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Title:
PREHEATING HRSG DURING IDLE
Document Type and Number:
WIPO Patent Application WO/2016/150458
Kind Code:
A1
Abstract:
The present invention describes a system (100) for storing thermal energy and a method for operating such a system (100). The system (100) comprises a thermal storage device (150) for storing thermal energy. A working fluid is streamable through the thermal storage device (150) in such a way that thermal energy is exchangeable between the thermal storage device (150) and the working fluid. The system (100) further comprises a steam generation device (110) for heating a steam turbine working fluid of a steam turbine system (120), a connecting line (140) which connects the thermal storage device (150) with the steam generation device (110) in such a way that the working fluid is feedable to the steam turbine system (120) for heating the steam turbine working fluid, and a preheat device (180). The preheat device (180) is connected between the thermal storage device (150) and the steam generation device (110) in such a way that the preheat device (180) and a bypass section (141) of the connecting line (140) are connected in parallel in such a way that a preheat portion of the working fluid is flowable through the preheat device (180), and a bypass portion of the working fluid is flowable through the bypass section (141) of the connecting line (140).

Inventors:
BARMEIER, Till Andreas (Kanalstraße 40, Hamburg, 22085, DE)
SEIDEL, Volker (Ebertallee 16, Hamburg, 22607, DE)
Application Number:
EP2015/055908
Publication Date:
September 29, 2016
Filing Date:
March 20, 2015
Export Citation:
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Assignee:
SIEMENS AKTIENGESELLSCHAFT (Wittelsbacherplatz 2, München, 80333, DE)
International Classes:
F01K3/16; F01K3/00; F01K3/14; F22B1/02
Foreign References:
EP2685101A12014-01-15
DE10260993A12004-07-08
Other References:
None
Download PDF:
Claims:
CLAIMS

1. System (100) for storing thermal energy, the system (100) comprising

a thermal storage device (150) for storing thermal en¬ ergy,

wherein a working fluid is streamable through the thermal storage device (150) in such a way that thermal energy is ex¬ changeable between the thermal storage device (150) and the working fluid,

a steam generation device (110) for heating a steam turbine working fluid of a steam turbine system (120),

a connecting line (140) which connects the thermal stor¬ age device (150) with the steam generation device (110) in such a way that the working fluid is feedable to the steam generation device (110) for heating the steam turbine working fluid, and

a preheat device (180),

wherein the preheat device (180) is connected between the thermal storage device (150) and the steam generation device (110) in such a way that the preheat device (180) and a by¬ pass section (141) of the connecting line (140) are connected in parallel in such a way that

a preheat portion of the working fluid is flowable through the preheat device (180), and

a bypass portion of the working fluid is flowable through the bypass section (141) of the connecting line

(140) . 2. System according to claim 1,

wherein the heat storage capacity of the preheat device (180) is about 5% to 25% of the heat storage capacity of the ther¬ mal storage device (150) . 3. System (100) according to claim 1 or 2, further comprising a fluid driving device (170), in particular a blower, for driving the working fluid, wherein the fluid driving device (170) is coupled to the thermal storage device (150) for driving the working fluid between the thermal storage device (150) and the steam gen¬ eration device (110).

4. System according to claim 3,

wherein the fluid driving device (170) is connected to the connecting line (140) . 5. System (100) according to claim 3 or 4, further comprising a first line (793) which is connected between the ther¬ mal storage device (150) and the fluid driving device (170), a first valve (783) ,

wherein the first valve (783) is connected to the first line (793) for controlling a flow of working fluid between the thermal storage device (150) and the fluid driving device (170) ,

a second line (795) which is connected between the fluid driving device (170) and the thermal storage device (150), a second valve (785) ,

wherein the second valve (785) is connected to the second line (795) for controlling a flow of the working fluid between the fluid driving device (170) and the thermal storage device ( 150 ) ,

a third line (797) which is connected between the fluid driving device (170) and the steam generation device (110), a third valve (787),

wherein the third valve (787) is connected to the third line (797) for controlling a flow of working fluid between the fluid driving device (170) and the steam generation device (110) ,

a fourth line (799) which is connected between the steam generation device (110) and the fluid driving device (170), and

a fourth valve (789),

wherein the fourth valve (789) is connected to the fourth line (799) for controlling a flow of working fluid between the steam generation device (110) and the fluid driving de¬ vice (170) .

6. System according to one of the claims 1 to 5, further com- prising

a heater device (130) for heating the working fluid, wherein the heater device (130) is connected between the thermal storage device (150) and the preheat device (180) . 7. System (100) according to claim 6, further comprising a further fluid driving device (171), in particular a blower, for driving the working fluid,

wherein the further fluid driving device (171) for driving the working fluid is coupled between the preheat device (180) and the heater device (130) .

8. System according to any one of the claims 1 to 7, further comprising

a preheat line (182) which is connected between the steam generation device (110) and the preheat device (180), wherein the preheat portion of the working fluid is selec¬ tively flowable through the preheat line (182) from the pre¬ heat device (180) to the steam generation device (110) . 9. System according to any one of the claims 1 to 7,

wherein the preheat device (180) is a water heat exchanger (280) for heating a feed water,

wherein the water heat exchanger (280) is connected with the steam turbine system (120) in such a way that the feed water is feedable to the steam turbine system from the water heat exchanger (280) .

10. System according to claim 9, further comprising

a feed water tank (381) for storing the feed water, wherein the feed water tank (381) is connected between the water heat exchanger (280) and the steam turbine system

(120) .

11. System according to claim 9 or 10,

wherein the feed water is the steam turbine working fluid.

12. System according to claim 9 or 10, further comprising

a further water heat exchanger (483) for preheating the steam turbine working fluid, and

a thermal transport fluid,

wherein the further water heat exchanger (483) is connected to the water heat exchanger (280) in such a way that the thermal transport fluid is feedable to the further water heat exchanger (483) for preheating the steam turbine working fluid .

13. System according to claim 9, further comprising

a preheat heater device (585) for heating up the feed water,

wherein the preheat heater device (585) is connected between the water heat exchanger (280) and the steam turbine system (120) .

14. System according to one of the claims 1 to 13,

wherein the steam generation device (110) is a heat recovery steam generator. 15. Method for operating a system (100) for storing thermal energy according to one of the preceding claims, the method comprises

guiding the working fluid through the thermal storage device (150) in such a way that thermal energy is exchanged between the heat storage device (150) and the working fluid, guiding the preheat portion of the working fluid through the preheat device (180), and

guiding the bypass portion of the working fluid through the bypass section (141) of the connecting line (140) .

