Abstract:
A device for the intermediate storage of thermal energy and to a system comprising a plurality of such devices is provided. The device has a solids store and a pipe system, which is formed of individual pipes and runs through the solids store and through which an energy carrier medium flows. In order to be able to quickly and uniformly charge the solids store with thermal energy or discharge thermal energy therefrom, heat conducting elements are provided, each forming heat transmission regions with the individual pipes and each extending into the regions of the solids store that are free of individual pipes. The heat conducting elements also have a higher heat conductivity than the solids store.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2010/007909, which was filed on Dec. 23, 2010, and which claims priority to German Patent Application No. DE 10 2009 060 911.3, which was filed in Germany on Dec. 31, 2009, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a device and system for the intermediate storage of thermal energy. 
         [0004]    2. Description of the Background Art 
         [0005]    In view of the dwindling primary raw materials worldwide as resources for energy production, regenerative and alternative concepts are becoming more and more important. Examples are the use of solar energy in solar thermal power plants or the use of waste heat from industrial manufacturing processes. Because these alternative forms of energy are coupled to solar radiation or to certain industrial processes, however, their continuous availability is not guaranteed. Their practical usability therefore depends greatly on the possible intermediate storage of energy accumulating at a certain time and the ability to provide it at a later time. The storage of thermal energy therefore has a key importance in the development and implementation of alternative concepts for energy recovery. 
         [0006]    Known systems for storing thermal energy comprise substantially a heat source, for example, a solar collector or an internal combustion engine, a heat accumulator with a thermally chargeable and dischargeable storage medium and at least one heat circuit for charging and discharging the heat accumulator, in which a working medium flows from the heat source to the heat point of use or from the heat accumulator to the heat point of use. 
         [0007]    The storage medium has central importance for the effectiveness of the entire system. It must satisfy substantially two requirements: namely, on the one hand, have a high thermal storage capacity, i.e., have as great a capability as possible to take up thermal energy per unit weight and unit volume, and, on the other, be characterized by a high thermal conductivity, i.e., the heat must be able to spread as rapidly as possible in the storage medium. 
         [0008]    Fluids that meet both of the above criteria are already known as a storage medium. For the low-temperature range to about 100° C., water is suitable as a storage medium, because it is available cost-effectively and is characterized by its high thermal storage capacity. A disadvantage, however, is the rapid increase in vapor pressure at temperatures above 100° C., which necessitates costly pressure vessels. For this reason, fluids with a higher boiling point are used for higher temperature ranges, e.g., heat transfer oils or salt melts. This is associated with a considerable increase in investment costs, however. A convective heat transfer results owing to the circulation of liquid storage media with the advantage of a rapid and uniform charging and discharging of the storage medium. 
         [0009]    Apart from liquid accumulators, solid storage media are also known, which may include, for example, metals such as steel or cast iron. Such metals are well suited as a storage medium because of their high specific weight and their high thermal conductivity but lead to high investment costs. 
         [0010]    DE 10 2008 047 557 A1 also discloses a solid storage medium made of a mineral material, through which a plurality of axis-parallel pipes run, in which an energy transfer medium flows in the heat circuit. The thermal energy of the energy transfer medium is introduced into the storage medium via the pipes and is distributed there gradually and uniformly over the entire volume. The mutual distance of the individual pipes and thereby their number are predetermined by the thermal conductivity of the storage material, because it must be assured for the practical usability of the heat accumulator that the thermal energy spreads within the entire storage device as rapidly and uniformly as possible in order to make possible a rapid charging and discharging of the energy storage device. 
         [0011]    Solid storage media made of a mineral material in fact have the great advantage of being able to be produced cost-effectively but during their use it must be accepted that they have only a limited thermal conductivity. In order to achieve a satisfactory heat conduction, nevertheless, the pipes are arranged at a relatively high density in the solid storage media, as a result of which their number and thereby again the production costs increase. A cost advantage achieved with the use of a mineral solid storage medium is thereby again partially nullified. 
