WO2023046767A1 - Composite for thermochemical reactor - Google Patents

Abstract

A composite (1) for a thermochemical reactor, comprising: - a metal foam (2) comprising a plurality of open cells (3), the average size of a cell (3) being between 50 μm and 500 μm; - crystallites (5) of hydrophilic salts that are able to react reversibly with water vapor in a hydration reaction to form crystallites (5) of salt hydrates, the crystallites (5) being retained within the cells (3) of the foam (2).

Description

Description

Title: Composite for thermochemical reactor

Technical area

The present invention relates to a composite for a thermochemical reactor, a method for manufacturing such a composite, a thermochemical energy storage and release unit and a method for using such an energy storage and release unit. energy by thermochemical means.

Prior technique

To generate heat, it is known to produce a thermochemical reactor module based on the exothermic reaction of hydration of salt powders by steam, which can provide several hundred kilojoules per kilogram of salt powder. The advantage of this reaction is that it is reversible, with the addition of heat.

However, as hydration cycles are carried out on the same salt powders, they become compacted, which leads to a loss of porosity. The loss of porosity impacts the hydration capacity by reducing the exchange surfaces and the water vapor transport kinetics.

Moreover, another disadvantage of this method is the low thermal conductivity of the salt.

There is thus a need to benefit from a thermochemical reactor module which does not have these two drawbacks and which allows good thermal efficiency, stable over time after implementation of a large number of hydration cycles.

There is still a need to have a unit for storing and restoring energy by thermochemical means using such a reactor module, as well as a method for using such an installation.

Disclosure of Invention

The invention meets all or part of these needs and it achieves this thanks to, according to one of its aspects, a composite for a thermochemical reactor comprising:

- a metal foam comprising a plurality of open cells, the average size of a cell being between 50 μm and 500 μm, in particular between 50 μm and 300 μm, - crystallites of hydrophilic salts capable of reversibly reacting with water vapor in a hydration reaction to form crystallites of salt hydrates, the crystallites being retained within the cells of the foam.

Thanks to the invention, there is a composite for a thermochemical reactor which makes it possible to preserve the arrangement of the crystallites so as to preserve the exchange surfaces, the crystallites being trapped in the open cells of the foam.

The crystallites are preferably attached along the reticles of the cells of the foam. This allows them to have a stable position within the composite. This allows at the same time to have a good circulation of water vapor through the cells.

The configuration of the composite thus makes it possible to ensure the transfer of mass, that is to say of water vapour, through the reactive medium formed by the crystallites, in order to allow their optimal hydration and the formation of hydrates of salts from hydrophilic salts.

In addition, the skeleton of the metal foam simultaneously ensures:

- the mechanical strength of the composite;

- conservation of the specific exchange surface of the reactive medium of the salt crystallites by maintaining the crystallites substantially isolated from each other and by limiting their agglomeration during the hydration cycles,

- the optimal permeability of the composite to the advection of water vapor which is the reactant, and

- the transport of heat generated within the composite by the hydration reaction of the salt crystallites to an external exchanger, which ensures thermal exploitation for domestic heating and hot water production applications, for example.

By “average size”, we mean the apparent diameter of a cell. It is obtained by the arithmetic mean carried out on a large number, that is to say greater than 10, of cells.

To measure the average size of a cell, that is to say the porosity of the foam, the linear intercept method can be used. It is a question of: i) tracing on a photographic plate of the foam (2D projection) segments of straight lines in random directions, ii) to count the number N of intersections of each line with the metallic crosshairs delimiting the cells of the foam.

The length of the intercept segments makes it possible to obtain from N an apparent diameter of the cells, which can be corrected by a factor of 1.6 to take into account the approximation by projection on a 2D image of the real 3D structure . A number of cells per cm is thus obtained, which makes it possible to define an average cell size. Cells are made up of open walls called reticles. The cells give the foam its porosity.

The crystallites preferably have an average size of between 10 μm and 150 μm, more preferably between 10 μm and 100 μm.

To measure the average crystallite size, the linear intercept method described above can be used.

The average cell size of less than 500 μm makes it possible to incorporate a large quantity of crystallites, advantageously limiting the average size of the crystallites to less than 100 μm in order to obtain large specific exchange surfaces.

The morphology of the porosity of the cells of the foam is for example of the tetrakaidecahedron, tetradecahedron or truncated octahedron type.

The metal can occupy more than 10% of the total volume of the foam, for example 15%. The porosity can therefore be between 90% and 95% of the total volume of the foam.

Hydrophilic salts preferably have an enthalpy of hydration greater than 200 kJ/kg of material.

