Thermal storage unit

A thermal storage unit has a thermal storage material which performs heat exchange through a fluid filled in a thermal storage body arranged within a vessel having an opening communicated to outside. The thermal storage body is a porous ceramic molding which contains the thermal storage medium.

BACKGROUND OF THE INVENTION 
This invention relates to a thermal storage unit which can recover, store 
and take out as desired heat energy including natural energy such as solar 
energy, terrestrial heat, etc., and artificial energy such as waste heat 
industrially generated, etc., thereby utilizing effectively said energy. 
Energy storage systems may be broadly classified into the sensible heat 
system in which solar heat, etc. is given to a thermal storage material 
such as water, sand, etc. to be stored therein and the temperature itself 
possessed by them is taken out to be utilized, the latent heat system in 
which melting heat or gasifying heat, etc. accompanied with phase change 
is utilized and the chemical reaction heat system in which thermal storage 
is effected by allowing a thermal storage material to undergo a 
endothermic-exothermic reaction to convert the energy into chemical 
energy. 
Among these energy storage systems, the thermal storage unit employing the 
latent heat system is higher in storage density and efficiency as compared 
with the thermal storage units of other systems, and also excellent 
economically with respect to a simple system, and further excellent in 
having also in combination the advantage of enabling enhancement of 
running efficiency of auxiliary equipment, because output and input of 
heat can be done at a constant temperature, and therefore has been 
utilized and investigated in various fields such as solar house, solar 
system for uses in industry, heat generating system and further cosmic 
heat generation. 
Such latent thermal storage unit generally includes the capsule type 9 as 
shown in FIG. 8, comprising a plurality of capsules 11 arranged within a 
vessel 10 having an introducing inlet 10a and a discharging outlet 10b for 
heat medium (fluid) and a thermal storage material 4 filled within said 
capsule 11, and the shell-tube type 12 as shown in FIG. 9 comprising heat 
transfer tubes 14 through which fluid passes between the thermal storage 
materials 4. 
Fluid progresses through the introducing inlet and is brought into contact 
with the thermal storage material through the capsules, etc. 
However, according to either heat exchange methods of the capsule type and 
the shell-tube type, there is involved the problem that the heat exchange 
capacity is remarkably lowered by heat resistance of the solid phase 
attached on the heat transfer surface during heat release. 
As a means for solving this problem, there has been proposed the method in 
which heat exchange is effected with good efficiency by direct contact 
with the heat medium instead of through capsules or heat transfer tubes. 
This effects direct contact with ethylene glycol as a heat medium by 
stabilizing the shape of high density polyethylene as a thermal storage 
material without flowing or sticking to each other even when melted, but 
it is difficult to stabilize the shape, and also there is the problem that 
this method is hardly applicable to thermal storage materials for high 
temperature. 
Also, most of organic polymeric substances and inorganic compounds to be 
used generally for thermal storage materials have large volume changes 
accompanied with phase change between solid phase and liquid phase. 
Accordingly, there is also involved the problem that deformation may occur 
in the vessel such as capsules, etc. holding the thermal storage material, 
or that cracks may occur in the solid phase, thereby lowering heat 
exchange capacity. 
To cope with such problem, as shown in FIG. 10 and FIG. 11, there has been 
proposed a device with the thermal storage material being housed in small 
sections to be scattered with little influence from volume change by 
providing a plurality of projections 16 or providing partitioning portions 
18, etc. as a thermal storage material housing chamber outside of a heat 
transfer tube 15 or 17, respectively. 
However, in this case, the heat transfer tube 15 or 17 and the thermal 
storage material housing chambers 16 or 18 provided therearound are 
required to be made of complicated structures, whereby the preparation 
steps become also complicated to a remarkable economical disadvantage. 
Further, enlargement of the contact area between the fluid and the thermal 
storage material is limited, and thus it has been desired to develop a 
thermal storage unit which can effect further improvement of heat exchange 
capacity. 
Further, in cosmos under minute gravitational force, when employing the 
capsule type and the shell-tube type of the prior art, the latent thermal 
storage material becomes apart from the heat transfer surface during 
melting, whereby there is also a fear that the heat exchange capacity may 
be extremely lowered. Also, for the thermal storage unit for use in 
cosmos, weight reduction thereof has been particularly desired. 
