accumulation modules 100; it returns towards the power cycle 200 at the temperature of about 300°C, controlled through a mixing step; current 6a is heated in the low temperature recovery unit RBT 230' up to about 200°C (7e) and joins 7d, forming current 7 of 150 kg/s (complete flow) at 215°C; current 7 is heated in the high temperature recovery unit RAT 230 up to about 400°C (8) and is sent to the solar array (or heat accumulation module system in an industrial plant) where it reaches the temperature of about 392 °C (9) due to the heat losses, and is heated on the heat accumulation modules 100 up to about 610°C (10); the current returns to the power cycle 200, reaching about 600°C (1) due to the heat losses; the temperature is controlled through a mixing step; the net electric power is about 16 MWe and 35-45% of energy efficiency.

In another alternative, said heat accumulator is the above-described heat accumulator 100 and supplies the thermal power alone used by an industrial, chemical manufacturing or food process and/or electric power produced according to what is described above.

According to an alternative embodiment of the invention shown in Figure 2C, there may be provided means for controlling the temperature of the receiving module 110 and possibly of the heat accumulation module 120 as well.

Such thermal control means may be represented by a first coil 101 in the receiving module 110 and possibly by a second coil 102, downstream of the first coil 101, in the heat accumulation module 120.

More specifically, said thermal control means are inserted in a secondary circuit of the working fluid 210' located upstream of the heat exchange module MST 130.

Due to said first coil 101 and possibly said second coil 102, a portion of the fluid entering the heat exchange module 130 is not sent to said module 130 but is deflected to the first coil 101 and/or the second coil 102 by means of the heat accumulation 120 in order to control the temperature of the receiving module 110 and of opening 111, and of the heat accumulation module (MAT) 120, respectively.

The flow is reintroduced into the power cycle 210 downstream of said first coil 101 and possibly said second coil 102.

According to a particular embodiment, a by-pass connection may be provided which, according to the temperature control needs, excludes the first coil 101 and/or the second coil 102 as well.

Those skilled in the art may understand the several advantages offered by the present invention from the description provided above.

Firstly, due to the configuration of the system, and in particular due to the structure of the heat accumulation module (MAT) , the whole mass forming the matrix thereof performs the function of accumulating the thermal energy, thus improving the energy efficiency overall .

The possibility of externally isolating such a module, and again the possibility of thermally isolating it towards the heat receiving module and towards the heat exchange module, allows such an efficiency to be further increased.

The use of renewable energy sources undoubtedly is advantageous from an environmental viewpoint, while the use of a gas represented for example, by a flare gas, syngas, tail gas after the separation from CO ₂ (biomethane), hydrogen, etc., or from the waste heat (thermal waste) from an industrial plant, is a clear indication of the wide flexibility of the system described and of the utility in making the most of otherwise lost heat sources.

The embodiment providing the use of a carrier fluid allows the advantage of collecting thermal energy from several utilities.

The absence of a heat-transmitting oil/molten salt carrier fluid for transporting the heat from the solar array/accumulation tank to the vapor power cycle eliminates the several problems of managing such fluids (for example, the molten salt may solidify) and also has benefits from an environmental viewpoint due to a reduction of pollution; indeed by using the heat- transmitting oil, there is a need to reclaim the ground at the end of the life of the plant.

The embodiment providing the use of the CO ₂ as working fluid in the critical or subcritical step has further particular advantages.

Indeed, the high density of the CO ₂ allows a compactness of the power cycle which reduces its weights, volumes and footprint.

In addition to being more compact, especially with respect to the technology of the patent EP 2,326,886 which requires tubes having a diameter of 16-18 inches (400-450 mm), and less expensive to be manufactured, the system generates lower heat losses due to the reduction of the diameter of the tubes, which low temperature heat exchange (SBT) inlet sections may have a diameter of about 3 inches (80 mm) and the outlet sections may be about 4 inches (100 mm), while the high temperature heat exchange (SAT) inlet sections may have a diameter of about 6 inches (150 mm) and the outlet sections of about 8 inches (200 mm) .

The greatest density of the CO2 with respect to the vapor moreover allows increasing the useful volume for the purposes of the thermal energy storage incremented by about 25%.

With regard to the advantages in terms of efficiency, the CO2 cycle allows an efficiency of 35-45% and higher with respect to the vapor cycles (which provide efficiency of 33.7% for the examined 12.5 MWe vapor turbine size) .

Considering the chains of efficiency of the thermal-electric conversion, the overall efficiency is in any case high at 29.4% The energy production is further increased up to by about 30%.