JP2008179709A - Heat storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat storage material capable of storing and releasing heat in a temperature region of 30 to about 200°C and having a large heat storage quantity. <P>SOLUTION: The heat storage material consists essentially of potassium dititanate hydrate represented by the general formula: K<SB>2-y</SB>Mg<SB>(y/2)</SB>Ti<SB>2</SB>O<SB>5-x</SB>-nH<SB>2</SB>O (wherein, 0≤x≤1 and 0<y<0.5) and the maximum of the n is a value exceeding 2.7 when hydration into the potassium dititanate hydrate is carried out. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heat storage material that can store waste heat and solar heat from boilers, air conditioning equipment, hot water supply equipment, and the like and can be effectively used as a timely heat source.

In recent years, cooling and heating equipment such as boilers, air conditioning equipment, and hot water supply equipment are indispensable in comfortable living spaces or industrial product production sites, and the increase in energy consumption by these equipment causes an increase in oil consumption. In addition, in order to bring about a great environmental load, various efforts for reducing energy consumption are being actively carried out.
There is a statistic that about 66% of the total primary energy consumed in Japan is unused in waste heat, but these waste heat is low in temperature and has the problem that waste heat of at least 200 ° C is difficult to use. ing.

  Waste heat recovery using a heat storage material uses a relatively inexpensive material, enables the recovery of waste heat with simple equipment, and allows the stored heat to be taken out as heat in a timely manner. Thermal storage materials are mainly classified into sensible heat storage materials and latent heat storage materials based on the difference in mechanisms for storing the heat.

The sensible heat storage material is a material that stores heat with a temperature change of a substance. Thermal energy flows into material of the low-temperature T _L, when it becomes the temperature T _H, the amount of heat stored material Q _{S (J)} is the specific heat of the material C, and the density [rho, when the volume is V, the following It is expressed by a formula. However, it is assumed that the specific heat C is constant.
_{Q S = CρV (T H -T} L) ········· (1)

That is, the process of increasing the temperature of the substance is heat storage, and the process of decreasing the temperature is heat dissipation. For waste heat whose temperature fluctuates, heat storage or heat dissipation occurs in conjunction with the temperature change. Since the heat storage amount Q _S changes in proportion to the temperature difference (T _H −T _L ) and the heat capacity CρV of the substance, the amount of stored heat increases if a large temperature difference is provided or a heat storage material having a large heat capacity is used. .

  As the sensible heat storage material, water (specific heat 4.18 J / gK), calorie HT43 (Exxon, specific heat 2.77 J / gK), Therminol T66 (Monsanto, specific heat 2.77 J / gK), around 288 K, Siltherm 800 (Dow Corning, specific heat 2.10 J / gK), concrete block (specific heat 0.88 J / gK), gravel (specific heat 0.88 J / gK), magnesia brick (specific heat 0.88 J / gK), etc. are known. It has been.

  Although water is chemically stable and has a large specific heat, it is a very excellent sensible heat storage material. However, since it has a low boiling point, it cannot be used with waste heat of 100 ° C. or higher in the open atmosphere. On the other hand, an organic heat medium such as Caloria HT43 has a high boiling point and can be used for waste heat around 200 ° C. However, since the specific heat is smaller than that of water, development of a material having a larger specific heat is eagerly desired. Furthermore, magnesia bricks and the like are excellent in heat resistance, and if the temperature difference is 500 ° C. or more, the heat storage amount becomes large, but the heat storage amount is low because the specific heat is very small for low-grade waste heat of 200 ° C. or less. It is a small and promising material.

  One latent heat storage material is a material that stores heat using latent heat associated with phase change or transition of a substance, and heat storage and heat dissipation are mainly performed at a melting point or a transition point unique to the material. Therefore, a large amount of heat storage cannot be obtained unless the melting point or transition point of the latent heat storage material is exceeded, and conversely, heat is not released unless the stored heat is less than the melting point or transition point. When the temperature of waste heat fluctuates, there are methods such as using several types of latent heat storage materials in combination, but there are problems such as inconsistencies in chemical properties and physical properties, and an efficient heat storage system can be constructed. could not. Recently, however, manganese nitrate hydrate has been proposed as a latent heat storage material whose melting point can be changed by a simple method (see Patent Document 1).

