HEAT STORAGE MATERIAL, METHOD FOR PRODUCTION OF HEAT STORAGE MATERIAL, AND CHEMICAL HEAT PUMP

Abstract

A heat storage material has a high hydration capacity, which does not readily deliquesce and can be effectively used. A method produces such a heat storage material, and a chemical heat pump and heat storage method use such a heat storage material. The heat storage material is a composite metal halide including a monovalent metal, a divalent metal, and a halogen. The method for producing the heat storage material includes preparing a mixture in which a monovalent metal halide and a divalent metal halide hydrate are mixed, and generating the composite metal halide by subjecting the mixture to a heat treatment. The chemical heat pump includes a water storage unit storing water as a working medium, a heat storage material retention unit retaining the heat storage material, and a water vapor flow path allowing water to flow vapor between the water storage unit and the heat storage material retention unit.

Claims

1. A heat storage material, being a composite metal halide including a monovalent metal, a divalent metal, and a halogen.

2. The heat storage material according to claim 1, wherein the composite metal halide has a perovskite structure of the composition of either Formula (1) or (2) below:
ABX.sub.3(1)
A.sub.2BX.sub.4(2) wherein A is a monovalent metal, B is a divalent metal, X is a halogen, and A, B, and X may each be one or a combination of a plurality of elements having the same valency.

3. The heat storage material according to claim 2, wherein the composite metal halide has the perovskite structure of the composition of Formula (1).

4. The heat storage material according to claim 1, wherein the monovalent metal is selected from the group consisting of alkali metals and transition metals.

5. The heat storage material according to claim 4, wherein the monovalent metal is selected from the group consisting of potassium and cesium.

6. The heat storage material according to claim 1, wherein the divalent metal is selected from the group consisting of alkaline earth metals and transition metals.

7. The heat storage material according to claim 6, wherein the divalent metal is selected from the group consisting of magnesium and calcium.

8. The heat storage material according to claim 1, wherein the halogen is selected from the group consisting of chlorine, bromine, and iodine.

9. The heat storage material according to claim 8, wherein the halogen is chlorine.

10. A method for the production of the heat storage material according to claim 1, comprising the following steps: preparing a mixture by mixing a monovalent metal halide and a divalent metal halide hydrate, and generating the composite metal halide by subjecting the mixture to a heat treatment.

11. A chemical heat pump, comprising: a water storage unit for storing water as a working medium, a heat storage material retention unit for retaining the heat storage material according to claim 1, and a water vapor flow path for allowing water vapor to flow between the water storage unit and the heat storage material retention unit.

12. A heat storage method, comprising performing, in the chemical heat pump according to claim 11, heat storage and heat dissipation by hydrating and dehydrating the working medium in the heat storage material.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0052] FIGS. 1A and 1B are schematic views of a chemical heat pump according to the present disclosure and a convention chemical heat pump.

[0053] FIG. 2 is a view showing X-ray diffraction measurement results of the samples obtained in Examples 1 to 3 and Comparative Examples 1, 2, 4, and 5.

DESCRIPTION OF EMBODIMENTS

[0054] The embodiments of the present disclosure will be described below. However, the present disclosure is not limited to the following embodiments.

[0055] <Heat Storage Material>

[0056] The heat storage material of the present disclosure is a composite metal halide comprising a monovalent metal, a divalent metal, and a halogen. The composite metal halide of the present disclosure is a compound in which both the monovalent metal and the divalent metal are ionically bonded to the halogen to constitute a single salt, and thus, the composite metal halide can be distinguished from, for example, a mixture of a monovalent metal halide and a divalent metal halide. Note that the form of the heat storage material is not limited and may typically be a particulate or granule.

[0057] The inventors of the present disclosure have discovered that composite metal halides comprising both a monovalent metal and a divalent metal (hereinafter referred to simply as the composite metal halide of the present disclosure) have low deliquescent properties and/or have advantageous properties, whereby the hydration capacity thereof can be effectively utilized, comparted to divalent metal halides such as alkaline earth metal compounds. Such low deliquescent properties are advantageous in view of the form of the heat storage material and the retention of reactivity.

