EP3759060A1

EP3759060A1 - Method for producing a ceramic material for thermal energy storage

Info

Publication number: EP3759060A1
Application number: EP19717516.9A
Authority: EP
Inventors: Rachid BOULIF; Driss Dhiba; Nawal SEMLAL; Alain Germeau; Claudia TOUSSAINT; Ange Nzihou; Doan PHAM MINH; Abdoul Razac SANE
Original assignee: Prayon SA; Association pour la Recherche et le Developpement des Methodes et Processus Industriels; OCP SA
Current assignee: Prayon SA; Association pour la Recherche et le Developpement des Methodes et Processus Industriels; OCP SA
Priority date: 2018-03-01
Filing date: 2019-02-28
Publication date: 2021-01-06
Also published as: US20200407616A1; FR3078533A1; SA520420078B1; IL277010B1; IL277010B2; WO2019166741A1; BR112020017789A2; MA52425A; IL277010A; CN112105590A; FR3078533B1; RU2020132263A

Abstract

The invention relates to a method for producing a ceramic material for thermal energy storage, characterised in that it comprises the production of a mixture of at least particles of clay and particles of natural and/or synthetic phosphate, and water, said mixture comprising between 0.5 and 40 mass % of phosphate in relation to the mass of the mixture, excluding the water, as well as the shaping and the firing of the mixture in order to produce the ceramic material. The invention also relates to a ceramic material for thermal energy storage, characterised in that it comprises a matrix of clay, and where appropriate, sand, and particles of a natural phosphate and/or a synthetic phosphate which are dispersed in said matrix, said ceramic material comprising between 0.5 and 40 mass % of phosphate in relation to the mass of the ceramic material. The invention also relates to a method for thermal energy storage in said ceramic material, characterised in that it comprises bringing a heat-transfer fluid into contact with said ceramic material, in such a way as to transfer heat from the heat-transfer fluid to the ceramic material in a charging phase and to transfer heat from the ceramic material to the heat-transfer fluid in a discharging phase.

Description

PROCESS FOR PRODUCING A CERAMIC MATERIAL

FOR THERMAL STORAGE OF ENERGY

FIELD OF THE INVENTION

The present invention relates to a thermal storage material, specifically for the storage of sensible heat, as well as a method of manufacturing such a material, and a thermal storage method using said material.

STATE OF THE ART

Thermal storage involves storing heat in a medium for later use. This medium consists of a specific material called thermal storage material.

There are three methods of thermal energy storage: sensible heat storage, latent heat storage and thermochemical storage [1]. As indicated above, the present invention relates to materials for storing sensible heat.

Storage of sensible heat consists of a simple rise in the temperature of the storage material. The amount of heat stored by the material is given by the following equation:

with Q the quantity of heat stored (J); T, and T _f the initial and final storage temperatures (K), respectively; m the mass of the storage material (g); C _P (T) the calorific value of the storage material (J / g K).

A sensible heat storage material may be a liquid or a solid.

The case of liquid materials has achieved industrial success with molten salts based on alkali nitrates which are used in concentrated solar power plants (CSP), the acronym for the English term "Concentrated Solar Power". Several CSP plants are currently operating [2] - [3]. However, molten salts have weaknesses related to their significant use as fertilizer in agriculture, the risk of complete chemical decomposition above 565 ° C, and their high price.

As a result, solid materials such as concrete have been tested by the German Center for Aeronautics and Astronautics, in German Deutsches Zentrum für Luft- und Raumfahrt, better known by its abbreviation DLR [4] Concrete is available in industrial quantity at competitive cost. However, its use is limited to about 400 ° C to prevent mechanical damage.

The development of more stable, more efficient and economically advantageous materials for thermal storage therefore remains necessary. SUMMARY OF THE INVENTION

An object of the invention is therefore to design a material for thermal storage that can be easily shaped by an industrial process, be available in industrial quantities, and be used in a wide range of temperatures, up to 1100 ° C.

For this purpose, the invention proposes a method of manufacturing a ceramic material for the thermal storage of energy, characterized in that it comprises the production of a mixture of at least particles of clay, particles of natural and / or synthetic phosphate, and water, said mixture comprising between 0.5% and 40% by weight of phosphate relative to the mass of the mixture with the exception of water. The method also includes the steps of shaping and baking the mixture to obtain the ceramic material.

Said natural and / or synthetic phosphate may in particular comprise hydroxyapatite.

In a particularly advantageous manner, the mixture comprises between 4% and 5% by weight of phosphate. In the remainder of the text, the mass contents are calculated with respect to the total mass of the mixture excluding water.

Said mixture advantageously comprises between 50 and 90% by mass of clay, preferably between 60 and 80%.

Preferably, the average size of the clay and phosphate particles is less than 1 mm.

According to one embodiment, the mixture further comprises up to 40% by weight of sand particles, preferably between 10 and 30% by weight.

The average size of the sand particles is advantageously less than 1.5 mm. Said method advantageously comprises forming the ceramic material by one of the following techniques: extrusion, granulation, molding, compacting or pressing the mixture.

The method may comprise, after the shaping step, drying the ceramic material at a temperature of 105 ° C or less.

The method may comprise, after the drying step, firing the ceramic material at a temperature between 800 and 1200 ° C, preferably between 900 and 1150 ° C.

Another object of the invention relates to a ceramic material for the thermal storage of energy, obtainable by the method as described above. Said ceramic material comprises a matrix of clay and, if appropriate, of sand, and particles of natural and / or synthetic phosphate dispersed in said matrix, said ceramic material comprising between 0.5% and 40% by weight of phosphate by relative to the mass of the ceramic material. Advantageously, the ceramic material is in the form of a cylinder, a sphere, a cube, a spiral, a flat plate, a corrugated plate, a hollow brick or of a Raschig ring.

