WO2008016216A1 - Fuel cell system and cooling control method thereof - Google Patents

Fuel cell system and cooling control method thereof Download PDF

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Publication number
WO2008016216A1
WO2008016216A1 PCT/KR2007/002515 KR2007002515W WO2008016216A1 WO 2008016216 A1 WO2008016216 A1 WO 2008016216A1 KR 2007002515 W KR2007002515 W KR 2007002515W WO 2008016216 A1 WO2008016216 A1 WO 2008016216A1
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Prior art keywords
fuel cell
cooling water
cell stack
water inlet
difference
Prior art date
Application number
PCT/KR2007/002515
Other languages
French (fr)
Inventor
Hyung-Mok Cho
Ho-Suk Kim
Sang-Yong Lee
Byung-Sun Hong
Mee-Nam Shinn
Original Assignee
Fuelcell Power, Inc.
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Publication date
Application filed by Fuelcell Power, Inc. filed Critical Fuelcell Power, Inc.
Priority to CN2007800011377A priority Critical patent/CN101356674B/en
Publication of WO2008016216A1 publication Critical patent/WO2008016216A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537—Electric variables
    • H01M8/04544—Voltage
    • H01M8/04559—Voltage of fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H01M8/04029—Heat exchange using liquids
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/0432—Temperature; Ambient temperature
    • H01M8/04358—Temperature; Ambient temperature of the coolant
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04746—Pressure; Flow
    • H01M8/04768—Pressure; Flow of the coolant
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0606—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants
    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H01M8/0618—Reforming processes, e.g. autothermal, partial oxidation or steam reforming
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M2008/1095—Fuel cells with polymeric electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H01M8/04059—Evaporative processes for the cooling of a fuel cell
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Sustainable Development (AREA)
  • Sustainable Energy (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Fuel Cell (AREA)

Abstract

The present invention relates to a fuel cell system, and to a cooling control method of the fuel cell system. The cooling control method of the fuel cell system includes measuring a voltage generated from the fuel cell stack in accordance with a time variation, and maintaining an internal temperature of the fuel cell stack at a predetermined value in accordance with the measured voltage. The method may further include controlling a cooling water temperature difference. Herein, when the measured voltage is equal to or greater than a predetermined value, the cooling water is normally supplied, and when the measured voltage is less than the predetermined value, a difference between cooling water inlet and outlet temperatures of the fuel cell stack is varied by a controller. The deterioration of the power generation performance due to the excessive moisture in the fuel cell stack can be prevented and thus the power generation can be effectively realized through the electrochemical reaction.

Description

[DESCRIPTION] [Invention Title]
FUEL CELL SYSTEM AND COOLING CONTROL METHOD THEREOF [Technical Field]
The present invention relates to a fuel cell system using a proton exchange membrane fuel cell (PEMFC). More particularly, the present invention relates to a fuel cell system that can stably generate electric power by effectively removing moisture from an electrode layer of a fuel cell stack or by effectively supplying a reaction gas to the fuel cell stack, and to a cooling control method of the fuel cell system. [Background Art]
A PEMFC uses a proton exchange membrane having hydrogen ion exchange properties as an electrolyte membrane. The PEMFC generates electricity and heat through an electrochemical reaction using a fuel gas containing hydrogen and air containing oxygen. The PEMFC has a quick start capability and can be designed in a small size. Therefore, the
PEMFC has been widely used in various applications such as a portable power source, an automotive power source, and a home cogeneration plant.
