GB2151249A - Heat storage material - Google Patents

Abstract

A heat storage material adapted to store thermal energy as latent heat comprises a hydrated compound dispersed throughout a polymeric matrix, and containing a chelating agent for polyvalent metal ions, e.g. ethylene diamine tetra-acetic acid or a soluble salt thereof.

Description

SPECIFICATION Heat storage materials and method of heat exchange therewith The present invention is concerned with materials which can store thermal energy as latent heat.

A large number of hydrated compounds, such as inorganic salts, are known which absorb latent heat at a characteristic transition temperature on transition to the anhydrous or a less hydrated crystal form, and release the latent heat on reversion to the more hydrated form on cooling to a temperature to below the transition temperature.

A typical inorganic salt suitable for latent heat storage is sodium sulphate decahydrate Na2SO4.10H2O, which undergoes transition to solid an hydros sodium sulphate and a saturated solution of sodium sulphate, with absorption of latent heat at a constant theoretical (equilibrium) temperature of 32.38 C on a heating cycle. When all the decahydrate has transformed to anhydrous salt and saturated solution, further heat input causes a rise in temperature (that is, the absorption of sensible heat). On cooling below the transition temperature, the decahydrate reforms, with the evolution of the absorbed latent heat.

A problem with the use of such hydrated salts for latent heat storage is the incongruency of the phases when the low temperature solid phase transforms to a two phase condition in which solid and liquid coexist. In the two phase condition, the difference in the densities of the two phases causes them to segregate and this reduces their ability to recombine and form the low temperature single solid phase. Consequently, the amount of heat recoverable on cooling is reduced.

Various means of alleviating this problem have been proposed, two proposals being in our European Patents 99 and 11411, in both of which there is disclosed at heat storage material in which a hydrated compound of the type described above is dispersed throughout a hydrogel matrix comprising a water-swollen cross-linked polymer. In European Patent 99, the cross-linked polymer is formed by cross-linking a water-soluble polymer having pendant carboxylic or sulphonic acid groups by means of cations of polyvalent metal (that is, cross-linking is via an ionic mechanism), while in European Patent 11411, the cross-linked polymer is formed by cross-linking a water-soluble or waterdispersible synthetic hydrophilic polymer by a covalent cross-linking mechanism.

Other cross-linked polymer matrices for thermal storage materials are disclosed in, for example, British Specification 2094333, European Specification 56591 and U.S. Patent 4003426.

It is economically desirable in production of such heat storage materials to be able to use commercial grades of their constituents which are available more cheaply than high purity grades and which contain varying concentrations of impurities.

Among these impurities may be compounds containing polyvalent transition metal ions. Similar impurities may also be introduced by the water used to produce the heat storage material or they may be introduced during the production process itself through contact with materials used in the construction of the production equipment. They may also be introduced after production through contact with the constructions materials of the vessel or container into which the storage material is placed for subsequent use.

We have now surprisingly found that the addition of a chelating agent for polyvalent transition metal ions to heat storage materials having a cross-linked polymer matrix results in improved rates of cross-linking and furthermore that the amount of cross-linking obtained is more complete as determined by standard solubility tests on the resulting heat storage material. We have further surprisingly found that the long term stability of the cross-linked polymer matrix incorporating such a chelating agent is substantially improved as judged by exposure of such heat storage material to elevated temperature. Similar results may be achieved with un-cross-linked polymer matrices.

The present invention accordingly provides a heat storage material comprising a hydrated compound dispersed throughout a polymeric matrix, the material containing a chelating agent for poiyvalent metal ions.

Currently preferred chelating agents are ethylene diamine tetra-acetic acid (EDTA) and soluble salts thereof (such as sodium salts). The chelating agent is preferably used in an amount of up to 2% by weight, based on the total weight of the heat storage material; at least 0.5% by weight is typically used. The precise amount to be used in any particular case depends on the amount of nature of the metal ion impurities introduced with the raw materials or likely to be introduced in subsequent use of the heat storage material.

The hydrated compound typically has a fusion point in the range of 0 C to 100"C and is preferably non-toxic, non-corrosive and readily available at low cost. Preferred hydrated compounds meeting some or all of these requirements include, for example, calcium chloride hexahydrate (the fusion point of which is 29"C); sodium sulphate decahydrate (the fusion point of which is 32"C); disodium hydrogen phosphate dodecahydrate (the fusion point of which is 35.5"C); sodium thiosulphate pentahydrate (the fusion point of which is 48 C); sodium acetate trihydrate (the fusion point of which is 58"C); barium hydroxide octahydrate (the fusion point of which is 75"C);; zinc nitrate hexahydrate (the fusion point of which is 35 C); potassium fluoride tetrahydrate (the fusion point of which is 18.5"C); sodium carbonate decahydrate (the fusion point of which is 35 C); and sodium pyrophosphate decahydrate (as disclosed, for example, in our European Patent Application 83304043.9).

