When the battery charges, sodium bromide sheds a sodium ion and becomes sodium tribromide. The positively charged ion passes through the membrane, combines with sodium polysulfide and forms sodium sulfide. During discharge, the process reverses.
A flow-cell battery being built by Regenesys Technologies, U.K. (www.regenesys.com), and the Tennessee Valley Authority could provide hours of backup power to the Columbus Air Force based in Mississippi. After that, the $25 million battery might just revolutionize the nation's power grid in which electricity being produced must be immediately used or lost. Flow batteries, unlike standard and rechargeable batteries, can charge and discharge without degrading. And unlike regular batteries, electrolytes for the cathode and anode are stored outside the battery in storage tanks. Electrolytes are pumped passed each other, separated by a semipermeable ion-exchange membrane, much like in a fuel cell. The membrane only passes positively charged ions.
The Columbus installation will use 24,000 cells, with up to 200 in a single module. The Air Force plans on keeping the battery charged, then using it during power outages caused by storms and tornadoes. The battery will output 65% of the energy it is fed. The rest goes to operating the pumps or dissipates as heat.