Water and electricity are the products of the chemical reaction of oxygen and hydrogen that takes place in the fuel cells. The water must be removed or the cells will become saturated with water, decreasing reaction efficiency. With an operating load of about 7 kilowatts, it takes only a few minutes to flood the fuel cell with produced water, thus effectively halting power generation. Hydrogen is pumped through the stack, reacting with oxygen and picking up and removing water vapor on the way. After being condensed, the liquid water is separated from the hydrogen by the hydrogen pump/water separator and discharged from the fuel cell to be stored in the ECLSS potable water storage tanks.
If the water tanks are full or there is line blockage, the water relief valves open at 45 psia to allow the water to vent overboard through the water relief line and nozzle. Check valves prevent water tanks from discharging through an open relief valve. An alternate water delivery path is also available to deliver water to the ECLSS tanks if the primary path is lost.
For redundancy, there are two thermostatically activated heaters wrapped around the discharge and relief lines to prevent blockage caused by the formation of ice in the lines. Two switches on panel R12, fuel cell H 2 O line htr and H2O relief htr , provide the flight crew with the capability to select either auto A or auto B for the fuel cell water discharge line heaters and the water relief line and vent heaters, respectively.
Thermostatically controlled heaters will maintain the water line temperature above 53 F, when required. The normal temperature of product water is approximately 140 to 150 F. The thermostatically controlled heaters maintain the water relief valve's temperature when in use between 70 to 100 F. Temperature sensors located on the fuel cell water discharge line, relief valve, relief line and vent nozzle are displayed on the CRT.
If the potassium hydroxide electrolyte in the fuel cell migrates into the product water, a pH sensor located downstream of the hydrogen pump/water separator will sense the presence of the electrolyte, and the crew will be alerted by an SM alert and display on the CRT.
During normal fuel cell operation, the reactants are present in a closed-loop system and are 100 percent consumed in the production of electricity. Any inert gases or other contaminants will accumulate in and around the porous electrodes in the cells and reduce the reaction efficiency and electrical load support capability. Purging, therefore, is required at least twice daily to cleanse the cells. When a purge is initiated by opening the purge valves, the oxygen and hydrogen systems become open-loop systems; and increased flows allow the reactants to circulate through the stack, pick up the contaminants and blow them out overboard through the purge lines and vents. Electrical power is produced throughout the purge sequence, although no more than 10 kilowatts should be required from a fuel cell being purged because of the increased reactant flow and preheater limitations.
Fuel cell purge can be activated automatically or manually by the use of fuel cell switches on panel R12. In the automatic mode, the fuel cell purge heater switch is positioned to GPC . The purge line heaters are turned on to heat the purge lines to ensure that the reactants will not freeze in the lines. The hydrogen reactant is the more likely to freeze because it is saturated with water vapor. Depending on the orbit trajectory and vehicle orientation, the heaters may require 27 minutes to heat the lines to the required temperatures. The fuel cell current is checked to ensure a load of less than 350 amps, due to limitations on the hydrogen and oxygen preheaters in the fuel cells. As the current output of the fuel cell increases, the reactant flow rates increase, and the preheaters raise the temperature of the reactants to a minimum of minus 40 F in order to prevent the seals in the dual gas regulator from freezing.
The purge lines from all three fuel cells are manifolded together downstream of their purge valves and associated check valves. The line leading to the purge outlet is sized to permit unrestricted flow from only one fuel cell at a time. If purging of more than one cell at a time is attempted, pressure could build in the purge outlet line and cause a decrease in the flow rate through the individual cells, which would result in an inefficient purge.
