CA2208695C

CA2208695C - Heat storage system and heat release control method

Info

Publication number: CA2208695C
Application number: CA002208695A
Authority: CA
Inventors: Kenji Saita
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1996-06-25
Filing date: 1997-06-24
Publication date: 2007-03-20
Anticipated expiration: 2017-06-24
Also published as: EP0816468B1; DE69737519D1; CA2208695A1; US6152212A; DE69737519T2; EP0816468A1

Abstract

A heat storage system comprising a heat storage material containing a salt hydrate, wherein the heat storage material comprises following three serial parts, (A) a seed crystal part in which a seed crystal is maintained, (B) a part which functions as a switch by heating, or heating and cooling and (C) a main body part which can store and release heat, and a method for heat release control, wherein crystal spread from the seed crystal part (A) to the main body part (C) is controlled by the heating or cooling operation of the part (B) in the above-described heat storage system.

Description

Heat Storage System and Heat Release Control Method Field of the Invention The present invention relates to a heat storage system and a heat release control method. More particularly, the present invention relates to a heat storage system and a heat release control method which are used for heating of buildings and the like.
Prior Art A heat storage material should satisfy a variety of conditions including a large amount of heat storage and heat release at a given temperature level, stability for a long period of time, inexpensiveness, no toxicity, no corrosion property and the like. As a heat storage material which satisfies these conditions, a salt hydrate having phase changing property has been most freguently investigated.
Representative examples thereof include sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate and the like, and some of them have been put into practice. In phase change these salt hydrates generate a large amount of heat and they have a suitable temperature of phase change. However, there is a problem that some of them results in high supercooling. High supercooling causes that the temperature drops too much until heat release occurs, and is considered as an obstacle to make a practical use. Supercooling is a undesirable property of a salt hydrate and a supercooling inhibitor has been investigated for a long time.
However, high supercooling is advantageous because heat release time can be selected. For the purpose of heat storage, several suggestions have been made such as the formerly known method of adding seed crystal to conduct heat release. This method provides reliable heat release, but there are problems that the composition thereof is changed by repeated addition of seed crystal, and that the system has to be opened when adding the seed crystal to cause water evaporation, and that this addition is complicated and must be conducted frequently.
Moreover, there has been a known method in which crystallization is conducted by immersing electrodes in an aqueous solution of sodium acetate trihydrate and by applying voltage between the electrodes. However, the problems with this method are that the possibilities of elution of an electrode material and generated gas by electric current occurs, and that repeated use is limited by the systems' life span.
Summary of the Invention The present inventors have intensively studied a heat storage system which does not have the problems as described above, and have found that a heat storage system comprising following three serial parts, (A) a seed crystal part, (B) a part which functions as a switch by heating, or heating and cooling and (C) a main body part which can store and release heat following in the order of (A), (B) and (C), is stable in a long period thermal cycle since the composition of the heat storage material does not change and one can easily control the time when heat release is conducted from the supercooling condition of the heat storage material since the seed crystal is maintained.
Based on this finding, the inventors have completed the present invention.
Namely, the present invention provides a heat storage system comprising a heat storage material containing a salt hydrate, wherein the heat storage material comprises following three serial parts, (A) a seed crystal part in which a seed crystal is maintained, (B) a part which functions as a switch by heating, or heating and cooling and (C) a main body part which can store and release heat.
The present invention also provides a method for controlling heat release from the heat storage system.
Brief Description of Drawings Fig. 1 is a schematic view of the heat storage system according to one embodiment according to the present invention; and Fig. 2 is a cross-sectional view of the heat storage system of Fig. 1 at the indicated line 2-2.
Detailed Description of the Invention The present invention will be described in detail below.
As shown in Figs. 1 and 2, the heat storage system of this particular embodiment comprises a flat plate vessel 1 filled with a heat storage material which is divided into three continuous parts in the following order: a seed crystal part (A) in which a seed crystal is maintained, a switch part (B) which is controlled by a heater 3 and a main body part (C) which is controlled by a second heater 2. A
heat insulating material 4 is provided to cover the vessel.
