Abstract:
A heat regenerator comprising at least one matrix made of a refractory material, to ensure the storage and recovery of heat. The matrix includes at least one through-channel capable of enabling the circulation of a fluid, said channel comprising at least two projections projecting into the space defined by said channel, said projections being positioned on two opposite surfaces of the channel.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a heat regenerator enabling to store and to recover heat. 
         [0002]    One of the fields of use of the present invention especially relates to installations enabling to store electric or solar energy in the form of heat. 
       BACKGROUND 
       [0003]    The short-, medium, or long-term storage of significant quantities of energy is a major issue, which has given rise to numerous studies relative, in particular, to the storage of electricity in the form of thermal energy. 
         [0004]    For this purpose, regenerators are currently used to temporarily store heat prior to its use in various applications. Typically, heat regenerators comprise enclosures where heat can be stored by heat transfer between a fluid such as smoke and the material of the elements forming the enclosures. These solid structures are crossed by channels through which smoke can circulate. The cross-section of these channels may vary according to the nature of the smokes, from a few millimeters to several tens of centimeters. Indeed, in the case of the recovery of heat conveyed by flue gases, the gases originating from dirty processes (glass furnaces, blast furnaces) require channels having a larger cross-section than gases originating from relatively clean processes such as gas turbines. 
         [0005]    Regenerators are generally defined according to the following parameters:
       the maximum heat storage capacity. This parameter is linked to the mass and to the physical properties of the material forming the enclosures or matrixes, and to the difference between the hot temperature and the cold temperature of operation of the regenerator;   the characteristic cycle time, which depends on the thickness of the solid partitions and of the channels forming the matrixes;   the thermal performance, that is, the ratio of the real storage capacity of the regenerator to its maximum capacity, which depends on the thermal transfer performance between the fluid and the matrix.       
 
         [0009]    Document FR 2916101 describes an electric energy storage method implementing the transformation of electric energy into heat stored in two enclosures of heat regenerator type. It comprises a thermodynamic cycle of heat pump type in storage mode, and of heat engine type in recovery mode, using argon as a fluid. This device causes practically no head loss during the fluid circulation and the heat exchange. The operating constraints due to such an electric energy storage installation imply storage/recovery times of a few hours and fast response times for the materials. In this device, the fluid crosses the regenerator via channels having a constant cross-section along their entire height. Such channels comprise no projections. 
         [0010]    Generally, three parameters can enable to improve the efficiency of a regenerator:
       the physical properties of the material storing thermal energy. It is the product of the density of the material by its specific heat. Generally, the optimal properties correspond to relatively expensive materials.   the porosity of the material storing thermal energy, that is, the volume left to the fluid within this material.   the thermal performance of the material storing thermal energy. This enables to promote an optimal use of the maximum storage capacity of the material.       
 
         [0014]    The object of the present invention falls within the framework of this last point relative to the improvement of heat exchanges. Further, the device developed by the Applicant also enables to considerably decrease the dimensions of heat regenerators. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    The present invention relates to a heat regenerator capable of being used in heat storage/recovery processes. It particularly enables to decrease the volume of regenerators while providing the same storage capacity as prior art regenerators. 
         [0016]    More specifically, the present invention relates to a heat regenerator comprising at least one matrix made of a refractory material, in order to ensure the storage and recovery of heat, said matrix comprising at least one through channel allowing the circulation of a fluid, said channel comprising at least two projections projecting into the volume defined by said channel. 
         [0017]    Said projections are positioned on two opposite surfaces of the channel. 
         [0018]    Generally, the refractory material has a density advantageously in the range between 2,000 and 10,000 kg/m 3 . 
         [0019]    Advantageously, the at least one channel is formed of a succession of repeated patterns, whereby the projections are preferably arranged at fixed intervals in the fluid circulation direction within the channel. 
         [0020]    The regenerator according to the invention may comprise many discretized elements in the form of bricks and forming the matrix of refractory material, said bricks being crossed by at least one channel comprising at least one projection projecting into the volume defined by said channel. 
