HEAT STORAGE APPARATUS

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2012-249105, filed Nov. 13, 2012, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described here relate to a heat storage apparatus.

BACKGROUND

There is known a heat storage apparatus in which a supercooling of a phase-changeable heat storage material is used to store heat in the heat storage material in a supercooled state, the supercooled state is released to change the heat storage material from a liquid-phase state to a solid-phase state when a heat dissipation is requested, and an object is heated by latent heat emitted accordingly. In the heat storage apparatus, regarding a nucleation operation of releasing the supercooling of the heat storage material, there has been proposed a method of providing scratches with a plurality of grooves in a copper electrode to which a voltage is applied for causing a nucleation, the grooves having a V-shaped cross-section that is gradually deeper from a circumferential surface near an end face of the copper electrode to the end face of the copper electrode. By applying a voltage to the copper electrode, a crystal nucleation occurs from a portion where the scratches of the copper electrode are given, and thus, the heat storage material can be nucleated.

Changing the heat storage material from the liquid-phase state to the solid-phase state by releasing the supercooled state of the heat storage material being in a liquid-phase state by supercooling is referred to as “nucleation”. If the nucleation operation is started when the heat storage material is in the liquid-phase state, the heat storage material being in the liquid-phase state by supercooling can be changed to the solid-phase state

In the heat storage apparatus using the heat storage material having supercooling, when the voltage is applied to the electrode by the nucleation operation, if a probability of generating crystal nuclei in the vicinity of the electrode is low, it may take a long time to generate crystal nuclei, or crystal nuclei may not be generated even though the voltage is applied for a predetermined time.

When the time taken to generate the crystal nuclei (in other words, time taken until the nucleation is started) is long, the controllability of the supercooling is reduced. That is, after the voltage is applied for nucleation, it takes a long time until the supercooled state of the heat storage material being in the liquid-phase state by supercooling is released. Therefore, latent heat accumulated in the heat storage material cannot be emitted rapidly after the nucleation operation.

When the crystal nuclei cannot be generated without regard to the application of the voltage, the control of releasing the supercooling of the heat storage material is impossible. That is, even after the nucleation operation is performed, the heat storage material does not change to the solid-phase state, and maintains the supercooled liquid-phase state. Since the heat storage material is not coagulated in this manner, the latent heat accumulated in the heat storage material cannot be emitted.

Therefore, increasing the probability of generating the crystal nuclei in the vicinity of the electrode to which the voltage is applied can contribute to improving the controllability of the supercooling and the reliability of releasing the supercooling. Therefore, there is a need for developing a heat storage apparatus that can solve these problems.

Embodiments are directed to provide heat a storage apparatus that can nucleate a supercooled heat storage material quickly and reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a configuration of a heat storage apparatus according to a first embodiment;

FIG. 2 is an enlarged side view illustrating a part of an anode electrode according to the first embodiment;

FIG. 3 is a cross-sectional view illustrating a first aspect of a nucleation start point portion of the anode electrode according to the first embodiment;

FIG. 4 is a cross-sectional view illustrating a second aspect of the nucleation start point portion of the anode electrode according to the first embodiment;

FIG. 5 is a cross-sectional view illustrating a third aspect of the nucleation start point portion of the anode electrode according to the first embodiment;

FIG. 6 is a cross-sectional view illustrating a fourth aspect of the nucleation start point portion of the anode electrode according to the first embodiment;

FIG. 7 is a cross-sectional view illustrating a fifth aspect of the nucleation start point portion of the anode electrode according to the first embodiment;

FIG. 8 is a graph obtained by measuring a relationship with nucleation start time of various samples having different shapes of nucleation start point portions;

FIG. 9 is a schematic view illustrating a configuration of a heat storage apparatus according to a second embodiment;

FIG. 10 is an enlarged side view illustrating a part of an anode electrode according to a third embodiment;

FIG. 11 is a cross-sectional view taken along line F11-F11 illustrated in FIG. 10;

FIG. 12 is an enlarged schematic view illustrating an apex portion (portion A) of the anode electrode illustrated in FIG. 11;

FIG. 13 is an enlarged side view illustrating a part of an anode electrode according to a fourth embodiment;

FIG. 14 is an enlarged side view illustrating a groove formed in the anode electrode illustrated in FIG. 13;

FIG. 15 is an enlarged side view illustrating a part of an anode electrode according to a fifth embodiment;

FIG. 16 is a cross-sectional view of the anode electrode illustrated in FIG. 15, taken along line F16-F16; and

FIG. 17 is an enlarged schematic view illustrating an apex portion (portion B) of the anode electrode illustrated in FIG. 16.

