Energy storage device having improved heat-dissipation characteristic

An energy storage device having improved heat-dissipating includes a cell assembly formed by connecting at least two cylindrical energy storage cells in series, a case having an accommodation portion shaped corresponding to an outer surface of the energy storage cells to accommodate the cell assembly, and a heat-dissipating pad installed between an outer surface of the energy storage cells of the cell assembly and an inner surface of the accommodation portion, wherein the case includes at least two case blocks, and wherein the accommodation portion is formed by coupling the case blocks.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a U.S. National Phase entry from International Application No. PCT/KR2015/006596, filed Jun. 26, 2015, which claims priority to Korean Patent Application Nos. 10-2014-0107939, 10-2014-0179732 and 10-2015-0086880, filed Aug. 19, 2014, Dec. 12, 2014 and Jun. 18, 2015, respectively, the disclosures of which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an energy storage device, and more particularly, to an energy storage device having an improved heat-dissipation characteristic.

2. Description of Related Art

Generally, an ultra-capacitor is also called a super capacitor and serves as an energy storage device having characteristics in between an electrolytic condenser and a secondary battery. The ultra-capacitor is a next-generation electric energy storage device which may be used together with, or instead of, a secondary battery due to high efficiency and semi-permanent life span.

The ultra-capacitor is used as a substitute for a storage battery when an application is not easy to maintain and require a long-term life span. The ultra-capacitor has a rapid charge/discharge rate and thus may be used as an auxiliary power source of a cellular phone, a notebook, a PDA or the like, which is a mobile communication information device. In addition, the ultra-capacitor is very suitable as a main or auxiliary power source of an electric vehicle, a pilot lamp on the road, an uninterrupted power supply (UPS) or the like, which demands a high capacity.

When the ultra-capacitor is applied, a high-voltage module of several thousand Farad or several hundred bolts is required in order to use the ultra-capacitor as a high-voltage battery. The high-voltage module may be configured by connecting a required number of ultra-capacitors in a case.

FIG. 1is a diagram showing an existing ultra-capacitor module.

As shown inFIG. 1, the existing ultra-capacitor module includes an ultra-capacitor array10, a case20accommodating the ultra-capacitor array10, and covers30,40covering upper and lower openings of the case20. The ultra-capacitor array10is configured by connecting electrode terminals of a plurality of ultra-capacitors by means of a bus bar11and coupling them by nuts.

The ultra-capacitor module may improve energy storage characteristics by operating a plurality of ultra-capacitors. However, the heat generated when operating the ultra-capacitor module also increases rapidly, which may deteriorate reliability or stability of the ultra-capacitor module.

The existing ultra-capacitor module as described above dissipates heat mainly through the bus bar11serving as a connection member connecting adjacent ultra-capacitors and the covers30,40made of metal and covering upper and lower surfaces of the case20. However, a side of the case20is made of a synthetic resin in order to reduce a weight of the ultra-capacitor module and lower a production cost thereof. In addition, the side of the case20has a plate shape, and thus a contact area with the ultra-capacitor is small and thus does not substantially dissipate heat.

In addition, in the existing technique, the ultra-capacitor may mainly dissipate heat through the bus bar11, but the bus bar11is not able to efficiently dissipate heat due to a small heat-dissipating area. Thus, due to the increased temperature in the case, the life span of the ultra-capacitor may be reduced.

SUMMARY OF THE INVENTION

The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to providing an energy storage device having an improved heat-dissipating characteristic, which may dissipate heat through a side of a case with a broad contact area when energy storage cells such as ultra-capacitors are accommodated in the case.

In one aspect of the present disclosure, there is provided an energy storage device, comprising: a cell assembly formed by connecting at least two cylindrical energy storage cells in series; a case having an accommodation portion shaped corresponding to an outer surface of the energy storage cells to accommodate the cell assembly; and a heat-dissipating pad installed between an outer surface of the energy storage cells of the cell assembly and an inner surface of the accommodation portion, wherein the case includes at least two case blocks, and wherein the accommodation portion is formed by coupling the case blocks.

The energy storage cells may contact the heat-dissipating pad with a central angle of 30 to 60 degrees.

The accommodation portion may form an are with a length greater than a length of the heat-dissipating pad.

