Abstract:
An internal coolant circulation system and method of homogenizing waste heat in a fuel cell stack using homogenous thermal coolant cycling is disclosed. The method includes operating a fuel cell stack, distributing a coolant through the fuel cell stack, terminating operation of the fuel cell stack, retaining the coolant in the fuel cell stack and circulating the coolant throughout the fuel cell stack.

Description:
FIELD OF THE INVENTION 
     The present invention relates to cooling systems for fuel cells. More particularly, the present invention relates to a homogenous thermal coolant cycling system and 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 
     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. 
     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. 
     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. 
     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 electro-chemical 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 in the stack is stationary, hot spots tend to occur in the fuel cell stack. Over time, these hot spots turn into pinholes, which ultimately render the stack non-functional. 
     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. 
     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 circulate even if the coolant pump is not in operation, especially if the stack is started in cold weather. This is due to the difference in density between hot and cold coolant. When coolant is heated in the stack, it rises into the coolant manifold 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. 
     Accordingly, a system and method is needed to circulate coolant within a stack during start-up of the fuel cell in order to retain waste heat in the stack and expedite attainment of the stack to operating temperatures. The circulated coolant maintains homogeneity in temperature among all regions of the stack, thus eliminating or reducing the formation of hot spots in the stack. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a novel internal coolant circulation system and method for warming a fuel cell stack to operating temperatures in a short period of time. The internal coolant circulation system includes a coolant circulation loop which is provided inside the fuel cell stack and circulates only the volume of coolant contained in the stack during start-up of the fuel cell. Internal circulation of the stack coolant retains waste heat in the stack and expedites attainment of the stack to operating temperatures. Furthermore, the circulated coolant maintains homogeneity in temperature among all regions of the stack, thus eliminating or reducing the formation of hot spots in the stack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an internal coolant circulation system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an illustrative embodiment of an internal coolant circulation system according to the present invention is generally indicated by reference numeral  10 . By using the Homogenous Thermal Coolant Cycling (HTCC) concept, the internal coolant circulation system  10  is designed to circulate coolant  26  throughout a PEM (polymer electrolyte membrane) fuel cell stack  12  in order to retain waste heat in the fuel cell stack  12  and rapidly warm the fuel cell stack  12  to operating temperatures upon initial start-up of the fuel cell stack  12 . The fuel cell stack  12  includes a coolant inlet  14  for receiving the coolant  26  and a coolant outlet  16  for discharging the coolant  26  from the fuel cell stack  12 . 
     The internal coolant circulation system  10  includes a coolant circulating mechanism  18 , such as a small pump, for example, which is placed in the fuel cell stack  12  typically on the coolant outlet side of the fuel cell stack  12 . The coolant circulating mechanism  18  is aligned with an imaginary line  24  which divides the fuel cell stack  12  into a low pressure region  20  and a high pressure region  22 . Accordingly, the low-pressure region  20  is on the inlet side  19   a  of the coolant circulating mechanism  18 , whereas the high-pressure region  22  is on the outlet side  19   b  of the coolant circulating mechanism  18 . 
     During operation of the fuel cell stack  12 , a coolant pump (not shown) which is exterior to the fuel cell stack  12  pumps the coolant  26  through a radiator (not shown), in which thermal energy from the coolant  26  is dissipated to air flowing through the radiator. The coolant  26  leaves the radiator and enters the fuel cell stack  12  through the coolant inlet  14 . As the coolant  26  is distributed throughout the fuel cell stack  12 , heat generated by the fuel cell stack  12  is absorbed by the coolant  26 , which then leaves the fuel cell stack  12  through the coolant outlet  16  and is again pumped through the radiator. 
     Upon subsequent shutdown of the fuel stack  12 , the coolant pump stops pumping coolant  26  through the fuel cell stack  12 . Accordingly, some of the coolant  26  remains in the fuel cell stack  12 . The coolant  26  which remains in the fuel cell stack  12  is initially hot due to the waste heat generated by the fuel cell stack  12 , but gradually cools non-uniformly such that some portions of the coolant  26  remain warm while other portions of the coolant  26  become cool. The coolant  26  remaining in the fuel cell stack  12  therefore contains portions of both warm and cool coolant  26 . 
     Upon subsequent start-up of the fuel cell stack  12 , the coolant inlet  14  and coolant outlet  16  are closed to prevent flow of coolant  26  into and out of the fuel cell stack  12 . The coolant circulation mechanism  18  is operated to draw coolant  26  through the inlet side  19   a  and out the outlet side  19   b  of the coolant circulation mechanism  18 . Consequently, within the fuel cell stack  12 , the coolant  26  circulates in a circular motion as it continually flows from the outlet side  19   b  of the coolant circulation mechanism  18 ; through the high pressure region  22  and low pressure region  20 , respectively, of the fuel cell stack  12 ; and back to the inlet side  19   a  of the coolant circulation mechanism  18 . This causes mixing of the warm and cold portions of the coolant  26 , thus raising the temperature of the fuel cell stack  12  and providing a substantially uniform temperature distribution of the coolant  26  throughout the fuel cell stack  12 . As a result, the initial temperature is closer to the operating temperature (typically in the 65˜80 degrees C. range) of the fuel cell stack  12 , and this decreases the amount of time necessary for the fuel cell stack  12  to reach normal operating temperatures. Furthermore, due to the homogeneity of the temperature in the fuel cell stack  12 , the formation of “hot spots” in the fuel cell stack  12  is eliminated or substantially reduced. This increases the lifetime of the fuel cell stack  12 . 
     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.