DETAILED DESCRIPTION
First Embodiment

Hereinafter, a heat storage apparatus according to a first embodiment will be described in detail with reference to FIGS. 1 to 8.

As in a schematic view of FIG. 1 illustrating a configuration of a heat storage apparatus, the heat storage apparatus 1 includes a heat storage unit 2 and a controller 11.

The heat storage unit 2 includes a heat storage tank 3, a heat storage material 4, an anode electrode 5, and a cathode electrode 6.

The heat storage material 4 is received in the inside of the heat storage tank 3. As the heat storage material 4, a latent heat storage material having supercooling (phase change material (PCM)) is used. The heat storage material 4 has a property that does not coagulate but maintains a liquid-phase state even when a temperature falls to below a melting point from the liquid-phase state. Such a heat storage material is referred to as a latent heat storage material or a phase change heat storage material having supercooling.

Examples of the heat storage material 4 may include sodium acetate such as sodium acetate hydrate, sodium sulfate such as sodium sulfate hydrate, and the like. When a heat storage temperature is high, sodium acetate hydrate is preferably used. A general physical property of sodium acetate hydrate is that a melting point is 40 to 58° C., latent heat is 100 to 264 kj/kg, and specific heat is 1 to 4 kJ/kg/K.

The anode electrode 5 is made of a material having a conductivity for applying a constant voltage to the heat storage material 4. The anode electrode 5 has a nucleation start point portion 7 at a part thereof. The nucleation start point portion 7 is integrally provided on a circumferential surface of a round-bar-shaped anode electrode body 5a, for example, while forming a spiral shape as in an enlarged side view of FIG. 2 illustrating a part of the anode electrode 5.

The nucleation start point portion 7 is provided with a tapered protrusion. As a specific example, as illustrated in FIG. 3, the nucleation start point portion 7 is provided with a protrusion having a triangular cross-section. A cross-sectional shape along a direction perpendicular to an extending direction of the spiral nucleation start point portion 7 is a triangle as illustrated in a first aspect of the nucleation start point portion of FIG. 3, and an angle θ between two oblique sides 7a and 7b of the triangle is 90° or less. In addition, a height h of the nucleation start point portion 7 is in a range from 5 μm or more to 100 μm or less. Herein, the height h of the nucleation start point portion 7 is defined as an arithmetic mean roughness Ra of JIS B0601-2001. That is, since h=Ra, the height h of the nucleation start point portion 7 can be represented by a value of the arithmetic mean roughness Ra.

The cross-sectional shape of the nucleation start point portion 7 illustrated in FIG. 3 is an isosceles triangle having a pointed apex, but is not limited thereto. The nucleation start point portion 7 may have a distorted shape at an apex portion thereof. Examples of such a shape may include shapes of the nucleation start point portions 7 illustrated in cross-sectional views of FIGS. 4 to 6 illustrating nucleation start points of the anode electrode according to second to fourth embodiments.

That is, the nucleation start point portion 7 formed to have a tapered protrusion as illustrated in FIG. 4 includes a rounded apical surface 7c at an apex. Also, the apical surface of the nucleation start point portion 7 in FIG. 4 may be a flat apical surface instead of a curved apical surface.

The nucleation start point portion 7 formed to have a tapered protrusion as illustrated in FIG. 5 includes a concave portion 7d at an apex portion thereof. The nucleation start point portion 7 formed to have a tapered protrusion as illustrated in FIG. 6 includes a curved apical portion 7e.

Also, the nucleation start point portion 7 can be formed to have a shape illustrated in a side view of FIG. 7 illustrating a fifth aspect of the nucleation start point portion of the anode electrode.

The nucleation start point portion 7 illustrated in FIG. 7 is not provided to protrude on a side surface of the anode electrode body 5a as illustrated in FIGS. 3 to 6, but is formed in a lower portion of the anode electrode body 5a. The nucleation start point portion 7 has a pointed conical shape. The angle θ of the nucleation start point portions 7 illustrated in FIGS. 4 to 7 is preferably 90° or less.

