DK157033B - Reversible liquid/solid phase change agent and application thereof - Google Patents

Description

DK 157033 B

The invention relates to reversible liquid / solid phase change agents consisting of CaCl2.6H2O, a site which prevents the formation of crystalline CaCl2 hydrate phases which are not CaCl2.6H2O, optionally one or more nucleating agents and optionally a 5 or more sodium salts and / or strontium salts, and wherein the semi-congruent melting behavior of CaCl2.6H2O is approximated to the congruent melting behavior of a congruent melting mixture, as well as using the agent as a heat alloying medium.

Phase change materials (PCMs), in which the melt heat of various hydrated mixtures are used, are well known in the literature. In the September 1974 ASHRAE Journal, entitled S0-LAR ENERGY STORAGE, Dr. M. Telkes PCM's thermal, physical and other relevant properties on the basis of economy, applicability, corrosion, toxicity and accessibility for use in large scale installations. Among the evaluated materials were various salt hydrates and their eutics including CaCl2.6H2O, which undergoes multiple phase transitions for materials of different crystal structure, ie. CaCl2f6H2O20 to CaCl2 / 4H2O + 2H2O at 29 ° C.

When the salt of CaCl2.6H2O is heated to a temperature above 33 ° C, it dissolves completely in its crystal water. When cooled, formation of four different crystal forms is possible, i.

And three forms of CaCl2.41 ^ 0. If any of the 4H 2 O crystals are formed, the heat of melting is much less than 193 J / g (CaCl 2/6 H 2 Despite the relatively low price of CaCl2, the formation of its four different crystal forms was considered to be a disadvantage.

Swedish Patent Specification No. 78.01037-8 (disclosure no. 410,004) discloses a method of suppressing tetrahydrate formation during repeated melting and crystallization of a System based on CaCl3. It was found that in readings in the concentration range from 48 to 53% by weight

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2

CaCl2 using the highest purity CaCl2.6H2Û, the crystallization temperatures of CaCl2.6H2O and CaCl2.4H2O were such that the solution melted incongruently and that the CaCl2 / 4H2Û crystallized and precipitated from the solution, thereby losing its heat storage capacity. Using a solution of the same concentration prepared from technical grade CaCl2 (road salt) and containing NaCl and KCl as impurities, the solubility of the tetrahydrate decreased and the solubility of the hexahydrate increased. With repeated melting and crystallization, the precipitation becomes significant and the system loses its heat storage capacity again. It can therefore be concluded that the use of a technical grade CaCl2 (road salt) results in a poorer effect due to a relative increase in tetrahydrate formation compared to a shark purity CaCl2 system.

It was also found that the addition of one or more compounds including ca. 2% by weight of SrCl2.6H2O pretreated the solubility of the tetrahydrate and suppressed the tetrahydrate formation by repeated melting and crystallization. The amount of addition was found to be dependent on the amount of impurity present in the system as in an example using road salt was determined to be 2.2% by weight.

The relative amounts of each impurity in the technical grade (road salt) salt were not determined, nor were they considered to be of significance to the result of the tests performed. Indeed, the use of road salt was found to be less solitary from the point of view of tetrahydrate formation compared to CaCl2 of high purity. There was also no recognition that, in the mild, impurities of NaCl and KCl could be beneficial in reducing tetrahydrate crystal formation in such phase change agents.

In U.S. Patent No. 4,613,444, it was shown that addition of KCl to CaCl2.6H2Û significantly reduces the possibility of forming the undesired CaCl2.4H20 crystal phase during recovery.

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3 of the stored heat by freezing the phase change agent.

It has now been found, surprisingly, in accordance with the present invention that the sole effect of reducing the formation of the unwanted CaCl2.4H2O crystal phase can also be obtained with other salts other than the chloride, i. KC1. In particular, the reduction of the formation of the CaCl2.4H2O crystal phase by recovering the stored heat by freezing the hydrated CaCl3 mixture can be achieved by the addition of a potassium salt in which the potassium salt of the used potassium forms a heavily soluble calcium salt. 10 seats, ie. a calcium salt which is substantially unresolved.

U.S. Patent No. 4,613,444 also discloses that the addition of NaCl and / or SrCl2 exacerbates the beneficial effect of adding KCl to the hydrated CaCl2 mixture to obtain an effective congruent melting agent. It has now been found that other sodium and / or strontium salts can also be used with the same favorable results.

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For some applications, it is preferred, for example. using potassium, sodium or strontium salts or mixtures of such salts rather than salts where the anion is chloride, to raise the pH of the mixture and thereby reduce the natural acidity of aqueous CaCl phase change agents, in which the anions of potassium, sodium and strontium salts form a substantially less soluble site with calcium, significantly improve compatibility with the metal containers of the heat storage agent.