       SUMMARY OF THE INVENTION 
       [0012]    It is therefore an object of the present invention to improve prior-art solid storage media in regard to their economy and function. 
         [0013]    The starting point for the invention comprises heat accumulators in which charging and discharging of the solid storage medium occurs via an energy transfer medium flowing in the pipe system, whereby the temperature gradient between the energy transfer medium and the solid storage medium is the driving force for the heat flow. Moreover, the thermal conductivity of the employed storage materials has a significant effect on the charging and discharging of the solid storage medium. For example, steel in comparison with concrete has a thermal conductivity that is higher by a factor of about 40, with the result that the thermal energy provided in the pipe system spreads only slowly in the solid storage medium. In a solid storage medium made of concrete, this has the result that areas, directly surrounding the individual pipes, of the solid storage medium are charged very rapidly thermally, however, with increasing distance from the individual pipes a great temperature drop is to be observed ( FIG. 12   a ), and therefore a relatively long time interval is necessary until a very largely equalized energy state over the entire volume of the solid storage medium has been achieved. To shorten the charging and discharging times for the solid storage medium, the radial distance of the individual pipes to one another would have to be reduced consistently, which, however, because of the associated higher number of individual pipes would negatively impact the economy of such a heat accumulator. 
         [0014]    The invention resolves this problem with the aid of heat conducting elements, which have a higher thermal conductivity compared with the material of the solid storage medium and extend proceeding from the individual pipes into the solid storage medium. Preferred materials for the heat conducting elements are metals such as, for example, steel, aluminum, copper, or graphite, which can be available both as ground or compressed natural graphite and expanded or compressed natural graphite (graphite film). The heat conducting elements in this way form flow pathways for the rapid transport of thermal energy over greater distances within the solid body, from which then a uniform loading of the storage volume occurs over only relatively short distances. This allows for a rapid and uniform charging and discharging of the solid storage medium with the best possible utilization of the storage volume. The invention is thus characterized by a high specific heat output of the solid storage medium also at relatively great radial distances of the individual pipes of the pipe system and thus combines the seemingly contradictory requirements for a high thermal conductivity, on the one hand, and a cost-effective storage material, on the other. 
         [0015]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0017]      FIG. 1  shows an oblique view of the heat accumulator of the invention; 
           [0018]      FIG. 2  shows a longitudinal section through the heat accumulator shown in  FIG. 1 ; 
           [0019]      FIG. 3  shows a cross section through the heat accumulator shown in  FIG. 2  along line III-III; 
           [0020]      FIG. 4  shows an oblique view of a first embodiment of the heat accumulator of the invention without a depiction of the solid storage medium; 
           [0021]      FIG. 5  shows an oblique view of a second embodiment of a heat accumulator according to the invention; 
           [0022]      FIG. 6  shows an oblique view of a third embodiment of a heat accumulator according to the invention; 
           [0023]      FIG. 7   a  and  b  show details for the embodiment illustrated in  FIG. 5 ; 
           [0024]      FIG. 8  shows an exploded view of a fourth embodiment of a device of the invention; 
           [0025]      FIG. 9  shows a detail of a fifth embodiment of a heat accumulator of the invention in the heat transfer region; 
           [0026]      FIG. 10  shows an oblique view of a sixth embodiment of a heat accumulator according to the invention; 
           [0027]      FIG. 11  shows an oblique view of a seventh embodiment of a heat accumulator according to the invention; 
           [0028]      FIG. 12   a  shows an oblique view of an eighth embodiment of a heat accumulator according to the invention; 
           [0029]      FIG. 12   b  shows a detail of the heat accumulator illustrated in  FIG. 12   a;    
           [0030]      FIG. 13   a  shows an oblique view of a ninth embodiment of a heat accumulator according to the invention; 
