The coolant (that which is not made to bypass) exits the fuel cells to the fuel cell heat exchanger, where it transfers its excess heat to be dissipated through the ECLSS Freon-21 coolant loop systems in the midfuselage.
In addition to thermal conditioning by means of the coolant loop, the fuel cell has internal startup and sustaining heaters. The 2,400-watt startup heater is used only during startup to warm the fuel cell to its operational level. The 1,100-watt sustaining heaters normally are used during low power periods to maintain the fuel cells at their operational temperature.
Two 160-watt end-cell electrical heaters on each fuel cell power plant were used to maintain a uniform temperature throughout the fuel cell power section. As an operational improvement, the end-cell electrical heaters on each fuel cell power plant were deleted due to potential electrical failures and were replaced by fuel cell power plant coolant (F-40) passages. This permits waste heat from each fuel cell power plant to be used to maintain a uniform temperature profile for each fuel cell power plant.
The hydrogen pump and water separator of each fuel cell power plant were also improved. To minimize excessive hydrogen gas entrained in each fuel cell power plant's product water, modifications were made to the water pickup (pitot) system. The centrifugal force of high-velocity water flowing around the pitot tube's bends separates the hydrogen gas and water. Pitot pressure then expels the hydrogen gas into the hydrogen pump's inlet housing though a bleed orifice.
A current measurement detection system was added to monitor the hydrogen pump load for each fuel cell power plant. Excessive load could indicate improper water removal, which could lead to flooding of the fuel cell power plant and eventually render that power plant inoperative.
The start/sustaining heater system for each fuel cell power plant was also modified. The modification was required specifically for fuel cell power plant No. 1, mounted on the port, or left, side. The No. 1 fuel cell power plant start/sustaining heater system added heat to that fuel cell power plant's F-40 coolant loop system during the startup of the power plant. Because of its orientation, any entrained gas in the coolant could enter the heater and become trapped at the heater elements. This would result in overheating of the heater elements, which could vaporize the F-40 coolant, causing heater failure and extensive damage to the fuel cell power plant. The F-40 coolant loop flow system within the start/sustaining heater of each fuel cell power plant was modified to prevent a gas bubble from developing or being trapped at the heater elements, preventing the loss of the start/sustaining heater.
A stack inlet temperature measurement was added to each fuel cell power plant. The temperature measurement was added to the in-flight system to provide full visibility of the thermal conditions of each fuel cell power plant (similar to the existing stack exit and condenser exit temperatures of each fuel cell power plant).
The product water from all three fuel cell power plants flows to a single water relief control panel. The water can be directed from the single panel to the ECLSS potable water tank A or to the fuel cell power plant water relief nozzle. Normally, the water is directed to water tank A. In the event of a line rupture in the vicinity of the single water relief panel, water could spray on all three water relief panel lines, causing them to freeze and prevent fuel cell power plant water discharge.
The product water lines from all three fuel cell power plants were modified to incorporate a parallel (redundant) path of product water to ECLSS potable water tank B in the event of a freeze-up of the single water relief panel. In the event of the single water relief panel freeze-up, pressure would build up and relieve through the redundant paths to water tank B. Temperature sensors and a pressure sensor installed on each of the redundant water line paths transmit data via telemetry for ground monitoring.
