JP2005135702A - Fuel cell power generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for generating fuel cell power, in which a stable humidification reactant gas is supplied by preventing heat radiation at the connecting part of the fuel cell power generation part and the gas circulation part, exhibiting stable battery performance over a long period. <P>SOLUTION: A heat radiation control part is arranged at the connecting part of the fuel cell power generation part and the gas circulation part, and by making the thermal conductivity of heat radiation control part the same or lower than the thermal conductivity of the fuel cell generation part and the gas circulation part, temperature drop of the reactant gas at the connecting part is prevented and a stable humidification reactant gas is supplied to the generation part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a power generation device including a fuel cell using a polymer electrolyte membrane.

A fuel cell such as a polymer electrolyte fuel cell obtains electric energy and thermal energy mainly from hydrogen and oxygen. Due to the catalytic action of Pt or the like, at the fuel electrode, the formula (1): H ₂ → 2H ⁺ + 2e ⁻
In the oxidant electrode, the formula (2): 1/2 O ₂ + 2H ⁺ → H ₂ O
The reaction represented by The total reaction is represented by the formula (3): H ₂ +1/2 O ₂ → H ₂ O
It is represented by
In addition to Pt, a mixture of Pt and Ru or a Pt—Ru alloy is used as the fuel electrode catalyst to prevent deterioration due to carbon monoxide or the like. Due to these actions, the fuel cell is characterized in that hydrogen and oxygen are reacted to produce only water as a reaction product, so that no environmentally affecting substances are discharged.

A fuel cell power generator generally includes a fuel cell power generation unit that obtains electric power by electrode reaction, a humidification unit that humidifies reaction gas (fuel gas and oxidant gas), a gas supply unit that supplies reaction gas, and a fuel It comprises a control unit that controls the extraction of electric power from the battery power generation unit and the supply of reaction gas. The control unit includes various power circuit units.
The fuel cell power generation device here means not only an actual system that uses electric power obtained from a polymer electrolyte fuel cell but also a performance evaluation device that evaluates the performance of a polymer electrolyte fuel cell. Both basically include the power generation unit, the humidification unit, the gas supply unit, and the control unit.

  The power generation efficiency of the polymer electrolyte fuel cell largely depends on the electrolyte membrane electrode assembly (MEA) included in the fuel cell (unit cell and unit cell stack) constituting the power generation unit. Therefore, in order to improve the power generation efficiency, it is necessary to effectively utilize the MEA performance. In order to effectively develop the performance of the MEA, the following two points are important in the power generator.

  The first is the purity of the humidified reaction gas. The polymer electrolyte and the noble metal catalyst used in the polymer electrolyte membrane and the catalyst reaction layer, which are constituent elements of MEA, are deteriorated by impurities contained in the humidified gas, and the conductivity and catalytic reaction are lowered. For example, metal ions are adsorbed on the hydrogen ion conducting portion of the polymer electrolyte membrane, and the conductivity is lowered. The organic matter adheres to the noble metal catalyst, reduces the reaction field, and inhibits the power generation reaction. In addition, when there is a metal weld in the humidifying part or when a metal material is used in the piping for supplying and discharging the reactive gas, the reactive gas reaches the power generation part via them, so that There is a problem that impurities are transported together by the reaction gas and the performance of the power generation section is degraded. In order to eliminate or reduce the influence of this impurity, it has been proposed to eliminate a metal welded part or use a non-metallic member in a humidifying part and a gas supply part (Patent Document 1).

The second is maintaining the humidified state of the reaction gas. The polymer electrolyte membrane, which is a constituent element of MEA, exhibits conductivity by retaining water. Therefore, the reaction gas needs to be humidified in advance and supplied to the fuel cell. Therefore, the power generation performance of the MEA depends on the humidified state of the reaction gas. As the reaction gas, a reaction gas humidified so as to have a dew point below the operating temperature of the fuel cell is supplied. In general, a reaction gas having a dew point at a temperature equivalent to the operating temperature of the fuel cell or a temperature of 5 to 10 ° C. or less from the operating temperature is supplied. The humidification unit needs to generate a reaction gas having a predetermined dew point, and the gas supply unit needs to stably supply the reaction gas to the power generation unit while maintaining the humidified state of the reaction gas.
JP 2001-176532 A

  However, in the conventional fuel cell power generation device, the humidification and retention of the reaction gas has been stabilized at each of the humidification unit, the gas supply unit, the power generation unit, and the like, in contrast to the above-described retention of the reaction gas in a humidified state. In this case, the humidified state of the reaction gas generated or transported at each part could be maintained, but on the other hand, the reaction gas transported between the parts, that is, at the connecting portion, due to the difference in heat conduction between them. There was a problem that a temperature change occurred and this changed the humidification of the reaction gas.