Another subject of the invention relates to a thermal energy storage method using such a material. The method comprises contacting a heat transfer fluid with the ceramic material described above, so as to transfer heat from the coolant to the ceramic material in a charging phase, and to transfer heat from the ceramic material to the heat transfer fluid in a discharge phase.

For the implementation of said method, the ceramic material is contained in a tank. Said reservoir is advantageously formed of at least one thermally insulating material.

The coolant is typically selected from air, water vapor, oil or molten salt.

During the charging phase and / or the discharge phase, the coolant is at a temperature between 20 and 1100 ° C.

Finally, the invention relates to a device for implementing said energy storage method. Said device comprises a reservoir containing the ceramic material and a circulation circuit of a heat transfer fluid in fluid connection with the reservoir so as to put said heat transfer fluid in contact with the ceramic material.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings in which:

- Figure 1 is a map of the elements present in a terracotta ceramic without addition of phosphate (referenced Ceraml in Table 1);

- Figure 2 is a map of the elements present in a ceramic containing 16.7% CP and cooked at 1100 ° C (referenced Ceram8 in Table 1);

- Figure 3 is a map of the elements present in a ceramic containing 16.7% of PN and cooked at 1100 ° C (referenced Ceram28 in Table 1);

FIG. 4 illustrates the thermal conductivity measured by the Hot Disk method for ceramics containing CP and baked at different temperatures;

FIG. 5 illustrates the thermal conductivity measured by the Hot Disk method for ceramics containing PN and cooked at different temperatures;

FIG. 6 illustrates the thermal conductivity measured dynamically on ceramics: phosphate-free (Ceraml); with 4.7% by weight of CP (Ceram8); with 5% by weight of PN (Ceram34); FIG. 7 illustrates the bending fracture stress of ceramics with or without addition of CP and baked at different temperatures;

FIG. 8 illustrates the bending fracture stress of ceramics with or without the addition of PN and baked at different temperatures;

FIG. 9 represents the mechanical resistance (Young's modulus) measured dynamically by acoustic resonance on a phosphate-free ceramic (Ceraml) or with the addition of 4.7% by weight of CP (Ceram8);

- Figure 10 illustrates the calorific value (specific heat) dynamically measured by the DSC 404 F1 Pegasus ™ on a ceramic without phosphate (Ceraml), with the addition of 4.7% by weight of CP (Ceram8), or with the addition of 5% by mass of PN (Ceram34);

FIG. 11 illustrates the bending fracture stress of ceramics prepared with different sizes (cf ₅ o) of PN particles and fired at different temperatures;

FIG. 12 illustrates a thermogravimetric analysis of ceramics containing 4.7% by weight of CP (Ceram8) or 5% by mass of PN (Ceram34) (the y-axis of the left is the variation of mass of the material (in% ), the ordinate axis on the right is the temperature (in ° C), and the x-axis is the time (in min);

FIG. 13 is a schematic diagram of the sensible heat storage tank used during the charging phase (a) and the discharging phase (b).

FIG. 14 relates to the charge phase during the test of storage of the sensible heat with the Ceram9 material at moderate temperatures (T _H around 340 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (q _C h _g ) as a function of charging time;

FIG. 15 relates to the discharge phase during the test of storage of the sensible heat with the material Ceram9 at moderate temperatures (T _H around 340-343 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and discharge rate (q _diS ) as a function of the discharge time;

FIG. 16 relates to the charge phase during the test of storage of the sensible heat with the Ceram9 material at moderately high temperatures (T _H around 520 ° C.): (a) evolution of the axial temperature as a function of the length storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (q _C h ₉ ) as a function of charging time;

FIG. 17 relates to the discharge phase during the test of storage of the sensible heat with the Ceram9 material at moderately high temperatures (T _H around 520 ° C.): (a) evolution of the axial temperature as a function of the length of the tank storage; (b) inlet (T1) and outlet (T2) temperature and discharge rate (h ^) as a function of charging time;

FIG. 18 relates to the charge phase during the test of storage of the sensible heat with the Ceram9 material at high temperatures (T _H around 760 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (n _Chg ) as a function of charging time;

FIG. 19 relates to the discharge phase during the test of storage of the sensible heat with the Ceram9 material at high temperatures (T _H around 760 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and discharge rate (n _diS ) as a function of the discharge time;

FIG. 20 relates to the charging phase during the test of storage of the sensible heat with the Ceram35 material at moderate temperatures (T _H around 350 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (n _Chg ) as a function of charging time;

FIG. 21 relates to the discharge phase during the test of storage of the sensible heat with the Ceram35 material at moderate temperatures (T _H around 350 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet temperature (T1) and outlet temperature (T2) and discharge rate (n _diS ) as a function of the discharge time;

FIG. 22 relates to the charging phase during the test of storage of the sensible heat with the Ceram35 material at moderately high temperatures (T _H around 580 ° C.): (a) evolution of the axial temperature as a function of the length storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (n _Chg ) as a function of charging time;

FIG. 23 relates to the discharge phase during the test of storage of the sensible heat with the MC / PN material at moderately high temperatures (T _H around 580 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet temperature (T1) and outlet temperature (T2) and discharge rate (n _diS ) as a function of the discharge time;

FIG. 24 relates to the charging phase during the test of storage of the sensible heat with the Ceram35 material at high temperatures (T _H around 850 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and charge rate (n _Chg ) as a function of charging time; FIG. 25 relates to the discharge phase during the test of storage of the sensible heat with the MC / PN material at high temperatures (T _H around 850 ° C.): (a) evolution of the axial temperature as a function of the length of the storage tank; (b) inlet (T1) and outlet (T2) temperature and discharge rate (q _d is) as a function of the discharge time.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The inventors have demonstrated the possibility of obtaining a ceramic material having excellent thermal energy storage ability by mixing particles of clay, sand, phosphate, and water. Said mixture may in fact have a favorable plasticity for the implementation of various techniques, such as extrusion, granulation, molding or pressing, which make it possible to put the ceramic material in a form suitable for the thermal storage of energy. .