As is well known, a conventional fuel cell system using the PEMFC includes a fuel cell stack, a fuel process unit, a voltage converter, and a heat recovery unit. The fuel cell system reforms an electricity generation material to supply hydrogen to an anode of the fuel cell stack. The fuel cell system supply air to a cathode of the fuel cell stack through using a blower. Then, an electrochemical reaction occurs on the electrode layer of the fuel cell stack, thereby generating the electric power. At this point, an electrochemical reaction formula is as follows. [Chemical Formula 1 ]
Anode: H2(g) → 2H+ + 2e
Cathode: 1/2O2(g) + 2H+ + 2e → H2O (1) Cell Reaction: H2(g) + 1/2O2(g) → H2O (1 )
The electrochemical reaction of the fuel cell system occurs on a three-phase intersurface where the electrode layer (containing a catalyst), an electrolyte, and a reaction gas meet simultaneously with each other. The electrode layer plays an important roll in the reaction gas diffusion and the moisture supply of the electrolyte. In order to adjust the moisture generated from the electrode layer, a hydrophilic property is adjusted by adding a functional polymer or the moisture is discharged by adjusting middle-sized pores and small-sized pores through density control of the electrode. However, when the fuel cell system starts or stops, a large amount of moisture may be supplied to the electrode layer of the fuel cell stack. In addition, even when the fuel cell system is driven in a rated operation, the moisture may be excessively supplied or it may suffer from a shortage of moisture. Accordingly, the fuel cell system experiences a difficulty in stably maintaining the power generation performance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. [Disclosure]
[Technical Problem] Embodiments of the present invention provide a fuel cell system that is improved to effectively supply reaction gases (oxygen and hydrogen) by removing a large amount of moisture existing in pores of an electrode layer and a gas diffusion layer of a fuel cell stack. Embodiments of the present invention also provide a cooling control method of the fuel cell system.
[Technical Solution]
In an exemplary embodiment of the present invention, a fuel cell system includes: a fuel cell stack in which an electrochemical reaction occurs; a fuel process unit that supplies hydrogen to the fuel cell stack by reforming a power generation material; an oxygen supply unit for supplying oxygen to the fuel cell stack; a cooling unit for absorbing heat from the fuel cell stack using cooling water; a heat recovery unit for recovering waste heat from the cooling water; a voltage detector for detecting a power voltage in the fuel cell stack in accordance with a time variation; and a controller for varying a difference between cooling water inlet and outlet temperatures in response to a time variation while maintaining a temperature of the cooling water flowing into the fuel ceil stack within a predetermined range in accordance with the voltage detected by the voltage detector.
In another exemplary embodiment of the present invention, a method of cooling a fuel cell stack of a fuel cell system includes measuring a voltage generated from the fuel cell stack in accordance with a time variation, and maintaining an internal temperature of the fuel cell stack at a predetermined value in accordance with the measured voltage. The method may further include, after maintaining the internal temperature, controlling a cooling water temperature difference. Herein, when the measured voltage is equal to or greater than a predetermined value, the cooling water is normally supplied, and when the measured voltage is less than the predetermined value, a difference between cooling water inlet and outlet temperatures of the fuel cell stack is varied by a controller.
In addition, maintaining the internal temperature may include determining if a reduction rate of the measured voltage in accordance with the time variation is within a predetermined range, and maintaining the internal temperature of the fuel cell stack at a predetermined value by adjusting the flow rate of the cooling water in accordance with the determining result of the reduction rate.
Further, controlling the cooling water difference may be performed by alternately performing a process for increasing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a first range, and a process for reducing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a second range, after a predetermined time has elapsed.
In addition, maintaining the internal temperature may include determining if a reduction rate of the measured voltage in accordance with a time variation is within a predetermined range, and maintaining the internal temperature of the fuel cell stack at a predetermined value by allowing the outlet cooling water of the fuel cell stack to heat-exchange or by disallowing the outlet cooling water of the fuel cell stack to heat-exchange but directly directing the cooling water to the fuel cell stack by changing a heat recovery path in accordance with the determined result of the reduction rate.
[Advantageous Effects]
In the fuel cell system and the cooling control method of the fuel cell system according to the exemplary embodiment of the present invention, since the moisture existing on the electrode layer and the gas diffusion layer can be properly discharged, the deterioration of the power generation performance due to the excessive moisture in the fuel cell stack can be prevented and thus the power generation can be effectively realized through the electrochemical reaction. [Description of Drawings]
FIG. 1 is a schematic diagram of a fuel cell system according to a first exemplary embodiment of the present invention.
FIG. 2 is a flowchart illustrating a cooling control method of the fuel cell system of FIG. 1.
FIG. 3 is a schematic diagram of a fuel cell system according to a second exemplary embodiment of the present invention. FIG. 4 is a flowchart illustrating a cooling control method of a fuel cell system of FIG. 3.
