Abstract:
A thermal energy storage and recovery device includes a heat exchanger arrangement for guiding a flow of a heat transfer medium between first and second ends thereof, and a heat storage material surrounding it, forming thermal interaction region between the heat transfer medium and the heat storage material. The heat exchanger arrangement transports the heat transfer medium from the first end to the second end when the heat storage material receives thermal energy from the heat transfer medium, and transports the heat transfer medium from the second end to the first end when the heat storage material releases thermal energy to the heat transfer medium. A controller operates the device such that that when storing or recovering thermal energy to or from the heat transfer medium within the device there exists a region where the inlet and outlet temperature of the heat transfer medium of this region is kept constant.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to the field of temporarily storing thermal energy. In particular, the present invention relates to a thermal energy storage and recovery device comprising a heat exchanger arrangement and a heat storage material. Further, the present invention relates to a thermal energy storage and recovery system comprising such a thermal energy storage and recovery device and to a method for storing and recovering thermal energy with such a thermal energy storage and recovery device. 
       ART BACKGROUND 
       [0002]    The production of electric power from various types of alternative energy sources such as for instance wind turbines, solar power plants and wave energy plants is not continuous. The production may be dependent on environmental parameters such as for instance wind speed (for wind turbines), sunshine intensity (for solar power plant) and wave height and direction (for wave energy plants). There is very often little or no correlation between energy production and energy demand. 
         [0003]    One known approach to solve the problem of uncorrelated electric power production and electric power demand is to temporally store energy, which has been produced but which has not been demanded, and to release the stored energy at times at which there is a high demand. In the past there have been suggested many different methods to temporarily store energy. Suggested methods are for instance (a) mechanical energy storage methods e.g. pumped hydro storage, compressed air storage and flywheels, (b) chemical energy storage methods e.g. electrochemical batteries and organic molecular storage, (c) magnetic energy storage, and (d) thermal energy storage. 
         [0004]    WO 9214054 A1 discloses a wind-powered energy production and storing system comprising a wind rotor in driving engagement with a power generator via transmission means, to which is also connected a heat pump for operation of at least one heat exchanger unit. The wind rotor is designed as a wind wheel having a rim in direct driving engagement with a main shaft positioned in a subjacent engine housing to which main shaft, in addition to the power generator and the heat pump, a dual circulation pump is coupled for conveying heated and cooled liquid, from a heating container and a cooling container, respectively, positioned in the engine housing to separate heat and cold storing stations. Via a steam separator and a pumping device, a steam generator may be connected to the heat storing station which via a steam turbine drives an additional power generator for power production during periods of slack winds. The storing stations may be buried in soil having a filling of sand, stones or similar materials. One disadvantage of this wind-powered energy production and storing system is that there is a direct mechanical connection between the energy storage and recovery system and the wind turbine and that all the energy storage equipment, besides the storing stations, is placed in the wind turbine resulting in a complex mechanical arrangement of the system components. This causes the known system to be not flexible. 
         [0005]    There may be a need for improving the temporal storage of thermal energy in particular with respect to the flexibility and the efficiency of a thermal energy storage and recovery system. 
       SUMMARY OF THE INVENTION 
       [0006]    This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims. 
         [0007]    According to a first aspect of the invention there is provided a thermal energy storage and recovery device comprising a heat exchanger arrangement, which is configured for guiding a flow of a heat transfer medium between a first end of the heat exchanger arrangement and a second end of the heat exchanger arrangement, a heat storage material, which surrounds the heat exchanger arrangement in such a manner that a thermal interaction region is formed for thermally coupling the heat transfer medium with the heat storage material and a control unit for controlling the operation of the thermal energy storage and recovery device. The heat exchanger arrangement is adapted to (a) transport the heat transfer medium from the first end to the second end, if the thermal energy storage and recovery device is in a first operational mode, in which the heat storage material is supposed to receive thermal energy from the heat transfer medium and (b) transport the heat transfer medium from the second end to the first end, if the thermal energy storage and recovery device is in a second operational mode, in which the heat storage material is supposed to release thermal energy to the heat transfer medium. Further, the thermal interaction region has at least such a physical length along a transport direction of the heat transfer medium and the control unit is configured for operating the thermal energy storage and recovery device in such a manner, that when storing thermal energy with a hot heat transfer medium or when recovering thermal energy with a cold heat transfer medium within the thermal energy storage and recovery device there exists a region where the inlet and outlet temperature of the heat transfer medium of this region is kept at least substantially constant. 
         [0008]    The described thermal energy storage and recovery device is based on the idea that by employing different transport directions of the heat transfer medium for different operational modes, wherein the physical length of the thermal interaction region is larger than a predetermined lengthwise extension, a highly efficient thermal energy storage can be realized. Specifically, the combination of (a) different transport directions for the two different operational modes and (b) a long physical length allow for achieving an outlet temperature of the heat transfer medium at least at some time during the second operational mode, which outlet temperature is not significantly smaller than the inlet temperature of the heat transfer medium at least at some time during the first operational mode. This means that the during the second operational mode the heat transfer medium can be received from the thermal energy storage and recovery device almost with the same (high) temperature as compared to the temperature with which the heat transfer medium is provided to the thermal energy storage during the first operational mode. In this way, if the hot heat transfer medium comprises heated steam, the heated up (originally) cold heat transfer medium may also comprise heated steam which then may be used directly to drive a steam turbine without any further heating means. Thereby, the efficiency of the heat storage process can be significantly increased. 
         [0009]    The described principle of using (a) a first transport direction from the first end to the second end for charging the heat storage material with thermal energy and (b) an opposite second transport direction from the second end to the first end for discharging the heat storage material from thermal energy can be interpreted as employing a counter current principle. 
         [0010]    Specifically, in the first operational mode (i.e. charging the heat storage material with thermal energy), a hot heat transfer medium is let into the first end. After having transferred at least a part of its thermal energy the at least partially cooled down heat transfer medium is returned at the second end. Correspondingly, in the second operational mode (i.e. discharging thermal energy from the heat storage material), a comparatively cold heat transfer medium is let into the second end. After having received thermal energy from the heat storage material the at least partially heated up heat transfer medium is returned at the first end. 
