THERMAL STORAGE TANK

Abstract

Said tank (1) comprises a chamber (10) including an inner surface (100) defining a space for heat storage, strata (2) 5 of solid elements made of a storage material, the storage material being suitable for storing heat from a heat-transfer fluid flowing in the chamber (10), the strata (2) occupying the space of the chamber (10), and at least one stack (3) of solid elements made of a heat-conductive material. The stack (3) is placed between two consecutive strata (2). The tank 10 (1) is characterized in that it comprises separation means designed to separate the solid elements of the strata (2) and the solid elements of the stack (3), and in that the separation means are designed to allow flow of the heat-transfer fluid through the stack (3).

Description

THERMAL STORAGE TANK The present invention relates to a thermal storage tank. More specifically, the invention concerns a dual thermocline type thermal storage tank. By way of non-limiting example, such a tank can be used in a solar power plant, particularly one of the Fresnel mirror type. A reservoir 1, known from the prior art and illustrated in Figure 1, comprises: - an enclosure 10 comprising an internal surface delimiting a volume for thermal storage, the internal surface of the enclosure 10 forming a cylinder having a height H and a diameter D; - 2 layers of solid elements of a storage material, the storage material being adapted to store the heat of a heat transfer fluid circulating in the enclosure 10, the 2 layers occupying the volume of the enclosure 10. Such a reservoir 1, according to the prior art, is of the thermocline type. Thermal stratification within the enclosure 10 leads to the presence of: - from a hot zone in an upper part of enclosure 10, - from a cold zone in a lower part of enclosure 10, - of a transition zone called thermocline between the hot zone and the cold zone. The heat transfer fluid is introduced from the top of tank 1, via an inlet Ec, during a storage phase to avoid natural convection currents. The heat transfer fluid is injected from the bottom of tank 1, via an inlet EF, during a destocking phase, and removed from the top of tank 1. Tank 1 has first and second distributors 11a, 11b of the heat transfer fluid arranged respectively in the lower and upper parts of enclosure 10. The heat transfer fluid is distributed in enclosure 10 at a sufficiently low velocity (on the order of 1 mm / s) to ensure efficient heat transfer between the heat transfer fluid and the layers 2. The principle of this type of heat storage is to create a kind of thermal piston, that is, a homogeneous thermal front advancing transversely, in order to maintain constant temperatures in the hot and cold zones during the storage and release phases. By "transversely," we mean in a direction perpendicular to the longitudinal axis of the tank. More precisely, such a state-of-the-art reservoir 1 is of the dual thermocline type. The solid elements of the layers 2 form a solid matrix. Thermal storage is provided by both the solid matrix and the heat transfer fluid. The use of a solid matrix and a heat transfer fluid significantly reduces costs compared to a thermal storage reservoir using only a heat transfer fluid. By "layer," we mean a set of solid elements extending in a plane perpendicular to the longitudinal axis of the reservoir (i.e., a horizontal plane when the longitudinal axis of the reservoir 1 is vertical). The solid elements of each layer 2 are disordered, mixed, and preferably have several particle sizes. Indeed, the storage material of the solid matrix is ​​"bulk" in the sense that it is inherently disordered and mixed.In addition, the solid matrix typically has several particle sizes in order to reduce the free space within the solid matrix. Such a state-of-the-art tank 1 is not entirely satisfactory because the thermal stratification is not perfectly uniform, leading to inhomogeneity of the thermal front. Preferential thermal paths then appear within the enclosure 10, resulting, for example, in the presence of a hot heat transfer fluid in portions of the cold zone of tank 1 during storage, thereby limiting the proper functioning of tank 1. Furthermore, such a prior art reservoir 1 is subject to the thermal ratcheting effect. During the various storage phases, the material of the container 10 expands differently from the solid matrix, creating a gap between the solid matrix and the inner surface of the container 10. This gap is then filled by solid elements of the solid matrix, thus reducing the height of the solid matrix. During the various unloading phases, the material of the container 10 contracts and is constrained by the solid elements of the solid matrix that filled the gap. The first distributor 1, arranged in the lower part of the container 10 and embedded in the solid matrix, is also subjected to significant stresses. It is known in the prior art to design a cylinder satisfying H / D < 1 in order to reduce the thermal ratcheting effect.However, this type of geometry makes it more difficult to obtain a homogeneous distribution of the heat transfer fluid along a transverse direction (i.e. horizontal when the longitudinal axis of the tank 1 is vertical). To overcome the problem of thermal front inhomogeneity, it is known from the