The heat storage material of the present invention comprises a salt hydrate. The above-described heat storage material is preferably used by packing in a vessel, and more preferably used by packing in a vessel which has no moisture permeating property. The shape of the vessel is not particularly restricted, and a vessel of any shape such as a - 3a -cylinder, coil, flat plate and the like can be used. When a heat storage apparatus is to be used by laying under the floor, the vessel preferably should be strong enough to stand a load.
The salt hydrate shows solid-liquid phase change by heating and cooling, and examples thereof include sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, calcium chloride hexahydrate, calcium nitrate tetrahydrate and the like. In the case of an incongruent salt hydrate, it is preferable to add a small excess of water and a solid-liquid separation inhibitor to avoid the formation of a lower hydrate. Small excess of water is selected from the stable range of a salt hydrate which is determined by solution equilibrium between a salt hydrate and water, and in many cases, it is from about 1/10 to 1/3 of the hydration mol number of the salt hydrate.
Example of the solid-liquid separation inhibitor include water-soluble polymers, water-swellable polymers, high water-absorption resins, silica based thickening agents and the like. In addition to these salt hydrates, melting point controlling agents, dispersing agents, defoaming agents, corrosion inhibitors, coloring agents and the like may be added.
The heat storage system of the present invention preferably includes an apparatus which can conduct heating and cooling, and an apparatus which can conduct heating, which are installed in the inner part or outer part of a vessel in which a heat storage material is packed. When the apparatus which can conduct heating and cooling, and the apparatus which can conduct heating, are installed in the inner part of the vessel, it is preferable that the apparatus is made of a material which causes no chemical reaction with the heat storage material.
The seed crystal part (A) comprises a salt hydrate crystaland isalways maintained at a temperature not higher than the melting point. To maintain the temperature not higher than the melting point, there may be adopted methods such as a forced cooling, heat release, heat insulation and the like according to the melting point thereof. The shape of the seed crystal part may be the shape of the whole vessel without any modification, may be a shape which is partially narrow, or may be a shape of which branch is taken out into an outer part .
The size of the seed crystal part may have a size which is required to maintain the temperature not higher than the melting point even if seed crystal part is under the influence of inflow of heat from the adjacent heating and cooling part. For example, when the vessel is a flat plate vessel of 10 mm thickness x 300 mm width x 600 mm length, the length of the seed crystal part is preferably not less than 20 mm, more preferably not less than 50 mm.
The part (H) is a part which functions as a switch by heating, or heating and cooling. The part (H) is preferably equipped with an apparatus which can conduct heating and cooling. For the heating, electric heating or hot water is usually used. For the cooling methods such as thermoelectric cooling, water cooling, air cooling and the like are usually adopted. The shape of the part (B) may be as same as or different from (A) seed crystal part. The part (B) has such a size that at least a part of part (B) is completely molten in heating. For example, in the case of the above-described flat plate vessel, the length of the part (B) is preferably not less than 10 mm, more preferably not less than 20 mm.
The main body part (C) is a main part of the heat storage and heat release system which can conduct heat storage and heat release. For the heat storage, an equipment such as electric heating, hot water or the like is preferably used.
When the heating is stopped after heat storage, the main body part (C) is cooled to a supercooling condition. When the main body part (C) is cooled to a considerable low temperature, and consequently there is a risk of the break of supercooling, it is preferable to conduct suitable heat insulation to maintain the supercooling condition.
The heat storage material comprises these three part (A) to (C) in the order of (A), (B), (C), is not separated by an air layer, heat insulation material and the like, and is of course continuous. Further, the heat storage material should have such a shape that a crystal can propagate from the seed crystal part (A) through the part (H) which functions as a switch of the main body part (C). The shape in section may have any shape such as circle, rectangle and the like, and is not particularly restricted. The sectional area is not particularly restricted if it has an area required for the propagation of a crystal.
The details of the heat release controlling method then will be described. In heat storage, the part (B) which functions as a switch and the main body part (C) are heated at a temperature not less than the melting point. After both parts are fully molten, the heating of the main body part (C) is stopped and the heating of the part (B) is continued.
Thus, the main body part (C) is cooled to a temperature not more than the melting point. In this process, the main body part (C) is in a supercooling condition. When, the heat release is demanded, the heating of the part (B) which functions as a switch is stopped or the part (B) is cooled.