         [0011]    In other words, when taking benefit from the described counter current principle the inlet end of the heat exchanger arrangement for hot heat transfer medium when charging the thermal energy storage and recovery device may be the same as the outlet end of the heat exchanger arrangement for heated up heat transfer medium when discharging the thermal energy storage and recovery device. Correspondingly, the outlet end of the heat exchanger arrangement for cooled down heat transfer medium when charging the thermal energy storage and recovery device may be the same as the inlet end of the heat exchanger arrangement for cold heat transfer medium when discharging the thermal energy storage and recovery device. 
         [0012]    The heat transfer medium may be a fluid, i.e. a liquid or a gaseous medium. Preferably, the heat transfer medium may be compressed air or a superheated steam at least when the heat transfer medium is at its higher temperature. This may mean that when charging thermal energy into the described thermal energy storage and recovery device, the heat transfer medium, which is inserted into the heat exchanger arrangement, is at least partially gaseous. When the heat transfer medium leaves the heat exchanger arrangement it may have become liquid again. Correspondingly, when discharging or extracting thermal energy from the described thermal energy storage and recovery device, a cold liquid fluid may be heated up such that it is converted into a gaseous or at least partially gaseous steam. This may be in particular advantageous if the extracted thermal energy is used for driving a steam turbine which itself drives an electric power generator. 
         [0013]    Because of the described long physical length of the thermal interaction region between the heat transfer medium and the heat storage material in combination with the above described counter current heat exchange principle it can be ensured that the temperature gradients of the thermal storage unit for both charging with a hot fluid (thermal energy storing fluid) and discharging with a cold fluid (thermal energy recovering fluid) are maintained nearly constant throughout the length of the stream of the counter current system. Further, it can be ensured that the inlet and outlet temperature of the thermal storage unit is also nearly constant. 
         [0014]    Having a long physical length of the heat exchanger arrangement together with the use of the counter current heat exchange principle may ensure that a nearly constant inlet and outlet temperature of the thermal energy storage and recovery device can be realized. This makes it quite easier to control connected equipment for recovering the stored energy and supplying electrical power to a power grid. 
         [0015]    By having a constant outlet temperature of the discharged heat transfer medium a high efficiency can be maintained during the complete thermal energy discharge cycle. This is a big advantage compared to other energy storage solutions such as batteries where the efficiency is reduced during discharge and varies at different discharge rates. The described thermal energy storage and recovery device is also much more efficient than known devices having a smaller interaction region, wherein typically the temperature of the whole heat storage material gradually decreases during a discharge cycle (=second operational mode) resulting in a reduced efficiency. 
         [0016]    According to an embodiment of the invention the described region, where the inlet and outlet temperature of the heat transfer medium of the region is kept constant, is longer than other regions of the thermal energy storage and recovery device, where the respective inlet and outlet temperature of the heat transfer medium is not substantially constant. 
         [0017]    According to an embodiment of the invention the physical length of the thermal interaction region is at least 200 m, preferably at least 500 m and in particular at least 1000 m. 
         [0018]    By having a long physical length of the interaction region between the heat transfer medium and the heat storage material, i.e. the region wherein the heat exchanger arrangement is placed in the heat storage material, and by employing the counter current heat exchange principle it can be ensured that during the second operational mode (i.e. stored thermal energy is recovered from the thermal energy storage and recovery device) the temperature of the cold fluid is increased to the same or nearly the same temperature as the inlet temperature of the hot fluid. In this way, if the hot fluid being provided to the thermal energy storage and recovery device during the first operational mode comprises a temperature as high as a heated steam, the heated up cold fluid may also comprise or be converted to heated steam which then may be used directly to drive a steam turbine without any further heating means. Thereby, a high efficiency of the thermal energy storage capability of the described thermal energy storage and recovery device can be realized. 
         [0019]    The first end and the second end may be located at one and the same side of the thermal energy storage and recovery device. By placing the inlet end and the outlet end of the heat exchanger arrangement relatively close to each other a heat loss caused by long fluid (heat transfer medium) feeding pipes and/or long fluid return pipes to or from the heat exchanger arrangement can be minimized. 
         [0020]    According to a further embodiment of the invention the heat exchanger arrangement comprises (a) a first heat exchange section being associated with the first end, (b) a second heat exchange section being associated with the second end, (c) a first connecting section connecting the first heat exchange section with the second heat exchange section and (d) a second connecting section connecting the first heat exchange section with the second heat exchange section parallel to the first connecting section. At least one of the connecting sections comprises a valve for controlling the flow of the heat transfer medium through the respective connecting section. This may provide the advantage that the heat exchange capacity of the heat exchanger arrangement and/or the thermal energy storage capacity of the thermal energy storage and recovery device can be adapted to actual operating conditions. For instance by closing (opening) the valve the effective amount or mass of the heat storage material, which contributes to the described thermal heat storage, can be reduced (increased). The same holds for the overall heat transfer rate between the heat exchanger arrangement and the heat storage material. 
         [0021]    Generally speaking, by changing the setting or the adjustment of the valve a decrease or an increase of the heat exchange capacity and of the thermal energy storage capacity may be realized. Thereby, the thermal energy storage and recovery device can be adapted to currently present operating conditions. 
         [0022]    It is mentioned that apart from one or more valves also heating means and/or cooling means like e.g. heating circuits and/or cooling circuits may be used for operating the thermal energy storage and recovery device within an energy storage and recovery system in order to optimize the inlet and/or outlet temperatures of the described thermal energy storage and recovery device. Thereby, the energy storage efficiency may be further increased. 
         [0023]    It is further mentioned that one or more of the valves may be thermostat controlled and/or remote controlled. 
         [0024]    According to a further embodiment of the invention the thermal energy storage and recovery device further comprises thermal insulating means (a) for thermally isolating the whole thermal energy storage and recovery device from its environment and/or (b) for thermally isolating different compartments of the thermal energy storage and recovery device from each other. This may provide the advantage that the thermal energy storage and recovery device can be at least partially thermally decoupled from its surrounding environment and/or different compartments or regions of the device can be at least partially thermally decoupled from each such that the effective size of the thermal energy storage and recovery device can be optimized in view of given operating conditions. 
         [0025]    The thermal insulation means may comprise e.g. mineral wool, glass wool, rock wool or other preferably similar insulating materials. 
         [0026]    According to a further embodiment of the invention the compartments are configured in such a manner that along the thermal interaction region a stepwise temperature gradient control can be realized such that in each compartment there is a constant temperature and the temperatures of different compartments are different from each other. 
