NZ202168A - Reversible liquid/solid phase change composition:heat storage device - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £02168 021 e>g Priority Date(s): Complete Specification Filed: Class: Publication Date: P.O. Journal, No: .J NO DRAW1H6S NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION 14 OCT 1982 REVERSIBLE PHASE CHANGE COMPOSITIONS OF CALCIUM CHLORIDE HEXAHYDRATE WITH A POTASSIUM SALT K/We, THE DOW CHEMICAL COMPANY, 2030 Dow Center, Abbott Road, Midland, Michigan 48640, United States of America., a corporation organized and existing under the laws of the State of Delaware, United States of America, hereby declare the invention for which X / we pray that a patent may be granted to rt4«/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - (followed-by la) 202168 REVERSIBLE PHASE CHANGE COMPOSITIONS OF CALCIUM CHLORIDE HEXAHYDRATE WITH A POTASSIUM SALT Background of the Invention solid phase change compositions. More particularly, the invention resides in phase change compositions 5 comprising a mixture of hydrated calcium chloride and a potassium salt whose anion forms a substantially less soluble salt with calcium. heat of fusion of various hydrated salt compositions is 10 employed are well known in the literature. In the ASHRAE Journal of September, 1974, entitled SOLAR ENERGY STORAGE, Dr. M. Telkes evaluated the thermal, physical and other pertinent properties of PCM's on the basis of economics, applicability, corrosion, toxicity 15 and availability for large scale installations. Among The invention relates to reversible liquid/- Phase change materials (PCM's) in which the 2 202168 3 the materials evaluated were various salt hydrates and their eutectics including CaCl2.6H20 which undergoes several phase transitions, to materials of different crystal structure, i.e. CaCl2.6H20 to CaCl2.4H20 + 2H20 at 29°C. the salt CaCl2.6H20 dissolves completely in its water of crystallization. When cooled, formation of four different crystal forms is possible, i.e., CaCl2.6H20 and three forms of CaCl2-4H20. If any of the 4H20 crystals form, the heat of fusion is much less than 46 cal/gm (CaCl2.6H20 in substantially pure form undergoes a liquid/solid phase transition at about 30°C releasing or alternately absorbing about 4 6 calories of heat per gram). Despite the relatively low cost of CaCl2, the formation of its four different crystal forms was deemed to be disadvantageous.

Number 200274 it was shown that the addition of KC1 to CaCl2.6H20 greatly reduces the possibility of forming the undesired CaCl2.4H20 crystalline phase during the retrieval of the stored heat on freezing of the phase change composition. The present invention now surprisingly shows that the desirable effect of reducing the formation of the undesired CaCl^. crystalline phase can also be achieved with salts other than the chloride, i.e., KC1. In particular, the reduction of the formation of the CaCl2.4H20 crystalline phase during the retrieval of stored heat on freezing of When heated to a temperature of above 33°C In our New Zealand Patent Specification 3 202168 the hydrated CaCl2 composition can be achieved by the addition of a potassium salt in which the anion of the potassium salt employed forms a sparingly soluble calcium salt, i.e., a calcium salt which is substantially insoluble.

Our New Zealand Patent Specification Number 2 00 274 also shows that the addition of NaCl and/or SrCl^ augments the beneficial effect produced by the addition of KC1 to the hydrated CaCl2 composition to thereby obtain a composition which is an effectively 10 congruently melting composition. It has now been discovered that other sodium and/or strontium salts can also be employed with the same beneficial results.

In some applications, for example, it is preferable to use potassium, sodium or strontium salts 15 or mixtures of such salts, other than salts in which the anion is chloride, to increase the pH of the composition and to thereby reduce the natural acidity of aqueous CaCl2- In systems, i.e., phase change compositions in which the anions of potassium, sodium and 20 strontium salts form a substantially less soluble salt with calcium, the compatibility of the heat storage composition in metal containers is substantially improved.

