Energy Storage Capacity Utilization in Redox Flow Batteries

Redox flow batteries (RFBs) are predicted to be a major contributing technology in the need for four-to-ten-hour energy storage. Other energy storage technologies, such as lithium-ion batteries, provide well-documented performance indicators such as efficiency and especially energy storage capacity utilization. While redox flow batteries are compared similarly based on cost and round-trip efficiency, they often neglect a comparison of their operational energy storage capacity utilization. The breakdown of operational losses that represent the current state of operation for RFBs are pivotal to further compare the development of these technologies to the standard of other electrochemical storage devices. Quantifying the current state-of-the-art in relation to the theoretical potential of RFBs allows us to assess the extent of development required to enhance their effectiveness in contributing to future energy storage goals. In this work, we set out to depict the operational energy storage density (W-h L -1 ) of various redox flow battery technologies based on consideration from experimental parameters and results from literature as key inputs. After reviewing five RFBs including the all-vanadium RFB, Hydrogen-Bromine RFB, All-Iron RFB, Zinc-Bromine RFB, and the Iron-Chromium RFB, results indicate that the VRFB is the most developed technology attaining around 60% of its theoretical energy storage density. While the Zinc-Bromine RFB has approximately the largest theoretical energy storage density (due to its high electroactive species concentration and standard potential difference), it is currently only able to operate at about a lowly 15% of it, dependent on its operational current density. We also assess the volumetric footprint for each RFB electrolyte system by considering a 6 MWh (6 MW, 1 hr) system. This assessment considers how large the electrolyte component of the RFB would be for operation. The Hydrogen-Bromine RFB would require electrolyte tanks to be more than 1600 m 3 based on operational parameters, but if it were able to operate at 100% of its capability, the electrolyte tanks would shrink to be about 400 m 3 .