The crystallite salt hydrates may be chosen from the group consisting of CaCh-(2 to 6)H ₂ O, MgCh-(1 to 6)H ₂ O, Na ₂ S-(0.5 to 9)H ₂ O , MgSO ₄ -7H ₂ O and Na ₂ SO ₄ -10H ₂ O, this list being non-exhaustive.

This list of salts/salt hydrates illustrates the different degrees of hydration considered. The salts can be used pure or mixed in variable proportions. The choice is essentially determined by economic constraints, in particular the cost per kilogram of salt, and technical constraints, in particular the control of the degree of hydration.

The constituent metal of the foam is for example chosen from the group consisting of aluminum, nickel, copper and their alloys.

The composite may comprise an organic binder, preferably insoluble in water, chosen in particular from the group consisting of thermoplastic polymers, suitable to withstand temperatures between 50°C and 150°C. Such a binder can cover, in particular by coating, at least part of the reticles of the cells of the foam, preferably at least 75% of the surface of the reticles, or even more than 90%, in particular the entire surface of the reticles of the cells. foam.

By the fact that it is insoluble in water, the organic binder makes it possible to fix micro-crystallites of salt (nuclei) on the reticles, which can subsequently grow from a solution saturated with salt, or else serve as nucleation sites for new crystallites that precipitate from solution.

The binder is preferably heat resistant over a temperature range of 50°C to 150°C.

The binder is advantageously thermoelastic in order to ensure the maintenance of the crystallites during thermal cycles during the use of the composite.

The presence of the binder, in particular thermoelastic, can ensure the protection of the foam against the corrosion which takes place in the presence of salts and water vapour.

The binder can be organic, hardening irreversibly by polymerization.

The foam has for example a plate shape, a parallelepiped shape or a cylindrical shape, suitable for packaging (containing) the composite (content), in order to constitute modular thermochemical modules. The form of sheets is suitable for packaging in platforms. The cylindrical shape is recommended for packaging in tubes. The shape of the foam can be arbitrarily defined by the use made of the composite. The same is true for the dimensions of the foam, which can vary between 5 and 50 cm, depending on the packaging and the desired application.

Another subject of the invention, according to another of its aspects, in combination with the foregoing, is a process for manufacturing a composite for a thermochemical reactor as defined above, the process comprising a foam manufacturing step consisting to impregnate, with the metal of the foam in the molten state, the porosity of a sacrificial granular matrix, in particular a sacrificial matrix in common salt (NaCl).

The shape of the foam cells can be determined by the nature of the sacrificial matrix, that is to say by the geometry of the NaCl crystallites. The metal foam is advantageously manufactured to specifically meet the technical constraints of controlling the open porosity and its average size.

Impregnation can be done by suction or by injection.

The foam manufacturing step is planned in such a way as to make it possible to obtain the average size of the cells desired for the foam, that is to say between 50 μm and 500 μm.

The method advantageously includes a step of inserting salt crystallites into the foam.

This step aims to substitute the common salt, which is used as a model material, an appropriate salt hydrate, such as: 1) the chlorides of general formula MCli or 2 -nLLO, where n denotes an integer number of molecules of water and M denotes an alkaline-earth or bivalent metal cation such as Ca, Mg, 2) sulphides, such as for example Na2S, 3) sulphates, such as for example MgSO4, Na2SO4.

The insertion step, according to a first embodiment, may comprise:

- soaking the foam in a bath of a saline solution, in particular a saturated saline solution,

- the evaporation of the solution, and

- the realization of a direct precipitation of the crystallites within the cells of the foam during the evaporation of the solution. This is advantageously stimulated by heating, the temperature of which is adapted to the precipitation kinetics, which depends on the type of salt.

This results in a heterogeneous spatial distribution in density, i.e. in quantity, and in crystallite sizes. Depending on the precipitation kinetics, the size of the crystallites can vary between a few pm and several hundred pm. Precipitation kinetics influence crystallite density, morphology and size. The kinetics are controlled by the temperature and the ambient humidity, which imposes the evaporation kinetics, as well as by the nature of the salt used.

In the case of this first embodiment, the method can also comprise a step, prior to the insertion step, of seeding the foam with micro-crystallites, comprising soaking the foam in a saline solution, including a saturated saline solution, and drying in ambient air. This results in a homogeneous distribution of fine micro-crystallites, with an average size of less than 100 μm along the reticles of the foam, but in small quantities. This preliminary step of seeding the foam can promote the growth of crystallites from the micro-crystallites seeded during the subsequent insertion step.

According to a second embodiment, the insertion step comprises:

- moistening the foam or coating the foam with an organic binder, preferably insoluble in water, and

- carrying out sieving/vibration of a powder of salt crystallites through the network of the foam.

