Abstract:
A system and method for warming a fuel cell on an aircraft, the system includes at least one fuel cell. The fuel cell includes an anode and a cathode for creating thermal and electrical energy. A temperature sensor measures a first temperature of the fuel cell. A control unit is coupled to the temperature sensor. The control unit increases the first temperature to a second temperature in response to the first temperature being at least equal to a selected temperature threshold. Increasing of the first temperature is indicative of the control unit operating in a warming mode. The second temperature is higher than the selected temperature threshold.

Description:
BACKGROUND OF THE INVENTION 
     This subject matter disclosed herein relates to a fuel cell system provided for use on board an aircraft and, in particular, to temperature regulation of such a fuel cell system. 
     Fuel cells generate electrical energy with low emissions and a high level of efficiency. The power generated by an aircraft fuel cell may be utilized to supplement or replace the primary power generation system. An aircraft fuel call may also be utilized for emergency power generation to supplement or replace an aircraft ram air turbine. 
     Thermal regulation is an important consideration for fuel cells utilized in aircraft emergency power generation systems. Due to the immediate demand for power in an emergency situation, delayed start times caused by a cold fuel cell are unacceptable. If a warming feature is not available for the fuel cell, a supplemental battery system will be necessary to provide power until the fuel cell has warmed to its operational temperature. The use of a battery system is both heavy and expensive. 
     Conventional fuel cells include an anode region and a cathode region separated by an electrolyte. When the fuel cell is operated, a fuel, for example hydrogen, is supplied to the anode side and an oxygen-containing oxidant, such as air, is supplied to the cathode side. In fuel cells where the electrolyte is a polymer electrolyte membrane (PEM), the hydrogen molecules react at an anode catalyst in the anode region to form positively charged hydrogen ions (H+) and transfer electrons to the electrode. The H+ ions, which are formed in the anode region, then diffuse through the electrolyte to the cathode where they react, at a cathode catalyst, with the oxygen supplied to the cathode and the electrons that are transferred to the cathode by way of an external circuit, forming water. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the invention, a system is provided for warming a fuel cell on an aircraft, the system including at least one fuel cell. The fuel cell includes an anode and a cathode for creating thermal and electrical energy. A temperature sensor measures a first temperature of the fuel cell. A control unit is coupled to the temperature sensor. The control unit increases the first temperature to a second temperature in response to the first temperature being at least equal to a selected temperature threshold. Increasing of the first temperature is indicative of the control unit operating in a warming mode. The second temperature is higher than the selected temperature threshold. 
     According to an alternate embodiment of the invention, a method of operating a fuel cell on an aircraft is provided wherein the fuel cell includes a membrane between an anode and a cathode and a sensor positioned to measure the temperature of the fuel cell. The method includes sensing the temperature of the fuel cell. If the temperature sensed is above a selected threshold, the fuel cell is operated in a first mode. If the temperature sensed is equal to or less than the selected threshold, the fuel cell is operated in a second mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic drawing of a thermal control subsystem of a fuel cell according to an embodiment of the invention; 
         FIG. 2  is a schematic drawing of an alternate thermal control subsystem of a fuel cell according to an embodiment of the invention; 
         FIG. 3  is a schematic drawing of an alternate thermal control subsystem of a fuel cell according to an embodiment of the invention; 
         FIG. 4  is a schematic drawing of an alternate thermal control subsystem of a fuel cell according to an embodiment of the invention; and 
         FIG. 5  is a schematic drawing of an alternate thermal control subsystem of a fuel cell according to an embodiment of the invention. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that a polymer electrolyte membrane (PEM) fuel cell shows high performance at temperatures between about 60° C. (333 Kelvin) and about 80° C. (353 Kelvin). Performance of the fuel cell is reduced at lower temperatures because the reaction activation and ion conductance of the electrolyte membrane are decreased. Particularly, if the ambient temperature falls below 0° C. (273 Kelvin) the temperature of the fuel cell stack may also fall below 0° C. (273 Kelvin). This can cause the water used for electrode activation and hydrogen ion transfer in an electrolyte membrane to freeze, resulting in degraded performance. For this reason, when a fuel cell starts at a low temperature, it is very important to raise the temperature to 0° C. (273 Kelvin) or greater in order to warm the inside of the fuel cell stack and melt any frozen water. 
