Patent Publication Number: US-2016223268-A1

Title: High-temperature heat accumulator

Description:
The invention relates to a heat accumulator for storing thermal energy in at least one heat accumulator block and at least one removal device for removing stored thermal energy, and to a method for controlling a removal rate from such a heat accumulator in an open-loop or closed-loop manner. 
     Such heat accumulators with a feeding device for the energy to be stored in the form of heat for delivering power uniformly and with long-term stability are known. They consist of a highly heat-resistant solid material which, when heated, is capable of storing heat, and a device for extracting the stored heat, preferably in the form of hot steam or superheated steam. The former can be used for example for district heating. The latter is necessary for the operation of steam turbines for power generation. 
     Extensive work is underway at present on many different technologies for storing energy. These technologies are necessary to allow advancement of the further development of renewable energy sources, for example solar, wind, hydro, tidal, etc. Although most of these energy sources are available in surplus, that is not always the case when they are needed. Therefore, they must be stored on a large scale, in order that even lengthy periods of undersupply can be covered by these stores. If this is not accomplished, the reliability of the supply must continue to be ensured by means of conventional standby power plants, which then in turn are not fully utilized over long periods. Maintaining the operational readiness of these standby power plants is a very expensive backup solution. 
     A heat accumulator of the generic type is known from DE 2117103 A. In this case, the stored thermal energy is accumulated in a single metallic block. This block is inductively heated; a spray nozzle protrudes into a cylindrical recess, which serves for the evaporation of the water introduced by the spray nozzle. The steam produced is condensed and recirculated to the spray nozzle. 
     A heating boiler for heating water is known from German utility model DE 7010442 U. It has a thermal storage core and means for heating the core to a temperature above the boiling point of water. Arranged above the core is a steam and water chamber. A tube extends from the steam and water chamber situated above the core into the core. Also provided are means that provide the chamber mentioned with a water supply source and a passageway into the tube. The water supply system is self-regulating in such a way that it maintains a pressure that is equal to the supply pressure, which may be very low. For this purpose, the water may be taken from an auxiliary reservoir and be supplied to the system. 
     The steam produced is condensed by the cold water supplied. The water is thereby heated and can be supplied from the chamber above the core to a heating unit as hot water. This previously known system restricts the removable temperature or the medium essentially to the removal of hot water. Steam is not made available. The amounts of heat removed consequently lie at a temperature level below the boiling point. 
     DE 6806870 U relates to an electrically heated heat storage stove. The storage core consists of accumulator blocks of ceramic material, which are passed through by metal channels, which form a heating register for the production of hot water, steam or the like. This document is based on the object of ensuring a good heat transfer from the storage core to the heating register even at low accumulator temperatures. Provided for this purpose are metal channels with enlarged surfaces that form the heating register. The metal channels forming the heating register are arranged vertically in the ceramic material, nothing being stated about the connection of the channels, let alone the existence and configuration of any steam collecting chamber. 
     GB 1344486 A describes a water heating system in which horizontal evaporator pipes are embedded in thermal storage blocks, which consist of refractory material. The storage blocks are electrically heated. The horizontally arranged evaporator pipes are connected by their open end to a vertical steam tube. The vertical steam tube forms a U, which ends with an open end in a closed condensate tank and ends with the other end in an open part of a tank. The tank and the closed condensate tank lie approximately at the same level. The heat is dissipated from the condensate chamber by way of a heat exchanger. Since the steam tube ends above the level of the condensate, the condensate produced is conducted by way of a separate line to the U-shaped end of the steam line, so that as a result a condensate cycle is obtained as soon as the level of the condensate in the condensate tank exceeds the upwardly open end of the condensate return line. 
     The invention is based on the object of drastically increasing the temperature level and thereby the heat storage amount of solid-body heat accumulators, without accepting the accompanying adverse effects of the materials that are likewise exposed to these high temperatures, for example steels, i.e. the steel pipes, which otherwise drastically restrict the long-term sustainability of such an energy store. 
     The object of the invention is also to propose a method which makes it possible for amounts of heat to be removed from the heat accumulator uniformly and with little adverse effect on parts of the plant. 
     [A01] The object in terms of the device is achieved in the case of a heat accumulator for storing thermal energy in at least one heat accumulator block and at least one removal device for removing stored thermal energy by the heat accumulator block and the removal device being formed separately and displaceably in relation to one another. The separation means that differences in temperature between the removal unit and the heat accumulator block cannot cause mechanical stresses between the two parts that are otherwise formed as a unit. The structural separation of the removal unit and the heat accumulator block allows an increase in the storage temperature to a level that is free from the limits of the materials used in the removal unit. 
     The invention presented here is consequently distinguished by a highest-possible storage temperature, and consequently energy density, and also by a highest-possible efficiency of the overall process, which is achieved by the output temperature/capacity that is constantly available over a great temperature range of the accumulator. 
     This is therefore achieved by a separation of the actual energy storage, i.e. the accumulator block, and the device that is necessary for withdrawing the energy, i.e. the removal unit. 
