Heat storage compositions

Abstract: Provided is a heat storage material composition comprising at least one sugar alcohol selected from erythritol, mannitol and galactitol, and a salt which is sparingly soluble in the sugar alcohol. The heat storage material composition of the present invention is very satble and can be used for a long period of time, as evidenced by maintained crystallization temperature after repeated heat absorption and heat radiation.

Description: The present invention relates to a heat storage composition which is composed mainly of erythritol, mannitol or galactitol or their mixture and which utilizes a latent heat of fusion of this or these compound(s). A latent heat storage material has a heat storage density which is higher than a sensible heat storage material and has a constant temperature during the phase change. Due to this nature, the latent storage material has been put to practical use for storing or releasing a heat. As a latent heat storage material, ice, sodium sulfate decahydrate, calcium chloride hexahydrate, sodium acetate trihydrate are known. However, the temperature of phase change of these latent heat storage materials is relatively low. Therefore, these materials are inappropriate as heat storage materials which use solar energy or waste heat of a hot water supply or boiler and which are required to have a relatively high temperature of phase change of from 90 to 190°C. It has been proposed that sugar alcohols such as erythritol, mannitol and galactitol be used as a composition of a heat storage material which has a high temperature of phase change [Japanese Laid-Open Patent Application (Kokai) No. 32,963/1993 and Japanese Patent Announcement No. 500,946/1988 (corresponding to EP 0236382 and U. S. Patent No. 4,795,580)]. A sugar alcohol has a large amount of heat of fusion, exhibits an excellent heat stability and is non-toxic. However, when the sugar alcohol first melts and then solidifies, supercooling occurs of the part which is not crystallized even at a temperature lower than the melting temperature. Japanese Laid-Open Patent Application (Kokai) No. 32,963/1993 describes that the supercooling phenomenon is alleviated by adding pentaerythritol. However, unless as large as from 10 to 30% by weight of pentaerythritol are added, no sufficient effect is obtained. In such a case, the proportion of the sugar alcohol present in the composition is decreased, and the amount of heat stored is reduced. In view of the above-mentioned problems, the present inventors have assiduously conducted studies, and have consequently found that when a sparingly-soluble salt as a supercooling inhibitor is added to a specific sugar alcohol, supercooling of a heat storage material is prevented. This finding has led to the completion of the present invention. That is, the present invention is to provide a heat storage material composition comprising at least one sugar alcohol selected from erythritol, mannitol and galactitol and a sparingly-soluble salt. The main component of the heat storage material composition in the present invention is a sugar alcohol selected from erythritol, mannitol and galactitol. These sugar alcohols may be used either singly or in combination. The present invention is characterized in that a salt which is sparingly soluble in water is added as a supercooling inhibitor to the sugar alcohol. In the present invention, the salt which is sparingly soluble in water refers to an anhydrous compound which has such a solubility in water at 25°C that the amount of the anhydrous compound contained in 100 g of a saturated solution is 10 g or less, and which is dispersed and maintained in the sugar alcohol as particles even at a use temperature of the heat storage material of from 90°C and preferably up to 190°C without being decomposed or dissolved. The salt which is sparingly soluble in water (these salt is hereinafter simply referred to as "sparingly-soluble salt") include inorganic or organic salts. Examples of the inorganic salts include phosphates, sulfates, pyrophosphates, carbonates, calcium salts of inorganic acids, aluminum salts of inorganic acids, silver salts of inorganic acids, and silver halides. Specific examples thereof include calcium tertiary phosphate [Ca 3 (PO 4 ) 2 ], calcium sulfate, calcium pyrophosphate (Ca 2 P 2 O 7 ), calcium fluoride, aluminum phosphate, silver iodide, silver phosphate, silver bromide, and silver sulfate. Of these, calcium tertiary phosphate, calcium sulfate, aluminum phosphate, silver iodide, silver phosphate and silver bromide are the preferred salts, because of their efficient supercooling inhibition and their stable crystallization temperature even after repeated melting and freezing. As the organic salts, polyvalent metal salts of fatty acids having at least 16 carbon atoms, preferably from 16 to 22 carbon atoms can be mentioned. Examples thereof include palmitates, stearates and behenylates. Specific examples thereof include calcium palmitate, calcium stearate, magnesium stearate, barium stearate, and calcium behenylate. The amount of the sparingly-soluble salt