Plant and method for energy production with thermal storage

Abstract

A plant (100) for energy production, particularly by means of a Rankine cycle, comprises: a boiler (1); a heat exchanger (2) where a process fluid receives heat produced by the boiler (1); at least one turbine (3) in which the process fluid circulates; an electric generator (4) driven by the turbine (3); a recirculation line (7) to return the process fluid from the turbine (3) to the heat exchanger (2); a series of preheaters (17) located along the recirculation line (7) to preheat the process fluid by heat exchange with process fluid drawn from the turbine (3) and / or the heat exchanger (2) via respective drawing lines (15a, 15b, 15c); and a thermal storage system (101), comprising one or more pressurized storage vessels (22a) containing process fluid at substantially the same temperature and pressure conditions to compensate for variations in the flow rate of the drawing lines (15a, 15b, 15c) by storing process fluid in the storage vessels (22a) and releasing process fluid from these.

Description

[0001] TRANSLATION (RULE 12.3)

[0002] 04 February 2026

[0003] PLANT AND METHOD FOR ENERGY PRODUCTION WITH THERMAL STORAGE

[0004] Cross-Reference to Related Applications This Patent Application claims priority from Italian Patent Application No . 102025000002196 filed on February 5, 2025, the entire disclosure of which is incorporated herein by reference .

[0005] Technical Field

[0006] This invention relates to a plant and method for energy production with thermal storage .

[0007] The invention finds a preferred application in connection with plants operating on a Rankine cycle and thus with pressurised water, such as nuclear reactors, biomass boilers, waste-to-energy plants and any other plant that needs to store heat in the form of hot water .

[0008] Background

[0009] Many plants, for example electricity generation plants, are preferably operated at constant rated power, that is, at maximum power . This is the case, for example, with nuclear power plants, which are programmable plants but characterised by high capital costs and low fuel costs, so that it is advisable to operate them continuously at maximum power, including so as to avoid transients that tend to fatigue the various reactor structures . This is also the case with biomass plants or waste-to-energy plants, which should preferably be operated at constant power . These programmable plants are destined to operate in an environment with increasing penetration of non-programmable plants that considerably complicate the management of the electricity grid. In this context, energy storage is increasingly in demand. Said storage can be done electrically, but in thermalplants, thermal storage is an opportunity that can be realised at a much lower cost, even if it requires the resolution of certain technical issues, in particular concerning the thermal cycle used.

[0010] Many plants adopt a Rankine cycle .

[0011] For example, as schematically illustrated in Figure 1, in nuclear reactors, the heat generated in a boiler 1 (nuclear reactor) is transferred to a primary fluid that can be water, molten sodium, molten lead, helium, molten salts, etc . , and transported to a steam generator 2 that generates steam V with an almost isobaric water-steam transformation. In biomass and waste-to-energy plants, the boiler acts directly as a steam generator .

[0012] In each case, the steam produced is sent to a turbine 3 that is the component where useful work is produced, that can be transformed into electricity by means of an electric generator 4 (alternator) . Steam, at a high pressure and enthalpy, enters the turbine 3 at the maximum cycle temperature and expands to the minimum pressure of a condenser 5, increasing specific volume and decreasing temperature .

[0013] An extraction pump 6 extracts condensate C from the condenser 5 and returns it to the steam generator 2 via a recirculation line 7 to repeat the cycle; the difference between the expansion and compression power is the net mechanical power obtained from the partial conversion of the heat input to the cycle .

[0014] In a diagram like Figure 1, the mechanical / electrical power is a well defined fraction of the steam generator ' s thermal power and defines the plant ' s production. When the electricity grid requires a change in load, it is thereforenecessary to change the power of the steam generator 2 and, in the case of a nuclear reactor, that of the reactor as well, which will therefore be operated under conditions that are not necessarily optimal .