16. Method according to claim 12, further comprising

providing a constant temperature of the steam generation device (110) by guiding the preheat portion of the working fluid through the preheat line (182) from the preheat device (180) to the steam generation device (110) when the system is in an idle mode.

Description:
DESCRIPTION

Preheating HRSG during idle

Field of invention

The present invention relates to the field of storing thermal energy. Particularly, the present invention relates to a system for storing thermal energy and to a method for operating a system for storing thermal energy.

Art Background

In today' s electricity system renewable energy sources and an integration of renewable energy sources in power supply net ¬ works have become more important than ever before. Conven ¬ tional power supply networks were designed for central power production with a continuous power supply. The continuous power supply may be ensured by conventional energies such as coal, liquid fuel, natural gas or nuclear energy. Energy pro ¬ vided by renewable energy sources such as wind or solar is difficult to forecast and depends on weather conditions such as wind speed and solar radiation. Thus, an integration of such fluctuating amounts of renewable energy in the power supply network challenges the power supply network. Further, the location of renewable energy production such as onshore or offshore wind or solar radiation does usually not coincide with the region of high power consumption.

In current solutions the base load in the power supply net ¬ work is provided by conventional energy such as for example fossil fuelled power plants. When a high amount of power is provided by renewable energy, the base load is curtailed or energy prices drop below the economical limits of power pro ¬ ducers . Another approach to handle the fluctuating production of renewable energy is to store excess energy in thermal energy storages. Thus, storing thermal energy plays a prominent role in an improvement of the stability of power supply networks. In times with no or low occurrence of wind or solar radiation or in times with high energy consumption, the stored thermal energy is extracted from the thermal energy storage and is used for producing electrical energy. The produced electrical energy is fed in the power supply network to satisfy peaks in power consumption or elevated power consumption over longer periods of time.

The response time of thermal energy storages and systems com ¬ prising thermal energy storages is crucial for an integration of such thermal energy storages in power supply networks. The power production of renewable energies may not be foreseen and thus the excess energy must be rapidly transferred and stored in the thermal energy storage. In times with low power production by renewable energy, the thermal energy stored in the thermal energy storage must rapidly be withdrawn from the thermal energy storage and fed to the power supply network.

Current steam cycle technologies need long start-up processes compared to for example a battery, a single cycle gas turbine or a pumped hydro plant. This is for example due to a re ¬ quired reduction of the thermal stress in a component of a current system for storing thermal energy such as a steam generator . A conventional technology for providing a relatively short response time is the once through steam generator (OTSG) which applies the Benson concept and the Sulzer concept, re ¬ spectively. The OTSG is a specialized heat recovery steam generator (HRSG) which does not comprise steam drums. The OTSG provides a short start-up time due to the absence of the steam drums, little wall thickness of its components, a consequently lower water volume and less thermal inertia. The advantages of the OTSG come along with certain disadvantages which may be associated with the geometry of the OTSG. These disadvantages may be significant. Rather, the OTSG requires a comprehensive control system to precisely manage the water content in its vaporizer as a reaction to the varying mass flow and temperature of the hot gas flow which is present in the OTSG. Thus, the OTSG needs a high number of sensors. This high number of sensors turns the OTSG fragile for failure. Conventional use of the OTSG in a system for storing thermal energy leads to more complex system with higher operational costs.

There may be a need for a robust and cost-efficient system for storing thermal energy which has its short response time in the start up process.

Summary of the Invention

It may be an object of the present invention to provide a simple, robust and cost-efficient system for storing thermal energy having a short response time in a start-up process.

This object may be solved by a system for storing thermal en ¬ ergy and by a method for operating a system for storing ther- mal energy according to the independent claims.

According to a first aspect of the present invention a system for storing thermal energy is disclosed. The system comprises a thermal storage device for storing thermal energy. A work- ing fluid is streamable through the thermal storage device in such a way that thermal energy is exchangeable between the thermal storage device and the working fluid. The system fur ¬ ther comprises a steam generation device for heating a steam turbine working fluid of a steam turbine system, a connecting line which connects the thermal storage device with the steam generation device in such a way that the working fluid is feedable to the steam generation device for heating the steam turbine working fluid, and a preheat device. The preheat de- vice is connected between the thermal storage device and the steam generation device in such a way that the preheat device and a bypass section of the connecting line are connected in parallel in such a way that a preheat portion of the working fluid is flowable through the preheat device, and a bypass portion of the working fluid is flowable through the bypass section of the connecting line.

The steam generation device may exchange thermal energy, e.g. by a heat exchanger, with a steam turbine system. In particular, the steam generation system heats up the steam turbine working fluid for driving the steam turbine system. In a further exemplary embodiment, the steam generation device may comprise a boiler or an evaporator. Heat exchangers are de- vices for transferring thermal energy from one medium to another in the direction of a temperature gradient. Heat ex ¬ changers are built for changing a state of a medium for exam ¬ ple by cooling, heating or changing the aggregation state of the medium. In a further exemplary embodiment, the steam gen- eration device comprises a direct heat exchanger or an indi ¬ rect heat exchanger. In a direct heat exchanger the heat is exchanged without solid walls separating the different media and thus by direct contact between the two media which are used. In an indirect heat exchanger the heat is transferred from one medium to another medium over solid walls separating the two media which are used, from each other. In a further exemplary embodiment, the steam generation device may comprise also a counter-flow heat exchanger, a parallel-flow heat exchanger, a double pipe heat exchanger, a shell and tube heat exchanger, a plate heat exchanger or a heat ex ¬ changer consisting of more than one stage of heat exchange for improving the efficiency.

The steam turbine working fluid describes the fluid which drives the steam turbine system. The steam turbine working fluid may be steam, water vapour or vapour with a high mass fraction of water in it, respectively. In a further exemplary embodiment, water vapour may be saturated but also unsatu- rated. Further, it may be possible to add supplements to the water vapour to influence physical characteristics of the wa ¬ ter vapour, such as for example the evaporation point or the condensation point or the chemical properties such as the PH- value.

