Patent Publication Number: US-2020284525-A1

Title: Buffer store

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Patent Application No. PCT/EP2018/071559, filed Aug. 8, 2018; which claims priority to German Patent Application No. 10 2017 118 952.1, filed Aug. 8, 2018. 
    
    
     FIELD OF DISCLOSURE 
     The invention relates to a buffer store, especially for heat storage. 
     A buffer store according to the invention may involve, in particular, a buffer store which works in the “pressureless region” or under atmospheric pressure with an operating temperature of up to 95 degrees Celsius and which is designed in particular to operate with heating water as the storage fluid. The buffer store in particular can be part of a heating plant. 
     The buffer store according to the invention may serve in particular for storing heat from biogas plants, biomass heating power stations, solar plants or other heat sources. Of course, excess heat of conventional power stations or incinerators can also be used to heat the storage fluid, especially the heating water. 
     BACKGROUND 
     Buffer stores of this kind generally have a vessel housing, which is filled with large amounts of a storage fluid, such as water, especially demineralized water, treated with a low oxygen content according to the prior art for heating plants. The water serves here as a storage medium for the buffering of heat which is generated for example from biogas plants, biomass heating power stations, solar plants or other heat sources. Such buffer stores generally have a useful volume of 40 to 10,000 cubic meters. 
     According to the present invention, by a buffer store is meant in particular a heat store not working in the pressure range, i.e., the buffer store is pressureless, working at ambient pressure, and is distinguished from so-called pressure stores. 
     It is known from practice that very large fluctuations in the volume of the storage fluid inside the vessel housing occur in buffer stores of this kind, especially those having a large useful volume, on account of temperature fluctuations in the range of 20 to 95 degrees Celsius. 
     In order to prevent a negative pressure from arising in the vessel housing, whereby unacceptably large forces would act on the shell of the vessel housing, the unoccupied volume in the vessel housing is generally occupied by a gas. In order to further avoid an excess pressure from arising inside the vessel housing due to a thermal expansion, the change in volume of the storage fluid due to thermal expansion is made possible by venting additional gas to the surroundings upon increase in volume of the storage medium. Then, when the temperature of the storage fluid drops, gas must be supplied once more in order to avoid a negative pressure in the vessel housing. 
     Furthermore, it is also known that oxidation processes of the metallic materials used for the components of a buffer store system and any connected heating plant, especially the heating lines and the heating bodies, should be avoided as much as possible. Traditional water contains free oxygen, which can combine with metallic materials, resulting in oxidation. In a closed system, the oxidation process is terminated when all of the free oxygen has been bound to the metallic materials. In order to avoid oxidation processes as much as possible, the storage fluid is often formed by specially treated water. Furthermore, in order to avoid corrosion, contact with the oxygen of air from the surroundings for example is prevented in the case of a storage fluid such as water. It is therefore known in practice how to use nitrogen, for example, as the gas to occupy the volume not occupied by the storage fluid. Consequently, the acquisition and providing of nitrogen in the case of changes in volume of the storage fluid involve an enormously high expense and associated additional costs. 
     If a gas such as nitrogen (an inert gas) is used to prevent oxidation, oxygen may even result in low concentrations in oxidation processes, whereby plants and components in the heating system will be damaged and the operation or the entire plant may be impaired or halted. 
     By contrast with this, however, high operating costs occur when a gas covering with an inert gas such as nitrogen is required. Thus, for example, it is known how to employ liquid nitrogen for the volume equalization, which is placed on the storage fluid. These facilities, in turn, are very costly in construction and operation. 
     SUMMARY OF INVENTION 
     Accordingly, one problem which the present invention proposes to solve is to provide an especially simple and cost-effective solution to solve the aforementioned problems at least in part. 
     Accordingly, the present invention proposes a buffer store having the features of claim  1 . 
     A buffer store according to the invention comprises a closed vessel housing to hold a storage fluid having at least one inlet opening for filling the vessel housing with the storage fluid, and at least one equalization opening, which connects the vessel housing fluidically to the surroundings and makes possible a fluidic equalization with the surroundings. Thus, for example, ambient air or also an inert gas such as nitrogen can get in through the equalization opening to the interior of the vessel housing and also be discharged once more to the outside or to an equalization tank or a corresponding system, such as a pressure equalization system, in which nitrogen is used in particular, when the volume of the storage fluid increases significantly. However, in order to avoid undesirable oxidation processes, especially when water is used as the storage fluid, a gas-tight cover is additionally provided in the interior of the vessel housing, which is capable of reliably protecting the storage fluid against mixing with other fluids, such as the ambient air. 
     Accordingly, in the context of the invention the term “gas-tight cover” is used to mean a gas-tight seal and/or a largely gas-tight seal. 
     The gas-tight cover thus divides the interior of the vessel housing into a storage space and an equalization space, where the equalization space or air space is fluidically connected to the surroundings of the vessel housing, so that in particular ambient air is sucked into the air space and can be discharged from it to the outside, respectively. 
     In the context of the invention, the term “surroundings” means in particular the surroundings of the vessel housing, especially the outside atmosphere or the outer region in which ambient air is located. However, the term “surroundings” also encompasses an equalization tank and/or an equalization system, especially a pressure equalization system, in which nitrogen or another inert gas is used as the fluid. An additional protection against oxidation is achieved when nitrogen or another suitable gas is introduced into the equalization space or the air space instead of the ambient air. 