The salt crystallite powder can be finely graded, with an average particle size of less than 100µm.

This embodiment results in a homogeneous distribution of agglomerated crystallites along the reticles of the foam. They can serve as nucleation sites, from which can grow crystallites by precipitate from a saturated solution, as described in the first embodiment, without having to resort to seeding by initial soaking in saturated solution and drying. .

Whatever the step of inserting the crystallites implemented among the two embodiments described above, this step makes it possible to keep the porosity of the foam open to allow the mass transfers, that is to say to water vapour, through the reactive material constituted by the crystallites.

Another subject of the invention, according to another of its aspects, in combination with the foregoing, is a thermochemical energy storage and release unit, comprising at least one thermochemical reactor module comprising at least one composite such as defined above and a heat exchanger enclosure allowing the circulation of a heat transfer fluid inside the enclosure and housing said at least one thermochemical reactor module.

Said at least one thermochemical reactor module advantageously comprises a container, preferably metallic, housing at least one composite and having at least one opening to allow the exchange of water vapor between the composite and a condensation tank of the steam. water.

The unit advantageously comprises a plurality of modules of thermochemical reactors. The modules of thermochemical reactors can be crossed by a or several perforated connecting tubes which have along their length at least one opening opening into each module. The or each connecting tube can be connected to said condensation tank and be configured to allow the exchange of water vapor between the composite and said tank. The containers of the modules of thermochemical reactors are advantageously in contact, externally, with the heat transfer fluid. The connecting tube(s) can pass through several modules of thermochemical reactors.

Another subject of the invention, according to another of its aspects, in combination with what has been defined above, is a method of using a unit for storing and restoring energy by thermochemical means as defined above. , comprising the following steps:

(a) Step a: step of storing energy in said unit comprising heating said at least one thermochemical reactor module so as to at least partially dehydrate the crystallites of salt hydrates and to release water vapor which is evacuated, in particular via the connecting tube or tubes, then condensed in a condensation tank,

(b) Step b: energy restitution step in said unit in which the water vapor is reintroduced, in particular through the connecting tube(s), into the composite(s) of said at least one thermochemical reactor module and in which the production of heat released by the hydration reaction of the salt hydrates heats the heat transfer fluid which is routed to one or more domestic installations.

The operating conditions, in particular the temperature range and the partial water vapor pressure, depend on the type of hydrate salt and the allowable degree of hydration. The melting temperature of salt hydrate decreases with the degree of hydration.

The operating conditions are therefore defined by the degree of hydration desired to avoid deliquescence and melting. These two states must be avoided to preserve the powdery character with open porosity of the composite. For example, if the CaCh salt is used at its maximum degree of hydration (6 molecules of water), the melting temperature is only 30°C.

The operating temperature ranges of the thermochemical reactor and the degrees of hydration of the recommended salt hydrate depend on the salt considered. By For example, if the CaCh salt is used, the degree of hydration is advantageously limited to 2 or 3 for a total hydration of degree 6, with temperature ranges of 50 to 150°C.

Brief description of the drawings

The invention may be better understood on reading the detailed description which follows, non-limiting examples of implementation thereof, and on examining the appended drawing, on which

[Fig 1] Figure 1 is a photograph of an example of a composite according to the invention,

[Fig 2] Figure 2 is an enlarged photograph of a portion of an example of foam for the production of the composite according to the invention,

[Fig 3] Figure 3 is a schematic perspective view of a portion of composite according to the invention,

[Fig 4] Figure 4 is a photograph of a portion of composite according to the invention during manufacture using the method according to the invention,

[Fig 5] Figure 5 is a photograph of a portion of composite according to the invention during manufacture using the method according to the invention,

[Fig 6] Figure 6 is a photograph of a portion of composite according to the invention during manufacture using the method according to the invention,

[Fig 7] Figure 7 is a photograph of a portion of composite according to the invention during manufacture using the method according to the invention,

[Fig 8] Figure 8 is a photograph of a portion of composite according to the invention during manufacture using the method according to the invention,

[Fig 9] figure 9 schematically illustrates an example of a set of thermochemical reactor modules respectively comprising composites according to the invention,

[Fig 10] Figure 10 is a schematic view of an example of a thermochemical energy storage and release unit using the thermochemical reactor modules illustrated in Figure 9, and seen in its thermochemical storage phase,

[Fig 11] figure 11 is a schematic view similar to figure 10, the unit being seen in its thermochemical restitution phase,