FIG. 5 shows graphs illustrating a performance variation of a fuel cell stack in accordance with a temperature variation of cooling water when the fuel cell systems of FIGS. 1 and 3 operate for a long period. 8? Description of Reference Numerals Indicating Primary Elements in the Drawings &
100, 200: Fuel Cell System 1 10, 210: Fuel Cell Stack 120, 220: Fuel Process Unit 130, 230: Oxygen Supply Unit 140, 240: Voltage converter 150, 250: Cooling Unit 160, 161 , 260, 261 : Controller
[Best Mode]
The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. FIG. 1 is a schematic diagram of a fuel cell system according to a first exemplary embodiment of the present invention.
As shown in FIG. 1 , a fuel cell system 100 of the present exemplary embodiment includes a fuel cell stack 110 in which an electrochemical reaction occurs, a fuel process unit 120 for reforming an electricity generation material and supplying hydrogen to the fuel cell stack 110, an oxygen supply unit 130 that supplies oxygen to the fuel cell stack 110 by cooperating with a compressor or a blower, and a voltage converter 140 for converting a direct current generated from the fuel cell stack 110 into an alternating current. Particularly, in order to remove moisture from an electrode layer and material supplying channel of the fuel cell stack 110, the fuel cell system 100 of the present exemplary embodiment is designed in the following structure.
The fuel cell system 100 further includes a cooling unit 150 using cooling water for absorbing heat generated from the fuel cell stack 110, and a heat recovery unit 151 for recovering waste heat from the cooling water of the cooling unit 150.
The cooling unit 150 includes a water tank 152, a first pump 153, and a second pump 154, which are installed on an inlet pipe connecting an inlet of the fuel cell stack 110 to the heat recovery unit 151. In more detail, the water tank 152 and the first pump 153 are installed between the inlet of the fuel cell stack 110 and a heat exchanger 155 that will be described later, and the second pump 154 is installed between the heat exchanger 155 and the heat recovery unit 151. At this point, the cooling unit 150 is installed such that inlet and outlet pipes thereof pass through the heat exchanger 155, where the heat exchange is realized between cooling water. In the case that heat is sufficiently recovered by the heat recovery unit 151 , the cooling unit 150 further includes an air cooled heat exchanger 156 that is connected between the cooling water inlet and outlet pipes.
First and second controllers 160 and 161 control a difference ΔT between temperatures at the cooling water inlet and outlet so that the difference ΔT becomes a pulse shape in accordance with a time variation. To realize this, the first controller 160 controls the operation of the first pump 153 and the second controller 161 controls the operation of the second pump 154. Subsequently, the first and second controllers 160 and 161 control the flow rate of the cooling water flowing into the fuel cell stack. Although the first and second controllers 160 and 161 are separately installed in this embodiment, the present invention is not limited to this case. For example, only one controller may be provided to control all of the constituent elements. In addition, the first and second controllers 160 and 161 may be designed to share or exchange data transmitted from a variety of sensors or a variety of constituent elements.
Temperature sensors 170 and 171 are respectively installed on the cooling water inlet and outlet pipes connected to the fuel cell stack 110 to measure the temperatures at the cooling water inlet and outlet pipes. The data on the cooling water inlet and outlet temperatures are transmitted to the first controller 160. If necessary, in this exemplary embodiment, a temperature sensor 172 may be further installed on the cooling water inlet pipe to measure the temperature of the cooling water discharged from the heat exchanger 155. The data on the temperature measured by the temperature sensor 172 are sent to the second controller 161. A pressure sensor 173 is provided to measure pressure in the cooling water outlet pipe and to send the measured pressure data to the first controller 160.
A voltage detector 180 measures a power voltage generated in the fuel cell stack 110 in accordance with a time variation and sends the measured data to the first controller 160.
The following will describe a cooling control method of the fuel cell system 100 with reference to FIG. 2.
FIG. 2 is a flowchart illustrating a cooling control method of the fuel cell system of FIG. 1.
Referring to FIGS. 1 and 2, a cooling control method of the fuel cell system according to an exemplary embodiment of the present invention will be described. In the first exemplary embodiment, when the generation of the electric power starts, the voltage detector 180 continuously measures a power voltage and the first controller 160 determines if a reduction rate ΔV of the measured voltage in accordance with a time variation is within a predetermined range in accordance with input data. According to the determining results of the first controller 160, the second controller 161 operates the second pump 154 of the cooling unit 150 to adjust a flow rate of the cooling water, thereby uniformly maintaining an internal temperature of the fuel cell stack 110. Next, in a state where the internal temperature of the fuel cell stack
110 is uniformly maintained, it is determined if a power voltage of the fuel cell stack 110 is equal to or greater than a predetermined value.