prior art, particularly from US document 4,405,010, to interpose at least one stack of solid elements of a thermally conductive material between two successive layers 80 of solid elements, as illustrated in Figure 2. Each stack of solid elements comprises a first stack of metal (or cordierite or mullite) matrices 78 and a second stack formed by a metal (or cordierite or mullite) rod 78, see Figure 2, and col. 9, 1.24-35. Each stack aims to strongly limit the transverse thermal gradient within the enclosure 10, using the high thermal conductivity of the matrices 78 to improve the homogeneity of the thermal front. However, such a state-of-the-art reservoir is not entirely satisfactory, as the metal matrices 78 are likely to interfere with the layers 80 of solid elements during storage / removal cycles. Preferential thermal paths can then appear within the enclosure 10, resulting in inhomogeneity of the thermal front and limiting the reservoir's performance. Thus, the present invention aims to remedy, in whole or in part, the aforementioned drawbacks, and to this end relates to a thermal storage tank comprising: - an enclosure comprising an internal surface delimiting a volume for thermal storage; - layers of solid elements of a storage material, the storage material being adapted to store the heat of a heat transfer fluid circulating in the enclosure, the layers occupying the volume of the enclosure; - at least one stack of solid elements of a thermally conductive material, the stack being interposed between two successive layers; the tank being remarkable in that it includes separation means configured to separate the solid elements of the strata and the solid elements of the stack, and in that the separation means are configured to permit circulation of the heat transfer fluid through the stack. By "stratum," we mean a set of solid elements extending in a plane perpendicular to the longitudinal axis of the reservoir (i.e., a horizontal plane when the longitudinal axis of the reservoir is vertical). The solid elements of each stratum 2 are disordered, mixed together, and preferably have several particle sizes. The solid elements of the strata 2 form a solid matrix. The storage material of the solid matrix is ​​"bulk" in the sense that it is inherently disordered and mixed together. The solid matrix advantageously includes several particle sizes in order to reduce the free space within the solid matrix. Thus, such a reservoir according to the invention eliminates the mixing of solid elements from the layers and the solid elements of the stack thanks to the separation means. In the prior art, there is a natural segregation in granular media governed by the "Brazil nut" effect. This effect causes the smaller solid elements of the stack to accumulate beneath the larger solid elements of the layers during successive storage / retrieval cycles. Preferential thermal paths therefore appear within the container, resulting in inhomogeneity of the thermal front and limiting the reservoir's performance. In other words, the separation means of the invention prevent this natural segregation of the solid elements of the layers and the stack, thereby maintaining good homogeneity of the thermal front throughout the storage / unstocking cycles. The separation means of the invention are hermetic in the sense that they prevent the passage or penetration of the solid elements of the layers and the solid elements of the stack. The storage material is separate from the thermally conductive material. By "thermally conductive", we mean that the material has a thermal conductivity at 20°C greater than or equal to 14 Wm~1.K~1. In one embodiment, the separation means include: - a rim extending around the perimeter of the inner surface of the enclosure; - a pressing element arranged to press the rim against the inner surface of the enclosure. Thus, such separation devices are held in position on the inner surface of the container by pressing them against it, without requiring any fasteners. Securing the separation devices to the inner surface of the container would be technically complex, especially for large tanks. Furthermore, the fastening areas would be subjected to increasingly greater mechanical stresses as the difference between the coefficients of thermal expansion of the storage material and the container material increases. According to one embodiment, the clamping element includes a spring mounted compressed against the rim. In one embodiment, the stack is interposed between two successive layers so as to delimit upper and lower layers on either side of the stack; and the separation means comprise: - an upper grid interposed between the upper layers and the stack, and - a lower grid interposed between the stack and the lower layers. According to one embodiment, the upper grid has meshes adapted for the retention of solid elements from the upper layers; the lower grid has meshes adapted for the retention of solid elements from the stack; the meshes of the upper and lower grids being adapted to allow the circulation of the heat transfer fluid through the stack. In one embodiment, the enclosure is of a material having a coefficient of thermal expansion CH; the storage material has a coefficient of thermal expansion <¾ satisfying <¾ < ai; the thermally