Patent Application
20200407616
The present invention relates to a thermal storage material, more specifically for the storage of sensible heat, as well as a method for manufacturing such a material, and a thermal storage method implementing said material.
Thermal storage consists in storing heat in a medium for later use. This medium is composed of a specific material called thermal storage material.
Three thermal energy storage methods exist: storage of sensible heat, storage of latent heat and storage by thermochemical process [1]. As indicated above, the present invention relates to materials for the storage of sensible heat.
The storage of sensible heat consists in a simple rise in the temperature of the storage material. The amount of heat stored by the material is given by the following equation:
Q=∫
T
T
mC
p(T)dT (1)
with Q the amount of heat stored (J); Ti and Tf the initial and final storage temperatures (K), respectively; m the weight of the storage material (g); Cp(T) the calorific value of the storage material (J/g·K).
A material for storing sensible heat may be a liquid or a solid.
The case of liquid materials has experienced industrial success with molten salts based on alkali nitrates which are used in CSP (Concentrated Solar Power) plants. Several CSP plants are currently in operation [2]-[3]. However, molten salts have weaknesses linked to their important use as fertilisers in agriculture, with a risk of complete chemical decomposition above 565° C., and to their high price.
Hence, solid materials such as concrete have been tested by the German Aerospace Centre, in German Deutsches Zentrum für Luft—and Raumfahrt, better known by the abbreviation DLR [4]. Concrete is available in industrial quantities at competitive cost. However, its use is limited to around 400° C. to avoid mechanical damage.
The development of materials that are more stable, more efficient and more advantageous economically thus remains necessary.
An aim of the invention is thus to design a material for thermal storage which can be easily shaped by an industrial method, be available in industrial quantities, and be used over a wide temperature range, being capable of going up to 1100° C.
For this purpose, the invention proposes a method for manufacturing a ceramic material for thermal energy storage, characterised 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 compared to the weight of the mixture with the exception of water. The method also comprises the steps of shaping and firing the mixture to obtain the ceramic material.
Said natural and/or synthetic phosphate may notably comprise hydroxyapatite.
In a particularly advantageous manner, the mixture comprises between 4% and 5% by weight of phosphate. In the remainder of the text, contents by weight are calculated compared to the total weight of the mixture excluding water.
Said mixture advantageously comprises between 50 and 90% by weight of clay, preferably between 60 and 80%.
Preferably, the average size of the clay and phosphate particles is less than 1 mm.
According to an 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 the shaping of the ceramic material by one of the following techniques: extrusion, granulation, moulding, compacting or pressing of the mixture.
The method may comprise, after the shaping step, the drying of the ceramic material at a temperature less than or equal to 105° C.
The method may comprise, after the drying step, the firing of the ceramic material at a temperature comprised between 800 and 1200° C., preferably between 900 and 1150° C.
Another object of the invention relates to a ceramic material for thermal energy storage, capable of being obtained by the method such as described above. Said ceramic material comprises a matrix of clay and, if appropriate, 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 compared to the weight 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 a Raschig ring.
Another object of the invention relates to a thermal energy storage method implementing such a material. Said method comprises placing a heat transfer fluid in contact with the ceramic material described above, so as to transfer heat from the heat transfer fluid to the ceramic material in a charge 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 tank is advantageously formed of at least one thermally insulating material.
The heat transfer fluid is typically selected from air, water vapour, an oil or a molten salt.
During the charge phase and/or the discharge phase, the heat transfer fluid is at a temperature comprised between 20 and 1100° C.
Finally, the invention relates to a device for the implementation of said energy storage method. Said device comprises a tank containing the ceramic material and a heat transfer fluid circulation circuit in fluidic connection with the tank so as to place said heat transfer fluid in contact with the ceramic material.
Other characteristics and advantages of the invention will become clear from the detailed description that follows, with reference to the appended drawings in which:
The inventors have demonstrated the possibility of obtaining a ceramic material having excellent aptitude to thermal energy storage by mixing particles of clay, sand, phosphate, and water. Said mixture may in fact have a plasticity favourable for the implementation of different techniques, such as extrusion, granulation, moulding or pressing, which enable the ceramic material to be shaped into a form suitable for thermal energy storage.
In the present text, ceramic is taken to mean a material in solid form having undergone a firing cycle.
Conventional earthenware ceramics are manufactured from a mixture of clay, sand and water.
