GB2169915A - Heat storage compositions - Google Patents

Abstract

Compositions comprising tetrasodium pyrophosphate which are useful as heat storage media further comprise a homopolymer or copolymer of polyethylene imine. The polymers, which are preferably cross-linked so as to form a three-dimensional matrix, act as a suspending agent and also to stabilise the pyrophosphate against hydrolytic degradation.

Description

SPECIFICATION Heat storage compositions This invention relatestocompositionswhich are useful for the storage of thermal energy, to processes for their manufacture and to their use as heat storage elements.

Thestorageofthermal energyfindsparticular application in the utilisation ofenergywhich is generated at a time of low energy utilisation in order to store that energy and subsequently release it. Examples of areas of interest are the utilisation of "off-peak" electricity and solar heating units. One possible type of heat storage medium which has been proposed is the crystals of inorganic salts which are capable of forming a discrete hydrated salt which preferably incorporates many molecules of water of crystaliisa- tion.The salts which are useful are those which can be heated to a transition point at which water of crystallisation is released and the anhydrous or a less hydrated saltformed thus taking up thermal energy as latent heat and which on cooling undergo a reversion to the hydrated salt thus releasing the latent heat.

Ideally the salt should be capable of undergoing a cycle involving the repeated transition between the two forms withoutany degradation that a heat storage unit containing this salt will perform efficiently for as long a period as possible.

One salt which has been proposed for use as a heat storage medium istetra sodium pyrophosphate (see for example European Patent Application 8220589 now published as publication no.0101181). Tetra sodium pyrophosphate (hereinafterfor convenience termed 4SP) forms a decahydrate which undergoes a transition to form the anhydrous salt at a temperature of approximately 70"C which is advantageous in that many domestic heating systems are designed to function at orjust belowthistemperature. (The transitioin temperature is commonly defined as the maximum temperature achieved aftersupercooling).

In our European patent application 84304274 now published as Publication Number 0140467 the addition of materials which increase the pH ofthe heat storage composition to at least 10.6 (expressed as the pH of a 1 % solution ofthe composition in water) is described. The addition of a caustic alkali e.g. sodium hydroxide or an alkali metal orthophosphate e.g.

trisodium orthophosphate and tripotassium orthophosphate provides a composition which is more stable on prolonged use apparently because the rate of hydrolysis ofthe 4SP is reduced.

These known heat storage compositions e.g. those which are described in European Patent 11411, and European PatentApplications 101181 and 0140467 preferably comprise a suspending agent in orderto reduce the incongruenceofthe phases during the transition between the low temperature hydrated formandthe high temperature anhydrous (or less hydrated) forms. In the absence of such a suspending agent the solid an hydrous 4SP tends to separate from the aqueous phase whilst the composition is at a temperaturewhich is above thetransition tempera- ture. The separation limitsthe ability ofthe phases to recombinetoform a single solid phasewhen the heat storage composition is cooled to a temperature below the transition temperature.

The presence of additives in the heat storage composition which do not themselves exhibit any heat storage properties (or which do possess such properties butto a lesser degreethan tetrasodium pyrophosphate decahydrate) detracts from the value of the composition in that the heat storage capacity of a unit volume of material is reduced. The need to incorporate these additives also complicates the manufacture ofthe compositions such appropriate quantity of the additives must be blended with the other ingredients in such a way that they are uniformly distributed in it.

We have now discovered a class of polymeric compounds which when added to a heat storage composition comprising 4SP act to stabilise the 4SP against hydrolysis and as suspending agents which red uces the incongruence ofthe phase transition.

The polymeric additives which have been discovered to be useful in this respect are homopolymers and copolymers of polyethylene imine which are capable of being cross-linked to form a three-dimensional matrix which will reduce the incongruence of the phases.

Accordingly from one aspect our invention provides a composition useful as a heat storage medium which comprises tetrasodium pyrophosphate decahydrate and at least one homopolymerorcopolymerof polyethylene imine.

The preferred polymeric additive for present use are homopolymers of polyethylene imine having a molecular weight of at least 10,000.

In use in the heat storage compositions those polymers are preferably crosslinked so as to form a three-dimensional matrix structure. Thus an appropriate cross linking agent such as formaldehyde, glutaraldehyde, epichlorhydrin oran epoxy resins is preferably added to the composition and the polymer cured by allowing itto age or by exposure to elevated temperatures or both as appropriate depending upon the natureofthe polymerandthechoiceofcross linking agent.

The preferred cross linking agents for use in the compositions ofthe present invention arethe epoxy resins.

The polymers are preferably cross-linked to substantiallythe maximum extent possible say at least 75% of the maximum. Thus the quantity of crosslinking agent which is used will be sufficient to achieve this degree of cross-linkage and preferably a substantial excess of cross-linking agent will be employed.

