Heat storage material

A heat storage material having a melting point in the range of 90.degree. to 100.degree. C. and is excellent in that it is neither corrosive, inflammable nor toxic. The heat storage material comprises a mixture of dimethyl terephthalate and one member selected from the group consisting of dimethyl fumarate and dihydroanthracene.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a heat storage material which consists of a 
mixture of dimethyl terephthalate and a compound selected from the group 
consisting of dimethyl fumarate and dihydroanthracene. More particularly, 
the invention relates to a heat storage material which is suitable for use 
in devices which utilize, for example, solar energy for room heating or 
cooling, hot-water supply and so forth. 
2. Description of the Prior Art 
The characteristic properties that are generally required of heat storage 
materials are large specific heat and/or large latent heat of fusion, 
thermal stability, noncorrosiveness, low vapor pressure, noninflammability 
and innoxiousness. 
When the latent heat of fusion is utilized for storing thermal energy, the 
amount of stored heat can be made large and the volume of the heat storage 
material can be made small, which are quite advantageous in practice. A 
material which is suitable for storing thermal energy by its latent heat 
of fusion is characteristically converted from one phase to another phase 
when it is heated to its own phase transition temperature. Various 
materials have been proposed as heat storage materials with the phase 
transition, for example, inorganic salt hydrates such as calcium chloride 
hexahydrate (U.S. Pat. No. 4,189,394--J. Schroder et al.), magnesium 
iodide hexahydrate, sodium sulfate decahydrate, barium hydroxide 
octahydrate and ammonium aluminum sulfate dodecahydrate; organic compounds 
or mixtures thereof such as tetradecane, pentadecane, decanol and a 
mixture of sodium acetate and sodium chloride; and low molecular weight 
organic salts. When inorganic salts or low molecular weight organic salts 
are used, however, the problem of corrosion of the thermal system becomes 
serious. Therefore, the tanks, pipes and other devices of the thermal 
system are generally made of noncorroding metals so that they are usually 
heavy and high in thermal conductivity. In addition, they are generally 
expensive, which increases the construction cost of the thermal energy 
storage system. Furthermore, there always remains the fear that the tanks 
or pipes may break, resulting in leakage of solution or fused salts. 
If an inorganic salt is mixed with another inorganic salt, a composition 
having a desired melting point can be obtained. However, if the 
composition deviates from the eutectic mixture of the inorganic salt 
components, separation of components occurs during solidification of the 
fused liquid. Accordingly, only eutectic mixtures are used as the heat 
storage materials. However, eutectic mixtures of inorganic salts are 
liable to supercool to temperatures considerably below the eutectic 
points. Therefore, it is necessary to add seed crystals or a nucleating 
agent, which causes the above-mentioned separation of components. 
Incidentally, when a paraffin mixture is employed as the heat storage 
material, it is not advantageous in practice because the range of melting 
point is wide and the production cost for pure paraffin is quite high. 
Furthermore, there are proposed mixtures containing higher fatty acids 
such as lauric acid, stearic acid and oleic acid (U.S. Pat. Nos. 
2,726,211--V.J. Schaefer and 4,100,092--H.O. Spauschus et al., and British 
Pat. No. 1,558,522--Ciba-Geigy AG). However, the mixtures do not meet the 
later-described requirement of 90.degree. to 100.degree. C. in melting 
point. 
With the increasing concern over the depletion of ordinary energy sources, 
considerable attention has been given to the use of solar energy, and 
various kinds of systems utilizing solar energy are being developed. Solar 
energy water heaters have already been used in practice. With them, water 
is heated by solar heat, and the obtained hot water is used as it stands 
or after being temperature-controlled, for room heating, bathing, cooking 
and washing. Besides the use of solar energy for heating and cooking with 
hot water, a new system is now being eagerly developed to drive a Rankine 
cycle engine by using the obtained hot water for the purpose of cooling. 
In order to drive the Rankine cycle engine of this system by using water 
as a heat transfer medium, it is desirable that the water temperature be 
as high as possible, about 90.degree. to 100.degree. C. Therefore, the key 
to accomplishing the above system is to develop an improved thermal energy 
storage material which can store much solar energy at high temperature and 
which can supply a large quantity of high temperature water. In other 
words, the heat storage material which is earnestly desired in this 
technical field is one which is free from the above-described 
difficulties, which is produced at low cost, and which has a large heat of 
fusion and a proper melting point in the range of 90.degree. to 
100.degree. C. 
