Lithium hydroxide-based thermal energy storage device

Abstract

A thermal energy storage device (1) comprises a storage unit, formed by a reservoir (3) for receiving a thermal energy storage material (5) and a closure device (2) configured to close the opening of the reservoir, and a sealing system for sealing the storage unit, said thermal energy storage material (5) consisting of anhydrous lithium hydroxide having a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56%.

Description

Description Title: THERMAL ENERGY STORAGE DEVICE BASED ON LITHIUM HYDROXIDE Technical field

[0001] The present invention relates to a thermal energy storage device based on a lithium hydroxide (LiOH) material for storing thermal energy at high temperature. Prior art

[0002] Many industrial installations use cyclic thermal processes that generate heat at high temperatures, for example in the cement industry, steel industry, paper industry, etc. In order to optimize the use of this energy, it would therefore be interesting to be able to recover this heat, store it and restore it as needed, in the form of heat or converted into electricity.

[0003] In the context of solar power plants, their operation is currently severely constrained by the intermittent nature of solar energy availability. Thus, one of the major problems with thermal and thermodynamic solar power plants concerns the impossibility of managing production peaks and adapting production to demand. For electricity production to be efficient and optimized, a thermal energy storage solution must be developed.

[0004] One of the additional advantages of the thermal storage solution is the development of nuclear power plants with high-temperature reactors, such as VHTR (Very High Temperature Reactor), AHTR (Advanced High Temperature Reactor), SFR (Sodium-cooled Fast Reactor) and LFR (Lead-cooled Fast Reactor). Since the operating conditions of this type of plant need to be constant and stable, the addition of a thermal energy storage system acts as a thermal regulator that will smooth out over- or under-production depending on demand. In addition, coupling a nuclear reactor to large-scale thermal energy storage can significantly improve the viability of the nuclear power plant in an electricity grid containing a significant fraction of renewable energy sources.

[0005] We can distinguish three modes of thermal energy storage.

[0006] Sensible heat storage allows thermal energy to be stored by increasing the temperature of a storage material. During the charging stage, thermal energy is transferred to the material to increase its temperature. During discharge, the material is cooled to recover the stored thermal energy. The technology for its implementation is relatively simple and it is the most mature thermal storage method. However, it has a very low energy density.

[0007] Thermochemical thermal energy storage exploits the reversibility of chemical reactions to store thermal energy. During the charging stage, thermal energy is used to drive the endothermic reaction. The products of this reaction are separated and stored. When the thermal energy is released, the stored products are brought into contact to carry out the reversible exothermic reaction and thus release the heat. Although this storage system has a high energy density, the technology for its implementation is relatively complex and does not currently allow for use on an industrial scale.

[0008] Latent heat storage allows thermal energy to be stored through a phase change of materials, solid / liquid or liquid / vapor. During the charging stage, the thermal energy supplied to the material allows it to heat up, then change phases. The material is stored at the charging temperature. When the energy is released, it changes phases again to release the stored energy and return to its initial state. Thermal energy storage based on phase change materials has a high capacity for storing and releasing thermal energy, within a narrow temperature range. As with sensible heat storage, the technology for its implementation is relatively simple, yet no latent heat thermal storage device is currently used on the market.

[0009] Furthermore, known phase change materials often exhibit chemical instability during use in a latent storage system. Indeed, some materials experience changes in their melting temperature and latent heat during use.

[0010] Among phase change materials, salts have been identified as potential candidates for the development of thermal energy storage systems. These compounds are interesting for storing energy at high temperatures. However, salts have low thermal conductivity.

[0011] Existing phase change materials that are known as potential materials for latent heat thermal energy storage are not exploited for industrial application due in particular to the complexity of the synthesis of these materials which are generally multi-component.

[0012] Document EP2444468 proposes a phase change material based on mixtures of lithium hydroxide LiOH and potassium hydroxide KOH salts comprising a porous carbon structure filled with the LiOH / KOH composite material to store thermal energy. The presence of the carbon structure improves thermal conductivity. However, the experimental tests described in this document seem to show a degradation of the thermal energy storage properties over time (in less than 10 applied thermal cycles). Thus, the device proposed in this document does not allow long-term thermal cycling of the materials, without chemical degradation of the material.

[0013] Document CN107699201 proposes a phase change material comprising a carbon structure filled with a lithium hydroxide monohydrate LiOH.HLO. However, the process for exploiting this material is relatively complex, long and requires the use of sulfuric acid. In addition, the useful volume obtained from the product is relatively low, only 20%.

[0014] An aim of the invention is therefore to provide a thermal energy storage device, based on phase change material, simple to implement, with a large storage capacity while occupying a small volume.

[0015] Another object of the invention is to propose a thermal energy storage device which is capable of storing thermal energy using sodium hydroxide LiOH, so as to exploit its potential for storing / removing thermal energy in a manner which is non-toxic, nor dangerous in its use and for the environment, while having good properties in terms of chemical, thermal, and energetic stability at a low manufacturing cost, operating in a narrow temperature range, at ambient pressure. Nevertheless, the possibility of exploiting the energy linked to the liquid and solid sensible heat, in addition to the energy linked to the latent heat, of the material must be taken into account for a device which is as optimized as possible from the energy point of view.

[0016] Another aim of the invention is to propose a thermal energy storage device that facilitates its integration into power plants producing electricity from conventional or renewable energy sources. The phase change material must not be corrosive and must be compatible with the materials used in the construction of the power plants.

[0017] Another aim of the invention is to overcome the aforementioned drawbacks, in particular by allowing a large storage capacity while ensuring physicochemical structural stability at high temperature.

