The Best Redox Flow Battery Tech

Batteries based on vanadium or zinc bromide represent the cutting edge of redox flow storage tech, an international research team has claimed. They have identified challenges and opportunities for about a dozen redox flow storage technologies, while providing estimates of their current and projected levelized costs of storage.

Researchers from the University of Burgos in Spain, Italy’s University of Padua, Finland’s Aalto University, the University of West Bohemia Pilsen in Czechia, and the Basque Research and Technology Alliance (BRTA) have conducted a comprehensive analysis of all redox flow battery (RFB) and hybrid RFP technologies. They argue that these technologies are promising alternatives to lithium-ion technologies in stationary storage applications.

Redox flow storage has advantages over other storage technologies, such as flexible modular design/operation, scalability, moderate maintenance costs, long-life cycling, high round-trip efficiency (RTE), depth of discharge (DoD), fast responsiveness, and negligible environmental impacts, the researchers said. Most of these positive factors are related to the technology’s unique ability to decouple energy and power.

“Power and energy density limitations in comparison to other technologies such as lithium-ion batteries are generally overcome by the more cost-effective scalability,”  the group said.

Vanadium redox flow batteries (VRFBs) and zinc-bromine redox flow batteries (ZBFBs) – the most representative kinds of hybrid flow batteries – are the real state of the art, the researchers claimed. However, the road to bringing them to commercial success and applicability is still a “long way to go,” they added.

Vanadium devices

The main barrier for VRFBs is low availability and the high cost of vanadium, as well as the need for a bidirectional DC/AC inverter to interface the grid. However, recent progress has been achieved with the electrolyte composition, membrane, and electrode, as well as improvements for efficiency and power and current densities.

Another big advantage with these technologies is the ability “to respond immediately to surge power demand from the grid, coping with power quality grid services such as sag compensation and frequency regulation,” the researchers said. The levelized cost of storage (LCOS) of such batteries has been estimated at €0.18 kW h-1 cycle-1.

“Present research aims at electrolytes capable of increased -active material concentrations and energy density, membranes with higher proton conductivity and lower ions crossover, porous electrodes capable of better hydraulic performance,” the scientists said. “Nevertheless, major issues remain. Low values of energy density make a VRFB system much bulkier than an equivalent Li-ion system.”

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