Heat storage compositions are ideally packaged in individual encapsulating means for use in 251i conjunction with solar heating systems. Exemplary of suitable known encapsulating means for the heat storage 202168 films or foils of plastic/metal laminates. Closed cell plastic foams have also been suggested in which the PCM may be encapsulated within the cells of the foam structure as illustrated in, for example, U.S. Patent 5 No. 4,003,426. Other useful encapsulating means are concrete, metal or plastic containers, pipes, and the like.

Summary of the Invention The invention resides in a reversible liquid/ 10 solid phase change composition comprising an admixture of hydrated CaCl2 and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaC^r wherein the potassium salt is added in an amount sufficient to modify the semi-congruent 15 melting behavior of CaCl2*6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate 20 phases other than CaCl2"6H20. liquid/solid phase change composition comprising a mixture of CaCl2*6H20 and a potassium salt, whose anion forms a substantially less soluble salt with 25 calcium than CaCl2, wherein the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2-6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, 30 during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2'6H20, and a reversible The invention also relates to a reversible 202168 liquid/solid phase change composition comprising a mixture of CaCl2-6H20 and a potassium salt whose anion forms a substantially less soluble salt with 5 calcium than CaCl2,wherein the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2*6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during 10 retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2 - CB.^0, and including a sodium and/or strontium salt, wherein the anion of said sodium and/or strontium salt forms a substantially less soluble 15 salt with calcium than CaCl2, said sodium and/or strontium salt being present in an amount sufficient to modify the CaCl2 -6H20 and potassium salt mixture to form an effectively congruently melting mixture.

The invention also relates to a heat storage 20 device comprising an encapsulating means having a reversible liquid-soli&i phase change composition hermetically sealed in said encapsulating means to prevent evaporation of water from the composition, said composition comprising an admixture of hydrated 25 CaCl2 and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaCl2, wherein the potassium salt is added in an amount such that the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2-6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2*6H20. 202168 The invention also relates to a method of storing heat, comprising the steps of preparing a reversible liquid-solid phase change composition by admixing hydrated CaCl2 and a potassium salt whose anion 5 forms a substantially less soluble salt with calcium than CaCl^/ adding said potassium salt in an amount sufficient such that the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2-6H20 to the extent that the mixture 10 approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2'6H20, introducing the composition into an 15 encapsulating means for use as a heat storage device, and hermetically sealing the encapsulating means to prevent evaporation of water from the composition.

Although the hydrated CaCl2/potassium salt mixture of the invention surprisingly reduces the 20 formation of crystal forms other than the hexahydrate form, it was found that it still retained the inherent characteristics of the supercooling properties of CaCl2-6H20. Accordingly, the present invention preferably provides for the addition of select nucleating 25 agents in order to effectively reduce supercooling in the hydrated CaC^/potassium salt system.

The avoidance of supercooling during the crystallization of hydrated CaCl2, as by the addition of various nucleating agents, is generally known in 30 the literature from, for example, -j- -a- 202168 U.S. Patent No. 4,189,394. However, nucleating agents were not intended as additives to achieve congruently melting salt hydrates.

Although the addition of a potassium salt in which the anion is present in an amount sufficient to effectively reduce the tendency of the phase change composition to form, on freezing, the undesired CaC^^^O phase, even a maximum amount present at 10 the solubility limit is not sufficient to completely prevent the formation of CaCl2-4H20. Accordingly, in a preferred embodiment of the invention, the formation of CaC^-^^O can be totally prevented from crystallizing if the potassium salt is used in combination with a 15 sodium and/or strontium salt in which the anion forms a substantially less soluble salt with calcium than CaC^.

Detailed Description of the Invention The present invention represents a significant improvement towards the goal of developing an 20 inexpensive yet highly effective reversible liquid/- solid phase change composition based on hydrated CaCl2 in admixture with a potassium salt.

Hydrated salt phase change materials exhibit three general types of phase/change behavior: congruent, 25 semi-congruent and incongruent melting. The most desirable behavior is congruent melting which occurs when the solid phase composition (ratio of salt to bound water) is the same as the liquid phase compositon. In that case, the hydration/dehydration process appears identical to 30 the melting and freezing process.