     The energy storage takes place in an accumulator block of a ceramic material, which makes very high storage temperatures possible with long-term stability. The ideal material has a maximum temperature resistance in order to be able to store more energy over higher temperatures, and also a high relative density/specific heat capacity in order to be able to store a maximum of energy per unit of space. Different materials are suitable for this, differing in the technical parameters, such as the specific heat capacity and the temperature resistance, and also in the costs. Fireclays with a high proportion of Al2O3 (for example of the grade A40t) for example already allow application temperatures of about 1450° C. At the same time, with a relative density of up to 2.15 g/cm 3 , these also offer a comparatively high specific heat capacity. It is also possible however to use technical ceramics, the temperature range of which may even be much higher, but which in return generally have disadvantages with regard to the specific heat capacity, which can fully or partially cancel out again the advantage of the higher temperature. 
     [A02] In a refinement of the invention it is provided that at least one heat-exchanger surface for a heat transfer from a fluid or an electrical heating element into the heat accumulator block is provided in the heat accumulator block. Alternatively, heat in the heat accumulator block may be introduced into the heat accumulator block for storage either by means of a fluid or by means of an electrical element, a so-called heating cartridge. The feeding-in takes place by way of the at least one heat-exchanger surface, which is advantageously formed as a surface of storage material, precluding the existence of differences in material that may lead to mechanical stresses. Both high-temperature gases and liquid metals may be used as the fluid. 
     However, at present the heat is preferably introduced into the accumulator block by way of electrical heating cartridges. However, as soon as technically suitable and economically viable fluids, for example liquid metals, are available, it is also possible to use fluids for introducing for storage the heat to be stored. The high temperatures can readily be produced for example by parabolic-trough solar installations. 
     For this, the liquid medium is passed through the heat accumulator under low pressure in ceramic pipes. The direct introduction of concentrated heat radiation by means of focusing and reflection is also possible. 
     In any event, it is advisable and necessary from the viewpoint of cost-effectiveness and achievable efficiency to achieve a storage temperature that is as high as possible. 
     [A03] In a further refinement of the invention it is proposed that, for the removal of thermal energy from the heat accumulator block, the removal unit has at least one evaporator surface for producing a phase change in a heat-transfer fluid or for further superheating of a fluid that has already exceeded the point of phase change. For the removal of the stored energy, evaporator surfaces on which a phase change of a liquid carried past takes place are provided in the removal unit. This liquid will generally be water, but with other temperature levels other liquids can also be used. If water is used as the liquid, it is of advantage for increasing the temperature level if this phase change takes place at higher pressures. 
     However, with the exception of the storage material itself, the materials that are available today are at odds with this basic principle. Mention may be made here primarily of the lack of creep strength of the steels available. This primarily has the effect that the maximum temperature to which the steels may be exposed is limited. Since the withdrawal of the stored heat generally takes place in the form of hot steam or superheated steam under very high pressures of several hundred bar, high-strength pipes are necessary. 
     [A04] Therefore, in a further refinement of the invention, it is advantageously provided that the evaporator surface is preferably formed by the inside surface of at least one pipe, which is preferably embedded in a block of which the material corresponds in particular to the material of the heat accumulator block. 
     This is where the basic principle of this invention comes in. The accumulator block, in which there are no metal components apart from the heating cartridge itself, is separated from the device for extracting the heat and can consequently be heated up to the maximum temperature of the accumulator material or the maximum temperature of the devices for introducing heat, for example the heating cartridges, without steels that are used in the device for extracting heat being damaged. 
     According to this invention, the removal of the stored heat then takes place through a removal device that is produced from a likewise highly heat-resistant block, i.e. the so-called heat extractor, embedded in which as heat exchangers are pipes in which the medium introduced, generally water, is evaporated or else in which for example steam introduced is further superheated. The superheated steam, under a very high pressure, is subsequently used for the operation of steam turbines for power generation. Of course, the heating of any other gases or liquids is also possible in this way. Alternatively, it is also possible to dispense with the heat-resistant block and to work only with a pipe manifold bundle for the heat removal. The pipe manifold bundle may be additionally provided with exchanger surfaces. 
     [A05] The measure that the heat accumulator block is formed as at least partially enclosing the removal device serves for increasing the removal rate. The shaping and dimensioning allows the degree of coupling between the heat accumulator block and the removal device to be influenced within wide ranges. The functions of the coupling in dependence on the distance between the heat accumulator block and the removal device can also be set by the design of the region in which the heat accumulator block encloses the removal device. For example, with a cylindrical enclosure, the coupling increases linearly with the reduction in the distance, whereas, with an insertion of the removal device in a conical depression, the coupling increases progressively with the distance. 
     At very high temperatures, the heat transfer between the heat accumulator block and the heat extractor takes place predominantly by heat radiation. The thermal power that is transferred to the heat extractor by heat radiation is controllable in a closed-loop and/or open-loop manner by a controllable distance between the movable heat extractor and the actual heat accumulator. One of the aims of this controllability is that of controlling the output capacity of the accumulator. Even more important, however, is not to increase the temperature of the heat extractor, independently of the actual temperature of the accumulator, beyond the limits that are dictated by the creep strength of the heat exchanger embedded therein. 