The steam turbine system may be a system which comprises a steam turbine and further devices for transforming thermal energy in mechanical energy and electricity, respectively. The subsequently described exemplary embodiment of the steam turbine system is only exemplary and not limiting. The steam turbine system comprises a steam turbine, a condenser, a gen ¬ erator, a first pump and a second pump. The steam turbine may be a multi stage steam turbine or a single stage turbine. In a multi stage turbine the steam turbine working fluid may be reheated between the different stages or a part of the steam turbine working fluid may be extracted from the steam turbine due to too excessive cooling of the steam turbine working fluid. The steam turbine comprises blades which are connected to a shaft. The energy of the steam turbine working fluid flowing through the steam turbine is transmitted by the blades of the steam turbine to the shaft. The generator is connected to the shaft of the steam turbine and converts the rotational energy of the shaft of the steam turbine into electrical energy. The generated electrical energy may then be transferred to a power supply network or any other end- user. After flowing through the steam turbine, the steam turbine working fluid flows through the condenser in which the steam turbine working fluid is condensed into its liquid state. The first pump and the second pump drive the steam turbine working fluid through the different components of the steam turbine system and through the steam generation device.

The working fluid may be a medium which flows through the system for storing thermal energy such as for example a gas or a liquid. Further, the medium may be a mixture consisting of a main component, such as water, with added supplements for influencing physical characteristics of the mixture, such as for example the evaporation point or the condensation point .

The thermal storage device may be a sensible thermal storage or a latent heat storage. The thermal storage device may be filled with a thermal storage material. The thermal storage material may be a solid or bulk material, such as stones, bricks, ceramics or other solid material which has the abil ¬ ity to be heated up, keep its temperature and thus store thermal energy over a predetermined period of time. It should be understood that also water may be used as the thermal en ¬ ergy storage material. The thermal storage material may be heated up by the working fluid. The connecting line may be a pipe or a duct. The connecting line enables the working fluid to flow from the thermal stor ¬ age device in the direction towards the steam generation device and vice versa.

The preheat device is connected in parallel to the connecting line. The section of the connecting line which is connected in parallel to the preheat device is the bypass section of the connecting line. The working fluid which flows between the thermal storage device and the steam generation device may be divided in two streams. A first of the two streams is guided through the preheat device and therefore is the pre ¬ heat portion. A second of the two streams is guided through the bypass section and is therefore the bypass portion. The ratio between the bypass portion and the preheat portion may depend on given boundary conditions such as the total pres- sure of the system or the required heat amount for the steam generation device.

In a further exemplary embodiment, it may be possible that the preheat portion is zero. That is, no working fluid is flowing through the preheat device and hence, all the working fluid flows through the bypass section of the connecting line. This may be useful for example if the steam generation device is already heated up and does not need any more addi- tional heating. On the other hand, the bypass portion may be zero. That is, no working fluid is flowing through the bypass section of the connecting line and all the working fluid flows through the preheat device. This may be useful for ex- ample when the steam generation device is not yet heated up at all, that is the temperature of the steam generation de ¬ vice is at ambient temperature.

The preheat device may be a sensible thermal storage or a la- tent heat storage. The preheat device may be filled with a thermal storage material. The thermal storage material may be a solid or bulk material, such as stones, bricks, ceramics or another solid material which has the ability to be heated up, keep its temperature and thus store thermal energy over a predetermined period of time. It should be understood that also water may be used as the thermal storage material. The thermal storage material may be heated up by the working fluid. Furthermore, the preheat device may also be a heat ex ¬ changer for preheating the steam turbine working fluid of the steam turbine system.

The preheat device being connected between the thermal stor ¬ age device and the steam generation device in such a way that the preheat device and the bypass section of the connecting line are connected in parallel means that the working fluid may either flow through the preheat device or through the bypass section contemporaneous.

Hence, in contrast to conventional approaches, by the present invention, the preheat device is connected between the ther ¬ mal storage device and the steam generation device. Hence, the preheat device and the bypass section are connected in parallel. Thus, the preheat portion of the working fluid is flowable through the preheat device and a bypass portion is flowable through the bypass section.

Hence, the thermal energy which may be still absorbed by the working fluid even after the working fluid may have been guided through the thermal storage device, may be used to be transferred to the preheat device. From the preheat device the thermal energy may be transferred for example to the steam generation device or the steam turbine system for pro- viding a predetermined temperature and thus a predetermined pressure in the steam generation device. Hence, the start-up time for the entire system and hence for storing thermal en ¬ ergy may be reduced. According to a further exemplary embodiment of the present invention, the heat storage capacity of the preheat device is about 5% to 25% of the heat storage capacity of the thermal storage device. The heat storage capacity specifies how much thermal energy must be absorbed by 1kg of a material so that the temperature of the material rises about IK. Hence, the unit of the heat storage capacity is J/ (kg*K) . Furthermore, the storage volume for the heat storage elements of the thermal excess storage device has less than approxi ¬ mately 50% of the storage volume for the heat storage ele ¬ ments of the thermal storage device. According to a further exemplary embodiment of the present invention, the system further comprises a fluid driving device, in particular a blower, for driving the working fluid. The fluid driving device is coupled to the thermal storage device for driving the working fluid between the thermal storage device and the steam generation device.

The fluid driving device may comprise a fan, a blower, a ven ¬ tilator, a super-charger or a device generating a pressure gradient in such a way that the working fluid which flows through the fluid driving device may be accelerated in a pre ¬ dominant and predetermined direction of the fluid driving de ¬ vice. The fluid driving device may also be a multi-stage de ¬ vice in which the working fluid is accelerated in multiple stages in such a way that an efficiency of the fluid driving device is improved.

It should be emphasized that one fluid driving device may be sufficient to drive the working fluid through the thermal storage device, the steam generation device and the preheat device. Is should be mentioned that it may also be possible to integrate more than one fluid driving device in the sys ¬ tem. The second, third or further fluid driving device may also be connected to the system. If a plurality of fluid driving devices is used, it may be advantageous to use fluid driving devices which may be identical in performance and/or form. The fluid driving devices may among themselves be con ¬ nected in line or parallel.

According to a further exemplary embodiment of the present invention, the fluid driving device is connected to the con ¬ necting line. The fluid driving device being connected to the connecting line means that the fluid driving device may also be con ¬ nected to the bypass section of the connecting line. The fluid driving device being connected to the connecting line may have the advantage that enough power may be ensured for driving the working fluid through the bypass section and contemporaneous through the preheat device.

According to a further exemplary embodiment of the present invention, the system further comprises a first line which is connected between the thermal storage device and the fluid driving device and a first valve. The first valve is con ¬ nected to the first line for controlling a flow of working fluid between the thermal storage device and the fluid driv ¬ ing device.

The system further comprises a second line which is connected between the fluid driving device and the thermal storage de ¬ vice and a second valve. The second valve is connected to the second line for controlling a flow of the working fluid between the fluid driving device and the thermal storage de ¬ vice . The system further comprises a third line which is connected between the fluid driving device and the steam generation device and a third valve. The third valve is connected to the third line for controlling a flow of working fluid between the fluid driving device and the steam generation device.