  In particular, in the gas supply unit and the power generation unit, the power generation unit often uses a metal material. Since the metal material has high thermal conductivity and a large heat capacity, the reaction gas temperature depends on the temperature of the power generation unit at the connecting part of the gas supply unit and the power generation unit, and stabilization of humidification has been a problem. That is, the heat release control of the reaction gas at the connecting portion between the gas supply unit and the power generation unit has been a problem. For example, the reaction gas temperature decreased due to heat radiation at the connecting portion, and accordingly, the humidification of the reaction gas decreased. Although it is possible to stabilize the reaction gas temperature by installing a heater or the like at the connecting portion, it is not desirable because it requires a new heat source, which complicates the fuel cell power generation device and reduces efficiency.

  Therefore, the present invention solves the above-mentioned conventional problems, and suppresses the humidification change of the reaction gas due to the difference in heat conduction at the connecting portion between the gas supply unit and the power generation unit in the fuel cell power generation device, thereby stabilizing the reaction. An object of the present invention is to provide a fuel cell power generation device that exhibits high power generation efficiency by supplying gas.

In order to solve the above problems, the present invention provides:
Supplying and discharging an electrolyte membrane, a pair of electrodes sandwiching the electrolyte membrane, and a fuel gas containing at least hydrogen to one of the pair of electrodes, and supplying and discharging an oxidizing gas containing at least oxygen to the other of the pair of electrodes A polymer electrolyte fuel cell power generation unit comprising at least one unit cell composed of a pair of separator plates having a gas flow path,
A humidifying unit for humidifying the fuel gas and the oxidant gas;
A fuel gas supply unit for supplying the fuel gas;
A fuel gas distribution unit connecting the fuel gas supply unit and the fuel cell power generation unit;
An oxidant gas supply unit for supplying the oxidant gas;
An oxidant gas distribution unit connecting the oxidant gas supply unit and the fuel cell power generation unit;
A control unit for controlling power extraction from the fuel cell power generation unit and supply of the fuel gas and the oxidant gas;
A fuel cell power generator comprising: a heat dissipation control unit provided between at least one of the fuel cell power generation unit and the fuel gas circulation unit and between the fuel cell power generation unit and the oxidant gas circulation unit. provide.

  The fuel cell power generation device according to the present invention includes the above-described heat dissipation control unit, thereby suppressing a change in humidification of the reaction gas due to a difference in heat conduction at a connection portion between the gas supply unit and the fuel cell power generation unit, and stabilizing High power generation efficiency can be expressed by supplying various reactive gases.

  The thermal conductivity of the heat dissipation control unit is equal to or lower than the thermal conductivity of the gas introduction part of the fuel cell power generation unit and is equal to or lower than the thermal conductivity of the fuel gas circulation part and / or the oxidant gas circulation part. Is effective. Thereby, the heat radiation by a connection part can be suppressed and the humidification state of a reactive gas can be stabilized.

  In addition, it is effective that the heat dissipation control unit is made of a non-metallic material having a thermal conductivity of 1.0 (W / (m · ° C.)) or less. Thereby, it is possible to realize a fuel cell power generator that supplies a stable reaction gas and exhibits high power generation efficiency.

  According to the fuel cell power generation device of the present invention, it becomes possible to supply the reaction gas to the power generation unit in a stable humidified state, and a fuel cell power generation device that exhibits high power generation efficiency can be realized.