[0058] Though not bound by theory, it is believed that the divalent metal is useful for hydration because it can easily coordinate with water, and conversely, the monovalent metal is less like to coordinate with water than the divalent metal. By primarily coordinating the halogen, the skeletal structure of the composite metal halide is maintained and deliquescence is prevented.

[0059] Thus, the relative humidity (RH) at which the composite metal halide of the present disclosure deliquesces at 20 C. is, for example, 35% or more, 40% or more, or 45% or more, and 80% or less, 70% or less, or 60% or less.

[0060] Note that divalent metal halide compounds such as alkaline earth metal compounds generally have a metastable intermediate hydration state between the anhydrous state and the saturated hydration state, and as a result, it may be difficult for the hydration to proceed beyond this stage. In the composite metal halide of the present disclosure, such a metastable intermediate state can be prevented. For example, the hydration reaction of the composite metal halide of the present disclosure can be a single stage.

[0061] (Crystal Structure)

[0062] In preferred embodiments, the composite metal halide of the present disclosure has a perovskite structure of the composition of either Formula (1) or (2) described below, in particular, has a perovskite structure of the composition of Formula (1) described below:

ABX.sub.3(1)

A.sub.2BX.sub.4(2)

wherein A is a monovalent metal, B is a divalent metal, X is a halogen, and A, B, and X may each be one or a combination of a plurality of elements having the same valency.

[0063] As the method for evaluating the presence or absence of a perovskite structure, known methods such as Rietveld analysis of X-ray diffraction measurement data have been considered. Furthermore, the presence or absence of a perovskite structure can also be confirmed by comparing the X-ray diffraction measurement data of a crystal sample with the X-ray diffraction profiles of known perovskite crystals.

[0064] In the perovskite structure, a stable crystal structure is formed due to a remarkable difference in ionic radius between the monovalent metal and the divalent metal, and as a result, it is believed that high hydration capacity and low deliquescent properties can be compatible with each other due to the presence of the monovalent metal having a large ionic radius, which is not easily hydrated, and the divalent metal having a small ionic radius, which is easily hydrated.

[0065] From the viewpoint of favorably forming a perovskite structure, the combination of the monovalent metal and the divalent metal is preferably selected so that the difference in ionic radius is large. Preferable examples thereof include KMgCl.sub.3, K.sub.2MgCl.sub.4, and CsMgCl.sub.3. Though it is known that MgCl.sub.2, which is a divalent metal halide, exhibits remarkable deliquescent properties, since KMgCl.sub.3, K.sub.2MgCl.sub.4, and CsMgCl.sub.3 further include a monovalent metal, each can exhibit reduced deliquescent properties as compared to MgCl.sub.2 while having a good hydration capacity.

[0066] (Monovalent Metal)

[0067] Monovalent metals are typically selected from the group consisting of alkali metals and transition metals, in particular, alkali metals. In the present disclosure, alkali metals means a Group I element. From the viewpoint of the ease of production of the composite metal halide, the alkali metal is selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Furthermore, examples of transition metals that can become monovalent metals include chromium (Cr) and copper (Cu).

[0068] The relatively large ionic radius of the monovalent metal icon is preferable from the viewpoint of easily forming a stable halide salt (e.g., a perovskite structure) in combination with the divalent metal, and from the viewpoint of the lack of ease of hydration thereof. From these viewpoints, the monovalent metal is preferably selected from the group consisting of potassium (K), rubidium (Rb), and cesium (Cs).

[0069] An alkali metal is more preferable as the monovalent metal from the viewpoint of the stability of the hydration behavior thereof as compared to transition metals, which have variable valences.

[0070] (Divalent Metal)

[0071] Divalent metals are typically selected from the group consisting of alkaline earth metals and transition metals, in particular, alkaline earth metals. In the present disclosure, alkaline earth metals means Group II elements. From the viewpoint of the ease of production of the composite metal halide, the alkaline earth metal is typically selected from the group consisting of magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, examples of transition metals that can become divalent metals include zinc (Zn), copper (Cu), iron (Fe), lead (Pb), nickel (Ni), manganese (Mn), and tin (Sn).