conductive material has a coefficient of thermal expansion <¾, the thermally conductive material being chosen such that a2 < 1≤ a3. Thus, through the careful selection of thermally conductive material, each stack creates zones of mechanical stress relief on the inner surface of the enclosure during storage and retrieval. The reduction in mechanical stress is greater when the difference (<¾ - CH) is small. These stress relief zones are located along the height of the lateral parts of each stack in contact with the inner surface of the enclosure. The thermal ratchet effect is reduced in these stress relief zones. Furthermore, when the separation means include a rim extending around the perimeter of the inner surface of the enclosure, the rim height partially compensates for the effect of the difference in coefficients of thermal expansion (<¾ - ai). During the various storage phases, the enclosure material expands, creating a space between the solid matrix and the inner surface of the enclosure. This space is closed by the rim of the separation means, which prevents solid elements from the layers from penetrating it. To achieve this, the rim is dimensioned based on the dimensions of the enclosure wall delimiting the inner surface. For example, for a cylindrical tank with a circular cross-section, the rim height must be greater than the maximum variation in the tank's radius due to thermal expansion. A safety factor of 1.5 for the rim height is recommended.During the various destocking phases, the enclosure material shrinks and is no longer constrained by the solid elements of the layers. The choice of thermally conductive material is critical. During the different storage phases, the container material expands differently from the solid matrix (<¾ < CH) and creates a gap between the solid matrix and the inner surface of the container. If the following relationship, O2 < O3 < ai, is verified, or if the following relationship, O3 < O2 < ai, is verified, then the rim of the separation means alone is insufficient to retain the storage material from the solid matrix. In one embodiment, the enclosure comprises a lower part and an upper part; the reservoir comprises: - first and second distributors of the heat transfer fluid arranged respectively in the lower and upper parts of the enclosure; - additional stacks of solid elements of the thermally conductive material, the additional stacks being arranged to envelop the first and second distributors. Thus, such additional stacking prevents the first and second dispensers from being subjected to significant mechanical stress during storage / retrieval phases. This results in an increased lifespan for the dispensers. According to one embodiment, the solid elements of each stack comprise balls, preferably having a diameter between 5 mm and 50 mm. Advantageously, each stack has a porosity between 30% and 50%. The homogeneity of the thermal front depends primarily on the porosity of the stack, the thermal conductivity of the thermally conductive material, and the circulation velocity of the heat transfer fluid within the enclosure. A porosity between 30% and 50% provides sufficient pore volume for the heat transfer fluid to fill. Higher porosity would require an excess of heat transfer fluid, resulting in a higher associated cost. Temperature homogeneity is achieved through remixing of the heat transfer fluid within the pores and by thermal conduction. The solid elements of each stack advantageously have a single particle size to improve temperature homogeneity through remixing and thermal conduction. The circulation velocity of the heat transfer fluid within the enclosure is preferably on the order of 1 mm / s. According to one embodiment, the thermally conductive material is a metallic material, preferably steel. Thus, such a thermally conductive material is particularly suitable when the enclosure wall, delimiting the internal surface of the enclosure, is made of a steel-type material, in order to reduce (<¾ - CH) as much as possible. According to one method of execution, the solid elements of the strata comprise rocks and preferably sand, the rocks being preferably alluvial. The rocks are preferentially rich in silica, preferably of the quartzite type. The solid elements of the layers advantageously have a double grain size in order to reduce the free space within the solid matrix, and thereby the proportion of heat transfer fluid in the tank. According to one embodiment, the heat transfer fluid is a liquid or a gas, the liquid preferably being an oil, the gas preferably being air. According to one embodiment, the internal surface of the enclosure forms a cylinder having a height H and a diameter D; the cylinder preferably satisfies 1 < ^ < 2.5; more preferably 1.5 < ^ < 2.5; even more preferably 2 < - < 2.5. D Thus, such enclosure shapes facilitate a homogeneous distribution of the heat transfer fluid along a transverse direction. These shapes are advantageously achieved with a thermally conductive material satisfying 2 < 1 ≤ a3 in order to avoid the thermal ratchet effect. Advantageously, each stack has a height h satisfying 0.05 < - < 0.15. D Thus, the height h of each stack is chosen sufficiently high to achieve satisfactory relief of the mechanical stresses on the inner surface of the