     Freezing of the water inside a fuel cell can occur often when the fuel cell is contained in a region of an aircraft that is not temperature controlled. Hereinafter, the fuel cell will be considered in an “idle” state when not generating power. A fuel cell on an aircraft may be idle when the aircraft is in-flight, taxiing, long-term parking, or undergoing service. Atmospheric temperature variations resulting from flight or environmental conditions, such as take-off and landing, may cause freezing and thawing cycles to occur within an idle fuel cell, thereby degrading the performance and life of the fuel cell. The critical layer of the fuel cell, which consists of the membrane and electrodes, may retain liquid water even below 0° C. (273 Kelvin). The electrodes are able to retain water up to approximately −10° C. (263 Kelvin) due to a lowered freezing point associated with capillary effects, and the membrane may have some “bound” water with a freezing point as low as −40° C. (233 Kelvin). As such, a low temperature for keeping the fuel cell warm to enable rapid startup and minimal loss of performance associated with freeze-thaw cycles is chosen to be approximately −10° C. (263 Kelvin). However, as fuel cell technology advancements are made, the coldest warming temperature could be as low as −30° C. (243 Kelvin). 
     Referring to the  FIG. 1 , a thermal control subsystem  10  for a fuel cell system located on an aircraft is illustrated. During operation, a fuel cell stack  20  and the associated subsystems in direct or fluid communication with the fuel cell stack  20 , are at an elevated temperature. For example, the operating temperature of a proton exchange membrane (PEM) fuel cell is commonly in the range of between about 60° C. (333 Kelvin) and about 85° C. (358 Kelvin). Therefore, the various subsystems in direct or fluid communication with the fuel cell stack  20  are similarly at an elevated temperature when the fuel cell system becomes idle. When the fuel cell system is in an idle state, assuming a sub-freezing ambient temperature, the various subsystems will cool rapidly. After a sufficiently long period of time, the fuel cell stack  20  will eventually cool to the sub-freezing ambient temperature. Positioned adjacent to or within the fuel cell stack  20  is a temperature sensor  44  connected to a control device  50  for monitoring the temperature of the stack  20 . When the temperature of the idle fuel cell stack  20  falls below a selected threshold, the control device  50  will shift the operation of the fuel cell system from an idle first mode to a second warming mode. An exemplary selected temperature threshold may be in the range of between about 4° C. (277 Kelvin) and −10° C. (263 Kelvin). More specifically, an exemplary selected temperature may be 0° C. (273 Kelvin). The fuel cell stack may include a series of fuel cells stacked and held together by pressure plates. The temperature of the fuel cell stack  20  may vary from fuel cell to fuel cell. Typically, during a cooling process, the ends of the fuel cell stack  20  nearest the pressure plates cool more quickly than the center of the stack  20 . Because of this, the temperature sensor  44  used to drive the control system may be arranged to measure the temperature of the coolest part of the fuel cell stack  20 , such as at the fuel cells adjacent the pressure plates for example. 
     Multiple methods exist for warming a fuel cell stack  20  in the second mode whereby a gas or a liquid is circulated through system  10 . In an embodiment, as shown in  FIG. 1 , a thermal control subsystem  10  increases the temperature of a fuel cell stack  20  using warm air from another aircraft subsystem. A warm air supply  30  is fluidly connected to an inlet  22  of the fuel cell stack  20  by a conduit  36 . Exemplary sources that may act as air supply  30  include the airplane&#39;s air supply management system, cabin air, and engine bleed air. The temperature of the air provided by the air supply  30  may be up to about 95° C. (368 Kelvin) depending on the source of the air. As the warm air A passes through the fuel cell stack  20 , thermal energy is transferred from the air A to the fuel cell stack  20 . The air A then exits through outlet  24  of the fuel cell stack  20  and is released in area  60 , such as a within another section of the aircraft or to the atmosphere, for example. In another embodiment, the air A may be returned to the air supply  30  from which it was taken by a conduit  38 . Disposed along the fluid path between the outlet  34  of the air supply  30  and the inlet  22  of the fuel cell stack  20  is a valve  42  operable between an open and a closed position. 