     It follows from the mechanical decoupling of the energy store and the heat extractor that the temperature of the store can be very much higher than in the case of all other known concepts, since here no steel-based metal materials are directly exposed to this high temperature. Thus, storage temperatures such as the aforementioned 1450° C. can be achieved with technology known from high-temperature furnace construction and can even be exceeded if suitable materials are used. 
     [A06] In a refinement of the invention it is proposed as an advantageous form of the enclosure that the heat accumulator block is concavely formed in the region of the enclosure and the removal device is at least partially convexly formed. 
     [A07] A reduction in heat losses is achieved if the heat accumulator block and the removal device are arranged in a common housing, which preferably has a negative pressure. The negative pressure brings about a better insulation with respect to the surroundings. Heat conduction and convection processes, which could lead to heat losses, are reduced. 
     [A08] In a further refinement of the invention it is provided that the heat accumulator block and the removal device are arranged in a common, preferably vertical axis, the distance from the heat accumulator block and the removal device being formed as changeable by motor and preferably the weight of the moved part of the heat accumulator block or the removal device being formed as at least partially compensated. The possibility of changing the distance between the removal device and the heat accumulator block makes closed-loop and/or open-loop control of the amounts of heat given off possible within wide ranges. 
     At the same time it is possible to keep the output capacity of the accumulator constant over a wide temperature range of the accumulator, since the storage temperature, decreasing due to heat removal, and consequently also the decreasing radiation output of the heat accumulator is counteracted by a reduction in the distance between the heat extractor and the heat accumulator. The ideal removal temperature is consequently achieved by means of a simple closed-loop control, the reference variable of which is the desired removal temperature and the actuating element of which is the device for changing the distance and the controlled variable of which is the actual temperature of the heat extractor. This goes on up to mechanical contact between the heat extractor and the heat accumulator, whereby in the course of the further cooling the decreasing heat radiation is increasingly taken over by direct heat conduction between the two bodies. If the storage temperature falls below the value of the defined and controllable ideal temperature for the removal, then there is also a fall in the output capacity as the temperature continues to fall, and a fall in the achievable maximum output temperature from the accumulator, and as a result also in the efficiency of the overall process. In the sense of an optimum overall efficiency, the output temperature should always be kept at the technically feasible maximum. If the main concern is the greatest possible storage range, the accumulator may also be cooled down further at the expense of efficiency. 
     In this case, the stored energy can be used up to an unavoidable minimum. For example by combined operation, in that very high storage temperatures are used to the benefit of the optimum efficiency for producing superheated steam. If the temperature of an accumulator block falls below the optimum temperature for steam superheating, the remaining temperature can be used by way of heat exchangers virtually without loss for water evaporation, in particular in the case of multistage steam superheating, or at the end of the temperature scale also for providing heat for heating purposes. 
     [A09] The physical laws of heat transfer from the heat accumulator block to the removal device can be adapted within wide ranges if an amount of a heat-conducting fluid is provided in a depression formed by the heat accumulator block. A liquid metal is preferably chosen as the fluid, so that the liquid metal transfers heat by means of convection. The physical laws of heat radiation are only determinative for those partial surface areas in which the interspace between the heat accumulator and the removal device is not filled with liquid metal. 
     [A10] The object in terms of the method is achieved by a method for controlling a removal rate from a heat accumulator in an open-loop or closed-loop manner in that a distance from the heat accumulator block and the removal device is changed. This produces the following advantages in particular:
         As long as the storage temperature lies in the range above the target temperature of the heat extractor, the highest efficiency that is technically possible at present is achieved for the power plant process as a whole.   The specific costs per kWh of stored heat fall considerably, since the in this way very much higher storage temperature has no appreciable influence on the costs of the accumulator itself. The only measure that has to be taken to allow for higher storage temperatures is increased insulation, if it is not wished to accept higher heat losses that would otherwise occur due to the higher temperature gradient with respect to the surroundings.   The accumulator may be structurally designed for very easy maintenance, so that, after removal of the insulation, all components possibly in need of maintenance or repair are accessible from above. Thus, both the heat extractor and the heating cartridges can be removed and exchanged without moving the accumulator down, while complying with corresponding safety measures.   The increased energy density due to the higher temperature makes smaller accumulators with the same energy content possible.   The service life of the metal materials used in the heat extractor can be determined by the controllable temperature of the heat extractor. By appropriate choice of the temperature, either the efficiency and the capacity or the lifetime can be favored.   Since the accumulator and the heat extractor consist of only a few simple components, their production costs are comparatively low. The necessary controlling and actuating elements can likewise be of a very simple design and construction. Thus, by the use of counterweights, the necessary forces and consequently the actuators for the movement of the heat extractor can be kept very small. The temperature of the accumulator and of the heat extractor is measured at intervals or permanently, in order to be able to derive from this the necessary manipulated variable for the correction.       

