Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a cooling fluid ventilation system for a fuel cell stack and, more particularly, to a cooling fluid ventilation system for a fuel cell stack, where the ventilation system includes a flame arrester within a coolant reservoir, and a compact multi-functional unit for venting and filling the coolant reservoir. 
     2. Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle. 
     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
     Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. 
     The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode reactant gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the MEA. Cathode reactant gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
     It is necessary that a fuel cell operate at an optimum relative humidity and temperature to provide efficient stack operation and durability. The temperature provides the relative humidity within the fuel cells in the stack for a particular stack pressure. Excessive stack temperature above the optimum temperature may damage fuel cell components, reducing the lifetime of the fuel cells. Also, stack temperatures below the optimum temperature reduce the stack performance. 
     Fuel cell systems employ thermal sub-systems that control the temperature within the fuel cell stack. Particularly, a cooling fluid is pumped through the cooling channels in the bipolar plates in the stack. Typically the cooling fluid is a liquid that inhibits corrosion within the stack, does not freeze in cold environments, and is non-conductive. One example of a suitable cooling fluid is a de-ionized water and glycol mixture. It is necessary that the cooling fluid be non-conductive so that current does not travel across the cooling fluid channels in the stack. 
     The thermal sub-system includes a coolant reservoir that equalizes the thermal expansion of the cooling fluid in the thermal sub-system, and replenishes the small losses of the cooling fluid that occur during stack operation. If the pressure of the gas within the reservoir exceeds a certain pressure, an over-pressure valve will open and release some of the gas to the atmosphere until the pressure is equalized. The coolant reservoir is typically positioned at the highest location in the thermal sub-system. 
     Hydrogen is a very thin gas and is difficult to contain within an enclosed environment. Because of this hydrogen typically permeates through stack and plate materials and seals within the fuel cell stack, especially around the plates of the stack. Hydrogen leaks into the cooling fluid channels where it is dissolved in the cooling fluid or is trapped in the cooling fluid as hydrogen bubbles. 
     The impeller of the pump creates cavitation that produces air bubbles that are trapped in the coolant loop. The system includes a ventilation line that allows the air bubbles to be removed from the coolant loop and enter the reservoir. In addition to the air bubbles, the hydrogen bubbles that are trapped within the cooling fluid are also vented to the reservoir, where they accumulate in an air pocket within the reservoir. 
     This accumulation of hydrogen and air within the reservoir is a combustible source that could ignite. Generally the reservoir includes movable parts, such as the over-pressure valve, that could cause a spark and ignite the combustible mixture of hydrogen and air. Also, the accumulation of hydrogen and air within the reservoir creates a pressure build up therein. The reservoir typically includes a cap that covers a fill port through which the reservoir is filled when necessary. If the cap is removed before the pressure in the reservoir is reduced, the cap may fly upwards under pressure. Thus, it is desirable to remove the pressure within the reservoir before the cap is removed. 
     Even if the pressure is released in the reservoir before the cap is removed, some ignitable gas may reach the environment. However, if the pressure is reduced, the amount of gas will be so small that it mixes with the ambient air fast enough so that it is not combustible. In other words, if the cap is removed shortly after the fuel cell is operated before the pressure in the reservoir is reduced, and ignitable gas is present in the reservoir, the gas may be released into the environment where it could ignite. If the cap is removed shortly after the fuel cell is operated, but after the pressure in the reservoir is reduced, any ignitable gas present in the reservoir escapes from the reservoir and will mix with the ambient air fast enough so that the danger of ignition does not exist. 
