CA2060438A1

CA2060438A1 - Heat storage composition and process for preparing the same

Info

Publication number: CA2060438A1
Application number: CA002060438A
Authority: CA
Inventors: Kenji Saita; Mitsuhiro Harada
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-01-31
Filing date: 1992-01-31
Publication date: 1992-08-01
Also published as: GB2252327A; JPH0525467A; JP3103927B2; KR920014914A; GB9202032D0

Abstract

ABSTRACT

The present invention is directed to a heat storage composition containing (1) at least one compound selected from the group consisting of sodium sulfate and its eutectic salts, (2) water and (3) a crosslinkable polymer which comprises a polyfunctional monomer and at least one monomer selected from the group consisting of unsaturated carboxylic acids, organic unsaturated sulfonic acids and salts thereof, wherein the molar ratio of water to sodium sulfate or its eutectic salt is from 13:1 to 27:1. The composition of the present invention has a large latent heat and maintains this large latent heat after a number of heating cycles. It is suitable for use in building air conditioning systems.

Description

206~38 HEAT STORAGE COMPOSITION AND PROCESS FOR PREPARING THE SAME

The present invention relates to a heat storage composition which is suitable for use in air conditioning systems for buildings and the like and to a process for preparing the same.
A heat storage material should have various properties, e.g. a large amount of stored heat, functioning at a predetermined temperature level, long term stability, low cost, non-toxicity, non-corrosiveness and the like. As a material which satisfies these properties, a phase changeable hydrated salt has been most extensively studied, and one typical example is sodium sulfate decahydrate.
Since sodium sulfate decahydrate has a melting point of 32C and a latent heat of 60 cal/g, many attempts have been made to utilize this salt as a heat storage material since sodium tetraborate decahydrate (Na2B407.10H20) was found to be an effective supercooling-preventing agent which is used in combination with sodium sulfate decahydrate in 1952. One of the problems which arises in the practical application of such a combination is that sodium sulfate decahydrate exhibits an incongruent melting behaviour. That is, upon melting, anhydrous sodium sulfate forms and precipitates at the bottom of a liquid system. When such a system is cooled, a surface layer of the anhydrous salt is rehydrated while an inner part of the precipitated salt remains in a dehydrated form. Since the remaining anhydrous sodium sulfate does not contribute to the phase change, the amount of stored heat decreases. To solve this problem, many methods have been studied to prevent the precipitation of the anhydrous salt and to disperse and maintain it in the liquid. Most of them comprise the addition of an organic or inorganic additive to increase viscosity and prevent the precipitation.
For example, the use of an inorganic compound was tried and reported (cf. Japanese Patent Kohyo Publication 35 No. 501180/1980 and Japanese Patent Kokai Publication No. 34687/1978). However, su~ficient precipitation prevention 2060~38 has not been achieved.
As the organic additive, organic polymers, for example, a water-soluble polymer (e.g. polysodium acrylate) and a crosslinkable polymer have been proposed (cf. Japanese Patent Publication Nos. 30873/1982 and 48027/1982 and Japanese Patent Kokai Publication Nos. 132075/1983 and 102977/1984). However, they are not necessarily satisfactory from the point of view of long term stability.
In a Glauber's salt base heat storage composition, it is known to suppress the decrease in the amount of stored heat by the addition of water containing a silicone defoaming agent and a chelating agent to the Glauber's salt (cf. Japanese Patent Kokai Publication No. 203687/1985). In this method, the silicone defoaming agent and the chelating agent are essential. In the absence of these two agents, the amount of stored heat decreases after 500 heating cycles.
An object of the present invention is to provide a heat storage composition which can solve the above problems.
Namely, an object of the present invention is to provide a heat storage composition which comprises sodium sulfate and water and does not suffer from a decrease in the amount of stored heat for a long time after repeated melting and freezing (heating cycle), and a process for preparing such a composition.
According to a first aspect of the present invention, there is provided a heat storage composition comprising (1) at least one compound selected from the group consisting of sodium sulfate and its eutectic salts, (2) water and (3) a crosslinkable polymer which comprises a polyfunctional monomer and at least one monomer selected from the group consisting of unsaturated carboxylic acids, organic unsaturated sulfonic acids and salts thereof, wherein the molar ratio of water to sodium sulfate or its eutectic salt is from 13:1 to 27:1.
According to a second aspect of the present invention, there is provided a process for preparing a heat storage composition, which comprises polymerizing a polyfunctional monomer and at least one monomer selected from the group 206043~