The present invention provides an electrolyte membrane, a pair of electrodes sandwiching the electrolyte membrane, and a fuel gas containing at least hydrogen on one of the pair of electrodes, and an oxidant gas containing at least oxygen on the other of the pair of electrodes. A polymer electrolyte fuel cell power generation unit comprising at least one unit cell comprising a pair of separator plates having gas flow paths for supplying and discharging
A humidifying unit for humidifying the fuel gas and the oxidant gas;
A fuel gas supply unit for supplying the fuel gas;
A fuel gas distribution unit connecting the fuel gas supply unit and the fuel cell power generation unit;
An oxidant gas supply unit for supplying the oxidant gas;
An oxidant gas distribution unit connecting the oxidant gas supply unit and the fuel cell power generation unit;
A control unit for controlling power extraction from the fuel cell power generation unit and supply of the fuel gas and the oxidant gas;
The present invention relates to a fuel cell power generator including a heat dissipation control unit provided between at least one of the fuel cell power generation unit and the fuel gas circulation unit and between the fuel cell power generation unit and the oxidant gas circulation unit. .
The fuel cell power generation device according to the present invention includes a heat dissipation control unit between the fuel cell power generation unit and the gas supply unit, thereby suppressing the influence of the reaction gas temperature due to heat dissipation by the connecting portion between the gas supply unit and the fuel cell power generation unit. Thus, the reaction gas can be provided in a stable humidified state up to the power generation unit.

  Moreover, the influence of the temperature of the fuel cell power generation unit can be eliminated by setting the heat conductivity of the heat dissipation control unit to be equal to or less than the heat conductivity of the gas introduction unit of the fuel cell power generation unit and the gas circulation unit of the gas supply unit. Similarly, heat dissipation can be suppressed at the connecting portion between the gas supply unit and the heat dissipation control unit. That is, this makes it possible to suppress the influence of the reaction gas temperature due to heat dissipation and stably maintain the humidified state. Further, it is possible to stabilize the humidification of the reaction gas without using an auxiliary device such as a heater, and it is possible to prevent complication of the apparatus and decrease in efficiency.

  Furthermore, the heat radiation control unit is made of a nonmetallic material having a thermal conductivity of 1.0 (W / (m · ° C.)) or less, so that the heat radiation effect due to the material of the fuel cell power generation unit or the gas supply unit can be reduced. The fuel cell power generator according to the present invention can be effectively operated, and the humidified state of the reaction gas can be stabilized.

Here, each part of the fuel cell power generator according to the present invention will be briefly described.
In the fuel cell power generation section, a catalytic reaction layer mainly composed of carbon particles carrying a noble metal catalyst on both sides of the polymer electrolyte membrane, and gas permeability and conductivity mainly composed of carbon particles on the outer surface of the catalyst reaction layer. An electrolyte membrane electrode assembly (MEA) having an electrode composed of a gas diffusion layer having both properties is used as a unit of power generation reaction.

  By disposing a conductive separator having a gas flow path for supplying water and excess gas generated by the reaction and supply of a reactive gas such as an oxidant gas and a fuel gas on both sides of the MEA. Is configured to constitute a power generation unit. Further, a high-power fuel cell power generation unit can be obtained by using a stacked battery (stack) in which a plurality of the single cells are stacked.

  The polymer electrolyte membrane, which is a constituent element of MEA, exhibits electrical conductivity through water. Therefore, it is necessary to humidify the fuel gas such as hydrogen and the oxidant gas such as air, which are reactive gases, and supply them to the unit cell or stack. By supplying the humidified reaction gas to the MEA, the single cell or the stack can obtain electric power.

  The humidification unit is a device for humidifying the reaction gas as described above. The humidifier is a humidifier that uses a bubbler, liquid vaporizer, etc., a method of humidifying the fuel gas by a reforming reaction that generates hydrogen from city gas when generating the fuel gas, or a heat generated from the exhaust gas or the power generation reaction. There is a method of humidifying the reaction gas by using it. In both cases, humidification is performed in advance before the reaction gas is introduced into the power generation section. Humidification is performed by controlling the dew point of the humidified reaction gas. The reactive gas is supplied at a dew point below the operating temperature of the unit cell or stack so that a voltage drop due to the flooding phenomenon does not occur. In general, a reactive gas humidified at the same dew point as the operating temperature is used.

  The gas supply unit communicates the humidification unit and the power generation unit, and carries the reaction gas from the humidification unit to the power generation unit while maintaining the humidified state. The gas supply unit controls the heat radiation of the gas supply unit by providing a heater, a heat insulating material, and the like in the gas pipe, and maintains a stable humidified state of the reaction gas.