           [0031]      FIG. 13   b  shows a detail of the heat accumulator illustrated in  FIG. 13   a;    
           [0032]      FIG. 14  shows an oblique view of a heat accumulator with horizontal bracing; and 
           [0033]      FIG. 15   a  and  b  show cross sections through a heat accumulator of the invention with an illustration of the heat distribution during charging of the heat accumulator. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  shows a heat accumulator  1  of the invention in an oblique view and  FIGS. 2 and 3  in associated sections. An essential element of heat accumulator  1  is a block-shaped solid storage medium  2  with a considerable longitudinal direction, whose longitudinal ends are formed by front faces  3  and  4 . Solid storage medium  2  in the present example is made of concrete, which can occur both in in-situ concrete casting and also, as will be described in greater detail below, with precast concrete parts. Other materials such as, for example, ceramic, brick, or fireclay are also within the scope of the invention. Free-flowing mineral material can also be used as the solid storage medium, which is then available as fill within a housing. The dimensions of heat accumulator  1  are not specified and are determined depending on the particular intended application. A preferred embodiment of a heat accumulator  1  has a length of about 18 m, a height of about 4 m, and a width of about 2.5 m to 3 m. 
         [0035]    Another element of the invention is the pipe system labeled with the number  5 , which comprises a plurality of individual pipes  6 . Individual pipes  6  go through solid storage medium  2  in its longitudinal direction in an axis-parallel position, which is made clear in  FIG. 1  by the omission of solid storage medium  2  in a middle longitudinal section. Individual pipes  6  in this case extend beyond front faces  3  and  4  with the formation of a projection. 
         [0036]    As is evident primarily from  FIG. 3 , individual pipes  6  are arranged preferably equidistantly in a plurality of horizontal levels, lying plane-parallel one over another, whereby individual pipes  6  of two adjacent levels may have a lateral offset by half the horizontal distance of two individual pipes  6 . In this way, a uniform distribution of individual pipes  6  over the cross section of solid storage medium  2  arises, which results in a uniform introduction of the thermal energy into solid storage medium  2 . In the case of solid storage media made of in-situ concrete, to maintain the above-described pattern over the entire length of individual pipes  6 , spacers, for example, made of steel mats  8  are arranged within solid storage medium  2  at predefined longitudinal distances in a cross-sectional level in each case; the cross and longitudinal rods of said spacers correspond to the predetermined pattern and are used for fastening individual pipes  6 . With greater longitudinal distances, the reinforcement of individual steel mats  8  by profile frames  9  is possible ( FIGS. 1 and 2 ). 
         [0037]    Individual pipes  6  end as already described in cross-sectional levels, which run at a clear distance to front faces  3  and  4 , for example, at a distance of 40 cm. Front plates  11  and  12 , which are provided with through-openings according to the pattern of individual pipes  6 , are arranged in these cross-sectional levels, therefore plane-parallel to front faces  3  and  4 . Individual pipes  6  open on the back of face plates  11  and  12  into collection channels, which in turn are connected via connecting pipe sections  16  to a distributor  17  or collector  18 , each of which have a pipe connection  19  for the inlet or outlet of heat accumulator  1  ( FIG. 2 ). 
         [0038]    A fluid energy transfer medium flows through pipe system  5 , for example, a heat transfer oil, which is supplied to the circulation and transports the thermal energy for charging heat accumulator  1  from a heat source, for example, a solar collector, to heat accumulator  1  or for discharging accumulator  1  the thermal energy present in accumulator  1  to a user. The thermal energy inherent to the energy transfer medium is thereby first transferred to pipe system  5 , from where it is fed into solid storage medium  2 . 
         [0039]    To avoid damage due to temperature-induced different linear expansions between solid storage medium  2  and pipe system  5 , a mechanical decoupling of these two components is provided, which can occur, for example, by providing a clear gap or a gap, filled with a thermally conductive material, between solid storage medium  2  and pipe system  5 . 