Examples of materials having a large latent heat include hydrocarbons such as 1-decanol (latent heat 206 J / g), C16 paraffin (latent heat 200 J / g), and the like at inorganic or inorganic transition temperatures of 2 ° C. to 94 ° C. For salt hydrates, Sr (OH) ₂ .8H ₂ O (latent heat 351 J / g), Ba (OH) ₂ .8H ₂ O (latent heat 293 J / g), Na ₂ SO ₄ .10H ₂ O (latent heat 200 J / g) Na ₂ CH ₃ COO.3H ₂ O (latent heat 251 J / g). Further, at 133 ° C. to 250 ° C., there are pentaerythritol (latent heat 322 J / g), LiOH-NaOH eutectic salt (latent heat 362 J / g), urea (latent heat 251 J / g), and the like (Non-patent Documents 1 to 3 and Patent Document 2). reference).
A material having a large latent heat is likely to be decomposed or altered due to repeated melting, solidification, or transition, and is disadvantageous in that there are many combustible or deleterious substances or corrosive substances.

  In order to eliminate the above-described drawbacks, a heat storage material containing aluminum oxide having an apparent specific gravity of 0.5 to 1.1 has been proposed. Unlike the conventional mechanism, the heat storage and heat dissipation of this heat storage material utilizes the adsorption and desorption of moisture to aluminum oxide. Since the basic principle uses heat storage by desorption of water molecules adsorbed on activated alumina and heat release by re-adsorption of water molecules, it is chemically stable and the amount of heat storage is desorption of water molecules by heating at 150 ° C. About 282 J / g is obtained (see Patent Document 3). However, since the apparent specific gravity is 0.5 to 1.1, the heat storage amount per volume is small.

And recently, it is represented by the general formula K ₂ Ti ₂ O _5−x · nH ₂ O (0 ≦ x ≦ 1, 0 ≦ n ≦ 2.7) as a heat storage material with improved heat storage amount per volume. A heat storage material mainly composed of potassium dititanate hydrate has been proposed. This heat storage material has a layered structure in which TiO ₅ triangular pyramidal bodies are chained, and uses heat storage by hydration and dehydration of K ⁺ ions arranged between the layers, and the heat storage amount is 306 J / g. It shows a very large value (780 J / cm ³ ) (see Patent Document 4). In addition, “n indicating H ₂ O content is not theoretical data, but the maximum value obtained from an experiment is 2.7” (Patent Document 4, page 5, paragraph 0016).
JP 2001-064635 A JP 59-134497 A JP 2002-162184 A JP-A-2005-36213 1 from Katsuhiko Narita, “Latent Heat Storage Material”, The Institute of Electrical Engineers of Japan, 101 (1), 1981, p. 15-22 Takeo Ozawa, 4 others, “Preliminary study 2 of heat storage materials using latent heat”, Electronic Technology Research Institute Bulletin 44 (11, 12), 1980, p. 707-p. 724 Kotaro Tanaka and 6 others, “Preliminary Study 3 on Thermal Storage Material Utilizing Latent Heat”, Vocabulary 51 (7), 1987, p. 469-p. 483

However, potassium dititanate hydrate represented by the general formula K ₂ Ti ₂ O _5-x · nH ₂ O (0 ≦ x ≦ 1, 0 ≦ n ≦ 2.7) described in Patent Document 4 above is used. When the heat storage material as the main component is also evaluated from the viewpoint of cost effectiveness, the amount of heat storage is not sufficient, and there is room for improvement. Development of a new heat storage material for further increase in the amount of heat storage or improvement of a conventional heat storage material is desired.

  An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a heat storage material that is easy to store and release heat and has a large amount of stored heat.

According to the present invention, dititanate indicated by the general formula _{K 2-y Mg (y /} 2) Ti 2 O 5-x · nH 2 O (0 ≦ x ≦ 1,0 <y <0.5) A heat storage material comprising potassium salt hydrate as a main component, wherein the maximum value of n exceeds 2.7 when the potassium dititanate hydrate is hydrated A heat storage material characterized by the above is provided.

  ADVANTAGE OF THE INVENTION According to this invention, heat storage and heat dissipation are easy, and the thermal storage material which has a big thermal storage amount is provided.