In the present text, ceramic means a material in solid form having undergone a firing cycle.

Conventional terracotta ceramics are made from a mixture of clay, sand and water.

The clays have a structure in the form of sheets allowing water molecules to interpose between said sheets. This gives them a plastic property and offers them the possibility of being used as plasticizing or structuring agents. The plastic property of clays is a decisive parameter for shaping ceramic earthenware materials.

Overall, clays exist in several mineralogical forms grouped into four families [5]. These are the kaolinites (AS ^ OsODH ^), the illites (K (AI, Mg, Fe) 2 (Si, Al) 40 [(OH) 2, H ₂ 0], the smectites ((Ca, Na) ₀ , 3

(Al, Mg, Fe) 2 (Si, Al) 40 (OH) 2.nH ₂ O) and chlorites.

Clay is a natural material, available in industrial quantities with good plasticity compared to several other binders such as polyvinyl alcohol, gelatin, polyethylene glycol or polyacrylic acid which are used in various industrial processes.

Sands are inert materials with no plasticity and are mainly composed of quartz and other minerals such as feldspars and micas. In the terracotta industry, sands are used as degreasers to facilitate the drying step. Their use makes it possible to obtain in the clay matrix a skeleton favorable to the dehydration of clay minerals. This prevents large shrinkage that can lead to cracking of materials.

There is a large family of phosphates, which are either natural (phosphate ores) or synthetic. They are formed from phosphate anions (orthophosphate (PO ₄ ) ³ ) and metal cations M where M can be an alkali, an alkaline earth metal or any metal of the periodic table of the elements. This diversity makes it possible to obtain phosphate products of very varied properties.

The phosphate used in the present invention may be a natural phosphate (that is to say a phosphate ore) or a synthetic phosphate such as hydroxyapatite (Caio (PO ₄ ) ₆ (OH) ₂ ), or a mixture of these two types of phosphate.

The presence of a phosphate incorporated in the clay matrix (which optionally also contains sand) makes it possible to improve the physical, thermal and mechanical properties of the ceramic material, in particular the density, the thermal conductivity and the heating value. or else the mechanical stability.

According to an advantageous embodiment, extrusion is a simple and controlled shaping technique for the large-scale production of ceramic materials for thermal energy storage, and which is suitable for the mixture described above. The extrusion consists in passing the mixture, at a controlled pressure, through a double helix, then an endless screw before extracting it by a die in a monolithic form. This technique makes it possible to obtain ceramic materials in different forms: cylindrical, alveolar, flat plate, corrugated plate, hollow brick, etc. The skilled person chooses the size and shape of ceramic materials to control heat exchange during storage and retrieval of sensitive heat.

However, this embodiment is not limiting and the mixture can be shaped by other techniques such as granulation, molding, compaction or pressing. For example, granulation is advantageous in that it makes it possible to obtain materials of spherical shape of different sizes.

In general, the ceramic material may have the following forms: cylinder, sphere, cube, spiral, flat plate, corrugated plate, hollow brick, Raschig ring (non-limiting list). Those skilled in the art will choose the shaping technique according to the desired shape.

The composition of the mixture is controlled to have good plasticity for the shaping step, and to obtain physical, thermal and mechanical properties suitable for the storage of sensible heat.

For this purpose, the added phosphate content can reach up to 40% by weight (ie 17% by mass in P ₂ 0 ₅ ), and is between 0.5% and 40%, preferably between 4% and 5% by weight (in the present text, the reference mass is that of the dry mixture (not including the added water)). In all cases, the phosphate content is non-zero. A phosphate content of at least 0.5% by weight makes it possible to significantly improve the thermal conductivity and the mechanical strength of the ceramics. A phosphate content of less than 40% by weight makes it possible to guarantee a good plasticity of the mixture of clay, phosphate, and water and facilitates its subsequent shaping.

The sand content may vary between 0 and 40% by weight, preferably between 10 and 30%. The sand content depends on the nature of the clay mixture (original deposit). Phosphate can replace all or part of the sand. Thus, it is possible to get rid of sand in the mixture, for example when large amounts of phosphate (of the order of 20 to 40%) are added. In the rest of the text, for the sake of brevity, the term "clay-sand matrix" covers a possible absence of sand.

The clay content can vary between 50 and 90% by weight, preferably between 60 and 80%.

The water content is adjusted so as to give the mixture a pasty consistency, the viscosity of which is adapted to the shaping technique adopted. This water will be removed during subsequent heat treatments (ie drying and cooking).

The particle size of the mixture is also controlled because it influences the final properties of the ceramic material. By size we mean in the present text the diameter of a sphere having the same volume as the particle considered; in the case of a spherical particle, the size is the diameter of the particle. To the extent that the particles generally have a variable size in a specific range, the median size is considered, denoted d ₅ o, that is to say the size for which 50% of the particles are smaller and 50% of particles have a larger size.

Thus, the size d ₅₀ of the phosphate particles is preferably less than 1 mm; that of clay and sand is preferably less than 1 and 1.5 mm, respectively.

After the shaping step, heat treatments by drying and baking are applied.