enclosure when the thermally conductive material satisfies a2 < 1 ≤ a3, and to obtain satisfactory homogeneity of the thermal front. The height h of each stack is chosen sufficiently low so as not to add weight to the tank and for cost reasons. Considering a first stack with a height hi and a second stack with a height h2, the first stack extending along a central portion of the tank and the second stack extending along a peripheral portion (i.e., a lower or upper portion), the height hi is advantageously chosen so that hi > h2 to most effectively reduce the thermal ratchet effect. Second stacks in the peripheral portion allow for transverse homogenization at the storage outlet, which smooths the evolution of the storage outlet temperature. A stable outlet temperature is more favorable for storage outlet performance. In one embodiment, the stack or stacks are interposed between two successive strata so as to delimit upper and lower strata on either side of the corresponding stack; the stack or stacks are distributed in the enclosure so that the upper and lower strata delimited by the stack or stacks occupy equal volumes in the enclosure with a tolerance of 15%. Other features and advantages will become apparent in the following description of various embodiments of the invention, given by way of non-limiting examples, with reference to the accompanying drawings in which: - Figure 1 (already discussed) is a schematic perspective view of a state-of-the-art reservoir, - Figure 2 (already discussed) is a schematic cross-sectional view of a state-of-the-art reservoir, - Figure 3 is a schematic cross-sectional view of a tank according to the invention, - Figure 4 is a schematic perspective view of a reservoir according to the invention, - Figure 5 is a schematic perspective view illustrating an additional stack surrounding a heat transfer fluid distributor, - Figure 6 is a schematic top view illustrating an additional stack surrounding a heat transfer fluid distributor, - Figure 7 is a schematic cross-sectional view of a tank according to the invention, - Figure 8 is a partial schematic cross-sectional view of a tank according to the invention, - Figure 9 is a schematic top view of the solid elements of the strata. For the different embodiments, the same reference numerals will be used for identical elements or elements performing the same function, for the sake of simplicity in the description. The technical characteristics described below for different embodiments are to be considered individually or in any technically feasible combination. The tank illustrated in Figures 3 to 8 is a thermal storage tank 1, comprising: - an enclosure 10 comprising an internal surface 100 delimiting a volume for thermal storage; - 2 layers of solid elements of a storage material, the storage material being adapted to store the heat of a heat transfer fluid circulating in the enclosure 10, the 2 layers occupying the volume of the enclosure; - at least one stacking 3 of solid elements of a thermally conductive material, the stacking 3 being interposed between two successive layers 2. Tank 1 includes separation means configured to separate the solid elements of strata 2 and the solid elements of stack 3. The separation means are configured to allow circulation of the heat transfer fluid through stack 3. The internal surface 100 of the enclosure 10 advantageously forms a cylinder with a height H and a diameter D. As an example, D is greater than 10 m, preferably greater than 15 m. The cylinder preferably satisfies 1 < ^ < 2.5; more preferably 1.5 < ^ < 2.5; even more preferably 2 < ^ < 2.5. The enclosure 10 is made of a material with a coefficient of thermal expansion CH. The material of the enclosure 10 is advantageously steel, preferably stainless steel. The steel has a coefficient of thermal expansion ai at 20°C between 10x10⁶ K⁻¹ and 20x10⁶ K⁻¹. The enclosure 10 comprises a lower part 10a and an upper part 10b. The reservoir 1 advantageously includes first and second distributors 11a and 11b for the heat transfer fluid, arranged respectively in the lower part 10a and the upper part 10b of the enclosure 10. The first distributor 11a has an inlet EF for the heat transfer fluid. The second distributor 11b has an inlet Ec for the heat transfer fluid. The heat transfer fluid is introduced from the top of the reservoir 1, via the inlet Ec, during a storage phase to avoid natural convection. For example, the temperature of the heat transfer fluid introduced via the inlet Ec is approximately 250°C. The heat transfer fluid is introduced from the bottom of the reservoir 1, via the inlet EF, during a removal phase. For example, the temperature of the heat transfer fluid introduced via the inlet EF is approximately 100°C.The heat transfer fluid is distributed in the enclosure 10 by the first and second distributors 11a, 11b at a speed in the enclosure 10 that is sufficiently low (on the order of 1 mm / s) to ensure efficient heat transfer between the heat transfer fluid and the layers 2. The storage material has a coefficient of thermal expansion <¾. The storage material is inert relative to the heat transfer fluid. As illustrated in Figure 9, the solid components of layers 2 advantageously consist of rocks, preferably sand, with the rocks preferably being