SUMMARY OF THE INVENTION 
An object of the present invention is to overcome the problems as described 
above and provide a thermal storage unit which is excellent in heat 
exchange capacity, light in weight and yet has mechanically strong 
stability to a heat stress and a repeated stress through volume change of 
a thermal storage material occurring during heat exchange, and also can be 
prepared with ease. 
The thermal storage unit of the present invention comprises a thermal 
storage material which exchanges heat through contact with a fluid filled 
in a thermal storage body arranged within a vessel having an opening 
communicated to the outside, characterized in that said thermal storage 
body is a porous ceramic molding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following, the present invention is described in detail by referring 
to drawing. 
FIG. 1 is a longitudinal sectional view showing schematically a thermal 
storage unit 1 comprising a capsule type thermal storage body 3 as the 
porous ceramic molding for holding the thermal storage material, a vessel 
2 having openings 2a and 2b communicated to outside, and a porous plate 2c 
for holding the thermal storage bodies 3. 
The capsule type thermal storage body 3 is, for example, a cylindrical 
hollow body as shown in FIG. 2 and FIG. 3, and after filling of the 
thermal storage material 4 within the porous cylinder 3a, sealing members 
3b are secured on both ends thereof. 
The thermal storage body 3 can be also constituted by use of flat plates 
with relatively smaller dimensions in place of the cylinder 3a or those 
with the same shape as the so-called artificial filling for a packed 
column, such as Raschich ring, etc., but cylindrical hollow bodies can be 
advantageously employed, because relatively more thermal storage material 
4 can be held therein. Further, as shown in FIG. 1, the above thermal 
storage bodies 3 may be arranged either with the respective center axes 
thereof being in parallel to each other or randomly within the vessel 2. 
Also, as in the molding 5 shown in a longitudinal sectional view in FIG. 4, 
a molding comprising a porous ceramic which is not a hollow body as 
different from the above, namely solid molding, can be also used as the 
thermal storage body. In this case, the thermal storage material is packed 
in the pores of the molding 5. 
FIG. 5 is a schematic longitudinal sectional view of the thermal storage 
unit 1 wherein a honeycomb structure body 6 comprises a porous ceramic 
member as the thermal storage unit. 
The honeycomb structure body 6 comprises a number of through-holes 6a and a 
number of closed hollow holes 6b sealed at both ends by securing of 
sealing members 6d through thin partitioning walls 6c along the axis 
direction, and further comprises the thermal storage material 4 filled 
within said hollow holes 6b before said sealing. 
The lateral sectional shape of the holes 6a and 6b of the honeycomb 
structure body 6 is not particularly limited, but may be circular, 
triangular, square, or any other shape. 
Reference numerals 7 and 8 are porous plates for holding the honeycomb 
structure body 6, and the porous plate 7 arranged on the fluid introducing 
inlet 2a side has a fluid introducing hole 7a located at the site 
corresponding to any desired through-hole 6a of the honeycomb structure 
body 6, while the porous plate 8 arranged on the fluid discharging outlet 
2b side has a fluid discharging outlet 8a located at the side 
corresponding to a throughhole 6a other than the through-hole 6a 
corresponding to the introducing hole 7a of the porous plate 7 on the 
introducing side. 
Accordingly, the fluid inflowed through the fluid introducing inlet 2a of 
the vessel 2 flows into the honeycomb structure body 6 through the fluid 
introducing hole 7a of the porous plate 7, and after being passed forcibly 
through the closed hollow hole 6b filled with the thermal storage material 
4, flows out through the fluid discharging outlet 2b via the fluid 
discharging outlet 8a of the porous plate 8. 
The porous ceramic molding such as the above capsule type thermal storage 
body 3 or the honeycomb structure body 6 should be preferably one formed 
into a desired shape such as cylinder or honeycomb by use of powder 
composed mainly of at least one selected from aluminum nitride, silicon 
carbide, silicon, titanium carbide, zirconium boride, titanium boride, 
boron carbide, boron nitride and carbon as the starting material, 
according to conventional method such as the extrusion molding method, the 
sheet molding method, the method of impregnating in an organic sheet, 
press molding method, etc., followed by sintering. Here, by referring to 
an example of silicon carbide molding of the present invention, its 
preparation process is to be described in detail. 