  The control unit controls the load current value, gas flow rate, reaction gas temperature, unit cell or stack temperature, reaction gas humidification, etc. required for the power generation reaction and measures the output voltage and power of the unit cell or stack obtained by the power generation reaction Make use.

Embodiments of the present invention will be described below.
The fuel cell power generation device means a performance evaluation device that evaluates the performance of a polymer electrolyte fuel cell and an actual system that uses electric power obtained from the polymer electrolyte fuel cell. Both of them basically have a configuration in which the above power generation unit, humidification unit, gas supply unit, and control unit are connected. A fuel cell power generation apparatus as a performance evaluation apparatus is shown in FIG. 1, and the present invention will be described based on this.

  The fuel cell power generator includes a fuel gas supply unit 16a and an oxidant gas supply unit 16c for supplying a raw material gas, humidification units 12a and 12c, a fuel gas circulation unit 13a and an oxidant gas circulation unit 13c, and a fuel cell power generation unit. 11 and a gas exhaust unit 18a and 18c for discharging surplus gas and generated water. And the heat radiation control parts 14a and 14c are arrange | positioned at the connection part of a fuel cell power generation part and a gas distribution part.

In the fuel cell power generation unit 11, a MEA having a pair of electrodes arranged on both sides of a polymer electrolyte membrane is generally installed in the order of a separator, a current collector plate, a heater, an insulating plate, and an end plate, and a plurality of MEAs are stacked. The bolt is fastened.
The humidification parts 12a and 12c consist of an airtight tank having a liquid phase part and a gas phase part. In the humidification of the reaction gas, the airtight tank is heated with a heater from the inside or the outside, and the temperature of the liquid phase part and the gas phase part is set to the dew point temperature of the reaction gas. In the actual system, humidification is performed by the reaction of the fuel gas generator and humidification with hot water or total heat exchange with exhaust gas.

  The reaction gas is supplied from the fuel gas supply unit 16a and the oxidant gas supply unit 16c through the mass flow valves 17a and 17c to the required flow rate and supplied to the humidification units 12a and 12c, and first through the liquid phase unit. Humidified to dew point temperature. Next, the humidified reaction gas flows through the gas phase portion to the fuel gas flow portion 13a and the oxidant gas flow portion 13c. At this time, the gas phase part has a function of removing excess water droplets or mist.

  FIG. 2 shows a diagram of a connecting portion between the fuel cell power generation unit and the gas distribution unit (oxidant gas distribution unit or fuel gas distribution unit). The gas circulation part 13 has a structure in which a heater 22 and a heat insulating material 23 are arranged around the gas circulation pipe 21 to keep the temperature of the reaction gas being conveyed constant. The heat dissipation control unit 14 connects the gas introduction part 24 and the gas circulation part 13 of the fuel cell power generation unit 11 with a structure as shown in FIG. The gas distribution unit 13 and the fuel cell power generation unit 11 have a structure in which direct heat transfer is not performed via the heat dissipation control unit 14.

  Here, as shown in FIG. 1, the fuel cell power generation unit 11 and the humidification unit 12 were connected via the gas circulation unit 13. In addition, the fuel cell power generation unit 11 and the gas circulation unit 13 are connected via a heat dissipation control unit 14. A single cell was used as the fuel cell of the fuel cell power generation unit 11. The unit cell was produced by the method shown below.

  Carbon powder acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd., particle size 35 nm) was mixed with an aqueous dispersion of polytetrafluoroethylene (PTFE) (D1 manufactured by Daikin Industries, Ltd.) and dried. A water repellent ink containing 20% by weight of PTFE was prepared. This ink is applied and impregnated on carbon paper (TGPH060H manufactured by Toray Industries, Inc.) serving as a base material for the gas diffusion layer, heat treated at 300 ° C. using a hot air dryer, and the gas diffusion layer (about 200 μm). ) Was formed.

  On the other hand, 66 parts by weight of a catalyst body (50 wt% Pt) obtained by supporting a Pt catalyst on Ketjen Black (Ketjen Black EC, Ketjen Black International Co., Ltd., particle size 30 nm), which is carbon powder. Is mixed with 33 parts by weight (polymer dry weight) of perfluorocarbon sulfonic acid ionomer (5% by weight Nafion dispersion manufactured by Aldrich, USA) which is a hydrogen ion conductive material and a binder, and the resulting mixture is molded. Thus, a catalyst layer (10 to 20 μm) was formed.

