Abstract:
Systems of checking thermal-induced circulation of a coolant in a fuel cell stack are disclosed. The system includes coolant inlet and outlet lines extending from a fuel cell stack. A pump and a radiator are confluently connected to the coolant inlet and coolant outlet lines. In one embodiment, a valve (either check type or automatic type) is provided in the coolant outlet line at the bottom of the fuel cell stack to prevent the flow of cold coolant from the coolant outlet line into the fuel cell stack upon start-up of the fuel cell stack. In another embodiment, a valve (either one-way flow control type or automatic type) is provided in the coolant inlet line at the top of the fuel cell stack. A method of checking thermal-induced circulation of a coolant in a fuel cell stack is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to cooling systems for fuel cells. More particularly, the present invention relates to a method for maintaining heat distribution in a fuel cell stack to decrease the time required to warm the stack on start-up and mitigate the formation of hot spots in the stack.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit only heat and water as by-products.  
         [0003]     Fuel cells include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode and passes only protons. Each electrode is coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through a drive motor and then to the cathode, whereas the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the drive motor and oxygen from the air to form water. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.  
         [0004]     While they are a promising development in automotive technology, fuel cells are characterized by a high operating temperature which presents a significant design challenge from the standpoint of maintaining the structural and operational integrity of the fuel cell stack. Maintaining the fuel cell stack within the temperature ranges that are required for optimum fuel cell operation depends on a highly-efficient cooling system which is suitable for the purpose.  
         [0005]     During startup of a PEM (polymer electrolyte membrane) fuel cell, the faster a fuel cell stack is able to reach operating temperatures, the better the performance of the fuel cell. Due to localized heating of the MEA (membrane electrode assembly) resulting from the electrochemical reaction of hydrogen and oxygen, adequate removal of heat from the MEA is required. Previous methods of terminating operation of the coolant pump have proven to help heat up the stack at a faster rate; however, because the coolant being heated will migrate out of the stack, arrival at operating temperature is delayed.  
         [0006]     The design operating temperature for a fuel cell stack is typically in the 65˜80 degrees C. range. During a cold start from a temperature of 5 degrees C., fuel cell stack waste heat is utilized to rapidly bring the temperature of the stack up to its design operating temperature. When the design operating temperature is reached, a coolant pump is started for rejecting waste heat and preventing temperature overshoot.  
         [0007]     It is important that the coolant pump not start too early since this will cause the desired operating temperature not to be reached or to be delayed. However, it has been discovered that coolant will migrate and circulate even if the coolant pump is not in operation, especially if the stack is started in cold weather. This is due to the thermally induced gradients of density, viscosity, and capillarity between hot and cold coolant. When coolant is heated in the stack, it migrates from the cells into the coolant manifold, where it then rises because it is lighter than the relatively cold coolant in the coolant system piping. The colder coolant, in turn, falls back down into the stack by gravity. This rising of the warm coolant and falling of the cold coolant in the system causes a “Ferris wheel” effect in which warm coolant flows freely from the stack to the system piping and cold coolant flows from the system piping into the stack.  