[0018] The Applicant, as part of its research aimed at developing a new high-performance thermal energy storage material for storing thermal energy, was able to demonstrate, through experimental tests, the unexpected energy, chemical and thermal stability of the pure LiOH body, which is superior to that of phase change materials of the state of the art and used in the same temperature range, while having a high latent heat of fusion (greater than 800 J / g) in order to be able to store a large amount of energy in a small volume of material. It has a high energy storage density, i.e. more than 350 kWh / m 3 (not including the part linked to the sensitive).

[0019] In these tests, it was found possible to cycle the LiOH material more than 150 times.

[0020] In particular, the applicant was able to demonstrate that the LiOH material has long-term chemical stability allowing the material to be cycled, without chemical degradation of the material.

[0021] In the context of this patent application, for a thermal energy storage system, the term “cycling” designates a succession of charges and discharges.

[0022] Unexpectedly and contrary to what is currently claimed, the applicant was able to demonstrate that it is possible to prevent its irreversible thermal decomposition, into Li2O or other IJ3O2 or IJ2CO3, and to be able to use it as a material for storing thermal energy at high temperatures, and in particular in a narrow range of temperature values.

[0023] Unlike composite materials proposed in the state of the art such as LiOH / LiBr, or LiOH / KOH, the use of pure LiOH does not require complex technology for its production, storage or use. It therefore allows the development of ultra-compact thermal energy storage devices by latent heat, with an attractive investment cost.

[0024] In addition, the use of LiOH material allows for the recycling of lithium-based materials and the recovery of recycled LiOH material that cannot be reused in its original application area. LiOH material is inexpensive and can be used to recycle lithium-based batteries. Summary

[0025] This disclosure improves the situation.

[0026] A thermal energy storage device is provided comprising a storage unit formed by a reservoir intended to receive a thermal energy storage material and a closing device configured to close the opening of the reservoir, a sealing system for sealing the storage unit, said thermal energy storage material being made of anhydrous lithium hydroxide of a purity greater than 96% or lithium hydroxide monohydrate of a purity greater than 56%.

[0027] According to one embodiment, the storage material is received in the reservoir so as to leave an empty space between the material and a lower face of the closure device.

[0028] According to another embodiment, the storage material is in the form of particles, or in a compacted form.

[0029] According to one embodiment, the device further comprises a porous and / or fibrous structure, said structure being filled with the anhydrous lithium hydroxide material or lithium hydroxide monohydrate.

[0030] Preferably, the porous and / or fibrous structure is made of a material chosen from a list of materials: carbon, metal, graphene, ceramic, carbon nanotubes.

[0031] According to one embodiment, said closing device comprises a head and a shutter body, the reservoir comprises an opening surrounded by an outer rim, a shoulder being arranged in the inner wall of the reservoir at the level of the opening, the shutter body being intended to engage in the opening to close the opening with the head bearing against an upper face of the outer rim and a lower face of the shutter body bearing against the shoulder, the lower face of the shutter body being separated from the material by an empty space.

[0032] The features set out in the following paragraphs may, optionally, be implemented, independently of each other or in combination with each other:

[0033] The sealing system includes a sealing disc made of graphite material interposed between the shutter body and the shoulder.

[0034] The device further comprises a thermal paste interposed between the shutter body and the sealing disc made of graphite material.

[0035] The sealing system includes a layer of weld interposed between the head and the upper face of the outer rim.

[0036] The closure device is secured to the tank by fasteners when the shutter body is engaged in the tank opening to close the opening.

[0037] According to one embodiment, the device further comprises a network for circulating a heat transfer fluid immersed in the lithium hydroxide material, said network comprising at least one inlet for introducing the fluid inside the material for the purpose of storing thermal energy and for the purpose of extracting thermal energy, and at least one outlet for guiding the fluid outside the material.

[0038] According to another aspect of the present invention, there is provided a method of storing and releasing thermal energy implementing the device as described above, the method comprising the following steps: Place the lithium hydroxide material in a storage tank; - Close the tank using the closing device so as to make the storage unit watertight; - Heating the lithium hydroxide material to a maximum temperature corresponding to the solid / liquid transition to store thermal energy; - Cool the lithium hydroxide material to a minimum temperature, below the softening temperature, to release thermal energy. Brief description of the drawings

[0039] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analyzing the attached drawings, in which: Fig. 1

[0040] [Fig. 1] Figure 1 represents a schematic sectional view of a thermal energy storage device according to one embodiment. Fig. 2

[0041] [Fig. 2] Figure 2 shows a schematic sectional view of a thermal energy storage device according to another embodiment. Fig. 3

[0042] [Fig. 3] Figure 3 shows a schematic sectional view of a thermal energy storage device according to another embodiment. Fig. 4

[0043] [Fig. 4] Figure 4 shows a schematic perspective view of the thermal energy storage device of Figure 3. Fig. 5

[0044] [Fig. 5] Figure 5 shows a schematic perspective view of the thermal energy storage device according to another embodiment. Fig. 6

[0045] [Fig. 6] Figure 6 shows a schematic perspective view of the thermal energy storage device according to another embodiment. Fig. 7

[0046] [Fig. 7] Figure 7 shows a thermal cycling protocol used for a given volume of lithium hydroxide material. Fig. 8

[0047] [Fig. 8] Figure 8 shows the evolution of fusion enthalpies (C1) and the evolution of solidification enthalpies (C2) during the 153 thermal cycles for a 60 mg LiOH sample. Fig. 9

[0048] [Fig. 9] Figure 9 schematically shows a solar thermal power plant comprising a thermal energy storage device according to one embodiment. Fig. 10

