Note: Descriptions are shown in the official language in which they were submitted.
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HEAT STORAGE COMPOSITION AND PROCESS FOR PREPARING THE SAME
The present invention relates to a heat storage
composition which is suitable for use in air conditioning
systems for buildings and the like and to a process for
preparing the same.
A heat storage material should have various properties,
e.g. a large amount of stored heat, functioning at a
predetermined temperature level, long term stability, low
cost, non-toxicity, non-corrosiveness and the like. As a
material which satisfies these properties, a phase changeable
hydrated salt has been most extensively studied, and one
typical example is sodium sulfate decahydrate.
Since sodium sulfate decahydrate has a melting point of
32C and a latent heat of 60 cal/g, many attempts have been
made to utilize this salt as a heat storage material since
sodium tetraborate decahydrate (Na2B407.10H20) was found to be
an effective supercooling-preventing agent which is used in
combination with sodium sulfate decahydrate in 1952. One of
the problems which arises in the practical application of such
a combination is that sodium sulfate decahydrate exhibits an
incongruent melting behaviour. That is, upon melting,
anhydrous sodium sulfate forms and precipitates at the bottom
of a liquid system. When such a system is cooled, a surface
layer of the anhydrous salt is rehydrated while an inner part
of the precipitated salt remains in a dehydrated form. Since
the remaining anhydrous sodium sulfate does not contribute to
the phase change, the amount of stored heat decreases. To
solve this problem, many methods have been studied to prevent
the precipitation of the anhydrous salt and to disperse and
maintain it in the liquid. Most of them comprise the addition
of an organic or inorganic additive to increase viscosity and
prevent the precipitation.
For example, the use of an inorganic compound was
tried and reported (cf. Japanese Patent Kohyo Publication
35 No. 501180/1980 and Japanese Patent Kokai Publication
No. 34687/1978). However, su~ficient precipitation prevention
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has not been achieved.
As the organic additive, organic polymers, for example, a
water-soluble polymer (e.g. polysodium acrylate) and a
crosslinkable polymer have been proposed (cf. Japanese Patent
Publication Nos. 30873/1982 and 48027/1982 and Japanese Patent
Kokai Publication Nos. 132075/1983 and 102977/1984). However,
they are not necessarily satisfactory from the point of view
of long term stability.
In a Glauber's salt base heat storage composition, it is
known to suppress the decrease in the amount of stored heat by
the addition of water containing a silicone defoaming agent
and a chelating agent to the Glauber's salt (cf. Japanese
Patent Kokai Publication No. 203687/1985). In this method,
the silicone defoaming agent and the chelating agent are
essential. In the absence of these two agents, the amount of
stored heat decreases after 500 heating cycles.
An object of the present invention is to provide a heat
storage composition which can solve the above problems.
Namely, an object of the present invention is to provide
a heat storage composition which comprises sodium sulfate and
water and does not suffer from a decrease in the amount of
stored heat for a long time after repeated melting and
freezing (heating cycle), and a process for preparing such a
composition.
According to a first aspect of the present invention,
there is provided a heat storage composition comprising (1) at
least one compound selected from the group consisting of
sodium sulfate and its eutectic salts, (2) water and (3) a
crosslinkable polymer which comprises a polyfunctional monomer
and at least one monomer selected from the group consisting of
unsaturated carboxylic acids, organic unsaturated sulfonic
acids and salts thereof, wherein the molar ratio of water to
sodium sulfate or its eutectic salt is from 13:1 to 27:1.
According to a second aspect of the present invention,
there is provided a process for preparing a heat storage
composition, which comprises polymerizing a polyfunctional
monomer and at least one monomer selected from the group
206043~
consisting of unsaturated carboxylic acids, organic
unsaturated sulfonic acids and salts thereof in the presence
of water and at least one compound selected from the group
consisting of sodium sulfate and its eutectic salts in a molar
5 ratio of 13:1 to 27:1.
In drawings which illustrate preferred embodiments of the
present invention:
Fig. 1 is a graph showing the change in the latent heat
of the heat storage composition of the present invention up to
5000 heating cycles in a temperature history test comprising
melting and freezing.