         [0021]    The regenerator according to the invention may comprise several identical reference bricks to keep the periodicity of the projections within the channel(s), said bricks being adjacent to one another to optimize the density of material without creating spaces. The regenerator may further comprise a stack of bricks. The stacking of the brick patterns is performed so that the channels are aligned with one another, to avoid altering the fluid circulation. Further, the brick may have a square, rectangular, hexagonal, or cylindrical cross-section. It will be within the abilities of those skilled in the art to determine means for advantageously attaching the bricks together. 
         [0022]    “Projection” means that the channel is not through in its entire cross-section along its entire length. The channel thus comprises at least two internal structures which obstruct part of its cross-section at least along part of its height. Said projections are made of a material identical to that of the matrix, and thus of the brick. They are advantageously integral with the matrix. In other words, in the brick or the matrix, the channel and the projections advantageously form a unit element. Indeed, it is important for the projections to be in contact with the heat-storing brick to allow a transfer of the heat received by the projections to the larger mass of the matrix. 
         [0023]    The projection may be parallelepipedal, and in particular have one dimension advantageously identical to the width of the at least one channel of the matrix or of the brick. 
         [0024]    Advantageously, the main dimension of the parallelepiped, and for example its length, is directed parallel to the fluid circulation direction, and thus to the main axis of the channel. 
         [0025]    The projections may also be trapezoidal, the longer side of said trapezoid being confounded with the lateral wall of the channel. Here again, one of the dimensions of the trapezoid is advantageously identical to the channel width. 
         [0026]    The projections present inside of the channels intensify heat transfers between the matrix and the fluid without for all this significantly altering the circulation of said fluid. Indeed, considering the configuration of the channels and of the projections according to the present invention, a low head loss is desired over the entire regenerator. “Head loss” means the pressure difference between the fluid pressure at the inlet of the regenerator and the fluid pressure at its outlet, and thus after having crossed the matrix. In other words, it designates the head loss after the fluid has passed through the regenerator, which may comprise a matrix of several bricks. 
         [0027]    The projections enable to create turbulence by disturbing the fluid circulation, and thus to intensify heat transfers. Indeed, in the case of a circulation in a channel comprising no projections, the only way to increase the heat exchange is to decrease the hydraulic diameter of the channel, thus resulting in channel continuity issues all along the height of the brick stack. Thus, the projections of the invention generate high-speed areas, direction change areas, and recirculation areas appear. Pressures losses are intensified as a consequence of the creation of such turbulence. 
         [0028]    According to a preferred embodiment, and to optimize the compactness of the regenerator relative to its thermal performance, the volume occupied by the fluid is in the range from 30 to 50% of the total volume of the matrix. It is the vacuum volume of the matrix, except for the volume of the pores of the refractory material, when said material comprises pores. 
         [0029]    When the volume occupied by the fluid is smaller than 30%, the solid mass is very large, and provides the matrix with a maximum energy storage capacity. However, heat transfers are more difficult and the real capacity in operation is much lower than this maximum value. 
         [0030]    When the volume occupied by the fluid is greater than 50%, the compactness of the regenerator and thus of the enclosure containing it is insufficient, and by all means incompatible with the envisaged applications. 
         [0031]    The channel(s) may advantageously have a rectangular or square cross-section. In this specific case, the channel width is preferably in the range between 4 and 15 millimeters, and its length is preferably in the range between 4 and 15 millimeters. The channel height depends on the regenerator height and thus on the quantity of energy to be stored. 
         [0032]    Advantageously, the regenerator height is between 5 and 50 meters. The matrix and the channel may have a height between 5 and 50 meters. 
         [0033]    As already mentioned, the channel comprises at least two projections advantageously positioned to be shifted in the fluid circulation direction and thus along the channel height, one of the projections being positioned on a first surface of the channel, and the other on a second opposite surface of the channel. More advantageously still, the channel comprises two projections respectively positioned on two opposite surfaces of the channel, so as to be shifted with respect to each other, in the general fluid flow direction within the channel. 