The drying is advantageously carried out in stages, at different temperatures which do not exceed 105 ° C. In a preferred embodiment, the drying comprises successively a first bearing at 25 ° C, a second bearing at 45 ° C, a third bearing at 70 and a fourth bearing at 105 ° C. Each step is applied for a fixed period of time which may be identical or different from one step to another. Preferably, the duration of each stage is 24 hours. Such stepwise drying makes it possible to evacuate the water in a progressive manner and thus to avoid generating stress in the material. After drying, the material does not contain any more water.

The cooking is carried out after the drying step. It can be done in a static oven or in a tunnel oven. A moderate rise ramp is applied, preferably 5 ° C / min, in order to avoid generating stress in the material. The firing temperature applied may vary between 800 and 1200 ° C, preferably between 900 and 1150 ° C. The bearing at the cooking temperature is between 0.5 and 5 h, preferably 1 h.

At the end of the drying step, the ceramic material has a clay-sand matrix in which are dispersed phosphate particles.

As the experimental results presented below demonstrate, said ceramic material has good thermal energy storage properties.

The ceramic material can therefore be used for the implementation of a thermal energy storage method. For this purpose, the ceramic material is brought into contact with a heat transfer fluid so as to allow heat exchange:

in a charging phase, the coolant is at an elevated temperature, higher than that of the ceramic material; the heat is transferred from the coolant to the ceramic material, and stored in said material for the desired storage time;

- In a discharge phase, the heat transfer fluid is at a low temperature, lower than that of the ceramic material; the heat stored in the ceramic material is then transferred to the coolant.

The heat thus discharged can be used for the generation of electricity, for the heating of a room, or for any other use.

The coolant can be a gas or a liquid. For example, but without limitation, the coolant may be air, water vapor, an oil or a molten salt.

For carrying out said thermal storage method, the ceramic material is in the form of a plurality of units which together constitute a lining. The size and shape of these units is chosen to maximize the contact surface with the coolant.

Said lining is arranged in a tank which is in one or thermally insulating material (s).

The reservoir is in fluid connection with a coolant circuit. Advantageously, the reservoir has an inlet and a heat transfer fluid outlet, arranged relative to each other so as to ensure a contact surface as large as possible between the heat transfer fluid and the ceramic material that makes up the packing. . For example, the reservoir has a cylindrical shape extending horizontally, and the inlet and the heat transfer fluid outlet are each arranged at one end of the reservoir.

Depending on the charge or discharge phase, the direction of circulation of the coolant within the tank can be reversed: the terms "input" and "output" are therefore relative. Such a device can in particular be implemented in a concentrated solar power station, but also in any installation requiring storage of sensitive energy. EXPERIMENTAL RESULTS

Several ceramic materials have been manufactured by extrusion as defined in Table 1. The parameters studied are: the composition of the mixture, the size of the phosphate particles, and the cooking temperature. As indicated above, the drying was carried out at 25, 45, 70 and 105 ° C with a 24 hour stage at each temperature. Materials containing no phosphate (CeramO, Ceraml, Ceram2) are considered as reference samples.

Table 1: List of prepared materials and associated characteristics

In the present text, the acronym CP designates synthetic hydroxyapatite of formula (Caio (P0 ₄₎ ₆ (OH) _2), whose size is d ₅ o of 5 pm; the acronym PN denotes a phosphate ore containing predominantly P ₂ O ₅ (30.4%), SiO ₂ (3.2%), Na ₂ O (0.7%), Al ₂ O ₃ (0.5% ), MgO (0.4%), Fe ₂ O ₃ (0.3%), K ₂ O (0.1%) (percentages by weight).

The distribution of the major elements present in some of these ceramics was studied by the MEB-EDX technique (Scanning Electron Microscopy associated with the X-ray Dispersive Energy Microanalysis) and the results are shown in Figures 1, 2 and 3. In Figure 1 (for the Ceraml sample), we find main elements of clay and sand such as Ca, Si, Al, Fe. Phosphorus is only in trace. On the contrary, in Figures 2 and 3, phosphorus is present in ceramics made with 16.7% by weight of CP or PN. It also appears that the phosphorus is homogeneously distributed in the clay-sand matrix when CP is used while it is less homogeneous when PN is used. Indeed, CP particles are smaller than those of PN and can therefore more easily fit into the clay-sand matrix.

Thermal conductivity is an important parameter of ceramic materials in storage of sensible heat. Indeed, it directly influences the heat transfer within the materials during the charging and discharging phases. Figures 4 and 5 show the evolution of the thermal conductivity as a function of the CP or PN content and the cooking temperature. The measurement was carried out by the Hot Disk method using a Kapton type probe (No. 5465). All measurements were carried out at 25 ° C on fired ceramics. Said Hot Disk method is based on the use of a probe placed between the samples to be characterized. Samples can be in powder form (in this case a sample holder is used) or in monolithic form. The probe is a resistive element acting both as a thin heat source, laterally limited, and as a temperature sensor. It consists of a 10 μm thick nickel film coated with a film of 25 to 30 μm thick kapton or 100 μm thick mica. On the metal film is drawn a double spiral circuit. During the measurement, the temperature rise in the sensor is accurately determined by the electrical resistance measurement. This increase in temperature strongly depends on the thermal transport properties of the material. By monitoring this temperature increase for a short time, it is possible to obtain accurate information on the thermal properties of the characterized material.

In general, the addition of phosphate makes it possible to increase the thermal conductivity of conventional terracotta ceramics. This increase can reach up to 20% compared to a ceramic tile without phosphate. The thermal conductivity can thus reach that of concrete which has a conductivity of the order of 1 to 1, 2 W / mK [4] The fact that the phosphate particles are inserted in the microstructure of the clay-sand matrix reduces the air pockets (porosity) in the structure of said matrix and therefore limit the resistance to the path of heat. This results in an improvement of the thermal conductivity. For a content of 5% by weight of phosphate, the thermal conductivity increases by about 7% (with PN, cooked at 1100 ° C.) and by 11% (with CP, baked at 1100 ° C.) compared with ceramic without phosphate. Thus, a phosphate content of at least 0.5% by weight makes it possible to significantly improve the thermal conductivity of ceramics.