One or more nucleation start point portions 7 may be provided. For example, each of FIGS. 3 to 6 illustrates a case where the anode electrode 5 has one nucleation start point portion 7 formed to have a spiral shape. Also, FIG. 7 illustrates a case where the anode electrode 5 has one nucleation start point portion 7 formed to have a conical shape.

In order to ensure the durability of the anode electrode 5 by potential corrosion, a plurality of nucleation start point portions 7 is preferably provided. In the case of providing a plurality of spiral nucleation start point portions 7 in the anode electrode 5, in each of FIGS. 3 to 6, spiral nucleation start point portions 7 are formed with multiple threads by shifting in an axial direction of the anode electrode body 5a by a predetermined pitch, like a multiple thread screw.

Also, the plurality of nucleation start point portions 7 can be formed to have not the spiral shape but an annular shape that is continuous around the circumferential surface of the anode electrode body 5a, and the plurality of nucleation start point portions 7 can be provided to protrude outward by shifting the position of the anode electrode body 5a in the axial direction. In a similar manner, the plurality of nucleation start point portions forming a tapered shape with a triangular cross-section protruding from the side surface of the anode electrode body 5a can be provided by shifting the position of the anode electrode body 5a in the axial direction or the circumferential direction.

When the shapes of the nucleation start point portions 7 have distortions as illustrated in FIGS. 4 to 6, the nucleation start point portions 7 have only to be shaped such that a volume S and a height h thereof satisfy the following conditions.

0<S≦(nh3)/3

5 μm≦h≦100 μm

Also, the height h of the nucleation start point portion 7 may be defined as the arithmetic mean roughness Ra of JIS as described above. Therefore, h=Ra.

In the anode electrode 5 having the nucleation start point portion 7 of the above-described shape, the surface of the nucleation start point portion 7 may be made of a metal material different from a metal constituting the anode electrode body 5a. In this case, as described above, as the voltage applied to the anode electrode 5 contacting the heat storage material 4 is increased, there are growing concerns that potential corrosion will occur in the nucleation start point portion 7. In order to prevent this corrosion, the surface of the nucleation start point portion 7 is preferably made of a metal material having high corrosion resistance. Examples of such a metal material may include silver and gold.

The anode electrode 5 having corrosion resistance can be obtained by making the anode electrode body 5a of stainless steel and forming a corrosion resistance layer by depositing silver or gold on the surface of the tapered protrusion (base portion of the nucleation start point portion 7) integrally provided in the anode electrode body 5a by sputtering. Employing this anode electrode reduces the use of gold or silver as compared with the entire anode electrode 5 is made of silver or gold. For this reason, it is preferable because the cost of the anode electrode 5 and therefore the cost of the heat storage apparatus 1 can be reduced.

As illustrated in FIG. 1, the anode electrode 5, for example, the entire anode electrode 5 is disposed in contact with the heat storage material 4. Therefore, the nucleation start point portion 7 is also in contact with the heat storage material 4.

The cathode electrode 6 is used to apply a negative voltage to the heat storage material 4. For example, the cathode electrode 6 is made of a metal material having conductivity, such as stainless steel or silver. As illustrated in FIG. 1, the cathode electrode 6 is spaced apart from the anode electrode 5. For example, the entire cathode electrode 6 is disposed in contact with the heat storage material 4. The cathode electrode 6 and the anode electrode 5, for example, extend in a vertical direction and are disposed substantially in parallel.

The controller 11 includes a voltage applying unit 12 and a controlling unit 13. The controller 11 constitutes, for example, a unit, and is disposed on, for example, an outer surface of the heat storage tank 3. Also, the controller 11 can be disposed away from the heat storage tank 3 and may be connected to or embedded in, for example, an air conditioner control system including the heat storage apparatus 1 of the present embodiment, or other systems such as a network household appliance control system that controls an air conditioner and other whole household appliances.

A positive electrode of the voltage applying unit 12 is connected to the anode electrode 5 through a first feed line 14, and a negative electrode of the voltage applying unit 12 is connected to the cathode electrode 6 through a second feed line 15. That is, the voltage applying unit 12 is provided to apply a voltage between the anode electrode 5 and the cathode electrode 6. In the voltage applying unit 12, a battery or a constant-voltage power supply can be used.