The heat-dissipating pad may have elasticity, and an interval between the accommodation portion and the energy storage cells may be smaller than a thickness of the heat-dissipating pad before being compressed and greater than a diameter tolerance of the energy storage cells.

The heat-dissipating pad may be attached to the energy storage cells.

The heat-dissipating pad may be a thermal conductive filler.

An adhesive layer may be provided at one side of the heat-dissipating pad.

The energy storage cells may be ultra-capacitors.

The case block may include a plurality of convex portions having the same arc shape as an outer shape of the energy storage cells; a convex portion connector configured to connecting the plurality of convex portions; and a concave portion formed between the convex portions and the convex portion connector.

At least one heat-dissipating plate may be formed at the concave portion to protrude perpendicularly.

The case block may have an ‘L’ shape or a ‘⊂’ shape.

When the case block has an ‘L’ shape, one of outermost convex portions of the plurality of convex portions may be connected so that the are shapes of the convex portions are connected.

The case block may further include a case block connector extending from one of the outermost convex portions and bent in a longitudinal direction of the case block.

When the case block has a ‘⊂’ shape, outermost convex portions of the plurality of convex portions may be connected so that the are shapes of the convex portions are connected.

The case block may further include a case block connector extending from each of the outermost convex portions and bent in a longitudinal direction of the case block.

A tab may be formed at the convex portion connector to cover a cover.

A distance between the energy storage cells and the case may be gradually increasing from an end point of the heat-dissipating pad so that the energy storage cells and the case are insulated from each other.

An insulation film may be further formed at an outer surface of the energy storage cell.

Advantageous Effects

In the present disclosure, heat is dissipated through not only connection members such as nuts and bus bars but also a heat-dissipating pad installed between a case and energy storage cells, and thus a contact area between the energy storage cells and the case increases, thereby improving a heat-dissipating characteristic.

In the present disclosure, since the case accommodating several energy storage cells is fabricated by coupling a plurality of case blocks, the heat-dissipating pad may be installed easily, and the case may be fabricated with a low cost.

In the present disclosure, it is possible to optimize a product mass of the energy storage device along with the improvement of the heat-dissipating characteristic.

In the present disclosure, a distance between the energy storage cell and the case is gradually increasing from both front ends of the heat-dissipating pad, and thus the case and the energy storage cell may be naturally insulated from each other, thereby improving product stability.

DETAILED DESCRIPTION OF THE INVENTION

In addition, in the present disclosure, if it is judged that detailed explanation on a known technique or configuration may unnecessarily make the essence of the present disclosure vague, the detailed explanation will be omitted.

FIG. 2is a diagram showing an energy storage device according to an embodiment of the present disclosure,FIG. 3is a diagram showing a connection between an energy storage cell according to another embodiment of the present disclosure, andFIG. 4is a sectional view, taken along the line II-II′ ofFIG. 2.

Referring toFIGS. 2 to 4, the energy storage device of this embodiment includes a cell assembly100having at least two energy storage cells110connected in series, and a case200accommodating the cell assembly100.

The cell assembly100may be formed by connecting at least two energy storage cells110in series. The energy storage cell110may be an ultra-capacitor, and in this embodiment, the energy storage cell employs an ultra-capacitor. However, the energy storage cell may be any cell capable of storing electric energy, for example a secondary battery, a battery cell or the like, without being limited thereto.

The ultra-capacitor110has a rapid charge/discharge rate and thus may be used as an auxiliary power source of a cellular phone, a notebook, a PDA or the like, which is a mobile communication information device. In addition, the ultra-capacitor may be used as a main or auxiliary power source of an electric vehicle, a hybrid electric vehicle, a power unit for a solar cell, an uninterrupted power supply (UPS) or the like, which demands a high capacity.

The ultra-capacitor110may have a cylindrical shape and may be connected to another ultra-capacitor in series in a longitudinal direction, where an electrode is formed, as shown inFIG. 2to configure the cell assembly100. At this time, the ultra-capacitor110may be connected to an adjacent ultra-capacitor by means of a connection member, for example a nut and a bus bar.

