Excess air valves for pressure control

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 the system.

Art Background

In a thermodynamic process a pressure change or a temperature change of a working fluid, which is used in the thermodynamic process, causes a density change and thus a change of the volume of the working fluid. In a closed loop configuration with a constant volume of gaseous working fluid the tempera ^¬ ture change results in a pressure variation. Thus, this re ^¬ sults in a pressure increase inside the closed loop configu ^¬ ration. Without any compensation of the pressure increase in ^¬ side the system, components such as pipes or in the worst case the thermal energy storage, the heat exchanger or an ^¬ other component which is of high importance for the function ^¬ ality of the whole system, may be damaged or even destroyed if not designed for high pressure usage. Both an expansion caused by an increase of the temperature of the working fluid and a shrinking caused by a decrease of the temperature have to be considered in a design of the closed loop configura ^¬ tion .

A conventional system for storing thermal energy comprises among other components a thermal energy storage, a blower, a heater device and a heat exchanger. Furthermore, a working fluid in the system is air. The conventional system may be designed in a closed loop configuration. That is, the working fluid is used in a charging cycle in which thermal energy is in a first step transferred to the working fluid by the heater device. In a second step, the thermal energy comprised in the working fluid is transferred to the thermal energy storage and stored therein. Additionally, the same working fluid is also used in a discharging cycle. In the discharging cycle the thermal energy is transferred from the thermal en ^¬ ergy storage to the working fluid by guiding the working fluid through the thermal energy storage. In the heat ex- changer the working fluid transfers the thermal energy to a steam turbine working fluid such that the thermal energy may be transformed in electricity by a steam turbine which is driven by the steam turbine working fluid. A first commonly known countermeasure for compensating pressure fluctuations may be to integrate an expansion chamber into the system. An exemplary embodiment of the expansion chamber may be a diaphragm expansion chamber. The diaphragm expansion chamber comprises a flexible diaphragm such as a flexible rubber diaphragm, which separates the expansion chamber in a first section and a second section. The first section contains a gaseous fluid and the second section con ^¬ tains a pressurized fluid. The pressurized fluid may be a gaseous fluid or a liquid fluid. The flexible diaphragm avoids that the pressurized fluid intermixes with the gaseous fluid for setting a pressure equilibrium. The diaphragm expansion chamber must be able to equilibrate all possible pressures that may occur in the system. Therefore, the dia ^¬ phragm expansion chamber may for example have a volume of up to 500 litres in a system operated at a temperature of about 130°C. Recapitulatory, for a system in closed loop configura ^¬ tion the diaphragm expansion chamber may cause enormous investments depending on a predetermined temperature swing and a predetermined volume of working fluid used in the system.

A second commonly known countermeasure may be to integrate a pressurized storage tank into the system. The thermal energy storage needs to be integrated entirely in the pressurized storage tank. Especially in a system comprising a geometrically large thermal energy storage or in a system comprising a plurality of thermal energy storages, the pressurized stor ^¬ age tank causes high investment costs to guarantee pressure tightness. Furthermore, a thermal expansion of the pressur ^¬ ized storage tank itself has to be taken into account.

A third commonly known countermeasure may be to integrate an open expansion system. The open expansion system is nowadays often used for heating systems in apartment houses. The open expansion system is integrated at the highest point of the system for storing thermal energy. It balances the pressure swings from the system by compensating them by a decrease or an increase of a fluid level of a working fluid in an open expansion chamber. An exemplary disadvantage of the open expansion system is that oxygen contained in the atmosphere may be dissolved in the working fluid. As the working fluid in open expansion systems is often water, the dissolving of oxygen in the working fluid may result in corrosion. Further- more, the open expansion chamber in an industrial process or a power plant process may be very large due to large pressure fluctuations .

A fourth commonly known countermeasure may be to integrate a compensation of possible pressure fluctuations by a pumped pressure conservation. The pumped pressure conservation is realized by pumping water from a non-pressurized container into the system for storing thermal energy if the pressure in the system decreases. Otherwise, if an overpressure is pre- sent in the system, valves may be used to guide water out of the system and back into the non-pressurized tank.

A fifth commonly known countermeasure may be to design the system not in a closed loop configuration but in an open loop configuration. This approach has the disadvantage that the efficiency of the system is significantly decreased due to the fact that the working fluid which still contains thermal energy is not guided back into a heater device of a charging cycle. Analogously, in a discharging cycle, the working fluid from the heat exchanger is guided into an exhaust stack in ^¬ stead of recuperating it in the thermal energy storage. Thus, systems with an open configuration often have a low effi- ciency.

There may be a need for providing a reliable system for storing energy which is able to work properly even when the system is subjected to significant pressure fluctuations.

Summary of the Invention

It may be an object of the present invention to provide a simple, reliable and efficient system for storing thermal en ^¬ ergy even when the system is subjected to significant pres ^¬ sure fluctuations.

This object may be solved by a system for storing thermal en- ergy and by a method for operating a system for storing thermal 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 for stor- ing thermal energy comprises a thermal storage device which comprises heat storage elements, a heater device for heating the working fluid, a steam generation device for heating a steam turbine working fluid of a steam turbine system, a thermal excess storage device for storing thermal energy and an excess valve device for controlling a flow of a working fluid between the thermal storage device and the thermal ex ^¬ cess storage device. The working fluid is streamable through the thermal storage device such that thermal energy is ex ^¬ changed between the heat storage elements and the working fluid. The thermal storage device is connected to the heater device for transferring the working fluid between the thermal storage device and the heater device. The steam generation device is connected to the thermal storage device such that the working fluid is feedable to the steam generation device for heating up the steam turbine working fluid. The thermal excess storage device is connected to the thermal storage de ^¬ vice .