The gas diffusion layer and the catalyst layer obtained as described above were joined to both surfaces of a polymer electrolyte membrane (Nafion 112 membrane manufactured by DuPont, USA) to produce an MEA.
Next, a rubber gasket plate was joined to the outer periphery of the MEA electrolyte produced as described above, and manifold holes for circulating cooling water, fuel gas, and oxidant gas were formed.

  On the other hand, using a conductive separator plate made of a graphite plate impregnated with a phenol resin, having an outer dimension of 12 cm × 12 cm × 1.3 mm and a gas flow path having a depth of 0.5 mm, Obtained.

Example 1
The end plate portion that becomes the gas introduction portion 24 of the fuel cell power generation unit 11 is made of SUS316 (thermal conductivity 16.0 W / (m · ° C.)), and the heat dissipation control unit 14 has a thermal conductivity of 0.96 W / ( m · ° C.) (1.0 W / (m · ° C. or less)), which is made of tetrafluoroethylene, which is a fluororesin. SUS316 was used for the gas flow pipe 21 of the gas flow section 13. Thus, the fuel cell power generator 1 was obtained.

Example 2
The end plate portion that becomes the gas introduction portion 24 of the fuel cell power generation unit 11 is made of SUS316 (thermal conductivity 16.0 W / (m · ° C.)), and the heat dissipation control unit 14 has a thermal conductivity of 0.96 W / ( m · ° C.) (1.0 W / (m · ° C. or less)), which is made of tetrafluoroethylene, which is a fluororesin. Fluorine resin tetrafluoroethylene was used for the gas flow pipe 21 of the gas flow section 13. In this way, a fuel cell power generator 2 was obtained.

《Comparative example》
The gas circulation part of the fuel cell power generation part 11 was directly connected without providing a heat dissipation control part. At this time, the end plate part which becomes the gas introduction part 24 of the fuel cell power generation unit 11 is made of SUS316, and the gas flow pipe 21 of the gas flow part 13 is made of fluororesin tetrafluoroethylene. In this way, a fuel cell power generator 3 was obtained.

The following evaluation was performed on the fuel cell power generators 1 to 3 manufactured as described above.
(1) Dew point measurement Humidified reaction gas obtained from the gas discharge unit in a state where the reaction gas is humidified to a dew point of 70 ° C, the heater temperature of the gas circulation unit is 80 ° C, the power generation unit is not operated and the temperature is 70 ° C. The dew point was measured using a dew point meter (SIM-12M, M2-Plus manufactured by GENERAL EASTERN). The results are shown in Table 1.

(2) Battery characteristics The battery characteristics were such that a gas mixture of 80% hydrogen and 20% carbon dioxide was supplied to the fuel electrode, and air was supplied to the air electrode. The battery temperature was 70 ° C., the fuel gas utilization rate (Uf) was 70%, and the air utilization rate (Uo) was 40%. The reactive gas was humidified by setting the dew point to 70 ° C. for the fuel gas and 70 ° C. for the air, and humidifying by an open air equivalent to the operating temperature of the power generation unit. A bubbler was used for the humidifying unit 12. The heater temperature in the gas circulation section was 80 ° C. for both the fuel electrode and the air electrode. The load was a current density of 0.2 A / cm ² . Under the above conditions, the change with time of the fuel cell characteristics using hydrogen and air as fuel was evaluated. The results are shown in Table 1.

  As shown in Table 1, the fuel cell power generators 1 and 2 according to Example 1 and Example 2 of the present invention having the heat dissipation control unit have a measurement result within 70 ± 0.2 ° C. with respect to the set dew point. Results were obtained. In contrast, in the fuel cell power generation device 3 according to the comparative example having no heat dissipation control unit, a dew point decrease of 2 ° C. or more was confirmed with respect to the set temperature.