[Fig 12] figure 12 schematically represents another example of a set of thermochemical reactor modules respectively comprising composites according to the invention, [Fig 13] Figure 13 is a schematic view of an example of a thermochemical energy storage and release unit using the thermochemical reactor modules illustrated in Figure 12, and shown in its thermochemical storage phase,

[Fig 14] figure 14 is a schematic view similar to figure 13, the unit being represented in its thermochemical restitution phase,

[Fig 15] Figure 15 represents four measurement cycles by differential scanning calorimetry of the melting temperature of CaC12 hydrates contained in a Ni foam. The result of 45° C. corresponds to the expected value for CaC12 tetrahydrate;

[Fig 16] Figure 16 represents the measurement of heat fluxes and temperature of CaC12 Ni-hydrate composites during 36 hours of hydration by ambient air flow (approx. 60% RH). The energy released during hydration is Q = 570 J for a composite with a total mass M = 506 mg, including 256 mg of CaC12 dihydrate. The corresponding hydration enthalpy, H = - 327 kJ/mol, is close to the experimental value in the literature (H = - 368 kJ/mol) for total hydration in the form of CaC12-6H2O (hexahydrate),

[Fig 17] Figures 17A and 17B represent thermochemical measurements of heat flux and temperature during the 1st and last 6h hydration phases, for the sample which underwent the sequence: 1) 6h hydration and dehydration , 2) 3h hydration and dehydration, 3) 1.5h hydration and dehydration, 4) 45min hydration and dehydration, 5) 6h hydration,

[Fig 18] figure 18 represents the maximum heat flux during the various hydration cycle tests of variable durations and humidity level of the ambient air used for hydration (in the figure the average value is 29.3mW),

[Fig 19A] Figure 19A represents the thermochemical energy released during hydration tests; the tests marked B are the hydration tests for 6 hours, those marked O for 3 hours, those marked V for 1.5 hours, those marked J for 1.5 hours;

[Fig 19B] figure 19B represents the thermochemical energy stored during dehydration,

[Fig 20] Figures 20A to 20C represent the thermochemical measurements of heat flux and temperature for monolithic CaC12 powder (without metal foam) during the first three phases of the sequence: 1) 6h hydration and dehydration, 2) 3h hydration and dehydration, 3) 1.5h hydration and dehydration. It can be seen that after an initial activation (6 h hydration) comparable to that of the composite, the performance of the monolithic powder drops rapidly during the sequence of cycles, with a maximum heat flux which gradually drops from 26 to 19 , then at 15 mW; FIG. 20D is a summary of the data in terms of maximum heat flux during the various tests of hydration cycles of variable durations and humidity level of the ambient air used for hydration. In the figure the average value is 19.4 mW; And

[Fig 21] Figure 21 is a summary table of composite performance versus powdered salt.

detailed description

There is illustrated in Figure 1 an example of composite 1 for thermochemical reactor according to the invention comprising a metal foam 2 comprising a plurality of open cells 3.

An example of foam 2, made of aluminum, is shown partially and in isolation in FIG. 2. The average size of a cell 3 is between 50 μm and 300 μm. Reticles 4 delimit the cells 3.

The composite 1 further comprises crystallites 5 of hydrophilic salts capable of reversibly reacting with water vapor in a hydration reaction to form crystallites 5 of salt hydrates, the crystallites 5 being retained, as illustrated very schematically in Figure 3, within the cells 3 of the foam 2.

The crystallites 5 have an average size between 10 μm and 100 μm, preferably, but a size up to 150 μm can also be suitable. The morphology of the cell porosity 3 of the foam 2 and in this example of type of truncated octahedron or tetrakai decahedron type.

In the example shown, the metal occupies approximately 15% of the total volume of foam 2 in the absence of crystallites 5.

The composite 1 has in the example of FIG. 1 a form of circular contour of small thickness. To produce the composite 1, it is possible to implement the method for manufacturing the composite 1 comprising the steps described below.

A first step, not illustrated, consists in manufacturing the foam 2 in metal like that which is visible in FIG. 2. To do this, one impregnates, by suction or by injection, with the metal - in this case aluminum - foam 2 in the molten state, the porosity of a sacrificial granular matrix, in this example made of common salt NaCl. The shape of the cells 3 of the foam 2 can be determined by the nature of the sacrificial matrix. It is not beyond the scope of the invention if the nature of the sacrificial granular matrix is different.

In a later step, we will seek to substitute the common salt, which is used as a model material, an appropriate salt hydrate of general formula MCh-nPLO, where n denotes an integer number of water molecules and M denotes a alkaline earth cation or bivalent metal such as Ca, Mg,. . .