[0072] The relatively small ionic radius of the divalent metal is preferable from the viewpoint of easily forming a stable halide salt (e.g., a perovskite crystal structure) in combination with the monovalent metal, and from the viewpoint of the ease of hydration thereof. From these viewpoints, the divalent metal is preferably selected from the group consisting of magnesium and calcium, more preferably magnesium.

[0073] Alkaline earth metals are more preferable as the divalent metal from the viewpoint of the stability of the hydration behavior thereof as compared to transition metals, which have variable valences.

[0074] (Halogen)

[0075] In the present disclosure, halogens means Group 17 elements. From the viewpoint of the ease of the production of the composite metal halide, the halogen is preferably selected from the group consisting of chlorine (Cl), bromine (Br), and iodine (I), more preferably chlorine (Cl).

[0076] <Heat Storage Material Production Method>

[0077] The method for production the heat storage material of the present disclosure includes the following steps: [0078] preparing a mixture by mixing a monovalent metal halide and a divalent metal halide hydrate, and [0079] generating the composite metal halide by subjecting the mixture to a heat treatment.

[0080] The use of a divalent metal halide hydrate as the supply source of the divalent metal rather than a divalent metal halide anhydride is advantageous for convenient production of the heat storage material of the present disclosure.

[0081] Specifically, when producing the composite metal halide of the present disclosure, if a divalent metal halide anhydride is used, after mixing the monovalent metal halide and the divalent metal halide anhydride, it is necessary to perform a heat treatment after performing a mechanical operation to compact the mixture, and it is necessary to repeat the compacting and heat treatment a plurality of times in order to increase the reaction yield. Thus, in the method of the present disclosure, since a divalent metal halide hydrate is used rather than a divalent metal halide anhydride, even if such a mechanical operation is not performed, the composite metal halide can be obtained at a high reaction yield.

[0082] The hydration number of the divalent metal halide hydrate used in the method of the present disclosure is preferably the maximum hydration number of the compound. For example, in the case of MgCl.sub.2, MgCl.sub.2.6H.sub.2O, which is a hexahydrate, is preferably used. Thus, in order to produce, for example, KMgCl.sub.3, which is a composite metal hydride, KCl can be used as the monovalent metal halide and MgCl.sub.2.6H.sub.2O can be used as the divalent metal halide hydrate.

[0083] The duration of the mixture of the monovalent metal halide and the divalent metal halide hydrate in the method of the present disclosure may be, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more, and may be 1 hour or less, 30 minutes or less, or 20 minutes or less.

[0084] The heat treatment in the method of the present disclosure can be performed in an inert atmosphere, for example, under a nitrogen atmosphere. The temperature of the heat treatment is, for example, a temperature close to the melting point of the desired composite metal halide, and may be, for example, a temperature which is 60% or more, 70% or more, or 80% or more of the melting point ( C.), and may be a temperature which is 100% or less, 95% or less, or 90% or less of the melting point ( C.). For reference, the melting point of KMgCl.sub.3 is about 487 C., the melting point of K.sub.2MgCl.sub.4 is about 429 C., and the melting point of CsMgCl.sub.3 is about 610 C. The duration of the heat treatment may be, for example 1 hour or more, 3 hours or more, 5 hours or more, or 10 hours or more, and may be 24 hours or less, 20 hours or less, or 15 hours or less.

[0085] <Heat Storage Material Applications>

[0086] The chemical heat pump of the present disclosure comprises, as shown in FIG. 1A, a water storage unit 10 for storing water as a working medium, a heat storage material retention unit 20 for retaining the heat storage material of the present disclosure, and a water vapor flow path 30 for allowing water vapor to flow between the water storage unit and the heat storage material retention unit.