consisting of unsaturated carboxylic acids, organic unsaturated sulfonic acids and salts thereof in the presence of water and at least one compound selected from the group consisting of sodium sulfate and its eutectic salts in a molar 5 ratio of 13:1 to 27:1.
In drawings which illustrate preferred embodiments of the present invention:
Fig. 1 is a graph showing the change in the latent heat of the heat storage composition of the present invention up to 5000 heating cycles in a temperature history test comprising melting and freezing.
Fig. 2 is a graph showing the relationship between the amount of water contained (moles per one mole of sodium sulfate) and the latent heat of a unit weight of the heat storage composition after 5000 heating cycles (A) and the relationship between the amount of water contained and the remaining rate of the latent heat after 5000 heating cycles (B).
In the heat storage composition of the present invention, sodium sulfate may be used in an anhydrous form or an eutectic salt form. In addition, sodium sulfate decahydrate may be used. Examples of compounds which form an eutectic salt with sodium sulfate are sodium chloride, potassium chloride, sodium nitrate, potassium nitrate, magnesium sulfate, urea ~nd the like. The compounds are used in an amount of 0.2 to 1.0 mole per one mole of sodium sulfate. The eutectic salt has a lower melting point than sodium sulfate as such and can be used to adjust the melting point.
One of the important characteristics of the composition of the present invention is the molar ratio of water to sodium sulfate in the heat storage composition. Water is used in an amount o~ 13 to 27 moles, preferably 15 to 25 moles, more preferably 16 to 24 moles per one mole of sodium sulfate (in an anhydrous form). By using water in this molar ratio range, the latent heat of the composition does not substantially decrease after heating cycles for a long time.
Therefore, the calculation of heat load is easy, and excessive 2060~38 loading of the heat storage composition to compensate for a decrease of latent heat is not necessary. When the heat storage composition of the present invention is used in a floor heating system, thickness of the floor can be made thin and floor weight is decreased.
When the amount of water is less than 13 moles per one mole of sodium sulfate, initial latent heat is large but the latent heat is decreased significantly by the repeated heating cycles. Then, contrary to the composition of the present invention, a composition is not practically acceptable in view of the equipment and the control.
When the amount of water exceeds 27 moles per one mole of sodium sulfate, the change of latent heat can be suppressed after the repeated heating cycles, but the composition has a small latent heat and a large amount of the composition should be used. Therefore, the equipment has some drawbacks, for example, an increase in floor thickness and it will withstand only a small load.
When the amount of water is from 16 moles to 24 moles per one mole of sodium sulfate, remaining rate of the latent heat is 95~ or higher after the 5000 heating cycles and its absolute value is sufficient for practical use.
The critical meaning of the amount of water in the composition of the present invention will be explained quantitatively by Examples and Comparative Examples below.
The crosslinkable polymer and its component monomers will be explained.
As the unsaturated carboxylic acid, a water-soluble unsaturated carboxylic acid is preferred. Specific examples of the unsaturated carboxylic acid are acrylic acid, methacrylic acid and itaconic acid. Among them, acrylic acid is preferred. A mixture of acrylic acid with methacrylic acid, itaconic acid or hydroxyethyl acrylate may be used.
Specific examples of the organic unsaturated sulfonic acid are 2-acrylamide-2-methylpropanesulfonic acid, p-styrenesulfonic acid, sulfoethyl methacrylate, allylsulfonic As the salt of the unsaturated carboxylic acid or the organic unsaturated sulfonic acid, a water-soluble salt, e.g.
an alkali metal salt and an ammonium salt, is used. Among them, a sodium salt is preferred. In particular, sodium acrylate and sodium methacrylate are most preferred.
It may be possible to use an unsaturated amide together with the above monomers. Examples of the unsaturated amide are acrylamide and methacrylamide.
The amount of the monomers, namely the polymer in the composition is 1 to 10% by weight, preferably 2 to 5% by weight based on the whole weight of the heat storage composition. When this amount is less than 1% by weight, the composition has a poor effect on prevention of anhydrous sodium sulfate precipitation caused by the phase change.
When it exceeds 10% by weight, the amount of stored heat decreases.
The polyfunctional monomer is used to crosslink the polymer. Preferably, a water soluble polyfunctional monomer is used. Specific examples are N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide, N,N'-dimethylenebisacryl-amide, N,N'-dimethylenebismethacrylamide and the like. Among them, N,N'-methylenebisacrylamide and N,N'-methylenebismeth-acrylamide are preferred. The amount of the polyfunctional monomer is from 0.01 to 1% by weight, preferably from 0.05 to 0.5% by weight based on the whole weight o~ the heat storage composition. When this amount is less than 0.01% by weight, the polymer has poor crosslinkability. Even when it exceeds 1~ by weight, the effect is not improved in comparison to the increased amount.
When the above monomer and the polyfunctional monomer are polymerized in the manner explained below, a crosslinkable polymer is obtained. The amount of the crosslinkable polymer in the heat storage composition is the same as the total amount of the monomers and is usually from 1 to 11~ by weight, preferably from 2 to 5.5% by weight.
As a polymerization initiator, any of the conventional radical polymerization initiators can be used. Examples are 2~60~38 diacyl peroxides, e.g. acetyl peroxide, lauroyl peroxide and benzoyl peroxide; hydroxyperoxides, e.g. cumenehydroxy-peroxide; alkyl peroxides, e.g. di-tert.-butylperoxide;
ammonium or potassium peroxydisulfate; hydrogen peroxide;
2,2-azobisisobutyronitrile and the like. Among them, a redox type polymerization initiator is preferred since it is active at a comparatively low temperature.
A preferred redox type polymerization initiator is a water-soluble one. As an oxidant, ammonium or potassium peroxydisulfate and hydrogen peroxide are exemplified. As a reducing agent, sodium thiosulfate, sodium sulfite, ferrous sulfate and the like are exemplified.
The crosslinking temperature is the same as or higher than the melting point of sodium sulfate decahydrate or its eutectic salt. Usually, it is from 20 to 50C.
The redox type polymerization initiator exhibits polymerization activity in a comparatively short time when the oxidant and the reducing agent are mixed. After the start of the polymerization activity, contact with air will deactivate the active species. Therefore, the mixture of the oxidant and the reducing agent should be charged into a polymerization reactor as quickly as possible without exposure to air.
The process of the present invention can be carried out bv various methods. For example, the polymerization is carried out in a comparatively large volume reactor and the produced heat storage composition is portioned and placed in a container which constitutes a heat storage part of the heating equipment. In this case, the internal atmosphere of the large volume reactor is replaced with nitrogen gas and then the raw materials are charged and reacted.
In the present invention, since the monomers are used as the raw material in place of the polymer, mixing is easy.
Alternatively, the polymerization can be carried out in a heat storage container of a heating unit. In particular, the characteristics of the present invention can be realized in this mode of polymerization.
Since the monomers are used as the raw materials in place 206~438 of the polymer, the mixture before the polymerization is a liquid composition having a low viscosity. Therefore, the raw material composition can be easily poured into a container having a complicated shape. By polymerization in the container, the heat storage material in a gel or solid state can be contained in the container having the complicated shape. When the raw material mixture is placed in the container and then polymerized, the interior of the container need not necessarily be replaced with nitrogen gas.
When the liquid mixture before polymerization is poured into the container for the heat storage material and a redox type polymerization initiator is used, it is preferred that the oxidant and the reducing agent are continuously mixed in a flow system of the composition and are poured in the container.
For example, the oxidant and the reducing agent are separately added while a liquid mixture of anhydrous sodium sulfate or its eutectic salt, water and the monomers is poured into the container. Either the oxidant or the reducing agent is dissolved in the liquid mixture and the other is added to the mixture when the mixture is poured into the container.
The liquid mixture is divided into two portions and the oxidant is added to one of them and the reducing agent is added to the other. Then, the two portions are mixed in a pouring conduit and poured into the container. It is possible to provide an in-line mixer in the pour~ng conduit to more sufficiently mix the components.
In the process of the present invention, it may be preferred to add a thickener or another additive to the mixture in order to prevent precipitation of anhydrous sodium sulfate after the raw materials are poured into the container and before the increase in the viscosity achieved by the polymerization of the monomers. As the thickener, any of the conventional ones may be used. Specific examples of the thickener are inorganic materials, e.g. fumed silica, fine silica produced by a wet process, various clays, etc., water-soluble polymers, e.g. polysodium acrylate and hydrogel. The 2060~38 amount of the thickener is from 0.1 to 7% by weight of the composition. In the case of the monomer, the thickener is added in such an amount that the viscosity of the mixture prevents the sedimentation of anhydrous sodium sulfate in a short time in which the crosslinking reaction proceeds and the viscosity of the composition increases.
To the heat storage composition, a supercooling-preventing agent is usually added. In the process of the present invention, the supercooling-preventing agent may be added to the liquid mixture before polymerization. When the polymerization of the raw material mixture is carried out in the container in which the heat storage composition is finally contained, the supercooling-preventing agent should be added to the mixture before polymerization.
In general, it is known that sodium tetraborate decahydrate is effective as the supercooling-preventing agent.
The amount of supercooling-preventing agent is usually from 2 to 5% by weight based on the whole weight of the heat storage compositioll. Since the pH range in which tetraborate decahydrate is stably present in an aqueous medium is neutral to basic, the mixture is preferably neutralized when the mixture is acidified by the monomer and/or the polymer.
The present invention will be illustrated by the following Examples.
Example 1 To a lO wt.% aqueous solution of sodium acrylate (150 g) which had been prepared by neutralizing acrylic acid with an aqueous solution of sodium hydroxide to pH of 7.5, water tl35 g) was further added. To the solution, N,N'-methylene-bisacrylamide (0.75 g), anhydrous sodium sulfate (142 g) and sodium tetraborate decahydrate (20 g) were added while stirring at 30C to obtain a homogeneous mixture containing no precipitate. In this mixture, the molar ratio of water to sodium sulfate (in the anhydrous form) is shown in the Table.
This mixture was divided into two portions. To one portion, ammonium peroxydisulfate (0.5 g) was added, and to the other, sodium thiosulfate pentahydrate (0.5 g) was added.