The excess valve device is coupled between the thermal stor ^¬ age device and the thermal excess storage device in such a way that, if a working pressure of the working fluid in the thermal storage device exceeds a predetermined set point pressure, an excess portion of the working fluid streams from the thermal storage device into the thermal excess storage device. Additionally, the excess valve device is coupled be ^¬ tween the thermal storage device and the thermal excess stor ^¬ age device in such a way that, if a working pressure of the working fluid in the thermal storage device falls below the predetermined set point pressure, a filling portion of the working fluid streams (e.g. from the thermal excess stor ^¬ age device) to the steam generation device. The thermal storage device may be a sensible thermal storage or a latent heat storage. The thermal storage device may be filled with heat storage elements. The heat storage elements may be solid or bulk elements, such as sand, stone, gravel, rubble, split, clinker, pebble, slag, ceramics, basalt, iron silicate slag or other solid material which has appropriate thermal capacity to store thermal energy over a predetermined period of time. It should be understood that also water or other liquids may be used as the thermal energy storage mate ^¬ rial. The thermal storage material may be heated up by the working fluid.

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 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 resistance or an inductive heater.

The steam generation device may exchange thermal energy, e.g. by a heat exchanger, with a steam turbine system. In particu- lar, 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 an- other 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 generation device comprises a direct heat exchanger or an indi- rect heat exchanger. In direct heat exchangers the heat is exchanged without solid walls separating the different media and thus by direct contact between the two used media. In in ^¬ direct heat exchangers the heat is transferred from one me ^¬ dium to another medium over solid walls separating the two used media 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 ex ^¬ changer, a plate heat exchanger or a heat exchanger consist- ing 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. 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 (low pressure) pump and a second (high pres ^¬ sure) pump. The steam turbine may be a multi stage steam tur ^¬ bine or a single stage turbine. In the 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 for feed water pre ^¬ heating of the steam working turbine fluid. After flowing through the steam turbine, the steam turbine working fluid flows through the condenser in which the steam turbine work- ing 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 thermal excess storage device stores thermal energy and may be a sensible thermal storage or a latent heat storage. The thermal storage device may be filled with heat storage elements. The heat storage elements may have the ability to be heated up, keep their temperature and thus store thermal energy over a predetermined period of time. It should be un ^¬ derstood that also water may be used as the thermal energy storage material. Furthermore, the thermal excess storage de ^¬ vice may also comprise a water heat exchanger and a water tank for providing the ability to storing thermal energy.

The excess valve device controls a flow of the working fluid between the thermal storage device and the thermal excess storage device. The excess valve device may comprise a single valve which may be a gate valve, a non-return valve or angle valve or a system of duct dampers. In a further exemplary embodiment, the excess valve device may also comprise a plural ^¬ ity of sub-valves to ensure the redundancy of a cut-off of the flow of the working fluid between the thermal storage de ^¬ vice and the thermal excess storage device.

The predetermined set point pressure (value) may be a set pressure value for the pressure of the working fluid in the system for storing thermal energy. The predetermined set point pressure defines e.g. a maximum pressure load limit of components of the system.

Normally, the predetermined set point pressure is set for a component which has the lowest pressure load limit. However, the predetermined set point pressure may be set under consid ^¬ eration of other constraints, such as thermodynamic con ^¬ straints. This ensures that the components of the system withstand a pressure load being of such a value and that the system works efficient.

The working pressure (value) is the pressure of the working fluid measured at a defined point of time in the system. The working pressure may be at different values and may vary dur- ing an operating of the system.

The excess portion of the working fluid is the amount of the working fluid which streams from the thermal storage device to the thermal excess device until the pressure of the work- ing fluid in the thermal storage device falls again below a limit and corresponds again to the predetermined set point pressure .

The filling portion of the working fluid is the amount of the working fluid which streams from the thermal excess device to the thermal storage device until the pressure of the working fluid in the thermal storage device rises again above a limit and corresponds again to the predetermined set point pres ^¬ sure .

The predetermined set point pressure may also define both a minimum set point pressure and a maximum set point pressure, so that in the pressure range between the minimum set point pressure and a maximum set point pressure no excess portion and no filling portion of the working fluid is guided through the excess valve device.

Hence, in contrast to conventional approaches, by the present invention, the excess valve device controls a flow of the working fluid between the thermal storage device and the thermal excess storage device in the above-mentioned manner, that is depending on the working pressure of the working fluid. Additionally, in the present invention the excess por ^¬ tion of the working or the thermal energy of the excess por ^¬ tion of the working fluid is stored in the excess thermal storage device such that the thermal energy may be re-guided to the thermal storage device by transferring the filling portion of the working fluid from the thermal excess storage device to the e.g. thermal storage device.

Hence, the thermal energy transferred by the excess portion and the filling portion, respectively, may be recaptured and used in the thermal storage device. Hence, the overall pres ^¬ sure of the system may be balanced in such a way, that the pressure fluctuations are small or at least insignificant while the thermal energy of the filling portion may be reused for the other components, i.e. the steam generation device, of the system.

According to a further exemplary embodiment of the present invention, the heat storage capacity of the thermal excess storage device is about 5% to 25% of the heat storage capac ^¬ ity 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 thermal excess storage device comprises heat storage elements. The heat storage elements may be solid or bulk elements, such as sand, stone, gravel, rubble, split, clinker, pebble, slag, ceramics, basalt, iron silicate slag or other 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 the thermal storage device and the thermal excess storage device may both comprise the same kind of thermal elements.