The term "effectively congruently melting mixture" herein used defines a mixture of ingredients, based on aqueous calcium chloride, for which, at the melting point, solid and liquid phases are in stable equilibrium: 5 the solid phase containing no hydrated calcium chloride material other than the hexahydrate or solid solutions thereof; and the liquid phase containing, for every mole of calcium chloride, six moles of water, plus sufficient water to form the stable hydrate of any additive materials 10 in solution.

Semi-congruent melting occurs when a phase change material has two or more hydrate forms with differing solid compositions and melting points. The material can be transformed into other hydrate forms 15 before either complete melting or freezing occurs, resulting in a broadened melting point range. In addition, there is a temporary loss in thermal storage capacity. Calcium chloride hexahydrate is an example of a semi--congruently melting phase change material.

Incongruently melting phase change materials yield two distinct phases upon melting: a saturated solution and a precipitate of an insoluble anhydrous salt. If the precipitate settles out of the solution, the anhydrous salt will not hydrate completely upon 25 cooling and some thermal storage capacity will be lost with each freezing/melting cycle. Incongruent melting, as observed with sodium sulfate decahydrate, for example, is a more serious problem because it can result in a continual loss of latent heat storage capacity.

The term "supercooling" refers to a discrepancy between the temperature at which freezing initiates and €«30,900 202168 the melting temperature of a given liquid/solid phase change material when cooled and heated under quiescent conditions.

The term "additives" includes, in addition to nucleating agents such as have been specified hereinbelow, precursors of such additives which are non-detrimental to the function of the phase change materials of the invention. More particularly, the additives herein referred to are either anhydrous or hydrated compositions of inorganic salts or precursor materials which would form the salt upon addition to hydrated calcium chloride.

The composition of the invention basically comprises a mixture of from 37 to 51 weight percent CaC^ and from 0 i 5 to 26 weight percent potassium salt, with the balance being ^0 (in an amount up to 10 0 weight percent) . The amount of potassium salt added to the hydrated CaCl2 should preferably be such that the potassium ion is present in an amount of 0.2 to 4.2 percent by weight of the composition. Examples of potassium salts are the following : potassium sulfate; potassium carbonate; potassium phosphate; potassium arsenate; potassium metaborate; potassium citrate; potassium fluoride; potassium hydroxide; potassium molybdate; potassium oxalate; potassium metaphosphate; potassium pyrophosphate; potassium silicate; potassium sulfite; potassium tartrate, and mixtures thereof.

Of the above listed potassium salts, potassium sulfate, potassium carbonate, and potassium phosphate are preferred. These salts are present in a composition in which the CaC^ comprises from 37 to 51 weight percent. The potassium sulfate is admixed with the CaCl^ in an amount of 0.5 to 19 percent by weight; the potassium carbonate is admixed with the CaC^ in an amount of from 0.5 to 15 percent by weight, and the potassium phosphate is admixed with the CaC^ in an amount of from 0.5 to 23 percent by weight, the balance in each instance being 202168 water in an amount of up to 10 0 percent by weight.

Examples of most preferred phase change compositions are the following : CaC^ ~ from 4 7 to 50 weight percent K^SO^ - from 8.4 to 10.8 weight percent £^0 - Balance of up to 100 weight percent CaCl2 - from 4 7 to 5 0 weight percent K2C03 ~ ^ram 8*7 to 10.6 weight percent - Balance of up to 100 weight percent CaCl2 - from 48 to 50 weight percent K^PO^ - from 7.6 to 9.1 weight percent H2O - Balance of up to 100 weight percent.

In a preferred application the phase change composition also includes the addition of a sodium and/or strontium salt to further modify the CaC^. ei^O/potassium salt mixture to thereby obtain a composition which is effectively a congruently melting composition. The sodium salt is preferably added to the hydrated CaC^ such that the sodium ion is present in an amount of from 0.04 to 2.0 percent by weight of the composition; the strontium salt is preferably added to the hydrated CaC^ such that the strontium ion is present in an amount of from 0.05 to 2.0 percent by weight of the composition.