     [A11] It is also of advantage however to use the heat accumulator for heating purposes in buildings. The advantages of the high storage temperature likewise apply here. The removal device can be controlled at any time such that the water introduced is not evaporated, but merely heated. Consequently, the accumulator assumes the function of a continuous-flow heater and can consequently replace the customary heating boiler together with the hot-water tank. 
    
    
     
       A preferred embodiment of the invention is explained by way of example on the basis of a drawing. Figures of the drawing specifically show: 
         FIG. 1  a schematized perspective view of a heat accumulator according to the invention, 
         FIG. 2  a schematic plan view of the heat accumulator according to the invention, 
         FIG. 3  a vertical section through the heat accumulator according to the invention under load according to sectional line A-A in  FIG. 2 , 
         FIG. 4  a vertical section through the heat accumulator according to the invention under load according to sectional line B-B in  FIG. 2 , 
         FIG. 5  a vertical section through the heat accumulator according to the invention according to sectional line C-C in  FIG. 6  in the uncoupled state and 
         FIG. 6  a schematic plan view of the heat accumulator according to the invention according to  FIG. 5 . 
     
    
    
     The schematized perspective view of the heat accumulator  1  according to the invention that is shown in  FIG. 1  shows the way in which the heat accumulator  1  is divided into a heat accumulator block  2  and a removal device  3 . The heat accumulator block  2  and the removal device  3  lie on a common center axis  16  and are displaceable in relation to one another in this axis  16 . For the sake of overall clarity, the drives for making this displaceability possible are not represented. The removal device  3  has on its upper side  21  a number of supply lines  22  for supplying water. The steam formed in the removal device  3  is removed from the removal device  3  by way of the steam line  23 . 
       FIG. 2  shows a plan view of the heat accumulator according to the invention. The water supplied through the supply line  22  for producing steam initially opens out into annular distributors  24 , from which it is directed into vertically oriented pipes  10 . The steam produced in the removal device  3  is then removed through the steam line  23 . 
     The heat accumulator block  2  is heated by means of four heating elements  6 , which are arranged equally distributed in the corner regions of the accumulator block  2 . 
     The detailed structure of the heat accumulator block  2  and of the removal device  3  can be taken from the vertical section through the heat accumulator according to the invention under load according to sectional line A-A of  FIG. 3 . The water supplied to the removal device  3  through the supply lines  22  is directed from the annular distributors  24  through pipes  10  initially vertically into a block  11  of a material  12 , in which the pipes  10  are embedded, and thus form an exchanger head  25 . There, the line  10  approximately follows the outer contour  26  of the exchanger head  25  in the vertical section, to then open out in a vertically upwardly rising manner in a steam chamber  27 . Arranged between the housing  15  of the removal device  3  and the exchanger head  25  is an insulator  28 , which reduces heat losses with respect to the surroundings of the heat accumulator. 