This subsequent step is a step of inserting the salt crystallites 5 into the foam 2. Several different embodiments are possible for this insertion step, described with reference to Figures 4 to 8.

In the example illustrated in FIG. 4 which represents the result obtained after implementation of the first embodiment, the direct precipitation of the crystallites 5 within the cells 3 of the foam 2 during the evaporation of a solution saturated saline in which the foam 2 has been previously immersed.

In the example illustrated in FIG. 5, which represents the result obtained after implementing the second embodiment, a sieved powder of salt crystallites is deposited on the previously moistened foam 2.

In the embodiment of FIG. 6 illustrating the result obtained, precipitation is carried out from the saturated solution remaining by capillarity on the foam 2 soaked then removed from the solution.

In the embodiment of FIGS. 7 and 8 also illustrating the result obtained, the salt crystallites 5 have been inserted into the structure of the metal foam 2 by sieving and vibration on the foam 2 previously coated with organic binder. Figure 7 is seen by optical microscopy while Figure 8 is seen by scanning electron microscopy. The step of inserting the crystallites 5 implemented makes it possible to keep the porosity of the foam 2 open to allow the transfer of water vapor through the composite 1. Whatever the embodiment implemented, it is observed in view of the figures that the crystallites are attached to the reticles 4 of the cells 3 and not in free circulation within the cells 3.

There is shown in Figures 9, 10 and 11, an example of implementation of the composite 1. In Figures 10 and 11, there is illustrated a unit 100 for storing and restoring energy by thermochemical means according to the invention, comprising at least one thermochemical reactor module 10 comprising at least one composite 1. The unit 100 further comprises a heat exchanger enclosure 110 allowing the circulation of a heat transfer fluid F and housing said at least one thermochemical reactor module 10.

In the example illustrated, the unit 100 comprises a number p greater than or equal to three of modules 10. The number p is advantageously adjusted according to the desired power of the thermochemical reactor.

As more particularly visible in FIG. 9, each module 10 comprises a metal container 11 housing a composite 1 in the form of a cylinder. The container 11 has at least one opening 12, in this example a plurality of openings 12, to allow the exchange of water vapour, as illustrated with the arrows in this figure 9, between the composite 1 and a reservoir 112 of condensation steam, as will be described below with respect to Figures 10 and 11.

The containers 11 are in this example housed in an enclosure 111, metal, itself in contact with the heat transfer fluid F, externally.

During the implementation of the method for using the unit 100 for storing and restoring energy by thermochemical means, an energy storage step is implemented, illustrated in FIG. 10, in the unit 100 This step consists in heating, for example using the heat transfer fluid F, the modules 10 of the thermochemical reactor so as to at least partially dehydrate the crystallites 5 of salt hydrates and to release water vapor E which is evacuated and then condensed in the condensation tank 112.

The heat transfer fluid F can be heated by thermal or photovoltaic solar panels so as to externally heat the enclosure 111 comprising the modules 10. An endothermic reaction takes place which dehydrates the salt hydrates contained in the composites of the modules 10. The water vapor E is evacuated to the storage tank condensation 112. This is a phase of thermochemical storage, also called charging, in times of excess energy availability.

The modules 10 are arranged side by side on grids 13 in this example, metallic and perforated to allow the water vapor to pass. Several levels of modules 10 are superimposed, being separated by the grids 13, as visible in Figures 10 and 11.

In a subsequent step illustrated in FIG. 11, the water vapor E is reinjected from the reservoir 112 into the enclosure 111 in this example with control of the partial pressure pEEO to control the degree of hydration of the salts, at the using a gas mixer 15, air A in this example placed at the outlet of the water vapor E from the tank 112. An exothermic hydration reaction of the salts contained in the composites 1 of the modules 10 has place, releasing heat which makes it possible to heat the enclosures 11 of the modules 10, and therefore the enclosure 111 comprising all the modules 10, so that the heat transfer fluid F, which circulates in the enclosure 110 of the exchanger thermal and is in contact with the wall of the enclosure 111, is heated and transported to the utility installations, for example central heating, sanitary water. This phase illustrated in FIG. 11 is a phase of thermochemical production, or discharge, during a period of energy consumption. The invention thus makes it possible to store energy in order to use it when necessary.

Another embodiment of the invention has been illustrated in Figures 12 to 14. In this embodiment, as seen in Figure 12, the composite 1 is inserted in the form of a thin plate between plates 14 metal forming the upper and lower walls of the container 11. The size of the plates 14 can be between 10 cm and several meters, in this example between 10 cm and 1 m. The modules 10 advantageously have a low thickness, of centimetric order, in particular between 0.5 cm and 5 cm. The enclosure 11 of each module 10 is sealed.