Moreover, whatever the nature of the phosphate, the thermal conductivity increases with the increase of the cooking temperature. This is due to the densification and sintering of ceramics. In general, the firing temperature is preferably between 900 and 1150 ° C.

In general, the thermal conductivity varies with the temperature at which the material is exposed. Measurements in dynamics between 30 and 1000 ° C were carried out with a machine LFA 547 ™ of NETSCH. The conditions of these measurements are as follows: atmosphere: air; heating rate: 5 ° C / min; temperature: 30-1000 ° C, flash: 1826 V; stabilization criterion: linear ("baseline"). Figure 6 shows the evolution of the thermal conductivity of ceramics with or without the addition of phosphate depending on the temperature. The ceramics were previously fired at 1100 ° C. It is clearly observed that the two ceramics containing phosphate have a higher thermal conductivity than that of the phosphate-free ceramic in the temperature range studied. An increase of about 20% is thus observed at 900 ° C. In this case, there is little influence of the phosphate nature on the thermal conductivity.

Mechanical strength is also an important parameter of ceramic materials for the storage of sensible heat. Figures 7 and 8 show the evolution of the three-point bending fracture stress of ceramics prepared with or without the addition of phosphate. The measurement was carried out at 25 ° C. on specimens 60mm x 15mm x 9mm in flexion using an INSTRON ™ machine. The characteristics of the three-point bending test are as follows: travel speed: 2 mm / min; cell: 500N; diameter of the support rollers: 5 mm; diameter of the central support roller: 5 mm; gap between the rollers: 40 mm; end of the test: rupture of the test piece; temperature: ambient (20 ° C). Whatever the cooking temperature used, the addition of CP made it possible to increase the mechanical strength of ceramics (see Figure 7). In particular, with reference to the graph of FIG. 7, a phosphate content of at least 0.5% by weight makes it possible to significantly improve the mechanical strength of ceramics. The insertion of the fine particles of CP into the clay-sand matrix develops a new microstructure and thus contributes to strengthening the overall structure by removing pores present in the initially phosphate-free ceramic. On the other hand, the addition of PN particles, whose particle size is 100 μm, weakly decreases the mechanical strength (see Figure 8).

On the other hand, measurements of the mechanical resistance in dynamics by acoustic resonance between 30 and 1050 ° C were carried out on ceramics with or without addition of phosphate, which were previously fired at 1100 ° C. These measurements were carried out with an FDA HT650 oven marketed by IMCE ™, equipped with a microphone whose sensitivity is from 20 Hz to 50 kHz; the tests were carried out in the air, with temperatures ranging from 30 to 1050 ° C., according to a heating rate of 5 ° C./min. Figure 9 shows the results obtained. The ceramic containing 4.7% CP is much more resistant than the one without phosphate in the temperature range studied. The difference is estimated at nearly 25%.

In the storage of sensible heat, the specific heat of a material is an important parameter because it is directly proportional to the quantity of heat stored (see equation (1)). Figure 10 shows the specific heat of non-phosphate ceramics or with the addition of 4.7 wt% CP and 5 wt% PN in the temperature range of 30 to 1000 ° C. The ceramics were previously fired at 1100 ° C. For the three materials, the specific heat increases with the increase of temperature. That of non-phosphate ceramic varies from 0.74 J / g. K at 30 ° C to 1.16 J / g. K at 1000 ° C; that of the ceramic containing CP varies from 0.77 J / g. K at 30 ° C at 1.19 J / g. K at 1000 ° C; and that of the ceramic containing PN ranges from 0.75 to 30 ° C to 1.16 J / gK at 1000 ° C.

Concerning the NP which is an ore, the fine particles are obtained by grinding.

Figure 11 shows the evolution of flexural fracture stress as a function of the average particle size of PN. The PN content was set at 4.7% by mass. The smaller the average size of the PN particles, the greater the flexural rupture stress.

In storage of sensible heat, the storage material must have good thermal stability during many heating and cooling cycles. The thermal stability has been studied by thermogravimetric analysis which makes it possible to follow the evolution of the mass during cycles of heating and cooling. The ceramics were previously fired at 1100 ° C. Figure 12 shows the results obtained with two ceramics containing respectively 4.7% by weight of CP and 5% by weight of PN. The analysis conditions are: heating rate of 10 ° C / min; air atmosphere at 100 NmL / min, free cooling, temperature range 30 to 1000 ° C. Both ceramics have a very good thermal stability in the temperature range studied. The mass variation is less than 0.2% during the 50 repeated heating and cooling cycles under air. Thus, these ceramics can be used in high temperature solar power plants such as tower plants with temperatures up to about 900 ° C, but also in plants at moderate temperatures such as parabolic power plants where temperatures rarely exceed 400 ° C. These ceramics can also be used to recover heat present in flue gases from industrial plants up to about 1000 ° C. In general, they can be in contact with a heat transfer fluid at any temperature up to 1100 ° C.

