Abstract:
A method of starting a polymer electrolyte membrane fuel cell (PEMFC) stack by rapidly increasing its temperature. The PEMFC stack includes: a first flow line connected to cooling plates; a second flow line connected to the cooling plates; a coolant reservoir; a heat exchanger; a by-pass line; a heating element; a first valve installed between the first flow line and the heat exchanger; and a second valve that selectively connects the coolant reservoir, the second flow line, and the by-pass line. The method of starting a PEMFC stack includes: closing the first valve and controlling the second valve so that the second flow line and the by-pass line are connected to each other, and the coolant in the coolant reservoir is not connected to the second flow line and the by-pass line; and heating the coolant in the by-pass line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 2007-7237, filed Jan. 23, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to a fuel cell, a fuel cell system, and a method of operating the same. 
     2. Description of the Related Art 
     A fuel cell is an electricity generation device that directly converts the chemical energy of hydrogen, included in a hydrocarbon material, such as, methanol, ethanol, or natural gas, into electrical energy through a chemical reaction with oxygen. 
     A polymer electrolyte membrane fuel cell (PEMFC) has a wide range of applications, for example, as a power source for automobiles, as a distributional power source for houses or public buildings, and as a small power source for electronic devices. A PEMFC has a superior power output, as compared to other fuel cells, a low operating temperature, a rapid start-up, and a short response time. 
     A conventional PEMFC mainly operates at a temperature below 100° C., for example, at approximately 80° C., to prevent the drying of a polymer electrolyte membrane included therein. However, a conventional PEMFC has problems, due to the low operating temperature of approximately 100° C., or less. For example, a conventional PEMFC uses a hydrogen-rich gas as a fuel, which is obtained by reforming an hydrocarbon containing material, such as, natural gas or methanol, which produces CO 2  and CO as by-products. The CO poisons a catalyst included in a cathode and anode of a conventional PEMFC. When the catalyst is poisoned by CO, an electrochemical activity of the catalyst is greatly reduced, and as a result, the operation efficiency and lifetime of the PEMFC are significantly reduced. Furthermore, as the operating temperature of a conventional PEMFC is decreased, the catalyst poisoning by the CO is increased. 
     A conventional high temperature PEMFC operates at a higher operating temperature of, for example, approximately 130° C., or higher, in order to reduce the poisoning of the catalyst by CO, and to allow for easier temperature control thereof. However, a conventional high temperature PEMFC requires a cooling system to cool a fuel cell stack thereof. The cooling system mainly uses de-ionized water as a coolant. 
     In a conventional high temperature PEMFC that uses a coolant, an internal temperature of a fuel cell stack must be brought to, for example, 120° C., when the fuel cell stack begins operation. To do this, the coolant that passes through the fuel cell stack must also be heated. However, due to the time required for the coolant to be heated, the normal operation of the PEMFC is delayed. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide a method to rapidly increase an internal temperature of a fuel cell stack of a fuel cell system, during a start-up (preheating) operation of the fuel cell system. 
     According to aspects of the present invention, there is provided a method of preheating a polymer electrolyte membrane fuel cell (PEMFC) stack of a fuel cell system. The fuel cell system comprises: a first flow line that is connected to upper parts of cooling plates installed in a plurality of unit cells of the PEMFC stack; a second flow line that is connected to lower parts of the cooling plates; a coolant reservoir installed between the first flow line and the second flow line; a heat exchanger installed between the first flow line and the coolant reservoir; a by-pass line that connects a point between the coolant reservoir and the second flow line, to the first flow line; a heating element that heats coolant in the by-pass line; a first valve installed between the first flow line and the heat exchanger; and a second valve that selectively connects the coolant reservoir, the second flow line, and the by-pass line 
     According to various embodiments, the method comprises: closing the first valve and controlling the second valve so that coolant in the second flow line can flow to the by-pass line, and so that coolant in the coolant reservoir cannot flow to the second flow line or the by-pass line; and increasing the temperature of the PEMFC stack, by heating the coolant in the by-pass line using the heating element. The method may further comprise opening the first valve and controlling the second valve, so that the second flow line is opened to the coolant reservoir, if the temperature of the PEMFC stack is increased to a predetermined temperature, or higher. 