The system further comprises a fourth line which is connected between the steam generation device and the fluid driving device and a fourth valve. The fourth valve is connected to the fourth line for controlling a flow of working fluid between the steam generation device and the fluid driving device.

The first line, the second line, the third line and the fourth line are structural components such as a pipe or a duct .

The respective valves may for example be a gate valve, a non ¬ return valve or angle valve or a system of duct dampers. The respective valves may also comprise a plurality of sub-valves to ensure the redundancy of a cut-off of the respective line. In a preferred exemplary embodiment the mechanics of the first valve is designed as simple as possible and addition ¬ ally only one single valve is connected to the first line. Hence, the first valve is formed to be as robust and simple as possible.

Controlling a flow of working fluid between the thermal storage device and the fluid driving device means that the first valve may regulate the flow of working fluid or may enable a flow of working fluid from the thermal storage device to the fluid driving device.

Controlling a flow of working fluid between the fluid driving device and the thermal storage device means that the second valve may regulate the flow of working fluid or may enable a flow of working fluid from the fluid driving device to the thermal storage device. Controlling a flow of working fluid between the fluid driving device and the steam generation device means that the third valve may regulate the flow of working fluid or may enable a flow of working fluid from the fluid driving device to the steam generation device.

Controlling a flow of working fluid between the steam generation device and the fluid driving device means that the fourth valve may regulate the flow of working fluid or may enable a flow of working fluid from the steam generation de- vice to the fluid driving device.

According to a further exemplary embodiment of the present invention, the system further comprises a heater device for heating the working fluid. The heater device is connected be- tween the thermal storage device and the preheat device.

The heater device may be an element which introduces heat en ¬ ergy to the working fluid. The working fluid may be in a gas or a liquid state. In an exemplary embodiment, the heater de- vice may comprise an electrical heater, e.g. a resistant heater or an inductive heater.

According to a further exemplary embodiment of the present invention, the system further comprises a further fluid driv- ing device, in particular a blower, for driving the working fluid. The further fluid driving device for driving the working fluid is coupled between the preheat device and the heater device. The further fluid driving device may be designed similar to the fluid driving device. Furthermore, the above-mentioned features for the fluid driving device may be likewise valid for the further fluid driving device According to a further exemplary embodiment of the present invention, the system further comprises a preheat line which is connected between the steam generation device and the pre- heat device. The preheat portion of the working fluid is se ¬ lectively flowable through the preheat line from the preheat device to the steam generation device.

The preheat line is a structural component such as a pipe or a duct. The preheat line enables the preheat portion of the working fluid to flow from the preheat device in a direction towards the steam generation device. Thus, the preheat por ¬ tion of the working fluid may transfer thermal energy from the preheat device to the steam generation device.

The term "selectively" flowable through the preheat line means that a user or a driver program may choose, e.g. by controlling a respective valve in the preheat line, if the preheat portion of the working fluid flows to the steam gen- eration device or if the preheat portion of the working fluid stays stored in the preheat device.

According to a further exemplary embodiment of the present invention, the preheat device is a water heat exchanger for heating a feed water. The water heat exchanger is connected with the steam turbine system in such a way that the feed wa ¬ ter is feedable to the steam turbine system from the water heat exchanger. The water heat exchanger is a heat exchanger in which the thermal energy of the preheat portion of the working fluid is transferred to the feed water. Thus, the feed water is heated up . The feed water may be mains water, destillated water or water which has supplements added to it to improve certain proper ¬ ties of the water, such as its boiling point or the chemical properties . The feed water being feedable to the steam turbine system from the preheat device means that there is a line for exam ¬ ple in form of a pipe or a duct such as the preheat line, which connects the preheat device and the steam turbine sys ¬ tem in such a way that the feed water may flow from the pre ¬ heat device to the steam turbine system.

According to a further exemplary embodiment of the present invention, the system further comprises a feed water tank for storing the feed water. The feed water tank is connected be ¬ tween the water heat exchanger and the steam turbine system.

After being heated up and hence after having absorbed thermal energy, the feed water may be stored in the feed water tank. Hence, the thermal energy from the excess portion may be storable in the feed water tank.

The feed water being feedable to the steam turbine system from the feed water tank means that there is a line for exam ¬ ple in form of a pipe or a duct which connects the feed water tank and the steam turbine system in such a way that the feed water may flow from the feed water tank to the steam turbine system. Hence, thermal energy which is absorbed in the feed water may thus be transferred into the steam turbine device and the steam generation device, respectively.

According to a further exemplary embodiment of the present invention, the feed water is the steam turbine working fluid.

According to a further exemplary embodiment of the present invention, the system further comprises a further water heat exchanger for preheating the steam turbine working fluid and a thermal transport fluid. The further water heat exchanger is connected to the water heat exchanger in such a way that the thermal transport fluid is feedable to the further water heat exchanger for preheating the steam turbine working fluid . The further water heat exchanger may be a heat exchanger in which the thermal energy of the feed water may be transferred to the steam turbine working fluid. Thus, the steam turbine working fluid is heated up in the further water heat ex ¬ changer .

The thermal transport fluid may be integrated in a closed loop between the water heat exchanger and the further water heat exchanger. Thus, the thermal transport fluid may be separated from the working fluid and from the steam turbine working fluid. Hence, possible contaminations contained in the feed water may not contaminate the working fluid or the steam turbine working fluid. The working fluid flows through the thermal storage device which may be sensible regarding contamination which may be contained in the working fluid. The steam turbine working fluid streams through the steam turbine which may also be sensible regarding contamination. If the feed water is in the separated closed loop the re- quirement regarding the water quality relating to contamina ¬ tion may for the feed water not be as high as for the steam turbine working fluid or the working fluid. The contamination may for example be particles or chemical substances. According to a further exemplary embodiment of the present invention, the system further comprises a preheat heater device for heating up the feed water. The preheat heater device is connected between the water heat exchanger and the steam turbine system.

The preheat heater device may be an element which introduces heat energy to the feed water. The feed water is in a liquid state. In an exemplary embodiment, the heater device may com ¬ prise an electrical heater, e.g a resistant heater or an in- ductive heater. The preheat heater device transfers addi ¬ tional thermal energy to the feed water in such a way that the feed water may be able to transfer more thermal energy into the steam turbine device as the thermal energy of the feed water recuperated from the preheat device.

According to a further exemplary embodiment of the present invention, the steam generation device is a heat recovery steam generator.