Sensitive heat storage experiments have been carried out on a pilot scale. The schematic diagram of the pilot used is shown in Figure 13. It consists of a storage tank R of dimension 1, 4 mx 0.3 mx 0.3 m or a nominal storage volume of 0.126 m ³ . The tank is made of vermiculite (an insulating and inert material, thickness 0.1 m) and is surrounded by a fibrous insulation layer of rockwool (thickness 0.25 m); everything is finally surrounded by a layer of stainless steel. The tank is installed horizontally. It is equipped with 37 thermocouples to follow the evolution of the axial temperature throughout the tank. The coolant used is air. The arrows indicate the flow direction of said fluid in the reservoir. For the charging phase (a), a blower generates a constant air flow to feed a hot air gun which then feeds the storage tank. This gun is positioned just before the entrance of the storage tank. The hot-air gun used makes it possible to obtain a temperature ranging from 100 ° C. to 900 ° C. at the barrel outlet. For the discharge phase (b), the blower injects ambient air into the cold part of the tank to recover initially stored heat. Two thermocouples, a mass flow meter and two pressure sensors are also installed to control the flow of heat transfer fluid during the charging and discharging phases.

To evaluate the performance of the charging and discharging steps, various terms are used which are defined below:

• T _L : Temperature of the storage material at the beginning of the charging phase; or low temperature of the coolant (air) used for the discharge phase (° C).

• T _H : Temperature of the coolant (air) at the inlet of the storage tank during the charging phase; or high temperature of the storage material at the beginning of the discharge phase (° C).

• T _amb : Ambient temperature (° C).

• m: mass flow rate of air (kg / h).

• T _{cut-off / chg} : Temperature _threshold at the outlet of the storage tank where the charging phase is stopped.

• T _{cut-off / dis} : Temperature threshold at the outlet of the storage tank where the discharge phase is stopped.

• b: Threshold temperature coefficient used for calculating the temperatures T _cut - o _{ff / chg} and T _{cut-off / dis} according to the following equations.

for a load: T _{cut-off / chg} = T _L + bx (T _H -T _L ) (2)

for a discharge: T _{cut-0ff / dis} = T _L + (1- b) x (T _H -T _L ) (3)

• _{breakthrough time} : The time required to reach a value of T _{Cut-off / chg} during the charging phase or T _{cut-off / dis} during the discharge phase.

• E _max : Quantity of thermal energy calculated theoretically by equation (1) between T _L and T _H (kWh).

• E _Chg : Quantity of thermal energy stored in the storage material during the charging phase where the outlet temperature of the storage tank is lower than T _{cut-off / chg} ; E _Chg is calculated by equation (1) (kWh).

• r | _chg : _Load rate which is the ratio between E _Ch and E _max (%).

• E _dis : Quantity of thermal energy recovered during the discharge phase where the outlet temperature of the storage tank is greater than T _{cut-off / dis} ; E _is is calculated by equation (1) (kWh).

• n _dis : Discharge rate which is the ratio between E _dis and E _Chg (%).

• E _in : Quantity of thermal energy sent to the storage tank during the charging phase (kWh). • E _out : Quantity of thermal energy lost at the outlet of the storage tank during the charging phase, calculated by equation (1) between TH and T _C ut-off / ch _g (kWh).

• n _wh : Thermal losses which are the ratio between E _out and E _in (%).

· E: porosity of the storage tank filled with cylinders of ceramic material 15 mm in diameter and 40 mm in length (%).

Two ceramics were used for the pilot scale tests. The first contains 4.7% by weight of CP (Ceram9). The second contains 5% by mass in PN (Ceram35). These ceramics were prepared by the extrusion method and fired at 1140 ° C. They are in cylindrical form 15 mm in diameter and 40 mm in length.

This form was chosen in order to have a good exchange surface within the thermal storage system. By exchange surface is meant the outer surface of the ceramic material directly in contact with the coolant. In addition, this cylindrical shape is easily obtained by the extrusion method. For each experiment, 160 kg of material is required to fill the storage tank. The porosity of the storage tank filled by these cylinders is about 40%.

Example 1

This test was performed with Ceram9 ceramics. The charging and discharging conditions are shown in Table 2.

Table 2: Test conditions for Ceram9 material at moderate temperatures (T _H around 340 ° C)

Figure 14 and Table 3 show the results obtained during the charging phase. Figure 14 (a) shows the axial temperature profiles at different charging times. At a given charging time, the axial temperature decreases with the increase in the length of the storage tank. At a given storage tank length, the axial temperature increases with increasing charging time. Figure 14 (b) shows the evolution of the inlet (T1) and outlet (T2) temperature of the storage tank, and the evolution of the charge rate. The tank inlet temperature was quickly stabilized around the T _H. The outlet temperature of the tank was maintained at room temperature for about 0.75 h load. As a result, all of the heat injected into the vessel was absorbed by the material. Then this output temperature increases. This indicates that some of the injected heat is coming out of the tank (E _out , heat not absorbed by the material). Table 3 summarizes the results obtained at different T _C ut-off / ch _g · With increasing charging time (t _pe rcée), the charging rate increases and reaches 86.9% after 2.28 h load . As a result, the thermal losses increase (increase of q _W h) · However, at a load rate of 86.9%, thermal losses are only 14.1% which is an excellent result and which demonstrates the effectiveness of this material for storing the heat provided by the coolant.

Table 3: Summary of the results obtained at different T _{mt-nff / rhn} during the charging phase of Ceram9 material at moderate temperatures (T _H around 340 ° C)

Figure 15 and Table 4 show the results obtained during the discharge phase. In Figure 15 (a), the axial temperatures are presented as a function of the discharge time or the length of the storage tank. At a given length of the storage tank, the increase in the discharge time leads to a decrease in temperature. And at a given discharge time, the temperature drops with the length of the storage tank. In Figure 15 (b), the increase in discharge time is accompanied by the decrease of the outlet temperature and the increase of the discharge rate. After 2.28 h, the discharge rate reaches 93.6% as specified

Table 4: Summary of the results obtained at different T ^ _/ ^ _H during the discharge phase of Ceram9 material at moderate temperatures (T _H around 340 ° C). Example 2

This test was carried out with the same material used for Example 1, but at moderately high values of T _H (around 520 ° C.). Table 5 shows the conditions used.

