Abstract:
A system and method for increasing the temperature of a fuel cell stack quickly, especially at cold stack start-up. The method includes determining whether the fuel cell stack is below a first predetermined temperature threshold, and, if so, starting a cooling fluid flow through the stack and engaging a shorting circuit across the stack to short circuit the stack and cause the stack to operate inefficiently. The method then determines a desired heating rate of the fuel cell stack and calculates a cathode airflow to the fuel cell stack based on the desired heating rate. The method reduces the flow of cathode air to the stack if a minimum cell voltage is below a predetermined minimum cell voltage threshold and disengages the shorting circuit and applies vehicle loads to the stack when the stack temperature reaches a predetermined second temperature threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a system and method for heating a fuel cell stack at system start-up and, more particularly, to a system and method for heating a fuel cell stack system start-up that includes electrically shorting the stack and using cathode air as a limiting reactant. 
     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. 
     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The 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 the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. 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 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, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. 
     During low temperature operation, such as below 50° C., a fuel cell stack generally operates with liquid water in the flow channels due to a low water saturation pressure. This liquid water can cause flow distribution problems, freeze start problems and electrode flooding. If the stack temperature was increased, many of these problems could be avoided. If the stack is below freezing, then ice may form in the flow channels, which needs to be quickly melted to liquid water or water vapor at system start-up so that it can be purged out of the flow channels to provide adequate flow distribution. At system shut-down, actions are taken to remove as much of the water from the stack as possible through flushing of liquid water droplets from channels and evaporate drying of the MEAs and diffusion media. However, it is generally not possible to remove as much of the water as desired from the MEAs and diffusion media, especially for low temperature starts. 
     It is known in the art to use the waste heat generated by a fuel cell stack to bring the system to its operating temperature, which can take a relatively long time because of the inherent efficiency of the fuel cell stack. It is also known to use a heater to heat the stack cooling fluid at system start-up so that the temperature of the stack increases more quickly. This heat put into the system is limited by the size of the cooling fluid heater and the area over which the heat transfer occurs. It is also known to inject hydrogen gas into the cathode air stream to the stack to allow for catalytic combustion of the hydrogen on the catalyst in the cathode side of the fuel cells to provide heat. However, there are limits as to the amount of hydrogen that can be sent to the cathode because of flammability and stack dry-out concerns. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a system and method are disclosed for increasing the temperature of a fuel cell stack quickly, especially at cold stack start-up. The method includes determining whether the fuel cell stack is below a first predetermined temperature threshold, and, if so, starting a cooling fluid flow through the stack at normal or lower flow rates and engaging a shorting circuit across the stack, or any type of electrical load, to clamp the stack voltage close to 0V during the hydrogen fill of the anode so as to short circuit the stack and cause the stack to operate inefficiently. The method then determines a desired heating rate of the fuel cell stack and calculates a cathode airflow to the fuel cell stack based on the desired heating rate. The method reduces the flow of cathode air to the stack if a minimum cell voltage is below a predetermined minimum cell voltage threshold and disengages the shorting circuit and applies vehicle loads to the stack when the stack temperature reaches a second predetermined temperature threshold. 
     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 block diagram of a fuel cell system employing a process for quickly heating a fuel cell stack, according to an embodiment of the present invention; and 
         FIG. 2  is a flow chart diagram showing a process for heating the fuel cell stack shown in  FIG. 1  at cold stack start-up, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a system and method for heating a fuel cell stack at cold stack start-up by shorting the terminals of the stack is merely exemplarary in nature, and is in no way intended to limit the invention or its applications or uses. 
     The present invention proposes a system and method for reducing the time required for a fuel cell stack to warm-up in cold temperatures to a desired minimum operating temperature. Cold system starts can be any start in which the temperature of the cooling fluid that cools the stack or the stack internal temperature on the bipolar plates or end plates is lower than the normal stack operating temperature. A predetermined cold-start temperature T cold-start  can be defined through experimentation. If the stack is above the cold-start temperature T cold-start , the fuel cell system will start with normal algorithms. However, if the cooling fluid temperature is below the cold-start temperature T cold-start , then the algorithm described herein can be employed to more quickly heat the fuel cell stack so that it can provide the desired power. 
