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Control

Pressure control in a fuel cell



Task

Electrical power is obtained from hydrogen and oxygen in a fuel cell. The two gases have to be controlled depending on current demand and the fuel cell pressure.

Solution

The fuel cell requires hydrogen and oxygen in order to produce energy. Oxygen is obtained from the air. More or less hydrogen is consumed depending on current demand. This quantity is measured by means of a mass flow meter at the inlet to the cell. The air quantity must be readjusted on the basis of the stoichiometric ratio of the process equation. This is made possible by a mass flow controller at the oxygen or air inlet to the cell whose set-point value results from the actual hydrogen quantity. Both gases are then available in the right ratio in the fuel cell. When the cell is in idle state, a minimum hydrogen flow must always be guaranteed, which is implemented by an additional mass flow controller at the outlet of the cell. Two closed-loop pressure control systems with proportional valves and compact pressure controllers are subordinate to these closed-loop flowrate control systems. This pressure control system maintains the pressures for hydrogen and air at the same level, ensuring that the cell's separating membrane is not destroyed and that the cell is maintained at its operating pressure. The pressure control for hydrogen is performed at the inlet to the cell while the air is controlled at the outlet.