When the power voltage is equal to or greater than the predetermined value, a current flow rate of the cooling water is normally maintained. When the power voltage is less than the predetermined value, the first controller 160 operates the first pump 153 to adjust the flow rate of the cooling water, thereby varying a cooling degree of the fuel cell stack 110. That is, a temperature difference control process for the cooling water is performed. For example, in the temperature difference control process, a process for increasing the flow rate of the cooling water is performed so that a difference ΔT between the inlet and outlet temperatures of the cooling water can be maintained within a relatively narrow range of 3-4 "C . In addition, in the temperature difference control process, a process for decreasing the flow rate of the cooling water is further performed so that a difference ΔT between the inlet and outlet temperatures of the cooling water can be maintained within a relatively wide range of 8-10°C . The processes for increasing and decreasing the flow rate of the cooling water are alternately performed at a predetermined period (2-5 times). Then, a graph of the difference ΔT between the inlet and outlet temperatures of the cooling water becomes a pulse shape.
However, the above-described cooling control method of the fuel cell system 100 may be performed when a reduction rate ΔV of the power voltage that is continuously measured in accordance with a time variation is within a predetermined range or when several hours (e.g., 8 hours) have been elapsed after the former cooling control was performed.
The following will describe a fuel cell system according to a second embodiment of the present invention.
FIG. 3 is a schematic diagram of a fuel cell system according to a second exemplary embodiment of the present invention.
A fuel cell system 200 of the present exemplary embodiment is substantially similar to the fuel cell system 100 of FIG. 1. Therefore, description of the identical constituent elements will be omitted herein and only different constituent elements will be described.
In the second exemplary embodiment, the air cooled heat exchanger 156 depicted in FIG. 1 is omitted. Instead, in this fuel cell system 200, a heat recovery pipe 257 is connected between cooling water inlet and outlet pipes such that the cooling water flowing along the cooling water outlet pipe is directed to the cooling water inlet pipe without passing through the heat recovery unit 251. Further, a first switch valve 258 is installed on the cooling water inlet pipe to selectively allow the flow of the cooling water from the heat recovery unit 251. A second switch valve 259 is installed on the heat recovery pipe 257 to selectively allow the flow of the cooling water. Then, when the cooling water is controlled not to pass through the heat recovery unit 251 , the first switch valve 258 is closed and the second switch valve 259 is opened. On the other hand, when the cooling water is controlled such that waste heat of the cooling water is recovered, the first switch valve 258 is opened while the second switch valve 259 is closed. That is, the fuel cell system 200 of this exemplary embodiment is configured such that the waste heat of the cooling water can be recovered as occasion demands by selectively opening/closing the first and second switch valves 258 and 259.
The following will describe a cooling control method of the fuel cell system 200 with reference to FIGS. 3 and 4. FIG. 4 is a flowchart illustrating a cooling control method of the fuel cell system of FIG. 3.
It is preferable that the fuel cell stack 210 maintains an internal temperature thereof within a predetermined range when operating. However, when the fuel cell stack 210 operates for a long duration, it becomes difficult to maintain a water balance therein by a variation of humidity or other external environmental variations. Therefore, condensed water is generated in a catalytic layer and a gas diffusion layer of the fuel cell stack 210, and this causes a water clogging phenomenon reducing a power voltage of the fuel cell system.
To solve this problem, a cooling control method of the fuel cell system of this second exemplary embodiment is performed according to the following processes. In the second exemplary embodiment, when the generation of the electric power starts, the voltage detector 280 continuously measures a power voltage and the first controller 260 determines if a reduction rate ΔV of the measured voltage in accordance with a time variation is within a predetermined range. Next, it is detected if a difference ΔT between cooling water inlet and outlet temperatures of the fuel cell stack 210 is within a predetermined temperature range of 3-40C . When the difference ΔT is not within the predetermined temperature range, the cooling water inlet temperature and the temperature of the cooling water introduced from the heat recovery unit 251 are measured. When the cooling water outlet temperature is equal to or greater than a predetermined value, the second controller 261 controls the first and second switch valves 258 and 259 such that the cooling water can pass through the heat recovery unit 251 , thereby lowering the temperature of the fuel cell stack 210. After the above, when the temperature difference ΔT reaches the predetermined value, the second controller 261 controls the first and second switch valves 258 and 259 such that the cooling water can be introduced into the heat recovery pipe 257. By the above-described process, the internal temperature of the fuel cell stack 210 can be maintained at a predetermined value.