     To prevent the accumulation of a combustible gas within the air pocket in the reservoir, it is known to periodically remove the hydrogen and gas mixture. Particularly, it is known to pump air into the air pocket in the reservoir, where the existing air/hydrogen mixture within the air pocket is vented from the reservoir through an outlet pipe. This operation removes the hydrogen from the reservoir, while maintaining the air pocket. However, by continually pumping air into the reservoir, the cooling fluid flow from the reservoir may become contaminated with dirt and the like, and therefore, a filter is required. Also, the air from the pump causes some of the cooling fluid to evaporate, which requires that the reservoir be filled more often. Further, the pump may have to be heated so that it does not freeze in sub-zero environments. Also, the ventilation system requires complex electrical systems to guarantee the operation of the ventilation system for safety purposes. 
     Another possible solution is to include a catalyst in the air pocket that converts the hydrogen and oxygen into water. However, catalysts will operate in this environment only if they are heated to a relatively high degree, where water droplets hitting the catalyst at the surface will immediately evaporate. Also, a device could be added that provides additional air to the reservoir in the event there is a shortage of oxygen in the air pocket to convert all of the hydrogen. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a coolant reservoir and associated multi-functional unit for a thermal sub-system of a fuel cell system is disclosed. The coolant reservoir includes a flame arrester positioned in a gas region of the reservoir where most of the gas is between the flame arrester and the cooling fluid within the coolant reservoir. The multi-functional unit includes a first pressure relief valve and a flow restrictor, where the pressure relief valve automatically opens if the pressure within the gas region exceeds a first predetermined pressure. The multi-functional unit may also include a second pressure release valve that quickly releases pressure in the coolant reservoir if the pressure within the gas region goes above a second higher predetermined pressure. The multi-functional unit also includes an air input line and a check valve for allowing ambient air to enter the coolant reservoir if the pressure within the gas region falls below the ambient pressure. The multi-functional unit also includes a coolant fill line having suitable plumbing that causes the coolant reservoir to vent prior to the cooling fluid cap being removed. In one embodiment, the coolant fill cap includes a pair of seals that allow the coolant reservoir to be vented as the cooling fill cap is being unthreaded. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fuel cell system including a coolant reservoir having a flame arrester therein and a multi-functional unit, according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a multi-functional unit including a manual 2/2 way valve, according to another embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of a multi-functional unit for a coolant reservoir in a thermal sub-system of a fuel cell system that includes integral pressure relief valves and associated lines, according to another embodiment of the present invention; and 
         FIG. 4  is a cross-sectional view of the multi-functional unit shown in  FIG. 3 , where the cap has been threaded partly off so that the gas inside the coolant reservoir can be vented to the environment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a multi-functional unit for a coolant reservoir in a fuel cell system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a schematic diagram of a fuel cell system  10  including-a fuel cell stack  12 . A pump  14  pumps a cooling fluid through the stack  12  and an external coolant loop  16 . A cathode exhaust from the stack  12  is output on line  18  through a back-pressure valve  20  to be exhausted to the environment, or recycled. The cooling fluid may provide heat to the stack  12  during start-up or remove heat from the stack  12  during fuel cell operation to maintain the stack at a desirable operating temperature, such as 80° C. The system  10  includes a coolant reservoir  24  that holds the cooling fluid  26  and provides the cooling fluid to the coolant loop  16  on line  32 . The coolant reservoir is typically positioned at the highest location in the thermal sub-system. A gas region  28  provides a compressible region within the reservoir  24 . The bubbles from the cooling fluid in the coolant loop  16  are vented on line  34  to the gas region  28 . A pressure sensor  36  measures the pressure in the line  32 , but can be positioned at any suitable location in the system  10  to provide the cooling fluid pressure. 
     According to the invention, a flame arrester  40  is positioned within the gas region  28  so that there is a minimal amount of gas above the flame arrester  40 . Because there are no moving parts between the flame arrester  40  and the cooling fluid  26 , there is no possibility of a spark occurring, which could ignite the combustible mixture of hydrogen and oxygen that may exist therein. Also, because the volume of the gas above the flame arrester  40  is so small, any combustion in this area of the gas region  28  will not cause any damage. Further, if there did happen to be combustion in the gas region  28 , the flame arrester  40  would prevent the flame from crossing the flame arrester  40  to the other side of the gas region  28 . Flame arresters that provide this function are known in the art, and are typically a metal mesh structure including holes therethrough. In one embodiment, the holes are about 0.4 μm to provide the flame retardation. 