Fig. 2 is a graph showing the relationship between
the amount of water contained (moles per one mole of sodium
sulfate) and the latent heat of a unit weight of the heat
storage composition after 5000 heating cycles (A) and the
relationship between the amount of water contained and the
remaining rate of the latent heat after 5000 heating cycles
(B).
In the heat storage composition of the present invention,
sodium sulfate may be used in an anhydrous form or an eutectic
salt form. In addition, sodium sulfate decahydrate may be
used. Examples of compounds which form an eutectic salt with
sodium sulfate are sodium chloride, potassium chloride, sodium
nitrate, potassium nitrate, magnesium sulfate, urea ~nd the
like. The compounds are used in an amount of 0.2 to 1.0 mole
per one mole of sodium sulfate. The eutectic salt has a lower
melting point than sodium sulfate as such and can be used to
adjust the melting point.
One of the important characteristics of the composition
of the present invention is the molar ratio of water to sodium
sulfate in the heat storage composition. Water is used in an
amount o~ 13 to 27 moles, preferably 15 to 25 moles, more
preferably 16 to 24 moles per one mole of sodium sulfate (in
an anhydrous form). By using water in this molar ratio
range, the latent heat of the composition does not
substantially decrease after heating cycles for a long time.
Therefore, the calculation of heat load is easy, and excessive
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loading of the heat storage composition to compensate for a
decrease of latent heat is not necessary. When the heat
storage composition of the present invention is used in a
floor heating system, thickness of the floor can be made thin
and floor weight is decreased.
When the amount of water is less than 13 moles per one
mole of sodium sulfate, initial latent heat is large but the
latent heat is decreased significantly by the repeated heating
cycles. Then, contrary to the composition of the present
invention, a composition is not practically acceptable in view
of the equipment and the control.
When the amount of water exceeds 27 moles per one mole of
sodium sulfate, the change of latent heat can be suppressed
after the repeated heating cycles, but the composition has a
small latent heat and a large amount of the composition should
be used. Therefore, the equipment has some drawbacks, for
example, an increase in floor thickness and it will withstand
only a small load.
When the amount of water is from 16 moles to 24 moles per
one mole of sodium sulfate, remaining rate of the latent heat
is 95~ or higher after the 5000 heating cycles and its
absolute value is sufficient for practical use.
The critical meaning of the amount of water in the
composition of the present invention will be explained
quantitatively by Examples and Comparative Examples below.
The crosslinkable polymer and its component monomers will
be explained.
As the unsaturated carboxylic acid, a water-soluble
unsaturated carboxylic acid is preferred. Specific examples
of the unsaturated carboxylic acid are acrylic acid,
methacrylic acid and itaconic acid. Among them, acrylic acid
is preferred. A mixture of acrylic acid with methacrylic
acid, itaconic acid or hydroxyethyl acrylate may be used.
Specific examples of the organic unsaturated sulfonic
acid are 2-acrylamide-2-methylpropanesulfonic acid,
p-styrenesulfonic acid, sulfoethyl methacrylate, allylsulfonic
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As the salt of the unsaturated carboxylic acid or the
organic unsaturated sulfonic acid, a water-soluble salt, e.g.
an alkali metal salt and an ammonium salt, is used. Among
them, a sodium salt is preferred. In particular, sodium
acrylate and sodium methacrylate are most preferred.
It may be possible to use an unsaturated amide together
with the above monomers. Examples of the unsaturated amide
are acrylamide and methacrylamide.
The amount of the monomers, namely the polymer in the
composition is 1 to 10% by weight, preferably 2 to 5% by
weight based on the whole weight of the heat storage
composition. When this amount is less than 1% by weight, the
composition has a poor effect on prevention of anhydrous
sodium sulfate precipitation caused by the phase change.
When it exceeds 10% by weight, the amount of stored heat
decreases.
The polyfunctional monomer is used to crosslink the
polymer. Preferably, a water soluble polyfunctional monomer
is used. Specific examples are N,N'-methylenebisacrylamide,
N,N'-methylenebismethacrylamide, N,N'-dimethylenebisacryl-
amide, N,N'-dimethylenebismethacrylamide and the like. Among
them, N,N'-methylenebisacrylamide and N,N'-methylenebismeth-
acrylamide are preferred. The amount of the polyfunctional
monomer is from 0.01 to 1% by weight, preferably from 0.05 to
0.5% by weight based on the whole weight o~ the heat storage
composition. When this amount is less than 0.01% by weight,
the polymer has poor crosslinkability. Even when it exceeds
1~ by weight, the effect is not improved in comparison to the
increased amount.