30

Heat storage agents are ideally packaged in individual encapsulants for use in conjunction with solar heating systems. Examples of suitable known encapsulants for the heat storage agents disclosed herein are water-impermeable films 35 or sheets of plastic / metal laminates. Also, closed-cell plastic foam has been proposed in which PCM may be encapsulated in the foam structure cells, such as e.g. illustrated in 4

DK 157033 B

U.S. Patent No. 4,003,426. Other useful encapsulants are concrete, metal or plastic containers, rudders and the like.

The invention relates to a reversible liquid / solid phase change agent of the kind mentioned in the preamble, which is characterized by 37-51% by weight CaCl2 mixed with 0.5-26% by weight of a potassium salt whose anion forms a site with calcium which is substantially less soluble than CaCl2, and with the remainder of the mixture H2O up to 100%.

The invention further relates to the use of said agent as heat storage medium.

Although the hydrated CaCl2 / potassium salt blend according to the invention surprisingly diminishes the formation of crystal forms other than the hexahydrate form, it was found that it still retained the inherent characteristics of the CaCl2.6H2O decay properties. Therefore, the invention is preferably directed to the addition of selected crystal nucleating agents to effectively reduce sub-precipitation in the hydrated CaCl2 / potassium salt system.

The avoidance of undercooling during the crystallization of hydrated CaCl2, e.g. by the addition of various nucleating agents is well known in the literature, e.g. from the U.S.S.R. Inventor Certificate No. 568,669, Japanese Patent No. 969,909, and U.S. Patent No. 4,189,394. However, nucleating agents were not intended as additives to obtain 30 congruent melting hydrates.

Even with the addition of a potassium salt in which the anion is present in a sufficient amount to effectively reduce the tendency of the phase change agent to form, by freezing, the unwanted phase CaCl2.4H2O, even a maximum amount present at the solubility limit is not sufficient to fully of preventing the formation of CaCl2 # 4H20. In a

DK 157033 B

Accordingly, the embodiment of the invention can be completely prevented from crystallizing if the potassium salt is used in combination with a sodium and / or strontium salt in which the anion forms a substantially less soluble site with calcium.

The present invention represents a significant step toward the development of an inexpensive but highly effective reversible liquid / solid phase change agent based on hydrated CaCl 2 in admixture with a potassium salt.

Hydrated site phase change materials exhibit three general types of phase change behavior: congruent, semi-congruent, and incongruent melting. The most desirable behavior is 15 congruent melting that occurs when the solid phase change agent (ratio of sait to bound water) is the same as the liquid phase agent. In that case, the hydration / dehydration process appears to be identical to the melting and freezing process.

20

The term "effective congruent melt blend" as used herein defines a blend of aqueous calcium chloride constituents for which solid and liquid phases are in stable equilibrium at the melting point. The solid phase contains no hydrated calcium chloride material other than the hexahydrate or solid solutions thereof, and for each mole of calcium chloride the liquid phase contains 6 moles of water plus ten sufficient water to form the stable hydrate of any added materials in solution.

30

Semi-congruent melting occurs when a phase change material has two or more hydrate forms with different solids compositions and melting points. The material can be converted to other hydrate forms for either complete melting or freezing, resulting in an extended melting point range. In addition, there is a temporary loss in heat storage capacity. Calcium chloride hexahydrate is an example of a semi-congruent melting phase change material.

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Incongruent melting phase change materials form two distinctly distinct phases of melting: a saturated solution and a precipitate of an insoluble anhydrous site. If the precipitate falls out of solution, the anhydrous site will not completely hydrate upon cooling and any heat storage capacity will be lost with each freeze / melt cycle. Incongruent melting observed with e.g. sodium sulfate decahydrate, is a more serious problem because it can result in a sustained loss of latent heat storage capacity.

10

The term "undercooling" refers to a difference between the temperature at which freezing begins and the melting temperature of a given liquid / solid phase change material when cooled and heated under calm conditions.

15

The term "additives" includes, in addition to nucleating agents, as specified in the following, precursors for such additives which are not detrimental to the operation of phase change materials according to the invention. More particularly, the additives mentioned herein are either anhydrous or hydrated sarcoma compositions of inorganic salts or precursor materials which would form the salt upon addition of hydrated calcium chloride.

The agent of the invention comprises, as mentioned, a mixture of from 37 to 51 wt% CaCl 2 and from 0.5 to 26 wt% potassium salt, the remainder being H 2 O (in an amount up to 100 wt%). Examples of potassium salts are as follows: Potassium arsenate, potassium metaborate, potassium citrate, potassium fluoride, potassium hydroxide, potassium molybdate, potassium oxalate, potassium metaphosphate, potassium pyrophosphate, potassium sulfate, potassium sulfite and potassium sulfate.