In addition, as shown inFIG. 3, it is also possible that ultra-capacitors110are located in parallel, and in this state, a positive electrode terminal of a first ultra-capacitor and a negative electrode terminal of a second ultra-capacitor are connected in series by means of a connection member such as a bus bar130and a nut150to form the cell assembly100. At this time, a plurality of ultra-capacitors110may configure the cell assembly100by connecting positive electrode terminals and negative electrode terminals by means of the bus bar130and coupling them by means of the nut150. The cell assembly100may be accommodated in the case200to configure an ultra-capacitor module.

The case200may accommodate the cell assembly100formed by connecting the ultra-capacitors110in series. The case200may have an accommodation portion shaped corresponding to an outer surface of the ultra-capacitors110so that the cell assembly100formed by connecting the ultra-capacitors110in series may be accommodated therein.

The case200may be formed by coupling at least two case blocks (510inFIG. 5 or 610inFIG. 6) having the same shape. The accommodation portion accommodating the cell assembly100may be formed by coupling the case blocks (510inFIG. 5 or 610inFIG. 6). The case blocks (510inFIG. 5 or 610inFIG. 6) will be described below in more detail with reference toFIGS. 5 and 6.

FIG. 5is a diagram showing a case block according to an embodiment of the present disclosure.

Referring toFIG. 5, the case block510may have an ‘L’ shape and includes an accommodation portion518shaped corresponding to an outer shape of the ultra-capacitor110. If the ultra-capacitor110has a cylindrical shape, the inner surface of the case block510contacting the outer surface of the ultra-capacitor110may have a rounded shape in a cylinder. The case200may be fabricated by coupling four case blocks with an ‘L’ shape, and accordingly the accommodation portion518may be formed.

In more detail, as shown inFIG. 5, the case block510includes a plurality of convex portions511having the same arc shape as the outer shape of the ultra-capacitor110, a convex portion connector513connecting the convex portions511, a concave portion512formed between the convex portion511and the convex portion connector513, and case block connectors514,515connecting the case blocks510.

The plurality of convex portions511have the same arc shape as the outer shape of the ultra-capacitor110to form the accommodation portion518accommodating the ultra-capacitor110, and a heat-dissipating pad210is attached to an inner side thereof. The heat-dissipating pad210emits heat generated from the ultra-capacitor110to the convex portion511and also gives an insulation function between the ultra-capacitor110and the convex portion511(namely, the case200). The convex portions511are connected by the convex portion connector513, and a tab is formed at the convex portion connector513to fix an upper cover and a lower cover which cover the case200. The tab is a structure for bolting, and a bolt for fixing the case200and the covers is inserted therein.

The case block510formed by connecting the plurality of convex portions511has an ‘L’ shape. In order to connect the case blocks510in a width direction, one of outermost convex portions is disposed and connected in a width direction, and the other convex portions are disposed and connected in a longitudinal direction. In other words, one of outermost convex portions of the plurality of convex portions511in a longitudinal direction is connected so that the arc shapes of the convex portions511are connected.

The concave portion512is formed between the convex portion511and the convex portion connector513. The concave portion512is formed by bending back a part of the convex portion511outwards in order to ensure an insulation distance, as described later. A plurality of heat-dissipating plates517are perpendicularly installed to the concave portion512at regular intervals to dissipate heat generated from the ultra-capacitor110. In other words, in order to enhance heat-dissipating efficiency by means of air flows among the heat-dissipating plates517, the heat-dissipating plates517are perpendicularly installed at regular intervals. In addition, in order to enlarge a heat-dissipating area, the plurality of heat-dissipating plates517are installed. At this time, the heat-dissipating plates517are formed to have the same height as the convex portion connector513. InFIG. 5, it is depicted that the concave portion512is not formed at both sides of the convex portion connector513located at a leftmost location in a longitudinal direction, but the concave portion512may be formed at both sides, similar to other convex portion connectors513.

The case block connectors514,515connect the cable blocks510. Among the case block connectors514,515, the case block connector514extends from the convex portion511and is bent in a longitudinal direction, and also the case block connector514connects the cases block510in a width direction. Among the case block connectors514,515, the case block connector515extends from the convex portion511and is bent in a width direction, and also the case block connector515connects the case blocks510in a longitudinal direction.

FIG. 6is a diagram showing a case block according to another embodiment of the present disclosure.

