Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WO 92/12217 PCT/DE 91/01014
STORAGE SALT MIXTURES
The invention relates to phase change materials
which comprise mixtures of perchlorates or perchlorate
hydrates and other inorganic salts, or salt hydrates, or
organic substances and to phase change material devices
filled with mixtures of this type.
It is known that when a substance is heated
it absorbs a specific amount of energy during the trans-
ition from the solid state into the liquid state.
Corresponding heating curves therefore show, after an
initial temperature increase, a temperature plateau at
the level of the melting point, before a further tempera-
ture increase occurs after complete melting of the
su~stance. For many substances this process is
reversible. During cooling, therefore, the substance
remains at the solidification temperature for a
corresponding period, while the heat previously absorbed
during melting is given off again. Because of the fact
that this energy of fusion may be about one hundred to
two hundred times larger than the specific heat of the
substance, it thus becomes possible to store a fairly
large amount of energy in a narrow temperature range with
a relatively small volume requirement.
Substances for the storage of energy by making
use of the solid/liquid phase transition and vice versa
are called phase change materials (PCMs). Generally, they
should have as high an enthalpy of fusion as possible;
what matters as a rule is the volume-specific enthalpy of
fusion (i.e. that relative to the volume), in order to
provide the maximum storage capacity per unit volume of
the available storage space. In addition, such PCMs must
be stable with regard to cycling, i.e. the solid-liquid-
solid phase transition must remain reversibly repro-
ducible over long periods of time and must not be
adversely affected by chemical reactions, separation,
elimination of water of crystallisation or similar
processes. Other important criteria may be the
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solidification beha~iour (e.g. the formation of a
metastable melt, the extent of a volume change during the
phase transition or the form of crystallisation), and
also compatibility with the construction materials,
physiological acceptability and availability at an accep-
table price. It has so far proved difficult to find PCMs
which meet all of these crit0ria in a manner optimal for
the particular application.
Many of the previously known PCMs were developed
for application in the field of space heating (e.g. in
conjunction with solar panels or heat pumps), and accoxd-
ingly have melting points in the process-water range.
Besides inorganic substances, such as salt hydxates and
salt mixtures, organic substances have also been proposed
for applications of this type.
Similar application areas for PCMs e~ist in
industrial processes, if waste heat which can be used in
another way is to be stored, or if heat reserves must be
held available to cope with load peaks.
PCMs can also be used for special applications
such as automotive preheating systems, thermal control
devices in satellites, hotplates for food, in medical
technology, or heat protection systems for measurement
electronics, for example in industrial processes, or for
borehole probes in geophysics.
In the case of the application areas listed as
examples there is still a xequirement for improved PCMs,
due on the one hand to the inadequacy of the substances
previously used and on the other hand to the striving for
constant improvement. In particular, it is the necessity
to accommodate as large an energy content as possible
into a given ~olume which establishes the object of
providing PCMs whose volume-specific storage capacity
reaches the physical limits.
According to previous accepted teachings, both
for organic and for inorganic substances, maximum energy
densities of the enthalpy of fusion of up to about
300 J/cm3 are to be e~pected in the temperature range up
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WO 92/12217 - 3 - PCT/DE 91/01014
to about 130C (cf. Dr. P. Kesselring, "Zur Energiedichte
im Latentwarmespeicher ~ einige grundsatzliche
Uberlegungen" ~On energy density in the PCM device - some
principal considerations], VDI-Berichte No. 288, 1977).
The Applicant's own investigations showed that on purely
physical grounds maximum values of the volume-specific
enthalpy of fusion are possible which correspond, at any
melting point, to about the value of the melting point of
the substance in degrees Kelvin multiplied by the factor
2, so that for example a substance with the melting
point 95C (corresponding to about 368K) could, in the
most favourahle case, have an enthalpy of fusion of
approximately 740 J/cm3.
Although a substance proposed in US Patent
lS 4,057,101, i.e. lithium perchloxate trihydrate, still
falls slightly short of this maximum value, said
substance does have comparatively very high calorific
values and also excellent melting and solidification
behaviour. Among the salt hydrates it has very probably
the best crystallisation properties. Its thermal
conductivity is also unusually high.