Preferably, the potassium salt is added to the hydrated CaCl 2 so that the potassium ion is present in an amount of 0.2 to 4.2% by weight of the mixture.

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Among the above mentioned potassium salts, potassium sulphate, potassium carbonate and potassium phosphate are preferred. These salts are present in an agent in which CaCl 2 is from 37 to 51% by weight. Potassium sulphate is mixed with CaCl2 in an amount of 0.5 to 19% by weight, 5 potassium carbonate is mixed with CaCl2 in an amount of 0.5 to 15% by weight, and potassium phosphate is mixed with CaCl2 in an amount of 0.5 to 23 % by weight, the remainder being water in each case up to 100% by weight.

Examples of most preferred phase change agents are the following:

CaCl2 - from 47.1 to 50% by weight K2SO4 - from 8.4 to 10.8% by weight H2O - the remainder up to 100% by weight 15

CaCl2 - from 47.5 to 50% by weight K2CO3 - from 8.7 to 10.6% by weight H2 O - the residue up to 100% by weight CaCl2 - from 48.5 to 50% by weight K3PO4 - from 7.6 to 9, 1 wt% H2O - the residue up to 100 wt%.

In a preferred application, the phase change agent 25 also includes the addition of a sodium and / or strontium salt to further modify the CaCl 2/6 H 2 O / potassium salt mixture to thereby obtain an agent which is effectively a congruent melting agent.

The sodium salt preferably adds to the hydrated CaCl2f such that the sodium ion is present in an amount of 0.04 to 2.0% by weight of the agent.

The strontium salt is preferably added to the hydrated CaCl 2 / so that the strontium ion is present in an amount of 0.05 to 2.0% by weight of the agent.

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8

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 pyrophosphate, sodium 5 Mixtures of these salts can also be used.

Effective amounts of the selected crystal nucleating agents for the hydrated CaCl 2 / potassium salt mixture according to the invention are determined by testing a given composition through repeated phase change cycles and in a manner similar to the procedures described in U.S. Patent No. 4,613,444. The nucleating agents disclosed therein have been found to provide similarly pronounced advantages in the CaCl2 / potassium salt system of the present invention. A nucleating agent in an amount of more than approx. 2.0 wt% of the weight of the mixture does not provide any added benefit, and thus the nucleating agent is preferably present in an amount of 0.005 to 2.0 wt%, based on the total weight to reduce undercooking. 5 ° C or less while recovering the heat stored by crystallization. The nucleating agent is more preferably present in an amount of 0.10 to 1.0% by weight.

Nucleating agents which have been found to be particularly advantageous in the CaCl2 / potassium salt mixture of the invention are Ba (OH) 2 / BaO, Bal2 »63 ^ 203, BaCOg, BaCl2, BaF2, BaF2, HF, Sr ( OH) 2, SrO, SrCO3 »SrF2, Srl2 or mixtures thereof. Germination agents selected from BaCl3, BaCl2, BaO, Ba (OH) 2 »30 Bal2, BaS Ba4, Sr (OH) 2r SrO or mixtures thereof are preferred.

Impurities may be present in the phase change material in smaller amounts of less than approx. 3.0% by weight, provided that these impurities have no detrimental effect on the function of the basic hydrated CaCl2 / potassium salt phase change agents of the invention, which may contain the nucleating agents and additives mentioned below, such as a sodium and / or

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9 strontium salt. Impurities may e.g. include LiCl, MgCl2 or other calcium salts, such as CaCO3 or CaSO4

The following examples illustrate the effectiveness of a potassium salt, whose anion forms a substantially less soluble calcium site, to suppress the formation of undesired hydrates in the CaCl2.6H2O / potassium salt phase altering agents of the invention.

In Example 4 of U.S. Patent No. 4,613,444, it was shown that at a H2O: CaCl2 molar ratio of 6: 1, without additives, the tetrahydrate melts at 32.8 ° C and the hexahydrate at 29.6 ° C . Therefore, in the 3.2 C range between these two temperatures, CaCl 2,4 H 2 O can be crystallized during the freezing process. In the following 15 examples, various potassium salts were added to aqueous CaCl 2 solutions in sufficient quantity to saturate the solution with potassium ions and the effect was determined at the tetrahydrate and hexahydrate melting points.

20

Example 1.