[0087] During use of this chemical heat pump, heat storage and heat dissipation are performed in the chemical heat pump by hydrating and dehydrating the working medium in the heat storage material. Specifically, for example, during use of this chemical pump, water vapor generated by evaporating the water in the water storage unit is supplied to the heat storage material retention unit, the water vapor reacts with the heat storage material retained in the heat storage material retention unit, and the heat energy generated from this hydration is discharged. Thereafter, coldness is generated in the water storage unit due to vaporization heat. Conversely, during regeneration of the heat storage material, water vapor is generated from the heat storage material by adding heat to the heat storage material retention unit, and this water vapor is condensed in the water storage unit to produce water.

[0088] The chemical heat pump can be used air condition systems, dehumidifying systems, etc.

EXAMPLES

[0089] The present disclosure will be further specifically described below by way of the Examples. However, the present disclosure is not limited to these Examples.

Sample Preparation

Example 1

[0090] Equimolar KCl and MgCl.sub.2.6H.sub.2O were mixed at room temperature for 20 minutes, and the mixture was then subjected to a heat treatment at 400 C. for 12 hours to obtain a KMgCl.sub.3 powder as the sample of Example 1. Note that preparation of the samples of Example 1 and the Examples and Comparative Examples below was performed in a nitrogen atmosphere.

[0091] Note that, for reference, equimolar KCl and MgCl.sub.2 were mixed at room temperature for 20 minutes and then compacted using a pressure of 10 MPa at room temperature. The obtained molded body was subjected to heat treatment at 400 C. for 12 hours. However, KMgCl.sub.3 powder could not be obtained by this process. When mixing, compacting, and heat treatment were repeated once again under the same conditions, a KMgCl.sub.3 powder the same as described above could be obtained (confirmed by X-ray diffraction profiling).

Example 2

[0092] MgCl.sub.2 and KCl at twice the molar number of the MgCl.sub.2 were mixed at room temperature for 20 minutes and then compacted using a pressure of 10 MPa at room temperature. The obtained molded body was subjected to heat treatment at 400 C. for 12 hours. Mixing, compacting, and heat treatment were repeated once again under the same conditions to obtained a K.sub.2MgCl.sub.4 powder as the sample of Example 2.

Example 3

[0093] Equimolar CsCl and MgCl.sub.2.6H.sub.2O were mixed at room temperature for 20 minutes and then subjected to heat treatment at 480 C. for 12 hours. Mixing and heat treatment were repeated once again to obtain a CsMgCl.sub.3 powder as the sample of Example 3.

Comparative Example 1

[0094] A commercially available MgCl.sub.2 powder was used as the sample of Comparative Example 1.

Comparative Example 2

[0095] A commercially available KCl powder was used as the sample of Comparative Example 2.

Comparative Example 3

[0096] Equimolar MgCl.sub.2 powder the same as that used in Comparative Example 1 and KCl powder the same as that used in Comparative Example 2 were mixed at room temperature for 20 minutes to obtain a composite powder as the sample of Comparative Example 3.

Comparative Example 4

[0097] A commercially available CsCl powder was used as the sample of Comparative Example 4.

Comparative Example 5

[0098] Equimolar MgCl.sub.2 powder the same as that used in Comparative Example 1 and CsCl powder the same as that used in Comparative Example 4 were mixed at room temperature for 20 minutes to obtain a composite powder as the sample of Comparative Example 5.

Comparative Example 6

[0099] A commercially available CaCl.sub.2 powder was used as the sample of Comparative Example 6.

[0100] <Evaluation>

[0101] 1. Confirmation of Crystal Structure

[0102] The crystal structures of the samples obtained in the Examples and Comparative Examples described above were confirmed by X-ray diffraction measurement under the following conditions:

Measurement Device: RINT RAPID II (manufactured by Rigaku Corporation)

Measurement Conditions: Voltage 50 V, Current 100 mA, Collimator 0.3, Sample Angle =15

[0103] In order to prevent the samples obtained in the Examples and Comparative Examples from reacting with moisture in the atmosphere, the sample surfaces were covered with adhesive tape for use as measurement samples.

[0104] FIG. 2 shows the X-ray diffraction measurement results of the samples obtained in Examples 1 to 3 and Comparative Examples 1, 2, 4, and 6.