     Represented in  FIG. 4  is a vertical section, which takes a diagonal section through the heat accumulator  1 , so that the heating cartridges or heating elements  6  provided in the heat accumulator block  2  can also be seen. The heating-up function of the heating element  6  may also be assumed by a fluid  5  passed through, which is at the appropriate temperature. The accumulator block  2  is also surrounded by a good thermal insulator  28  in the housing  15 , so that an uncontrolled heat loss from the heat accumulator block  2  is also avoided. The housings  15  of the removal device  3  and the heat accumulator block  2  partially enclose one another. As can be taken from  FIGS. 3 and 4 , they are displaceable in relation to one another telescopically in one another. As a result, the distance  17  between the heat accumulator block  2  and the removal device  3  can be changed. Suitable drives, which a person skilled in the art can form for example as piston-cylinder units or spindle-nut drives that are synchronized with one another, are provided for carrying out the change. 
     The steam produced is removed from the steam chamber  27  by means of a collecting bell  30  and is passed to a turbine by way of steam line  23 , for example for generating electrical energy. 
     The material  13  of the heat accumulator block  2  is preferably a dense high-temperature-resistant compound, which is suitable for taking up a correspondingly large amount of thermal energy. The material  12  of the removal device  3  may consist of the same material, so that both materials have a similar expansion behavior. The outer contour of the exchanger head  25  is convexly shaped in the form of a bell and inserts into a depression correspondingly shaped to match in the material  13  of the heat accumulator block  2 . The heat transfer between the heat accumulator block  2  and the removal device  3  is initially based essentially on heat radiation. It may however also be based on convection, if the interspace between the removal device  3  and the heat accumulator block  2  is filled by a fluid, preferably liquid metal. In  FIG. 3 , such a liquid medium  31  is represented in form by blackening. The upper level of the medium  31  is located below the upper edge of the depression  18 . 
     The representations of  FIGS. 3 and 4  consequently show the state in which heat is removed from the heat accumulator block  2  by way of the exchanger head  25 . This means that the exchanger head  3  is thermally coupled to the heat accumulator block  2 . 
     The uncoupled state, in which no appreciable heat is removed from the heat accumulator block  2 , is represented in  FIGS. 6 and 5 .  FIG. 6  shows a vertical section, as previously already represented in  FIG. 3 , but with a considerably greater distance  17  of the removal unit  3  from the heat accumulator block  2 . In this representation is the convex profile of the block  11  of the removal device  3  and the region  14  correspondingly concavely formed depression  18  of the accumulator block  13 . 
     While in  FIG. 3  the liquid medium  31  took up almost the entire interspace between the material  12  of the removal device  3  and the material  13  of the heat accumulator  2 , in  FIG. 6  this medium  31  is collected in the lower part of the depression  18 . A bulkhead  32  forms the upper termination of the depression  18 . Since the exchanger head  25  is completely separated from the heat accumulator  2  by the bulkhead  32 , there is also no heat transfer between the accumulator block  2  and the removal device  3 . 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Heat accumulator 
           2  Heat accumulator block 
           3  Removal device 
           4  Heat-exchanger surface 
           5  Fluid 
           6  Heating element 
           7  Evaporator surface 
           8  Heat-transfer fluid 
           9  Inner surface of the pipe 
           10  Pipe 
           11  Block 
           12  Material 
           13  Material 
           14  Region 
           15  Housing 
           16  Axis 
           17  Distance 
           18  Depression 
           19  Fluid 
         
           20 
         
           21  Upper side 
           22  Supply lines 
           23  Steam line 
           24  Distributor 
           25  Exchanger head 
           26  Outer contour 
           27  Steam chamber 
           28  Insulator 
           29  Distance 
           30  Collecting bell 
         
           31 
         
           32  Bulkhead