Examples of sodium and strontium salts are the following : sodium arsenate; sodium metaborate, sodium citrate; sodium fluoride; sodium hydroxide, strontium hydroxide; sodium molybdate; sodium oxalate; sodium metaphosphate; sodium pyrophosphate; sodium silicate; sodium sulfite; sodium tartrate, and strontium tartrate. Mixtures of these salts may also be employed. 202168 It is to be understood that the above list of potassium, sodium and strontium salts are not intended to be all inclusive but that persons skilled in the art may readily substitute equivalent potassium, sodium or strontium salts in order to achieve similar beneficial results.

Effective amounts of selected nucleating agents for the hydrated CaCl2/potassium salt mixture of the invention are determined by testing a given composition over repetitive phase change cycles and in "a manner similar to the procedures employed in New Zealand Patent Specification No. 200274. The nucleating agents of that specification have been found to produce similar marked benefits in the CaCl2/potassium salt system of the present invention. A nucleating agent in an amount in excess of 2.0 weight percent of the weight of the composition does not provide any increase in the benefit and, accordingly, the nucleating agent is preferably present in an amount of from 0.005 to 2.0 weight percent, based on the total weight, to reduce supercooling to 5°C -or less during retrieval of the stored heat by crystallization. More preferably, the amount of the nucleating agent in the phase change composition is from 0.01 to 1.0 weight percent.

Nucleating agents which have been found to be of particular benefit in the CaC^/potassium salt mixture of the present invention are Ba(OH)2, BaO, Bal2, BaSC>4, BaS203,BaCC>3, BaCl2, BaF2, BaF2 .HF, Sr(OH)2, SrO, SrC03, SrF2, Srl2, or mixtures thereof. Nucleators selected from BaCC>3; BaCl2; BaO; Ba(OH)2; Bal2; BaSC>4; Sr(OH)2; or mixtures thereof are preferred. 2021 Impurities may be present in the phase change composition in minor amounts of less than about 3.0 weight percent and provided that such impurities do not detrimentally affect the function of the basic hydrated 5 CaCl2/potassium salt phase change compositions of the invention which may include the hereinafter specified nucleating agents and additives such as a sodium and/or strontium salt. Impurities may include, for example, LiCl, MgCl2, or other calcium salts such as CaCO^ or 10 CaS04.

The following examples illustrate the effectiveness of a potassium salt whose anion forms a substantially less soluble salt with calcium for suppressing the formation of unwanted hydrates in 15 the CaCl2•6H20/potassium salt phase change compositions of the invention.

In Example 4 of our New Zealand Patent Specification Number 200274, it was shown that for a 6:1 I . mole ratio of H20:CaCl2, without additives, the tetra-20 hydrate melts at 32.8°C and the hexahydrate at 29.6°C. Thus, in the 3.2°C span between these two temperatures/ CaCl2-4H20 can be crystallized during the freezing process. In the following examples, various potassium salts were added to aqueous CaCl2 compositions, suf-25. ficient to saturate the solution with potassium ion, and the effect was determined on the tetrahydrate and hexahydrate melting points.

Example 1 29.5 Grams of K2SC>4 was added to 500 g of 30 a stirred solution of 47 percent CaCl2 and 53 percent H20. This first mixture was heated to 60°C, cooled 202168 slowly to 23.8°C, seeded with a crystal of CaCl2*6H20, and allowed to come to equilibrium. A concentrate was also prepared by adding 31.9 g of I^SO^ to 569.7 g of a solution of 55 percent CaCl2 and 45 percent H20 5 and heating to 55°C. This concentrate was added stepwise to the first mixture in 70-150 g increments, allowing equilibrium to be restablished after each increment. Before each addition, the equilibrium temperature was determined and a small sample of 10 the liquid phase was withdrawn and analyzed. These data showed that at a molar ratio of 6:1 for the H20:CaCl2, the melting point of CaCl2»6H20 is 27.1°C. Further incremental additions of the concentrate caused the hexahydrate crystals to change to alpha 15 tetrahydrate crystals. Data obtained from these additions showed that at a molar ratio of 6:1 for the H20:CaCl2, the melting point of CaCl2-4H20 is 29.°c.