         [0027]    According to a further embodiment of the invention the heat storage material comprises a solid material such as in particular sand, soil, ashes, stones and/or gravel. Of course, also other materials which are preferably also relatively cheap and which comprise similar thermal properties may be used. 
         [0028]    According to a further embodiment of the invention the first end comprises a single first opening and the second end comprises a single second opening, wherein (i) in the first operational mode the first opening is used for receiving hot fluid and the second opening is used for emitting a cold fluid representing cooled down hot fluid and (ii) in the second operational mode the second opening is used for receiving cold fluid and the first opening is used for emitting a hot fluid representing heated up cold fluid. This may provide the advantage that a single heat exchanger arrangement is sufficient for realizing the described counter current heat exchange principle. 
         [0029]    According to a further embodiment of the invention the thermal energy storage and recovery device further comprises a further heat exchanger arrangement, which is configured for guiding a flow of a further heat transfer medium between a further first end of the further heat exchanger arrangement and a further second end of the further heat exchanger arrangement, and a further heat storage material, which surrounds the further heat exchanger arrangement in such a manner that a further thermal interaction region is formed for thermally coupling the further heat transfer medium with the further heat storage material. The further heat exchanger arrangement is adapted to (a) transport the further heat transfer medium from the further first end to the further second end, if the thermal energy storage and recovery device is in a further first operational mode, in which the further heat storage material is supposed to receive thermal energy from the further heat transfer medium and (b) transport the further heat transfer medium from the further second end to the further first end, if the thermal energy storage and recovery device is in a further second operational mode, in which the further heat storage material is supposed to release thermal energy to the further heat transfer medium. Furthermore, the further thermal interaction region has at least such a further physical length along a further transport direction of the further heat transfer medium and the control unit is further configured for operating the thermal energy storage and recovery device in such a manner, that when storing thermal energy with a hot heat transfer medium being guided within the further heat exchanger arrangement or when recovering thermal energy with a cold heat transfer medium being guided within the further heat exchanger arrangement within the thermal energy storage and recovery device there exists a further region where the inlet and outlet temperature of the heat transfer medium of this further region is kept constant. Thereby, this further region is longer than other further regions of the thermal energy storage and recovery device where the respective inlet and outlet temperature of the heat transfer medium is not constant. 
         [0030]    This may provide the advantage that a further heat transfer medium can be used for charging and/or discharging the described thermal energy storage and recovery device. Thereby, the further heat transfer medium may be a different fluid than the heat transfer medium. Alternatively, the further heat transfer medium and the heat transfer medium may be the same fluid, which however are guided through different heat transfer pipes through the heat storage material. 
         [0031]    The further heat storage material being associated with the further heat exchanger arrangement may be the same or may be a different material as compared to the heat storage material being associated with the above described heat exchanger arrangement. 
         [0032]    The various inlet ends and outlet ends of both the heat exchanger arrangement and the further heat exchanger arrangement may also just be used to let originally cold fluid and originally hot fluid flow in separate chambers or tubes of the thermal energy storage and recovery device. Thereby, in order to recover stored energy only the originally cold fluid is flowing through the device and in order to store energy the originally hot fluid is flowing through the device. 
         [0033]    According to a further embodiment of the invention the heat exchanger arrangement and the further heat exchanger arrangement form a counter current heat exchanger system. Thereby, the further heat transfer medium and the heat transfer medium are transportable simultaneously and the further heat transfer medium is transportable in an opposite direction with respect to the heat transfer medium. 
         [0034]    Generally speaking, the described counter current heat exchanger system may let both heat transfer media travel through the respective pipes of the heat exchanger arrangement respectively of the further heat exchanger arrangement at the same time but in opposite directions with respect to each other. In this way the heat transfer media move in opposite directions along each other in separate chambers or tubes of the counter current heat exchanger system. Thereby, the velocity of the hot inlet flow into the counter current heat exchanger system may differ from the velocity of the cold inlet flow into the counter current heat exchanger system. This may provide the advantage that the stored thermal energy can be slowly tapped or slowly stored depending on the velocity of the cold and/or the hot inlet flows. 
         [0035]    According to a further aspect of the invention there is provided a thermal energy storage and recovery system comprising (a) a thermal energy storage and recovery device as defined above, (b) a heat generating arrangement, which is connected directly or indirectly to the thermal energy storage and recovery device and which is adapted to heat up the heat transfer medium, which has been received from the thermal energy storage and recovery device and which is supposed to be transported to the thermal energy storage and recovery device, and (c) a heat consumption arrangement, which is connected directly or indirectly to the thermal energy storage and recovery device and which is adapted to receive thermal energy from heat transfer medium, which has been heated up by the thermal energy storage and recovery device. 
         [0036]    The described thermal energy storage and recovery system is based on the idea, that when the above described thermal energy storage and recovery device co-operates with a heat generating arrangement and with a heat consumption arrangement a highly efficient temporal heat storage and heat recovery can be realized. 
         [0037]    The heat generating arrangement may be any device which is capable of converting energy, in particular electric energy, into thermal energy. The generated respectively converted thermal energy is then transferred to the thermal energy storage and recovery device via the heat transfer medium. 
         [0038]    In case of a direct (thermal) connection between the thermal energy storage and recovery device and the heat generating arrangement, the heat transfer medium being used by the thermal energy storage and recovery device is the same as the operating medium of the heat generating arrangement. In case of an indirect connection different fluids may be used for the heat transfer medium and for the operating medium. The thermal connection between the two fluids may then be realized by means of a heat exchanger and/or by means of a condenser. 
         [0039]    The heat consumption arrangement may be any device, which is capable of converting thermal energy into mechanical and/or electric energy which can be fed for instance into a power grid. 
         [0040]    In case of a direct (thermal) connection between the thermal energy storage and recovery device and the heat consumption arrangement, the heat transfer medium being used by the thermal energy storage and recovery device is the same as the operating medium of the heat converting arrangement. In case of an indirect connection different fluids may be used for the heat transfer medium and for the operating medium. The thermal connection between the two fluids may then be realized for instance by means of a heat exchanger and/or by means of an evaporator. 
