A humidity control system for a fuel cell stack includes a gas supply and a humidifier
including an outlet and an inlet connected to the gas supply. A fuel cell stack
includes an inlet that is connected to the outlet of the humidifier. A bypass line
and a valve bypass gas around the humidifier to control the humidity of gas entering
the fuel cell stack. The valve is located in the bypass line, between the gas supply
and the humidifier, or between the humidifier and the fuel cell stack. The valve
is a gas restriction valve, a throttle valve, or a directional valve. A humidity
sensor generates a humidity signal based on humidity of gas entering the fuel cell
stack. A controller connected to the humidity sensor and the valve controls the
valve based on the humidity signal. The inlet of the fuel cell stack is one of
a cathode flow line and an anode flow line of the fuel cell stack.