     In one embodiment, a pump  40  is located along the conduit  38  for circulating the air from the fuel cell stack  20  back to the air supply  30 . The pump  40  and the valve  42  are connected to a control device  50 . Once a temperature sensor  44  determines that the temperature of the fuel cell stack  20  is below a selected threshold, the control device  50  will begin to operate in the second mode by opening the valve  42 , so that air will flow from the air supply  30  to the fuel cell stack  20 . In addition, the control device  50  may run the pump  40  to circulate the cool air exiting the fuel cell stack  20  back to the air supply  30 . In one embodiment, the warm air A is circulated around the outside of the fuel cell stack  20  rather than through it. For fuel cells constructed with porous bipolar plate technology, circulating warming air around the fuel cell, as opposed to through it, will prevent dehydration of the membrane. For fuel cells constructed with a solid bipolar plate technology, where the coolant loop is fluidly isolated from the membrane electrode assembly, the warm air may be circulated through the coolant channels of the fuel cell. Both solid plate and porous plate technology fuel cells may be warmed by circulating warm air around the outside of the fuel cell stack  20 . 
     Referring now to  FIG. 2 , in yet another embodiment, a thermal control subsystem  110  warms a fuel cell stack  120  using one or more resistive electrical elements  160 . The resistive electrical element  160  may be positioned adjacent the fuel cell stack  120  or alternately may be embedded within the end plates or pressure plates of the fuel cell stack  120 . In one embodiment, the resistive electrical element  160  is a heater. In one embodiment, thermal insulation  180 , such as an insulating jacket for example, surrounds the fuel cell stack  120 . The thermal insulation will slow the cooling of the idle fuel cell stack  120  by retaining some of the heat generated during operation. The thermal insulation  180  illustrated may be used in conjunction with any of the thermal control subsystems disclosed herein. When a temperature sensor  144  connected to the fuel cell stack  120  determines that the temperature of the fuel cell stack  120  has fallen below a selected threshold, the control device  150  switches from a first mode to a second warming mode. In the second warming mode, the control device  150  sends a signal to activate the resistive electrical element  160 . In one embodiment, power is provided to the resistive electrical element  160  by a power source  170 . The power source  170  is the electrical grid of the aircraft. In another embodiment, the power source  170  is a battery. In yet another embodiment, when the aircraft is parked, ground power may be used to power the resistive electrical element  160 . 
     Referring now to the exemplary thermal control subsystem  210  of  FIG. 3 , an outlet  224  of a fuel cell stack  220  is fluidly connected to an inlet  232  of a heat exchanger  230  by a first conduit  226 . Similarly, a second conduit  236  connects the outlet  234  of the heat exchanger  230  with the inlet  222  of the fuel cell  220  to form a thermal management loop. In one embodiment, a fluid C, such as air or a coolant including water or antifreeze for example, circulates within the thermal management loop. If fluid C is antifreeze, thermal control subsystem  210  may not be applied to porous bipolar plate fuel cells. As illustrated, a control valve  242  is provided along conduit  226  between the outlet  224  of the fuel cell stack  220  and the inlet  232  of the heat exchanger  230 . The valve  242  is operable between a first position and a second position to control the circulation of the fluid C within the thermal management loop. 
     Additionally a bypass conduit  227  connects conduit  226  to conduit  236  adjacent the heat exchanger  230  in order to redirect a flow of fluid. The end of bypass conduit  227  is connected to valve  242 . In one embodiment, valve  242  is a three way valve such that when the fuel cell  220  is idle, the flow of fluid C may be redirected through bypass conduit  227 , rather than through the heat exchanger  230 . The fluid C is circulated by a pump  240  under control of the control device  250 . In one embodiment, the fluid C has a temperature generally in the range of between −10° C. (263 Kelvin) and the temperature of the fuel cell  220 . If water is used as a coolant fluid, the fluid C temperature would be limited to 0° C. (273 Kelvin). An accumulator  238  may be provided upstream from the pump  240 . In one embodiment, a heating device  239 , such as a wire mesh heater for example, may be positioned within the accumulator  238  to increase the temperature of the fluid C being circulated to the fuel cell stack  220 . The pump  240 , the valve  242 , and the heating device  239  are operably coupled to the control device  250 . 