The process for preparing the porous silicon carbide molding of the present 
invention is a process for producing a porous silicon carbide molding 
having open pores with a three-dimensional network structure, comprising 
the first step by use of silicon carbide powder as the starting material 
added with, if necessary, a crystal growth aid to obtain a mixture; the 
second step of adding a binder for molding to said mixture and molding the 
mixture into a molding with a desired shape; and the third step of 
inserting said molding into a heat-resistant vessel and sintering it 
within a temperature range of 1700.degree. to 2500.degree. C. while 
shutting out penetration of the outer air, characterized in that, in 
obtaining the molding in said second step, at least one element selected 
from aluminum, boron, calcium, chromium, iron, lanthanum, lithium, 
yttrium, silicon, nitrogen, oxygen and carbon, and compounds thereof 
(hereinafter sometimes referred to as "transition layer forming aid") is 
permitted to exist within the molding. 
In the above first step, it is preferred to use the .beta.-type silicon 
carbide powder as the starting material. This is because the .beta.-type 
silicon carbide crystal has the specific feature of readily forming plate 
crystals and also is excellent in growth of crystals. Particularly, by use 
of a starting material of which 60% by weight or more comprises the 
.beta.-type silicon carbide, the porous body intended by the present 
invention can be prepared suitably. Among them, it is advantageous to use 
a starting material containing 70% by weight or more of the .beta.-type 
silicon carbide. 
As the crystal growth aid, for example, aluminum, boron, magnesium, carbon, 
etc. may be employed. 
Next, in the second step, to the mixture obtained in the first step is 
added a binder for molding such as methyl cellulose, polyvinyl alcohol, 
water glass, etc., and a molding shaped in a desired shape such as hollow 
cylinder or honeycomb is obtained according to such method as extrusion 
molding, sheet molding, press molding, etc. 
If the above transition layer forming aid remains in a large amount in the 
sintered body, the characteristics inherent in silicon carbide will be 
lost, and therefore its residual amount in the sintered body should be 
desirably as small as possible, preferably 10 parts by weight or less, 
above all preferably 5 parts by weight or less, per 100 parts by weight of 
silicon carbide. 
Next, as the third step, the molding obtained is sealed within a 
heat-resistant vessel and sintered within a temperature range of 
1700.degree. to 2500.degree. C. while shutting out penetration of the 
outer air. 
The reason why the molding is sealed within a heat-resistant vessel and 
sintered while shutting out penetration of the outer air is because the 
adjacent silicon carbide crystals can be fused together and the growth of 
plate crystals can be promoted, whereby the plate crystals can be 
entangled under a complicated state to form a three-dimensional network 
structure. 
Growth of plate crystals can be promoted because evaporation-recondensation 
of silicon carbide between silicon carbide grains and/or migration through 
surface diffusion can be considered to be promoted. 
As the above heat-resistant vessel, it is preferred to use a heat-resistant 
vessel comprising at least one of graphite, silicon carbide, tungsten 
carbide, molybdenum, molybdenum carbide. 
Also, the reason why the sintering temperature is made 1700.degree. to 
2500.degree. C. is because the growth of grains is insufficient if it is 
lower than 1700.degree. C. to form a porous body having high strength with 
difficulty, while if it is higher than 2500.degree. C., sublimation of 
silicon carbide is accelerated and the plate crystal developed will become 
contrariwise slender, whereby a porous body having high strength can be 
obtained with difficulty. A more preferred range may be 1800.degree. to 
2300.degree. C. 
In the above molding, without forming pores with uniform pore diameter and 
pore ratio within the molding, gradients may be also created in pore 
diameter and pore ratio internally of the molding. 
For example, as shown in FIG. 4, a solid cylindrical porous body rather 
than a hollow body is formed, and the pores are formed with the pore 
diameters and pore ratios becoming smaller stepwise or continuously from 
the central portion of said porous body toward the outer peripheral 
portion. Alternatively, in the case of hollow bodies as shown in FIG. 2 
and FIG. 3, they may be also formed with the pore diameters and pore 
ratios becoming smaller stepwise or continuously from the inner surface 
toward the outer surface of a cylinder 3a. In the case of a honeycomb 
structure body 6 as shown in FIG. 5, they can be also formed with pore 
diameters, pore ratios becoming smaller similarly as described above from 
the thermal storage material side of each partitioning wall 6c toward the 
outside. 
