Thermal energy storage material

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a transition temperature to the anhydrous or a less hy

THERMAL ENERGY STORAGE MATERIAL drated form in the range 10 to 100° C. is dispersed in a hydrogel formed from a water-soluble synthetic poly

The present invention is concerned with thermal mer having pendant carboxylic or sulphonic acid energy storage materials and heat-exchange devices groups cross-linked with cations of a polyvalent metal. containing such materials. One advantage of the material according to the in Thermal energy storage materials may store thermal vention is that the hydrated inorganic salt is immobi energy as specific heat and/or as latent heat. It is often lized in close proximity in small volumes throughout desirable to use materials which store thermal energy as the gel. This minimises any segregation which could latent heat, since this enables the volume occupied by 10 arise, after fusion of the hydrate phase, by any solid the storage material to be minimised. This is advanta sinking to the bottom of the mixture. There is no need to geous, for example, in materials operating in the temper use the material according to the invention in flat hori ature range 10 C. to 100 C. for the storage of solar Zontal trays; the material can be arranged in vertical energy or of heat extracted during refrigeration. columns of substantial height (for example, 50 cm. to Materials which are useful for the storage of thermal 15 one meter).

energy as latent heat undergo reversible transition from A further advantage of the material according to the one form to another on heating to a characteristic tran invention is that the cross-linked hydrogel can be pre sition temperature. This transition may be from solid pared in situ by reaction between the respective water phase to liquid phase (fusion) or from one crystal form to another (this latter transition also being referred to as 20 soluble thereof polymer or an alkali metal or ammonium salt and a water-soluble salt of the polyvalent metal.

fusion). Suitable polyvalent metals include, for example, A number of hydrated inorganic salts are known which undergo transition to the anhydrous or a less chromium, iron, tin, magnesium and aluminium. Alu hydrated form at a characteristic temperature on heat minium and magnesium are preferred in view of the ready availablility of water-soluble salts thereof. Suit ing and revert to the more hydrated form on cooling. 25 able water-soluble salts of the above metals include, for A potential drawback in the use of many of these hydrated salts is incongruency of the phase transition, example, minium chlorides, nitrates or sulphates, of which alu sulphate and magnesium sulphate are preferred.

that is, the transformation of the low-temperature solid The polyvalent metal phase to a two-phase condition where a solid and liquid sufficient to react withis all preferably present in an amount coexist. In the two-phase condition, the difference in mer to form ionic crosslinks.theThe densities of the two phases causes segregation thereof, sary to achieve complete reactionactual amount neces depends on factors which limits their ability to recombine and form the low-temperature single solid phase. Consequently the such as the valency of the metal, the proportion of acid amount of heat recoverable on cooling is reduced. groups in the polymer and the amount of polymer in the Attempts can be made to avoid the formation of two 35 material. Typical amounts of polyvalent metal are 0.5 to -phases above the transition point by controlling the 5% (expressed as the weight of water-soluble salt, based initial composition of the material, but, even for materi on the weight of the storage material). als with a congruent phase transition there remains the The water-soluble polymer preferably has a back problem that the solid phase tends to settle out in time. bone containing units of acrylic acid or methacrylic acid, for example, a homopolymer or copolymer or

This limits both the kinetics of transformation and the acrylic acid or methacrylic acid, partially hydrolysed uniformity of energy storage density within a container, and results in deterioration of the material on repeated polyacrylamide or polymethacrylamide, or an alkali heating/cooling cycles. metal or ammonium salt thereof. In some embodiments, Thermal energy storage materials have been pro the polymer preferably contains 5 to 50% (for example posed in which the hydrated inorganic salt is thickened 10 to 40%) carboxylic groups, the percentages being by an organic thickening agent, for example, cellulosic based on the number of repeating units in the polymer polymers, starch, alginates or an inorganic thickening backbone.