[0049] [Fig. 10] Figure 10 schematically shows a nuclear power plant comprising a thermal energy storage device according to one embodiment. Fig. 11

[0050] [Fig. 11] Figure 11 shows the evolution of heat flux (C3) and temperature evolution (C4) for a 60 mg LiOH sample in a crucible without a hole during thermal cycles. Fig. 12

[0051] [Fig. 12] Figure 12 shows the evolution of the fusion enthalpy (C5) and the evolution of the solidification enthalpy (C6) during the 70 thermal cycles for a sample of 60 mg of LiOH contained in a crucible closed by a ceramic glue. Fig. 13

[0052] [Fig. 13] Figure 13 shows the evolution of the solidification enthalpy (C7) and the evolution of the fusion enthalpy (C8) during the 170 thermal cycles for a sample of 60 mg of LiOH contained in a crucible made watertight by a layer of solder. Fig. 14

[0053] [Fig. 14] Figure 14 shows the evolution of the solidification enthalpy (C9) and the evolution of the fusion enthalpy (C10) during the 500 thermal cycles for a sample of 60 mg of LiOH contained in a crucible made watertight by a layer of solder. Description of the embodiments

[0054] For the purposes of the description, an upper direction and a lower direction will be considered along a vertical axis Z of the device of the invention. The different elements of the storage device must be interpreted according to this reference.

[0055] A thermal energy storage device is described below with reference to Figures 1 to 6. Like reference numbers are used to describe like elements.

[0056] According to the invention, the thermal energy storage device comprises a storage unit formed by a reservoir intended to receive a thermal energy storage material 5 and a closing device configured to close the opening of the reservoir, a sealing system for sealing the storage unit. The closing device is fixed to the reservoir by fixing elements.

[0057] According to the invention, the thermal energy storage material 5 consists of anhydrous lithium hydroxide (LiOH) with a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56%.

[0058] The material comprises, at least 96% by mass, LiOH which is used to store energy in applications using heat transfer fluids having temperatures higher than that of the melting temperature of LiOH, which corresponds to 474 ± ​​2 °C.

[0059] LiOH is available in the form of small particles in the tank or in compacted form. Compact form, in the form of tablets, regardless of size, can be considered.

[0060] According to one embodiment of the invention, the LiOH is used alone and received in the sealed storage unit of the thermal energy storage device to be able to exploit the thermal energy storage capacity of the LiOH without degradation. Advantageously, the storage device can be made of any type of material chemically compatible with LiOH and adapted to the usage temperatures of LiOH.

[0061] According to another embodiment, it is also possible to use a porous or fibrous structure 51 filled with the anhydrous lithium hydroxide material or lithium hydroxide monohydrate. This structure is chosen so as to allow maximum filling of the LiOH material. This porous or fibrous structure is made of a material chosen from a list of the following materials: carbon, metal, graphene, ceramic, carbon nanotubes.

[0062] In one embodiment, the filling of the porous or fibrous structure is carried out by infiltration. The LiOH material infiltrates into the pores of the structure by gravity.

[0063] According to another embodiment, the filling of the porous or fibrous structure is carried out by impregnation. The LiOH material fills the pores of the structure under the effect of pressure applied by a device external to the structure.

[0064] The various possible embodiments of the storage device will be described below with reference to figures 1 to 6.

[0065] With reference to Figure 1 and according to one embodiment, the thermal energy storage device 1 comprises a reservoir 3 in which the lithium hydroxide material 5 is received and a closing device 2.

[0066] The closing device 2 comprises a head 2A and a shutter body 2B having a substantially cylindrical shape.

[0067] The tank 3 comprises a main body 3B enclosing a chamber in which the material 5 is received. The upper end of the main body defines an opening surrounded by an outer rim 3A. A shoulder 11 is provided in the inner wall of the tank near the opening.

[0068] The shutter body 2B is intended to close the opening by insertion into the opening of the tank. In this closed position, the head 2A bears against an upper face of the outer rim 3A and a lower face of the shutter body 2B bears against the shoulder 11.

[0069] According to the invention, the volume of the thermal energy storage material 5 received in the chamber of the main body 3B of the tank is defined so that when the shutter body 2B is placed in the opening to close the tank, the lower face of the shutter body 2B is separated from the thermal energy storage material 5 by an empty space 6.

[0070] The presence of this empty space 6 makes it possible to respect the occupation volume required by the material during its reversible transition between the solid state and the liquid state in order to avoid overpressure.

[0071] In the embodiment shown in Figure 1, the assembly formed by the closure device 2 and the reservoir 3 is made watertight by the presence of a weld layer 4 produced and interposed between the head 2A and the outer rim 3A. The presence of this weld layer 4 also makes it possible to ensure the attachment of the closure device 2 to the reservoir 3.

[0072] The storage device further comprises a network for circulating a heat transfer fluid immersed in the lithium hydroxide material as illustrated in Figures 9 and 10. This heat transfer fluid is suitable for transferring thermal energy to the LiOH material contained in the reservoir chamber and for withdrawing thermal energy from the LiOH material. The storage device comprises one or more inlets for introducing, for example, the heated fluid into the reservoir into which it transfers thermal energy. The fluid having released the thermal energy is then guided via one or more outlets outside the reservoir. In the phase where the energy storage device releases thermal energy, a cooled fluid is introduced into the reservoir via the inlet to withdraw thermal energy. The Fluid having absorbed thermal energy is then guided through one or more outlets outside the tank.

[0073] As an example and with reference to Figure 1, the storage device comprises an inlet 7 and an outlet 9 for the circulation of the fluid. The direction of flow is indicated by the arrows.