         [0040]      FIGS. 4 to 11  show different embodiments of the invention, which enable a high specific heat output of solid storage medium  2  in the case of simultaneously large radial distances of individual pipes  6  of pipe system  5 . To clarify the construction method and functioning of a heat accumulator  1  of the invention, only small partial details of solid storage medium  2  with its essential features are shown on a larger scale there.  FIGS. 4 to 11  are therefore to be viewed together with  FIGS. 1 to 3 . 
         [0041]      FIG. 4  shows a first embodiment of heat conducting elements according to the invention in an oblique view of a partial detail of pipe system  5 , whereby for a clearer illustration solid storage medium  2  itself is not shown. Seen are individual pipes  6  in horizontally stacked levels and running axis-parallel to one another, said pipes which are arranged from one level to another with a lateral offset in the height of half the mutual lateral distance, so that the distance between two individual pipes  6  is uniform at each place in solid storage medium  2 . In addition, individual pipes  6  are mechanically decoupled from solid storage medium  2 , for example, by sheathing with a graphite film (not shown). 
         [0042]    Whereas primarily a heat distribution in solid storage medium  2  occurs in the axial direction in individual pipes  6  due to the energy transfer medium flowing therein, horizontal heat conducting elements  20  and vertical heat conducting elements  21  are provided for lateral distribution of the thermal energy. Heat conducting elements  20 ,  21  in the present exemplary embodiment has flat metal bars. 
         [0043]    Horizontal heat conducting elements  20  are placed with their longitudinal axis transverse to individual pipes  6  of a level on these and due to their length extend over several individual pipes  6 . The axial distance of horizontal heat conducting elements  20  is within a range from 5 cm to 30 cm and is 15 cm in the present example. The contact-based placement of horizontal heat conducting elements  20  on individual pipes  6  forms a substantially linear heat transfer region via which the thermal energy is introduced from individual pipes  6  in to horizontal heat conducting elements  20 . 
         [0044]    In their simplest embodiment, because of their own weight, heat conducting elements  20  rest on individual pipes  6  without other securing measures. Preferred, however, is their fixation at a predetermined place, for example, by welding or binding with binding wire. Another type of fixation can also occur by the interlacing of horizontal heat conducting elements  20  in individual pipes  6  lying in a level, whereby heat conducting element  20  alternates the attachment side from individual bar  6  to individual bar  6 , therefore is guided once above and once below past individual bars  6 . Because of the elastic properties of heat conducting elements  20 , the restoring forces in this case result in a pressing of heat conducting elements  20  against individual pipes  6 . 
         [0045]    For heat distribution in the vertical direction, the embodiment of the invention shown in  FIG. 4  provides vertical heat conducting elements  21 , which in the present example also includes flat metal bars and which because of their length bridge at least the vertical distance of two individual pipes  6  lying one above another. The fixation of vertical heat conducting elements  21  can occur as previously described, namely, by welding, binding, or interlacing. The exemplary embodiment shown in  FIG. 4  discloses, moreover, another option in which the upper end of vertical heat conducting elements  21  is bent into a U-shape to form a hook  22  and is hung with hook  22  on individual pipes  6 , whereas the opposite end lies against individual pipe  6  lying below. Because hook  22  in part follows the perimeter of individual pipes  6 , an enlargement of the heat transfer region and thereby an improved heat transfer result. 
         [0046]    A pipe system  5  prepared in this way can be provided for the completion of solid storage medium  2 , for example, in a closed formwork and concreted. In this way a solid storage medium  2  of concrete forms, which is run through in the longitudinal direction by individual pipes  6  of pipe system  5  and in the horizontal and vertical lateral direction in addition by horizontal heat conducting elements  20  and vertical heat conducting elements  21 . This type of solid storage medium  2  can be charged or discharged uniformly with thermal energy with a very short time despite the limited thermal conductivity of the storage material. 