[0015] Various solutions have been proposed to overcome this drawback. For example, the solutions described in WO2023 / 078825A1 solve this problem by introducing an intermediate circuit between the nuclear boiler and the steam generator . The intermediate circuit is equipped with a thermal storage tank for storing hot fluid and a thermal storage tank for storing cold fluid. With this construction configuration, it is possible to operate the nuclear boiler at a constant power and at the same time change the power sent to the steam generator, in particular by filling the hot storage tank and emptying the cold storage tank to reduce the power required by the steam generator, or by emptying the hot storage tank and filling the cold storage tank to increase the power required by the steam generator .

[0016] This solution, too, is not without its drawbacks for various reasons, in particular because the addition of an intermediate circuit and heat exchanger, in addition to entailing greater intrinsic complexity, depresses the temperature of the fluid at the inlet to the steam generator and thus also the production of the cycle and the mechanical / electrical power that can be generated. The presence of a hot storage tank and a cold storage tank also doubles the required process volume with respect to the volume of the storage fluid.

[0017] US2013056170A1 also proposes to solve the same problem, but with a plant that appears extremely inefficient because it consists of a cascade of modules with a limitedtemperature difference between inlet and outlet . Since the energy stored depends on the product of the mass of fluid by its thermal capacity by its temperature difference, if a cascade of modules is used to fractionate the temperature difference, the volume required and the cost of storage will be amplified.

[0018] Summary

[0019] One purpose of this invention is to provide a plant and method for energy production with thermal storage that overcome the drawbacks highlighted by the known solutions and have construction and cost advantages .

[0020] This invention thus relates to a plant and method for energy production with thermal storage as defined in claims 1 and 10, respectively.

[0021] Additional features of the invention are defined in the dependent claims .

[0022] In essence, the thermal cycle according to this invention does not require an additional circuit for thermal storage, instead using the same process fluid for thermal storage, generally water, as is used as the process fluid of the steam generator and turbine . The power of the steam generator remains constant over time, while the power of the turbine can vary due to the different timing with which the water is preheated. To increase the power of the turbine, hot stored water is used and the flow rate of the drawing is reduced; to reduce the power of the turbine, the drawing flow rate is increased to preheat additional hot water .

[0023] Furthermore, the solution of this invention does not require a hot storage and a cold storage, but a single storage in the lower part of which cold water is contained and hot water in the upper part, with the hot-cold interfacerising when more power is required and falling when less power is required.

[0024] Brief Description of the Drawings

[0025] The invention is described in the following nonlimiting embodiment, with reference to the figures in the accompanying drawings in which:

[0026] - Figure 1 is a simplified diagram of a Rankine cycle applied to a plant for energy production of the prior art;

[0027] - Figure 2 is a more detailed diagram of a plant for energy production according to the prior art, operating with a Rankine cycle;

[0028] - Figure 3 is a diagram of a plant for energy production in accordance with a first embodiment of the invention;

[0029] - Figure 4 is a diagram of a plant for energy production in accordance with a second embodiment of the invention;

[0030] - Figure 5 exemplifies a possible development of the power produced by the plant in Figure 3 or 4 during the 24-hour day.

[0031] Description of Embodiments

[0032] Figure 1 is a simplified schematic representation of a plant for energy production using a Rankine cycle : a boiler 1 for heat production ( for example, a nuclear reactor) supplies heat to a steam generator 2 ; steam V produced in the steam generator 2 feeds a turbine 3, which is the component where useful production takes place that can be converted into electricity by means of an electric generator 4 ( for example, an alternator) . The steam leaving the turbine 3 is condensed in a condenser 5; an extraction pump 6 extracts condensate C from the condenser 5, which is sent back to the steam generator 2 via a recirculation line 7 to repeat the cycle .In industrial practice, the basic diagram in Figure 1 is incorporated into a much more complex plant to improve the cycle production, as depicted in Figure 2 .

[0033] The steam V produced in the steam generator 2 with the heat provided by the boiler 1 is sent via a steam line 8 to the turbine 3, which for large plants of tens or hundreds of MWe consists of at least two turbine bodies, for example (as in the example in Figure 2 ) a high-pressure turbine body 3a and a low-pressure turbine body 3b . Steam VI is extracted from the high-pressure turbine body 3a, which is sent to a dehumidifier 9 where a flow of condensate Cl is subtracted from it; subsequently, the dehumidified steam V2 leaving the dehumidifier 9 is superheated in a superheater 12 by thermal exchange with steam V3 taken directly from the steam generator 2 or from an initial part of the high-pressure turbine body 3a; the steam V3 from the superheater 12 is for the most part transformed into a condensate C2 as a result of heat exchange with the steam V2 at a lower pressure and lower temperature .