This is because, reduction of the thermal storage material through external 
dissipation can be made extremely small with increase of the inside 
thermal storage material when the thermal storage material is used as 
impregnated in said molding. 
For forming such porous molding, in obtaining a molding in the second step 
of the above preparation process, the above transition layer forming aid 
is permitted to exist so as to create a concentration gradient in the 
molding, and open pores of the above network structure are formed so that 
their average pore diameter may become smaller, for example, from the 
central portion toward the peripheral portion of the molding stepwise or 
continuously. 
Formation of such pores may be practiced by coating the above molding 
directly with a solution containing the above compound, or by removing the 
binder for molding the above molding to make it porous, followed by 
similar impregnation. 
Concentration gradient is created because, of the above substances, 
aluminum, boron, calcium, chromium, iron, lanthanum, lithium and yttrium 
have a role of accelerating the rate of silicon carbide crystal grain 
growth, and extremely many nuclei of plate crystals are formed at the 
sites where these substances exist and developments of plate crystals 
occur at the respective portions, whereby the sizes of the plate crystals 
formed are restricted and three-dimensional network structures with finer 
textures are formed at the sites where these substances exist in more 
amounts. 
In contrast, silicon, nitrogen, oxygen and carbon have contrariwise a role 
of retarding the rate of silicon crystal grain growth, and at the sites 
where these substances exist, nucleus formation of plate crystal is 
inhibited and the number of plate crystals becomes smaller, whereby the 
respective plate crystals are grown relatively larger and therefore 
greater three-dimensional network structure can be formed at the site 
where these substances exist in more amounts. 
Accordingly, for obtaining a molding in which the open pores of the network 
structure are formed with an average pore diameter becoming smaller 
stepwise or continuously, for example, from the central portion toward the 
peripheral portion of the molding, there may be employed the method in 
which, of the above transition layer forming aids, aluminum, boron, 
calcium, chromium, iron, lanthanum, lithium or yttrium is contained in the 
vicinity of the peripheral portion of the molding, followed by sintering 
according to the method as described above, or the method in which 
silicon, nitrogen, oxygen or carbon is contained in the vicinity of the 
central portion of the molding, followed by sintering according to the 
method as described above, or further the method in which both methods are 
used in combination. 
The sealing members 3b and 6d to be secured at the predetermined ends of 
the porous ceramic molded product as shown in FIG. 2 and FIG. 3 or FIG. 5 
are preferably of the plate sintered body comprising the same material as 
described above. The securing method may include the adhesion method, the 
mechanical bonding method by screw, etc., but is not particularly limited. 
The sintered body comprising the material as described above has relatively 
higher mechanical strength even if the pore ratio may be larger, and can 
stand sufficiently the thermal stress through volume change occurring by 
phase change of the thermal storage material 4, and yet can effect heat 
exchange with good efficiency due to high thermal conductivity. 
Among them, one comprising at least one of silicon carbide, boron carbide, 
silicon and carbon as the main component with a mechanical strength of 500 
kg/cm.sup.2 or higher may be more preferred. 
The pore ratio of the above porous ceramic molding is preferably 80 to 30% 
by volume. If it is larger than 80% by volume, the mechanical strength of 
the molding becomes smaller, while if it is smaller than 30% by volume, 
the ratio of the thermal storage material 4 filled in the pores becomes 
smaller, whereby no efficient heat exchange occurring on the molding 
surface can be effected. Above all, 55 to 35% by volume is more preferred. 
Further, the average pore size is preferably 50 .mu.m or less. This is 
because the thermal storage material 4 will not be leaked out from pores 
of the molding when the thermal storage material 4 undergoes phase change 
to become liquid. Above all, it is more preferably 30 .mu.m or less. 
