Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO91/00324 PCT/AU90/00264
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~ TITLE: "CALCIUM CHLORIDE HEXAHYDRATE FORMULATIONS
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FOR LOW TEMPERATURE HEAT STORAGE APPLICATIONS"
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Technical Field
This invention concerns heat storage systems. More
5 particularly, it concerns phase change materials
; based upon calcium chloride hexahydrate for use in
thermal storage systems (such as low energy
greenhouses).
Background
10 Low temperature heat storage systems have been the
subject of considerable development over the years.
For some years, work was directed primarily to
improvinq rock bed regenerative heating systems, such
as the system described by C D Baird, W E Waters and
15 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 l to l;8. Rock bed
20 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 30C. Such phase
2S 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
t` fusion when they solidify again as their temperature
~ is lowered.
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The phase change material first used in low energy
heat storage systems was GIauber's salt, sodium
sulphate decahydrate ~Na2SO4.10H2O), which has a
phase change temperature of about 32C. However, as
5 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
10 strong "undercooling" (called "supercooling" by some
workers in this field) before it solidifies
spontaneously. Undercooling by as much as 11C is
reported by Stein in his specification. This
undercooling problem can be overcome by the addition
15 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, CaC12.6H2O, are now
preferred. However, his own invention, aimed at
20 avoiding ~he 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
25 material to avoid precipitation problems has also
been proposed by Mario Stiffler for the latent heat
accumulator described in the specification of his
Australian p~tent No 559,354 Among Stiffler's
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W O 91/00324 P ~ /A U90/00264
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preferred phase change materials are sodium sulphate
decahydrate, bisodium phosphàte dodecahydrate and
bisodium phosphate heptahydrate.
The storage of phase change materials in small
5 parcels - or microencapsulation of such materials -
was a solution adopted by other workers and referred
to by B Carlsson, H Stymne and G Wettermaxk in their
paper entitled "An incongruent heat-of-fusion system
- CaC12.6H20 - made congruent through modification of
10 the chemical composition of the system", which was
published in Solar Energy, volume 23, 1979, pages 343
to 350. Carlsson et al, however, explained the
physical chemistry involved when the phase change
material calcium chloride hexahydrate is cycled
15 repeatedly through its melting/freezing point, and
showed that the formation of the tetrahydrate
CaC12.4H20 can be inhibited by the inclusion of up to
2 per cent of strontium chloride hexahydrate
(SrC12.6H20), which acts as both a nucleating agent
20 (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
25 freezing point of the calcium chloride hexahydrate.
Carlsson et al also observed that impurities in
technical grade calcium chloride hexahydrate, such as
sodium chloride and potassium chloride, :increase the
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WO91/00324 PCT/AU90/00264
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incongruity of the CaC12.6H2O 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
5 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 CaC12.6H2O without additives as a heat
10 storage phase change material due to its
- "undercooling" tendency and its breakdown with the
formatior,l of the tetrahydrate. This paper also draws
attention to the use of additives to overcome these
problems, notably strontium chloride hexahydrate
15 (SrC12.6H2O~ as a nucleating agent and furned 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
20 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
I change materials in buildings include (i) the
25 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
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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
5 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
lO 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
15 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
20 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
25 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
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barium sulphide, from 0.001 to 5 per cent barium
chlQride dihydrate and from 0.001 to 0.1 per cent
strontium chloride hexahydrate, with a bromide
~potassium bromide, sodium bromide or ammonium
5 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.
10 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
15 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
20 ~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
25 (c) extra water above the stoichiometric quantity
included in the calcium chloride hexahydrate in the
range of from 1.0 per cent to S.0 per cent (by
weight) of the calcium chloride hexahydrate.
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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
5 4.0 per cent tby 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)
10 of the càlcium 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
15 of calcium chloride hexahydrate phase change
materials in melt/freeze cycling experiments.
i
Strontium chloride hexahydrate is known to be
isomorphous with calcium chloride hexahydrate and to
be capable of forming nearly ideal solid solutions
20 with CaC12.6H2O; 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
25 is 0.1 per cent. Any lower concentration of
strontium chloride hexahydrate is close to the limit
of dissolution of SrC12.6H2O in calcium chloride
hexahydrate. An increase in the concentration of
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SrC12.6H20 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
5 phase change material formulation. Increasing the
concentration of SrC12.6H20 to more than 2.0 per Gent
of the CaC12.6H2O produces no improvement in
performance of the phase change material but it adds
significantly to the cost of the formulation. At the
10 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
15 CaC12.6H20, the inert nature of the strontium
chloride hexahydrate, with its lower thermal capacity
than that oP the CaC12.6H2O, is expected to reduce
the efficacy oP the phase change material
formulation, and this factor, combined with the high
? cost of SrC12.6~2O, 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
25 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 ~uantity
of at least 0.02 per cent (by weight) was required to
30 ensure long-term, multi-cycle stability of the phase
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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
5 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
10 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
15 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
20 showed that the éxtra 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
1 25 water is about 5.0 per cent, which corresponds to a
I degree of hydration of 6.61, which was determined on
` the basis of storage efficiency considerations.
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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
5 (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
CaC12.6H2O has sodium chloride as one of its
10 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 ~odium chloride is
15 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
20 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
25 ~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;
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the percentages being relative to the calcium
chloride hexahydrate.
Such a formulation has a solid/liquid transition
temperature of 29.6 :L O.2 C. This transition
5 temperature can be reduced down to about 22C 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
10 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
15 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
20 (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.
25 Thus an optional, but preferred, variation of the
phasè change material formulation of the present
invention is the additlon of a chemically inert
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colouring agent. The presence of a colouring agent
(preferably a black, or at least a dark, colo~ring
agent) has been found to improve the ability of the
phase change material to absorb and release heat by
5 radiation.
.
A convenient technique for tinting the phase change
material is to mix black drawing ink into the
formulation, using ultrasonic activation to ensurè a
substantially uniform distribution of colour within
10 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
15 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 ROTRIN~ ttrade mark) black
drawing ink was added to the sample of the
formulation. The formulation and the glass jar were
20 held in warm water for S 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.
25 The phase change material containing the black
drawing ink was then subjected to freezing ~at about
10C), then melting (at about 45C). The formulation
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WO91/00324 PCT/AU90/00264
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was then held in its molten state or 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
5 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
10 samples to melt. Measurements of the temperatures of
the formulations during heating showed that the dark
phase change material was up to 8C hotter than the
untinted control formulation.
In over 20 melt/freeze cycles, the tinted formulation
15 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
20 radiant energy when stored in a transparent
container.
To test the formulations of the present invention,
continuous calorimetric measurements o a range of
formulations have been made over a period of several
25 years. Samples of formulations of the present
invention, and some samples of phase change materials
not in accordance with the present invention,
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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
5 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
10 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
15 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
20 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.
25 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 C-nberra, Australia. In t~ct, one ~t
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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
5 aforementioned paper by A Brandstetter, entitled
'IPhase change storage for greenhouses".
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