         [0047]      FIG. 5  shows a partial detail of a second embodiment of a heat accumulator  1  of the invention. In this embodiment, solid storage medium  2  has prefabricated elements  23 , which are placed one on top of another in horizontal layers  24  in a modular manner, whereby individual pipes  6  of pipe system  5  run in the horizontal butt joints of adjacent layers  24 . 
         [0048]    As already mentioned, only the functional principle of heat accumulator  1  is to be clarified with the type of presentation selected in  FIG. 5 , which is why only a small partial detail of heat accumulator  1  is shown. In reality, prefabricated elements  23 , depending on the size of solid storage medium  2 , extend over the entire width and/or length of solid storage medium  2  or only over a part thereof when a number of prefabricated elements  23  are strung together. The thickness of prefabricated elements  23  corresponds to the vertical distance of individual pipes  6  of pipe system  5 . 
         [0049]    Groove-shaped recesses  25  are formed in the top side of prefabricated elements  23  for receiving individual pipes  6  in the butt joint. Groove-shaped recesses  25  have a rounded bottom and a depth and width somewhat larger than the diameter of individual pipes  6 , which results in a U-shaped cross section of groove-shaped recesses  25 . 
         [0050]    Groove-shaped recesses  25  extend over the entire length of solid storage medium  2 , so that if a number of prefabricated elements  23  are placed one behind the other in the longitudinal direction, groove-shaped recesses  25  run aligned over the entire length. The lateral distance of groove-shaped recesses  25  among one another corresponds to the lateral distance of individual pipes  6 , whereby depending on the width of prefabricated elements  23  a prefabricated element  23  may have up to a plurality of groove-shaped recesses  25 . 
         [0051]    In addition, in the butt joint of two prefabricated elements  23  an upper horizontal heat conducting element  26  and a lower horizontal heat conducting element  27  can be seen, each of which has an thin-walled, planar structure and may include, for example, sheet metal or a graphite film. Heat conducting elements  26  and  27  extend over the entire width and/or length of prefabricated elements  23  or also only over a partial width and/or partial length, whereby in the latter case the stringing together of a number of heat conducting elements  26  and  27  is possible. 
         [0052]    Whereas the upper heat conducting element  26  is formed planar over its entire surface, the lower heat conducting element  27  in the regions assigned to groove-shaped recesses  25  has U-shaped bent areas to form through-shaped seats  28  for individual pipes  6 . In this way seats  28  with their outer circumference fit form-fittingly in groove-shaped recesses  25  of prefabricated elements  23  and with their inner circumference on individual pipes  6 . 
         [0053]    The building of such a solid storage medium  2  occurs by the sequential layering of the individual components, as is shown in  FIG. 5  in the right exploded illustration; the finished state is shown in  FIG. 5  in the left section. In this state, individual pipes  6  are embedded between the upper heat conducting element  26  and lower heat conducting element  27 . It is assured by the weight of overlying layers  24  of solid storage medium  2  that contact is created, on the one hand, between heat conducting elements  26  and  27  and prefabricated elements  23  and, on the other, between heat conducting elements  26  and  27  and individual pipes  6 . Thus, the thermal energy provided in individual pipes  6  can be introduced via heat conducting elements  26  and  27  deep into prefabricated elements  23 , or vice versa for the discharge process. 
         [0054]    The additional embodiment of the invention, shown in  FIG. 6 , corresponds very largely to the embodiment described for  FIG. 5 , so that the statements made in that regard apply. In contrast, the lower horizontal heat conducting element  27 ′ is expanded by a vertical heat conducting bar  29 , which is disposed along an outer surface line of seat  28  of heat conducting element  27 ′ and is fixedly connected to seat  28 . Heat conducting bar  29  thus extends at right angles to the main extension plane of horizontal heat conducting element  27 ′. 