[0074] Figure 2 shows another embodiment of the thermal energy storage device.

[0075] This device 10 also comprises a reservoir 13 in which the lithium hydroxide material 5 is received and a closing device 12.

[0076] The closing device 12 comprises a head 12A and a shutter body 12B.

[0077] The tank 13 comprises a main body 13B enclosing a chamber in which the material 5 is received. The upper end of the main body defines an opening surrounded by an outer rim 13A. A shoulder 16 is provided in the inner wall of the tank near the opening.

[0078] In the closed position, the head 12A bears against an upper face of the outer rim 13A and a lower face of the shutter body 12B bears against the shoulder 16.

[0079] The assembly formed by the closing device 12 and the reservoir 13 is made watertight by the presence of a layer of welding made between the head 12A and the outer rim 13A. The presence of this layer of welding also ensures the fixing of the closing device to the reservoir.

[0080] In the embodiment shown in Figure 2 which is particularly advantageous, the sealing of the assembly formed by the closing device 12 and the reservoir 13 is also ensured by the presence of a sealing disc made of perfectly smooth graphite material 8 interposed between a lower face of the closure body 12B and the shoulder 16. The presence of this disc 8 makes it possible to ensure the sealing of the storage unit, in addition to the weld 4.

[0081] As illustrated in Figure 2, the volume of the thermal energy storage material 5 received in the reservoir chamber is defined so that when the shutter body is placed in the opening to close the reservoir, the lower face of the sealing disc 8 made of graphite material is separated from the storage material 5 by an empty space 6. The presence of this disc is particularly advantageous, because during the solid / liquid phase transition, the LiOH material even if it comes into contact with the graphite material disc, there is no adhesion between them. LiOH does not wet the graphite disc which repels the latter, thereby promoting the sealing of the device.

[0082] Figure 3 shows yet another embodiment of the thermal energy storage device.

[0083] This device 20 also comprises a reservoir 23 in which the lithium hydroxide material 5 is received and a closing device 22.

[0084] The closing device 22 comprises a head 22A and a shutter body 22B.

[0085] The tank 23 comprises a main body 23B enclosing a chamber in which the thermal energy storage material 5 is received. The upper end of the main body defines an inlet opening surrounded by an outer rim 23A. A shoulder 26 is provided in the inner wall of the tank proximate the opening.

[0086] In the closed position, the head 22A bears against an upper face of the outer rim 23A and a lower face of the shutter body 22B bears against the shoulder 26.

[0087] As in the case of Figure 2, the sealing of the assembly formed by the closing device 22 and the reservoir 23 is also ensured by the presence of a sealing disc made of graphite material 8 interposed between a lower face of the shutter body 22B and the shoulder 26. The volume of the material 5 received in the chamber of the reservoir 23B is defined so that when the shutter body 22B is placed in the opening to close the reservoir, the lower face of the sealing disc 8 made of graphite material is separated from the storage material 5 by an empty space 6.

[0088] The storage device 20 further comprises mechanical fastening elements 24, 25 for fastening together the closure device 22 and the reservoir 23. By way of example and with reference to FIG. 4, a plurality of fastening holes 25 are provided in the outer rim 23A and a plurality of through fastening holes in the head 22A. When the closure device 22 is in the closed position, the fastening holes in the head and the holes in the outer rim are opposite each other to receive fastening rods 24. The advantage of this variant allows the device to be opened and closed at will.

[0089] Figure 5 shows another embodiment of the thermal energy storage device.

[0090] This device 40 also comprises a reservoir 43 in which the lithium hydroxide material 5 is received and a closing device 42.

[0091] The closure device 42 comprises a head 42A and a shutter body 42B.

[0092] The reservoir 43 comprises a main body 43B enclosing a chamber in which the material 5 is received. The upper end of the main body 43B defines an inlet opening surrounded by an outer rim 43A. A shoulder 46 is provided in the inner wall of the reservoir near the opening.

[0093] As in the case of figures 2 and 3, the sealing of the assembly formed by the closing device 42 and the reservoir 43 is also ensured by the presence of a sealing disc made of graphite material 8 interposed between a lower face of the shutter body 42B and the shoulder 46. The volume of the material 5 received in the chamber of the reservoir 43B is defined so that when the shutter body 42B is placed in the opening to close the reservoir, the lower face of the sealing disc 8 made of graphite material is separated from the storage material 5 by an empty space 6.

[0094] Furthermore, the storage device 40 comprises a thermal paste 47 interposed between the lower face of the shutter body 42B and the sealing disc 8. Preferably, this thermal paste 47 covers an outer periphery of the disc 8. The thermal paste acts here as a sealing complement to reinforce the sealing.

[0095] For example, the storage device 40 further comprises mechanical fastening elements 44 for fastening together the closure device 42 and the reservoir 43, as in the embodiment of FIG. 3. According to another variant, the closure device 42 is fastened to the reservoir 43 by a layer of solder as in the case of FIGS. 1 and 2.

[0096] Generally, the closing device 42 can be fixed to the tank 43 by any suitable securing means.

[0097] Figure 6 shows another embodiment of the thermal energy storage device.

[0098] This device 30 also comprises a reservoir 33 in which the lithium hydroxide material 5 is received and a closing device 32.

[0099] The closure device 32 comprises a head 32A and a shutter body 32B.

[0100] The reservoir 33 comprises a main body 33B enclosing a chamber in which the material 5 is received. The upper end of the main body defines an inlet opening surrounded by an outer rim 33A. A shoulder 36 is provided in the inner wall of the reservoir near the opening.