     It may further be provided that the gas-tight cover is flexible at least for a portion. In this way, a relative movement of the gas-tight cover relative to the vessel housing is facilitated. 
     Moreover, the gas-tight cover may have a filmlike structure at least for a portion. Such a filmlike structure is characterized in particular by low weight and relatively low material costs. 
     Furthermore, it may be provided that the gas-tight cover is temperature-resistant at least in a temperature range of 0 to 100° C. In particular, the temperature resistance may lie in a temperature range of 20° C. to 95° C. 
     Furthermore it may be provided that the gas-tight cover is firmly joined to the vessel housing in the area of its outer circumference, for example, being clamped, glued, screwed on, or secured in a water channel, as is further described below. 
     In one conceivable embodiment, which is independent of the specific structure of the gas-tight cover and its mounting, it can be provided that the gas-tight cover is doublewalled and/or multilayered at least for a portion. 
     Alternatively or additionally, the gas-tight cover may at least for a portion have a lower density than the storage fluid. 
     In the case of an at least partly double-walled gas-tight cover, the lower density can be accomplished by filling a medium, such as air or the like, into the area between the two walls of the gas-tight cover. Thus, at least the double-walled section of the gastight cover may lie on the storage fluid, that is, virtually float on it. 
     Alternatively or additionally, the material can of course also be chosen in a singlewalled formation of the gas-tight cover so that it has a lower density than the storage fluid, so that the described effects of being placed upon or floating on the storage fluid can be achieved. 
     Furthermore, the vessel housing may have on its inner circumference a mounting rail, especially a clamping rail, for the gas-tight mounting of the gas-tight cover. Tensioning systems or tension springs may also be provided. For the mounting of the gas-tight cover, there are preferably provided holding, clamping or tensioning systems or rings, which encircle the inner circumference of the vessel housing as a closed ring. Furthermore, a water channel, as described below, can also be used for the mounting of the gas-tight cover. 
     The gas-tight cover may have at least one compensation area, which can compensate for a change in volume of the storage fluid by expanding or contracting the gas-tight cover in this area. For this, the compensation area may be flexible and/or elastic. Thus, the compensation area may be formed for example in the manner of one or more bulges, making possible an expansion or contraction in the manner of a bellows. A material apron or a material bag is also conceivable as the compensation area, which in event of an expansion of volume of the storage fluid can become filled with the fluid and can give back the fluid once again when the storage fluid again contracts. 
     The gas-tight seal may also have a compensation area within which a change in volume of the storage volume can be equalized in that the gas-tight cover can be raised or lowered at least in the compensation area by the storage fluid. That is, the gas-tight cover especially in an embodiment with a filmlike structure, especially when formed entirely of film, can be movably mounted inside a compensation area such that a defined change in volume of the storage fluid in the vessel housing can be compensated in that the gas-tight cover can be raised or lowered because it is floating on the storage volume. 
     It is advantageous when the gas-tight cover is formed as a film which floats at least for a portion on the storage fluid, especially the water. 
     It is advantageous when the film is secured in the vessel housing in such a way that the film can follow a change in volume of the storage fluid at least in the compensation area, in particular it can be raised and lowered by floating on the storage fluid. 
     Because the film floats on the storage fluid and is mounted accordingly, the film is raised when the storage fluid rises and is lowered again accordingly when the volume of the storage fluid decreases. This creates a reliable separation in simple manner in the interior of the vessel housing, such that the vessel housing comprises on the one hand a storage space and on the other hand an equalization space, especially an air space. 
     It is advantageous when the compensation area is chosen such that, when the vessel is filled with storage fluid having a temperature of 20 degrees Celsius, the nominal filling height in the vessel housing defines a lower end (minimum filling height) of the compensation area, while an upper end (maximum filling height) of the compensation area is defined by a fill level of the storage fluid in the vessel housing that is produced when the storage fluid is heated to a temperature of 95 degrees Celsius. 
     The nominal filling height at the lower end, i.e., the minimum filling height, can also be defined by a temperature of the storage fluid, especially water, of 10 degrees Celsius. Water typically has this temperature during the first filling of the buffer store. The temperature of the storage fluid defining the minimum filling height may also encompass a range which results when the temperature of the storage fluid, especially water, lies between 10 degrees Celsius and 20 degrees Celsius. 
     Especially in the preferably intended use of the buffer store for operation with heating water, the aforementioned values can define a compensation area, i.e., the area within which the gas-tight seal, especially the film, should equalize a change in volume. The aforementioned values have proven to be especially suitable; of course, a filling of the vessel with storage fluid having a different temperature, especially a lower temperature such as 5 to 20 degrees Celsius, especially 10 to 20 degrees Celsius, further for example 15 degrees Celsius, preferably 10 degrees Celsius, can also be provided, especially in order to determine the lower end of the compensation area. 
     It may be provided that the buffer store according to the invention comprises an emergency vent valve, which opens when the storage fluid goes beyond a particular temperature or when a pressure rise occurs beyond a defined level. 
     It is advantageous when the compensation area is chosen such that volume changes of the storage fluid can be equalized up to 20% of the storage volume of the buffer store. 
     The solution according to the invention makes it possible in particular to equalize temperature-related fluctuations in the storage volume of the buffer store. Furthermore, fluctuations in the heating grid may also be equalized, if the buffer store is connected directly to it. The aforesaid compensation area is also suitable for this. 
     It is advantageous when nitrogen is introduced or can be channeled in an air space of the vessel housing above the gas-tight cover and/or a system is provided to introduce and take away nitrogen, especially to produce a pressure equalization. 