alluvial, silica-rich, and quartzite. The rocks have a coefficient of thermal expansion <¾ at 20°C between 2 x 10⁶ K⁻¹ and 7 x 10⁶ K⁻¹. The heat transfer fluid is a liquid or a gas. The liquid heat transfer fluid is preferably an oil, such as a synthetic oil. The liquid heat transfer fluid can also be a molten salt. The gas heat transfer fluid is preferably air. The storage material is advantageously chosen according to: - its thermal storage capacity p Cp, where p is the density, and Cp the specific heat, - its thermal conductivity, - its compatibility with the heat transfer fluid, - its maximum operating temperature, - its price. The solid elements of the two layers form a solid matrix. Thermal storage is provided by both the solid matrix and the heat transfer fluid. Using a solid matrix and a heat transfer fluid significantly reduces costs compared to a thermal storage tank using only a heat transfer fluid. The storage material for the solid elements of the two layers is "bulk" in the sense that it is inherently disordered, a jumbled mass. Furthermore, the solid matrix typically includes several particle sizes (for example, a few centimeters and a few millimeters in average diameter) to reduce the free space for the heat transfer fluid within the solid matrix. As an example, each two layer has a porosity between 25% and 30%, preferably 27%. The thermally conductive material has a coefficient of thermal expansion < ¾. The thermally conductive material is inert relative to the heat transfer fluid.The thermally conductive material is advantageously chosen such that a2 < 1 ≤ a3. Each stacking 3 contributes to a release of the mechanical stresses experienced by the internal surface 100 of the enclosure 10. The thermally conductive material is advantageously suited for thermal storage so as not to reduce the storage capacity of the tank 1. The thermally conductive material is advantageously chosen such that its thermal storage capacity is greater than or equal to the thermal storage capacity of the solid matrix storage material. The thermally conductive material is advantageously a metallic material, preferably steel, so that the difference (<¾ - CH) is as small as possible. Each stack 3 has a height h satisfying 0.05 < ^ < 0.15. The solid elements of each stack 3 advantageously comprise balls, preferably with a diameter between 5 mm and 50 mm. The balls may be from discarded ball bearings. The balls may be shot. Each stack 3 advantageously has a porosity between 30% and 50%. Considering a first stack 3 with a height hi and a second stack 3' with a height h2, the first stack 3 extending along a central part of the reservoir 1, the second stack 3' extending along a peripheral part of the reservoir 1 (i.e., a lower or upper part), the height hi is advantageously chosen so that hi > h2 in order to most effectively reduce the thermal ratchet effect, as illustrated in Figure 8.Moreover, such a geometric configuration of the first and second stacks 3, 3' is also advantageous from a thermal point of view in order to improve the homogenization of the thermal front to the peripheral parts of the tank 1. The reservoir 1 advantageously includes additional stacks 3a, 3b of solid elements of the thermally conductive material, the additional stacks 3a, 3b being arranged to respectively enclose the first and second distributors 1 1 a, 1 1 b. As illustrated in Figure 7, each stack 3 is interposed between two successive layers 2 so as to delimit upper and lower layers 2 on either side of the corresponding stack 3. The stacks 3 are advantageously distributed within the enclosure 10 so that the upper and lower layers 2 delimited by the stacks 3 occupy equal volumes within the enclosure, with a tolerance of 15%. The means of separation advantageously include: - a rim 4 extending around a perimeter of the internal surface 100 of the enclosure 10; - a pressing member 5 arranged to press the rim 4 against the internal surface 100 of the enclosure 10. The clamping element 5 advantageously comprises a spring mounted compressed against the rim 4. The spring is advantageously made of a metallic material. The spring advantageously has a toroidal shape. The means of separation advantageously include: - an upper grid 6b interposed between the upper strata 2 and the corresponding stacking 3, and - a lower grid 6a interposed between the corresponding stack 3 and the lower strata 2. The upper grid 6b has mesh sizes adapted for retaining solid elements from the upper layers 2. The lower grid 6a has mesh sizes adapted for retaining solid elements from the corresponding stack 3. The mesh sizes of both the upper grid 6b and the lower grid 6a are adapted to allow the heat transfer fluid to circulate through the corresponding stack 3. As illustrated in Figure 7, reservoir 1 has additional grids 6' interposed between strata 2 and additional stacks 3a, 3b. The rim 4 extends in line with each grid 6a, 6b, 6'. Each grid 6a, 6b, 6' has a circular horizontal cross-section with a diameter greater than D, such that the corresponding rim 4 bears against the internal surface 100 of the enclosure 10. Each grid 6a, 6b, 6' is made of a material compatible with the storage material and the heat transfer fluid. The material of each grid 6a, 6b, 6' is advantageously steel, preferably stainless steel.