The Applicant's own investigations on the DSC
apparatus on the pure substance showed, at a melting
point of 94.3C, an enthalpy of fusion of 306 J/g and a
specific heat of 1.5 J/g/K in the solid state and 1.98
J/g/K in the liquid state. By means of the density given
in the literature of 1.89 g/cm3 a volume-specific enthalpy
of fusion of 578 J/cm~ is thus calculated, and a specific
heat relative to volume of 2.84 J/cm3/K in the solid state
and 3.74 J/cm3/K in the liquid st2te.
Unfortunately the melting point of lithium-
perchlorate trihydrate, at 94.2C, is very close to the
boiling point of water, which makes it virtually unusable
in unpressurised system~ in which, for example, water is
used for the heat transfer medium~ Its melting tempera-
ture is also too high for it to be used as a heat storage
medium within preheating units for automotive cooling-
water or heating~water circulation systems. Even in the
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wo 92/12217 - 4 - PC~/DE 91/01014
case of the application intended in said patent, as a
heat sink for electronic subassemblies, a lower melting
point would be desirable in a number of instances.
The object was therefore to provide phase change
materials which have similarly positive properties as has
lithium perchlorate trihydrate and whose melting points
should also cover a lower temperature range as widely as
possible.
The ob~ect of the invention is achieved by mi~ing
of specific perchlorates or perchlorate hydrates with
other inorganic salts or salt hydrates or with organic
su~stances.
The invention provides a whole family of mixtures
of this type which cover a wide melting point range and
which have, at the same time, very high enthalpies of
melting. For these mixtures having at least one component
from the group of the perchlorates or perchlorate
hydrates there are proposed as additional components
other perchlorates, the chlorides, bromides, nitrates and
hydroxides, or in each case their hydrates, these addi-
tional components preferably being selected from those
groups which contain the elements lithium, sodium,
potassium, magnesium, calcium, stxontium or barium.
A further family, those mixtures are proposed
which, in at least one component, comprise a perchlorate
or perchlorate hydrate, and as an additional component of
at least one admixed organic substance, the admixed
organic substances being selected from those groups which
cannot be oxidised by perchlorates.
It was found that the perchlorates or perchlorate
hydrates form, with one another or with the other sub
stances mentioned, binary, ternary and multinary systems
with eutectic or invariant melting points, thus allowing
the selection of a series of diferent melting points,
depending on the mixture. It is particularly surprising
that some of these mixtures have decidedly high
enthalpies of melting which are not very much lower than,
for example, that of pure lithium perchlorate trihydrate.
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WO 92/12217 - 5 PCT/DE 91/01014
An additional advantage of the proposed mixtures
resides in the formation of a very fine structure of the
solidified mixture, as a result of which the detrimenkal
and obstructive formation of coarse crystals, as it is
known of the pure substances, is reliably inhibited.
Although some of the perchlorates contained in
the mixtures have, with regard to physiological
acceptability, been classified as irritants, and as a
rule also as detrimental to health on swallowingr no MAC
values have been established. For the technical
applications envisaged, the classifications of the
perchlorates are relatively favourable, because they may
thus be able to replace other substances having
considerably greater hazardous potential. It should
lS however be borne in mind that there is a risk of
e~plosion, in particular if anhydrous perchlorates are
mixed with combustible materials. In the case of
industrial applications, therefore, appropriate design
safeguards are required to preclude reliably any contact
with such materials.
On the other hand it is an advantage that
perchlorates and perchlorate hydrates do not attack
aluminium and its alloys, thus allowing the cost-
effective construction of appropriate containers and the
like from aluminium-containing materials.
On the basis of the investigations so far, the
mixtures provided by the invention may, for the time
being, be divided into various groups. The most important
group is formed by those mixtures which comprise
predominantly, in terms of molar fractions, lithium
per~chlorate trihydrate, which mixtures fuse and solidify
congruently and on the whole, apart from having different
melting points, show virtually no difference from ~he
behaviour of pure lithium perchlorate trihydrate, thus,
even in the case o~ a coxre~pondingly eutectic mixture
composition, uniformly undergo the phase transition at a
constant temperature. They also have in common that they
return from the liquid to the solid skate without
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WO 92/12217 - 6 - PCT/DE 91/01014
supercooling.