The general formula of potassium dititanate in the present invention is usually expressed as K ₂ Ti ₂ O ₅ , but is exactly K ₂ Ti ₂ O _5-x , where x is the synthesis temperature and oxygen partial pressure in the synthesis atmosphere. It depends on.
As the synthesis temperature is lower and the oxygen partial pressure in the synthesis atmosphere is higher, x approaches 0.
Further, by setting x to 1 or less, a crystal structure as potassium dititanate can be obtained, and physical properties that can achieve the object of the present invention can be achieved. In order to make x larger than 1, it is necessary to synthesize in a strong reducing atmosphere.

The crystal structure of potassium dititanate has a layered structure in which TiO ₅ triangular pyramidal bodies are linked, and it is considered that K ⁺ ions are arranged between the layers made of TiO ₅ triangular pyramidal bodies. ing. Inferring from the crystal structure, x is considered to be a stable state with fewer lattice defects as it is closer to 0, and is most suitable for the heat storage material of the present invention.

In such a layered structure crystal, K ⁺ ions arranged between the layers are easily hydrated, and potassium dititanate hydrate represented by the general formula K ₂ Ti ₂ O _5-x · nH ₂ O Become. At this time, H ₂ O molecules enter as a guest between the layers made of TiO ₅ triangular pyramidal bodies so as to be induced by K ⁺ ions. When H ₂ O molecules enter, the distance between the layers composed of TiO ₅ triangular pyramidal bodies expands, but the crystal structure does not change.

Further, when the above potassium dititanate hydrate is heated, the H ₂ O molecules that have invaded as a guest between the layers composed of the TiO ₅ triangular pyramidal bodies are desorbed from the layers, and are composed of the expanded TiO ₅ triangular pyramidal bodies. The interlayer shrinks and returns to its original state.
This reaction is considered to be a kind of topochemical reaction and is not accompanied by a change in crystal structure. Therefore, potassium dititanate hydrate is extremely stable against repeated hydration and dehydration.
That is, a heat storage material that is stable and safe against repeated heat dissipation and heat absorption can be obtained.

The hydration amount n of potassium dititanate hydrate in the present invention varies with hydration and dehydration.
The minimum value of n when dehydrated is preferably a value greater than zero. By doing so, even in the dehydrated state, the H ₂ O molecule remains between the layers made of TiO ₅ triangular pyramidal bodies, so that the layers are expanded, thereby slowing the hydration rate of K ⁺ ions arranged between the layers. Can be prevented. Furthermore, the minimum value of n when dehydrated is more preferably a value of 1.3 or more. By doing this, the hydration rate is faster.
The maximum value of n when hydrated is preferably greater than 2.7. By doing so, it is possible to obtain a larger amount of heat storage than conventional heat storage materials. Furthermore, the maximum value of n when hydrated is more preferably a value greater than 3.5. By doing so, a larger amount of heat storage can be obtained.

Furthermore, some of the K ⁺ ions arranged between the layers can be replaced with Mg ²⁺ ions having the closest ionic radius among the alkaline earth metals. Since the hydration energy of Mg ²⁺ ions is approximately 18% greater than the hydration energy of K ⁺ ions, the amount of heat storage increases when substitution is made within a composition range in which the crystal structure of potassium dititanate hydrate can be maintained.

In the general formula _{K 2-y Mg (y /} 2) Ti 2 O 5-x · nH 2 O in the present invention, y is preferably larger than 0. By doing so, K ⁺ ions are replaced with Mg ²⁺ ions, and the amount of heat storage is increased.
Moreover, y is preferably smaller than 0.5. By doing so, it is possible to prevent MgTiO _{3 from} being mixed into the potassium dititanate hydrate and the heat storage amount to rapidly decrease.

  In order to increase thermal conductivity, potassium dititanate may be mixed with metal fillers, graphite particles, etc., and Teflon (registered to prevent decomposition of potassium dititanate by adhering water) (Trademark) You may use it in the state kneaded with fine powder.

The heat storage material in the present invention does not decompose or change due to repeated heat storage and heat dissipation, and can be used safely. In particular, it is easy to store and dissipate heat in the temperature range from 30 ° C. to 200 ° C.
Furthermore, not only is it suitable for a cogeneration system that collects and reuses low-grade thermal energy discharged from boilers, air-conditioning equipment, or hot-water equipment in homes, but it also collects solar thermal energy for daytime cooling and nighttime use. It is also effective for use in heating.

  It should be noted that the present invention is not limited to the above-described embodiments, and modifications, improvements, and the like within the scope that can achieve the object of the present invention are included in the present invention.