         [0041]    Preferably, the thermal energy storage and recovery device comprises two heat exchanger arrangements, in particular the above described heat exchanger arrangement and the above described further heat exchanger arrangement, wherein one heat exchanger arrangement is associated with the heat generating arrangement and the other heat exchanger arrangement is associated with the heat consumption arrangement. 
         [0042]    According to an embodiment of the invention the heat generating arrangement comprises (a) a compressor for feeding the thermal energy storage and recovery device with compressed hot heat transfer medium and (b) a turbine for receiving from the thermal energy storage and recovery device cooled down heat transfer medium. This may provide the advantage that any gas such as for instance compressed air can be used as the heat transfer medium for loading the thermal energy storage and recovery device with thermal energy. Since the thermal energy storage and recovery device will cool down the air during its passage through the heat exchanger arrangement of the thermal energy storage and recovery device, the air pressure at the outlet of the thermal energy storage and recovery device will be smaller than the pressure of the compressed air at the input of the thermal energy storage and recovery device. 
         [0043]    According to a further embodiment of the invention the heat generating arrangement further comprises a motor driving the compressor. Thereby, the turbine is mechanically connected to the motor. This may provide the advantage that a high efficiency of the heat generating arrangement can be achieved. 
         [0044]    Specifically, if the hot heat transfer medium or fluid comprises a hot compressed air as an inlet to the heat exchanger arrangement in the thermal energy storage and recovery device then a cooled compressed air may be returned at the outlet of the heat exchanger arrangement, wherein the cooled compressed air may be fed into an air-turbine which may be mechanically connected to a shaft being common for the air-turbine and for a compressor helping driving the compressor and thereby increasing the efficiency of the described thermal energy storage and recovery device. 
         [0045]    The heat generating arrangement may comprise an electric boiler and/or a heat pump. This may provide the advantage that electric energy, which has been generated in particular by an alternative energy source such as a wind turbine, can be converted into heat which can be stored as thermal energy within the above described thermal energy storage and recovery device. In particular a heat pump may provide the advantage of a very efficient heat generation. When using a heat pump electric energy may be first converted into mechanical energy of a compressor, which in accordance with the well known physical principle of a heat pump compresses a gaseous heat pump medium and circulates the same around a closed loop comprising inter alia a condenser and an evaporator. Thereby, the energy being released within the condenser may be used to heat up the heat transfer medium which is then forwarded to the thermal energy storage and recovery device. In this respect it is mentioned that the described evaporator may be driven by air, by a further cooling means and/or by pumped return water e.g. from a district heating installation. 
         [0046]    According to a further embodiment of the invention the heat consumption arrangement comprises a steam turbine, which in the second operational state receives hot heat transfer medium from the thermal energy storage and recovery device. This may provide the advantage that a highly efficient conversion of the recovered thermal energy can be achieved. 
         [0047]    In this respect “hot heat transfer medium” may mean that because of its previous passage through the thermal energy storage and recovery device the originally cooler or cold heat transfer medium has been heated up. 
         [0048]    A rotating shaft of the steam turbine may be connected to an electric power generator, which is capable of converting the mechanical energy being provided by the steam turbine into electric energy, which can be easily fed to a power grid and/or which can be directly consumed by at least one electric consumer. 
         [0049]    The steam turbine may be connected to a condenser, wherein the operating medium of the steam turbine, after it has been delivered its energy to the steam turbine, is converted into its liquid phase. 
         [0050]    The described condenser may be a part of a further closed loop, which apart from the steam turbine and the condenser may comprise inter alia a pump and an evaporator. Thereby, energy being released from the thermal energy storage and recovery device may be transferred to the steam turbine via the mentioned evaporator, wherein the operating medium of the steam turbine is transferred from the liquid phase into the gaseous phase. 
         [0051]    The described condenser may be driven by air, by a further cooling means and/or by pumped return water from a district heating installation. 
         [0052]    According to a further embodiment of the invention the heat consumption arrangement further comprises a circulation pump for feeding a cold heat transfer medium to the thermal energy storage and recovery device. 
         [0053]    In this respect “cold heat transfer medium” may mean that during its following passage through the thermal energy storage and recovery device the cold heat transfer medium will be heated up. 
         [0054]    According to a further embodiment of the invention the heat consumption arrangement further comprises a district heating installation system, which (a) receives heat transfer medium from the steam turbine and (b) provides heat transfer medium to the circulation pump. 
         [0055]    The district heating installation system may comprise a heat exchanger system which thermally connects the heat transfer medium with a fluid such as for instance water. Thereby, the district heating installation may receive comparatively cold water from a water installation via a water inlet and may provide hot or warm water to the water installation via a water outlet. 
         [0056]    It is mentioned that the thermal energy storage and recovery system may further comprise a control unit, which is connected to at least one of (a) the thermal energy storage and recovery device, (b) the heat generating arrangement and (c) the heat consumption arrangement. Thereby, the control unit is adapted to control the operation of the thermal energy storage and recovery system. 
         [0057]    Specifically, the control unit may be coupled to one or more of the following components: (a) compressor of the heat generating arrangement, (b) a valve of the heat generating arrangement, (c) at least one valve of the thermal energy storage and recovery device, (d) at least one circulation pump driving the heat transfer medium through the thermal energy storage and recovery device, (e) a (steam) turbine of the heat consumption arrangement, (f) a feed pump of the heat converting arrangement, (g) a circulation pump for a cold medium being cycled within a cold reservoir cycle, wherein the cold medium drives (g 1 ) an evaporator of the heat generating arrangement (realized by means of the above described heat pump) and/or (g 2 ) a condenser of the heat consumption arrangement (comprises inter alia a steam turbine). 
         [0058]    According to a further aspect of the invention there is provided a method for storing and recovering thermal energy with a thermal energy storage and recovery device having a heat exchanger arrangement, which comprises a first end and a second end, and a heat storage material, which surrounds the heat exchanger arrangement in such a manner that a thermal interaction region is formed for thermally coupling a heat transfer medium being guided within the heat exchanger arrangement with the heat storage material. The provided method comprises (a) transporting the heat transfer medium from the first end to the second end, if the thermal energy storage and recovery device is in a first operational mode, in which the heat storage material is receiving thermal energy from the heat transfer medium, and (b) transporting the heat transfer medium from the second end to the first end, if the thermal energy storage and recovery device is in a second operational mode, in which the heat storage material is releasing thermal energy to the heat transfer medium. Further, the thermal energy and storage device is operated in such a manner and the thermal interaction region has at least such a physical length along a transport direction of the heat transfer medium, that when storing thermal energy with a hot heat transfer medium or when recovering thermal energy with a cold heat transfer medium within the thermal energy storage and recovery device there exists a region where the inlet and outlet temperature of the heat transfer medium of this region is kept at least substantially constant. 
