Patent Application
20070287041
The following discussion of the embodiments of the invention directed to a control system for a fuel cell stack that maintains the cathode inlet air relative humidity above a predetermined value by doing one or more of decreasing the stack cooling fluid temperature, increasing the cathode pressure, decreasing the cathode stoichiometry and/or limiting the power output of the stack when necessary is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses.
The system 10 includes a pump 24 that pumps a cooling fluid through a coolant loop 28 that flows through a stack 12. The heated cooling fluid from the stack 12 is sent through a radiator 30 where it is cooled to be returned to the stack 12 through the coolant loop 28. The system 10 also includes a backpressure valve 42 positioned in the cathode exhaust gas line 14 after the WVT device 20 for controlling the pressure of the cathode side of the stack 12.
The system 10 includes several sensors for sensing certain operating parameters. Particularly, the system 10 includes an RH sensor 36 for measuring the relative humidity of the cathode inlet air in the line 14, and a temperature sensor 34 for measuring the temperature of the cathode inlet air in the line 14. It is known in the art to use a dew point sensor instead of the combination of the RH sensor 36 and the temperature sensor 34. A temperature sensor 38 measures the temperature of the cooling fluid in the coolant loop 28 entering the stack 12, and a temperature sensor 26 measures the temperature of the cooling fluid exiting the stack 12. A pressure sensor 32 measures the pressure of the cathode exhaust gas in the line 16. As is known in the art, the measured relative humidity of the cathode inlet air needs to be corrected because the temperature of the stack 12 is different than the temperature of the air in the inlet line 14. By knowing the inlet RH and the temperature of the cooling fluid entering the stack 12, the corrected relative humidity of the cathode air can be calculated.
A controller 40 receives the mass flow signal from the mass flow meter 22, the relative humidity signal from the RH sensor 36, the temperature signal from the temperature sensor 34, the temperature signal from the temperature sensor 38, the temperature signal from the temperature sensor 26 and the pressure signal from the pressure sensor 32. The controller 40 also controls the backpressure valve 42.
According to the invention, the controller 40 attempts to maintain the corrected relative humidity above a predetermined percentage by performing one or more of decreasing the cooling fluid temperature, increasing the cathode pressure, and/or decreasing the cathode stoichiometry when necessary to increase the relative humidity of the cathode exhaust gas that is used by the WVT device 20 to humidify the cathode inlet air. The controller 40 can also limit the power output of the stack 12 to keep the relative humidity of the cathode inlet air above the predetermined percentage.
The controller 40 may decrease the stack cooling fluid temperature by increasing the speed of the pump 24 and/or the cooling ability of the radiator 28, such as by cooling fans. The controller 40 may increase or decrease the cathode pressure within the stack 12 by closing and opening the backpressure valve 42. The pressure sensor 32 will measure the change in the cathode pressure. Further, the controller 40 may decrease the cathode stoichiometry by decreasing the speed of the compressor 18 for a particular output current. The signal from the mass flow meter 22 is read by the controller 40 and based on this signal, the controller 40 controls the speed of the compressor 18 to the desired cathode stoichiometry set-point. The combination of one or more of these operations should increase the relative humidity of the cathode exhaust gas on the line 16, thus providing more humidity in the WVT device 20 for humidifying the cathode inlet air.
If one or more of these three operations does not increase the corrected relative humidity of the cathode inlet air above the desired percentage, then the controller 40 may limit the power output from the stack 12. This can be done by changing a “maximum current available” variable between the fuel cell stack 12 and the stack load. The value of the variable is decreased an appropriate amount until the cathode inlet humidification is sufficient. By reducing the variable, the stack load should draw less power, which reduces by-product water that could flood flow channels. Also, the cathode airflow set-point for the compressor 18 will decrease, resulting in a slower airflow through the WVT device 20, and more cathode inlet air humidification.
If the relative humidity of the cathode exhaust gas in the line 16 is increased to satisfy the inlet air relative humidity, then the output voltage of the fuel cells in the stack 12 are monitored to determine whether the cells may be flooded, especially the end cells. If there is an indication that water is accumulating in the flow channels, then the controller 40 can decrease the relative humidity of the cathode exhaust gas by any of the operations discussed above.
With this control design, it may be possible to reduce the size of the WVT device 20 over those typically used in the industry without sacrificing the minimum cathode inlet humidification needed for long stack life. Therefore, the cost, weight and space requirements required for the WVT device 20 can be reduced.
Equations are known in the art for calculating the cathode outlet relative humidity, the cathode stoichiometry and the cathode inlet RH for the control algorithm of the invention discussed above. Particularly, the cathode output relative humidity can be calculated by:
The cathode stoichiometry can be calculated by:
The cathode inlet relative humidity percentage can be calculated by:
Where CS is the cathode stoichiometry, T1 is the stack cooling fluid outlet temperature in degrees Celcius, P1 is the cathode outlet pressure in kPa, T2 is the cathode inlet temperature in degrees Celcius, P2 is the cathode pressure drop in kPa, which is calculated based on a known model, and T3 is the stack cooling fluid inlet temperature in degrees Celcius.
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.