Referring toFIG. 6, the case block610has a ‘⊂’ shape and also has an accommodation portion618shaped corresponding to an outer shape of the ultra-capacitor110. If the ultra-capacitor has a cylindrical shape, the inner surface of the case block610contacting the outer surface of the ultra-capacitor110may have a rounded shape in a cylinder. The case200may be fabricated by coupling two case blocks with an ‘⊂’ shape, and accordingly the accommodation portion618may be formed.

In more detail, as shown inFIG. 6, the case block610includes a plurality of convex portions611having the same arc shape as the outer shape of the ultra-capacitor110, a convex portion connector613connecting the convex portions611, a concave portion612formed between the convex portion611and the convex portion connector613, and a case block connector614connecting the case blocks610.

The plurality of convex portions611have the same are shape as the outer shape of the ultra-capacitor110to form the accommodation portion618accommodating the ultra-capacitor110, and a heat-dissipating pad210is attached to an inner side thereof. The heat-dissipating pad210emits heat generated from the ultra-capacitor110to the convex portion611and also gives an insulation function between the ultra-capacitor110and the convex portion611(namely, the case200). The convex portions611are connected by the convex portion connector613, and a tab is formed at the convex portion connector613to fix an upper cover and a lower cover which cover the case200. The tab is a structure for bolting, and a bolt for fixing the case200and the covers is inserted therein.

The case block610formed by connecting the plurality of convex portions611has a ‘⊂’ shape. In order to connect two case blocks610in a width direction, outermost convex portions are disposed and connected in a width direction, and the other convex portions are disposed and connected in a longitudinal direction. In other words, the outermost convex portions of the plurality of convex portions611are connected so that the arc shapes of the convex portions611are connected.

The concave portion612is formed between the convex portion611and the convex portion connector613. The concave portion612is formed by bending back a part of the convex portion611outwards in order to ensure an insulation distance, as described later. A plurality of heat-dissipating plates617are perpendicularly installed to the concave portion612at regular intervals to dissipate heat generated from the ultra-capacitor110. In other words, in order to enhance heat-dissipating efficiency by means of air flows among the heat-dissipating plates617, the heat-dissipating plates617are perpendicularly installed at regular intervals. In addition, in order to enlarge a heat-dissipating area, the plurality of heat-dissipating plates617are installed. At this time, the heat-dissipating plates617are formed to have the same height as the convex portion connector613. InFIG. 6, it is depicted that the concave portion612is not formed at both sides of the convex portion connector613located at an outermost location in a longitudinal direction, but the concave portion612may be formed at both sides, similar to other convex portion connectors613.

The case block connector614connects the cable blocks610. The case block connector614extends from the convex portion611and is bent in a longitudinal direction, and also the case block connector614connects the case blocks610in a width direction.

The case200formed by coupling the case blocks510,610as described above with reference toFIGS. 5 and 6may be made of metal. The accommodation portion518,618formed in the case200is fabricated to conform to the shape of the ultra-capacitor110as much as possible so that its shape corresponds to the outer surface of the ultra-capacitor110. Therefore, a contact surface between the case200and the ultra-capacitor110is maximized to increase an area through which heat is dissipated, thereby enhancing heat-dissipating effects.

As described above, in order to further improve the heat-dissipating effect, in this embodiment, the heat-dissipating pad210is attached to the inner surface of the accommodation portion518,618. In other words, the heat-dissipating pad210may be attached to the inner surface of the accommodation portion518,618so that the heat-dissipating pad210is located between the cell assembly100and the accommodation portion518,618when the cell assembly100is inserted into the accommodation portion518,618. The heat-dissipating pad210may be attached to the inner surface of the accommodation portion518,618in a longitudinal direction of an electrode of the ultra-capacitor110. The width of the heat-dissipating pad210is smaller than a length of an arc formed by the accommodation portion518,618. If the width of the heat-dissipating pad210is greater than the length of the arc formed by the accommodation portion518,618, a part of the heat-dissipating pad210does not contact the accommodation portion518,618and thus does not dissipate heat. On the contrary, the accommodation portion518,618should have an arc with a length greater than the width of the heat-dissipating pad210.