Heat recovery steam generator is cut short HRSG. The HRSG may be an energy recovery heat exchanger which recovers energy from a stream. The HRSG may comprise e.g. four principal components. For example, the four principal components are an economizer, an evaporator, a super-heater and a pre-heater. The four principal components are put together to meet several requirements such as for example operating

requirements or a given efficiency of the HRSG. Different

HRSGs may be distinguished by the direction of an exhaust gas flow or the number of pressure levels integrated into the HRSG. In an exemplary embodiment, the HRSG may be a vertical type HRSG, a horizontal type HRSG, a single pressure HRSG or a multi pressure HRSG.

According to a further aspect of the present invention a method for operating a system for storing thermal energy is disclosed. The method comprises guiding the working fluid through the thermal storage device in such a way that thermal energy is exchanged between the heat storage device and the working fluid, guiding the preheat portion of the working fluid through the preheat device, and guiding the bypass por ¬ tion of the working fluid through the bypass section of the connecting line.

A charging cycle may describe a flow direction of the working fluid in which the thermal storage device is charged with thermal energy which is transferred from the working fluid to the thermal storage device. In the charging cycle, the working fluid flows first through an inlet into the thermal storage device, in such a way that the inlet may be defined as a hot end of the thermal storage device. After flowing through the thermal storage device, the working fluid leaves through an outlet and is therefore colder than during

entering the thermal storage device through the inlet. Hence, in the charging cycle the outlet may be colder than the inlet and may therefore be defined as the cold end of the thermal storage device. A temperature level at the hot end (i.e. the inlet) may be 600°C and a temperature level at the cold end (i.e. at the outlet) may be 200°C. A discharging cycle may describe a flow direction of the working fluid in which the thermal storage device is

discharged from thermal energy, wherein the working fluid absorbs thermal energy from the thermal storage device.

Hence, in the discharging cycle, the working fluid is heated up by the thermal storage device. The working fluid which is heated up, heats up the steam turbine working fluid in the steam generation device in such a way that e.g. steam as steam turbine working fluid is generated. Thus, guiding the working fluid through the thermal storage device may be performed in a direction of the charging cycle or in a direction of the discharging cycle.

In the charging cycle or in the discharging cycle, the preheat portion of the working fluid is guided through the preheat device for storing thermal energy in the preheat device. Simultaneously, the bypass portion of the working fluid is guided through the bypass section of the connecting line. The ratio between the bypass portion and the preheat portion may depend on given boundary conditions such as the total pressure of the system or the required heat amount for the steam generation device.

According to a further exemplary embodiment of the present invention, the method further comprises providing a constant temperature of the steam generation device by guiding the preheat portion of the working fluid through the preheat line from the preheat device to the steam generation device when the system is in an idle mode.

An idle mode may describe the condition when the system is neither in the charging cycle nor in the discharging cycle. Thus, the thermal storage device is neither charging thermal energy nor discharging thermal energy. In the idle mode, the thermal storage device may be fully or partly charged or discharged, but no charging or discharging may take place. Hence, the thermal storage device may actually be storing thermal energy. The idle mode may be maintained for long time periods, for example the thermal energy may be stored several days inside the thermal storage device. Providing a constant temperature means that the preheat portion flows from the preheat device to the steam generation device in such a way that the preheat portion always

transfers enough thermal energy to the steam generation device such that the steam generation device may have the constant temperature.

The constant temperature may not only be one temperature of a certain value but also be defined as a predetermined range of temperature values. Hence, the temperature may be seen as constant if the temperature value is in the predetermined range of temperature values.

Contrary to the conventional known systems for storing thermal energy, the system disclosed in the present invention provides the preheat device which directly or indirectly preheats the steam generation device in the idle mode.

It has to be noted that embodiments of the present invention have been described with reference to different subject- matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodi ¬ ments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination be ¬ tween features relating to different subject-matters, in par- ticular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ¬ ment but to which the invention is not limited.

Fig. 1 shows a schematical view of a system for storing ther- mal energy comprising a preheat device according to an exemplary embodiment of the present invention.

Fig. 2 shows a schematical view of a system for storing thermal energy comprising a water heat exchanger according to a further exemplary embodiment of the present invention.

Fig. 3 shows a schematical view of a system for storing thermal energy comprising a water heat exchanger and a feed water tank according to an exemplary embodiment of the present in- vention.

Fig. 4 shows a schematical view of a system for storing thermal energy comprising a water heat exchanger and a feed water tank according to a further exemplary embodiment of the pre- sent invention.

Fig. 5 shows a schematical view of a system for storing thermal energy comprising a water heat exchanger, a feed water tank and a preheat heater device according to a further exemplary embodiment of the present invention.

Fig. 6 shows a schematical view of a system for storing ther- mal energy according to a further exemplary embodiment of the invention .

Fig. 7 shows a schematical view of a system for storing thermal energy according to a further exemplary embodiment of the invention.

Detailed Description The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ¬ ments are provided with the same reference signs.

Fig. 1 shows a system 100 for storing thermal energy according to an exemplary embodiment of the present invention. The system 100 comprises a thermal storage device 150 for storing thermal energy. A working fluid is streamable through the thermal storage device 150 in such a way that thermal energy is exchangeable between the thermal storage device 150 and the working fluid. The system 100 further comprises a steam generation device 110 for heating a steam turbine working fluid of a steam turbine system 120, a connecting line 182 which connects the thermal storage device 150 with the steam generation device 110 in such a way that the working fluid is feedable to the steam turbine system 120 for heating the steam turbine working fluid, and a preheat device 180. The preheat device 180 is connected between the thermal storage device 150 and the steam generation device 110 in such a way that the preheat device 180 and a bypass section 141 of the connecting line 140 are connected in parallel in such a way that a preheat portion of the working fluid is flowable through the preheat device 180, and a bypass portion of the working fluid is flowable through the bypass section 141 of the connecting line 140.

The steam generation device 110 is a device for exchanging thermal energy, in particular a heat exchanger. In Fig. 1 the steam generation device 110 may be a HRSG (heat recovery steam generator) .

The steam turbine system 120 comprises a steam turbine 127, a condenser 123, a generator 125, a first pump 121 and a second pump 122. After flowing through the steam turbine 127, the steam turbine working fluid flows through the condenser 123 in which the steam turbine working fluid is condensed e.g. in its liquid state. The first pump 121 and the second pump 122 drive the steam turbine working fluid through the steam turbine system 120 and through the steam generation device 110.

The thermal storage device 150 may be a sensible thermal storage or a latent heat storage. The thermal storage device 150 may be filled with a thermal energy storage material.

