Latent heat storage device for cooling purposes

A latent heat-storage device for cooling purposes includes a non-deformable closed container, and a heat-storage medium comprising water or a eutectic aqueous solution of a salt contained in the container. The heat-storage medium expands in volume as its temperature is lowered through its phase transition from fluid to solid. The amount of the heat-storage medium is such that a part of the container volume equals or is slightly larger than the maximum volume expansion of the heat-storage medium during operation of the device. A gas condensing between room temperature and the melting temperature of the heat-storage medium fills the container part at atmospheric pressure and room temperature.

This invention relates to a latent heat storage device for cooling purposes 
having a non-deformable closed container which comprises water or a 
eutectic mixture of water with at least a salt as a storage medium as well 
as a means for the compensation of volume variations of the storage 
medium, and of pressure variations in the storage device. 
For the storage of latent heat between .+-.0.degree. C. and approximately 
-50.degree. C., water and a few eutectic mixtures of water with a salt are 
particularly readily suitable. These storage media, in contrast with 
nearly all other substances, expand in the phase transition from fluid to 
solid by several percent by volume. Another disadvantage is that said 
volume expansion or increase occurs very non-uniformly and, as described 
in U.S. Pat. No. 4324287, leads to local bulges. As a result of the 
additions described in the said United States Patent said non-uniform 
expansion, however, can be considerably avoided. The expansion in volume 
of the storage medium, however, does not disappear. The resulting pressure 
variations occurring in a closed storage container would deform the 
container already in the first storage cycle and would destroy it if, in 
addition to the said homogenization additions, suitable measures would not 
be taken to compensate for said volume variation. In the said United 
States Patent the expansion in volume is compensated by the use of 
flexible storage containers. 
For certain applications, however, it is required that the shape and the 
dimensions of the storage container during operation should remain 
unchanged. As an example is mentioned a machine for the production of 
ice-cream: a cylindrical storage container filled with a cold storage 
medium rotates horizontally in the ice-cream mixture and the ice-cream 
film freezing to the container is stripped off by a stationary scraper 
sliding over the cylinder surface. The storage container, as a moving part 
of the machine, must hence be entirely stable as regards shape and 
dimensions. 
The simplest measure for compensation of the volume variation is to leave 
in the storage container a free space which corresponds to the expansion 
of the storage medium. Since, however, the air present in the said free 
space would be compressed to a very high pressure during the expansion of 
the storage medium, the volume must be larger than the increase in volume 
of the storage medium. With a double free space the pressure would still 
rise, for example, to 2 bar. When the free space would be evacuated, the 
container would be loaded externally by 1 bar excessive pressure. A 
compromise between the two measures is most favourable. The said pressure 
differences can then be compensated for by suitable wall thicknesses of 
the steel container. 
Remaining disadvantages of these measures, however, are 
a. the comparatively large wall thicknesses required by the pressure 
differences, 
b. a storage capacity which is reduced by the amount of the free space, 
c. a very poor thermal conductivity in the places of the container where 
the free space is present. 
In the above mentioned example this is disadvantageous especially upon 
freezing in the freezer compartment and in the initial phase of the ice 
cream preparation. 
The said disadvantages can be removed at least partly by container 
constructions as they are known from the U.S. Pat. No. 1,380,987 and 
Published German Application OS No. 2828902. According to these 
publications the volume variation is compensated for by compression of a 
concentric hose of rubber, a silicone or other elastic synthetic resin. In 
this manner atmospheric pressure is substantially maintained in the 
storage container. 
In addition to the still remaining disadvantage of a comparatively large 
free space (hose+inner volume) new difficulties appear with this measure: 
a. a restriction of the keeping qualities and reliability (sealing problem) 
required by the strong mechanical load and deformation. 
b. more difficult cleaning since humidity of the air may condense or, for 
example, ice-cream mixture may penetrate and freeze in the interior of the 
hose. 
c. a comparatively complicated construction which is susceptible to 
disturbances. 
It is an object of the present invention to provide a simple and reliable 
measure to compensate for pressure variations which occur by volume 
expansion of the aqueous latent heat-storage in the phase transition in 
non-deformable closed storage containers. 