[0034] Steam V4 escapes from the turbine 3 and specifically from the low-pressure turbine body 3b, which is condensed in the condenser 5 to obtain the condensate C that is sent via the recirculation line 7 equipped with the extraction pump 6 to the steam generator 2 .

[0035] It is well known that the production of a cycle such as the one described here depends on the temperature at which heat is supplied to the process fluid in the steam generator 2 and the temperature at which the steam is condensed in the condenser 5.

[0036] For this reason, the condensate C is not sent directly to the steam generator 2 but is pre-heated by means ofsubsequent heat exchanges with steam drawn, by means of drawing lines 15a, 15b equipped with respective control valves 16a, 16b, from the high-pressure turbine body 3a and, respectively, from the low-pressure turbine body 3b; the drawing lines 15a, 15b feed respective preheaters 17 placed in series along the recirculation line 7 and where the condensate C is heated before returning to the steam generator 2. In particular, the preheaters 17 comprise a first preheater 17a that the condensate C passes through first along the recirculation line 7 and is supplied with steam at a lower pressure and temperature than the other preheaters 17, taken in particular from an intermediate part of the low-pressure turbine body 3b .

[0037] In some cases, such as in liquid-lead-cooled nuclear reactors, a final preheater 17c can also be added, which directly utilises steam V taken from the steam generator 2 via an additional drawing line 15c (which branches off, for example, from the steam line 8 ) equipped with a control valve 16c .

[0038] In industrial practice, a condensate C degasser 19 is also provided, located along the recirculation line 7 between two groups of preheaters 17, for example, between the preheaters 17 fed by the high-pressure turbine body 3a and the preheaters 17 fed by the low-pressure turbine body 3b .

[0039] The condensate Cl coming from the dehumidifier 9 and the condensate C2 coming from the superheater 12 are preferably sent to the degasser 19, via respective condensate lines 19a, 19b, or to one or more of the preheaters 17.

[0040] The extraction pump 6 generally has limited head to feed the degasser 19; a second high-head pump 20, located along the recirculation line 7 downstream of the degasser19, feeds the steam generator 2.

[0041] Figure 3 illustrates a plant 100 for energy production according to this invention, wherein the general diagram described above is combined with a thermal storage system 101 .

[0042] It is understood that although reference is made in this description to a process fluid consisting of water (in liquid and steam form) and thus the heat supplied by the boiler is used in a steam generator, the plant 100 may operate with a different process fluid. Here and below, the term steam generator indicates a heat exchanger not necessarily operating with steam.

[0043] The thermal storage system 101 comprises at least one pressurized storage vessel 22a having an upper opening 23a connected to a hot line 24a and a lower opening 25a connected to a cold line 26a; the hot line 24a and the cold line 26a are connected to the recirculation line 7 at a first connection point 27a and a second connection point 27b, respectively .

[0044] In general, the first connection point 27a is located downstream of all the preheaters 17 (Figure 3) , or between a first and a second set of preheaters 17 (Figure 4 ) , and the second connection point 27b is located upstream of at least some and preferably all the preheaters 17. Here and below, the terms "downstream" and "upstream" are understood to refer to the normal direction of circulation of the process fluid in the plant (in particular, from the turbine 3 to the heat generator 2 ) .

[0045] In the embodiment in Figure 3, the connection point 27a is located downstream of all the preheaters 17 and upstream of the steam generator 2, and the connection point 27b islocated downstream of the extraction pump 6 and upstream of the first preheater 17a and then upstream of all the preheaters 17 .

[0046] The cold line 26a is equipped with a pair of valves 28a, 29a placed in parallel along the cold line 26a on respective parallel branches of the cold line 26a, and a pump 30a connected with the valve 29a, that is, placed in series with the single valve 29a on the same branch of it .

