Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K5/00—Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
  • C09K5/02—Materials undergoing a change of physical state when used
  • C09K5/06—Materials undergoing a change of physical state when used the change of state being from liquid to solid or vice versa
  • C09K5/063—Materials absorbing or liberating heat during crystallisation; Heat storage materials

Definitions

• 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) .
• phase change material first used in low energy heat storage systems was Glauber's salt, sodium sulphate decahydrate (Na 2 S0 4 .lOH-0) , which has a phase change temperature of about 32 C.
• 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.
• phase change materials based on calcium chloride hexahydrate, CaCl ? .6H_0 are now preferred.
• 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.
• phase change material sodium sulphate decahydrate, bisodium phosphate dodecahydrate and bisodium phosphate heptahydrate.
• 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.
• phase change materials in buildings have 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".
• phase change materials have been proposed for use in a variety of situations.
• 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.)
• 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.
• a bromide potassium bromide, sodium bromide or ammonium bromide
• 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.
• 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.
• 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 CaCl 2 .6H 2 0; 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-. ⁇ H-O in calcium chloride hexahydrate.
• extra water that is, water in excess of the quantity required stoichiometrically for the hexahydrate formulation
• 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.
• 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.
• sodium chloride is preferably included in concentrations of from 0.001 per cent to 1.0 per cent (by weight).
• 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 CaCl 2 .6H 2 0 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.
• 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;
• 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.
• 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.
• 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.
• phase change material formulation of the present invention is the addition of a chemically inert colouring agent.
• a colouring agent preferably a black, or at least a dark, colouring agent
• 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.
• 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.
• an ultrasonic cleaner model FX 10 having an output of 100 watts at 40 kHz
• 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.
• tinted and untinted phase change materials 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.
• the tinted formulation In over 20 melt/freeze cycles, the tinted formulation has shown no indication of deterioration in its performance as a heat storage medium.
• 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.
• phase change material 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.
• 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”.

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• Chemical & Material Sciences (AREA)
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• Physics & Mathematics (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Combustion & Propulsion (AREA)
• Thermal Sciences (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

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• 1990
  • 1990-06-22 EP EP19900909576 patent/EP0478637A4/en not_active Withdrawn
  • 1990-06-22 WO PCT/AU1990/000264 patent/WO1991000324A1/en not_active Application Discontinuation

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