Abstract:
A circuit configuration provides a simple device that can be used to monitor the voltage potential between any two points within the fuel cell coolant system without loading (i.e. decreasing) the voltage being monitored. The monitoring circuit include a lamp electrically coupled to the coolant system and arranged in a light-tight package with a photoresistor. When the ionization voltage for the lamp is reached, a fully isolated signal is provided in the form of an abrupt resistance change in the photoresistor. Visual and audible indicators can be coupled to the monitoring circuit to indicate a coolant contamination condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to conductivity monitoring circuits, and more particularly to coolant conductivity monitoring circuits for fuel cell systems with a fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cells are emerging as a viable power source for many applications. In a fuel cell, an active element referred to as a membrane electrode assembly is sandwiched between sheets of porous, gas-permeable, conductive material which serve as the primary current collectors for the MEA and provide mechanical support therefore. The MEA/primary current collector assembly is pressed between a pair of non-porous, electrically conductive separator plates or metal sheets which serve as the secondary current collectors for the primary current collectors and conduct current between adjacent cells with the fuel cell stack. 
     The MEAs must operate within a desired temperature range. Since the chemical reaction within the fuel cell is exothermic, a fuel cell stack often utilizes some internal cooling system to extract heat therefrom. To this end, a separate cooling layer is in thermal contact with individual fuel cells within the stack. A coolant fluid is circulated through the coolant layer to remove the heat from the adjacent cells. 
     As the coolant ages it collects contaminates that cause it to become electrically conductive. Some stack designs depend on plate insulation coatings for electrical isolation of the coolant fluid. If the plate coatings crack or begin to leak electrically, the stack coolant could conduct a leakage current throughout the coolant loop. Thus, it is often desirable to monitor the conductivity of the coolant to detect such conditions. 
     Conventional means for monitoring the conductivity of a coolant loop employ a resistor connected between two conductive points in the coolant loop with a volt meter connected across it. However, this method creates an alternate current path in the circuit and ends up loading the voltage that is being monitored. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple device that can be used to monitor the voltage potential between any two points within the fuel cell coolant system without loading (i.e. decreasing) the voltage being monitored. 
     In a first preferred embodiment, the device combines a gas-filled lamp electrically coupled to the coolant system and arranged in a light-tight package with a photoresistor. This device does not induce any detectable leakage path until the ionization voltage or glow point of the lamp is reached. When the ionization voltage is reached, a fully isolated signal is provided in the form of an abrupt resistance change in the photoresistor. The resistance change ranges from nearly open when the voltage being monitored is below the ionization voltage to just a few hundred ohms when the voltage exceeds the ionization voltage of the lamp. 
     Accordingly, in one aspect of the present invention, a coolant contamination monitoring circuit for a fuel cell system is provided which includes a lamp operable in a non-illuminated state when a voltage build-up is less than a threshold value and an illuminated state when said voltage build-up is at least equal to the threshold value, a photoresistor located adjacent to the lamp that varies in resistance in response to illumination of the lamp so as to function as a switch that is open when the lamp is not illuminated and that is closed when the lamp is illuminated, and an optical isolator isolating the lamp and the photoresistor from ambient light. Visual and audible indicators may be coupled to the monitoring circuit to indicate a coolant contamination condition. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic showing the coolant contamination monitoring circuit according to the present invention; 
         FIG. 2  is a schematic showing a battery-driven audible alarm circuit used with the monitoring circuit; 
         FIG. 3  is a schematic showing a battery-driven visual alarm circuit used with the monitoring circuit; and 
         FIG. 4  is a schematic showing two lamps in series providing a two-stage alarm output in another feature of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , a coolant contamination monitoring circuit  10  is connected to a fuel cell system  12  with a coolant loop  14 . Coolant enters the fuel cell stack  16  for removing heat therefrom. The coolant continues through the other components in the coolant loop  14 , including the radiator  18  (where the coolant is cooled) and pump  20 , before returning to the stack  16 . An electrical load  22  is powered by the positive terminal  24  and negative terminal  26  of the fuel cell stack  16 . 
     The coolant contamination monitoring circuit  10  contains a lamp  28  that operates in a non-illuminated state when an applied voltage is less than a threshold value. The lamp  28  operates in an illuminated state when the applied voltage is at least equal to the threshold value. The lamp  28  is connected to the fuel cell system  12  by a first monitoring resistor  30  and a second monitoring resistor  32 . The first monitoring resistor  30  is shown connected from one side of the lamp  28  to the positive terminal  24  of the fuel cell stack  16 . The second monitoring resistor  32  is shown connected from the opposite side of the lamp  28  to a metal elbow  34  in the coolant loop  14  that is in conductive contact with the coolant. A skilled practitioner will recognize that the monitoring resistors  30  and  32  can connect to the fuel cell system  12  at any two points that are conductive with the coolant. For example, the first monitoring resistor  30  could be connected to another conductive point along the coolant loop  14  in a manner such as the second monitoring resistor  32 . Alternatively, one of the monitoring resistors  30  and  32  could be connected to a point grounded to the vehicle chassis. 