According to a further exemplary embodiment of the present invention, the thermal excess storage device comprises a wa ^¬ ter heat exchanger for heating a feed water, and a feed water tank for storing the feed water. The feed water tank is connected with the steam turbine system such that the feed water is feedable to a feed water preheating system of the steam turbine system from the feed water tank.

The water heat exchanger is a heat exchanger in which the thermal energy of the excess portion of the working fluid is transferred to the feed water. Thus, the feed water is heated up. After being heated up, the feed water is stored in the feed water tank. Hence, the thermal energy from the excess portion is stored in the feed water tank. The feed water may be mains water, demineralized water, des- tillated water or water which has supplements added to it to improve certain properties of the water, such as the boiling point or chemical properties such as the PH-value.

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 feed water preheating system such that the feed water may flow from the feed water tank to the steam turbine system.

According to a further exemplary embodiment of the present invention, the feed water is used as the steam turbine work- ing fluid.

This means that the feed water may flow into the steam tur ^¬ bine system and may be used as the steam turbine working fluid or it may be intermixed with the steam turbine working fluid in the feed water preheating system. Thus, the feed wa ^¬ ter and the steam turbine working fluid may have the same composition. Hence, the feed water and the steam turbine working fluid may be intermixed without any separation or dispersion of individual components of either the feed water or the steam turbine working fluid.

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 drives the working fluid between the thermal storage device and the heater device..

The fluid driving device may be, aside from a blower, a fan, a ventilator, a compressor, a super-charger or a device gen- erating a pressure gradient such that the working fluid which flows through the fluid driving device may be accelerated in a predominant and predetermined direction of the fluid driv ^¬ ing device. The fluid driving device may also be a multi- stage device in which the working fluid is accelerated in multiple stages such that an efficiency of the fluid driving device is improved. 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 which is connected to the first line for controlling a flow of working fluid between the thermal storage device and the fluid driving 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 which is connected to the second line for controlling a flow of the working fluid between the fluid driving device and the thermal storage device.

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

The system further comprises a fourth line which is connected between the excess valve device and the fluid driving device and a fourth valve which is connected to the fourth line for controlling a flow of working fluid between the excess valve 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 first line enables the working fluid to flow through the first line from the thermal storage device in a direction towards the fluid driving device. The second line enables the working fluid to flow through the second line from the fluid driving device in a direction towards the thermal storage device. The third line enables the working fluid to flow through the third line from the fluid driving device in a direction towards the excess valve device. The fourth line enables the working fluid to flow through the fourth line from the excess valve device in a direction to ^¬ wards the fluid driving device.

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 from the thermal storage device to the fluid drive 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 from the fluid driving device to 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 from the fluid driving device to the excess valve 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 excess valve device.

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

The second valve and the third valve form the excess valve device. The thermal excess storage device is connected to the second valve and the third valve.

According to a further exemplary embodiment of the present invention, the system further comprises a further fluid driving device, in particular a blower, for driving the working fluid. The further fluid driving device drives the working fluid between the steam generation device and the thermal storage 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 thermal excess storage device is further con ^¬ nected to an environment such that the excess portion is streamable to the environment and the filling portion is streamable from the environment into the thermal excess stor ^¬ age device.

The environment may be the space surrounding the system, a machine in which the system is integrated, a room in which the machine or the system is integrated or a building in which the system is integrated. Furthermore, it should be mentioned that the system may also be surrounded by air, wa ^¬ ter, a gas such as an inert gas, or a mixture of the afore ^¬ mentioned, depending on the requirements to the system.

According to a further exemplary embodiment of the present invention, the system further comprises a preheat line which is connected between the thermal excess storage device and the steam generation device in such a way that the working fluid selectively flows from the thermal excess storage de ^¬ vice to the steam generation device.

The preheat line is a structural component such as a pipe or a duct. The preheat line enables the working fluid to flow through the preheat line from the thermal excess storage de ^¬ vice in a direction towards the steam generation device.

The term "selectively" flowing means that a user or a driver program may choose whether the working fluid flows through the preheat line from the thermal excess storage device to ^¬ wards the steam generation device or the working fluid stays stored in the thermal storage device. The working fluid which flows through the preheat line from the thermal excess storage device towards the steam genera ^¬ tion device may be used to transfer the thermal energy stored in the working fluid to the steam generation device. Hence, the steam generation device may be preheated just before op ^¬ erating the system or the steam generation device and thus the system may be kept at a predetermined temperature and pressure during an idle mode. The idle mode describes an operation mode of the system where the heater device, the steam generation device and/or the steam turbine system is/are not operating. Thus, in the idle mode the components of the system are generally not subjected to excessive pressure fluctuations.

According to a further exemplary embodiment of the present invention, the system further comprises a further preheat line which connects the thermal excess storage device and the steam turbine system in such a way that the feed water selec- tively flows from the thermal excess storage device to the steam turbine system.

The further preheat line is a structural component such as a pipe or a duct. The further preheat line enables the working fluid to flow through the further preheat line from the thermal excess storage device in a direction towards the steam turbine system.

The term "selectively" flowing means that a user or a driver program may choose whether the working fluid flows through the further preheat line from the thermal excess storage de ^¬ vice towards the steam turbine system or the working fluid stays stored in the thermal storage device. The working fluid which flows through the preheat line from the thermal excess storage device towards the steam turbine system may be used to transfer the thermal energy which is still be stored in the working fluid to the steam turbine system. Hence, the steam turbine system may be preheated just before operating. The steam turbine system may be kept at a predetermined pressure during the idle mode. According to a further aspect of the present invention, the steam turbine system comprises a first pump and a second pump which are connected to each other by a pump line of a feed water preheating system of the steam turbine system. The further preheat line is connected to the pump line in the feed water preheating system.