     It will be understood by a person of ordinary skill that this thermal management loop uses fluid C to transfer heat to the fuel cell stack  220  from the heating device  239  when the fuel cell  220  is idling. When the fuel cell stack  220  is in normal operation and not idling, this same thermal management loop  210  may serve as a cooling loop that transfers thermal energy from the fuel cell stack  220  to the environment via the heat exchanger  230 . When the temperature sensor  244  embedded within the fuel cell stack  220  falls below a selected value, the control device  250  may be programmed to operate the heating device  239  within accumulator  238 , causing the fluid C to become heated. Additionally, the control device  250  opens the valve  242  at the end of bypass conduit  227  and activates the pump  240  to circulate the fluid C through the fuel cell stack  220 . Circulation of the fluid C will transfer thermal energy from the heating device  239  to the fuel cell stack  220 . As the fluid C exits the outlet  224  of the fuel cell stack  220 , the cooled fluid C flows through bypass conduit  227  and into the accumulator  238  where the fluid C may be heated by the heating device  239 . 
     In an alternate embodiment, illustrated in  FIG. 4 , the bypass conduit  227  of thermal management system  210  includes a heat exchanger  229  rather than a heating device  239  while all other components remain substantially the same as the system shown and described with reference to  FIG. 3 . The heat exchanger  229  is located in a temperature controlled area of the aircraft, such as in the cabin for example. As the fluid C circulates through the heat exchanger  229 , heat from the surrounding air will transfer to the fluid C. 
     The thermal control subsystem  210  illustrated in  FIG. 5  is a variation of the thermal control subsystem of  FIG. 4  for warming a fuel cell stack  220 . The fuel cell in  FIG. 5  includes a dedicated cooling loop  300  and a separate, dedicated heating loop  310 . The outlet  224  of the fuel cell stack  220  is fluidly connected to an inlet  268  of a heat exchanger  270  by a first conduit  260 . A second conduit  274  connects the outlet  272  of the heat exchanger  270  with the inlet  222  of the fuel cell stack  220  to form a heating loop. In one embodiment, the heat exchanger  270  is located in a temperature controlled area in the aircraft, such as in the cabin for example. A warming fluid H, such as water or air for example, circulates within the heating loop  310 . A cooling fluid C, such as water for example, circulates within the cooling loop  300 . The heating loop  310  includes a control valve  262  and a pump  280  for circulating the fluid H. The cooling loop  300  includes a control valve  242  and a pump  240  for circulating the fluid C. When the fuel cell  220  is in normal operation and generating heat, control valve  242  of the cooling loop  300  will be open and the control valve  262  of the heating loop  310  will be closed. Opening of control valve  242  and operation of pump  240  will circulate the cooling fluid C and will remove heat from the fuel cell  220 . 
     During normal operation, heat from the fuel cell  220  is released to the ambient atmosphere through conduit  226  and the heat exchanger  230 . In the illustrated embodiment, the cooling loop  300  does not include a bypass conduit, or a heating device disposed within the accumulator  238 . When the fuel cell  220  is idling and above a selected temperature threshold, both control valves  242 ,  262  are closed such that neither the heating loop  310  nor the cooling loop  300  is operating. When the temperature sensor  244  detects that the temperature of the fuel cell stack  220  is below a selected threshold, the control device  250  will initiate the operation of the heating loop  310  by opening control valve  262  and miming pump  280  to circulate the fluid H through the fuel cell  220 . As the cooled fluid H exits the outlet  224  of the fuel cell stack  220 , it passes through heat exchanger  270 , where heat is transferred to the fluid H from the surrounding environment. In embodiments where fluid H is water, the heating loop  310  may additionally include an accumulator  282 . 
     In another embodiment, the fuel cell stack of any of the prior embodiments may be warmed by operating the fuel cell at a low or minimum load. The heat produced as a byproduct of the electrochemical process of the fuel cell warms the fuel cell stack to a temperature above the minimum threshold. To warm the fuel cell stack using this method for long periods of time, a large supply source of both hydrogen and oxygen is required. 
     For any of the disclosed embodiments, when the fuel cell is idle, power is required to operate the associated devices, such as a control device, a pump, or a heating device for example. Because the fuel cell is not generating power when it is idle, the power for the associated devices is supplied by an alternate source. Exemplary power sources include a battery, the power grid of the aircraft, ground power is the aircraft is parked, and any other means known to a person skilled in the art. 
     Any of the embodiments of the invention discussed above may be applied to a fuel cell system of an aircraft individually or in combination with any of the other embodiments. For example, when the fuel cell system operates in a second mode, the fuel cell may operate at a minimum load and include a resistive electrical heating element. When applied together to the fuel cell system, the resistive electrical heating element may be driven using the electrical power generated by the fuel cell stack. In addition, a thermal insulation, such as an insulating jacket, may surround any of the fuel cell stacks in the previously described thermal control systems. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.