         [0034]    According to the invention, there is a plurality of projections along a channel crossing the regenerator, advantageously more than 100. 
         [0035]    “Channel” also means the channel formed when two bricks are stacked. The channel runs all throughout the matrix height. 
         [0036]    According to an advantageous embodiment, when the brick or the regenerator comprises at least two channels, the latter are substantially parallel. Further, the channels are advantageously separated by a distance between channels in the range between 2 and 15, more advantageously still equal to 4 millimeters. 
         [0037]    According to a specific embodiment, in the case where the heat regenerator comprises at least two adjacent bricks, the distance between the channels of a first brick and the channels of a second brick is preferably in the range from 2 to 15 millimeters, and more advantageously still equal to 4 millimeters. 
         [0038]    In a preferred embodiment, the heat regenerator has one and the same inter-channel distance between channels of a same brick and between channels of two different bricks, the bricks being adjacent. 
         [0039]    The distance between channels depends on the storage and recovery time, given that the entire matrix thickness has to rise from the cold temperature to the hot temperature, and conversely. If the thickness is too large, a temperature gradient appears between the surface and the core of the matrix. In this case, the storage capacity decreases, since the refractory material cannot achieve a complete cycle between the two operating temperatures. If the thickness is too low, there is not enough refractory material to store the thermal energy. 
         [0040]    Typically, the internal projections extend across the entire width of the channel when it has a square or rectangular cross-section. In the case of a cylindrical channel, the projections advantageously extend on one quarter of the perimeter of the channel cross-section. The height of the projections advantageously amounts to from 5 to 50% of the channel dimension along which they are directed. The ratio of the height of the projections to the channel length is advantageously between 0.05 and 0.5, and more advantageously between 25% and 35%. 
         [0041]    It should be noted that the two dimensions do not extend along the same direction. Further, in the regenerator according to the invention, distance P between two successive projections on a same side of a channel wall advantageously amounts to between one and five times the length of channel L. Advantageously, distance P between projections is constant along the entire height of the channel crossing the matrix and the regenerator, thus creating a repetitive channel portion pattern, repeated along the entire height of the matrix and of the regenerator. It should also be noted that distance P between projections comprises the length of a projection. 
         [0042]    Thus, according to a development of the invention, the regenerator comprises a matrix comprising a plurality of through channels, each channel being provided with a plurality of projections respectively originating from a first wall and from the wall opposite to the first wall of a channel, each projection being spaced apart from the next one by a fixed distance P to form a projection pattern repeated along the channel, along the regenerator height. 
         [0043]    “Plurality of channels” means from 10 to 100,000 channels, especially from 100 to 10,000 channels. 
         [0044]    The length of the projections along the fluid circulation direction, that is, along the regenerator height, is adjusted to obtain a geometric shape adapted to the manufacturing process, while providing a sufficient contact surface area between the projections and the channel wall to transfer the heat exchanged by conduction. It advantageously is in the order of the channel width. 
         [0045]    Advantageously, the brick and the projections are made of a non-porous refractory material, advantageously of ceramic based on aluminum or cordierite ceramic. 
         [0046]    Further, the non-porous refractory material advantageously has no or almost no porosity, preferably below 5%. 
         [0047]    The present invention also relates to the use of a heat regenerator such as described hereabove in a heat storage and recovery installation comprising at least one fluid admission orifice. Further, the invention also relates to a heat storage and recovery installation comprising at least one heat regenerator such as described hereabove. Said heat may advantageously originate from electric power or from concentrating solar power. 
         [0048]    In a particularly preferred embodiment, in said heat storage and recovery installation, at least the lateral portions and the bottom of the heat regenerator are covered with a thermal insulator layer. 
         [0049]    This installation may further comprise a grid interposed between the fluid inlet portion and the openings of the channels of the heat regenerator. Said grid provides a better distribution of the fluid between the channels. 