20~0438 They were caused to flow through respective flow conduits and brought in contact with each other to mix them. The mixture was then poured into a polyethylene bag having a width of 40 mm and a length of 600 mm.
The bag was hung in an air oven at 40C. After one hour, the crosslinking proceeded and the contents in the bag formed a homogeneous gel form elastic composition. This composition phase changed at about 32C.
The obtained composition (50 g) was charged in a lo cylindrical glass container having a diameter of 30 mm and a height of 100 mm and was subjected to the heating cycle test comprising repeating heating and cooling between 40OC and 10C. After 5000 heating cycles, the composition was stable and no phase separation was observed. Before the heating cycle, the latent heat was 44.5 cal/g. With this value being 'llO0", a relative latent heat after 5000 heating cycles was 91 ~an absolute latent heat being 40.5 cal/g), which means that the composition maintained the high latent heat for a long time.
ExamPles 2 to 5 and Comparative Example In the same manner as in Example :l but using the components shown in the Table, a heat storage composition was prepared. In Example 5, sodium chloride formed an eutectic salt with sodium sulfate.
The results of the heating cycle test are shown in the Table.
The results are also plotted in the graphs of Figs. 1 and 2, in which the numerals 1 to 5 and 6 stand for "Examples 1 to S" and "~omparative Example".