The first pump and the second pump drive the steam turbine working fluid through the steam turbine system and through the steam generation device.

The above-mentioned explanations for the preheat line or the further preheat line is applicable and valid for the pump line as well. According to a further aspect of the present invention, the steam generation device is a heat recovery steam generator which is cut short HRSG.

A HRSG may be an energy recovery heat exchanger which recov- ers 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 as aforementioned is disclosed. The method comprises guiding the working fluid through the thermal storage device such that thermal energy is exchanged between the heat storage elements and the working fluid, and transferring the working fluid be- tween the thermal storage device and the heater device. The method further comprises guiding the excess portion of the working fluid e.g. from the thermal storage device into the thermal excess storage device, and guiding the filling por ^¬ tion of the working fluid from the thermal excess storage de- vice to the steam generation device.

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 elements which are comprised in the thermal storage device. In the charging cycle, the working fluid flows first through an inlet into the thermal storage device, such 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, the outlet may be in the charging cycle 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 elements, for example. Hence, in the discharging cycle, the working fluid is heated up by the thermal storage device. The heated up working fluid heats up the steam turbine working fluid in the steam generation device such 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 the charging cycle or in the discharging cycle. During the charging cycle or the discharging cycle the volume of the working fluid changes due to the transferring of thermal energy between the thermal storage device and the working fluid. Especially, during the charging cycle, the volume of the working fluid increases considerably. The system is designed as a closed cycle. Hence, the pressure inside the system also increases considerably and pressure sensible components of the system may fail due to a pressure overload. For avoiding this, the excess portion of the working fluid is guided into the thermal excess storage device.

During the idle mode the pressure of the working fluid decreases and therefore the volume of the working fluid which is present in the system decreases as well. Hence, the pressure in the system may become too low. This may result in poor stream characteristics or if the system comprises leaks ambient air that has a lower temperature than the low

temperature of the system may enter the system. In the worst case this may result in a damage of a component of the system. For avoiding a decrease in the efficiency of the system or any damages of the system, the filling portion of the working fluid is guided into the thermal excess storage device . Contrary to the conventional known systems for storing thermal energy, the system disclosed in the present invention provides the thermal excess storage device and the excess valve device for controlling the working pressure (value) of the working fluid depending on the predetermined set point pressure (value) .

Summarizing, by the present invention, a combination of a complete pressure tight thermal energy storage system with a concept for pressure compensation with integrated excess working fluid (e.g. air) valves is provided. These excess valves regulate the pressure of the working fluid to the de ^¬ sign pressure of the system that may be the ambient pressure and avoid damages to the system and hence leakage at not de ^¬ sired locations. The working fluid (e.g. air), which is heated with the heater device, is guided through the thermal storage device. Due to the volume increase of the working fluid as a result from the temperature increase of the work- ing fluid, a part of the air (i.e. the excess portion) is di ^¬ rected via the excess valve devices into a second thermal storage, i.e. the thermal excess storage device. With this measure, the pressure in the system is maintained at ambient pressure, which is defined by the predetermined set point pressure.

The second thermal energy storage stores the thermal energy e.g. at medium temperature. The excess valve devices are in ^¬ stalled between a cold end of the thermal storage device and the fluid driving device, which is a blower. It is also pos ^¬ sible to use a plurality of excess valve devices at different positions in the air (i.e. working fluid) -guiding system. The excess valve devices regulate passively or actively over con ^¬ tinuous measurements e.g. of differential pressure of the system. This increases the efficiency of the whole thermal storage device. Especially the charging cycle is improved, because the filling portion of the working fluid from the thermal excess storage device can be used to preheat the sys ^¬ tem (in particular the steam generation device and/or the steam turbine system) or to keep the system at a constant temperature. Then the system does not need to be heated up from ambient conditions which is inefficient.

In an idle time past a charge cycle, the temperature in the ducting system, e.g. in the lines and ducts connecting the components of the system, and in the components of the system reduces except the temperature in the thermal storage device. This leads to a pressure reduction of the working fluid. For compensation of the pressure change ambient air enters the thermal excess storage and is thereby heated up. The ambient air functions as the filling portion of the working fluid, which is guided through the excess valve devices to the sys- tern. In this idle period of the system, a small mass flow constantly transfers thermal energy from the thermal excess storage device to the ducting system and compensates partly the heat losses of the ducting system and the steam genera ^¬ tion device. This thermal excess storage device is constantly discharged, wherein the discharge state depends on the idle time .

When discharging the high temperature thermal storage device, the air (working fluid) flows in reverse direction compared to the charge cycle. It enters the thermal storage device with a temperature higher than ambient e.g. approx. T=200°C from an outlet of the steam generation device (HRSG) and thermal energy stored in the heat storage elements is trans ^¬ ferred to the air and leaves the storage with approx.

T=600°C. This working fluid is used in the HRSG to generate steam and to produce electrical power via a steam turbo gen ^¬ erator of the steam turbine system.

Prior to the discharge cycle, the working fluid temperature in the ducting system is reduced due to heat losses after the previous discharge or charge cycle. The temperature depends on the idle time. In the discharge mode the air temperature in the ducting system increases and the pressure is regulated due to the gas expansion filling portion of the air (working fluid) which is guided through the thermal storage device and enters the ducting system via the excess valve device.