Example 2 In the same manner as Example 1, 23 g of K2CC>2 was added to 500 g of a stirred solution of 50 percent CaCl2 and 50 percent H20. The concentrate contained 17.7 g of K2C03 and 385 g of a solution of 55 percent CaCl2 and 45 percent H20. The data obtained 25 showed that at a molar ratio of 6:1 for the H20:CaCl2, the melting point of CaCl2*6H20 is 27.0°C and that of CaCl2-4H20 is 28.0°C.

Example 3 In the same manner as Examples 1 and 2, 30 23.6 g of K3P04 w^s added to 503.7 g of a stirred solution of 49.6 percent CaCl2 and 50.4 percent H20. The concentrate contained 35.6 g of K3PC>4 C-30,960 -14- in 759 g of a solution of 55 percent CaCl2 and 45 percent H20. The data showed that at a mole ratio of 6:1 for the H20:CaCl2, the melting point of CaCl2«6H20 is approximately 27.8°C and that of 5 CaCl2*4H20 is 30.1°C.

They show that the addition of K2SC>4, I^CO^, or K^PO^ reduces the temperature range over which CaCl2«4H20 is stable, and thus decreases the 10 tendency to form this undesired crystalline phase during the freezing process.

Effect of Potassium Salts on Calcium Chloride Hydrate Table I summarizes these experiments.

TABLE I Salt b none CaCl2*4H20 m.p.,°C CaCl2-6H20 m.p.,°C CaCl2-4H20 Stable Span 32.8 29.1 28.0 .1 29.6 27.1 27.0 27.8 3 .2°C 2.0°C 1.0°C 2 .3°C a - 6:1 mole ratio H20:CaCl2 b - Not an example of this invention m.p. - melting point -€—30,900 2021 £8

Claims (31)

WHAT WE CLAIM IS:

1. A reversible liquid/solid phase change composition comprising an admixture of hydrated CaCl^ and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaCl2, wherein the potassium salt is added in an amount sufficient to modify the semi-congruent melting behavior of CaCl^^H^O to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2.-

6H20.

2. The composition of Claim 1, wherein the potassium salt is added to the hydrated CaCl2 such that the potassium ion is present in an amount of from 0.2 to 4.2 percent by weight of the composition.

3. The composition of Claim 1, comprising an admixture of from 37 to 51 weight percent CaCl2 and from 0.5 to 26 weight percent of the potassium salt,

with the balance being H20 (up to 100 weight percent).-.

4. The composition of Claim 1,2 or 3 wherein the potassium salt is selected from the group consisting of potassium sulfate; potassium carbonate; potassium phosphate; potassium arsenate; potassium metaborate; potassium citrate; potassium fluoride; potassium hydroxide; potassium molybdate; potassium oxalate; potassium metaphosphate; potassium pyrophosphate; potassium silicate; potassium sulfite; potassium tartrate, and mixtures thereof.

28 JUNI9853

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2 02

5. The composition of Claim 4, wherein the potassium salt is potassium sulfate, potassium carbonate or potassium phosphate, or a mixture thereof.

6. The composition of Claim 4, wherein the potassium salt is K^SO^, and the composition comprises from 37 to 51 weight percent CaC^, from 0.5 to 19 weight percent I^SO^, with the balance being (up to 100 weight percent).

7. The composition of Claim 6, comprising from 47 to 50 weight percent CaC^, from 8.4 to 10.8 weight percent I^SO^, with the balance being 1^0 (up to 100 weight percent).

8. The composition of Claim 4, wherein the potassium salt is f^CO^, and the composition comprises from 37 to 51 weight percent CaCl2, from 0.5 to 15 weight percent I^CO^, with the balance being H^O (up to 100 weight percent).