     According to some embodiments, the second valve V 2  may be a 3-way valve that selectively allows the coolant to flow between the coolant reservoir, the second flow line, and the by-pass line. The heating element may be a burner or an electric heater. 
     According to other aspects of the present invention, there is provided a method of preheating a PEMFC stack of a fuel cell system. The fuel cell system comprises: a first flow line that is connected to upper parts of cooling plates installed in a plurality of unit cells of the PEMFC stack; a second flow line that is connected to lower parts of the cooling plates; a coolant reservoir installed between the first flow line and the second flow line; a heat exchanger installed between the first flow line and the coolant reservoir; a by-pass line that connects a point between the coolant reservoir and the second flow line, to the first flow line; a heating element that heats coolant in the by-pass line; a third valve connected to the coolant reservoir, to discharge coolant to the outside; and a fourth valve connected to a coolant supply line, to control a flow of coolant to the coolant reservoir from the coolant supply line. 
     According to some aspects of the present teachings, the method comprises: opening the third valve, in order to discharge the coolant in the coolant reservoir to the outside; and increasing the temperature of the PEMFC stack by operating the heating element. The method may further comprise supplying coolant to the coolant reservoir by opening the fourth valve, when the temperature of the PEMFC stack reaches a predetermined temperature, or higher. The fourth valve may be located at a position higher than the coolant reservoir. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram of a fuel cell system, according to an exemplary embodiment of the present invention; 
         FIG. 2  is a perspective view of a portion of a cooling system of a fuel cell stack of the fuel cell system of  FIG. 1 , according to an exemplary embodiment of the present invention; 
         FIG. 3  is an exploded perspective view of the flow of a fluid between the cooling plate and the unit cells of the fuel cell stack of  FIG. 2 , according to an exemplary embodiment of the present invention; 
         FIG. 4  is a schematic drawing of a cooling system having a heating device for initially heating the fuel cell stack of the fuel cell system of  FIG. 1 , according to an exemplary embodiment of the present invention; 
         FIG. 5  is a schematic drawing of a cooling system, to which a method of starting a fuel cell stack, according to an exemplary embodiment of the present invention is employed; and 
         FIG. 6  is a schematic drawing of a cooling system, to which a method of starting a fuel cell stack, according to an exemplary embodiment of the present invention is employed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 1  is a block diagram of a fuel cell system  50 , according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the fuel cell system  50  includes a fuel processor  200 , a polymer electrolyte membrane fuel cell (PEMFC) stack  100 , and a cooling system  60  to cool the PEMFC stack  100 . 
     The fuel processor  200  includes a desulfurizer  210 , a reformer  220 , a burner  230 , a water supply pump  260 , first and second heat exchangers H 1  and H 2 , and a carbon monoxide (CO) removing unit  250 . The CO removing unit  250  comprises a CO shift reactor  251  and a CO remover  252 . 
     Hydrogen is generated in the reformer  220 , which is heated by the burner  230  through a chemical reaction between a hydrocarbon containing gas, which is a fuel source supplied from a fuel tank  270 , and steam, which is supplied from a water tank  280  by the water supply pump  260 . CO 2  and CO are generated as by products from the chemical reaction. If a fuel containing 10 ppm, or more, of the CO is supplied to the PEMFC stack  100 , catalysts of electrodes of the PEMFC stack  100  become poisoned, resulting in a rapid reduction in the performance of the fuel cell system  50 . Therefore, the content of the CO in the fuel supplied to the PEMFC is reduced to 10 ppm, or less, by installing the CO shift reactor  251  and the CO remover  252  at an outlet of the reformer  220 . The CO 2  is mainly produced in the CO shift reactor  251  by a chemical reaction between the CO and the steam. An oxidation reaction between the CO and oxygen mainly occurs in the CO remover  252 . 
     The desulfurizer  210 , installed at the inlet of the reformer  220 , removes sulphur components included in the fuel source. In the first and second heat exchangers H 1  and H 2 , the water supplied by the water supply pump  260  absorbs heat from a combustion gas exhausted from the burner  230  and the reformer  220 . 