     The coolant reservoir  24  and the coolant loop  16  are filled through a fill port  42 . Therefore, the flame arrester  40  may prevent the coolant reservoir  24  and the coolant loop  16  from being filled fast enough. Therefore, a separate fill tube (not shown) may be provided to accommodate the initial filling of the coolant reservoir  24 . Any subsequent addition of the cooling fluid to the coolant reservoir  24  can be provided through the fill port  42  because it will be a minimal amount, as will be discussed in more detail below. 
     A multi-functional unit  50  is mounted to the coolant reservoir  24  over the fill port  42 . The unit  50  includes a manual  3 / 3  way valve  52  that connects a fill line  54  to the fill port  42  and the fill port  42  to a vent line  56 . In the left position of the valve  52  that is shown, both the fill line  54  and the vent line  56  are disconnected from the fill port  42 . If an operator wishes to fill the coolant reservoir  24  with more cooling fluid through the fill line  54 , he will slide the manual valve  52  from the left position to the right position through the middle position. When the valve  52  reaches the middle position, the fill port  42  is connected to the vent line  56  so that gas within the gas region  28  is vented out of the coolant reservoir  24  on line  58  to the cathode exhaust. Therefore, the pressure in the gas region  28  will be reduced so that the reservoir  24  can be opened. As the valve  52  continues to move to the right position, the vent line  56  is disconnected from the fill port  42  and the fill line  54  is connected to the fill port  42 . The operator can then safely remove a fill cap  60  to fill the coolant reservoir  24  through the fill line  54 . 
     After the fuel cell system  10  is shut down and the temperature of the system  10  decreases, the temperature within the reservoir  24  will also decrease, and thus, the pressure within the reservoir  24  will decrease. The unit  50  includes an air-line  64  and a check valve  66  that allows ambient air at a higher pressure to enter the fill port  42  through line  68 , but not allow the gas region  28  to be vented through the line  68 . In an alternate embodiment, the line  64  can be coupled to the cathode exhaust line. 
     During normal operation of the system  10 , the pressure within the gas region  28  may increase above a predetermined level, such as 0.6 bar, as a result of the bubbles in the coolant being vented to the reservoir  24 . If the pressure within the gas region  28  does increase beyond this predetermined level, it is desirable to reduce the pressure within the gas region  28  for safety purposes. To accomplish this, a vent valve  72  is provided between the vent line  58  and the line  68 . The vent valve  72  is calibrated so that it automatically opens if the pressure in the gas region  28  exceeds the predetermined pressure and gas in the gas region  28  is exhausted to the environment or to the cathode exhaust line. A flow restrictor  74  is also positioned between the line  58  and the line  68  to limit the gas flow. The vent valve  72  will close automatically when the pressure difference is lower than the opening pressure of the valve  72  plus a certain valve hysteresis. 
     It is possible that a malfunction could occur that may cause too much hydrogen to leak into the fuel cell stack  12  and collect in the gas region  28 , significantly increasing the pressure therein. The vent valve  72  will open in this situation, however, the flow restrictor  74  may prevent it from venting quickly enough to reduce the pressure before the reservoir  24  cracks or otherwise fails. Therefore, a second vent valve  76  is positioned in line  78  between the line  58  and the line  68  that automatically opens if the pressure within the reservoir exceeds a second predetermined pressure, such as 1.2 bar. 