[0101] As in the case of figures 2, 3 and 4, the sealing of the assembly formed by the closing device 32 and the reservoir 33 is also ensured by the presence of a sealing disc made of graphite material 8 interposed between a lower face of the shutter body 32B and the shoulder 36. The volume of the material 5 received in the chamber of the reservoir 33B is defined so that when the shutter body 32B is placed in the opening to close the reservoir, the lower face of the sealing disc 8 made of graphite material is separated from the storage material 5 by an empty space 6.

[0102] The storage device 30 further comprises mechanical fastening elements 34, 35 for fastening together the closure device 32 and the reservoir 33.

[0103] As illustrated in Figure 6 and by way of example, the external surface of the closure body 32B is provided with an external thread 34 and the internal wall of the reservoir 33 at the opening is provided with a complementary internal thread 35 which extends from the upper end of the opening to the shoulder 36. The closure device is thus fixed to the reservoir by screwing the closure body 32B into the opening.

[0104] The experimental results conducted by the applicant unexpectedly show the thermal energy storage capacities of the pure LiOH body which are superior to those of the salt-based phase change materials currently used and proposed in the same operating temperature range.

[0105] Experimental results also show high energetic, chemical and thermal stabilities of LiOH which can undergo numerous thermal cycles without decomposing.

[0106] Reference is now made to Figure 1 to describe an experimental protocol applied in the experimental tests carried out by the applicant to show the possibility of cycling LiOH alone as a thermal storage material by latent heat.

[0107] In the experimental tests, a differential scanning calorimeter is used to determine the transition temperatures of interest and the associated mass enthalpies.

[0108] Differential Scanning Calorimetry (DSC) is a well-known analytical technique for characterizing phase-change materials. It involves subjecting two identical crucibles, one containing the sample and the other empty, serving as a reference. Both crucibles are subjected to identical thermal stresses in order to measure the difference in energy required to change their temperature in an identical manner. This makes it possible to characterize the exothermic or endothermic behavior of materials, particularly LiOH. In the tests conducted, the applicant uses a DSC produced by Setaram.

[0109] Figure 7 illustrates an example of a thermogram applied by the calorimeter to characterize a LiOH sample. The set temperature of each phase can be expressed as a function of time in the form of a linear function: Te (t) = T0 + axt, with T0 the initial temperature, a the temperature variation rate and t the time.

[0110] The temperature program consists of a series of cycles and each cycle consists of four phases: a heating ramp, an isothermal melting phase with a being zero, a cooling ramp and an isothermal solidification phase.

[0111] The thermal cycle is described as follows. In the first phase of the cycle (1), a heating rate, varying from 0.5 °C / min to 50 °C / min starting from room temperature up to the maximum temperature of 520 °C. In another embodiment, the maximum temperature can be higher than this value. In a second phase, a plateau of 520 °C is applied, the duration of which depends on the quantity of LiOH and the heating rate applied. In a third phase, cooling is applied with a cooling rate varying from 0.5 °C / min to 50 °C / min. It is not necessary to go down to room temperature between each cycle.The minimum temperature set must not be lower than the first-order structural transition temperature of LiOH (called the softening temperature of LiOH which occurs just before melting and just after solidification, determined for the cooling rate applied. In the case of the 60 mg sample shown below, the minimum temperature applied is 250 °C.

[0112] A first series corresponds to LiOH samples with masses between 20 mg and 100 mg which aim to demonstrate the stability of the material and the enthalpy associated with the material as a function of the number of cycles. The thermal cycling tests were carried out in Differential Scanning Calorimeter crucibles whose sealing is ensured by the use of ceramic glue and / or thread lock glue at the screw threads.

[0113] A second series, of more than 50 thermal cycles, corresponds to LiOH samples with masses between 30 g and 160 g, which aim to demonstrate the possibility of cycling the LiOH material on a larger scale with a view to industrial scaling up. The thermal cycling tests were carried out in a standard tank whose sealing is ensured by the use of a graphite paper seal and a high-temperature thermal paste at the closure device as in the case of Figure 5. Examples

[0114] An experimental study has demonstrated the storage performance of lithium hydroxide.

[0115] The characteristics of the LiOH material used are shown in Table 1 below.

[0116] [Table 1] ;0117] The evolution of the fusion and solidification enthalpies measured during 153 thermal cycles applied according to the protocol of figure 7 on a sample of 60 mg of LiOH is represented as a function of the number of cycles in figure 8. The fusion enthalpy corresponds to the latent heat of fusion and the solidification enthalpy to the latent heat of solidification.

[0118] In Figure 8, curve C1 represents the evolution of the fusion enthalpies and curve C2 represents the evolution of the solidification enthalpies of a 60 mg LiOH sample. The values ​​of the enthalpies obtained vary according to the heating and cooling rate applied. The fusion enthalpy values ​​vary between 630 and 1374 J / g and the solidification enthalpy values ​​vary between 644 and 877 J / g.

[0119] The protocol parameters used for each of the steps numbered 1 to 11 in Figure 2 are summarized in Table 2 below.

[0120] [Table 2]

[0121] The results of the top five are described below.

[0122] Step 1

[0123] The 60 mg sample was subjected to 22 cycles. The specific heat of fusion value for an applied heating rate of 21 °C / min varies between 947 J / g and 1063 J / g. The specific heat of solidification value for an applied cooling rate of 6 °C / min varies between 768 J / g and 877 J / g. The weighings before and after the 22 cycles show a loss of approximately 0.26%. No trace of LiOH is observed on the walls of the DSC furnace crucible.