         [0059]    The described method is based on the idea that when a thermal energy and recovery device is operated in such a manner that a region develops, wherein the inlet and outlet temperature of the heat transfer medium of this region is kept substantially constant, a maximum temperature difference between the inlet temperature of the heat transfer medium entering this region and the outlet temperature of the heat transfer medium leaving this region can be achieved. Thereby, the efficiency of the energy storage and recovery procedure can be maximized. 
         [0060]    Specifically, when charging the heat storage material with thermal energy the inlet temperature of the originally hot heat transfer medium entering this region will be at least almost the same as the temperature of the (hot) heat transfer medium entering the whole thermal energy storage and recovery device. Further, the outlet temperature of the cooled down heat transfer medium leaving this region will be at least almost the same as the temperature of the heat transfer medium leaving the whole thermal energy storage and recovery device. 
         [0061]    Correspondingly, when discharging the heat storage material the originally cold heat transfer medium entering this region will be at least almost the same as the temperature of the (cold) heat transfer medium entering the whole thermal energy storage and recovery device. Further, the outlet temperature of the heated up heat transfer medium leaving this region will be at least almost the same as the temperature of the heat transfer medium leaving the whole thermal energy storage and recovery device. 
         [0062]    It is mentioned that by increasing the physical length of the thermal interaction region along the transport direction of the heat transfer medium this region having a constant inlet and outlet temperature will become larger. Therefore, by increasing the physical length of the thermal interaction region the efficiency of the whole thermal energy storage and recovery procedure can be significantly increased. 
         [0063]    It has to be noted that embodiments of the invention have been described with reference to different subject matters. Specifically, some embodiments have been described with reference to claims being directed to a thermal energy storage and recovery device whereas other embodiments have been described with reference to claims being directed to a thermal energy storage and recovery system or to a method for storing and recovering thermal energy with such a thermal energy storage and recovery device. 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 between features relating to different subject matters is considered as to be disclosed with this document. Further, when taking benefit of the disclosure of this document the person skilled in the art will understand the operation of the described thermal energy storage and recovery device and system. 
         [0064]    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 embodiment but to which the invention is not limited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0065]      FIG. 1  shows a sectional top view of a thermal energy storage and recovery device with a heat exchanger arrangement, which comprises a first heat exchange section being associated with the first end, a second heat exchange section being associated with the second end and three connecting sections connecting in parallel the first heat exchange section with the second heat exchange section. 
           [0066]      FIG. 2  shows a sectional top view of the thermal energy storage and recovery device depicted in  FIG. 1 . 
           [0067]      FIG. 3  shows a thermal energy storage and recovery device with a lengthy heat exchanger arrangement and the corresponding temperature behavior along the pipe of the lengthy heat exchanger arrangement. 
           [0068]      FIG. 4  shows a schematic illustration of a thermal energy storage and recovery system in accordance with a first embodiment of the invention. 
           [0069]      FIG. 5  shows a schematic illustration of a thermal energy storage and recovery system in accordance with a second embodiment of the invention. 
           [0070]      FIG. 6  illustrates the temperature behavior along the pipe of a heat exchanger arrangement having a long thermal interaction length with the surrounding heat storage material when the thermal energy storage and recovery device is charged in several steps by a hot inlet heat transfer medium. 
           [0071]      FIG. 7  illustrates a stepwise temperature behavior along a pipe of a heat exchanger arrangement, wherein the thermal energy storage and recovery device comprises different compartments being thermally isolated from each other. 
           [0072]      FIG. 8  illustrates a temperature gradient along the pipe of a heat exchanger arrangement, wherein during a thermal charging the temperature gradient moves in one direction and during a thermal discharging the temperature gradient moves in the opposite direction. 
       
    
    
     DETAILED DESCRIPTION 
       [0073]    The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit. 
         [0074]      FIG. 1  shows a sectional top view of a thermal energy storage and recovery device  100 . The thermal energy storage and recovery device  100  comprises a casing  102 , which comprises an insulating material. Therefore, the casing  102  represents an outer insulation wall  108  of the thermal energy storage and recovery device  100 . The thermal energy storage and recovery device  100  further comprises inner insulation walls  104 , which segment the volume of the thermal energy storage and recovery device  100  into different regions. According to the embodiment described here both the outer insulation walls  108  and the inner insulation walls  104  comprise a mineral wool. 
         [0075]    The casing  102  is filled with a heat storage material  108 . The heat storage material  108  may comprise sand, soil, ashes, gravel, stone and/or other kind of solid material, which preferably comprises a large specific heat capacity. The whole thermal energy storage and recovery device  100  is embedded within ground  120 , which may also comprise soil, gravel, stones, rock, ashes and/or sand or similar materials. 
         [0076]    The thermal energy storage and recovery device  100  further comprises a heat exchanger arrangement  110 . The heat exchanger arrangement  110  is embedded with the heat storage material  108 . The heat exchanger arrangement  110  comprises a first heat exchange section  112  being associated with a first end  112   a  of the heat exchanger arrangement  110 , a second heat exchange section  114  being associated with a second end  114   a  of the heat exchanger arrangement  110  and three connecting sections  116 ,  117  and  118  connecting in parallel the first heat exchange section  112  with the second heat exchange section  114 . Within each connecting sections  116 ,  117  and  118  there is provided a valve  116   a,    117   a  and  118   a,  respectively. The valves  116   a,    117   a  and  118   a  can be controlled by a non depicted control unit such that each of the three connecting sections  116 ,  117  and  118  can be opened, closed or partially opened/closed individually. By closing one or more of the valves  116   a,    117   a  and  118   a  a flow of heat transfer medium through the heat exchanger arrangement  110  can be controlled. Thereby, sub-regions of the thermal energy storage and recovery device  100 , which are associated with a closed valve, can be effectively decoupled from the remaining regions of the thermal energy storage and recovery device  100 . This means that by opening one valve and closing one or more of the other valves the energy storage capacity may be decreased or increased as the used capacity of the heat exchanger arrangement  100  is decreased or increased. 