         [0055]    Prefabricated element  23  is formed in a corresponding manner; i.e., it has a vertical slot  30 , which extends from the bottom of groove-shaped recess  25  into prefabricated element  23 , as far as is possible for structural reasons. In the present case, slot  30  extends over half the thickness of prefabricated element  23 . 
         [0056]    It becomes clear from  FIG. 7   a  and  b  that the invention for receiving individual pipes  6  provides not only solutions according to  FIGS. 5 and 6 , where groove-shaped recesses  25  completely receiving individual pipes  6  are arranged in only one of the prefabricated elements  23  forming the joint ( FIG. 7   b ). As  FIG. 7   a  shows, a design, symmetric to the joint plane, of the seats is also within the scope of the invention in which groove-shaped recesses  25 ″, whose depth is slightly more than half of the diameter of individual pipes  6 , are provided both on the lower side of an upper prefabricated element  23  and also on the upper side of a lower prefabricated element  23 . The U-shaped seats  28 ″ of horizontal heat conducting elements  26 ″ and  27 ″, arranged in the joint, fit here form-fittingly into longitudinal grooves  25 ″. As also in the previously described embodiments of the invention, heat conducting elements  26 ″ and  27 ″ advantageously can already be connected form-fittingly to prefabricated element  23  during the production of prefabricated elements  23 , for example, by insertion into the formwork before the concreting. 
         [0057]    The advantage of this embodiment of the invention is that individual pipes  6  of pipe system  5 , after being placed in seat  28 ″ of a lower heat conducting element  27 ″ with their lower circumference form a projection in the butt joint in a lower prefabricated element  23 . After the prefabricated elements  23  of overlying layer  24  are placed on top, thus a centering of the two overlying prefabricated elements  23  occurs via a form fit. A centering of the prefabricated elements can also be achieved by separate form-fitting component in the butt joint, such as, for example, groove bars and female connectors or pin and indentation. 
         [0058]    In the embodiments of the invention according to  FIGS. 5 to 7 , the longitudinal grooves can also be made larger in cross section than the individual pipes running through therein. The resulting gap between the individual pipe and heat conducting element makes it possible to compensate for production- and assembly-related tolerances. For an effective heat transfer between the individual pipe and solid storage medium to be nevertheless assured, the gap is filled with a thermally highly conductive material such as, e.g., ground natural graphite or metal filings or a suitable fluid. 
         [0059]      FIG. 8  discloses another embodiment of the invention in an exploded illustration. A single pipe  6 , from which heat conducting elements  31  extend to the left and right and up and down, is visible in the center. Heat conducting elements  31  can be both surface elements and strip elements and are welded, for example, to the outer circumference of individual pipes  6 . In the quadrants formed thereby, prefabricated elements  32  are inserted along individual pipes  6 ; these can have a bevel at the edge facing individual pipes  6  to assure complete contact between heat conducting elements  31  and prefabricated elements  32 . As an alternative to the use of prefabricated elements  32 , the production of solid storage medium  2  also by in-situ concrete casting is an option here as well. 
         [0060]    The detail of another embodiment of the invention, as shown in  FIG. 9 , has a heat conducting element  33 , which includes a central heat conducting pipe  34 , to which the radially upper and lower and left and right bars  35  are connected. This type of heat conducting element  33  is used advantageously in conjunction with prefabricated elements  32 , whereby it is already concreted during their production and is thus an integral component of prefabricated element  32 . 
         [0061]    Heat conducting pipe  34  surrounds individual pipes  6  of pipe system  5  in a coaxial manner, whereby the annular gap between heat conducting pipe  34  and individual pipe  6  is filled with a thermally conductive material  41  such as, e.g., ground natural graphite or metal filings, to decouple mechanically solid storage medium  2  and pipe system  5  from each other and at the same time to assure the heat transfer from individual pipes  6  to heat conducting element  33 . According to a variation of this embodiment of the invention, this function can also be assumed by fluids with which the annular gap sealed in each case on the front side is filled. Such embodiments of the invention are capable of compensating for dimensional differences between pipe system  5  and solid storage medium  2 , which can greatly facilitate the assembly of heat accumulator  1 . 
