TABLE OF CONTENTS A regenerative fuel cell (RFC) is a device that can operate alternately as an electrolyzer and as a fuel cell. The electrolyzer and fuel cell functions can be integrated into a single stack of cells (unitized), or two separate (discrete) stacks may be used, one as a fuel cell and one as an electrolyzer. In an electrolysis mode, the stack generates hydrogen (and oxygen) from electricity, and in a fuel cell mode it generates electricity from stored hydrogen (and oxygen, possibly extracted from air). It is therefore an energy storage device (similar to a rechargeable battery) with hydrogen as a storage medium. The regenerative fuel cell has the highest achievable specific energy of all rechargeable energy storage options. Theoretical specific energy of H 2/O 2fuel cell systems is 3600Wh/kg (accounting only for the mass of reactants and assuming theoretical fuel cell efficiency), but when the masses of storage tanks and the fuel cell system are taken into account with practical fuel cell efficiencies, specific energy in excess of 400Wh/kg may be achieved [ [39]]. As such, it may be used in applications where relatively large amounts of electricity need to be stored, such as:
energy storage for remote off-grid power sources;
in conjunction with highly intermittent renewable energy sources (solar and wind);
emergency or backup power generation (as discussed in the section Backup Power);
unmanned underwater vehicles;
high-altitude, long-endurance solar rechargeable aircraft;
hybrid energy storage/propulsion systems for spacecraft.
Regenerative fuel...