[0047] The vessel 22a contains a process fluid that is the same process fluid circulating in the plant 100, that is, water .

[0048] Optionally, the thermal storage system 101 comprises multiple pressurized storage vessels 22a, all containing the same process fluid and all operating in parallel under the same pressure and temperature conditions; the vessels 22a all contain process fluid under essentially the same temperature and pressure conditions, all being connected in parallel to the recirculation line 7 via the hot line 24a and the cold line 26a, which connect to the recirculation line 7 at the first connection point 27a and the second connection point 27b, respectively, which are common to all of the vessels 22a .

[0049] In particular, the vessel 22a (or each of the vessels 22a if more than one) contains hot process fluid in an upper zone at the temperature of the inlet to the steam generator 2 and cold process fluid in a lower zone at the temperature of the outlet from the condenser 5, separated by a hot-cold interface 32a .

[0050] The vessel 22a is maintained in such a condition that it essentially contains only single-phase liquid phase fluid, in particular liquid water, and does not contain steamor, in any case, process fluid in the gaseous phase .

[0051] The thermal storage system 101 also comprises at least one expansion vessel 22c, preferably at atmospheric pressure, having a lower opening 25c connected via a compensation line 31 to the cold line 26a, on which it engages between the connection point 27b and the valves 28a, 29a; the compensation line 31 is provided with a pair of valves 28c, 29c placed in parallel along the compensation line 31 on respective parallel branches of the compensation line 31, and a pump 30c connected with the valve 29c, that is, placed in series with the single valve 29c on the same branch as the valve 29c . The vessel 22c is connected to the rest of the plant 100 via only the opening 25c, being in particular only connected to the cold line 26a via the compensation line 31, and has no other inlets or outlets connected to the plant 100 other than the opening 25c .

[0052] The vessel 22c contains process fluid (the same as that circulating in the plant 100 and vessel 22a, namely water) up to a level 33.

[0053] The actuation and opening of the control valves 16a, 16b supplying the preheaters 17 and the valves 28a, 28c, 29a, 29c connected to the vessels 22a, 22c are controlled, for example by means of a control system (not illustrated) , in order to regulate the flow rate of the drawing lines 15a, 15b and the flows of process fluid in the vessels 22a, 22c .

[0054] The control valves 16a, 16b, 16c define flow control means 36 acting on the drawing lines 15a, 15b, 15c to regulate the flow rate of the drawing lines 15a, 15b, 15c and consequently the output of the turbine 3. In other embodiments, the flow-rate regulation of the drawing lines 15a, 15b, 15c can be carried out, instead of using therespective control valves 16a, 16b, 16c, by controlling the condensate discharge from the various preheaters 17 , by means of condensate discharge lines 37 (shown in dotted lines) .

[0055] The operation of the plant 100 in executing the method of the invention is as follows .

[0056] When the plant 100 is operating at the rated output, that is, both the boiler 1 and the electric generator 4 are operating at the rated output, the valves 28a, 29a on the cold line 26a connected to the vessel 22a (or to each of the vessels 22a) are closed; the hot-cold interface 32a in the vessel 22a does not change level over time . The valves 28c, 29c connected to the expansion vessel 22c are also closed.

[0057] The condensate C leaving the condenser 5 circulates in the recirculation line 7 back to the steam generator 2, effectively without circulating in the thermal storage system 101 .

[0058] The dehumidifier 9 operates at the rated capacity; the superheater 12 and pre-heaters 17 operate at the rated output .

[0059] When less mechanical / electrical power is required from the plant 100, the valves 28a, 28c are opened in flow-rate control (while the valves 29a, 29c are closed) to release cold process fluid from the vessel 22a to the first preheater 17a and to the expansion vessel 22c, where the level 33 increases . In the vessel 22a, the volume of hot water increases, which, having a lower density, needs the expansion vessel 22c to absorb the total expansion volume of the cycle .

