Flow Battery

The vanadium redox flow battery is a promising technology for grid scale energy storage. The tanks of reactants react through a membrane and charge is added or removed as the catholyte or anolyte are circulated. The large capacity can be used for load balancing on grids and for storing energy from intermittent sources such as wind and photovoltaics. The UET flow battery is the size of a shipping container and has 600kW power and 2.2MWh in capacity.

UniEnergy Flow Battery

UniEnergy Flow Battery Source: http://www.uetechnologies.com/




The Chemistry

A flow battery consists of two tanks filled with chemicals in different oxidation states that react through a membrane. Charge is added or removed through two electrodes. Source: https://commons.wikimedia.org/wiki/File:Redox_Flow_Battery_English.png

Experimental Flow battery membrane at PNNL

Experimental Flow battery membrane at PNNL Source: CEI

One tank contains V5/4  mixture and the other contains a V2+/3+ mixture.

During discharge in the negative half cell V2+ is oxidized to V3+ and the electron that is freed travels to the external circuit to perform work. At the same time in the positive half cell V5+ is reduced to V4+ in the form of VO2+.  H+ ions pass through the membrane to maintain charge balance.


VO2+  +H2O    <->  VO2+ + 2H+ +e


V3+ + e  <-> V2+

During charging each reaction is reversed. The charge state of the battery is described by the ratio of the species in each oxidation state in the two tanks. Power and energy are decoupled so either can be optimized. Power can be increased by changing the number or area of the membrane stacks so that more reactants can participate. Energy is controlled by the size of the storage tanks.



Experimental Flow Battery at PNNL

Experimental Flow Battery at PNNL Source: CEI


UniEnergy Technology



Simplied animation



“Providing clean energy to the inhabitants of our planet is a major challenge to future generations. The University of Washington is to be congratulated for establishing an Institute where faculty and students can work together to tackle the difficult global challenge of energy sustainability.”
– Mildred Dresselhaus, Professor of Physics and Electrical Engineering, Emerita and Institute Professor, Massachusetts Institute of Technology
“Energy competition is opening up in a variety of ways, the push for carbon control will continue, and the rate of technology advancement is exponential. All the things I’ve seen at the CEI are just perfect for the way we see things going in energy. You guys are at the cutting edge. We’re counting on you.”
– Ronald Litzinger, President, Edison Energy