Table 5: Test conditions for Ceram9 material at moderately high temperatures (T _H around 520 ° C)

Figure 16 and Table 6 summarize the results obtained for the charging phase. The inlet temperature is rapidly stabilized between 500 and 528 ° C after 30 minutes of charging. The output temperature stays close to room temperature for the first 30 min, then starts to increase. The charge rate increases with charging time and reaches 86.4% after 3.27 h. At this charge rate,

Table 6: Summary of the results obtained at different _{nff / rhns} during the phase of

loading Ceram9 material at moderately high temperatures (T _H around 520 ° Q)

Figure 17 and Table 7 show the results obtained during the discharge phase. The increase in discharge time leads to a consequent decrease in the outlet temperature and a consequent increase in the discharge rate (see Figure 17). After 3.9h of discharge, 94.2% of the amount of heat stored was returned

(Table 7) The results obtained for the two phases of charging and discharging show that the studied material is effective for the storage of sensible heat at moderately high temperatures.

Table 7: Summary of results obtained at different _{nff /} rii during the phase of

discharge of Ceram9 material at moderately high temperatures (T _H around 520 ° Q)

Example 3

This test was carried out with the same material as that used for Examples 1 to 2, but at high values of T _H (around 760 ° C.). Table 8 shows the conditions used.

Table 8: Test conditions for Ceram9 material at high temperatures (T _H around 760 ° C)

Figure 18 and Table 9 show results obtained for the charging phase. The inlet temperature is rapidly stabilized around 760 ° C after 60 min of charging. The output temperature stays close to room temperature for the first 60 minutes and then begins to increase. The charge rate increases with charging time and reaches 86.9% after 3.76 h. At this loading rate, heat losses are relatively low (q _W h of 13.9% only).

Table 9: Summary of the results obtained at different T _{mt-nff / rhn} during the charging phase of Ceram9 material at high temperatures (T _H around 760 ° C) Figure 19 and Table 10 show the results obtained for the discharge phase. The increase in discharge time is accompanied by a consequent decrease in the outlet temperature and a consequent increase in the discharge rate (Figure 19). After 4.58h discharge, 96.7% of the amount of heat stored was restored (Table 10). The results obtained for the two charging and discharging phases show that the studied material is effective for the storage of sensible heat at high temperatures around 760 ° C.

Table 10: Summary of the results obtained at different T _r "t _{nff /} rii during the discharge phase of Ceram9 material at high temperatures (T _H around 760 ° C)

Example 4

This test was carried out with Ceram35 ceramics, which contains 5% by mass of PN, at moderate temperatures T _H. Table 11 summarizes the conditions used for the charging and discharging phases.

Table 1 1: Test conditions for Ceram35 material at moderate temperatures (T _H around 350 ° C)

Figure 20 and Table 12 show results obtained for the charging phase. The inlet temperature is rapidly stabilized around 340-350 ° C after 30 min of charging. The output temperature stays close to room temperature for about 0.75h and then starts to increase. Charge rate increases with charging time and reaches 89.9% after 2.43 hours. At this loading rate, heat losses are relatively low (q _W h of 14.6% only).

Table 12: Summary of the results obtained at different T _{mt-nff / rhn} during the charging phase of Ceram35 material at high temperatures (T _H around 350 ° C)

Figure 21 and Table 13 show the results obtained for the discharge phase. The increase in discharge time is accompanied by a consequent decrease in the outlet temperature and a consequent increase in the discharge rate (Figure 21). After 2.06h discharge, the discharge rate is 84.2% (Table 13). The material Ceram35 is therefore effective for storage and retrieval of sensible heat at moderate temperatures (around 350 ° C).

Table 13: Summary of the results obtained at different T _r "t _{nff /} rii during the discharge phase of Ceram35 material at moderate temperatures (T _H around 350 ° C)

Example 5

The test of this example was carried out with Ceram35 material at moderately high temperatures (around 580 ° C). Table 14 shows the conditions used for this test.

Table 14: Test conditions for Ceram35 material at moderately high temperatures (T _H around 580 ° C) Figure 22 and Table 15 show results obtained for the charging phase. The inlet temperature is rapidly stabilized around 550-580 ° C after 60 min of charging. The output temperature remains close to room temperature for about 1, 25h and then starts to increase. The charge rate increases with charging time and reaches 89.6% after 3.50 h. At this load rate, heat losses are relatively low (n _Wh of 14.0% only).

Table 15: Summary of the results obtained at different T _{mt-nff / rhn} during the loading phase of Ceram35 material at medium high temperatures (T _H around 580 ° C)

Figure 23 and Table 16 show the results obtained for the discharge phase. The outlet temperature drops with the charging time. At the same time, the discharge rate increases (Figure 23). After 3.35h of discharge, the amount of heat initially stored was discharged at a rate of 92.8% (Table 16). These results show that the Ceram35 material is effective for storage and retrieval of sensible heat at moderately high temperatures (around 580 ° C).

Table 16: Summary of results obtained at different _{nff /} rii during the phase

discharge of Ceram35 material at moderately high temperatures (T _H around 580 ° C)

Example 6

The same material used for Examples 4 and 5 was tested at elevated temperatures (T _H around 850 ° C). The experimental conditions of this test are summarized in Table 17.

Table 17: Test conditions for Ceram35 material at high temperatures (T _H around 850 ° C)

Figure 24 and Table 18 show results obtained for the charging phase. The inlet temperature is stabilized around 800-850 ° C after 45 minutes of charging. The outlet temperature is close to room temperature for about 1 hour indicating that all of the injected heat has been absorbed by the material. Then this output temperature starts to increase. The charge rate increases with the charging time and reaches 86.3% after 2.91 h. At this loading rate, the thermal losses are relatively low (q _W h of 8.9% only).

