AU640154B2

AU640154B2 - Calcium chloride hexahydrate formulations for low temperature heat storage applications

Info

Publication number: AU640154B2
Application number: AU59275/90A
Authority: AU
Inventors: Aharon Brandstetter; Stephen Kaneff
Original assignee: Australian National University
Current assignee: Anutech Pty Ltd
Priority date: 1989-06-23
Filing date: 1990-06-22
Publication date: 1993-08-19
Anticipated expiration: 2010-06-22
Also published as: AU5927590A

Description

TITLE: "CALCIUM CHLORIDE HEXAHYDRATE FORMULATIONS

FOR LOW TEMPERATURE HEAT STORAGE APPLICATIONS"

Technical Field

This invention concerns heat storage systems. More particularly, it concerns phase change materials based upon calcium chloride hexahydrate for use in thermal storage systems (such as low energy greenhouses) .

Background Low temperature heat storage systems have been the subject of considerable development over the years. For some years, work was directed primarily to improving rock bed regenerative heating systems, such as the system described by C D Baird, W E Waters and D R Mears in their paper entitled "Greenhouse solar heating system utilizing underbench storage", which was published in 1977 Annual Meeting of the American Society of Agricultural Engineers, North Carolina State University, June 1977, pages 1 to 18. Rock bed systems, however, are bulky and awkward to assemble, and most recent development of low temperature heat storage systems has been concentrated on those systems which incorporate phase change materials operating at temperatures around 30 C. Such phase change materials absorb heat from the environment when they change from their solid phase to their liquid phase and they release the latent heat of fusion when they solidify again as their temperature is lowered. The phase change material first used in low energy heat storage systems was Glauber's salt, sodium sulphate decahydrate (Na₂S0₄.lOH-0) , which has a phase change temperature of about 32 C. However, as noted by Charles Stein in the specification of his International patent application No PCT/US84/01005 (WIPO publication No WO 85/00212), sodium sulphate decahydrate changes its composition when cycled through a number of phase changes, and it exhibits a strong "undercooling" (called "supercooling" by some workers in this field) before it solidifies spontaneously. Undercooling by as much as 11 C is reported by Stein in his specification. This undercooling problem can be overcome by the addition of a nucleating agent (borax) and a thickening agent (fine silica) to the sodium sulphate decahydrate. Stein also observes that phase change materials based on calcium chloride hexahydrate, CaCl_?.6H_0, are now preferred. However, his own- invention, aimed at avoiding the known problems of sodium sulphate decahydrate, involves the use of another phase change material - paraffin wax - in small structures with metal wool dividers.

The use of small cells filled with a phase change material to avoid precipitation problems has also been proposed by Mario Stiffler for the latent heat accumulator described in the specification of his Australian patent No 559,354. Among Stiffler's preferred phase change materials are sodium sulphate decahydrate, bisodium phosphate dodecahydrate and bisodium phosphate heptahydrate.

The storage of phase change materials in small parcels - or microencapsulation of such materials - was a solution adopted by other workers and referred to by B Carlsson, H Stymne and G Wettermark in their paper entitled "An incongruent heat-of-fusion system - Ca.Cl~ . 6E~0 - made congruent through modification of the chemical composition of the system", which was published in Solar Energy, volume 23, 1979, pages 343 to 350. Carlsson et a_l, however, explained the physical chemistry involved when the phase change material calcium chloride hexahydrate is cycled repeatedly through its melting/freezing point, and showed that the formation of the tetrahydrate CaCl_.4H₂0 can be inhibited by the inclusion of up to 2 per cent of strontium chloride hexahydrate (SrCl_.6H_0) , which acts as both a nucleating agent (thus reducing the supercooling tendency) and as an additive that raises the solubility of the tetrahydrate while lowering the solubility of the hexahydrate, thereby preventing the tetrahydrate from solidifying as the temperature decreases towards the freezing point of the calcium chloride hexahydrate. Carlsson et j also observed that impurities in technical grade calcium chloride hexahydrate, such as sodium chloride and potassium chloride, increase the incongruity of the CaCl-.βH-O system, but their effect can be countered by the addition of calcium hydroxide (Ca(OH)₂).

A more recent paper by H Feilchenfeld, J Fuchs and S Sarig, entitled "A calorimetric investigation of the stability of stagnant calcium chloride hexahydrate melt", published in Solar Energy, volume 30, 1984, pages 779 to 784, has also emphasised the degradation of CaCl₂.6H₂0 without additives as a heat storage phase change material due to its "undercooling" tendency and its breakdown with the formation of the tetrahydrate. This paper also draws attention to the use of additives to overcome these problems, notably strontium chloride hexahydrate (SrCl₂.6H₂0) as a nucleating agent and fumed silica as a thickening agent.

