[0001] The present invention is concerned with thermal energy storage materials and heat-exchange
devices containing such materaials.
[0002] Thermal energy storage materials may store thermal energy as specific heat and/or
as latent heat. It is often desirable to use materials which store thermal energy
as latent heat, since this enables the volume occupied by the storage material to
be minimised. This is advantageous, for example, in materials operating in the temperature
rango 10°C to 100°C for the storage of solar energy or of heat extracted during refrigeration.
[0003] Materials which are useful for the storage of thermal energy as latent heat undergo
reversible transition from one form to another on heating to a characteristic transition
temperature. This transition may be from solid phase to liquid phase (fusion) or from
one crystal form to another (this latter transition also being referred to as fusion).
[0004] A number of hydrated inorganic salts are known which undergo transition to the anhydrous
or a less hydrated form at a characteristic temperature on heating and revert to the
more hydrated form on cooling.
[0005] A potential drawback in the use of many of these hydrated salts is incongruency of
the phase transition, that is, the transformation of the low-temperature solid phase
to a two-phase condition where a solid and liquid coexist. In the two-phase condition,
the difference in densities of the two phases causes segregation thereof, which limits
their ability to recombine and form the low-temperaturo singlo solid phase. Consequently
the amount of heat ro- coverable on cooling is reduced.
[0006] Attempts can bo made to avoid the formation of two phascs above the transition point
by controlling the initial composition of the material, but, even for materials with
a congruent phase transition there remains the problem that the solid phase tends
to settle out in time. This limits both the kinetics of transformation and the uniformity
of energy storage density within a'container, and results in deterioration of the
material on repeated heat- ing/cooling cycles.
[0007] Thermal energy storage materials have been proposed in which the hydrated inorganic
salt is thickened by an organic thickening agent, for example, cellulosic polymers,
starch, alginates or an inorganic thickening agent, such as a clay (as disclosed in
U.S. Patent 3 986 969). The above-mentioned organic thickening agents are natural
polymers (or derivatives thereof) and are therefore unstable to hydrolysis and bacterial
and enzyme action, which considerably shortens the life of the material. The above-mentioned
inorganic thickening agents are more stable, but it appears that thermal energy storage
materials containing such thickening agents can only be used in very shallow depths
(for example, about one inch) and must therefore be disposed horizontally.
[0008] We have now found that the above problems are alleviated according to the invention
by the use of a thermal energy storage material in which a hydrated inorganic salt
having a transition temperature to the anhydrous or a less hydrated form in the range
10 to 100°C is dispersed and suspended in a hydrogel formed from a water-soluble synthetic
polymer having pendant carboxylic or sulphonic acid groups cross-linked with cations
of a polyvalent metal.
[0009] One advantage of this material is that the hydrated inorganic salt is immobilised
in close proximity in small volumes throughout the gel. This minimises any segregation
which could arise, after fusion of the hydrate phase, by any solid sinking to the
bottom of the mixture. There is no need to use the material according to the invention
in flat horizontal trays; the material can be arranged in vertical columns of substantial
height (for example 50 cm. to one metre).
[0010] A further advantage of the material according to the invention is that the cross-linked
hydrogel can be prepared in situ by reaction between the respective water-soluble
polymer or an alkali metal or ammonium salt thereof and a water-soluble salt of the
polyvalent metal.
[0011] Suitable polyvalent metals include, for example, chromium, iron, tin, magnesium and
aluminium. Aluminium and magnesium are-preferred in view of the ready availability
of water-soluble salts thereof. Suitable water-soluble salts of the above metals include,
for example, chlorides, nitrates or sulphates, of which aluminium sulphate and magnesium
sulphate are preferred. The polyvalent metal is preferably present in an amount sufficient
to react with all the acid groups in the polymer to form ionic crosslinks. The actual
amount necessary to achieve complete reaction depends on factors such as the valency
of the metal, the proportion of acid groups in the polymer and the amount of polymer
in the material. Typical amounts of polyvalent metal are 0.5 to 5% (expressed as the
weight of water-soluble salt, based on the weight of the storage material).
[0012] The water-soluble polymer preferably has a backbone containing units of acrylic acid
or methacrylic acid, for example; a homopolymer or copolymer of acrylic acid or methacrylic
acid, partially hydrolysed polyacrylamide or polymethacrylamide, or an alkali motal
or ammonium salt thereof. In some embodiments, the polymer preferably contains 5'
to 50% (for example 10 to 40%) carboxylic groups, the percentages being based on the
number of repeating units in the polymer backbone.
[0013] The molecular weight of the polymer may vary over a wide range. For some applications
it may be advantageous to use polymers of relatively low molecular weight (for example,
100,000 to 500,000), while for other applications, higher molecular weights (for example
1 million to 8 million) may be preferred.
[0014] The water-soluble polymer is preferably present in the thermal energy storage material
in a relatively minor amount, such as from 0.5 to 10% (for example, about 5%), based
on the weight of the material.
[0015] Suitable hydrated inorganic salts for use in the material according to the present
invention include, for example, calcium chloride hexahydrate (the fusion point of
which is 29°C); sodium sulphate decahydrate (the fusion point of which is 32°C); disodium
hydrogen phosphate dodecahydrate (the fusion point of which is 35.5°C); sodium thiosulphate
pentahydrate (the fusion point of which is 50°C); sodium acetate trihydrate (the fusion
point of which is 58°C); barium hydroxide octahydrate (the fusion point of which is
75°C) and zinc nitrate hexahydrate (the fusion point of which is 35°C).