         [0062]      FIG. 10  shows an alternative embodiment of the invention, in which prefabricated star-shaped heat conducting elements  36  are attached at axial distances to individual pipes  6 . Heat conducting elements  36  are made up of a cylindrical section  37 , to which the radial bars  38  connect at a uniform angular distance of 45°. Cylindrical section  37  has an inside diameter, which corresponds somewhat to the outer diameter of individual pipes  6 , so that heat conducting elements  36  can be pushed onto individual pipes  6 , before solid storage medium  2  is completed in in-situ concrete casting. 
         [0063]    The particular feature of the embodiment of the invention as shown in  FIG. 11  is the use of planar heat conducting elements  39 , which are equipped with openings  40 , which correspond in size and arrangement to the pattern of individual pipes  6  of pipe system  5 . It is possible as a result to slip heat conducting elements  39  axially onto individual pipes  6 , which can occur either before concreting in the case of in-situ concrete solid storage media, or by sandwich-like insertion of heat conducting elements  36  between two prefabricated elements  41 , as shown in  FIG. 11 . The heat transfer region between individual pipes  6  and heat conducting element  39  is formed by the reveal surfaces of openings  40 , which lie against the outer circumference of individual pipes  6 . 
         [0064]      FIGS. 12   a  and  b  show another embodiment of the invention. A solid storage medium  2  is evident which includes a plurality of concrete prefabricated elements  46 . Concrete prefabricated elements  46  are stacked one above the other in horizontal layers, whereby a planar horizontal heat conducting element  47  is arranged in the butt joints of two overlying layers in each case. This produces a structure of solid storage medium  2 , in which concrete prefabricated elements  46  and heat conducting elements  47  are arranged alternately in the vertical direction. Heat conducting element  47  in this case corresponds in structure and material selection to those described in regard to  FIGS. 1 to 11  and can include, for example, a sheet or a film. 
         [0065]    Concrete prefabricated elements  46  of a horizontal layer have among one another a horizontal lateral distance to the neighboring concrete prefabricated element  46 ; this results in a longitudinal gap  49  aligned in the horizontal direction and extending over the entire height of concrete prefabricated elements  46 . Longitudinal gap  49  is used to receive individual pipes  6  of pipe system  5 , which run at half the height of a longitudinal gap  49  in the middle between the horizontal heat conducting elements  47 . The width of longitudinal gap  49  therefore corresponds at least to the diameter of individual pipes  6 . 
         [0066]    To transfer the thermal energy from individual pipes  6  to horizontal heat conducting elements  47  and vice versa, in each case strip-shaped heat conducting elements  48  which enable a vertical heat transport and whose long side  50 , assigned to individual pipe  6 , is made concave in order to create as great a heat transfer region is possible, are arranged in longitudinal gaps  49 . The opposite long side  51  of vertical heat conducting elements  48  is made planar, in order to form as large a contact region as possible with horizontal heat conducting elements  47 . In cross section, in each case two such heat conducting elements  48  fill longitudinal gap  49  above and below an individual pipe  6 . 
         [0067]    During charging of solid storage medium  2 , therefore, the thermal energy supplied in individual pipes  6  is taken up linearly via vertical heat conducting elements  48  and further fed into the planar horizontal heat conducting elements  47 , where a rapid and extensive distribution of the thermal energy in solid storage medium  2  occurs. Proceeding from heat conducting elements  47 , the supplying of concrete prefabricated elements  46  with thermal energy for its storage then occurs. 