[0060] The control valves 16a, 16b of the drawing lines 15a, 15b (and possibly also the control valve 16c of the drawing line 15c) release more steam to preheat more water that feeds the steam generator 2 with a constant flow rate and partlyfeeds the vessel 22a from above, via the hot line 24a, with a consequent reduction in the level of the hot-cold interface 32a .

[0061] When more mechanical / electrical power is required from the plant 100, the valves 28a, 28c are closed, and the valves 29a, 29c are opened to allow the pumps 30a, 30c to feed the vessel 22a from below with condensate C from the condenser 5 and, respectively, with compensation fluid from the vessel 22c, resulting in a reduction of its level 33.

[0062] The control valves 16a, 16b of the drawing lines 15a, 15b (and possibly also the control valve 16c of the drawing line 15c) are open but with a smaller opening than the rated operation in order to subtract less power from the turbine 3 and increase the power of the electric generator 4. Under extreme conditions, all the valves 16a, 16b, 16c are fully closed to maximise the power of the turbine 3 and electric generator 4 .

[0063] In this case, which needs to be evaluated on a case-by-case basis depending on the thermal cycle adopted, the maximum peak power can be up to about 40% higher than the rated power . The minimum power is limited by the operation of the turbine 3 against the increased extraction of steam from the drawing lines 15a, 15b .

[0064] As already shown, according to some alternative embodiments, the flow-rate regulation of the drawing lines 15a, 15b, 15c can be carried out, instead of using the respective control valves 16a, 16b, 16c, by controlling the condensate discharge from the various preheaters 17 , by means of condensate discharge lines 37 (shown in dotted lines) .

[0065] The configuration just described is the one which allows the greatest increase in electrical output, but aconfiguration is also possible in which the vessel 22a is not directly connected to the steam generator 2 outlet but is inserted between the preheaters 17, for example between the last preheater 17c and the penultimate preheater 17 (immediately preceding the last preheater 17c) .

[0066] In another variant, illustrated in Figure 4, the vessel 22a (or multiple vessels 22a operating in parallel) is connected, via its opening 23a and the hot line 24a, with the outlet of the degasser 19; the connection point 27a is located along the recirculation line 7 between a first and a second group of preheaters 17 (and not downstream of all the preheaters 17 as in the embodiment in Figure 3) , while the connection point 27b is still located upstream of all the preheaters 17 ; compared with the solution illustrated above, this variant allows the use of a less efficient pump 30a, and also allows the pressure of the vessel 22a to be kept relatively low and is, therefore, economically more attractive, despite having a lower peak power .

[0067] According to this variant, the pressurised storage vessel 22a is connected via the upper opening 23a and the hot line 24a to the outlet of the degasser 19, and via the lower opening 25a and the cold line 26a to the inlet of the first preheater 17a; the cold line 26a is connected to the recirculation line 7 upstream of the extraction pump 6 (that is, between the condenser 5 and the extraction pump 6) ; the cold line 26a is still provided with a pair of valves 28a, 29a placed in parallel along the cold line 26a on respective parallel branches of the cold line 26a, and with a pump 30a connected with the valve 29a, that is, placed in series with the single valve 29a on its same branch.

[0068] The vessel 22a (or each of the vessels 22a if more thanone) contains hot process fluid in an upper zone at the temperature of the inlet to the pump 20 and cold process fluid in a lower zone at the temperature of the outlet from the condenser 5, separated by the hot-cold interface 32a .

[0069] In this case too, the vessel 22a is maintained in such a condition that it essentially contains only single-phase liquid phase fluid, in particular liquid water, and does not contain steam or, in any case, process fluid in the gaseous phase .

[0070] The thermal storage system 101 still comprises at least one expansion vessel 22c, as described above .

[0071] When the plant 100 is operating at rated power, the valves 28a, 29a, 28c, 29c connected to the cold line 26a of the vessel 22a are closed and the hot-cold interface 32a in the vessel 22a does not vary in level over time .

[0072] The dehumidifier 9 operates at the rated capacity; the superheater 12 and pre-heaters 17 operate at the rated output .

