TABLE OF CONTENTS A system is defined as a group of units, objects, or items, so combined as to form a whole and to operate in unison. In case of a fuel cell, the system includes all the necessary components needed to operate a fuel cell stack and deliver electrical current. A fuel cell stack is obviously the heart of a fuel cell system; however, without the supporting equipment the stack itself would not be very useful. The fuel cell system typically involves the following subsystems:
oxidant supply (oxygen or air),
fuel supply (hydrogen or hydrogen rich gas),
heat management,
water management,
power conditioning,
instrumentation and controls.
Depending on the available or chosen fuel and oxidant, the fuel cell systems may be categorized as:
hydrogen oxygen systems,
hydrogen air systems,
reformate air systems.
Because of technical difficulties, the added size and weight of oxygen storage, and related safety concerns, pure oxygen systems are typically used only in applications where air is not available, such as in submarines and space applications.
Stored oxygen is already under pressure, so the supply of oxygen to a fuel cell involves a pressure regulator to reduce the pressure to the fuel cell operating pressure. Oxygen should be supplied in excess of stoichiometrically required, typically with stoichiometric ratios of 1.2 to 1.3 ( i.e., 20% to 30% excess). The reason for this is that the excess oxygen has to carry the product water from the cell. Excess oxygen may be vented at the fuel...