The heat-dissipating pad210may include a thermal conductive filler for heat transfer, for example metal powder or ceramic powder. The metal powder may be selected from aluminum, silver, copper, nickel, tungsten, and mixtures thereof. In addition, the ceramic powder may be selected from silicone, graphite and carbon black. In an embodiment of the present disclosure, the heat-dissipating pad210is not limited to specific materials. In addition, the heat-dissipating pad210may also be made of silicon composite rubber.

The heat-dissipating pad210may plays a role of fixing the ultra-capacitor110accommodated in the case200. In other words, when the ultra-capacitor110is accommodated in the case200, the heat-dissipating pad210may direct contact the ultra-capacitor110to prevent the ultra-capacitor110from moving. Even though the accommodation portion518,618is fabricated with a shape corresponding to the outer surface of the ultra-capacitor110, it is possible that the accommodation portion518,618does not closely contact the ultra-capacitor110and thus may mot suitably dissipate heat. Therefore, if the heat-dissipating pad210is attached to the inner surface of the accommodation portion518,618which contacts the ultra-capacitor110, the heat-dissipating pad210may fix the ultra-capacitor110in the case200and also enlarge a contact area between the case200and the ultra-capacitor110, thereby enhancing the heat-dissipating effect.

In addition, the heat-dissipating pad210may have elasticity. A plurality of ultra-capacitors110are inserted into the case200, and the ultra-capacitors110may have different diameters. Accordingly, the ultra-capacitors110may not perfectly compressed to the heat-dissipating pad210. For this reason, considering the difference in diameters of the ultra-capacitors110, an elastic heat-dissipating pad210may be used so that all ultra-capacitors110may be sufficiently compressed to the heat-dissipating pad210. At this time, a thickness of the heat-dissipating pad210before being compressed may be greater than a diameter tolerance of the ultra-capacitors110. For example, if the ultra-capacitors110have a standard diameter of 60.7 mm and a tolerance of ±0.7 mm, the heat-dissipating pad210before being compressed may have a thickness greater than 1.4 mm (0.7 mm×2), and may have a thickness of, for example, 2 mm.

If the heat-dissipating pad210has elasticity, when the ultra-capacitor110is inserted into the case200, the heat-dissipating pad210is deformed according to the outer shape of the ultra-capacitor110, and thus the adhesion to the ultra-capacitor110may be enhanced, thereby increasing the contact area. Therefore, as the contact area increases, the heat-dissipating efficiency may be further enhanced.

Meanwhile, when the heat-dissipating pad210is used, an interval between the accommodation portion518,618of the case200and the ultra-capacitor110may be smaller than the thickness of the heat-dissipating pad210before being compressed and greater than the diameter tolerance of the ultra-capacitors110. Here, the interval between the accommodation portion518,618and the ultra-capacitor110represents an interval when the energy storage device is assembled without using the heat-dissipating pad210. The interval should be greater than the diameter tolerance of the ultra-capacitors110because the case is unstably assembled to create a gap when the interval is smaller than the diameter tolerance. In addition, the interval should be smaller than the thickness of the heat-dissipating pad210before being compressed in order to ensure the ultra-capacitors110to be sufficiently compressed to the heat-dissipating pad210. If the interval is smaller than the thickness of the heat-dissipating pad210before being compressed, when the case is assembled, the ultra-capacitors110compress the heat-dissipating pad210to fix the ultra-capacitors110in the case200and enlarge a contact area between the ultra-capacitor110and the heat-dissipating pad210, thereby enhancing the heat-dissipating effect.

In addition, though not shown in the figures, an adhesive layer may be provided at one side of the heat-dissipating pad210so that the heat-dissipating pad may be easily adhered to the accommodation portion518,618of the case200. Here, the adhesive layer may further include a thermal conductive filler, for example metal powder or ceramic powder, to prevent the thermal conductivity from deteriorating through the adhesive layer.

In this embodiment, since the heat-dissipating pad210is attached to the inner surface of the case200, namely the inner surface of the accommodation portion518,618which corresponds to the outer surface of the ultra-capacitor110, heat is dissipated through the side of the case200, thereby further enhancing the heat-dissipating performance. In addition, since the case200is made of material with excellent thermal conductivity such as copper or aluminum, the heat generated in the case200may be effectively transferred and dissipated to the outside.

