ABSTRACT

Figure 7.1 Functional schematic of a regenerative fuel cell system.When the regenerative fuel cell system is producing power, it is operating in the discharge cycle. During this discharge cycle, the fuel cell subsystem is active. In a fuel cell, hydrogen is fed to the anode and oxygen is fed to the cathode, and as power is drawn from the fuel cell, hydrogen is oxidized and oxygen is reduced to water (“Discharge Cycle,” Fig. 7.1). The byproduct of a hydrogen/oxygen fuel cell is water, which in this case is stored for later electrolysis and regeneration back to these gases. Hydrogen/oxygen fuel cells can operate in either a flow-through or non-flow-through mode [2]. Non-flow-through fuel cells are also referred to as “dead-ended,” which means that the reactants are not allowed to sweep through the fuel cell. In a terrestrial fuel cell system, the hydrogen feed (anode compartment) is typically operated dead-ended. The oxidant

feed (cathode compartment), which in a terrestrial system typically operates on air, is allowed to flow through the cell. When the oxidant is allowed to flow through the cathode, it will sweep product water out of the cathode compartment. A flow-through fuel cell system would require components such as gas-liquid separators and condensers for water handling, which can increase system complexity [2]. Non-flow-through fuel cell systems are being developed for space use; these systems use passive components to remove product water from the fuel cell reactions [2]. Some advantages of using non-flow-through fuel cells include: passive balance of plant components, operation at stoichiometric feed rates of reactants, and minimal system complexity [2].Several options for the development of regenerative fuel cell systems have been considered, including unitized and discrete systems. In a discrete regenerative fuel cell system, the fuel cell and electrolyzer stacks (and their associated MEAs) are separate units, each performing a single function (power generation or reactant production). The fuel cell stack is the power-producing unit and the electrolysis stack is the reactant-producing unit of a regenerative fuel cell system. In a unitized regenerative fuel cell system, the fuel cell and electrolysis stacks are combined into one unit and as such must serve both functions of producing power and regeneration of reactants. Unitized regenerative fuel cells require MEAs that can sustain both the fuel cell and electrolysis reactions. An advantage of a unitized regenerative fuel cell system is that it can be made very compact. An advantage of a discrete regenerative fuel cell system is that the individual fuel cell stack and the electrolyzer stack can each be optimized to maximize system efficiency. Discrete regenerative fuel cell systems are being pursued by NASA [3]. 7.3 Space-Rated Regenerative Fuel Cell Systems

The round-trip (charge/discharge cycle) efficiency goal for a regenerative fuel cell system developed by the NASA Energy Storage Program is 54% with a success threshold of 43% [3]. The regenerative fuel cell systems efficiencies include 5% and 10% allocations for parasitic inefficiencies for the program goal and threshold values, respectively. To meet the program threshold values, the fuel cell and electrolyzer MEAs would need to operate at 0.9 and 1.46 V, respectively, at an operating current of 200 mA/cm2.