Patent Publication Number: US-2022228813-A1

Title: Thermal energy storage device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/EP2020/067645, having a filing date of Jun. 24, 2020, which claims priority to EP Application No. 19183257.5, having a filing date of Jun. 28, 2019, the entire contents both of which are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The following relates to a storage device for storing thermal energy. 
     BACKGROUND 
     It is known to store fluctuating electrical energy as heat inside heat storages. The electrical energy may be generated in renewable and/or traditional power plants running on fossil fuels. The electrical energy from such plants is stored in heat storages when the electricity demand is low. The stored heat is reconverted back to electrical energy in times when the demand is higher than the production. The heat storages are usually part of thermal energy storage plants which typically further comprise a heater, a steam generator, a steam turbine, a heat transporting fluid, a storage material inside the heat storage and a piping system. The storage material may be a granular material, for example comprising a plurality of stones. The granular material is housed inside a hollow housing extending between an inlet and an outlet. The inlet and an outlet need to be open to allow the flowing of a heat transporting fluid, which exchanges heat with the granular material. It is known to provide grated structures at the inlet and outlet of the heat storage to contain the granular material inside the hollow housing of the heat storage. The thermo-mechanical forces originating from the storage material may require thick and heavy grated structures to be constructed to withstand such forces and contain the storage material inside the hollow housing. Thick and heavy grated structures may be associated with undesired manufacturing complexity, costs and weight of the heat storage. 
     There may be a need for providing a heat storage device where the thermo-mechanical forces originating from the storage material are limited as much as possible to avoid the above-mentioned inconveniences. 
     SUMMARY 
     An aspect relates to a heat storage for a thermal energy storage plant, the heat storage comprising:
         a hollow housing comprising an inlet and an outlet,   a granular material for storing heat housed in the hollow housing between the inlet and the outlet,   the hollow housing defining a fluid passage for the circulation of a heat transporting fluid between the inlet and the outlet and through the granular material,   wherein the granular material subject to the gravity force forms at least one free surface respectively facing the inlet or the outlet the at least one free surface including a border in contact with the hollow housing and being inclined with respect to the gravity direction, the respective inlet or outlet being with respect to the gravity direction at a higher level than a lowest point of the at least one free surface.       