When the above monomer and the polyfunctional monomer are
polymerized in the manner explained below, a crosslinkable
polymer is obtained. The amount of the crosslinkable polymer
in the heat storage composition is the same as the total
amount of the monomers and is usually from 1 to 11~ by weight,
preferably from 2 to 5.5% by weight.
As a polymerization initiator, any of the conventional
radical polymerization initiators can be used. Examples are
2~60~38
diacyl peroxides, e.g. acetyl peroxide, lauroyl peroxide and
benzoyl peroxide; hydroxyperoxides, e.g. cumenehydroxy-
peroxide; alkyl peroxides, e.g. di-tert.-butylperoxide;
ammonium or potassium peroxydisulfate; hydrogen peroxide;
2,2-azobisisobutyronitrile and the like. Among them, a redox
type polymerization initiator is preferred since it is active
at a comparatively low temperature.
A preferred redox type polymerization initiator is a
water-soluble one. As an oxidant, ammonium or potassium
peroxydisulfate and hydrogen peroxide are exemplified. As a
reducing agent, sodium thiosulfate, sodium sulfite, ferrous
sulfate and the like are exemplified.
The crosslinking temperature is the same as or higher
than the melting point of sodium sulfate decahydrate or its
eutectic salt. Usually, it is from 20 to 50C.
The redox type polymerization initiator exhibits
polymerization activity in a comparatively short time when the
oxidant and the reducing agent are mixed. After the start of
the polymerization activity, contact with air will deactivate
the active species. Therefore, the mixture of the oxidant and
the reducing agent should be charged into a polymerization
reactor as quickly as possible without exposure to air.
The process of the present invention can be carried out
bv various methods. For example, the polymerization is
carried out in a comparatively large volume reactor and the
produced heat storage composition is portioned and placed in a
container which constitutes a heat storage part of the heating
equipment. In this case, the internal atmosphere of the large
volume reactor is replaced with nitrogen gas and then the raw
materials are charged and reacted.
In the present invention, since the monomers are used as
the raw material in place of the polymer, mixing is easy.
Alternatively, the polymerization can be carried out in a
heat storage container of a heating unit. In particular, the
characteristics of the present invention can be realized in
this mode of polymerization.
Since the monomers are used as the raw materials in place
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of the polymer, the mixture before the polymerization is a
liquid composition having a low viscosity. Therefore, the raw
material composition can be easily poured into a container
having a complicated shape. By polymerization in the
container, the heat storage material in a gel or solid state
can be contained in the container having the complicated
shape. When the raw material mixture is placed in the
container and then polymerized, the interior of the container
need not necessarily be replaced with nitrogen gas.
When the liquid mixture before polymerization is poured
into the container for the heat storage material and a redox
type polymerization initiator is used, it is preferred that
the oxidant and the reducing agent are continuously mixed in a
flow system of the composition and are poured in the
container.
For example, the oxidant and the reducing agent are
separately added while a liquid mixture of anhydrous sodium
sulfate or its eutectic salt, water and the monomers is poured
into the container. Either the oxidant or the reducing agent
is dissolved in the liquid mixture and the other is added to
the mixture when the mixture is poured into the container.
The liquid mixture is divided into two portions and the
oxidant is added to one of them and the reducing agent is
added to the other. Then, the two portions are mixed in a
pouring conduit and poured into the container. It is possible
to provide an in-line mixer in the pour~ng conduit to more
sufficiently mix the components.
In the process of the present invention, it may be
preferred to add a thickener or another additive to the
mixture in order to prevent precipitation of anhydrous sodium
sulfate after the raw materials are poured into the container
and before the increase in the viscosity achieved by the
polymerization of the monomers. As the thickener, any of the
conventional ones may be used. Specific examples of the
thickener are inorganic materials, e.g. fumed silica, fine
silica produced by a wet process, various clays, etc., water-
soluble polymers, e.g. polysodium acrylate and hydrogel. The
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amount of the thickener is from 0.1 to 7% by weight of the
composition. In the case of the monomer, the thickener is
added in such an amount that the viscosity of the mixture
prevents the sedimentation of anhydrous sodium sulfate in a
short time in which the crosslinking reaction proceeds and the
viscosity of the composition increases.