[0073] When less mechanical / electrical power is required, the valves 28a, 28c are open and release water from the vessel 22a to the inlet of the extraction pump 6 and to the expansion vessel 22c, respectively, resulting in an increase in its level 33. In the vessel 22a, the volume of hot water increases, which, having a lower density, needs the expansion vessel 22c to absorb the total expansion volume of the cycle .

[0074] The control valves 16b of the drawing lines 15b, and possibly also the valves placed along the supply lines of the degasser 19, release more steam to preheat more water that feeds the steam generator 2 with a constant flow rate and partly feeds the vessel 22a from above, via the hot line 24a, with a consequent reduction in the level of the hot-cold interface 32a .

[0075] When more mechanical / electrical power is required, the valves 28a, 28c are closed, and the valves 29a, 29c are opened to allow the pumps 30a, 30c to feed the vessel 22a from below with, respectively, condensate C from the condenser 5 and with compensation fluid from the vessel 22c, respectively, resulting in a reduction of its level 33.

[0076] The control valves 16b of the drawing lines 15b and the valves placed on the feeding lines of the degasser 19 are open but with a smaller opening than the rated operation in order to subtract less power from the turbine 3 and increase the power of the electric generator 4. Under extreme conditions, all the valves 16b are fully closed to maximise the power of the turbine 3 and electric generator 4.

[0077] An example of use of the plant 100 described above equipped with the thermal storage system 101 according to the invention is shown in Figure 5.

[0078] Assuming, by way of example, that the plant 100 is plugged into an electricity grid with a strong presence of photovoltaic plants, the electricity grid will face a production shortage in the morning before the photovoltaic power goes up and in the late evening following the shutdown of photovoltaic production. The thermal storage system of the invention will be able to compensate for the grid demand with greater production in the two periods mentioned above by reducing production at night when there is usually less demand for energy and towards the middle of the day when photovoltaic production becomes predominant .

[0079] The production diagram reproduced in Figure 5 refers to such a situation, with 12 hours of operation at rated power P, two phases of operation at reduced power Rl, R2 ofapproximately 40% of rated power, and two phases of increased power Wl, W2 of approximately 40% of rated power, for a daily energy production almost identical to that of a plant operating 24 hours at rated power .

[0080] The above reveals clear advantages of this invention: - the thermal storage system can use the same process fluid as the plant with which it is connected, in particular the same water;

[0081] - the thermal storage system can store both cold and hot process fluid in the same volume with a change in the proportion of the hot-cold interface; the expansion vessel compensates for the change in volume due to the different density of the process fluid when changing from cold to hot and vice versa;

[0082] - the boiler (however powered) and the steam generator can continue to operate under rated conditions, while the output of the turbine and electric generator can be varied;

[0083] - assuming, by way of example, that the plant is plugged into an electricity grid with a strong presence of photovoltaic plants, the electricity grid will face a production shortage in the morning before the photovoltaic power goes up and in the late evening following the shutdown of photovoltaic production. The system of the invention will be able to compensate for the grid demand with greater production in the two periods mentioned above by reducing production at night when there is usually less demand for energy and towards the middle of the day when photovoltaic production becomes predominant;

[0084] the example in Figure 5 shows step and maximum amplitude variations in the plant ' s power output, but slower and smaller amplitude variations are also possible; thispossibility when applied to a nuclear plant allows the intervention of the control rods to be minimised and the transients of the nuclear boiler to be minimised;

[0085] - with reference to a period comprising the same amount of thermal storage as storage recirculation as in the case of Figure 5, the energy exchanged within the process, including that of the drawing, is constant compared to the case with rated operation and only the timing in which this energy is exchanged varies, so that there is no degradation of the cycle and the total energy produced during the period remains almost constant .

[0086] Finally, it is understood that modifications may be made to the plant and method for energy production described and illustrated herein, and variants produced thereto, without thereby departing from the scope of the attached claims .