Although not illustrated, the controlling unit 13 is provided with a memory, an arithmetic section, and an applied voltage controller. A variety of data necessary for controlling the heat storage apparatus 1 are stored in the memory of the controlling unit 13. The applied voltage controller of the controlling unit 13 and the voltage applying unit 12 are electrically connected through a signal line 16.

The heat storage apparatus 1 having the above configuration can be provided in a heat pump type air conditioner (not illustrated) capable of performing a heating operation by a known refrigeration cycle. In this case, in order for a heat exchange with a compressor included in the air conditioner, the heat storage apparatus 1 is disposed in contact with a sidewall of the heat storage tank 3 in at least a portion of a circumferential surface of the compressor.

Therefore, during the heating operation of the air conditioner in winter, the heat storage material 4 inside the heat storage tank 3 is heated to a temperature above a melting point of the heat storage material 4 by the compressor that becomes a high temperature. In other words, during the operation of the air conditioner, the temperature of the heat storage material 4 is increased by exhaust heat of the compressor. Therefore, the heat storage material 4 is dissolved and becomes a liquid-phase state.

If a stopped state of the heating operation is maintained, the heat storage material 4 is lowered a temperature below a melting point thereof. As described above, the heat storage material 4 is made of a material that can be supercooled. Therefore, the heat storage material 4 does not coagulate but maintains a liquid-phase state even when a temperature falls to below a melting point from the liquid-phase state, and thus, accumulates latent heat.

When the heat storage material 4 is maintained in the supercooled state, the controlling unit 13 controls the stop of the voltage applying operation by the voltage applying unit 12 such that a preset voltage is not applied between both electrodes (the anode electrode 5 and the cathode electrode 6). Due to this control, while the heat storage material 4 is maintained in the supercooled state, the heat storage material 4 is stabilized in the supercooled liquid-phase state, and thus, the heat storage material 4 is not crystallize (nucleated). Therefore, the latent heat accumulated in the heat storage material 4 is not emitted.

When an instruction to take out the latent heat from the heat storage material 4 is given from the outside to the controlling unit 13, the controlling unit 13 sets an applied voltage to the voltage applying unit 12 and controls application of the set voltage between both electrodes through the voltage applying unit 12. In this case, the application may be performed while continuously applying the set voltage for a predetermined time, or may be performed by applying an applied voltage in a pulsed form a plurality of times.

As the voltage applied between both electrodes by the voltage applying unit is increased, there are concerns that the anode electrode 5 and the cathode electrode 6 contacting the heat storage material 4 will be degraded by potential corrosion. Therefore, the voltage applied between both electrodes by the voltage applying unit 12 preferably uses a minimum voltage necessary for releasing the supercooling of the heat storage material 4 being in the liquid-phase state (in other words, necessary for nucleating the heat storage material 4).

When a predetermined voltage is applied between the anode electrode 5 and the cathode electrode 6 contacting the heat storage material 4 made of sodium acetate hydrate, the surface of the nucleation start point portion 7 causes a silver reduction reaction in at least the silver anode electrode 5 and causes a water reduction reaction in the cathode electrode 6, and thus, a current flows through the heat storage material 4.

The nucleation of the heat storage material 4 is initiated from the nucleation start point portion 7 included in the anode electrode 5. When the nucleation occurs, the heat storage material 4 being in the supercooled state phase-changes from a liquid-phase state to a solid-phase state and emits the latent heat accordingly. Since the emitted latent heat is supplied to the compressor of the air conditioner that has initiated the heating operation, the temperature of the compressor is rapidly increased by the latent heat emitted from the heat storage material 4. Since this rapidly increases a temperature of a coolant, it is possible to shorten time required for hot air to be blown from an indoor unit of the air conditioner from the start of the heating operation.

The present inventors have investigated a relationship between various samples of anode electrodes 5 having different shape of the nucleation start point portion 7 and a nucleation start time. Conditions, that is, shape, angle θ, and height h (or depth h) of the nucleation start point portions included in samples A to J prepared for this investigation are shown in Table 1 below.