     In order to increase the safety, an inert gas, especially nitrogen, can be brought into the air space above the gas-tight seal, especially in place of the ambient air or in place of oxygen. In this way, it is ensured in especially reliable manner that no oxygen gets into the storage fluid, especially a heating water. 
     It is advantageous when the gas-tight cover is a film made of ethylene-propylene-diene terpolymer (EPDM) or comprises ethylene-propylene-diene terpolymer (EPDM). 
     A forming of the film made of ethylene-propylene-diene terpolymer or at least a partial forming made of this material has proven to be especially suitable. 
     It is advantageous when the film comprises a thickness of 0.5 to 5 mm, preferably 1 mm to 4 mm, especially preferably 1.5 to 3 mm and most especially preferably 2 mm+/−0.2 mm. 
     It is advantageous when the vessel housing comprises a channel to hold a liquid, especially water, on its inner circumference and the gas-tight cover is mounted in the channel, in particular, clamped or screwed or pinched. 
     The mounting of the gas-tight cover, especially when it is formed as a film, in a channel which is filled with a liquid, preferably water, has proven to be especially suitable to making sure that no gas, especially no oxygen, can get into the storage space of the vessel housing in which the storage fluid is present. Thanks to the formation of such a water channel, the safety can be further enhanced. Preferably, the channel runs entirely around an inner wall of the vessel housing. A known system can be used for the mounting of the film, especially a tensioning system. However, a screw fastening, a clamping, a gluing, a welding, or another procedure can also be provided in particular to secure the film in the channel. 
     The vessel housing of the buffer store may furthermore have a maintenance access in the area above the gas-tight cover, as well as a catwalk arranged optionally on its inner circumference at least above and/or below the gas-tight cover. In this way, a repair can be done in event of damage to the gas-tight cover without this requiring a draining of the entire storage fluid of the buffer store. The buffer store according to the invention may have a maintenance access, especially a roof hatch, so that maintenance personnel can get inside the buffer store. In particular, it may be provided that the servicing or the replacement of the gas-tight cover, especially when this cover is a film, can be done via a maintenance access, especially a roof hatch, which is integrated in a lid or roof of the buffer store. 
     Moreover, it is advantageous when the buffer store furthermore has a manhole, which is preferably arranged in the vessel housing, preferably such that the manhole is located in a lower region of the vessel housing, so that easy access is possible. 
     The cover according to the invention makes it possible to divide the interior of the vessel housing into a storage space for the storage fluid, especially water, and an equalization space sealed off from it in gas-tight manner. Thanks to the at least partial flexible formation of the cover, especially when it has a filmlike structure, the cover rises with the storage fluid, especially water, or falls when the volume of the storage fluid expands or becomes reduced. 
     Since the storage fluid in a buffer store can have different temperatures, the expansion of the storage fluid and thus also the storage space is of different size during the operation of the buffer store. 
     The cover according to the invention adapts to the space requirement of the storage fluid. Upon expansion or reduction of the volume of the storage fluid, the cover will be accordingly raised or lowered by the storage fluid, so that the volume of the equalization space will change accordingly. The equalization space is connected to the surroundings or a separate pressure space at least by one equalization opening, as already described, so that gas, especially air, can flow from the equalization space or again flow back in. 
     Instead of air, nitrogen or another gas can be brought into the equalization space, and in this case it is advantageous to drain or remove the gas not into the surroundings, but into a pressure store or pressure space. 
     The cover, especially when configured as a film, as already described, is joined in gastight manner to an inner wall of the vessel housing, for which various fastening methods can be used, including a clamping element or a film holder, such as a clamping rail. As already described, it may be provided that the film is fastened above the minimum filling height, such as 50 cm to 2 m, preferably 1.5 m above the minimum filling height, and hangs down in the manner of a bag open toward the top. The film is then correspondingly lifted or displaced when the storage fluid or the water level rises. 
     In an alternative embodiment, it is also possible for the cover or the film to be fastened in the area of the minimum filling height to the inner wall of the vessel with the already described procedures. The cover or the film has a configuration making it possible for the film to be lifted up or displaced when the storage fluid expands or the water level in the buffer store rises. 
     Of course, it is also possible to fasten the cover or the film to the inner wall of the vessel at a height corresponding to the height roughly midway in the compensation area, i.e., the area between a minimum filling height and a maximum filling height. 
     The cover or the film preferably has a material surplus or a configuration such that it is possible for the film to be raised or lowered in the desired manner by a change in volume of the storage fluid or a rising or falling of the water level so that the volume of the storage space is adapted accordingly. 
     In known manner, the buffer store comprises an inlet opening in order to admit the storage fluid, especially prior to the first use. For this, a basically familiar filling system can be used. Preferably, the filling system is situated in the area of the lower end of the vessel housing and serves to fill the storage space up to a minimum filling height. The storage fluid being introduced, preferably water, has at least a temperature which is significantly lower than the temperature of the storage medium in the later operation of the buffer store. Insofar as the storage fluid is water, this usually has a temperature of around 10 degrees Celsius during the filling. 
     The buffer store according to the invention preferably comprises an overflow for the so-called first filling. That is, once the minimum filling height has been reached, the storage fluid during the filling of the buffer store with the storage fluid, especially water, drains off through the overflow for the first filling. An attendant will thus notice when the buffer store has been filled to the minimum filling height. The filling process can then be stopped. For this, it may also be provided that the overflow for the first filling is closed, so that when the storage fluid expands during the operation of the buffer store the storage fluid does not drain off through the overflow for the first filling, but instead remains in the storage space. The overflow for the first filling serves primarily to ensure that the storage space during the first filling with the storage fluid is filled only up to the minimum filling height and not beyond. 
