Abstract:
A device is provided for the temporary storage of thermal energy with a solid storage medium and a pipe system made of individual pipes. The pipe system extends through the solid storage medium and an energy transfer medium flows through the pipe system. The solid storage medium and the pipe system being mechanically decoupled from one another.

Description:
[0001]    This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application Nos. 10 2008 026 017.7, which was filed in Germany on May 30, 2008, and to 10 2008 047 557.2, which was filed in Germany on Sep. 16, 2008 and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a device and system for storing 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 or 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 possibility of temporarily storing energy accumulating at a certain time and being able 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]    For the low-temperature range to about 100° C., heat accumulators in the form of water reservoirs are known. Water as a heat-storing medium is notable for its high specific heat capacity and low cost. A disadvantage, however, is the rapid increase in vapor pressure at temperatures above 100° C., which necessitates costly pressure vessels. For this reason, liquids with a higher boiling point are used for higher temperature ranges, such as, for example, oils, which is associated with a considerable increase in cost, however. 
         [0007]    Apart from liquid accumulators, solid storage media are also known, which may include, for example, of mineral fill materials but also of steel, cast iron, fireclay, and the like. Whereas in the case of steel and cast iron the high cost for the storage material is a disadvantage, the other solids have only a limited available performance due to the low heat conductivity. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the present invention to provide a thermal storage medium with as high a heat capacity as possible and a high charging and discharging capacity, which permits both reliable and economic storage of thermal energy. 
         [0009]    The invention is based on the idea of storing thermal energy in a solid storage medium. The loading of the thermal energy into the solid storage medium occurs via a gaseous or liquid energy transfer medium, which is carried within a pipe system. 
         [0010]    Concrete can be used as the material for the solid storage medium, because it is generally and economically available and complex forms can also be produced with concrete. It is possible further to adapt the concrete of the solid storage medium to specific requirements with use of suitable aggregate, cement, additives, admixtures, and incorporation of additives. 
         [0011]    For example, the harmless removal of water released when the concrete of the solid storage medium is heated can be promoted by providing structural measures. According to embodiments of the invention, the following options are proposed here, which may be used alternatively or cumulatively: 
         [0012]    A first option is the production of concrete for the solid storage medium with a water-cement value of 0.5 or higher, as a result of which the permeability of the concrete storage medium is increased. 
         [0013]    Another option is the addition of fibers made of meltable materials such as, for example, wax or suitable plastics, which melt during the first startup of the heat accumulator and increase the permeability of the concrete with the remaining hollow spaces. 
         [0014]    To remove water vapor it is also possible during concreting of the solid storage medium to provide areas with an increased permeability or flow channels, which by taking up and removing the vapor lead to stress relief within the solid storage medium. 
         [0015]    Another option for creating hollow spaces in the solid storage medium arises with the use of molded articles of meltable materials, such as, for example, waxes or suitable plastics. These melt during the first startup of the heat accumulator and in this way leave behind local vapor flow and vapor relaxation zones. 
         [0016]    Another embodiment provides for the concreting of molded articles, which have a greater permeability compared with the concrete of the other solid storage medium. Such molded articles, for example, sand-filled textile tubes or precast concrete parts made of single-grain concrete, can remain permanently in the solid storage medium and there also cause removal or release of the vapor. 
         [0017]    An at least partially increased permeability of the solid storage medium makes it possible to dimension the medium larger, without a too high concrete-damaging vapor pressure developing due to the associated increase in the flow path. 
         [0018]    The loading of the thermal energy into the solid storage medium occurs via a gaseous or liquid energy transfer medium, which is carried within a pipe system. The individual pipes forming the pipe system in this case penetrate the solid storage medium in an axis-parallel position, so that an energy input uniform over the cross section of the solid storage medium is achieved. This type of pipe system can be prefabricated as a pipe register or produced on site. After the formwork is set up, the pipe register is concreted, whereby mechanical decoupling of the pipe register and solid storage medium is achieved simultaneously by means of measures described hereinafter. This produces a simple and very cost-effective production of the heat accumulator of the invention. 
         [0019]    To achieve the most uniform input possible of thermal energy into the solid storage medium, the individual pipes of the pipe system within the solid storage medium can run in a predefined pattern, in which the individual pipes are arranged in plane-parallel levels. The levels in this case can be congruent in regard to the position of the pipes or be arranged with a lateral offset from level to level. Spacers are used to assure placement of the pipes at a uniform distance. 