TABLE 1

Angle of
Height of

Shape of
Nucleation Start
Nucleation Start

Nucleation Start
Point Portion
Point Portion h

Point Portion
θ (°)
(μm)

Sample A
Convex Portion
60
5

(Protrusion)

Sample B
Convex Portion
30
12

(Protrusion)

Sample C
Convex Portion
60
12

(Protrusion)

Sample D
Convex Portion
60
25

(Protrusion)

Sample E
Convex Portion
60
100

(Protrusion)

Sample F
Convex Portion
60
200

(Protrusion)

Sample G
Convex Portion
120
4

(Recess)

Sample H
Convex Portion
120
12

(Recess)

Sample I
Convex Portion
120
25

(Recess)

Sample J
Convex Portion
160
100

(Recess)

In this investigation, sodium acetate hydrate was used in the heat storage material 4, and both the anode electrode 5 and the cathode electrode 6 were made of silver. The prepared samples A to J were used in the anode electrode 5 of the heat storage apparatus illustrated in FIG. 1. The convex portions (protrusions) formed in the samples A to F are the nucleation start point portions 7 having the shape illustrated in FIGS. 2 and 3, and the nucleation start point portions 7 were formed by shaving the anode electrodes. The concave portions (recesses) formed in the anode electrodes of the samples G to J are grooves formed by shaving in a V shape so as to reduce the thickness of the anode electrode body. Therefore, the angle θ of the nucleation start point portion in the samples G to J is an angle of the V-shaped groove, and the height h of the nucleation start point portion in the same samples is a depth of the V-shaped groove.

Also, in this inspection, a fixed voltage of 1.7 V was applied in a pulse form between both electrodes (the anode electrode 5 and the cathode electrode 6) in the heat storage apparatus 1 of FIG. 1, and a time until the crystallization of the heat storage material 4 was started from the time point when the application was initiated was measured as a nucleation start time. A case where the nucleation start time has elapsed 60 seconds was determined as crystallization inability (nucleation inability).

In addition, in this inspection, a nucleation start time of an initial 1 cycle and a nucleation start time at 50 cycles were measured. 1 cycle refers to a process until the heat storage material 4 is supercooled after the heat storage material 4 is crystallized and becomes a solid-phase state and then the crystal is dissolved to change the heat storage material 4 to a liquid-phase state. The initial 1 cycle refers to the first cycle at which the first measurement is performed, and the 50 cycles refer to 50 times repetition of the above process.

In the above conditions, the result obtained by measuring the relationship between the samples A to J and the nucleation start time is illustrated in FIG. 8.

It was confirmed by FIG. 8 that the nucleation start time of the samples B to D is significantly short at the initial 1 cycle and after the 50 cycles, as compared with the samples A and E to J. In addition, the crystallization (nucleation) of the heat storage material 4 is observed in the samples A to E in which the nucleation start point portion is made of the protrusion, whereas the crystallization (nucleation) of the heat storage material 4 is not observed in the samples F to J in which the nucleation start point portion is made of the groove.

It is estimated that the reason why the nucleation start point portion 7 made of the tapered protrusion can contribute to the crystallization (nucleation) of the heat storage material 4 is that a voltage applied to the anode electrode 5 concentrates on the nucleation start point portion 7 and a strong electric field is formed between the nucleation start point portion 7 and the cathode electrode 6, which increases the certainty of generation of crystal nuclei in the vicinity of the nucleation start point portion 7.

It was confirmed by the result shown in FIG. 8 and the conditions of Table 1 that when the tapered nucleation start point portion 7 included in the anode electrode 5 has an angle θ of 90° or less and a height h of 5 μm or more to 100 μm or less, it is preferable to crystallize the heat storage material 4 by releasing the supercooled state of the heat storage material 4. Also, it was confirmed by the results of the samples B to D that it is preferable to form the nucleation start point portion 7 having a height h of 12 μm or more to 25 μm so as to obtain a shorter nucleation start time.

Therefore, according to the heat storage apparatus 1 of the first embodiment which includes the anode electrode 5 in which the angle θ of the nucleation start point portion 7 formed to have the tapered shape is 0°<θ≦90° and the height h of the nucleation start point portion 7 is 5 μm≦h≦100 μm as described above, the supercooled heat storage material 4 can be quickly and reliably nucleated.