The modules 10 are stacked together as shown in Figure 12, at a distance from each other, and connected to each other by openwork connecting tubes 16 which allow the composites 1 of the different modules 10 to communicate with each other. while remaining insulated in a sealed manner vis-à-vis the environment comprising the heat transfer fluid F which circulates, located around the modules 10. The connecting tubes 16 each comprise at least one opening 17 opening inside a module 10. This all of the modules 10 and the connecting tubes 16 is directly bathed in the heat exchanger enclosure 110 housing the heat transfer fluid F which circulates therein.

The principle of operation of such a unit 100 is illustrated in FIGS. 13 and 14. During the loading illustrated in FIG. 13, the heat transfer fluid F heated by an external source of available energy heats the modules 10. The salt hydrates dry and release water vapor E which is evacuated by the connecting tubes 16 and condensed in the attached reservoir 112 (not illustrated in this figure but similar to that of the embodiment of FIGS. 10 and 11).

During the use of the unit 100, illustrated in FIG. 14, the water vapor E is reintroduced through the connecting tubes 16 into the composite 1 of the modules 10. The release of the heat of hydration reaction of the salt hydrates heats the heat transfer fluid F which is routed to the domestic installations to be heated.

The advantage of this embodiment is to make it possible to have units of large lateral size and to directly offer a large surface for exchange with the heat transfer fluid F formed by the surfaces of the plates 14 of the modules 10.

In addition, the industrial manufacture of foam is often, that is to say in a standard way, carried out in thin layers and of extended dimensions, which makes it possible to simplify the manufacture of the unit 100.

Thermomechanical behavior tests

Mechanical compression tests, with in situ observation SEM (scanning electron microscope" were carried out on the Al and Cu foams and on the corresponding "foam - CaCl2 hydrate" composites. The mechanical behavior of the composite is conditioned by the structure of the metal foam, which performs its role as a framework and allows the salt crista Hites to remain unaffected by mechanical loading.In particular, there is no decohesion of the crista Hites and the porosity of the composite structure remains open, even after strong deformations (10%).This result ensures the kinetic efficiency of mass transfer (H2O vapour).Thermomechanical tests in thermal cycles with in situ observation SEM (scanning electron microscope" were carried out on Al foams and on the corresponding "foam - CaCl2 hydrate" composites. The stability of the composites to thermomechanical stresses is demonstrated following several cycles of temperature (up to 150°C). In particular, the crista Hites do not undergo decohesion and do not migrate into the porous structure.

corn trials

Thermochemical tests were carried out on composites by differential scanning calorimetry, in order to determine the crystallization and melting temperatures of CaCl2 hydrates, as well as the hydration reaction enthalpies.

An example of four cycles of CaCl2-4H2O melting temperature measurements is shown in Figure 15, the result of which at 45°C confirms the tetra-hydrated form of the CaCl2 crystallites.

Shown in Figure 16 are the results of the measurements of the heat and temperature fluxes of the “Ni foam - CaCl2 hydrates” composite during 36 hours of hydration by ambient air flux (approx. 45-65% RH). The integration of the heat flux makes it possible to calculate the quantity of energy released during hydration, which is Q = 570 J for a composite of total mass M = 506 mg, including 256 mg of CaCl2 dihydrate (form initially introduced in composite). By considering this energy and based on the molar mass of the CaCl2 dihydrate, we obtain an enthalpy of hydration H = - 327 kJ/mol. This value is close to the experimental value of the literature (H = - 368 kJ/mol) for the total hydration in the form of CaCl2-6H2O (hexahydrate)

Partial hydration tests were carried out by consecutive drying and hydration cycles, using a very dry nitrogen flow (laboratory quality) and an ambient air flow, respectively. The same sample was subjected several times to the following sequence of cycles:

1) 6h hydration and initial dehydration,

2) 3 hour hydration and dehydration (3 tests),

3) 1.5 h hydrations and dehydration (4 tests), 4) 45 min hydration and dehydration (3 tests),

5) 6 hour hydration and final dehydration.

Figure 17 shows the thermochemical measurements of heat flux and temperature during the 1st and last 6h hydration phase. A very important observation shows that there is an initial peak of heat flow, at the start of hydration. The maximum value of the initial peak (Fig. 17A) is about 23 mW. It is linked to an activating structural modification of the reactive medium, which is probably linked to a reorganization of the crista Hites during hydration and which increases the specific exchange surface. It is also important to note that once this first activation has been carried out, the values of the heat flux in general are greater during the following cycles. In particular, the height of the hydration start peak during the various following cycles corresponds to values ranging from 29 to 39 mW. This height at the end of the sequence of cycles is approximately 29 mW (FIG. 17B). This thermochemical behavior indicates that the initial structural modification, activating the reactive medium, is permanent. The results of all the tests are summarized in Figure 18. It can be seen that the maximum heat flux during hydration is on average 29.3 mW.s

By calculating for each test the integration of the heat flows over time, during hydration and dehydration, we obtain the quantity of heat released during the hydration process, as well as the quantity of heat stored during the dehydration process.