         [0008]     Accordingly, a check system and method is needed to prevent circulation due to thermal gradients in a fuel cell stack system.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention is generally directed to a novel circulation check system and method to prevent thermally-induced circulation of coolant due to the presence of thermal gradients in a fuel cell stack system. In one embodiment, the gravity circulation check system includes a valve which is placed at a bottom coolant outlet of the fuel cell stack. During circulation of coolant, the coolant is pumped from the coolant outlet, through the valve and into a top coolant inlet of the fuel cell stack, respectively. When coolant circulation stops, the valve prevents the coolant from re-entering the stack through the coolant outlet. In another embodiment, the circulation check system includes a valve positioned in a coolant discharge conduit at the top of the fuel cell stack. When circulation of the coolant stops, the valve closes and prevents gravity-induced circulation of the coolant. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0011]      FIG. 1  is a schematic view of a circulation check system according to a first embodiment of the present invention; and  
         [0012]      FIG. 2  is a schematic view of a circulation check system according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring initially to  FIG. 1 , a circulation check system according to one embodiment of the present invention is generally indicated by reference numeral  30  and is designed for a fuel cell stack  32  in which a coolant  48  is pumped into the top of the stack  32 . The circulation check system  30  is designed to prevent flow of the coolant  48  due to thermal gradients which remain in the coolant  48 , particularly during start-up of the fuel cell stack  32 . The circulation check system  30  includes a coolant outlet line  34  which extends from a coolant outlet  33  at the bottom of the fuel cell stack  32 . A valve  36  (either a check type or automatic type) is provided in the coolant outlet line  34 , and a pump inlet line  38  extends from the check valve  36 . The pump inlet line  38  is provided in fluid communication with a coolant pump  40 , which is connected to a radiator  44  through a pump outlet line  42 . A coolant inlet line  46  connects the outlet of the radiator  44  to a coolant inlet  45  at the top of the fuel cell stack  32 .  
         [0014]     During operation of the fuel cell stack  32 , after the fuel cell stack  32  reaches the design operating temperature of typically about 65˜80 degrees C., the coolant pump  40  pumps the coolant  48  from the fuel cell stack  32  through the bottom coolant outlet  33  and then through the coolant outlet line  34 , valve  36 , pump inlet line  38 , pump outlet line  42  and radiator  44 , respectively. In the radiator  44 , thermal energy from the coolant  48  is dissipated to air flowing through the radiator  44 . The coolant  48  flows from the radiator  44  through the coolant inlet line  46 , and enters the fuel cell stack  32  through the top coolant inlet  45 . As the coolant  48  is distributed throughout the fuel cell stack  32 , heat generated by the fuel cell stack  32  is absorbed by the coolant  48 . The coolant  48  then leaves the fuel cell stack  32  through the coolant outlet  33 , and the cycle is repeated.  
         [0015]     During start-up of the fuel cell stack  32 , thermal gradients are induced in fuel cell stack  32 . Consequently, the coolant  48  in the fuel cell stack  32  includes both warm portions and cold portions. Due to differences in density, viscosity, and capillarity between cold coolant  48  and warm coolant  48 , the non-pumped coolant  48  has a tendency to migrate and circulate. Warm coolant  48  then rises from the stack  32  and enters the coolant inlet line  46 , due to thermal gradients between the warmed coolant  48  and the cold coolant  48 . Similarly cold coolant  48  enters stack  32  and replaces the warmed coolant. Were it not for the presence of the valve  36  in the coolant outlet line  36 , this would result in movement of warm coolant  48   a  from the stack  32  into the coolant inlet line  46  and movement of the cold coolant  48   b  from the coolant outlet line  34  into the stack  32 , as indicated by the dashed arrows. The outflow of warm coolant  48   a  from the fuel cell stack  32  and the influx of cold coolant  48   b  from the coolant outlet line  34  into the fuel cell stack  32  would therefore tend to cool the stack  32  upon start-up of the fuel cell stack  32 , whereas rapid heating of the coolant  48  upon start-up is desired to attain operating temperatures as rapidly as possible.  
         [0016]     During start-up of the fuel cell stack  32 , valve  36  prevents the reverse flow of cold coolant  48   b  from the coolant outlet line  34  and into the coolant outlet  33  of the fuel cell stack  32 , as well as the flow of warm coolant  48   a  from the fuel cell stack  32  through the coolant inlet line  46 . Consequently, due to the waste heat which remains in the coolant  48 , the coolant  48  which remains in the fuel cell stack  32  is closer to the operating temperatures of the stack  32  at startup, thereby reducing the time required to bring the temperature of the coolant  48  up to the operating temperature and increasing the performance of the fuel cell.  