Also, in the heat storage apparatus 1 of the first embodiment as described above, there are concerns that as the voltage applied between the anode electrode 5 and the cathode electrode 6 is increased, the nucleation start point portion 7 of the anode electrode 5 contacting the heat storage material 4 will be degraded by potential corrosion. However, the nucleation start point portion 7 of the anode electrode 5 is single instead of a protrusion such as an independent peak, but is provided in a spiral shape.

Therefore, the nucleation start point portion 7 of the anode electrode 5 can be regarded as substantially including a plurality of portions facing the cathode electrode 6. As compared with a case where the potential corrosion concentrates on one nucleation start point portion, the potential corrosion is dispersed all over the long nucleation start point portion 7. As a result, the degradation by the potential corrosion of the nucleation start point portion 7 having the spiral shape is suppressed. Therefore, like the case where a plurality of nucleation start point portions 7 is provided in the anode electrode 5, the durability can be improved.

Second Embodiment

FIG. 9 illustrates a second embodiment. A heat storage apparatus of a second embodiment is identical to the first embodiment, except for the following description. Therefore, the same reference numerals as those used in the first embodiment are assigned to configurations identical to or similar to the functions of the first embodiment, and a description thereof will be omitted.

The second embodiment differs from the first embodiment in a configuration of a cathode electrode 6. That is, the cathode electrode 6 has, for example, a cylindrical shape, such as a cylinder, both ends of which are opened in an axial direction. An anode electrode 5 is disposed in an inner central portion of the cathode electrode 6. In other words, the cathode electrode 6 is disposed to surround the anode electrode 5. In this case, it is preferable that the cathode electrode 6 is disposed such that a distance between a nucleation start point portion 7 having a spiral shape and an inner circumferential surface of the cathode electrode 6 is substantially equal.

According to the second embodiment including the cathode electrode 6, the distance between the nucleation start point portion 7 having the spiral shape and the inner circumferential surface of the cathode electrode 6 becomes substantially equal. Therefore, with an application of a voltage between the anode electrode 5 and the cathode electrode 6, no strength or weakness occurs in an electric field formed between each part of the nucleation start point portion 7 and the inner circumferential surface of the cathode electrode 6.

In this regard, in the configuration of the first embodiment, a distance between the cathode electrode 6 and a portion of the nucleation start point portion 7 of the spiral shape facing the cathode electrode 6 is shorter than a distance between the following other portion and the cathode electrode 6. Herein, “other portion” is a portion of the nucleation start point portion 7 that is disposed on an opposite side to the above-described portion with respect to an anode electrode body 5a and thus does not face the cathode electrode 6. Therefore, an electric field formed by a voltage applied between both electrodes becomes strong when the distance is relatively short, and becomes weak when the distance is long.

According to the second embodiment in which the distance between each part of the nucleation start point portion 7 of the anode electrode 5 and the inner circumferential surface of the cathode electrode 6 is substantially equal, the nucleation of the supercooled heat storage material 4 can be accelerated by a strong electric field formed between the cathode electrode 6 and each part of the nucleation start point portion 7, which is continued in a circumferential direction of the anode electrode 5, by the application of the voltage between both electrodes.

Configurations other than those described above in the heat storage apparatus 1 of the second embodiment are identical to the first embodiment, including configurations that are not illustrated in FIG. 9. Therefore, even in the second embodiment, since the anode electrode 5 includes the tapered nucleation start point portion 7, the supercooled heat storage material 4 can be quickly and reliably nucleated.

The example in which the nucleation start point portion 7 is formed by a shaving process has been described above, but the method for manufacturing the nucleation start point portion 7 is not limited thereto. As in the following embodiment, the nucleation start point portion 7 can also be formed by a shearing process and a polishing process.

Third Embodiment

A third embodiment of forming a nucleation start point portion 7 by a shearing process will be described below with reference to FIGS. 10 to 12.

A nucleation start point portion 7 having a convex shape is formed by crushing a rod-shaped anode electrode 5 in a direction indicated by an arrow of FIG. 10 by using a nipper or the like and elongating a shaving portion in a longitudinal direction of the anode electrode 5. For example, in the case of using soft silver as a material of the anode electrode 5, such an elongation is formed when shearing is done at a low speed. Therefore, a cut surface is distorted and small convex portions (nucleation start point portions 7) are formed at a plurality of portions as illustrated in FIG. 11.