The polymer is preferably cured fora sufficienttime and under such conditions that this degree of cross- linkage will be achieved. Conveniently where crosslinking proceeds at an adequate rate at ambient temperature the polymer and cross-linking agent will be separately added to the heat storage composition and the composition allowed to age until cross-linking is at least substantially complete.

The amount of polymeric additive present in the heat storage compositions of the present invention may be from 1.0 to 10.0% by weight ofthe weight of anhydrous 4SP present. In general sufficient additive toensurethatthe pH ofthecomposition(measured as a 1% solution) is at least 10.6 preferably at least 10.7 and more preferably at least 10.9. The increase in the pH is maybe achieved solely byadding the necessary quantity of the polymeric additive but the composition may contain other additives which increase its pH e.g.

caustic alkalis such as sodium hydroxide and potassium hydroxide and alkali metal orthophosphates such astrisodium orthophosphate. In a preferred embodimentthe composition comprise at least 1% and preferably at least 3.0% of trisodium orthophosphate. The quantity of polymeric additive should also be sufficient to suppress the incongruence ofthe phase transition to the desired degree. In general a quantity of at least 3.0% by weight ofthe weight of anhydrous4SPwill besufficientto achieve this. The quantity of polymeric additive needed to provide a desired degree of stabil ity varies with the nature of the cross-linking agent.In general the use of epoxy resin cross-linking agents provides a more stable composition and inthiscasefrom4.0to 5.0% ofthe polymeric additive will typically be employed. Where other cross linking agents are used the quantity of polymeric additive needed will be increased to e.g. about 5.5 to 7.0% byweight. The incongruence cannot be measured as such but manifests itself in a reduction of the capacity of the heat storage medium caused by increasing volumes of anhydrous 4SPwhich are not forming the decahydrate when the composition is heated to a temperature above the transition temperature.The polymeric additives ofthe present invention may be used in combination with other known suspending agents but such compositions represent a less preferred aspect of the present invention since the compositions will have a lowerpH.

The pyrophosphate which is employed in the heat storage compositions ofthe present invention need notto be a pure material butthe presence of impurities which tend to promote its degradation should be avoided.

Commercial products sold as4SP are commonly produced by the calcination of disodium hydrogen orthophosphate. Such product normally contain minorquantities of other phosphatese.g. unchanged disodium hydrogen orthophosphate, trisodium orthosphosphate (carried overfromthe neutralisation of phosphoric acid to produce the sodium hydrogen orthophosphate) and the salts of higher condensed phosphoric acids especially sodium tripolyphosphate.

Typical commercial products sold as anhydrous4SP contain up to 0.5% byweightoftrisodium orthophosphate (hereinafter for convenience termed TSP).

The presence of impurities which tend to promote the degradation of the 4SP should be avoided in so far as is possible. We have discovered thatthe presence ofthesalts of highercondensed phosphoric acids especially tripolyphosphoric acid tends to accelerate the degradation ofthe pyrophosphate and accordingly pyrophosphates which contain such salts are less preferred for present use. Preferably the pyrophos phatewill contain less than 5% and more preferably less than 3% e.g. less than 1 % of tripolyphosphate.

The presence of these higher condensed polyphosphates reducesthe pH ofthecomposition and where they are present it is necessary to compensate for this by the addition of greater quantities of alkaline materials e.g. the orthophosphatesoralkali metal hydroxides.

The presence of disodium hydrogen orthosphosphate is also disadvantageous but to an lessee degree.

Preferablythe4SPwill contain less than 10% and more preferably less than 5% disodium hydrogen orthophosphate.

The heat storage compositions preferably comprise TSP in a quantity offrom 2.5 to 30.0% by weight ofthe anhydrous4Spwhich is present. The quantity of TSP which is added will preferably be sufficientto ensure thatthe pH of the composition is at least 10.6.

4SP may conveniently be manufactured by the addition of a basic derivative of sodium (which is usually sodium hydroxide) to phosphoric acid in a quantity which is sufficient to form 4SP ie. to form disodium hydrogen orthophosphate; drying the slurry or solution thus formed to produce solid disodium hydrogen orthophosphate and calcining this solid.

In orderto produce a product having a consistent pH conventional 4SP products have been produced by neutralizing the phosphoric acid with a quantity of base which is not morethan that which is required to form disodium hydrogen phosphate. The calcination of such materials produces a product comprising relatively large quantities ie. up to 5% of higher condensed phosphates and smaller quantities ie. less than 0.5% byweightoftrisodium phosphate.

The calcination may be effected by heating the dried disodium hydrogen phosphate attemperatures of from 300 to 7000Cfor periods offrom 1 to 6 hoursthe shortertimes referring to the highertemperatures. In general we prefer two utilise 4SP products prepared by calcination attemperatures offrom 400 to 6000for periods offrom 1 to4 hours in the compositions of our invention. The most preferred process comprise calcination attemperatures of from 450 to 550for periods offrom 2 to 4 hours.