29.5 g was added to 500 g of a stirred solution of 47% CaCl2 and 53% H2O. This first mixture was heated to 60 ° C, slowly cooled to 23.8 ° C, seeded with a crystal of CaCl2r6H2O and allowed to equilibrate. A concentrate was also prepared by adding 31.9 g of K 2 SO 4 to 569.7 g of a solution of 55% CaCl 2 and 45% H 2 O and heating to 55 ° C. This concentrate was gradually added to the first mixture in progressively increasing amounts of 30 to 70 g, equilibrating after each addition. Before each addition, the equilibrium temperature was 35

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10 and a small sample of the liquid phase was taken and analyzed. These data show that, at a 6 OrCaC molar ratio of 6: 1, the CaCl 2 melting point is 27/1 ° C. Further incremental additions of the concentrate caused the 5 hexahydrate crystals to be changed to alpha-tetrahydrate crystals. Data obtained from these additions showed that at a O OiCa C

Example 2.

In the same manner as in Example 1, 23 g to 500 g of a stirred solution of 50% CaCl 2 and 50% 10 O were added. The concentrate contained 17.7 g ° 9 385 9 of a solution of 55%

CaCl2 and 45% H2O. The obtained data showed that at a 1 C, Ca 2 O molar ratio of 6: 1, the CaCl 2 E 2 O melting point is 27.0 ° C and the 15 CaC 2 E 2 O melting point is 28.0 ° C.

Example 3

In the same manner as in Examples 1 and 2, 23.6 g of K3PC> 4 were added to 503.7 g of a stirred solution of 49.6% CaCl 2 and 50.4% I₂O. The concentrate contained 35.6 g of K3 PO4 in 759 g of a solution 20 of 55% CaCl2 and 45% H2 O. Data showed that at an E ^CkCaC ^ molar ratio of 6: 1, the CaC ^ / O₂O melting point is about 27.8 ° C, and the CaC₂ fiF ^O melting point is 30.1 ° C.

The following table summarizes these attempts. They show that the addition of 2 SO 2, K 2 CO 3 or K 3 PO 4 reduces the temperature range within which CaCl 2,4 H 2 O is stable and thus decreases the tendency to form this undesirable crystalline phase during the freezing process.

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1.1

Table

Effect of potassium salts on calcium chloride hydrates

CaCl2.4H20 CaCl2.6H20 CaCl2.4H20 Sait_Smp «° C_Smp. ° C_Stable range

netb 32.8 29.6 3.2 ° C

K 2 SO 4 29.1 27.1 2.0 ° C

K 2 CO 3 28.0 27.0 1.0 ° C

K3 PO4 30.1 27.8 2.3 ° C

a - H2O: CaCl2 mole ratio = 6: 1 b - not an example of the invention m.p. - melting point.

Claims (8)

1. Reversible liquid / solid phase change agent consisting of 2. An agent according to claim 1, characterized in that the potassium salt is added to the hydrated CaCl 2 in an amount such that the potassium ion is present in an amount of approx. 0.2 to approx. 4.2% by weight of the mixture. 20 3. An agent according to claim 1, characterized in that the potassium salt is K2SO4 and the mixture contains from 37 to 51% by weight CaCl2 and from 0.5 to 19% by weight K2SO4, the remainder of the mixture being H2O (up to 100% by weight). %). 25 An agent according to claim 1 or 2, characterized in that the potassium salt is K2CO3 and the mixture contains from 37 to 51% by weight CaCl2 and from 0.5 to 15% by weight K2CO3, the remainder of the mixture being H2O (up to 100% by weight). %). 30 Agent according to claim 1 or 2, characterized in that the potassium salt is K3PO4 and the mixture contains from 37 to 51% by weight CaCl2 and from 0.5 to 23% by weight K3PO4, the remainder of the mixture being H2O (up to 100% by weight). %). 35 CaCl2 / 6H2O, a site which prevents the formation of crystalline CaCl2 hydrate phases which are not CaCl2 / 6H2Û, optionally one or more nucleating agents and optionally one or more sodium salts and / or strontium salts, and wherein the semi-congruent melting behavior of CaCl2, 6H2O is approximated to the congruent melting behavior of a congruent melting mixture, characterized in that 37-51% by weight of CaCl2 is mixed with 0.5-26% by weight of a potassium salt whose anion forms a site with calcium, which is substantially less soluble than CaCl2i and with the remainder of the mixture H2O up to 100%. 15 Agent according to one or more of the preceding claims, characterized in that the nucleating agent is selected from Ba (OH) 2, BaO, Bal2, BaSO4, BaCO3 and BaCl2 · DK 157033 B An agent according to claim 6, characterized in that the nucleated agent (s) is used in an amount of from 0.005 to 2.0% by weight. Use of the agent according to any one of the preceding claims as a heat storage medium. 10 15 20 25 30 35