       FIG. 2  is a perspective view of a portion of a portion of the cooling system  60  of the fuel cell stack  100 , according to an exemplary embodiment of the present invention, and  FIG. 3  is an exploded perspective view of the flow of a fluid between a cooling plate  5  and unit cells  10  of the PEMFC stack  100 , of  FIG. 2 , according to an exemplary embodiment of the present invention. 
     Referring to  FIGS. 2 and 3 , the unit cells  10  are stacked in the PEMFC stack  100 . Each of the unit cells  10  includes an electrolyte membrane  2 , and a cathode electrode  1 , an anode electrode  3 , and an electrolyte membrane  2  disposed therebetween. Separators  4 , having flow channels  4   a  to supply an oxidant or hydrogen gas to the cathode and anode electrodes  1  and  3 , are installed between the unit cells  10 . Gaskets  6  to seal each of the unit cells  10  are installed between the cathode and anode electrodes  1  and  3 , and between the separators  4 . 
     The PEMFC stack  100  includes a plurality of cooling plates  5  and a heat exchanger H 5  (refer to  FIG. 1 ). The cooling plates  5  channel a coolant through the PEMFC stack  100 . One cooling plate  5  is disposed on each of the unit cells  10 . For example, a cooling plate  5  can be disposed between each of the unit cells  10 . The coolant absorbs heat from the PEMFC stack  100 , by passing through flow channels  5   a  of the cooling plate  5 . The heated coolant is cooled in the heat exchanger H 5 , by a secondary coolant. Afterwards, the coolant from the heat exchanger H 5  is re-circulated in the PEMFC stack  100 , via a coolant reservoir  130  (refer to  FIG. 1 ). 
     End plates  21  and  22  are respectively installed on opposing ends of the PEMFC stack  100 . An oxygen (air) supply hole, an oxygen (air) recovery hole, a fuel (hydrogen gas) supply hole, and a fuel (hydrogen gas) recovery hole are formed in the end plate  21 . A coolant supply hole and a coolant recovery hole are formed in the end plate  22 . Each of the cooling plates  5 , the unit cells  10 , and the separators  4  includes the coolant supply and recovery holes, the oxygen (air) supply and recovery holes, and the fuel (hydrogen) supply and recovery holes. The fuel (air, and hydrogen gas) or the coolant is supplied and discharged through the respective above-described holes. 
     Referring to  FIG. 1 , a coolant stored in the coolant reservoir  130  is supplied to the cooling plates  5  in the PEMFC stack  100 , in a liquid state, and exits the PEMFC stack  100  in a vapour state, after absorbing heat in the PEMFC stack  100 . The coolant is cooled through a heat exchange, in the heat exchanger H 5 , with the secondary coolant supplied from a water tank  140  (refer to  FIG. 1 ). The coolant is stored in the coolant reservoir  130 . A process burner  110  may use surplus hydrogen that is not consumed in the anode electrode  3  of the PEMFC stack  100 , and/or may use a fuel supplied from the fuel tank  270  during a normal operation state of the PEMFC stack  100 . The process burner  110  heats the water supplied from the water tank  140  at a heat exchanger H 3 . Water heated by the process burner  110  is guided to a warm water storage  120 . 
     Air is supplied to the cathode electrodes  1  of the PEMFC stack  100 . A mixture of air and steam is guided to a heat exchanger H 4 , where the steam is condensed into water, which is recovered in the water tank  280 . 
       FIG. 4  is a schematic drawing of a cooling system  400 , including a heating element  310  to preheat the PEMFC stack  100 , according to an exemplary embodiment of the present invention. Like reference numerals are used to indicate elements that are substantially identical to the elements of  FIGS. 1 through 3 , and thus, the detailed description thereof will not be repeated. 