     Moreover, it can be provided according to the invention that the buffer store comprises an emergency overflow for the maximum filling (maximum filling height). The emergency overflow for the maximum filling should ensure that storage fluid can drain from the buffer store when the volume of the storage fluid expands beyond the maximum filling height. The emergency overflow for the maximum filling height preferably has a siphon system, preferably a double siphon system, so that on the one hand no penetration of air is possible, but on the other hand it is possible to release storage fluid to the outside surroundings when the storage fluid has risen beyond the maximum filling height. 
     Preferably, the emergency overflow for the maximum filling height extends from an upper region of the vessel to a lower region of the vessel, so that surplus storage fluid can drain automatically. Preferably a siphon is provided at both the upper and the lower end of the overflow in order to avoid air getting in from the surroundings in a reliable manner. 
     It is advantageous when the buffer store according to the invention comprises an emergency air inlet. The emergency air inlet system preferably comprises a vacuum relief valve, which opens an air inlet into the storage space, preferably into an upper region of the storage space, when the pressure in the storage space rises beyond a defined level. The buffer store is usually operated in pressureless manner, i.e., the storage space has atmospheric pressure. However, a pressure change in the storage space may occur when there is a pipe breakage or when the storage fluid is accidentally or deliberately drained from the storage space for other reasons. In this case, the cover at first moves downward with the storage fluid. But when the cover has reached the lower end of the compensation area or its maximum freedom of movement, the cover can no longer drop downward. Thus, upon further draining of the storage fluid, the pressure in the storage space above the storage fluid would drop and a vacuum might be created, which could result in damage or tearing of the cover, especially the film. For this event, it is provided that the emergency air inlet opens and air can flow into the storage space to equalize the pressure. 
     Preferably the emergency air inlet is combined with a siphon system and/or arranged such that no air can flow in during regular operation. 
     Preferably the emergency air inlet is combined with the emergency overflow and arranged such that access to the interior of the vessel is located usually below the surface of the storage fluid, especially the water level, so that for this reason alone no air can flow in. 
     In basically familiar fashion, the buffer store according to the invention can have an outlet line and an inlet line, which are connected in particular to a heating plant. In usual manner, the outlet line is positioned in an upper region of the buffer store, the outlet line in the context of the invention being situated preferably beneath, especially preferably directly beneath, the minimum filling height. The inlet line, i.e., the line by which the storage fluid is led back to the buffer store, in particular from a heating plant, is preferably situated in the lower region of the vessel. 
     It is especially advantageous when the outlet line situated at the top in the buffer store (the upper line) serves both as an outlet and an inlet, especially in order to bring in warm or heated storage fluid or to remove it from the buffer store. Warm storage fluid which is removed from the upper line is preferably taken to a heating plant. However, it may also be desirable to bring already heated storage fluid through the upper line to the buffer store, especially storage fluid that has been heated externally, e.g., by surplus energy. 
     Moreover, it is especially advantageous when the inlet line (lower line) situated at the bottom in the buffer store serves as both an inlet and an outlet, especially to bring in cold or not yet heated storage fluid or to remove it from the buffer store. A removal of storage fluid through the lower line may be primarily useful for heating the storage fluid externally, e.g., using surplus energy. 
     According to the invention, two, three, four, five or more upper lines and/or lower lines can be provided respectively for the supply and removal of storage fluid. 
     Preferably both the inlet line and the outlet line are arranged centrally or in the middle of the buffer store in regard to its horizontal positioning. Preferably the inlet line and the outlet line have a respective inlet and outlet, having a suitable flat extension. Preferably at least the outlet comprises a disk or plate at its upper end, below which the outlet openings are formed. The disk or plate serves for the cover, especially when formed as a film, to lie against it without the film covering the outlet openings. 
     In an especially advantageous embodiment, it may be provided that the outlet and/or the inlet has two disks or plates at a spacing from each other, between which the outlet openings and the inlet openings are respectively situated. 
     The disks or plates extend preferably substantially parallel to a horizontal cross section area of the buffer store and preferably have a diameter of at least 50 cm, especially preferably at least 1 m. The plate or disk is preferably flat, but it may also be curved. Thanks to this configuration, the outlet especially when situated at the top in the buffer store has a suitable bearing surface on which the cover or the film may optionally lie. 
     It is advantageous when the buffer store comprises at least one brace, which extends in a preferably horizontal cross section plane through the buffer store, preferably from one inner wall of the buffer store to an oppositely situated inner wall of the buffer store, the brace serving for the outlet and/or the inlet to be supported or lie thereupon, so that the outlet or the inlet is respectively held in a stable manner. 
     The brace may run both above and below the outlet. In an arrangement of the brace above the outlet, the outlet may be suspended from or otherwise connected to the brace. 
     The cover is preferably a film or filmlike. The film may be composed of one layer, or also from multiple layers of different materials. It is provided that at least one of the layers is impermeable to gas. Aluminum foil for example is especially suitable for this. According to the invention, it may be provided that the film has two, preferably three or more layers, wherein a lower membrane layer, a middle gas impermeable layer, especially an aluminum foil, and an upper membrane layer may be provided. 