is usually between 0.01 and 30% by weight, preferably between 0.3 and 15% by weight, more preferably between 0.3 and 10% by weight based on a total amount of the sugar alcohol and the sparingly-soluble salt. When the amount of the sparingly-soluble salt is larger than 30% by weight, the proportion of the sugar alcohol in the heat storage material composition becomes correspondingly smaller, with the consequence of decreasing the amount of heat of fusion. Meanwhile, when it is less than 0.01% by weight, the effect of preventing supercooling cannot be obtained. The average particle diameter of the sparingly-soluble salt is usually between 0.01 and 1,000 µm. When the average particle diameter is too large, the salt is not uniformly dispersed in the heat storage material composition, making it impossible to obtain a sufficient effect of preventing supercooling. The heat storage material composition in the present invention may be used together with known heat storage materials, for example, organic materials such as paraffins, polyethylene glycol, polyvinyl alcohol, polyethylene, crosslinked polyethylene and glycerin. Further, the composition may contain, besides the specific sugar alcohol and the sparingly-soluble salt, a thickner such as a water-insoluble super-absorbent polymer, carboxymethyl cellulose, sodium alginate, potassium alginate or fumed silica, an antioxidant such as phenols, amines or hydroxyamines, and a metal corrosion inhibitor such as chromates, polyphosphates or sodium sulfite. The heat storage material composition in the present invention is packed into a stainless steel container having a diameter of 40 mm and a height of 80 mm , and the container is dipped in an oil bath. The container is heated until the temperature of the sample reaches 130°C so that this sample is completely dissolved. Then, the container is allowed to cool while it is dipped in the oil bath. The temperature at which crystallization starts is measured by means of a thermocouple positioned 10 mm above the bottom in the center of the container. At this stage, the above-mentioned temperature of the composition of the present invention is at least 5°C, preferably at least 10°C higher than that of any one of constituting sugar alcohols, when no sparingly-soluble salt is added. A method of preparing the heat storage material composition in the present invention is not particularly limited. At least a sugar alcohol, a sparingly-soluble salt and optionally additives and a known heat storage material may be mixed and uniformly dispersed. In order to disperse them uniformly, the sugar alcohol is heated up to a temperature above the melting point thereof, and is mixed with the sparingly-soluble salt and the additives while being stirred. The heat storage material composition of the present invention may be used in an encapsulated or micro-encapsulated form. When the composition is to be put into a capsule, the heat storage material composition is poured into a type of heat storage container, fills it and the container is then sealed. The capsule may be formed of a material which is not deformed nor dissolved within the range of the use temperature. Examples thereof include metals such as stainless steel and aluminum, glass, and engineering plastics such as polycarbonates. The shape of the capsule is not particularly limited. Examples of the shape include a sphere, a flat plate, a pipe, a compressed cylinder, a twin sphere, and a wavy plate. It is appropriately selected depending on the use. When a micro-capsule is to be prepared, the above type of heat storage container is usually not used. Instead, fine particles of the heat storage composition or an aggregate thereof are coated with a resin which is not melted nor deteriorated within the range of the use temperature. Since the total surface area of the microcapsules formed is much larger than that of the capsule, a heat transfer efficiency is advantageously high. In the heat storage system, a heating medium is passed around the capsule or the microcapsules. The capsule or the resin coating the microcapsules acts as a heat exchanger for heat storage or heat radiation. Examples of the heating medium include water, water vapor and gas such as air. The heat storage material composition of the present invention is very stable and can be used for a long period of time as evidenced by maintained crystallization temperature after repeated heat storage and heat radiation. The above and other objects, features and advantages will be made apparent from the following description of the preferred embodiments, given as a non-limiting example, with reference to the accompanying tables and drowings, in which: Table 1 shows crystallisation temperatures of the heat-storage composition obtained in Examples 1 to 4 and Comparative Examples 1 to 8; Table 2 shows an evolution of crysatllsation-starting temperatures of the heat-storage compositions obtained in Example 5 and Comparative Examples 9 to 12, when crystallisation is repeated; Table 3 shows crystallisation temperatures of the heat-storage compositions obtained in Examples 6 to 