  This is because the temperature of the reaction gas decreases just before reaching the fuel cell power generation unit due to heat dissipation at the connecting portion between the fuel cell power generation unit and the gas circulation unit, and the dew point of the reaction gas is condensed by condensation of moisture in the reaction gas. When the temperature reaches MEA, the temperature is lower than the set temperature. That is, it is shown that by providing the heat dissipation control unit of the present invention, a decrease in the reaction gas temperature due to heat dissipation in the power generation unit and the gas supply unit can be prevented, and stable humidification can be maintained.

  The evaluation results of the battery performance are shown in FIG. The fuel cell power generators 1 and 2 according to Example 1 and Example 2 of the present invention having the heat dissipation control unit exhibited a stable battery voltage of 3000 hours or more, as indicated by a and b, respectively. On the other hand, in the case of the fuel cell power generation device 3 according to the comparative example that does not have the heat dissipation control unit, as shown by c, the cell voltage is not stabilized in a short time unit, and a rapid voltage drop occurs in 2000 hours or less. Indicated.

  From the results of dew point measurement, it was shown that the battery characteristics deteriorated due to the destabilization of the battery characteristics due to a decrease in the dew point of the reaction gas and the drying of the MEA by long-time low humidification operation contrary to the set humidification. That is, in the present invention, by providing the heat dissipation control unit, it is possible to prevent a decrease in the reaction gas temperature due to heat dissipation in the fuel cell power generation unit and the gas circulation unit, to maintain stable humidification, even in a long-time operation. It shows that stable battery characteristics can be exhibited.

  Moreover, from Example 1 and Example 2, it was shown that an equivalent result will be obtained if the thermal conductivity of a heat dissipation control part is below the thermal conductivity of a fuel cell power generation part and a gas distribution part. In this embodiment, tetrafluoroethylene, which is a fluororesin, is used for the heat dissipation control unit. However, the present invention is not limited to this. Other experiments have shown that similar results can be obtained with acrylic resins, polycarbonate resins, nylon resins, polyphenylene sulfonate resins, and the like.

  The fuel cell power generator according to the present invention has an effect that a stable humidified reaction gas can be supplied, and is useful for a power generator or a device using a polymer electrolyte membrane.

It is a figure showing composition of a fuel cell power generator concerning the present invention. In the fuel cell power generation device according to the present invention, it is a schematic cross-sectional view of a portion connecting a fuel cell power generation unit and a gas distribution unit. It is a graph which shows the time-dependent change of the battery characteristic in the fuel cell power generators 1-3 which concern on Example 1, Example 2, and a comparative example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Fuel cell power generation part 12 Humidification part 13 Gas distribution part 14 Heat radiation control part 15 Control part 16 Gas supply part 17 Mass flow valve 18 Gas discharge part 21 Gas distribution pipe 22 Heater 23 Heat insulating material 24 Gas introduction part

Claims (3)

Supplying and discharging an electrolyte membrane, a pair of electrodes sandwiching the electrolyte membrane, and a fuel gas containing at least hydrogen to one of the pair of electrodes, and supplying and discharging an oxidizing gas containing at least oxygen to the other of the pair of electrodes A polymer electrolyte fuel cell power generation unit comprising at least one unit cell composed of a pair of separator plates having a gas flow path,
A humidifying unit for humidifying the fuel gas and the oxidant gas;
A fuel gas supply unit for supplying the fuel gas;
A fuel gas distribution unit connecting the fuel gas supply unit and the fuel cell power generation unit;
An oxidant gas supply unit for supplying the oxidant gas;
An oxidant gas distribution unit connecting the oxidant gas supply unit and the fuel cell power generation unit;
A control unit for controlling power extraction from the fuel cell power generation unit and supply of the fuel gas and the oxidant gas;
A fuel cell power generator comprising: a heat dissipation control unit provided between at least one of the fuel cell power generation unit and the fuel gas circulation unit and between the fuel cell power generation unit and the oxidant gas circulation unit.   The thermal conductivity of the heat dissipation control unit is equal to or lower than the thermal conductivity of the gas introduction portion of the fuel cell power generation unit and is equal to or lower than the thermal conductivity of the fuel gas circulation portion and / or the oxidant gas circulation portion. Item 4. The fuel cell power generator according to Item 1.   3. The fuel cell power generator according to claim 1, wherein the heat dissipation control unit is made of a nonmetallic material having a thermal conductivity of 1.0 (W / (m · ° C.)) or less.

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