The preheat device 180 may be a sensible thermal storage or a latent heat storage. The preheat device 180 may be filled with a thermal storage material. The thermal storage material may be a solid or bulk material, such as stones, bricks, ce ¬ ramics or another solid material which has the ability to be heated up, keep its temperature and thus store thermal energy over a predetermined period of time. The connecting line 140 enables the working fluid to flow from the thermal storage device 150 in the direction towards the steam generation device 110 and vice versa.

The preheat device 180 is connected in parallel to the con- necting line 140. A section of the connecting line 140 which is connected in parallel to the preheat device 180 is the by ¬ pass section 141 of the connecting line 140. The working fluid which flows between the thermal storage device 150 and the steam generation device 110 may be divided in two sepa ¬ rate streams/portions. A first one of the two

streams/portions is guided through the preheat device 180 and is therefore the preheat portion. A second one of the two streams/portions is guided through the bypass section 141 of the connecting line 140 and therefore is the bypass portion. The ratio between the bypass portion and the preheat portion may depend on given boundary conditions such as the total pressure of the system 100 or the required heat amount for the steam generation device 110. However, also a control valve may be coupled to the preheat device 180 and the bypass section 141 for controlling the amount of the preheat portion and the bypass portion. The system 100 further comprises a fluid driving device 170 for driving the working fluid. The fluid driving device 170 is coupled to the thermal storage device 150 for driving the working fluid between the thermal storage device 150 and the steam generation device 110. More precisely, the fluid driv- ing device 170 is connected to the connecting line 140.

The system 100 further comprises a heater device 130 for heating the working fluid. The heater device 130 is connected between the thermal storage device 150 and the preheat device 180.

The system 100 further comprises a further fluid driving device 171 for driving the working fluid. The further fluid driving device 171 is coupled between the preheat device 180 and the heater device 130.

The heater device 130 may be an element which introduces heat energy to the working fluid. The working fluid may be in a gas or a liquid state. In an exemplary embodiment, the heater device 130 may comprise an electrical heater, e.g. a resis ¬ tant heater or an inductive heater. The system 100 further comprises a preheat line 182 which is connected between the steam generation device 110 and the preheat device 180. The preheat portion of the working fluid is selectively (e.g. by coupling a control valve to the con- nection line 140) flowable through the preheat line 182 from the preheat device 180 to the steam generation device 110.

The preheat line 182 enables the preheat portion of the work ¬ ing fluid to flow through the preheat line 182 from the pre- heat device 180 in the direction towards the steam generation device 110. Thus, the preheat portion may transfer thermal energy from the preheat device 180 to the steam generation device 110. In a charging cycle the working fluid may be driven by the further fluid driving device 171 and hence may flow through the heater device 130 (in which the working fluid may further be heated up) and further through the thermal storage device 150. In the thermal storage device 150 the working fluid transfers thermal energy to the thermal storage device 150. Next, the working fluid is divided into the preheat portion and the bypass portion. The preheat portion flows through the preheat device 180 and the bypass portion flows through the bypass section 141 of the connecting line 140. The preheat portion transfers thermal energy to the preheat device 180. Afterwards, the bypass portion and the preheat portion are remerged again for continued streaming as the working fluid.

In Fig. 1, the preheat portion of the working fluid is stored inside the preheat device 180. Hence, only the bypass portion continues to stream through the heater device 130.

In a discharging cycle the working fluid flows from the steam generation device 110 via the fluid driving device 170 to the thermal storage device 150. The working fluid further passes through the thermal storage device 150 and thermal energy is transferred from the thermal storage device 150 to the work ¬ ing fluid. Next, from the thermal storage device 150 the working fluid flows through the steam generation device 110. In the steam generation device 110 the working fluid heats up the steam turbine working fluid of the steam turbine system 120.

In an idle mode the preheat portion of the working fluid which may be stored in the preheat device 182 is guided through the preheat line 182 towards the steam generation de ¬ vice 110. The preheat portion may hence heat up the steam generation device 110.

Hence, in the system 100 shown in Fig. 1 air may be used as the working fluid. If the working fluid has a temperature higher than an ambient temperature but lower than a design temperature of the thermal storage device 150 when leaving the thermal storage device 150 during the charging cycle or the discharging cycle, the working fluid may be used for a preheating propose. If the thermal storage device 150 is al ¬ most fully charged, the temperature of the working fluid which streams out of the thermal storage device 150 rises. The working fluid which comprises thermal energy may be guided via a valve to the preheat device 180. The thermal en ¬ ergy may be stored in the preheat device 180 and may be used to preheat the steam generation device 110 by driving the working fluid with a low mass flow. A further smaller fluid driving device may be installed in the preheat line 182 be ¬ tween the preheat device 180 and the steam generation device 110 (e.g. a HRSG) . Fig. 2 shows a system 100 according to a further exemplary embodiment of the present invention. The system 100 comprises the thermal storage device 150 for storing thermal energy, the heater device 130 for heating the working fluid, the steam generation device 110 for heating the steam turbine working fluid of the steam turbine system 120, the steam turbine system 120, the fluid driving device 170, the further fluid driving device 171, the connecting line 140 comprising the bypass section 141 of the connecting line 140, the water heat exchanger 280 and a preheat line 282 which is connected between the water heat exchanger 280 and the steam generation device 110. The preheat line 282 is indirectly connected to the steam generation device 110 in such a way that the preheat line 282 is directly connected to the steam turbine system 120. The steam turbine system 120 is in thermal contact with the steam generation device 110 in such a way that the steam turbine working fluid of the steam turbine system 120 is heated up by the steam generation device 110. Hence, the steam turbine working fluid or a portion of the steam turbine working fluid may flow through the preheat pipe 282 and may be heated up by the water heat exchanger 280. Hence, in the idle mode, the steam turbine working fluid may be directly preheated.

In the charging cycle and in the discharging cycle, respec ¬ tively, the working fluid is guided through the system 100 shown in Fig. 2 in the respective manner as it was described for the charging cycle and the discharging cycle, respec ¬ tively, in Fig. 1.

The preheat device may be the water heat exchanger 280 for heating the feed water. The water heat exchanger 280 is con- nected with the steam turbine system 120 by the preheat line 382 and in such a way that the feed water is feedable to the steam turbine system 120 from the water heat exchanger 280.