         [0050]    In a preferred embodiment, the heat regenerators according to the present invention are used in a method for storing electric power in the form of heat involving a perfectly clean neutral gas rather than fouling and corrosive flue gases, such as are likely to come out of blast furnaces or of glass furnaces. Thus, the use of a neutral gas such as argon as a fluid enables to decrease the dimensions of the bricks of the heat regenerator. 
         [0051]    Further, the heat regenerator which is the object of the present invention may be formed according to a brick manufacturing process such as previously described, according to shaping methods known by those skilled in the art, by casting in a mold or by pressing of a basic component. Several bricks may then be assembled to form a heat regenerator. 
         [0052]    In the context of the invention, it will be within the abilities of those skilled in the art to adapt the dimensions of the brick and of the regenerator, but also the dimensions of the channels and of the projections according to the fluid temperature, to the storage time, and to the amount of energy to be stored. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0053]    The invention and the resulting advantages will better appear from the following non-limiting drawings and examples, provided as an illustration of the invention. 
           [0054]      FIG. 1A  illustrates a brick comprising four through channels according to the invention. 
           [0055]      FIG. 1B  illustrates a channel portion crossing a matrix portion of a regenerator according to the invention. 
           [0056]      FIG. 2A  illustrates a portion of a channel according to the present invention, comprising two parallelepipedal internal projections positioned on two opposite surfaces of the channel. 
           [0057]      FIG. 2B  also illustrates a portion of a channel according to the present invention, comprising two trapezoidal internal projections positioned on two opposite surfaces of the channel. 
           [0058]      FIG. 3  shows a portion of a channel according to a specific embodiment of the present invention, comprising two internal projections. 
           [0059]      FIG. 4  shows the graph corresponding to the head loss observed during the circulation of a fluid in a regenerator of height R according to the invention, according to the ratio of distance P between projections of the regenerator to the regenerator length (L) according to  FIG. 3 . 
           [0060]      FIG. 5  shows the graph obtained by plotting the Nusselt number according to the channel geometry, and especially to the ratio of distance P between projections of the regenerator and the regenerator length (L). 
           [0061]      FIG. 6  illustrates a heat storage enclosure comprising a regenerator according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0062]    Brick  1  of refractory material shown in  FIG. 1A  comprises four parallel through channels  2 . The channels emerge on either side at the upper surface and at the lower surface of brick  1 . Channels  2  are characterized by their width  b , their length  L , and their height  R . Said channels are spaced apart by a distance  c  along a first direction and a distance  c ′ along a second direction perpendicular to the first one. Distances c and c′ between channels are advantageously identical. 
         [0063]    Of course, brick  1  may comprise a single channel or a plurality of channels without departing from the scope of the invention. 
         [0064]      FIG. 1B  schematically shows a portion of the matrix of the regenerator crossed by a channel. 
         [0065]      FIGS. 2A and 2B  schematically show in perspective view two channels  2  of rectangular cross-section according to the invention. The fluid circulation direction within these channels has been shown by arrows. The arrow direction indicates either a storage, or an extraction. 
         [0066]    The channel of  FIG. 2A  comprises two parallelepipedal projections  3  of same width than that of channel  2 , but having a height h smaller than its length, to avoid obstructing it. Thus, lateral surfaces  7  of projection  3  define a 90° angle with the channel wall in contact with the projection or from which it projects. Longitudinal surface  8  of projection  3  is perpendicular to lateral surfaces  7  of said projection and extends along the channel height, that is, in the fluid circulation direction. 
         [0067]    The two projections  3  have a substantially identical shape, but are shifted from each other and also project from two opposite surfaces of the channel 
         [0068]    The channel of  FIG. 2B  has two trapezoidal projections  3 . The principle is identical to that of  FIG. 2A . In the case in point, the longer side of the trapezoid is confounded with the lateral wall of the channel from which projection  3  projects. 
         [0069]    In this specific case, although the disturbance to the fluid circulation generated by the projections is lower than that of the channel of  FIG. 2A , it however provides an efficient heat transfer, improved with respect to prior art. 