     A construction of a film having the following layers or plies or combinations of these may be especially suitable:
         a. a polyethylene terephthalate (PET) layer and an aluminum (Al) layer or a composite film consisting of these materials   b. two polyethylene terephthalate (PET) layers and an aluminum (Al) layer situated between them   c. a polyethylene terephthalate (PET) layer, a polyester layer, and an aluminum (Al) layer situated between them   d. two polypropylene (PP) layers and an aluminum (Al) layer situated between them.       

     Instead of the aluminum (Al) layer, a layer of other metals may be used, especially copper, silver or gold as a barrier or metal layer. 
     Moreover, in place of the aluminum (Al) layer it is also possible to use a layer of lithium and/or magnesium, especially a layer of ethylene-vinyl alcohol copolymers (EVOH) as a barrier or metal layer. 
     The films respectively constructed according to a) through d) may also be constructed in combination with an ethylene-propylene-diene terpolymer (EPDM) or comprise one or more layers of ethylene-propylene-diene terpolymer (EPDM). This also holds when the aluminum (Al) layer has been replaced by another barrier or metal layer. 
     The film may have even further layers to supplement the already mentioned layers. 
     The cover, especially in a film configuration, may also furthermore have an insulation. In this case, the insulating of the lid of the buffer store may be unnecessary. 
     In an especially advantageous embodiment, it may be provided furthermore that alternatively or additionally to a configuration, the film is provided with an insulation, that an insulation is provided on the top side of the cover or the film, that is, an insulation is provided on the side where the cover or the film borders on the equalization space. In an especially advantageous manner, this can be accomplished by filling insulation beads, such as styrofoam beads, in the equalization space, so that they lie as a movable layer against the cover, especially the film, and can move along with the film. 
     Basically any desired fill or insulation is possible on the film. 
     The bottom of the vessel or the base can preferably be made from stainless steel plates or traditional steel plates having been given a corresponding corrosion resistant treatment, for example, a corresponding painting. 
     As already discussed, the buffer store according to the invention may have a maintenance hatch, especially in the lid. Alternatively or additionally, a maintenance hatch may also be provided in the side wall of the buffer store especially in the region of the equalization space. 
     Finally, the invention also relates to a buffer store system having a buffer store especially for operation with heating water as the storage fluid for a heating plant having the aforementioned features and optionally a rubber boat for an easier maintenance and/or a pump system and/or a stirring system and/or a vessel heating and/or a connection system for connecting to further system components in proximity to the buffer store. 
     Further features and benefits of the invention will emerge from the following description of an exemplary embodiment of the invention as well as the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described more closely below with reference to the enclosed figures. 
       The figures show multiple features of the invention in combination with each other. Of course, the person skilled in the art may also consider them in isolation from each other and also combine them into other meaningful combinations without needing an inventive step for this. 
       There are shown schematically: 
         FIG. 1  is a buffer store according to the invention in a front view; 
         FIG. 2  is the buffer store according to the invention of  FIG. 1  in a partial sectioned representation; 
         FIG. 3  is an optional configuration of the buffer store according to the invention; 
         FIG. 4  is a buffer store according to the invention in a second, slightly modified embodiment, in a sectional representation; 
         FIG. 5  is the buffer store according to the invention of  FIG. 4  in a second sectional representation; 
         FIG. 6  is an enlarged cutout of region VI of  FIG. 5  in a schematic representation; 
         FIG. 7  is a view of the overflow for the first filling and the respective emergency overflow from arrow direction VII of  FIG. 6 ; and 
         FIG. 8  is a schematic representation of an outlet of the buffer store, especially for the feeding of a heating plant. 
     
    
    
     DETAILED DESCRIPTION 
     The figures show a buffer store according to the invention, which is denoted generally by reference number  10 . It comprises a vessel housing  12 , which stands on a base  14  and is closed with a lid  16 . 
     The buffer store  10  according to the invention preferably comprises an insulation and a jacketing in the exemplary embodiment. It is preferably provided that the vessel housing  12  comprises an inner shell, forming the actual vessel, and an outer jacketing with an insulation to keep heat in the buffer store  10 . In  FIG. 2 , the outer jacketing with the insulation is schematically given the reference number  36 . 
     Moreover, it is provided that the bottom and/or the roof of the buffer store  10  or the base  14  and/or the lid  16  of the buffer store  10  are also insulated per the prior art, although this is not otherwise denoted in  FIG. 2 . 
     In its upper region, the vessel housing  12  comprises an equalization opening  18 , by which the vessel housing  12  is fluidically connected to the surroundings. Finally, in the lower region of the vessel housing  12 , one or more outlet cocks  20  are provided, making possible a draining of the storage fluid from the interior of the vessel housing  12  to the outside. These may also be used for the filling or for the operation of the vessel housing  12  with storage fluid, depending on the design. 
     In the upper region of the vessel housing  12  there is furthermore provided a gas-tight cover  22  beneath the equalization opening  18 . This is joined firmly by its outer circumference to the inner circumference of the vessel housing  12  and it divides the interior of the vessel housing  12  into a space for the storage fluid (storage space  24 ) and an equalization space or air space  26 . 
     The cover  22  is connected gas-tight to the inner circumference of the vessel housing  12 . 
     The storage space  24  and the air space  26  (equalization space) are thus separated from each other gas-tight in the exemplary embodiment. 
