FOR LOW TEMPERATURE HEAT STORAGE APPLICATIONS"

Technical Field

This invention concerns phase change materials for use in heat storage systems. More particularly, it concerns phase change material formulations based on (i) Glauber's salt (sodium sulphate decahydrate - Na ₂ SO ₄ .10H ₂ O), and (ii) sodium acetate trihydrate (CH ₃ C0 ₂ Na.3H ₂ 0). The former have a solid/liquid transition temperature in the range of from 5°C to 15°C. The latter have a solid/liquid transition temperature in the range of from 50 ^β C to 58°C.

Background

Low temperature heat storage systems have been the subject of considerable development in recent years. For some time, work in this field 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 regenerative heating systems, however, are bulky and awkward to assemble, and the most recent developments in the field of low temperature heat storage systems have concentrated on those systems which incorporate phase change materials which have a solid/liquid transition temperature of about 30 ^β C. Such phase change materials absorb heat from the environment when they change from their solid phase to their liquid

phase and they -eleaε^ the latent heat of fusion when they solidify again as the r emperature is lowered.

The phase change material first used in low energy heat storage systems was Glauber's salt, sodium sulphate decahydrate (Na ₂ S0 ₄ .10H ₂ O), which has a phase change temperature of about 32°C. Glauber's salt has two features which make it attractive for commercial heat storage systems, namely, it has a high latent heat of fusion (about 250 kJ/kg) and a low cost.

The use of Glauber's salt for heat storage purposes was first proposed (it is believed) by Maria Telkes in 1954 in the specification of her US patent No 2,677,664. Subsequently, a substantial amount of theoretical and experimental work was undertaken in order to produce improved formulations of high latent heat capacity, based on sodium sulphate decahydrate. Examples of such work are reported in the papers by

(a) S B Marks, entitled "The effect of crystal size on the thermal energy storage capacity of thickened Glauber's salt", which appeared in Solar Energy. Volume 30, pages 45-49, 1983;

(b) M Telkes, entitled "Solar energy storage", which was published in ASHRAE Transactions 80. Part 2, page 38, 1974; (c) M Telkes, entitled "Latent heat storage technology", which formed part of the Symposium on Energy from the Sun, held by the Institute of Gas Technology (USA), April 1978;

(d) A D Solomon, entitled "Melt time and heat flux for a simple PCM body", which was published in Solar Energy. Volume 22, page 251, 1979; and

(e) H G Lorsch, K W Kaufman and J C Denton, entitled "Thermal energy storage for solar heating and off-peak air conditioning", which was published in Energy Conversion. Volume 15, pages 1 to 8, 1975.

These publications have shown that, despite the advantages of relatively low cost and high latent heat of fusion of sodium sulphate decahydrate, all formulations based on this material suffer from incongruent melting. Another problem with the use of Glauber's salt as a phase change material was commented upon by Charles Stein in the specification of his International patent application No PCT/US84/01005 (WIPO Publication No WO 85/00212). That problem is that sodium sulphate decahydrate changes its composition when cycled through a number of phase changes, and this change in composition results in a strong "undercooling" (called "supercooling" by some workers in this field) before the formulation solidifies spontaneously. Undercooling by as much as 11 ^β C is reported by Charles Stein in WIPO Publication No WO 85/00212. Although he asserts that this undercooling problem can be overcome by the addition of fine silica and a thickening agent to the sodium sulphate decahydrate, Charles Stein concludes, in WIPO Publication No WO 85/00212, that phase change materials based on calcium chloride hexahydrate (CaCl ₂ .6H ₂ 0) are now preferred over those based on Glauber's salt. Incidentally, Stein's own invention, which allegedly avoids the known problems associated with the use of sodium sulphate decahydrate as

a phase change material, involved the use of a panel-like energy storage str sture with horizontal dividers, preferably made from metal wool.

Thus the opinion, now held by many workers in this field, of the value of Glauber's salt as a basis for phase change materials is as stated in the aforementioned paper by Lorsch et al;

"sodium sulphate decahydrate mixtures should no longer be considered as high quality storage materials for heating and cooling of buildings".

The more recent work on phase change materials based on calcium chloride hexahydrate has been described by Stephen Kaneff and Aharon Brandstetter in the specification of their International patent application No PCT/AU90/00264 (WIPO Publication No WO 91/00324). The contents of WIPO Publication No WO 91/00324, including the International Search Report that forms part of WIPO Publication No WO 91/00324, are incorporated into this specification by this reference thereto.