     A photoresistor  36  is located adjacent to the lamp  28 . The photoresistor  36  varies in resistance in response to the illumination of the lamp  28 . When the lamp  28  is not illuminated, the resistance of the photoresistor  36  is very high, on the order of a mega ohm (10 6 Ω) or more, which prevents useful current from flowing through the circuit  38  to which the photoresistor  36  is connected. When the lamp  28  illuminates, the resistance of the photoresistor  36  lowers enough, on the order of about a few 100 ohms, so that current may flow through the photoresistor  36  as well as the circuit  38  it is connected to. Actual values depend on the type of photoresistor used and its response to the intensity and color temperature of the light emitted by lamp  28 . Both the photoresistor  36  and the lamp  28  are contained in an optical isolator  40  to shield both components from ambient light. The steep change in resistance of the photoresistor  36  can be thought to function as a switch that is open when the lamp  28  is not illuminated and is closed when the lamp  28  is illuminated. 
     The applied voltage that controls the illumination of the lamp  28  is a voltage build-up between the two monitoring resistors  30  and  32 . The voltage build-up is a potential difference between the conductive points of the monitoring resistors  30  and  32 . When the voltage build-up reaches the threshold value, the lamp  28  illuminates. The threshold value varies in accordance with a given application and is chosen to correspond with a voltage at which the conductivity in the coolant loop  14  becomes too high for the fuel cell system  12  to function efficiently. 
     The lamp  28  remains an open circuit and non-illuminated when the voltage build-up is below the threshold value. This prevents a leakage current from flowing through the coolant contamination monitoring circuit  10  until the voltage build-up reaches the threshold value and the lamp  28  illuminates. While the lamp  28  remains non-illuminated, loading (lowering) of the voltage that is being monitored is also prevented. The combination of the lamp  28  and photoresistor  36  allows for a fully isolated signal to be produced in the form of the abrupt resistance change in the photoresistor  36  when the lamp  28  illuminates. This makes it possible for an isolated alarm circuit to be connected to the photoresistor  36  that activates when the lamp  28  illuminates. 
     In a preferred embodiment, the lamp  28  is a gas-filled lamp that illuminates when the voltage build-up reaches the ionization voltage of the gas-filled lamp. The ionization voltage of the gas-filled lamp corresponds with the threshold value of the coolant contamination monitoring circuit  10 . The ionization voltage is a value at which the gas in the gas-filled lamp will conduct current. In a highly preferred embodiment, the gas-filled lamp is comprised primarily of neon gas. 
     Referring now to  FIG. 2 , a battery-driven audible alarm circuit  48  is shown connected to the photoresistor  36 . When the voltage build-up reaches the threshold value and the lamp  28  illuminates, the abrupt resistance change in the photoresistor  36  activates the audible alarm circuit  48 . The audible alarm circuit  48  comprises a battery  50  and an audible indicator  52  in series with the photoresistor  36 . In  FIG. 2 , the audible indicator  52  is a battery-driven piezo-type alarm. Additionally, a test button  54  is connected in parallel with the photoresistor  36 . When pressed, the test button  54  shorts the photoresistor  36 , allowing the operation of the audible alarm circuit  48  to be tested. 
     If the battery  50  has a long shelf-life and the test button  54  is included, the audible alarm circuit  48 , along with the photoresistor  36  and lamp  28 , may be manufactured as a disposable and completely packaged product  56 . In this case, the first and second monitoring resistors  30  and  32  could be placed in the lead wires  58  and  60  close to the monitoring points  24  and  34 . This eliminates the customary need for protective fuses and allows for the use of smaller connection wires while monitoring high voltage and high current potentials. The first and second monitoring resistors  30  and  32  are needed to limit the current through the lamp  28 . When the lamp  28  is not illuminated, it acts as an open circuit. Once the voltage build-up reaches the threshold value, the lamp  28  becomes a closed circuit and allows current to flow. The first and second monitoring resistors  30  and  32  prevent excessive levels of current from going through the lamp  28  and damaging it. 