As illustrated in FIG. 12, an angle θ between oblique sides 20a and 20b of an apex portion of a nucleation start point portion 7 having a substantially triangular shape (or, an angle between tangent lines 10 of the apex portion of the nucleation start point portion 7) is a predetermined value ranging from, for example, greater than 0° to, for example, 90° or less.

For example, the anode electrode 5 of the present embodiment may be prepared by manually shaving a silver rod having a diameter of 2 mm at a low speed over a period of 3 seconds by using a nipper or the like. Thus, the nucleation start point portion 7 can be easily formed as compared with a drilling method. As a result of evaluating the nucleation performance of the anode electrode 5 prepared in the above-described manner, it was confirmed that nucleation stably occurred.

Fourth Embodiment

A fourth embodiment will be described below with reference to FIGS. 13 and 14. In the fourth embodiment, as in the third embodiment, a nucleation start point portion 7 having a convex shape is formed by a shearing process (press process).

In the fourth embodiment, convex portions 7a and 7b forming an angle θ of, for example, greater than 0° to 90° or less, that is, a nucleation start point portion 7 having a convex portion, are formed by engraving a groove 21 in a shape with respect to a rod-shaped anode electrode 5 by using a cutter. For example, in the case of using silver in the anode electrode 5, the groove 21 can be easily formed by a press process due to a soft characteristic of silver. An angle of a cutter blade is set such that an angle θ formed by oblique sides 20a and 20b of the nucleation start point portion 7 (7a, 7b) is in a range from greater than 0° to 90° or less. Preferably, a height h of the anode electrode 5 (depth of the groove 21) is in a range from 5 μm or more to 100 μm or less.

The anode electrode 5 of the present embodiment, for example, can be formed by a press process of pressing a cutter blade having a blade width of 0.3 mm against a silver rod having a diameter of 2 mm. As a result of evaluating the nucleation performance of the anode electrode 5 prepared in the above-described manner, it was confirmed that nucleation stably occurred from the nucleation start point portion 7 (7a, 7b).

The protrusions described in the present invention include those formed in three dimensions and also include protrusions (planar protrusions) formed within a certain surface (in the present embodiment, an outer circumferential surface of the rod-shaped anode electrode 5).

Fifth Embodiment

A fifth embodiment of forming a nucleation start point portion 7 by a polishing process will be described below with reference to FIGS. 15 to 17.

In the fifth embodiment, an anode electrode 5 is made of a silver plate having a thin plate shape. Nucleation start point portions 7 having a convex shape are formed at a plurality of positions on a surface of the anode electrode 5. The plurality of nucleation start point portions 7 is formed by forming a plurality of grooves 31 by polishing the surface of the anode electrode 5 by using a file. In order to form the nucleation start point portion 7 having a height h in a range from 5 μm or more to 100 μm or less as described above, the surface of the anode electrode 5 is polished by using a coarse cloth file of #80 to #200. Of the nucleation start point portions 7, the nucleation start point portion 7 in which an angle θ between oblique sides 20a and 20b is in a range from 0° or more to 90° is suitable for stably causing nucleation.

According to the present embodiment, the nucleation start point portion 7 can be formed easily and cheaply with respect to the drilling method. Also, in order to increase the number of nucleation start points, the durability can be increased.

A silver plate (anode electrode 5) polished by a coarse cloth file of #80 and #200 includes a convex portion having a height h in a range from 5 μm or more to 100 μm or less, and it was confirmed that the nucleation stably occurred. On the other hand, as a result of evaluating the surface roughness and the nucleation performance in the method of polishing the silver plate (anode electrode 5), the silver plate polished by a fine file of #1000 became a convex portion of 5 μm or less, and it was confirmed that the nucleation was not stable.

Particles of the heat storage material may be attached in advance before the process of forming the nucleation start point portion 7 having the spiral shape on the circumferential surface of the anode electrode body 5a as in the first and second embodiments, the process of forming the nucleation start point portion 7 having a small convex shape by using a nipper or the like as in the third embodiment, the process of forming the nucleation start point portion 7 by the cutter as in the fourth embodiment, or the process of forming the nucleation start point portion 7 by the file as in the fifth embodiment.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

HEAT STORAGE APPARATUS

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