     In the embodiment shown, the gas-tight cover  22  is flexible and comprises a filmlike structure with a compensation area  22   a . In the exemplary embodiment, the gas-tight cover  22  is formed as a film. The gas-tight cover  22  is configured as a bag open at the top (opened at the top toward the air space  26 ) and situated inside the vessel housing  12  such that it is connected by its upper edge firmly to the inner wall of the vessel housing  12  or to a mounting rail  12   a  formed or arranged inside the vessel housing  12  (denoted by reference number  28 ). 
     For this, the gas-tight cover  22  may be everted in its edge region across the mounting rail  12   a  and/or glued, welded, clamped, screwed and/or otherwise fastened to it. 
     Alternatively, the gas-tight cover  22  may also be joined directly to the vessel wall, without an additional mounting rail  12   a  or the like being needed for this. The connection to the inner wall of the vessel housing  12  may also be produced with the aid of known tensioning systems. A further alternative possibility for fastening the gas-tight cover  22 , especially when it is formed as a film, is presented below with the aid of  FIG. 3 , again only schematically. 
     Upon change in volume of the storage fluid in the storage space  24 , the preferably bag-shaped compensation area  22   a  of the gas-tight cover  22  is forced in its lower region in the direction of the air space  26  (everted, so to speak) or lifted up and thereby making possible the compensation for the change in volume in the storage space  24 . At the same time, air in the air space  26  is compressed in this way or vented through the equalization opening  18  to the surroundings. If the volume of the storage fluid in the storage space  24  then decreases once more, the compensation area  22   a  of the gaseous cover  22  rebounds once again due to the negative pressure formed in the storage space  24 . The resulting negative pressure in the air space  26  means that ambient air is drawn in through the equalization opening  18  into the air space  26  and thus prevents an unwanted negative pressure inside the vessel housing  12 . 
     In the schematic representation of  FIG. 2 , the bottom side of the bag-shaped gastight cover  22  lies flat on the water surface of the storage fluid (preferably a specially treated water such as is used to avoid corrosion in heating plants). Of course, however, the support surface may also differ from the one shown, depending on the fill level of the storage fluid. 
     As can be seen from  FIG. 2 , the buffer store  10  according to the invention may also have an emergency overflow  38 . The emergency overflow  38  is preferably formed as a siphon, so that no air can get in from the outside. The emergency overflow  38  makes it possible for storage fluid to drain out from the storage space  24 , if the fill level of the storage fluid should expand beyond the maximum water fill level. The emergency overflow  38  is therefore located preferably, as shown in  FIG. 2 , at a level or in a plane which corresponds to or defines the maximum water fill level. The maximum water fill level corresponds preferably to the upper end of the compensation area  22   a , as is likewise represented in  FIG. 2 . The lower end of the compensation area  22   a  is preferably chosen such as was already explained in the general part of the specification (nominal fill level of the storage fluid at a temperature of 20 degrees Celsius or optionally also at 10 degrees Celsius or defined by a range at a temperature between 10 degrees Celsius and 20 degrees Celsius). 
     Thanks to the gas-tight cover  22  according to the invention, an especially simple and cost-effective solution is provided, which on the one hand compensates for a pressure equalization due to a change in volume of the storage fluid inside a vessel housing  12  of a buffer store  10  and on the other hand prevents an unwanted oxidation of metallic materials of the buffer store  10 , the buffer store system and/or a heating plant, which may be part of the buffer store system. In this case, no additional cost-intensive arrangements are needed, such as a device for spraying an inert gas into the air space  26 . 
     To enhance the safety, however, it may be provided that instead of the ambient air an inert gas such as nitrogen is brought into the air space  26 . It can then be provided preferably that the gas is not given off to the outside atmosphere through the equalization opening  18 , but instead a basically known pressure equalization system or an equalization tank is used. Of course, nitrogen for example may also be given off to the outside atmosphere through the equalization opening  18 . In this case, systems are then preferably provided to ensure a refilling of the air space  26  with nitrogen or another inert gas if the volume of the storage fluid in the storage space  24  is again reduced. 
       FIG. 3  shows as an example another advantageous possibility of connecting a gastight cover  22 , especially a film, in gas-tight manner to the vessel housing, especially the inner wall of the vessel housing. For this, a channel  30  is provided, being a water channel filled with water in the exemplary embodiment. In order to fasten or tension or clamp the gas-tight seal  22  or the film, clamping, fastening or tensioning means are provided inside the channel  30  (not described in greater detail). Another kind of fastening may also be provided. The water introduced in the channel  30  reliably prevents a gas from getting in, especially air from the air space  26  getting into the storage space  24 . 
       FIG. 3  also shows as an example a catwalk  32  above and a catwalk  32  beneath the gas-tight seal  22 , by which maintenance work can be facilitated. The use of only one catwalk  32  above or beneath the gas-tight seal is also possible. The catwalks  32  are preferably positioned such that the channel  30  or in general the region where the gastight seal  22  is attached to the vessel housing  12  can be reached. 
     The figures also indicate a maintenance access  34 , for example in order to bring in a rubber boat with which the gas-tight seal  22  can be driven for maintenance purposes, especially when it involves a film. Moreover, a manhole  40  can be provided (see  FIG. 2 ) in order to get into the interior of the storage space  24 . 
       FIGS. 4 to 8  show a buffer store  10  in a second embodiment, although the features shown with respect to the buffer store of  FIGS. 1 to 3  can also be used in the buffer store of  FIGS. 4 to 8 , and vice versa. 
       FIGS. 4 and 5  show the buffer store according to the invention in two different viewing directions, each time in a sectional representation. 