       FIG. 1  is a block diagram of the fuel cell system  10  including a fuel cell stack  12 . Cathode input air is provided by a compressor  14  and is sent to the cathode side of the fuel cell stack  12  on cathode input line  16 . The flow of the cathode input air is measured by a mass flow meter  18 , and the cathode exhaust gas is output from the fuel cell stack  12  on cathode exhaust gas line  20 . The measured cathode airflow by the mass flow meter  18  is used to control how much of the cathode air will flow into the fuel cell stack  12  and how much of the cathode air will by-pass the fuel cell stack  12  on by-pass line  22 . A by-pass valve  24  can be provided in the by-pass line  22  to selectively control the amount of cathode input air that will by-pass the stack  12  based on the mass flow measurement and be directly sent to the cathode exhaust gas line  20 . The by-pass valve  24  is optional and is not necessary for proper operation of the system  10 . 
     Hydrogen gas is provided to the anode side of the fuel cell stack  12  from a hydrogen source  38  on anode input line  26 . An injector or suitable flow control valve  28  is provided in the anode input line  26  to control the amount of hydrogen gas that is received by the stack  12 . Anode exhaust gas is output from the fuel cell stack  12  on anode exhaust gas line  30 . The stack output pressure is kept higher than atmospheric pressure so that there is always excess hydrogen available and enable anode purge to remove nitrogen and other inert gases from the stack  12 . Without excess hydrogen, the stack  12  may fuel starve, which results in damage to the electrodes in the fuel cells in the stack  12 . A pressure sensor  32  provides a measurement of the anode side pressure of the fuel cell stack  12  to maintain positive driving for bleeding and provide feedback for the hydrogen feed system. Additionally, the method could use a hydrogen injector duty cycle or mass flow meter to ensure that excess hydrogen is provided to the fuel cell stack  12 . 
     The fuel cell system  10  also includes a thermal sub-system that controls the temperature of the fuel cell stack  12  during operation of the stack  12 . Particularly, the fuel cell system  10  includes a coolant loop  34  outside of the stack  12  where a cooling fluid is pumped through the coolant loop  34  by a pump  36  and cooling fluid flow channels within the stack  12 . A cooling fluid temperature sensor  46  measures the temperature of the cooling fluid in the coolant loop  34  out of the stack  12 . A radiator (not shown) is typically provided in the thermal sub-system to cool the cooling fluid that flows out of the stack  12 . Additional temperature sensors at the coolant inlet and a coolant flow-meter can be used to improve system reliability. 
     The fuel cell stack  12  includes a positive terminal  40  and a negative terminal  42  from which the output power of the stack  12  is provided. According to the invention, a shorting switch  44  is provided that selectively electrically couples and disconnects the terminals  40  and  42 . The switch  44  is closed to short the fuel cell stack  12  and reduce the stack voltage to at or near 0V during cold starts so that the fuel cell stack  12  operates inefficiently and generates significant heat. The shorting switch  44  has a much lower resistance than the stack&#39;s high frequency resistance so that the fuel cells will be at a limiting current. Although the shorting switch  44  is provided in this embodiment, other embodiments may employ a low ohm resistor that is switched into and out of the circuit or some type of voltage regulation device for controlling the output voltage of the stack  12  so that it is at or near 0 volts. The shorting resistor can be coupled across the stack  12  or shorting resistors coupled across each fuel cell or groups of fuel cells in the stack  12 . 
     If the stack voltage is at or near 0 volts, and air and hydrogen are applied to the fuel cell stack  12 , the stack  12  operates to generate almost all heat. When the stack  12  is in this configuration with the switch  44  closed, certain control procedures need to be taken to operate the stack  12  safely so that the distribution of air and hydrogen throughout the stack  12  is properly provided. Particularly, it is desirable that each fuel cell of the stack  12  have an adequate amount of hydrogen. By controlling the operation of the compressor  14 , the amount of heat generated by the stack  12  with the switch  44  closed can be control. A small amount of hydrogen would be distributed to the flow channels in the stack  12  while the compressor speed was controlled to slightly less than the stoichiometric amount. 
       FIG. 2  is a flow chart diagram  50  showing one non-limiting process of how the fuel cell stack  12  can be quickly heated at system cold-starts, according to an embodiment of the present invention. The algorithm determines whether there is a fuel cell system start request at box  52 , and if so, detects the fuel cell stack temperature at box  54  using, for example, the cooling fluid temperature sensor  46  or an internal stack temperature measurement. For example, the algorithm can determine the temperature of the fuel cell stack by measuring the temperature of the cathode airflow or anode flow stream or measuring the temperature of a bipolar plate, gas diffusion layer(GDL), MEA or end plate in the fuel cell stack  12 . The algorithm determines whether the stack temperature is above the predetermined temperature threshold T cold-start  at decision diamond  56 , and if so, the algorithm starts the system normally at box  58  because the cold-start algorithm is not needed. 
