Liquid anode (anolyte) and cathode (catholyte). Electrolytes flow through reaction cell and charge transfer occurs at a membrane. Vanadium-based chemistry is most mature, other chemistries being developed.
Benefits:
Power (reactor size) decoupled from Energy (tank size)
Limited impact of cycling on degradation
Higher fire safety than lithium ion
Challenges:
Lower energy density
Potential environmental spill risk
OK to poor efficiency observed to-date
Added system complexity with pumps etc.
Technology Variations:
Vanadium Redox
Zinc Bromine
Coupled iron-chrome
Zinc/Chlorine
Organic
Applications:
Energy shifting for renewable integration, T&D deferral, potential for longer duration
AC RTE Efficiency:
50-75%
Cycle Life:
20 years, >100,000 cycles (claimed)
Technology Readiness Level (TRL):
8 - deployed (for Vanadium redox). Early deployment / continued R&D.