The fuel cell start/stop switch on panel R1 for each fuel cell is used to initiate the start sequence or stop the fuel cell operation. When this switch is held in its momentary start position, the ECU connects the three-phase ac power to the coolant pump and hydrogen pump/water separator (allowing the coolant and the hydrogen-water vapor to circulate through these loops) and connects the dc power to the internal startup and sustaining heaters and the flow control bypass valve. The switch must be held in the start position until the coolant pump P talkback shows gray in approximately three to four seconds, which indicates that the coolant pump is functioning properly by creating a differential pressure across the pump. When the coolant pump P talkback indicates barberpole, it indicates the coolant pump is not running.
The ready for load talkback for each fuel cell will show gray after the 30-second timer times out and the stack-out temperature is above 187 F (which can be monitored on panel O2 in conjunction with the 1, 2, 3 switch located beneath the fuel cell stack out temp meter). This indicates that the fuel cell is up to the proper operating temperature and is ready for loads to be attached to it. It should not take longer than 25 minutes for the fuel cell to warm up and become fully operational, the actual time depending on the fuel cell's initial temperature. The ready for load indicator remains gray until the fuel cell start/stop switch for each fuel cell is placed to stop, the FC cntlr switch is placed to off , or the essential bus power is lost to the ECU.
The startup heater enable/inhibit switch on panel R12 for each fuel cell provides the crew control of the off/on status of the startup heaters during fuel cell startup. The inhibit position allows the startup heaters to remain off and would be used only when immediate power is required from a shutdown fuel cell.
Fuel cell 1, 2 or 3 dc voltage and current (amps) can be monitored on the dc volts and dc amps meters on panel F9, using the fuel cell volts/amp rotary switch to select a specific fuel cell.
The fuel cells will be on when the crew boards the vehicle, and the vehicle is powered by the fuel cells and load sharing with the ground support equipment power supplies. Just before lift-off (T minus three minutes and 30 seconds), the GSE is powered off and the fuel cells take over all of the vehicle's electrical loads. Indication of the switchover can be noted on the CRT display and the dc amps meter. The fuel cell current will increase to approximately 220 amps; the oxygen and hydrogen flow will increase to approximately 4 and 0.6 pound per hour, respectively; and the fuel cell stack temperature will increase slightly.
Fuel cell standby consists of removing the electrical loads but continuing operation of the fuel cell pumps, controls, instrumentation and valves, with the electrical power being supplied by the remaining fuel cells. A small amount of reactants is used to generate power for the fuel cell internal heaters.
Fuel cell shutdown, after standby, consists of stopping the coolant pump and hydrogen pump/water separator by positioning that fuel cell start/stop switch on panel R1 to the stop position. If the temperature in the fuel cell compartment beneath the payload bay is lower than 40 F, the fuel cell should be left in standby instead of being shut down to prevent it from freezing.
Each fuel cell power plant is 14 inches high, 15 inches wide and 40 inches long and weighs 255 pounds.
The voltage and current range of each is 2 kilowatts at 32.5 volts dc, 61.5 amps, to 12 kilowatts at 27.5 volts dc, 436 amps. Each fuel cell is capable of supplying 12 kilowatts peak and 7 kilowatts maximum continuous power. The three fuel cells are capable of a maximum continuous output of 21,000 watts with 15-minute peaks of 36,000 watts. The average power consumption of the orbiter is expected to be approximately 14,000 watts, or 14 kilowatts, leaving 7 kilowatts average available for payloads. Each fuel cell will be serviced between flights and reused until each accumulates 2,000 hours of on-line service.