Clays have a structure in the form of lamina enabling water molecules to be interposed between said lamina. This confers on them a plastic property and offers them the possibility of being used as plastifiers or structuring agents. The plastic property of clays is a decisive parameter for the shaping of earthenware ceramic materials.
Globally, clays exist in several mineralogical forms grouped together into four families [5]. They are kaolinites (Al2Si2O5(OH)4), illites (K(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,H2O], smectites ((Ca,Na)0.3 (Al,Mg,Fe)2(Si,Al)4O10(OH)2,nH2O) and chlorites.
Clay is a natural material, available in industrial quantities with good plasticity compared to several other binders such as polyvinyl alcohol, gelatine, polyethylene glycol or polyacrylic acid, which are used in different industrial methods.
Sands are inert materials without plasticity which are essentially composed of quartz and other minerals such as feldspaths and micas. In the earthenware industry, sands are used as tempers to facilitate the drying step. Their use makes it possible to obtain in the clayey matrix a skeleton conducive to the dehydration of clayey minerals. This prevents important shrinkages which can lead to fissuring of the materials.
An important family of phosphates exists, which are either natural (phosphate ores), or synthetic. They are formed from phosphate anions (orthophosphate (PO4)3−) and metal cations M where M may be an alkali, an alkaline-earth or any metal of the periodic table of elements. This diversity makes it possible to obtain phosphated products with highly 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 (Ca10(PO4)6(OH)2), or even a mixture of these two types of phosphate.
The presence of a phosphate incorporated within the matrix of clay (which further optionally contains sand) makes it possible to improve the physical, thermal and mechanical properties of the ceramic material, notably the density, the thermal conductivity, the calorific value or the mechanical stability.
According to an advantageous embodiment, extrusion is a simple and well-mastered shaping technique for the production on a large industrial scale of ceramic materials intended for thermal energy storage, and which is suited for the mixture described above. Extrusion consists in passing the mixture, at a controlled pressure, through a double helix, then a worm screw before making it come out through a die in monolithic form. This technique makes it possible to obtain ceramic materials of different shapes: cylindrical, alveolar, flat plate, corrugated plate, hollow brick, etc. Those skilled in the art choose the size and the shape of the ceramic materials in order to control heat exchanges during the storage and the de-storage of sensible heat.
However, this embodiment is not limiting and the mixture may be shaped by other techniques such as granulation, moulding, compacting or pressing. For example, granulation is advantageous in that it makes it possible to obtain materials of spherical shape of different sizes.
Generally speaking, the ceramic material may have the following shapes: 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 as a function of the desired shape.
The composition of the mixture is controlled to have good plasticity with a view to the shaping step, and to obtain physical, thermal and mechanical properties appropriate for the storage of sensible heat.
For this purpose, the added phosphate content may reach up to 40% by weight (i.e. 17% by weight of P2O5), and is comprised between 0.5% and 40%, preferably comprised between 4% and 5% by weight (in the present text, the reference weight is that of the dry mixture (not including added water)). In all cases, the phosphate content is not zero. A phosphate content of at least 0.5% by weight makes it possible to improve significantly the thermal conductivity and the mechanical strength of ceramics. A phosphate content less than 40% by weight makes it possible to guarantee good plasticity of the mixture of clay, phosphate, and water and facilitates the later shaping thereof.
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 clayey mixture (source deposit). Phosphate may replace all or part of the sand. Thus, it is possible to be free of sand in the mixture for example when important amounts of phosphate are added (of the order of 20 to 40%). In the remainder of the text, for the sake of brevity, the term “clay-sand matrix” covers a possible absence of sand.
The clay content may vary between 50 and 90% by weight, preferably between 60 and 80%.
The water content is adjusted in such a way as to confer on the mixture a pasty consistency, the viscosity of which is suited to the retained shaping technique. This water will be eliminated in the course of later thermal treatments (namely drying and firing).
The size of the particles of the mixture is also controlled because it influences the final properties of the ceramic material. Size is taken to mean in the present text the diameter of a sphere having the same volume as the considered particle; in the case of a spherical particle, the size is the diameter of the particle. In so far as the particles generally have a variable size within a determined range, the median size, noted d50, is considered that is to say the size for which 50% of the particles have a smaller size and 50% of the particles have a larger size.