     The buffer store  10  once again comprises a base  14 , which is preferably formed by stainless steel plates or metal plates provided with a corrosion layer. The vessel housing  12  may have an insulation  36 . Moreover, a lid  16  is likewise shown in  FIGS. 4 and 5 , having an advantageous tilt toward the horizontal. The maintenance access  34  is preferably situated in a side wall of the vessel housing  12  in the area of the equalization space  26 . 
     As can be seen moreover from  FIGS. 4 and 5 , the buffer store  10  comprises an inlet line  42  with a preferably central inlet  44  and an outlet line  46  with a preferably central outlet  48 . Basically, multiple inlet and outlet lines  42 ,  46  or inlets  44  and outlets  48  can also be provided. 
       FIG. 8  shows schematically an outlet  48  with an outlet line  46 . The outlet  48  and the outlet line  46  serve for taking the storage fluid present in the buffer store to a use, especially to feed a heating plant with it. For this, the outlet  48  has a relatively large-area extension in a horizontal direction, preferably substantially parallel to the base  14 . The outlet  48  may have at least one upper plate  48   a , which is preferably circular shaped. The cover  22  or the film may lie on the upper plate  48   a , as shown schematically, when the level of the storage fluid has dropped correspondingly. Preferably the outlet  48  has a second plate  48   b , as shown, being preferably substantially identical to or the mirror image of the plate  48   a . The two plates  48   a ,  48   b  run plane parallel to each other, but at a spacing from each other, so that outlet openings  48   c  are formed between the plates  48   a ,  48   b , feeding the outlet line  46 . Between the outlet openings  48   c , webs  48   d  may be provided to keep the plates  48   a ,  48   b  apart and to stabilize them. 
     The represented construction of the outlet  48  prevents the cover  22 , especially when designed as a film, from closing the outlet  48  or the outlet openings  48   c  when the film has dropped down far enough. 
     It may be provided that the inlet  44  is designed accordingly. 
       FIG. 8  moreover shows schematically a brace  50 , being fastened at its ends to preferably oppositely situated inner walls of the vessel housing  12  and serving to support and stabilize the outlet  48 , respectively. 
     The outlet  48  preferably has an extension in the horizontal direction with a diameter of at least 1 m. 
     As can be seen from the representation of  FIGS. 4 and 5 , the buffer store  10  may preferably comprise a vessel wall of a metal band, extending from a lower end of the vessel in encircling manner with a pitch up to the lid  16 . Such a construction is basically familiar. 
       FIGS. 4 and 5  also show the compensation area  22   a . The lower line of the compensation area  22   a  denotes a minimum filling height (Min) and the upper line of the compensation area  22   a  denotes the maximum filling height (Max), as shall be further presented more closely in  FIGS. 6 and 7  below. 
     In the exemplary embodiment of  FIGS. 4 to 7 , it is provided that the cover  22  in the embodiment of a film is connected gas-tight by means of a mounting rail  12   a  or the like to the vessel housing  12  in the area of the maximum filling height. The film  22  thus hangs down or floats accordingly on the storage fluid when the storage space  24  beneath the maximum filling height is filled with the storage fluid. 
     Basically, however, it is also possible for the film  22  to be fastened to the vessel housing  12  lower down than is shown in  FIGS. 4 to 6 . The film  22  may be fastened at the height of the compensation area  22   a , i.e., between the minimum filling height and the maximum filling height. However, other configurations are also possible here. 
       FIG. 5  shows schematically a filling opening  20 ,  52 , which can serve among other things for filling the buffer store  10  with the storage fluid. The filling opening  52 , however, may also be configured such, as is further explained in the following, that it stands in communication with an overflow  54  for the first filling or an emergency overflow  38 , so that storage fluid can be drained. Preferably, the inlet opening  52  has a siphon in this area (not otherwise shown), especially in connection with the emergency overflow  38 . 
       FIGS. 6 and 7  show an especially advantageous configuration of the overflow  54  for the first filling or the emergency overflow  38  basically already mentioned in regard to  FIG. 2 . 
     The invention is not confined to the following configuration, but it is especially suited to forming both an overflow  54  for the first filling and an emergency overflow  38 . 
     The overflow  54  and the emergency overflow  38  are formed by a preferably doublewalled pipe. As is also seen from viewing  FIGS. 6 and 7 , the overflow for the first filling emerges in the region of the minimum filling height in the storage space  24 . In the exemplary embodiment, the overflow  54  emerges just above the minimum filling height in a vertically extending pipe  56 , having a lower piece  56   a  and an upper piece  56   b , on which respective openings are formed. The lower piece  56   a  of the vertically extending pipe  56  is situated distinctly below the minimum filling height of the storage fluid and is therefore generally surrounded by storage fluid. The upper piece  56   b  of the vertically extending pipe  56  is situated in the region of the hanging of the film  22  or the mounting rail  12   a . In the exemplary embodiment, the upper piece  56   b  is moreover situated in the region of the maximum filling height. The upper piece  56   b  is preferably angled and extends, for reasons yet to be explained below, at least for a portion of its length also in the horizontal direction and preferably has multiple openings, distributed along the horizontal segment. However, it may also be provided that an opening is located only at the end of the horizontal segment. 
     During the first filling of the buffer store or generally during the filling of the buffer store with storage fluid, the storage fluid rises in the buffer store and reaches the lower end of the lower piece  56   a  of the pipe  56 . The storage fluid then rises in the lower piece  56   a  until it reaches an entrance  58 , preferably having a siphon. The storage fluid then flows from the entrance  58  into the overflow  54  and from there to an outlet opening, preferably to an outlet opening which is combined with the inlet opening  52  or is preferably at least situated in spatial proximity to it. 