12 and Comparative Examples 13 to 18; Table 4 shows an evolution of crystallisation-starting temperatures of the heat-storage compositions obtained in Examples 13 to 20, when crystallisation is repeated; and, Fig. 1 shows an evolution of crystallisation-starting temperatures of the heat-storage compositions obtained in Example 5 and Comparative Example 9, when the compositions are cooled for the 5th time, where ▪ indicates Example 5 and □ Comparative Example 9. Examples Erythritol (made by Nikken Kagaku K.K.), sorbitol, calcium tertiary phosphate having an average particle diameter of 7.9 µm, calcium sulfate having an average particle diameter of 2.8 µm, aluminum phosphate having an average particle diameter of 15.2 µm, calcium pyrophosphate having an average particle diameter of 6.2 µ m, silver iodide having an average particle diameter of 5.0 µm (made by Kishida Kagaku K.K.), mannitol, xylose, xylitol (made by Towa Kasei Kogyo K.K.), galactitol, silver bromide having an average particle diameter of 70 µm, silver chloride having an average particle diameter of 200 µm (made by Nacalai Tesque Co.), and silver phosphate having an average particle diameter of 7.9 µm (made by Mitsuwa Kagaku Yakuhin) were used in these Examples. The average particle diameter of the sparingly-soluble salt alone was measured upon pulverizing this salt in a mortar. The average particle diameters of silver bromide and silver chloride were measured using a microscope. The average particle diameters of the other components were measured by means of a particle diameter measuring device (HORIBA LA-500, manufactured by Horiba K.K.) using distilled water (or a saturated solution of 25°C in the case of the components which were partially dissolved in water) as a solvent. Example 1 Erythritol (4.95 g) and 0.05 g of calcium tertiary phosphate were uniformly pulverized and mixed in a mortar. The crystallization temperature of the resulting heat storage material composition was measured by means of a differential scanning calorimeter (DSC-220C, manufactured by Seiko Denshi Kogyo K.K.) using an aluminum sealed cell. The results are shown in Table 1. Example 2 Example 1 was repeated except that 4.75 g of erythritol and 0.25 g of calcium tertiary phosphate were used. The crystallization temperature of the resulting heat storage material composition is shown in Table 1. Comparative Example 1 Example 1 was repeated except that 5.00 g of erythritol alone were used. The crystallization temperature of the resulting heat storage material composition is shown in Table 1. Examples 3 and 4 Example 2 was repeated except that erythritol was replaced with mannitol (Example 3) and galactitol (Example 4). The crystallization temperatures of the resulting products are shown in Table 1. Comparative Examples 2 and 3 Examples 3 and 4 were repeated except that 5.00 g of mannitol (Comparative Example 2) and 5.00 g of galactitol (Comparative Example 3) alone were used. The crystallization temperatures of the resulting products are shown in Table 1. Comparative Examples 4 to 6 Example 2 was repeated except that erythritol was replaced with sorbitol (Comparative Example 4), xylose (Comparative Example 5) and xylitol (Comparative Example 6). The crystallization temperatures of the resulting products are shown in Table 1. Comparative Examples 7 and 8 Example 2 was repeated except that calcium phosphate was replaced with sodium chloride (Comparative Example 7) and potassium chloride (Comparative Example 8). The crystallization temperatures of the resulting products are shown in Table 1. Example 5 Erythritol (47.5 g) and 2.5 g of calcium tertiary phosphate were uniformly pulverized and mixed in a mortar to obtain a heat storage material composition. This composition was used as a sample, and was packed in a stainless steel container having a diameter of 40 mm and a hight of 80 mm. The container was dipped in an oil bath, and heated until the temperature of the sample reached 130°C to completely dissolve this sample. Subsequently, with the container being kept in the oil bath, the temperature of the oil was allowed to cool down. Then, the temperature at which crystallization of the sample starts was measured by means of a thermocouple. The temperature of the sample was measured in a position 10 mm above the bottom in the center of the container. When the temperature of the sample reached 60°C, the sample was reheated to 130°C in the oil bath. The heating and the cooling were repeated for a total of 100 times. The temperatures at which start the 1st, 2nd, 5th, 10th and 100th crystallizations are shown in Table 2. As an Example, the temperature evolution in the 5th cooling is shown in Fig. 1. Comparative Example 9 Example 5 was repeated except that 50 g of erythritol alone were used. The results are shown in Table 2 and Fig. 1. From Fig. 1, it became apparent that the curves of the temperature decrease in Examples 5 and Comparative Example 9 were approximately consistent, but that the temperature at which crystallization starts, in the case of Comparative Example 9 in which erythritol alone is used, was 64°C whereas