Vanadium redox battery
The Vanadium redox flow battery (VRB) constitutes a promising renewable energy storage technology that may boost a powerful rise on environmentally friendly energy production like that generated by photovoltaic panels or eolic turbines. Apart from the extremely high response times of this battery, which make them ideal for energy leveling, the absence of polluting metals allows this battery to be that with the lowest ecological impact of all storage technologies based on batteries.
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This battery works with only one electroactive element, vanadium, thanks to the fact that vanadium can exist in 4 different oxidation states forming water soluble and stable species: vanadyl (VO2+), dioxovanadium (V) (VO2+), vanadium (II) and vanadium (III). Two electrolytes are used, each one containing a redox-pair based on the above mentioned vanadium species dissolved in sulfuric acid. These two electrolytes are stored in tanks, from which they are pumped through PVC pipes to a cell stack containing graphite felt electrodes and proton exchange membranes.
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When the battery is being discharged, the following half-reactions take place:
V2+ → V3+ + 1e-
VO2+ + 2H+ + 1e- → VO2+ + H2O
global: V2+ + VO2+ + 2H+ → V3+ + VO2+ + H2O
VO2+ + 2H+ + 1e- → VO2+ + H2O
global: V2+ + VO2+ + 2H+ → V3+ + VO2+ + H2O
Each cell provides 1.26 V at 25 °C. The simplified scheme only shows a cell, but in reality there are cell stacks, consisting of many cells, each of which containing two half-cells separated by the proton exchange membrane.
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When the vanadium redox flow battery is being charged, the electrochemical processes take place the other way round:
V3+ + 1e- → V2+
VO2+ + H2O → VO2+ + 2H+ + 1e-
global: V3+ + VO2+ + H2O → V2+ + VO2+ + 2H+
VO2+ + H2O → VO2+ + 2H+ + 1e-
global: V3+ + VO2+ + H2O → V2+ + VO2+ + 2H+
Advantages of the vanadium redox flow battery:
- Virtually unlimited capacity. The capacity can be increased simply by using larger and larger storage tanks, as the amount of stored energy depends only on the tank size.
- It can be left completely discharged for a long time with no negative effects.
- It can be quickly recharged by replacing the electrolyte if no power is available to charge it.
- If the electrolytes are accidentaly mixed, there are no serious consequences of cross-contamination and the battery suffers no permanent damage.
- Very little environmental impact and no health risks.
- No memory effect and indefinite life-span.
Disadvantages of the vanadium redox flow battery:
- Vanadium redox flow batteries have still a lower energy density (25 W h Kg-1) if compared with other rechargeable batteries, such as lead-acid (30 - 40 W h Kg-1) or lithium-ion battery (80 - 200 W h Kg-1).
- Higher complexity of the system in comparison with standard storage batteries.
Applications of vanadium redox flow batteries:
- Large power applications helping to level the production of highly variable generation sources such as eolic or solar power.
- UPC (uninterruptible power supply) systems. They can be used to replace lead-acid batteries and even diesel generators.
This is a short video on Vanadium Redox Battery and its main developer, Professor Maria Skyllas-Kazacos, from Australia.
Vanadium redox batteries are being used in a 275 kW output balancer in Tomari Wind Hills, Hokkaido, Japan.
The Canadian company Prudent Energy Inc has developed this technology to commercial status.