     As soon as storage fluid drains out from the overflow  54 , the attendant recognizes that the minimum filling height has been reached and the filling process can be terminated. The overflow  54  for the first filling can then also be blocked, at least in a lower region (not shown) where the outlet is situated, preferably in the region of the inlet opening  52 . This will prevent storage fluid from emerging in normal operation through the overflow  54  for the first filling. This is not desirable. In normal operation, the storage fluid, usually having a temperature of 10 degrees Celsius during filling, will expand any way through the entrance  58 , so that a flowing of the storage fluid through the overflow  54  should be prevented. 
     In the exemplary embodiment an emergency overflow  38  is designed such that it is also connected to the entrance  58  and thus to the vertical pipe  56 . However, the emergency overflow  38  extends from the entrance  58  at first in the vertical direction upward, and far enough so that the upper end of the emergency overflow  38  is located at the maximum filling height. This ensures that no storage fluid will flow out through the emergency overflow  38  during normal operation below the maximum filling height. Only when the storage fluid has risen so far in the buffer store  10  that the maximum filling height is reached will storage fluid drain out through the emergency overflow  38 . It is provided that a siphon is formed in the region of an outlet opening (not shown) of the emergency overflow  38 , especially in the lower region of the vessel housing  12 , so that no oxygen can get in through the emergency overflow  38 . Thus, a double siphon system is created together with the entrance  58 , which likewise preferably has a siphon, which reliably prevents oxygen from getting into the storage space  24  during normal operation. 
     The outlet of the emergency overflow  38  is once again preferably combined with the inlet opening  52  or preferably situated at least in spatial proximity to it. 
     The buffer store  10  represented in the exemplary embodiment moreover has an emergency air inlet  60 . The emergency air inlet  60  has a vacuum relief valve  62 . The emergency air inlet  60  serves for preventing a damaging of the film  22  in event of a pipe rupture or if too much storage fluid has accidentally been drained off. If the storage fluid drops below the minimum filling height or drops so far that the film  22  can no longer keep up with it, a negative pressure or vacuum would be formed in the storage space  24  beneath the film  22 , so that the film  22  might become damaged. In this case, it is provided that the emergency air inlet  60  opens, for which the vacuum relief valve  62  will serve. Basically, the emergency air inlet  60  can be realized in various designs. The embodiment described below is, however, especially suitable. 
     In the exemplary embodiment it is provided that the emergency air inlet  60  at first emerges into the emergency overflow  38 , which in turn stands in communication with the entrance  58 . The entrance  58 , as described, is connected to the pipe  56 . As soon as the storage fluid has dropped so far that the entrance  58  lies above the storage fluid, which in the exemplary embodiment also means that the storage fluid has dropped below the minimum filling height, the entrance  58  upon further dropping on account of the negative pressure in the buffer store  10  will be free of fluid, even if it has a siphon, to such an extent that a connection will exist to the emergency air inlet  60  and to the vacuum relief valve  62 , respectively. Upon dropping of the pressure below a defined level, the vacuum relief valve  62  opens, so that air can flow in, getting through the entrance  58  into the pipe  56  and from there to the upper piece  56   b , which is already clear and exposed. From here, the air then flows into the storage space  24 . In order to prevent the film  22  from covering the upper piece  56   b  of the pipe  56 , the latter preferably runs for a partial length also in the horizontal direction, preferably in the region of the maximum filling height, and it preferably comprises multiple openings, so that the air can flow out in suitable manner. 
     The equalization opening  18  shown in  FIG. 6  leads in the simplified representation directly to the outside or constitutes a connection to the surroundings or to a pressure vessel. However, the equalization opening  18  may also preferably be led downward by a line (not shown) and emerge from the buffer store  10  preferably in spatial proximity to the inlet opening. 
     In the exemplary embodiment, it is preferably provided that the outlet line  46  or the outlet  48  situated at the top in the buffer store  10  is used both to supply and to remove storage fluid, especially to bring in warm or heated storage fluid or to remove it from the buffer store  10 . 
     The outlet line  46  or the outlet  48  may in particular serve for supplying externally heated storage fluid and/or removing warm storage fluid from the buffer store in order to supply a heating plant with it. 
     In the exemplary embodiment it may be provided (not shown) that the buffer store  10  has two, three, four, five or more outlet lines  46  and/or outlets  48 , especially multiple upper plates  48   a  and lower plates  48   b , on which the cover  22  can be supported. 
     In the exemplary embodiment it is preferably provided that the inlet line  42  or the inlet  44  situated at the bottom in the buffer store  10  can be used both to supply and to remove storage fluid, in particular to bring in cold or unheated storage fluid or to remove storage fluid from the buffer store  10 . 
     The inlet line  42  or the inlet  44  may in particular serve for supplying storage fluid and/or removing storage fluid from the buffer store in order to heat it externally, especially by surplus energy. 
     The film  22  may basically have a suitable construction. It is provided in the exemplary embodiment that the film  22  has at least two layers, preferably three layers, where the second, preferably middle layer is gas impermeable, for example being formed by an aluminum foil. 
     In a manner not otherwise shown, the film  22  may also have an insulation. Moreover, alternatively or additionally in not otherwise shown manner it may also be provided that an insulation is placed on the surface of the film  22  facing toward the equalization space  26 , preferably in the form of beads, preferably styrofoam beads.