The material according to the invention may also contain a fusion temperature depressing compound, for example, a salt as described in our British Specification 2110708.

In one embodiment of the present invention, the polymeric matrix comprises a water-swollen crosslinked polymer hydrogel. Such a hydrogel may be formed by cross-linking a water-soluble or waterdispersible synthetic hydrophilic polymer, or it may be formed by polymerisation in situ of appropriate monomers or pre-polymers. When formed from a hydrophilic polymer, the latter is preferably linear and thermoplastic, and may have functional groups which are cross-linkable by a covalent cross-linking mechanism (in which case the polymer and cross-linking agent therefor are preferably as described in detail in the above-mentioned European Patent 11411).

Hydrogel matrices for heat storage materials are also disclosed in British Specification 2094333 and Japanese Patent Application Publication No. 57/ 82696 (Application No. 55/158224).

It is preferred that the cross-linked polymer in such a hydrogel matrix retains active hydrophilic groups (that is, they are not blocked during crosslinking). For example, where the hydrogel is produced from a polymer having carboxy groups (such as polyacrylamide), the polymer preferably contains at least 20% of units containing carboxy groups.

A heat storage material according to the invention containing a hydrogel matrix may, if desired, contain a dispersant which facilitates uniform and rapid solution of the polymer. Examples of such dispersants include organic liquids which are miscible in water. Particularly preferred such organic liquids are lower aliphatic alcohols, such as methanol or ethanol.

When such a water-miscible organic liquid is included (for example when the polymer is not highly water-soluble, but only sparingly water-soluble or water-dispersible), it is preferably present in a relatively minor amount, compared with water, for example from 5 to 25% based on the weight of the water.

The polymer matrix may also be an open-cell foam structure, for example, as disclosed in the above-mentioned European Specification 56591. A preferred polymer for such a structure is an elastic polyurethane (again, as disclosed in European Specification 56591); alternatively, the polymer may be one of those disclosed in the above-mentioned U.S. Patent 4003426. In any case, the polymer matrix should not be one which includes essentially closed cells containing the hydrated compound: the latter regions should be substantially interconnected in order to minimise supercooling effects.

A further example of a polymer matrix which may be used in the material according to the invention is produced by polymerisation of a monomer/prepolymer syrup (such as a methyl methacrylate syrup) in an aqueous emulsion. Such a syrup is available from ICI under the designation MDR-80.

The polymer matrix and the hydrated compound are preferably used in such amount that the storage material contains a major proportion, by weight, of the hydrated compound and a minor proportion, by weight, of the polymer matrix whereby the resulting material may have an advantageously high heat capacity per unit volume.

For example, when the polymer is in the form of a hydrogel, the proportion thereof may be 0.1 or 10%, preferably 2 to 8% (for example, about 5%) based on the weight of the material according to the invention.

In order to minimise supercooling, the material may be nucleated, for example, by a heat-transfer method as disclosed in U.S. Patent 2677243, by careful control of the proportions of the ingredients of the composition, or by addition of a nucleating agent. Sometimes the polymer matrix may act as the nucleating agent.

The heat storage material according to the invention preferably contains water in an amount sufficient to hydrate all the an hydros form of the hydrated compound. Water may be present in excess of some circumstances, but this reduces the heat storage capacity of the material.

The material according to the invention is preferably used in a method of heat exchange which comprises alternately heating the material to a temperature above its transition temperature and allowing the heat storage material to cool below its transition temperature, with evolution of heat.

The alternate heating and cooling of the material can be repeated for many cycles. In use, the material is preferably contained in a sealed container formed of a gas-, vapour-, and liquid-impermeable material.

In a preferred embodiment, the material according to the invention is retained in sealed plastics tubes, such as polypropylene tubes. Such tubes may be stacked vertically in a tank through which a heat exchange fluid is periodically heated, for example, in a heater operating low tariff periods.

In order that the present invention may be more fully understood, the following examples and comparative tests are given by way of illustration only.

Example 1 10 kg. of water-soluble polyacrylamide available commercially from Allied Colloids Ltd. as WN33 was dissolved in 96.6 litres of water at room temperature (WN33 has an average molecular weight of about 6 million and a ratio of carboxyl: amide radicals of 7:3).

The solution was mixed at about 80"C with 2.6 kg. of the disodium salt of ethylene diamine tetraacetic acid, 150 kg. of anhydrous sodium pyrophosphate and one litre of formalin (an aqueous solution containing approximately 40% by weight of formaldehyde and 14% by weight of methanol).