Claims

Demands 1. Thermal storage tank (1), comprising: - an enclosure (10) comprising an internal surface (100) delimiting a volume for thermal storage; - layers (2) of solid elements of a storage material, the storage material being adapted to store the heat of a heat transfer fluid circulating in the enclosure (10), the layers (2) occupying the volume of the enclosure (10); - at least one stack (3) of solid elements of a thermally conductive material, the stack (3) being interposed between two successive layers (2); the reservoir (1) being characterized in that it comprises separation means configured to separate the solid elements of the layers (2) and the solid elements of the stack (3), and in that the separation means are configured to permit circulation of the heat transfer fluid through the stack (3).

2. Reservoir (1) according to claim 1, wherein the separation means comprise: - a rim (4) extending around a perimeter of the internal surface (100) of the enclosure (10); - a clamping element (5) arranged to clamp the rim (4) against the internal surface (100) of the enclosure (10).

3. Reservoir (1) according to claim 2, in which the clamping member (5) has a spring mounted compressed against the rim (4).

4. A reservoir (1) according to any one of claims 1 to 3, wherein the stack (3) is interposed between two successive layers (2) so as to delimit upper layers (2) and lower layers (2) on either side of the stack (3); and the separation means comprise: - an upper grid (6b) interposed between the upper strata (2) and the stack (3), and - a lower grid (6a) interposed between the stack (3) and the lower strata (2).

5. Tank (1) according to claim 4, in which the upper grid (6b) has meshes adapted for the retention of solid elements from the upper layers (2); the lower grid (6a) has meshes adapted for the retention of solid elements from the stack (3); the meshes of the upper grid (6b) and the lower grid (6a) being adapted to allow the circulation of the heat transfer fluid through the stack (3).

6. Tank (1) according to any one of claims 1 to 5, wherein the enclosure (10) is of a material having a coefficient of thermal expansion ai; the storage material has a coefficient of thermal expansion <¾ satisfying <¾ < en; the thermally conductive material has a coefficient of thermal expansion <¾, the thermally conductive material being chosen such that 2 < a1≤ a3.

7. A reservoir (1) according to any one of claims 1 to 6, wherein the enclosure (10) comprises a lower part (10a) and an upper part (10b); the reservoir (1) comprises: - the first and second distributors (11a, 11b) of the heat transfer fluid arranged respectively in the lower part (10a) and in the upper part (10b) of the enclosure (10); - additional stacks (3a, 3b) of solid elements of the thermally conductive material, the additional stacks (3a, 3b) being arranged to envelop the first and second distributors (1 1 a, 1 1 b).

8. Reservoir (1) according to any one of claims 1 to 7, in which the solid elements of each stack (3) comprise balls, preferably having a diameter between 5 mm and 50 mm.

9. Reservoir (1) according to any one of claims 1 to 8, in which each stack (3) has a porosity of between 30% and 50%.

10. Tank (1) according to any one of claims 1 to 9, wherein the thermally conductive material is a metallic material, preferably a steel. 1 1. Reservoir (1 ) according to any one of claims 1 to 10, in which the solid elements of the strata (2) comprise rocks and preferably sand, the rocks being preferably alluvial.

12. Tank (1) according to any one of claims 1 to 1 1, in which the heat transfer fluid is a liquid or a gas, the liquid preferably being an oil, the gas preferably being air.

13. Tank (1) according to any one of claims 1 to 12, in which the internal surface (100) of the enclosure (10) forms a cylinder having a height H and a diameter D; the cylinder preferably satisfies 1 < ^ < 2.5; more preferably 1.5 < ^ < 2.5; even more preferably 2 < ^ < 2.

5.

14. Tank (1) according to claim 13, in which each stack (3) has a height h satisfying 0.05 < ^ < 0.

15.

15. Tank (1) according to any one of claims 1 to 14, in which the stack or each stack (3) is interposed between two successive strata (2) so as to delimit upper strata (2) and lower strata (2) on either side of the corresponding stack (3); the stack or stacks (3) are distributed in the enclosure (10) so that the upper strata (2) and lower strata (2) delimited by the stack or stacks (3) occupy equal volumes in the enclosure (10) with a tolerance of 15%.