     If the temperature of the fuel cell stack  12  is below the temperature threshold T cold-start  at the decision diamond  56 , then the algorithm starts the cooling fluid pump  36  and begins cooling fluid flow through the stack  12  at box  60 . Providing a cooling fluid flow through the stack  12  during the heating process helps improve the heating uniformity eliminates any damaging hotspots within the stack  12 . The algorithm also engages the shorting switch  44  at box  62  to provide a short across the terminals  40  and  42  so that the stack voltage is at or near 0V and the stack  12  operates inefficiently. When the switch  44  is closed, any reactants within the stack  12  are immediately consumed. By closing the switch  44 , the stack voltage will be near 0V, which will generate the most heat. The algorithm then causes hydrogen gas to be sent to the anode side of the fuel cell stack  12  where the hydrogen gas supply is at least at stoichiometric amounts to prevent fuel starvation. The hydrogen gas flow must be limited, however, because too much hydrogen gas flow could result in excess hydrogen gas pumping from the anode to the cathode side of the stack  12 . 
     The algorithm then determines the necessary heating rate of the stack  12  at box  66  to bring the stack  12  to the desired temperature as quickly as possible. Various factors go into determining the heating rate of the stack  12 , including the thermal mass of the stack  12 , the ambient temperature, etc. The algorithm will determine how much heat is necessary based on these factors to allow the vehicle driver to operate the vehicle within some predetermined minimum period of time for the current system factors. That predetermined period of time will vary depending on the factors, such as the ambient temperature and the maximum rate at which the stack  12  can be heated. The algorithm calculates the necessary cathode air at box  68  to be provided to the stack  12  to meet determined heating rate. The cathode air is metered into the cathode side of the fuel cell stack  12  at a controlled rate that is dictated by the heating requirements. Current that the stack  12  generates is based on the moles of air. The heat generated by this reaction is the maximum possible because only a small fraction of the free energy of the reaction is used to produce electric current as a result of the shorting switch  44 . The rest of the energy goes directly to heat, which is taken away by the cooling fluid flowing in the coolant loop  34 . 
     Once the proper amount of cathode air is calculated at the box  66 , that amount is flowed to the fuel cell stack  12  at box  70 . The hydrogen gas flow to the anode side of the stack  12  is controlled at box  72  to prevent excess hydrogen pumping. The algorithm controls the amount of hydrogen flowing to the stack  12  so that there is not excess hydrogen that could flow through the MEA&#39;s in the fuel cells to the cathode side of the stack  12 . Typically, it is desirable to maintain a stoichiometric excess of hydrogen flow over air flow to the stack  12 . The algorithm then measures the stack cooling fluid temperature at box  74 , and determines whether the cooling fluid temperature is above a predetermined temperature threshold, which can be higher than the cold start temperature T cold-start . 
     If the temperature of the cooling fluid is not above the predetermined threshold at the decision diamond  76 , then the algorithm determines the minimum cell voltage at box  78 , and determines whether the minimum cell voltage is below a minimum cell voltage threshold at decision diamond  80 . By monitoring the minimum cell voltage as the stack  12  is being heated, fuel cells in the stack  12  can be prevented from being damaged as a result of the heating process if they tend to reverse their electrical potential. If the minimum cell voltage is below the minimum threshold at the decision diamond  80 , then the algorithm reduces the cathode air set-point at box  82 , and then returns to the box  70  to provide the new cathode flow rate to the stack  12 . If the minimum cell voltage is not below the minimum threshold at the decision diamond  80 , then the algorithm increases the cathode airflow a predetermined amount at box  92  until a maximum flow is reached. The newly calculated cathode airflow is then sent to the cathode side of the stack at the box  70 . 
     If the stack temperature is above the threshold at the decision diamond  76 , then the stack  12  is heated properly, and the shorting switch  44  is disengaged at box  84 . The electrical vehicle loads are then applied at box  86  and the fuel cell system  10  begins normal operation at box  88 . The driver is then allowed to operate the vehicle at box  90 . 
     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.