By this procedure, the part (B) is cooled to a temperature not more than the melting point and a crystallization is propagated from the seed crystal part (A) through the part (B) to the main body part (C). Thus, heat of crystallization is released with the progress of the crystallization. When large time difference of heat release from one end to the other end of the main body part (C) comes into question, the length of the main body part (C) is made shorter or divided into small sections and the like are preferred. To the contrary, when the difference in the heat release times is to be utilized positively, it is preferred to make the length of the main body part (C) longer.
According to the present invention, it becomes possible to control the heat release of a heat storage system which comprises a heat storage material containing a salt hydrate, and it becomes possible to generate heat when necessary to heat buildings, consequently, it becomes possible to operate the heat storage system more economically.
Example The following examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof.
Example 1 (An example of heat release control by operating the part (B) as a switch) An aqueous solution (7.50 g) of sodium acrylate (20~
by weight) obtained by neutralizing acrylic acid with sodium hydroxide to pH 7.0, 24.00 g of water, 15.84 g of disodium _ g _ hydrogen phosphate anhydride were mixed and heated in a water bath of 50°C to obtain a clear solution. Into this solution were dissolved 0.025 g of N,N'-methylenebisacrylamide and0.03 g of potassium peroxodisulfate, then, to the resulting solution was added a solution prepared by dissolving 0.03 g of sodium sulfite into 2.01 g of water, and the resulting solution was immediately poured into a polyethylene bag having a size of 25 mm width X 300 mm length of which one end had been sealed , to obtain a water- containing gel { 245 mm length, mm thickness ) having no flowability after 2 minutes . This gel was maintained overnight at 40°C, and was cooled to I5°C
to solidify the whole body. The mol number of water per 1 mol of disodium hydrogen phosphate anhydride in this composition was 16. The solidified composition was divided into three parts, (A) 95 mm length, (B) 50 mm length and (C) 100 mm length, respectively. Thermocouples were adhered to each center part of them, respectively. Plane heaters were installed under the parts ( B ) and ( C ) , respectively, and the upper and lower sides of the parts (B) and (C) were wrapped with heat insulating materials having a thickness of 20 mm. The parts ( B ) and ( C ) were heated at 45°C for 4 hours to be fully molten, and then the power source of the plane heater of (C) was turned off to lower the temperature of the center part of (C) to room temperature (21°C) after 1 hour. However, the center part of (B) was maintained at 45°C. Next, the power source of the plane heaters of (B) was turned off after 2 hours, the temperature of the center part of (B) decreased to 30°C then immediately increased again to 33 °C, then this temperature was maintained for 90 minutes and then the temperature decreased gradually to room temperature. The temperature of center part of ( C ) steeply increased to 29°C after 2 minutes of re-increase of the temperature of (B).
Comparative Example 1 (An example in which heat release is not controlled since the part (B) does not function as a switch) The same composition was used as in Example I, and (B) and (C) were heated at 45°C for 4 hours to be fully molten, and the power sources of plane heater of (B) and (C) were simultaneously turned off . Then the temperature of the center part of ( B ) decreased to 31°C then immediately increased again to 32 °C. The temperature of center part of (C) steeply increased ( from 30°C to 31°C ) after 2 minutes of re-increase of the temperature of the center part of (B).
Example 2 (An example of heat release control by operating the part (B) as a switch) Sodium acetate anhydride (27.51 g) and 22.51 of water were mixed and heated in a water bath of 60°C to obtain a clear solution. Into this solution was added 1.50 g of "Sumiflock FN-15H" (manufactured by Sumitomo Chemical Co., Ltd.) which was a partially hydrolized polyacrylamide and stirred for 5 minutes, then the resulting solution was poured into a polyethylene bag having a size of 25 mm width x 270 mm length of which one end had been sealed, and the bag was sealed. This bag was immersed in a water bath of 60°C for 2 hours, then cooled to room temperature, opened and added several grains of seed crystals into this bag to crystallize whole body and the bag was re-sealed. The mol number of water per 1 mol of sodium acetate anhydride in this composition was 3.7.
The solidified composition was divided into 3 parts in the same manner as in Example 1, and plane heaters and heat insulating materials were installed. The parts (B) and (C) were heated at 65°C for 4 hours to be fully molten and then the power source of the plane heater of (C) was turned off, then the temperature of the center part of (C) lowered to I9°C
after 6 hours . However, the center part of ( H ) was maintained at 65°C. Next, the power source of the plane heater of (H) was turned off after 6 hours, the temperature of the center part of (H) decreased to 46°C then immediately increased again to 48 °C. The. temperature of center part of (C) steeply increased to be 44°C after I1 minutes of re-increase of the temperature of (H).
*Trade-mark