BRIEF SUMMARY OF THE INVENTION 
In order to comply with the above requirements, the inventors of the 
present application have carried out extensive investigations with regard 
to various kinds of materials and, as a result, the heat storage material 
of the present invention has been found. 
It is, therefore, the primary object of the present invention to provide an 
improved heat storage material which meets the foregoing requirements. 
Another object of the present invention is to provide a heat storage 
material which has a melting point in the range of 90.degree. to 
100.degree. C. for advantageously utilizing its latent heat of fusion. 
A further object of the present invention is to provide a heat storage 
material which is neither corrosive, inflammable nor toxic, and which can 
be used quite safely. 
Still a further object of the present invention is to provide a heat 
storage material which has excellent thermal and chemical stability so as 
to be used repeatedly for a long period of time. 
In accordance with the present invention, the heat storage material 
comprises a mixture of dimethyl terephthalate and a compound selected from 
the group consisting of dimethyl fumarate and dihydroanthracene. When a 
heat storage material having a melting point in the range of 90.degree. to 
100.degree. C. is required, the mixture is made of 3 to 35% by weight of 
dimethyl terephthalate and 97 to 65% by weight of dimethyl fumarate, or 20 
to 40% by weight of dimethyl terephthalate and 80 to 60% by weight of 
dihydroanthracene. 
The above heat storage material is, of course, suitable for various kinds 
of uses such as body warmers, foot warmers and rollers of hair curlers as 
well as for devices which utilize solar energy for room heating or cooling 
and hot-water supply.

DETAILED DESCRIPTION OF THE INVENTION 
The characteristics of component compounds used for the heat storage 
material mixture of the present invention, i.e. dimethyl terephthalate, 
dimethyl fumarate and dihydroanthracene are given in the following Table 
1. 
TABLE 1 
______________________________________ 
Dimethyl Dimethyl Dihydro- 
Items terephthalate 
fumarate anthracene 
______________________________________ 
Structural formula 
##STR1## 
##STR2## 
##STR3## 
Molecular 
194 144 180 
weight 
Melting 140.degree. C. 
101.degree. C. 
108.degree. C. 
Point 
Boiling 288.degree. C. 
192.degree. C. 
305.degree. C. 
point 
Heat of 39.8 54.3 31.8 
fusion 
(cal/g) 
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In the above Table 1, the melting points and heats of fusion were 
determined with using the Perkin-Elmer Differential Scanning Calorimeter 
DSC-2 for a sample size of about 6.0 mg and a temperature rise of 
10.degree. C./min. 
The melting points of the mixtures of these compounds depend upon the 
compositions thereof. Shown in FIG. 1 is the relation between the melting 
point and the composition of a mixture of dimethyl terephthalate and 
dimethyl fumarate (solid line) and a mixture of dimethyl terephthalate and 
dihydroanthracene (chain line). In the drawing, the abbreviations "DMT", 
"DMF" and "DHA" represent dimethyl terephthalate, dimethyl fumarate and 
dihydroanthracene, respectively. For example, when a mixture of 50% by 
weight of dimethyl terephthalate and 50% by weight of dimethyl fumarate 
was fused and gradually cooled, precipitation of dimethyl terephthalate 
first occurred from the Point A in FIG. 1, and the precipitation proceeded 
accompanied by changes in the composition of the fused liquid phase. When 
the mixture reached the Point B, precipitation of both components occured 
at the fixed temperature of 91.5.degree. C. and the phase change into 
solid state was completed as it stands. In other words, the melting point 
of a mixture like the one above is not fixed, but falls within a range of 
melting point. However, when 25 parts by weight of dimethyl terephthalate 
and 75 parts by weight of dimethyl fumarate were mixed together to obtain 
a eutectic mixture, the mixture melted or solidified at a constant 
temperature of 91.5.degree. C. (Point B in FIG. 1). 