     In the case of a malfunction in the fuel cell stack  12  where hydrogen leaks into the cooling fluid, the pressure may be between the opening pressure of the vent valve  72  and the opening pressure of the vent valve  76 . Because the pressure sensor  36  is monitoring the pressure in the reservoir  24 , it can send a signal to the electronic control unit (not shown) in the system  10  to indicate a potential problem. Although the pressure sensor  36  is shown in the line  32 , it can be positioned in the gas region  28 . Also, because the pressure sensor  36  is an existing component in the system  10 , using it for this purpose eliminates the need for a hydrogen sensor, which are expensive. If there is no need to detect for a malfunction, the vent valve  76  can be eliminated. 
       FIG. 2  is a diagram of a multi-functional unit  90  that can replace the multi-function unit  30  in the system  10 , where like elements are identified by the same reference numeral. In this embodiment, the 3/3 way valve  52  is replaced with a manual 2/2 way valve  92 . Further, the line  56  is directly connected to the fill line  54 . In the left position of the valve  92 , the fuel line  54  is blocked. When the valve  92  is switched from the left position to the right position to connect the fill line  54  to the fill port  42 , the gas region  28  is able to vent to the line  56 . A blocking mechanism  94  prevents the cap  60  from being opened until after the valve  92  has been switched to the right position. 
     The multi-functional unit  50  shows a functional operation of the various components therein for the purposes described above. In a practical design, those components can be integrated in a simple, small and cheap assembly.  FIG. 3  is a cross-sectional view of a multi-functional unit  100  that can replace the multi-functional unit  50 . The unit  100  includes a valve body  102  defining a first chamber  104  and a second chamber  106 . The first chamber  104  is in fluid communication with a passageway  108  and a passageway  110 . The chamber  106  is in fluid communication with a passageway  112 , a passageway  114  and a passageway  116 . The passageways  108  and  112  are in fluid communication with a passageway  118  in the body  102 . The passageways  110 ,  114  and  116  are in fluid communication with the fill port  42 . A spring  122  and associated sealing plate  124  are positioned within the chamber  104  and represent the check valve  66 . Likewise, a spring  126  and associated sealing plate  128  are positioned within the chamber  106  and represent the vent valve  72 . A flow restrictor  134  is positioned within the passageway  116  and represents the flow restrictor  74 . A spring  136  and associated sealing plate  138  are positioned within the chamber  106  and represent the vent valve  76 . In an alternate embodiment, the passageways  110 ,  114  and  116  can be in fluid communication with the gas region  28  through one or more separate ports. 
     If the pressure within the coolant reservoir  24  is less than ambient pressure, the ambient pressure pushes against the sealing plate  124 , and compresses the spring  122 , which allows air to enter the reservoir  24  and increase the pressure within the gas region  28 . The spring  126  is calibrated so that if the pressure within the gas region  28  goes above the first pressure, the pressure within the gas region  28  pushes the sealing plate  128  away from the passageway  116  to allow gas within the gas region  28  to vent to the passageway  118  through the flow restrictor  134  to reduce the pressure in the region  28 . The spring  136  is calibrated so that if the pressure within the gas region  28  exceeds the second pressure level, then the pressure within the gas region  28  will push the sealing plate  138  away from the passageway  114  to allow the gas region  28  to quickly vent to the passageway  118  to reduce the pressure therein. 
     The unit  100  also includes a fill cap  140  including a cap top  142  and a cap body  144 . The cap top  142  is threaded onto an extended annular portion  146  so that a flat seal  148  seals against a shoulder  150  to seal a chamber  152  in fluid communication with the fill port  42 . Additionally, an O-ring  154  is positioned within an annular channel in the cap body  144  to provide additional sealing. 
     When the fill cap  140  is unthreaded from the annular portion  146 , the flat seal  152  will unseal from the shoulder  150 , as shown in  FIG. 4 . Once the cap  140  reaches a certain level, the chamber  152  is in fluid communication with the passageway  118 . Further, the O-ring  154  continues to maintain a seal between ambient and the chamber  152 . Therefore, the gas region  28  is allowed to vent to the exhaust through the vent line  118  before the cap  140  is completely removed from the unit  100 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.

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