The use of such phase change materials in buildings has been suggested on a number of occasions. Some workers have recognised the problems inherent in the use of phase change materials and have suggested techniques or special arrangements to overcome the problems. Others have tended to ignore the problems. Examples of proposals involving the use of phase change materials in buildings include (i) the specification of Australian patent application No 47850/85 in the name of R K Prudhoe (a proposal for controlling the temperature fluctuations in buildings which contain electronic equipment and the like); (ii) the specification of Australian patent application No 49046/85 in the name of Kubota Ltd, which describes a greenhouse in which a phase change material is stored in a tank structure; and (iii) the paper by A Brandstetter, entitled "Phase change storage for greenhouses", published in Advances in Solar Energy Technology (Pergamon Press, 1988), pages 3353 to 3357, which describes a low energy greenhouse in which the heat storage medium is calcium chloride hexahydrate "appropriately formulated against supercooling and degradation".

In the above-mentioned specifications and papers, a number of different phase change materials have been proposed for use in a variety of situations. Thus it is clear that the concept of the use of phase change materials as low temperature heat storage media, in greenhouses and other buildings, in heat pumps, in solar energy storage tanks and in industrial waste heat utilisation facilities, is now well accepted. (This list is not exhaustive.) However, the production of a satisfactory phase change material, which can be cycled numerous times through the melting and freezing point, has posed many problems to researchers in this field.

The specification of Australian patent application No 55769/86 describes a number of phase change materials which have been investigated by N Yano, T Ueno and S Tsuboi. The preferred composition disclosed in that specification is a calcium chloride hexahydrate with additives including up to 5 per cent barium sulphide, from 0.001 to 5 per cent barium chloride dihydrate and from 0.001 to 0.1 per cent strontium chloride hexahydrate, with a bromide (potassium bromide, sodium bromide or ammonium bromide) added as a solidification point modifier and relatively large quantities of ultrafine silica powder and glycerine added as thickening agents. With such additives, however, the phase change material becomes a costly material to produce.

Disclosure of the Present Invention

It is an object of the present invention to provide a relatively low cost formulation of a phase change material which can be used successfully for numerous melting and freezing cycles without significant departure from its performance as a heat storage medium.

This objective is achieved by the inclusion of selected quantities of specific additives to calcium chloride hexahydrate. The additives are (a) strontium chloride hexahydrate, as a nucleator, in quantities upward from 0.1 per cent (by weight) of the calcium chloride hexahydrate, (b) fumed silica in quantities ranging from 0.02 per cent to 1.0 per cent (by weight) of the calcium chloride hexahydrate, and (c) extra water above the stoichiometric quantity included in the calcium chloride hexahydrate in the range of from 1.0 per cent to 5.0 per cent (by weight) of the calcium chloride hexahydrate. In addition, from 0.001 per cent to 1.0 per cent (by weight) of sodium chloride may also be added.

Preferably, the strontium chloride hexahydrate is present in quantities ranging from 0.1 per cent to 4.0 per cent (by weight) of the calcium chloride hexahydrate. More preferably the upper concentration of the strontium chloride hexahydrate is about 2.0 per cent, and most preferably the strontium chloride hexahydrate comprises about 0.3 per cent (by weight) of the calcium chloride hexahydrate.

The selection of these additives and the ranges of their concentrations to produce compositions which are stable phase change materials is the outcome of a long-term investigation of the performance parameters of calcium chloride hexahydrate phase change materials in melt/freeze cycling experiments.

Strontium chloride hexahydrate is known to be isomorphous with calcium chloride hexahydrate and to be capable of forming nearly ideal solid solutions with CaCl₂.6H₂0; and is also known to be a nucleator of the solidification of calcium chloride hexahydrate. The investigation showed that the minimum amount of strontium chloride hexahydrate that is required to sustain long-term nucleation stability is 0.1 per cent. Any lower concentration of strontium chloride hexahydrate is close to the limit of dissolution of SrCl-.δH-O in calcium chloride hexahydrate. An increase in the concentration of SrCl_.6H₂0 to more than about 1.0 per cent (by weight) of the calcium chloride hexahydrate results in little significant improvement in the performance or in the long-term stability of the heat storage phase change material formulation. Increasing the concentration of SrCl₂.6H₂0 to more than 2.0 per cent of the CaCl_.6H₂0 produces no improvement in performance of the phase change material but it adds significantly to the cost of the formulation. At the time of writing this specification, in Australia, strontium chloride hexahydrate costs about $20.00 per kilogram whereas calcium chloride hexahydrate costs about $0.20 per kilogram. At concentrations of greater than about 4.0 per cent by weight of the CaCl_?.6H₂0, the inert nature of the strontium chloride hexahydrate, with its lower thermal capacity than that of the CaCl₂.6H_0, is expected to reduce the efficacy of the phase change material formulation, and this factor, combined with the high cost of SrCl_.6H₂0, sets a practical upper limit to the concentration of strontium chloride hexahydrate.

