Method for reducing fuel cell output voltage to permit low power operation

In a method for permitting the lowering of the power output of a plurality of fuel cells when the average cell voltage at a present actual power output is at the maximum permitted, wherein the fuel cells are connected electrically in series and each cell includes an anode and a cathode wherein air as the oxidant is fed to the cathodes via a common manifold and hydrogen as the fuel is fed to the anodes and both the anode and cathode exhaust streams are continuously vented, and wherein means is provided to recycle a portion of the cathode exhaust stream to the cathodes, the steps of:

• (1) comparing the desired power output to the actual power output and, if the desired power output is less than the actual power output, increasing the cathode exhaust stream recycle rate to lower the average cell voltage;
• (2) decreasing the actual power output until the desired power output or the maximum permitted voltage is reached, whichever occurs first;
• (3) comparing again the desired power output to the actual power output and, if the desired power output is still less than the actual power output, increasing the oxygen utilization of the cells by decreasing the rate of air flow to the cells to lower the average cell voltage;
• and
• (4) repeating steps (2) and (3) until either the actual power output equals the desired power output or until a maximum allowable oxygen utilization is reached.