[0105] From the X-ray diffraction results shown in FIG. 2, it could be understood that the diffraction peak of Example 1 (KMgCl.sub.3) was different from the simple summation of the diffraction peaks of Comparative Example 1 (MgCl.sub.2) and Comparative Example 2 (KCl), i.e., the diffraction peak of Example 1 (KMgCl.sub.3) was a composite metal halide peak rather than the peak of a mixture of an alkaline earth metal halide and an alkali metal halide.

[0106] 2. Water Vapor Adsorption (Hydration) Isotherm Measurement

[0107] Water vapor adsorption isotherms were obtained for the samples before and after the following pretreatment:

Pretreating Device: BELSORP-vacII (manufactured by Microtrac-Bel Corporation)
Pretreatment Conditions: Degree of Vacuum: 10.sup.2 Pa or less, Heated at 150 C. for 6 hours
Measurement Device: BELSORP-max (manufactured by Microtrac-Bel Corporation)
Measurement Conditions: Temperature 20 C., the water vapor adsorption amount was measured from a relative pressure of 0 to the relative pressure at deliquescence

[0108] (1) Hydration Reaction Stage No.

[0109] In the isotherms described above, when the isotherm changed stepwise as the relative humidity increased, the number of stages was evaluated as the number of hydration reaction stages. The evaluation results are shown in Table 1 below.

[0110] (2) Deliquescent Humidity

[0111] In the isotherms described above, when the water vapor adsorption amount increased abruptly at a certain relative humidity and the water vapor adsorption amount continued to increase at that relative humidity, it was assumed that the sample deliquesced at that relative humidity. The evaluation results are shown in Table 1 below.

[0112] 3. Hydration Capacity (Thermogravimetric Analysis)

[0113] The samples of the Examples and Comparative Examples were hydrated to the stage prior to deliquescing to obtain hydrated samples. The hydrated samples were introduced to the measurement device described below, and the change in weight when the temperature was raised under the elevated temperature conditions described below was measured. The evaluation results are shown in Table 1 below.

[0114] Measurement Device: Pyris TGA (manufactured by Perkinelmer Corporation) Heating Conditions: Temperature 30 to 500 C., Heating Rate 2 C./min

TABLE-US-00001 TABLE 1 Water Vapor Adsorption Isotherm Thermogravimetric Analysis Heat Storage Hydration Deliquescence Hydration Capacity Material Reaction Stage No. Humidity (% RH) (g-water/g-sample) Example 1 KMgCl.sub.3 1 49 Actual 0.374 Example 2 K.sub.2MgCl.sub.4 1 49 Measurement 0.295 Example 3 CsMgCl.sub.3 1 49 Value 0.29 Comparative MgCl.sub.2 4 32 Theoretical 0.215 (at maximum Example 1 Value.sup.(1) hydration number) Comparative KCl 0 (Non- 75 0 (Non-Hydratable) Example 2 Hydratable) Comparative MgCl.sub.2 + KCl 31 Example 3 Comparative CsCl 0 (Non- 59 0 (Non-Hydratable) Example 4 Hydratable) Comparative MgCl.sub.2 + CsCl 30 Example 5 Comparative CaCl.sub.2 4 33 0.197 (at maximum Example 6 hydration number)

[0115] As shown in Table 1, in the samples of the Examples, which correspond to the composite metal halide of the present disclosure, hydration capacity and deliquescent humidity were higher as compared to the samples of the Comparative Examples.

[0116] Specifically, in, for example, the magnesium chloride (MgCl.sub.2) of Comparative Example 1, the hydration capacity was 0.215 g-water/g-sample and the deliquescent humidity was 32% RH. In the potassium chloride (KCl) of Comparative Example 2, the hydration capacity was 0 g-water/g-sample (non-hydratable) and the deliquescent humidity was 75% RH. Contrary thereto, in the samples of Examples 1, 2, and 3 (KMgCl.sub.3, K.sub.2MgCl.sub.4, and CsMgCl.sub.3), the hydration capacities were higher than those of the samples of the Comparative Examples and the deliquescent humidities were 49% RH. From these results, it could be understood that the composite metal halide of the present disclosure achieved both good hydration capacity and low deliquescent properties.