9. The composition of Claim 8, comprising from 47 to 5 0 weight percent CaCl2, from 8.7 to 10.6 weight percent K2CO^, with the balance being H^O (up to 100 weight percent).

10. The composition of Claim 4, wherein the potassium salt is K^PO^, and the composition comprises from 37 to 51 weight percent CaCl2/ from 0.5 to 23 weight percent K^PO^, and the balance being H20 (up to 100 weight percent).

11. The composition of Claim 10, comprising from 48 to 50 weight percent CaCl2, from 7.6 to 9.1 weight percent K3PC>4 , with the balance being H20 (up to 100 weight percent).

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202)69

12. The composition of any one of Claims 1 to 11, including a sodium and/or strontium salt, wherein the anion of said sodium and/or strontium salt forms a substantially less soluble salt with calcium than CaCl^, said sodium and/or strontium salt being present in an amount sufficient to modify the CaC^•6H20 and potassium salt mixture to form an effectively congruently melting mixture.

13. The composition of Claim 12, wherein said sodium and/or strontium salt is/are selected from the group consisting of sodium arsenate; sodium metaborate;

sodium citrate; sodium fluoride; sodium hydroxide;

strontium hydroxide; sodium molybdate; sodium oxalate;

sodium metaphosphate; sodium pyrophosphate; sodium silicate; sodium sulfite; sodium tartrate; strontium tartrate, and mixtures^ thereof.

14. The composition of Claim 12 or 13 , wherein the sodium salt is added to the hydrated CaC^ such that the sodium ion is present in an amount of from 0.04 to 2.0 percent by weight of the composition.

15. The composition of Claim 12 or 13 wherein the strontium salt is added to the hydrated CaC^ such that the strontium ion is present in an amount of from 0.0 5 to 2.0 percent by weight of the composition.

16. The composition of any one of the preceding claims, including the addition of one or more nucleating agent or agents to said composition in an amount of from 0.005 to 2.0 weight percent to reduce supercooling to 5°C or less during retrieval of the stored heat by crystallization.

*

17. The composition of claim 16, wherein the nucleating agent is present in an amount of from 0.10 1.0 weight percent.

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202168

18. The composition of Claim 16 or 17, wherein the nucleating agent is selected from the group consisting of Ba(OH)2, BaO, Bal^, BaSO^, BaS^^, BaCO^, BaCl2, BaF2, BaF2.Hr, Sr(OH)2/ SrO, SrCO^ SrF2' SrI2' and mixtures thereof.

19. The composition of Claim 18, wherein the nucleating agent is selected from Ba(OH)2; BaO; Bal2; BaSO^; BaCO^; BaCl2; Sr(0H)2, and mixtures thereof.

composition comprising a mixture of CaCl2.6H20 and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaCl2, wherein the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2.6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2.6H20, and including a sodium and/or strontium salt, wherein the anion of said sodium and/or strontium salt forms a substantially less soluble salt with calcium than CaCl2, said sodium and/or strontium salt being present in an amount sufficient to modify the CaCl2.6H20 and potassium salt mixture to form an effectively congruently melting mixture.

CaCl2 in an amount of from 37 to 51 weight percent and from 0.5 to 26 weight percent of the potassium salt, with the balance being H20 (up to 100 weight percent).

22. The composition of Claim 20 or 21, wherein said sodium and/or strontium salt is/are selected from the

20. A reversible liquid/solid phase change

21. The composition of Claim 20, comprising

202168

20 -

group consisting of sodium arsenate; sodium metaborate;

sodium citrate; sodium fluoride; sodium hydroxide;

strontium hydroxide; sodium molybdate; sodium oxalate;

sodium metaphosphate; sodium pyrophosphate; sodium silicate; sodium sulfite; sodium tartrate; strontium tartrate, and mixtures thereof.

23. The composition of Claim 20, 21 or 22,

wherein the potassium ion is present in an amount of from 0.2 to 4.2 weight percent.