In the existing technique, heat is mainly dissipated through a connection member, namely a bus bar, connecting ultra-capacitors110adjacent to each other, but the bus bar has so small area to dissipate heat sufficiently and thus has unsatisfactory heat-dissipating effect. For example, when the bus bar has a longitudinal length of 100 (mm) and a vertical length of 28 (mm), an area capable of dissipating heat through the bus bar for a single ultra-capacitor may be 100*28/2 (area of the bus bar for a single ultra-capacitor)*2 (top and bottom sides)=2800 (mm2).

However, in this embodiment, as described above, the heat-dissipating area increases by means of the side of the case200, and thus the heat in the case200may be dissipated out more effectively. In addition, since a heat-dissipating member having excellent thermal conductivity, namely the heat-dissipating pad210, is attached to the inner surface of the case200which contacts the ultra-capacitor110, the heat-dissipating performance may be improved further.

For example, if a contact angle, namely a central angle, of the ultra-capacitor contacting the heat-dissipating pad210is 60 degrees as shown inFIG. 4, a heat-dissipating area for a single ultra-capacitor may be 2*3.14*(60 (diameter of the ultra-capacitor)/2)*130 (length of the heat-dissipating pad) (mm)*60 (angle)*2/360=8164 (mm2). At this time, the angle is multiplied by 2 since the heat-dissipating pads210are attached to two points in this embodiment. Generally, the central angle is an angle formed by two radii at a circle or a fan shape, and in this embodiment of the present disclosure, the central angle represents an angle formed by two radii connecting from the center of the ultra-capacitor110to both ends of a contact portion between the heat-dissipating pad210and the ultra-capacitor110.FIG. 7is a diagram showing a central angle formed when the heat-dissipating pad210and the ultra-capacitor110make contact according to an embodiment of the present disclosure. As shown inFIG. 7, the central angle α is an angle formed by two radii connecting from the center of the ultra-capacitor110to both ends of a contact portion between the heat-dissipating pad210and the ultra-capacitor110. In addition, both ends represent both ends when the heat-dissipating pad210is compressed between the ultra-capacitor110and the case200.

Meanwhile, the contact angle, namely the central angle α, of the ultra-capacitor110contacting the heat-dissipating pad210may be 30 degrees to 60 degrees. The heat-dissipating efficiency when the central angle α is 30 degrees or above is much greater than the heat-dissipating efficiency when the central angle α is less than 30 degrees. In addition, if the contact area of the heat-dissipating pad210and the ultra-capacitor110increases, namely if the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210is greater, the heat-dissipating efficiency becomes better, but the product mass of the energy storage device increases as much. If the central angle α is 30 degrees to 60 degrees, the product mass increases gently, but if the central angle α increases greater than 60 degrees, the product mass increases rapidly. Therefore, the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210may be 30 degrees to 60 degrees. This will be described below with reference to the drawings.

FIG. 8is a diagram showing a contact shape, heat-dissipating efficiency and a product mass of the heat-dissipating pad and the energy storage cell depending on the contact angle, according to an embodiment of the present disclosure, andFIG. 9is a graph showing the change of heat-dissipating efficiency and product mass depending on the contact angle according to an embodiment of the present disclosure.

First, calculation conditions of the heat-dissipating efficiency are as in Table 1 below, and 18 ultra-capacitors are used as energy storage cells.

The heat-dissipating efficiency is calculated using the following equation.

The product mass is calculated by adding a total weight of the ultra-capacitors, a mass of the case, a mass of the heat-dissipating pad and masses of other components.

Referring toFIG. 8, the energy storage cells, namely the ultra-capacitors110, are inserted into the accommodation portion518,618formed between the case blocks, and the heat-dissipating pad210contacting the ultra-capacitors110is attached to the inner surface of the accommodation portion518,618. In order to increase the contact area between the ultra-capacitor110and the heat-dissipating pad210, the heat-dissipating pad210should have a greater width, and accordingly the length of the arc of the accommodation portion518,618should also be increased. If the contact area between the ultra-capacitor110and the heat-dissipating pad210increases as described above, the central angle of the ultra-capacitor110contacting the heat-dissipating pad210also increases. In addition, the concave portion512,612formed at the outer surface of the case200between adjacent ultra-capacitors110also has an increased depth.

