Oxidant and Fuel Flow Metering

The basic controls of one cell in a fuel cell stack. 

Fuel flow control in low-temperature cells is relatively simple. The control valves used are servomotor-driven quarter-turn valves, and the flow sensors can be the types used on gas furnaces, such as thermal dispersion type mass flow meters. 

The previously used flow sensor option in research fuel cells were the integrated circuit (IC)-type differential sensors, which were limited by their low tolerance for water contamination. This was a serious limitation in reforming-type fuel cell applications, because the presence of water is essential for the operation of the system, and no protection against water contamination was provided in the smaller and cheaper IC sensors. 

In spacecraft and similar closed-circuit fuel cell applications, partial pressure reductions are not a problem, other than for the production of waste-water. Therefore, constant supply pressure can be used, which greatly simplifies the control requirements, as the system becomes self-regulating on the demand side. In such applications, the waste can simply be blown down to a reservoir, based on time and current draw. 

In high-temperature cells, special valve designs are required to handle the fuel gas, which is up to 1000°C. These valves are used to balance the gas flows between stacks and for stack isolation to permit continued operation when some of the stacks fail. High-temperature servomotor drives (Mitsubishi and Hitachi) can be used on these valves. 

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