On the other hand, as the thermal storage material 4, it is advantageously 
at least one selected from LiF, NaF, KF, MgF.sub.2, CaF.sub.2, LiH and 
eutectic mixtures containing at least one of these as the component, such 
as KF--MgF.sub.2, NaF--MgF.sub.2, NaF--KF--MgF.sub.2, CaF.sub.2 
--MgF.sub.2, LiF--CaF.sub.2, LiF--MgF.sub.2, CaF.sub.2 --MgF.sub.2 --NaF, 
LiF--KF--MgF.sub.2, NaF--KF, LiF--NaF--MgF.sub.2, NaF--KF--MgF.sub.2, 
LiF--LiH, NaF--FeF.sub.2, MgF.sub.2 --BeF.sub.2, LiF--NaF, 
LiF--NaF--CaF.sub.2, KCl--LiF--NaF, LiF--NaF--NaCl, LiF--KF, LiF--LiCl, 
LiF--BeF.sub.2, LiF--NaF--KF, LiF--LiCl--LiH, LiF--NaF--KF--MgF.sub.2, 
LiF--LiOH and NaF--BeF.sub.2. This is because these compounds have greater 
latent heat to be excellent in thermal storage efficiency. 
Among them, it is more preferred to use at least one selected from LiF, 
NaF, KF, MgF.sub.2, CaF.sub.2 and eutectic mixtures containing one of 
these as the component. 
Also, the above compound should be preferably one having excellent adhesion 
to the porous ceramic molding, and among those as mentioned above, LiF, 
MgF.sub.2 and eutectic mixtures containing at least one of these as the 
component are particularly preferred. 
Further, these thermal storage materials 4 are preferably filled in the 
pores forming the cylindrical tube 3a of the hollow body or the 
partitioning wall 6c forming the hollow hole when filled within the hollow 
body or within hollow holes of the porous ceramic molding shown in FIG. 2 
and FIG. 3 or FIG. 5. This is because heat exchange occurring on the 
molding surface can be made more efficient. Particularly, 40% by volume or 
more of all the thermal storage material 4 are advantageously filled 
within the pores. 
EXAMPLE 1 
To 100 parts by weight of silicon carbide fine powder of a purity of about 
98% with an average particle size of 0.25 .mu.m were added 5 parts by 
weight of methyl cellulose and 35 parts by weight of water, and after 
thoroughly kneaded, the mixture was subjected to extrusion molding through 
an extruder having a dice outer diameter of 10 mm and an inner diameter of 
8 mm into a hollow cylinder as shown in FIG. 2. 
On the other hand, the both end surfaces of the above extruded molding 
shaped in a hollow cylinder were sealed by use of the above starting 
materials. Subsequently, the molding was dried, defatted under an 
oxidative atmosphere and then sintered by holding in a Tanman furnace 
under argon atmosphere at 1800.degree. C. for 3 hours. The sintered body 
obtained was porous with a porosity of 45% by volume, an average pore 
diameter of 2.5 .mu.m, substantially without shrinkage and having a high 
strength of 18 kg/mm.sup.2. 
Subsequently, the hollow cylindrical porous sintered body was vacuum 
impregnated with molten LiF under vacuum of 0.2 Torr. 
The hollow cylindrical sintered body which is a thermal storage body 
obtained was found to be filled with 88% by volume of LiF at the spatial 
portion internally of the cylinder and at the pore portion of the porous 
body. 
Next, 15 thermal storage bodies holding the thermal storage material thus 
obtained were arranged within a vessel of 30 cm in diameter and 1 m in 
length, and air of a temperature of 945.degree. C. was delivered at 1 
m.sup.3 /min from the inlet side to effect thermal storage, and the change 
in the discharged air temperature at the outlet was measured. The result 
is shown in FIG. 6. 
Then, conversely, air of 745.degree. C. was delivered at 1 m.sup.3 /min. 
from the outlet side, and the temperature of the discharged air from the 
inlet side was measured. The result is shown in FIG. 7. 
EXAMPLE 2 
The fine powder of silicon carbide employed as the starting material was 
one of which 80% by weight comprised .beta.-type crystals. In the starting 
material were contained as impurities were contained 0.01 of B, 0.5 of C, 
0.01 of Al, 0.2 of N and 0.08 of Fe in atomic weight part, respectively, 
and traces of other elements, and the total amount of these impurities was 
0.81 atomic weight part. The starting material had an average particle 
size of 0.3 .mu.m and a specific surface area of 18.7 m.sup.2 /g. 