Claims

1. A heat storage system, comprising:
(i) a vessel in which a heat storage material containing a salt hydrate having a melting point is packed, wherein the heat storage material comprises the following three parts arranged continuously in series in this order, (A) a seed crystal part in which a seed crystal of the salt hydrate is maintained, (B) a switch part which functions as a switch by heating, or by heating and cooling and (C) a main body part which stores and releases heat, and (ii) an apparatus for heating the switch part (B), wherein the vessel has a cylinder, coil or flat plate shape whereby a crystallization of the salt hydrate propagates from the said crystal part (A) through the switch part (B) to the main body part (C) and is controlled by heating the switch part (B) to above, or cooling the switch part (B) to below, the melting point of the salt hydrate.

2. The heat storage system according to claim 1, wherein the apparatus (ii) also cools the switch part (B).

3. The heat storage system according to claim 1 or 2, which further comprises:
(iii) an apparatus for heating the main body part (C).

4. The heat storage system according to claim 1 or 2, wherein the apparatus (ii) for heating, or heating and cooling the switch part (B) is installed outside the switch part (B).

5. The heat storage system according to claim 3, wherein the apparatus (iii) is installed outside the main body part (C).

6. A heat storage system, comprising:
(i) a flat plate vessel in which a heat storage material containing a salt hydrate is packed, wherein the salt hydrate shows a solid-liquid phase change by heating and cooling and has a melting point and wherein the heat storage material comprises the following three parts arranged continuously in series in this order:
(A) a seed crystal part in which a seed crystal of the salt hydrate is always maintained at a temperature not higher than the melting point of the salt hydrate;
(B) a switch part which functions as a switch by heating or by heating and cooling and in which at least a part of the heat storage material is completely molten; and (C) a main body part which stores and releases heat;
(ii) an apparatus for heating and cooling the switch part (B); and (iii) an apparatus for heating the main body part (C), whereby a crystal propagation from the seed crystal part (A) through the switch part (B) to the main body part (C) is controlled by heating the switch part (B) to above, or cooling the switch part below, the melting point of the salt hydrate.

7. The heat storage system according to claim 6, wherein the temperature of the seed crystal part (A) is maintained not higher than the melting point of the salt hydrate by a heat insulation.

8. The heat storage system according to claim 6 or 7, wherein the main body part (C) is heat insulated to maintain a supercooling condition of the heat storage material.

9. The heat storage system according to any one of claims 1 to 8, wherein the salt hydrate is a member selected from the group consisting of sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, calcium chloride hexahydrate and calcium nitrate tetrahydrate.

10. The heat storage system according to any one of claims 1 to 8, wherein the salt hydrate is disodium hydrogen phosphate dodecahydrate.

11. The heat storage system according to any one of claims 1 to 8, wherein the salt hydrate is sodium acetate trihydrate.

12. The heat storage system according to any one of claims 1 to 11, wherein the heat storage material contains, in addition to the salt hydrate, water in an amount of 1/10 to 1/3 of a hydration mol number of the salt hydrate.

13. The heat storage system according to any one of claims 1 to 12, wherein the heat storage material further contains a solid-liquid separation inhibitor selected from the group consisting of water-soluble polymers, water-swellable polymers, high water-absorption resins and silica-based thickening agents.

14. A method for controlling heat release from a heat storage system, comprising:

(i) a vessel in which a heat storage material containing a salt hydrate having a melting point is packed, wherein the heat storage material comprises the following three parts arranged continuously in series in this order, (A) a seed crystal part in which a seed crystal of the salt hydrate is maintained, (B) a switch part which functions as a switch by heating, or by heating and cooling and (C) a main body part which stores and releases heat, and (ii) an apparatus for heating the switch part (B), in which the seed crystal part is always maintained at a temperature not higher than the melting point of the salt hydrate and which comprises:
(I) heating the parts (B) and (C) to a temperature not less than the melting point until the both parts are fully molten, to store heat;
(II) heating only the switch part (B) while stopping the heating of the main body part (C) to cool the main body part (C) to a temperature not more than the melting point, thereby bringing the main body part (C) into a supercooling condition; and (III) stopping the heating of the switch part (B) or cooling the switch part (B), thereby lowering a temperature of the switch part (B) to not more than the melting point and propagating a crystallization of the salt hydrate from the seed crystal part (A) through the switch part (B) to the main body, resulting in release of heat of crystallization.

15. A method for controlling heat release from the heat storage system as defined in any one of claims 6 to 8, in which the seed crystal part is always maintained at a temperature not higher than the meltingpoint of the salt hydrate and which comprises:
(I) heating the parts (B) and (C) to a temperature not less than the melting point until the both parts are fully molten, to store heat;
(II) heating only the switch part (B) while stopping the heating of the main body part (C) to cool the main body part (C) to a temperature not more than the melting point, thereby bringing the main body part (C) into a supercooling condition; and (III) stopping the heating of the switch part (B) or cooling the switch part (B), thereby lowering a temperature of the switch part (B) to not more than the melting point and propagating a crystallization of the salt hydrate from the seed crystal part (A) through the switch part (B) to the main body, resulting in release of heat of crystallization.

16. The method according to claim 14 or 15, wherein the salt hydrate is a member selected from the group consisting of sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate, sodium acetate trihydrate, calcium chloride hexahydrate and calcium nitrate tetrahydrate.

17. The method according to claim 14 or 15, wherein the salt hydrate is disodium hydrogen phosphate dodecahydrate.

18. The method according to claim 14 or 15, wherein the salt hydrate is sodium acetate trihydrate.

19. The method according to any one of claims 14 to 18, wherein the heat storage material contains, in addition to the salt hydrate, water in an amount of 1/10 to 1/3 of a hydration mol number of the salt hydrate.

20. The method according to any one of claims 14 to 19, wherein the heat storage material further contains a solid-liquid separation inhibitor selected from the group consisting of water-soluble polymers, water-swellable polymers, high water-absorption resins and silica-based thickening agents.