Thus, the size d50 of the phosphate particles is advantageously less than 1 mm; that of the clay and the sand is preferably less than 1 and 1.5 mm, respectively.
After the shaping step, thermal treatments by drying and by firing are applied.
Drying is advantageously carried out in stages, at different temperatures which do not exceed 105° C. According to a preferred embodiment, the drying comprises successively a first stage at 25° C., a second stage at 45° C., a third stage at 70° C. and a fourth stage at 105° C. Each stage is applied for a determined duration which may be identical or different from one stage to the next. Preferably, the duration of each stage is 24 h. Such a drying by stages makes it possible to evacuate water progressively and thus to avoid generating strains in the material. At the end of drying, the material does not in principle contain any more water.
Firing is carried out after the drying step. It may be carried out in a static oven or in a tunnel oven. A moderate temperature rise ramp is applied, preferably 5° C./min, in order to avoid generating strains in the material. The firing temperature applied may vary between 800 and 1200° C., preferably between 900 and 1150° C. The stage at the firing temperature is comprised 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 described hereafter demonstrate, said ceramic material has good thermal energy storage properties.
The ceramic material may thus be used for the implementation of a thermal energy storage method. For this purpose, the ceramic material is placed in contact with a heat transfer fluid in such a way as to enable an exchange of heat.
The heat thus discharged may be used for the generation of electricity, for the heating of a room, or for any other use.
The heat transfer fluid may be a gas or a liquid. For example, but in a non-limiting manner, the heat transfer fluid may be air, water vapour, an oil or a molten salt.
For the implementation of said thermal storage method, the ceramic material is in the form of a plurality of units which together constitute a packing. The size and shape of these units is chosen to maximise the contact surface with the heat transfer fluid.
Said packing is arranged in a tank which is made of one or more thermally insulating material(s).
The tank is in fluidic connection with a heat transfer fluid circuit. Advantageously, the tank has a heat transfer fluid inlet and outlet, arranged with respect to one another in such a way as to ensure as large a contact surface as possible between the heat transfer fluid and the ceramic material which composes the packing. For example, the tank has a cylindrical shape extending horizontally, and a heat transfer fluid inlet and outlet are each arranged at one end of the tank.
Depending on the charge or discharge phase, the direction of circulation of the heat transfer fluid within the tank may be reversed: the terms “inlet” and “outlet” are thus relative.
Such a device may notably be put in place in a concentrated solar power plant, but also in any installation requiring sensible energy storage.
Several ceramic materials were manufactured by extrusion as defined in Table 1. The studied parameters were: the composition of the mixture, the size of the phosphate particles, and the firing temperature. As indicated above, drying was carried out at 25, 45, 70 and 105° C. with a 24 h stage at each temperature. The materials not containing phosphate (Ceram0, Ceram1, Ceram2) are considered as reference samples.
In the present text, the acronym CP designates synthetic hydroxyapatite of formula (Ca10(PO4)6(OH)2), of which the size d50 is 5 μm; the acronym PN designates a phosphate ore mainly containing P2O5 (30.4%), SiO2 (3.2%), Na2O (0.7%), Al2O3 (0.5%), MgO (0.4%), Fe2O3 (0.3%), K2O (0.1%) (weight percentages).
The distribution of major elements present in certain of these ceramics was studied by the SEM-EDX (Scanning Electron Microscopy associated with Energy Dispersive X-ray spectroscopy) technique and the results are shown in
Thermal conductivity is an important parameter of ceramic materials for the storage of sensible heat. Indeed, it directly influences the transfer of heat within materials, during the charge and discharge phases.
In general, the addition of phosphate makes it possible to increase the thermal conductivity of conventional earthenware ceramics. This increase may reach up to 20% compared to an earthenware ceramic without phosphate. The thermal conductivity may thus reach that of concrete, which has a conductivity of the order of 1 to 1.2 W/m.K [4]. The fact that the phosphate particles are inserted in the microstructure of the clay-sand matrix makes it possible to reduce the air pockets (porosities) in the structure of said matrix and consequently to limit the resistance to heat travel. The result is an improvement in thermal conductivity. For a phosphate content of 5% by weight, the thermal conductivity increases by around 7% (with PN, fired at 1100° C.) and 11% (with CP, fired at 1100° C.) compared to a ceramic exempt of phosphate. Thus, a phosphate content of at least 0.5% by weight makes it possible to improve significantly the thermal conductivity of ceramics.