To the starting material were added 10 parts by weight of methyl cellulose 
as the binder for molding and 20 parts by weight of water. The mixture was 
kneaded and a solid cylindrical silicon carbide material molding with a 
diameter of 10 mm and a length of 1.2 m was obtained according to the 
extrusion molding method. 
The molding was heated in an oxidative atmosphere up to 500.degree. C. at a 
temperature elevation rate of 1.degree. C./min to remove the above organic 
binder by oxidation. Subsequently, an aqueous solution of BN fine powder 
(0.2 .mu.m particle) was added at the portion 2 mm from the outer 
peripheral portion of the molding, followed by drying. As the result, B 
was found to be contained at 0.1% in the portion 2 mm from the outer 
peripheral portion, being gradually and continuously reduced toward the 
inner side, until B was contained at 0.01% in the portion 5 mm from the 
outer peripheral portion. 
Then, the molding was placed in a graphite crucible with a porosity of 20% 
and sintered in Ar gas atmosphere of 1 atm. 
Sinteringation was effected by elevating the temperature at 2.degree. 
C./min up to 2150.degree. C. and holding the maximum temperature for 4 
hours. 
The sintered body obtained was porous with a porosity of 40% by volume, an 
average pore diameter of 2 .mu.m at the outer peripheral portion, and a 
porosity of 50% by volume, an average pore diameter of 12 .mu.m at the 
central portion, having a high strength of 9.5 kg/mm.sup.2. 
Subsequently, the thermal storage material LiF was filled into the pores of 
the sintered body according to the same method as in Example 1, and 15 
cylinders were arranged in parallel within a vessel of 30 cm in diameter 
and 1 m in length in the same manner as in Example 1, followed by testing 
according to the same measurement method. The results are shown in FIG. 6 
and FIG. 7. 
COMATIVE EXAMPLE 
Into a vessel made of Ni having the same shape as the silicon carbide 
sintered body of Example 1 was filled the same thermal storage material 
LiF as used in Example 1. The filling percentage was found to be 95% by 
volume. 
Subsequently, the cylinders were arranged within a vessel similarly as 
described in Example 1, and the same operations were conducted to measure 
the temperatures on the inlet side and the outlet side. The results are 
also similarly shown in FIG. 6 and FIG. 7. 
From the above results, as shown in FIG. 6, it can be understood that in 
the thermal storage unit of Example 1, the outlet temperature of the 
vessel became 945.degree. C. which was the same as the inlet temperature 
after about 80 minutes, while being heat stored in the thermal storage 
body with lapse of time, thus indicating thermal storage in LiF during 80 
minutes. 
On the other hand, in the case of Comparative example, the outlet 
temperature of the vessel was 935.degree. C. even after elapse of 110 
minutes, indicating the state in which heat was not sufficiently stored, 
namely during thermal storage, thus indicating lower heat exchange ability 
as compared with Example 1. 
Also, in Example 1, thermal storage is effected better correspondingly than 
in Comparative example, and the corresponding rate from the initial value 
of the housing vessel temperature is more rapid. For example, the outlet 
temperature after elapse of 20 minutes is about 840.degree. C. in Example 
1, while about 870.degree. C. in Comparative example. Thus, in Example 1, 
thermal storage is effected surely with good correspondence. 
Further, in Example 1, thermal storage state in latent heat is exhibited 
from 30 minutes to 50 minutes after high temperature gas flowing, namely 
during about 20 minutes, indicating that output and input of heat at a 
constant temperature (about 850.degree. to 860.degree. C.), namely heat 
exchange at a constant temperature, is possible. 
In FIG. 7, similarly as in Example 6, Example 1 is rapid in corresponding 
rate, and heat is taken out sufficiently from the thermal storage body 
during about 50 minutes after inflow to be discharged as high temperature 
fluid, indicating that more heat is absorbed than at the initial time. 
Similarly as described above, in Example 1, it is shown that heat exchange 
through latent heat is effected with good efficiency under the state of a 
constant temperature of about 850.degree. to 860.degree. C.. Accordingly, 
if heat cycle (output and input of heat) is performed in this range, it 
can be effectively used as the latent heat system thermal storage unit. 
Also, the above physical properties did not change even after 100 repeated 
operations, and no leak of molten salt and ceramic breaking was observed. 