The results are shown in figure 19, and allow some observations: The processes of hydration and dehydration during the same duration are reproducible and present a relatively low standard deviation.

Heat release during hydration is not a linear process. For example, the amount released for 1.5 hours is not half of that released for 3 hours. The same goes for the dehydration process. After 11 cycles, the heat released during 6 h of hydration (189.02 J) did not decrease compared to that of the first hydration of 6 h (152.82 J), which proves the high performance and stability of the composite.

Even with half the hydration time, the average heat released for 3h (182.14 J) is still higher than that of the first 6h hydration (152.82 J), indicating that the composite needs initial activation to reach their optimum level of performance.

It can be concluded that one and a half hours is insufficient to completely hydrate the reactive material. On the other hand, 1.5 h to 3 h are sufficient for the completion of the reaction. The fluctuations, however, let us understand that the level of hygrometry can also affect the kinetics, but not in an intuitive way. In particular, the highest RH level = 64%, for which the highest hydration kinetics are expected, does not lead to the release of the greatest amount of energy. In other words, too fast hydration kinetics can inhibit the completion of the process, perhaps due to local structural modifications, at the level of the arrangement of the clusters of crista Hites of hydrates on the reticles of the foam. metallic.

Comparative tests were also carried out with monolithic reactive CaCl2 material, that is to say without the structure of the metallic foam. In this case, CaCl2 powder alone is used, in an identical quantity to that contained in the composite. The protocol for the sequence of cyclic hydration and dehydration tests is strictly the same as previously shown for the composite.

The preliminary results are shown in Figure 20. It is seen that after an initial activation (6 h hydration) comparable to that of the composite, the performance of the monolithic powder drops rapidly during the sequence of cycles, with a maximum of heat flux which gradually decreases from 26 to 19, then to 15 mW. With the exception of the first cycle, hydration and dehydration of the composite releases/stores more heat than monolithic salt powder, for the same amount of time.

The average heat released during 3 hours of hydration for the composite is 182 J, while for salt the value is 143 J, which means that for 3 hours of hydration, the composite is about 30% more efficient.

The average heat stored during 3h of dehydration for the composite is 145 J, while for salt the value is 105 J, which means that for 3h of dehydration, the composite increased the stored heat by 38%. The average heat released during 1.5h of hydration for the composite is 147 J, while for salt the value is 75 J, which means that for 1.5h of hydration, the composite performs better d about 97%.

The average heat stored during 1.5h of dehydration for the composite is 106 J, while for salt the value is 54 J, which means that for 1.5h of dehydration the composite increased the stored heat by 97%.

The table in Figure 21 summarizes the performance of the composite compared to powdered salt.

In conclusion, for the same hydration and dehydration protocol and with the same quantity of reactive material (CaCl2) the "metal foam - hydrates of CaCl2" composite always shows 1) a maximum heat flux considerably higher than that of the monolithic CaCl2 powder (both during hydration and dehydration), 2) a quantity of energy released (hydration) or stored (dehydration) greater than that of the monolithic CaCl2 powder.

In addition, the composite shows a great stability of operability, and this during a dozen cycles carried out over a month. Its initial “activation” is carried out from the first 6-hour cycle. Subsequently, the performance of the composite stabilizes at values higher than those of the first cycle. After activation, the composite shows superior performance compared to salt powder. The thermochemical energy charge and discharge kinetics are also improved in the composite, which stores the same amount of energy more quickly.

3.

Corrosion behavior tests

Hydration/dehydration tests showed a strong propensity for oxidation of Cu foams (formation of verdigris). On the other hand, the Al and Ni foams remain intact for standard laboratory tests from a few hours to a few days (optical observation).

Of course, the invention is not limited to the examples which have just been described.

In particular, the foam may have a different shape and/or be made of another metal, such as nickel or copper or an alloy.