[0124] Step 2

[0125] The same 60 mg sample was then subjected to 14 cycles. The specific heat of fusion value for an applied heating rate of 22 °C / min varies between 925 J / g and 965 J / g. The specific heat of solidification value for an applied cooling rate of 6 °C / min varies between 750 J / g and 774 J / g. The weighings before and after the 14 cycles show a loss of approximately 0.26%. No trace of LiOH is observed on the walls of the DSC furnace crucible.

[0126] Step 3

[0127] The same 60 mg sample was then subjected to 14 cycles. The specific heat of fusion value for an applied heating rate of 23 °C / min varied between 898 J / g and 939 J / g. The specific heat of solidification value for an applied cooling rate of 6 °C / min varied between 663 J / g and 686 J / g. The weighings before and after the 14 cycles showed a loss of approximately 0.021%. No trace of LiOH was observed on the walls of the DSC furnace crucible.

[0128] Step 4

[0129] The same 60 mg sample was then subjected to 14 cycles. The specific heat of fusion value for an applied heating rate of 24 °C / min varies between 899 J / g and 939 J / g. The specific heat of solidification value for an applied cooling rate of 6 °C / min varies between 649 J / g and 663 J / g. The weighings before and after the 14 cycles show a loss whose value is included in the precision error of the balance. No trace of LiOH is observed on the walls of the DSC furnace crucible.

[0130] Step 5

[0131] The same 60 mg sample was then subjected to 6 cycles. The specific heat of fusion value for an applied heating rate of 22 °C / min varies between 825 J / g and 844 J / g. The specific heat of solidification value for an applied cooling rate of 6 °C / min varies between 706 J / g and 713 J / g. The weighings before and after the 14 cycles show a loss whose value is included in the precision error of the balance. No trace of LiOH is observed on the walls of the DSC furnace crucible.

[0132] After 70 thermal cycles with a heating rate varying between 21 and 24 °C / min and a solidification rate set at 6 °C / min, the LiOH sample has a fusion enthalpy varying between 1063 J / g and 825 J / g and a solidification enthalpy varying between 877 J / g and 649 J / g.

[0133] The results of thermal cycling tests also show a high energy capacity of the mass enthalpies of fusion and solidification, with a very high stability of the solidification enthalpy, of the LiOH sample after 153 applied thermal cycles.

[0134] For the second series of experimental tests, with samples having masses between 30 g and 160 g, a similar thermal cycling protocol is applied and temperature curves as a function of time are obtained, with high reproducibility, thus indicating that the energetic behavior of LiOH is identical to that determined by DSC. A DSC test on a sample of the second series taken after 30 cycles of 160 g of LiOH showed that the fusion and solidification enthalpies obtained are of the order of 825 J / g.

[0135] Figure 9 is a perspective view of a solar thermal power plant 100 comprising a thermal energy storage device according to one of the embodiments described above. The power plant is configured to supply heat to an installation 104. The installation may be, for example, an industrial electricity production installation, an urban heat network, or any other structure. The power plant thus comprises a hydraulic circuit (comprising pipes, valves, pumps, etc.) in order to be connected to the installation.

[0136] In the example illustrated in Figure 9, the solar thermal power plant comprises a solar reflective panel 101. The solar thermal power plant advantageously comprises a plurality of reflective panels.

[0137] Solar radiation is reflected by the reflector panel towards a solar receiving panel 102.

[0138] The irradiation of the receiving panel makes it possible to heat a heat transfer fluid, such as air, water vapor, glycol water, thermal oil, liquid sodium or molten salt, arranged in tubes. The heat transfer fluid circulates in a hydraulic circuit up to the installation 104.

[0139] One of the main problems with solar energy is the availability of sunlight depending on weather conditions. In addition, the heat demand from the installation associated with the power plant can also vary depending on needs.

[0140] Thus, in nominal operation, the plant must be able to produce enough heat to power the installation without exceeding a limit temperature, called the target temperature. Exceeding the target temperature is otherwise likely to cause the plant to overheat.

[0141] Overheating can have several causes, such as: - stagnation of the heat transfer fluid in the power plant circuit, for example due to a technical failure, leading to a sudden increase in temperature and pressure; or - a poor match between the heat produced by the power plant and the heat consumption of the installation. The phenomenon of overheating is then linked to the inability to evacuate the excess heat produced, which leads to a progressive rise in temperature.

[0142] Overheating can lead to significant adverse consequences due to the resulting high pressure or high temperature, or a combination of both. Overheating can cause the degradation of certain components of the plant or the vaporization of the heat transfer fluid.

[0143] In order to remedy the phenomenon of overheating but also to overcome the problem of sunshine which could disrupt the quantity of heat available for the installation, the solar thermal power plant advantageously comprises a thermal energy storage device, 10, 20, 30, 40 as described above. The thermal energy storage device comprises a reservoir adapted to receive a phase change material which comprises anhydrous lithium hydroxide with a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56%. The reservoir is sealed by a closure device according to the methods described previously. An empty space necessary for the volume expansion of LiOH estimated at less than 3% and linked to its reversible transition between the solid phase and the liquid phase must be taken into account in the filling of the reservoir.A heat transfer fluid distribution network 105 is immersed in the lithium hydroxide material. Said network comprises an inlet and an outlet intended to be connected respectively to an inlet and an outlet of a heat exchanger 104 which is connected to the hydraulic circuit for distributing heat transfer fluid of the solar thermal power plant.