As described above, in the fuel cell system 200, it can be detected that the reduction rate ΔV of the power voltage in accordance with the time variation due to the water clogging phenomenon is within a predetermined range. Then, in the second exemplary embodiment, the second controller 261 stops operating the second pump 254 to increase the internal temperature of the fuel cell stack 210, thereby removing the moisture condensed in the catalytic layer and the gas diffusion layer. Next, when the cooling water outlet temperature is maintained at the predetermined value, the second controller 261 operates again the second pump 254 so that the temperature difference ΔT can be maintained within a range of 8-100C .
Further, it is determined if the power voltage of the fuel cell stack 210, which is detected by the voltage detector 280, is equal to or greater than a predetermined value.
When the power voltage is equal to or greater than the predetermined value, a current flow rate of the cooling water is normally maintained. When the power voltage is less than the predetermined value, the first controller 260 operates the first pump 153 to adjust the flow rate of the cooling water, thereby varying a cooling degree of the fuel cell stack 210. That is, a temperature difference control process for the cooling water is performed. For example, in the temperature difference control process, a process for increasing the flow rate of the cooling water is performed so that a difference ΔT between the inlet and outlet temperatures of the cooling water can be maintained within a relatively narrow range of 3-4 °C. In addition, in the temperature difference control process, a process for decreasing the flow rate of the cooling water is further performed so that a difference ΔT between the inlet and outlet temperatures of the cooling water can be maintained within a relatively wide range of 8-1 O0C . The processes for increasing and decreasing the flow rate of the cooling water are alternately performed at a predetermined period (2-5 times). Then, a graph of the difference ΔT between the inlet and outlet temperatures of the cooling water becomes a pulse shape.
FIG. 5 shows graphs illustrating a performance variation of a fuel cell stack in accordance with a temperature variation of cooling water when the fuel cell systems of FIGS. 1 and 3 operate for a long period.
The graphs of FIG. 5 show a relationship between a voltage k of the fuel cell stack, a cooling water inlet temperature I, and a cooling water outlet temperature j when the fuel cell system operates for a long period. As shown in the graphs, the voltage of the fuel cell stack is prone to increase as the cooling water inlet and outlet temperatures increase. This can be understood according to the following Equation 1. [Equation 1]
Figure imgf000017_0001
π ) E= E 9+ RT nF In
HtQ
Here, G is Gibb's free energy, R is a Molar gas constant, P is partial pressure, n is the number of electrons per molecule, F is Avogadro's number, T is a temperature, and E is Potential measured in volts. According to the graphs of FIG. 5, it can be noted that the voltage of the fuel cell stack keeps increasing until the cooling water inlet and outlet temperatures increase up to a point c. This can be understood from Equation 1 where the voltage is proportional to the temperature. Further, a reaction speed of a reaction material depends on the temperature and thus the reaction material actively reacts on a surface of the electrode, thereby increasing the voltage of the fuel cell stack. Further, when the heat exchange is normally performed again when the temperature of the cooling water increases up to the point c, the temperature of the cooling water is steeply reduced. Then, the voltage of the fuel cell stack reaches the highest value when the difference between the cooling water inlet and outlet temperatures is within the temperature range of 8-10°C . The voltage increase a of the fuel cell stack until the temperature difference changes to be within the temperature range can be realized since the moisture from the small pores of the catalytic layer and the supply passage can be removed due to the increase of saturated vapor pressure, and thus a reaction point of metal catalysts of the cathode and anode is obtained. Therefore, since the vapor pressure is reduced again in a section where the temperature is reduced and thus the gas supply is effectively realized, the voltage of the fuel cell stack can increase. While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

[CLAIMS] [Claim 1 ]
A fuel cell system comprising: a fuel cell stack in which an electrochemical reaction occurs; a fuel process unit that supplies hydrogen to the fuel cell stack by reforming a power generation material; an oxygen supply unit for supplying oxygen to the fuel cell stack; a cooling unit for absorbing heat from the fuel cell stack using cooling water; a heat recovery unit for recovering waste heat from the cooling water; a voltage detector for detecting a power voltage in the fuel cell stack in accordance with a time variation; and a controller for varying a difference between cooling water inlet and outlet temperatures in response to a time variation while maintaining a temperature of the cooling water flowing into the fuel cell stack within a predetermined range in accordance with the voltage detected by the voltage detector.