Claims

Claims 1. Composite (1) for thermochemical reactor comprising: - a metal foam (2) comprising a plurality of open cells (3), the average size of a cell (3) being between 50 μm and 500 μm, - crystallites (5) of hydrophilic salts capable of reversibly reacting with water vapor in a hydration reaction to form crystallites (5) of hydrates of salts, the crystallites (5) being retained within cells (3) of the foam (2). 2. Composite (1) according to claim 1, in which the crystallites (5) have an average size of between 10 μm and 150 μm, preferably between 10 μm and 100 μm. 3. Composite (1) according to claim 1 or 2, in which the morphology of the porosity of the cells (3) of the foam (2) is of the tetrakaidecahedron, tetradecahedron or truncated octahedron type. 4. Composite (1) according to any one of the preceding claims, in which the metal occupies more than 10% of the total volume of the foam (2), with a porosity established in particular between 90% and 95% of the total volume of the foam (2). 5. Composite (1) according to any one of the preceding claims, in which the hydrophilic salts have an enthalpy of hydration greater than 200 kJ/kg of material. 6. Composite (1) according to any one of the preceding claims, in which the hydrates of crystallite salts (5) are chosen from the group consisting of CaCh- (2 to 6) H ₂ O, MgCh- (1 to 6 )H ₂ O, Na ₂ S-(0.5 to 9)H ₂ O, MgSO ₄ -7H ₂ O and Na ₂ SO ₄ -10H ₂ O. 7. Composite (1) according to any one of the preceding claims, in which the metal constituting the foam (2) is chosen from the group consisting of aluminium, nickel, copper and their alloys, preferably aluminum and nickel, and their alloys. 8. Composite (1) according to any one of the preceding claims, comprising an organic binder, preferably insoluble in water, chosen in particular from the group consisting of thermoplastic polymers, capable of withstanding temperatures between 50° C. at 150°C. 9. Composite (1) according to any one of the preceding claims, in which the foam (2) has a plate shape, a parallelepipedic shape or a cylindrical shape. 10. A method of manufacturing a composite (1) for a thermochemical reactor according to any one of the preceding claims, comprising a step for manufacturing the foam (2) consisting in impregnating, with the metal of the foam (2) at the molten state, the porosity of a sacrificial granular matrix, in particular a common salt (NaCl) sacrificial matrix. 11. Method according to the preceding claim, comprising a step of inserting salt crystallites (5) into the foam (2), the insertion step comprising: - soaking the foam (2) in a bath of a saline solution, in particular a saturated saline solution, - the evaporation of the solution, and - the realization of a direct precipitation of the crystallites (5) within the cells (3) of the foam (2) during the evaporation of the solution. 12. Method according to the preceding claim, comprising a step, prior to the insertion step, of seeding the foam (2) with micro-crystallites, comprising soaking the foam (2) in a saline solution, including a saturated saline solution, and drying in ambient air. 13. Method according to claim 10, comprising a step of inserting salt crystallites (5) into the foam (2), the insertion step comprising: - moistening the foam (2) or coating the foam (2) with an organic binder, preferably insoluble in water, and - sifting/vibrating a powder of crystallites (5) of salt through the network of the foam. 14. Unit (100) for storing and restoring energy by thermochemical means, comprising at least one thermochemical reactor module (10) comprising at least one composite (1) according to any one of claims 1 to 9 and an enclosure ( 110) heat exchanger allowing the circulation of a heat transfer fluid (F) inside the enclosure and housing said at least one thermochemical reactor module (10). 15. Unit (100) according to claim 14, said at least one thermochemical reactor module (10) comprising a container (11), preferably metallic, housing at least one composite (1) and having at least one opening (12) to allow the exchange of water vapor (E) between the composite (1) and a water vapor condensation reservoir (112). 16. Unit (100) according to claim 15, comprising a plurality of modules (10) of thermochemical reactors traversed by one or more connecting tubes (16) perforated which have along their length at least one opening (17) opening into each module (10), the or each connecting tube (16) being connected to said condensation tank (112) and being configured to allow the exchange of water vapor (E) between the composite (1) and said tank (112), the containers (11) of the modules (10) of thermochemical reactors being in contact, externally, with the heat transfer fluid (F). 17. Method of using a thermochemical energy storage and release unit (100) according to any one of claims 14 to 16, comprising the following steps: (a) Step a: step of storing energy in said unit (100) comprising heating said at least one thermochemical reactor module (10) so as to at least partially dehydrate the crystallites (5) of salt hydrates and to release water vapor (E) which is evacuated, in particular via the connecting tube(s) (16), then condensed in a condensation tank (112), (b) Stage b: energy restitution stage in said unit (100) in which the water vapor (E) is reintroduced, in particular via the connecting tube(s) (16), into the composite(s) (1 ) of said at least one thermochemical reactor module (10) and in which the production of heat given off by the hydration reaction of the salt hydrates heats the heat transfer fluid (F) which is routed to one or more domestic installations.