[0016] For the storage of solar energy, the hydrated salt preferably has a fusion point
in the range 20° to 90°C and is preferably non-toxic, non-corrosive and readily available
at low cost. Preferred hydrated salts meeting some or all of the above requirements
are sodium sulphate decahydrate, disodium hydrogen phosphate dodcca- hydrmte, sodium
thiosulphate pcntahydrato and sodium carbonate decahydrate.
[0017] Some of the above-mentioned hydrated salts, whan cooled below the fusion point thereof,
tend to undergo supercooling (that is they do not transform back to the hydrated form
until the temperature is below the theoretical fusion point). This may result in less
hydrated forms of the salt being formed, with consequent reduction in the amount of
energy released. In order to avoid supercooling, the material may be nucleated, for
example, by a heat- transfer method as disclosed in U.S. Patent 2 677 243, by careful
control of the proportions of the ingredients of the composition, or by addition of
an insoluble nucleating agent. A preferred nucleating agent for sodium sulphate decahydrate
is borax, as proposed in U.S. Patent 2 677 664.
[0018] When a nucleating agent is present, this agent, like the inorganic salt, is dispersed
and suspended in the hydrogel and effectively immobilized therein. This wide dispersion
of immobilized nucleating agent ensures efficient nucleation of the hydrate phase
during cooling cycles, thereby inhibiting supercooling.
[0019] The thermal energy storage material according to the invention preferably contains
the hydrated salt in an amount of from 66% to 95% by weight and, optionally, a nucleating
agent in an amount of from 1 to 10%, based on the weight of the hydrated salt.
[0020] Substantially all the balance of the thermal energy storage material according to
the invention is preferably water and, optionally, an organic .liquid which is miscible
with water. A particularly preferred such organic liquid is a lower aliphatic alcohol,
such as ethanol (for example, when the hydrated salt is sodium sulphate decahydrate).
The water is preferably present in an amount sufficient to hydrate all the anhydrous
inorganic salt, and is preferably present in a small excess. The material may contain
water in an amount of, for example, from 25 to 75% by weight. When a water-miscible
organic liquid is included, it is preferably present in a relatively minor amount,
compared with water, for example, from 5 to 25%, based on the weight of water.
[0021] The material according to the invention is preferably, used in a method of heat exchange
in which the material is first heated to a temperature above the transition temperature
of the hydrated salt, and the heat is extracted from the material by passing a fluid
at a temperature below the abovementioned transition temperature in heat-asohange
relationship therewith. The alternate heating and cooling of the material can be repeated
for many cycles.
[0022] The present invention also comprises a heat-exchange device, which comprises a tank
containing the thermal energy storage material according to the invention and means
for supplying a cooling fluid in heat-exchange relationship with the thermal energy
storage material.
[0023] In order that the invention may be more fully understood, the following Examples
are given by way of illustration only.
EXAMPLE 1
[0024] 397 gm. of anhydrous sodium sulphate, 10 gm. of solid aluminium sulphate Al
2(SO
4)
314H
2O, 40 gm. of borax Na
2B
4O
710H
2O, and 50 gm. of the sodium salt of an acrylamide polymer containing acrylic acid
units were thoroughly mixed together while adding 70 ml. of ethanol. The polymer,
which had an average molecular weight of about 7.5 million, and had a ratio of carboxyl:
amide radicals of about 1:9, was a material commercially available from Allied Colloids
Ltd. as WN23.
[0025] 503 ml. of water was then added while vigorously agitating the mixture at a temperature
of approximately 35°C. In a few seconds the mixture gelled to a uniformly thick but
smooth consistency of density about 1.4 gsn. /cm3. This mixture contained no excess
of water over that required to completely hydrate all the sodium sulphate present
in the final mixture, and on cooling it fully transformed to a solid.
[0026] A sample of the solid was sealed in a square section tube measuring 5 cm. x 5 cm.
x 50 cm. long, made of inert plastics. The ends of the tube were aealed by cast epoxy
resin plugs.
[0027] The tube was disposed vertically and alternately heated to about 60°C (the heating
time being about one hour) and cooled to about 20°C by heat-exchanging with water
circulating outside the tube (the cooling time being three to four hours). Reproducible
thermal arrests were obtained for more than 500 cycles of heating and cooling.
EXAMPLE 2
[0028] Example 1 was repeated, except that the aluminium sulphate was replaced by the same
amount of MgSO
4. 7H
2O.
[0029] In the thermal cycling test, reproducible thermal arrests were obtained for more
than 500 cycles.
1. A thermal energy storage material comprising at least one hydrated inorganic salt
which has a transition temperature to the anhydrous or a less hydrated form in the
range 10° to 100°C, characterised in that the hydrated inorganic salt is dispersed
and suspended in a water-insoluble hydrogel formed from a water-soluble synthetic
polymer having pendant carboxylic or sulphonic acid groups cross-linked with cations
of a polyvalent metal.
2. A thermal energy storage material according to claim 1, characterised in that the
polyvalent metal is magnesium or aluminium.
3. A thermal energy storage material according to claim 1 or 2, characterised in that
the polymer is a homopolymer or copolymer of acrylic or methacrylic acid or partially
hydrolysed polyacrylamide or polymethacrylamide.
4. A thermal energy storage material according to any of claims 1 to 3, characterised
in that the water-insoluble hydrogel is formed in situ by reaction between a water-soluble
salt of the polyvalent metal and the water-soluble polymer or an'alkali metal or ammonium
salt thereof.
5. A thermal energy storage material according to any of claims 1 to 4, characterised
in that the hydrogel contains water in an amount sufficient to hydrate all the inorganic
salt and a water-miscible organic liquid in a minor amount, relative to the amount
of water.