[Claim 2] The fuel cell system of claim 1 , wherein a cooling water inlet pipe for directing the cooling water from the heat recovery unit to the fuel cell stack is disposed between the fuel cell stack and the heat recovery unit; a cooling water outlet pipe for directing the cooling water from the fuel cell stack to the heat recovery unit is disposed between the fuel cell stack and the heat recovery unit; and temperature sensors for measuring the respective cooling water inlet and outlet temperatures are installed on the cooling water inlet and outlet pipes, respectively.
[Claim 3]
The fuel cell system of claim 2, wherein an air cooled heat exchanger for dissipating the waste heat of the cooling water to an external side is disposed between the cooling water inlet and outlet pipes.
[Claim 4]
The fuel cell system of claim 2, wherein a heat recovery pipe is connected between the cooling water inlet and outlet pipes so that the cooling water can be directed from the cooling water outlet pipe to the cooling water inlet pipe without passing through the heat recovery unit.
[Claim 5]
The fuel cell system of claim 4, wherein a first switch valve is installed on the cooling water inlet pipe to selectively allow the cooling water to be discharged from the heat recovery unit and a second switch valve is installed on the heat recovery pipe to selectively allow the cooling water to flow.
[Claim 6] A method of cooling a fuel cell stack of a fuel cell system, comprising: measuring a voltage generated from the fuel cell stack in accordance with a time variation; maintaining an internal temperature of the fuel cell stack at a predetermined value in accordance with the measured voltage; and controlling a cooling water temperature difference, wherein, when the measured voltage is equal to or greater than a predetermined value, the cooling water is normally supplied, and when the measured voltage is less than the predetermined value, a difference between cooling water inlet and outlet temperatures of the fuel cell stack is varied by a controller.
[Claim 7]
The method of claim 6, wherein maintaining the internal temperature comprises: determining if a reduction rate of the measured voltage in accordance with the time variation is within a predetermined range; and maintaining the internal temperature of the fuel cell stack at a predetermined value by adjusting the flow rate of the cooling water in accordance with the determined result of the reduction rate.
[Claim 8]
The method of claim 7, wherein controlling the cooling water difference is performed by alternately performing a process for increasing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a first range, and a process for reducing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a second range, after a predetermined time has elapsed.
[Claim 9]
The method of claim 8, wherein the first range is 3-40C and the second range is 8-100C .
[Claim 10] The method of claim 8, wherein the difference between the cooling water inlet and outlet temperatures is controlled to be a pulse shape in accordance with a time variation. [Claim 11 ]
The method of claim 6, wherein maintaining the internal temperature comprises: determining if a reduction rate of the measured voltage in accordance with a time variation is within a predetermined range; and maintaining the internal temperature of the fuel cell stack at a predetermined value by allowing the outlet cooling water of the fuel cell stack to heat-exchange or by disallowing the outlet cooling water of the fuel cell stack to heat-exchange but directly directing the cooling water to the fuel cell stack by changing a heat recovery path in accordance with the determined result of the reduction rate. [Claim 12]
The method of claim 11 , wherein controlling the cooling water difference is performed by alternately performing a process for increasing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a first range, and a process for reducing the flow rate, thus allowing the difference between the cooling water inlet and outlet temperatures to reach a second range, after a predetermined time has elapsed. [Claim 13]
The method of claim 12, wherein the first range is 3-40C and the second range is 8-100C . [Claim 14]
The method of claim 12, wherein the difference between the cooling water inlet and outlet temperatures is controlled to be a pulse shape in accordance with a time variation.
PCT/KR2007/002515 2006-08-02 2007-05-23 Fuel cell system and cooling control method thereof WO2008016216A1 (en)

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