The sodium pyrophosphate used included 7.5 kg of anhydroud trisodium phosphate (Na3PO4) and was essentially free of pentasodium tripolyphosphate.

The mixture was pumped into a number of polypropylene tubes, which were sealed.

Separate groups of tubes were maintained at various constant temperatures between 65"C and 85"C and the development of cross-linking in the polymeric matrix was observed. The tubes were stacked vertically in a tank through which heat-exchange liquid was circulated. The fluid was periodically heated to 84"C and allowed to cool to 40"C; reproducible thermal arrests were obtained in the heat storage material in the tubes for many cycles and the degree of material breakdown after a long period of use was assessed.

Comparative Test 1 Example 1 was repeated, but omitting the disodium EDTA salt.

Comparison of the rate of cross-linking of the polymeric matrix on holding the mixture at various temperatures from 65" to 85"C, showed that in the presence of the EDTA salt (Example 1) cross-linking was at least four times more rapid and was more complete (at each of the temperatures used) than in the absence of the EDTA salt (Comparative Test 1).

The long term thermal cycling of the materials showed that the material with the EDTA salt (Example 1) suffered less breakdown than that without (Comparative Test 1).

Example 2 25g. of water-soluble polyacrylamide available commercially from Allied Colloids Ltd. as WN27 were dissolved in 500 cm3 of water at room temperature to form a polyacrylamide gel (WN27 has a molecular weight of 12-15 x 106 and is 20% carboxylated).

The gel was mixed with 14g of the disodium salt of ethylene diamine tetra-acetic acid, 100 cm3 of ethylene glycol, 7609. of anhydrous sodium acetate, 13g. of dipotassium hydrogen phosphate (K2HP04), and 2.5 cm3 of formalin (35% aqueous solution of formaldehyde).

The mixture was transferred to containers which were sealed. The containers were maintained at 65"C and the time taken for substantially complete cross-linking of the polymeric matrix was noted; it was 3 days.

Comparative Test 2 Example 2 was repeated, but the disodium EDTA salt was omitted.

The time taken for cross-linking was more than 6 days.

Claims (16)

1. A heat storage material adapted to store thermal energy as latent heat, which comprises a hydrated compound dispersed throughout a polymeric matrix, and containing a chelating agent for polyvalent metal ions.

2. A heat storage material according to claim 1, in which the chelating agent is ethylene diamine tetra-acetic acid or a soluble salt thereof.

3. A heat storage material according to claim 2, in which the chelating agent is a sodium salt of ethylene diamine tetra-acetic acid.

4. A heat storage material according to any of claims 1 to 3, which contains from 0.5 to 2% of the chelating agent, based on the total weight of the material.

5. A heat storage material according to any of claims 1 to 4, in which the hydrated compound has a fusion point of from 0 to 100"C and is non-toxic and non-corrosive.

6. A heat storage material according to any of claims 1 to 4, in which the hydrated compound is calcium chloride hexahydrate, sodium sulphate decahydrate, disodium or dipotassium hydrogen phosphate dodecahydrate, sodium thiosulphate pentahydrate, sodium acetate trihydrate, barium hydroxide octahydrate, zinc nitrate hexahydrate, potassium fluoride tetrahydrate, sodium carbonate decahydrate, or sodium pyrophosphate decahydrate.

7. A heat storage material according to any of claims 1 to 6, in which the polymeric matrix is a water-swollen, cross-linked polymer hydrogel.

8. A heat storage material according to claim 7, in which the polymer hydrogel is formed from a linear thermoplastic hydrophilic polymer.

9. A heat storage material according to claim 8, in which the hydrophilic polymer has pendant functional groups which are cross-linked by a covalent mechanism.

10. A heat storage material according to claim 9, in which not all the functional groups are reacted during cross-linking so that the hydrogel retains a proportion of such groups.

11. A heat storage material according to any of claims 1 to 10, which comprises a major proportion, by weight, of the hydrated compound and a minor proportion, by weight, of the polymeric matrix.

12. A heat storage material according to claim 11, in which the polymeric matrix is a hydrogel and the material comprises from 2 to 8%, by weight, of the hydrogel.

13. A heat storage material according to any of claims 1 to 12, contained in a sealed container formed of a gas-, vapour- and liquid-impermeable material.

14. A heat storage material according to claim 13, in which the container is a sealed polypropylene tube.

15. A method of heat exchange which comprises alternately heating a heat storage material according to any of claims 1 to 14 to a temperature above its transition temperature and allowing the heat storage material to cool below its transition temperature with evolution of heat.

16. A heat storage material substantially as herein described in either of the Examples.