         [0020]    According to an embodiment of the invention, the pipe system at its two longitudinal ends is each closed with a front plate, which is provided with through-openings for the individual pipes. In this case, a defined free space, which due to constraints prevents temperature-induced different expansions, is maintained between the front plates and the front faces of the solid storage medium 
         [0021]    The distribution or collection of the energy transfer medium on the outside of the front plates after its entry into or exit from the individual pipes is preferably effected by half-shells, which are welded along their longitudinal edges with the front plates with the formation of channels and in this case comprise several through-openings in each case. 
         [0022]    Another principle of the invention is to decouple mechanically the solid storage medium from the pipe register by providing a gap or a sliding layer between these two elements. This succeeds in preventing constraints, which can lead to damage up to failure of the heat accumulator due to different thermal exposures and different coefficients of expansion for the solid storage media and the pipe register. The following variants are considered for generating a gap or a sliding layer: 
         [0023]    Coating, sheathing, or wrapping of the pipes of the pipe register with melting materials, e.g., polyethylene, polypropylene, zinc, or tin. 
         [0024]    Due to the relatively low melting points (e.g., polyethylene about 110° C.; polypropylene about 160° C.), decoupling occurs between the pipe and concrete even at the start of the heating, in that the melting materials first enter a plasticized transitional state, which already enables relative movements between the pipe register and the solid storage medium, before they leave behind a gap between the pipe and solid storage medium after achieving the liquid aggregate state. To assure a high heat transfer despite the gap, it is advantageous to select the thickness of the gap as small as possible, for example, between 0.1 mm and 0.5 mm, preferably 0.2 mm. The coating of the pipes can occur economically with large-scale systems. 
         [0025]    Installation of a slide layer between the pipes of the pipe register and the concrete of the solid storage medium, e.g., coating with graphite-containing material. 
         [0026]    In this variant, to assure optimal heat transfer from the pipe to the solid storage medium the gap is filled with a solid, highly heat-conducting material, e.g., a graphite-containing material. This material is notable, on the one hand, for its high heat conductivity and, on the other, for its low friction coefficient and its formability. One succeeds in this way to combine initially contradictory requirements for mechanical decoupling, on the one hand, and optimal heat transfer, on the other, in order to attain the object. Expanded graphite is used with particular advantage in regard to the manufacture of a heat accumulator according to the invention. This can be applied, for example, in film form as a coating to the pipe system. Fixation to the pipe system can occur by means of a self-adhesive back of the graphite film, application of an adhesive to the pipe and/or to the graphite film, or by means of additional external fixation. The gap between the solid storage medium and pipe system is completely filled by subsequent concreting of the thus prepared pipe system. The thickness of the coating can be, for example, 0.2 mm to 3 mm, preferably 0.5 mm. 
         [0027]    To keep heat losses as low as possible, the heat accumulator can be surrounded by thermal insulation. 
         [0028]    It is possible according to the invention to form greater storage units with greater storage capacities to connect heat accumulators of this type module-like in series and/or parallel. This has the advantage that adaptations can be made to different storage volumes even during planning but also afterwards by varying the number and size of the individual heat accumulators. In this embodiment of the invention, the formation of the collector and distributor between the individual heat accumulators can be omitted and instead the individual pipes of two pipe registers, connected in series, are connected with appropriately curved connection pipes or with special connection tubes, which compensate for the expansions of the register pipes during temperature variations. 
         [0029]    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 
         [0030]    In the figures: 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: 
           [0031]      FIG. 1  shows an oblique view of a heat accumulator of the invention; 
           [0032]      FIG. 2  shows a longitudinal section through the heat accumulator shown in FIG.  1 ,; 
           [0033]      FIG. 3  shows a cross section through the heat accumulator shown in  FIG. 2  along line III-III; 
           [0034]      FIG. 4  shows a front view of the heat accumulator shown in  FIGS. 1 and 2 ; 
           [0035]      FIG. 5  shows a detail of the cross section shown in  FIG. 3 ; 
           [0036]      FIG. 6  shows a detail of the longitudinal section shown in  FIG. 2 ; 
           [0037]      FIG. 7  shows an oblique view of an aggregate accumulator; and 
           [0038]      FIG. 8  shows a detail of the connection of two individual heat accumulators connected in series. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]      FIG. 1  shows a heat accumulator  1  of the invention in an oblique view, and  FIGS. 2 to 4  in the associated sections and a front view. An element of the heat accumulator  1  is a rectangular solid storage medium  2  with a considerable longitudinal direction, whose longitudinal ends are formed by front faces  3  and  4 . Solid storage medium  2  is fabricated of concrete, which can occur by on-site concrete casting or with precast concrete parts. 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. 