The phosphoric acid from which the 4SP is derived may be thermal acid, ie. acid obtained by burning phosphorus and absorbing the phosphorus pentoxide produced in water or a dilute solution of phosphoric acid or wet process acid ie. acid obtained by the acidulation of phosphate rock or a purified acid obtained therefrom e.g. by solvent extraction or by precipitation of impurities. In general the use of thermal acid or purified wet process acid is preferred in thatthey are more pure grades of acid but wet process acid may be used. Both thermal and wet process acid contain at leasttrace amounts of metallic impurities e.g. iron, chromium manganese and copper ions butthese do not exert a significant effect upon the stability ofthe 4SPand their presence is thereby tolerable.

The 4SP product may be utilised as a component of a heat storage medium by packing it into watertight containers with at least sufficientwaterto form the decahydrate of 4SP. These containers conveniently taketheform of long cylindrical tubes which are packed with the producttoforma heat storage element. These elements may be used e.g. in a domestic central heating system wherein the element is immersed in water within a water circulation system. The packing of the tubes is preferably achieved by admixing the solid mixture of 4SP with at least sufficient water as is required to form the decahydrate of 4SP.

Heat storage elements containing the novel heat storage compositions are believed to be novel and form another aspect of our invention. Thus from another aspect the invention provides a heat storage element which comprises a heat storage composition comprising tetrasodium pyrophosphate and at least one homo or copolymer of polyethylene imine as hereinbefore described in a liquid tight container.

Heating systems which contain heat storage elements according to the invention form another aspect of the invention. Accordingly the invention provides a storage heating system comprising athermal reservoir, means for supplying heated the reservoir and meansforwithdrawing heatfromthe reservoir wherein the reservoir contains at least one heat storage element as hereinbefore described.

During the mixing of the ingredients ofthe heat storage composition the temperature of the mixture is maintained at a level which is above the transition temperature between the 4SP and its decahydrate. At these temperatures the mixture takestheform of a slurryorpastewhich can be pumped into liquid tight containers wherein it will form a dry mixture comprising 4SP decahydrate.

The polymeric additive may conveniently be added to the slurry comprising 4SP and water priorto its being packed into the heating elements. Where appropriate the polymeric additive may be produced in situ thus ensuring the intimate admixture of the 4SP and the polymer. Thus it is preferred to premix a polymer or prepolymer, any curing or cross linking agent and the 4SP with thewaterso asto form a slurry or paste which is then pumped into the heat storage elements and subsequentlyto cure the polymere.g.

by allowing itto age or by heating itto an elevated temperatu re say 40 to 70"C.

The amount of polymeric additivewhich is employed is preferably keptto a minimum in orderto maximise the heat storage capacity of a unit volume of the composition. The amount of agent required to safeguard against the incongruence ofthe phase transition varies with the nature ofthe agent.

The novel heat storage compositions may contain other additives if desired. We have discovered that the addition ofwatersoluble salts which are otherwise inert such as sodium chloride and sulphate is advan tageous in so far as it appears to reduce the rate of I hydrolysis ofthe pyrophosphate. Howeverthis addi tion reduces the heat storage capacity of the composi tion and tends to lowerthetransition temperature between the anhydrous and the hydrated phases.

The novel heat storage compositions may further ; comprises other conventional ingredients of such compositions e.g. insoluble nucleating agents selected to minimise anysupercooling e.g. borax and water miscible dispersants such as methanol or ethanol which assistthe production of a homogeneous composition.