On the other hand, in Example 2, although similar as in Example 1, the 
product was made a solid cylinder and therefore the amount of LiF filled 
was increased, and the heat exchange capacity through latent heat was 
further increased in the state at a constant temperature of about 
850.degree. to 860.degree. C., whereby good results can be obtained. 
EXAMPLE 3 
Application of a thermal storage unit for generation of electricity by 
cosmic heat 
As a constitutional example of a thermal storage unit for generation of 
electricity by cosmic heat, an exemplary apparatus is shown in FIG. 12, 
and an enlarged portion of which is shown in FIG. 12A. This apparatus was 
arranged many numbers o thermal storage units 19 so as to surround a heat 
exchanger tube 20. In FIG. 12, 21 is an insulation, 22 is an apperture, 23 
is reflectors, 24 is an outlet mainfold, 25 is an inlet mainfold, 26 is a 
fluid inlet and 27 is a fluid outlet. By this constitution, preparation of 
the thermal storage unit, and maintenance and control thereof can be 
carried out easily and improvement in characteristics can be made. 
A structure for cosmic at a height of 450 km or so goes around the earth 
for about 90 minutes. When getting the sunlight, a part of the thermal 
storage unit is heated by the collected sunlight, and while heating a heat 
medium, a thermal storage material is fused to conduct thermal storage. At 
eclipse of sunlight, the heat medium is heated by taking a latent heat 
from the thermal storage material while solidifying it. Simulation results 
of an outlet temperature of the heat medium (mixed medium of herium and 
xenon) when thermal storage and dissipation were repeatedly carried out is 
shown in FIG. 13. In this simulation, LiF is used as a thermal storage 
material. 
As seen from FIG. 13, it can be understood that substantially constant 
outlet temperature of the heat medium can be obtained. 
EXAMPLE 4 
Example of thermal storage system 
As an example of a thermal storage system using the thermal storage unit of 
the present invention, porosity of the ceramic molding is changed to 
change the ratio of a thermal storage material, or kinds of a molding or a 
thermal storage material are changed which can be varied depending on the 
place constituted of the thermal storage system. 
As one example, at a portion which is the most easily heated, a thermal 
storage unit having a high compositional ratio of a thermal storage 
material, a thermal storage material having high melting point or a 
molding having high thermal conductivity is used, and at a portion which 
is hardly heated, a thermal storage unit having a low compositional ratio 
of a thermal storage material or a thermal storage material having a low 
melting point is used. By the above consitution, it can be clarified by 
the result of simulation that effective utilizing ratio of the thermal 
storage material can be improved. 
EXAMPLE 5 
Prevention of evaporation of molten salt 
When a thermal storage material having a high vapor pressure is used, 
evaporation of the thermal storage material becomes serious problem 
particularly when it is used under high vacuum condition such as cosmos. 
This problem can be solved by covering the surface of a ceramic molding 
with a dense material. 
As a method of covering the surface of the ceramic molding, there may be 
mentioned a chemical vapor deposition method, a physical vapor deposition 
method, a sintering method and a flame spraying method. 
In case of a thermal storage unit comprising silicon carbide and lithium 
fluoride, a phase of silicon carbide is formed on a surface of the thermal 
storage unit by a chemical vapor deposition method with a film thickness 
of several microns to several ten microns. By this treatment, evaporation 
of lithium fluoirde can be prevented. Further, a method in which a surface 
of a porous ceramic is coated with a dense phase apart from a part 
thereof, and a thermal storage material is impregnated therein from 
uncoated portion and finally the uncoated portion is coated with a dense 
material is also effective as prevention of evaporation. 
According to the thermal storage unit of the present invention, since a 
porous ceramic molding having high mechanical strength and high thermal 
conductivity used as the vessel for holding the thermal storage material, 
heat exchange capacity is excellent, and also it has sufficient mechanical 
strength to thermal stress through volume change of the thermal storage 
material occurring during heat exchange and repeated stress thereof. Also, 
since the structure of the porous molding is simple, it can be prepared 
with ease. 
Also, since the molten salt can be held within the pores of the porous 
molding through capillary phenomenon, heat exchange capacity is good also 
in cosmos under minute gravitational force. Further, since no heat 
transfer tube is required, the weight can be reduced to great extent as 
compared with one of the prior art.