Humidification control during shutdown of a fuel cell system
View Patent ↗A process for controlling the length of a purge and the purge rate of a fuel cell stack at system shut-down so as to provide the desired amount of stack humidity. The membrane humidification is measured at system shut-down by a high frequency resistance sensor that detects membrane humidification and provides the measurement to a controller. The controller controls the compressor that provides cathode input air to the fuel cell stack so that the time of the purge and the flow rate of the purge provide a desired membrane humidity for the next start-up.
1. A fuel cell system comprising:
a fuel cell stack;
a compressor for providing cathode inlet air to a cathode side of the fuel cell stack and purge air to both an anode side and the cathode side of the fuel cell stack;
at least one high frequency resistance measurement device for measuring the resistance of a high frequency component on an electrical load of the stack; and
a controller configured to use the resistance measurement to identify an amount of humidity in the fuel cell stack at system shut-down, said controller further configured to control the compressor during a stack purge based on the measurement from the high frequency resistance measurement device, wherein the controller ends the stack purge when the high frequency resistance measurement is greater than a threshold.
2. The system according to claim 1 wherein the fuel cell stack is split sub-stacks and the at least one high frequency resistance measurement device is one high frequency resistance measurement device for each sub-stack, each high frequency resistance measurement device measuring the resistance of a high frequency component for a separate one of the sub-stacks.
3. The system according to claim 2 wherein the controller ends the stack purge when an average high frequency resistance measurement of the sub-stacks reaches a predetermined threshold.
4. The system according to claim 2 wherein the controller ends the stack purge when a minimum high frequency resistance measurement of one of the sub-stacks reaches a predetermined threshold.
5. The system according to claim 2 wherein the controller ends the stack purge when a maximum high frequency resistance measurement of one of the sub-stacks reaches a predetermined threshold.
6. The system according to claim 1 wherein the controller ends the stack purge when a changing slope of the high frequency resistance measurement reaches a predetermined slope.
7. The system according to claim 1 where the controller controls stack temperature during the stack purge.
8. The system according to claim 1 further comprising a heater for heating the stack during the stack purge.
9. The system according to claim 1 wherein the controller employs a dynamic membrane humidification model in combination with the high frequency resistance measurements to determine stack purge duration.
10. The system according to claim 1 wherein the controller employs a water buffer model to determine humidification across fuel cell membranes in combination with the high frequency resistance measurements to determine stack purge duration.
11. A fuel cell system comprising:
a fuel cell stack;
a compressor for providing cathode inlet air to a cathode side of the fuel cell stack and purge air to both an anode side and the cathode side of the fuel cell stack; and
a controller configured to use an amount of humidity in the fuel cell stack at system shut-down to control a purge duration and a purge rate by the compressor during a stack purge, wherein the controller uses a high frequency resistance measurement of a high frequency component on an electrical load of the stack to determine stack relative humidity at system shut-down, and wherein the controller ends the stack purge when the high frequency resistance measurement reaches a target set-point.
12. The system according to claim 11 further comprising a heater for heating the stack during the stack purge.
13. A method for determining a fuel cell stack purge rate and duration at fuel cell system shut-down, said method comprising:
measuring the resistance of a high frequency component on an electrical load of the stack; and
controlling a compressor during a stack purge based on the high frequency component measurement to control purge air sent to a cathode and an anode of the fuel cell stack so that membranes within the stack have a desired relative humidity at the system shut-down, wherein the stack purge is ended when the high frequency resistance measurement reaches a target set-point.
14. The method according to claim 13 further comprising heating the stack during the stack purge.
15. The method according to claim 13 further comprising employing a dynamic membrane humidification model in combination with the high frequency resistance measurements to determine stack purge duration.
16. The method according to claim 13 further comprising employing a water buffer model to determine humidification across fuel cell membranes in combination with the high frequency resistance measurements to determine stack purge duration.