It has also been found that, to produce a phase change material based on calcium chloride hexahydrate which possesses long term stability, only a small quantity of fumed silica is required as a thickener. Using fumed silica marketed under the trade mark CAB-O-SIL by Cabot Chemical Company, the above-mentioned investigation showed that a quantity of at least 0.02 per cent (by weight) was required to ensure long-term, multi-cycle stability of the phase change material, but a concentration in excess of 1.0 per cent (by weight) added unnecessarily to the cost of the phase change material, with no improvement in stability or other performance parameter.

The use of "extra water" (that is, water in excess of the quantity required stoichiometrically for the hexahydrate formulation) was not contemplated in the disclosures in the aforementioned specification of Australian patent application No 55769/86. The use of "extra water" has been proposed in relation to sodium sulphate decahydrate and some other salt hydrates by S Furbo in the article entitled "Heat Storage Units Using Salt Hydrates", which was published in Sunworld, volume 6, No 5, pages 134 to 139, 1982. In the context of calcium chloride hexahydrate, however, water in excess of the quantity required stoichiometrically was not proposed in this paper. The investigation by the present inventors showed that the extra water is required to ensure the long-term stability of the phase change material. The minimum quantity of extra water is 1.0 per cent (by weight), which corresponds to a degree of hydration of 6.123, and the maximum quantity of extra water is about 5.0 per cent, which corresponds to a degree of hydration of 6.61, which was determined on the basis of storage efficiency considerations. Variations to the basic formulation of the present invention are possible. For example, as already indicated, sodium chloride is preferably included in concentrations of from 0.001 per cent to 1.0 per cent (by weight). Indeed, sodium chloride has been an implicit additive in most of the prior art formulations of phase change materials based on calcium chloride hexahydrate, • for technical grade CaCl₂.6H₂0 has sodium chloride as one of its impurities. Up to approximately 0.4 per cent (by weight) of sodium chloride can form a solid solution with calcium chloride hexahydrate in the temperature range in which phase change materials are normally used. The preferred addition of sodium chloride is in the range of from 0.2 per cent to 1.0 per cent (by weight) or the calcium chloride hexahydrate.

It is also advantageous, in some circumstances, to add up to 10 per cent (by weight) each of ammonium chloride and potassium chloride to the formulation of the present invention, to reduce the melt/freeze transition temperature of the formulation.

The preferred formulation of the present invention comprises calcium chloride hexahydrate to which has been added (a) about 0.3 wt per cent strontium chloride hexahydrate;

(b) about 0.1 wt per cent fumed silica;

(c) about 1.5 wt per cent extra water; and

(d) about 0.4 wt per cent sodium chloride; - li ¬

the percentages being relative to the calcium chloride hexahydrate.

Such a formulation has a solid/liquid transition temperature of 29.6 JL 0.2 C. This transition temperature can be reduced down to about 22 C by the addition of up to 10 wt per cent each of ammonium chloride and potassium chloride.

Most inorganic phase change materials have a light colour. The formulations of the present invention which have been discussed above have a light colour in the solid state and are colourless in the liquid state. Thus those formulations, like the other phase change materials, are not good absorbers of radiant energy. Indeed, most phase change materials used in the past have been stored in opaque containers and the heat transfer to, and from, the phase change materials has occurred by conduction.

It has now been discovered that improved heat transfer to and from phase change materials generally (including the formulations of the present invention) can be achieved by colouring the materials so that they have a dark colour, preferably black, and by holding the materials in transparent containers, such as containers made from glass or perspex.

Thus an optional, but preferred, variation of the phase change material formulation of the present invention is the addition of a chemically inert colouring agent. The presence of a colouring agent (preferably a black, or at least a dark, colouring agent) has been found to improve the ability of the phase change material to absorb and release heat by radiation.

A convenient technique for tinting the phase change material is to mix black drawing ink into the formulation, using ultrasonic activation to ensure a substantially uniform distribution of colour within the material.

To demonstrate the effectiveness of this modification of phase change materials, 60 grams of a calcium chloride hexahydrate formulation containing additives against incongruent melting and supercooling, in accordance with the present invention as described above, was placed in an 80 ml glass jar. 0.12 gram (0.2 wt per cent) of ROTRING^" (trade mark) black drawing ink was added to the sample of the formulation. The formulation and the glass jar were held in warm water for 5 minutes in a 3-litres tank of an ultrasonic cleaner (model FX 10 having an output of 100 watts at 40 kHz). The resultant ultrasonic activation distributed the ink uniformly throughout the phase change material.

The phase change material containing the black drawing ink was then subjected to freezing (at about 10°C), then melting (at about 45°C). The formulation was then held in its molten state for several days. At the end of this period, no segregation of the ink from the other components was observed.