Phase change material formulations based on calcium chloride hexahydrate, such as those described and claimed in WIPO publication No WO 91/00324, are particularly useful for the control of the temperature within structures such as greenhouses (glasshouses) and electronic equipment shelters, which should not significantly exceed about 30°C. However, it has long been recognised that phase change materials which function effectively at temperatures which

are considerablys lower, or higher, than 30°C, would be useful for other applications.

Formulations for phase change materials which have a solid to liquid transition temperature in the range of from 5°C to 15°C have been proposed. Such formulations have been based on organic compounds such as paraffins and olefins, clathrate and semi-clathrate hydrates, and also sodium sulphate decahydrate containing additives which lower its melting point. The organic materials tried were all unsatisfactory, for one or more reasons (the flammability of the material being a problem in some instances). The clathrate and semi-clathrate hydrates have solid to liquid transition temperatures in the desired region, but a heterogeneous nucleating agent which ensures an essentially constant phase change temperature after repeated freeze/melt cycles has yet to be discovered. And, as noted above, there has been very little success with phase change materials based on sodium sulphate decahydrate, at any phase change temperature.

In the field of higher temperature phase change materials, sodium acetate trihydrate has been known as a potentially useful material for more than a decade. This material is a relatively low cost material which has a high latent heat of fusion (about 260 kJ/kg, or 340 kJ/litre) and a solid/liquid transition temperature of about 58 ^β C. Unfortunately, in common with most phase change materials, it suffers from incongruent melting.

One of the first nucleators of crystallisation that was used in an attem to avoid the problem of incongruent melting of sodium acetate trihydrate was sodium carbonate decahydrate (Na ₂ CO ₃ .10H ₂ O). Although P F Barnett and B R Best, in their paper entitled "Supercooled mixtures with Na ₂ S ₂ 0 ₃ .5H ₂ 0", which was published in Materials Chemistry and Physics. Volume 12, page 529, 1985, reported that Na ₂ CO ₃ .10H ₂ O acts as an effective nucleator, it was later shown that after repeated cycling of formulations comprising sodium acetate trihydrate and sodium carbonate decahydrate through melting and freezing, the nucleation effect ceased. This was apparently due to the self-decomposition of the sodium carbonate decahydrate, to form anhydrous sodium carbonate. Other nucleators reported to be effective in ensuring congruent melting of CH ₃ C0 ₂ Na.3H ₂ 0 (see the paper by A Ulman and B Valentin, entitled "Investigations of sodiur acetate trihydrate for solar latent heat storage; controlling the melting point", which was published in Solar Energy Materials. Volume 9, pages 177 to 181, 1983) are 2-4 dinitrobenzoic acid and gum arable. Even if these chemicals do produce, with sodium acetate trihydrate, a phase change material with long term congruent melting, the cost of producing the formulations would mean that they are uneconomic for use in space heating (for example, in greenhouse heating), unless the dosages of the additives are minimal.

Other reported nucleating catalysts for sodium acetate trihydrate are anhydrous disodium hydrogen phosphate and tetrasodium pyrophosphate decahydrate. These nucleators are referred to by

(a) T Wada and R Yamamoto, in their paper entitled "Studies on salt hydrates for latent heat storage. 1. Crystal nucleation of sodium acetate trihydrate catalyzed by tetrasodium pyrophosphate decahydrate", which was published in the Bulletin of the Chemical Society of Japan. Volume 55, page 3603, 1982;

(b) T Wada, R Yamamoto and Y Matsuo, in their paper entitled "Heat storage capacity of sodium acetate trihydrate during thermal cycling", which appeared in Solar Energy. Volume 33, pages 373 to 375, 1984; and

(c) H Kimura, in the paper entitled "Nucleating agents for sodium acetate trihydrate", which was published in the Journal of the Japanese Association of Crystal Growth. Volume 9, issue 3, page 73, 1982.

However, in a more recent review article by J Guion and M Teisseire, entitled "Nucleation of sodium acetate trihydrate in thermal heat storage cycles", which was published in Solar Energy, Volume 46, pages 97 to 100, 1991, the conclusion reached, after considering the various proposed nucleators, is that the active nucleating catalyst species, if any, may not be easily identified, and that in further studies of the stability of formulations on cycling, identification of the solid phases in equilibria is necessary.

Thus it is the opinion of many workers in this field that formulations based on sodium acetate trihydrate are not suitable as practical phase change materials.

Disclosure of the Present Invention

It is an object of a first aspect of the present invention to provide a phase change material formulation which has a solid/liquid transition temperature in the range of from 5°C to 15 ^β C, which is stable and does not exhibit incongruent melting with repeated cycling through the solid/liquid transition temperature, and which is economical to produce.

It is an object of a second aspect of the present invention to provide a phase change material formulation which has a solid/liquid transition temperature of about 58°C, which is stable and does not suffer incongruent melting with repeated cycling through the solid/liquid transition temperature, and which is economical to produce.