[0144] The thermal energy storage device is thus located upstream of the installation that uses all or part of the solar energy. The LiOH phase change material exchanges energy with the heat transfer fluid through the exchanger for the storage and restitution of energy. thermal. When storing thermal energy, the heat transfer fluid from the exchanger exchanges heat with the LiOH phase change material and heats the LiOH material until it changes phase, i.e., from a solid phase to a liquid phase. When the LiOH material reaches its phase change temperature, it absorbs a quantity of heat energy to complete its transformation. The heat supplied to the material to complete the phase change is thus stored isothermally in the material. When releasing thermal energy, the heat transfer fluid from the exchanger cools the LiOH material until it changes phase, i.e., from a liquid phase to a solid phase. During this phase change, heat is released by the LiOH material to heat the heat transfer fluid.

[0145] Thanks to the thermal energy storage device, it is possible to oversize the power plant's capacity without the risk of overheating. Since the power plant has a large capacity, the heat produced by it can be managed by the thermal energy storage device according to the installation's needs. The thermal energy storage device also helps solve the problems of intermittency of solar energy.

[0146] Figure 10 is a perspective view of a high-temperature reactor nuclear power plant 200 comprising a thermal energy storage device according to one of the embodiments described above. The power plant is configured to supply heat to a facility 204. The facility may be, for example, an industrial electricity production facility, a district heating network, or any other structure. The power plant thus comprises a hydraulic circuit (comprising pipes, valves, pumps, etc.) in order to be connected to the facility.

[0147] In the example illustrated in Figure 10, the nuclear power plant comprises a reactor 201 whose heat produced makes it possible to heat a heat transfer fluid, arranged in tubes. The heat transfer fluid circulates in a hydraulic circuit to the installation 204.

[0148] In order to manage the heat produced by the power plant according to the needs of the installation, the nuclear power plant advantageously comprises a thermal energy storage device 1, 10, 20, 30, 40 according to one embodiment. The thermal energy storage device comprises a reservoir adapted to receive anhydrous lithium hydroxide phase change material with a purity greater than 96% or monohydrated lithium hydroxide with a purity greater than 56%. The reservoir is sealed by a closure device according to the methods described above. A heat transfer fluid distribution network 205 is immersed in the lithium hydroxide material. Said network comprises an inlet and an outlet intended to be connected respectively to an inlet and an outlet of a heat exchanger 204 which is connected to the hydraulic heat transfer fluid distribution circuit of the nuclear power plant 200.

[0149] The thermal energy storage device is thus located upstream of the installation 204 which uses all or part of the nuclear energy. Similarly, the LiOH phase change material exchanges energy with the heat transfer fluid through the exchanger for the storage and restitution of thermal energy. During the storage of thermal energy, the heat transfer fluid from the exchanger exchanges heat with the LiOH phase change material and heats the LiOH material until it changes phase, i.e. from a solid phase to a liquid phase. When the LiOH material reaches its phase change temperature, it absorbs a quantity of thermal energy to carry out its transformation. The heat supplied to the material to accomplish the phase change is thus stored isothermally in the material. When the thermal energy is released, the heat transfer fluid in the exchanger cools the LiOH material until it changes phase, i.e. from a liquid phase to a solid phase. During this phase change, the heat is released by the LiOH material to heat the heat transfer fluid.

[0150] Thanks to the thermal energy storage device of the present invention, it is possible to oversize the capacity of the thermal power plant without the risk of overheating. Since the power plant has a large capacity, the heat produced by it can be managed by the thermal energy storage device according to the needs of the installation. In the case of the solar thermal power plant, the thermal energy storage device also makes it possible to solve the problems of intermittency of solar energy and to smooth out electricity production as much as possible.

[0151] Example of testing the energy stability of LiOH contained in a sealed storage unit

[0152] A series of experimental studies were carried out by the applicant to demonstrate the energetic, chemical and thermal stability of LiOH which can undergo numerous thermal cycles without decomposing and degrading, when the material is contained in a sealed storage unit, unlike existing technical solutions.

[0153] Figure 11 shows the results of a first study of the energy stability of LiOH using a differential scanning calorimeter (DSC). Conventional commercial crucibles are equipped with a millimeter-sized hole to avoid any overpressure problems. The crucible used in this first study is modified from the commercial crucible. It does not have a hole, unlike the conventional crucible, thus providing a first improvement in the mechanical seal between the cap and the body of the crucible in which 60 mg of LiOH material is placed.

[0154] The characteristics of LiOH material are shown in Table 1 above.

[0155] Curve C3 represents the heat flux as a function of time and curve C4 represents the temperature as a function of time. The experimental results show that from the first cycle, a drastic decrease in enthalpy until its decomposition into Li2O in the third cycle is observed. This decrease is attributed to the loss of LiOH during heating and the condensation of LiOH vapor on the walls of the calorimeter furnace leads to the decomposition of LiOH.

[0156] Figure 12 shows the experimental results of a second energy stability study of LiOH in which a ceramic glue is placed between the stopper and the body of the crucible used in the first experimental study to improve the sealing.

[0157] Curve C5 represents the enthalpy of fusion as a function of the number of thermal cycles applied and curve C6 represents the enthalpy of solidification as a function of the number of cycles applied thermal cycles. The C5 melting and C6 solidification enthalpy curves show that due to the presence of the ceramic glue, enthalpy losses were observed after applying fourteen cycles.

[0158] Figure 13 shows the experimental results of a third LiOH energy stability study in which the assembly formed by the closure device 2 and the reservoir 3 is sealed by a solder layer as described above with reference to Figures 1 and 2.

[0159] Curve C7 represents the solidification enthalpy as a function of the number of thermal cycles applied and curve C8 represents the fusion enthalpy as a function of the number of thermal cycles applied. Curves C7 and C8 show that there is no enthalpy loss after 170 thermal cycles. The experimental results thus show that it is possible to thermally cycle the LiOH material without any enthalpy loss, i.e. without degradation of the material.