The invention is illustrated bythefollowing exam ples: Example 1 A composition was made up as follows: 193 gum ofanhydrous4SP 101 gms de-ionisedwater 44.i8gmsofa 50% aqueous solution of polyethyleneimine having a molecularweightof40,000 15.58 gm of a 40% solution offormaldehyde The rate of hydrolysis of 4SP in this composition was studied by storing a sample in a liquid tight container at a temperature of 85"C. Samples were analysed at intervals using the auto-analyser chroma tographic method described in Greenfield and Clift in "The Analytical Chemistry of Condensed Phosphates" published by the Pergamon Press in 1975.A sample of the mixture was also tested by subjecting it to a heating/cooling cycle and measuring the temperature at which the transition between the hydrated and the anhydrous phase took place. lnthistestthe ingre dients of the composition were thoroughly mixed whilst being maintained at a temperature of at least 75"C. The resulting slurry was packed into a series of cylindrical plastic tubes which were fitted with a thermocouple and fully sealed. The tubeswere maintained ata temperature of 85 C and atset intervals a tube was allowed to cool the temperature being recorded at intervals during the cooling. The temperature ofthetubes dropped until the phase transition commenced whereupon it rose before falling away.The maximum temperature attained afterthe transition commenced was recorded as the transition temperature. The results were as follows: Days % Hydrolysis Cycles Transition Temperature 1 1.2 6 68.4 7 2.9 42 66.2 10 3.6 20 5.5 23 6.1 138 63.3 35 8.3 50 10.5 300 58.3 Example 2 The composition of Example 1 was reformulated using 17.6% epoxy resin based on PEI in place ofthe formaldehyde.The results ofthe sametestwere as follows: Days % Hydrolysis Cycles Transition Temperature 1 1.4 6 66.9 7 1.9 42 65.5 20 2.6 23 2.7 138 63.0 Example 3 The composition of Example 1 was formulated and tested using 17.6% glutaraldehyde based on PEI in place oftheformaldehyde. The results were as follows: Days % Hydrolysis Cycles Transition Temperature 1 1.4 6 68.2 7 1.7 42 66.7 20 2.1 23 2.4 138 63.2 Example 4 Theformulation of Example 1 wasformulated with the addition of 1% and 3% oftrisodium orthophosphate (which was added in the form of its dodecahydratewith a consequential reduction in the amount of waterwhich was employed). The results were as follows: Days % Hydrolysis Cycles Transition Temperature (a) 1% TSP 10 3.5 50 7.7 (b) 3% TSP 1 3.4 6 7 3.6 42 10 3.7 23 3.9 138 33 4.1 50 4.25 55 4.3

Claims (22)

1. A composition useful as a heat storage medium which comprises tetrasodium pyrophosphate and at least one polymeric additive which is a homopolymer orco-polymerof polyethylene imine.

2. Acomposition according to claim 1,characte- rised in thatthe polymeric additive is a homopolymer of polyethylene imine having a molecular weight of at least 10,000.

3. A composition according to either of claims 1 or 2, characterised in thatthe composition further comprises a cross-linking agent.

4. A composition according to claim 3, characterised in that the cross-linking agent is selected from the group comprising formaldehyde, glutaraldehyde, epichlorhydrin and epoxy resins.

5. Acomposition according to eitherof claims 3 or 4, characterised in that the quantity of cross-linking agent is at least sufficient as is required to provide at least 75% ofthe maximum possible degree of cross-linking.

6. Acomposition according to anyofthe preceding claims, characterised in thatthe polymeric additive has been cross-linked to form athree- dimensional matrix.

7. Acomposition according to claim 6,characte- rised in thatthe composition has been cured in order to achieve the maximum possible degree of crosslinkina.

8. A composition according to claim 7, characterised in thatthe composition has been cured to allowing itto age atambienttemperature.

9. A composition accordingto any ofthe preceding claims, characterised in that the polymeric additive is present in a quantity offrom 1 .Oto 10.0% by weight ofthe weight of tetrasodium pyrophosphate (expressed as the an hydros material).

10. Acomposition according to claim 9,characte- rised in that the cross-linking agent is an epoxy resin and the quantity of polymeric additive isfrom 4.0to 5.5% by weight of the weight of tetrasodium pyrophosphate.

11. A composition according to any ofthe preceding claims, characterised in thatthe pH ofthe composition (measured as a 1 % solution) is at least 10.6.

12. Acomposition according to claim 11, characterised in thatthe pH ofthe composition is at least 10.9.

13. Acomposition according to any of the preced- ing claims, characterised in thatthe composition comprises at least 1 % byweightoftrisodium orthophosphate.

14. Acompositionaccordingtoclaim 13, char- acterised in that it comprises at least 3% by weight of trisodium orthophosphate.

15. Acomposition according to any of the preced- ing claims, characterised in that it comprises from 2.5 to 30.0% by weight oftheweight of anhydrous tetrasodium pyrophosphate oftrisodium orthophosphate.

16. A composition according to anyofthe preceding claims characterised in that the composition comprises less than 5% by weight ofthe weight of tetrasodium pyrophosphate of sodium tripolyphosphate.

17. Acomposition according to claim 16, char- acterised in that it comprises less than 1 % by weight of sodium tripolyphosphate.

18. A composition according to any ofthe preceding claims, characterised in that it comprises less than 10% by weight of the weight of tetrasodium pyrophosphate of disodium hydrogen orthophosphate.

19. A composition according to claim 18, characterised in that it comprises less than 5% by weight of disodium hydrogen orthophosphate.

20. A heat storage element which comprises a composition comprising tetrasodium pyrophosphate and at least one homopolymer or co-polymer of polyethylene imine according to any ofthe preceding claims in a liquid-tight container.

21. Astorage heating system comprising a ther- mal reservoir, means for supplying heat to the reservoir and means for withdrawing heatfrom the reservoir, characterised in that the reservoir contains at least one heat storage element according to claim 20.

22. A heat storage composition according to claim 1 substantially as hereinbefore described with reference to the foregoing examples.