agent, such as a clay (as disclosed in U.S. Pat. No. The molecular weight of the polymer may vary over 3,986,969). The above-mentioned organic thickening a wide range. For some applications it may be advanta agents are natural polymers (or derivatives thereof) and 50 geous to use polymers of relatively low molecular are therefore unstable to hydrolysis and bacterial and weight (for example, 100,000 to 500,000), while for enzyme action, which considerably shortens the use of other applications, higher molecular weights (for exam . . the material. The above-mentioned inorganic thicken ple 1 million to 8 million) may be preferred. ing agents are more stable, but it appears that thermal The water-soluble polymer is preferably present in energy storage materials containing such thickening 55 the thermal energy storage material in a relatively agents can only be used in very shallow depths (for minor amount, such as from 0.5 to 10% (for example, example, about one inch) and must therefore be dis about 5%), based on the weight of the material. posed horizontally. Suitable hydrated inorganic salts for use in the mate It is an object of the invention to provide a thermal rial according to the present invention include, for ex energy storage material comprising a hydrated inor 60 ample, calcium chloride hexahydrate (the fusion point ganic salt which can be used in vertical columns of of which is 29 C); sodium sulphate decahydrate (the substantial height, fusion point of which is 32 C.); disodium hydrogen It is a further object of the invention to provide a phosphate dodecahydrate (the fusion point of which is thermal energy storage material which is stable to hy 35.5° C); sodium thiosulphate pentahydrate (the fusion drolysis and biological breakdown. 65 point of which is 50 C); sodium acetate trihydrate (the These and other objects are achieved according to fusion point of which is 58 C.); barium hydroxide octa the invention by the provision of a thermal energy stor hydrate (the fusion point of which is 75° C) and zinc age material in which a hydrated inorganic salt having nitrate hexahydrate (the fusion point of which is 35° C).

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For the storage of solar energy, the hydrated salt borax Na2B4O710H2O, and 50gm. of the sodium salt of preferably has a fusion point in the range 20' to 90° C. an acrylamide polymer containing acrylic acid units and is preferably non-toxic, non-corrosive and readily were thoroughly mixed together while adding 70 ml. of available at low cost. Preferred hydrated salts meeting ethanol. The polymer, which had an average molecular some or all of the above requirements are sodium sul weight of about 7.5 million, and had a ratio of carboxyl phate decahydrate, disodium hydrogen phosphate do amide radicals of about 1:9, was a material commer decahydrate, sodium thiosulphate pentahydrate and cially available from Allied Colloids Ltd. as WN23. sodium carbonate decahydrate. 503 ml. of water was then added while vigorously Some of the above-mentioned hydrated salts, when agitating the mixture at a temperature of approximately cooled below the fusion point thereof, tend to undergo 10 35 C. In a few seconds the mixture gelled to a uni supercooling (that is they do not transform back to the formly thick but smooth consistency of density about hydrated form until the temperature is below the theo 1.4 gm./cm3. This mixture contained no excess of water retical fusion point). This may result in less hydrated over that required to completely hydrate all the sodium forms of the salt being formed, with consequent reduc sulphate present in the final mixture, and on cooling it tion in the amount of energy released. In order to avoid 15 fully transformed to a solid. supercooling, the material may be nucleated, for exam A sample of the solid was sealed in a square section ple, by a heat-transfer method as disclosed in U.S. Pat. tube measuring 5 cm.X5 cm.X50 cm. long, made of No. 2,677,243, by careful control of the proportions of inert plastics. The ends of the tube were sealed by cast the ingredients of the composition, or by addition of an epoxy resin plugs.

insoluble nucleating agent. A preferred nucleating 20 The tube was disposed vertically and heated to about agent for sodium sulphate decahydrate is borax, as pro 60' C. (the heating time being about one hour) and posed in U.S. Pat. No. 2,677,664. cooled to about 20 C. by heat-exchanging with water When a nucleating agent is present, this agent, like circulating outside the tube (the cooling time being the inorganic salt, is dispersed and suspended in the three to four hours). Reproducible thermal arrests were hydrogel and effectively immobilized therein. This 25 obtained for more than 500 cycles of heating and cool wide dispersion of immobilized nucleating agent en ing.

sures efficient nucleation of the hydrate phase during EXAMPLE 2 cooling cycles, thereby inhibiting supercooling.

The thermal energy storage material according to the Example 1 was repeated, except that the aluminium invention preferably contains the hydrated salt in an 30 sulphate was replaced by the same amount of MgSO4.7- amount of from 66% to 95% by weight and, optionally, H2O.