         [0040]    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  over an average longitudinal section. Individual pipes  6  in this case extend beyond front faces  3  and  4  with the formation of a projection. 
         [0041]    As is evident primarily from  FIG. 3 , individual pipes  6  can be arranged in a plurality of horizontal levels, lying plane-parallel one over another and arranged substantially equidistantly, whereby individual pipes  6  of two neighboring 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 . 
         [0042]    To maintain the above-described pattern over the entire length of individual pipes  6 , spacers 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 the spacers corresponding to the predefined pattern and are used for fastening individual pipes  6 . At greater longitudinal distances, individual steel mats  8  are reinforced and supported in addition by profile frames  9  ( FIGS. 1 ,  2 , and  3 ). 
         [0043]    Individual pipes  6  end as already described in cross-sectional levels, which run at a clearance 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  10  according to the pattern of individual pipes  6 , are arranged in these cross-sectional levels, therefore plane-parallel to front faces  3  and  4 . 
         [0044]    It is evident from  FIGS. 5 and 6  that individual pipes  6  on the back of front plates  11  and  12  open into openings  10  and are there closely welded with front plates  11  and  12 . A plurality of half-shells  13 , which are welded in the horizontal position and axis-parallel to their longitudinal edges  14  with front plates  11  and  12 , are provided on the opposite front side of the front plates  11  and  12 , so that half-shells  13  together with front plates  11  and  12  form horizontal channels  15 . The relative position of half-shells  13  is selected in this case so that individual pipes  6  of two overlying levels open via openings  10  into a single channel  15 . 
         [0045]    In an embodiment, all channels  15  can be connected via connecting pipe sections  16  to a distributor  17  or collector  18 , each of which is equipped with a pipe connection  19  for the supplying or discharging of heat accumulator  1  ( FIG. 2 ). 
         [0046]    For a simplified and economic manufacture of heat accumulator  1 , individual pipes  6 , steel mats  8 , profile frames  9 , front plates  11  and  12 , half-shells  13 , pipe sections  16 , distributors  17 , collectors  18 , and pipe connections  19  are prefabricated as a pipe register. Heat accumulator  1  is then manufactured in a simple manner by setting up the formwork and concreting of solid storage medium  2 . 
         [0047]    So that, in this case, no frictional connection forms between the concrete of solid body  2  and individual pipes  6 , a gap  20  is provided for mechanical decoupling between individual pipes  6  and solid body  2 ; said gap can remain free or, as shown in the present exemplary embodiment, be filled with a solid, highly heat-conducting, and moldable material  21  ( FIG. 5 ). In the present example, the solid, highly heat-conducting, and moldable material  21  includes expanded graphite in film form, which has self-adhesive properties on one side and thus can be applied before the concreting in a simple manner by tightly wrapping on the outer circumference of individual pipe  6 . During introduction of the concrete for solid body  2 , therefore a form closure to the solid, highly heat-conducting, and moldable material  21  occurs. It is assured in this way that relative movements between individual pipes  6  and solid storage medium  2  are not impeded, but simultaneously optimal heat transfer from individual pipes  6  to solid storage medium  2  is maintained. 
         [0048]    Heat accumulator  1  described in  FIGS. 1 to 6  can be used both as a single accumulator to take up thermal energy and as part of an aggregate accumulator built in a modular manner. This type of aggregate accumulator  22 , shown in  FIG. 7 , therefore has a plurality of individual heat accumulators  1 , which are connected in series or parallel and through which the heat transfer medium flows from left to right in the figure. The storage capacity of aggregate accumulator  22  is adapted to predefined requirements by suitable selection of the size and/or number of heat accumulators  1 . In the present example, aggregate accumulator  22  includes nine parallel storage lines  23 , which are loaded in parallel and are each formed by seven heat accumulators  1  connected in series. 
         [0049]    Two neighboring heat accumulators  1  of a storage line  23  are connected to one another, as shown in detail in  FIG. 8 . In this case, the individual pipes  6 , assigned to one another, of the two pipe registers are each connected by means of pipe elbows  24  or tubes bent twice at about 90 degrees. Expansions of individual pipes  6  during temperature variations are compensated by the S-form of pipe elbows  24  or the flexibility of the tubes. It is possible with this type of construction to omit the construction of collectors and distributors between the respective heat accumulators  1 . Distributor  17  is present only at the beginning and a collector  18  at the end of a line  23  ( FIG. 7 ). 
         [0050]    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.