Furthermore, whatever the nature of the phosphate, the thermal conductivity increases with the increase in the firing temperature. This is explained by the densification and the sintering of ceramics. Generally speaking, the firing temperature preferably is between 900 to 1150° C.
In general, the thermal conductivity varies with the temperature to which the material is exposed. Dynamic measurements between 30 and 1000° C. were performed with a NETSCH LFA 547™ apparatus. The conditions for these measurements were the following: atmosphere: air; heating rate: 5° C./min; temperature: 30-1000° C., flash: 1826 V; stabilisation criterion: linear (baseline).
Mechanical strength is also an important parameter of ceramic materials for the storage of sensible heat.
Furthermore, dynamic mechanical strength measurements by acoustic resonance between 30 and 1050° C. were carried out on the ceramics with or without addition of phosphate, which were fired beforehand at 1100° C. These measurements were carried out with a FDA HT650 furnace sold by IMCE™, equipped with a microphone of which the sensitivity was 20 Hz to 50 kHz; the tests were carried out in air, with temperatures varying from 30 to 1050° C., according to a heating rate of 5° C./min.
In sensible heat storage, the specific heat of a material is an important parameter because it is directly proportional to the amount of heat stored (cf. equation (1)).
Concerning PN, which is an ore, fine particles were obtained by grinding.
In sensible heat storage, the storage material must have good thermal stability during numerous heating and cooling cycles. The thermal stability was studied by thermogravimetric analysis which makes it possible to monitor the evolution of the weight during heating and cooling cycles. The ceramics were fired beforehand at 1100° C.
Sensible heat storage experiments were carried out at the pilot scale. A schematic diagram of the pilot used is shown in
To evaluate the performance of the charge and discharge steps, different terms are used which are defined hereafter:
for a charge: Tcut-off/chg=TLβ×(TH−TL) (2)
for a discharge: Tcut-off/dis=TL+(1−β(3)×(TH−TL) (3)
Two ceramics were used for the tests at the pilot scale. The first contained 4.7% by weight of CP (Ceram9). The second contained 5% by weight of PN (Ceram35). These ceramics were prepared by the extrusion method and fired at 1140° C. They were of cylindrical shape of 15 mm diameter and 40 mm length. This shape was chosen in order to have a good exchange surface within the thermal storage system. Exchange surface is taken to mean the outer surface of the ceramic material directly in contact with the heat transfer fluid. In addition, this cylindrical shape is easily obtained by the extrusion method. For each experiment, 160 kg of material were needed to fill the storage tank. The porosity of the storage tank filled by these cylinders was around 40%.
This test was carried out with the ceramic Ceram9. The charge and discharge conditions are shown in Table 2.
This test was carried out with the same material used for example 1, but at moderately high values of TH (around 520° C.). Table 5 shows the conditions used.
This test was carried out with the same material as that used for examples 1 to 2, but at high values of TH (around 760° C.). Table 8 shows the conditions used.
This test was carried out with the ceramic Ceram35, which contains 5% by weight of PN, at moderate temperatures TH. Table 11 summarises the conditions used for the charge and discharge phases.
The test of this example was carried out with the material Ceram35 at moderately high temperatures (around 580° C.). Table 14 shows the conditions used for this test.
The same material used for examples 4 and 5 was tested at high temperatures (TH around 850° C.). The experimental conditions of this test are summarised in Table 17.
tbreakpoint
Other storage and de-storage tests were carried out with the two materials Ceram9 and Ceram35 at different values of TH and mass flow rate of the heat transfer fluid (air). Tables 20 and 21 summarise the experimental conditions and the main results obtained for these tests. Whatever the temperature TH tested and at a given mass flow rate of the heat transfer fluid, the results are reproducible. At a given temperature TH, the increase in the mass flow rate of the heat transfer fluid makes it possible to reduce the charge time to reach the same level of charge. This observation is similar for the discharge phase. For the charge phase, in all cases, thermal losses are relatively low (less than 19%). In other words, the materials used are efficient for the transfer of heat with the heat transfer fluid in the conditions used.
[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 economic 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 clay mineralogy, 1st Edition, Elsevier Science, 2007 (Hardcover ISBN: 9780444517012).
| Number | Date | Country | Kind |
|---|---|---|---|
| 1851786 | Mar 2018 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2019/050459 | 2/28/2019 | WO | 00 |