In the processes described above the thermal energy in the working fluid that leaves the system through the excess valve device is stored in the thermal excess storage device. This stored thermal energy can be further used to preheat the feed water for a steam generator, as described, or other purposes. Furthermore it is also possible to use the stored thermal en- ergy to keep the system on a constant temperature level for the next charge cycle or to preheat the steam generation de ^¬ vice to reduce the start-up time. In this case a small air flow is guided through the steam generation device. This im- proves the dynamic behaviour of a thermal energy plant com ^¬ prising the described system in the discharge cycle and in ^¬ creases the efficiency and flexibility of the whole system.

It is also possible to charge the thermal storage device with the heating device and the HRSG in one gas path. In this case the hot end of the thermal storage device is connected with the heater in line with the HRSG. The fluid driving device (blower) is connected to a system of valves before the work ^¬ ing fluid path is connected to the cold end of the closed thermal storage device. The valve positions in the system are designed such that the fluid can be guided in both directions using only one fluid driving device. The air that leaves the HRSG still contains thermal energy and this air is guided back into the cold end of the thermal storage device and to recover the energy. When the thermal storage device is charged with the hot air flow of the working fluid the gas volume increases. The surplus excess portion of the working fluid is guided over the excess valve devices through the thermal excess storage device and the thermal energy which is contained in the excess portion is stored in the thermal ex ^¬ cess storage device.

When discharging the thermal storage device in this configu ^¬ ration with heater and HRSG in line, the working fluid is guided also in reverse direction through the thermal storage device. In this mode the working fluid volume increases analogously to the charge mode and is guided through the thermal excess storage device. During idle mode the working fluid volume will decrease and the excess portion and the filling portion, respectively, which is stored in the thermal excess storage device is guided back into the system and could also been used for feed water preheating or keep the system on its temperature level at e.g. 200°C.

It is important to mention that the position of the thermal excess storage device is not limited to the one drawn in the figures. Since it is a closed loop system it is possible to place it in between any other component of the system, for both the charging cycle and the discharging cycle. It has to be noted that embodiments of the invention have been described with reference to different subject-matters. In particular, some embodiments have been described with ref ^¬ erence to apparatus type claims whereas other embodiments 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 noti ^¬ fied, in addition to any combination of features belonging to one type of subject-matter also any combination between features relating to different subject-matters, in particular 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 thermal excess storage device accord ^¬ ing to an exemplary embodiment of the present invention. Fig. 2 shows a schematical view of a system for storing thermal energy comprising a thermal excess storage device and a preheat line according to an exemplary embodiment of the pre ^¬ sent invention.

Fig. 3 shows a schematical view of a system for storing thermal energy comprising a water heat exchanger according to an exemplary embodiment of the present invention. Fig. 4 shows a schematical view of a closed charging cycle of a system for storing thermal energy according to an exemplary embodiment of the present invention.

Fig. 5 shows a schematical view of a further closed charging cycle of a system for storing thermal energy according to an further exemplary embodiment of the present invention.

Fig. 6 shows a schematical view of a system for storing thermal energy comprising a thermal excess storage device accord- ing to an exemplary embodiment of the present invention.

Fig. 7 shows a schematical view of a system for storing thermal energy comprising a thermal excess storage device accord ^¬ ing to an exemplary embodiment of the present 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 if the present invention. The system 100 comprises a thermal storage device 150 which com ^¬ prises heat storage elements, a heater device 130 for heating a working fluid, a steam generation device 110 for heating a steam turbine working fluid of a steam turbine system 120, a thermal excess storage device 180 for storing thermal energy and an excess valve device 181 for controlling a flow of the working fluid between the thermal storage device 150 and the thermal excess storage device 180. The working fluid is streamable trough the thermal storage device 150 such that thermal energy is exchanged between the heat storage elements and the working fluid. The thermal storage device 150 is con ^¬ nected to the heater device 130 for transferring the working fluid between the thermal storage device 150 and the heater device 130. A fluid driving device 170 is connected between the thermal storage device 150 and the heater device 130 for driving the working fluid.

The steam generation device 110 is connected to the thermal storage device 150 such that the working fluid is feedable to the steam generation device 110 for heating up the steam turbine working fluid. The thermal excess storage device 180 is connected to the thermal storage device 150. The excess valve device 181 is coupled between the thermal storage device 150 and the thermal excess storage device 180. A further fluid driving device 171 is connected between the steam generation device and the thermal storage device 150 for driving the working fluid. If a working pressure of the working fluid in the thermal storage device 150 exceeds a predetermined set point pres ^¬ sure, the excess valve device 181 is coupled between the thermal storage device 150 and the thermal excess storage de ^¬ vice 180 in such a way that an excess portion of the working fluid streams e.g. from a duct or line connecting the thermal storage device 150 and the steam generation device 110 into the thermal excess storage device 180.

The excess valve device 181 is coupled between the thermal storage device 150 and the thermal excess storage device 180 in such a way that a filling portion of the working fluid streams from the thermal excess storage device 180 into the thermal storage device 150, if a working pressure of the working fluid in the thermal storage device 150 falls below a predetermined set point pressure.

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 heater device 130 heats up the working fluid. The thermal storage device 150 and/or the thermal excess storage device 180 may be a sensible thermal storage or a la ^¬ tent heat storage. The thermal storage device 150 may be filled with a thermal energy storage material. For example, the thermal storage device 150 and/or thermal excess storage device 180 may be filled with thermal heat storage elements.