     Referring to  FIG. 4 , for convenience of explanation, the cooling plates  5  of the PEMFC stack  100  are simplified, by omitting the unit cells  10  disposed between the cooling plates  5 . A first flow line  301  that passes by the upper parts of the cooling plates  5  is a flow line to channel a cooling medium (coolant) that exits the cooling plates  5 . A second flow line  302  is connected to the lower parts of the cooling plates  5 , and is a flow line to channel the cooling medium than enters the PEMFC stack  100 . The first flow line  301  is connected to the coolant reservoir  130  and the heat exchanger H 5 . The second flow line  302  is connected to the coolant reservoir  130 , in order to receive coolant. The heat exchanger H 5  can cool the coolant received from the first flow line  301 , using a secondary coolant other than the coolant that circulates in the PEMFC stack  100 , for example, the water from the water tank  140 . The coolant can be deionized (DI) 
     A heating element  310 , for example, an electric heater, is connected to the coolant reservoir  130 . The heating element  310  preheats the PEMFC stack  100  to a predetermined temperature, for example, 120° C., when the fuel cell system  50  is started. The heating element  310  installed in a by-pass line  303 , disposed between the coolant reservoir  130  and the first flow line  301 . The by-pass line  303  is used for the preheating when the fuel cell system  50  is started. 
     In order to rapidly heat the PEMFC stack  100 , using the heating element  310 , the capacity of the heating element  310  can be increased. However, in this case, the loss of electricity is large. Also, in order to reduce a volume of the coolant, the capacity of the coolant reservoir  130  can be reduced. However, a minimum capacity of the coolant reservoir  130  should be maintained. 
     For example, if the capacity of the coolant reservoir  130  is 500 ml, and the rated power consumption of the heating element  310  (electric heater) is 500 W, it takes approximately 15 minutes to raise the temperature of all of the coolant in the coolant reservoir  130 , from room temperature to 120° C., using the heating element  310 . Also, if a total amount of coolant, except the coolant in the coolant reservoir  130 , is 750 ml, it takes approximately 23 minutes to increase the temperature of the PEMFC stack  100 , by heating all of the coolant in the cooling system  400 . This can lead to a delay in the start-up operation of the fuel cell system. 
       FIG. 5  is a schematic drawing of a cooling system  500 , to which a method of starting (preheating) a fuel cell stack, according to an exemplary embodiment of the present invention, is employed. Referring to  FIG. 5 , a first valve V 1  is installed between the first flow line  301 , which is connected to the upper parts of the cooling plates  5 , and the heat exchanger H 5 . A second valve V 2 , which is a 3-way valve, is installed in the by-pass line  303 . The second valve V 2  has a first position to allow the coolant flow from the coolant reservoir  130  to the second flow line  302  (a first position). The second valve V 2  has a second position to open the second flow line  302  to the by-pass line  303  (a second position), while blocking the coolant from flowing from the coolant reservoir  130  to the second flow line  302 . The first valve V 1  controls a flow rate of the coolant. 
     A method of heating the PEMFC stack  100 , for starting-up of the fuel cell system, will now be described. Firstly, the first valve V 1  is closed and the second valve V 2  is set to the second position. In this case, the coolant in the coolant reservoir  130  is blocked from entering the by-pass line  303  and the second flow line  302 . Accordingly, an amount of the coolant heated by the electric heater  310  is reduced, and a time required to preheat the PEMFC stack  100  is reduced. For example, the time required can be reduced by approximately 15 minutes, when an amount of coolant stored in the coolant reservoir  130  is 500 ml, and the rated power consumption of the heating element  310  is 500 W. 
     If the coolant in the by-pass line  303  is heated using the heating element  310 , a portion of the coolant is vaporized and moves upward into the first flow line  301 . The coolant in the first flow line  301  is blocked by the first valve V 1  and heats the cooling plates  5 , while passing through the flow channels  5   a  of the cooling plates  5 . The coolant in the flow channels  5   a  moves to the second flow line  302 , then to the by-pass line  303  through the second valve V 2 , then to the heating element  310 , where the coolant is heated. The heated coolant is then re-circulated to the cooling plates  5 . The temperature of the PEMFC stack  100  is increased to an operating temperature, due to the circulation of the coolant, and the fuel cell system can be started, in accordance with an operation condition of the CO shifter  251 . 