Samples of the tinted and untinted phase change materials (of otherwise identical formulations) were placed in identical transparent containers and the containers were exposed to full solar radiation. The tinted (black) formulation consistently melted in less than one third the time taken for the untinted samples to melt. Measurements of the temperatures of the formulations during heating showed that the dark pphhaassee cchhaannggee mmaatteerriiaall wwaass uupp to 8 C hotter than the untinted control formulation.

In over 20 melt/freeze cycles, the tinted formulation has shown no indication of deterioration in its performance as a heat storage medium.

It will be apparent that a tinted phase change material is particularly suitable for use within greenhouses, where it will be able to be exposed to radiant energy when stored in a transparent container.

To test the formulations of the present invention, continuous calorimetric measurements of a range of formulations have been made over a period of several years. Samples of formulations of the present invention, and some samples of phase change materials not in accordance with the present invention, typically 0.4 kg in weight, have been subjected to daily (and sometimes more frequent) melting and freezing cycles. The behaviour of the materials has been monitored comprehensively and recorded. This testing has shown that formulations in accordance with the present invention have not deteriorated during the test period, and their heat storage capacity has remained substantially constant within statistically reasonable limits. One sample of a formulation in accordance with the present invention, for example, has retained its heat storage capacity at around 200 kJ/kg over 1000 melt/freeze cycles, with no indication of a deterioration in performance.

In another experiment, some 300 kg of calcium chloride phase change material, held in 6 litre plastic containers, was used as the basis of an off peak heating system for a laboratory. In this experiment, the phase change material was heated with off-peak electricity and, with the aid of a water circulation heat transport system, delivered its stored heat when required. At the time of writing this specification, the material is still in satisfactory working order in the fourth winter season of the use of the heating system.

Phase change materials having untinted formulations in accordance with the present invention have also been tested in a low energy greenhouse mounted on a roof of a building of The Australian National University, in Canberra, Australia. In fact, one of the formulations of the present invention was the calcium chloride hexahydrate "appropriately formulated against supercooling and degradation" used to obtain the experimental data reported in the aforementioned paper by A Brandstetter, entitled "Phase change storage for greenhouses".

Claims

1. A phase change material comprising calcium chloride hexahydrate containing the additives strontium chloride hexahydrate and fumed silica, characterised in that:

(a) the quantity of strontium chloride hexahydrate added is at least 0.1 per cent by weight of the calcium chloride hexahydrate;

(b) the fumed silica additive comprises from 0.02 per cent by weight to 1.0 per cent by weight of the calcium chloride hexahydrate; and

(c) water in excess of the stoichiometric quantity included in the hexahydrates is also added, the added water comprising from 1.0 per cent by weight to 5.0 per cent by weight of the calcium chloride hexahydrate.

2. A phase change material as defined in claim 1, in which the strontium chloride hexahydrate additive is present in the range of from 0.1 to 4.0 per cent by weight of the calcium chloride hexahydrate.

3. A phase change material as defined in claim 2, in which the strontium chloride hexahydrate additive is present in the range of from 0.1 to 2.0 per cent by weight of the calcium chloride hexahydrate.

4. A phase change material as defined in claim 3, in which the strontium chloride hexahydrate additive comprises about 0.3 per cent by weight of the calcium chloride hexahydrate.

5. A phase change material as defined in any preceding claim, further characterised in that the phase change material includes added sodium chloride, the added sodium chloride comprising from 0.001 per cent by weight to 1.0 per cent by weight of the calcium chloride hexahydrate.

6. A phase change material as defined in claim 1, in which

(i) the added strontium chloride comprises about 0.3 per cent by weight of the calcium chloride hexahydrate;

(ii) the added fumed silica comprises about 0.1 per cent by weight of the calcium chloride hexahydrate;

(iii) the excess water over the stoichiometric quantity included in the calcium chloride hexahydrate comprises about 1.5 per cent by weight of the calcium chloride hexahydrate; and

(iv) sodium chloride is also added, the added sodium chloride being about 0.4 per cent by weight of the calcium chloride hexahydrate.

7. A phase change material as defined in any preceding claim, to which has been added up to 10 per cent by weight of ammonium chloride.

8. A phase change material as defined in any preceding claim, to which has been added up to 10 per cent by weight of potassium chloride.

9. A phase change material as defined in any preceding claim, further characterised in that a chemically inert colouring agent has also been added.

10. A phase change material as defined in claim 9, in which the colouring agent is a dark colouring agent.

11. A phase change material as defined in claim 9, in which the colouring agent is black drawing ink.

12. A phase change material as defined in claim 11, in which the colouring agent comprises about 0.2 per cent by weight of the phase change material prior to the addition of the colouring agent.

13. A phase change material as defined in any one of claims 9 to 12, in a transparent container.