         [0077]    The described thermal energy storage and recovery device  100  may be of the size of more than 1000 m long, 100 m wide and 5 m deep. This results in a volume of 500,000 m 3 . As has already been mentioned above the heat storage material may be sand filled with sand, which has a specific heat capacity of 0.8 kJ/(kg K) and a sand density of 1740 kg/m 3 . When the sand  108  is heated up from a temperature of 20° C. to 200° C. (=temperature difference of 180° C.), this results in a heat storage capacity of up to 125280 GJ. This corresponds to 34.8 GWh. 
         [0078]    Of course, also thermal energy storage and recovery devices having other sizes and other operating temperatures may be used in order to get other heat storage capacities. 
         [0079]    When operating the thermal energy storage device  100  different operational modes are used (a) for charging the thermal energy storage and recovery device  100  with thermal energy and (b) for discharging the thermal energy storage and recovery device  100 , i.e. for retrieving thermal energy from the thermal energy storage and recovery device  100 . Specifically, in a first operational mode in which the thermal energy storage and recovery device  100  is charged by receiving thermal energy from the heat transfer medium, the heat transfer medium is transported from the first end  112   a  to the second end  114   a.  In a second operational mode in which the thermal energy storage and recovery device  100  is discharged by providing thermal energy to the heat transfer medium, the heat transfer medium is transported from the second end  114   a  to the first end  112   a.  This dependency of the transport direction of the heat transfer medium from the operational state can be seen as employing a counter current principle. By using this counter current principle, when thermal energy is recovered from the thermal energy storage and recovery device  100 , it is possible to heat up the cold heat transfer medium to nearly the same temperature as the inlet temperature of the hot heat transfer medium when thermal energy is inserted into the thermal energy storage and recovery device  100 . This makes the thermodynamic efficiency of the described thermal energy storage and recovery device  100  very high. 
         [0080]    It is mentioned that according to the embodiment described here the thermal energy storage and recovery device  100  further comprises a further not depicted further heat exchanger arrangement having a further first heat exchange section with a further first end and a further second heat exchange section with a further second end. Hot fluid may then be fed into one of the first ends and returned in one of the second ends and a cold fluid may be fed into the other first end and returned in the other second end using the counter current principle. Thereby, with respect to the heat exchanger arrangement  110  the further heat exchanger arrangement may comprise separate cavities or tubes. 
         [0081]      FIG. 2  shows a sectional top view of the thermal energy storage and recovery device  100 . The ends  112   a  and  114   a  of the heat exchanger arrangement  100  can be seen on the front side of the thermal energy storage and recovery device  100 . Further, in accordance with the embodiment described above, on the right side of the thermal energy storage and recovery device  100  there are provided the first end  112   a  and a further first end  112   b  of the further heat exchanger arrangement. Accordingly, on the left side of the thermal energy storage and recovery device  100  there are provided the second end  114   a  and a further second end  114   b  of the further heat exchanger arrangement. 
         [0082]    It is mentioned that in the orientation depicted in  FIG. 2  the thermal energy storage and recovery device  100  may be placed down into the ground  120 . 
         [0083]      FIG. 3  shows a thermal energy storage and recovery device  300  according to a further embodiment of the invention. The thermal energy storage and recovery device  300  comprises a heat exchanger arrangement  310  and a further heat exchanger arrangement  311 . Both heat exchanger arrangements  310  and  311  have a long physical interaction length with heat storage material  308 . 
         [0084]    As can be seen from  FIG. 3 , the thermal energy storage and recovery device  300  is divided into several compartments  305 , which are separated from each other via inner insulation walls  304 . 
         [0085]    The heat exchanger arrangement  310  comprises an inlet end  312   a  and an outlet end  314   a.  The further heat exchanger arrangement  311  comprises an inlet end  314   b  and an outlet end  312   b.  For storing thermal energy into the thermal energy storage and recovery device  300  a hot charging fluid with a temperature t 1  is fed into the inlet end  312   a  and is returned via the outlet end  314   a  with the temperature t 2 . For recovering thermal energy from the thermal energy storage and recovery device  300  a comparatively cold discharging fluid with a temperature t 3  is fed into the inlet end  314   b  and is returned via the outlet  312   b  with a temperature t 4 . 
         [0086]    As can be seen in the bottom of  FIG. 3 , the discharging fluid reaches almost the same temperature t 4  as the inlet temperature t 1  of the charging fluid. This advantageous temperature behavior is realized because of two reasons:
   (A) The long thermal interaction length between (a) the heat exchanger arrangement  310  and the further heat exchanger arrangement  311  and (b) the heat storage material  308 . In the embodiment described here this thermal interaction length is 1000 m.   (B) The use of a counter current heat exchange principle for charging/discharging the thermal energy storage and recovery device  300 , where the temperature gradients for both charge and discharge of the thermal energy storage and recovery device  300  are maintained at least approximately constant throughout the length of the stream of the counter current system. The temperature curve for the discharge fluid looks almost the same as the temperature curve for the charge fluid, it is just displaced by a distance d while the inlet temperature and outlet temperature are the same or nearly the same for the two fluids.   
 
         [0089]      FIG. 4  shows a schematic illustration of a thermal energy storage and recovery system  430  in accordance with a first embodiment of the invention. To store energy within a thermal energy storage and recovery device  400  a heat generating arrangement  470  is used. To recover energy from the thermal energy storage and recovery device  400  a heat consumption arrangement  490  is used. 
         [0090]    As can be seen from  FIG. 4 , the heat generating arrangement  470  comprises a compressor  472 , which is driven be a motor  476 . The compressor  472  comprises an air inlet  472   a.  The air in the air inlet  472   a  may have a temperature of e.g. 20 Celsius degrees and a pressure of e.g. 1 bar. During compression of the air the pressure may rise to e.g. 25 bar and the temperature may rise to e.g. 500 Celsius degrees. This heated up and compressed air is fed into an inlet of a heat exchanger arrangement  410  of the thermal energy storage and recovery device  400 . The compressed air then returns via an outlet of the heat exchanger arrangement  410  now having a temperature of e.g. 20 Celsius degrees and a pressure of still nearly 25 bar. 