According to the invention this object is achieved in that the storage 
container contains only so much storage medium that a part of the 
container volume which is equal to or slightly larger than the maximum 
expansion in volume of the storage medium during operation of the storage 
device remains free from storage medium at room temperature and is filled 
at atmospheric pressure with a gas which condenses in the temperature 
range between room temperature and the melting temperature of the storage 
medium. 
Examples of gases which are suitable according to the invention are: 
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boiling 
point 
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trichlorofluoromethane (Freon 11) CCl.sub.3 F 
23.8.degree. C. 
1,2-dichlorotetrafluoroethane (Freon 114) 
3.5.degree. C. 
CClF.sub.2 CClF.sub.2 
butane C.sub.4 H.sub.10 * 
-0.4.degree. C. 
1,1-difluoro-1-chloroethane (Genetron 142 B) 
-9.2.degree. C. 
CH(CH.sub.3).sub.3 
isobutane CH (CH.sub.3).sub.3 * 
-11.7.degree. C. 
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*Butane and isobutane have a low solubility in the aqueous storage media, 
which must also be taken into account for their use. The Freons and 
Genetron 142 B on the contrary are substantially insoluble in water.

Reference numeral 1 in FIG. 1 denotes a non-deformable storage container 
which at room temperature is partially filled with liquid storage medium M 
and has a gas space V corresponding to the expansion of the storage medium 
upon phase transition. Reference numeral 2 denotes the same storage 
container after cooling below the melting temperature and phase transition 
of the storage medium M. The gas space V in this case is filled entirely 
with solid storage medium N and the gas has condensed substantially. O are 
sealed filling apertures 
In FIG. 2 
A=CCl.sub.3 F trichlorofluoromethane (F 11) 
B=C.sub.2 Cl.sub.2 F.sub.4 1,2-dichlorotetrafluoroethane (F 114) 
C=C.sub.4 H.sub.10 butane 
D=C.sub.2 H.sub.3 ClF.sub.2 1,1-difluoro-1-chloroethane (G 142 B) 
E=C.sub.4 H.sub.10 isobutane 
Tmin 
lowest temperature in the freezer compartment 
Ts 
melting point of the NH.sub.4 Cl/H.sub.2 O eutectic solution (19.5% by 
weight NH.sub.4 Cl+80.5% by weight H.sub.2 O) 
To room temperature (filling temperature) 
Tmax highest temperature upon rinsing 
Pmin gas pressure at Tmin 
Ps gas pressure at the melting point 
Po gas pressure at room temperature 
Pmax gas pressure at Tmax. 
The relationships which exist in the storage container with such a gas 
filling will be illustrated with reference to the following example. 
In the non-deformable storage container 1 (FIG. 1) with NH.sub.4 Cl/H.sub.2 
O as the storage medium M, a free space V of 5% is left which was filled 
with Genetron 142 B at a room temperature of 24.degree. C. and a 
atmospheric pressure (1013 mbar). In the diagram (FIG. 2) this corresponds 
to the situation at point F. 
When the storage container is now laid in a freezer compartment the gas 
pressure also decreases with temperature to p=Po. (1+T/273), i.e. by 1/273 
per .degree.C., since the volume to a first approximation remains 
constant. In the diagram this is a movement from point F on the straight 
line to the left up to point K where the vapour pressure curve D of the 
Genetron is contacted. From here, upon further cooling, more and more 
Genetron condenses and the pressure decreases further along the vapour 
pressure curve of the Genetron. 
At point S the actual storage temperature of -16.degree. C. is achieved at 
which the storage medium (NH.sub.4 Cl/H.sub.2 O) freezes. The temperature 
now remains constant until the whole storage medium has solidified. As a 
result of the 4.4% expansion of the storage medium in the phase transition 
the free space decreases from 5% to 0.6%. However, the gas pressure does 
not vary but a quantity of gas corresponding to the decrease in volume 
condenses. 
Upon further cooling of the fully charged storage container, i.e. of the 
fully solidified storage medium, the vapour pressure further decreases 
along the vapour pressure curve of the Genetron until at G a minimum 
pressure of approximately 4.67 bar is achieved for the conventionally 
lowest temperatures of freezers. 
When after use the storage container is cleaned, it may be exposed, for 
example, in a rinsing machine, to temperatures of nearly 100.degree. C. 
for a short period of time. The gas pressure in the free space moves along 
the straight line p=Po. (1+T/273) to the right and reaches maximally 1.29 
bar. 