The system 100 further comprises a fluid driving device 170, in particular a blower, for driving the working fluid. 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. In Fig. 1 the fluid driving de ^¬ vice 170 is connected between the thermal storage device 150 and the heater device 130 for driving the working fluid from the thermal storage device 150 to the heater device 130. As shown in Fig. 1, the thermal excess storage device 180 is further connected to an environment such that the excess por ^¬ tion of the working fluid is streamable to the environment after transferring at least a part of its thermal energy to the thermal excess storage device 180. The filling portion of the working fluid is streamable from the environment into the thermal excess storage device 180 such that the filling por ^¬ tion is heated up inside the thermal excess storage device 180.

If the excess portion exceeds the volume of the thermal ex ^¬ cess storage device 180, the excess portion may be streamed to the environment to avoid an overpressure in the thermal excess storage device 180 and thus potential damages of the thermal excess storage device 180.

In contrast, if the thermal excess storage device 180 is empty or does not contain enough working fluid, the filling portion may be guided from the environment through the ther- mal excess storage device 180 directly into the thermal stor ^¬ age device 150.

In a charging cycle the working fluid may flow from the fluid driving device 170 through the heater device 130 (in which the working fluid may further be heated up) and further through the thermal storage device 150 and transfers thermal energy to the thermal storage device 150. The predetermined set point pressure may be an exact value of pressure. It should be emphasized that the predetermined set point pres- sure may also be a range of pressure values.

Afterwards the streaming of the working fluid depends on the working pressure. That is, if the working pressure equals the predetermined set point pressure, a valve 141 is switched in such a way, that the working fluid may flow from the thermal storage device 150 to the heater device 130. If the working pressure exceeds the predetermined set point pressure, the valve 141 is switched in such a way that the excess portion of the working fluid may stream from the thermal storage device 150 to the excess valve device 181. The excess valve device 181 is then switched in such a way that the excess portion flows into the thermal excess storage de ^¬ vice 180. The remaining working fluid is guided to the heater device 130 by the valve 141. If the working pressure falls below the predetermined set point pressure, the valve 141 and the excess valve device 181 are switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 181 and through the valve 141. At the valve 141 the filling portion intermixes with the working fluid and is then guided together with the working fluid to the heater device 130.

In a discharging cycle the working fluid flows from the steam generation device 110 via the further fluid driving device 171 the valve 141 to the thermal storage device 150. The working fluid further passes through the thermal storage de ^¬ vice 150 and thermal energy is transferred from the thermal storage device 150 to the working fluid. Next, from the ther- mal storage device 150 the working fluid flows through the steam generation device 110. Afterwards the streaming of the working fluid depends on the working pressure.

That is, if the working pressure equals the predetermined set point pressure, the excess valve device 181 is switched in such a way, that the working fluid may flow from the steam generation device 110 via the further fluid driving device 171 and via the valve 141 to the thermal storage device 150. If the working pressure exceeds the predetermined set point pressure, the excess valve device 181 is switched in such a way that the excess portion of the working fluid streams from the steam generation device 110 through the excess valve de- vice 181 into the thermal excess storage device 180. The re ^¬ maining working fluid is guided to the thermal storage device 150. If the working pressure falls below the predetermined set point pressure, the excess valve device 181 is switched in such a way that the filling portion of working fluid streams from the thermal excess storage device 180 through the excess valve device 181 and through the valve 141. At the valve 141 the filling portion intermixes with the working fluid and is then guided together with the working fluid via the further fluid driving device 171 to the thermal storage device 150.

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 which comprises heat storage elements, 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 thermal excess storage device 180 for storing thermal energy and an excess valve device 281 for controlling a flow of the working fluid between the thermal storage device 150 and the thermal excess storage device 180. In the discharging cycle, the excess valve device 281 is con ^¬ nected between the steam generation device 110 and the thermal storage device 150. In the charging cycle, the excess valve device 281 is connected between the thermal storage de ^¬ vice 150 and the fluid driving device 170. The excess valve device 281 is formed as one physical unit through which the working fluid flows in both the charging cycle and the dis ^¬ charging cycle. The excess valve device 281 may be single valve or may comprise a plurality of sub-valves and is switched depending on the working pressure in the system 100.

If the working pressure equals the predetermined set point pressure, the excess valve device 281 is switched in such a way, that the working fluid may flow through the excess valve device 281 towards the thermal storage device 150 and towards the fluid driving device 170, respectively.

If the working pressure exceeds the predetermined set point pressure, the excess valve device 281 is switched in such a way that the excess portion of the working fluid may stream from the steam generation device 110 and the thermal storage device 150, respectively, through the excess valve device 281 into the thermal excess storage device 180. The remaining working fluid is guided to the thermal storage device 150.

If the working pressure falls below the predetermined set point pressure, the excess valve device 281 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 281. At the excess valve device 281 the filling portion intermixes with the working fluid and the filling portion together with the working fluid is guided into the thermal storage device 150 and the fluid driving de- vice 170, respectively.

The system further comprises a preheat line 285 which is con ^¬ nected between the thermal excess storage device 180 and the steam generation device 110 in such a way that the working fluid selectively flows from the thermal excess storage de ^¬ vice 180 to the steam generation device 110. Hence, the ther ^¬ mal energy which may still be stored in the thermal excess storage device 180 may be used for a pre-heating of the steam generation device 110. A pre-heating of the steam generation device 110 may shorten the time needed by the system 100 be ^¬ tween the charging cycle and the idle mode and between the discharging cycle and the idle mode, respectively.