This group includes a eutectic mixture of 79 mol
of lithium perchlorate trihy~rate with 21 mol of its own
monohydrate which melts at 91.9C and whose enthalpy of
fusion barely differs from that of the pure trihydrate.
In addition, eutectic mixtures of lithium
perchlorate trihydrate and 9 mol% of sodium chloride or
14 mol% of sodium hromide belong to this group (see
Figure 1 and Figure 2); the melting points of these
mixtures were found to be 87.7C and 82.5~C respectively,
and their enthalpies of melting are also surprisingly
high.
Mixtures of lithium pexchlorate trihydra~e and
potassium perchlorate, and of magnesium perchlorate
hexahydrate, magnesium chloride hexahydrate, lithium
nitrate and sodium nitrate may well also belong to this
group. In the case wh~re lithium nitrate is admixed it is
particularly surprising that it forms, without water of
crystallisation, a eutectic with lithium perchlorate at
about 80.6C (see Figure 3), although lithium nitrate
itself normally melts at 29.9C as the trihydrate. In the
case of this eutectic the volume-specific enthalpy of
fusion, in spite of the depression of the melting point,
is again virtually identical to that of pure lithium
perchlorate trihydrate.
A second group is formed by those mixtures which
also for the greater part consist of lithium perchlorate
trihydrate, but which in previous in~estigations gave
somewhat indifferent results. Those are admixtures of
potassium chloride, or lithium chloride monohydrate. In
both cases the curves obtained by the DSC equipment do
not show good reproducibility and are subject to constant
change. It is thought that in the case of the admixture
o lithium chloride monohydrate, the extreme
hygroscopicity of the substance, produces an excess of
water in the mixture, which may he the cause of an
indifferent melting behaviour. Although this mix~ure may
possibly melt incongruently, the quasi-binary system it
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WO 92/12217 - 7 - PCT/DE 91/01014
foxms appears to have at least an invariant melting point
at about 73.0C. Although the molar composition has so
far not been determined exactly, a ~alue for the enthalpy
of fusion was measured which, in relation to the weight,
is virtually identical with that of pure lithium
perchlorate trihydrate.
The third group comprises mixtures which have a
considerable molar proportion of lithium perchlorate
trihydrate, but which, because of the added component,
melt incongruently or peritectically, some of these
mixtures tending to exhibit the behaviour which was found
by the Applicant and which has be~n called ~'dry melting"
(see EP ~pplication 90730008). Among these are in par-
ticular admixtures of lithium hydroxide monohydra~e or
strontium hydroxide octahydrate. During investigations in
the DSC equipment, a melting point of 40.0C and an
enthalpy of fusion of about 300 J/g were found for such
a quasi-eutectic mixture of 2 mol of lithium perchlorate
trihydrate and 1 mol of strontium hydroxide octahydrate.
2G In the case of a mixture of 38 mol% of lithium
perchlorate trihydrate and 62 mol~ of lithium hydroxide
monohydrate, the melting point was dete~mined as 66.8C
and the enthalpy of fusion as 288 ~/g.
Said mixture of lithium perchlorate trihydrate
with stron~ium hydroxide octahydrate can for example be
used in the field of heating of buildings, for example to
store solar energy. When dimensioning the corresponding
containers, a slightly increased expansion space, for
example by means of elastic walls, should be taken into
account for this mixture because it still shows a slight
volume increase after solidification.
The next group includes those mixtures which
consist of a smaller fraction of a perchlora~e hydrate
and a predominant residual fraction or example of
chlorides or bromides, hydroxides or nitrates, or the
respective hydrate, of the elements lithium, sodium,
potassium, magnesium, calcium, strontium or barium.
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WO 92/12217 - 8 - PCT/DE 91/01014
A further group i5 essentially fo~ned by ~hose
mixtures which consist of a usually fairly small fraction
of potassium perchlorate, the remaining fraction consist-
ing of selected chlorides or bromides, hydroxides or
nitrates, or their respective hydrates, of the elemenks
lithium, sodium, potassium, magnesium, calcium, strontium
or barium.