     As “granular material” it is meant any conglomerate of discrete solid elements or particles, for example stones or rocks, having a convenient thermal capacity for storing thermal energy at a desired temperature range. The discrete solid elements which constitute the granular material may a spheroidal shape or polyhedral shape, for example comprising a plurality of flat surfaces and/or curved surface. The type, shape and dimensions of the discrete solid elements which constitute the granular material may be chosen to achieve the desired level of friction between such solid elements. This may permit to control expansion and contraction of the granular material, for example during thermal exchanges with the heat transporting fluid. As “heat transporting fluid” it is meant any suitable fluid for transporting thermal energy, for example air. 
     The heat storage according to embodiments of the present invention is shaped in such a way that the granular material arranges itself near the inlet and the outlet in a naturally forming heap angle, preventing the granular material subject to the gravity force to exit the hollow housing through the inlet and/or the outlet. The granular material weight is supported by the hollow housing so that no grates are required to contain the granular material between the inlet and the outlet of the heat storage. The geometry of the heat storage prevents the granular material subject to the gravity force to exit the hollow housing through the inlet and/or the outlet in all conditions, including thermal expansion of the granular material towards the inlet and/or the outlet. 
     The fluid passage comprises at least a first portion crossing the inlet and a last portion crossing the outlet. According to embodiments of the present invention, any of the first or last portions of the fluid passage may orthogonal or parallel to the gravity direction or inclined with respect thereto. 
     According to embodiments of the present invention, the hollow housing comprises at least a bottom wall and a top wall, the top wall being with respect to the gravity direction at a higher level than the bottom wall, the at least one free surface extending between the lowest point in contact with the bottom wall and a highest point in contact with the top wall. The bottom wall may have a higher curvature than the top wall. When seen from the inside of the heat storage, the bottom wall may be substantially convex, i.e., bent towards the outside of the housing, and the top wall may be also substantially convex or planar. According to other embodiments of the present invention, when seen from the inside of the heat storage, the bottom wall may be substantially convex and the top wall may be substantially concave, i.e., bent towards the inside of the housing. 
     According to embodiments of the present invention, the top wall comprises a flexible foil. A flexible foil can compensate expansions or contractions of the granular material. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a schematic sectional view of a heat storage, according to a first exemplary embodiment of the present invention; 
         FIG. 2  shows a detailed view of the heat storage of  FIG. 1 ; 
         FIG. 3  shows a detailed view of a heat storage, according to a second exemplary embodiment of the present invention; 
         FIG. 4  shows a schematic sectional view of a heat storage, according to a third exemplary embodiment of the present invention; 
         FIG. 5  shows a schematic sectional view of a heat storage, according to a fourth exemplary embodiment of the present invention; 
         FIG. 6  shows a schematic sectional view of a heat storage, according to a fifth exemplary embodiment of the present invention; 
         FIG. 7  shows a schematic sectional view of a heat storage, according to a sixth exemplary embodiment of the present invention; and 
         FIG. 8  shows a schematic sectional view of a heat storage, according to a seventh exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  schematically show a heat storage  100  for a thermal energy storage plant (not shown as whole). The heat storage  100  comprises a hollow housing  170  comprising an inlet  101  and an outlet  102  and a granular material  160  for storing heat. The granular material  160  is housed in the hollow housing  170  between the inlet  101  and the outlet  102 . The granular material comprises a plurality of discrete solid elements or particles, for example stones or rocks, having a convenient thermal capacity for storing thermal energy at a desired temperature range. The granular material  160  occupies at least a portion of the volume of the hollow housing  170  comprised between the inlet  101  and the outlet  102 . The hollow housing  170  defines a fluid passage for the circulation of a heat transporting fluid between the inlet  101  and the outlet  102  and through the granular material  160 . The fluid passage comprises a first portion  111  crossing the inlet  101 , an intermediate portion  113  crossing granular material  160  and a last portion  112  crossing the outlet  102 . The intermediate portion  113  is oriented according to a horizontal or substantially horizontal direction, i.e., orthogonal or substantially orthogonal to the gravity direction. The first portion  111  and the last portion  112  are also both oriented according to a horizontal or substantially horizontal direction, i.e., orthogonal or substantially orthogonal to the gravity direction. The hollow housing  170  comprises a bottom wall  171  and a top wall  172  joined together, in order to laterally, i.e., in a direction orthogonal to sections in the attached figures, contain the granular material  160 . The top wall  172  is with respect to the gravity direction at a higher level than the bottom wall  171 . The bottom wall  171  and the top wall  172  are both substantially convex, when seen from the inside of the heat storage  100 . The bottom wall  171  may have a higher curvature than the top wall  172 . According to other embodiments of the invention (not shown), the bottom wall  171  is substantially convex and the top wall  172  is substantially concave, when seen from the inside of the heat storage  100 . The bottom wall  171  collects and supports the weight of the granular material  160 . In a direction transversal to the fluid passage  111 ,  112 ,  113 , the inlet  101  and the outlet  102  are closer to the top wall  172  than to the bottom wall  171 . In the embodiment of  FIGS. 1 and 2  the inlet  101  and the outlet  102  are intermediate, along the direction transversal to the fluid passage  111 ,  112 ,  113 , between the top wall  172  than to the bottom wall  171 . 
     In a direction transversal to the fluid passage  111 ,  112 ,  113 , the granular material  160  extends in the hollow housing  170  between a bottom surface  163 , in contact with the bottom wall  171  and a top surface  164 , which may be in contact with the top wall  172 . The granular material  160  subject to the gravity force forms a first free surface  161  and a second free surface  162 , respectively facing the inlet  101  and the outlet  102 . Each of the two free surfaces  161 ,  162  includes a border A-B in contact with the hollow housing  170 . The border A-B is a closed line of which only the linear projection is visible in attached figures. The border A-B may be circular or include on or more curved or linear edges, depending on the shape of the hollow housing  170  on a sectional view transversal to the ones of the attached figures. The border A-B is inclined, i.e., not parallel, with respect to the gravity direction. Following the gravity force G the discrete solid elements or particles which constitute the granular material  160  naturally form the two free surfaces  161 ,  162  inclined according to a heap angle W with respect to a horizontal direction X. During charging, i.e., while the granular material  160  receives heat, and discharging, i.e., while heat is transferred from the granular material  160 , of the heat storage  100  the granular material  160  may expand and contract and the heap angle W may change. In situations where the granular material  160  slides and the heap angle W begins to flatten the shape of the hollow housing  170 , in particular close to the inlet  101  and the outlet  102 , can still provide an accommodation for the granular material  160 . The border A-B of each of the two free surfaces  161 ,  162  extends, with respect to the gravity direction, between a lowest point A and the highest point B. The lowest point A is in contact with the bottom wall  171  and a highest point B is in contact with the top wall  172  The inlet  101 , with or respect to the gravity direction, is at a higher level than the lowest point A of the first free surface  161 . The outlet  102 , with respect to the gravity direction, is at a higher level than the lowest point A of the second free surface  162 . The relative positions along the gravity direction between the border A-B of each of the two free surfaces  161 ,  162  and the inlet  101  and the outlet  102 , respectively, prevent the granular material subject to the gravity force to exit the hollow housing through the inlet  101  and the outlet  102 . The granular material weight is supported by the bottom wall  171  of the hollow housing  170 . The weight forces F are directed towards the bottom wall  171  and not towards the two free surfaces  161 ,  162 . In the embodiment of  FIGS. 1 and 2 , the portion of the volume of the hollow housing  170  occupied by the granular material  160 , i.e., comprised between the bottom wall  171 , the top wall  172  and the two free surfaces  161 ,  162  forms a structure with substantially convex top and bottom parts. 
       FIG. 3  schematically shows another embodiment of the heat storage  100 . The embodiment of  FIG. 3  differentiates itself from the previous one in that the top wall  172  comprises a flexible foil. 
       FIG. 4  schematically shows a further embodiment of the heat storage  100 . The embodiment of  FIG. 4  differentiates itself from the embodiment of  FIGS. 1 and 2  in that:
         the top wall  172  is planar;   the inlet  101  and the outlet  102  are, along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than the top wall  172  and the bottom wall  171 ;   the first portion  111  and the last portion  112  of the fluid passage are both inclined with respect to the gravity direction of two respective angles respectively comprised between 0° and 90° and between 90° and 180°;   the two free surfaces  161 ,  162  are parallel to the horizontal direction X, i.e., orthogonal to the gravity direction, and are along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than both the top wall  172  and the bottom wall  171 .       