a nucleating agent in an amount of from 1 to 10%, based In the thermal cycling test, reproducible thermal on the weight of the hydrated salt. arrests were obtained for more than 500 cycles. Substantially all the balance of the thermal energy What is claimed is:

storage material according to the invention is preferably 35 1. In a thermal energy storage material which com water and, optionally, an organic liquid which is misci prises at least one hydrated inorganic salt having a tran ble with water. A particularly preferred such organic sition temperature to a less hydrated form in the range liquid is a lower aliphatic alcohol, such as ethanol (for 10 to 100 C., the improvement comprising that the example, when the hydrated salt is sodium sulphate inorganic salt hydrate is dispersed and suspended in a decahydrate). The water is preferably present in an water-insoluble hydrogel formed from a water-soluble amount sufficient to hydrate all the anhydrous inor polymer having pendant acidic groups selected from ganic salt, and is preferably present in a small excess. the group consisting of carboxylic acid and sulphonic The material may contain water in an amount of, for acid groups cross-linked with cations of a polyvalent example, from 25 to 75% by weight. When a water-mis metal.

cible organic liquid is included, it is preferably present 45 2. A thermal energy storage material according to in a relatively minor amount, compared with water, for claim 1, wherein said polyvalent metal is selected from example, from 5 to 25%, based on the weight of water. the group consisting of magnesium and aluminum. The material according to the invention is preferably 3. A thermal energy storage material according to used in a method of heat exchange in which the material claim.1, wherein said polymer has a backbone contain is first heated to a temperature above the transition 50 ing units of acrylic acid.

temperature of the hydrated salt, and the heat is ex 4. A thermal energy storage material according to tracted from the material by passing a fluid at a temper claim 1, wherein said water-insoluble hydrogel is ature below the abovementioned transition temperature formed in situ by reaction between a water-soluble salt in heat-exchange relationship therewith. The alternate of said polyvalent metal and an alkali metal salt of said heating and cooling of the material can be repeated for 55 water-soluble polymer.

many cycles. 5. A thermal energy storage material comprising at The present invention also comprises aheat-exchange least one hydrated inorganic salt selected from the device, which comprises a tank containing the thermal group consisting of sodium sulphate decahydrate, diso energy storage material according to the invention and dium hydrogen phosphate dodecahydrate, sodium thio means for supplying a cooling fluid in heat-exchange sulphate pentahydrate and sodium carbonate decahy relationship with the thermal energy storage material. drate, wherein said inorganic salt hydrate is dispersed In order that the invention may be more fully under and suspended in a water-insoluble, ionically cross stood, the following Examples are given by way of linked hydrogel which is a salt of a polyvalent metal illustration only. and a water-soluble polymer having pendant acidic 65 groups.

Example 1

6. A thermal energy storage material according to 397 gm. of anhydrous sodium sulphate, 10 gm. of claim 5, wherein said polyvalent metal is selected from solid aluminium sulphate Al2(SO4)314H2O, 40 gm. of the group consisting of magnesium and aluminum.

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7. A thermal energy storage material according to 10. In a method of extracting heat from a thermal claim 5, wherein said polymer contains acrylamide re energy storage material comprising sodium sulphate peating units and acrylic acid repeating units. decahydrate and a minor amount of borax by passing a 8. A thermal energy storage material according to fluid in heat-exchange relationship with said material, the fluid being at a temperature below 32 C., the im claim 5, wherein said water-insoluble salt is formed in provement comprising that the sodium sulphate deca situ by reaction between a water-soluble salt of said hydrate is dispersed and suspended in a water-insoluble polyvalent metal and an alkali metal salt of said water hydrogel formed from a water-soluble polymer having soluble polymer. pendant carboxylic groups cross-linked with cations of 9. A thermal energy storage material in accordance 10 a polyvalent metal.

with claim 5 wherein the water soluble polymer is se 11. A method in accordance with claim 10 wherein lected from the group consisting of water soluble: the water soluble polymer is selected from the group homopolymers of acrylic acid, consisting of water soluble:

copolymers of acrylic acid, homopolymers of acrylic acid, homopolymers of methacrylic acid, 15 copolymers of acrylic acid, copolymers of methacrylic acid, homopolymers of methacrylic acid, partially hydrolyzed polyacrylamide, copolymers of methacrylic acid, partially hydrolyzed polymethacrylamide, and partially hydrolyzed polyacrylamide, alkali metal and ammonium salts thereof, partially hydrolyzed polymethacrylamide, and 20 alkali metal said polymer having pendant acidic groups, and the said polymer having and ammonium salts thereof, pendant carboxylic groups, and the polyvalent metal is a member selected from the group polyvalent metal is a member of the group consisting of consisting of chromium, iron, tin, magnesium, and alu chromium, iron, tin, magnesium, and aluminum. minum. sk k x 2k is