         [0017]     Referring next to  FIG. 2 , a circulation check system according to a second embodiment of the present invention is generally indicated by reference numeral  50  and is designed for a fuel cell stack  52  in which a coolant  74  is pumped into the bottom of the stack  52 . The circulation check system  50  is designed to prevent gravity flow of the coolant  74  due to thermal gradients which remain in the coolant  74 , particularly during subsequent start-up of the fuel cell stack  52 . The circulation check system  50  includes a coolant outlet line  64  which extends from valve  54  (either flow control type or automatic type) at the top of the fuel cell stack  52 . Valve  54  is provided in fluid communication with a coolant outlet  63  of the fuel cell stack  52 . A radiator  66  is connected to the coolant outlet line  64 , and a pump inlet line  68  connects the radiator  66  to a coolant pump  70 . A coolant inlet line  72  connects the outlet of the coolant pump  70  to a coolant inlet  71  at the bottom of the fuel cell stack  52 .  
         [0018]     If valve  54  is of the flow control type it includes a valve housing  55  in which is provided a valve stem  56 . A valve weight  58  is slidably mounted on the valve stem  56 . A valve seat  60  is provided in the bottom of the valve housing  55 . A coolant opening  62  extends through the valve seat  60  and normally establishes fluid communication between the coolant outlet  63  and the valve housing  55  when the flow control valve  54  is in the open position. As indicated by the dashed lines, the valve weight  58  is capable of seating against the valve seat  60  to block the coolant opening  62  during shutdown of coolant pump  70 . During operation of the coolant pump  70 , the weight  58  slides upwardly on the valve stem  56  to unblock the coolant opening  62  and allow flow of the coolant  74  from the fuel cell stack  52  to the coolant outlet line  64 , as will be hereinafter further described. An example of a valve  54  (flow control type) which is suitable for the circulation check system  50  is the SA (straight or angle) flow control valve available from the Bell &amp; Gossett Co.  
         [0019]     During operation of the fuel cell stack  52 , after the fuel cell stack  52  reaches the design operating temperature of typically about 65˜80 degrees C., the coolant pump  70  pumps the coolant  74  from the fuel cell stack  52  through the top coolant outlet  63  and valve  54  respectively, and into the coolant outlet line  64 . Upward pressure of the rising coolant  74  flowing upwardly through the coolant opening  62  of the valve (flow control type) seat  60  pushes the valve weight  58  upwardly on the valve stem  56 . This facilitates flow of the coolant  74  from the coolant outlet  63 , through valve  54  and into the coolant outlet line  64 , respectively.  
         [0020]     From the coolant outlet line  64 , the coolant  74  flows through the radiator  66 , the pump inlet line  68 , the coolant pump  70  and the coolant inlet line  72 , respectively, and enters the bottom coolant inlet  71  of the fuel cell stack  52 . In the radiator  66 , thermal energy from the coolant  74  is dissipated to air flowing through the radiator  66 . As the coolant  74  is distributed throughout the fuel cell stack  52 , heat generated by the fuel cell stack  52  is absorbed by the coolant  74 . The coolant  74  again leaves the fuel cell stack  52  through the coolant outlet  63  and flow control valve  54 , respectively, and the cycle is repeated.  
         [0021]     During start-up of the fuel cell stack  52 , thermal gradients are induced in the fuel cell stack  52 . Consequently, coolant  74  includes both warm portions and cold portions. Due to differences in density, viscosity and capillarity between cold coolant  74  and warm coolant  74 , coolant  74  has a tendency to migrate and circulate. Warm coolant  74  then rises in the stack  52 , whereas cold coolant  74  falls through the stack  52 . However, flow of warm coolant  74  from the fuel cell stack  52  and into the coolant outlet line  64  is prevented by valve  54 , since upon shutdown of the coolant pump  70 , the warm coolant  74  does not push upwardly against the valve (flow control type) weight  58  with a force which is necessary to raise the valve weight  58  from the valve seat  62  to unblock the coolant opening  62 . This maintains the warm coolant  74  in the fuel cell stack  52  and prevents cooling of the stack  52  during start-up of the fuel cell stack  52 , thereby facilitating rapid heating of the coolant  74  upon start-up to attain operating temperatures as rapidly as possible.  
         [0022]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.