Claims

CLAIMS1 . A plant ( 100) for energy production, particularly by means of a Rankine cycle, comprising a boiler ( 1 ) ; a heat exchanger (2 ) where a process fluid receives heat produced by the boiler ( 1 ) ; at least one turbine (3) in which the process fluid circulates; an electric generator (4 ) driven by the turbine (3) ; a recirculation line (7 ) to return the process fluid from the turbine (3) to the heat exchanger (2 ) ; a series of preheaters ( 17 ) located along the recirculation line (7 ) to preheat the process fluid upstream of the heat exchanger (2 ) by heat exchange with process fluid drawn from the turbine (3) and / or the heat exchanger (2 ) via drawing lines ( 15a, 15b, 15c) equipped with flow control means (36) configured to regulate the flow rate of the drawing lines ( 15a, 15b, 15c) and consequently the output of the turbine (3) ; and a thermal storage system ( 101 ) , comprising one or more pressurized storage vessels (22a) containing process fluid, consisting of the same process fluid circulating in the plant (100) , wherein said one or more storage vessels (22a) all contain process fluid at substantially the same temperature and pressure conditions, all being connected in parallel to the recirculation line (7 ) via a hot line (24a) and a cold line (26a) which connect to the recirculation line (7 ) at a first common connection point (27a) and a second common connection point (27b) respectively, to compensate for variations in the flow rate of the drawing lines ( 15a, 15b) by storing process fluid in the one or more storage vessels (22a) and releasing process fluid from the one or more storage vessels (22a) .2 . The plant according to claim 1, wherein the flow control means (36) comprise respective control valves ( 16a,16b) located along the drawing lines ( 15a, 15b) and operable to vary the opening of the control valves ( 16a, 16b) and thereby adjust the flow rate of the drawing lines ( 15a, 15b) and consequently the output of the turbine (3) ; and / or condensate discharge lines (37 ) exiting from respective preheaters ( 17 ) and configured to control the discharge of condensate from the preheaters ( 17 ) .

3. The plant according to claim 1 or 2, wherein the process fluid is water and the heat exchanger (2) is a steam generator, in which steam (V) is produced by heat supplied from the boiler ( 1 ) and feeds the turbine (3) ; and the plant comprises a condenser (5) in which the steam circulated in the turbine (3) condenses to form a condensate (C) which returns to the steam generator via the recirculation line (7 ) .

4. The plant according to any one of the preceding claims, wherein the storage vessel (22a) or each of the storage vessels (22a) contains hot process fluid in an upper zone and cold process fluid in a lower zone, separated by a hot-cold interface (32a) , the process fluid being essentially a single-phase liquid phase fluid.

5. The plant according to claim 4, wherein the hot line (24a) joins the recirculation line (7 ) downstream of at least some preheaters ( 17 ) to selectively release hot process fluid to the heat exchanger (2 ) and receive hot process fluid from the preheaters ( 17 ) ; and the cold line (26a) is connected to the recirculation line (7 ) upstream of at least some preheaters ( 17 ) to selectively receive cold process fluid from the recirculation line (7 ) and release cold process fluid to the recirculation line (7 ) .

6. The plant according to claim 5, wherein the coldline (26a) is provided with a pair of valves (28a, 29a) placed in parallel along the cold line (26a) on respective parallel branches of the cold line (26a) , and with a pump (30a) placed in series with only one of said valves (29a) .

7. The plant according to any one of claims 4 to 6, wherein the cold line (26a) is connected to the recirculation line (7 ) upstream of all the preheaters ( 17 ) .

8. The plant according to any one of claims 4 to 7, wherein the first connection point (27a) is located along the recirculation line (7 ) between a first and a second group of preheaters ( 17 ) , in particular downstream of a degasser ( 19) located along the recirculation line (7 ) between a first group of high-pressure preheaters ( 17 ) and a second group of low-pressure preheaters ( 17 ) .

9. The plant according to any one of the preceding claims, wherein the thermal storage system ( 101 ) comprises at least one expansion vessel (22c) , preferably at atmospheric pressure, connected via a compensation line (31 ) to the cold line (26a) for exchanging cold process fluid with the expansion vessel (22c) .

10. The plant according to claim 9, wherein the compensation line (31 ) has a pair of valves (28c, 29c) placed in parallel along the compensation line (31 ) on respective parallel branches of the compensation line (31 ) , and a pump (30c) placed in series with only one of said valves (29c) .