[0160] Figure 14 shows the experimental results of a fourth LiOH energy study in which the same crucible as in the third study was used and the number of thermal cycles applied was increased.

[0161] Curve C9 represents the solidification enthalpy as a function of the number of thermal cycles applied and curve C10 represents the fusion enthalpy as a function of the number of thermal cycles applied. Curves C9 and C10 show that there is no loss of enthalpy after 500 thermal cycles. The experimental results thus demonstrate the high energy stability of the LiOH material, thanks to the presence of the sealing system put in place to ensure the sealing of the reservoir unit in which the LiOH material is stored. This sealing system can be a solder layer. According to certain embodiments, this solder disc can be used in combination with a sealing disc made of graphite material. The presence of this graphite material, in the form of a more or less thick sheet available on the market, prevents LiOH in the liquid phase, but also the LiOH vapor that forms in the crucible during heating, from adhering to the cover.The non-wettability of LiOH with this graphite disc further reinforces the sealing of the device, LiOH being unable to deposit and condense between the cover and the body of the tank. Industrial application

[0162] The results from the cycling tests conducted by the applicant show that it is possible to cycle anhydrous LiOH with a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56%, contrary to what is currently claimed, using a suitable sealed storage unit. By preventing its irreversible thermal decomposition into Li2O or any other compound, thermal energy can be charged and discharged by anhydrous LiOH with a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56% for conversion or not into another form of energy, such as electrical energy.

[0163] The use of the phase change material LiOH as a new thermal energy storage material by latent heat with a temperature at least equal to that of LiOH melting, at atmospheric pressure, allows simplified integration into power plants exploiting renewable energies such as solar power plants, in order to manage the storage of thermal energy.

[0164] The use of anhydrous LiOH with a purity greater than 96% or lithium hydroxide monohydrate with a purity greater than 56% makes it possible to develop ultra-compact latent heat thermal energy storage devices at a more attractive manufacturing cost than currently available storage devices. Furthermore, only one sealed storage unit is required for latent heat exploitation, unlike the two (minimum) required for sensible heat exploitation.

[0165] This disclosure is not limited to the embodiments described above and provided as examples, but it encompasses all the variants that a person skilled in the art may envisage within the framework of the protection sought.

Claims

Claims

1. Thermal energy storage device (1) comprising a storage unit formed by a reservoir (3) intended to receive a thermal energy storage material (5) and a closing device (2) configured to close the opening of the reservoir, a sealing system for sealing the storage unit, said thermal energy storage material (5) being made of anhydrous lithium hydroxide of a purity greater than 96% or lithium hydroxide monohydrate of a purity greater than 56%.

2. A device according to claim 1, wherein the storage material (5) is received in the reservoir (3) so as to leave an empty space (6) between the material (5) and a lower face of the closure device (2).

3. A device according to claim 1 or 2, wherein the storage material is in particulate form, or in compacted form.

4. Device according to one of claims 1 to 3, further comprising a porous and / or fibrous structure, said structure being filled with the material (5) of anhydrous lithium hydroxide or lithium hydroxide monohydrate.

5. Device according to claim 4, in which the porous and / or fibrous structure is made of a material chosen from a list of materials: carbon, metal, graphene, ceramic, carbon nanotubes.

6. Device according to one of claims 1 to 5, wherein said closing device (2, 12, 22, 32, 42) comprises a head (2A, 12A, 22A, 32A, 42A) and a shutter body (2B, 12B, 22B, 32B, 42B), the reservoir (3, 13, 23, 33, 43) comprises an opening surrounded by an outer rim (3A, 13A, 23A, 33A, 43A), a shoulder (11, 16, 26, 36, 46) being arranged in the inner wall of the reservoir at the opening, the shutter body (2B, 12B, 22B, 32B, 42B) being intended to engage in the opening to close the opening with the head (2A, 12A, 22A, 32A, 42A) bearing against an upper face of the outer rim (3A, 13A, 23A, 33A, 43A) and a lower face of the shutter body (2B, 12B, 22B, 32B, 42B) bearing against the shoulder (11, 16, 26, 36, 46), the lower face of the shutter body being separated from the material by an empty space (6).

7. Device according to claim 6, wherein said sealing system comprises a sealing disc made of graphite material (8) interposed between the shutter body (12B, 22B, 32B) and the shoulder (16, 26, 36).

8. Device according to claim 7, further comprising a thermal paste (47) interposed between the shutter body (42B) and the graphite material sealing disc (8).

9. Device according to one of claims 6 to 8, in which the sealing system comprises a layer of welding (4) interposed between the head (2A, 12A) and the upper face of the outer rim (3A, 13A).

10. A device according to one of claims 6 to 8, wherein the closure device (22, 32, 42) is secured to the reservoir by fastening elements when the closure body is engaged in the opening of the reservoir to close the opening.

11. Device according to one of claims 1 to 10, further comprising a network for circulating a heat transfer fluid (105, 205) immersed in the lithium hydroxide material (5), said network comprising at least one inlet (7) for introducing the fluid inside the material for the purpose of storing thermal energy and for the purpose of extracting thermal energy, and at least one outlet (9) for guiding the fluid outside the material.

12. A method of storing and releasing thermal energy using the device according to one of claims 1 to 11, the method comprising the following steps: Placing the lithium hydroxide material in a storage tank; - Close the tank using the closing device so as to make the storage unit watertight; - Heating the lithium hydroxide material to a maximum temperature corresponding to the solid / liquid transition to store thermal energy; - Cooling the lithium hydroxide material to a minimum temperature, below the softening temperature, to release thermal energy.

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