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 which comprises heat storage elements, 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, a thermal excess storage device 380 for storing thermal energy and an excess valve device 381 for controlling a flow of the working fluid between the thermal storage device 150 and the thermal excess storage device 380.

In Fig. 3 the excess valve device 381 is a multi-way valve.

The excess valve device 381 is coupled to the thermal storage device 150, the steam generation device 110 and to the ther ^¬ mal excess storage device 380.

If the working pressure equals the predetermined set point pressure, the excess valve device 381 is switched in such a way, that the working fluid may flow through the excess valve device 381 from the steam generation device 110 towards the thermal storage device 150 via the further fluid driving de ^¬ vice 171 and from the thermal storage device 150 towards the fluid driving device 170, respectively.

If the working pressure exceeds the predetermined set point pressure, the excess valve device 381 is switched in such a way that the excess portion of the working fluid may stream from the steam generation device 110 and the thermal storage device 150, respectively, through the excess valve device 381 into the thermal excess storage device 380. The remaining working fluid is guided via the further fluid driving device 171 to the thermal storage device 150 and the fluid driving device 170, respectively.

If the working pressure falls below the predetermined set point pressure, the excess valve device 381 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 380 through the excess valve device 381 and afterwards together with the working fluid into the thermal storage device 150 and the fluid driving device 170, respectively. In Fig. 3, the thermal excess storage device 380 comprises a water heat exchanger 383 for heating a feed water, and a feed water tank 385 for storing the feed water. The feed water tank 385 is connected with a feed water preheating system of the steam turbine system 120 in such a way that the feed wa ^¬ ter is feedable to the steam turbine system 120 from the feed water tank 385.

The system 100 further comprises a further preheat line 387 which connects the thermal excess storage device 380 and the feed water preheat system of the steam turbine system 120 in such a way that the feed water selectively flows from the thermal storage device 380 to the steam turbine system 120.

The steam turbine system 120 further comprises the first pump 121 and the second pump 122 which are connected to each other by a pump line 329. The further preheat line 387 is connected to the pump line 329.

The excess portion streams through the water heat exchanger 383 and thus heats up the feed water. The feed water compris ^¬ ing the thermal energy transmitted to the feed water is stored in the feed water tank 385. The water heat exchanger 383 may further comprise a storage for also storing the ex ^¬ cess portion of the working fluid (not shown in Fig. 3) . If the steam turbine system 120 should be pre-heated or heated up additionally, feed water from the feed water tank 385 may transmit thermal energy to the steam turbine system 120 by the further preheat line 387. Fig. 4 shows the charging cycle of the system 100 for storing thermal energy in which the thermal storage device 150 is charged with thermal energy and the working pressure exceeds the predetermined set point pressure. The system 100 com ^¬ prises the thermal storage device 150, the excess valve de- vice 181, the heat device 130, the fluid driving device 170 and the thermal excess storage device 180. The working fluid flows from the fluid driving device 170 to the heat device 130 and to the thermal storage device 150. After flowing through the thermal storage device 150, the ex ^¬ cess portion of the working fluid flows through the excess valve device 181 into the thermal excess storage device 180 and if necessary the excess portion flows from the thermal excess storage device 180 to the environment. The remaining working fluid streams through the excess valve device 181 to the fluid driving device 170.

The exemplary embodiment shown in Fig. 4 shows an open cycle system, wherein the excess portion is exhausted to the environment after passing the thermal excess storage device 180 and wherein the filling portion is injected from the environ- ment into the thermal excess storage device 180.

Fig. 5 shows the charging cycle of the system 100 for storing thermal energy in which the thermal storage device 150 is e.g. in the idle mode and the working pressure falls below the predetermined set point pressure. The system 100 com ^¬ prises the thermal storage device 150, the excess valve de ^¬ vice 181, the heat device 130, the fluid driving device 170 and the thermal excess storage device 180. The working fluid flows from the fluid driving device 170 to the heat device 130 and to the thermal storage device 150. After flowing through the thermal storage device 150, the working fluid is intermixed with the filling portion of the working fluid and both flow to the fluid driving device 170. The filling portion flows from the thermal excess storage device 180 to the excess valve device 181.

The exemplary embodiment shown in Fig. 5 shows a closed cycle system, wherein the excess portion is not exhausted to the environment and the filling portion is not injected from the environment into the thermal excess storage device 180.

Fig. 6 shows a system 200 which comprises the same components as the system 100 which is shown in Fig. 1 and further a first line 693 which is connected between the thermal storage device 150 and the fluid driving device 170, a second line 695 which is connected between the fluid driving device 170 and the thermal storage device 150, a third line 697 which is connected between the fluid driving device 170 and the excess valve device 281 and a fourth line 699 which is connected be ^¬ tween the excess valve device 281 and the fluid driving de ^¬ vice 170. The first valve 683 is connected to the first line 693 for controlling a flow of working fluid between the thermal storage device 150 and the fluid driving device 170. The second valve 685 is connected to the second line 695 for controlling a flow of the working fluid between the fluid driving device 170 and the thermal storage device 150. The third valve 687 is connected to the third line 697 for controlling a flow of working fluid between the fluid driving device 170 and the excess valve device 281. The fourth valve 689 is connected to the fourth line 699 for controlling a flow of working fluid between the excess valve device 281 and the fluid driving de ^¬ vice 170.