this temperature in Example 5 in which calcium phosphate is used, was 109°C. Thus, the supercooling phenomenon was alleviated in Example 5. Comparative Examples 10 to 12 Example 5 was repeated except that the heat storage composition used was prepared by adding pentaerythritol to. erythritol in an amount of 10% by weight (Comparative Example 10), 20% by weight (Comparative Example 11) or 30% by weight (Comparative Example 12) . The results are shown in Table 2. Examples 6 to 12 Example 1 was repeated except that erythritol was replaced by mannitol (Examples 6 to 9) and galactitol (Examples 10 to 12) and the sparingly-soluble salts were used in amounts shown in Table 3. The temperature at which crystallization starts is shown in Table 3 as a crystallization temperature. Comparative Examples 13 to 18 Example 1 was repeated except that erythritol was replaced by sorbitol (Comparative Examples 13 to 15), xylose (Comparative Examples 16 and 17), and xylitol (Comparative Example 18), and the sparingly-soluble salts were used in amounts shown in Table 3. Examples 13 to 15 Example 5 was repeated except that calcium tertiary phosphate was replaced by calcium sulfate (Example 13), calcium pyrophosphate (Example 14) and aluminum phosphate (Example 15). The heating and the cooling were repeated for a total of 100 times, and the temperatures at which start the 1st, 5th, 10th and 100th crystallizations are shown in Table 4. Example 16 Example 5 was repeated except that 49.75'g of erythritol and 0.25 g of silver iodide were used. The heating and the cooling were repeated for a total of 100 times, and the temperatures at which start the 1st, 5th, 10th and 100th crystallizations are shown in Table 4. Examples 17 to 20 Example 16 was repeated except that silver iodide was replaced by silver phosphate (Example 17), silver bromide (Example 18), silver chloride (Example 19) and calcium stearate (Example 20). The heating and the cooling were repeated for a total of 100 times, and the temperatures at which start the 1st, 5th, 10th and 100th crystallizations are shown in Table 4.

Claims: A heat storage composition comprising a sugar alcohol and a supercooling inhibitor, characterised in that the sugar alcohol comprises at least one compound selected from the group consisting of erythritol, mannitol and galactitol and that the supercooling inhibitor comprises a salt which has such a solubility in water at 25°C that the amount of the anhydrous compound contained in 100 g of saturated solution is 10 g or less, provided that said salt is different from carbonates of calcium, magnesium, zinc or aluminum ions. The heat storage composition of claim 1, wherein the salt is an inorganic salt selected from phosphates, sulfates pyrophosphates, silver salts of inorganic acids or silver halides. The heat storage composition according to claim 1 or 2, wherein the inorganic salt is calcium phosphate, calcium sulfate, calcium pyrophosphate, aluminium phosphate, silver phosphate, silver sulfate, silver chloride or silver iodide. The heat storage composition of claim 1, wherein the salt is an organic salt. The heat storage composition of claim 4, wherein the organic salt is a polyvalent metal salt of an aliphatic acid having at least 16 carbon atoms. The heat storage composition of claim 5, wherein the organic salt is a palmitate, a stearate or a behenate. The heat storage composition according to any one of claims 4 to 6, wherein the organic salt is calcium palmitate, calcium stearate, magnesium stearate, barium stearate or calcium behenate. The heat storage composition according to any one of claims 1 to 7, wherein the average particle diameter of the salt is between 0.01 and 1,000 µm. The heat storage composition according to any one of claims 1 to 8, wherein the amount of the salt is between 0.01 and 30% by weight. The heat storage composition according to any one of claims 1 to 9, wherein the temperature at which the heat storage material composition starts to crystallise is at least 5°C higher than that of a corresponding composition omitting said salt. Use of the heat storage composition according to any one of claims 1 to 10 at a temperature above 90°C. Use of a heat storage composition comprising a sugar alcohol and a supercooling inhibitor, in which the sugar alcohol comprises at least one compound selected from the group consisting of erythritol, mannitol and galactitol and the supercooling inhibitor comprises a salt which has such a solubility in water at 25°C that the amount of the anhydrous compound contained in 100 g of saturated solution is 10 g or less, for a device in which either solar energy or waste heat is used as a heat source.

Inventors: CHIHARA SHOICHI, ISAKA TSUTOMU, KAKIUCHI HIROYUKI, YAMAZAKI MASANORI, CHIHARA, SHOICHI, ISAKA, TSUTOMU, KAKIUCHI, HIROYUKI, YAMAZAKI, MASANORI, Chihara, Shoichi, c/o Mitsubishi Chem. Corp., Isaka, Tsutomu, c/o Mitsubishi Chem. Corp., Kakiuchi, Hiroyuki, c/o Mitsubishi Chem. Corp., Yamazaki, Masanori, c/o Mitsubishi Chem. Corp.
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1996
Main classification: C09K5
IPC codes: C09K5