The first of these objectives is achieved by providing a novel formulation based upon Glauber's salt, which includes compounds that (i) reduce the solid/liquid transition temperature to a temperature in the range of from 5°C to 15°C, and (ii) nucleate the crystallisation of the sodium sulphate decahydrate. In addition, a compound which effectively encapsulates the active formulation in an essentially solid matrix to prevent incongruent melting is added.

In particular, the first aspect of the present invention provides a phase change material having a formulation such that the material changes from a solid phase to a liquid phase at a temperature in the range of from 5 ^β C to 15 ^β C, said formulation comprising an intimate mixture of sodium

sulphate decahydrate, borax (sodium tetraborate decahydrate), fumed silica, ammonium chloride, potassium chloride, and water in excess of the stoichiometric quantity included in the hydrates, to form an active formulation, and also calcium sulphate to provide an essentially solid matrix, in the following proportions:

(a) sodium sulphate decahydrate : 100 units by weight;

(b) borax (sodium tetraborate decahydrate) : from 0.1 to 5.0 units by weight; (c) fumed silica : from 0.1 to 5.0 units by weight;

(d) ammonium chloride : from 35.0 to 5.0 units by weight;

(e) potassium chloride : from 5.0 to 35.0 units by weight;

(f) calcium sulphate : from 10.0 to 40.0 units by weight;

(g) excess water : from 2.0 to 25.0 units by weight.

The second of the above-noted objectives is achieved by providing a phase change material having a formulation based on sodium acetate trihydrate. This formulation includes tetrasodium pyrophosphate decahydrate (Na ₄ P ₂ O ₇ .10H ₂ O), fumed silica, and water in excess of the stoichiometric quantity required for the hydrates.

Thus, according to the second aspect of the present invention, there is provided a phase change material having a solid to liquid transition temperature of about 58 ^β C, which comprises an intimate mixture of sodium acetate trihydrate, tetrasodium pyrophosphate decahydrate, fumed silica, and water in excess of the stoichiometric quantity required for the hydrates, in the following proportions:

(a) sodium acetate trihydrate : 100 units by weight;

(b) tetrasodium pyrophosphate decahydrate : from 0.1 to 10.0 units by weight;

(c) fumed silica : from 0.1 to 5.0 units by weight;

(d) excess water : from 0.05 to 0.6 mole per mole of the sodium acetate trihydrate.

The solid to liquid transition temperature of the formulation of the second aspect of the invention may be lowered by the addition of either urea or lithium acetate dihydrate.

The way in which the phase change material formulations of the first and second aspects of the present invention were developed and tested will now be described.

Detailed Description of the Development and Testing of the Present Invention

The present inventors designed and built test facilities for evaluating the performance of phase change materials. The most recent version of these test facilities consisted of three water baths, each containing a heater, a cooler, stirrers, four calorimeters and four cells. Samples of phase change materials were placed in the cells and were daily subjected to either two or four heating and cooling cycles, through the expected solid/liquid transition temperature of the formulations. The operation of heaters and coolers, to control the temperatures of the water baths, was automated. A Chessell 4500 data acquisition system (incorporating a Chessell 4001 multi-channel recorder) was used to obtain a complete record of the parameters monitored during the experiments.

Using these test facilities, the inventors investigated the performance of a wide range of phase change materials. The data obtained from the early formulations tested provided an indication of the most promising phase change material formulations. Further experiments with modified versions of the promising formulations were undertaken so that the most useful formulations could be identified and tested thoroughly.

As a consequence of this experimentation, it was shown that, for materials having a solid/liquid transition temperature in the range of from 5°C to 15 ^β C, the use of gels and gum-like materials to thicken the formulations does not prevent incongruent melting, but the addition of fumed silica and water in excess of the stoichiometric quantity required for the hydrate, and the inclusion of sufficient calcium sulphate to form, in effect, a solid matrix within which the formulation is supported, does avoid this problem. The fumed silica used was that marketed under the brand name CAB-0-SIL.

After detailed experimentation, the present inventors determined a range of formulations, in both the "low" and "high" solid/liquid transition temperature ranges, which could be cycled through melting and solidification more than 500 times with no apparent change in their properties. In the "low temperature range" of from 5 ^β C to 15 ^β C, the formulations which make apparently reliable phase change materials were found to have the following compositions:- Glauber's salt (Na ₂ S0 ₄ .10H ₂ 0) : 100 units by weight; borax (sodium tetraborate decahydrate - Na ₂ B ₄ O ₇ .10H ₂ O) : 0.1 to 5.0 units by weight;

fumed sil- * (CAB-O-SIL brand)

0.1 to 5.0 units by weight; ammonium chloride 35.0 to 5.0 units by weight; potassium chloride 5.0 to 35.0 units by weight; calcium sulphate 10.0 to 40.0 units by weight; and water (in excess of the stoichiometric quantity required for the hydrates)

: 2.0 to 25.0 units by weight.