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. In the charging cycle the working fluid may flow from the downstream end of the fluid driving device 170 via the third line 697, the third valve 687 and the excess valve device 281 through the heater device 130 (in which the working fluid is heated up) and further through the thermal storage device 150. Next, the working fluid flows through the first line 693 to the upstream end of the fluid driving device 170 and closes the loop. The working fluid is guided through the ex ^¬ cess valve device 281 dependent on a working pressure. If the working pressure equals the predetermined set point pressure, the excess valve device 281 is switched in such a way, that the working fluid may flow through the excess valve device 281 from the downstream end of the fluid driving de ^¬ vice 170 towards the heater device 130.

If the working pressure exceeds the predetermined set point pressure, the excess valve device 281 is switched in such a way that the excess portion of the working fluid may stream from the downstream end of the fluid driving device 170 through the excess valve device 281 into the thermal excess storage device 180. The remaining working fluid is guided to the heater device 130.

If the working pressure falls below the predetermined set point pressure, the excess valve device 281 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 281 and afterwards together with the working fluid into the heater device 130.

In the discharging cycle the working fluid may flow from the fluid driving device 170 via the second line 695 and the sec- ond valve 685 through the thermal storage device 150 and fur ^¬ ther through the steam generation device 110. Next, the working fluid flows through the excess valve device 281 and then through the fourth line 699 and the fourth valve 689 to the fluid driving device 170 and closes the loop. The working fluid is guided through the excess valve device 281 dependent on a working pressure.

If the working pressure equals the predetermined set point pressure, the excess valve device 281 is switched in such a way, that the working fluid may flow through the excess valve device 281 from the steam generation device 110 to the up ^¬ stream end of the fluid driving device 170. If the working pressure exceeds the predetermined set point pressure, the excess valve device 281 is switched in such a way that the excess portion of the working fluid may stream from the steam generation device 110 through the excess valve device 281 into the thermal excess storage device 180. The remaining working fluid is guided to the fluid driving device 170.

If the working pressure falls below the predetermined set point pressure, the excess valve device 281 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 281 and afterwards together with the working fluid to the fluid driving device 170.

Fig. 7 shows a system 700 for storing thermal energy according 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 thermal excess storage device 180 and a fluid driving device 170, in particular a blower, for driv- ing the working fluid.

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 693 which is connected between the thermal storage device 150 and the downstream end of the fluid driving device 170 and a first valve 683 which is connected to the first line 693 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 695 which is con ^¬ nected between the upstream end of the fluid driving device 170 and the thermal storage device 150 and a second valve 685 which is connected to the second line 695 for controlling a flow of the working fluid between the fluid driving device 170 and the thermal storage device. The system further comprises a third line 697 which is con ^¬ nected between the upstream end of the fluid driving device 170 and the steam generation device 110 and a third valve 687 which is connected to the third line 697 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 699 which is connected between the steam generation device 110 and the downstream end of the fluid driving device 170 and a fourth valve 689 which is connected to the fourth line 699 for controlling a flow of working fluid between the steam generation device 110 and the fluid driving device 170.

The second valve 685 and the third valve 687 form an excess valve device 181.

The thermal excess storage device 180 is connected to the second valve 685 and the third valve 687. In the charging cycle the working fluid may flow from the up ^¬ stream end of the fluid driving device 170 via the third line 697 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 693 to the fluid driving device 170 and closes the loop. The working fluid is guided through the third valve 687 which is part of the excess valve device 181 dependent on a working pressure.

If the working pressure equals the predetermined set point pressure, the excess valve device 181 is switched in such a way, that the working fluid may flow through the excess valve device 181 from the upstream end of the fluid driving device 170 towards the steam generation device 110. If the working pressure exceeds the predetermined set point pressure, the excess valve device 181 is switched in such a way that the excess portion of the working fluid may stream from the thermal storage device 150 through the excess valve device 181 into the thermal excess storage device 180. The working fluid is guided from the fluid driving device 170 to the steam generation device 110.

If the working pressure falls below the predetermined set point pressure, the excess valve device 181 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 181 together with the working fluid via the valve 683 to the fluid driving device and afterwards into the steam generation device 110.

In the charging cycle the first valve 683 and the third valve 687 are in an open position and the second valve 685 and the fourth valve 689 are in a closed position. Thus, the working fluid flows through the first line 693, the fluid driving de- vice 170 and subsequently the third line 697.

In the discharging cycle the working fluid flows from the fluid driving device 170 via the second line 695 and the sec- ond valve 685 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 699 and the fourth valve 689 to the fluid driving device 170 and closes the loop. The working fluid is guided through the second valve 695 which is part of the ex ^¬ cess valve device 181 dependent on a working pressure.

If the working pressure equals the predetermined set point pressure, the excess valve device 181 is switched in such a way, that the working fluid may flow through the excess valve device 181 from the upstream end of the fluid driving device 170 towards the thermal storage device 150. If the working pressure exceeds the predetermined set point pressure, the excess valve device 181 is switched in such a way that the excess portion of the working fluid may stream from the steam generator 110 through the excess valve device 181 into the thermal excess storage device 180. The working fluid is guided from the fluid driving device 170 to the thermal storage device 150.

If the working pressure falls below the predetermined set point pressure, the excess valve device 181 is switched in such a way that the filling portion of working fluid may stream from the thermal excess storage device 180 through the excess valve device 181 together with the working fluid via the valve 689 to the fluid driving device and afterwards to the thermal storage device 150.

In the discharging cycle the second valve 685 and the fourth valve 689 are in the open position and the first valve 683 and the third valve 687 are in the closed position. Thus, the working fluid flows through the fourth line 699, the fluid driving device 170 and subsequently the second line 695.

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 be noted that reference signs in the claims should not be con- strued as limiting the scope of the claims.