Table 18: Summary of the results obtained at different T _{mt-nff / rhn} during the charging phase of the Ceram35 material at high temperatures (T _H around 850 ° C) Figure 25 and Table 19 show the results obtained for the phase discharge. With the charging time, the outlet temperature drops and at the same time the discharge rate increases (Figure 25). After 3.95h of discharge, 94% of the stored heat is returned (Table 19). These results show that the Ceram35 material is effective for storing and retrieving sensitive heat at high temperatures (T _H around 850 ° C).

Table 19: Summary of the results obtained at different T _r "t _{nff /} rii during the discharge phase of Ceram35 material at high temperatures (T _H around 850 ° C)

Other storage and retrieval tests were carried out with the two materials Ceram9 and Ceram35 with different T _H values and mass flow rate of the coolant (air). Tables 20 and 21 summarize the experimental conditions and the main results obtained for these tests. Whatever the temperature T _H tested and at a given mass flow rate of the heat transfer fluid, the results are reproducible. At a given temperature T _H , the increase in the mass flow rate of the heat transfer fluid makes it possible to reduce the charging time to reach the same charge rate. This observation is similar for the discharge phase. For the charging phase, in all cases, the thermal losses are relatively low (less than 19%). In other words, the materials used are effective for heat transfer with the heat transfer fluid under the conditions used.

Table 20: Experimental conditions and main results for all load and discharge tests obtained with Ceram9 ceramics (160 kg of ceramic, ceramic in the form of cylinders 15 mm in diameter and 40 mm in length).

Table 21: Experimental conditions and main results for all load and discharge tests obtained with Ceram35 ceramics (160 kg of ceramic, ceramic in the form of cylinders 15 mm in diameter and 40 mm in length)

REFERENCES

[1] Kuravi S., Trahan J., Goswami D.Y., Rahman M.M., Stefanakos E.K. Thermal energy storage technologies and systems for concentrating solar power plants. Progress in Energy and Combustion Science 39 (2013) 285-319.

[2] Dintera, F., Gonzalez, D.M. Operability, reliability and economy benefits of CSP with thermal energy storage: first year of operation of ANDASOL 3. Energy Procedia 49 (2014) 2472-2481.

[3] Rellosoa, S., Garcia, E. Tower technology cost reduction approach after Gemasolar experience. Energy Procedia 69 (2015) 1660-1666.

[4] D. Laing and S. Zunft. 4 - Using Concrete and Other Solid Storage Media in Thermal Energy Storage (TES) Systems. Advances in Thermal Energy Storage Systems, pp 65-86.Woodhead Publishing, 2015.

[5] Murray H., Applied Mineralogy Clay, 1 ^st Edition, Elsevier Science, 2007 (Hardcover ISBN: 9780444517012).

Claims

1. A method of manufacturing a ceramic material for the thermal storage of energy, characterized in that it comprises the production of a mixture of at least clay particles and particles of natural phosphate and / or synthetic , and water, said mixture comprising between 0.5% and 40% by weight of phosphate relative to the mass of the mixture with the exception of water, and the shaping and baking of said mixture to obtain the ceramic material.

2. Method according to one of claim 1, wherein the mixture comprises between 4% and 5% by weight of phosphate relative to the mass of the mixture with the exception of water.

3. A process according to claim 1 or claim 2, wherein the mixture comprises between 50 and 90% by weight of clay, preferably between 60 and 80% by weight.

4. Method according to one of claims 1 to 3, wherein the average size (d ₅ o) of clay particles and phosphate is less than 1 mm.

5. Method according to one of claims 1 to 3, wherein the mixture further comprises up to 40% by weight of sand particles, preferably between 10 and 30% by weight.

The method of claim 5, wherein the average size (d ₅ o) of the sand particles is less than 1.5 mm.

7. Method according to one of claims 1 to 6, further comprising forming the ceramic material by one of the following techniques: extrusion, granulation, molding, compacting or pressing the mixture.

The method of one of claims 1 to 7, further comprising, after the shaping step, drying the ceramic material at a temperature of 105 ° C or less.

9. The method of claim 8, wherein the firing of the ceramic material is carried out at a temperature between 800 and 1200 ° C, preferably between 900 and 1150 ° C.

Ceramic material for storing thermal energy, characterized in that it comprises a matrix of clay and, if appropriate, of sand, and particles of a natural phosphate and / or a synthetic phosphate dispersed in said matrix, said ceramic material comprising between 0.5% and 40% by weight of phosphate relative to the mass of the ceramic material.

1 1. Material according to claim 10, in the form of a cylinder, a sphere, a cube, a spiral, a flat plate, a corrugated plate, a brick hollow or Raschig ring.

12. A method for storing thermal energy in a ceramic material, characterized in that it comprises contacting a heat transfer fluid with the ceramic material according to one of claims 10 or 1 1, so as to transfer the heat of the heat transfer fluid to the ceramic material in a charging phase, and transferring heat from the ceramic material to the heat transfer fluid in a discharge phase.

13. The method of claim 12, wherein the ceramic material is contained in a reservoir.

14. The method of claim 13, wherein the reservoir is formed of at least one thermally insulating material.

15. Method according to one of claims 12 to 14, wherein the coolant is selected from air, water vapor, oil or molten salt.

16. Method according to one of claims 12 to 15, wherein, during the charging phase and / or the discharge phase, the heat transfer fluid is at a temperature between 20 and 1100 ° C.

17. Thermal energy storage device for carrying out the method according to one of claims 12 to 16, comprising a reservoir containing the ceramic material and a circulation circuit of a heat transfer fluid in fluid connection with the reservoir. so as to put said coolant in contact with the ceramic material.