Improvements in freezing and sub-freezing startup time for an
electrochemical fuel cell system may be observed by having a passive
coolant microloop using two gear pump heads. During a cold start, a stack
valve is initially fully closed and causes all system coolant arriving at
the module to run through a driving gear pump head. This causes a driven
gear pump head to recirculate coolant inside the module. In an
embodiment, the stack valve is a thermostatic valve that begins to open
as the coolant begins to warm up. This allows a small bleed of system
coolant into the module and decreases the percentage of microloop
recirculation. When operating temperature is reached, the thermostatic
valve is fully open and system coolant inlet pressure on the driving gear
pump head counteracts the pressure on the driven gear pump head, thus
stopping recirculation.