         [0070]    Channel  2  illustrated in  FIG. 3  comprises two cubic projections  3  of height  h  and of width  b . Lateral surfaces  7  of the projections define a straight angle with the channel wall in contact with the projection or from which it projects. The channel is further characterized by its length  L  and  P , the distance between two successive projections on a same side. As illustrated in  FIG. 3 , distance P between projections comprises the length of a projection. The length of the projection is defined by the length of the surface in contact with the channel, according to the fluid circulation direction, which direction is indicated by an arrow in  FIG. 3 . Further, to better show distance P between projections, projection  3  of a stacked channel is shown in dotted lines. 
         [0071]      FIG. 6  shows a heat storage enclosure comprising a heat regenerator according to the invention. The regenerator comprises, in particular, an assembly of bricks  1 , having their lateral portions, in the channel height direction, covered with a thermal insulator layer  4 . Thermal insulator layer  4  enables to limit energy losses towards the outside of the regenerator between storage and recovery areas. 
         [0072]    Although, for clarity, this has not been shown in  FIG. 6 , channels  2  comprise projections projecting into the volume defined by said channels. The regenerator comprises in the described example 35 parallel channels, all separated by an identical distance c. 
         [0073]    Further, two fluid inlets  6  enable the fluid to penetrate and to be discharged from the enclosure comprising the bricks. After it has been introduced into the enclosure, the fluid is distributed in the channels after having been distributed through a grid  5 , installed between fluid inlet  6  and bricks  1 . Grid  5  has a multiplicity of through openings ensuring a homogeneous distribution of the fluid. 
         [0074]    “Fluid inlet” also designates a fluid outlet or discharge port. 
       EXAMPLES OF EMBODIMENT 
       [0075]    The heat regenerator according to the present invention advantageously enables to store a quantity of electric energy in the range between 1 and 100 GW.h −1  for a storage time between 2 and 6 hours. Further, from 60 to 70% of the energy is recovered. 
         [0076]    The regenerators of examples A and B comprise a stack of adjacent identical bricks. The bricks comprise at least one through channel. 
         [0077]    The channels have the following dimensions:
       height R=10 meters;   width b=6 mm;   length L=12 mm   distance c between channels=4 mm.   height of projection h=3.5 mm.       
 
       Regenerator A (Prior Art) 
       [0083]    In the case of an installation having a 100-MW power and a 600-MWh capacity, heat regenerator A according to prior art (FR 2916101) comprises two enclosures for which the brick volume is 11,100 m 3 , and straight channels. The volume defined by the channels is equal to 4,900 m 3 , that is, approximately 44% of the total volume defined by the bricks. The volume defined by the material forming the enclosures is equal to 6,200 m 3 , that is, approximately 56% of the total volume defined by the bricks. 
       Regenerator B (Invention) 
       [0084]    For an installation having the same characteristics, the heat regenerator according to the invention comprises two enclosures with a 6,900-m 3  brick volume, and straight channels comprising projections projecting into the volume defined by said channels. The vacuum volume defined by the channels is equal to 2,700 m 3 , that is, approximately 39% of the total volume defined by the bricks. The volume defined by the material forming the enclosures and the projections is equal to 4,200 m 3 , that is, approximately 61% of the total volume defined by the bricks. 
         [0085]    The present invention thus enables to optimize the enclosure geometry and in particular to decrease their volume by close to 38% in this case. Such modifications thus enable to decrease the quantity of material used and manufacturing costs. 
       Head Loss According to Ratio P/L 
       [0086]    The fluid undergoes a head loss ΔP after having circulated through a regenerator according to the invention having a height  R  of 10 m. 
         [0087]    It is typically admitted that in such conditions, a pressure loss lower than or equal to 0.1 bar remains acceptable. 
         [0088]    In the context of the invention, a head loss of 0.1 bar for a ratio P/L of 1.70 corresponds to a Nusselt number greater than 13. This value reflects the amplification of the heat exchange due to the turbulence created by the projections. Indeed, in the case of a channel of similar dimensions but having no projections, the obtained Nusselt number is equal to 3.4.