Within this range of "low temperature" phase change material formulations, the preferred formulation (having a solid/liquid transition temperature of about 9°C) comprises:

Glauber's salt - 100 units by weight borax - 2 units by weight fumed silica - 1 unit by weight ammonium chloride - 25 units by weight potassium chloride - 10 units by weight calcium sulphate - 20 units by weight excess water - 10 units by weight.

At the time of writing this specification, one sample of this preferred formulation has undergone more than 1,000 solid/liquid/solid cycles, without apparent variation in performance. However, the present inventors are not able to predict the useful lifetime of "low temperature" phase change material formulations which are in accordance with the first aspect of the present invention.

In the "high temperature" solid/liquid transition range of about 58°C, the effective (long term) phase change materials were found to have formulations based on sodium acetate trihydrate, withe;the addition of tetrasodium pyrophosphate decahydrate, fumed silica, and water in excess of the stoichiometric quantity required for the hydrates in the formulation. The most reliable formulations were those in the following range of compositions: sodium acetate trihydrate (CH ₃ C0 ₂ Na.3H ₂ 0)

: 100 units by weight; tetrasodium pyrophosphate decahydrate (Na ₄ P ₂ O ₇ .10H ₂ O)

: 0.1 to 10.0 units by weight; fumed silica : 0.1 to 5.0 units by weight; and excess water : 0.05 to 0.60 mole per mole of the sodium acetate trihydrate.

Within this range of compositions, the preferred formulation comprises: sodium acetate trihydrate - 100 units by weight;

Na ₄ P ₂ 0 ₇ .10H ₂ 0 - 2 units by weight; fumed silica - 2 units by weight' excess water - 0.27 mole per mole of sodium acetate trihydrate.

At the time of writing this specification, one sample of this preferred formulation based on sodium acetate trihydrate has undergone more than 500 solid/liquid/solid cycles, without any apparent variation in performance. However, the present inventors are not able to predict the

useful lifetime of "high temperature" phase change material formulations which are in accordance with the second aspect of the present invention.

The experiments conducted by the present inventors also showed that a progressive reduction below 58°C of the solid/liquid transition temperature of the "high temperature" phase change material formulations can be obtained by adding either urea or lithium acetate dihydrate to the formulations. Such a lowering of the solid/liquid transition temperature was noted by Ul an and Valentin in their aforementioned 1983 paper. Ulman and Valentin, however, were unable to prevent supercooling of their phase change materials. In the present invention, the addition of urea or lithium acetate dihydrate can be used to lower the melting point of the formulation to below 50 ^β C without the onset of supercooling or incongruent melting. With the addition of up to 10 per cent (by weight) of lithium acetate dihydrate, the lowering of the solid/liquid transition temperature is linear with respect to the quantity added.

The fumed silica in all of the formulations of the present invention appears to act as a nucleator. The quantities of fumed silica that are used would not thicken the liquid phase of the formulations. If the fumed silica is omitted from the formulations based on sodium acetate trihydrate, the liquid/solid transition temperature varies widely and sporadically. The addition of the fumed silica completely avoids this behaviour, and produces a stable and reliable phase change material.

Samples of the phase change material formulations of the present invention have been encapsulated and used successfully in trials of the encapsulated materials.

High-molecular-density polyethylene (HMDPE) tubes with end caps which were subsequently welded to the tubes were first used to encapsulate the new formulations. These tubes have an outside diameter of 40 mm, an inside diameter of 36 mm and a length of 1 metre. These tubes comply with the encapsulation requirements of Telecom Australia for phase change materials for use in equipment shelters.

Other forms of encapsulation which were successfully tested were (i) encapsulation in transparent, heat-sealed, polyethylene sachets; and (ii) encapsulation in mild steel cans of the usual commercial type. Encapsulation in black polyethylene modules having a capacity of 7.5 litres (which have previously been used for the temperature control of greenhouses when filled with phase change materials based on calcium chloride hexahydrate) has not been tried at the time of writing this specification. However, the inventors foresee no problem in using the formulations of the present invention in such modules.

In summary, the present invention provides relatively low cost phase change material formulations with solid/liquid transition temperatures of (i) about 9°C, and (ii) about 58°C, which can be used for long-term heat storage applications without the fear of deterioration of performance due to incongruent melting or supercooling.