         [0091]    The compressed outlet air is then fed into an air turbine  474 . According to the embodiment described here the air turbine  474  and the motor  476  and the compressor  472  have a common shaft  477 . This provides the advantage that the air turbine  474  will help the motor  476  driving the compressor  472  such that the efficiency of the heat generating arrangement  470  will be increased. 
         [0092]    The temperature of expanded outlet air being provided by the air turbine  474  via an air outlet  474   a  may e.g. fall to minus one degrees Celsius (−1° C.) when the air is expanded from a pressure of 25 bar to 1 bar. This makes the expanded outlet air from the air turbine  474  suitable for cooling purposes, e.g. for air conditioning of the surrounding air in one or more rooms in one or more buildings. 
         [0093]    To recover stored energy a cold fluid with a temperature of e.g. 20 degrees Celsius is fed into an inlet of a further heat exchanger arrangement  411  of the thermal energy storage and recovery device  400 . According to the embodiment described here this is done by a circulation pump  492 . The circulation pump  492  collects water from a district heating installation  498  which comprises a water inlet  498   a.    
         [0094]    After passage through the further heat exchanger arrangement  411  the fluid has an outlet temperature, which is significantly larger than the inlet temperature of the fluid at the inlet of the further heat exchanger arrangement  411 . Due to (a) the described counter current fluid flow within the thermal energy storage and recovery device  400  and (b) the long physical interaction length between the heat exchanger arrangement  411  and the heat storage material of the thermal energy storage and recovery device  400  the outlet temperature of the fluid leaving the further heat exchanger arrangement  411  is almost the same as the inlet temperature of the hot compressed air, which has entered the heat exchanger arrangement  410 . 
         [0095]    In this way the cold fluid is converted to steam which may be further overheated by heating means (not depicted) before the steam is let into a steam turbine  494  which drives an electric power generator  496  through a shaft connection. Optionally, the steam may further be let into a condenser (not shown) where it turns into water. This condenser may be driven by air (ambient air, stationary or ventilation). 
         [0096]    Alternatively or in combination return water from the district heating installation  498  may be pumped through the condenser in order to cool the steam. The condensed water may by pumped back to the district heating installation  498  and returned by a water outlet  498   b  of the district heating installation  498 . The electric power generator  496  may be connected to a utility grid (not shown) as well as a wind turbine or other kind of alternative energy resources (not shown). 
         [0097]    In this way electrical energy produced by e.g. a wind turbine may be used by the motor  476  to drive the compressor  472  and to feed compressed air through the thermal energy storage and recovery device  400  and to store, the thermal energy in the heat storage material such as sand or other similar solid material with high heat capacity. In periods with no or with little wind or perhaps in periods with too high wind speeds where the wind turbines stands still water may be pumped through the thermal energy storage and recovery device  400  heating it up to steam which then drives the steam turbine  494 . The steam turbine  494  drives the electrical power generator  496  which supplies electrical energy to the utility grid. 
         [0098]      FIG. 5  shows a schematic illustration of a thermal energy storage and recovery system  530  in accordance with a second embodiment of the invention. In this embodiment, a district heating installation or a thermal power generation plant  535  is connected to a utility grid  550  and to a thermal energy storage and recovery device  500 . According to the embodiment described here the district heating installation or power generation plant  535  comprises a steam turbine  540  with a condenser (not shown) and a connected electrical power generator  545  and a compressor  572  with a built in motor. The compressor  572  may be replaced by an electrical boiler or may be supplemented by means of a heat pump system or other heating means. 
         [0099]    The district heating installation or thermal power generation plant  535  is connected to the thermal energy storage and recovery device  500  both for energy storage and for recovering of stored energy. Also here a wind turbine  560  or other kind of alternative energy resources may be connected to the utility grid  550 . 
         [0100]    The compressor  572  with a built-in electrical motor may also comprise a mechanical connected air turbine (not shown) helping driving the compressor  572  together with the electrical motor. The air turbine may be connected to an outlet of a heat exchanger arrangement of the thermal energy storage and recovery device  500  receiving the cooled compressed air in the outlet. 
         [0101]      FIG. 6  illustrates the temperature behavior along the pipe of a heat exchanger arrangement having a long thermal interaction length with the surrounding heat storage material when the thermal energy storage and recovery device is charged in several steps by an originally hot inlet heat transfer medium. On the abscissa there is plotted the length L of the heat exchanger arrangement running through the thermal energy storage and recovery device from an inlet end (first end) at a position L 1  to an outlet end (second end) L 2 . On the ordinate there is plotted the temperature T of the heat storage material. 
         [0102]    In  FIG. 6  the charging steps are indicated with encircled numbers “1”, “2”, “3”, “4”, “5” and “6”. Thereby, the step numbers reflect the sequence of the steps. Step 1 is performed at an initial state of the thermal energy storage and recovery device wherein all the heat storage material is at an initial low temperature. According to the embodiment described here this initial temperature is 20° C. Further, in this embodiment the temperature of the heat transfer medium, which is entering the thermal energy storage and recovery device at its first end L 1  is 500° C. It is mentioned that these temperatures are exemplary and that of course also other temperatures may be employed for operating the thermal energy storage and recovery device. 
         [0103]    In the first shown three steps “1”, “2” and “3” the charging heat transfer medium gets rid of all its thermal energy from a temperature of 500° C. down to 20° C. until the temperature of the outlet of the thermal energy storage device begins to rise beginning with step “4” from the initial temperature of 20° C. up towards 500° C. due to the fact that the thermal energy storage and recovering device gets more and more saturated by thermal energy. In the embodiment described here a full thermal saturation will show up shortly after step “6”. 
         [0104]    As can be seen from  FIG. 6 , the most efficient charging region is a region R where the whole temperature difference can be used. In this region R the inlet temperature of this region R is at least approximately the same as the temperature (here 500° C.) of the heat transfer medium which is supplied to the thermal energy storage and recovery device at its inlet end at the position L 1 . Further, the outlet temperature of the heat transfer medium leaving this region R is at least approximately the same as the temperature (here 20° C.) of the heat transfer medium which is released from the thermal energy storage and recovery device at its outlet end at the position L 2 . 
         [0105]    It can be elucidated from  FIG. 6  that a longer physical length of the thermal interaction region along the transport direction of the heat transfer medium of the thermal energy storage and recovery device increases the size of the most efficient charging region R. 