11. An method for energy production, in particular using a Rankine cycle, comprising the steps of : producing heat in a boiler ( 1 ) ; transferring the heat to a process fluid in a heat exchanger (2 ) ; feeding the process fluid to a turbine (3) ; driving an electric generator (4 ) by means of the turbine (3) ; returning the process fluid from the turbine(3) to the heat exchanger (2 ) via a recirculation line (7 ) ; subj ecting the process fluid to one or more preheating stages in respective preheaters ( 17 ) located along the recirculation line (7 ) by heat exchange with process fluid taken from the turbine (3) and / or the heat exchanger (2 ) by respective drawings in respective drawing lines ( 15a, 15b) ; adjusting the flow rate of the drawing lines (15a, 15b) to adjust the output of the turbine (3) accordingly; and compensating variations in the flow rate of the drawing lines ( 15a, 15b) by varying the flow rate of the recirculation line (7 ) to keep the temperature of the process fluid circulating in the recirculation line (7 ) at the outlet of each preheater ( 17 ) constant; and compensating for the variation in flow rate of the recirculation line (7 ) by alternatively storing process fluid in and releasing process fluid from one or more pressurized storage vessels (22a) all containing process fluid, consisting of the same process fluid circulating in the turbine (3) , at substantially the same temperature and pressure conditions, said one or more storage vessels (22a) being all connected in parallel to the recirculation line (7 ) via a hot line (24a) and a cold line (26a) which connect to the recirculation line (7 ) at a first common connection point (27a) and a second common connection point (27b) respectively.

12. The method according to claim 11, wherein the process fluid is water and the heat exchanger (2) is a steam generator, wherein steam (V) is produced by heat supplied from the boiler ( 1 ) and feeds the turbine (3) ; and the steam circulated in the turbine (3) is condensed to form a condensate (C) that is recirculated to the steam generator .

13. The method according to claim 11 or 12, whereinthe storage vessel (22a) or each of the storage vessels (22a) contains hot process fluid in an upper zone and cold process fluid in a lower zone, separated by a hot-cold interface (32a) , the process fluid being essentially a single-phase liquid phase fluid; and wherein the hot process fluid enters or exits the storage vessel (22a) or each of the storage vessels (22a) from above, and the cold process fluid enters or exits the storage vessel (22a) or each of the storage vessels (22a) from below.

14. The method according to any one of claims 11 to 13, comprising an operating condition at nominal power, in which the boiler ( 1) and the electric generator (4 ) operate at nominal power and the storage vessel (22a) or each of the storage vessels (22a) remains with the hot-cold interface (32a) at a substantially constant level and do not receive or release process fluid.

15. The method according to any one of claims 11 to 14, comprising an operating condition at a power lower than the nominal power, in which cold process fluid is released from the storage vessel (22a) or each of the storage vessels (22a) to the recirculation line (7 ) and the amount of process fluid withdrawn through the drawing lines ( 15a, 15b) is increased to preheat a greater amount of the process fluid that goes to feed the heat exchanger (2 ) and partly feeds from above the storage vessel (22a) or each of the storage vessels (22a) , resulting in a reduction in the level of the respective hot-cold interfaces (32a) .

16. The method according to claim 15, wherein cold process fluid is released from the storage vessel (22a) or each of the storage vessels (22a) to an expansion vessel (22c) , resulting in an increase in the level of process fluidwithin the expansion vessel (22c) .

17. The method according to any one of claims 11 to 16, comprising an operating condition at a power higher than the nominal power, in which the storage vessel (22a) or each of the storage vessels (22a) is fed from below with process fluid drawn from the recirculation line (7) resulting in an increase in the level of the respective hot-cold interfaces (32a) ; and wherein the drawing lines ( 15a, 15b) draw from the turbine (3) a flow rate of process fluid that is lower with respect to the nominal operation to subtract less power from the turbine (3) and increase the power of the heat exchanger ( 2 ) .

18. The method according to claim 17, wherein the storage vessel (22a) or each of the storage vessels (22a) is also supplied with process fluid from an expansion vessel (22c) .

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