Finally, according to the invention, there exists
a separate group of those mixtures which are mixed from
a proportion of at least one perchlorate or perchlorate
hydrate and a further proportion of an organic substance,
the organic substance being selected from that group
which cannot be oxidised by perchlorates. The mixtures of
this group are distinguished by a pronounced tendency to
supercooling. They are therefore advantageously intended
for those applications where heat is to be available, for
example, on demand (by means of deliberate initiation of
crystallisation). Typical examples from this group are
mixtures of lithium perchlorate trihydrate and mannitol
or pentaerythritol.
Of the various groups~ the use of the higher-
melting mixtures having melting points from about 70 to
about 92C may be advantageous, for example, for a
storage heater using off-peak electricity, whilst those
mixtures with melting points from about 40 to about 70C
appear appropriate in conjunction with solar panels.
The~e mixtures also offer advantages in the case
of latent heat sinks for heat protection systems for the
purpose of thermal protection of temperature-sensitive
measurement electronics, for example in continuous
furnaces or in borehole probes for the geophysical
investigation of deep boreholes.
According to another aspect of the invention,
those of the abovementioned mixtures with a melting
point between 60 and 80C are proposed for use in auto-
motive preheating units. The object in this case is to
charge a PCM device by means of the engine cooling water
while the engine is running, and to store this amount of
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WO 92/12217 - 9 - PCT/DE 91/01014
heat over several days with as little loss as possible
in order, in the case of a cold start, either to allow
the immediate operation of the car heating or to short~n
the cold start phase of the engine, in order to reduce
wear and emissions. Units of this type are currently
being developed, the PCM barium hydroxide octahydrate
being used experimentally in prototyp~s at present.
Barium hydroxide octahydrate has the dràwbacks that it is
very toxic and that, with a melting point of 78C, it is
slightly above the temperature of about 70C considered
most suitahle. Particularly serious however is the fact
that barium hydroxide octahydrate~ like all alkali metal
hydroxides, reacts violently with aluminium and its
alloys, generating heat and fission gases~ rapidly
destroying components made of said light metals. Since
not only the radiator but also in some cases the cylinder
heads and the engine blocks in modern motor vehicles are
made from aluminium or aluminium alloys, such
subassemblies would be damaged in the event of barium
hydroxide octahydrate discharging into the cooling water
circuit due to a leak of the storage container~ A risk of
this type is eliminated according to the invention by the
use of aluminium-compatible mixtures based on
perchlorates or perchlorate hydrates. The comparatively
higher cost of mixtures of this type does however have to
be taken into account.
The invention further opens up various
possibilities here of using particular mixtures above or
below the eutectic point, so as to define a melting range
instead of a fixed melting point. As an example of an
adaptation of this type, a mixture of lithium perchlorate
trihydrate and sodium nitrate may be mentioned, which
melts eutectically at 64.0C at a molar composition of 62
to 38 (see Figure 4). By reducing the proportion of
sodium nitrate a melting range extending to higher
temperatures is formed, which can be determined exactly
by defining the proportions o the mixture. In this way
it is possible to adapt exactly, for said application
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Wo 92/12217 - 10 - PCT/DE 91/01014
example, the melting point range of the PCM to the
temperature conditions in the cooling water circulation
system of various engine types (for example diesel or
petrol).
From the larqe number of mixtures provided by the
invention, a mixture of strontium perchlorate and
strontium hydroxide octahydrate again appears to be of
interest for automotive application, because this mixture
has a very favourable melting point of 71.2C.
The object of reducing the melting point of
barium hydroxide octahydrate, in the case where this i5
used as an automotive PCM, is also achieved according to
the invention in several ways which consist in admixing
perchlorates or perchlorate hydrates~ As an example of an
adaptation of this type, a mixture of 86 mol of barium
hydroxide octahydrate and 14 mol of po~assium perchlorate
is to be mentioned here, which mixture has a melting
point of 76.5C and whose volume-specific enthalpy of
fusion is even slightly higher than that of pure barium
hydroxide octahydrate.
For an overview of the invention a table has been
appended whose upper section lists data for various pure
substances. Selected examples of mixtures according to
the invention are compiled in the lower section. Fox the
sake of simplicity the abbreviations in parentheses in
the upper section have been used in the lower section.