InFIG. 9, a left Y axis represents heat-dissipating efficiency, and a right Y axis represents a product mass. InFIG. 9, a reference symbol910represents a graph of the heat-dissipating efficiency, and a reference symbol920represents a graph of the product mass. As shown inFIGS. 8 and 9, if the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210increases, the energy storage device has better heat-dissipating efficiency. In particular, if the central angle α increases to 30 degrees or above, the heat-dissipating efficiency is improved rapidly in comparison to the case where the central angle α is less than 30 degrees. For example, when the central angle α is 10 degrees, the heat-dissipating efficiency is 90.66%, but when the central angle α is 30 degrees, the heat-dissipating efficiency is 97.28%. Thus, if the central angle α becomes 30 degrees, the heat-dissipating efficiency is improved greatly. InFIG. 9, numbers marked along the graph of heat-dissipating efficiency represents an increment of the heat-dissipating efficiency per 1 degree. For example, when the central angle α increases from 10 degrees to 20 degrees, the heat-dissipating efficiency increases by 0.36% point (3.6%÷10) per 1 degree on average. When the central angle α increases from 20 degrees to 25 degrees, the heat-dissipating efficiency increases by 0.30% point per 1 degree on average. As shown inFIG. 9, the heat-dissipating efficiency increases greatly until the central angle α becomes 30 degrees, and the increment of the heat-dissipating efficiency becomes smaller if the central angle α is over 30 degrees. Therefore, the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210may be 30 degrees or above.

However, if the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210increases over 30 degrees, the product mass of the energy storage device increases as much. In this case, since the width of the heat-dissipating pad210increases, the mass of the heat-dissipating pad210increases, and also the length of the arc of the accommodation portion518,618increases. Thus, the concave portion512,612formed at the outer surface of the case200between adjacent ultra-capacitors110increases, and thus the mass of the case200also increases. As shown inFIGS. 8 and 9, until the central angle α becomes 60 degrees, the product mass increases gently, but if the central angle α increases over 60 degrees, the product mass increases rapidly. In other words, an increase rate of the product mass at a central angle α greater than 60 degrees is greater than an increase rate of the product mass at a central angle α smaller than 60 degrees. InFIG. 9, numbers marked along the graph of the product mass represents an increment of the product mass per 1 degree. For example, when the central angle α increases from 10 degrees to 20 degrees, the product mass increases by 0.25% point (2.5%÷10) per 1 degree on average. When the central angle α increases from 20 degrees to 22.5 degrees, the product mass increases by 0.23% point per 1 degree on average. As shown inFIG. 9, until the central angle α becomes 60 degrees, the product mass increases gently, but if the central angle α increases over 60 degrees, the product mass increases rapidly. For example, when the central angle α increases from 55 degrees to 60 degrees, the product mass increases by 0.34% point per 1 degree, but if the central angle α increases from 60 degrees to 65 degrees, the product mass greatly increases by 0.45% point per 1 degree. Therefore, the central angle α of the ultra-capacitor110contacting the heat-dissipating pad210may be 30 degrees to 60 degrees.

FIG. 10is an enlarged view showing a portion A ofFIG. 2. Referring toFIG. 10, a distance1010between the case200and the ultra-capacitor110is gradually increasing from the concave portion512,612formed by bending back the case200. In other words, based on the front end of the concave portion512,612, the distance1010between the case200and the ultra-capacitor110is gradually increasing. The case200gradually spaced apart from adjacent concave portions512,612of a specific cell encounters a case200gradually spaced apart from adjacent concave portions512,612of an adjacent cell, at the convex portion connectors513,613, thereby configuring the case. As described above, the heat-dissipating pad210functions to insulate the case200and the ultra-capacitor110from each other along with the heat-dissipating function. At a portion where the heat-dissipating pad210is absent, namely from a point where the heat-dissipating pad210ends, the distance1010between the case200and the ultra-capacitor110is gradually increasing so that the case200and the ultra-capacitor110are indirectly insulated from each other. In addition to the insulation distance, an outer surface of each cell may also be covered by an insulation film or coated with an insulating material. In addition, as shown inFIG. 10, a space1020is formed between adjacent ultra-capacitors110and the case200so that a harness may be installed therein for sensing and balancing. The harness is provided through the space1020, and also the flow of air present in the space1020gives additional heat-dissipating.