Hence, it is possible to integrate the preheat line 282 in the water heat exchanger 280 and circulate the steam turbine working fluid continuously during the idle mode to preheat exchanger surfaces of a super heater and a vaporizer of the steam generation device 110 and the steam turbine working fluid itself. Additionally, a low mass flow of the working fluid may be used to maintain an elevated temperature of the components of the system 100 which are not in contact with the steam turbine working fluid but with the working fluid. Fig. 3 shows a system 100 according to a further exemplary embodiment of the present invention. The system 100 comprises the thermal storage device 150 for storing thermal energy, the heater device 130 for heating the working fluid, the steam generation device 110 for heating the steam turbine working fluid of the steam turbine system 120, the steam turbine system 120, the fluid driving device 170, the further fluid driving device 171, the connecting line 140 comprising the bypass section 141 of the connecting line 140, the water heat exchanger 280, a preheat line 382 which is connected be ¬ tween the preheat device and the steam generation device 110, and a feed water tank 381.

The preheat device may be the water heat exchanger 280 for heating the feed water. The water heat exchanger 280 is connected with the steam turbine system 120 by the preheat line 382 and in such a way that the feed water is feedable to the steam turbine system 120 from the water heat exchanger 280. The water heat exchanger 280 may additionally comprise a thermal storage (not shown in Fig. 3) which may be a thermal storage similar to the preheat device 180 (shown in detail in Fig. 1) .

The feed water tank 381 is connected to the preheat line 382 and is connected between the water heat exchanger 280 and the steam turbine system 120. The feed water may be the steam turbine working fluid.

In the charging cycle and in the discharging cycle, respec- tively, the working fluid is guided through the system 100 shown in Fig. 3 in the respective manner as it was described for the charging cycle and the discharging cycle, respec ¬ tively, in Fig. 1. The preheat portion of the working fluid is guided through the water heat exchanger 280. In the water heat exchanger 280 the preheat portion transfers thermal energy to the feed wa ¬ ter. The feed water which may transfer the thermal energy is then guided through the preheat line 382 towards the steam turbine system 120. If there is less thermal energy or feed water needed in the steam turbine system 120, the feed water is stored in the feed water tank 381. If necessary, the stored feed water is guided from the feed water tank 381 to the steam turbine system 120. Hence, the feed water tank 381 provides a possibility to react in a fast manner to a need of thermal energy for preheating the steam turbine system 120 or the steam generation device 110.

It is possible to bypass the preheat portion to the water heat exchanger 280 which is e.g. installed in parallel to the bypass section 141 of the connecting line 140 downstream (in the charging cycle) of the thermal storage device 150. The water heat exchanger 280 may be used to preheat pressurized feed water, to store the feed water in the feed water tank 381 and to preheat the steam turbine system 120 and accord ¬ ingly the steam generation device 110 in the idle mode. This may be realized directly with the stored feed water which may be used in the steam turbine system 120 as the steam turbine working fluid.

Fig. 4 shows a system 100 according to a further exemplary embodiment of the present invention. The system 100 comprises the thermal storage device 150 for storing thermal energy, the heater device 130 for heating the working fluid, the steam generation device 110 for heating the steam turbine working fluid of the steam turbine system 120, the steam turbine system 120, the fluid driving device 170, the further fluid driving device 171, the connecting line 140 comprising the bypass section 141 of the connecting line 140, the water heat exchanger 280, the preheat line 382 which is connected between the water heat exchanger 280 and the steam turbine system 120, the feed water tank 381 and a further heat ex- changer 483 for preheating the steam turbine working fluid.

The further water heat exchanger 483 is connected to the wa ¬ ter heat exchanger 280 in such a way that the thermal trans- port fluid is feedable to the further water heat exchanger 483 for preheating the steam turbine working fluid.

In the charging cycle and in the discharging cycle, respec- tively, the working fluid is guided through the system 100 shown in Fig. 4 in the respective manner as it was described for the charging cycle and the discharging cycle, respec ¬ tively, in Fig. 1. However, the preheat portion of the working fluid is guided through the water heat exchanger 280. In the water heat exchanger 280 the preheat portion transfers thermal energy to the thermal transport fluid. The thermal transport fluid which may be able to transfer the thermal energy is then guided through the preheat line 382 towards the further water heat exchanger 483 which indirectly preheats the steam tur ¬ bine working fluid which is the feed water. If there is less thermal energy or feed water needed in the steam turbine sys ¬ tem 120, the thermal transport fluid may be stored in the feed water tank 381. If necessary, the stored thermal trans ¬ port fluid is guided from the feed water tank to the further water heat exchanger 483.

Hence, the further water heat exchanger 483 provides the pos- sibility for having a closed thermal transport fluid circuit which may come in indirect contact with the preheat portion of the working fluid when the thermal transport fluid flows through the water heat exchanger 280 and which may come in indirect contact with the steam turbine working fluid when the thermal transport fluid flows through the further water heat exchanger 483. Hence, the thermal transport fluid does not need to fulfil as strict requirements for the purity as for example the steam turbine working fluid or the working fluid .

It is possible to bypass the preheat portion to a water heat exchanger 280 which is installed in parallel to the bypass section 141 of the connecting line 140 downstream (in the charging cycle) of the thermal storage device 150. The water heat exchanger 280 may be used to preheat pressurized thermal transport fluid, store it in the feed water tank 381 and pre ¬ heat the steam turbine system 120 and accordingly the steam generation device 110 in the idle mode. This may be realized indirectly with the further water heat exchanger 483 to sepa ¬ rate the thermal transport fluid circuit from the steam tur ¬ bine system 120. Fig. 5 shows a system 100 according to a further exemplary embodiment of the present invention. The system 100 comprises the thermal storage device 150 for storing thermal energy, the heater device 130 for heating the working fluid, the steam generation device 110 for heating the steam turbine working fluid of the steam turbine system 120, the steam turbine system 120, the fluid driving device 170, the connecting line 140 comprising the bypass section 141 of the connecting line 140, the water heat exchanger 280, a preheat line 582 which is connected between the water heat exchanger 280 and the steam turbine system 120 and a preheat heater device 585 for heating up the feed water.

The preheat heater device 585 is connected between the water heat exchanger 280 and the steam turbine system 120. The pre- heat heater device 585 may be an element which introduces heat energy to the feed water. In an exemplary embodiment, the preheat heater device 585 may comprise an electrical heater e.g a resistant heater, , an inductive heater or a fuel fired heater. Hence, the preheat heater device 585 may additionally increase the thermal energy transferred by the feed water. In a further exemplary embodiment, the preheat heater device 585 may compensate the thermal energy which may be lost in the preheat line 582. It is also possible to keep the temperature of the feed water by using the preheat heater device 585 which may for example be an electric heater or a fuel fired heater. This consumes electrical energy or fuel but if the ability of a thermal en- ergy storage in a short time period is important, it may be beneficial .