Thus it may be derived from the diagram that the pressure in the storage 
container itself at 100.degree. C. can rise only be 280 mbar above 
atmospheric pressure. Upon cooling, the pressure of the Genetron at the 
melting-point of the NH.sub.4 Cl/H.sub.2 O solution of -16.degree. C. 
still is 758 mbar and remains constant at this value in spite of the 
drastic decrease in volume. Even at the lowest temperature of 
approximately -28.degree. C. which may occur in practice still 465 mbar 
gas pressure remain in the storage container. Per storage container with 1 
liter of storage medium approximately 50 cm.sup.3 of gas filling are used. 
Advantages of the invention are: 
a. small pressure variation under operating conditions, enabling use of 
non-deformable storage container of simple construction and small wall 
thickness; 
b. optimum use of space with correspondingly optimum storage capacity; 
c. good and uniform heat transitions. 
Only a comparatively small free space of approximately 5% in the melted 
state remains which upon freezing, however, goes towards 0%. 
When this small free space is still to be avoided, the gas may be provided, 
instead of directly in the free space, into a flexible sysnthetic resin 
container (for example a rubber or a silicone hose closed at each end) of 
the same contents as the expansion of the storage medium. This is provided 
in the storage container (optionally held in the center of the storage 
device by supports) which is then filled entirely with storage medium and 
is then closed. 
Further examples for suitable latent storage media and their physical data 
are recorded in the table below. From the stated values for the volume 
variation the required gas mixture can easily be derived. 
Table: latent heat storage medium for cooling purposes (x=melting point; 
eut=eutectic; s=solid; 1=liquid). 
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volume 
% by melting density 
(20.degree. C.) 
varia- 
weight point D.sub.s 
D.sub.1 
tion 
composition 
of salt .degree.C. 
g/cm.sup.3 
g/cm.sup.3 
V.sub.1 s 
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Al(NO.sub.3).sub.3 /H.sub.2 O 
30.5 eut -30.6 1.251.sup.x 
1.283 +4.1% 
NH.sub.4 F/H.sub.2 O 
32.3 eut -28.1 1.001.sup.x 
1.096 +9.4% 
NH.sub.4 SCN/H.sub.2 O 
42.0 eut -25.5 1.065.sup.x 
1.097 +4.7% 
KF/H.sub.2 O 
21.5 eut -21.6 1.168.sup.x 
1.194 +5.8% 
NaCl/H.sub.2 O 
22.4 eut -21.2 1.108.sup.x 
1.165 +6.4% 
(NH.sub.4).sub.2 SO.sub.4 / 
39.7 eut -18.5 1.166.sup.x 
1.227 +6.0% 
H.sub.2 O 
NaNO.sub.3 /H.sub.2 O 
36.9 eut -17.7 1.211.sup.x 
1.29 +7.7% 
NH.sub.4 Cl/H.sub.2 O 
19.5 eut -16.0 1.020.sup.x 
1.059 +4.4% 
K.sub.2 HPO.sub.4 /H.sub.2 O 
36.8 eut -13.5 1.303.sup.x 
1.357 +5.1% 
KCl/H.sub.2 O 
19.5 eut -10.7 1.052.sup.x 
1.126 +7.3% 
ZnSO.sub.4 /H.sub.2 O 
27.2 eut -6.5 1.252.sup.x 
1.34 +7.3% 
KHCO.sub.3 /H.sub.2 O 
16.95 eut -5.4 1.018.sup.x 
1.115 +9.2% 
NH.sub.4 H.sub.2 PO.sub.4 / 
16.5 eut -4.0 1.044.sup.x 
1.092 +4.8% 
H.sub.2 O 
MgSO.sub.4 /H.sub.2 O 
19.0 eut -3.9 1.152.sup.x 
1.208 +5.3% 
NH.sub.4 HCO.sub.3 /H.sub.2 O 
9.5 eut -3.9 0.992.sup.x 
1.036 +4.7% 
NaF/H.sub.2 O 
3.9 eut -3.5 0.958.sup.x 
1.040 +8.3 
KNO.sub.3 /H.sub.2 O 
9.7 eut -2.8 0.992.sup.x 
1.068 +7.6% 
Na.sub.2 CO.sub.3 /H.sub.2 O 
5.9 eut -2.1 0.984.sup.x 
1.062 +7.8% 
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