24. The composition of any one of Claims 20 to 23, comprising one or more nucleating agents in said composition in an amount of from 0.005 to 2.0 weight percent to reduce supercooling to 5°C or less during retrieval of the stored heat by crystallization, wherein the nucleating agent is selected from the group consisting of Ba(OH)2,

BaO, Bal2, BaS04, BaS203, BaC03, BaCl2, BaF2, BaF2.HF, .

-Sr(OH)2, SrO, SrCO^, SrF2, Srl2, and mixtures thereof.

25. The heat storage device comprising an encapsulating means having a reversible liquid/solid phase change composition hermetically sealed in said encapsulating means to prevent evaporation of water from the composition, said composition comprising an admixture of hydrated CaCl2 and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaCl2, wherein the potassium salt is added in an amount such that the potassium ion is present in an amount sufficient to modify the semi-congruent melting behavior of CaCl2.6H20 to the extent that the mixture approaches the congruent melting behavior of a congruently melting mixture and to

9

reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl2 hydrate phases other than CaCl2-6H^O,

2021

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26. The heat storage device of Claim 25,

wherein said composition includes a sodium and/or strontium salt, wherein the anion of said sodium and/or strontium salt forms a substantially less soluble salt with calcium than CaC^/ said sodium and/or strontium salt being present in an amount sufficient to modify the CaC^'SH 0 and potassium salt mixture to form an effectively congruently melting mixture.

27. The heat storage device of Claim 26, wherein said sodium and/or strontium salt is/are selected from the group consisting of sodium arsenate; sodium metaborate; sodium citrate; sodium fluoride; sodium hydroxide; strontium hydroxide; sodium molybdate;

sodium oxalate; sodium metaphosphate; sodium pyrophosphate; sodium silicate; sodium sulfite; sodium tartrate;, strontium tartrate, and mixtures thereof.

28. The heat storage device of Claim 25 , 2 6 or 27, wherein said composition includes one or more nucleating agents in an amount of from 0.00 5

to 2.0 weight percent to reduce supercooling <•

to 5°C or less during retrieval of the stored heat by crystallization, said nucleating agents being selected from Ba(OH)2^ BaO, Bal^, BaSO^, BaS20^, BaCO^, BaC^, BaF2/ BaF2;HF, Sr(0H)2/ SrO, SrCO^, SrF2, Sr^, and mixtures thereof.

29. A method of storing heat, comprising the steps of preparing a reversible liquid-solid phase change composition by admixing hydrated CaC^ and a potassium salt whose anion forms a substantially less soluble salt with calcium than CaC^/ adding said potassium* salt in an amount sufficient to modify the semi-congruent melting. _ behavior of CaClo.6Ho0 to the extent that the mixture-' ^

' Va• O\

t 'V .-A

<K\

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202168

approaches the congruent melting behavior of a congruently melting mixture and to reduce, during retrieval of the stored heat by crystallization of the mixture, the formation of crystalline CaCl^ hydrate phases other than CaC^^I^O, introducing the composition into an encapsulating means for use as a heat storage device, and hermetically sealing the encapsulating means to prevent evaporation of water from the composition.

30. The method of Claim 29, including the step of adding an amount of a sodium and/or strontium salt, wherein the anion of said sodium and/or strontium salt forms a substantially less soluble salt with calcium than CaCl2/ said sodium and/or strontium salt being present in an amount sufficient to modify the CaC^-SI^O and potassium salt mixture to form an effectively congruently melting mixture.

31. The method of Claim 29 or 30,

including the step of adding one or more nucleating agents in said composition in an amount of from 0.005 to 2.0 weight percent to reduce supercooling to

5°C or less during retrieval of the stored heat by crystallization, said nucleating agents being selected from Ba (OH) BaO, Bal^, BaSO^, BaS203, BaCO^ / BaC^, BaF2' BaF2'HF, SrCOH^/ SrO, SrCO^/ SrF2/ Srl^, and mixtures thereof.

OATED THK / OAY

a. j. park ft fir A

PEK

AGENTS for the appucahts