Fig. 6 shows a system 100 according to a further exemplary embodiment of the present invention. The system 100 comprises the thermal storage device 150 for storing thermal energy, the heater device 130 for heating the working fluid, the steam generation device 110 for heating the steam turbine working fluid of the steam turbine system 120, the steam tur- bine system 120, two fluid driving devices 170, 670, a pre ¬ heat line 682 which is connected between the thermal storage device 150 and the steam generation device 110, and a further smaller fluid driving device 687 for driving the working fluid in the idle mode between the thermal storage device 150 and the steam generation device 110.

In the charging cycle the working fluid flows from the thermal storage device 150 through the fluid driving device 670. Next, the working fluid flows through the heater device 130. Afterwards, the working fluid streams through the thermal storage device 150 for transferring thermal energy to the thermal storage device 150.

In the discharging cycle the working fluid flows from the thermal storage device 150 through the steam generation de ¬ vice 110 for heating up the steam turbine working fluid of the steam turbine system 120. Next, the working fluid streams through the fluid driving device 170. Afterwards, the working fluid streams through the thermal storage device 150 which transfers thermal energy to the working fluid.

In the idle mode the working fluid may be driven by the fur ¬ ther smaller fluid driving device 687 from the thermal storage device 150 to the steam generation device 110 in such a way that the steam generation device 110 may be preheated by the working fluid. A constant low flow of working fluid may be applied to con ¬ tinuously preheat the steam generation device 110, mainly a super heater heat exchanger surface and an evaporator heat exchanger surface and the ducting of the system. Therefore, it may be possible to use the further fluid driving device

687 which is smaller (i.e. lower fluid driving capacity) than the fluid driving device 170 for idle. Another possibility may be to omit the further smaller fluid driving device 687. Then, the fluid driving device 170 may be used with a reduced power for driving the working fluid during the idle mode from the thermal storage device 150 to the steam generation device 110.

Another configuration for a constant low discharge flow for preheating may be a connection from the heating device 130 and the steam generation device 110 (e.g. the HRSG) in-line. The hot end if the thermal storage device 150 may be con ¬ nected with the heating device 130. The heating device 130 and the steam generation device 110 are connected in-line and the fluid driving device 170 is interconnected with an array of vents before the gas path is ending in the cold end of the thermal storage device 150. To preheat the heat exchanger surface of the super heater and the evaporator it may be con ¬ siderable to use a separate small fluid driving device (i.e. lower fluid driving capacity) or to use the fluid driving de ¬ vice 170 also with reduces power (i.e. lower fluid driving capacity) .

Fig. 7 shows a system 700 for storing thermal energy accord- ing to an exemplary embodiment of the present invention. The system 700 comprises a steam generation device 110 for heat ¬ ing a steam turbine working fluid of a steam turbine system 120. The steam generation device 110 is feedable by a working fluid for heating the steam turbine working fluid. The system 700 further comprises a heater device 130 for heating the working fluid and a thermal storage device 150. The steam generation device 110 is connected to the heater device 130 for transferring the working fluid between each other. The steam generation device 110, the heater device 130 and the thermal storage device 150 are connected in line. The system 700 further comprises a fluid driving device 170, in particu ¬ lar a blower, for driving the working fluid, and a preheat line 682 which is connected between the thermal storage de ¬ vice 150 and the steam generation device 110.

The system further comprises a further smaller fluid driving device 687 for driving the working fluid in the idle mode be- tween the thermal storage device 150 and the steam generation device 110.

The working fluid is driven and accelerated by the fluid driving device 170 in a predominant and predetermined flow direction of the fluid driving device 170. The predominant and predetermined flow direction of the fluid driving device 170 is from a downstream end of the fluid driving device 170 to an upstream end of the fluid driving device 170. The system 700 further comprises a first line 793 which is connected between the thermal storage device 150 and the downstream end of the fluid driving device 170 and a first valve 783 which is connected to the first line 793 for con ¬ trolling a flow of working fluid between the thermal storage device 150 and the fluid driving device 170.

The system further comprises a second line 795 which is con ¬ nected between the upstream end of the fluid driving device 170 and the thermal storage device 150 and a second valve 785 which is connected to the second line 795 for controlling a flow of the working fluid between the fluid driving device 170 and the thermal storage device 150.

The system further comprises a third line 797 which is con- nected between the upstream end of the fluid driving device

170 and the steam generation device 110 and a third valve 787 which is connected to the third line 797 for controlling a flow of working fluid between the fluid driving device 170 and the steam generation device 110.

The system further comprises a fourth line 799 which is con- nected between the steam generation device 110 and the downstream end of the fluid driving device 170 and a fourth valve 789 which is connected to the fourth line 799 for controlling a flow of working fluid between the steam generation device 110 and the fluid driving device 170.

In the charging cycle the working fluid may flow from the up ¬ stream end of the fluid driving device 170 via the third line 797 and the third valve 687 through the steam generation device 110, further through the heater device 130 (in which the working fluid is heated up) and further through the thermal storage device 150 and transfers thermal energy to the ther ¬ mal storage device 150. Then, the working fluid flows through the first line 793 to the fluid driving device 170 and closes the loop.

In the charging cycle the first valve 783 and the third valve 787 are in an open position and the second valve 785 and the fourth valve 789 are in a closed position. Thus, the working fluid flows through the first line 793, the fluid driving de- vice 170 and subsequently the third line 797.

In the discharging cycle the working fluid flows from the fluid driving device 170 via the second line 795 and the sec ¬ ond valve 785 to the thermal storage device 150 and thermal energy is transferred from the thermal storage device 150 to the working fluid. Next, from the thermal storage device 150 the working fluid flows through the heater device 130 and af ¬ terwards through the steam generation device 110, where a steam turbine fluid is heated up by the thermal energy pro- vided by the working fluid. Next, after having passed the steam generation device 110, the working fluid flows through the fourth line 799 and the fourth valve 789 to the fluid driving device 170 and closes the loop. In the discharging cycle the second valve 785 and the fourth valve 789 are in the open position and the first valve 783 and the third valve 787 are in the closed position. Thus, the working fluid flows through the fourth line 799, the fluid driving device 170 and subsequently the second line 795.

In the idle mode the working fluid may be driven by the fur ¬ ther smaller fluid driving device 687 from the thermal stor- age device 150 to the steam generation device 110 in such a way that the steam generation device 110 may be preheated by the working fluid.

It should be noted that the term "comprising" does not ex ¬ clude other elements or steps and "a" or "an" does not ex ¬ clude a plurality. Also elements described in association with different embodiments may be combined. It should also noted that reference signs in the claims should not be con strued as limiting the scope of the claims.