To the heat storage composition, a supercooling-
preventing agent is usually added. In the process of the
present invention, the supercooling-preventing agent may be
added to the liquid mixture before polymerization. When the
polymerization of the raw material mixture is carried out in
the container in which the heat storage composition is finally
contained, the supercooling-preventing agent should be added
to the mixture before polymerization.
In general, it is known that sodium tetraborate
decahydrate is effective as the supercooling-preventing agent.
The amount of supercooling-preventing agent is usually from
2 to 5% by weight based on the whole weight of the heat
storage compositioll. Since the pH range in which tetraborate
decahydrate is stably present in an aqueous medium is neutral
to basic, the mixture is preferably neutralized when the
mixture is acidified by the monomer and/or the polymer.
The present invention will be illustrated by the
following Examples.
Example 1
To a lO wt.% aqueous solution of sodium acrylate (150 g)
which had been prepared by neutralizing acrylic acid with an
aqueous solution of sodium hydroxide to pH of 7.5, water
tl35 g) was further added. To the solution, N,N'-methylene-
bisacrylamide (0.75 g), anhydrous sodium sulfate (142 g) and
sodium tetraborate decahydrate (20 g) were added while
stirring at 30C to obtain a homogeneous mixture containing no
precipitate. In this mixture, the molar ratio of water to
sodium sulfate (in the anhydrous form) is shown in the Table.
This mixture was divided into two portions. To one
portion, ammonium peroxydisulfate (0.5 g) was added, and to
the other, sodium thiosulfate pentahydrate (0.5 g) was added.
20~0438
They were caused to flow through respective flow conduits and
brought in contact with each other to mix them. The mixture
was then poured into a polyethylene bag having a width of
40 mm and a length of 600 mm.
The bag was hung in an air oven at 40C. After one hour,
the crosslinking proceeded and the contents in the bag formed
a homogeneous gel form elastic composition. This composition
phase changed at about 32C.
The obtained composition (50 g) was charged in a
lo cylindrical glass container having a diameter of 30 mm and a
height of 100 mm and was subjected to the heating cycle test
comprising repeating heating and cooling between 40OC and
10C. After 5000 heating cycles, the composition was stable
and no phase separation was observed. Before the heating
cycle, the latent heat was 44.5 cal/g. With this value being
'llO0", a relative latent heat after 5000 heating cycles was 91
~an absolute latent heat being 40.5 cal/g), which means that
the composition maintained the high latent heat for a long
time.
ExamPles 2 to 5 and Comparative Example
In the same manner as in Example :l but using the
components shown in the Table, a heat storage composition was
prepared. In Example 5, sodium chloride formed an eutectic
salt with sodium sulfate.
The results of the heating cycle test are shown in the
Table.
The results are also plotted in the graphs of Figs. 1
and 2, in which the numerals 1 to 5 and 6 stand for
"Examples 1 to S" and "~omparative Example".
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Table
_
Exam- Molar Heating cycle test:
ple ratio to Latent heat (cal/g) and
No. Na2S4 its re~ aining ra~e (%) in brackets
After Heat Cycles of
Water NaCl Before 1000 2000 3000 4000 5000
Hcycalteng
1 15.0__ 44.5 40.6 41.8 39.6 40.5 40.5
(100) (91) (94) (89) (91) (91)
_
2 17.0__ 40.6 40.2 41.4 42.2 40.6 42.2
(100) (99) (102) (104) (100) (104)
3 13.0__ 49.8 47.8 43.8 36.4 38.8 36.9
(100) (96) (88) (73) (78) (74)
I _
4 19.0__ 36.3 37.3 40.0 39.9 40.6 40.6
(100) (103) (~10) (110) (112) (112)
23.00.5 32.3 32.0 31.0 31.3 32.6 32.3
(100) (99) (96) (97) (101) (100)
Comp. 11.0 __ 55.2 45.8 48.0 37.5 32.0 30.4
¦EX. (100) (83) (87) (68) (58) (55)