2~438 Table _ Exam- Molar Heating cycle test:
ple ratio to Latent heat (cal/g) and No. Na2S4 its re~ aining ra~e (%) in brackets After Heat Cycles of Water NaCl Before 1000 2000 3000 4000 5000 Hcycalteng 1 15.0__ 44.5 40.6 41.8 39.6 40.5 40.5 (100) (91) (94) (89) (91) (91) _ 2 17.0__ 40.6 40.2 41.4 42.2 40.6 42.2 (100) (99) (102) (104) (100) (104) 3 13.0__ 49.8 47.8 43.8 36.4 38.8 36.9 (100) (96) (88) (73) (78) (74) I _ 4 19.0__ 36.3 37.3 40.0 39.9 40.6 40.6 (100) (103) (~10) (110) (112) (112) 23.00.5 32.3 32.0 31.0 31.3 32.6 32.3 (100) (99) (96) (97) (101) (100) Comp. 11.0 __ 55.2 45.8 48.0 37.5 32.0 30.4 ¦EX. (100) (83) (87) (68) (58) (55)

Claims

1. A heat storage composition comprising (1) at least one compound selected from the group consisting of sodium sulfate and its eutectic salts, (2) water and (3) a crosslinkable polymer which comprises a polyfunctional monomer and at least one monomer selected from the group consisting of unsaturated carboxylic acids, organic unsaturated sulfonic acids and salts thereof, wherein the molar ratio of water to sodium sulfate or its eutectic salt is from 13:1 to 27:1.

2. The heat storage composition according to claim 1, wherein the amount of said crosslinkable polymer is from 1 to 11% by weight based on the whole weight of said composition.

3. A process for preparing a heat storage composition, which comprises polymerizing a polyfunctional monomer and at least one monomer selected from the group consisting of unsaturated carboxylic acids, organic unsaturated sulfonic acids and salts thereof in the presence of water and at least one compound selected from the group consisting of sodium sulfate and its eutectic salts in a molar ratio of 13:1 to 27:1.

4. The process according to claim 3, wherein the total weight of said polyfunctional monomer and said monomer is from 1 to 11% by weight based on the whole weight of said composition.

5. The process according to claim 3, wherein said monomer is a water-soluble monomer.

6. The process according to claim 3, wherein said monomer is sodium acrylate or sodium methacrylate.

7. The process according to claim 3, wherein said polyfunctional monomer is a water-soluble polyfunctional monomer.

8. The process according to claim 3, wherein said polyfunctional monomer is N,N'-methylenebisacrylamide or N,N'-methylenebismethacrylamide.