Vanadium flow batteries (VRB™) offer a set of compelling advantages over other energy storage technologies but face several technological challenges. They are not perhaps the best fit for certain applications being rather bulky, but for most commercial and industrial and some utility scale applications are ideal.
Economically the Vanadium flow battery almost certainly has the lowest LCOE (Levelized Cost of Energy) of all electrochemical energy storage given that the electrolyte lasts indefinitely and retains a very high residual value, has low O&M costs and is reasonably efficient at >70% RTE (Round Trip Efficiency) at any DOD (Depth of Discharge), as well as having unlimited cycle capability.
The greatest challenge that has faced the adoption of this technology over the past three decades has been in development of the cell stack (Note: Lithium has had a longer development history since its origins with Dr John Goodenough in the early 1980’s). The cell stack comprises of multiple individual cells sandwiched together to form a stack. This is an electrical series configuration but incorporates a shunt or parallel electrolyte feed component. Unlike all other batteries, there is therefore no need to balance the individual cells given that the electrolyte is common to ALL cells, so control is very simple. If you limit the highest allowable voltage of 1.6 Volts for example, which is measured on line continuously, then the battery remains stable and not overcharged.
Each individual cell contains two graphite or graphite impregnated plastic electrodes, which are electrically conductive but which are not porous to electrolyte. In a cell stack the electrode of one cell forms the opposite polarity electrode for the next cell and are hence called bipoles.
What has proven to be the greatest challenge is that there is an absolute stringent requirement to maintain a minimum flow rate across the entire individual cell surface areas. This is in order to ensure that in the REDOX reaction there is never a shortage of the most critically important V4+ species. If this occurs the reaction results in an attack of the carbon in the positive electrode. This results in the development of holes in the bipole material and failure of the cell stack. Several factors in addition to the even flow distribution exacerbate this degradation phenomenon, including shunt currents, which are a function of the number of cells connected in series.
Having said this, a properly designed cell stack with sufficient and even flow distribution, and limited shunt currents last for 10 years or more as evidenced by many installed units over the last 15 years. There are many VRB systems in operation for at least a decade without any performance degradation while cycling multiple times per day at varying DOD in applications such as wind and PV power smoothing. Tens of thousands of full and partial cycles have occurred so the technology is well proven.
The question is can this type of performance be attained in a cost effective and repeatable manner using commercially available materials.
eChemion has developed a material treatment process that takes commercially available bipole materials and treats them in such a way as to limit chemical attack and degradation, hence extending the life of cell stacks. It is cost effective and can be adapted to manufacturing and cell stack assembly processes.