EXAMPLES Hereinafter, although this invention is demonstrated based on an Example, this invention is not limited to this. Moreover, although demonstrated based on potassium dititanate salt hydrate, the heat storage material which has potassium dititanate hydrate as a main component may be sufficient.
Potassium dititanate hydrate is prepared, for example, as follows.
In the general formula _{K 2-y Mg (y /} 2) Ti 2 O 5-x · nH 2 O, and y = 0.0,0.1,0.2,0.4,0.5 and 1.0 Thus, potassium carbonate (K ₂ CO ₃ ), magnesium carbonate (MgCO ₃ ) and titanium oxide (TiO ₂ ) were weighed, and these powders were thoroughly mixed and pulverized in an agate mortar. The obtained mixed powder is put into a platinum crucible and heated at a rate of 10 ° C./min from 30 ° C. to 950 ° C. in air at a temperature of 20 ° C. and a relative humidity of 50% (water vapor pressure of about 1.2 kPa) using a muffle furnace. The potassium dititanate was synthesized by maintaining at 950 ° C. for 48 hours.
In order to perform a process of repeating hydration and dehydration, the potassium dititanate synthesized as described above was transferred to a thermo-hygrostat, and the water vapor pressure was controlled to 1.2 kPa at 30 ° C. for 24 hours. After leaving it to stand, the temperature-and-humidity chamber was heated and left at 300 ° C. for 0.5 hour to lower the temperature. The treatment according to this temperature program was performed once, and this treatment was repeated three times to obtain potassium dititanate hydrate as a heat storage material.

(result)
The heat storage amount of the potassium dititanate hydrate obtained as described above was measured as follows.
Hydration by allowing potassium dititanate hydrate to stand in air at a temperature of 20 ° C. and a relative humidity of 50% for 72 hours, followed by using a differential thermal analyzer in air at a temperature of 20 ° C. and a relative humidity of 50%. The temperature was raised from 1 ° C. to 300 ° C. at 1 ° C./min. The endothermic amount (J / g) associated with dehydration due to this temperature rise was measured, and this was defined as the heat storage amount (J / g).
Hydration of potassium dititanate hydrate was performed by leaving it in air at a temperature of 20 ° C. and a relative humidity of 50% for 72 hours. At this time, the value of the hydration amount n is the maximum value. The dehydration of potassium dititanate hydrate is carried out at a rate of 1 ° C./min from 30 ° C. to 300 ° C. in air at a temperature of 20 ° C. and a relative humidity of 50% using a differential thermal analyzer after the above hydration. I went. At this time, the value of the hydration amount n becomes the minimum value.
The weight of the potassium dititanate hydrate when hydrated and dehydrated was measured, and the change Δn in the amount of hydration was determined from the weight difference. Further, the maximum value of n was calculated from the weight when hydrated, and the minimum value of n was calculated from the weight when dehydrated.
These results are shown in Table 1 below.

Example 1 is y = 0.1, Example 2 is y = 0.2, Example 3 is y = 0.4, Comparative Example 1 is y = 0, Comparative Example 2 is y = 0.5, Comparative Example 3 is y = 1.0.
The amount of stored heat was 0 <y <0.5, which exceeded that of Comparative Example 1, and the maximum value was 408 J / g when y = 0.2. Moreover, the change Δn in the amount of hydration was 2.7 to 2.8 when 0 ≦ y ≦ 0.4.

FIG. 1 shows the relationship between the heating temperature and the hydration amount n for Comparative Example 1 and Example 2.
In both Comparative Example 1 and Example 2, the maximum value of hydration amount n was 4.1, and the minimum value was 1.4.

It is a graph which shows the relationship between heating temperature and the amount of hydration n.

Claims (4)

Formula _{K 2-y Mg (y /} 2) Ti 2 O 5-x · nH 2 O (0 ≦ x ≦ 1,0 <y <0.5) the main two potassium titanate hydrate, denoted by A heat storage material characterized by being an ingredient,
The heat storage material, wherein the maximum value of n when the potassium dititanate hydrate is hydrated exceeds 2.7.   2. The heat storage material according to claim 1, wherein the minimum value of n when dehydrating the potassium dititanate hydrate is a value exceeding 0. 3.   The heat storage material according to claim 1, wherein the minimum value of n when the potassium dititanate hydrate is dehydrated is 1.3 or more. The heat storage material according to any one of claims 1 to 3,
A heat storage material having a heat storage amount of 360 J / g or more.

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