       FIG. 3  shows a visual alarm circuit  68  connected to the photoresistor  36 . When the voltage build-up reaches the threshold value and the lamp  28  illuminates, the abrupt resistance change in the photoresistor  36  activates the visual alarm circuit  68 . The visual alarm circuit  68  includes a battery  50  and a visible indicator  70  connected in series with the photoresistor  36 . In  FIG. 3 , the visible indicator  70  is a battery-operated lamp. Additionally, a test button  54  is connected in parallel with the photoresistor  36 . Pressing the test button  54  shorts the photoresistor  36  to test the operation of the visual alarm circuit  68 . If the battery  50  has a long shelf-life and the test button  54  is included, the visual alarm circuit  68 , like the audible alarm circuit  48 , may be manufactured as a disposable and completely packaged product  56 . 
     In another embodiment, a second lamp is used to provide a basic two-stage alarm having two monitoring voltage thresholds. Referring to  FIG. 4 , a first lamp  28  and a second lamp  78  are connected in series and both lamps  28 ,  78  operate in a non-illuminated state when the voltage build-up is less than the threshold value of the first lamp  28 . The first lamp operates in an illuminated state when the voltage build-up is at least equal to a first threshold value associated with the first lamp  28  but below a second threshold value associated with the second lamp  78 . The second lamp  78  remains non-illuminated. The first lamp  28  is located adjacent to the first photoresistor  36 , in an optically coupled configuration inside a light tight optical coupler package that shields the lamp  28  and photoresistor  36  from all outside ambient light. The second lamp  78  is located adjacent to a second photoresistor  80 , in a second light tight optical coupler package. Like the single lamp circuit, when lamp  28  illuminates, the resistance of photoresistor  36  lowers allowing current to flow in the first output. Since the second lamp  78  remains non-illuminated, no current will flow in the second output. As the voltage build-up increases to the threshold value of the second lamp  78 , lamp  28  remains illuminated and lamp  78  will illuminated. With both lamps  28 ,  78  illuminated, both photoresistors  36 ,  80  will have lower resistance and current will flow in both alarm outputs. 
     The lamps  28 ,  78  remain an open circuit and non-illuminated when the voltage build-up is below the threshold value for the first lamp  28 . This prevents a leakage current from flowing through the coolant contamination monitoring circuit  10  until the voltage build-up reaches the threshold value for lamp  28 . While the lamps  28 ,  78  remain non-illuminated, loading (i.e. lowering) of the voltage that is being monitored is also prevented. The combination of the lamps  28 ,  78  and photoresistors  36 ,  80  allow for two fully isolated signals to be produced in the form of the abrupt resistance change in the photoresistors  36 ,  80  when the lamps illuminate. This makes it possible for two separate isolated alarm circuits to be driven—one from the photoresisotor  36 , and the other from the photoresistor  80 . Each alarm circuit will activate when the voltage build-up being monitored reaches the illumination point or threshold value of their respective lamps. This configuration will provide a two-stage monitor output, where the first stage (lower voltage) could be used as a warning signal and the second stage (higher voltage) could be used as a system shut down signal. 
     Having two lamps simply in series nearly doubles the threshold voltage to illuminate the lamps. As the voltage increases, the lamp with the lowest ionization voltage will illuminate first, compromising the two-stage operation of the coolant contamination monitoring circuit. To insure which lamp illuminates first, a high value resistor  86  is added in parallel with the second lamp  78 . Resistor  86  allows the voltage to build-up across lamp  28  first, thereby insuring that the lower voltage alarm output conducts current first as the voltage being monitored increases. 
     In a preferred embodiment, the lamps  28 ,  78  are gas-filled lamps combined with a high value resistor to illuminate when two different voltage levels are reached. In a more preferred embodiment, the gas-filled lamps are primarily comprised of neon gas. Photoresistor  36  may be connected to an audible alarm circuit  48  as shown in FIG.  2 . This would provide an audible warning that the voltage being monitored is increasing past its initial limits. Photoresistor  80  may be connected to a visual alarm circuit  68  as shown in FIG.  3 . This would provide a visual and audible indication that the voltage being monitored has surpassed its initial limits and has reached its higher limit. Alternately, photoresistor  80  may be connected to an external circuit to provide a fuel cell system shut down in the event that the voltage being monitored reaches its upper limit. 
     The resistance of the first and second monitoring resistors  30  and  32 , which are both equal, depends on the voltage to be monitored. The resistance limits the maximum current through the lamps after the threshold voltage has been reached. In the highly preferred embodiment of using neon gas-filled lamps, the coolant contamination monitoring circuit  10  is limited to monitoring voltage values that cause standard production neon lamps to ionize and glow. By creating new gas lamps with different gas compositions (such as neon, argon, krypton, xenon), pressures, and electrodes, new ionization voltage can be realized. These new ionization voltages could be designed to work directly with a variety of leakage current applications including lower voltage stack cooling systems. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.