     In such embodiment, the portion of the volume of the hollow housing  170  occupied by the granular material  160 , i.e., comprised between the bottom wall  171 , the top wall  172  and the two free surfaces  161 ,  162  forms a structure with a substantially convex bottom part and a substantially concave top part. 
       FIG. 5  schematically shows yet another embodiment of the heat storage  100 . The embodiment of  FIG. 5  differentiates itself from the embodiment of  FIGS. 1 and 2  in that:
         the top wall  172  is planar;   the inlet  101  and the outlet  102  are, along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than the top wall  172  and the bottom wall  171 .       

       FIG. 6  schematically shows yet another embodiment of the heat storage  100 . The embodiment of  FIG. 6  differentiates itself from the embodiment of  FIGS. 1 and 2  in that:
         the top wall  172  is planar;   the inlet  101  and the outlet  102  are, along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than the top wall  172  and the bottom wall  171 ;   the first portion  111  and the last portion  112  of the fluid passage are both parallel to the gravity direction.       

       FIG. 7  schematically shows yet another embodiment of the heat storage  100 . The embodiment of  FIG. 7  differentiates itself from the embodiment of  FIGS. 1 and 2  in that:
         the top wall  172  is planar;   the inlet  101  is, along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than the top wall  172  and the bottom wall  171 ;   the first portion  111  of the fluid passage is parallel to the gravity direction.       

       FIG. 8  schematically shows yet another embodiment of the heat storage  100 . The embodiment of  FIG. 8  differentiates itself from the embodiment of  FIGS. 1 and 2  in that:
         the top wall  172  is planar;   the outlet  102  is, along the direction transversal to the fluid passage  111 ,  112 ,  113 , at a higher position than the top wall  172  and the bottom wall  171 ;   the last portion  112  of the fluid passage is parallel to the gravity direction.       

     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.