The molar proportions of each mixture are given where
they have already been determined. In the case of
virtually eutectic mixture proportions an ~E" has been
added in parenthesesO After the temperature stated for
each melting point determined, a parenthetic indication
of the melting behaviour observed is also given. The
abbreviations and the symbol used are explained at the
foot of the table.
The figures show, as an example, four phase
diagrams, obtained from DSC melting curves, of the
following quasi-binary systems:
Fig. 1 ~ithium perchlorate trihydrate/sodium chloride
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W0 Y2/12217 ~ PCT/DE 91/01014
Fig. 2 Lithium perchlorate trihydrate/sodium bromide
Fig, 3 Lithium perchlorate trihydrate/lithium nitrate
Fig. 4 ~ithium perchlorate trihydrate/sodium nitrate
It can be clearly seen that these systems behave
eutectically, that section of the curve which runs
towards the anhydrous component in each case naturally
being very steep.
Overall the invention provides a family of
storage hydrate mixtures which allows the selection of a
wide variety of melting points in the process water
range, has very nigh volume specific enthalpies of
melting and is stable with regard to cycling. In terms of
its physiological acceptability it appears relatively
acceptable. ~he main mixture components are available in
large quantities at favourable costs. Most of the mix-
tures proposed have no, or only a small, tendency to
segregate. The solidification structure is advantageously
finely crystalline, the volume change upon melting is
minimal. A chemical attack of aluminium and its alloys
either does not happen at all or, in the case of some of
the mixtures proposed, is relatively mild. The vapour
pressure of some of these mixtures is distinctly lowered.
This provides the conditions for improved performance and
improved properties of phase change material devices.
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TABLE
Data
Substance Melting p~int Density Enthalpy of ~usion
(~C) (g/cm3) (J/g)(J/cm3)
5 Ca(ClO4)2 x 6H~O (CPH) 71.9 125 --
Ba(OH)2 x 8H2O (BHO) 78.0 2.16 272 588
Sr(OH)2 x 8H2O (SHO) 85.0 1.91 344 657
LiCl04 x 3H2O (LPT) 94.3 1.89 305 576
LiCl x H2O (LCM) 95 1.73 230 398
LiOH x H20 (LHM) 106.8 1.51 419 633
MgCl2 x 6H2O (MCH) 117 1.57 169 265
Mannitol (NA) 166 1.50 306 459
Pent~erythritol (PE) (T) 189 1.55 294 456
Mg(ClO4)2 x 6H2O (MPH) 193 1.98 --- ---
LiNO3 (LN) 252.7 2.36 367 866
NaNO3 (NN) 306 2.26 172 389
~aBr (NB) 741 3.20 224 717
KCl (KC) 772 1.98 342 677
NaCl (NC) 800 2.16 493 1065
Mixtures (mol/~ol)
LPT/SHO 20/lO (E) 40,0 (*) 1.90 300 570
LPT/NA --/-- 48.1 (c,s) ---- --- ---
LPT/PE --/-- 57.2 (c,s) ---- --~
LPT/CPH --/-- 60.9 (c) ---- --- ---
LPT/NN 62/38 (E) 64.0 (c) 1.98 243 481
LPT/L~N 38/62 (E) 66.8 (*) 1.77 288 510
LPT/LCN 56/44 73.0 (i) 1.85 305 564
LPT/KC --/-- 73.6 (i) ---- --- --- :-
BHO/KP 86/14 76.5 (i) 2.18 275 600
LPT/LN 70/30 (E) 80.6 (c) 1.96 296 580
LPT/MCH 90/10 82.3 (c) 1.85 274 507
LPT/NB 86/14 (E) 82.5 (c) 2.01 281 565
L~T/KC 90/10 85.2 (i) 1.89 267 505
LPT/NC 91/9 (E) 87.7 (c) 1.90 304 578
LPT/MPH 90/lO 90.0 (c) 1.91 264 504
LPT/LPM 79/21 (E) 91.9 (c) 1.94 294 570
LPT/KP 90/10 93.8 (c) 1.95 284 554
(c) ~ congruent melting
(i) - incongruent melting
(s) 3 supercooling
(*) ~ "dry" melting
(T) ~ transition point
(E) ~ eutectic
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