Patent Application
20130288082
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.
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.
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:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
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
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
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
Referring now to the exemplary thermal control subsystem 210 of
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
The thermal control subsystem 210 illustrated in
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.