Similar phase change occured in the mixture of dimethyl terephthalate and 
dihydroanthracene, in which the weight ratio of dimethyl terephthalate to 
dihydroanthracene of the eutectic mixture was 33 to 67, and the melting 
point thereof was 93.5.degree. C. (Point C in FIG. 1). 
Shown in FIG. 2 is the DSC thermogram on the eutectic mixture of the 
foregoing dimethyl terephthalate and dimethyl fumarate. As seen from FIG. 
2, the eutectic mixture had a sharp peak in the range of 90.degree. C. to 
100.degree. C., with the temperature of the peak being about 95.degree. C. 
The heat of fusion of the mixture was 46.5 cal/g. Further, the other 
eutectic mixture of dimethyl terephthalate and dihydroanthracene was 
tested likewise and similar results were obtained, in which the peak 
temperature of the DSC thermogram was about 98.0.degree. C. and the heat 
of fusion, 33.5 cal/g. From this, it is understood that the latter 
eutectic mixture can provide the same effects as the former eutectic 
mixture. 
Further, the foregoing mixture of dimethyl terephthalate and dimethyl 
fumarate was fused by being heated to 100.degree. C. and then allowed to 
cool, and the temperature of the mixture was measured by a thermocouple. 
The results of this test are shown in FIG. 3. As seen from the figure, the 
temperature of the above mixture drops to about 90.degree. C. into a 
supercooled state and, when the supercooled state is broken, the heat with 
phase change is released at a high temperature of 91.degree. to 93.degree. 
C. When this phase change is finished, the temperature of the mixture 
again drops. The temperature during this phase change is the value 
suitable for the purpose of the present invention. Therefore, it will be 
understood that the above mixture has quite excellent characteristics to 
be used as a heat storage material according to the present invention. 
Furthermore, the mixing ratio of the dimethyl terephthalate and dimethyl 
fumarate in the present invention may be so determined that the obtained 
mixture has a desirable melting point, that is, a temperature in the range 
of 90.degree. to 100.degree. C. As shown by the dashed lines in FIG. 1, 
the preferred mixing ratio is 3-35% by weight of dimethyl terephthalate to 
97-65% by weight of dimethyl fumarate. In like manner, in the case of the 
mixture of dimethyl terephthalate and dihydroanthracene, 20-40% by weight 
of the former is preferably mixed with 80-60% by weight of the latter. 
EXAMPLES 
In order to confirm the practical effect of the above-described mixtures as 
heat storage materials, the following tests were carried out. 
(a) Preparation of Mixtures 
Mixtures M-1, M-2 and M-3 were prepared in accordance with the formulae in 
the following Table 2. 
TABLE 2 
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Mixture 
Component (wt %) 
M-1 M-2 M-3 
______________________________________ 
dimethyl terephthalate 
25 33 32 
dimethyl fumarate 
75 -- 68 
dihydroanthracene 
-- 67 -- 
______________________________________ 
In the above mixtures, the Mixtures M-1 and M-2 were eutectic mixtures and 
the Mixture M-3 was not a eutectic mixture but included in the scope of 
the present invention. 
(b) Heating Test 
9.8 kg of each mixture was dividedly put into 10 of stainless steel made 
cylindrical tubes of 5 cm in diameter and 50 cm in length and the tubes 
were sealed up. The tubes were placed in a closed container. This 
container was then fed with hot water of 100.degree. C. to heat up and 
fuse the test mixture in the cylindrical tubes. The fusion of text mixture 
was confirmed by previously inserting a thermometer into the cylindrical 
tube. After the fusing, heat exchange was carried out by supplying 100 
ml/min of 80.degree. C. water in place of the 100.degree. C. hot water. 
The temperatures of heated water and the times (hrs.) of discharge were 
measured which are shown in the following Table 3. 
TABLE 3 
______________________________________ 
Mixture M-1 M-2 M-3 
______________________________________ 
Temperature (.degree.C.) 
90 92 90-97 
Time (hours) 5.4 4.0 4.5 
______________________________________ 
It will be understood from the above disclosure that the heat storage 
material of the present invention is quite useful. 
While preferred examples of the invention have been described, it will be 
understood by those skilled in the art that changes and modifications may 
be made without departing from the spirit and scope of the invention.