         [0068]    A variation of this embodiment is shown in  FIG. 13   a  and  b . Solid storage medium  2  shown there corresponds substantially in it basic structure with its alternate arrangement of horizontal layers of concrete prefabricated elements  52  and horizontal heat conducting elements  47  to the medium described in  FIG. 12   a  and  b . Solid storage medium  2  according to  FIG. 13   a  and  b , however, differs in that concrete prefabricated elements  52  of a horizontal layer lie against one another with contact on the side; therefore there is no continuous longitudinal gap  49 . Nevertheless, to be able to guide individual pipes  6  through solid storage medium  2 , the opposite long sides of two concrete prefabricated elements  52  in the area of their upper and/or lower longitudinal edges each have an offset  53 . Offsets  53  lying opposite in this way form, in the butt joint of two concrete prefabricated parts  52 , channel  54 , which is intended to receive individual pipes  6  and is open only to horizontal heat conducting element  47 . To improve the heat conduction between individual pipes  6  and horizontal heat conducting elements  47 , heat-conducting molded parts  55  are inserted in channel  54 ; these molded parts with their concave side form a bearing surface for individual pipes  6  and with their opposite planar side a bearing surface toward horizontal heat conducting element  47 . 
         [0069]      FIG. 14  discloses solid storage medium  2  made up of prefabricated elements  56 . To stabilize solid storage medium  2 , prefabricated elements  56  are held together by horizontal prestressing anchors  57 , which extend from the one vertical long side of solid storage medium  2  to the opposite side. To even out the load transfer, load distribution plates  58  are arranged between solid storage medium  2  and the anchor heads of prestressing anchors  57 . 
         [0070]    The effective mode of action of a heat accumulator  1  of the invention compared with conventional heat accumulators comes across clearly in  FIGS. 15   a  and  b .  FIG. 15   a  shows the heat distribution over the cross section of a solid storage medium  2  without the heat conducting elements of the invention during thermal charging. Lines  42  to  45  represent in each case places with the same temperature, also called isotherms. The distance of isotherms  42  to  45  is a measure of the temperature gradient within solid storage medium  2 . An approximately square region is evident, which is surrounded by isotherm  42  and surrounds the central individual pipe  6  and describes the zone with the highest temperature within solid storage medium  2 . The temperature in solid storage medium  2  declines steadily with increasing distance from central individual pipe  6 . Only regions directly adjacent to other individual pipes  6  have local, narrowly limited zones with a higher temperature. 
         [0071]    In contrast, the temperature profile shown in  FIG. 15   b  of a solid storage medium  2  of the invention within isotherm  42  shows an extensive zone of a maximum temperature, which extends over nearly the entire section surrounded by all individual pipes  6 . A temperature drop is determined substantially between outer individual pipes  6  and the surface of solid storage medium  2 , where isotherms  42  to  45  lie relatively close together and thereby indicate a large temperature gradient. It becomes clear as a result that the heat conducting elements of the invention contribute extremely effectively to a rapid and uniform supplying of the solid storage medium with thermal energy. 
         [0072]    Not shown in the drawing but still within the scope of the invention are embodiments of the invention, in which the heat conducting elements includes a paste-like or free-flowing material, for example, of metal filings or metal powder, which is applied like the already described sheets or graphite films in a uniform thickness between two layers of the solid body. These materials have the advantage that with the application of the load from the overlying layers a deformation and adaptation of the heat conducting elements to the surface contour of the layers occur and thus despite possible tolerances a snug butting of the heat conducting elements against the solid storage medium and thereby optimal heat transfer are assured. So that these materials do not escape from the solid storage medium in the edge regions, a sheathing of these materials can be provided. 
         [0073]    According to another embodiment of the invention, which is not shown, it is provided to design the heat conducting elements as a grid structure, which can be achieved in a simple way, for example, by the use of a wire-mesh-like network. Here as well, an automatic adaptation to possible irregularities occurs in the butt joint during the placement of two prefabricated parts one on top of another. Furthermore, the manageability and economy of the invention can be increased further with the saving of weight and materials. 
         [0074]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.