     After the PEMFC stack  100  is preheated to the operating temperature, the first valve V 1  is opened, and the second valve V 2  is adjusted from the second position to the first position, in a stepwise fashion. Thus, the by-pass line  303  is closed, and the coolant circulates between the coolant reservoir  130  and the cooling plates  5 . The stepwise adjustment of the second valve V 2  controls the flow rate of coolant entering the PEMFC stack  100 . Thus, the PEMFC stack  100  can be switched to a normal operation. In the normal operation of the PEMFC stack  100 , the second valve V 2  is adjusted to the first position. 
     The heating element  310  can be, for example, a burner, and thus, the present invention is not limited to the heating element  310  being an electric heater. The second valve V 2  may be a 3-way valve, however, the present invention is not limited thereto. For example, a valve (not shown) can be installed between the coolant reservoir  130  and the second flow line  302  and another valve (not shown) can be installed between the coolant reservoir  130  and the by-pass line  303 , as long as similar results can be obtained. 
       FIG. 6  is a schematic drawing of a cooling system  600 , to which a method of starting (preheating) a fuel cell stack, according to another exemplary embodiment of the present invention, is employed. Referring to  FIG. 6 , a third valve V 3 , to control the discharge of the coolant to the outside, is installed on the coolant reservoir  130 , and a fourth valve V 4 , to control a flow of the coolant from the outside, is installed on the first flow line  301 . Except for the third and fourth valves V 3  and V 4 , the rest of the elements in  FIG. 6  are substantially identical to the elements in  FIG. 5 , and thus, like reference numerals are used in  FIGS. 5 and 6  and the detailed descriptions thereof, will not be repeated. The fourth valve V 4  may control a flow rate of the coolant. 
     A method of heating the PEMFC stack  100 , for starting up (preheating) the fuel cell system, will now be described. The fourth valve V 4  closed, and the coolant in the coolant reservoir  130  is discharged to the outside, by opening the third valve V 3 . After the coolant is discharged from the reservoir  130 , the third valve V 3  is closed. Accordingly, the amount of coolant to be heated by the heating element  310  is reduced, and the time required to increase the temperature of the PEMFC stack  100  can be reduced. 
     When the coolant in the by-pass line  303  is heated using the heating element  310 , the temperature of the coolant rises, and a portion of the coolant turns into vapor. The vapor moves to the first flow line  301 . A portion of the heated coolant in the first flow line  301  moves to the coolant reservoir  130 , and the rest of the heated coolant heats the cooling plates  5 , while the heated coolant flows through the flow channels  5   a  of the cooling plates  5 . The coolant in the flow channels  5   a  of the cooling plates  5  flows down the second flow line  302 , to the by-pass line  303 , and then from the by-pass line to the heating element  310 . The coolant is heated in the heating element  310 , flows to the PEMFC stack  100 , and is then circulated therein. Due to the circulation of the coolant as described above, the temperature of the PEMFC stack  100  increases to an operating temperature, and the fuel cell system can be started, in accordance with the operating condition of the CO shifter  251 . 
     When the PEMFC stack  100  is preheated to the operating temperature, the fourth valve V 4 , which is connected to a coolant supply line, is opened. The coolant can then flow to the coolant reservoir  130 , via the by-pass line  303 . At this point, an amount of coolant that is supplied through the fourth valve V 4  may be appropriately controlled. Since the fourth valve V 4  is located at a higher position than the coolant reservoir  130 , the coolant can be supplied to the coolant reservoir  130 , due to hydraulic pressure. If the fourth valve V 4  is located at a lower position than the coolant reservoir  130 , an additional pump may be used. 
     As described above, in a method of starting (preheating) a PEMFC stack, according to aspects of the present invention, the amount of coolant that is to be heated using a heating element is reduced, the temperature of the PEMFC stack can be rapidly increased, and the time required for a fuel cell system to be started (preheated) can be reduced. The exemplary method of starting the PEMFC stack can be realized by installing two additional valves. Accordingly, this modification can be achieved in a conventional system with a minimum cost. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, scope of which is defined in the claims and their equivalents.