         [0106]    It is pointed out that the inlet and outlet temperature of this efficient charging region R is substantial constant as long as the thermal energy storage and recovery device is in a state which corresponds to the thermal regime being represented by step “3”. 
         [0107]    When discharging, the area to the left of the efficient charging region R should be avoided as the temperature here drops down from the shown 500° C. to the initial temperature of 20° C. corresponding to the ambient temperature and/or the inlet temperature of the discharging fluid and it will require some thermal energy charging to reach again the fluid inlet temperature (here 500° C.) 
         [0108]    In other words a longer physical length of the thermal interaction region along a transport direction of the heat transfer medium of the thermal energy storage and recovery increases the region where the charging and the discharging of the thermal energy storage and recovery device is performed without reaching a thermal energy saturation level limiting the efficiency of the thermal energy storage and/or thermal energy recovery procedure. 
         [0109]      FIG. 7  illustrates a stepwise temperature behavior along a pipe of a heat exchanger arrangement, wherein the thermal energy storage and recovery device comprises different compartments being thermally isolated from each other. For charging thermal energy into the respective thermal energy storage and recovery device a hot fluid is fed into an inlet end being located on the left side of  FIG. 7  and cooled down fluid is outputted at an outlet end being located at the right side of  FIG. 7 . As a consequence, compartments being located more to the left side will have a higher temperature than compartments being located more to the right side of  FIG. 7 . Specifically, the compartment being located directly at the left input end will adopt a temperature t 1  (e.g. 560° C.) and the compartment being located directly at the right output end will adopt a lower temperature t 2  (e.g. 20° C.). 
         [0110]      FIG. 8  illustrates a temperature gradient along the pipe of a heat exchanger arrangement  810  which is surrounded by a heat storage material  808 . As has already been mentioned above, the heat storage material  808  may comprise for instance sand, soil or spoil or any combination of these substances. A thermal energy storage and recovery device  800  being formed by the heat exchanger arrangement  810  and the surrounding heat storage material  808  is charged with thermal energy by inputting a hot fluid into the left end of the heat exchanger arrangement  810  and by outputting the cooled down fluid from the right end of the heat exchanger arrangement  810 . Correspondingly, thermal energy is released from the thermal energy storage and recovery device  800  by inputting a cold fluid into the right end of the heat exchanger arrangement  810  and by outputting heated up fluid at the left end of the heat exchanger arrangement  810 . 
         [0111]    The thermal energy storage and recovery device  800  has such a physical length that when the thermal energy storage and recovery device  800  is partially loaded with thermal energy there has been developed a hot region  810   a  being located next to the left end of the heat exchanger arrangement  810 , wherein the temperature within the hot region  810   a  is at least approximately constant at e.g. 560° C. Accordingly, there is a cold region  810   c  being located next to the right end of the heat exchanger arrangement  810 , wherein the temperature within the cold region  810   c  is at least approximately constant at e.g. 20° C. In between the regions  810   a  and  810   c  there is an intermediate region  810   b,  wherein there is a comparatively strong temperature gradient between the hot temperature of the hot region  810   a  and the cold temperature of the cold region  810   c.  This situation is depicted in the insert diagram given directly below the thermal energy storage and recovery device  800 . 
         [0112]    When the thermal energy storage and recovery device  800  is further charged with thermal energy, the location of the intermediate region  810   b  comprising the described temperature gradient is shifted towards the right side. The resulting temperature profile is illustrated in the insert diagram being located on the bottom left side of  FIG. 8 . 
         [0113]    When the thermal energy storage and recovery device  800  is further discharged from thermal energy, the location of the intermediate region  810   b  comprising the described temperature gradient is shifted towards the left side. The resulting temperature profile is illustrated in the insert diagram being located on the bottom right side of  FIG. 8 . 
         [0114]    The temperature gradient may preferably develop within a length of 10 to 20 meters or more depending on different physical parameters like e.g. the flow speed of the fluid passing the heat storage medium. 
         [0115]    The thermal interaction region between the fluid and the heat storage medium  808  may have a length of 80 m, though preferably 500 m up to 1000 m or more. 
         [0116]    It should be noted that the term “comprising” does not exclude other elements or steps and the use of articles “a” or “an” does not exclude 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 construed as limiting the scope of the claims. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
         
           
               100  thermal energy storage and recovery device 
               102  casing/outer insulation wall 
               104  inner insulation wall 
               108  heat storage material 
               110  heat exchanger arrangement 
               112  first heat exchange section 
               112   a  first end 
               112   b  further first end 
               114  second heat exchange section 
               114   a  second end 
               114   b  further second end 
               116  first connecting section 
               116   a  valve 
               117  second connecting section 
               117   a  valve 
               118  third connecting section 
               118   a  valve 
               120  ground 
               300  thermal energy storage and recovery device 
               304  inner insulation walls 
               305  compartments 
               308  heat storage material 
               310  heat exchanger arrangement 
               311  further heat exchanger arrangement 
               312   a  inlet end 
               312   b  outlet end 
               314   a  outlet end 
               314   b  inlet end 
             T temperature 
               400  thermal energy storage and recovery device 
               410  heat exchanger arrangement 
               411  further heat exchanger arrangement 
               430  thermal energy storage and recovery system 
               470  heat generating arrangement 
               472  compressor 
               472   a  air inlet 
               474  air turbine 
               474   a  air outlet (for air conditioning) 
               476  motor 
               477  common shaft 
               490  heat consumption arrangement 
               492  circulation pump 
               494  steam turbine 
               496  electric power generator 
               498  district heating installation 
               498   a  water inlet 
               498   b  water outlet 
               500  thermal energy storage and recovery device 
               530  thermal energy storage and recovery system 
               535  district heating installation/thermal power generation plant 
               540  steam turbine 
               545  electric power generator 
               550  utility grid 
               560  wind turbine 
               572  compressor 
             L length 
             L 1  inlet end for heat transfer medium into thermal energy storage and recovery device 
             L 2  outlet end for heat transfer medium from thermal energy storage and recovery device 
             R region with constant inlet and outlet temperature 
               800  thermal energy storage and recovery device 
               808  heat storage material 
               810  heat exchanger arrangement 
               810   a  hot region 
               810   b  intermediate region 
               810   c  cold region